Thermal conductivity of ceramics in the ZrO 2 -GdO 1.5 system
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Paul G. Klemens Department of Physics, Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269-3136
Maurice Gell Department of Metallurgy and Materials Engineering, Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269-3136
Eugenio Garcı´a, Pilar Miranzo, and Maria I. Osendi Instituto de Cera´mica y Vidrio, CSIC, Cantoblanco, 28049 Madrid, Spain (Received 27 August 2002; accepted 25 September 2002)
Low thermal conductivity ceramics in the ZrO2–GdO1.5 system have potential in structural (refractories, thermal barrier coatings, thermal protection) and nuclear applications. To that end, the thermal conductivities of hot-pressed xGdO1.5 䡠 (1 − x)ZrO2 (where x ⳱ 0.05, 0.15, 0.31, 0.50, 0.62, 0.75, 0.89, and 1.00) solid solutions were measured, for the first time, as a function of temperature in the range 25 to 700 °C. On the ZrO2-rich side, the thermal conductivity first decreased rapidly with increasing concentration of GdO1.5 and then reached a plateau. On the GdO1.5-rich side, the decrease in the thermal conductivity with increasing concentration of ZrO2 was less pronounced. The thermal conductivity was less sensitive to the composition with increasing temperature. The thermal conductivity of pyrochlore Gd2Zr2O7 (x ⳱ 0.5) was higher than that of surrounding compositions at all temperatures. A semiempirical phonon-scattering theory was used to analyze the experimental thermal conductivity data. In the case of pure ZrO2 and GdO1.5, the dependence of the thermal conductivity to the absolute temperature (T) was less than 1/T. Therefore, the minimum thermal conductivity theory was applied, which better described the temperature dependence of the thermal conductivity of pure ZrO2 and GdO1.5. In the case of solid solutions, phonon scattering by cation mass fluctuations and additional scattering by oxygen vacancies on the ZrO2-rich side and by gadolinium vacancies on the GdO1.5-rich side seemed to account for the composition and temperature dependence of the thermal conductivity.
I. INTRODUCTION
There is a growing demand for high-temperature structural ceramics with low thermal conductivities for a variety of thermal-insulation applications. These applications include nuclear reactor components,1 furnace walls, crucible linings, thermal barrier coatings (TBCs),2 and thermal insulators. Rare-earth zirconates, which have one of the lowest thermal conductivities of all ceramics, are being explored for use in the above applications. Rare-earth zirconates, with a general formula M2Zr2O7 (M ⳱ rare earth), crystallize in the ordered pyrochlore
a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 17, No. 12, Dec 2002
structure over a narrow composition range.3,4 At elevated temperatures the disordered fluorite is the stable phase, as it is also outside that composition range.3,4 Vaßen et al.5 have measured the thermal conductivity of hot-pressed, monolithic La2Zr2O7 ceramics (approximately 97% dense, pyrochlore phase) to
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