YTTRIUM SEGREGATION AND YAG PRECIPITATION AT SURFACES OF YTTRIUM-DOPED alpha Al2O3.

Auger electron spectroscopy and low-energy ion scattering were used to assess surface composition of yttrium-doped polycrystalline and single-crystal alpha -aluminas which were heated under vacuum to temperatures above 1500 degree C. Extensive surface and intergranular precipitation of yttrium aluminum garnet was observed in a compact of nominal Y content of 210 wt ppm, while apparent Y segregation to surface cation sites predominated in a second compact with a Y content of 160 wt ppm. In the latter case, the segregation enthalpy of yttrium to the alumina surface in the temperature range 1700 degree to 1900 degree C was estimated to be of the order minus 44 kJ/mol with a maximum surface cation site occupancy of approximately equals 13%.