研究者データベース

青木 芳尚(アオキ ヨシタカ)
工学研究院 応用化学部門 機能材料化学分野
准教授

基本情報

所属

  • 工学研究院 応用化学部門 機能材料化学分野

職名

  • 准教授

学位

  • 博士(工学)(北海道大学)

論文上での記載著者名

  • Yoshitaka Aoki

ホームページURL

科研費研究者番号

  • 50360475

J-Global ID

研究キーワード

  • Solid Oxide fuel cells   Solid oxide electrolysis cells   電気化学   固体イオニクス   

研究分野

  • ナノテク・材料 / 機能物性化学 / エネルギー材料

職歴

  • 2010年06月 - 現在 北海道大学 准教授
  • 2013年10月 - 2017年03月 JST さきがけ「再生可能エネルギーからのエネルギーキャリアの製造とその利用のための革新的基盤技術の創出」 さきがけ研究員
  • 2011年01月 - 2012年06月 RWTH-Aachen 博士研究員
  • 2008年07月 - 2010年05月 北海道大学 助教
  • 2002年04月 - 2008年07月 理化学研究所 博士研究員
  • 2003年07月 - 2005年06月 Massacusetts University 博士研究員

学歴

  • 1997年04月 - 2002年03月   北海道大学大学院
  • 1993年04月 - 1997年03月   北海道大学

所属学協会

  • ヨーロッパ材料学会   アメリカ電気化学会   固体イオニクス学会   日本電気化学会   

研究活動情報

論文

  • Yuki Sato, Yoshitaka Aoki, Kentaro Takase, Hisao Kiuchi, Damian Kowalski, Hiroki Habazaki
    ACS Applied Energy Materials 3 6 5269 - 5276 2020年06月22日 [査読有り][通常論文]
  • Yuki Sato, Damian Kowalski, Yoshitaka Aoki, Hiroki Habazaki
    Applied Catalysis A: General 597 117555 - 117555 2020年05月 [査読有り][通常論文]
  • Nan Sheng, Ruijie Zhu, Kaixin Dong, Takahiro Nomura, Chunyu Zhu, Yoshitaka Aoki, Hiroki Habazaki, Tomohiro Akiyama
    Journal of Materials Chemistry A 2019 7 4934 - 4940 2019年09月 [査読有り][通常論文]
  • Cheong Kim, Chunyu Zhu, Yoshitaka Aoki, Hiroki Habazaki
    Electrochimica Acta 314 173 - 187 2019年08月10日 [査読有り][通常論文]
     
    © 2019 Elsevier Ltd Biomass-derived porous carbons with very high specific surface area (SSA) and heteroatom-doping are important for obtaining high performance supercapacitor. In this study, we report a simple and straightforward strategy, which involves the efficient and exothermic pyrolysis of Mg/K/Mg·K-nitrate-urea-cellulose mixture with subsequent high temperature carbonization and washing treatment, to produce N-doped porous carbon with tunable pore structure. The vigorous exothermic pyrolysis of Mg/K/Mg·K-nitrate-urea-cellulose induces the formation of large macropores. The subsequent high temperature carbonization and washing treatments remove Mg,K compounds and facilitate the creation of numerous micro and mesopores. The pore size distribution of the obtained carbon is quite dependent on the Mg·K ratio in the precursors. In this manner, the three-dimensional hierarchical porous carbon with SSA larger than 2700 m2 g−1 is obtained. The obtained porous carbon as the electrodes for supercapacitor in two-electrode measurement shows high specific capacitance (279 F g−1 at 1 A g−1 in a 6 M KOH electrolyte), excellent cycling stability (larger than 89% capacitance retention after 10,000 cycles at 2 A g−1) and good rate capability (235 F g−1 at even 30 A g−1). These results indicate that biomass cellulose-derived heteroatom-doped hierarchical porous carbon is a promising material for supercapacitor.
  • Fadillah Laras, Takase Kentaro, Kobayashi Hikaru, Turczyniak-Surdacka Sylwia, Strawski Marcin, Kowalski Damian, Zhu Chunyu, Aoki Yoshitaka, Habazaki Hiroki
    ELECTROCHIMICA ACTA 309 274 - 282 2019年06月20日 [査読有り][通常論文]
  • Hajime Toriumi, Taisei Kobayashi, Satoshi Hinokuma, Toshiaki Ina, Takashi Nakamura, Koji Amezawa, Chunyu Zhu, Hiroki Habasaki, Yoshitaka Aoki
    Inorganic Chemistry Frontiers 6 1587 - 1597 2019年05月 [査読有り][通常論文]
  • Cheong Kim, Chunyu Zhu, Yoshitaka Aoki, Hiroki Habazaki
    Industrial and Engineering Chemistry Research 58 8 3047 - 3059 2019年02月27日 [査読有り][通常論文]
     
    © 2019 American Chemical Society. In this work, high-value-added N-doped hierarchical porous carbon (NHPC) for oxygen reduction reaction (ORR) electrocatalysis was prepared using biomass cellulose as the raw material. The pyrolysis of cellulose is accelerated by a redox combustion reaction of magnesium nitrate-carbohydrates (urea and cellulose) as absorbed in cellulose fibers, which endows doping with nitrogen, exfoliates the cellulose to highly porous particles, and creates numerous pores simultaneously. After being further carbonized at high temperature and washed with acid, NHPCs were produced that have hierarchical porous structure and large specific surface area. These features are beneficial to the ORR. The influence of four preparation parameters, including species of magnesium salt, carbonization temperature, urea amount, and magnesium salt amount, on the porous characteristics and ORR performance is comprehensively investigated.
  • Atsushi Kasuga, Akira Koyama, Katsutoshi Nakayama, Damian Kowalski, Chunyu Zhu, Yoshitaka Aoki, Hiroki Habazaki
    ISIJ International 59 2 345 - 350 2019年02月01日 [査読有り][通常論文]
     
    © 2019 ISIJ. Stainless steels is practically important corrosion-resistant metallic materials, and additional surface functionalities including self-cleaning, anti-fouling, anti-ice and snow sticking and fluid drag reduction by the introduction of superhydrophobic and superoleophobic surfaces are of recent interest. Here, we report the micro-/nano-hierarchical roughening of type 304 stainless steel surface by chemical and electrochemical etching and anodizing. Chemical etching in HCl + FeCl 3 aqueous solution containing a surfactant introduces surface roughness of several tens micrometers scale and the electrochemical etching in HCl + HNO 3 aqueous solution produces a number of etch pits of ~1 μm in size. Then, a porous anodic layer of the pore size of ~20 nm is formed on the etched surface by anodizing in ethylene glycol electrolyte containing 0.1 mol dm −3 NH 4 F and 0.1 mol dm −3 H 2 O. After fluoroalkylsilane (FAS) coating of the hierarchically rough surface to reduce the surface energy, the surface becomes superhydrophobic and superoleophobic; the advanced contact angle for hexadecane (surface tension of 27.6 mN m −1 ) is ~160° and the contact angle hysteresis is less than 10°. Since the FAS-coated flat surface is oleophilic, so that such hierarchically rough surface is of significant importance to achieve the superoleophobicity even for low surface tension liquids.
  • Naohito Yamada, Damian Kowalski, Akira Koyama, Chunyu Zhu, Yoshitaka Aoki, Hiroki Habazaki
    RSC Advances 9 7 3726 - 3733 2019年01月01日 [査読有り][通常論文]
     
    © The Royal Society of Chemistry. In this study, platelet-type carbon nanofibers prepared by the liquid phase carbonization of polymers in the pores of a porous anodic alumina template were used to prepare the Co 3 O 4 /carbon electrocatalysts. For comparison, Co 3 O 4 nanoparticles were also deposited on multiwall carbon nanotubes (MWCNTs). Both the nitrogen-free platelet-type carbon nanofibers (pCNFs) and the nitrogen-containing analogue (N-pCNFs) exhibited better dispersion and higher amount of deposited Co 3 O 4 nanoparticles compared to the MWCNTs. In addition, many individual Co 3 O 4 nanoparticles were deposited separately on pCNF and N-pCNF, whereas aggregated deposition was commonplace on MWCNTs. The results indicated that the side wall of the pCNFs, which consisted of carbon edge planes, was the preferential nucleation site of Co 3 O 4 nanoparticles rather than the basal planes of carbon that predominated the surface of the MWCNTs. The oxygen reduction reaction (ORR) activity of the Co 3 O 4 /pCNF composite in 0.1 mol dm −3 KOH solution was better than that of Co 3 O 4 /MWCNTs. The N-pCNF further enhanced the ORR activity of the Co 3 O 4 /pCNFs even though the dispersion and supported amount of Co 3 O 4 nanoparticles were negligibly affected by the presence of the nitrogen species. Synergistic interactions of the Co 3 O 4 nanoparticles with N-doped CNFs contributed to the increased ORR activity.
  • Chunyu Zhu, Manami Takata, Yoshitaka Aoki, Hiroki Habazaki
    Chemical Engineering Journal 350 278 - 289 2018年10月15日 [査読有り][通常論文]
     
    Porous carbon has attracted great interest because of its distinctive structure and superior properties for designing electrochemical energy storage devices. In this work, we report a novel and facile method to fabricate nitrogen-doped porous carbon (NPC) with designed hierarchical multi-porous structure. The NPC is prepared using a facile and scalable MgO template method as-mediated by a magnesium nitrate-glycine solution combustion synthesis (SCS), in which the preparation conditions of calcination temperature and glycine-nitrate ratio are carefully studied. The NPC shows a high specific surface area up to 1958 m2 g−1 and contains hierarchical multi-pores of interconnected macro-, meso- and micro-pores. When applied as ORR electrocatalyst, the NPC-n2-1000 sample shows the highest activity, good durability and high tolerance against methanol, which is among the best reported carbon-base catalysts. The NPC samples also shows great potential as the active electrode material for supercapacitors. NPC-n2-900 sample exhibits a high specific capacitance of 232 F g−1 at 1 A g−1 in 6 M KOH electrolyte, a superior rate capability (205 F g−1 at 10 A g−1) and a good cycling performance. The present work demonstrates that the combination of glycine-nitrate SCS and template introduction in one-step can turn the carbohydrate fuel to advanced porous carbon with prospective applications in high-performance energy storage devices.
  • La0.7Sr0.3Mn1−xNixO3−δ electrocatalysts for the four-electron oxygen reduction reaction in concentrated alkaline media
    Y. Aoki, E. Tsuji, T. Motohashi, D. Kowalski, H. Habazaki
    Journal of the Physical Chemistry C 122 39 22301 - 22308 2018年10月 [査読有り][通常論文]
  • Jinhui Cao, Chunyu Zhu, Yoshitaka Aoki, Hiroki Habazaki
    ACS Sustainable Chemistry and Engineering 6 6 7292 - 7303 2018年06月04日 [査読有り][通常論文]
     
    The development of green and clean synthetic techniques to produce carbon materials for energy storage and conversion applications has motivated researchers to use sustainable biomass. In this study, hierarchical porous carbon (HPC) with very high specific surface area and controlled porosity is synthesized by a novel and facile method, which employs an exothermic pyrolysis process of starch-magnesium nitrate raw materials with subsequent high temperature thermal treatment and acid washing. The biomass starch acts as both a reductant and carbon source, while magnesium nitrate is an oxidant and provides MgO template as pore creator. The vigorous exothermic pyrolysis of starch-magnesium nitrate mixture introduces MgO@C precursor with a highly 3D porous network containing meso- and macropores. After MgO template is removed, plenty of micro- and mesopores are further created. Experimental parameters including calcination temperature, starch-nitrate ratio, and magnesium salt species are comprehensively evaluated. The HPC shows a very large specific surface area up to about 2300 m2 g-1 and a hierarchical porous architecture composed of interconnected micro-, meso- and macropores. As an electrode material for supercapacitors, the HPC exhibits high specific capacitance (229 F g-1 at 1 A g-1 in a 6 M KOH electrolyte), good rate capability (211 F g-1 at even 10 A g-1) and outstanding cycling stability (94% capacitance retention after 10.000 cycles at 2 A g-1). The superior electrochemical performance of the HPC stems from both high surface area and the hierarchical multiporous structure, which provides an accessible pathway for electrolyte transport. These results demonstrate a very effective and low-cost method for scalable preparation of HPC using green biomass carbon source for supercapacitors, which also has potential applications such as adsorbent for water/gas treatment.
  • 常温でも働く水素分離膜の開発
    青木芳尚
    クリーンエネルギー 27 6 44 - 51 2018年06月 [査読有り][招待有り]
  • K. Shahzad, D. Kowalski, C. Zhu, Y. Aoki, H. Habazaki
    ChemElectroChem 5 4 610 - 618 2018年02月01日 [査読有り][通常論文]
     
    Anodizing involves a high-voltage electrochemical conversion process that forms barrier-type oxide layers or self-organized nanoporous/nanotubular structures. So far, Al2O3-like nanopores and TiO2-like nanotubes could be successfully synthesized on many metals and alloys. The proposed models of anodic oxide nanotubes growth, however, sacrifice from lack of evidence of the transition from nanopores to nanotubes. The present study demonstrates a missing piece of this anodizing puzzle, which is responsible for the formation of nanotubes in fluoride-containing organic electrolytes. For this purpose, we choose an anodic oxide formed on iron, as a model case, because both nanotubes and nanopores can be formed and slow kinetics of transition between those two forms allows us to observe, ex situ, a fluoride-rich-layer upon nanopores/nanotubes transition. The compositional fingerprints of this transition shed a light on the general mechanism of nanotubes growth in fluoride-containing electrolytes.
  • Taisei Kobayashi, Kosuke Kuroda, SeongWoo Jeong, Hyuna Kwon, Chunyu Zhu, Hiroki Habazaki, Yoshitaka Aoki
    Journal of the Electrochemical Society 165 5 F342 - F349 2018年 [査読有り][通常論文]
     
    Anodic reaction pathways in proton-conducting solid oxide electrolyzer cells (H+−SOECs) were investigated using electrochemical impedance spectroscopy with a cell structure of Sm0.5Sr0.5CoO3 (anode) | BaZr0.4Ce0.4Y0.2O3-δ | Pt (cathode). Densely sintered BaZr0.4Ce0.4Y0.2O3-δ ceramics (> 97% relative density) were fabricated by a reactive sintering process with a 2 mol% Zn(NO3)2 additive and were applied as the electrolyte. The impedance spectra were measured while the oxygen (pO2) and water partial pressures (pH2O) in the anode side were systematically varied, which revealed that the SOECs have two polarization resistances at the anode side, one proportional to pO2 −1/4 and pH2O 0 and another insensitive to pO2 and pH2O. A comparison between the experimental results and elementary step modeling revealed that the actual anode reactions could be described by the reverse mode H+-SOFC cathode reactions, and, thus, the elementary steps dominating the anodic polarization resistance were assigned.
  • Jeong Seongwoo, Kobayashi Taisei, Kuroda Kosuke, Kwon Hyuna, Zhu Chunyu, Habazaki Hiroki, Aoki Yoshitaka
    RSC ADVANCES 8 46 26309 - 26317 2018年 [査読有り][通常論文]
  • Etsushi Tsuji, Shiki Matsuura, Yoshitaka Aoki, Hiroki Habazaki
    Electrochemistry 86 184 - 189 2018年01月01日 [査読無し][通常論文]
     
    © The Electrochemical Society of Japan, All rights reserved. This paper reports unusual diffusion-controlled growth of TiO2 mesoporous anodic films on titanium in hot phosphate/glycerol electrolytes. The formation behavior was investigated by cyclic voltammetry (CV) between 0 and 5 V vs. Pt at 433 K. The current density became almost constant above 1.5 V vs. Pt during the positive potential sweep, and was maintained even during the negative potential sweep. This is contrast to a drastic decrease in current density in changing the direction of potential sweep from the positive to negative in fluoride-containing ethylene glycol electrolyte. The constant current density between 1.5 and 5 V vs. Pt increased with an increase in the basicity of the hot phosphate electrolyte, suggesting that the rate-determining step of the film formation in the hot phosphate electrolyte was diffusion process of oxygen sources in the electrolyte, not the ion migration in the thin barrier layer under the high electric field. When CV measurements were conducted to higher potentials up to 20 V vs. Pt, anatase was developed above 7 V vs. Pt, leading to generate oxygen gas. The film morphology was also potential-dependent and the diffusion current was also influenced by the film morphology as well as oxygen gas generation.
  • Strong lanthanoid substitution effect on electrocatalytic activity of double-perovskite-type BaLnMn2O5 (Ln = Y, Gd, Nd, and La) for oxygen reduction reaction
    E. Tsuji, T. Motohashi, H. Noda, Y. Aoki, H. Habazaki
    Journal of Physical Chemistry C 121 13 7081 - 7087 2018年 [査読有り][通常論文]
  • Chiharu Kura, Sho Fujimoto, Yuji Kunisada, Damian Kowalski, Etsushi Tsuji, Chunyu Zhu, Hiroki Habazaki, Yoshitaka Aoki
    Journal of Materials Chemistry A 6 6 2730 - 2741 2018年 [査読有り][通常論文]
     
    Hydrogen permeability based on mixed hydride ion electron conduction was demonstrated for hafnium nitride HfNx (film thickness of 100-500 nm, x = 0.8 and 1.0) nanocrystalline membranes. Nanocrystalline films with a (100) orientation and crystallite sizes of a few tens of nanometers were prepared on porous alumina supports by radio frequency (RF) reactive sputtering. Combined spectroscopic, permeability, and microbalance analysis suggests that the nanocrystalline matrices were readily hydrogenated by the formation of Hf-H terminal groups on the internal grain surfaces at ambient temperature and thus efficient hydrogen permeation took place due to an enhanced diffusion of hydridic defects through the grain boundaries this was further aided by the Hf-H bond exchange process. Hence, membranes with an average crystallite size of 11 nm yielded a hydrogen flux of 6 × 10-7 mol cm-2 s-1 at 25 °C at an applied hydrogen partial pressure of 50 kPa this value is higher than those exhibited by the current state-of-the-art Pd membranes. These findings establish a new concept for Pd alternatives based on the pronounced hydric conductivity of transition metal nitride nanomaterials.
  • Anode reaction analysis of solid oxide electrolyzer cells at BaZr0.4Ce0.4Y0.2O3-/Sm0.5Sr0.5CoO3- interfaces
    T. Kobayashi, K. Kuroda, S. W. Jeong, H. Kwon, C. Zhu, H. Habazaki, Y. Aoki
    Journal of the Electrochemical Society 165 5 F342 - F349 2018年 [査読有り][通常論文]
  • High efficiency direct ammonia fuel cells based on BaZr0.1Ce0.8Y0.2O3-/Pd meta-oxide junction
    Y. Aoki, T. Yamaguchi, S. Kobayashi, D. Kowalski, C. Zhu, H. Habazak
    Global Challenge 2 2 1700088/1 - 10 2018年 [査読有り][通常論文]
  • Rapid and Repeatable Self‐Healing Superoleophobic Porous Aluminum Surface Using Infiltrated Liquid Healing Agent
    K. Nakayama, A. Koyama, C. Zhu, Y. Aoki, H. Habazaki
    Adv. Mater. Interface 5 1800566  2018年 [査読有り][通常論文]
  • Electrochemical Oxidation of Hf–Nb Alloys as a Valuable Route to Prepare Mixed Oxides of Tailored Dielectric Properties
    A. Zaffora, F. D. Quarto, C. Kura, Y. Sato, Y. Aoki, H. Habazaki, M. Santamaria
    Adv. Electron. Mater. 4 1800006  2018年 [査読有り][通常論文]
  • Highly increased breakdown potential of anodic films on aluminum using a sealed porous layer
    J.H. Cao, Y. Sato, D. Kowalski, C. Zhu, Y. Aoki, H.Habazaki
    J. Solid State Electrochem. 22 2073 - 2081 2018年 [査読有り][通常論文]
  • Evaluation of thin film fuel cells with Zr-rich side BaZrxCe0.8-xY0.2O3- electrolytes fabricated by single step cofiring process
    S. W. Jeong, T. Kobayashi, K. Kuroda, H. Kwon, C. Zhu, H. Habazaki, Y. Aoki
    RSC Advances 8 7 26309 - 26317 2018年 [査読有り][通常論文]
  • Chiharu Kura, Sho Fujimoto, Yuji Kunisada, Damian Kowalski, Etsushi Tsuji, Chunyu Zhu, Hiroki Habazaki, Yoshitaka Aoki
    Journal of Materials Chemistry A 6 6 2730 - 2741 2018年 [査読有り][通常論文]
     
