Microstructure control of porous Si₃N₄ whisker scaffolds by in-situ carbothermal reduction and nitridation reaction

Si3N4 whisker scaffolds have great potentials owing to the superior properties of Si3N4 and their unique porous structures. Obviously, the properties of the scaffolds are closely related to the morphology and distribution of phases, and the bonding between whiskers. This study proves that the above-mentioned microstructural features can be tailored to a great extent by the in-situ growth of secondary Si3N4 products via carbothermal reduction and nitridation (CRN) of SiO2 inside a beta-Si3N4 whisker network. Specifically, the yield and morphology (grain size, aspect ratio, etc.) of the CRN product are found to be strongly dependent on the composition and amount of the reactants (SiO2 and C/SiO2 molar ratio) and reaction temperature. When reacted at 1300 degrees C, a distinctive microstructure is formed in which nano sized and CRN-derived Si3N4 whiskers are uniformly distributed in the original micron sized whisker network. The whisker scaffolds reveal improved flexural strength if compared with that without CRN-derived Si3N4. This finding indicates the formation of a strong bonding between the beta-Si3N4 whiskers by the growth of the CRN product. The measured low dielectric constant (2.6 similar to 3.1) and low dielectric loss (<1.1 x10(-2)) of the scaffolds makes them suitable for wave transmission application.