Hot isostatic pressing of transparent nd:Yag ceramics

This paper demonstrates that fine-grained (2-3 μm), transparent Nd:YAG can be achieved at SiO ₂ doping levels as low as 0.02 wt% by the sinter plus hot isostatic pressing (HIP) approach. Fine grain size is assured by sintering to 98% density, in order to limit grain growth, followed by HIP. Unlike dry-pressed samples, tape-cast samples were free of large, agglomerate-related pores after sintering, and thus high transparency (i.e., >80% transmission at 1064 nm) could be achieved by HIP at <1750°C along with lower silica levels, thereby avoiding conditions shown to cause exaggerated grain growth. Grain growth was substantially limited at lower SiO ₂ levels because silica is soluble in the YAG lattice up to ∼0.02-0.1 wt% at 1750°C, thus allowing sintering and grain growth to occur by solid-state diffusional processes. In contrast, liquid phase enhanced densification and grain growth occur at ∼0.08-0.14 wt% SiO ₂, especially at higher temperatures, because the SiO ₂ solubility limit is exceeded.