    Hydrogen permeability based on mixed hydride ion electron conduction was demonstrated for hafnium nitride HfNx (film thickness of 100-500 nm, x = 0.8 and 1.0) nanocrystalline membranes. Nanocrystalline films with a (100) orientation and crystallite sizes of a few tens of nanometers were prepared on porous alumina supports by radio frequency (RF) reactive sputtering. Combined spectroscopic, permeability, and microbalance analysis suggests that the nanocrystalline matrices were readily hydrogenated by the formation of Hf-H terminal groups on the internal grain surfaces at ambient temperature and thus efficient hydrogen permeation took place due to an enhanced diffusion of hydridic defects through the grain boundaries this was further aided by the Hf-H bond exchange process. Hence, membranes with an average crystallite size of 11 nm yielded a hydrogen flux of 6 × 10-7 mol cm-2 s-1 at 25 °C at an applied hydrogen partial pressure of 50 kPa this value is higher than those exhibited by the current state-of-the-art Pd membranes. These findings establish a new concept for Pd alternatives based on the pronounced hydric conductivity of transition metal nitride nanomaterials.
  • 窒化チタン微結晶膜におけるヒドリドイオン伝導の発現とそれによる常温水素膜分離
    青木芳尚
    真空技術協会誌 132 32 - 41 2017年12月 [査読有り][招待有り]
  • Chunyu Zhu, Yoshitaka Aoki, Hiroki Habazaki
    PARTICLE & PARTICLE SYSTEMS CHARACTERIZATION 34 11 1700296/1 - 1700296/7 2017年11月 [査読有り][通常論文]
     
    Electrochemical oxygen reduction reaction (ORR), using nonprecious metal catalysts, has attracted great attention due to the importance in renewable energy technologies, such as fuel cells and metal-air batteries. A simple and scalable synthetic route is demonstrated for the preparation of a novel 3D hybrid nanocatalyst consisting of Co9S8 nanoparticles which are incorporated in N, S-doped carbon (N, S-C) with rational structure design. In particular, the hybrid catalyst is prepared by direct pyrolysis and calcination of a gel mixture of Mg, Co nitrate-thiourea-glycine under Ar atmosphere, with subsequent HCl washing. The properties of obtained hybrid catalyst are quite dependent on calcination temperature and added glycine amount. Under a molar ratio of Co5-Mg15-tu10-gl45 and a calcination temperature of 900 degrees C, Co9S8 nanoparticles are embedded in a well-developed carbon matrix which shows a porous 3D few-layer graphene-like N, S-C with open and hierarchical micro-meso-macro pore structure. Because of the synergistic effect between Co9S8 nanoparticles and well-developed carbon support, the composite exhibits high ORR activity close to that of commercial Pt/C catalyst. More importantly, the composite displays superior long-term stability and good tolerance against methanol. The strategy developed here provides a novel and efficient approach to prepare a cost-effective and highly active ORR electrocatalyst.
  • Katsutoshi Nakayama, Takuya Hiraga, Chunyu Zhu, Etsushi Tsuji, Yoshitaka Aoki, Hiroki Habazaki
    APPLIED SURFACE SCIENCE 423 968 - 976 2017年11月 [査読有り][通常論文]
     
    Herein we report simple electrochemical processes to fabricate a self-healing superhydrophobic CeO2 coating on Type 304 stainless steel. The CeO2 surface anodically deposited on flat stainless steel surface is hydrophilic, although high temperature-sintered and sputter-deposited CeO2 surface was reported to be hydrophobic. The anodically deposited hydrophilic CeO2 surface is transformed to hydrophobic during air exposure. Specific accumulation of contaminant hydrocarbon on the CeO2 surface is responsible for the transformation to hydrophobic state. The deposition of CeO2 on hierarchically rough stainless steel surface produces superhydrophobic CeO2 surface, which also shows self-healing ability; the surface changes to superhydrophilic after oxygen plasma treatment but superhydrophobic state is recovered repeatedly by air exposure. This work provides a facile method for preparing a self-healing superhydrophobic surface using practical electrochemical processes. (C) 2017 Elsevier B.V. All rights reserved.
  • C. Zhu, C. Kim, Y. Aoki, H. Habazaki
    Advanced Materials Interfaces 4 19 1700583/1 - 1700583/8 2017年10月 [査読有り][通常論文]
     
    Hierarchical porous carbon has attracted great interest because of its distinctive structure and superior properties for designing electrochemical energy storage and conversion devices. In this work, a novel method to fabricate nitrogen-doped hierarchical porous carbon (NHPC) is reported, which is incorporated with Co nanoparticles and carbon nanotubes (CNTs). The NHPC is prepared using a facile and scalable MgO-Co template method. Metal nitrate-glycine solution combustion synthesis, followed by a high temperature calcination, is used to prepare MgO-Co/N-doped carbon precursor. CNTs are formed by the in situ Co-catalytic growth during heat treatment; at the same time, localized graphitic layers are also formed around the Co nanoparticles. After acid washing, NHPC with hierarchical multipores and ultrafine Co nanoparticles is obtained. When applied as oxygen reduction reaction (ORR) catalyst, the NHPC displays high catalytic activity not only in terms of onset potential and current density, but also superior durability and tolerance to methanol crossover in alkaline electrolyte. The remarkable ORR activity is originated from the cooperative effects of high specific surface area, hierarchical pore structure, ultrasmall Co nanocrystals, localized graphitic layers, CNTs, and N-doping.
  • H. Habazaki, K. Kobayashi, E. Tsuji, C. Zhu, Y. Aoki, S. Nagata
    JOURNAL OF SOLID STATE ELECTROCHEMISTRY 21 10 2807 - 2816 2017年10月 [査読有り][通常論文]
     
    Heat treatment of Zr-24 at% Ti alloy with barrier-type dielectric anodic oxide films was conducted at 473 K in air to examine the thermal stability of the dielectric oxide films for possible electrolytic capacitor application. The anodic oxide film was formed by anodizing of the alloy at 50 V for 30 min in 0.1 mol dm(-3) ammonium pentaborate electrolyte. The anodic oxide film of 125 nm thickness was crystalline, containing both monoclinic and tetragonal ZrO2 phase. It was found that marked thickening of the oxide film with generation of cracks occurred during heat treatment at 473 K. Thus, the dielectric loss was largely increased along with the capacitance increase. In contrast, the anodic oxide film formed on the oxygen-incorporated alloy remained uniform, and no significant increase in dielectric loss was observed even after the heat treatment. The capacitance of the anodic film became as high as 4.8 mF m(-2), which was nearly twice that on Ta. The high capacitance was associated with the preferential formation of tetragonal ZrO2 phase in the anodic oxide film on the oxygen-incorporated alloy. Findings indicated that the oxygen-incorporated Zr-Ti alloy is a promising novel material for capacitor application.
  • Chiharu Kura, Yuji Kunisada, Etsushi Tsuji, Chunyu Zhu, Hiroki Habazaki, Shinji Nagata, Michael P. Mueller, Roger A. De Souza, Yoshitaka Aoki
    NATURE ENERGY 2 10 786 - 794 2017年10月 [査読有り][通常論文]
     
    The production of pure hydrogen for use in energy applications and related industries often relies on the permeation of hydrogen through palladium-based membranes. However, the scarcity of Pd reserves necessitates the development of affordable alternatives with high hydrogen permeability. Here we report room-temperature hydrogen permeability of titanium nitrides (widely used as tough and inert coating materials) enabled by mixed hydride ion-electron conductivity. Combined spectroscopic, permeability and microgravimetric measurements reveal that nanocrystalline TiNx membranes feature enhanced grain-boundary diffusion of hydride anions associated with interfacial Ti cations on nanograins. Since the corresponding activation energies are very low (<10kJmol(-1)), these membranes yield a considerably higher room-temperature hydrogen flux than Pd membranes of equivalent thickness. Overall, the current study establishes general guidelines for developing hydride ion transport membranes based on a simple transition metal nitride for hydrogen purification, membrane reactors and other applications.
  • Y. Aoki, K. Kuroda, S. Hinokuma, C. Kura, C. Zhu, E. Tsuji, A. Nakao, M. Wakeshima, Y. Hinatsu, H. Habazaki
    Journal of the American Chemical Society 139 32 11197 - 11206 2017年08月 [査読有り][通常論文]
     
    The oxygen storage capability and related defect structure of tetrahedral orthochromite(V) compound YCr1-xPxO4 (x = 0, 0.3, 0.5, and 0.7) were investigated by employing thermal gravimetry and in situ X-ray spectroscopy for reversible oxygen store/release driven by healing-cooling cycles in the temperature range from 50 to 600 degrees C. YCr1-xPxO4 started releasing oxygen as heated from 50 degrees C under ambient atmosphere, with reduction of Cr-V to Cr-IV, while the reduced YCr1-xPxO4-delta phase was significantly reoxidized via absorbing oxygen by cooling to 50 degrees C under ambient atmosphere, recovering the original stoichiometric phase. Operando X-ray adsorption spectroscopy and first-principles calculations demonstrate that nonstoichiometric YCr1-xPxO4-delta phases were stabilized by forming linking polyhedral (Cr2O76-)-O-IV via corner sharing between oxygen-deficient (CrO32-)-O-IV and adjacent (CrO44-)-O-IV. YCr1-xPxO4 was found to have an extremely low reduction enthalpy of about 20 kJ mol(-1) probably due to the relatively high reduction potential of high-valence-state Cr(V)/Cr(IV) redox pairs, thereby resulting in reversible oxygen storage in such a low-temperature region.
  • Etsushi Tsuji, Teruki Motohashi, Hiroyuki Noda, Damian Kowalski, Yoshitaka Aoki, Hajime Tanida, Junji Niikura, Yukinori Koyama, Masahiro Mori, Hajime Arai, Tsutomu Ioroi, Naoko Fujiwara, Yoshiharu Uchimoto, Zempachi Ogumi, Hiroki Habazaki
    CHEMSUSCHEM 10 14 2864 - 2868 2017年07月 [査読有り][通常論文]
     
    Here, we report remarkable oxygen evolution reaction (OER) catalytic activity of brownmillerite (BM)-type Ca2FeCoO5. The OER activity of this oxide is comparable to or beyond those of the state-of-the-art perovskite (PV)-catalyst Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) and a precious-metal catalyst RuO2, emphasizing the importance of the characteristic BM structure with multiple coordination environments of transition metal (TM) species. Also, Ca2FeCoO5 is clearly advantageous in terms of expense/laboriousness of the material synthesis. These facts make this oxide a promising OER catalyst used in many energy conversion technologies such as metal-air secondary batteries and hydrogen production from electrochemical/photocatalytic water splitting.
  • Y. Aoki, T. Yamaguchi, S. Kobayashi, C. Zhu, H. Habazaki
    ECS Transactions 78 1 1511 - 1515 2017年05月30日 [査読有り][通常論文]
     
    Hydrogen membrane fuel cells, HMFC, were fabricated by RF sputtering deposition of BaZr0.1Ce0.7Y0.2O3-δ thin films on Pd solid anode. The HMFC can give OCV of 1.0 V and peak power density of 0.8 W cm-2 at 600°C by using H2 fuels. The direct NH3 fueled HMFC reveals OCV of 0.92 V and peak power density of 0.58 W cm-2 at 600°C. These performances are superior to the best data of direct ammonia PCFC.
  • Cheng-Gong Han, Chunyu Zhu, Nan Sheng, Yoshitaka Aoki, Hiroki Habazaki, Tomohiro Akiyama
    ELECTROCHIMICA ACTA 235 88 - 97 2017年05月 [査読有り][通常論文]
     
    Iron oxide has been considered as one of the most promising anode materials due to its high theoretical capacity, low cost and environmental friendliness. However, few simple and effective method is explored for preparing iron oxides with high electrochemical performance via alleviating the volume change and agglomeration of active particles. In this work, FeOx/carbon/graphene composites are fabricated by a facile solution combustion synthesis within several minutes in one step. Characterization demonstrates that FeOx nanoparticles are well-dispersed in the graphene matrix. The presence of graphene effectively alleviates the agglomeration of FeOx nanoparticles, and accommodates the volume changes during the cycling process, thereby resulting in the excellent electrochemical performance. FeOx/carbon/graphene (31.4 wt.% graphene) delivers a higher discharge capacity of 824 mAh g (1) after 100 cycles at 0.4 A g (1), in comparison to the value of 301 mAh g (1) for the composite without graphene. This easily prepared FeOx/carbon/graphene composite with excellent electrochemical performance can be considered as one promising anode material used for lithium-ion batteries. (C) 2017 Elsevier Ltd. All rights reserved.
  • 金属/酸化物ヘテロ接合からなる中温作動燃料電池の分極挙動」,「環境・エネルギーに関わるセラミックスの界面設計
    青木芳尚
    セラミックス協会誌 52 2 82 - 86 2017年02月 [査読有り][招待有り]
  • R. Tomizawa, C. Zhu, E. Tsuji, Y. Aoki, H. Habazaki
    OXIDE FILMS: A SYMPOSIUM IN HONOR OF MASAHIRO SEO 75 27 67 - 75 2017年 [査読有り][通常論文]
     
    In this study, a nanoporous anodic film was successfully formed on Fe-41 mass% Ni alloy at a constant voltage of 40 V in ethylene glycol electrolyte containing 0.1 mol dm-(3) NH4F and 0.5 mol dm(-3) H2O. The resultant anodic film contained fluoride species and was rather soluble even in de-ionized water. The heat treatment in an argon atmosphere at and above 673 K converted a poorly crystalline anodic film to a crystalline film with a (FeNi)(3)O-4 spinel phase with the nanoporous morphology remaining unchanged. The activity of the heattreated specimens for oxygen evolution reaction (OER) was examined and the results were compared with the anodized iron specimens after heat treatment. The Fe-Ni alloy specimen heattreated at optimized temperature showed the improved activity in comparison with the corresponding iron specimen. Anodizing is promising to form a highly porous electrode directly on metal substrate.
  • MnO/NC anode materials for lithium-ion batteries prepared by cotton-templated combustion synthesis
    C. G. Han, C. Zhu, Y. Aoki, H. Habazaki, T. Akiyama
    Green Energy & Environment 2 377 - 386 2017年 [査読有り][通常論文]
  • D. Quintero, M. A. Gomez, J. G. Castano, E. Tsuji, Y. Aoki, F. Echeverria, H. Habazaki
    SURFACE & COATINGS TECHNOLOGY 310 180 - 189 2017年01月 [査読有り][通常論文]
     
    Spark anodizing of Ti6A14V alloy has been performed in alkaline electrolytes containing aluminate to form wear resistant coatings. Coatings obtained under potentiostatic conditions exhibit a better wear resistance, with a wear rate approx. 10 times lower than galvanostatic coatings, due to the formation of coatings with low porosity since potentiostatic mode controls the size of the micro-discharges. The coatings consist mainly of gamma-Al2O3, whilst Al2TiO5 is present in minor proportions. For some coatings, the formation of Al6KO5,5 is observed due to the incorporation of potassium into the coating. EDS analysis shows that aluminum is the main constituent of the coatings, being mainly incorporated from the anodizing solution and homogeneously distributed into the coating. A decrease in pore density between 15 and 30% and roughness reduction of 10 to 60% was obtained with KOH and Na2WO4 additions, improving the coating wear resistance in pin-on-disc wear tests, especially by Na2WO4 addition. (C) 2016 Elsevier B.V. All rights reserved.
  • Y. Aoki, T. Yamaguchi, E. Tsuji, H. Habazaki
    Journal of the Electrochemical Society 164 6 F577 - F581 2017年 [査読有り][通常論文]
     
    Fuel cell performances at intermediate temperature 600 degrees C were examined for the hydrogen membrane fuel cell (HMFC) designed by rf sputter deposition of a BaCe0.8Y0.2O3-delta thin film electrolyte (1 mu m thickness) on a Pd1-xAgx (x = 0, 0.2 and 0.4) solid anode. The maximum power density reached to 1.2 W cm(-2) at 600 degrees C with the Pd0.8Ag0.2 anodes, and this value was much higher than the champion data of the recent anode-supported proton-conducting ceramic fuel cells (PCFCs). By electrochemical impedance analysis, it was demonstrated that the hydrogen permeability was crucial to the anode polarization losses of the HMFC because the charge transfer resistances and concentration overpotentials at BaCe0.8Y0.2O3-delta/Pd oxide-metal interfaces are simultaneously improved with the alloys having higher hydrogen permeability. (C) 2017 The Electrochemical Society. All rights reserved.
  • Y. Aoki, S. Kobayashi, T. Yamaguchi, E. Tsuji, H. Habazaki, K. Yashiro, T. Kawada, T. Ohtsuka
    The Journal of Physical Chemistry C 120 29 15976 - 15985 2016年07月 [査読有り][通常論文]
     
