Fabrication And Performance Analysis Of Low-dimensional Nanophase Composite Telluride Thermoelectric Materials

Thermoelectric material is a kind of green functional material that have the capability of converting heat into electricity.Thermoelectric power generations or refrigeration devices,which made of such type of material,have important application prospects in environmental energy recovery,special power sources and refrigeration due to the advantages of no noise,no pollution,simple structure,etc.telluride-based thermoelectric materials?such as Bi₂Te₃,Sb₂Te₃,AgSbTe₂,etc.?is the earliest researched and one of the best performing room temperature thermoelectric materials currently.However,limited by low thermoelectric performance,which the average ZT value is still hovering around 1 for a long time,telluride-based thermoelectric materials cannot reach the level of large-scale commercial applications.Therefore,improving the comprehensive performance of telluride-based thermoelectric materials and the conversion efficiency of devices is of great significance for the large-scale application of thermoelectric materials.Related researches have shown that nanocrystallization of thermoelectric materials and the introduction of nano-second phase are effective ways to achieve thermoelectric materials microstructure control and improve their thermoelectric properties.In this paper,taking the high-quality of telluride-based?Sb₂Te₃,Bi₂Te₃,AgSbTe₂,?Bi,Sb?₂Te₃?thermoelectric materials prepared by hydrothermal method combined with spark plasma sintering?SPS?process as the research objects,high quality thermoelectric nano-powders and thermoelectric matrix materials with high performance were obtained by optimizing the hydrothermal synthesis conditions and sintering process.Based on the above,the thermoelectric properties of telluride-based thermoelectric materials is further enhanced through the introduction of nanosecond phases such as copper coated with silver nanoparticles?Cu@Ag?,one-dimensional material carbon nanotubes,two-dimensional material graphene,and the new two-dimensional material MXene.By systematically studying the effects of the composite method and the content of the second phase on the properties of the thermoelectric materials,telluride-based composite thermoelectric materials with excellent properties were obtained.Finally,thermoelectric devices was fabricated with the best performance thermoelectric composite material,and the conversion efficiency of the device was greatly improved,reaching 7.8%.The main research results and conclusions obtained in this paper are as follows:High quality Sb₂Te₃,Bi₂Te₃,?Bi,Sb?₂Te₃,and AgSbTe₂ thermoelectric nanopowders were successfully prepared by establishing the best preparation conditions though studying the influence of hydrothermal preparation process conditions on powder phase and morphology.The thermoelectric materials were obtained by spark plasma sintering?SPS?,and the effects of different sintering processes on thermoelectric properties were investigated.It is found that the preferred orientation of Sb₂Te₃ nanosheets along the pressure direction makes its bulk materials exhibit monocrystal-like electrical conductivity and polycrystalline thermal conductivity,and the ultimate ZT value is up to 1.1 in the out-of-plane direction at 525K.When the Sb₂Te₃ bulk obtained by the first sintering was mechanically broken and sintered for a second time,the obtained second sintered samples finally exhibited similar thermoelectric properties along the in-plane and out-of-plane directions due to the poor grain orientation.Using the Bi₂Te₃nano-powder prepared by the hydrothermal method as the raw material,the best thermoelectric performance was obtained when the sintering temperature is 350? and 70MPa,which ZT value is0.42 at 475K.With the same preparation method,the obtained Bi_xSb_2-xTe₃ has the best electrical transport performance when x=0.4.At the same time,owe to the orientation of the Bi_0.4Sb_1.6Te₃nanosheets in the pressure direction,the best ZT value about 0.95 was acheived at 500K in the out-of-plane direction.For the preparation of AgSbTe₂,when the sintering temperature is 340? and 50MPa,ZT value of AST is 0.6 at 400K.The stable preparation of nano-thermoelectric powders and bulk materials with high performance has laid a good foundation for the preparation and research of thermoelectric composites.The thermoelectric powders have poor dispersibility,which are easy to precipitate and separate from the second phase of nanoparticles due to the lack of macromolecular groups on the surface,causing the second phase to be unevenly dispersed in the matrix.In this study,the compound method of“ultrasonic dispersion-flash freezing-freeze drying”combined with SPS technology was adoped to successfully prepare Cu@Ag/Bi₂Te₃ and Cu@Ag/Bi_0.4Sb_1.6Te₃composite materials.It was found that Cu@Ag nanoparticles can effectively reduce the thermal conductivity by inhibit the growth of the matrix grains.In the n-type composite,the Cu atoms of Cu@Ag are more easily intercalated in Bi₂Te₃ to provide interlayer conductive channels,which improving the