A narrow range of composition exists along both the ZrO2–YTaO4 and ZrO2–YbTaO4 quasi-binaries over which the tetragonal zirconia phase could be retained on cooling. Unlike other stabilized zirconia materials which had low thermal conductivity as a result of phonon scattering by oxygen vacancies, these compositions did not contain oxygen vacancies and yet an equimolar YO1.5 + TaO2.5 composition was reported to also exhibited low thermal conductivity. It was found that zirconia compositions along the quasi-binaries had low and temperature-independent thermal conductivities, and that the thermal conductivities and their temperature dependence were 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 about 1.4Wm/K at 900C. The low thermal conductivity and phase stability made these systems promising candidates for low conductivity applications, such as thermal barrier coatings.

Low Thermal Conductivity Without Oxygen Vacancies in Equimolar YO1.5 + TaO2.5- and YbO1.5 + TaO2.5-Stabilized Tetragonal Zirconia Ceramics. Y.Shen, R.M.Leckie, C.G.Levi, D.R.Clarke: Acta Materialia, 2010, 58[13], 4424-31