    A hydrogen membrane fuel cell (HMFC) consisting of a Pd solid anode, 1 mu m thick BaCe0.8Y0.2O3-delta thin-film electrolyte, and La0.6Sr0.4Co0.2Fe0.8O3 cathode was examined. A single-phase BaCe0.8Y0.2O3-delta thin film was successfully deposited by radio frequency cosputtering with BaCe0.8Y0.2O3 and Ce0.9Y0.1O2 double targets, as checked by X-ray diffraction, transmission electron microscopy, and wavelength dispersive X-ray analysis. The maximum power density reached 1.05 W cm-(2) at 600 degrees C, and this value was higher than the champion data of the recently reported proton-conducting ceramic fuel cells (PCFCs). Electrochemical impedance analysis was performed to characterize the anode and cathode polarization behavior. The impedance responses of HMFC were explicable with an equivalent circuit built by a series connection of cathode charge-transfer elements and anode mass-transfer elements. The contribution of the mass transfer in Pd bulk was found to be relatively small in comparison to cathode polarization and ohmic loss in normal fuel cell atmosphere. Moreover, the cathodic charge-transfer resistance of HMFC was found to be 25 times smaller than those of the recent PCFC systems. The current results demonstrated that the HMFC retained relatively large gas-proton-electron triple-boundary zones near the interface between the BaCe0.8Y0.2O3-delta electrolyte and porous La0.6Sr0.4Co0.2Fe0.8O3-delta cathode.
  • Katsutoshi Nakayama, Etsushi Tsuji, Yoshitaka Aoki, Soo-Gil Park, Hiroki Habazaki
    JOURNAL OF PHYSICAL CHEMISTRY C 120 29 15684 - 15690 2016年07月 [査読有り][通常論文]
     
    This paper reports the formation of hierarchically structured aluminum mesh by a combination of simple chemical etching and anodizing. The former introduced micrometer-sized etch pits, and the latter produced nanopores of the order of 10 nm on the mesh with 150 mu m mesh openings. Coating the mesh with a monolayer of fluoroalkyl phosphate made the surface superoleophobic to practically any liquid, including hexane with a surface tension as low as 18.4 mN m(-1). The hierarchical triple similar to 100 mu m/similar to 1 mu m/similar to 10 nm pore surface morphology improved the superoleophobicity compared to the dual similar to 100 mu m/similar to 10 nm and similar to 1 mu m/similar to 10 nm pore structures. When the aluminum mesh was coated with a fluorine-free alkylphosphate monolayer, the surface was superhydrophobic, but superoleophilic. The noncoated aluminum mesh was superhydrophilic and superoleophilic with a liquid contact angle close to 0 degrees. Using the aluminum mesh with an alkylphosphate coating, a water/oil mixture was successfully separated by allowing only the oil to pass through the mesh. In addition, the mixture was separated using noncoated mesh wetted with water, since oil was not able to pass through the mesh in this case.
  • H. Habazaki, T. Kimura, Y. Aoki, E. Tsuji, T. Yano, K. Shimizu, A. W. Hassel
    ELECTROCHIMICA ACTA 201 311 - 319 2016年05月 [査読有り][通常論文]
     
    Highly corrosion-resistant, similar to 65-nm-thick, layer-by-layer aluminosilicate coatings have been prepared by multiple spin casting on Type 430 stainless steel. These coatings have been characterized by field emission scanning electron microscopy, transmission electron microscopy, conductive atomic force microscopy, and micro-electrochemical measurements using a microcapillary cell. The coatings annealed at 400 degrees C are non-uniform and contain fine iron oxide nodules, which are formed in high densities on the {111} grain surface of the steel. The iron oxide nodules arise from the outward diffusion of the oxidized iron from the substrate through the coating. The coatings annealed at 400 degrees C are more insulating compared with those before annealing; however, the nodule sites are less insulating owing to the development of more conductive iron oxide channels in the coatings. A microcapillary cell study reveals that the coated specimens prepared from diluted precursor solutions by a layer-by-layer process exhibit higher pitting potential in 3.5 wt% NaCl solution compared with those prepared through a single-layer process. Moreover, the coated specimen obtained from the layer-by-layer process exhibits similarly high pitting potential even at the flaw sites in the coating; in contrast, the pitting potential in the flaw-containing regions of the coated specimen obtained from a single-layer process shifts towards the less noble direction. The layer-by-layer coating is also effective in suppressing the corrosion of the scratched region of the coated specimens, owing possibly to the excellent adhesion between the coating and the substrate. (C) 2015 Elsevier Ltd. All rights reserved.
  • Yoshitaka Aoki, Kosuke Kuroda, Etsushi Tsuji, Hiroki Habazaki
    Solid State Ionics 285 175 - 179 2016年02月01日 [査読有り][通常論文]
     
    Redox transformation of NdCrVO4 involved by insertion/desertion of protons was examined by XRD, XPS, in situ FT-IR and fuel cell tests. It was found that NdCrO4 caused bulk reduction with H2 gas at around 400 °C, changing to amorphous Nd(III)-Cr(III) hydroxide phase. This hydroxide phase, however, could recover to the original zircon type oxide phase by air oxidation at the same temperature region. Proton-conducting ceramic fuel cells having NdCrO4 film as an interlayer between cathode and electrolyte was fabricated and the cell was confirmed to exhibit remarkable power generation giving rise to OCV of more than 1.0 V at 500 °C. It was speculated that the interlayer facilitates proton transfer from electrolyte to cathode because redox cycling of NdCrO4 and the corresponding hydroxide was driven by the high proton chemical potential at the interface with electrolyte and plenty of oxygen in cathode gases.
  • Yoshiki Konno, Ahmed A. Farag, Etsushi Tsuji, Yoshitaka Aoki, Hiroki Habazaki
    Journal of the Electrochemical Society 163 7 C386 - C393 2016年 [査読有り][通常論文]
     
    The growth behavior of nanoporous anodic films on carbon steel containing 0.213 mass% carbon has been examined. The films were grown by anodizing in an ethylene glycol (EG) electrolyte containing 0.1 mol dm-3 NH4F and 0.5 mol dm-3 H2O. The steel contains carbide precipitates with sizes in the range 50-800 nm. The anodic film formed on the carbide phase grew more slowly and was more chemically soluble during anodizing, resulting in submicrometer pits on the anodic film. The nanoporous morphology of the anodic films formed on an á-Fe matrix resembled those formed on iron. Heat treatment of the anodized specimens caused transformation of the chemically soluble fluoride-containing amorphous or poorly crystalline anodic films to crystalline oxide films containing α-Fe2O3 and Fe3O4. Polypyrrole (PPy)was electropolymerized on the transformed surfaces to form a corrosion-protective composite coating. The resultant specimens coated with the composite coating showed improved durability compared to passivated steel with a PPy coating.
  • Khurram Shahzad, Chunyu Zhu, Etsushi Tsuji, Yoshitaka Aoki, Shinji Nagata, Hiroki Habazaki
    Materials Transactions 57 9 1552 - 1559 2016年 [査読有り][通常論文]
     
    In this study, we report the formation of barrier-type anodic films on magnetron-sputtered magnesium films at a constant current density of 10 A m-2 in ethylene glycol (EG)-H2O electrolytes containing 0.1 mol dm-3 ammonium fluoride and 0.1 mol dm-3 dipotassium hydrogen phosphate. The growth efficiency is close to 100% up to 10 vol% H2O, but decreases to 52% in the EG-free aqueous electrolyte. Even at such a low efficiency in the aqueous electrolyte a uniform barrier-type anodic film with flat and parallel metal/film and film/electrolyte interfaces is developed over 100 V. This is contrast to the non-uniform film growth and low breakdown voltage in the phosphate-free aqueous electrolyte containing ammonium fluoride. The anodic films appear to be amorphous regardless of H2O concentration in the phosphate-containing electrolytes, and consist of phosphate-incorporated oxy fluoride. The phosphate incorporation is suppressed by an increase in H2O concentration. In addition, the anodic films consist of two layers with an inner layer containing less amount of phosphate. The outer layer is probably formed at the film/electrolyte interface by the migration of Mg2+ ions outwards, while the inner layer is formed at the metal/film interface. The film formation at the former interface even in the aqueous electrolyte at low efficiency is likely to contribute to the formation of barrier films, not porous anodic films.
  • Chiharu Kura, Yoshitaka Aoki, Etsushi Tsuji, Hiroki Habazaki, Manfred Martin
    RSC ADVANCES 6 11 8964 - 8970 2016年 [査読有り][通常論文]
     
    Resistive switching gallium oxide base thin films with tailored oxygen deficiency were fabricated by rf cosputtering of Ga2O3 and Cr. XPS and STEM-EDX analyses were used to determine that the resultant film was made of a homogeneous oxide glass layer with mixed valance states of Ga(III)-Ga(I). The amount of Ga(I) and the corresponding oxygen deficiency was precisely controlled because the following redox reaction subsequently progresses within the deposited films: 3Ga(III) + 2Cr(0) -> 3Ga(I) + 2Cr(III). The on/off resistance ratio was largely varied by changing the Ga(I) fraction in relation to the oxide ion conductivity, and Ga0.82Cr0.18O1.2 thin film was found to exhibit an optimal switching performance. The film resistance state was tunable by 100's of ms pulse biasing and was incrementally changed by increasing the applied pulse numbers. The strongly time-dependent switching events and area dependent current level of Cr-GaOx films were distinct from the abrupt switching behavior of the filamentary mechanism TiOx thin film devices. It was demonstrated that rf cosputtering of the metal oxides and the corresponding oxygen scavenging metals was a powerful technique to design the bulk state resistive switching devices based on nonstoichiometric metal oxide thin films.
  • Y. Aoki, K. Kuroda, E. Tsuji, H. Habazaki
    Solid State Ionics 285 175 - 179 2016年 [査読有り][通常論文]
  • Hiroki Habazaki, Fumitaka Kataoka, Khurram Shahzad, Etsushi Tsuji, Yoshitaka Aoki, Shinji Nagata, Peter Skeldon, George E. Thompson
    ELECTROCHIMICA ACTA 179 402 - 410 2015年10月 [査読有り][通常論文]
     
    This study demonstrates the formation of uniform barrier-type anodic films on magnetron-sputtered magnesium films at high current efficiency in ethylene glycol electrolytes containing 0.1 mol dm (3) NH4F and various concentrations (0.1-28 mol dm (3)) of H2O. The anodic films containing a crystalline MgF2 phase develop both at the metal/film and film/electrolyte interfaces due to simultaneous migrations of anions inwards and cations outwards, respectively. When a Mg - 1.2 at% Au/Mg bilayer film is anodized, initial prior oxidation of magnesium proceeds with gold atoms accumulating in a thin layer beneath the anodic film. The accumulated gold atoms are incorporated into the anodic film as a band when the alloy layer is completely anodized. Fluoride-containing gold species are formed by the incorporation and the gold species migrate outwards at a rate of 0.4 times the rate of Mg2+ ions. The addition of phosphate in the electrolyte results in the formation of an amorphous anodic film, and the phosphate incorporated into the anodic film is distributed throughout the film thickness. The transport number of cations is also influenced by the phosphate incorporation. (C) 2015 Elsevier Ltd. All rights reserved.
  • Etsushi Tsuji, Takenori Yamasaki, Yoshitaka Aoki, Soo-Gil Park, Ken-ichi Shimizu, Hiroki Habazaki
    CARBON 87 1 - 9 2015年06月 [査読有り][通常論文]
     
    We report the preparation and characterization of highly durable platinum catalysts supported on platelet-structure carbon nanofibers (Pt/p-CNFs) for the oxygen reduction reaction. The p-CNFs were prepared by liquid phase carbonization of polyvinyl chloride using a porous anodic alumina template at 600 degrees C; their degree of graphitization was increased by the subsequent heat treatment at higher temperatures of up to 1400 degrees C. The platinum nanoparticles with similar to 3 nm diameter were deposited more uniformly on the p-CNFs compared with those on the commercial Ketjen black (KB). The catalytic activity and durability of the Pt/p-CNFs for the oxygen reduction reaction (ORR) in H2SO4 solution were improved by increasing the heat-treatment temperature of p-CNFs. The durability of the Pt/p-CNFs was much higher than that of Pt/KB; in particular, a loss of less than 10% was observed in the ORR activity of Pt/p-CNF heat-treated at 1400 degrees C after potential cycling from 0.5 to 1.5 V vs. RHE for 200 cycles in an argon-saturated H2SO4 aqueous solution. (C) 2015 Elsevier Ltd. All rights reserved.
  • Y. Aoki, S. Kobayashi, T. Yamaguchi, E. Tsuji, H. Habazaki
    ECS Transactions 68 1 2735 - 2738 2015年 [査読有り][通常論文]
     
    Hydrogen membrane fuel cells, HMFC, were fabricated by RF sputtering deposition of BaCe< sup> 0.8< /sup> Y< sup> 0.2< /sup> O< sup> 3-δ< /sup> thin films on Pd solid anode. The HMFC can give OCV of 1.1 V and peak power density of 300 mW cm< sup> -2< /sup> at 600°C with H< inf> 2< /inf> fuels. On the other hand, the direct NH< inf> 3< /inf> fueled HMFC reveals OCV of 0.95 V and peak power density of 80 mW cm< sup> -2< /sup> at 600°C. The impedance spectroscopy reveals that anodic polarization is the main contribution to the loss in HMFC.
  • H. Habazaki, K. Shahzad, T. Hiraga, E. Tsuji, Y. Aoki
    ECS Transactions 69 2 211 - 223 2015年 [査読無し][通常論文]
     
    High purity iron and commercial Type 304 and 430 stainless steels were anodized in ethylene glycol electrolytes containing fluoride and water. When iron was anodized in the electrolytes containing relatively high water concentrations (≥1.5 mol dm-3) at 50 V, porous anodic films with scalloped metal/film interface were developed. In contrast, the anodic films formed in the electrolytes with low water concentrations (≤0.5 mol dm-3) resulted in the flat metal/film interface. In addition, the interpore distance became smaller with respect to the thickness of barrier layer when the water concentration is reduced. These morphological changes may be associated with the change in the composition of the anodic films. Anodizing of stainless steels was dependent upon the type of stainless steel a steady-state current continued up to ~160 V on Type 430 stainless steel, while exponential current increase occurred above ~50 V on Type 304 stainless steel during potentiodynamic anodizing. The thickness of the barrier layer and the morphology of the porous layer were largely dependent upon the type of stainless steel. Based on such findings, the mechanism of film growth was discussed.
  • H. Habazaki, M. Uemura, K. Fushimi, E. Tsuji, Y. Aoki, S. Nagata
    Journal of The Surface Finishing Society Japan 66 670 - 672 2015年 [査読有り][通常論文]
  • Khurram Shahzad, Etsushi Tsuji, Yoshitaka Aoki, Shinji Nagata, Hiroki Habazaki
    ELECTROCHIMICA ACTA 151 363 - 369 2015年01月 [査読有り][通常論文]
     
    Magnetron-sputtered iron films were potentiodynamically anodized at two different sweep rates to 50 V in an ethylene glycol electrolyte containing ammonium fluoride and water. At a high sweep rate of 1.0 V s(-1), a barrier-type anodic film was formed even though the current efficiency was as low as similar to 50%. In contrast, a nanoporous anodic film developed at a low sweep rate of 0.05 V s(-1), and the film-formation efficiency reduced to 37%. The main part of the anodic films consists of iron (III) hydroxyfluoride with a thin inner layer composed of FeF3. The inner fluoride layer is formed owing to the faster inward migration of fluoride ions compared to that of the oxygen species. During immersion or re-anodizing of the iron specimen with an approximately 100-nm-thick, barrier-type anodic film at and below 15 V, thinning of the anodic film proceeded uniformly and film dissolution was enhanced by applying an electric field. The impact of the electric field on film formation and dissolution is discussed. (C) 2014 Elsevier Ltd. All rights reserved.
  • Yoshiki Konno, Etsushi Tsuji, Yoshitaka Aoki, Toshiaki Ohtsuka, Hiroki Habazaki
    FARADAY DISCUSSIONS 180 479 - 493 2015年 [査読有り][通常論文]
     
    Conducting polymers (CPs), including polypyrrole, have attracted attention for their potential in the protection of metals against corrosion; however, CP coatings have the limitation of poor adhesion to metal substrates. In this study, a composite coating, comprising a self-organized porous anodic oxide layer and a polypyrrole layer, has been developed on iron. Because of electropolymerization in the pores of the anodic oxide layer, the composite coating showed improved adhesion to the substrate along with prolonged corrosion protection in a NaCl aqueous corrosive environment. The anodic oxide layers are formed in a fluoride-containing organic electrolyte and contain a large amount of fluoride species. The removal of these fluoride species from the oxide layer and the metal/oxide interface region is crucial for improving the corrosion protection.
  • Ryo Nonaka, Yoshitaka Aoki, Etsushi Tsuji, Shinji Nagata, Hiroki Habazaki
    ELECTROCHEMISTRY 82 10 859 - 864 2014年10月 [査読有り][通常論文]
     
    An electrochemical analysis was conducted with respect to a hydrogen membrane fuel cell (HMFC) comprising proton-conducting, amorphous zirconium phosphate, a-ZrP2.5O8.9H1.3, thin film electrolyte supported on a dense Pd anode. The HMFC gave rise to an OCV of 1.0 V, but the maximum power density was limited and was about 1 mW cm(-2) at 400 degrees C. The impedance spectroscopy revealed that the interfacial polarizations were decreased by two orders of magnitude when the cell configuration was changed from the fuel cell setup to the hydrogen concentration cell with an anode symmetric configuration. These results indicated that the polarization losses at the solid-solid anode interface are not a main contribution to the voltage loss of the HMFC. The large cathode polarization might be attributed to the lessened conductivity of amorphous zirconium phosphate electrolyte thin film formed on a precious metal electrode. (C) The Electrochemical Society of Japan, All rights reserved.
  • Hiroki Habazaki, Fumitaka Kataoka, Etsushi Tsuji, Yoshitaka Aoki, Shinji Nagata, Peter Skeldon, George E. Thompson
    ELECTROCHEMISTRY COMMUNICATIONS 46 30 - 32 2014年09月 [査読有り][通常論文]
     
    The present study reports, for the first time, the highly efficient formation of barrier-type anodic films, with flat and parallel metal/film and film/electrolyte interfaces, on magnesium in ethylene glycol electrolytes containing ammonium fluoride and water. The anodizing voltage increases linearly with time during galvanostatic anodizing at 10Am(-2) up to 350 V.The anodic film formed to 200V is 247 nm thick, containing a crystalline MgF2 phase. Analysis by Rutherford backscattering spectroscopy discloses the film composition of MgF1.8O0.1 and Pilling-Bedworth ratio (PBR) of 1.67. The PBR value greater than unity and the formation of chemically stable fluoride-based films may contribute to the film growth at high current efficiency. (C) 2014 Elsevier B.V. All rights reserved.
  • E. Tsuji, Y. Taguchi, Y. Aoki, T. Hashimoto, P. Skeldon, G. E. Thompson, H. Habazaki
    APPLIED SURFACE SCIENCE 301 500 - 507 2014年05月 [査読有り][通常論文]
     
