Thermal transport properties of ultra-high-temperature ceramic superlattices

The superlattice (SL) structure, which can efficiently suppress phonon thermal transport, has important implications for materials design in thermal insulating and thermoelectric applications. In this work, we prepare periodic ultra-high-temperature ceramic SLs made of transition metal carbides HfC and TaC with SL interface spacing ranging from 9.5 to 84.5 nm. The measured cross-plane phonon thermal conductivity displays a crossover dependence on SL interface spacing, achieving a minimum value of 0.84 W m⁻¹ K⁻¹ at room temperature. Moreover, the SLs with small interface spacing (9.5 and 14.7 nm) even show higher thermal conductivity than the constituent materials. The interfacial thermal resistance for HfC/TaC interface is determined from both a prepared single-interface double-layer sample and the thermal conductivity data of SLs, which is found to be a considerably large value for ceramic material interfaces. We further resolve the electron and phonon components of the interfacial thermal resistance. Finally, thermal stability of SLs is evaluated at 1200 °C in air, and a thin HfO₂ cap layer is shown to effectively protect against high-temperature oxidation and preserve the thermal insulating property to a good extent.