Modeling experiments on continuous microwave processing of thermal runaway materials

Summary form only given. Microwave processing of metal oxide ceramic fibers offers several potential advantages. However, work so far has revealed several difficult problems; typically, these materials have very low dielectric loss coefficients at room temperature which increase rapidly with temperature. Although microwave heating of such low-loss materials can be accomplished by using high field strengths in high-Q applicators, this approach usually fails because of thermal runaway. We seek to demonstrate that custom designed microwave applicators can overcome the problem of thermal runaway and produce heating patterns which lead to temperature profiles tailored to specific processing objectives. The first step is to develop a verified model of the electromagnetic and temperature fields in an applicator and load. We report results from experiments on well-characterized ceramic rods and a heat transfer model of the experiments which assumes uniform electromagnetic fields. The experimental system consists of a single-mode TE10n cavity coupled to a broad-band traveling wave tube (TWT) amplifier with a nominal frequency range of 2.5-8 GHz. For these experiments, the frequency was maintained near 2.93 GHz. The resonant cavity consists of a water-cooled section of copper WR284 waveguide with a coupling iris and "beyond-cutoff" coupler at the inlet, and an adjustable short. During each experiment, the forward and reflected power from the cavity were measured and recorded, along with the temperature data, at one second intervals.