Cerâmicas termoelétricas de CaMnO3 dopadas com La3+ e V5+ produzidas por método químico e sinterizadas por micro-ondas

Abstract

Nowadays, the use of thermoelectric materials could be a promising solution to a global energy crisis, due to its ability to convert thermal energy into electrical energy, based on the Seebeck effect. Due to its intrinsic properties, calcium manganite (CaMnO3 or CMO) is an auspicious n-type semiconductor oxide for thermoelectric applications. Furthermore, the adoption of sintering by microwave irradiation favors shorter processing times, microstructures with reduced grain sizes, in some cases, properties equal to or even superior to those of conventionally sintered ceramics. Thus, in this study, the influences of a chemical synthesis method, doping with La3+ and V5+, and sintering time on the thermoelectric properties of CaMnO3 ceramics are reported. Ceramic powders of CaMnO3, Ca0.90La0.10MnO3, and CaMn0.96V0.04O3 stoichiometric compositions were produced by the modified Pechini method. According to X-ray diffraction analysis, it was confirmed that the CMO phase, which can be pure or doped, was predominant (>98%) for the calcined powders of all compositions. From the production of these powders, samples in discs form were pressed uniaxially, under a pressure of 175 MPa, and then sintered by microwave irradiation, in air, without a permanence time, at temperatures of 500 °C, 700 °C, 900 °C, and 1100 °C. Still, ceramics were sintered by microwave irradiation, in air, without a soaking time (0 min), for 15 min and for 30 min at a temperature of 1300 °C, 1200 °C, and 1300 °C, for ceramics of CMO, CMO-La, and CMO-V compositions, in the respective order, these temperatures were determined by dilatometric analyses. The apparent densities reached values greater than 65 % of the theoretical densities, for ceramics of all studied compositions. Characterization by X-ray diffraction confirmed the formation of the CMO crystalline phase as the only phase for ceramics of all stoichiometric compositions produced. Through scanning electron microscopy characterization, larger grains were found for CMO ceramics, intermediate grains for CMO-V, and smaller grains for CMO-La. In addition, the average grain size increased when the sintering time was raised. Seebeck coefficient values and thermal and electrical conductivities were measured between 25 °C and 600 °C. The thermoelectric properties most promising results are reported next. The sintered CMO sample without soaking time showed the highest values, in modulus, of the Seebeck coefficient. The CMO-La sample sintered for 15 min showed the highest electrical conductivity (~14600 S/m, at 600 °C), meanwhile, CMO samples sintered without soaking time and for 30 min (~250 S/m, at 600 °C). The sample of CMO-La sintered without soaking time showed the lowest values of thermal conductivity, on the other hand, this of CMO sintered for 15 min reached the highest values. The highest 𝑍𝑇 value, ~0.098, was obtained for the CMO-La-15min sample, at 600 °C. This sample presented the highest 𝐸𝑇 (~0.94 %) either. Therefore, the CMO-La ceramic sintered by microwave irradiation for 15 min is the most prominent for thermoelectric applications.