Low oxygen transparency of Hf6Ta2O17 and homologous ZrxTa2O2x+5 series revealed by electrochemical impedance spectroscopy

Abstract

Hf₆Ta₂O₁₇ and Zr₆Ta₂O₁₇ are promising thermal barrier coating (TBC) candidates due to their high melting points and low oxygen diffusivity. Electrochemical impedance spectroscopy was used to study dielectric and ionic properties of these materials, with higher measurement temperatures required and no clear distinction between the bulk and grain boundaries. Activation energies for oxygen diffusion were determined from Arrhenius plots of the measured conductivities: 1.76–1.89 eV for the Zr_xTa₂O_2x+5 (x = 5, 6, 7, 8) homologous series and 2.26 eV for Hf₆Ta₂O₁₇. Frequency mismatch between the peak imaginary impedance and peak imaginary modulus points to a local relaxation process as the dominant mode of charge transfer rather than a long-range (vacancy migration) mode. A table of activation energies for various TBC ceramics and these materials are presented. Hf₆Ta₂O₁₇ and Zr₆Ta₂O₁₇ are verified to be effective oxygen barriers protecting underlying substrates from oxidative corrosion at high temperatures.