Effects of RE2O3-MgO (RE = La, Gd, Yb, Sc) additives on microstructure and thermal conductivity of SiC ceramic

SiC ceramics were fabricated by spark plasma sintering of β-SiC powder with various rare-earth (RE = La, Gd, Yb, Sc) oxides and magnesia additives. The phase compositions, microstructures, lattice oxygen contents, and thermal conductivities were systematically investigated. A decrease in the cationic radius of rare-earth element resulted in an increased cationic field strength. This enhanced attraction to oxygen anion led to higher viscosity and improved high-temperature stability of the intergranular RE-Mg-Si-O liquid phase. After sintering at 1850°C, influenced by liquid-phase viscosity, the degree of β→α phase transformation in SiC and the grain size decreased with decreasing cationic radius of rare-earth element. After sintering at 2050°C, the liquid-phase viscosity significantly decreased, but volatilization became pronounced. The extent of β→α phase transformation and the grain size were affected by both liquid-phase viscosity and volatility. Larger grain size and higher affinity of rare-earth cations for oxygen anions contributed to reduce lattice oxygen content. The thermal conductivities of the SiC ceramics ranged from 192 to 229 W/mK. Notably, the SiC ceramic with Yb 2 O 3 -MgO additives exhibited the largest grain size, the lowest lattice oxygen content, and the elimination of β/α heterophase boundaries, thereby achieving the highest thermal conductivity of 229 W/mK.