Advanced Ceramics Sintering Using High-Power Millimeter-Wave Radiation

The results of ceramics sintering experiments using high-power millimeter-wave radiation are reported. Sintering of silicon nitride with 5%Al2O3 and 5%Y2O3 was performed in a multi-mode applicator using a 10-kW 28-GHz gyrotron in CW operation. It was found that the silicon nitride samples sintered with 28 GHz radiation at 1650'C for 30 min reached to as high as theoretical density (TD), while the conventionally sintered samples at 1700°C for 60 min resulted in the density as low as 90% TD. Focusing experiments of millimeter-wave radiation from the high-power pulsed 60-GHz gyrotron have been performed using a quasi-optical antenna system (two-dimensional ellipso-parabolic focusing antenna system) to demonstrate the feasibility of the power density of as high as 100 kW/cm2. Typical heating characteristics using the focused beam were made clear for this system. It was found that the densification of yttria-stabilized zirconia (ZrO2-8mol%Y2O3) samples to as high as 97% TD was obtained from the sintering with focused 60 GHz beam in pulse operation with a 10-ms pulse duration at a 0.5Hz repetition. The densification temperature for the zirconia could be lowered by 200°C than that expected conventionally.