    We report the size-selective photocatalytic decomposition of organic molecules using crystalline anodic TiO2 nanochannel films as the photocatalyst. The porous TiO2 films were formed by anodizing titanium at 20 V in glycerol electrolyte containing various amounts of K3PO4, K2HPO4, and KH2PO4 at 433K. Regardless of the electrolyte composition, the as-formed TiO2 films had a crystalline anatase structure. The basicity of the electrolyte markedly influenced the morphology of the TiO2 nanochannel films; more regular nanochannels developed with increasing basicity of the electrolyte. Because the diameter of the nanochannels in the films formed in a basic electrolyte was as small as similar to 10 nm, the anodic TiO2 nanochannel films with a thickness of 5 mu m revealed a selective photocatalytic decomposition of methylene blue (MB) in a mixture of MB and direct red 80 (DR) kept under UV irradiation. The importance of the diameter of the nanochannels and their uniformity for size-selective decomposition of organic molecules were investigated. (C) 2014 Elsevier B.V. All rights reserved.
  • Yoshitaka Aoki, Carsten Wiemann, Vitaliy Feyer, Hong-Seok Kim, Claus Michael Schneider, Han Ill-Yoo, Manfred Martin
    NATURE COMMUNICATIONS 5 3 3473/1 - 3473/9 2014年03月 [査読有り][通常論文]
     
    In thin films of mixed ionic electronic conductors sandwiched by two ion-blocking electrodes, the homogeneous migration of ions and their polarization will modify the electronic carrier distribution across the conductor, thereby enabling homogeneous resistive switching. Here we report non-filamentary memristive switching based on the bulk oxide ion conductivity of amorphous GaOx (x similar to 1.1) thin films. We directly observe reversible enrichment and depletion of oxygen ions at the blocking electrodes responding to the bias polarity by using photoemission and transmission electron microscopies, thus proving that oxygen ion mobility at room temperature causes memristive behaviour. The shape of the hysteresis I-V curves is tunable by the bias history, ranging from narrow counter figure-eight loops to wide hysteresis, triangle loops as found in the mathematically derived memristor model. This dynamical behaviour can be attributed to the coupled ion drift and diffusion motion and the oxygen concentration profile acting as a state function of the memristor.
  • Katsutoshi Nakayama, Etsushi Tsuji, Yoshitaka Aoki, Hiroki Habazaki
    RSC ADVANCES 4 58 30927 - 30933 2014年 [査読有り][通常論文]
     
    This study demonstrates the fabrication of hierarchical surfaces with super-repellency even for low-surface-tension liquids, including octane (surface tension of 21.7 mN m(-1)). Dual-pore surfaces were prepared by a combination of practical wet processes on an aluminium substrate: chemical etching, anodizing, and organic monolayer coating. The size of the larger pores formed by the chemical etching of aluminium is controlled by the concentration of HCl in the CuCl2/HCl etching solution. The etched aluminium is then anodized to introduce nanopores, followed by a pore-widening treatment that controls the nanopore size and porosity. The repellency for low-surface-tension liquids is enhanced by increasing the size of the larger pores as well as the porosity of the walls of the larger pores in this dual-pore morphology. Under optimized morphology with a fluoroalkyl-phosphate monolayer coating, an advancing contact angle close to 160 degrees, a contact angle hysteresis of less than 5 degrees and a sliding angle of 10 degrees is achieved even for octane.
  • Hiroki Habazaki, Taiki Kimura, Yoshitaka Aoki, Etsushi Tsuji, Takayoshi Yano
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY 161 1 C57 - C61 2014年 [査読有り][通常論文]
     
    In this study, aluminosilicate sol-gel coatings were deposited on Type 430 stainless steel by multiple spin casting cycles. Amorphous aluminosilicate coatings, 65 nm thick, were prepared from precursor solutions with 50, 100, and 500 mmol dm(-3) total concentrations of aluminum and silicon species (molar ratio of Al/Si = 1/4) by 10, 5, and 1 spin casting cycles, respectively. Although the obtained coatings had a similar composition with a molar ratio of Al/Si = 25/75, the coatings with increased density were formed by reducing the concentration of precursor solution. The pitting potentials of the coated stainless steels, measured by potentiodynamic polarization in 3.5% NaCl solution, increased remarkably (to 1.1 V vs Ag/AgCl) with a decrease in the concentration of precursor solution. Cyclic corrosion tests, consisting of 30 cycles of spraying of 5% NaCl solution at 35 degrees C for 2 h, drying at 60 degrees C for 4 h, and wetting at 50 degrees C and >95% relative humidity for 2 h, revealed highly improved corrosion resistance. Layer-by-layer sal-gel deposition using diluted precursor solutions is an effective way to obtain highly protective coatings. (C) 2013 The Electrochemical Society.
  • Yoshitaka Aoki, Kazumasa Kikutani, Etsushi Tsuji, Hiroki Habazaki
    ECS ELECTROCHEMISTRY LETTERS 3 10 F61 - F64 2014年 [査読有り][通常論文]
     
    This is the first report to investigate on the electrical conductivity of unusual high valence state pentavalent chromium oxides in terms of defect thermodynamics. Calcium chromium(V) apatite was prepared as a single phase, and the electrical conductivity was examined as a function of oxygen partial pressure p(O2). The conductivity was increased at higher p(O2) in wet atmosphere but it was not dependent on p(O2) in dry atmosphere. The water concentration cell gives stable electromotive force. The features disclosed that the chromium(V) apatite reveals proton electron mixed conductivity under wet oxidative condition. (C) 2014 The Electrochemical Society. All rights reserved.
  • Masatoshi Ishizuka, Etsushi Tsuji, Yoshitaka Aoki, Hiroki Habazaki
    Electrochemistry 81 10 840 - 844 2013年10月 [査読有り][通常論文]
     
    Barrier-type anodic films are formed on magnetron sputtered Ta-W alloy films to various formation potentials in 0.1 moldm-3 ammonium pentaborate electrolyte. The anodic films consist of two layers, comprising an outer thin Ta2O5 layer free from tungsten species and an inner layer containing both tantalum and tungsten species. Slower migration of W6+ ions with respect to Ta5+ ions results in the formation of the two-layered films. Because of the absence of more soluble tungsten species in the outer layer, the anodic films grow at high current efficiency. The reciprocal of capacitance of the anodic films changes linearly with formation voltage, as a consequence of linear thickening of the anodic films with the formation potential. The capacitance is enhanced by the addition of tungsten, particularly at low formation potential. The shift of the potential, at which the anodic film growth commences, to the noble direction, contributes to the enhanced capacitance at the low formation voltages. © The Electrochemical Society of Japan, All rights reserved.
  • H. Sato, T. Fujii, E. Tsuji, Y. Aoki, K. Shimizu, P. Skeldon, G. E. Thompson, H. Habazaki
    SURFACE AND INTERFACE ANALYSIS 45 10 1441 - 1445 2013年10月 [査読有り][通常論文]
     
    Self-assembled alkyl phosphate layers have been formed on a flat, anodized aluminum substrate in dilute ethanol solution containing 2wt% n-tetradecylphosphonic acid (TDP) and examined by low-voltage scanning electron microscopy as well as atomic force microscopy and X-ray photoelectron spectroscopy. Locally, multi-layered alkyl phosphate films have been formed on aluminum, being clearly observed by a low-voltage scanning electron microscope operated at less than 1kV. Atomic force microscopy observations disclosed that bilayers of tetradecylphosphonic acid are stacked on the substrate to form multilayers. The present study revealed that the uniform self-organized monolayer is not always formed readily on an oxidized aluminum surface. Copyright (c) 2013 John Wiley & Sons, Ltd.
  • H. Habazaki, H. Sato, E. Tsuji, Y. Aoki, K. Shimizu
    ECS Transactions 58 41 75 - 83 2013年 [査読有り][通常論文]
     
    Organic layers of tetradecylphosphonic acid (TDP) and octadecyltrietoxysilane (OES) are formed on flat anodized aluminum surfaces by a simple immersion process. The organic layers are characterized by dynamic contact angle measurements for water, AFM, low-voltage SEM and angle-resolved XPS. The results indicate that a self-assembled monolayer is formed using TDP, but not for OES. Plasma surface treatment prior to the coating was found to significantly influence the formation of organic layers. © 2014 The Electrochemical Society.
  • Ishizuka M, Tsuji E, Aoki Y, Hyono A, Ohtsuka T, Sakaguchi N, Nagata S, Habazaki H
    ELECTROCHEMICAL CAPACITORS 50 43 245 - 254 2013年 [査読有り][通常論文]
  • Y. Aoki, R. Nonaka, H. Habazaki
    ECS Transactions 57 1 911 - 916 2013年 [査読有り][通常論文]
     
    Novel thin film fuel cell based on the 100 nm-thick electrolyte of amorphous ZrP2.5Ox, working at 400°C, was demonstrated. The hydrogen permeable membrane fuel cell (HMFC) using a Pd foil as a nonporous solid anode was fabricated. Ni interlayer of several hundreds nm thickness was introduced between the Pd anode and the ZrP2.5O x electrolyte in order to suppress the deterioration of the electrolyte nanofilm by the deformation of the Pd anode during hydrogen absorption. In the ZrP2.5Ox electrolyte the transport number of proton was unity at 400°C as determined by an EMF measurement. The modification of the Ni anode surface by an ultrathin Pt or Pd layer effectively decreased the anode/electrolyte interfacial polarization. Consequently, the HMFC revealed the OCV of 1.0 V and the maximum power density of 3.6 mW cm -2 at 400°C. © The Electrochemical Society.
  • Ke Ye, Yoshitaka Aoki, Etsushi Tsuji, Shinji Nagata, Hiroki Habazaki
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY 160 9 F1096 - F1102 2013年 [査読有り][通常論文]
     
    Novel proton-conducting ZrO2-WO3-SiO2 nanofilms of various compositions and thicknesses (similar to 50 to similar to 300 nm) have been prepared by anodizing of magnetron-sputtered Zr-W-Si alloys in 0.1 mol dm(-3) phosphoric acid electrolyte at 20 degrees C. All the anodic oxide nanofilms examined reveal efficient proton conductivity after post-annealing at 250 degrees C. Further increase in the post-annealing temperature results in the conductivity degradation for the anodic oxide nanofilms on the alloy containing only 5 at% silicon, while the high conductivity is maintained even after post-annealing at 300 degrees C for those containing 15 at% or more silicon. The proton conductivity is dependent upon tungsten content; the conductivity of 5 x 10(-6) S cm(-1) for the similar to 100 nm-thick films on the Zr31W55Si14 at 100 degrees C is approximately 10 times that on the Zr48W37Si15. The anodic oxide nanofilms consist of two layers, comprising a thin outer ZrO2 layer and an inner ZrO2-WO3-SiO2 layer. Both layers show thickness-dependent conductivity and the proton conductivity of the two-layer anodic films is enhanced one order of magnitude by reducing the film thickness from similar to 300 nm to similar to 100 mu. Different mechanisms are proposed for the thickness dependence of the conductivity of the outer and inner layers. (C) 2013 The Electrochemical Society. All rights reserved.
  • Etsushi Tsuji, Naoto Hirata, Yoshitaka Aoki, Hiroki Habazaki
    MATERIALS LETTERS 91 39 - 41 2013年01月 [査読有り][通常論文]
     
    In this study, mesoporous TiO2 films were formed on ITO conductive glasses by anodizing of titanium thin films without post-annealing. The titanium thin films, similar to 200 nm thick, sputter-deposited on to the ITO substrates were anodized at 20 V in a phosphate/glycerol electrolyte at 433 K. A completely. transparent TiO2 specimen was obtained by anodizing for 60 s. The as-anodized mesoporous,TiO2 films consisted of anatase, even without post-annealing. The transparent mesoporous TiO2 films anodized for a suitable time showed much better photoelectrochemical performance than the post-annealed TiO2 nanotubular film. (C) 2012 Elsevier B.V. All rights reserved.
  • Takashi Fujii, Hina Sato, Etsushi Tsuji, Yoshitaka Aoki, Hiroki Habazaki
    JOURNAL OF PHYSICAL CHEMISTRY C 116 44 23308 - 23314 2012年11月 [査読有り][通常論文]
     
    This work reports the importance of nanopore morphology in designing super liquid-repellent submicropillar/nanopore hierarchical surfaces. The hierarchical surfaces were fabricated using a combined process of oblique angle sputter deposition of aluminum with subsequent anodizing, and the surfaces were coated with a fluorinated alkyl phosphate layer to reduce the surface energy. The size of the nanopores, the interpore distance, and the porosity of the anodic films on the submicrometer pillars were controlled by varying the anodizing and pore-widening conditions. The present study demonstrates that super liquid repellency can be achieved on intrinsically oleophilic surfaces by introducing hierarchical submicropillar/nanopore morphology even for oils with surface energies as low as similar to 25 mN m(-1). The porosity in the submicrometer pillars was a key factor in influencing the contact angle hysteresis; higher porosity is needed to reduce the contact angle hysteresis.
  • Yoshiaki Taguchi, Etsushi Tsuji, Yoshitaka Aoki, Hiroki Habazaki
    APPLIED SURFACE SCIENCE 258 24 9810 - 9815 2012年10月 [査読有り][通常論文]
     
    In this study, anatase crystalline TiO2 mesoporous film was formed by anodizing of titanium specimens without annealing procedures. The specimens were anodized at 3 and 20 V in 0.6 mol dm(-3) K2HPO4 and 0.2 mol dm(-3) K3PO4/glycerol electrolyte at 433 K. The obtained films had mesoporous structures with pore diameters as small as similar to 10 nm. The mesoporous film formed at 20 V without annealing (MP-20V) was a mixture of amorphous phase and nanograined anatase, which clearly showed strong < 0 0 1 > preferred orientation, whereas that at 3 V was completely amorphous. Even without annealing, the MP-20V showed high photocatalytic activities for decomposition of water and methylene blue. In contrast, the anodic TiO2 nanotube film formed in NH4F/ethylene glycol electrolyte revealed photocatalytic activities only after annealing at 723 K, because of the amorphous nature of the as-anodized nanotube film. The MP-20V film also showed superhydrophilicity with UV light irradiation. (C) 2012 Elsevier B. V. All rights reserved.
  • K. Ye, Y. Aoki, E. Tsuji, S. Nagata, H. Habazaki
    Journal of Power Sources 205 194 - 200 2012年05月 [査読有り][通常論文]
     
    Amorphous ZrO2-WO3-SiO2 nanofilms are simply prepared by anodizing of sputter-deposited Zr37W47Si16 alloy at several formation voltages for 1.8 ks in 0.1 mol dm(-3) phosphoric acid electrolyte at 20 degrees C. Efficient proton conductivity was observed after thermal treatment at 250 degrees C with the conductivity enhanced by reducing the film thickness. The conductivity is enhanced more than one order of magnitude by reducing the thickness from 300 to 140 nm. The anodic oxide films consist of two layers, comprising a thin outer ZrO2 layer free from silicon and tungsten species and an inner main layer containing all zirconium, silicon and tungsten species. The thickness-dependent conductivity of the anodic ZrO2-WO3-SiO2 films is associated with the conductivity of the outer ZrO2 layer, which increases exponentially with reducing the film thickness. The area-specific resistivity of 0.14 Omega cm(2), which is below the minimum requirement (0.2 Omega cm(2)) for a practical electrolyte membrane in commercial fuel cells, is achieved at a temperature at 225 degrees C for 100 nm-thick anodic ZrO2-WO3-SiO2 films. (C) 2012 Elsevier B.V. All rights reserved.
  • S. Komiyama, E. Tsuji, Y. Aoki, H. Habazaki
    Journal of Solid State Electrochemistry 16 4 1595 - 1604 2012年04月 [査読有り][通常論文]
     
    The growth behavior of amorphous anodic films on Ta-Nb solid solution alloys has been investigated over a wide composition range at a constant current density of 50 A m(-2) in 0.1 mol dm(-3) ammonium pentaborate electrolyte. The anodic films consist of two layers, comprising a thin outer Nb2O5 layer and an inner layer consisting of units of Ta2O5 and Nb2O5. The outer Nb2O5 layer is formed as a consequence of the faster outward migration of Nb5+ ions, compared with Ta5+ ions, during film growth under the high electric field. Their relative migration rates are independent of the alloy composition. The formation ratio, density, and capacitance of the films show a linear relation to the alloy composition. The susceptibility of the anodic films to field crystallization during anodizing at constant voltage increases with increasing niobium content of the alloy.
  • Masahiro Seo, Koji Fushimi, Yoshitaka Aoki, Hiroki Habazaki, Masayuki Inaba, Mitsutoshi Yokomizo, Takazumi Hayakawa, Takenori Nakayama
    JOURNAL OF ELECTROANALYTICAL CHEMISTRY 671 7 - 15 2012年04月 [査読有り][通常論文]
     
    In situ X-ray absorption spectroscopy (XAS) using a synchrotron radiation was applied to identify the Pb species adsorbed on Ni surface in acidic perchlorate solution containing 10(-4) M Pb2+ with relation to the Pb-induced stress corrosion cracking (Pb-SCC) of Ni-base alloys used as tubing materials of steam generators in nuclear power plants. The periodical emersion method under potentiostatic polarization, using the Ni plate with large specific surface area (surface roughness, S-r = 78.3) as a working electrode, was developed to detect sensitively the sub-monolayer coverage of Pb on Ni. The Pb L-III absorption spectra in a scanning XAS mode were measured by monitoring the Pb L alpha fluorescence line. The Pb L-III absorption near-edge structure (XANES) at the potential more positive than -0.245 V (SHE) corresponding to the equilibrium potential of Pb-electrodeposition in 10(-4) M Pb2+ solution has revealed that the Pb species adsorbed on the Ni surface are metallic, providing the clear evidence of underpotential deposition (UPD) of Pb on the Ni surface. Moreover, the extended X-ray absorption fine structure (EXAFS) analysis was performed with a two-shell fit involving Pb-Ni and Pb-Pb interactions assuming that the Ni (1 1 1) plane is mainly exposed to the solution. The EXAFS results were convincingly explained in terms of the Pb-UPD model which represents the coexistence of the surface alloy phase and the adlayer with the same p(2 x 2) structure in the narrow potential range of -0.185 V (SHE) to -0.245 V (SHE). (C) 2012 Elsevier B.V. All rights reserved.
  • S. P. Sah, E. Tsuji, Y. Aoki, H. Habazaki
    Corrosion Science 55 90 - 96 2012年02月 [査読有り][通常論文]
     
    Sequential anodic and cathodic pulse voltages were applied on anodised Al micro-electrodes in alkaline silicate electrolyte to explore the role of cathodic pulse in AC or bipolar plasma electrolytic oxidation (PEO) process. SEM observation was carried out to observe the sites of anodic and cathodic breakdown and their morphologies. The prior anodic breakdown accelerated the cathodic breakdown at -50 V, and the acceleration was associated with the preferential cathodic breakdown at the anodic breakdown sites. However, the succeeding anodic breakdown during applying anodic pulse of 420 V for 2 ms was highly suppressed at the cathodic breakdown sites. This would randomise the anodic breakdown sites. Such role may contribute to the formation of rather uniform coatings on aluminium in this electrolyte without large discharge channels when larger cathodic current is applied with respect to the anodic current in AC PEO. (C) 2011 Elsevier Ltd. All rights reserved.
  • H. Habazaki, T. Fujii, E. Tsuji, Y. Aoki
    ECS Transactions 50 37 217 - 224 2012年 [査読有り][通常論文]
     
