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

Abstract

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, Nb 5+ and Td 5+ 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 Ta 5+ doping than Nb 5+ doping. Finally, Nb 5+/Ta 5+–Y 3+ 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.