AbstractsEngineering

Transition metal oxide Ca3Co4O9+δ has demonstrated to be one of the most promising p-type thermoelectric materials for power regeneration. In this dissertation, the evolution of crystal structure with various dopants (i.e., Fe, Ga and Bi) was characterized by employing synchrotron powder diffraction and X-ray absorption spectroscopy. Experimental results show that both Ga and Fe were preferentially located on Co site in the rock-salt type ([Ca2CoO3]) layer rather than the hexagonal CdI2-type ([CoO2]) layer, as previously assumed in the literatures. The doped Bi atoms were found to be located on both Ca and Co site in the [Ca2CoO3] layer, with the solubility of ~5-6% on each site. It is also found that Bi substitution did not change the average Co valence states but affected the Co coordination environment in the [Ca2CoO3] layer, in which the Co ions showed more preference to reside in the tetrahedral site upon Bi doping. Based on the crystallographic information, the influence of Fe, Ga and Bi substitution on the thermoelectric properties was systematically investigated. Experimental results show that both Fe and Ga substitution decreases electrical resistivity and thermal conductivity of Ca3Co4O9+δ while less affects the Seebeck coefficient. Phonon density of states was investigated by means of inelastic neutron scattering and the results revealed that the suppressed optical phonon vibration, which was responsible for the reduced thermal conductivity with Bi doping. In addition, it is the first time to show the key role of Bi, which substantially improved electrical transport properties of this material, is to reduce the hopping energy that drives the charge carriers to transfer from the [Ca2CoO3] layer to [CoO2] layer. The highest figure of merit ZT achieved in present study is ~0.72 at 966K, for the Bi/Ga co-doped sample with ~ 40% porosity. This value is comparable to that of Ca3Co4O9+δ single crystal and is among the highest ZT values reported so far for polycrystalline oxide-based thermoelectric materials.