Microstructural effects on the transmittance of translucent AlN ceramics by SPS

Two kinds of translucent aluminum nitride (AlN) ceramics with CaF2 and Y2O3 as sintering additives, respectively, are fabricated by spark plasma sintering (SPS) at 1800 °C for 10 min under the same sintering conditions and the microstructures are characterized using X-ray diffractometer (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) attached with an energy dispersive X-ray microanalyzer (EDX). The content of additives, sintered density and average grain size are fairly similar for the two samples, however, the transmittances are quite different. SEM and TEM studies show that the existence of secondary phase and its distribution along the grain boundaries have a major influence on the optical properties of the sintered translucent AlN ceramics. No secondary phase is observed at the grain boundaries or triple grain junctions in the sample doped with CaF2 while YAG in the sample doped with Y2O3 is found to disrupt the connections of AlN grains and cause great light diffraction at grain boundaries.