Part I:
A TM011 microwave cavity was used to measure the radially averaged electron density n¯e and the electron-neutral collision frequency νen in d.c. glow discharge positive columns of pure rare gases and their binary mixtures. Electric field E was measured by measuring the floating potential across pairs of probes in the plasma. The pure gases studied were He, Ne and Ar at pressures of 1-8 torr and discharge currents I of 5-20 mA. The calculated values of the electron drift velocity vd in He and Ne are substantially lower than the Bradbury and Nielson drift tube values, indicating substantial ionization via metastables in the range of p and I investigated. Discharges in He-Ar and He-Ne mixtures at p = 2-5 torr and I = 15 mA exhibit time and space dependent values of n¯e, νen due to the effect of cataphoresis. The results indicate that the Shair and Remer model is a reasonable description of the phenomenon of cataphoresis in these mixtures where the impurity (Ar or Ne) content is in the range of 2-20%.
Part II:
By employing a positive column flow reactor with low residence times (< 100 msec) in d.c. discharges with low currents (< 5 mA), it is possible to convert up to 5% of the feed methane into ethane, ethylene and hydrogen with negligible formation of solid and liquid products. The percent methane in the argon or helium feed Co affects the product distribution, with good selectivity for Co > 5%. Average steady-state electron densities in the discharge were simultaneously measured by perturbation of the resonance of a TM010 microwave cavity.A pulsed d.c. discharge enabled the residence time to be effectively reduced even further. The pulsed discharge experiments were conducted at 3-9 torr with pulse durations ~ 1 msec and pulse intervals of 20-50 msec. Pulse current was measured as a function of time with a current probe. The TM010 cavity was used to obtain transient electron densities in the discharge, which varied between 109 and 1010 electrons/cm3.
The kinetic results are consistent with a free radical mechanism with electron-impact dissociation as the initiating step. Derived values of the integrated cross-section for CH4 + e →α CH3 + H + e and CH4 + e → β CH2 + H2 + e are in the range 3.6 - 15.5 x 10-11cm3sec-1, and that for C2H4 + e →γ C2H2 + H2 + e is ~ 9 x 10-10cm3sec-1. α andβ fall with Co in the range 4.0 - 12.6%, indicating a shift of the electron energy distribution function fe(ε) to lower values of ε. Calculations are presented for the case of H2 dissociation to illustrate the strong effect of the tail end of fe(ε) on the rate of chemical reaction.
