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 1wt%Nd2O3-2wt%NdF3 (1Nd(2)Nd sample) and 1wt%Gd(2)O(3)2-wt%GdF3 (1Gd(2)Gd sample) additive exhibited the highest thermal conductivity of 187.8W/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. 1Nd(2)Nd 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 1Gd(2)Gd 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. (C) 2021 The Ceramic Society of Japan. All rights reserved.