Realizing Record-Low Thermal Conductivity and Diffusivity at High Temperatures Using Stable High-Entropy Spinel Oxide Nanoparticles

Achieving efficient thermal insulation at high temperatures is important for concentrating solar thermal (CST) and other thermal energy applications. Recent progress in high-entropy ceramics has offered an opportunity to engineer thermal conductivity while maintaining excellent stability at high temperatures. In this study, we report the realization of ultra-low thermal conductivity and diffusivity in ambient air using high-density nanoparticle (NP) assembles composed of a series of high-entropy spinel oxide (HESO) with a large number of cations (>5). Unlike conventional porous thermal insulation materials, the HESO-8 NP pellets maintain a relatively high packing density while suppressing all three major heat transfer pathways—solid conduction, gas conduction, and thermal radiation. Our measurements show that the thermal conductivity of these HESO-8 NP pellets exhibits a low value of around 0.1 W/m/K at high temperatures, which is comparable to the conductivity of air at this temperature. This low thermal conductivity is attributed to reduced solid conduction due to minimal contact between nanoparticles, reduced gas conduction via small interstitial spaces, and suppressed thermal radiation thanks to the IR-absorbing metallic spinel oxides. Furthermore, due to the relatively high packing density, the thermal diffusivity of the HESO NP pellets is much lower than that of aerogels, making it effective at delaying heat propagation under transient heat fluxes. In addition to their thermal insulation properties, the HESO NP pellets exhibit good stability at elevated temperatures in air. The high-entropy spinel structure provides excellent resistance to coarsening (sintering), ensuring that the particle size and thermal conductivity remain stable even after exposure to high temperatures. These results show the potential of high-entropy oxide nanostructures for future applications in thermal insulation and other high-temperature processes.