| The thermoelectric(TE) material is a kind of functional material for thermal and electric energy conversion. The material can not only improve the conversion efficiency between the heat energy and electric energy, but also recycle the waste heat which directly transform the waste heat into electrical energy without any moving part, and improve the using ratio of energy sources. The layered calcium cobalt oxides attract the majority of the researchers’ attention due to the greens environmental protection, low cost, easy-preparation, good-thermal stability and high thermoelectric properties. But at present, compared with the practical application materials, TE material still has very great disparity. And people are constantly striving to seek a new method to improve its thermoelectric properties.In this paper, Fe, Ni and Ce doped Ca3Co4O9+δ materials are synthesized under two-step external magnetic field during the progresses of sol-gel and pre-pressing. Phase structures of samples are examined by using X-ray diffraction(XRD) patterns. The size, morphology and structure of the samples are observed by a scanning electron microscope(SEM) with an attached energy dispersed spectrometer(EDS).The Seebeck coefficient S, resistivity ρ and powerfactor PF are tested in the temperature range of 300~1073 K. The effects of the doping concentration and external magnetic field on the organization structure and electrical properties are studied deeply.The deviation of diffraction peak position and the calculated lattice parameter show that, Fe, Ni and Ce are doped into the lattice of Ca3Co4O9+δ materials, respectively. According to the calculated results of F values, the F values decrease with the increasing doping concentration slightly. It means that doping elements increase the lattice scattering, and make against the preferred orientation of grain. The F values of the samples with external magnetic field are larger, which shows the magnetic field can induce grains alignment. Compared with samples without external magnetic field, the samples with external magnetic field show more lamellar microstructures, which also explains that the application of magnetic field in the sample fabrication progress can prompt the internal structure of the sample preferred orientation, which is consistent with the XRD analysis.Study on the electrical properties of metal Fe and Ni doped Ca3Co4O9+δ based materials shows that the PF values of all the samples increase with increasing temperature. Because of the simultaneous increasing in the resistivity(ρ) and thermopower(S), the PF values of Fe and Ni doping materials decrease. All the samples with magnetic field exhibit much higher PF values than those without magnetic field. Among the Fe dope samples, a maximum value of 0.51 m W/m K2 is achieved at 1073 K for the Ca3Co3.75Fe0.25O9+δ sample, approximately 76% higher than that without magnetic field.Study on the electrical properties of the rare Ce doped Ca3Co4O9+δ based materials shows that Ce-doping has evidently increased PF, especially at higher temperature region. Of all the Ce doped samples without external magnetic field, Ca2.9Ce0.1Co4O9+δ has a maximum PF value of 0.34 m W/m K2 at 1023 K,which is 21% higher than that of pure Ca3Co4O9+δ. Compared with the Ce doping samples without magnetic field, the samples with external magnetic field show larger power factor values. Ca2.9Ce0.1Co4O9+δ with external magnetic field has a maximum PF value of 0.49 m W/m K2 at 1023 K,which is 48 % higher than that of samples without external magnetic field. |
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