From high-entropy ceramics to compositionally-complex ceramics: A case study of fluorite oxides

• Published in: Journal of the European Ceramic Society Vol. 40; no. 5; pp. 2120 - 2129
• Main Authors: Wright, Andrew J., Wang, Qingyang, Huang, Chuying, Nieto, Andy, Chen, Renkun, Luo, Jian
• Format: Journal Article
• Language: English
• Published: United Kingdom Elsevier Ltd 01-05-2020; Elsevier
• Subjects: Compositionally-Complex ceramics (CCCs); High-Entropy ceramics (HECs); Multi-Principal cation ceramics (MPCCs); Thermal barrier coatings; Thermal conductivity; Compositionally-Complex ceramics (CCCs); Thermal barrier coatings; Thermal conductivity; High-Entropy ceramics (HECs); Multi-Principal cation ceramics (MPCCs)
• ISSN: 0955-2219; 1873-619X
• DOI: 10.1016/j.jeurceramsoc.2020.01.015

•Nine compositionally-complex fluorite oxides (CCFOs) are made and investigated.•CCFOs exhibit reduced thermal conductivity and increased cubic phase stability.•Lower thermal conductivity is achieved in medium-entropy non-equimolar CCFOs.•High modulus and hardness retain in CCFOs with reduced thermal conductivity.•Non-equimolar CCFOs exhibit amorphous-like T-dependent thermal conductivity.
Using fluorite oxides as an example, this study broadens high-entropy ceramics (HECs) to compositionally-complex ceramics (CCCs) or multi-principal cation ceramics (MPCCs) to include medium-entropy and/or non-equimolar compositions. Nine compositions of compositionally-complex fluorite oxides (CCFOs) with the general formula of (Hf1/3Zr1/3Ce1/3)1-x(Y1/2X1/2)xO2-δ (X = Yb, Ca, and Gd; x = 0.4, 0.148, and 0.058) are fabricated. The phase stability, mechanical properties, and thermal conductivities are measured. Compared with yttria-stabilized zirconia, these CCFOs exhibit increased cubic phase stability and reduced thermal conductivity, while retaining high Young’s modulus (∼210 GPa) and nanohardness (∼18 GPa). Moreover, the temperature-dependent thermal conductivity in the non-equimolar CCFOs shows an amorphous-like behavior. In comparison with their equimolar high-entropy counterparts, the medium-entropy non-equimolar CCFOs exhibit even lower thermal conductivity (k) while maintaining high modulus (E), thereby achieving higher E/k ratios. These results suggest a new direction to achieve thermally-insulative yet stiff CCCs (MPCCs) via exploring non-equimolar and/or medium-entropy compositions.