High-entropy ceramics: Synthesis, properties, and applications

High-entropy ceramics (HECs) embrace compositional complexity and configurational entropy to reinvent the science of traditional ceramic design. They display high thermal, mechanical, electrical, and chemical resilience and, to achieve this incredible feat, incorporate four or more elements in near-equiatomic ratios to stabilize single-phase solid solutions. By beginning with the conceptual development of high-entropy alloys, the review critically examines the role of configurational entropy in overcoming phase separation. A comprehensive discussion on the implications of ionic radius mismatch, lattice distortion, and electronegativity changes on microstructure and the tuning of functional properties is presented. Analysis of various synthesis processes and their influence on grain size, densification, phase composition, and the resulting material properties is also discussed. The intricate details of fine-grain engineering and the role of entropy in homogenizing microstructure and mechanical properties, such as improved hardness, fracture toughness, thermal stability, and oxidation resistance, have been discussed at length. The review considers grain boundary dynamics and configurational disorder to understand the chemical and electrical properties, highlighting the potential of HECs in energy storage and conversion devices. The exceptional potential of hydroxyapatite as an HEC, with improved bioactivity, controlled resorption rates, and mechanical stability for biomedical applications, is considered. HECs as multipurpose diffusion barriers and coatings with demonstrated resilience to wear, corrosion, and heat are also included in the review. It concludes with perspectives on the latest characterization methods, limitations in existing processing technologies, the role of machine learning, high-throughput synthesis, and additive manufacturing in paving the way for future breakthroughs.