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Open Access Research Article Issue
Low thermal conductivity of dense (TiZrHfVNbTa)Cx high-entropy carbides by tailoring carbon stoichiometry
Journal of Advanced Ceramics 2023, 12 (1): 49-58
Published: 02 December 2022
Downloads:480

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)Cx (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)C0.6 possesses low thermal conductivity of 6.4 W·m−1·K−1, thermal diffusivity of 2.3 mm2·s−1, as well as electrical resistivity of 165.5 μΩ·cm. The thermal conductivity of (TiZrHfVNbTa)C0.6 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.

Open Access Research Article Issue
Texture and anisotropy of hot-pressed h-BN matrix composite ceramics with in situ formed YAG
Journal of Advanced Ceramics 2022, 11 (4): 532-544
Published: 17 March 2022
Downloads:159

Textured hexagonal boron nitride (h-BN) matrix composite ceramics were prepared by hot- pressing using different contents of 3Y2O3-5Al2O3 (molar ratio of 3:5) as the sintering additive. During hot-pressing, the liquid Y3Al5O12 (YAG) phase showing good wettability to h-BN grains was in situ formed through the reaction between Y2O3 and Al2O3, and a coherent relationship between h-BN and YAG was observed with [010]h-BN// [1¯11]YAG and (002)h-BN//(321)YAG. In the YAG liquid phase environment formed during hot-pressing, plate-like h-BN grains were rotated under the uniaxial sintering pressure and preferentially oriented with their basal surfaces perpendicular to the sintering pressure direction, forming textured microstructures with the c-axis of h-BN grains oriented parallel to the sintering pressure direction, which give these composite ceramics anisotropy in their mechanical and thermal properties. The highest texture degree was found in the specimen with 30 wt% YAG, which also possesses the highest anisotropy degree in thermal conductivity. The aggregation of YAG phase was observed in the specimen with 40 wt% YAG, which resulted in the buckling of h-BN plates and significantly reduced the texture degree.

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