| In nature and human activities,a huge energy dissipation waste heat is generated and not been utilized.However,the temperature difference power generation technology by using Seebeck effect can directly convert waste heat into high-quality electricity.As a new type of clean energy technology,thermoelectric power generation is widely used for its features of no moving parts,no emission of combustion pollutants,and wide application temperature range.Excellent thermoelectric materials are the basis for obtaining high performance thermoelectric devices.PbTe and Bi2Te3 systems are currently used commercially as high-performance thermoelectric materials,but their applications are limited by their toxic,expensive raw materials,and low conversion efficiency.Therefore,Cu2Se thermoelectric materials based on cheap,environmentally friendly and earth-rich elements have attracted attention.Cu2Se as a typical“phonon-liquid electron-crystal”material,has excellent thermoelectric properties,but due to its poor thermal stability,it cannot be used in continuous medium and high temperature power generation systems.This paper attempts to prepare single crystal Cu2Se thermoelectric materials and nano-layered Cu2Se thermoelectric materials by flux method,and studies the effects of element composition,flux ratio and element doping on the structure,crystal quality,thermoelectric performance and thermal stability of the material.Through experimental exploration,the main conclusions are as follows:?-Cu2Se single crystal was prepared by the Bi-flux method according to the stoichiometric ratio of Cu2Se(1+x)Bi3(x=0,0.01,0.02 and 0.03).The experimental results show that the single crystal ?-Cu2Se thermoelectric material prepared by the Bi flux method has a high power factor and good thermal stability.Among them,the single crystal ?-Cu2Se1.01 sample achieves the highest power factor 1.53×10-3W·m-1·K-2 at 690 K,which is significantly higher than that of polycrystalline samples.The TG-DTA shows that no weight loss occurred below the melting point.After three continuous thermal cycles,the power factor drops down to-0.04×10-3 W·m-1·K-2 at 660 K.In comparison with the polycrystalline sample prepared by hot pressing,the thermal stability is improved obviously.Nano-layered Cu2Se thermoelectric materials were prepared using the NaCl-flux method based on the stoichiometric ratios of Cu2Se(NaCl)x(x=1.5,2,2.5,3,and 3.5)We found that the NaCl-flux can effectively modulate the structure of Cu2Se materials,when 1.5?x?2.5,samples possess a single ?-phase structure,whereas samples consist of a mixture of ?-and ?-phase structures when x?3.Moreover,Cu2Se thermoelectric materials grown using the NaCl-flux method have lower thermal conductivity compared with the bulk samples prepared by other methods due to its thin lamellar structure.Eventually,Cu2Se(NaCl)2.5 shows the best thermoelectric performance among all samples and the maximum ZT value reaches 1.42 at 700 K.A series of Cu2-xSeCdx(x=0,0.0075,0.01,and 0.02)samples were prepared by NaCl flux method.The X-ray powder diffraction results indicated that the solubility of Cd atoms in Cu2Se is very low,and a small amount of dissolved Cd atoms can significantly reduce the thermal conductivity of Cu2Se materials,thereby improving the thermoelectric properties of the materials.However,excessive Cd doping will cause the second phase of CdSe in the sample,its high electrical resistivity and thermal conductivity had an opposing effect,which lowered the thermoelectric performance of the system.The final Cu1.9925SeCd0.0075 sample obtained a maximum ZT value of 1.61 at 700 K.Compared with the undoped sample,the ZT value was increased by about 12%. |
PDF Full Text Request