Síntese e caracterização estrutural da cerâmica termoelétrica ca(1-x)bixmno3

Abstract

The present work proposes to analyze how the substitution of Bismuth (Bi) can influence the structure and thermal and electrical properties of Calcium Manganite (CaMnO3), which is among the main thermoelectric materials studied. This material is considered one of the most promising among the thermoelectric oxides due to its high Seebeck coefficient value, low thermal conductivity and considerable electrical conductivity, fundamental characteristics for an efficient thermoelectric material. The choice of Bi is due to the ionic radius greater than Ca and due to the valence of Bi being 3+, this causes a partial reduction from Mn4+ to Mn3+ increasing the concentration of the charge carriers. The Ca(1-x) BixMnO3 system samples were synthesized using the solid state reaction method considering x = 0, 2, 3 and 5%. They were calcined at 1000 °C for 20 hours in a muffle furnace and sintered at 1150 °C for 10 hours. Later the characterization techniques were applied: X-ray diffraction (XRD), which made it possible to identify its crystalline structure and determine the network parameters; Two-Point Method, which allowed verifying the electrical resistivity, and finally, the methods of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) that made it possible to investigate the thermal properties. In addition, unit cell volume and density values were calculated; where the values were compared with the JCPDS database (Joint Committee for Powder Diffraction Studies). The XRD results revealed that the materials crystallized in an orthorhombic structure of the spatial group Pnma. The network parameters, volume and theoretical density were consistent with the literature, and there was an increase in these parameters with the replacement of bismuth. A significant reduction in thermal conductivity was observed in the sample with 3% Bi compared to the other concentrations. The behavior of Electrical resistivity was not as expected, since in the literature there was a reduction in electrical resistivity while in the present work an increase was noted. Although the electrical resistivity behavior was not as expected in relation to the pure sample, authors who sintered the Ca (1-x) BixMnO3 system obtained similar values for a 3% sample.