From stoichiometric to non-stoichiometric high-entropy carbide: A case study of hafnium addition

High-entropy carbide (Ti _{0.2 x} Zr _{0.2 x} Hf _{0.2 x + (1 -x)} Nb _{0.2 x} Ta _{0.2 x} )C _x (HEC- (1-x)Hf, x = 0.6-1) ceramics with 0-40 at.% nominal carbon vacancy were prepared by adding metal Hf into (Ti _0.2 Zr _0.2 Hf _0.2 Nb _0.2 Ta _0.2 )C (HEC) as a case study. The samples prepared by spark plasma sintering at 2000 ^o C exhibit relative densities higher than 99%, single-phase rock salt structure, and homogeneous metal elements distribution. Their lattice parameters increase gradually with the added Hf content. The lattice parameter of HEC-0.4Hf is 0.86% higher than HEC's. The average grain size of the samples decreased initially, followed by an increase as Hf content increased, which is controlled by the comprehensive effect of Hf addition and carbon vacancy. The thermal conductivities of the samples decrease with the increase of Hf addition. HEC-0.4Hf has a very low thermal conductivity of 5.6 W/m·K at room temperature, which decreases 67.8% compared to HEC. The increased phonon scattering by the carbon vacancies can be the main reason for the reduced thermal conductivity.