Architectural design and cryogenic synthesis of Si₃N₄@(TiN–Si₃N₄) for high conductivity

Si₃N₄@(TiN–Si₃N₄) composites with heteroshelled structure were designed for enhanced conductivity and successfully synthesized through the simultaneous reduction and in-situ cocoating process in liquid ammonia at around −40°C. The heteroshells were composed of nanosized TiN and Si₃N₄ particles, which were amorphous with the size ranging from 10 to 40 nm. Using spark plasma sintering, dense bulk composite with >98.1% relative density of theoretical value were obtained and their electrical conductivity were increased to an adequate value (6.62 × 10² S·cm⁻¹) for electrical discharge machining by compositing 15 vol% TiN to Si₃N₄, which is superior to the previous reports. The excellent electric performance could be attributed to the heteroshelled structure which guarantees the conductive network can be formed and kept with minimal TiN content. The nanosized Si₃N₄ powders in the shells reduce the content of conductive powders and limit the growth of TiN particles.