Effects Of Element Doping On Cu_2-xSe Microstructure And Thermoelectric Properties

Now we are tying up a wide variety of energy sources,but no any energy source is perfect.Since the 20th century peoples were thinking the question of howto maintain the balance between energy and environment,and strived to create new technologies to overcome energy defects.Researchers were not only constantly exploring novel clean energy,but also were trying to solve the problem of low energy efficiency.In order to maintain the balance between development and environment,more attentions should be payed.At present,how to improve energy efficiency has become one of the core issues of energy research.With the development of thermoelectric materials,thermoelectric conversion technology can be a feasible technical solution to solve the problem of how to improve energy efficiency.In this paper,a high-performance thermoelectric material Cu2-x Se is selected as the research object,which has typical phononic liquid electronic crystal characteristics.Cu2-xSe nanopowders were synthesized using a hydrothermal method and hot-pressed into bulk pellets to assess the effects of doping on the thermoelectric properties of Cu2-xSe.The main achievements are followed:(1)A series of Cu2-xSe solid solutions were designed and synthesized.The preparation scheme combining a chemical hydrothermal synthesis method and hot pressed sintering realizes low cost,less device-dependence and large-scale synthesis.In the present work,ZT values of 1.5 and 1.32 were obtained for polycrystalline Cu2Se and Cu1.98Se,respectively.These values are one of better results so far reported in Cu2Se based materials.(2)Powder X-ray diffraction(XRD),scanning electron microscopy(SEM)and X-ray photoelectron spectroscopy(XPS)were utilized to study the phase transition and crystal structure evolution of alkali metal doped Cu2Se compounds.The results show that the material after doping the crystal structure and the phase change characteristics of Cu2Se are still maintained,but the microscopic morphologies change.The SEM result indicates that the morphology of the grains structure is somehow larger,and that a small number of nanopores appear in the doped materials.The relevant thermoelectric performance test analysis shows that the material resistivity and Seebeck are improved by doping.The nanopores together with point defects decrease the thermal conductivity.The thermoelectric properties of the material are improved by alkali metal doping,resulting in optimized ZT values of 2.14 and 2.1 for Cu1.98Li0.02Se and Cu1.96Na0.04Se at 973 K,respectively.The experimental data indicate that an optimal level of alkali metal doping improves the thermoelectric performance of Cu2Se effectively.(3)The formation of microporous structure in Cu2Se after doping with alkali metal elements Li,Na and K was investigated.Experiments show that microporous structure can occur in Cu1.97Li0.03Se with increasing sintering temperature.Sintering temperature is one of the reasons for the formation of microporous structure.Therefore,due to the decrease of the melting point of the material after doping the alkali metal element,the microporous structure is introduced into the material.The introduction of the microporous structure can effectively reduce the lattice thermal conductivity.(4)Non-stoichiometric Cu2-xSe is considered as a potential "phonon liquid electronic crystal" thermoelectric material.Cu1.98Se has higher thermal conductivity than Cu2Se,but the power factor of Cui.98 Se is about twice that of Cu2Se.The alkali metal Na and heavy metal element Pb are selected as doping elements to modify the Cui.98Se material.There are more Cu vacancies in Cu1.9sSe,which provides more positional possibilities for doping elements.The doping elements may play a more complicated role than simple doping upon incorporation into the Cu1.98Se matrix.The intermixing of doping elements at the various Cu sites within the Cu1.9sSe lattice results in the formation of a hybrid "Cu1.98 Se like" structure.In such a hybrid "Cu1.98Se-like"structure,may help to improve material stability.As noted,whether before or after doping the electrical properties of Cu1.98Se-based materials are superior to those of Cu2Se-based materials,but not the thermal properties.After analysis and comparison,it is considered that the microporous structure plays an important role in optimizing the performance of Cu2Se.(5)Based on the experimentally-determined resistivity values,Seebeck coefficients and thermal conductivity data,the temperature dependence of ZT for the Cui.98Se doped with Na and Pb elements has been calculated,the ZT values of the materials can reach 1.2 and 1.52 respectively at 973K.These results show that the thermoelectric properties of the doped Cu1.98Se materials are still sufficient at high temperatures.