Synthesis Of Two Selenium-based Compounds Of Tin And Lead And Their Thermoelectric Properties

Thermoelectric materials,as an environmentally friendly,clean,pollution-free new energy material,are the best choice to replace traditional energy materials.The material can directly convert thermoelectricity,and its efficiency is measured by the optimal value of thermoelectricity.Its research and development are aimed at solving problems such as effective use of energy and environmental pollution.It is a new type of energy material that can directly convert thermoelectricity and whose efficiency is measured by the optimal value of thermoelectricity.This thesis mainly focused on the synthesis of two selenides:Pb Se and Sn Se₂.The morphology and structure of the synthesized selenide were characterized;then the thermoelectric properties of the selenide were improved by doping or compounding.The results mainly include the following aspects:Ⅰ.Preparation of In-doped Pb Se and its thermoelectric propertiesThe effects of a series of different concentrations of In element doping on the electrical conductivity,Seebeck and thermal conductivity of cubic phase Pb Se were studied.The results show that the thermal conductivity of the Pb Se sample is reduced by the In element due to doping in different proportions,the Seebeck coefficient of the 5%In-doped sample is improved,and the electrical conductivity of the sample is reduced by the doping.Finally,the highest ZT value of the pure phase is about 0.16.Ⅱ.Preparation and thermoelectric properties of Pb Se-Ag nanocompositeFirst,sea urchin-like Pb Se nanoparticles are synthesized,and then self-synthesized Ag nanoparticles are compounded in order to improve the electrical conductivity,reduce the thermal conductivity of the sample,and enhance the thermoelectric performance finaly.The introduction of Ag into the Pb Se matrix will produce two phenomena:（i）partial substitution of Pb by Ag and（ii）formation of a secondary phase:Ag₂Se.The electrical conductivity of the Pb Se-Ag nanocomposite is improved highly due to the contribution of Ag hole carriers.At the same time,the conduction behavior is changed from the p-n to p-type.Pb Se-Ag nanocomposites are also found with reduced lattice thermal conductivity due to effective phonon at grain boundaries,secondary phase,and point defect.The research results shows that the best ZT value of the Pb Se-5 wt%Ag nanocomposite reachs 0.97,which is 8.8 times higher than the pure phase Pb Se at 723 K.Ⅲ.Preparation and thermoelectric properties of Sn Se₂nanosheet composite graphiteUniform SnSe₂nanosheets were successfully synthesezed using Schlenk-Line method at temperature of 40℃;the composition,morphology,phase and structure of sample were characterized by X-ray diffraction（XRD）,scanning electron microscope（SEM）and transmission electron microscope（TEM）and other means.Sn Se₂nanosheets present a layered structure,and the thermoelectric properties of the material may have anisotropy.The thermoelectric parameters of sample in perpendicular and parallel directions were tested,and the electrical conductivity of the material was improved by compounding an appropriate amount of expanded graphite.The results show that the electrical conductivity of the composite sample was improved,and the Sn Se₂composite sample with 5 wt%G has the highest electrical conductivity.After performance testing,the thermoelectric performance in the parallel direction has been greatly improved.At 723 K,the ZT value had increased from0.11 for the pure phase to highest 0.26 for the composite 3 wt%G in parallel direction.