Annealing induced discoloration of transparent YAG ceramics using divalent additives in solid-state reaction sintering

Abstract

Tetraethyl orthosilicate (TEOS) was commonly served as a sintering additive to promote the densification of transparent Y3Al5O12 (YAG) ceramics. However, Si4+ that decomposed from TEOS would restrain the conversion of dopants into a higher valence state (e.g., Cr3+→Cr4+). In this study, by using divalent sintering additives (CaO and MgO), the colorless and highly transparent YAG ceramics (T=84.6%, at 1064nm) were obtained after vacuum sintering at 1840°C for 8h and without subsequent annealing in air. An absorption peak centered at ∼320nm was observed before annealing, and it extended to ∼550nm after annealing at 1450°C for 10h in air. A discoloration phenomenon occurred and more scattering centers were observed with the formation of new [Mg/Ca2+F+] color centers. Air annealing did not improve the optical quality of the as-fabricated YAG ceramics with divalent dopants as sintering additives, owing to the formation of scattering centers.