Thermal properties of zirconia co-doped with trivalent and pentavalent oxides

Zirconia doped with 6-8 wt% (3.2-4.2 mol%) yttria (6-8YSZ), the most common thermal barrier coating material, relies mostly on oxygen vacancies to provide the phonon scattering necessary for low thermal conductivity. The present study examines whether specific substitutional defects - in addition to, or instead of, oxygen vacancies - can provide similar or greater reductions in conductivity. To this end a series of zirconia samples co-doped with varying levels of yttrium (trivalent) and tantalum/niobium (pentavalent) oxides were synthesized, thereby allowing oxygen vacancy and substitutional atom concentration to be varied independently. The results show that Nb-Y and Ta-Y co-doped zirconia samples containing only substitutional defects produce stable single-phase tetragonal materials with thermal conductivities very close to that of the conventional 6-8YSZ. In these samples, Nb⁵⁺ and Td⁵⁺ are similarly effective in lowering thermal conductivity, in contradiction to phonon scattering theories that consider primarily mass effects and thereby predict significantly greater conductivity reduction due to Ta⁵⁺ doping than Nb⁵⁺ doping. Finally, Nb⁵⁺/Ta⁵⁺-Y³⁺ doped samples, which contain both oxygen vacancies and substitutional defects, are found not to be stable in single-phase form; however, the thermal conductivities of the two-phase tetragonal+cubic mixtures are again as low as that of the conventional 6-8YSZ.