A stable plasma source for high-density fluids, i.e. liquids, supercritical fluids (SCFs), and high-pressure gases, was developed by utilizing carbon nanotube (CNT) sheets that possess a high supply capability for field electron emission. Discharges could be generated in high density N2 and Ar on glassy carbon (GC) electrodes covered by CNT sheets (CNT electrodes), but not on bare GC electrodes. The similarity of the discharge characteristics with those reported by previous studies implies that the discharges in these high density fluids on the CNT electrodes were driven by field electron emission. The plasma produced in the high density N2 is strongly suggested to comprise high-density metastable molecules reacting with the dielectric aluminoborosilicate glass, which causes the auroral emission of atomic oxygen lines in the optical emission spectrum. Although the discharge of N2 or Ar gas under atmospheric pressure at room temperature destroyed the CNT sheets in terms of visual appearance, the discharges under the higher density conditions of high pressure gas, SCF, and their liquids, did not cause any observable damage to the CNT sheets. Following the exposure to discharges under several high density conditions of N2, the chemical changes in the CNT electrodes were investigated using x-ray photoelectron spectroscopy (XPS). The results of the XPS analysis revealed that few chemical changes were caused in the electrodes by the exposure to discharges under the high pressure N2 gas and gas-like N2 SCF, whereas the discharge in the liquid-like SCF oxidized the CNT sheets.
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