(327f) Control of Thermal Transport at Ultrahigh Temperatures By Immiscible Oxide Heterostructures

Degradation of materials under harsh, high-temperature conditions is one of the grand challenges of high-performance energy conversion. Current approaches make use of refractory materials that do not possess ideal properties but are resistant to decomposition by heat. To suppress undesired thermal transport, multiple refractory materials are shaped and arranged with varying degrees of complexity, ranging from multilayers to plasmonic arrays and 3D photonic crystals. Ultimately these structures progress toward more thermodynamically favorable configurations and result in intermixing, reaction or new phase formation, and coarsening. This severely limits their performance in extreme conditions. Here, we introduce the concept of using immiscible refractory oxides with high crystallinity as building blocks of functional ultrahigh-temperature materials. We demonstrate epitaxial oxide heterostructures made from perovskite BaZr_0.5Hf_0.5O₃ (BZHO) and rocksalt MgO as a photonic crystal (PhC) that can suppress undesired thermal emission at a desired cutoff wavelength. The PhC is implemented as surface filter to suppress mid-infrared thermal emission from the best intrinsic, spectrally selective emitter operating at 1400°C in air. The heterostructure exhibits coherent atomic registry that retains clearly separated refractive index mismatched layers without interface coarsening, interdiffusion, phase change, or decomposition up to at least 1100°C in dry air. The use of dissimilar crystal structures, with low lattice and thermal expansion mismatch, allows for high crystallinity superlattices with interlayer immiscibility at high temperatures. The understanding gained from this study can be used to develop thermal barrier coatings and selective emitters that are resistant to instabilities in air. Beyond BZHO/MgO, we computationally identify ~10³ potential oxide pairings that fit our design criteria, demonstrating the vast potential of this approach.