Thermal and electrical properties of a high entropy carbide (Ta, Hf, Nb, Zr) at elevated temperatures

Abstract

The thermal and electrical properties were measured for a high entropy carbide ceramic, consisting of (Hf, Ta, Zr, Nb)C. The ceramic was produced by spark plasma sintering a mixture of the monocarbides and had a relative density of more than 97.6%. The resulting ceramic was chemically homogeneous as a single‐phase solid solution formed from the constituent carbides. The thermal diffusivity (0.045–0.087 cm2/s) and heat capacity (0.23–0.44 J/g•K) were measured from room temperature up to 2000°C. The thermal conductivity increased from 10.7 W/m•K at room temperature to 39.9 W/m•K at 2000°C. The phonon and electron contributions to the thermal conductivity were investigated, which showed that the increase in thermal conductivity was predominantly due to the electron contribution, while the phonon contribution was independent of temperature. The electrical resistivity increased from 80.9 μΩ•cm at room temperature to 114.1 μΩ•cm at 800°C.

Low thermal conductivity of dense (TiZrHfVNbTa)Cxhigh-entropy carbides by tailoring carbon stoichiometry

“…This indicates the predominant role of the electron conduction in the thermal transport mechanism, in agreement with those of monocarbides and quaternary high-entropy carbide, as reported in Refs. [11,22,23].…”

“…7. The thermal diffusivity of the (TiZrHfVNbTa)C 0.6 ceramic increases from 2.3 mm 2 •s −1 at room temperature to 4.3 mm 2 •s −1 at 1473 K, which are far lower than those of the constituted individual carbides and other high-entropy carbides over the entire temperature range ((HfTaZrNb)C of 4.5 mm 2 •s −1 and (HfZrTaNbTi)C of 3.6 mm 2 •s −1 at ambient temperature) [8,11]. This is speculated to be the combined influence of component number and carbon vacancies, which also reflects that the sluggish diffusion effect plays a role [8,28].…”

“…Although the thermal conductivity of ZrC x can be effectively reduced by tailoring the carbon stoichiometry, the large-scale application of the transition metal carbides is still restricted due to the stringent requirement for applications in an ultra-high-temperature insulation system. Recently, the emerging high-entropy carbides have evoked extensive research, possessing excellent mechanical properties [4,5], corrosion and oxidation resistance [6,7], low thermal conductivity [8][9][10], prominent electrical properties [11], and electromagnetic wave absorbing properties [12]. Therefore, adjusting the carbon stoichiometry of the high-entropy carbides is feasible to further lower the thermal conductivity and maintain moderate mechanical properties at the same time, making them promising to be candidates for thermal protection materials.…”

“…Fig.8Thermal conductivity of (TiZrHfVNbTa)C x compared with those of the reported carbides with different numbers of cations and carbon stoichiometries at ambient temperature[8][9][10][11]15,20,[29][30][31][32][33][34][35]. Relative density is also attached to make an accurate comparison.…”

Direct selective laser sintering of high-entropy carbide ceramics

“…HECCs such as (Hf 0.2 Zr 0.2 Ta 0.2 Nb 0.2 Ti 0.2 )C were first reported by our group [8] and Castle et al [9] in 2018, which were synthesized using spark plasma sintering (SPS) from monocarbide powders in equimolar concentrations. Since then, many unique physical properties have been reported in the literature such as higher hardness and high-temperature strength [10], lower thermal conductivity [8,11], and improved irradiation resistance than the constitute monocarbides [12,13]. The promising properties of HECCs have been attributed to the atomic-level disorder, compositional complexity, lattice distortion, etc., although the fundamental mechanisms governing the microstructure-property relationship remain to be elucidated [14].…”