Investigation of microwave cavity applicators for plasma assisted CVD diamond synthesis and plasma assisted combustion

The objective of this research was to design, build, optimize, and experimentally evaluate microwave applicators that operate at high pressures for two specific applications: (a) microwave plasma assisted chemical vapor deposition (MPACVD) and (b) microwave plasma assisted combustion (MPAC).;Microwave plasma assisted chemical vapor deposition was experimentally investigated using a cylindrical plasma source, high purity, 2-5% H2 /CH4 input gas chemistries and operating at high pressures of 180-250 Torr for diamond synthesis. A microwave cavity plasma reactor was specifically modified to be experimentally adaptable and tunable in order to enable operation with high input microwave plasma absorbed power densities within this higher pressure regime. Uniform polycrystalline diamond films were synthesized on 2.54 cm diameter silicon substrates and single crystal diamonds were deposited on HPHT diamond seeds at substrate temperatures of 950-1282 °C. The polycrystalline growth rates ranged from 3 to 21 mum/hr at 2-5% CH4/H2 while single crystal diamond growth rate varied from 8 to 36 mum/hr at 3-5% CH4/H2. Higher operating pressures, absorbed power densities, and methane concentrations resulted in higher diamond growth rates. FTIR transmission and Raman measurements indicated the synthesized diamond at these high pressures was of excellent quality.;Microwave plasma assisted combustion was also investigated using cylindrical and coaxial microwave cavity applicators using premixed gas chemistries of 02/CH4. These applicators were developed to enable the efficient coupling of microwave energy into gases/plasmas/flames at pressures of one atmosphere. The mechanical tuning of the applicators allowed for the efficient matching of microwave power into the flame and also allowed the optimal positioning of the flame with respect to the impressed electric field. The addition of a few Watts of microwave power to a combustion flame with a flame power of 10-40 W served to extend the flammability limits under fuel rich and fuel lean conditions, increased the flame length and intensity, and also increased the number density and mixture of excited radical species in the flame vicinity. Optical emission spectroscopy measurements showed gas rotational temperatures in the range of 2300 - 3600 K.;This thesis research has led to two experimental applicator designs and associated systems that allow experimental investigation of microwave energy interaction with combustion flame and a microwave applicator that enables MPACVD diamond synthesis at 180-250 Torr pressure regime. This MSU MPACVD reactor design has recently been commercialized.