Phase stability, sintering, and electrical properties of the (Mg,Ni,Co,Cu,Zn)O entropy stabilized oxide system

Abstract

This thesis focuses on the synthesis and characterization of the recently discovered (Mg,Ni,Co,Cu,Zn)O entropy stabilized oxide. In this system, oxygen populates the anion sublattice of the FCC rock-salt structure while the five cations randomly populate the cation sublattice. Configurational entropy drives the formation of a single-phase solid solution, overcoming the enthalpy of mixing of the five components. The temperature required for this transition is minimized for the (Mg0.2Ni0.2Co¬0.2Cu0.2Zn0.2)O composition because configurational entropy is maximized for equimolar amounts of each cation. Various techniques are employed to characterize the formation of the single phase and its stability under different thermal treatments. Upon heating a single-phase sample at temperatures between 600 and 875 °C, a copper-rich tenorite precipitate forms, providing another method for controlling the microstructure. The reversibility of the formation of the single-phase solution is paramount to the entropy stabilization. Octahedrally coordinated Cu2+ ions exhibit a Jahn-Teller distortion and have been shown to cluster within the rock-salt lattice to relieve the stress induced by these distorted bonds. The global structure of this entropy-stabilized single phase is rock-salt; under certain processing conditions, selective peak broadening appears which suggests the presence of lattice distortion. This is hypothesized to be the result of copper-rich nanoscale defects where the cations are displaced from ideal lattice sites, due to the tetragonal distortion around these ions. Samples were sintered to >95% density using mixed oxide processing and solid-state sintering, which is an improvement on previous attempts to densify this material by similar methods. Dilatometry revealed two distinct temperature regimes where densification occurs at different rates, indicating the presence of different active sintering mechanisms. DC and AC measurements were employed to characterize the electrical properties of dense samples. The single-phase entropy stabilized oxide is insulating at room temperature with resistivity values on the order of 109 Ω m and possesses several thermally activated conduction mechanisms. When heat treated at lower temperatures to precipitate tenorite, the electrical resistivity drops by six orders of magnitude.