Design models, synthesis methods, atomic scale structure characterization, properties and applications of high entropy ceramics are reviewed.

A lead‐free high‐Curie‐point Aurivillius‐phase ferroelectric ceramic, CaBi₂Nb₂O₉, has been fabricated by spark plasma sintering (SPS). It has a Curie point (943 °C) that is much higher than previously reported (650 °C). The piezoelectric constant d₃₃ of the SPS textured material was nearly three times that of ordinary‐fired (OF) materials and was stable up to 800 °C (see Figure). These textured ceramics are promising candidates for high‐ temperature piezoelectric applications.

High-entropy compounds with compositional complexity can be designed as new thermoelectric materials. Here a data-driven model was developed, which chose suitable elements to reduce the enthalpy of formation and hence to increase the chance of single phase formation. Using this model, two high-entropy sulfides were designed, metallic Cu₅SnMgGeZnS₉ and semiconducting Cu₃SnMgInZnS₇. They were then successfully fabricated as single-phase dense ceramics with homogeneously distributed cations, and their phase stability and atomic local structures were investigated using density functional theory calculations. Finally, a zT value of 0.58 at 773 K was obtained for Cu₅Sn_1.2MgGeZnS₉, where additional Sn was used to tune the carrier concentration. This work provides a simple approach to find new high-entropy functional materials in the largely unexplored multielement chemical space.

The thermal depoling behavior of several different Aurivillius phase ferroelectric ceramics has been studied. This includes two-layer $(\mathrm{Ca}{\mathrm{Bi}}_2{\mathrm{Nb}}_2{\mathrm{O}}_9,{\mathrm{Ca}}_{0.9}{\mathrm{Ba}}_{0.1}{\mathrm{Bi}}_2{\mathrm{Nb}}_2{\mathrm{O}}_9,{\mathrm{Bi}}_3\mathrm{Nb}\mathrm{Ti}{\mathrm{O}}_9,{\mathrm{Bi}}_3{\mathrm{Nb}}_{1.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_9)$ , three-layer $\left({\mathrm{Bi}}_4{\mathrm{Ti}}_3{\mathrm{O}}_{12}\right)$ , and four-layer $[\mathrm{Ca}{\mathrm{Bi}}_4{\mathrm{Ti}}_4{\mathrm{O}}_{15},{\mathrm{Ca}}_{0.94}{(\mathrm{Na},\mathrm{Ce})}_{0.03}{\mathrm{Bi}}_4{\mathrm{Ti}}_4{\mathrm{O}}_{15}]$ compounds. All of them have a high Curie point $(T_c⩾675°\mathrm{C})$ . The orthorhombic structured materials show good resistance to thermal depoling up to temperatures close to their Curie points. However, ${\mathrm{Bi}}_4{\mathrm{Ti}}_3{\mathrm{O}}_{12}$ , which has a monoclinic structure, shows a significant reduction in $d_{33}$ well before its Curie point. The monoclinic distortion produces more non-180° ferroelectric domain structures, and it is the thermal instability of these that accounts for their thermal depoling behaviour. Excess Nb doping of ${\mathrm{Bi}}_3\mathrm{Nb}\mathrm{Ti}{\mathrm{O}}_9$ produces a significant reduction in its resistance to thermal depoling, suggesting that the doping produces a lowering of the crystallographic symmetry.

This paper reports the electrochemical, optical and thermal effects occurring during flash sintering of 8 mol % yttria-stabilized zirconia (8YSZ). In-situ observations of polycrystalline and single crystal specimens revealed electrochemical blackening/darkening during an incubation period prior to flash sintering. The phenomenon is induced by cathodic partial reduction under DC fields. When using a low frequency AC field (0.1–10 Hz) the blackening is reversible, following the imposed polarity switching. Thermal imaging combined with sample colour changes and electrical conductivity mapping give a complete picture of the multi-physical phenomena occurring during each stage of the flash sintering event. The partial reduction at the cathode causes a modification of the electrical properties in the sample and the blackened regions, which are close to the cathode, are more conductive than the remainder of the sample. The asymmetrical nature of the electrochemical reactions follows the field polarity and causes an asymmetry in the temperature between the anode and cathode, with the positive electrode tending to overheat. It is also observed that the phenomena are influenced by the quality of the electrical contacts and by the atmosphere used.