Effects of RE ₂ O ₃ ‐MgO (RE = La, Gd, Yb, Sc) additives on microstructure and thermal conductivity of SiC ceramic

Abstract 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.