carrier mobility and Seebeck coefficient of the matrix.However,Cu atoms of Cu@Ag are more doped in p-type Bi_0.4Sb_1.6Te₃,which provides more carrier concentration for the matrix.In addition,it has been found that Cu@Ag in Bi₂Te₃ or Bi_0.4Sb_1.6Te₃ matrix can improve the thermoelectric performance effectively at high temperature by suppressing bipolar diffusion.Finally,when the Cu@Ag composite content is 1vol%,the thermoelectric properties of Bi₂Te₃composites are the best,which the ZT up to 0.69.Besides,the composite of Cu@Ag nanoparticles adjusts the best thermoelectric performance of Bi_0.4Sb_1.6Te₃ to the middle temperature range,which the highest ZT value about 0.95 was obtained at 475K when the Cu@Ag composite content is 1vol%.In order to solve the problem that one-dimensional two-dimensional carbon materials such as carbon nanotubes and graphene are easy to agglomerate and cannot be uniformly dispersed when they are used as the nanosecond phase composite,aqueous self-assembly composite method was proposed.CNTs/AgSbTe₂ and rGO/AgSbTe₂ composite materials were prepared by aqueous self-assembly and SPS sintering.By using the electrostatic force between the nanosecond phase and the matrix,this method successfully solves the problem of dispersing the one-dimensional/two-dimensional material in the thermoelectric matrix,and does not cause damage to the matrix and the second phase.It was found that the CNTs/AgSbTe₂ composite had the highest ZT value about 1.03 when the CNTs content is 0.5vol%,which is 43%higher than that of the matrix.And the maximum ZT of GO/AgSbTe₂ composite is 1.25 when the rGO content is0.9vol%,which is 69%higher than that of the matrix.The study also found that the uniform dispersion of rGO in the matrix effectively improved the stability of the material by slowing down the precipitation of the second phase Ag₂Te.A thin layer of high quality two-dimensional MXene material Ti₃C₂T_x was prepared by ion intercalation and delamination,which has good thermal stability within the telluride sintering temperature range.On this foundation,the homogenous dispersion of two-dimensional materials Ti₃C₂T_x in Ti₃C₂T_x/BST composites was achieved by aqueous self-assembly composite method combined with SPS process.For comparison,rGO/BST thermoelectric composites were prepared by the same method.It was found that a peak ZT of up to 1.31 and an average ZT value of 1.23from 300 to 475K are realized for the 1 vol%Ti₃C₂T_x/BST composite,which is 17%higher than the BST matrix.Comparing the effect of rGO and MXene on the thermoelectric properties of the matrix BST with the same compounding amount,it was found that the high thermal conductivity and high density defects of rGO led to a sharp increase in the thermal conductivity and a decrease in the electrical conductivity of composite.Finally,when the content of rGO and MXene all was1vol%,the thermoelectric properties of MXene composites were better than that of rGO.This work strongly demonstrates that the two-dimensional MXene material is a very effective nanocomposite second-phase material for regulating the thermoelectric properties of the matrix.The working mechanism of MXene in the matrix is that its work function will increase with the increase of the oxygen content of the surface end group.The energy band bending at the interface can be achieved by changing the oxygen content,which enhances the scattering of low-energy carriers and improves the Seebeck coefficient.In addition,MXene's high electrical conductivity can also inject holes into the matrix at the hetero-interface to increase the electrical conductivity of the matrix.Moreover,the alignment of Ti₃C₂T_x with the layered structure significantly suppresses the phonon transport,resulting in lower lattice thermal conductivity.Thus realizing the comprehensive regulation of thermal and electrical transport,and improving the thermoelectric properties.Using unannealed MXene/BST as p-type terminal and commercial Bi₂Te_2.7Se_0.3 as n-type terminal,a record-high thermoelectric conversion efficiency of up to 7.8%is obtained under a temperature gradient of 237K with rational device design.In this study,high-quality telluride-based thermoelectric matrix was successfully prepared by hydrothermal method combined with the SPS process.Through the introduction of nano-second phases such as nano particles of silver coated copper?Cu@Ag?,one-dimensional material carbon nanotubes,two-dimensional material graphene and new two-dimensional material MXene,a high-performance telluride-based thermoelectric material is obtained,and the performance of the prepared thermoelectric device is also greatly improved,which is expected to promote the further large-scale commercial application of thermoelectric materials.In addition,the methods of“Ultrasonic dispersion-flash freeze-freeze drying”composite method for the dispersion of nanoparticle phase and the aqueous self-assembly composite method aims at the dispersion of one dimensional and two dimensional nano-phases are also proposed,which can provide a new idea for the preparation and development of other composite thermoelectric materials.