Microstructure And Thermoelecttric Properties Of The Cu Doping Bi₂Te₃ Alloys Of Directional Solidification

Thermoelectric materials provide a useful solid-state conversion technology for refrigeration and power generation which can be used in numerous fields, such as the new energy resource, environment protection and so on. As the best candidate thermoelectric material, bismuth chalcogenide alloys, possessing a unique topological layered structure, exhibiting lower thermal conductivity but higher electrical conductivity at room temperature, promoting it as a promising thermoelectric material in a sustainable energy solution. Recently, more and more reports towards this promising emerging material have been reported, one of the most popular and effective ways to promote its thermoelectric propert ies is doping and nano-structuring, by this way a large number of defects can be introduced into the matrix, which can build a strong scattering effect to the phonon and lead to a low thermal conductivity. In fact, the thermoelectric properties are mostly depended on the crystal structure, composition, doping and other factors in the process of manufacture, so it is crucially important to figure out the interaction and mechanism during this process. Therefore, this work mainly focus on to fabricate a Bi₂Te₃ based alloy with copper doped by means of direction solidification, expecting to gain a composite with grain growing towards its best properties direction, so the whole experiment will provide a new thinking and theoretical basis to optimization of the Bi₂Te₃ systems.In this paper, we choose two different doping formats as the study subject--Cux Bi₂Te₃+x/2 and CuxBi₂Te₃. Firstly we found that the copper would solid solute fine in the matrix, then with an increase of Cu content, there will be a solid phase transformation, after which the nano-structure second-phase emerging around the boundaries of the matrix grain. The second phase can be confirmed by EDS and XRD experiments, the result turn out to be Cu2 Te. With the continuous increase of Cu content, eutectic phase of Cu2 Te and Bi₂Te₃ can be found in the same places.Then the Bi-Te-Cu alloy was directionally solidified at growth rates of 5 μm/s, and the compound ingredients are uniform after direction solidification, meanwhile more copper can be solidified in the Bi₂Te₃ matrix, and almost all the grains are grows towards their best growth direction--（015）,（1010） and（110）--in this way.Then we explore the thermoelectric performance of the Bi-Te-Cu alloy, with the amount of second phase increasing, the electrical conductivity decreased first and then increased, which is in the contrast trend to the Seebeck coefficient. For Cux Bi₂Te₃ samples, its Seebeck coefficient is higher than the other components, and owning to its lower thermal conductivity, the ZT of this alloy finally reaching its summit at 0.69 when temperature was 496 K, which is two times as much as the Bi₂Te₃ without copper doped and direction solidification, and is also 37. 7 percent than the same alloy that fabricated by powder metallurgy and melt spin rejection method.