Experimental studies of microplasmas from diamond substrates

Mitea, Sebastien (2015). Experimental studies of microplasmas from diamond substrates. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f853

Abstract

The birth of the new field of Microplasma Physics, at the turn of the century, follows decades of miniaturization of plasma sources. The empirical Paschen law from the early twentieth century ensures that increasing the pressure allows a reduction in the reactor dimensions and vice versa. However stable operation of a direct current microscopic discharge had been elusive, until the end of the 1990s. At microscopic dimensions the importance of surface reactions is magnified, emphasizing the role of the reactor materials. Diamond, obtained synthetically by Chemical Vapour Deposition, offers unprecedented versatility and robustness. By selecting, during deposition, between insulating and semiconducting thin films, diamond-based micro-reactors were fabricated and operated for the first time. The ignition conditions were similar to results reported with other microplasma sources, in argon and helium at pressures ranging from a few torr to atmospheric pressure. The same abnormal and normal glow modes were obtained by comparing the V-I characteristics with those obained with more common microplasma reactor materials. The dielectric spacer was shown to drive heat transfer through the reactor. Its role on the microplasma was studied via gas temperature measurements. Measurements in diamond were compared with glass-based results. Occupying opposite ends of the thermal conductivity spectrum, they led to significantly different results. Owing to the excellent thermal conductivity of diamond, gas temperature decreased with reactor diameter. That is, heat transfer through the dielectric prevailed over that through the gas phase. To the contrary, glass-based microdischarges were hotter at smaller diameters, when heat conduction through the dielectric was too poor. Finally, diamond outlived glass and is poised to become a material of choice for microplasma research. Indeed, no diamond-based reactors suffered any failure from the microplasma operation, showing signs of long lifetime.

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