Development of high-performance thermoelectric calcium cobaltate for power regeneration

Abstract

Thermoelectric materials can convert waste heat into usable electricity without a high operating cost and useless by-product, providing an alternative strategy to overcome the energy crisis. However, conventional high-performance thermoelectric materials are mainly alloys which are toxic and unstable at high temperature. Owing to its excellent thermal stability and high efficiency compared with other oxide materials, Ca3Co4O9+δ is one of the most promising high-temperature thermoelectric materials. On the other hand, thermoelectric materials with nanostructure have been proved to have better ZT performance as a result of low thermal conductivity due to more phonon scattering on grain boundaries. This thesis reports polycrystalline Ca3Co4O9+δ synthesized by high energy ball milling and spark plasma sintering. Different high energy milling times were selected to fabricate Ca3Co4O9+δ with different grain size. The high energy ball milling process not only controlled the average grain size down to the nanoscale, but this high energy mechanical process also introduces defects into the lattice which contribute to reducing thermal conductivity. A high value of zT =0.5 was obtained at 973 K for Ca3Co4O9+δ with average grain size 66nm which was the highest value ever reported for pure phase Ca3Co4O9+δ. To further optimize the grain growth and densification process, different spark plasma sintering parameters including sintering temperature and dwelling time were also applied to prepare polycrystalline Ca3Co4O9+δ ceramics. Sample sintered at 1123K for 1mins and 1073K for 5mins performed best which showed very high power factor of 5×10-4 Wm-1K-2. At 1123K, densification dominated the SPS process while at 1073K grain grown was dominated. After durability testing, polycrystalline Ca3Co4O9+δ fabricated by high energy ball milling and SPS showed an excellent chemical and thermal stability even keep excellent thermoelectric properties. To optimize resistivity to improve thermoelectric properties of polycrystalline Ca3Co4O9+δ, Ga and Bi were selected as dopants. The substitution of Ga at the Co site effectively decreased resistivity by increasing carrier concentration. The existence of Bi2O3 further decreased resistivity by optimizing grain boundary conductivity and enhancing carrier mobility. The power factor of Ca3Co3.95Ga0.05O9+δ+Bi0.3 reaches the highest value of 4.75×10-4 Wm-1/mK2 at 973 K which is an 88.07% improvement compared to pure Ca3Co4O9+δ.