Unusual glass-like thermal conductivity trend in solid-state sintered polycrystalline CaO-doped zirconia ceramics

The general trend of thermal conductivity for crystalline solids shows a monotonic decrease with increasing temperature, while glasses typically exhibit an increase in thermal conductivity due to dominant radiative heat transfer and changes in atomic vibrations. For the present work, CaO-stabilized zirconia (CaSZ) polycrystalline ceramics showed a stark deviation from their expected thermal conductivity behavior at elevated temperatures. This anomalous deviation in thermal conductivity is different from the monotonically decreasing trend shown by the Y2O3-stablized zirconia (YSZ) ceramics and is similar (in shape) to the behavior of glasses in terms of both specific heat and thermal conductivity. Our investigation reveals that, at intermediate temperatures (300–600 °C) an interplay of mechanisms: (i) local structural disorder (higher compared to YSZ), (ii) increasing specific heat at elevated temperatures, and (iii) moderate translucency—synergistically suppresses the phonon transport and produces a plateau-like thermal conductivity. At elevated temperatures (beyond 600 °C), due to the translucency of CaSZ ceramics in both the visible and mid-infrared range, it allows radiative transport to become significant, leading to an overall increase in the thermal conductivity. This interplay between lattice (phonon) and radiative (photon) parts of thermal transport is enabled by the unique defect chemistry and translucency of the CaSZ ceramics.