The development of manganite-based materials for thermoelectric power generation

Abstract

CaMnO3 is one of the promising n-type thermoelectric (TE) materials for power generation applications due to its superior TE properties and stability at high temperature. Nevertheless, efficiency is still the major limiting factor for its common use. It is essential to understand the mechanisms associated with carrier and phonon transports for further enhancing the TE properties of this material. The main focus in this dissertation was to improve the TE properties of CaMnO3 by optimizing microstructure using different synthesis techniques and doping technology. It was found that the microstructure have remarkable influence on the electrical properties and thermal transport properties. The relative density of the sample was identified to be 86 (±3)% to achieve the high TE performance irrespective of their synthesis technique (solid state reaction, coprecipitation). The experimental results may provide a guideline for the variation in performance of this material system under different processing. In addition, it was found that doping in either Ca or Mn site with higher valence elements increases the carrier concentration, leading to the reduction of electrical resistivity ρ and Seebeck coefficient S. Furthermore, doping decreased the thermal conductivity κ of CaMnO3 up to a critical doping level, which is believed to associate with the decrease of phonon mean path and doping-induced MnO6 octahedral distortion. It was found that distortion mainly affects the equatorial bonds in the a-c plane in manganite system. Substitution of Bi was found to be effective to reduce κ due to its large mass difference with Ca. Doping in both Ca and Mn site was also studied and found that the appropriate doping element and doping concentration are the determining factors to obtain positive influence from the codoping technology. Bi was found as an effective substituent for a critical doping content (x=0.03) which simultaneously provides the high power factor (4.67×10-4 W m-1 K-2 at 423 K) and low κ (1.4 W m-1 K-1 at 973 K). These led to obtain maximum ZT value ~0.25 at 973 K for Ca0.97Bi0.03MnO3, which is three times larger than that for undoped CaMnO3.