High-entropy ceramics: Review of principles, production and applications

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Abstract

High-entropy ceramics with five or more cations have recently attracted significant attention due to their superior properties for various structural and functional applications. Although the multi-component ceramics have been of interest for several decades, the concept of high-entropy ceramics was defined in 2004 by producing the first high-entropy nitride films. Following the introduction of the entropy stabilization concept, significant efforts were started to increase the entropy, minimize the Gibbs free energy and achieve stable single-phase high-entropy ceramics. High-entropy oxides, nitrides, carbides, borides and hydrides are currently the most popular high-entropy ceramics due to their potential for various applications, while the study of other ceramics, such as silicides, sulfides, fluorides, phosphides, phosphates, oxynitrides, carbonitrides and borocarbonitrides, is also growing fast. In this paper, the progress regarding high-entropy ceramics is reviewed from both experimental and theoretical points of view. Different aspects including the history, principles, compositions, crystal structure, theoretical/empirical design (via density functional theory, molecular dynamics simulation, machine learning, CALPHAD and descriptors), production methods and properties are thoroughly reviewed. The paper specifically attempts to answer how these materials with remarkable structures and properties can be used in future applications.

Introduction

Entropy is a thermodynamic parameter which represents the degree of anarchy or disorder in a material. Entropy is influenced by different configurations, such as magnetic moment, atomic vibration and atomic arrangement, while the latter one is usually the most effective configuration regarding entropy changes. Entropy has not been considered as effective as enthalpy for material design until recent years, when the concept of high-entropy alloys (HEAs) was introduced [1,2]. In 2004 and following some earlier ground studies, the concept of HEAs was introduced by the parallel studies of Yeh et al. [1] and Cantor et al. [2], who introduced a novel family of multi-component alloys. The HEAs are usually defined as multi-principal element alloys (MPEAs) with a high configuration entropy which are formed from five or more elements with equal or close to equal atomic fractions [1]. These alloys exhibit a unique combination of composition, microstructure and properties [[3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. Similar to HEAs, high-entropy ceramics are defined as the solid solution of five or more cations or anions sublattices with a high configuration entropy [31]. The high-entropy ceramics now include a wide range of materials including high-entropy oxides (HEOs), nitrides (HENs), carbides (HECs), borides (HEBs), hydrides (HEHs), silicides (HESis), sulfides (HESs), fluorides (HEFs) phosphides (HEPs), phosphates (HEPO4s), oxynitrides (HEONs), carbonitrides (HECNs) and borocarbonitrides (HEBCNs).

In this paper, the development in the new field of high-entropy ceramics is reviewed with a special attention on principles, historical and publication trends, crystal structure, theoretical/empirical design, production methods, properties and potential applications. Moreover, a summary of publications in this field are given in the Appendix.

Section snippets

Principles

Stability of an elemental mixture in the form of a solid solution is related to the Gibbs free energy changes (ΔGmix).ΔGmix=ΔHmix-TΔSmixwhere ΔHmix is the enthalpy of the mixture, T is the absolute temperature and ΔSmix is the entropy of mixing. When the mixing entropy increases, the Gibbs free energy decreases and the solid solution becomes more stable [32,33]. The entropy is influenced by the temperature, number of elements and atomic fraction of each element in the composition. The

Historical and publication trends

Fig. 3 is an image showing that there has been a continuous trend to increase the number of elements in engineering materials since the start of civilization: from bronze to cast iron, aluminum alloys, magnesium alloys, amorphous alloys and currently to HEAs [42,43]. There have been some scientific movements in this direction within centuries. The German scientist, Karl Franz Achard, is one of the first people who tried to fabricate alloys with up to seven principal elements. His work, which

Crystal structures

The crystal structures of high-entropy ceramics have a significant effect on their properties. In this section, the crystal structure of HEOs is first discussed due to the significant information reported about these ceramics. The crystal structures of other ceramics are explained altogether in the second subsection. The main crystal structures of the high-entropy ceramics are summarized in Table 1.

