UC Irvine

High entropy oxide (HEO) materials consist of five or more oxide components in equimolar amounts, which form a single-phase state after processing. The first HEO developed was a transition metal HEO (CoCuMgNiZn)O (referred to here as TM-HEO). These materials exhibit interesting functional properties and hold excellent promise in battery applications. However, the influence of processing on the mechanical and electrical behavior of TM-HEOs is not well studied, the understanding of which is critical for implementation into practical applications. Here we first investigate the role of microstructure on mechanical properties in TM-HEO. Vickers indentation and nanoindentation methods were used to explore the influence of grain size on hardness, elastic modulus, and fracture toughness of single-phase TM-HEO. Hardness values exhibit a Hall-Petch relationship at larger grain sizes and an inverse Hall-Petch relationship at nanocrystalline grain sizes. Variations in elastic modulus and fracture toughness are attributed to several failure mechanisms discussed. This grain-size dependent mechanical behavior emphasizes the necessity of microstructure control when designing TM-HEO materials for applications such as batteries.Concerns with the safety and sourcing of lithium-ion batteries have prompted significant research into sodium-based systems. TM-HEOs are well suited for sodium battery applications due to their ability to accommodate a substantial quantity of mobile charge carriers, while also demonstrating promising cycling stability, conductivity, and battery capacity retention. The second investigation focuses on the underexplored influence of sodium doping, processing, and microstructure on charge transport in bulk sintered (CoCuMgNiZn)1-xNaxO (Na-HEO). We find that the conductivity increases with increasing dopant amount, up to 1.4x10-5 S∙cm-1 at x=0.33. Much of this increase is attributed to the high grain boundary conductivity, which originates from a NaxCoO2 layered structure that forms in the grain boundaries. The relative contributions of the grain boundaries and the grain bulk to the charge transport are discussed, along with how processing conditions and composition can be used to engineer conductive grain boundaries in Na-HEO. The third study introduces additional electrical property tuning by attempting to control secondary phase formation, particularly NaxCoO2, through heat treatment. This phenomenon of secondary phase formation is well studied in the undoped TM-HEO but has not been investigated in conjunction with Na-HEO. We find that Na-HEO shows evidence of the layered grain boundary phase after heat treatment, but also exhibits similar TM-HEO copper rich tenorite. Notably, secondary phase volume fraction reaches up to 13%, with evidence of large variations in microstructure and cation electronic state based on the heat treatment temperature. Multi-phase samples also demonstrate the highest reported conductivity of 2.03 x 10-4 S∙cm-1 for any room temperature Na-HEO study. Together, these findings underscore the potential for property control in TM-HEO systems, both mechanical and functional. Further analysis of multi-phase Na-HEO grain boundaries and composition is recommended to confirm grain boundary charge transport contributions.