Nanostructured Nb-substituted CaMnO 3 n-type thermoelectric material prepared in a continuous process by ultrasonic spra
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One way to further optimize the thermoelectric properties toward a higher ZT is a temperature stable nanoengineering of materials, where the thermal conductivity is reduced by increasing the phonon scattering at the grain boundaries. To study this, Nb-substituted CaMnO3 perovskite-type material was synthesized by ultrasonic spray combustion (USC). The grain growth has been characterized by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Finally, the thermoelectric properties of compacted and sintered bulk samples from powder prepared by a continuous scalable USC process were measured up to 1050 K. The thermoelectric legs were prepared by an adapted sintering process. Here, a compromise between enhanced porosity to reduce the thermal conductivity and securing of mechanical stability and low resistivity should be obtained. Based on the grain growth mechanisms, an advanced sintering process for additional interconnection of the particles without particle growth is needed to further increase the thermoelectric performance.
I. INTRODUCTION
The expected decline of fossil-fuel resources is promoting growing interest in renewable energy technologies such as solar energy converters and waste heat recovery techniques. In manufacturing industry, automobiles, and thermal power plants, a huge amount of energy is lost as waste heat, which could be recovered and converted into electricity by a new generation of improved thermoelectric devices. This requires thermoelectric materials that are mechanically and temperature stable, nontoxic, and inexpensive. Conventional thermoelectric modules based on Bi2Te31 exhibit relatively high conversion efficiencies but limited temperature stability in air. The perovskite-type structure, CaMn0.98Nb0.02O3 (n-type), is a promising candidate for the production of thermoelectric oxide devices. One possibility to improve the performance of the thermoelectric materials toward a higher ZT is the nanoengineering of bulk materials. Nanostructuring effects in bulk materials are a promising way to reduce the thermal conductivity j by increasing the phonon scattering at the grain boundaries.2 The motivation of this study is to investigate the deviation of ZT values between very promising solid state chemistry experiments and the much lower ZT values of the thermoelectric legs in the application. Prominent examples are the materials showing ZT . 1, which was never reproduced in a device a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.140 J. Mater. Res., Vol. 26, No. 15, Aug 14, 2011
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up to now. This deviation might be caused by differences in morphology of the material due to the different production process. Nanocrystalline CaMn0.98Nb0.02O3 was synthesized by a continuous ultrasonic spray combustion (USC) process, a low-cost, scalable, and environmentally benign method. The effect of this synthesis method suitable for large scale production on the grain growt
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