Controllable Phase Heterogeneity in High Entropy Oxides

High entropy oxides (HEOs) consist of five or more oxide components which form a single-phase solid-solution structure after processing. Despite their recent discovery, HEOs already show promise in a variety of applications, particularly in electronics and battery components. A unique characteristic of HEOs is their reversible entropy-driven phase transformation between the single-phase and multiphase states. The phase transformation manifests as the formation of secondary phases, whose volume fractions can be controlled through heat treatment. This feature presents an opportunity to produce oxide materials with highly controlled phase states, microstructures, and behavior.<br/><br/>Here, we explore the behavior and consequences of this phase transformation in (CoCuMgNiZn)O. First, we show that solid-state synthesis and sintering can be used to consolidate fully dense HEO ceramics with grain sizes spanning several orders of magnitude. After heat treatment, we observe that the phase transformation manifests as the formation of Cu-rich tenorite and Co-rich spinel secondary phases. We demonstrate that the phase heterogeneity can be controlled through heat treatment, while the as-consolidated grain size significantly influences the secondary phase evolution and morphology. Furthermore, we discuss our efforts to characterize the secondary phases using atom probe tomography (APT), X-ray diffraction (XRD), and electron microscopy. From these investigations, we find that the secondary phases exhibit several interesting behaviors, such as a complex nucleation sequence and a size induced morphological transition.<br/><br/>We then explore how the controllable phase heterogeneity can be leveraged to tailor the electrical and mechanical behavior. We observe that the electrical conductivity and dielectric constant increase significantly with increasing amount of secondary phase, due to the formation of a conductive percolative network. Conversely, the interplay between the various secondary phases and their morphological evolution results in complex changes to the mechanical behavior and failure mechanisms. Our results demonstrate that the entropic transformation is a powerful tool for engineering the microstructure and behavior of high entropy oxide ceramics.