Pulsed 35 GHz gyrotron with overmoded applicator for sintering ceramic compacts

Summary form only given, as follows. The use of high frequency microwaves for processing ceramic materials is currently under investigation at NRL. This paper will discuss an overmoded microwave furnace based on a 35 GHz pulsed gyrotron which is being used for experiments on microwave sintering of small (/spl sim/1 cm diameter) compacts of fine grain and ultra-fine grain ceramics such as alumina. Unlike most microwave furnaces which utilize cw 2.45 GHz microwave sources, the present millimeter wave system operates at relatively high peak power (30-100 kW) and low duty factor (<1%) to produce average powers up to 200 W. The TE/sub 01/ mode gyrotron operates at 70 kV and currents up to 10 A with an electronic efficiency of about 20%. It is driven by a hard-tube, variable pulse length (1-10 /spl mu/s) modulator at repetition rates up to 1 kHz. The gyrotron output power is transported via pressurized K/sub a/-Band waveguide to an overmoded resonator containing the workpiece. In initial experiments, the resonator consists of a 17 cm section of WR-284 waveguide. The WR-284 waveguide section is terminated at one end by a step transition to the K/sub a/-Band waveguide, and at the other end by a copper screen backed by a hollow plastic water-filled window. The copper screen/water window serves as a low-loss viewing port for a two-color pyrometer. The compact is placed in an insulating, low-loss fiberboard casket. This system is being used to assess the advantages of using 35 GHz microwave heating to densify ceramic compacts. In preliminary operation the system has heated high purity, nanocrystalline alumina compacts to /spl sim/1300/spl deg/C with an input power of 160 W without the need for hybrid heating techniques. The application of the system to controlled sintering experiments will be described and factors impacting compact heating uniformity and power coupling efficiency will be discussed.