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SHIRAI Naoki

Faculty of Engineering　Applied Quantum Science and Engineering　Quantum Engineering for MaterialsAssociate Professor

Naoki Shirai was born in Aizuwakamatsu city Japan in 1980. He completed his doctoral program at Tokyo Institute of Technology (Department of Electrical and Electronic Engineering, Graduate School of Science and Engineering) in 2008 and was appointed as a Research fellow by the Japan Society for the Promotion of Science (JSPS). He served as an Assistant Professor at Tokyo Metropolitan University from 2009 and has been an Associate Professor at Hokkaido University since 2016.He holds a Ph.D. in Engineering and is a member of the Institute of Electrical Engineers of Japan (IEEJ), the Japan Society of Applied Electrical Engineers (JSAP), the Japan Society for Plasma Fusion and Nuclear Energy (JSPF), and the Institute of Electrical and Electronics Engineers (IEEE).

He is interested in research on atmospheric pressure plasma and plasma-liquid interactions.

Researcher basic information

■ Degree
  • Ph D, Tokyo Institute of Technology
■ URL
researchmap URLホームページURL■ Various IDs
Researcher number
  • 80552281
J-Global ID■ Research Keywords and Fields
Research Keyword
  • 大気圧グロー放電
  • 液体電極
  • パルス放電
  • 大気圧プラズマ
  • プラズマ気液界面現象
  • 分子動力学
  • 非熱平衡大気圧プラズマ
  • 流体シミュレーション
  • プラズマシミュレーション
  • 電解反応
Research Field
  • Energy Engineering, Fundamental plasma
  • Energy Engineering, Applied plasma science
■ Educational Organization

Career

■ Career
Career
  • Apr. 2016 - Present
    Hokkaido University, 工学研究院量子理工学部門, Associate Professor
  • Apr. 2009 - Mar. 2016
    Tokyo Metropolitan University, 大学院 理工学研究科電気電子工学専攻, Assistant Professor
  • Apr. 2007 - Mar. 2009
    Japan Society for the Promotion of Science, JSPS Fellow
Educational Background
  • Apr. 2003 - Mar. 2008, Tokyo Institute of Technology, Graduate School of Science and Engineering, Electrical and electronic engineering
  • Apr. 1999 - Mar. 2003, Tokyo Institute of Technology, Faculty of engineering, Electrical and electronic engineering
Committee Memberships
  • Apr. 2024 - Present
    プラズマ核融合学会, 年会・応用領域プログラム委員
  • Apr. 2023 - Present
    プラズマ核融合学会, 代議員, Society
  • Apr. 2020 - Mar. 2022
    応用物理学会プラズマ若手チャプター代表
  • Apr. 2016 - Mar. 2021
    応用物理学会講演会 プログラム編集員(2016年4月-), Society
  • Mar. 2018 - Jul. 2019
    XXXIV ICPIG & ICRP-10 現地実行委員, Society
  • Apr. 2017 - Mar. 2019
    応用物理学会 プラズマエレクトロニクス会 幹事, Society
  • Mar. 2018 - Jan. 2019
    第36回プラズマプロセッシング研究会 第31回プラズマ材料科学シンポジウム 現地実行委員, Society
  • Apr. 2016 - Jan. 2017
    第34回プラズマプロセッシング研究会 第29回プラズマ材料科学シンポジウム 現地実行委員, Society
  • May 2015 - Dec. 2015
    国際学会 9th APSPT 28th SPP WEB担当, Society
  • Apr. 2012 - Aug. 2014
    電気学会A部門委員, Society
  • Apr. 2012 - Mar. 2014
    電気学会 プラズマ技術委員会, 幹事補, Society

