2024 IRG-1: Conductivity enhancement in sodium-doped multi-phase high entropy oxides
Our previous work revealed that the (Co,Cu,Mg,Ni,Zn)1-xNaxO system exhibits an electrically conductive layered grain boundary phase at high-temperature processing conditions (Figure A). This boundary phase is a P2 type NaxCoO2 layered structure that has not been reported for other promising high entropy oxides and allows for the intercalation of Na+ ions along the grain boundaries. To control this boundary phase, single-phase equimolar Na-HEOs were processed with varying grain sizes (fine and coarse grain size) and heat treated up to 12 hours at 600, 700, and 850°C.
After heat treatment at 600°C, we observed the formation of copper-rich tenorite in fine grain (FG) Na-HEO, accompanied by a decrease in lattice parameter and grain boundary conductivity (Figure B). Heat treatment at 700°C induced significant tenorite formation and a decrease in lattice parameter. Both FG and coarse grain (CG) Na-HEO exhibited substantial increases in grain boundary conductivity by several orders of magnitude and evidence of NaxCoO2. Heat treatments at 850°C revealed minimal tenorite formation until after 12 hours of heat treatment. FG Na-HEO showed a marked increase in grain boundary conductivity, coupled with a decrease in lattice parameter. Conversely, CG Na-HEO displayed the opposite trend in conductivity and lattice parameter.
X-ray absorption spectroscopy revealed similar behavior to undoped TM-HEO for copper, nickel, and zinc cations. Successful linear combination fitting of cobalt spectra showed that highly conductive samples had significant cobalt oxidation from the 2+ to the 3+ state (Figure C). These data also correlate well with samples that exhibit decreasing lattice parameter and increasing grain boundary conductivity. The high degree of cobalt oxidation is likely occurring in the bulk and from the formation of the boundary layer, which may facilitate enhanced sodium transport. From this work we’ve demonstrated the ability to control the grain boundary structure and enhance the conductivity of this compositionally complex ceramic by several orders of magnitude.
Students: Justin Cortez (Schoenung); Post-doc: Alexander Dupuy (Schoenung)
PI: Julie Schoenung, William Bowman; University of California, Irvine
Cortez J, Dupuy A, Schoenung J. “Conductivity enhancement in sodium-doped multi-phase high entropy oxides.” In preparation