High Pressure Synthesis And Thermoelectric Performance Of CoSb₃-based And Bi₂Te₃-based Compounds

With the continuous development of human society,the demand for energy is growing with each passing day.Traditional fossil energy is faced with many crises,such as resource exhaustion and environmental destruction,so it is very important to improve its efficiency.Thermoelectric material is a kind of functional material which can realize the conversion of heat energy and electric energy directly.Thermoelectric devices based on the Seebeck effect can use waste heat to achieve thermoelectric generation and improve energy utilization,while those based on Peltier effect can realize electroheating or cooling.Two kinds of thermoelectric materials will be selected to carry out research.The elemental filled skutterudites,with the characteristics of phonon-glass electron-crystal(PGEC),is a kind of important thermoelectric materials in intermediate temperature region due to its low cost,easy synthesis and excellent thermoelectric properties.Bi₂Te₃-based compounds with layered crystal structure are the only room temperature thermoelectric material to achieve commercial scale application at present.In this dissertation,the elemental filled CoSb₃-based compounds were prepared by a high pressure synthesis method,and their microstructure and thermoelectric properties were characterized and analyzed.The regulation of different types of filling elements on the thermoelectric properties of CoSb₃-based compounds was studied.The Bi Sb Te alloys were prepared by high pressure synthesis,and then combining with ball milling,spark plasma sintering and other experimental methods to regulate the microstructure of the bulks so as to synthesize porous Bi Sb Te thermoelectric bulks.The microstructure and thermoelectric properties were characterized,and the influence of microstructure on its thermoelectric and mechanical properties was investigated.p-Type Ca-filled Co-rich Ca_xFe_1.3Co_2.7Sb₁₂ skutterudites were synthesized by a high pressure synthesis method followed by spark plasma sintering.The structure,composition and thermoelectric properties were investigated.All samples possess a dominant skutterudite structure of Im3 symmetry.In this skutterudite materials,Fe replaces Coto introduce holes,and all the samples show p-type conductivity.While the introduction of Ca can provide electrons to further regulate the hole concentration,and achieve fine control of the carrier concentration,so as to optimize the electrical transport properties of the samples.Besides,Fe substitution at the lattice point and Ca filling at the cage vacancy introduce additional phonon scattering mechanisms(alloy scattering and"rattling effect",etc.),which effectively reduce the thermal conductivity of the materials.Under the combined action of these two aspects,the ZT of Ca_0.6Fe_1.3Co_2.7Sb₁₂ sample reaches 0.70 at 820 K,and the average ZT of the whole temperature range is 0.4.n-Type S-filled Te-substituted S_xCo₄Sb_12-yTe_y skutterudites were synthesized by high pressure synthesis combining with spark plasma sintering.The structure,chemical composition,and thermoelectric properties were analyzed and studied.All samples were dominated by a skutterudite structure of Im3 symmetry.Te replaces Sb to produce electrons,while the high electronegativity element S,which is filled into the cage of skutterudite structure,attracts electrons and reduces the carrier concentration.By changing the content of Te and S,the electron concentration of the material can be effectively controlled and the electrical transport performance can be improved.Furthermore,the substitution of Sb by Te leads to point defects,and the local disordered oscillation caused by the filling of S atom into the cage vacancy effectively reduces the thermal conductivity.The optimal thermoelectric properties are achieved in S_0.2Co₄Sb_11.2Te_0.8 sample,with lattice thermal conductivity as low as 0.60 Wm^-1K^-1,figure of merit as high as 1.6 at 823 K,and an average ZT of 1.0 within the temperature region of 298–823 K.Porous Bi_xSb_2-xTe₃ alloys were synthesized from sequential high pressure synthesis,ball milling,and spark plasma sintering.The structure,thermoelectric properties and mechanical properties of porous Bi_xSb_2-xTe₃ samples were investigated.The porosity of the samples is as high as 0.15,and the microstructure of porous Bi_xSb_2-xTe₃ samples is distinctly different from that of dense samples.The porous Bi_xSb_2-xTe₃ bulks are composed of micron to submicron size grains,which exhibit obvious characteristics of equiaxed grain.The grain boundaries are tightly bonded with no visible cracks,and a large number of twins and dislocations are found in the grains.Unique microstructure endows the bulks with excellent thermoelectric and mechanical properties.The average thermoelectric figure of merit of the samples is more than 1.0 in the temperature range of 298–463 K.The optimal Bi_0.42Sb_1.58Te₃sample reaches the highest ZT of 1.27 at 350 K while the average ZT of 1.15 is achieved in the temperature range of 298–463 K,which is competitive with the best dense bulks.The mechanical properties of the porous bulks are also significantly improved,thanks to micron-sized,nearly equiaxed grains and tightly bonded grain boundaries.The Vickers hardness and compressive strength of Bi_0.42Sb_1.58Te₃ sample are 0.56 GPa and 130 MPa,which is even better than that of the dense sample.The Bi_xSb_2-xTe₃ alloys were synthesized by high pressure synthesis,melting and mechanical alloying,and then the Bi_xSb_2-xTe₃ bulks with different microstructures were prepared by ball milling and spark plasma sintering.The formation path of porous structure of Bi_xSb_2-xTe₃ bulks was determined,and the relationship between the microstructure of Bi_xSb_2-xTe₃ bulks and the thermoelectric and mechanical properties was analyzed.Micron powders agglomerated by a large number of randomly oriented nanocrystals(grain size about 10 nm)were obtained by ball milling.After adjusting the SPS sintering conditions,Bi Sb Te bulks with different porosity were synthesized.The micron-sized grains in the bulks showed obvious equiaxed grain characteristics.It is found that the mechanical properties are closely related to the porosity and grain size of the sample,i.e.,the lower the porosity and the smaller the grain size,the better the mechanical properties.The Vickers hardness of the sample sintered at 50 MPa is more than 1 GPa(4.9 N load)and the compressive strength is as high as 234 MPa.The thermoelectric properties of this sample are also excellent,whose maximal ZT and the average ZT in the 298–463 K temperature range are 1.20 and 1.09,respectively.