A facile method to produce rodlike β‐Si3N4 seed crystallites for bimodal structure controlling

Abstract

Abstractβ‐Si3N4 rodlike seed crystallites were successfully produced by single‐step heat treatment of commercial α‐Si3N4 powder at 1900°C for 20 h under an N2 gas pressure of 980 kPa. The average diameter, length, and aspect ratio of the seed crystallites were 0.73 μm, 1.37 μm, and 1.86, respectively. The α‐ ⇀ β‐Si3N4 phase transformation proceeded mainly at 1900°C, and this temperature was lower than the theoretical α‐Si3N4 dissociation temperature (1933°C) under N2 gas pressure of 980 kPa. The formation of metastable solid solution due to the dissolution of O impurity into the α‐Si3N4 crystal lattice was suggested as the driving force for the present oxide additive‐free α‐ ⇀ β‐Si3N4 phase transformation. β‐Si3N4 ceramics were fabricated by liquid phase sintering promoted by an additive system of 1 wt% MgO with 3 wt% Gd2O3. Starting α‐Si3N4 powder with 10 vol% rodlike β‐Si3N4 seed crystallites prepared in this study and an extended sintering time for up to 20 h at 1950°C resulted in the formation of bimodal microstructure composed of fine matrix grains and large elongated grains originated from the seed crystallites. The β‐Si3N4 ceramics exhibited improved fracture toughness and thermal conductivity of 5.9 ± 0.8 MPa m−1/2 and 109.3 ± 0.4 W m−1 K−1, respectively, retaining a high fracture strength of about 1 GPa.