No abstract

Oxide materials based on calcium manganite show clear prospects as thermoelectrics, provided by their stability at high temperatures and inherent flexibility in tuning the relevant electrical and thermal transport properties. Donor-doped CaMnO 3 is an n-type semiconductor with a perovskite structure and relatively high thermoelectric performance. In this work, the precursor powders have been modified through potassium carbonate additions to produce Ca 0.9 Gd 0.1-MnO 3 pellets without the usual delamination problems occurring during the compaction process. In order to demonstrate the relevant effects, several samples with different amounts of potassium carbonate (0-15 wt%) have been prepared. The results showed that potassium additions significantly facilitate the compaction procedure, while also improving the thermoelectric performances. The results also highlight the importance of porosity control for improving ZT, by decreasing the thermal conductivity without reduction of the electrical performance. The highest ZT values were observed for the samples processed at 15 wt% of potassium carbonate addition, exhibiting an improvement at least 30% at 800°C when compared to the pure samples.

Materials with strong magneto‐volume coupling can present a coupled first‐order magnetic and structural transition and a giant magnetocaloric effect. The trade‐off of the resulting increased entropy change is hysteresis, and incomplete transition when the applied field is <2 T, implying that the full cooling capacity of the refrigerant is not harnessed. In this work, the behavior of the magnetic entropy change as a function of temperature is simulated, considering the Bean–Rodbell model, for various spin systems with Curie temperatures close to room temperature. Increasing magneto‐volume coupling parameter (η) results in higher entropy change peak value (ΔSMmax), together with a narrowing of entropy change function ΔSM(T), limiting the working temperature range. Accordingly, cooling capacity behaves non‐monotonously, reaching a peak value at ηMAX, which departs from the tri‐critical point as H increases up to 2 T. By tuning η, the cooling capacity can be considerably larger than the uncoupled system (up to 15–40% for low applied fields). Such tuning can be accomplished experimentally through proper chemical substitution. A universal behavior of cooling capacity as a function of ηMAX underlines that the performance of magnetic refrigerant is optimized, by tuning magneto‐volume coupling within the second order range, rather than the tri‐critical region.

No abstract

No abstract