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 behaviour at elevated temperatures. This anomalous deviation in thermal conductivity is different from the monotonic trend shown by the Y2O3-stablized zirconia (YSZ) ceramics and is similar in shape to the behaviour of glasses in terms of both specific heat and thermal conductivity. Our investigation reveals that, at intermediate temperatures (300 - 600 {degree sign}C) an interplay of mechanisms, viz., (i) local structural disorder (higher compared to YSZ); (ii) increasing specific heat at elevated temperatures; and (iii) moderate translucency - all of these factors synergistically suppresses the phonon transport and produces a plateau-like thermal conductivity without any amorphization. At elevated temperatures (beyond 600 {degree sign}C), due to the translucency of CaSZ ceramics in both 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 optical response of the CaSZ ceramics.