| Devices made from thermoelectric materials can convert thermal energy directly into electric energy. These devices are characterized by small volume, low cost, zero pollution, no moving parts and long-term durability which enjoy a wide application in aspects like the recovery of industrial waste heat. In this case, these devices have been paid attention all the time by scientific researchers and engineering technicians. The Ca-Co-O layered compound comparatively has large Seebeck and low thermal conductivity so that it is recognized as a type of oxide thermoelectric materials with particular application potential. As targeting to solve the extremely high resistivity of the ceramic material, this paper figures out the method to improve the area and properties of the interfaces in the composite material by composite and doping. The effect of interface on the thermoelectric properties of the composites has been preliminary explored, and the experimental basis for further optimization of the thermoelectric properties has also been established.Firstly, the influence of the amount of Bi2O3 on Ca3Co3.9Cu0.1O9/Bi2Ca2Co2 Oy material has been researched on its microstructure, the area of interfaces and thermoelectric properties.In the experiment, Ca3Co3.9Cu0.1O9 powder is synthesized by flux method, by adding Bi2O3 in different volumes and utilizing technologies like rapid hot pressing and liquid-phase reaction sintering, the block sample of composite material can be obtained. XRD and SEM results show that Bi2Ca2Co2 Oy phase is formed after adding Bi2O3. When the Bi2Ca2Co2 Oy phase exists at the interface of Ca3Co4O9, the thermoelectric properties have been improved obviously, especially the significantly decrease of resistivity. The thermoelectric properties show that when the mole ratio of Bi2O3 is added to 20% the resistivity of Ca3Co3.9Cu0.1O9/Bi2Ca2Co2 Oy thermoelectric materials decreases to the minimum, 8.58×10-5?·m?973 K? and its corresponding power factor reach 4.12×10-4 W·m-1K-2.The influence of interface properties on thermoelectric performance probably holds a dominant position when the interface area achieves the best. Based on this reason, this paper further research the influences of Fe-doped on Co site and La-doped on Ca site on the thermoelectric properties of Ca3Co3.9Cu0.1O9/Bi2Ca2Co2 Oy materials. Results show that the Fe-doped makes the Fermi level of Ca3Co4O9 and Bi2Ca2Co2 Oy all move toward valence bands, but the barrier height decreases because of the discrepantly close distance. While Seebeck coefficient increases at first and then decreases as Fe-doped grows. Power factor ofCa3Co3.8Fe0.1Cu0.1O9/Bi2Ca2Co2 Oy thermoelectric material reaches the maximum, 4.24×10-4W·m-1K-2?973 K?. Fe-doped makes the Fermi level near the valence band and La-doped leads the Fermi level to deviate, so as to increase the interface barrier, and the Seebeck coefficient and resistivity of the material increase at the same time. As phonon scattering is enhanced by scattering center of heavy ion La3+, the thermal conductivity has been dropped off, and thermoelectric properties of the whole thermoelectric material have been improved.When adding Bi2O3 into Ca3Co3.9Cu0.1O9 to form a composite material, the superfluous Co O can be formed which will increase the resistivity of the material. To that end,Bi2Ca1.2Sr0.8Co2 Oy and Bi1.6Pb0.4Ca2Co2 Oy is combined into Ca3Co3.9Cu0.1O9 in a direct way respectively with the expectation of promoting the thermoelectric properties of Ca3Co3.9Cu0.1O9. Results demonstrate that when the content of a composite phase is risen up,the resistivity and Seebeck coefficient both decrease at first and then increase. At 973 K, the maximum power factor is 3.7×10-4W·m-1K-2and 3.04×10-4W·m-1K-2, respectively. |
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