THERMAL PROPERTIES OF ZIRCONIA CO-DOPED WITH TRIVALENT AND PENTAVALENT OXIDES

摘要

Zirconia doped with 6-8 wt/100 (3.2-4.2 mol/100) 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 tantalumiobium (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 substi- tutional defects produce stable single-phase tetragonal materials with thermaI 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.