Low thermal conductivity without oxygen vacancies in equimolar YO1.5 + TaO2.5- and YbO1.5 + TaO2.5-stabilized tetragonal zirconia ceramics

A narrow range of composition exists along both the ZrO2-YTaO4 and ZrO2-YbTaO4 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 YO1.5 + TaO2.5 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 Y3+ ions and the heavier Yb3+ and Ta5+ ions. The lowest thermal conductivity measured was similar to 1.4 W mK(-1) at 900 degrees C. The low thermal conductivity and phase stability makes these systems promising candidates for low conductivity applications, such as thermal barrier coatings. (C) 2010 Published by Elsevier Ltd. on behalf of Acta Materialia Inc.