Atmospheric Microwave Discharge for Portable and Ozone-Free Air Plasma

Abstract

In this dissertation, researches on development of microwave air plasma devices are reported. Simulation and experiment for investigating characteristics of atmospheric pressure microwave plasma have been conducted to find its efficacy and safety. Through these researches, microwave plasma system is suggested for developing ozone free air plasma device. To find the characteristics of atmospheric pressure microwave plasmas for their kinetics and RONS production efficacy, One-dimensional Particle-in-Cell Monte Carlo Collision simulations have been conducted. Profitable property changes happened after 300 MHz for atmospheric pressure argon plasma at microsize. Breakdown voltages decreased rapidly and it means that microwave plasmas (>0.3 GHz) can be free from electric shock or arc. As the driving frequency increases, most of power dissipations were observed for electrons. Effective electron temperature and the proportion of high energetic electrons responsible for bond-breaking energy of OH (~2 eV) and O (4.8 eV) near electrode increase significantly. Electron densities and temperatures obtained from particle simulations can be provided to zero-dimensional global simulation as larger electron temperature can be thought as microwave plasmas case. Comparing before and after 0.3 GHz phase properties, two major reactive species, OH and O which is useful for biomedical applications, were expected to be generated more than lower frequency plasmas. For investigating the driving frequency effects in experiments, the properties and effectiveness for biocompatibility of microwave-powered and low-frequency (LF) argon plasmas were compared. The optical emission observation of these two plasmas showed that more useful reactive species were generated by microwave plasma than by LF plasma with the intensities being higher by factors of about 9, 6, and 3 for OH (309 nm), O (777 nm), and O (844 nm), respectively. For effectiveness for biocompatibility, surface hydrophilic modification experiments were also conducted. A SUS304 steel plate became hydrophilic after 5 s of microwave plasma treatment. This is more than ten times faster than in the case of LF plasma treatment, an action related to the generation of reactive species as measured by optical emission spectroscopy. Observations of surface morphology by SEM and chemical composition by XPS indicate that microwave plasma does not induce physical damages and the modification is mainly due to change of chemical composition by plasma generated RONS. High-energy electrons were considered a major factor for microwave plasma characteristics. To overcome limitations of gas-based plasmas and conventional air plasmas, a 1.29-GHz microwave excited atmospheric pressure ambient air plasma jet has been developed as a portable plasma device. The compact air plasma system has no ozone, electric shock, and temperature problems. A 10 mm stable air plasma jet was generated with a pin-to-hole structure coaxial line resonator. Ozone problem, most significant issue of air plasma, was also solved by high temperature at local generating spot of microwave air plasma. Appropriate ambient air flow was supplied to reduce the plasma gas temperature up to 36°C at treating point. Excited NO lines in the ultraviolet region between 200 and 300 nm were observed. Oxygen related plasma species at 777.4 and 844.6 nm, which have potential in several physical and biological phenomena, were also observed. The ozone-free nonthermal portable air plasma system is expected to be useful in biomedical area for its components investigated by optical emission analysis.