Controlling thermal gradient during sintering of polycrystalline laser host materials with a millimeter-wave beam

Millimeter-wave sintering of ceramic laser host materials has been under investigation at the Naval Research Laboratory (NRL) for high-energy laser (HEL) applications. Potential advantages of polycrystalline, compared to single-crystal, laser host materials include lower processing temperature, higher gain from higher dopant concentration, cheaper fabrication, and larger devices. Successful production of laser quality polycrystalline laser host requires a high purity material and the sintering process must achieve nearly perfect micro structure as imperfections quickly result in unacceptable light absorption and scattering. Transparent, laser quality, polycrystalline Nd:YAG has recently been achieved in conventional vacuum furnaces using a reactive sintering process based on commercially available powders, and we have been trying to replicate these results in the NRL millimeter-wave materials processing facility. Compacts prepared from mixtures of the oxide precursors are placed in an open or closed crucible in a vacuum environment and are heated directly by the 83 GHz beam to temperatures of 1000- 1800degC. Over 99% theoretical density has been achieved at the highest temperatures with moderate grain growth. Fluorescence lifetime studies of the Nd+3 ion are in good agreement with published results for laser quality material. Our X-ray diffraction studies of the phases present at various temperatures are generally in agreement with previous work. However, full transparency has not been achieved and investigation of the sintered compact microstructure has revealed trapped pores and other imperfections. There has also been unexpected variability in the results. New modeling results for the open casket configuration, which was chosen for temperature measurement and heating efficiency considerations, indicate that large thermal gradients are likely. These gradients can produce non-uniform sintering and complicate temperature measurements. New closed casket configurations are being developed that are expected to result in smaller thermal gradients and more accurate temperature measurement while retaining good heating efficiency. The results of model calculations and available experimental results will be discussed.