Effect of Silica on the Reactive Sintering of Polycrystalline Nd:YAG Ceramics

Abstract

In the present work, the role of silica as sintering aids in the densification and the grain growth of yttrium aluminum garnet doped by neodymium (Nd:YAG) ceramics has been investigated. The samples were prepared by ball milling of pure oxides (Al2O3, Y2O3, Nd2O3, SiO2), shaped by cold uniaxial pressing, and sintered in vacuum between 1473 and 1973 K. After cooling, the specimens were annealed under air or vacuum. Their microstructure and the chemical composition of the secondary phases were examined using electron probe microanalysis, scanning electron microscopy, or transmission electron microscopy techniques. From these results, silica addition proved to be efficient on the densification kinetics between 1673 and 1873 K, especially when SiO2 content exceeds 0.05 wt%. Indeed, the solid‐state reaction between SiO2 and Nd:YAG particles in the vicinity of 1660 K leads to a liquid phase that adopts a eutectic composition in the Al2O3–Y2O3–SiO2 system. This phase enhances the densification by improving rearrangement of particles and mass transport at the grain boundaries. At a high temperature, liquid phase and silica were partially removed and some intergranular and intragranular inclusions of residual silica remained after cooling. The most promising thermal treatment consists of sintering Nd:YAG ceramics under vacuum at a high temperature (T≥1973 K) to reach fully dense pieces and to decrease the volume fraction of secondary phases (i.e., liquid phase or silica‐based phases). Finally, these conditions would allow to produce transparent YAG pieces due to their good homogeneity with regard to their microstructural properties (grain size, volume fraction of secondary phases).