Effect of processing variables on synthesis of β-Si3N4 particles

Synthesis of highly anisotropic rod-like β-Si3N4 particles with different morphological characteristics was accomplished using commercial α-Si3N4. A number of powder mixtures were prepared with additions of various oxides (Al2O3 + RE2O3, being RE = Y, La or Yb) that were subsequently heat treated in a loose powder state in the temperature range of 1750–1950 °C and dwell times from 10 to 180 min under high pressure of N2 (60 MPa) in a hot isostatic press. The obtained aggregated porous bodies were lightly crushed by hand using an alumina mortar and pestle and subsequently subjected to sequential chemical treatments to eliminate the glassy phases and possible residual crystalline compounds, and to separate the synthesized β-Si3N4 particles. Resulting β-Si3N4 particles were characterized by X-ray diffraction and examined by scanning and transmission electron microscopy. Morphological characteristics of the synthesized β-Si3N4 particles were determined through quantitative dimensional analysis. The effect of amount and chemical composition of oxide additives responsible for liquid phase formation as well as heat treatment parameters on β-Si3N4 particles size and morphology are established. Possible mechanisms of β-Si3N4 particles growth are discussed.