A facile method to produce rodlike β‐Si 3 N 4 seed crystallites for bimodal structure controlling

Abstract β‐Si 3 N 4 rodlike seed crystallites were successfully produced by single‐step heat treatment of commercial α‐Si 3 N 4 powder at 1900°C for 20 h under an N 2 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‐Si 3 N 4 phase transformation proceeded mainly at 1900°C, and this temperature was lower than the theoretical α‐Si 3 N 4 dissociation temperature (1933°C) under N 2 gas pressure of 980 kPa. The formation of metastable solid solution due to the dissolution of O impurity into the α‐Si 3 N 4 crystal lattice was suggested as the driving force for the present oxide additive‐free α‐ ⇀ β‐Si 3 N 4 phase transformation. β‐Si 3 N 4 ceramics were fabricated by liquid phase sintering promoted by an additive system of 1 wt% MgO with 3 wt% Gd 2 O 3 . Starting α‐Si 3 N 4 powder with 10 vol% rodlike β‐Si 3 N 4 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‐Si 3 N 4 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.