    In this study, hierarchical dual pore surfaces have been developed by two-step anodizing of aluminum. The first anodizing has been carried out at 390 V in 2 wt% citric acid, and then, the resultant anodic film has been dissolved to disclose the scalloped aluminum substrate with a cell size of ∼900 nm. Subsequently, the second anodizing has been carried out at 25 V in 0.3 mol dm-3 sulfuric acid electrolyte to generate smaller nanopores. The size of nanopores has been further controlled by pore-widening treatment in 5 wt% phosphoric acid at 303 K. After coating with a fluoroalkyl phosphate monolayer to reduce the surface energy, the dual pillar surfaces with enlarged nanopores show super-repellency even for rapeseed oil as well as for water. © The Electrochemical Society.
  • Ke Ye, Yoshitaka Aoki, Etsushi Tsuji, Shinji Nagata, Hiroki Habazaki
    ECS Transactions 50 27 193 - 201 2012年 [査読有り][通常論文]
     
    The proton conductivity of the anodic ZrO2-WO 3-SiO2 nanofilms prepared by anodizing of sputter-deposited Zr37W47Si16 alloy at several formation voltages for 1.8 ks in 0.1 mol dm-3 phosphoric acid electrolyte at 20°C has been examined below 250°C in various environments. The proton conductivity activated by thermal treatment at 250°C is not influenced by the presence of O2 and H2O in the atmosphere. However, the conductivity is enhanced by one order of magnitude in H2-containing atmosphere. The H2-induced conductivity enhancement is reversible the enhanced conductivity is returned to an original value by exposing again in an O 2- containing atmosphere. © The Electrochemical Society.
  • Y. Konno, S. Yang, E. Tsuji, Y. Aoki, P. Skeldon, G. E. Thompson, H. Habazaki
    ECS Transactions 50 37 183 - 190 2012年 [査読有り][通常論文]
     
    The growth behavior of nanoporous anodic films on carbon steel containing 0.213 mass% carbon has been examined during anodizing in ethylene glycol containing 0.1 mol dm-3 NH4F and 0.5 mol dm-3 H2O. The nanopores in the anodic films are generated preferentially along with steps and at "kink-like" sites of the steel surface generated by rf-GD sputtering. The steel contains carbide precipitates with a size of 50-800 nm. The anodic oxide formed on the carbide phase is more chemically soluble during anodizing, producing pits in the anodic films. © The Electrochemical Society.
  • Yoshitaka Aoki, Akihisa Harada, Aiko Nakao, Toyoki Kunitake, Hiroki Habazaki
    PHYSICAL CHEMISTRY CHEMICAL PHYSICS 14 8 2735 - 2742 2012年 [査読有り][通常論文]
     
    The finite size effect of proton conductivity of amorphous aluminosilicate thin films, a-AlnSi1-nOx (n = 0.07, 0.1, 0.2, 0.3 and 0.45), prepared by a sol-gel process was investigated by experimental and numerical techniques. High-resolution TEM clarified that a-AlnSi1-nOx films had the heterogeneous nanoscale microstructures comprised of the ion-conducting, condensed glass microdomain and the poor-conductive, uncondensed glass microdomain. s of the films with n <= 0.1 exponentially increased upon decreasing thickness in the sub-100 nm range because the volume fraction of conductive domains was less than the percolation threshold and cluster size scaling of the conductive domain was operative. The numerical simulation suggested that conductance of the condensed domain was higher than that of the uncondensed domain by 2 orders of magnitude. Volume fractions of the condensed domain increased with increasing Al/Si molar ratio and were over the percolation threshold (24.5%) with n >= 0.2. However, conductance of the condensed domain decreased with increasing Al/Si ratio with n >= 0.2 because the aluminosilicate glass framework made of 4-fold-connected MO4 tetrahedra was deformed by forming the octahedral AlO6 moieties, as checked by Al K-edge XAS. It was found that the optimal Al/Si composition in terms of the conductance of the condensed domain is not in coincidence with that in terms of the average conductivity of the films.
  • Shun Koyama, Yoshitaka Aoki, Shinji Nagata, Hiroki Habazaki
    JOURNAL OF SOLID STATE ELECTROCHEMISTRY 15 10 2221 - 2229 2011年10月 [査読有り][通常論文]
     
    Zr-Al alloys containing up to 26 at.% aluminum, prepared by magnetron sputtering, have been anodized in 0.1 mol dm(-3) ammonium pentaborate electrolyte, and the structure and dielectric properties of the resultant anodic oxide films have been examined by grazing incidence X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy, and AC impedance spectroscopy. The anodic oxide film formed on zirconium consists of monoclinic and tetragonal ZrO(2) with the former being a major phase. Two-layered anodic oxide films, comprising an outer thin amorphous layer and an inner main layer of crystalline tetragonal ZrO(2) phase, are formed on the Zr-Al alloys containing 5 to 16 at.% aluminum. Further increase in the aluminum content to 26 at.% results in the formation of amorphous oxide layer throughout the thickness. The anodic oxide films become thin with increasing aluminum content, while the relative permittivity of anodic oxide shows a maximum at the aluminum content of 11 at.%. Due to major contribution of permittivity enhancement, the maximum capacitance of the anodic oxide films is obtained on the Zr-11 at.% Al alloy, being 1.7 times than on zirconium at the formation voltage of 100 V.
  • Takashi Fujii, Yoshitaka Aoki, Hiroki Habazaki
    LANGMUIR 27 19 11752 - 11756 2011年10月 [査読有り][通常論文]
     
    Hierarchical dual pillar surfaces with optimized pillar intervals are fabricated by a novel combined process of the c oblique angle magnetron sputtering deposition of Al-Nb alloys and their anodizing. The pillar intervals are controlled by the deposition angle and cell size of a scalloped substrate for oblique angle deposition. Anodizing of the deposited pillar surfaces develops a nanopillar oxide layer, producing the hierarchical dual pillar surfaces. After being coated with a fluoroalkyl phosphate layer to reduce the surface free energy, hierarchical surfaces with submicrometer pillar intervals greater than 400 nm show super liquid repellency even for hexadecane with a low surface tension of 27.5 mN m(-1), although the submicrometer pillar surfaces with smaller submicrometer pillar intervals and without nanopillars were not super-oil-repellent. In contrast, the dual pillar surfaces show superhydrophobicity regardless of the submicrometer pillar intervals. Thus, the present study demonstrates the importance of the pillar intervals (gap size between pillars) to realize the superoleophobicity.
  • Shu Yang, Hiroki Habazaki, Takashi Fujii, Yoshitaka Aoki, Peter Skeldon, George E. Thompson
    ELECTROCHIMICA ACTA 56 22 7446 - 7453 2011年09月 [査読有り][通常論文]
     
    We report the fabrication of superhydrophobic surfaces with a hierarchical morphology by self-organized anodizing process. Simply by anodizing of niobium metal in hot phosphate-glycerol electrolyte, niobium oxide microcones, consisting of highly branched oxide nanofibers, develop on the surface. The size of the microcones and their tip angles are controlled by changing the applied potential difference in anodizing and the water content in the electrolyte. Reduction of the water content increases the size of the microcones, with the nanofibers changing to nanoparticles. The size of microcones is also reduced by increasing the applied potential difference, without influencing the tip angle. The hierarchical oxide surfaces are superhydrophilic, with static contact angles close to 0 degrees. Coating of the anodic oxide films with a monolayer of fluoroalkyl phosphate makes the surfaces superhydrophobic with a contact angle for water as high as 175 degrees and a very small contact angle hysteresis of only 2 degrees. The present results indicate that the larger microcones with smaller tip angles show the higher contact angle for water. (C) 2011 Elsevier Ltd. All rights reserved.
  • S. Yang, Y. Aoki, H. Habazaki
    APPLIED SURFACE SCIENCE 257 19 8190 - 8195 2011年07月 [査読有り][通常論文]
     
    Nanoporous niobium oxide films with microcone-type surface morphology were formed by anodizing at 10 V in glycerol electrolyte containing 0.6 mol dm(-3) K(2)HPO(4) and 0.2 mol dm(-3) K(3)PO(4) in a temperature range of 428-453 K. The microcones appeared after prolonged anodizing, but the required time was largely reduced by increasing electrolyte temperature. The anodic oxide was initially amorphous at all temperatures, but crystalline oxide nucleated during anodizing. The anodic oxide microcones, which were crystalline, appeared on surface as a consequence of preferential chemical dissolution of initially formed amorphous oxide. The chemical dissolution of an initially formed amorphous layer was accelerated by increasing the electrolyte temperature, with negligible influence of the temperature on the morphology of microcones up to 448 K. (C) 2011 Elsevier B. V. All rights reserved.
  • Takashi Fujii, Yoshitaka Aoki, Hiroki Habazaki
    APPLIED SURFACE SCIENCE 257 19 8282 - 8288 2011年07月 [査読有り][通常論文]
     
    A combined process of oblique angle magnetron sputtering and anodizing has been developed to tailor superhydrophobic surfaces with hierarchical morphology. Isolated submicron columns of single-phase Al-Nb alloys are deposited by magnetron sputtering at several oblique deposition angles on a scalloped substrate surface, with the gaps between columns increasing with an increase in the deposition angle from 70 degrees to 110 degrees. Then, the columnar films have been anodized in hot phosphate-glycerol electrolyte to form a nanoporous anodic oxide layer on each column. Such surfaces with submicron-/nano-porous structure have been coated with a fluoroalkyl phosphate layer to reduce the surface energy. The porous surface before coating is superhydrophilic with a contact angle for water is less than 10 degrees, while after coating the contact angles are larger than 150 degrees, being superhydrophobic. The beneficial effect of dual-scale porosity to enhance the water repellency is found from the comparison of the contact angles of the submicron columnar films with and without nanoporous oxide layers. The larger submicron gaps between columns are also preferable to increase the water repellency. (C) 2011 Elsevier B. V. All rights reserved.
  • M. Tauseef Tanvir, T. Fujii, Y. Aoki, K. Fushimi, H. Habazaki
    APPLIED SURFACE SCIENCE 257 19 8295 - 8300 2011年07月 [査読有り][通常論文]
     
    For electrolytic capacitor application of the single-phase Ti alloys containing supersaturated silicon, which form anodic oxide films with superior dielectric properties, porous Ti-7 at% Si columnar films, as well as Ti columnar films, have been prepared by oblique angle magnetron sputtering on to aluminum substrate with a concave cell structure to enhance the surface area and hence capacitance. The deposited films of both Ti and Ti-7 at% Si have isolated columnar morphology with each column revealing nanogranular texture. The distances between columns are similar to 500 nm, corresponding to the cell size of the textured substrate and the gaps between columns are 100-200 nm. When the porous Ti-7 at% Si film is anodized at a constant current density in ammonium pentaborate electrolyte, the growth of a uniform amorphous oxide film continues to similar to 35 V, while it is limited to less than 6 V on the porous Ti film. The maximum voltage of the growth of uniform amorphous oxide films on the Ti-7 at% Si films is similar for both the flat and porous columnar films, suggesting little influence of surface roughness on the amorphous-to-crystalline transition of growing anodic oxide under the high electric field. Due to the suppression of crystallization to sufficiently high voltages, the anodic oxide films formed on the porous Ti-7 at% Si film shows markedly improved dielectric properties, in comparison with those on the porous Ti film. (C) 2011 Elsevier B. V. All rights reserved.
  • Habazaki H, Koyama S, Aoki Y, Sakaguchi N, Nagata S
    ACS applied materials & interfaces 3 7 2665 - 2670 7 2011年07月 [査読有り][通常論文]
     
    Anodic oxide films with nanocrystalline tetragonal ZrO2 precipitated in an amorphous oxide matrix were formed on Zr-Si and Zr-Al alloys and had significantly enhanced capacitance in comparison with those formed on zirconium metal. The capacitance enhancement was associated with the formation of a high-temperature stable tetragonal ZrO2 phase with high relative permittivity as well as increased ionic resistivity, which reduces the thickness of anodic oxide films at a certain formation voltage. However, there is a general empirical trend that single-phase materials with higher permittivity have lower ionic resistivity. This study presents a novel material design based on a nanocrystalline-amorphous composite anodic oxide film for capacitor applications.
  • S. Tsunekawa, Y. Aoki, H. Habazaki
    SURFACE & COATINGS TECHNOLOGY 205 19 4732 - 4740 2011年06月 [査読有り][通常論文]
     
    Ti-15V-3Al-3Cr-35n (Ti-15-3) is one of the important practical titanium alloys with high cold deformability and high mechanical strength, but its wear resistance is poor. This paper reports the formation of wear-resistant and adhesive ceramic coatings on Ti-15-3 by two-step plasma electrolytic oxidation (PEO). The PEO of Ti-15-3 has been carried out first in alkaline aluminate electrolyte to form a wear-resistant oxide layer and then in acid electrolyte containing both phosphoric acid and sulfuric acid to improve adhesion of the coating. The coating formed in the alkaline aluminate electrolyte is more than 10 mu m thick, and highly crystalline. The main phase is Al(2)TiO(5). This coating shows high wear resistance, but is not adherent to substrate due to the development of a number of voids and pores in the oxide layer close to the substrate. A new oxide layer with amorphous structure is formed next to the substrate in the subsequent PEO in the acid electrolyte, during which the voids are filled with a new oxide formed in the acid electrolyte, reducing the porosity. As a consequence, the adhesion of the coating is markedly improved without deteriorating the high wear resistance. (C) 2011 Elsevier B.V. All rights reserved.
  • Yoshitaka Aoki, Hiroki Habazaki, Toyoki Kunitake
    SOLID STATE IONICS 192 1 93 - 96 2011年06月 [査読有り][通常論文]
     
    Amorphous hafnium silicate, a-Hf0.1Si0.9Ox, thin film with thickness of 32, 41, 55, 80, 110, 120, 180 and 320 nm was prepared by multiple spin-cast process and the proton conductivity across the films was measured at intermediate temperatures (100-400 degrees C) in dry atmosphere. The morphologically- and compositionally-uniform films were prepared on a substrate as confirmed by SEM. RBS and XPS measurements. a-Hf0.1Si0.9Ox thin film clearly revealed the H/D isotope effect on ionic conductivity, indicating that protonic conduction is dominant in the measured temperature range. The films did not reveal thickness-dependent proton conductivity in dry air and the sigma at given temperatures is almost constant at any thickness. No increment of sigma in a-Hf0.1Si0.9Ox thin films by reduction of thickness might be related to the absence of the highly-conductive acid network with mesoscopically-sized length because of the relatively low concentration of Bronsted acid sites inside films. (C) 2010 Elsevier B.V. All rights reserved.
  • Santosh Prasad Sah, Yasuhiro Tatsuno, Yoshitaka Aoki, Hiroki Habazaki
    CORROSION SCIENCE 53 5 1838 - 1844 2011年05月 [査読有り][通常論文]
     
    Single pulse anodizing of aluminium micro-electrode has been employed to study the behaviour of dielectric breakdown and subsequent oxide formation on aluminium in alkaline silicate and pentaborate electrolytes. Current transients during applying pulse voltage have been measured, and surface has been observed by scanning electron microscopy. Two types of current transients are observed, depending on the electrolyte and applied voltage. There is a good correlation between the current transient behaviour and the shape of discharge channels. In alkaline silicate electrolyte, circular open pores are healed by increasing the pulse width, but such healing is not obvious in pentaborate electrolyte. (C) 2011 Elsevier Ltd. All rights reserved.
  • S. Yang, Y. Aoki, P. Skeldon, G. E. Thompson, H. Habazaki
    JOURNAL OF SOLID STATE ELECTROCHEMISTRY 15 4 689 - 696 2011年04月 [査読有り][通常論文]
     
    Growth of porous anodic alumina films has been examined at 10 V in hot phosphate-containing glycerol electrolyte containing 0.1 to 0.57 mass% water. The growth rate of the films is highly dependent upon the water content of the electrolyte, reducing markedly at a water content of 0.1 mass%, an opposite trend to that found previously for the formation of porous films on titanium and niobium. Chemical dissolution of the anodic alumina is also suppressed in electrolyte of low water content. GDOES depth profiles revealed that an increased water content of the electrolyte promoted the incorporation of phosphorus species into the films, although chemical dissolution reduced the amounts of phosphorus in the outer regions. Carbon species also appeared to be present in films, particularly at lower water content. Using a niobium oxide outer layer to suppress chemical dissolution resulted in films that were about 1.2 times the thickness of the consumed aluminium for an electrolyte containing 0.25 mass% water. The expansion suggests a possible contribution of field-assisted flow of film material in the growth of the porous anodic film.
  • Yoshitaka Aoki, Hiroki Habazaki, Shinji Nagata, Aiko Nakao, Toyoki Kunitake, Shu Yamaguchi
    JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 133 10 3471 - 3479 2011年03月 [査読有り][通常論文]
     
    The finite size effect of proton conductivity of amorphous silicate thin films, a-M0.1Si0.9Ox (M = Al, Ga, Hf, Ti, Ta, and La), was investigated. The proton conductivity across films, sigma, was measured in dry air by changing the thickness in the range of 10-1000 nm. sigma of the films with M = Al, Ga, and Ta was elevated in a power law by decreasing thickness into less than a few hundred nanometers, and the increment was saturated at a thickness of several 10's of nanometers. On the other hand, a of the films with M = Hf, Ti, and La was not related to the decrease of the thickness in the range of >10 nm. Thickness-dependent conductivity of the former could be numerically simulated by a percolative resistor network model that involves the randomly distributed 3 4 5 6 7 8 2 3 4 5 6 7 8 array of two kinds of resistors R-1 and R-2 (R-1 > R-2) in the form of a simple cubic-type lattice. High-resolution TEM clarified that a-M0.1Si0.9Ox films involved heterogeneous microstructures made of the condensed domain and the surrounding uncondensed matrix due to the fluctuation of glass networks on the nanometer scale. The condensed domain had a wormlike shape with an average length of several 10's of nanometers and performed the role of the proton conduction pathway penetrating through the poorly conducting matrix. It was concluded that the thickness-dependent conductivity could be identical to finite-size scaling of the percolative network of the interconnected domains in the nanometer range.
  • Ke Ye, Yoshitaka Aoki, Etsushi Tsuji, Shinji Nagata, Hiroki Habazaki
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY 158 11 C385 - C390 2011年 [査読有り][通常論文]
     
    Novel proton-conducting amorphous anodic ZrO2-WO3-SiO2 films, 200 nm thick, are prepared by anodizing of sputter-deposited Zr37W47Si16 at 100 V with current decay for 1.8 ks in 0.1 mol dm(-3) phosphoric acid electrolyte at 20 degrees C. The resultant anodic films have been characterized using electrochemical impedance spectroscopy, transmission electron microscopy, glow discharge optical emission spectroscopy and Rutherford backscattering spectroscopy. The addition of silicon species to the anodic ZrO2-WO3 film significantly enhanced the thermal stability. Even after thermal treatment at 300 degrees C in dry Ar atmosphere, the anodic ZrO2-WO3-SiO2 films revealed stable proton conductivity in the temperature range of 50-225 degrees C, while the anodic ZrO2-WO3 on the Zr43W57 loses the proton conductivity by annealing at 250 degrees C. The anodic film on the Zr37W47Si16 consisted of two layers, comprising an outer thin ZrO2 layer, free from tungsten and silicon species, and an inner main layer containing all zirconium, tungsten and silicon species. The results in this study suggest that the conductivity deterioration at high annealing temperatures is associated with the diffusion-induced formation of a poorly-conducting layer near the alloy/anodic oxide interface. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.068111jes] All rights reserved.
  • Y. Aoki, S. Hirata, H. Habazaki
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY 158 4 P41 - P44 2011年 [査読有り][通常論文]
     
