생의학 응용을 위한 마이크로파 플라즈마의 특성에 관한 연구

Abstract

Distinctive characteristics of microwave excited plasmas (> 0.5 GHz) are observed. Microwave Ar plasmas form a plasma plume, whereas microwave He plasmas are confined and ring-shaped on the contrary to low frequency plasmas (~ kHz). As the frequency increases from 0.9 GHz to 2.45 GHz, the Ar plasma exhibits radial contraction and filamentation. The He plasma is constricted in all directions with increasing frequency. The He plasmas are more strongly affected by the electric field than the Ar plasmas. The breakdown and sustain powers yields opposite results from those for low-frequency plasmas (~kHz). Penning ionization and the contribution of secondary electrons in sheath region reduce as the frequency increases, leading to less efficient ionization of He because its ionization and excitation energies are higher than those of Ar. Emission spectra of the microwave plasmas show the frequency effect on the generation of reactive species including ROS and RNS in terms of biomedical applications. The frequency effect of properties such as the plasma impedance, electron density, and device efficiency are presented. Another interesting behavior is observed when the 0.9 GHz microwave Ar plasma jet (uAPJ) contacts a human finger or grounded metals. No plasma channel and no attraction of the jet to the targets are observed in contrast to low frequency plasmas. Glow-to-arc transition does not occur even at a very small target distance (< 1 mm) to bare electrodes. There has been no report of the behavior in previous microwave plasmas including surface wave sustained plasmas. Measured and calculated values of S11, current and electric field are investigated to find the mechanism. This unique phenomenon is caused by the characteristic of microwave frequency systems. Decrease of target distance induces impedance mismatching leading to reduction of net input power. This prevents changes in the discharge regime including glow-to-arc transition, similar to a feedback system or a ballast. The mechanism is different from the arc prevention methods including dielectric barrier discharge in low frequency systems.With the distinctive characteristics, the microwave plasmas have higher efficacy in biomedical applications such as sterilization. The uAPJ exhibits a synergistic sterilization effect when it is combined with hydrogen peroxide (H2O2), distilled water (DW), and titanium dioxide (TiO2) photocatalyst. The sterilization efficacy of H2O2-uAPJ increases as the H2O2 concentration increases. The addition of TiO2 also remarkably increases sterilization efficacy. Optical emission spectra and degradation rate of methylene blue solution are measured. Numerical analysis, newly developed global modeling, is conducted together to discover mechanisms. Both experimental measurements and global modeling results suggest that combinations of H2O2, DW, and TiO2 increase the generation of hydroxyl radicals (•OH), which are known as strong bactericidal agents. It is revealed that charged species, especially electrons, have a dominant role to increase •OH. These studies are expected to be helpful for the field of biomedical plasmas and understanding of the behavior of microwave plasmas.