In this paper, Si₃N₄ was used as a novel solid-state sintering additive to prepare AlON transparent ceramics with high transparency and flexural strength via the pressureless pre-sintering and hot isostatic pressing (HIP) method at a relative low HIP temperature (1800°C). The effect of Si₃N₄ content on the phase, microstructure, optical property and flexural strength was investigated. The experimental results showed that Si element was homogenously distributed in both pre-sintered and HIPed AlON ceramics. The densification enhanced and grain grew with increasing Si₃N₄ content in the pre-sintered AlON ceramics and all the samples became pore-free after HIP, which favor for transparency. The AlON ceramics doped with 0.1wt% Si₃N₄ had the highest transmittance of 83.8 % at 600 nm and 85.6 % at 2000 nm (4 mm thickness), with a flexural strength of 404 MPa, which was higher than previous reports.

AlN/epoxy composites with high thermal conductivity were successfully prepared by infiltrating epoxy into AlN porous ceramics which were fabricated by gelcasting of foaming method. The microstructure, mechanical, and thermal properties of the resulting composites were investigated. The compressive strengths of the AlN/epoxy composites were enhanced compared with the pure epoxy. The AlN/epoxy composites demonstrate much higher thermal conductivity, up to 19.0 W/(m·K), compared with those by the traditional particles filling method, because of continuous thermal channels formed by the walls and struts of AlN porous ceramics. This study demonstrates a potential route to manufacture epoxy-based composites with extremely high thermal conductivity.