    The proton conductivity sigma across 100 nm thick, amorphous aluminum phosphate thin films under anhydrous conditions was investigated. The densely packed glass films were uniformly formed without formation of pinholes and clacks over the electrode substrate by multiple spin-coating with a mixed precursor sol, as checked by transmission electron microscopy. X-ray absorption spectroscopy, and Fourier transform infrared indicated that the Al-rich films were mainly composed of alumina and phosphate mixed glass phase, but the P-rich films involved a large amount of the aluminum metaphosphate glass moiety. sigma abruptly changed with Al/P ratio owing to this variation of glass network structure. The Al-rich films revealed the large activation energy E-a of about 1.0 eV, but the P-rich films revealed the small E-a of about 0.2 eV above 200 degrees C and they kept the value on the order of 10(-5) S cm(-1) at the temperatures. Consequently, the sigma of P-rich films were one order of magnitude higher than that of the others at around 300 degrees C. (C) 2011 Electrochemical Society. [DOI: 10.1149/1.3551572] All rights reserved.
  • Y. Aoki, Y. Fukunaga, H. Habazaki, T. Kunitake
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY 158 8 B866 - B870 2011年 [査読有り][通常論文]
     
    Novel thin film fuel cell based on the 100 nm-thick electrolyte of amorphous ZrP(2.6)O(x), working at 400 degrees C, was demonstrated. The hydrogen permeable membrane fuel cell (HMFC) using a Pd foil as a nonporous solid anode was fabricated. Ni interlayer of several hundreds nm thickness was introduced between the Pd anode and the ZrP(2.6)O(x) electrolyte in order to suppress the deterioration of the electrolyte nanofilm by the deformation of the Pd anode during hydrogen absorption. In the ZrP(2.6)O(x) electrolyte the transport number of proton was unity at 400 degrees C as determined by an EMF measurement. The modification of the Ni anode surface by an ultrathin Pt or Pd layer effectively decreased the anode/electrolyte interfacial polarization. Consequently, the HMFC revealed the OCV of 1.0 V and the maximum power density of 1.8 mW cm(-2) at 400 degrees C. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3595388] All rights reserved.
  • Hiroki Habazaki, Yoshiki Konno, Yoshitaka Aoki, Peter Skeldon, George E. Thompson
    JOURNAL OF PHYSICAL CHEMISTRY C 114 44 18853 - 18859 2010年11月 [査読有り][通常論文]
     
    The growth of porous anodic films on iron has been examined at a constant current density of 50 A m(-2) in 0.1 mol L(-1) NH(4)F-elhylene glycol electrolytes containing 0.1-1.5 mol L(-1) water. Nanoporous films are formed in all the electrolytes, with the growth rate increasing with the decrease in the water content of the electrolyte. A barrier layer, in which a high electric field is applied during anodizing, thickens in proportion to the formation voltage at a ratio of 1.9 nm V(-1), regardless of the water content of the electrolyte. However. there is a transition water content between 0.3 and 0.5 mol L(-1). at which growth behavior changes. Above the transition level, the formation voltage is constant after an initial voltage rise, with the constant voltage slightly rising with a decrease in water content. In contrast, the formation voltage increases continuously to more than 150 V when the water contents are below the transition level. The anodic films are poorly crystalline and contain a significant amount of fluoride species. A high enrichment of fluoride species occurs near the metal/film interface when the water content in the electrolyte is below the transition level. Such enrichment is not as significant, or possibly absent, in electrolytes of increased water content.
  • Yoshitaka Aoki, Kota Ogawa, Hiroki Habazaki, Toyoki Kunitake, Yuanzhi Li, Shinji Nagata, Shu Yamaguchi
    CHEMISTRY OF MATERIALS 22 19 5528 - 5536 2010年10月 [査読有り][通常論文]
     
    Amorphous zirconium phosphate thin films, a-ZrP2.5Ox, revealed unique proton conductivity transition induced by reducing thickness due to the formation of highly conductive, hydrated nanolayer. The dense films made of a metaphosphate glass phase were uniformly formed over the electrode substrate by multiple spin-coating with a mixed precursor sol. as checked by TEM and RBS. When thickness d was larger than 60 nm, the proton conductivity across film and the activation energy E were not variable with d. sigma abruptly increased 200 times and E-a decreases from 0,9 to 0.7 eV when d decreased from 60 to 40 nm. and it became thickness-independent again in d < 40 nm. a of 100 mm-thick film is increased to the similar value as that of the 40 nm thick by annealing at 400 degrees C in H2O/air. It was concluded that the conductivity transition could be associated with the hydration of metaphosphate nanolayer. The hydrated. high-conductive phase was very stable Only when the thickness was less than 100 nm. Therefore, the films of more than hundreds nm thickness cannot change to the high-conducting hydrated phase throughout the film thickness. These unprecedented behaviors could not be explicable with a simple model based on the core space charge or continuum structural relaxation at hetrointerface.
  • Takashi Fujii, Yoshitaka Aoki, Koji Fushimi, Takeshi Makino, Shoji Ono, Hiroki Habazaki
    NANOTECHNOLOGY 21 39 395302  2010年10月 [査読有り][通常論文]
     
    Nanopillar films of Al-Nb alloys have been fabricated on substrates with a regular concave cell structure by oblique angle physical vapor deposition. The concave cell structure of the substrate increases the shadow region for the flux of depositing atoms, assisting the formation of an isolated nanopillar on each cell. Depending upon the alloy composition and deposition angle, the pillar shape changes from horn-like nanopillars through triangular nanoprisms to nanoplates. The Al-Nb alloy nanoplate films with wide gaps between plates are of interest as electrodes for capacitor applications. The dielectric oxide film formed on the nanoplate film showed a capacitance more than ten times larger than that on the respective flat film, due to the enlarged surface area.
  • H. Habazaki, M. Teraoka, Y. Aoki, P. Skeldon, G. E. Thompson
    ELECTROCHIMICA ACTA 55 12 3939 - 3943 2010年04月 [査読有り][通常論文]
     
    The present study reveals the formation of porous anodic films on titanium at an increased growth rate in hot phosphate/glycerol electrolyte by reducing the water content. A porous titanium oxide film of 12 mu m thickness, with a relatively low content of phosphorus species, is developed after anodizing at 5 V for 3.6 ks in 0.6 mol dm(-3) K(2)HPO(4) +0.2 mol dm(-3) K(3)PO(4)/glycerol electrolyte containing only 0.04% water at 433K. The growth efficiency is reduced by increasing the formation voltage to 20V, due to formation of crystalline oxide, which induces gas generation during anodizing. The film formed at 20V consists of two layers, with an increased concentration of phosphorus species in the inner layer. The outer layer, comprising approximately 25% of the film thickness, is developed at low formation voltages, of less than 10 V, during the initial anodizing at a constant current density of 250 A m(-2). The pore diameter is not significantly dependent upon the formation voltage, being similar to 10 nm. (C) 2010 Elsevier Ltd. All rights reserved.
  • S. Koyama, Y. Aoki, S. Nagata, H. Kimura, H. Habazaki
    ELECTROCHIMICA ACTA 55 9 3144 - 3151 2010年03月 [査読有り][通常論文]
     
    Sputter-deposited zirconium and Zr-16 at.% Si alloy have been anodized to various voltages at several formation voltages in 0.1 mol dm(-3) ammonium pentaborate electrolyte at 298 K for 900 s. The resultant anodic films have been characterized using X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy, glow discharge optical emission spectroscopy, and electrochemical impedance spectroscopy. The anodic oxide films formed on Zr-16 at.% Si are amorphous up to 30 V, but the outer part of the anodic oxide films crystallizes at higher formation voltages. This is in contrast to the case of sputter-deposited zirconium, on which the crystalline anodic oxide films, composed mainly of monoclinic ZrO2, are developed even at low formation voltages. The outer crystalline layer on the Zr-16 at.% Si consists of a high-temperature stable tetragonal phase of ZrO2. Due to immobile nature of silicon species, silicon-free outermost layer is formed by simultaneous migrations of Zr4+ ions outwards and O2- ions inwards. An intermediate crystalline oxide layer, in which silicon content is lower in comparison with that in the innermost layer, is developed at the boundary of the crystalline layer and amorphous layer. Capacitances of the anodic zirconium oxide are highly enhanced by incorporation of silicon due to reduced film thickness, even though the permittivity of anodic oxide decreases with silicon incorporation. (C) 2010 Elsevier Ltd. All rights reserved.
  • Yoshitak Aoki, Hiroki Habazaki, Toyoki Kunitake
    SOLID STATE IONICS 181 3-4 115 - 121 2010年02月 [査読有り][通常論文]
     
    Ceramic electrolytes operating in the temperature range of 200 to 500 degrees C under dry atmosphere are a key material for the next-generation fuel cell and related applications. We discovered that nanometer-thick films of amorphous hafnium silicate (Hf(n)Si(1-n)O(x)) exhibited efficient ionic conduction at 100-400 degrees C in dry air. When the fraction of hafnium doping was around 0.1, the nanofilm showed a low area-specific-resistance (<0.15 Omega cm(2)) at around 350 degrees C that was small enough for the practical fuel cell application. The sub-100 nm-thick membranes of Hf(0.13)Si(0.87)O(x), could be fabricated on porous Pt/alumina substrate to provide gas concentration cells. The electromotive force observed with H(2) concentration cell indicated that the ceramic nanomembrane acted as predominant proton conductor without permeation of H(2) gas. In addition, the Hf(0.13)Si(0.87)O(x) and Zr(0.11)Si(0.89)O(x) membranes responded to the change of O(2) pressures in O(2) concentration cells, while the Al(0.16)Si(0.84)O(x) and Ce(0.06)Si(0.94)O(x) membranes did not produce the electrical voltage by gradient of O(2) pressure. We conclude that Hf(0.13)Si(0.87)O(x) nanomembrane is promising as electrolyte material for fuel cells and related ionics devices. (C) 2009 Published by Elsevier B.V.
  • Santosh Prasad Sah, Yoshitaka Aoki, Hiroki Habazaki
    MATERIALS TRANSACTIONS 51 1 94 - 102 2010年01月 [査読有り][通常論文]
     
    Plasma electrolytic oxidation of AZ80 magnesium alloy in alkaline aluminate electrolytes develops MgAl2O4-based highly crystalline oxide coatings with the morphology changing largely with phosphate concentration in electrolyte. The thickness of the coatings increases with phosphate concentration from 5 mu m in phosphate-free electrolyte to similar to 70 mu m in the electrolyte containing 0.1 mol.dm(-3) phosphate after anodizing for 900 s. The formation of the latter thick coating is associated with intense sparking during, anodizing. The thick coatings formed in the electrolytes containing 0.075 and 0.1 mol.dm(-3) phosphate are highly cracked. In contrast, the coating formed in the electrolyte containing 0.05 mol.dm(-3) phosphate is relatively uniform, showing the highest corrosion protection in 0.5 mol.dm(-3) NaCl solution. The coatings consist of two layers, comprising an outer thick layer with high concentration of aluminum and an inner thin magnesium-rich layer. [doi:10.2320/matertrans.M2009226]
  • Yuanzhi Li, Emi Muto, Yoshitaka Aoki, Toyoki Kunitake
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY 157 8 B1103 - B1108 2010年 [査読有り][通常論文]
     
    Uniform, defect-free nanofilms (around 100 nm thick) of hafnium phosphate were prepared via layer-by-layer deposition of precursor solution of metal alkoxides and were shown to give practically useful proton conductivity at 300-400 degrees C as fuel cell electrolyte membrane. Annealing of the deposited precursor film at 500 degrees C gave a lower area specific resistance (R(AS)) than that at 400 degrees C, and this effect was coincident with the formation of the pyrophosphate unit. The best (lowest) R(AS) value of less than 0.1 Omega cm(2) for hafnium phosphate membrane (annealing at 500 degrees C, 291 nm thick, conductivity of 3 x 10(-4) S cm(-1) at 320 degrees C) was comparable to that of a nanofilm of an yttrium-doped zirconium phosphate. The RAS values of nanofilms of zirconium sulfate and zirconium borate were 2-3 orders higher than that of the corresponding zirconium phosphate. It is clear from these results that nanofilms of various solid acids are promising candidates for the electrolyte membrane of fuel cells operating at the intermediate temperatures of 300-400 degrees C. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3432600] All rights reserved.
  • S. Koyama, Y. Aoki, N. Sakaguchi, S. Nagata, H. Habazaki
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY 157 12 C444 - C451 2010年 [査読有り][通常論文]
     
    Capacitance enhancement of anodic oxide films on zirconium by adding silicon is reported here with correlation to the phase transformation of the oxide. The anodic oxide film formed on zirconium consists mainly of monoclinic ZrO2, which changes to tetragonal ZrO2 phase on the Zr-5.5 atom % Si. Further increase in the silicon contents to 10 and 16 atom % results in the formation of amorphous oxide up to 30 V, above which two-layered films, comprising an outer crystalline tetragonal-phase oxide layer and an inner amorphous layer, are developed. The relative thickness of the outer crystalline layer to the total film thickness increases with formation voltage. The highest capacitance of the anodic oxide films is obtained on the Zr-10 atom % Si. The changes in capacitance, permittivity and formation ratio of anodic oxide films with alloy composition are discussed with phase transformation and growth process of anodic oxides. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3503592] All rights reserved.
  • M. Tauseef Tanvir, Y. Aoki, H. Habazaki
    THIN SOLID FILMS 517 24 6711 - 6716 2009年10月 [査読有り][通常論文]
     
    The present work demonstrates the formation of porous niobium films with separated columnar structures by oblique angle magnetron sputtering for capacitor application. The mobium films deposited on textured aluminium substrates, which had concave cell structures with the cell sizes ranging from 125 nm to 550 nm, consist of isolated columns of niobium with wider gaps between columns developing on the substrates with larger cell sizes. The surface areas of the deposited films. evaluated by the capacitance of the anodic films formed at several voltages, increased with an increase in the cell size of substrate. The surface area decreases with an increase in the formation potential of anodic films from 2 V to 10 V vs Ag/AgCl, because the gaps are filled with anodic oxide as a consequence of the large Pilling-Bedworth ratio of 2.6 for the Nb/Nb(2)O(5) system. The reduction of the surface area is suppressed when the substrate with larger cell size is used, due to the formation of niobium columns with wider gaps, which are not filled with anodic oxide. The high surface area even at higher formation voltages of the anodic films is a requisite for capacitor application. (C) 2009 Elsevier B.V. All rights reserved.
  • Yoshitaka Aoki, Hiroki Habazaki, Toyoki Kunitake
    JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 131 40 14399 - 14406 2009年10月 [査読有り][通常論文]
     
    Amorphous aluminosilicate nanofilms, a-Al(0.1)Si(0.9)O(x), exhibit unique size-enhancement of the proton conductivity along the thickness direction because of the presence of the zeolite-like, acid site network with the mesoscopically sized dimension inside glass matrix. The dense films with the thickness of 22-1400 nm were uniformly formed over the electrode substrate in nanometer thickness precision by multiple spin-coating with a mixed precursor Sol. XANES measurements indicated that the basic framework of a-Al(0.1)Si(0.9)O(x) films was similar to the zeolitic one, consisting of the corner-linkage Of SiO(4) and AlO(4) tetrahedral units. These films revealed the complex temperature- and humidity-dependency of proton conductivity by the existence of two kinds of protonic carriers: Bronsted acidic protons and Lewis acidic protons. The Bronsted acidic protons could be persistent in amorphous films at around 500 degrees C, as checked by thermal desorption spectroscopy, so that the film exhibited the humidity-independent proton conductivity at temperatures above 300 degrees C. Furthermore, the conductivity across the film a increased in a power low by reduction of the film thickness of to less than 120 nm as sigma proportional to d-(tau), and it was saturated when the thickness become less than 40 nm. The observed scaling index tau was 2.2 in agreement with the value of the theoretical index (2.3) of cluster size scaling in a three-dimensional percolation system. This conduction behavior is explicable by finite size-scaling of the highly conductive pathway based on the interconnected Bronsted acid centers in the range of a few tens to hundreds of nanometers.
  • Y. Oikawa, T. Minami, H. Mayama, K. Tsujii, K. Fushimi, Y. Aoki, P. Skeldon, G. E. Thompson, H. Habazaki
    ACTA MATERIALIA 57 13 3941 - 3946 2009年08月 [査読有り][通常論文]
     
    Porous anodic niobium oxide with a pore size of similar to 10 nm was formed at 10 V in glycerol electrolyte containing 0.6 mol dm(3) K(2)HPO(4) and 0.2 mol dm(-3) K(3)PO(4) at 433 K. After prolonged anodizing for 5.4 ks, niobium oxide microcones develop on the surface. X-ray diffraction patterns of the anodized specimens revealed that the initially formed anodic oxide is amorphous, but an amorphous-to-crystal-line transition occurs during anodizing. As a consequence of the preferential chemiccal dissolution of the initially formed amorphous oxide, due to different solubility of the amorphous and crystalline oxides, crystalline oxide microcones appear on the film surface after prolonged anodizing. The surface is superhydrophilic. After coating with fluorinated alkylsilane, the surface becomes superhydrophobic with a contact angle of 158 degrees for water. The surface is also oil repellent, with a contact angle as high as 140 degrees for salad oil. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
  • M. Uemura, T. Yamamoto, K. Fushimi, Y. Aoki, K. Shimizu, H. Habazaki
    CORROSION SCIENCE 51 7 1554 - 1559 2009年07月 [査読有り][通常論文]
     
    Thin passive films formed on highly corrosion-resistant type-312L stainless steel, containing 20 mass% chromium and 6 mass% molybdenum, in 2 mol dm(-3) HCl solution at 293 K have been analyzed by glow discharge optical emission spectroscopy (GDOES). The stainless steel does not suffer pitting corrosion even in this aggressive solution, showing a wide passive potential region. The depth profiles obtained clearly show a two-layer structure of the air-formed and passive films: an outer iron-rich layer and an inner layer highly enriched in chromium. Alloy-constituting molybdenum is deficient in the inner layer of the passive films and is enriched in the outer layer, particularly at the active dissolution potential. The molybdenum species in the outer layer may retard the active dissolution of stainless steel, promoting the formation of stable passive films highly enriched in chromium. Chloride ions are present only at the outermost part of the passive films, not penetrating into the interior part. (C) 2008 Elsevier Ltd. All rights reserved.
  • M. T. Tanvir, Y. Aoki, H. Habazaki
    Applied Surface Science 255 20 8383 - 8389 2009年07月 [査読有り][通常論文]
     
