Millimeter-wave reactive sintering of neodymium-doped yttrium aluminum garnett

Millimeter-wave sintering of ceramic laser host materials has been under investigation for high-energy laser (HEL) applications. 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. We are currently investigating the solid-state reactive sintering of neodymium-doped yttrium aluminum garnet (Nd:YAG) using a high power millimeter-wave beam as the heat source. The 83 GHz beam is generated in the Naval Research Laboratory (NRL) High Frequency Materials Processing Facility that is powered by a 15 kW, CW, 83 GHz GYCOM gyrotron. The starting powder is a mixture of commercially available alumina, yttria, and neodymia powders. Near transparency and over 99% theoretical density have been achieved with grain sizes of 5 to 10 μm. The fluorescence lifetime of the Nd ⁺³ 1.06 μm lasing transition was measured to be about 200 μs, in good agreement with other work. SEM studies of the sintered microstructure show residual porosity caused by trapped pores that must be eliminated to produce fully transparent material.