Research activity information

■ Awards
  • Mar. 2026, Outstanding Reviewers Awards 2025 Journal of Physics D: Applied Physics.
  • Mar. 2023, Outstanding Reviewers of 2022 Plasma Sources Science and Technology
  • Apr. 2020, Outstanding Reviewers of 2019 Plasma Science and Technology
  • Mar. 2018, Outstanding Reviewers of 2017 (Plasma Sources Science and Technology )
  • Nov. 2017, プラズマ核融合学会第1回フォトコンテスト最優秀賞
  • Mar. 2017, Outstanding Reviewers of 2016 Journal of Physics D: Applied Physics.
    Naoki SHIRAI
  • Sep. 2015, JSAP Autumn Meeting, Poster Award
    Naoki SHIRAI
  • Feb. 2014, ICRP-8/SPP-31, Young Scientist Awards: Gold Medal
    Naoki SHIRAI
  • Mar. 2013, SPM1 (International Wokshop on Solution Plasma and Molecular Technology), The Best Poster Presentation Award
    Naoki SHIRAI
  • Jan. 2012, IEEJ, IEEJ Excellent Presentation Award
    Naoki SHIRAI
  • Nov. 2011, JSAP, Young Scientist Oral Presentation Award
    Naoki SHIRAI
  • Mar. 2010, IWPL2010 (International Workshop pn Plasma with Liquids), Outstanding Poster Paper Award
    Naoki SHIRAI
  • Dec. 2009, MRS-J, Award for Encouragement of Research in Materials Science
    Naoki SHIRAI
  • Aug. 2007, ISPC-18 (18th International Symposium on Plasma Chemistry ), Best Paper Awards for Young Researchers
    Naoki SHIRAI
  • Mar. 2006, IEEJ, IEEJ Excellent Presentation Award
    Naoki SHIRAI
■ Papers
■ Other Activities and Achievements
■ Syllabus
  • プラズマ生成工学特論, 2024年, 修士課程, 工学院
  • プラズマ生成工学特論, 2024年, 博士後期課程, 工学院
  • 応用電子工学, 2024年, 学士課程, 工学部
  • メカトロニクス実習, 2024年, 学士課程, 工学部
  • 応用数学Ⅰ, 2024年, 学士課程, 工学部
  • 一般教育演習(ﾌﾚｯｼｭﾏﾝｾﾐﾅｰ), 2024年, 学士課程, 全学教育
■ Affiliated academic society
  • Jan. 2026 - Present
    静電気学会
  • Sep. 2024 - Present
    IEEE(Institute of Electrical and Electronics Engineers)
  • Apr. 2020 - Present
    プラズマ核融合学会
  • Nov. 2007 - Present
    THE JAPAN SOCIETY OF APPLIED PHYSICS
  • Dec. 2002 - Present
    THE INSTITUTE OF ELECTRICAL ENGINEERS OF JAPAN
  • Dec. 2019 - Dec. 2022
    物理学会
  • 2005 - 2016
    米国電気電子学会
■ Research Themes
  • Development of hydrogen production technology using plasma-assisted water electrolysis
    Grants-in-Aid for Scientific Research
    27 Jun. 2025 - 31 Mar. 2029
    白井 直機
    Japan Society for the Promotion of Science, Grant-in-Aid for Challenging Research (Pioneering), Hokkaido University, 25K21663
  • 大気圧直流グロー放電生成時に観測される発光の自己組織化メカニズムの解明
    科学研究費助成事業
    01 Apr. 2023 - 31 Mar. 2027
    白井 直機; 佐々木 浩一; 富田 健太郎
    大気圧中で直流電圧駆動の放電プラズマを生成するとある条件において、陽極表面上で自己組織化された発光が観測されるが、その生成因子を実験的アプローチにより調査した。自己組織化パターン形成には、負性ガスである酸素の有無が重要であること、数学的に自己組織化形状が得られる反応拡散系ではある反応物質の密度が増減しながら拡散して模様を形成することを考慮すると、プラズマによるパターン形成は負イオンの有無と電界による輸送が重要であると予想される。2023年度はレーザー光脱離法による負イオン種の同定、レーザー誘起蛍光法による窒素イオン密度の計測、OHラジカルの密度分布・回転温度の計測パターン構造の外部電場制御を行った。その結果パターン構造が生じた際にOH-等の負イオンは確かに観測されるものの、液体陽極の場合と金属陽極の場合で観測される負イオン種の比率が異なっていてもパターン構造が観測されることから負イオンは直接的な生成因子でないことが示唆された。一方、レーザー誘起蛍光法によるOHラジカルの密度ならびに回転温度の空間分布の結果からパターン形成が観測されているときに放電プラズマ領域の温度が高くなっていることが観測された。これは温度の上昇によりプラズマ領域の密度分布が低くなっていることを示唆している。計算シミュレーションや低気圧環境でのプラズマにおいては希ガスのみでも
    パターン形成が得られていることから、大気圧直流放電によるパターン形成においても負イオンよりも温度上昇による密度の低下或いは、空間密度分布の変化が重要である可能性が見出された。
    日本学術振興会, 基盤研究(B), 北海道大学, 23K26083
  • 大気圧直流グロー放電生成時に観測される発光の自己組織化メカニズムの解明
    科学研究費助成事業 基盤研究(B)
    01 Apr. 2023 - 31 Mar. 2027
    白井 直機
    日本学術振興会, 基盤研究(B), 北海道大学, 23H01388
  • Understanding scientific principle of plasma-liquid interface
    Grants-in-Aid for Scientific Research
    01 Apr. 2020 - 31 Mar. 2023
    Sasaki Koichi
    The uniqueness of plasma-liquid interaction, in comparison with conventional liquid-phase chemistry, originates in short-lived species located in the vicinity of the liquid surface. In this work, we succeeded in the experimental investigation of the vicinity of the plasma-water interface by detecting OH radicals using luminol chemiluminescence, by measuring the surface tenson of water interacting with a plasma, by observing redox reactions at the water surface interacting with a plasma, and by detecting hydrated electrons at the plasma-liquid interface using laser-indued desolvation.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (A), Hokkaido University, 20H00135
