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 [1].
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.