Controlled Transient Solid Second‐Phase Sintering of Yttria

Compositions of La 2 O 3 ‐Y 2 O 3 were sintered to full density by a process which uses a controlled transient solid phase to retard grain growth sufficiently that pores remain on grain boundaries and are eliminated by solid‐state diffusion. The temperature is then decreased into a single‐phase region and the structural transition yields a pore‐free, single‐phase body. Optimum compositions are 8.1 to 12.1 mol% La 2 O 3 for a sintering temperature of 2170°C and annealing temperatures of 1920° to 2020°C, resulting in materials with near‐theoretical total transmittances and specular transmittance within 6% of theoretical.