Synthesis of silicon carbide ceramics by novel microwave methods

The work described in this thesis was carried out on the synthesis of silicon carbide using microwave processing and aimed to develop procedures to reduce processing complexity and cut processing times. Si-C/SiO2-C systems were first studied due to the ready availability at reasonable cost of the starting powders and the excellent microwave absorption properties of carbon. Silicon carbide was synthesised from silicon or silica combined with activated carbon or graphite via microwave heating over timescales from minutes to seconds without the need for inert atmospheres or subsequent purification. In the reactions performed in a MMC, graphite was found fundamental not only as a microwave susceptor, but also as a reductant, preventing the oxidation of silicon carbide. Another important beneficial factor was water, used as a binder in the pellet making process, it minimised the intergrain void space between particles and possibly acted as a polar liquid microwave susceptor. It was found the carbide morphology and phase purity can be controlled by the microwave cavity used, the power applied and hence by the heating rate. Short irradiation times (ca. 5 minutes) in a multimode cavity using silicon and activated carbon powders produced single phase β-SiC nanofibres as small as 5 nm in diameter while large crystallites of β-SiC can obtained in ~1 minute using high power, single mode cavity microwave techniques. Furthermore, similar microwave cavity systems shown that the removal of the susceptor, starting from silica and carbon mixtures, is possible and the successful conversion to silicon carbide can be performed using loose powders. This represented a major step with respect to designing a flow process and reducing carbon contamination. Studies of microwave processing of silicon carbide were then extended to x-aerogels, to probe whether the produced silicon carbide would mimic the porous microstructures offered by the precursor. This indeed resulted in the production of porous silicon carbide (in 15 minutes) and also sintered crystallites of micrometre sizes (after 3.5 minutes) whether MMC or SMC systems were employed