  • Measurement of surface tension change at the liquid interface in contact with plasma
    Grants-in-Aid for Scientific Research
    Apr. 2018 - Mar. 2021
    Shirai Naoki
    The plasma-liquid interface is highly chemically reactive, however, the reactive species and radicals that contribute to the reaction are short-lived, and their measurement has not progressed much. In this study, we focused on the surface tension of the liquid and investigated how it is changed by plasma. Since the surface tension is determined by van der Waals forces and hydrogen bonds between molecules, reactive species and radicals transported by atmospheric pressure plasma may affect the change in surface tension. We first established a means of measuring the surface tension of water interacting with plasma and confirmed that the surface tension value of water increased when it was irradiated with plasma. It was confirmed that the surface tension value of water increased when it was irradiated with plasma. It was clarified that the cause of the increase may be the substances produced by OH radicals.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (C), Hokkaido University, Principal investigator, Competitive research funding, 18K03596
  • Construction of Scientific Principle in Plasma-Liquid Interaction
    Grants-in-Aid for Scientific Research
    Apr. 2016 - Mar. 2020
    Sasaki Koichi
    We carried out fundamental investigation which contributes to the academic progress of plasma-liquid interaction. We proposed 1) distortion in the shape of liquid surface in strong electric field, 2) reaction between liquid and nanoparticle of alkali metal, and 3) the formation of gas in liquid after the neutralization of irradiated ions as the production mechanisms of droplets from liquid surfaces interacting with plasmas. We observed the production of metal atoms from droplets when they were evaporated in plasma. We have shown that the chemiluminescence of luminol can be utilized for the detection of OH radicals in liquid. In addition, we examined the reaction frequency of solvated electrons in liquids interacting with plasmas using CTTS transition and transient absorption spectroscopy.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (A), Hokkaido University, Competitive research funding, 16H02121
  • Challenge of diagnostic methods for plasma-liquid interaction
    Grants-in-Aid for Scientific Research
    01 Apr. 2015 - 31 Mar. 2018
    Shusuke Nishiyama; SASAKI Koichi; SHIRAI Naoki
    We investigated diagnostic methods which can be used for fundamental studies of plasma-liquid interaction. We confirmed that two-dimensional laser-induced fluorescence kept its sensitivity at the vicinity to the interface between plasma and solid (or liquid). It was impossible to observe the region with high densities of liquid-phase reactive species, which are transported from plasma, by laser-induced fluorescence of phenol and the color reaction of titanium sulfate. On the other hand, we observed blue chemiluminescence from alkaline solutions with the addition of luminol. The region with the chemiluminescence was very thin, and was located just below the plasma-irradiated interface. It has been suggested that the chemiluminescence of luminol is useful for the detection of plasma-induced short-lived reactive species in liquids.
    Japan Society for the Promotion of Science, Grant-in-Aid for Challenging Exploratory Research, Hokkaido University, 15K13388
  • Foundation of plasma electrochemistry and its application to material process in liquid
    Grants-in-Aid for Scientific Research
    01 Apr. 2015 - 31 Mar. 2018
