Effect of Processing Temperature on the Thermoelectric and Optical Properties of Copper Manganite-based Spinels

This research is focused on investigation of the effect of doping and processing on the microstructure and phase evolution, and the effect of the microstructure and phases on the thermoelectric and optical properties of doped Copper Manganites. Undoped CuMn2O4 and four doped compositions, with nickel and cobalt, were investigated. Detailed processing protocols, including green state processing, were developed to make samples with a range of densities between 65 to 95%. Densification kinetics studies (dilatometry), together with phase and microstructural evolution (XRD, SEM and EDS) were used to demonstrate the role of second phases and especially liquid phase in controlling the densification. It is shown that the Cu content of the oxides plays an important role in controlling the densification and optimum sintering temperature. X-ray diffraction analysis showed that all the majority phase in all materials maintained a cubic spine] structure at all sintering temperatures.;The thermoelectric and optical properties were also characterized. Electronic conduction in these oxides is through polaron hopping between Mn3+ and Mn4+. Thus, the thermoelectric properties were controlled largely by the nature of the primary phase, as the secondary phase was too Mn-deficient to support such conduction. Seebeck coefficient ranged from 16 to 185 microV/K, with both composition and the sintering temperature affecting it. The electrical resistivity spanned a wide range of 1.4--2537 O-cm, and was also affected by both the composition, and sintering temperature. Although no general trend was established for the thermal conductivity, it was apparent that the thermal diffusivity was dependent on the density as expected. The FTIR results show that these materials are transparent in the mid-infrared region, showing appreciable transmittance in the range of 12--18 microm. Finally, XPS analysis indicated that the Mn valence states ranged from Mn4+ to Mn2.5+ and generally decreased as sintering temperature increased, in agreement with the results of previous studies on thin films.