Thermal conductivity of yttria-stabilized zirconia–hafnia solid solutions

The thermal conductivity of the yttria-stabilized zirconia–hafnia solid solution series as a function of temperature is reported. For all the compositions studied, the thermal conductivity of the fully dense materials is almost temperature independent up to 1000 °C, the limit of our measurements. A strong effect of solid solution alloying is reported with the mixed zirconia–hafnia compositions having lower thermal conductivity than either yttria-stabilized zirconia or yttria-stabilized hafnia end members of the solid solution series. The compositions with the lowest thermal conductivity are approximately equimolar zirconia–hafnia, and these have a thermal conductivity approximately 25% lower than that of the standard 8YSZ composition currently used in thermal barrier coatings. Although solid solutions generally have lower thermal conductivity at low temperatures, where the conductivity is temperature dependent, this is the first report of solid solution alloying lowering the value of the minimum thermal conductivity, the high-temperature limit of thermal conductivity. The effect is attributed to mass disorder on the cation sublattice of zirconia in which Zr4+ ions are replaced and mixed with Hf4+ ions, decreasing the phonon mean free path. This is in addition to the disorder in the anion sublattice created by the vacancies introduced by the Y3+ stabilizer.