Transmission electron microscopy observation of second-phase particles in β-Si₃N₄ grains

Silicon nitride was fabricated by adding Y₂O₃ and Nd₂O₃ as sintering additives, sintering for 8 h at 1900 °C, and heat treating for 4 h at 2200 °C to enhance grain growth. The microstructure was investigated by scanning electron microscopy, high-resolution electron microscopy, energy dispersive x-ray spectroscopy (EDS), and electron microdiffraction. This material had a duplex microstructure composed of many fine grains and a few coarse grains. In β-Si₃N₄ grains, second-phase particles with the composition of liquid phase, Y-Nd-Si-O or Y-Nd-Si-O-N, in the size of 10-30 nm were observed. EDS spectra and microdiffraction patterns revealed that those were amorphous or crystalline particles of Y-Nd-apatite, (Y,Nd)₁₀Si₆O₂₄N₂. These particles were presumably formed during cooling by the precipitation of Y-Nd-Si-O-N, which was trapped in the β-Si₃N₄ grains as solid solution or trapped liquid. The results suggest that attention should be paid to the trace amounts of trapped elements in β-Si₃N₄ grains in trying to improve the thermal conductivity of sintered silicon nitride.