    In the present study, porous Nb-Si alloy films with isolated nano-column morphology have been successfully developed by oblique angle magnetron sputtering on to aluminum substrate with concave cell structure. The deposited films are amorphous with the 15 at% silicon supersaturated into niobium. The porous Nb-15 at% Si films, as well as niobium films with similar morphology, are anodized at several voltages up to 50 V in 0.1 mol dm (3) ammonium pentaborate electrolyte. Due to the presence of sufficient gaps between neighboring columns, the gaps are not filled with anodic oxide, despite the large Pilling-Bedworth ratio (for instance, 2.6 for Nb/Nb(2)O(5)) and hence, a linear correlation between the reciprocal of capacitance and formation voltage is obtained for the Nb-15 at% Si. From the comparison with the anodic films formed on porous niobium films, it has been found that silicon addition improves the thermal stability of anodic niobium oxide; the change in capacitance and increase in leakage current become small for the Nb-Si. The findings indicate the potential of oblique angle deposition to tailor porous non-equilibrium niobium alloy films for high performance niobium-base capacitor. (C) 2009 Elsevier B.V. All rights reserved.
  • Jozsef Hakl, F. Peter de Chatel, Sandor Meszaros, Kalman Vad, Zoltan Klencsar, Zoltan Nemeth, Erno Kuzmann, Zoltan Homonnay, Attila Vertes, Athanassios Simopoulos, Eamonn Devlin, Yoshitaka Aoki, Hidetaka Konno, Subodh Kumar De
    SOLID STATE SCIENCES 11 4 852 - 860 2009年04月 [査読有り][通常論文]
     
    Transport and magnetic properties of LaCoO3-based compounds, doped with 20% Sr and 2.5, 5,10,15, 20 and 30% Fe, were investigated by means of magnetization, resistivity and magnetoresistance measurements as well as by Fe-57 Mossbauer spectroscopy. While the temperature dependence of the dc and ac magnetic susceptibilities reveals the presence of magnetic phase separation accompanied by spin-glass and cluster-glass behavior, the electrical resistivity and magnetoresistance characteristics indicate that the mesoscopic structure of the present compounds is rather well described as consisting of ferromagnetic, metallic grains embedded in an insulating matrix. The effect of the partial Co -> Fe substitution on the bulk magnetic and transport properties, as well as on the local state of Co and Fe ions is discussed. (C) 2009 Elsevier Masson SAS. All rights reserved.
  • H. Habazaki, Y. Oikawa, K. Fushimi, Y. Aoki, K. Shimizu, R. Skeldon, G. E. Thompson
    ELECTROCHIMICA ACTA 54 3 946 - 951 2009年01月 [査読有り][通常論文]
     
    Niobium has been anodized at a constant current density to 10 V with a current decay in 0.8 mol dm(-3) K(2)HPO(4)-glycerol electrolyte containing 0.08-0.65 mass% water at 433 K to develop porous anodic oxide films. The film growth rate is markedly increased when the water content is reduced to 0.08 mass%; a 28 mu m-thick porous film is developed in this electrolyte by anodizing for 3.6 ks, while the thickness is 4.6 and 2.6 mu m in the electrolytes containing 0.16 and 0.65 mass% water respectively. For all the electrolytes, the film thickness changes approximately linearly with the charge passed during anodizing, indicating that chemical dissolution of the developing oxide is negligible. SIMS depth profiling analysis was carried for anodic films formed in electrolyte containing similar to 0.4 mass% water with and without enrichment of H(2) (18)O. Findings disclose that water in the electrolyte is a predominant source of oxygen in the anodic oxide films. The anodic films formed in the electrolyte containing 0.65 mass% water are practically free from phosphorus species. Reduction in water content increased the incorporation of phosphorus species. (c) 2008 Elsevier Ltd. All rights reserved.
  • T. Fujii, Y. Aoki, K. Fushimi, T. Makino, S. Ono, H. Habazaki
    Hyomen Gijutsu 60 3 166 - 169 2009年 [査読有り][通常論文]
  • Damian Kowalski, Yoshitaka Aoki, Hiroki Habazaki
    ANGEWANDTE CHEMIE-INTERNATIONAL EDITION 48 41 7582 - 7585 2009年 [査読有り][通常論文]
  • Yoshitaka Aoki, Emi Muto, Aiko Nakao, Toyoki Kunitake
    ADVANCED MATERIALS 20 23 4387 - 4393 2008年12月 [査読有り][通常論文]
     
    Nanometer-thick membranes of a large variety of silica-based double oxides commonly show gas-tightness (H(2) and O(2)) and efficient proton conduction at 100-400 degrees Cunder dry atmosphere.
  • Yoshitaka Aoki, Mineo Hashizume, Shinya Onoue, Toyoki Kunitake
    JOURNAL OF PHYSICAL CHEMISTRY B 112 46 14578 - 14582 2008年11月 [査読有り][通常論文]
     
    The Brunauer-Emmett-Teller (BET) Surface area of 15 nm-thick films made of TiO2,/polyelectrolyte bilayer was determined by quartz crystal microbalance (QCM) measurement of N-2 and Ar adsorption isotherms at 77 K. The measurements were carried out using a home-built vacuum chamber that includes built-in 9 MHz QCM and cryostat units. As little as 1 ng of the adsorbed gas could be detected, and the BET surface area of a flat Au film (ca. 0.5 cm(2)) on an oscillator was determined within an experimental error of +/- 5%. The titania/polymer composite film gives N-2 and Ar adsorption isotherms consisting of a less-pronounced type-I curve and a break at around p/p(0) = 0.7. This behavior is ascribed to the presence of irregular micropores and 6 nm (phi-mesopores in the composite film. An analysis of the isotherms shows that the porosity of the composite film is about 12%. which is much smaller than that of bulk titania (gel powder. The greater density appears to be related to the reported superior properties (robustness and resistance to electrical breakdown) of the organic/inorganic multilayer film. We conclude that the QCM-based, high-precision measurement of Gas adsorption is a powerful tool for investigation of the detailed morphology of nanometer-thick films.
  • M. Tauseef Tanvir, K. Fushimi, Y. Aoki, H. Habazaki
    MATERIALS TRANSACTIONS 49 6 1320 - 1326 2008年06月 [査読有り][通常論文]
     
    Niobium films with isolated columnar morphology have been prepared by oblique angle magnetron sputtering for capacitor application. Anodizing of the deposited mobium to form dielectric mobium oxide reduces the surface roughness, since the gaps between the neighboring columns are filled with the oxide due to large Pilling-Bedworth ratio for Nb/Nb2O5. To increase the gaps between neighboring columns, the influences of the angle of mobium flux to substrate and substrate surface roughness on the columnar morphology of the deposited films have been investigated using scanning electron microscopy and the electrochemical measurements. The deposition on the textured rough substrate surface and at higher angle of the mobium flux from normal to the substrate surface fabricates the mobium films with higher surface roughness.
  • Yuanzhi Li, Toyoki Kunitake, Yoshitaka Aoki, Emi Muto
    ADVANCED MATERIALS 20 12 2398 - + 2008年06月 [査読有り][通常論文]
     
    composition of Y(0.9)5/Y-0.05/P-2.64, exhibit high proton conductivities with an area-specific resistance of 0.085 Omega cm(2). The films are very stable, even when operating in the flow of dry air at 340 degrees C for a prolonged period of time (see figure).
  • Kentaro Miyoshi, Yoshitaka Aoki, Toyoki Kunitake, Shigenori Fujikawa
    LANGMUIR 24 8 4205 - 4208 2008年04月 [査読有り][通常論文]
     
    The fabrication of metallic nanostructures is one of the main issues in nanotechnology. This article describes the fabrication of a silver nanofin array by combining microlithography, electroless plating, and an etching technique. Fabricated Ag nanofins have a high aspect ratio (height/width = 10, width = 60 nm, height = 600 nm), and their widths and heights can be controlled by the period of electroless plating and the height of the original line pattern. An isolated Ag nanofin was proven to show metallic electrical conductivity. The current process provides a rapid and shape-designable fabrication method of metallic nanostructures.
  • Y. Aoki, H. Habazakia, T. Kunitake
    ECS Transactions 16 51 407 - 412 2008年 [査読有り][通常論文]
     
    AlxSi1-xOn, films exhibit a drastic change of proton conductivity across the film by reducing its thickness to less than 100 nm. The temperature- and humidity-dependence of conductivity of the sub-100 nm films is quite different from those of the thicker films. Furthermore, in the former thickness range, the value of conductivity markedly increases with reducing the film thickness, and its thickness dependence follows a power law with a fixed index of -2.1. This size-scaling effect can be explained by the percolation conductivity model that the probability for percolating of the conductive moiety in AlxSi1-xOn, films increases with decreasing the thickness. ©The Electrochemical Society.
  • Y. Oikawa, K. Fushimi, Y. Aoki, H. Habazaki
    ECS Transactions 16 3 345 - 351 2008年 [査読有り][通常論文]
     
    Porous anodic films have been developed on niobium at constant formation voltages between 5 and 50 V in glycerol electrolyte containing 0.8 mol dm -3 dibasic potassium phosphate at 433 K. Except at the formation voltage of 5 V, at which the current density is approximately constant during anodizing, the current shows the minimum and then maximum. After the current maximum, the current density decreases gradually, which follows the Cottrell equation. Thus, diffusion of some species in the pores should control the growth rate of the anodic films. The porous films developed are practically Nb 2O5 with a small amount of carbon contamination. Incorporation of phosphorus species, which often occurs in aqueous electrolytes, is negligible at all the formation voltages in the present electrolyte. Based on the results obtained, the growth process of the porous anodic films on niobium in this hot organic electrolyte is discussed. © The Electrochemical Society.
  • Yoshitaka Aoki, H. Habazaki, Toyoki Kunitake
    ELECTROCHEMICAL AND SOLID STATE LETTERS 11 11 P13 - P16 2008年 [査読有り][通常論文]
     
    AlxSi1-xOn films exhibit a drastic change of proton conductivity across the film by reducing their thickness to less than 100 nm. The temperature and humidity dependence of conductivity of the sub-100 nm films is quite different from those of the thicker films. Furthermore, in the former thickness range, the value of conductivity markedly increases with reducing the film thickness, and its thickness dependence follows a power law with a fixed index of -2.1. This size-scaling effect can be explained by the percolation conductivity model that the probability for percolating of the conductive moiety in AlxSi1-xOn films increases with decreasing the thickness.
  • Z. Németh, Z. Homonnay, F. Árva, Z. Klencsár, E. Kuzmann, A. Vértes, J. Hakl, S. Mészáros, K. Vad, P. F. de Châtel, G. Gritzner, Y. Aoki, H. Konno, J. M. Greneche
    The European Physical Journal B 57 3 257 - 263 2007年06月 [査読有り][通常論文]
     
    Emission and transmission Mossbauer studies of La0.8Sr0.2CoO3-delta perovskites doped with similar to 0.02 stoichiometric units of oxygen vacancy or 2.5% iron corroborate the occurrence of electronic phase separation in these systems. The effect of the small perturbation of the CoO3 sublattice with either iron ions or oxygen vacancies on the bulk magnetization as well as on the Mossbauer spectra is in good agreement with the double exchange based cluster model. The magnetoresistance does not show any peak near the Curie temperature, but reaches -84% in a field of 7.5 T at T = 8 K. Below T-C approximate to 180 K the Mossbauer spectra distinctly include the contribution from paramagnetic and ferromagnetic regions, providing direct evidence for phase separation. No contribution to the spectra from Fe4+ ions can be observed, which is an unambiguous evidence that at low concentration iron (either directly doped or formed from Co-57 by nuclear decay) is accommodated in the cobaltate lattice as Fe3+ ion.
  • Yuanzhi Li, Toyoki Kunitake, Yoshitaka Aoki
    CHEMISTRY OF MATERIALS 19 3 575 - 580 2007年02月 [査読有り][通常論文]
     
    A uniform V2O5 nanostructured ultrathin film has been prepared by spin-coating followed by heat treatment. This V2O5 nanostructured ultrathin film is composed of 4 nm closely connected V2O5 domains and has lamellar structure. Compared with a thicker V2O5 film (78 nm), the 17 nm V2O5 film showed a very good electrochemical performance of fast charge/discharge and excellent electrochemical stability. The excellent electrochemical performance of the V2O5 nanostructured ultrathin film is attributed to its nanostructure, which results in short Li+ diffusion distance and free Li+ accessibility to intercalation active sites. A facile approach to preparing a freestanding V2O5/Pt/PVA multilayer film on a large scale was developed. The thicknesses of the V2O5, Pt, and PVA are 22, 57, and 704 nm, respectively. The freestanding multilayer is stable in air, flexible, and robust and exhibits excellent electrochemical performance.
  • Yoshitaka Aoki, Emi Muto, Shinya Onoue, Nakao Aiko, Toyoki Kunitake
    CHEMICAL COMMUNICATIONS 23 2396 - 2398 2007年 [査読有り][通常論文]
     
    Amorphous aluminosilicate nanofilms as prepared by a sol-gel process and post-annealing exhibit proton conduction at a wide temperature range in dry air with a lowest area specific resistance of 0.24 Omega cm(2) at 400 degrees C.
  • Y Aoki, JG Huang, T Kunitake
    JOURNAL OF MATERIALS CHEMISTRY 16 3 292 - 297 2006年01月 [査読有り][通常論文]
     
    Free-standing, nanotubular ITO sheet with different In/Sn ratios was fabricated by the surface sol-gel process using cellulose filter paper as template. The resulting materials have a hierarchical structure originating from the morphology of cellulosic paper, and the ITO nanotubes are composed of interconnected layers of ITO nanocrystals of a few nanometres. Semiconducting behavior was observed at temperatures above room temperature, probably because electronic migration is strongly affected by the scattering at the boundary between nanocrystals. The ITO sheet with In/Sn ratio of 93.5/6.5 showed an intrinsic electronic conductivity (0.53 S cm(-1)) that is higher than those of other nanostructured ITO, and a very low apparent density (3.3% of neat ITO). These features indicate that the nanotubular ITO sheet has a potential use for chemical sensors and other devices.
  • 青木 芳尚
    化学と工業 = Chemistry and chemical industry 58 12 2005年12月01日 [査読無し][通常論文]
  • Y Aoki, T Kunitake, A Nakao
    CHEMISTRY OF MATERIALS 17 2 450 - 458 2005年01月 [査読有り][通常論文]
     
    Solution-based fabrication of a high-quality metal oxide nano-film (similar to10-nm thickness) by the surface sol-gel process and postannealing is reported. Hafnium(IV) n-butoxide in toluene-ethanol was chemisorbed onto hydroxylated Si wafer to give a uniform gel layer, of which alkoxide group was then hydrolyzed and subjected to a second cycle of chemisorption/hydrolysis. Annealing of a 10-cycle film at 500 degreesC produced uniform, void-free HfO2 layer of 5.7-nm thickness. Its electrical properties, dielectric constant, leakage current, and dielectric breakdown were comparable to the HfO2 film as prepared by the conventional vapor deposition method. On the other hand, a similar HfO2 nano-film prepared by spin-coating provided a less homogeneous layer in a high-resolution TEM image. The electrical properties of the latter film were much inferior to those of the surface sol-gel film. We concluded that the solution-based method is suitable for fabrication of dielectric nanofilms of metal oxide.
  • Y Aoki, H Habazaki, H Konno
    CHEMISTRY LETTERS 33 8 992 - 993 2004年08月 [査読有り][通常論文]
     
    The novel sensing property of neodymium (III) chromate(V), NdCrO4, to methanol has been demonstrated. The electrical response of this compound did not rely on the surface potential change by the interaction with methanol gas, but was instead due to the redox interconversion of zircon-type (NdCrO4)-O-V to KDP-type (NdH2CrO4)-O-III. The NdCrO4 film prepared on alumina substrates with 2.5-mum thickness could respond to methanol of ppm order concentration with an apparent increase in electrical resistance.
  • Y. Aoki, T. Kunitake
    Advanced Materials 16 2 118 - + 2004年01月 [査読有り][通常論文]
     
    The layer-by-layer adsorption of precursor metal alkoxides in solution and post-annealing at 400degreesC affords an alternate technique to the atomic layer chemical vapour deposition method for fabrication of next-generation high-kappa gate dielectrics. A void-free TiO(2)-La(2)O(3) composite film (see Figure) with 18 nm thickness is readily fabricated, and shows a dielectric constant higher than 30.
  • Y Aoki, H Habazaki, H Konno
    CHEMISTRY OF MATERIALS 15 12 2419 - 2428 2003年06月 [査読有り][通常論文]
     
    Rare-earth metal(III) chromates(V), LaCrO(4), NdCrO(4), and Y(0.9)CrO(3.85), were found to show reversible structural changes by reduction with methanol and oxidation in air. This anomalous behavior was investigated by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared, in situ Raman spectroscopy, and other methods. After less than 30 min of reduction at 543 K, zircon-type NdCrO(4) in the bulk scale changed to the low-crystalline II-KDP-type phase, which has the same arrangement of metal atoms as zircon. The reduced phase was quickly reverted to the zircon-type phase by air oxidation at 543 K for a few minutes. NdCrO(4) was reduced by hydrogen from methanol by forming hydrogen bonds between CrO(4) units, which caused the oxygen displacement and the formation of pseudo-octahedral CrO(6-m)(OH)(m) units, leading to the distorted II-YDP-type structure. Defect zircon-type Y(0.9)CrO(3.85) changed more quickly to the low-crystalline II-KDP-type phase than NdCrO(4), and the structure was not completely restored by the oxidation at 543 K for 1 h, because of the formation of small amounts of YCrO(3) and Cr(2)O(3). Monazite-type LaCrO(4) changed to an amorphous phase with the formation of the Schottky defects of oxide ions because of the structural differences between the monazite-type and II-KDP-type phases and was restored by air oxidation at 723 K.
  • Y. Aoki, T. Kunitake
    Extended Abstracts of International Workshop on Gate Insulator, IWGI 2003 40 - 41 2003年 [査読有り][通常論文]
     
    In this paper, we conducted fabrication of ultrathin films from conventional dielectric materials MOx (M=Ti, Zr) as well as from binary oxide composites MOx-MOy (M=Ta, La).
  • H Konno, A Sudoh, Y Aoki, H Habazaki
    MOLECULAR CRYSTALS AND LIQUID CRYSTALS 386 15 - 20 2002年 [査読有り][通常論文]
     
    B/C composites containing crystalline B4C particles were synthesized at 1400degreesC from the precursor composed of sugar-borate complexes. The precursors prepared from aqueous solutions of glucose and boric acid were found to be advantageous to the formation of B4C at low temperatures.
  • H Konno, Y Aoki, Z Klencsar, A Vertes, M Wakeshima, K Tezuka, Y Hinatsu
    BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 74 12 2335 - 2341 2001年12月 [査読有り][通常論文]
     
