称号及び氏名

博士（工学） Tran Trung Nguyen

学位授与の日付

令和 4 年 9 月 23 日

論

「Optimization of an Atmospheric-Pressure Plasma Source

文

名for Biomedical and Environment Applications（バイオ及び環境応用のための大気圧プラズマ源の最適化）」

論文審査委員

主査

松浦 寛人

副査

川又 修一

副査

梅澤 憲司

論文要旨

Non-thermal plasmas (NTPs) have exhibited great potential for various fields,such as agriculture, environmental, and biomedical due to their operational
flexibility. NTPs sources can produce a chemically rich environment at close toroom temperature both at reduced and ambient pressures. This unique conditionenables the delivery of highly reactive plasma species in a non-destructive andbeneficial way to even extremely heat-sensitive surfaces. Electric field, as well asreactive oxygen and nitrogen species produced by NTPs, may inactivate bacteria,

and stimulate skin regeneration (dermatology), tumor reduction (oncology), seedgermination (agriculture), and wastewater purification (environment). These new
fields of research, based on plasma-liquid chemistry are very promising anddeveloping quickly. However, due to the very high level of complexity,
understanding is still poor concerning mechanisms involved in biologicalprocesses, in particular, the respective role of reactive species and electric fields.
To help elucidate these effects, advanced approaches have been brought to bear on
many issues. In addition, with the aim of enhancing efficient treatment andreducing operation costs under similar operating conditions, it is important todevelop control methods that allow for the minimization of the effects ofenvironmental conditions while at the same time optimizing the treatment byadjusting the electrode configuration or using a variety of gas mixtures, etc. Themain objective of this thesis is based on both fundamental and applied research in
non-thermal atmospheric pressure plasma interacting with water for biological andenvironment applications and aims at a better understanding of the physical andchemical processes of plasma-liquid interaction.
The thesis has been divided into six chapters.
Chapter 1 describes namely basic knowledge about physical plasmas andintroduces the different types of NTPs. A special focus addresses the atmosphericpressure plasma jets (APPJs), as the main core of our work, are discussed. Finally,
the emerging fields of APPJs applications for environmental and biomedical.
Chapter 2 presents the fundamentals of plasma-liquid interaction as well since theinteraction between plasmas and the biological targets usually involves liquids as

an interface. A special focus addresses the transport process of reactive species
from gas to liquid phase and several measurement methods to evaluate density anddistribution of reactive species.
In Chapter 3, the main effect of insulating oil was investigated on ring powerelectrode and needle power electrode configurations. To reduce operation costs,
our group proposed using a cheaper working gas—argon (Ar), as a replacement
for helium (He). When using available low-cost Ar at atmospheric pressure, the Ar
plasma discharge also needs to higher breakdown voltage, leading to a higher gas
temperature. Therefore, the Ar plasma jet is known to be difficult to sustain, andunstable. The combination of a needle power electrode configuration and Ar gas
was applied as an alternative solution. However, the difficulty in sustaining a glow
discharge under a high supplied voltage during a long irradiation time leads to a
new set of challenges. A high temperature at the ground electrode leads to thermal
damage to the quartz tube. Recently, studies have illustrated that two-ringelectrodes immersed in an electrically insulating oil bath could prevent arc
formation. Based on these ideas, our group confirmed that the ring powerelectrodes plasma source could operate steadily for a long time due to the electricalfield modifying the function of the insulating oil. This chapter shows positive
results as insulating oil prevents arc formation, while it improved the suppliedpower and plasma jet length and increased radical production. Furthermore, theheat balance property of the insulating oil was observed with the needle powerelectrode configuration. Using the needle power electrode configuration, the heatdamage could be prevented by covering the ground electrode with insulating oil.
In this study, the heat balance property of the insulating oil was observed, and theoperating parameters of the needle power electrode plasma source are investigatedusing Ar gas.
Chapter 4 proposes Polyvinyl Alcohol-Potassium Iodine (PVA–KI) as a newchemical probe in the form of gel, to visualize the distribution of reactive species.
Our group has reported the development and biomedical application of PVA-KI
liquid-type in previous research as a novel chemical probe to study free radicals.
In particular, the liquid-type PVA–KI was used to observe the liquid flow andobtain the relative reactive species concentration distribution. Recently, reactivespecies' transport and distribution phenomena into biological tissue following nonthermal plasma treatment have received much attention. Therefore, the gel-type
PVA-KI is developed to give information regarding the distribution of reactive
species by coloration on the gel surface. The effects of the gas-phase surrounding
plasma jet on the distribution and diffusion of reactive species have been
investigated using plastic shielding put on the PVA-KI gel surface. The
concentration of reactive species produced by Ar plasma sources distributed on thesurface of PVA-KI gel in the different nozzle to target distances was measured
from visualized color distributions by a color meter device.
In Chapter 5, the aim of this chapter is to test the efficacy of the Ar-ethanol plasma
jet in transporting the reactive species to the target surface. Our group investigatedthe effect of ethanol addition on the discharge characteristics of Ar plasma jet inprevious research. However, Ar-ethanol plasma jets' reactive species distribution
has not been studied enough. Based on previous research combined with a newchemical probe, Ar-ethanol plasma jets' reactive species distribution can bevisualized. To estimate the total amount of reactive species generation, we employ

