Effects of Melt State and Subsequent Processing on Microstructure and Properties of (Bi,Sb)2(Te,Se)3 Thermoelectric Materials

Author:Wang Xiao Yu

Supervisor:zu fang zuo


Degree Year:2019





Due to the two crises of energy and environment,thermoelectric(TE)materials that can realize thermal-electrical mutual conversion have become one of the hotspots of national development strategies.In recent years,the basic theory of thermoelectrics and the preparation methods have made a lot of progress,but there are still many optimized spaces due to the unsatisfactory TE performance.Previous studies have found that by controlling the melt state,the solidification behavior,the microstructures and defects in the as-cast ingot can be optimized,hence the TE properties can be improved.Further more,after ball milling+SPS sintering,the properties of the material can be enhanced again.However,the laws and mechanisms involved in these processes are still unclear.In order to further understand the generation of microstructures and defects in materials,the evolution in subsequent processing and the variation of properties,we took(Bi,Sb)2(Te,Se)3 alloys as the example to explore the rules and mechanisms of preparation factors such as melting,ball milling and subsequent processing from multiple aspects.The contents include:the effects of melting process,melt state,solidification rate,particles size and heterogeneous PN junctions on the microstructure and properties of as-cast ingots or as-sintered bulks were explored.Furthermore,the evolution regularity of microstructures in subsequent processes was also investigated.The main conclusions obtained are as follows:(1)The changes of solidification behavior,microstructure and properties are regular for P-type Bi0.3Sb1.7Te3 alloys prepared with different melting processes.With the prolongation of melting time and the increase of melting temperature,the solidification structure is refined,and higher density nanocrystals,lattice distortion regions,dislocations and antistructure defects are produced in the matrix.Therefore,a significantly improved electrical conductivity and maintaining Seebeck coefficient are obtained.Furthermore,the lattice thermal conductivity of the material are decreased,because the phonons scattering is enhanced by the increased concentration of the microstructures and defects.As a result,sample B3a with the melt state treatment and melting for 4 h has the best TE properties,which is 14.3%higher than that of A1a without melt treatment and melting for 2 h.In addition,the Vickers hardness of B3a is19.2%higher than that of A1a,due to the increase of microstructures and defects.(2)The as-cast ingots were obtained by solidifying in air,oil and water.The effects of different cooling rates and melt states on the structure and properties of Bi0.3Sb1.7Te3bulking prepared by ball milling+SPS sintering were discussed.The results show that increasing the solidification cooling rate combined with the melt treatment can effectively increase the concentration of the antistructure defect in the material and reduce the interplanar spacing of(015)planes the powders and the as-sintered bulks.Besides,the concentration and effective mass of carriers increase,thus the electrical properties at room temperature are improved.In addition,since the increased microstructures and defects concentration enhance the phonon scattering,the lattice thermal conductivity is significantly reduced.Compared to the sample without melt state treatment,the TE and mechanical properties of the as-sintered sample with the melt treatment and the same cooling rate were significantly improved.(3)The effect of melt treatment on the microstructure of as-cast N-type binary Bi2Te3 alloy and the evolution during subsequent processes were investigated.The results show that a large number of small-scale nanocrystals and high density dislocations and lattice distortion regions exist in the ball-milled powder of BT-LSM sample with the melt treatment.During the sintering process,the small-scale nanocrystals will grow into a large-scale ones.As cores of nucleation,the high-density dislocations and lattice distortions will grow into small-scale nanocrystals.Consequently,a large number of grain boundaries,high density nanograins and dislocations were observed in the matrix of as-sintered BT-LSM bulk.Moreover,the sample of BT-LSM has a higher carrier concentration,a larger effective mass and a wider band gap,leading to an improved electrical properties.Meanwhile,phonons are also strongly scattered by multi-scale microstructures,resulting in an reduced thermal conductivity.Hence,ZT of sample BT-LSM is significantly improved with 22%compared to the one without melt state treatment.(4)By adjusting the particle size of as-prepared powders,the changes of structure orientation,microstructure for the as-sintered bulks were investigated,and the change of the resulting properties of the as-sintered bulks were also discussed.It is found that with the increase of mechanical deformation degree in preparing powders process,the particle size of powders and grain size for the as-sintered bulks are refined,and the orientation of the material along(00l)direction is weakened in the direction of vertical SPS sintering pressure.More interestingly,the donor-like effect is enhanced in the bulks,leading to an decreased carrier concentration in P-type alloys and an increased concentration in N-type ones.In addition,the weak orientation,reduced grain size and increased density grain boundaries and other crystal structure defects enhance the scattering of carriers and phonons,resulting in the decrease of carrier mobility and lattice thermal conductivity.Furthermore,the mechanical properties of the bulks are also optimized.As a result,by adjusting the degree of mechanical deformation in the powders preparing process,the microstructure of the as-sintered bulk can be effectively adjusted and the TE and mechanical properties can be modified simultaneously.(5)With theoretical analysis,the TE properties of the alloy with high intrinsic carrier concentration can be optimized by PN junctions.In this work,we constructed PN junctions by mixing P-type Sb2Te3 and N-type Bi2Te3 alloys and investigated the effect of PN junctions on the microstructures and electrical and thermal transport properties.The results show that with the content of PN junctions increasing,the microstructure of the as-sintered bulks is refined,the carrier concentration and mobility decreases.Hence,the electrical conductivity and thermal conductivity decrease,simultaneously.When the content of N-type Bi2Te3 is less than 0.20,there is only hole transport channel in the material.Due to the enhanced energy filtering effect and the decrease of carrier concentration,Seebeck coefficient of the bulks increases.However,Seebeck coefficient for the sample with Bi2Te3 content of 0.25 is seriously deteriorated,due to the formed electron and hole independent transport channels.Therefore,when the content of Bi2Te3 is 0.15,the as-sintered bulk has a 131%increase of ZTmax1.03,compared with pure Sb2Te3.Meanwhile,the mechanical properties of the bulks increase with the increase of the content of PN junctions.The Vickers hardness of samples with Bi2Te3 of 25%shows a higher value of 90.29 HV.These results do indicate that the proper introduction of PN junctions can optimize the microstructure and properties of the materials,which opens a new way to improve the TE properties of the materials.In this work,by adjusting the melt state,melting process,solidification rate,the size of powders and introducing PN junctions,the formation and the evolution in the subsequent process of microstructures and defects for(Bi,Sb)2(Te,Se)3 alloys were detailed explored.Besides,the effects of microstructures on the electrical,thermal and mechanical properties were analyzed.It provides a detailed technical basis and meaningful academic reference for the development and actual production of TE materials.