Multiscale phase homogeneity in bulk nanocrystalline high entropy oxides

Abstract

X-ray diffraction, energy dispersive X-ray spectroscopy, and atom probe tomography (APT) were used to assess the phase homogeneity of bulk sintered (Cu, Co, Mg, Ni, Zn)O transition metal high entropy oxides (TM-HEOs) across multiple length scales. These three methods were applied to both coarse-grain and nanocrystalline TM-HEO samples sintered using two different processing routes. Coarse-grain samples were prepared by reaction sintering of unreacted nanopowders using conventional sintering, while nanocrystalline samples were prepared from solid-state synthesized nanopowders consolidated using spark plasma sintering. Despite the different processing methods and grain sizes, the TM-HEO samples exhibit little-to-no chemical or phase segregation across all length scales. Critically, APT confirms that there is no chemical segregation at the nm scale even after solid-state synthesis and high temperature consolidation.