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

Abstract

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.