Micro Discharge Under Supercritical Conditions -- Physics and Application to Materials Processing

Recently, micro discharge or discharge microplasma have attracted much attention. Miniaturization of discharge or plasma allows us easy generation of discharge or discharge plasma under a high-pressure condition even up to supercritical fluid (SCF). Applying plasma to SCF processing may yield a high efficiency due to a combination of advantages of plasma and SCF. In addition, plasma generated in SCF is anticipated to contain radical and ion clusters, which may lead to novel physical/chemical phenomena and reactions, comparing to typical gas plasma. In our previous works, we succeeded in generating discharge or discharge plasma in high-pressure CO$_{2}$, H$_{2}$O, Xe up to supercritical conditions, and discovered novel phenomena, such as the drastic decrease in the breakdown voltages of 1 micrometer-gap electrodes near the critical point. Furthermore, we have applied SCF plasma to materials synthesis processings, and succeeded in fabricating carbon materials including carbon nanotubes (CNTs), under milder condition (31.1-70$^{\circ}$C, 7.38-12 MPa) of scCO$_{2}$, as a processing media and starting raw material, with no catalyst [3], comparing to conventional thermal equilibrium processing. In this study, we generate stable low-temperature plasma using DBD (dielectric barrier discharge) in supercritical CO$_{2}$ and Xe conditions. In addition to its diagnosis by optical emission spectroscopy and Raman spectroscopy, application of it to film depositions of carbon nanomaterials and Cu will be discussed.