Theoretical/empirical design

High-entropy materials including both alloys and non-metallic ceramics are interesting due to their enhanced properties resulting from their unique crystal structures. Although various high-entropy materials have been introduced so far, there are rather limited publications about the theoretical simulation and computation of the high-entropy materials due to their complex chemistry [85]. For the prediction of the structure of a high-entropy material, empirical models based on a descriptor are

Production methods

The synthesis methods play an important role in development of high-entropy ceramics. The selection of an appropriate synthesis route is based on several aspects including phase stability, elemental distribution, product shape, product size, desirable property and application. There are some other parameters which should also be considered in selecting a synthesis process, such as time, temperature and complexity of process. Although bulk samples and thin films are quite important for

Properties and potential applications

Ceramics, which mainly contain ionic or covalent bonds, have various applications as structural and functional materials. They have a higher hardness, higher melting temperature and high oxidation and corrosion resistance than metallic materials which result in their application as coating or high-temperature materials. Unlike metallic materials, which are usually conductors, ceramics can have different electronic features as conductors, semi-conductors or insulators, leading to their

Concluding remarks and outlook

The concept of entropy stabilization by increasing the number of principal elements to five or more has been resulted in the introduction of various high-entropy alloys and ceramics. Despite the significant studies of high-entropy alloys, research activities regarding high-entropy ceramics have been enhanced only in recent years. There is now a wide range of high-entropy ceramics including oxides, nitrides, carbides, borides, hydrides, silicides, sulfides, fluorides phosphides, phosphates,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The author KE acknowledges the MEXT, Japan for Grants-in-Aid for Scientific Research on Innovative Areas (No. 19H05176 and No. 21H00150).

Saeid Akrami obtained a MSs degree in Chemical Engineering and he is currently a PhD candidate in Applied Chemistry and Life Sciences in Nagoya Institute of Technology, Japan. The topic of his PhD study is the development of highly-strained and high-entropy ceramics for CO2 conversion.

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    Saeid Akrami obtained a MSs degree in Chemical Engineering and he is currently a PhD candidate in Applied Chemistry and Life Sciences in Nagoya Institute of Technology, Japan. The topic of his PhD study is the development of highly-strained and high-entropy ceramics for CO2 conversion.

    Parisa Edalati is a PhD candidate in the Advanced Ceramics Research Center, Nagoya Institute of Technology, Japan. She currently works on the design and synthesis of advanced high-entropy ceramics for water splitting and battery applications. As a visiting researcher in Kyushu University, she worked and published a few papers on high-entropy alloys for biomedical application, high-entropy hydrides for hydrogen storage and high-entropy oxides/oxynitrides for photocatalysis.

    Masayoshi Fuji is currently a professor of Advanced Ceramics Research Center, Nagoya Institute of Technology, Tajimi, Japan and a visiting professor and joint researcher in Beijing University of Chemical Technology, the Chinese Academy of Sciences and Osaka University. He received his BSc, MSc and PhD degrees in the field of industrial chemistry from Tokyo Metropolitan University. He worked in Tokyo Metropolitan University and the University of Florida before joining Nagoya Institute of Technology in 2002. He is the editor-in-chief of the journal of Advanced Powder Technology. His research interests are surface/bulk chemistry of various kinds of multifunctional ceramics and their processing. Among his research activities, the development of hollow particle ceramics and the introduction of non-firing solidification technique for oxides have received high scientific and industrial attention and led to several awards including an award from the minister of education, culture, sports, science and technology of Japan in 2013.

    Kaveh Edalati obtained a PhD degree in materials physics and chemistry from Kyushu University, Fukuoka, Japan, in 2010. He is currently an associate professor at the International Institute for Carbon-Neutral Energy Research, Kyushu University, Fukuoka, Japan. His current research interest is the development of functional energy materials for hydrogen-related applications such as hydrogen storage and photocatalytic hydrogen generation. He specifically employs severe plastic deformation through the high-pressure torsion process to develop new functional materials including high-entropyy alloys and ceramics. He is the author of over 150 journal papers and serves as the editor of several journals.

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