Enhanced thermal conductivity of Si3N4 ceramics through pyrolytic carbon prepared by gel casting using DMAA

Abstract

To reduce the lattice oxygen content of Si3N4 green bodies, samples were prepared using an N,N-dimethylacrylamide (DMAA) system. The endothermic and exothermic peaks are related to the decomposition of organic matter and the reaction between pyrolytic carbon and impurity oxides in the sintering process. Multi-step heat preservation during the thermal degreasing process reduced the carbon content and defects. The α→β phase transformation and aspect ratio of grains changed from 89.2% to 4.84:1 at 1640 °C to 96.3% and 7.80:1 at 1700 °C, respectively. The relative density, hardness, fracture toughness, and thermal conductivity of the Si3N4 samples reached 98.5%, 1353 HV, 8.51 MPa·m1/2, and 97.1 W·m−1·K−1, respectively. Pyrolytic carbon can react with SiO2, which reduces the content of the glass phase in the green body and affects the dissolution and precipitation process and lattice oxygen content. In the structure of the Si3N4 sample, carbon particles with sizes of 100–500 nm and near-spherical shapes were observed. Intergranular-phase YMgSi2O5N was formed during the sintering process, which was conducive to improving the thermal conductivity. This article uses lower temperature and no pressure conditions to prepare Si3N4 ceramics, which can provide reference for batch and low-cost production.

Keywords

• Pyrolytic carbon
• Thermogravimetric analysis
• Aspect ratio
• Dissolution precipitation mechanism
• Thermal conductivity