    Single phase zircon type EuCrO4 (S.G. I4(1)/amd) was synthesized and the structure including atomic positions was precisely determined by X-ray Rietveld refinement. The electronic and magnetic properties of the compound were studied based on the electric conductivity, Seebeck coefficient, Raman spectroscopy, Mossbauer spectroscopy, magnetic susceptibility, and specific heat. The CrO43- tetrahedra in EuCrO4 were slightly elongated compared with that in NdCrO4, which caused splitting of the degenerated vibration modes of CrO43-, nu(2) and nu(3), in the Raman spectra. The Eu-151 Mossbauer spectra showed not a trace of the Eu(II) species in the compounds measured in the present work. The Debye temperature, Theta(D), estimated from Mossbauer spectra suggested that the vibrational state of Eu(III) in EuCrO4 was similar to that in the perovskite-type EuCrO3. However, the isomer shift of Mossbauer spectra revealed that the electron density of the 6s orbital of Eu(III) in EuCrO4 was very low and almost the same as that in the zircon type EuVO4, that is, Eu(III) in these compounds has highly ionic character, whereas the electron density of the 6s orbital of Eu(III) in EuCrO3 was higher and nearly the same as that in Eu2O3. EuCrO4 was found to be an n-type semiconductor similar to NdCrO4, while zircon type REMO4 (RE: rare earth metal(III); M: V, P, As, etc.) are insulators. Magnetic susceptibility and specific heat measurements showed that antiferromagnetic transitions took place at around 15 K and was solely due to Cr(V) from the magnetic entropy value of 5.54 J mol(-1) K-1.
  • Y Aoki, H Konno
    JOURNAL OF SOLID STATE CHEMISTRY 156 2 370 - 378 2001年02月 [査読有り][通常論文]
     
    Single phase Nd1-xCaxCrO4 (x = 0-0.20) oxides were synthesized by the pyrolysis of precursors prepared from Nd-III-Ca-II-Cr-VI mixed solutions. Nd1-xCaxCrO4 having x greater than or equal to 0.25 was not obtained as a single phase. All Nd1-xCaxCrO4 were zircon type (tetragonal, I4(1)/amd), and the composition was almost stoichiometric without any essential defects, which was determined by chemical analyses. The lattice constants and atomic positions was refined by the X-ray Rietveld method. The calculated densities of Nd1-xCaxCrO4 (x = 0-0.20) were in good agreement with the ones measured by the picnometry. XPS and Raman spectra indicated that Nd1-xCaxCrO4 (x = 0.02-0.20) are mixed valence oxides containing two types of tetrahedra, (CrO43-)-O-V and (CrO42-)-O-VI) having D-2d symmetry in the structure, and this compensates the decrease of positive charges introduced by Ca-II ions. Though two types of tetrahedra were not distinguishable by XRD, lattice constants a and c decreased almost linearly with x. The values for x = 0.02-0.20, however, were not on the line expected by Vegard's law between NdCrO4 and CaCrO4 but larger. The calculated O-Cr-O bond angles, however, did not change monotonously as lattice constants and other crystallographic parameters such as Cr-O bond length did, indicating that CrO4 tetrahedra in Nd1-xCaxCrO4 (x = 0.02-0.20) are more elongated than in NdCrO4 and CaCrO4. It was deduced that the limit of x (about 0.25) may be determined by the difference in geometry between (CrO42-)-O-IV and (CrO43-)-O-V tetrahedra, (C) 2001 Academic Press.
  • Y Aoki, H Konno, H Tachikawa
    JOURNAL OF MATERIALS CHEMISTRY 11 4 1214 - 1221 2001年 [査読有り][通常論文]
     
    The electric conductivity of monazite-type LaCrO4 and zircon-type Nd1-xCaxCrO4 (x = 0-0.2) were characterized by measurements of dc electric conductivity and the Seebeck coefficient in the 300-600 K range. The LaCrO4 and NdCrO4 were found to be n-type semiconductors, while commonly known monazite- or zircon-type oxides are insulators. In mixed valence compounds of Cr-V and Cr-VI, Nd1-xCaxCrO4 (x = 0.1, 0.2), hole hopping conduction arising from the mixed valency was also observed. The Nd1-xCaxCrO4 (x = 0.1, 0.2) compounds obeyed Curie-Weiss behavior above the Neel temperature, and the observed magnetic moments for the Cr-V ions were in good agreement with the theoretical values. Experimental results and the calculated spin densities by the UHF method indicated that most of the unpaired electrons from Cr-V were localized on Cr atoms. The ab initio MO calculations for CrO43- clusters in NdCrO4 revealed that the SOMO is the degenerate d pi* state and that the LUMO is the p sigma* state (O 2p origin): the SOMO forms the degenerate states at the top of the valence band and the LUMO forms a wide conduction band. For NdCrO4 and LaCrO4 the electronic conduction mechanism as semiconductors was explained by the band model. For Nd1-xCaxCrO4 (x = 0.1, 0.2) electronic conduction was described by the band model combined with hopping conduction of holes in degenerate d pi* states. It is concluded that the electronic conductivity of these compounds is caused by an intermixing of the ligand-to-metal charge-transfer (LMCT) state into the ionic configuration.
  • Y Aoki, H Konno
    JOURNAL OF MATERIALS CHEMISTRY 11 5 1458 - 1464 2001年 [査読有り][通常論文]
     
    Single phase zircon type rare earth (REIII) chromates(v), containing Gd-III, Yb-III and Y-III ions, were synthesized by pyrolysis of the precursors prepared from the mixed solutions of REIII and Cr-VI in the ratio of RE/Cr = 0.90/1.00. With other RE/Cr ratios, RE/Cr greater than or equal to 0.95 or RE/Cr less than or equal to 0.85, the pyrolysis products contained a secondary phase of RE2O3 or Cr2O3. The composition of the compounds was determined to be RE0.9CrO3.85 by chemical analyses, with small standard deviations. The ESR spectrum of Y0.9CrO3.85 was typical of Cr-V ions in solids with g = 1.968 and DeltaH = 140 mT, and did not show the signals of Cr-II or Cr-III. These compounds have a defect structure which differs from other zircon type compounds containing REIII of larger ionic radius than Gd-III. The detailed structure was determined by the powder X-ray Rietveld method. The measured densities suggested that the defect structure is RE0.9(CrO4)(0.85)(CrO3)(0.15) (RE = Gd, Yb and Y). Apparently this structure contains CrO3. with a dangling bond. Based on the structural analyses and ab initio MO calculations, these species might be stabilized by forming Cr2O7 (CrO3-O-CrO3) units in RE0.9CrO3.85 due to the oxygen sharing between CrO3. and the neighboring CrO4 tetrahedron. The present results conclude that the phase diagram of rare earth chromate(v) series reported so far should be corrected so that the compounds containing Gd-III and smaller ions have the defect zircon type structure.
  • H Konno, T Erata, K Fujita, Y Aoki, K Shiba, N Inoue
    CARBON 39 5 779 - 782 2001年 [査読有り][通常論文]
  • Y Aoki, H Konno, H Tachikawa, M Inagaki
    BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 73 5 1197 - 1203 2000年05月 [査読有り][通常論文]
     
    Single phase LnCrO(4) (Ln = La-Nd) compounds were attempted to synthesize by the pyrolysis of precursors prepared from Ln(III)-Cr-VI mixed solutions. Only LaCrO4 and NdCrO4 were obtained as a single phase. LaCrO4 was monazite type (monoclinic, P2(1)/n), as reported, and the cell parameters were refined to be a = 0.70369, b = 0.72348, c = 0.66918 nm and beta = 104.950 degrees by an X-ray Rietveld method. NdCrO4 was zircon type (tetragonal, I4(1)/amd) and the eel parameters were a = 0.73107 and c = 0.66999 nm. It was found that CrO43- tetrahedra in LaCrO4 have C-1 symmetry and the four Cr-O bond lengths are different, whereas those in NdCrO4 have D-2d symmetry and all Cr-O bond lengths are 0.1702 nm. Detailed Raman spectra of LaCrO4 and NdCrO4 were measured and assigned to the vibrational modes of the CrO(4)(3-)tetrahedra. Molecular orbital calculations indicated that the charge density on the chromium ions in the tetrahedra is much less than the nominal 5+ due to the migratory electrons from oxygen atoms, and nearly the same irrespective of the symmetry of tetrahedron, suggesting that the Cr-O bonds are strongly covalent in nature.
  • REDUCE-3.2 IAPX86/286-BASED PERSONAL COMPUTERS
    T YAMAMOTO, Y AOKI
    LECTURE NOTES IN COMPUTER SCIENCE 378 134 - 135 1989年 [査読有り][通常論文]
  • HOLOGRAPHIC B-SCOPE IMAGING USING WIDEBAND CHIRPED ULTRASOUND
    T YAMAMOTO, Y AOKI
    IEEE TRANSACTIONS ON SONICS AND ULTRASONICS 31 4 362 - 366 1984年 [査読有り][通常論文]

講演・口頭発表等

その他活動・業績

特許

  • 特願PCT/JP2018/ 41555:酸素発生電極触媒とその利用法  2018年11月08日
    幅崎浩樹, 青木芳尚, Damian Kowalski
  • 特願2018-210890:酸素発生電極用ナノクラスター触媒  2018年11月08日
    幅崎浩樹, 青木芳尚, Damian Kowalski
  • 特願PCT/JP2018/ 31116:酸素発生電極用導電助剤  2018年08月23日
    幅崎浩樹, 青木芳尚, Damian Kowalski
  • 特願2018-032702:プロトンセラミックス燃料電池及びその製造方法  2018年02月17日
    青木芳尚, 山口智之, 幅崎浩樹
  • 特願2017-353650:窒化チタンナノ結晶膜による水素分離膜  2017年11月22日
    青木芳尚, 倉千晴, 幅崎浩樹
  • 特願2017-21778:酸素発生電極触媒とその利用法  2017年11月20日
    幅崎浩樹, 青木芳尚, Damian Kowalski

共同研究・競争的資金等の研究課題

  • 実用的中温作動型水素膜燃料電池の開発
    科学技術振興事業団:未来社会創造事業 「地球規模での低炭素社会の実現」領域探索加速型
    研究期間 : 2017年11月 -2022年03月 
    代表者 : 青木芳尚
  • ヒドリド伝導性水素膜とそれに基づくメンブレンリアクターの基盤技術創成
    日本学術振興会:科学研究費助成事業(基盤(B))
    研究期間 : 2018年04月 -2021年03月 
    代表者 : 青木芳尚
  • 亜鉛空気電池用実用的空気極触媒の開発
    新エネルギー・産業技術開発機構:革新型実用蓄電池基盤技術開発(RISING2)
    研究期間 : 2016年10月 -2021年03月 
    代表者 : 幅崎浩樹
  • 高原子価Mn(V)酸化物による水の酸化反応の開拓
    日本学術振興会:科学研究費助成事業(萌芽研究)
    研究期間 : 2015年04月 -2017年03月 
    代表者 : 青木芳尚
  • 中温領域で作動する直接アンモニア形水素膜燃料電池の創製
    JST:さきがけ再生可能エネルギーからのエネルギーキャリアの製造とその利用のための革新的基盤技術の創製」領域
    研究期間 : 2014年10月 -2017年03月 
    代表者 : 青木芳尚
  • プロトン伝導性アノード酸化膜を新規電解質とする中温燃料電池の創製
    日本学術振興会:科学研究費助成事業(基盤(B))
    研究期間 : 2012年04月 -2015年03月 
    代表者 : 幅崎浩樹
  • 水素透過膜燃料電池におけるアモルファス電解質膜/金属電極―ヘテロ界面の設計
    東京応化財団:研究助成
    研究期間 : 2013年04月 -2014年03月 
    代表者 : 青木芳尚
  • 中温域新規プロトン伝導性酸化物ナノ薄膜の電気化学合成
    日本学術振興会:科学研究費助成事業(萌芽研究)
    研究期間 : 2011年04月 -2013年03月 
    代表者 : 幅崎浩樹
  • Investigation of size-scaling behavior in insulator-metal transition of highly-nonstoichiometric, amorphous gallium oxide thin film
    Alexander von Humbold Fundation:AvH Fellowship
    研究期間 : 2011年02月 -2012年05月 
    代表者 : 青木芳尚
  • 日本学術振興会:科学研究費補助金(若手研究(A))
    研究期間 : 2010年04月 -2012年03月 
    代表者 : 青木 芳尚
     
    Brφnsted酸性電子-プロトン混合伝導体(EPmix)薄膜を創出するのための検討課題は以下の2点である。第一に高い酸性度と混合伝導率をもつEPmix複合酸化物薄膜の合成方法を最適化すること。第二に高いシート伝導度と垂直水素透過速度を同時に満たす、混合パーコレーション伝導体の設計方法を確立することである。本研究ではNb-P-Co混合アルコキシド溶液を用いた多層スピンキャスト法により、400℃ドライ雰囲気下で高い垂直混合伝導率を示すNb_<1.2>Co_<1.4>P_2O_x-アモルファス複合酸化物薄膜を合成することに成功した。この薄膜(100nmd)は明らかな水素透過能を示し、水素輸送に関する活性化エネルギーは約0.4eVと比較的小さな値を示した。また400℃における水素透過率は約10^<-7>molm^<-2>Pa^<-0.5>オーダーであり、これは燃料電池において2.0Acm^2の電流密度を生成するのに十分な値である。水素透過Pd合金をアノード基板、アモルファスZrP_<2.6>O_x薄膜(130nmd)を電解質膜、およNb_<1.2>Co_<1.4>P_2O_x-EPmix薄膜(200nmd)をカソードに用いた薄膜燃料電池を作製し、出力特性を調べた。薄膜燃料電池は350℃において理論値に匹敵する開回路電圧1.05V、最高出力2mWcm^<-2>を示した。セル抵抗の主な原因はアノードにおける分極であることが分かった。来年以降は、電極界面における分極抵抗低減とセル発電効率のさらなる向上を目指し研究を進める予定である。
  • 中温作動型薄膜燃料電池のための電解質薄膜/金属アノード接合技術の開発
    JST:シーズ発掘試験A(発掘型)
    研究期間 : 2009年06月 -2010年03月 
    代表者 : 青木芳尚
  • 文部科学省:科学研究費補助金(若手研究(B))
    研究期間 : 2008年04月 -2010年03月 
    代表者 : 青木 芳尚
     
    a-M_nSi_<1-n>O_x薄膜の膜厚が100nm以下に減少すると、プロトン伝導率がべき乗則で増加する、いわゆるサイズスケーリング現象が発見された。a-M_nSi<1-n>O_xは、低伝導マトリクスと高伝導パス(Bronsted 酸点:-O-Si^<4+>-O-M^-O-ヘテロ結合)からなる不均一構造を有している。この高伝導パスのつながり、いわゆる'クラスター'がプロトンチャンネルとして働く。膜厚dが最大クラスターのサイズζより大きい場合(バルク状態)、伝導パスのパーコレーションは起こらない。dがζにほぼ等しくなると(臨界点)、伝導パスのパーコレーションが起こり、σは増加する。更にdが減少すると、より小さなクラスターもパーコレーションするためその数N_p はd_<-μ>に従い増加し、こうしてσのスケーリングが起こる。本研究の結果は、アモルファスイオン伝導体にはバルク平均値より高いコンダクタンスの微細構造(伝導パス)が存在することを示唆しており、a-Al_<0.1>Si^<0.9>O_x薄膜の場合、伝導パスは平均長約20nmのアルミノシリカ鎖であることが示された。本研究の結果は、アモルファス酸化物の分子組成などを変えることにより、さらに新しい伝導率増強効果が発見される可能性が示唆された。
  • 日本学術振興会:科学研究費補助金(若手研究(B))
    研究期間 : 2003年04月 -2005年03月 
    代表者 : 青木 芳尚
     
    本研究は表面ゾル-ゲル法とテンプレート合成法を組み合わせることにより、導電性金属酸化物のナノ構造体により骨格形成された、規則多孔体を作成することを目的としている。前年度までの研究において、我々はセルロース繊維(ろ紙).をテンプレートに用いて表面ゾル-ゲル法をおこなうことにより、壁厚:10-20nm、直径:100-300nm、長さ:2-15μmのチューブ状インジウム-スズ複合酸化物(ITO)が3次元的に絡み合った多孔質構造体を生成することに成功した。これらのITO多孔質構造体は、高い気孔率(>80vol%)であるにもかかわらず室温において高い導電率を示し、中でもインジウムとスズの元素比(In/Sn)が9/1のものは最も高い導電率、0.53Scm^<-1>を示した。In/Sn=9/1の試料に対し、空間充填率を用いて理論密度100%のときの見かけの伝導率を計算したところ、20Scm^<-1>となり、これはスパッタリング法で作成した厚さ20nmの高品質ITO薄膜の導電率とほぼ等しいことがわかった。本研究で作成したITO多孔質材料では、セルロース繊維におけるマクロスケールのネットワーク構造がそのままITOネットワークとして転写されている。従って、そのITOネットワーク構造が電子のパーコレーション経路を形成し、高い気候率にもかかわらず良い導電性を生み出すものと考えられる。これらのITO多孔質構造体は、その構造的・電気的特徴から、センサー、光電変換システムなど、様々な次世代デバイスへの応用が期待される。

教育活動情報

主要な担当授業

  • 物理化学演習
    開講年度 : 2018年
    課程区分 : 学士課程
    開講学部 : 工学部
  • 応用物質化学(電子材料化学特論)
    開講年度 : 2018年
    課程区分 : 修士課程
    開講学部 : 総合化学院
    キーワード : 燃料電池,半導体,イオン伝導体,ヘテロ界面,欠陥熱力学
  • 物理化学Ⅰ
    開講年度 : 2018年
    課程区分 : 学士課程
    開講学部 : 工学部
  • 応用化学学生実験Ⅰ
    開講年度 : 2018年
    課程区分 : 学士課程
    開講学部 : 工学部
    キーワード : 実験技術、安全、酸化還元滴定、キレート滴定、酸塩基平衡、錯形成平衡、吸光光度法、クラジウス-クラペイロンの式、蒸発エンタルピー、電荷移動錯体、分子間相互作用、電極現象、PC計測制御、吸収・蛍光スペクトル、分子軌道論、反応速度、アレニウスの式

大学運営

委員歴

  • 2017年11月 - 現在   JST未来社会技術創造事業   研究代表者
  • 2011年07月 - 現在   電気化学会北海道支部   常任幹事
  • 2015年04月 - 2018年03月   北海道大学工学部国際連携委員
  • 2014年04月 - 2016年03月   北海道大学安全衛生委員
  • 2014年04月 - 2016年03月   日本電気化学会   編集委員
  • 2015年04月 - 2015年12月   第41回固体イオニクス討論会実行委委員
  • 2015年04月 - 2015年06月   第31回ライラックセミナー・第21回若手研究者交流会・産官学フォーラム講演会合同大会
  • 2014年04月 - 2014年09月   2014年電気化学秋季大会実行委員


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