UC Irvine

Entropy-stabilized oxides (ESO) consist of five or more oxide components that form a random solid solution structure after processing. ESO materials have garnered significant attention due to their interesting functional properties and the promise of undiscovered properties that may lie within the expanded compositional space that these materials possess. Multiple entropy stabilized oxides have been discovered and are being investigated around the world. In this study, (CoCuMgNiZn)O is investigated. This ESO composition, which was discovered in 2015, is known to exhibit a reversible phase transformation between the single-phase entropy stabilized rocksalt phase to a multi-phase state consisting of the primary rocksalt phase, a copper-rich tenorite secondary phase, and a cobalt-rich spinel secondary phase. The phase transformation between the single-phase state and the multi-phase state, and the specific phase concentrations, has been previously demonstrated to be a function of heat treatment temperature and grain size. In this study, we consider an additional variable – starting composition – in an effort to further understand the phase transformation behavior, as well as to explore the electrical behavior of this material. Thus, the goal of this research was to investigate the influence of the secondary phases on the electrical behavior of ESO materials, by manipulating the copper content in the materials, and consequently the phase composition of ESO materials, to better understand the structure-property relationships and the mechanisms that control electrical behavior in these versatile materials. To achieve this goal, the concentration of copper oxide (CuO) was varied during synthesis, and the post sintering heat treatment temperatures and times were also varied, in an effort to control the formation of secondary phases in the ESO material. Fully dense, bulk ESO samples were synthesized using solid-state powder processing and elevated temperature sintering, prior to secondary heat treatment. The electrical conductivity in the bulk samples was evaluated as a function of heat treatment temperature (450-900°C), time (0-48 hours), and frequency (500 mHz to 1 MHz), and the phase state and microstructure were characterized using various microscopy, diffraction, and spectroscopy techniques. By comparing equimolar (20% CuO), Cu-deficient (10% CuO), and Cu-enriched (30% CuO) compositions, the conductivity mechanisms are elucidated.The results of this experimental investigation indicate that the concentration and distribution of the secondary phases can be controlled by varying starting composition, heat treatment temperature and heat treatment time. The Cu-rich tenorite secondary phase ranged in concentration from 0-21 vol% and was distributed both at the grain boundaries and within the grains. The Co-rich spinel secondary phase ranged in concentration from 0-10 vol% and was distributed within the grains. The total conductivity ranged from approximately 10-12 S/cm to 10-5 S/cm, and the dielectric constant ranged from approximately 10 to 104. The highest values for both properties were observed in the 30% CuO sample heat treated at 750°C for 12 hr, and the lowest values were observed in the single-phase samples. The results of the frequency response and activation energy analysis, in combination with the microstructural characterization, indicate that the electrical conductivity enhancement is due to the presence of the Cu-rich tenorite secondary phase, which creates a conductive percolation path, or grain boundary network, along which charge transport is enhanced. Additionally, the Cu-rich tenorite secondary phase creates a capacitive effect at the interface regions between the primary rocksalt phase and the Cu-rich tenorite secondary phase, thereby increasing the dielectric constant. Overall, the results of this study highlight the ability to tailor the microstructure and phase state of ESO materials, and thereby manipulate their electrical behavior.