The Influence of Tri-Structural Isotropic Fuel on the Microstructure and Thermal Conductivity of SiC Tri-Structural Isotropic Composite Fuels

Thermal conductivity is the key property of SiC-TRISO composite fuel. This study investigated the relationship between SiC phase transition, thermal conductivity, and microstructure across different temperatures. The physical phase, morphology, and microstructure of SiC and SiC-TRISO composite fuels were characterized by XRD and SEM. Meanwhile, EDS was employed to determine the chemical composition within SiC grains. The results showed the transformation of the β-SiC phase to α-SiC in the matrix with increasing sintering temperature, while Al, Y, and Ca concentrations within the SiC grains decreased. The highest λ value of SiC was achieved at a sintering temperature of 1750 °C, measuring 75.51 W m · K at room temperature and 43.36 W m · K at 500 °C. The incorporation of TRISO fuel lowered the λ value of SiC-TRISO composite fuel, yielding 57.96 and 34.51 W m · K at room temperature and 500 °C, respectively. The outermost carbon layer of TRISO fuel interacts with the silicon carbide matrix and liquid phase, facilitating the phase transition from 3C-SiC to 6H-SiC and, subsequently, to 4H-SiC. This process accelerates the depletion of Al, Y, and Ca within the silicon carbide grains, encourages grain growth, and raises the free-carbon content, thereby decreasing the λ of the composite fuel.