Preparation And The Study Of Thermoelectric Performance Of Cu₂Se-based Composite Materials

Thermoelectric material,which can directly convert heat energy into electric energy through thermoelectric effect,is a kind of environment-friendly material that has been looking as possible solution to the current energy problem.Cu₂Se-based thermoelectric materials have attracted great attention to the researchers due to their good thermoelectric properties.It has been confirmed in previous studies that the non-stoichiometric material of Cu_2-xSe with Cu vacancy has better properties than those of stoichiometric Cu₂Se.However,Cu_2-xSe undergoes a structural phase transition at around 127℃,from the low-temperatureαphase to the high-temperatureβphase or reversal.The good thermoelectric performance of Cu_2-xSe at moderate and high temperatures is due to the"liquid-like behavior"of copper ions inβphase.In contrast,the thermoelectric performance ofαphase is poor and needs to be further improved.In this thesis,various composite materials composed of Cu_1.95Se and metal selenide（Ni,Fe,Al）were prepared by high-energy ball milling combined with direct-current hot press approach to improve the thermoelectric performance.First,the effect of the 5 vol.%Ni,Fe or Al in Cu_1.95Se on the thermoelectric properties of the composites was studied,respectively.On this basis,the effects of various concentrations of selenides of Ni or Fe in the composites were further studied.The main results are as follows:1.Cu_1.95Se/M_xSe_y-5 vol.%composites were successfully prepared by high-energy ball milling combined with direct-current hot press approach.All samples are crystalline in low-temperatureαphase at room temperature.The metal selenides in Cu_1.95Se/M _xSe_y-5 vol.%（M=Ni,Fe,Al）are Ni₂Se₃,CuFeSe₂,and AlCuSe₂,respectively.Compared with Cu_1.95Se,although the presence of metal selenides reduced the electrical conductivity of the composite and dropped those electrical performances,the substantial reduction in thermal conductivity makes the ZT of the composite improved.As the result,the ZT values of Cu_1.95Se,Cu_1.95Se/M_xSe_y-5 vol.%（M=Ni,Fe,Al）at room temperature are 0.09,0.65,0.21,and 0.07.2.Cu_1.95Se/Ni_xSe_y-f vol.%（f=1,3,5）composites were successfully prepared.All samples are crystalline in low-temperatureαphase at room temperature.In Cu_1.95Se/Ni_xSe_y-f vol.%（f=1,3,5）,the nickel-selenium phase in the f=1 sample is NiSe₂,and the nickel-selenium phase in the f=3,5 sample is Ni₃Se₂.Compared with Cu_1.95Se,the presence of the nickel-selenium compounds significantly reduces the thermal conductivity of the composites,and also changes the sample carrier concentration so that its conductivity and Seebeck coefficient have been changed.The thermoelectric properties have been improved to a certain extent.The ZT values of Cu_1.95Se and Cu_1.95Se/Ni_xSe_y-f vol.%（f=1,3,5）are 0.09,0.26,0.31,and 0.66,respectively.The maximum ZT value of the sample with f=1 is 1.91 at600℃,which is nearly 65%higher than that of the substrate at 1.16.3.Cu_1.95Se/Fe_xSe_y-f vol.%（f=1,3,5）composites were successfully prepared.All samples are crystalline in low-temperatureαphase at room temperature.In Cu_1.95Se/Fe_xSe_y-f vol.%（f=1,3,5）,no second phase exists in the sample with f=1,and the second materials in the samples with f=3,5 are the ternary compound FeCuSe₂.Compared with Cu_1.95Se,the electrical properties of composites with FeCuSe₂ are poor,which is the result of FeCuSe₂reducing the carrier concentration of the samples.Except for the sample with f=1,the electronic thermal conductivity and lattice thermal conductivity of the other samples are reduced,and their thermoelectric performance at room temperature has been improved to a certain extent.The ZT values of Cu_1.95Se/Fe_xSe_y-f vol.%（f=1,3,5）are 0.07,0.33 and 0.24,respectively.Among them,the sample with f=3 has a ZT value as high as 1.63 at 600℃,which is the highest value among all samples,which is nearly 40%higher than the 1.16 of Cu_1.95Se at this temperature.