Microstructural design of CaMnO3 and its thermoelectric proprieties.
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Calcium manganate (CMO) is an n-type semiconductor with promising thermoelectric proprieties. Solid state synthesis was employed to synthesise the desired material in two phases: i) reduced rock-salt phase of CaO-MnO (ss) and ii) oxidized phase of CaMnO3 with secondary phase of CaMn2O4 (marokite). In addition to stoichiometric CMO, three two-phase materials consisting of 2.5vol%, 5vol% and 10vol% of marokite were synthesised. The secondary phase was introduced to investigate its proprieties as a phonon scattering agent with the aim on lowering of the thermal conductivity and enhancement of the thermoelectric figure of merit, zT. Structural and thermoelectric proprieties of the materials were investigated in order to determine correlation between material s microstructure, composition and TE proprieties. Investigated CMO system produced dense samples with over 90% of the theoretical density. Resulting material consists of large grains with secondary phase precipitating on grain boundaries and triple points. Increased amount of secondary phase reduced material s strength and lead to formation of microcracks on the surface. Transformation of the rock-salt into perovskite is a very rapid reaction and it follows the topotactic reaction mechanism. Formation of marokite is a two step reaction with formation of Ca2Mn3O8 at about 570degC and its transformation to marokite at about 850degC. Introduction of marokite as secondary phase resulted in enhancement of electrical and thermal conductivity and lowering of the absolute value of Seebeck coefficient. Thermal conductivity was enhanced due to large grains that are beneficial for thermal transport and good thermal conductivity proprieties of the secondary phase. Electrical conductivity was enhanced due to change in [Mn3+]/[Mn4+] ratio that is governs charge carrier density. Phase transitions between the two secondary phases lead to formation of oxygen vacancies in CMO that increase its electrical proprieties through generation of Mn3+ ions that supply free charge carriers at lower temperatures. Seebeck coefficient values increase with increasing volume of secondary phase as the density of charge carrier increases. High thermal conductivity and lower than expected electrical conductivity resulted in relatively low power factor (PF) and zT. From the investigated samples CMO with 2.5vol% marokite is the most promising one as it exhibits highest PF and zT = 0.0062 at 900degC.