Effect of Re₂O₃–MgO additives on the microstructure evolution and properties of β-Si₃N₄ ceramics

β-Si₃N₄ ceramics were prepared by gas pressure sintering using α-Si₃N₄ powders, β-Si₃N₄ whiskers and Re₂O₃–MgO (Re = Y, La, Gd, Ho, Yb) additives. The phase transformation from α-Si₃N₄ to β-Si₃N₄ was completed after sintering. The distribution of oxide particles gets more homogeneous with the increase of the atomic number of the Re element, resulting in an increase of the grain size and the formation of elongated β-Si₃N₄ grains, which significantly influence the fracture mode and the mechanical properties of β-Si₃N₄ ceramics. The thermal conductivity of β-Si₃N₄ ceramics is determined by both the grain size and the distribution homogeneity of oxide particles, thus showing no regular trend between different specimens.