Reduced lattice thermal conductivity of perovskite-type high-entropy (Ca0.25Sr0.25Ba0.25RE0.25)TiO3 ceramics by phonon engineering for thermoelectric applications

High thermal insulation properties of A2FeCoO6−δ (A = Ca, Sr)

“…Similar dimensions have already been reported for thermal conductivity measurements of other materials that were also synthesized by solid state reactions. 11,13 The detailed thermal conductivity measurement method and conditions for our materials are described in the ESI. † Our materials, which are oxygen-deficient perovskites, have shown thermal conductivities lower than those of the reported fully occupied perovskite oxides.…”

“…Thus, different factors play a role in the reduction of thermal conductivity of perovskite oxides. Widely discussed factors are crystal structure variations, grain size, 20 grain boundaries 11,21 and oxygen vacancies, which are associated with the increased phonon scattering, resulting in reduced thermal conductivity of the lattice. Many of the previous studies on polycrystalline materials, such as perovskite oxides, discuss oxygen vacancies or defect generation that increase the phonon scattering, resulting in the thermal conductivity reduction.…”

“…1,22 Some reports clearly discussed the oxygen vacancy generation and its effectiveness in reducing thermal conductivity. 11,12 Some reports considered the reduction of mean free path of phonons as a responsible factor for the thermal conductivity reduction. 1,22,24 All these reports discussed that oxygen vacancies acted as strong scattering centers for phonons.…”

“…So, the grain boundary contribution to thermal resistance can be considered insignificant though it could still influence the thermal conductivity/ resistance. 1,11,21,34 Co 2+ , Co 3+ and Co 4+ spins may play a role in the thermal conductivity because these cations act as electron holes and electrons are, in most cases, considered the thermal energy carrier. However, this is not the only factor that controls thermal conductivity.…”

“…These materials were synthesized by solid state reaction at high temperature in the same way and under the same environments as other previously reported perovskite oxides. 11–13 Our materials were sintered at 1200 °C in air. The detailed synthesis conditions and techniques are described in the ESI †.…”