Thermophysical properties of Yb³⁺-doped nonstoichiometric gadolinium zirconate ceramics

Low thermal conductivities and high thermal expansion coefficients are desirable for rare-earth zirconate thermal barrier coating materials. Introduction of cation doping and adjustment of the nonstoichiometry are efficient methods for enhancing the thermophysical characteristics of materials. Herein, the thermophysical properties of Yb³⁺-doped nonstoichiometric gadolinium zirconate ceramic materials were investigated using first-principles calculations and solid-state reaction techniques. As the Yb³⁺ content increases, additional cations enter the interstitial spaces (for excess Gd³⁺ gadolinium zirconate) and form vacancy defects (for excess Zr⁴⁺ gadolinium zirconate), which serve as phonon scattering centres to decrease the thermal conductivity. When Yb³⁺ is doped at the same concentration, gadolinium zirconate with excess Zr⁴⁺ exhibits a lower thermal conductivity. Specifically, Gd_1.875Yb_0.25Zr_2.125O₇ shows the lowest minimum thermal conductivity of 1.133 W/(m⸱K) (according to the Clark model) and 1.241 W/(m⸱K) (according to the Cahill model). In addition, the experimental results also suggest that the optimal content of Yb³⁺ is 0.25 excess Zr⁴⁺ gadolinium zirconate, which has the lowest intrinsic thermal conductivity of 1.037 W/(m⸱K) at 1300 K. Moreover, the thermal expansion coefficient of the Yb³⁺-doped excess Gd³⁺ nonstoichiometric material is greater than that of the doped excess Zr⁴⁺ material, this is due to the greater electronegativity difference between Yb³⁺ and Zr⁴⁺ than that between Yb³⁺ and Gd³⁺ and the lower binding energy of Gd-O than that of Zr-O. The experimental results and the calculated results are in good agreement. The aim of this work is to enhance the thermophysical characteristics of gadolinium zirconate ceramics for use as thermal barrier coating materials.