Controllable Synthesis And Properties Study Of Chalcogenide Thermoelectric Materials

Thermoelectric material is a kind of functional material that intensively matches the requirement of the society and has broad prospects. However, the efficiencies of most of the thermoelectric materials are so mediocre that the application is hindered. Thus, the studies to enhance the efficiencies of thermoelectric materials retain most of the attentions. In addition, there are some works else important to the development of thermoelectric materials, such as the efforts to improve the maximum output power of the thermoelectric generators and the mechanical performance of thermoelectric materials, the works to decrease the cost and the use of harmful elements, etc.Cu_2-xS（0≤x≤0.2） is a sort of potential thermoelectric material that behaves good properties under moderate or high temperature, with advantage of low cost and no toxicity. However, the existing methods to synthesize Cu_2-xS thermoelectric material all use high purity element Cu and S as starting materials, which ensures the good thermoelectric properties but increases the cost. Furthermore, no good thermoelectric performance has been reported on the Cu_2-xS that synthesized by using analytic reagents or more impure materials, which partly restricts the development of Cu_2-xS thermoelectric material. Here, we propose a new approach to synthesize Cu_2-xS thermoelectric material economically and efficiently. CuS powder was first synthesized through a simple chemical reaction by using analytic compound reagents as starting materials. Then Cu_2-xS powder was obtained through the decomposition of CuS powder under high temperature. The bulk Cu_2-x S was made by direct-current hot pressing. We studied the impacts of different decomposition temperature on the phase and the thermoelectric properties of Cu_2-xS. When the decomposition temperature increases from 873 to 1273 K, the element S that volatilizes from Cu_2-xS increases, leading to the reduction of the concentration of Cu defect in Cu_2-xS. The concentration of carrier decreases with the reduction of the concentration of Cu defect, resulting in the increasing resistivity and Seebeck coefficient, as well as the reductive power factor and thermal conductivity. The largest ZT value about 1.3 is observed at 800 K on the bulk Cu_2-xS sintered using the powder obtained from the decomposition of the CuS at 1203 K, which is comparable to the best performance that has been detected on Cu_2-xS.In the other aspect, composites show great potential in enhancing the properties of thermoelectric materials（not only thermoelectric but also mechanical properties）. At present, few studies have been done on the thermoelectric properties of Cu_2-xS composites. There is lack of a complete and surely instructional theory on how and which materials to composite with Cu_2-xS to enhance the thermoelectric properties. More experimental investigation should be done to help the establishing and verification of the theoretical model. Herein, we respectively synthesized the Cu_2-xS/SnS composites, Cu_2-xS/SnS2 composites and Cu₂S/CNT（carbon nanotube） composites, to study the influence of different ratio of Cu_2-x S and SnS（ or SnS2, or CNT） on the phase and the thermoelectric properties of the composites. To the Cu_2-xS/SnS composites, the moderate addition of SnS can increase the dimensionless figure of merit. When the mass ratios of SnS and Cu_2-xS are 0.0025 and 0.01, the ZT value of Cu_2-xS/SnS composite raises with the temperature increasing, and it reaches 0.8 at 800 K, bigger than that of the pure Cu_2-xS. The Cu_2-xS/SnS2 composite with a mass ratio of SnS2 and Cu_2-xS of 0.003 has a maximum ZT value of 0.57 at 650 K, which is bigger than that of the pure Cu_2-xS and may increase when the temperature is higher than 650 K. To the Cu₂S/CNT composites, when the mole ratio of CNT and Cu₂ S is 0.1, the Cu₂S/CNT composite has a maximum ZT value of 0.74 at 750 K, which is bigger than that of the pure Cu₂ S and may increase when the mole ratio of CNT and Cu₂ S increases.