Analysis of Phase Stability and Defect Mobility in Functional Oxides Exposed to Extreme Conditions

Abstract

With materials being pushed to further and further extremes in the name of improved performance, understanding failure mechanisms through phase stability and defect mobility in materials subjected to extreme conditions is more important than ever before. While high temperatures may be the most synonymous extreme condition, there are a number of other external stimuli that could be considered extreme, including strain, electric field, and chemical environment. For each condition, there are many unique failure mechanisms that can occur, such as phase segregation, corrosion, and degradation of functional properties. These failure mechanisms can be understood through the mobility of defects such as oxygen vacancies, or the relative stability of each phase. Metal oxide materials often exhibit exciting and unique functional properties, such as magnetism and ferroelectricity. The primary aim of this work is to understand the stability of these materials under numerous extreme conditions and to relate this stability to the underlying failure mechanism. The first material, a high entropy mixed oxide, Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O, is predicted to be highly stable, due to the configurational entropy in the system, that is believed to dominate the Gibbs’ free energy at high temperatures. The stability is investigated in bulk and thin film composite systems, where tunable optical properties would allow for the production of reflecting filters. The stability is lower than expected, with bulk failure occurring due to phase segregation of the oxide components that possess an unfavorable enthalpy of formation, CuO and ZnO, following annealing above 1300 ºC. By 1700 ºC, this results in the formation of a nearly pure rocksalt oxide, retaining < 5% Cu cation concentration and negligible Zn concentration, this rocksalt phase is evidently highly stable, but lacks the functional properties of the high entropy oxide. Failure of the thin film optical system occurs through issues with miscibility and dewetting at even lower temperatures. The failure of the Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O thin film systems motivates the investigation of a different oxide system. This system, consisting of alternating thin films of BaZr0.5Hf0.5O3 and MgO, is shown to possess suitable optical properties, and to retain these properties following annealing at 1100 ºC in air. The failure of this system occurs through the loss of Ba ions from the BaZr0.5Hf0.5O3 layer, resulting in the formation of an (Hf, Zr)O2 phase, occurring preferentially around strain-induced defect structures formed during the deposition. Nonetheless, this system demonstrated a 10% improvement in thermophotovoltaic spectral control (relative to the prior state-of-the-art value) at an extreme temperature in an oxidizing environment. The observed formation of an (Hf, Zr)O2 phase under these extreme conditions illustrates the high stability of this system. As a ferroelectric material of interest to the semiconductor industry, the stability of Hf0.5Zr0.5O2 under extreme electric fields was investigated, with failure occurring through the formation of a conductive filament. The filament is able to form due to the lowering of the activation barrier for charged particle motion under an electric field. For oxygen vacancies in Hf0.5Zr0.5O2, this work reports a lowering of the barrier by ∼ 0.8 eV, compared to the thermal diffusion barrier, under a field of 3 MV cm-1. The outcome of this work is an improved understanding of the failure of highly stable functional oxide materials subjected to extreme conditions. This understanding should be utilized to further the development of novel highly stable oxide materials.