Research And Improvement Of The Thermoelectric Properties Of Some Cobalt Base Oxides And Cu₂Se

The thermoelectric material is a kind of function material, which can directly realize the convention between thermal energy and electrical energy through the transportation of electric carriers in the materials. These materials are regard as one of the promising medium for energy source, because devices that are made by thermoelectric materials have many advantages such as small sizes, no pollution, quiet, no moving part and etc. Therefore, the thermoelectric materials will have important applications in fields such as power generation using waste heat and refrigeration in solid-state. The judgment of the quality of one thermoelectric material is according to its dimensionless figure of merit(ZT). The value of ZT is determined by the electrical resistivity, Seebeck coefficient and the thermal conductivity of the material. The promising thermoelectric material with high ZT value should have a low electrical resistivity, a large Seebeck coefficient, and a low thermal conductivity simultaneously. The oxide materials are not regarded as promising thermoelectric material by the researchers because of the high resistivities of the oxide materials. But since the discovery of the good thermoelectric properties of the oxide NaCo2O4 and also due to the fact that the oxides have high chemical and thermal stability and can be applied at a large temperature gradient across the materials in air, therefore they can reach a high Carnot efficiency that will compensate in some extent the relative lower ZT value of the oxides as comparing with the thermoelectric alloy. The discovery of the NaCo2O4 arouses a research stirring of the oxides.In this paper, we studied mainly the influence of the elements doping on the thermoelectric properties of the cobalt based oxides. Firstly, the influence of Ba, Na, and K partially substitution for Bi on thermoelectric properties of the Bi2Sr2Co2 Oy were sdudied. It was found that the doping of Ba, Na, and K can reduce the resistivity of the samples remarkably. But Ba or K substitute for Bi will reduce the seebeck coefficient. However, the doping of Na has the trend to increase the seebeck coefficient in the low temperature range. The element doping can reduce the thermal conductivities slightly. As the combined results of element doping, the thermoelectric properties of the samples are all enhanced by these three elements’ doping. The highest ZT value of doped samples is 0.3 at 973 K caused by Na or K doping, this ZT value is much higher than the undoped sample.Secondly, we studied the influence of different earth elements on the thermoelectric properties of the double perovskite structure compounds RBaCo2O5+δ. We found that with the change of the earth elements, the resistivity, Seebeck coefficients, and power factors of the samples will also change greatly. At room temperature 300 K, the power factor(PF) of the GdBaCo2O5+δ sample is relatively larger which can reach 50.3 μWm-1K-2. We also studied the influence of Fe replacing Co on the lattice structure, electrical resistivity, Seebeck coefficients and thermal conductivity. It was found that with the increase of the doping amount of Fe, the phase of the sample will transform from the orthorhombic phase to the tetragonal phase gradually. The doping of Fe also reduces the carrier concentration resulting in an increase in the electrical resistivity and Seebeck coefficients. Due to the lattice misfit and decrease in carrier concentration, the thermal conductivity of the doped samples is reduced. As a whole effect of the Fe doping, the ZT values at above 355 K are enhanced.We also studied the effects of the ion size of rare earth R3+ on the thermoelectric properties of the RBaCo4O7+δ oxides. It was found that the electric resistivity decreases with decrease of the R3+ ion sizes. While the R3+ ion sizes have little influence on the Seebeck coefficient and the thermal conductivity. As a total result the ZT values of RBaCo4O7+δ increase with decrease of the R3+ ion sizes. Among the RBaCo4O7 samples the InBaCo4O7+δ is a much good one, its ZT value can reaches 0.1 at 973 K.In 2011 the Chinese Academy of Science and their partners proposed a new conception “phonon-liquid-electric-crystal”, when they studied the Cu2 Se. This material has excellent properties in nature, which is worth to further study. In this paper we synthesized Cu2 Se nanopowders by the hydrothermal method and studied the thermoelectric properties of the bulk samples hot pressed by the nanopowders. It is found that the powders will be spherical nanoparticles if the solvent is the H2 O. The sizes of the nanoparticles are different if the sources of the Cu are different. Few of the intermediates Cu3Se2 would be produced when the reaction time was insufficient. If we used the mixture H2 O and ethylene glycol as solvent and added some EDTA, the nanosheets would be produced. The width and thickness of the nanosheets are about several hundreds and tens nanometers, respectively. The Cu2 Se bulks samples were prepared by hot press using the spherical nanoparticles powders. The phase structures of the material will transform from a cubic one to the orthogonal one after hot pressing. The microstructure of the hot-pressed bulk has clear layered shape structure. The size of the grain is easily affected by the hot pressing temperature and time. Because when the hot pressing temperature increases, some Se elements will be volatile, which will cause the increase of the resistivity, as a result of the increase of the resistivity the seebeck coefficient will also increase. Among the samples made at different hot pressing temperature, the sample hot pressed at 973 K for 30 min will have the highest ZT value 1.2 as measured at 873 K. In order to improve the ZT values of the samples, we studied the doping effects of Ni, Ag and Hg. The Ni and Hg were added in the progress of the hydrothermal synthesis, while the Ag nanoparticles were added by grinding after hydrothermal synthesis. It is shown that the method of the Ag doping is not as effective as the Ni and Hg doping method. We chose 973 K and 15 min as the hot pressing conditions for keeping the microstructure of samples. It was found that the doping of Ni and Hg can improve the thermoelectric properties. The ZT values of Cu1.9Hg0.1Se and Cu1.9Ni0.05 Se bulk samples can reach 1.5 and 1.53 respectively as measured at 873 K.