Thermoelectric Properties Of Cu₂Se Prepared By Self-propagating High-temperature Synthesis

Thermoelectric materials were realized mutual conversion between heat and electricity directly, which can beused for thermoelectric power generation and spot cooling. Recently, Cu2 Se compounds have aroused interests of the world due to its excellent thermoelectric performance(ZTmax=1.8 @ 1000 K) with low cost. Since self-propagating high-temperature synthesis(SHS) was introduced for the synthesis of Cu2 Se, however, so far no research on the correlation between the phase transformation mechanism and microstrucuture evolution during the SHS processhas been reported. In addition, the role of dopant elements during ultra-fast process of SHS for Cu2 Se remains unknown. In order to precisely control the synthesis process and optimize the thermoelectric performance, it is of crucial significance to explore the correlation and figure out the SHS process of Cu2 Se.In this study, the phase transformation process during SHS were figured out via combustion front quenching technique. Besides, the effects of several key parameters, such as particle size of raw materials and compact density on the SHS process were investigated. Furthermore, the effects of the size of raw materials and relative density of reactant on thermoelectric properties of Cu2 Se were realised from the perspective of industrial large-scale production. The doping effect of Cd on the Cu site and Te on the Se site on the phase composition, phase transition, microstructure, component and thermoelectric properties of Cu2 Se based compounds were revealed. The main research contents and results are as following:Through the combustion front quenching technique, the phase transformation process of Cu2 Se during SHS was investigated and the results were obtained via characterizations of phase composition and microstructure of each layer of quenching product. The results revealed that Cu2 Se was formed through several steps instead of forming directly. A series of intermediate phases were formed during SHS. The whole process was that CuSe phase formed first from Cu and Se, with the reaction going on, the CuSe phase transformed to phases with larger Cu content, such as Cu3Se2, Cu1.8Se and Cu2-xSe until Cu2 Se formed at last. Therefore, the SHS process of Cu2 Se was a non-equilibrium process.The effects of particle size of raw materials and compact relative density on the SHS process of Cu2 Se were investigated systematically, and the results indicated that with the reduction of particle size of raw materials, the speed of combustion wave increased ascribed to the larger specific surface area and high surface energy of fine powders. However, with the increasing compact density, the speed of combustion wave decreased due to the higher thermal conductivity of the pellet which make it easy to lose the heatin the reaction zone during the SHS process. Combustion temperature of the samples with different particle size and compact density is almost the same in SHS process. After SHS, densified Cu2 Se bulks were prepared by PAS, and the effects of particle size of raw materials and compact relative density on the thermoelectric performance of Cu2 Se were studied systematically. The results indicated that particle size of raw materials and compact density did no effects on the thermoelectric properties of Cu2 Se.The effects of dopants at Cu sites and Se sites on the phase composition, elemental distribution, microstructure and thermoelectric performance have been investigated respectively. After Cd doping on the Cu site, single phase sample with elemental distributed homogeneously was obtained. No noticeable changes of phase composition, microstructure and component distribution were found after Cd doping, which, however, the phase transition temperature between ?-Cu2 Se and ?-Cu2 Se raised. With the increasing of Cd content, the electrical conductivity declined first, and then remained unchanged after the doping level reached 0.006. The trend of Seebeck coefficient is inverse to that of the electrical conductivity. The power factor decreased after Cd doping in comparison with undoped Cu2 Se. The maximum power factor of undoped Cu2 Se is 1.09 mWm-1K-2 at 873 K. The thermal conductivity of all Cd doped samples decrease significantly compared with undoped Cu2 Se, and the sample with x=0.006 possessed the minimum thermal conductivity about 0.5 Wm-1K-1. After Cd doping, the maximum ZT of 1.34 is obtained for the sample with x=0.006 at 873 K, which increased by 4.7% compared to undoped Cu2 Se, which the maximum ZT is 1.28 at 873 K.After Te doping on the Se site, single phase sample with elemental distributed homogeneously was obtained. No noticeable changes of phase composition, microstructure and component distribution were found after Te doping, which, however, the phase transition temperature between ?-Cu2 Se and ?-Cu2 Se raised. With the increasing of Te content, the electrical conductivity declined first, and then increased. The trend of Seebeck coefficient is inverse to that of the electrical conductivity. The power factor decreased after Te doping in comparison with undoped Cu2 Se.The thermal conductivity of all Te doped samples decrease significantly compared with undoped Cu2 Se, and the sample with x=0.02 possessed the minimum thermal conductivity about 0.57 Wm-1K-1. After Te doping, the maximum ZT of about 1.28 is obtained for the sample with x=0.02 at 873 K, which is almost the same with undoped Cu2 Se.