A facile method to produce rodlike beta-Si3N4 seed crystallites for bimodal structure controlling

beta-Si3N4 rodlike seed crystallites were successfully produced by single-step heat treatment of commercial alpha-Si3N4 powder at 1900 degrees 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 mu m, 1.37 mu m, and 1.86, respectively. The alpha- -> beta-Si3N4 phase transformation proceeded mainly at 1900 degrees C, and this temperature was lower than the theoretical alpha-Si3N4 dissociation temperature (1933 degrees C) under N-2 gas pressure of 980 kPa. The formation of metastable solid solution due to the dissolution of O impurity into the alpha-Si3N4 crystal lattice was suggested as the driving force for the present oxide additive-free alpha- -> beta-Si3N4 phase transformation. beta-Si3N4 ceramics were fabricated by liquid phase sintering promoted by an additive system of 1 wt% MgO with 3 wt% Gd2O3. Starting alpha-Si3N4 powder with 10 vol% rodlike beta-Si3N4 seed crystallites prepared in this study and an extended sintering time for up to 20 h at 1950 degrees C resulted in the formation of bimodal microstructure composed of fine matrix grains and large elongated grains originated from the seed crystallites. The beta-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.