A novel approach for the composition design of high-entropy fluorite oxides with low thermal conductivity

Current researches show that the standard deviation of the cationic radii, configuration entropy, or maintenance of Ce⁴⁺ have a certain impact on the formation of single-phase HEFOs, but the discovered rules are only applicable to partially synthesized material systems and have significant limitations. Furthermore, the range of elements used in the synthesized materials is relatively narrow, which restricts the potential to fully exploit the advantages of high-entropy materials and their vast compositional space.

“Inspired by the synthesized HEFOs and the stabilization mechanism of doped fully stabilized zirconia, we believe that the mean cation radius (r) and oxygen vacancy concentration (O_vac) should be important factors affecting the formation of HEFOs.” Cuiwei Li, the leader of the research team said, “We chose (Ce_0.2Zr_0.2Hf_0.2Sn_0.2Ti_0.2)O₂ as the main object, and CaO was used to introduce oxygen vacancies. The effects of the CaO content on the formability of single-phase (Ca_x(Ce_0.2Zr_0.2Hf_0.2Sn_0.2Ti_0.2)_1−x)O_2−δ (x = 0–0.3) were firstly investigated. Then, a grid search was performed near the two single-phase compositions (Ca_x(Ce_0.2Zr_0.2Hf_0.2Sn_0.2Ti_0.2)_1−x)O_2−δ (x = 0.2 and 0.22)) to systematically study the influence of the r  and O_vac on the formability of single-phase (Ca_xCe_y1Zr_y2Hf_zSn_zTi_z)O_2−δ. We found that an appropriate increase in r  and the introduction of O_vac contribute to the formability of HEFOs. The above research findings were further validated in three new HEFOs designed based on the valence combination strategy, which is composed of elements with different valences ranging from +3 to +6.”

It is well known that thermal energy is transported through phonons in electrical insulating materials, and phonon movement is mainly restricted by various phonon-scattering processes, including phonon-phonon scattering, grain boundary scattering, and defect scattering. And research has shown that the low thermal conductivity of HEFOs could be attributed to the large number of oxygen vacancies, as well as quality fluctuations and stress field fluctuations caused by chemical complexity. “In this paper, we considered the influence of phonon scattering on thermal conductivity from the aspects of mass disorder, size disorder, and O_vac. For samples with the same O_vac, a negative correlation exists between thermal conductivity and size disorder, as well as mass disorder. Aggregation of oxygen vacancies resulting from high O_vac levels can weaken the phonon scattering effect.” said Cuiwei Li.

“Based on the findings of our study, we propose a new design approach for HEFOs with low thermal conductivity. This approach involves two steps: first, determining the element combinations based on the valence combination and element properties; second, within the range of r  and O_vac capable of forming HEFOs, mass disorder and size disorder should be increased as much as possible, and O_vac should be adjusted to a level that prevents agglomeration. This approach emphasizes the careful consideration of the content of high-valence cations in the composition. This is because high-valence cations typically have relatively small radii, and their introduction can lead to a reduction in O_vac.” said Cuiwei Li.

This new design approach will contribute to the composition expansion and performance research of HEFOs, and also provide a reference for the composition design of other high-entropy oxides. The prepared HEFOs with low thermal conductivity are expected to be applied in the field of thermal protection.

The team published their work in Journal of Advanced Ceramics on September 29, 2024.

This work was supported by the Fundamental Research Funds for the Central Universities (Nos. 2023YJS062 and 2022JBZY025), the State Key Laboratory of New Ceramic and Fine Processing, Tsinghua University (No. KFZD201902), and the Beijing Government Funds for the Constructive Project of Central Universities.

About Author

Cuiwei Li, Professor, is currently working at the School of Mechanical and Electronic Control Engineering, Beijing Jiaotong University. Her research mainly focuses on the composition design and characterization of high-entropy oxides, the structural design and application of porous ceramics.

About Journal of Advanced Ceramics

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

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