Consolidation of polycrystalline yttria powder by MILLIMETER-WAVE sintering for laser host applications

We report recent results of an investigation of millimeter-wave processing of yttria (Y2O3) for fabrication of transparent, high strength polycrystalline ceramic laser hosts for High Energy Laser (HEL) applications.^1,2 The objective is to produce polycrystalline materials with optical quality comparable to that of a single crystal. It is difficult to produce yttria single crystals because of the phase transformation around 2000°C and the high melting temperature which is over 2400°C. While single crystals have high thermal conductivity and can operate at high powers, they are costly and limited in size and dopant concentration. Significant advantages of polycrystalline materials compared to single crystals, are lower processing temperature, higher gain as a result of higher dopant concentrations, faster and less expensive fabrication, and the possibility of larger devices. Millimeter-wave processing has been proposed as an alternative method to solve the problems of both conventional vacuum sintering and low frequency microwave sintering, such as low heating rates, poor coupling, and unfavorable thermal gradients. A major component of the NRL millimeter-wave processing facility is a 20-kW, continuous-wave (CW), 83-GHz gyrotron oscillator (GYCOM, Ltd.). Translucent yttria has been successfully sintered with millimeter-wave beams with up to 99% theoretical density. A partially transparent yttria ceramic sample has also been achieved using the millimeter-wave sintering process. Several factors impact the quality of the sintered material including the presence of agglomerates, impurities, processing atmosphere, sintering aids, and thermal gradients. Efforts to improve the transparency are in progress.