Effects of rare earth oxides and fluorides on flexural strength and thermal conductivity of hot-pressed SiC ceramics

Dense silicon carbide (SiC) ceramics were fabricated through hot-pressing with a novel combination of rare earth oxides and fluorides (CeO2–CeF3, Nd2O3–NdF3, Gd2O3–GdF3 and Y2O3–YF3) as additive. Effects of these additive combinations on microstructure, phase composition, flexural strength and thermal conductivity of SiC ceramics were evaluated. The samples with 1 wt %Nd2O3–2 wt %NdF3 (1Nd2Nd sample) and 1 wt %Gd2O3–2 wt %GdF3 (1Gd2Gd sample) additive exhibited the highest thermal conductivity of 187.8 W/m·K and highest flexural strength of 607.6 MPa respectively. Impedance spectroscopy analysis was employed to further investigate the variations of defects and impurities in SiC ceramics. 1Nd2Nd sample exhibited a higher fitting grain and grain boundary resistance that suggested a lower concentration of V''''Si vacancies than other samples, which resulted in a higher thermal conductivity. On the other hand, the highest flexural strength of 1Gd2Gd sample was attributed to a combined effect of its small grain size, contiguous microstructure and low content of grain boundary phases. All in all, Re2O3–ReF3 additive combinations are suitable for tailoring and improving the thermal conductivity and flexural strength of SiC ceramics.