Phase Stability and Thermal Transport in Yttria-Stabilized Zirconia: A Challenge for First-Principles Theory
Final Report Abstract
Zirconia-based materials plays a prominent role in various high-temperature applications, among which, most prominently, in thermal barrier coatings (TBCs) for gas and aircraft turbines. Due to the low thermal conductivity of this material class, such zirconia-based coatings are able to protect the turbine’s components from the extreme temperatures generated during combustion. This improves the durability, but also enables an increase of the operation temperature and with that a significantly higher fuel efficiency. Over the last decades, advances in this field have been mainly driven by the optimization of TBCs, e.g., by the use of doped compounds such as yttria-stabilized zirconia (ZrO2 doped with ~8 mol-% YO1.5), which feature a lower thermal conductivity, higher toughness, and longer durability (i.e. long-term phase stability). Understanding the fundamental mechanisms that determine these properties in these materials and to exploit this knowledge to design improved TBCs is thus an import goal in this research field. In this project, we have used theoretical and computational techniques to shed light on the mechanisms that drive the thermodynamic equilibrium (phase stability and dynamics) and non-equilibrium properties (thermal conductivity) of this material class. To ensure that we achieve an accurate description of the interactions in these compounds, we used a so called ab initio method (namely density-functional theory), which does not rely on any empirical parameters or idealized models, but only on fundamental physical laws, constants, and well-defined approximations. To validate the chosen approach, we have carefully benchmarked it against computationally much more involved, higher-order techniques first. To enable the envisioned studies, also extensive code development was performed to ensure that the required forces acting on the lattice degrees of freedom (i.e. the axes of the crystal structure) are computed as accurately and as efficiently as possible. In turn, these efforts revealed that the mechanism that drives the high-temperature, tetragonal-cubic phase transition in zirconia involves a complex and novel dynamics (ferroelastic switching), in which the atomic and lattice degrees of freedom spontaneously reorient themselves along a different Cartesian direction. This finding, which differs significantly from the models discussed in literature so far, provided valuable insights for the design of TBCs: First, our calculations support the models that ascribe the durability of TBCs to a ferroelastic toughening mechanism that so far lacked an atomistic explanation. Second, we were able to show that the occurrence of such ferroelastic switches can be controlled and tailored by cation (co-)doping, which in turn allowed us to rationalize the increased durability and toughness of TBCs doped with yttrium and titanium. Third, further investigations regarding the thermal conductivity of these compounds revealed that the anomalously low thermal conductivity of this material class can also be traced back to the occurrence of ferroelastic switches, which disrupt heat transport by deflecting the flux perpendicularly to the transport direction. This enabled us to explain the curious, non-monotonic dependence of the thermal conductivity on the yttrium concentration. These findings allowed us to provide insights and guidelines that facilitate the design of improved TBCs.
Publications
- “Ferroelastic switching of doped zirconia: Modeling and understanding from first principles”, Physical Review B 90, 144109 (2014)
C. Carbogno, C. G. Levi, C. G. Van de Walle, and M. Scheffler
(See online at https://doi.org/10.1103/PhysRevB.90.144109) - “All-electron Formalism for Total Energy Strain Derivatives and Stress Tensor Components for Numeric Atom-Centered Orbitals”, Comp. Phys. Comm. 190, 33 (2015)
F. Knuth, C. Carbogno, V. Atalla, V. Blum, and M. Scheffler
(See online at https://doi.org/10.1016/j.cpc.2015.01.003)
