Propriedades termoelétricas de cerâmicas à base de CaMnO3 dopadas produzidas por rota química

Abstract

The thermoelectric materials have been very investigated due to its capacity to direct conversion of thermal energy into electrical energy, focusing on the search for materials that have high values of Figure of Merit (ZT), used in calculations of energy conversion efficiency. Among these oxides, the CaMnO3(CMO) is a strong candidate for thermoelectric applications because in addition to the mentioned stability, these ceramics can acquire conductive properties under suitable conditions of synthesis and processing, as well as by the addition of dopants, which consequently gives them superior thermoelectric properties. Thus, in this work, the influence of dopants and different sintering times on the thermoelectric properties was studied for the CMO ceramics obtained by a simplified chemical route, developed in this work. The powders of the pure (CMO) and doped compositions with 10mol% La3+ (CMO-La) and 4mol% V5+ were produced by modified Pechini method. The amounts of the adopted dopants were based on the literature for V5+ and on work developed in the master’s degree for doping with La3+. From these powders, the samples in the form of disks were uniaxially pressed, and after, the samples were sintered in air resistive electric furnace for the 1, 3, 6, 12 and 24 h. The sintering process were realized at 1270°C to CMO composition, at 1100°C for the CMO-La and 1400 °C to CMO-V, these temperatures being defined from the dilatometric analysis. The apparent densities, determined by Archimedes’ Principle reached values greater than 80% for the CMO, CMO-La and CMO-V compositions. The X-ray diffraction, scanning electron microscopy assisted by energy dispersive spectroscopy were conducted to realization of structural, microstructural, and semi-quantitative chemical analysis, respectively. From these characterizations, it was verified the formation of the CMO phase for all the sintered compositions and the presence of a small amount of the secondary phase (CaMn2O4) was found in the CMO and CMO-V composition. Results of Seebeck coefficient, 4-probe DC electrical conductivity and the thermal conductivity recorded for samples between 25ºC and 600ºC, were used in the calculation of Figure of Merit (ZT) and Power Factor, characterizing the thermoelectric properties of the samples. The CMO, sintered for 12h (CMO (12h)) presented the highest Seebeck coefficient, which is -350μV/K. The CMO-La sample (3h) presented the highest electrical conductivity, reaching values of 14000S/m, at 400°C, while the CMO-La (1,3 e 6h) registered the lowest thermal conductivity reaching a maximum value of 4.7 W/mK. The best values of thermoelectric properties were achieved for CMO-La (3h), presenting a result of a power factor of 4.1x10-4 W/mK² and CMO-V (1h) 3.25x10-4 W/mK²). From these results, the highlighted composition for thermoelectric application, CMO-La (3h) was subjected to sintering in oxidizing and reducing atmospheres. The sintered sample in a reducing atmosphere showed promising results, presenting the highest value of Figure of Merit. Thus, the CMO-La sample sintered for 3h in an H2 atmosphere at 600 °C reached ZT values of 0.12.