    Tochikubo Fumiyoshi; SHIRAI NAOKI; SHINOHARA MASANORI
    The purpose of this work is the academic foundation of plasma electrochemistry, which is regarded as the liquid-phase reaction induced by glow discharge electrolysis, and its application to material processes in liquid. For that purpose, we carried out (1) investigation of liquid-phase reaction in plasma electrochemistry, (2) magnetic nanoparticle generation, and (3) surface treatment of solid materials. It is clarified that the liquid-phase reaction is started from the very thin layer from the plasma-liquid interface by the electrons/ions/radicals irradiation of liquid surface from the glow discharge. For this application, Au/Ag composite nanoparticles and magnetite nanoparticles were synthesized and their synthesis processes were investigated. In addition, hydrophilization treatment of polypropylene as a target material was performed using plasma electrochemistry.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (B), Tokyo Metropolitan University, 15H03584
  • Study on synthesis of metal nanoparticles in mist-containing plasma
    Grants-in-Aid for Scientific Research
    Apr. 2014 - Mar. 2017
    Tochikubo Fumiyoshi; SHIRAI Naoki
    Metallic nanoparticles such as gold nanoparticles and silver nanoparticles can be formed in liquid by irradiating a chloroauric acid aqueous solution or a silver nitrate solution with a nonequilibrium plasma. In this research, we focused on the use of mist in plasma from the viewpoint of reaction efficiency at the gas - liquid interface and control of particle size. We studied the method for the generation of mist-containing plasma, and developed a discharge reactor using ultrasonic atomization method and dc glow discharge, and a discharge reactor using electrostatic atomization method and corona discharge, and evaluated their characteristics. We also attempted to synthesize gold nanoparticles and silver nanoparticles using mist-containing plasma and confirmed its feasibility.
    Japan Society for the Promotion of Science, Grant-in-Aid for Challenging Exploratory Research, Tokyo Metropolitan University, Competitive research funding, 26600128
  • Generation of atmospheric negative corona discharge using Taylor cone and its application for plasma reactor
    Grants-in-Aid for Scientific Research
    2011 - 2011
    SHIRAI Naoki
    We examined characteristics of atmospheric negative corona discharge using liquid needle cathode. As a liquid needle cathode, we adopted Taylor cone with conical shape. A nozzle with inner diameter of 10 mm is filled with liquid, and a plate electrode is placed at 10 mm above the nozzle. By applying a dc voltage between electrodes, Taylor cone is formed. To change the liquid property, we added sodium dodecyl sulfate to reduce the surface tension, sodium sulfate to increase the conductivity, and polyvinyl alcohol to increase the viscosity, in distilled water. The liquid, with high surface tension such as pure water could not form a Taylor cone. When we reduced surface tension, a Taylor cone was formed and the stable corona discharge was observed at the tip of the cone. When we increased viscosity, a liquid filament protruded from the solution surface was formed and corona discharge was observed along the filament at position 0.7-1.0 mm above fromthe tip of the cone. Increasing the conductivity resulted in the higher light intensity of corona and the lower corona onset voltage. When we use the metal needle electrode, the corona discharge depends on the voltage and the gap length. Using Taylor cone, different types of discharges were observed by changing the property of the liquid.
    Japan Society for the Promotion of Science, Grant-in-Aid for Young Scientists (B), Tokyo Metropolitan University, Principal investigator, Competitive research funding, 23740409
  • Influence of Particle and Heat Transportation on Plasma-Material Interactions
