Research Of Thermoelectric And Mechanical Properties Of SnSe-based Thermoelectric Materials

Thermoelectric is a new type of energy conversion technology,which can enable the direct conversion between thermal and electrical energy without any pollution,noise,moving part,and has been extensively investigated over the past decades.The efficiency of the thermoelectric material is characterized by the dimensionless figure of merit ZT,ZT=S²?T/?,in which S,?,?and T are the Seebeck coefficient,electrical conductivity,thermal conductivity,and absolute temperature,respectively.SnSe is a medium-temperature semiconductor thermoelectric material with good thermal stability,abundant and eco-friendly raw materials.Due to its special crystal and band structures,the thermoelectric figure of merit?ZT?of single crystal SnSe reaches a maximum value of 2.62 at 923 K,which is the best thermoelectric performance for bulk thermoelectric materials.However,due to the easy cleavage along b-c plane direction,low mechanical strength and low yield,SnSe single crystals are difficult to be employed in practical thermoelectric devices.Besides,the techniques used for growing single crystals are also limited for industrial scale-up.Therefore,the preparations of polycrystalline SnSe materials with high performance have become the hotspots and key points in the field of thermoelectricity.In this thesis,the thermoelectric properties and mechanical properties of polycrystalline SnSe materials prepared by high temperature and high pressure sintering?HPS?are studied.The main research contents and results are as follows:?1?In this paper,the synthesis conditions of polycrystalline SnSe by melting are firstly explored and optimized.The dense polycrystalline SnSe bulks were successfully consolidated by HPS technology.The working pressure keeps 2.5 GPa,the holding time is 7 min.The effect of sintering temperature on the thermoelectric properties of polycrystalline SnSe was studied by designing different sintering temperatures.The maximum ZT 0.45 was obtained at the sintering temperature of623 K.?2?K₂Ti₆O₁₃ whiskers were introduced into the polycrystalline SnSe matrix by the best HPS process.The evenly dispersed whisker SnSe+x vol%K2Ti6O13?x=0,1,3,5?samples were prepared by ultrasonic dispersion and ball milling processes before sintering.The ZT values of the composites decrease with the increase of the K₂Ti₆O₁₃ whisker volume fraction.It is found that the K₂Ti₆O₁₃ whiskers dispersed almost evenly and most were embedded in the matrix,as shown in the fracture scanning electron micrographs.The introduction of K₂Ti₆O₁₃ whiskers significantly enhanced the mechanical properties of the materials.Compared with the K₂Ti₆O₁₃whiskers-free samples,the Vickers hardness,flexural strength,compressive strength,and fracture toughness of the composites with 5 vol%K2Ti6O13 are improved by31%,98%,86%,and 71%,respectively.?3?A series of SnSe+x vol%K₂Ti₆O₁₃?x=0,1,3,5?samples were prepared by melting and spark plasma sintering?SPS?.The effects of the introduction of K₂Ti₆O₁₃ whiskers on their thermoelectric properties and mechanical properties were investigated.With the increase of K₂Ti₆O₁₃,the thermoelectric performance of composites decreases slightly;the mechanical properties increases initially,and reaches a maximum value at a K₂Ti₆O₁₃ content of 3 vol%,and decreases thereafter.Compared to the sample with x=0,the Vickers hardness of the sample with x=3increased by 33%,the flexural strength increased by 20%,the compressive strength increased by 39%,and the fracture toughness increased by 22%.