Low thermal conductivity without oxygen vacancies in equimolar YO 1.5 + TaO 2.5- and YbO 1.5 + TaO 2.5-stabilized tetragonal zirconia ceramics

Abstract

A narrow range of composition exists along both the ZrO 2–YTaO 4 and ZrO 2–YbTaO 4 quasi-binaries over which the tetragonal zirconia phase can be retained on cooling. Unlike other stabilized zirconia materials which have low thermal conductivity as a result of phonon scattering by oxygen vacancies, these compositions do not contain oxygen vacancies and yet an equimolar YO 1.5 + TaO 2.5 composition has been reported to also exhibit low thermal conductivity [1]. We find that zirconia compositions along the quasi-binaries have low and temperature-independent thermal conductivities, and that the thermal conductivities and their temperature dependence are consistent with a defect scattering model that takes into account a minimum phonon mean free path due to the inter-atomic spacing. Furthermore, the conductivities of the Yb and Y trivalent-doped compositions scale in a predictable manner with atomic site disorder effects on the cation sub-lattice associated with the lighter Y 3+ ions and the heavier Yb 3+ and Ta 5+ ions. The lowest thermal conductivity measured was ∼1.4 W mK –1 at 900 °C. The low thermal conductivity and phase stability makes these systems promising candidates for low conductivity applications, such as thermal barrier coatings.