Ref.: MceMge06-003
Apresentador: Pedro Miguel Neto Silveira
Autores (Instituição): Silveira, P.M.(Universidade Federal de Itajubá); Freire, L.A.(Universidade Federal de Itajubá); Rosa, J.P.(UNIFEI); Thomazini, D.(Universidade Federal de Itajubá); Gelfuso, M.V.(Universidade Federal de Itajubá);
Resumo:
In the current landscape, the perovskite CaMnO3 (CMO), an n-type semiconductor and thermoelectric material shows promise in addressing the energy crisis by converting residual heat into electricity through the Seebeck effect. The control of oxygen partial pressure in ceramics processing has proven pivotal in modifying electronic and thermal properties of this perovskite oxide, resulting in presence of the distinct phases, altering the final properties and thermoelectric efficiency. In this work, CMO was synthesized using the modified Pechini method (MPM). High purity (over 99 %) precursors CaCO3 and MnO were weighted in stoichiometric ratio. The former precursor was solubilized in citric acid [2.5 M], and the latter in nitric acid [6 M]. These solutions were then mixed with magnetic stirring at 343 K until a champagne coloration was achieved. The product of the route was dried at 343 K for 24 h, yielding a non-crystalline powder. Based in differential thermal analysis (DTA), the powder was firstly calcined for 0.5 h at 823 K and subsequently calcined, for 3 h, 6 h, 12 h or 24 h at 1073 K under oxygen (O2), air, or hydrogen (H2) atmospheres. DTA showed the decomposition of both acids between 403 K and 509 K, the formation of Ca2Mn3O8 between 553 K and 750 K, and the formation of the crystalline phase CMO and the secondary phases (Mn2O3, Mn3O4, Ca2MnO4 and CaMn3O6) around 1073 K. The mentioned secondary phases and the formation of Ca2Mn3O8 on the first calcination of the MPM powders were identified through X-ray diffraction (XRD). The structural analysis of the calcined powders and the quantification of the mass fraction of the formed phases were performed by Rietveld Refinement. The most favorable condition for the formation of the interest phase CMO was achieved in 3 h in H2, with 78.7% mass fraction, and the least favorable condition occurred in 3 h in O2, with 15.4%. The lattice parameters of the formed CMO were obtained for all produced powder samples. Evaluations in terms of calcination time and atmosphere revealed no standardized behavior, without any influence of the calcination time or atmosphere on the resulting orthorhombicity of CMO. On the other hand, CMO calcined with oxygen atmosphere showed higher crystallite size (45.56 nm) with the increasing in calcination time. The results of the scanning electron microscopy (SEM) show different particles morphology for the different phases found on the CMO powder calcined, confirming XRD results.