Microstructure and optical properties of hot isostatically pressed Nd:YAG ceramics

Neodymium-doped transparent yttrium-aluminum garnet (Y3Al5O12, YAG) (Nd:YAG) ceramics for solid-state laser material were fabricated by a solid-state reaction method using high-purity powders (Al2O3, Y2O3, and Nd2O3) as starting materials and capsule-free hot isostatic pressing (HIP). The mixed powder compacts were presintered at 1,600 C for 3 h under vacuum, hot isostatically pressed at 1,500--1,700 C for 3 h under 9.8 or 1.96 MPa of argon gas pressure, and then sintered again at 1,750 C for 20 h under vacuum. Although the presintered specimen approached full density after HIP, its optical transmittance was quite low (∼5% at 1,000 nm) because of lack of grain growth. Grain growth was observed in the specimens that were hot isostatically pressed and vacuum sintered at 1,750 C for 20 h, but numerous pores occurred around the surface of these specimens. Consequently, the optical transmittance of Nd:YAG ceramics that were treated by HIP was inferior to that of the same ceramics that were sintered under vacuum only because of light scattering that was caused by the pores (at the grain boundaries) that were produced during the HIP treatment