New high-entropy ceramics for very high temperature insulation

Thermal insulation material is an essential component of the thermal protection system (TPS) of hypersonic vehicles. Recently, high-entropy ceramics have attracted great attention in thermal insulation due to their low thermal conductivity due to phonon scattering by multiple components and distorted lattices. Among the various available options, porous high-entropy carbide ceramics (PHECs) have emerged as promising candidates for TPS due to their inherent characteristics such as high melting point, excellent high-temperature stability, low density, and superior thermal insulation properties.

Generally, high-entropy porous carbide ceramics are fabricated using various methods such as template, direct foaming, and partial sintering. To synthesize these ceramics, carbides or metal oxides are commonly used as raw materials by solid-state methods. This requires intensive grinding to disperse the various components and extremely high processing temperatures to accelerate the diffusion of atoms. However, this approach is energy-intensive and introduces impurities during the ball milling process, which destroys the stoichiometry and affects the configurational entropy. In addition, high-temperature sintering leads to grain growth and the removal of nano-sized pores, making it difficult to control the porosity and pore structure of PHEC ceramics. In addition, the casting and processing of high-entropy carbides are limited by this solid-state method.

Recently, a team of materials scientists led by Haibo Ouyang from Shaanxi University of Science and Technology, China, reported the fabrication, microstructure, compressive strength and thermal conductivity of the porous material (Ta_0.2Nb_0.2You_0.2Zr_0.2Hf_0.2)C high entropy ceramic by self-foaming method.

This work not only explains the formation mechanism of the unique hierarchical porosity structure and the superior thermal insulation and compression properties of the porous material (Ta_0.2Nb_0.2You_0.2Zr_0.2Hf_0.2)C high entropy ceramics, but also provides a cost-effective and easy strategy to produce ultrahigh temperature porous ceramics.

The team published their work in Journal of Advanced Ceramics May 6, 2024.

“In this report, we synthesized the porous (Ta_0.2Nb_0.2You_0.2Zr_0.2Hf_0.2)C high-entropy ceramic by a self-foaming method using commercially available metal chloride and furfuryl alcohol as a precursor. This method was inspired by the self-foaming behavior of FA. Polymer foam containing elements such as Ti, Zr, Ta, Nb, and Hf can be produced by the self-condensation of FA using metal chlorides as catalysts. The polymer foam transformed into porous ceramic by pyrolysis and carbothermic reduction processes,” said Haibo Ouyang, a professor at the School of Materials Science and Engineering, Shaanxi University of Science and Technology, China, an expert whose research interests focus on the field of ultrahigh-temperature ceramic materials and composites.

“PHEC ceramics consist of microspheres with a size of 2 µm, which leads to a high porosity of 91.3% and an interconnected framework. These microspheres are made of high-entropy carbide grains (20 nm), which results in an abundant interface and nano-sized pores in PHEC ceramics,” Ouyang said.

“Due to its unique hierarchical structure, the prepared PHEC ceramic exhibits exceptional compressive strength (28.1±2 MPa) and exceptionally low thermal conductivity at room temperature (0.046 W m−1 K−1). This makes it a promising thermal insulation material for ultra-high temperature applications,” Ouyang said.

However, more delicate research work is still needed to explore the oxidation resistance of the porous material (Ta_0.2Nb_0.2You_0.2Zr_0.2Hf_0.2)C high entropy ceramic as a new thermal insulation material. In this regard, Ouyang also presented two major works, including ultra-high temperature oxidation/ablation resistance and high temperature mechanical properties.

Other contributors include Cuiyan Li, Ruinan Gao, Tianzhan Shen, Zihao Chen, and Yanlei Li from the School of Materials Science and Engineering, Shaanxi University of Science and Technology, China.

This work was supported by the National Natural Science Foundation of China (No. 52173299 and 52372087) and the Natural Science Foundation of Shaanxi Province (No. 2021JZ-51).

About the Author

Haibo Ouyang is a professor in the School of Materials Science and Engineering, Shaanxi University of Science and Technology. He received his PhD in materials science from Northwestern Polytechnic University in 2009. He was a visiting scholar at the University of Delaware from 2018 to 2019. His current research interests and areas are the design, fabrication, and understanding of structure-property relationships of ceramics/composites for ultrahigh temperature applications.

About Journal of Advanced Ceramics

Journal of Advanced Ceramics (JAC) is an international academic journal that presents cutting-edge results of theoretical and experimental studies on the processing, structure, and properties of advanced ceramics and ceramic-based composites. JAC is open access, published monthly by Tsinghua University Press and available exclusively through SciOpen. The JAC 2023 IF is 18.6, ranking in the Top 1 (1/31, Q1) among all journals in the category “Materials Science, Ceramics”, and its 2023 CiteScore is 21.0 (top 5%) in the Scopus database. ResearchGate homepage: https://www.researchgate.net/journal/Journal-of-Advanced-Ceramics-2227-8508

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