    Grants-in-Aid for Scientific Research
    2009 - 2011
    TOCHIKUBO Fumiyoshi; SHIRAI Naoki; UCHIDA Satoshi; ODA Akinori
    We investigated the phenomena at plasma-liquid interface with nanometer scale interactions theoretically and experimentally, using atmospheric pressure dc glow discharge with liquid electrode. We found that electron/positive ion irradiation from plasma to liquid surface induced variety of redox reactions in liquid by changing not only charged species but also liquid composition and gas condition experimentally. As an application of reactions at plasma-liquid interface, we demonstrated magnetic particle generation (magnetite) as well as Au and Ag nanoparticle generation in liquid. Numerical simulation of glow discharge with liquid electrode was carried out by considering the transport and reactions of charged and active species both in plasma and in liquid with Poisson's equation. The effect of low energy ion irradiation to liquid surface on water molecule kinetics was investigated by classical molecular dynamics simulation.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research on Innovative Areas (Research in a proposed research area), Tokyo Metropolitan University, Coinvestigator not use grants, Competitive research funding, 21110007
  • Control of atmospheric glow discharge using liquid electrode and its application for plasma reactor
    Grants-in-Aid for Scientific Research
    2009 - 2010
    SHIRAI Naoki
    An atmospheric DC glow discharge using liquid (electrolyte : NaCl solution) electrodes and axial miniature helium flow was generated stably in ambient air. In the case of liquid cathode discharge, when the discharge current was increased further, yellow light emission which was originated from sodium atoms vaporized from the electrolyte surface was observed in the negative glow region. To examine the effect of temperature of liquid electrode, we controlled the electrolyte cathode temperature using injection type cooler or heater. The intensity of the sodium emission decreased with the refrigerated electrolyte cathode, while it increased with the heated electrolyte cathode. When we use pulse modulated DC voltage, the sodium emission appeared with a delay time from the start of the discharge, while the emission of nitrogen molecular lines appeared and reached their peaks immediately. The temperature of liquid cathode is important factor to control the plasma-liquid interaction from the discharges and to resolve the detailed mechanism of the electrolyte cathode discharges. When we use electrolyte as anode, self-organized anode patterns were observed on the liquid or metal surface when atmospheric dc glow discharge with helium flow is generated. The pattern formation depends on the current, gap length and helium flow rate.
    Japan Society for the Promotion of Science, Grant-in-Aid for Research Activity Start-up, Tokyo Metropolitan University, Principal investigator, Competitive research funding, 21840042
  • 微小な液体を用いたマイクロプラズマの生成とプラズマ化学への応用
    科学研究費助成事業
    2007 - 2008
    白井 直機
    微小な液体を用いて大気圧下で新たなマイクロプラズマを生成する手法を考案した生成した。大気圧下で液体を利用したマイクロプラズマの手法は多岐にわたるが、液体を電極としたグロー放電、コロナ放電を生成し、基礎特性と応用法を検討した。電解質溶液を用いて、微小電極間で放電を生成した際、その不安定さが問題であったが、電極間に微細ヘリウムガス流を導入して放電を形成すると低電流でも安定に放電を形成することに成功した。液体を陰極とすると、特性は液体の表面状態によって変化し、電流が低いときには電極間の気体が主成分となる放電を形成するが、電流が高く液体陰極の表面温度が上昇したときには、液体の成分が放電部に現れてくることを明らかにした。放電を制御するための電源にも着目し、FET等の半導体デバイスを用いたパルス電源を作成した結果、パルス幅を調節することで液体陰極からの発光を制御することができた。放電部の極性を逆にして液体を陽極とした際には、液体表面の発光部が規則正しい自己組織化模様を形成することを明らかにした。これは他の文献にも掲載されていない新規の結果である。液体陽極放電は、近年ナノサイズの金属材料生成プロセスとして注目されており、本研究で得られた自己組織化模様と組み合わせることで、新たな材料プロセスが期待できる。また、微小サイズの液体は静電界による力でテイラーコーンと呼ばれる円錐形状を形成するが、それを利用してコーンの先端にコロナ放電を形成できることも示した。コロナ放電は、負コロナのときのみ安定に生成され、特性は液体の導電率が高く、表面張力が低いときほど安定に生成できる。PVAのような粘性の高い液体を用いるとコロナ放電はコーンの先端よりも高い位置に局戸的に生成され、液体の表面張力、粘性、導電率を調整することで様々な形状のコロナ放電が形成できることを見出した。
    日本学術振興会, 特別研究員奨励費, 東京工業大学, 07J09239