Propriedades termoelétricas de cerâmicas de CaMnO3 produzidas por diferentes rotas de síntese em atmosferas oxidante e redutora

Abstract

In the pursuit of clean and sustainable energy, In the quest for clean and sustainable energy, thermoelectric materials have been extensively studied and improved, with calcium manganite, CaMnO3 (CMO), being one of the most relevant materials. In this context, a study was conducted on the production of CaMnO3 ceramics in different atmospheres to optimize their thermoelectric properties. CMO powders were synthesized using solid-state reaction (SSR) and the modified Pechini chemical method (QUI). Reducing and oxidizing atmospheres (air, O2, or 10% H2/N2) were used during the sample preparation stages, calcination, and sintering. The influence of the synthesis route was investigated by modifying the calcination and sintering times (3, 6, 12, and 24 hours), evaluating their effects on the formed phases as well as the properties of the produced ceramics. Differential thermal analysis conducted in different atmospheres identified 800 ºC as the crystallization temperature of the CMO phase, but it resulted in excessive undesirable secondary phases. Therefore, batches of powders were calcined at 1000 ºC for 3 or 24 hours in different atmospheres. Scanning electron microscopy (SEM/EDS) observations were carried out to evaluate the morphology, chemical microanalysis, and particle size distribution of the particles obtained in SSR and QUI syntheses. X-ray diffraction (XRD) analysis, coupled with Rietveld refinement, was used to quantify the formed phases. Measurements of the Seebeck coefficient and thermal and electrical conductivities as a function of temperature were performed to characterize the thermoelectric properties and calculate the Figure of Merit (zT) values for each studied condition. Little variation was observed between the phases and their respective amounts under O2 and air atmospheres; however, using the H2/N2 atmosphere during sintering resulted in the complete reduction of CMO, thus not forming the desired phase. Calcination at 1000 ºC (in air or O2) favored the formation of the CMO phase, especially for SSR powders, which presented mass fractions of the desired phase between 78% and 100%, while for QUI powders, the CMO phase fraction ranged between 60% and 70%. The powders calcined at 1000 ºC were relevant in forming the microstructure of the sintered ceramics, as the quantities of the CMO phase in the powders influenced the densification and grain growth processes, whereas using powders calcined at 800 ºC resulted in ceramics with the lowest relative densities (64.1%). Ceramics sintered in H2/N2 were found to be highly electrically resistive, whereas ceramics sintered in oxygen exhibited the highest electrical conductivity values (4117 S/m) and the highest zT values (0.08) among the produced samples. CMO ceramics obtained by the QUI route presented very low electrical conductivity compared to those produced by SSR, and therefore, achieved the lowest zT values, even though they exhibited the desirable low thermal conductivity values (1.42 W/mK) compared to those obtained for SSR CMO ceramics (4.02 W/mK).