Characterization and Diagnostics of Multiphase AC Arc for Innovative Material Processing

Thermal plasmas have attracted extensive attention due to their unique advantages, and are expected to be utilized for a number of innovative industrial applications such as decomposition of harmful materials, recovery of useful materials from wastes, and synthesis of high-quality and high-performance nanoparticles. Thermal plasmas have advantages such as high enthalpy to enhance reaction kinetics, high chemical reactivity, and oxidation or reduction atmospheres depending on the required chemical reaction, making them effective for s innovative processing.<br/>The experimental and modeling efforts on thermal plasma characteristics has been devoted to industrial application. However, despite these efforts, the thermal plasma characteristics remain to be elucidated. The electrode phenomena are one of the most important issues, because it determines the processing performance in thermal plasmas. The objective of the study is to investigate the physical and chemical phenomena in thermal plasma processing for industrial application.<br/>The multiphase AC arc is one of the most attractive thermal plasmas because of its advantages for material processing, such as large plasma volume and low gas velocity. It also has the advantages of high energy efficiency and low cost compared to other thermal plasmas. Therefore, multiphase AC arcs have been applied to innovative material processing, such as in-flight glass melting technology and nanomaterials manufacturing processes. The stability of the arc, the temporal and spatial characteristics of the arc discharge, and the electrode phenomena are the most important phenomena to understand. The study of the characteristics of the multiphase AC arc will be useful for industrial applications.<br/>The physics of electrodes was elucidated based on high-speed visualization of electrode phenomena in a multiphase AC arc. An optical system including a band-pass filter and a high-speed camera were combined to observe images of different wavelengths synchronously. Visualization of metal evaporation from the electrodes was performed with the high-speed visualization system using a bandpass filter with a selected wavelength to observe tungsten vapor in the arc. The tungsten electrode started to evaporate just after the peak top of the arc current in the anodic period. The tungsten metal vapor becomes the main species in the arc during the anodic period when it started to evaporate. In contrast, a small amount of tungsten evaporation was observed during the cathodic period.<br/>The electrode temperature of the multiphase AC arc was estimated by the same system with bandpass filters of selected wavelengths to observe the thermal radiation from the electrodes. Two-color pyrometry was applied to measure the electrode temperature. The temperature around the electrode tip was higher than the melting point of tungsten (3,695 K).<br/>The measurement of the excitation temperature of the multiphase AC arc was performed by the same high-speed visualization system. The excitation temperature was estimated from the intensity at each wavelength obtained by the atomic-to-light ratio method. The temperature of the arc varied from 7,000 to 10,000 K during the cathode period and from 7,000 to 9,000 K during the anode period. The arc from the cathode was more constricted than the arc from the anode. This constriction increases the current density of the arc and promotes heating by Joule heating.<br/>Thermal plasma has been used simply as a high temperature source for conventional processing. For innovative materials processing and waste treatment, thermal plasmas need to be effectively utilized as chemically reactive gases with higher capabilities. In order to develop innovative processes, it is important to conduct further research on plasma generation, electrode phenomena, and advanced numerical analysis of reactive plasmas.