Phase stability studies and evaluation of thermal & mechanical properties of (MgNiCoCuZn)O entropy stabilized transition metal oxides

Entropy stabilized oxides (ESOs) have received considerable attention in recent years due to their unique functional properties. Entropy stabilization of a multi-component system relies on the possibility of altering the phase stability of solid solutions through precise control of configurational entropy. A multi-component ESO system consisting of MgO, NiO, CoO, CuO and ZnO is synthesized to form a nanocrystalline single-phase rocksalt structure using a lowtemperature solution combustion synthesis (SCS) route. However, the phase transformation and stability studies under different heat treatment conditions for these compositions have not been investigated or rather limited. We have performed a recent study to understand the critical role of local stress fields around the cations on the stabilization of a single phase. The current work is therefore aimed at gaining insights on the interplay between heat treatment, phase segregation and its impact on the thermal and mechanical properties of (MgNiCoCuZn)O ESO. The phase transformation and stability studies of the ESO system using systematic thermal treatments coupled with extensive structural characterization techniques like XRD, EPR and Raman spectroscopy revealed the role of Cu2+ ions in the stabilization of a single phase rocksalt structure. Quenching the sample from high temperatures results in the formation of a single phase whereas furnace cooling results in the segregation of CuO. The material exemplified a very low thermal conductivity of ~ 2.7 Wm-1K -1 which is similar to that of zirconia. A drastic reduction in the thermal conductivity was observed in comparison to its CuO-free 4-component ESO counterpart resulting from the strong sublattice distortion induced by Cu2+ ions. Lowering of lattice distortion due to the segregation of CuO contributes to the reduction in the coefficient of thermal expansion value from 21 to 14 × 10-6 K-1. The elastic modulus values measured are in accordance with the general trend of rocksalt type divalent binary oxides and a reasonable increase in the elastic modulus value from 132 to 183 GPa was observed in the slow cooled samples. The whole study helped in resolving the knowledge gaps on the phase stability and the influence of CuO segregation on the thermal and mechanical properties of the ESO system.