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

Abstract

Transition metal carbides are promising candidates for thermal protection materials due to their high melting points and excellent mechanical properties. However, the relatively high thermal conductivity is still a major obstacle to its application in an ultra-high-temperature insulation system. In this work, the low thermal conductivity of dense (TiZrHfVNbTa)C x (x = 0.6-1) high-entropy carbides has been realized by adjusting the carbon stoichiometry. The thermal conductivity gradually decreases from 10.6 W•m −1 •K −1 at room temperature to 6.4 W•m −1 •K −1 with carbon vacancies increasing. Due to enhanced scattering of phonons and electrons by the carbon vacancies, nearly full-dense (97.9%) (TiZrHfVNbTa)C 0.6 possesses low thermal conductivity of 6.4 W•m −1 •K −1 , thermal diffusivity of 2.3 mm 2 •s −1 , as well as electrical resistivity of 165.5 μΩ•cm. The thermal conductivity of (TiZrHfVNbTa)C 0.6 is lower than that of other quaternary and quinary high-entropy carbide ceramics, even if taking the difference of porosity into account in some cases, which is mainly attributed to compositional complexity and carbon vacancies. This provides a promising route to reduce the thermal conductivity of high-entropy carbides by increasing the number of metallic elements and carbon vacancies.

Lattice rigidity in high‐entropy carbide ceramics with carbon vacancies

“…High‐entropy carbides are the solid solutions of monocarbides, composed of five or more metal elements in roughly equal equimolar proportions and inherit the properties of the related monocarbides, which shows high thermal stability and high hardness 7–27 . High‐entropy carbides also have the feature of forming nonstoichiometric composition 16–23 . However, carbon vacancies, acting as structural defects in the monocarbides and high‐entropy carbides, may play different roles in the two materials due to the high‐entropy effect and severe lattice distortion present in the latter one.…”

“…High-entropy carbides also have the feature of forming nonstoichiometric composition. [16][17][18][19][20][21][22][23] However, carbon vacancies, acting as structural defects in the monocarbides and high-entropy carbides, may play different roles in the two materials due to the high-entropy effect and severe lattice distortion present in the latter one.…”

“…High‐entropy carbides are the solid solutions of monocarbides, composed of five or more metal elements in roughly equal equimolar proportions and inherit the properties of the related monocarbides, which shows high thermal stability and high hardness 7–27 . High‐entropy carbides also have the feature of forming nonstoichiometric composition 16–23 .…”

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

“…The result showed that the added Si with a low melting point and the formed carbon vacancy could obviously decrease the densification temperature from 1900 to 2000°C for stoichiometric high-entropy carbides to 1600°C for the non-stoichiometric ones. Very recently, Chen et al [14] and Zhang et al [30] prepared non-stoichiometric high-entropy carbide ceramics by carbothermal reduction reaction followed by pressureless sintering and found that the thermal conductivity decreased with the increased content of carbon vacancy.…”

“…The two sublattices not only affect the calculation of the configurational entropy but also influence the diffusion-related behaviour of each other. Especially for the high-entropy transition metal carbides, the generally existing non-stoichiometry in the carbon site of the monocarbides will result in a similar nonstoichiometry in the high-entropy carbides [12][13][14][15][16]. For instance, zirconium carbide (ZrC) has a broad non-stoichiometry ranging from 0.49 to 1 [17].…”