Thermoelectric Performance Of P-Type PbTe-Based Compounds And Polycrystalline SnSe Synthesized Under High Pressure

With the acceleration of the industrialization process,people are enjoying the convenience while facing the increasingly serious energy crisis and environmental problems.Therefore,the development of new energy materials has been paid more and more attention.Thermoelectric materials,which can transform the heat and electricity directly and vice verse,are a kind of promising energy materials.PbTe-based thermoelectric materials have a long research history,and have been widely applied in the fields of aronautics,astronautics,military and so on.SnSe is a new type of thermoelectric materials,which has been paid much attention since it was discovered.During fabrication of the two materials,introduction of pressure can effectively regulate the reaction equilibrium,shorten the reaction time and optimize the carrier concentration.In this dissertation,PbTe and SnSe thermoelectric materials were prepared by means of high pressure synthesis?HPS?,and their thermoelectric properties were investigated.The high pressure synthesis procedure of pure phase PbTe was explored,and the thermoelectric properties of the pure phase PbTe with different synthetic pressure were analyzed and tested.It is found that the pure phase PbTe can be synthesized under high pressure when the ratio of Pb/Te is 1:1.2 and the synthesis temperature is no less than1073 K.The HPS pure phase PbTe samples present an inferior thermoelectric performance.The ZT value of the sample synthesized at 2 GPa is less than 0.2 at 420 K,which is attributed to the poor electrical transport properties resulting from low carrier concentration due to undoping.Na doped PbTe samples were synthesized with high pressure synthesis combining with SPS method.The structure,chemical composition,microstructure and thermoelectric properties of the samples were analyzed and studied.The implementation of high pressure effectively improves the solid solubility of Na in PbTe materials,and greatly increases the carrier concentration.The actual Na doping amount of Na_0.03Pb_0.97Te?raw material ratio?is 0.027,the carrier concentration is 3.2×10²⁰ cm^-3,and the electrical transport performance is obviously improved.Doping Na atoms introduce a large number of point defects to enhance the scattering of heat transport phonons.The microstructure analysis shows that a large number of dislocations and lattice distortion exist in the Na_0.03Pb_0.97Te samples synthesized under high pressure,which enhances the scattering of the medium frequency phonon and further reduces the thermal conductivity of the material.Lattice thermal conductivity of the sample is reduced to 0.67 W/mK above 700 K.The ZT value of Na_0.03Pb_0.97Te sample synthesized at high pressure is increased to 1.7 at 740 K.Na doped PbTe-SrTe samples were synthesized with high pressure synthesis combining with SPS.The structure,chemical composition and thermoelectric properties of the samples were analyzed.The carrier concentration of Na_0.03Sr_0.04Pb_0.93Te sample reaches 2.2×10²⁰ cm^-3,which is higher than those synthesized at ambient pressure.Obvious Na-rich areas exist in PbTe-SrTe system and the enhancd diffusion of Na at high temperature makes power factor stable and not attenuate.The highest power factor of Na_0.03Sr_0.04Pb_0.93Te sample is 3100?Wm^-1K^-2 at 620 K.Increased solid solubility of Na brings more point defects.Meanwhile,a special cellular texture forms in the sample.Both of them scatter the phonons effectively and reduce the lattice thermal conductivity,Na_0.03Sr_0.04Pb_0.93Te sample has the lowest lattice thermal conductivity of 0.56 W/mK at770 K.With synergistic optimization of electrical and thermal properties,the highest ZT value of 2.1 at 870 K is achieved in the sample.The high pressure synthesis procedure of polycrystalline SnSe was explored.Polycrystalline Na-doped SnSe bulks were produced with high pressure synthesis combining with SPS.The structure,chemical composition and thermoelectric properties of the samples were investigated,and the relation of orientation on the thermoelectric properties of the samples was also discussed.Na doping obviously improves the electrical transport properties of the samples.The point defects introduced by Na doping significantly decrease the lattice thermal conductivity.The ZT value of Na_0.02Sn_0.98Se samples reaches 0.87 at 798 K,and the average ZT value also reaches 0.31.The SPS densified polycrystalline Na_xSn_1-xSe bulks reveal obvious orientation,and present better thermoelectric properties in the direction perpendicular to the SPS compressing direction.