Variations in the thermal conductivity of La₂Zr₂O₇ and Gd₂Zr₂O₇ with variable La/Gd concentrations

Abstract

The phonon density of states and thermal conductivity of La₂Zr₂O₇ and Gd₂Zr₂O₇ are tuned to get materials with improved thermal insulation properties. In this study, pure and doped models of La₂Zr₂O₇ and Gd₂Zr₂O₇ were developed and simulated based on density functional theory. Doping Gd at La sites in La₂Zr₂O₇ was observed to modify the band structure. The density of states analysis showed that Gd d states were introduced in the band gap. With increasing Gd concentration, the band gap of La₂Zr₂O₇ was initially populated and then, a clear band gap was observed at high Gd/La ratios. Smoothness in the phonon density of states curves was observed with an increase in Gd concentration. However, irregular phonon density of states curves was obtained for the Gd₂Zr₂O₇ system. Among the simulated models, the La_16-5Gd₅Zr₁₆O₅₆ system with Gd/La ratio of 0.454 provided minimum thermal conductivity.

Introduction

Rare-earth zirconium oxides (RE₂Zr₂O₇; RE = rare earth elements) are widely investigated and found to be potential candidates for thermal barrier coatings applications [[1], [2], [3], [4], [5], [6]]. Such materials were reported to provide good thermal insulation in gas turbine engine due to its excellent properties such as physical stability, thermal compatibility, low thermal conductivity, and low migration energy [[7], [8], [9], [10], [11]].

The lanthanum zirconium oxide (La₂Zr₂O₇) possesses lower thermal conductivity and Young modulus values compared to yttria stabilized zirconia [7]. Looking to the structure of La₂Zr₂O₇, the Zr⁴⁺ and O²⁻ form ZrO₆ octahedra while the La³⁺ are present at the holes in the octahedral structure. Creation of point defects in this structure is possible by replacing La³⁺ by foreign ions.

Recent studies of Gd₂Zr₂O₇ based materials demonstrate their application potential for the ceramics top of thermal barrier coatings [4,[12], [13], [14]]. Defects alter the thermal conductivity of ceramic materials. Point defects enhance the phonon scattering and indirectly influence thermal conductivity [15,16]. By incorporating different divalent and trivalent substitutional dopants (at La and Zr sites) in La₂Zr₂O₇, it was reported that these dopants modified thermal conductivity of the final material [17]. Both the Gd₂Zr₂O₇ and La₂Zr₂O₇ are potential candidate materials for thermal barrier coatings. It would be interesting to introduce some substitutional dopants into this compound and investigate the thermal conductivity of the product.

In the present study, substitutional defects were created at La sites in the LZO structure. The doping concentration of Gd in LZO was started from 6.6% and increased sequentially till all the La ions in the LZO were replaced by Gd ions forming the Gd₂Zr₂O₇ structure. The developed models were optimized to get a stable configuration and finally, the properties of the final material were determined.