| Large thermopower is essential for highly efficient thermoelectric materials which are of great interest from the technological point of view. Since Terasaki et al. reported that NaCo2O4 had a large thermopower and a high conductivity, the layer cobalt oxides have been studied extensively. Up to now, the origin of its large thermopower has drawn much attention. Many investigations disclose that the spin entropy plays an important role in enhancing the thermopower in cobalt oxides and some experimental evidences support the spin entropy theory. In particular, Wang et al. observed a suppression of the thermopower under a longitudinal magnetic field at low temperatures in NaCo2O4 and suggested the spin entropy as the likely source for the observed large thermopower. In cobalt oxides, the spin-entropy contribution to the thermopower is related to the Co4+ concentration and degeneracy of Co ions. Doping with metal atoms is one way to change the Co4+ concentration, which is important for the spin entropy, or provides an alternative hopping model for transporting the spin entropy. In this respect, investigating the doping effects of metal atoms on the spin entropy may provide an important guidance for improving the thermopower. In this dissertation, we devote out attention to the cobalt oxides materials. By both magnetothermopower and magnetic measurements, we study the doping effects of metals on the spin entropy of cobalt oxides. The main results are summarized as follows:The effects of Ni doping on the spin entropy in Na1.2Co2O4 has been carefully studied. A strong magnetic-field suppression of the thermopower indicates the emergence of large spin-entropy effect. The magnetothermopower increases with increasing Ni doping level, suggesting that Ni doping improves the spin entropy of Na1.2Co2O4. We find a new transport mechanism on spin entropy and propose a spin entropy competition model.Through the measurements of magnetothermopower on NaCo2-xFexO4, we find that the spin entropy is suppressed by Fe doping. Mossbauer spectra and magnetic properties give support to that high-spin Fe3+ ions (s=5/2) present in these materials. The Co4+ concentration is enhanced by Fe doping, which results in the spin suppression.Firstly, we investigate the thermoelectric properties of Ce doped Ca3Co4O9+δ. With partial Ce substitution, the thermopower increases, while the thermal conductivity decreases. The dimensionless figure of merit ZT=0.016 is achieved at 335 K for Ca2.9Ce0.1Co4O9+δ, demonstrating that heavy atoms Ce doping may promise an effective way for improving thermoelectric properties of Ca3Co4O9+δsystem. We also present the measurements of thermopower as a function of temperature under different magnetic fields, for Ce-doped Ca3Co4O9+δ. A strong magnetic field suppression of the thermopower indicates a large spin entropy contribution. The magnetothermopower is enhanced in all doped samples, which provides an experimental evidence for the enhanced spin entropy by Ce doping. Our magnetic results allow us to determine the decrease in Co4+ concentration which results in the spin-entropy enhancement. We adopt a suitable theoretical model to explain our experimental observations. This investigation gives strong evidence that the enhanced thermopower mainly originates from enhancement of the spin entropy. Finally, we have investigated the effect of Ce doping on the magnetic properties of Ca3Co4O9+δ. A small shoulder occurring at about 23 K in the dχ-1/dT curve indicates an occurrence of spin-density-wave transition, which is confirmed by resistance characteristics.A series of Ca3-xLuxCo4O9+δ(x=0,0.1,0.2 and 0.4) has been prepared by sol-gel method. The effects of Lutetium substitution on thermoelectric properties of Ca3Co4O9+δhave been systematically investigated from 4 to 335 K. With the partial substitution of Lu3+ for Ca2+, the resistivity and thermopower for Lu-doped samples increase, while their thermal conductivity decreases. The dimensionless figure of merit for Ca2.8Lu0.2Co4O9+δmaterial (ZT=0.032) is about five times better than that for Ca3Co4O9+δ(ZT=0.007) at 335 K. In addition, the magnetothermopower have been studied in Lu-doped Ca3-xLuxCo4O9+δ. A strong magnetic field suppression of thermopower indicates a large spin entropy contribution. The magnetothermopower for doped samples are overall enhanced compared with that for undoped Ca3Co4O9+δ, providing an experimental evidence for the enhancement of spin entropy. Magnetic results confirm that Co4+ concentration is reduced by Lu doping. We suggest that the reduction in Co4+ concentration results in the enhanced spin entropy.
|
PDF Full Text Request