Carrier Regulation And Thermoelectric Performance Optimization Of N-type Bismuth Telluride Based Materials Under High Pressure Synthesis

Thermoelectric material is a kind of functional material which can realize the direct conversion of heat energy and electric energy by using the motion of carrier and phonon inside solid.It has a wide application prospect in the field of clean energy and refrigeration.Bi₂Te₃-based al oy is the best thermoelectric material at room temperature,which has attracted much attention.In recent years,the thermoelectric properties of p-type Bi₂Te₃-based thermoelectric materials have been greatly improved,while that of n-type Bi₂Te₃-based thermoelectric materials has been less improved.As a result,the conversion efficiency of the thermoelectric devices composed of p and n legs is difficult to be significantly improved,which restricts the large-scale application of thermoelectric devices.How to effectively improve the thermoelectric properties of n-type Bi₂Te₃-based thermoelectric materials has become the focus of this dissertation.In this dissertation,by optimizing the composition of the al oy and changing the synthesis process,the melting method and high pressure synthesis method combined with SPS technology were adopted respectively.And it was compared between the corresponding n-type Bi₂Te₃ based al oy synthesized by melting method and high pressure synthesis.Bi₂Te₃ materials were al oyed with Se to prepare Bi₂Te_3-xSe_x samples.The influence of different Se content on Bi₂Te_3-xSe_xsamples under the two synthesis methods was studied.On the other hand,the donor dopant SbI₃ and the acceptor dopant Pb Te were used to doping the n-type Bi Te Se al oys,and the effects of SbI₃ and Pb Te dopants on the matrix properties were studied.The specific work is as follows:(1)n-Type Bi₂Te_3-xSe_x al oys were synthesized by melting method and high pressure synthesis method,combined with spark plasma sintering(SPS)technology.The phase structure and microstructure of the samples were characterized by XRD,SEM and TEM.The thermal and electrical transport characteristics of the samples were studied and analyzed by measuring the resistivity,Seebeck coefficient,thermal conductivity and Hall coefficient of the samples.This study shows that compared with Bi₂Te_3-xSe_x,the lattice thermal conductivity of Bi₂Te₃ is reduced due to the scattering of the al oy.In the two different synthesis methods,Se doping solution has different regulation rules on the carrier concentration.The carrier concentration of high pressure synthesis samples increases with the increase of Se content,while the carrier concentration of melt synthesis samples decreases.And when Se content x=0.3,the relative optimal carrier concentration of both samples can be reached.The maximum ZTs reach 0.65 at 383 K by melting method and0.47 at 403 K by high pressure synthesis method,which is 80.6%and 147.4%higher than that of pure Bi₂Te₃,respectively.(2)SbI₃ doped Bi₂Te_2.7Se_0.3 al oys were successful y synthesized by melting method and high pressure synthesis method,combined with SPS technology.As the donor doping,SbI₃ increases the carrier concentration of the materials,the power factor of the material is improved with appropriate doping level.Due to the enhancement of phonon scattering by heteroion I^-and carriers,the lattice thermal conductivity is reduced effectively.The maximum ZT of the melting sample with 0.10 wt.%doping is 0.75 at 423 K,the average ZT is 0.67;and the the high pressure synthesized sample doped with 0.03 wt.%SbI₃ is 0.60 at463 K,the average ZT is 0.55.Compared with the melting sample,the high pressure synthesized sample can cause a greater change in the thermoelectric properties with small amount of SbI₃ doping,and the effect on improving the properties of the thermoelectric materials is more obvious.(3)The high pressure synthesis method combined with SPS technology was used to synthesize Pb Te doping of Bi₂Te_2.6Se_0.4 al oys.As the acceptor doping,Pb Te can optimize the carrier concentration of the materials and improve the power factor.The maximum power factor of 0.10 wt.%doping sample is 2.39 m Wm^-1K^-2 at 343 K.Meanwhile,the thermal conductivity decreases due to the enhancement of the scattering effect.Finally the maximum ZT of the sample doped with 0.10 wt.%Pb Te is 0.81 at 423 K,which is 37.3%higher than that of the undoped sample at the same temperature,and the average ZT is 0.72,35.8%higher than that of the undoped sample.