Tue, 27 April, 2021
Compositionally complex ceramics (CCCs) are a relatively new class of materials. The CCCs were proposed in 2020 by Wright et al. as an extension of the also relatively new material class the High entropy ceramics (HECs) [1, 2].
The HECs are defined as single-phase ceramics consisting of at least four different anions or cations . The composition of the different components should be equimolar in a single-phase solid . Ideally, this results in a unit cell with the same number of the different elements homogeneously distributed.
In 2015, the first high entropy ceramic with the rocksalt structure was produced from the oxides MgO, NiO, CoO, CuO, and ZnO [4, 5]. In 2016, the production of HECs with borides was reported . This was followed in 2018 by the production of HECs with carbides or nitrides . Meanwhile, the preparation of HECs with boride-carbides, carbonitrides, sulfides, and fluorides was also reported . Mostly, the HECs were produced by solid-state reaction. Alternatively, they could be produced by wet-chemical processes and epitaxial growth .
Properties of HECs include high dielectric constants, high lithium-ion conductivity, tunable magnetism, high hardness, and low thermal conductivity . The high hardness and low thermal conductivity appear to be general properties of HECs . Therefore there is a large number of potential applications for HECs. These include the use as catalysts, as battery materials, as thermoelectronics, as thermal barrier coatings, and as wear- and corrosion-resistant coatings .
The CCCs extend the range of the HEC-compounds to include medium entropy and non-equimolar compositions . This allows a more specific adjustment of the various properties, which can be achieved by entropy stabilization .
Wright et al. have prepared fluorite oxides CCCs consisting of the oxides of hafnium, zirconium, cerium, and yttrium, and one of the oxides of ytterbium, calcium, and gadolinium . They found that the medium entropy CCCs with non-equimolar composition can achieve even lower thermal conductivities than their highly entropic counterparts . The mechanical properties of the HECs can also be outperformed by the CCCs .
Another type of CCCs was presented by Hassan et al. in 2021 . They have replaced part of the Zn atoms in a ZnO lattice with five other elements . The five dopant elements were all incorporated together in an equimolar composition. The elements used were Ba, Sr, Mn, Fe, and Ni . Thin films were prepared with the compositions (BaxSrxMnxFexNix) Zn1-5xO with x=0, 0.01, 0.02 and 0.03 via spray pyrolysis . All the films exhibited a wurtzite crystal structure due to entropy stabilization. Hassan et al. have concluded that CCC doping can significantly affect properties such as structure, morphology, and optical properties .
Due to the wide field of possible compositions, CCCs are an promising field to develop new materials with excellent and precisely adjusted properties. In project Forge, CCC coating materials will be developed. The properties of the CCC coatings, such as corrosion resistance, coefficient of thermal expansion, etc., will be adjusted in such a way that the service life of refractories in ceramic kilns will be significantly increased.
 Wright, Andrew J.; Luo, Jian (2020): A step forward from high-entropy ceramics to compositionally complex ceramics: a new perspective. In J Mater Sci 55 (23), pp. 9812–9827. DOI: 10.1007/s10853-020-04583-w.
 Wright, Andrew J.; Wang, Qingyang; Huang, Chuying; Nieto, Andy; Chen, Renkun; Luo, Jian (2020): From high-entropy ceramics to compositionally-complex ceramics: A case study of fluorite oxides. In Journal of the European Ceramic Society 40 (5), pp. 2120–2129. DOI: 10.1016/j.jeurceramsoc.2020.01.015.
 Zhang, Rui-Zhi; Reece, Michael J. (2019): Review of high entropy ceramics: design, synthesis, structure and properties. In J. Mater. Chem. A 7 (39), pp. 22148–22162. DOI: 10.1039/c9ta05698j.
 Oses, Corey; Toher, Cormac; Curtarolo, Stefano (2020): High-entropy ceramics. In Nat Rev Mater 5 (4), pp. 295–309. DOI: 10.1038/s41578-019-0170-8.
 Rost, Christina M.; Sachet, Edward; Borman, Trent; Moballegh, Ali; Dickey, Elizabeth C.; Hou, Dong et al. (2015): Entropy-stabilized oxides. In Nature communications 6, p. 8485. DOI: 10.1038/ncomms9485.
 Wright, Andrew J.; Huang, Chuying; Walock, Michael J.; Ghoshal, Anindya; Murugan, Muthuvel; Luo, Jian (2021): Sand corrosion, thermal expansion, and ablation of medium‐ and high‐entropy compositionally complex fluorite oxides. In J. Am. Ceram. Soc. 104 (1), pp. 448–462. DOI: 10.1111/jace.17448.
 Mohammad Rahat Al Hassan, Aungkan Sen, Mohammad Khalid Hasan, Mohammad Abdul Matin, Structural (2021): Morphological and Optical Properties of Spray Deposited Multi-doped (Ba, Sr, Mn, Fe and Ni) Compositionally Complex ZnO Thin Films. American Journal of Nanosciences. Vol. 7, No. 1, 2021, pp. 6-14. DOI: 10.11648/j.ajn.20210701.12.