Synthesization And Properties Of Bi₂Te₃ Based Nanocomposite Thermoelectric Materials

Thermoelectric materials interconvert heat and electricity directly. They are used in power generation and cooling. Bi₂Te₃ based materials are the best thermoelectric material at room temperature. The figure of merit ZT of Bi₂Te₃ is about unit for decades. In recent years, the research of thermoelectric material has made new progress by nanotechology. High figure of merits over unit are reported. Nanostructured materials have a large amount of boundaries that will strongly scatter the phonons and carries. Therefore the Seebeck coefficient is improved, the thermal conductivity reduced, and the thermoelectric properties improved. In this paper, various Bi₂Te₃ based alloys were hydrothermally synthesized and hot pressed to make bulk samples. The mechanisms and properties are systematically investigated.In this paper, the influences of excess Te and doped Ag on the properties are systematically studied. The result shows that the doping levels can be optimized by changing the proportion of Bi and Te. The conductivities are 8.0×10³ to 1.03×10⁵ Sm^-1 for the nominal compositions of Bi₂Te_x(x=2.85～3.45). The conductivities also can be optimized by coating Ag on the hydrothermally synthesized nanopowders by glucose.Ternary Bi₂Te₃ based alloys have the lower thermal conductivities due to the stronger scattering to the short wave phonons, and the optimized electronic structures. Therefore, the ternary alloys have the higher properties than binary alloys. In this paper, ternary Bi₂Te₃ based alloys were synthesized by hydrothermal method. The strucucture analysis shows that the products are composed of two phases, both of which of are Bi₂(Te, Se)₃ alloys with different lattice constants. Sample Bi₂Te_2.70Se_0.45 and Bi₂Te_3.00Se_0.45 are nearly single phase. The property analyses show that changing the Te content can optimize the doping levels. The samples close to single phase have the stronger scattering. In (Bi, Sb)₂Te₃ alloys, it become difficult to form single phase alloys with increasing the Sb content. Bi_1.8Sb_0.2Te₃ is single phase after hydrothermal synthesis. BiSbTe₃ is single phase after hot pressing. But Bi_0.5Sb_1.5Te₃ can not form single alloys.Single phase ternary Bi₂Te₃ based alloys are synthesized by synthesizing binary alloys of Bi₂Te₃, Bi₂Se₃ and Sb₂Te₃ followed by hot pressing. The results show that single phase alloys were formed in the whole Bi₂Te₃-Bi₂Se₃ quasi binary series. However no single phase alloys were formed in the whole Bi₂Te₃-Sb₂Te₃ quasi binery series. The crystal sizes of Bi₂Te₃ and Bi₂Se₃ were both tens of nanometers, so it is easier to form single phase alloys by diffusion,while the sizes of part of Sb₂Te₃ crystals were on the micrometer level, so it is difficult to form single phase.The hydrothermally synthesized powders have a large amount of boundaries that strongly scatter the phonons and carriers. Seebeck coefficient is improved and thermal conductivity reduced. However it is difficult to industrially produce nanomaterials because of the technical and commercial barriers. In this paper, micro-nano-composite Bi₂Te₃ alloys are synthesized by mixing the nanopowders hydrothermally synthesized at 120-250℃ into the industrial materials and hot pressing. The crystal sizes of the nanopowders have a great influence on the properties. The smaller is the nanopowders, the stronger is the scattering. The sample hot pressed with nanopowders synthesized at 120℃ and N type industrial powders has the highest ZT value of 0.83.The micro zone distribution of Seebeck coefficient was analyzed. The distributions of Seebeck coefficient are generally believed to be even. The usually tested are the average value. However, the result here shows that the distribution of Seebeck coefficient in the samples is unhomogeneous, with a fluctuation of 40 μVK^-1. The distribution has no relationship to the composition, morphology and crystal orientation.Waste heat power generation is an important thermoelectric use. Because of its low efficiency, the thermoelectric generation device should be of low cost in production, installation, running and maintenance. Industry chimney is an important waste heat source. To the heat source, an inside thermoelectric generator without cooling driver is made. The device has low effect to the chimney structure, that is easy for installing, and reduces the cost. The device is cooled without a driver, which simplifies the structure and further reduces the cost. The water-cooling runs continuously and the output power is 0.027 W in testing.