Millimeter-Wave Assisted Sintering of Polycrystalline Yttria for Laser Host Material
We report an investigation of millimeter-wave processing of yttria for fabrication of transparent, high-strength polycrystalline laser hosts for high energy laser (HEL) applications. Advantages of polycrystalline, compared to single-crystal laser host materials, include lower processing temperature, higher gain with flexibility of higher dopant concentrations, cheaper fabrication, and larger-size devices. Millimeter-wave processing is an alternative method to solve the problems of both conventional vacuum and low-frequency microwave sintering, such as low heating rate, poor coupling and thermal gradients. A major component of the millimeter-wave processing facility is a 20-kW, continuous-wave, 83-GHz gyrotron oscillator. Yttria has been successfully sintered with millimeter-wave beams with up to 99% theoretical density. A partially transparent yttria 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 will be discussed.
Young Won Chang, Nack J. Kim and Chong Soo Lee
M. K. Hornstein et al., "Millimeter-Wave Assisted Sintering of Polycrystalline Yttria for Laser Host Material", Materials Science Forum, Vols. 561-565, pp. 511-514, 2007