a PVA-KI liquid type. The absorbance of the PVA-KI solution near 490 nm can bemeasured by a UV-Vis spectrometer. On the other hand, the gel-type of PVA-KI
can easily detect reactive species distribution on the gel surface. Then, the RGB
value is recorded by using a color meter device to determine color distribution on
the surface of PVA-KI gel. The amount of reactive species distribution is strongly
dependent on the amount of ethanol addition.
In Chapter 6, the general conclusion of this study summarizes the main outcomesand points out the future perspective.

審査結果の要旨
本論文は、最近になってプラズマ応用の対象となってきた、生体や環境物質の処理に今日する事の可能な、大気圧下で運用可能な非平衡プラズマ源の改良につながる研究をすすめたものである。プラズマ生成のコストは主に動作ガスと放電電力であり、特に環境応用については大規模化の可能なシステムが好ましい。他方、プラズマが誘起する反応は、プラズマ源の置かれた環境で、あるいはプラズマ照射ターゲットとの相互作用で生成される化学活性種の種類と量で支配される。本研究では、誘電体バリア放電で生成されるプラズマジェットをベースとして、プラズマ源の最適化につながる研究を行った。
特に、注目すべき成果は以下のとおりである。
(1)プラズマ源に低周波高電圧を印加するパワー電極をリング電極から針電極に変更することにより、安価なアルゴンガスを使ったプラズマ生成が可能になっていた。しかしながら、誘電体としてアーク放電の発生を防止するガラスチューブへの熱ダメージの低減が新たな課題となっていた。本研究では、韓国の研究グループがリング電極の装置に適用を提案していた絶縁油での電極の被覆が、針電極の装置にも適用可能であり、その安定化のメカニズムが、電磁気学的な効果ではなく、伝熱的な冷却効果によるものであることを明らかにすることができた。
(2)ポリビニールアルコールーヨウ化カリウム（PVA-KI）混合物は新しい放射線化学線量計や紫外線の照度計として注目されている。同様に、プラズマ照射された液中に生成された活性酸素種の定量化に利用することも行われている。本研究では、ゲル状の PVA-KIサンプルを利用し、プラズマ源内部で生成され、ガス流で輸送される活性酸素種を可視化する可能性が示されている。予備的な試験の結果として、名古屋大学の狭ギャップガス流路を備えた大型プラズマ源では、照射ターゲットを近づけすぎるとプラズマ処理の効果が低減することが予想されている。
(3)放電ガスに微量のアルコールを添付すると、ぺニング効果で放電維持電圧が低下することが報告されているが、化学活性種の生成へのアルコール添付の影響はまだ明確にされていない。本研究では、PVA-KI を化学プローブとして利用し、アルコールの効果を明確にすることを試みた。プラズマ電子分布の最適化とラジカル生成の最適化が異なる可能性を初めて示すことができた。

以上の諸成果は、大気圧放電プラズマ源の内部及び照射環境で起こっている物理化学過程についての重要な知見を与える。本研究はこのように特定のプラズマ装置の改良のみならず、プラズマ化学やプラズマ生体反応の上からも有益な情報を提供したものであり、関連分野の学術的・産業的な発展に貢献するところ大である。また、申請者が自立して研究活動を行うのに必要な能力と学識を有することを証したものである。
学位論文審査委員会は、本論文の審査および最終試験の結果から、博士（工学）の学位を授与することを適当と認める。