Exploring New Methods To Improve Thermoelectric Performances Of N-type Bi-Te-Se Semiconductors Based On The Melt State

Thermoelectric(TE)materials,which can directly convert heat into electricity or vice versa,has been regarded as one of the best candidates for meeting energy crisis and environment pollution.However,the low energy conversion efficiency for TE materials limits its application.Generally,the conversion efficiency is governed by the dimensionless figure of merit,ZT = S2?6T/?,where S,?,?and T are the Seebeck coefficient,electrical conductivity,thermal conductivity and absolute temperature,respectively.Bi2Te3-based alloys are one of the best TE materials for commercial application near room temperature.Currently,commercial Bi2Te3-based ingots are fabricated by direction-solidification techniques,such as zone melting,Bridgman and Czochralski,its ZT is about 1.0 and efficiency is lower than 10%,which is apparently not competitive with other energy conversion techniques.Therefore,i2 is especially important to improve ZT for developing Bi2Te3-based TE materials.Rencently,nanostructure techniques has been reported to successfully improve the TE properties of P-type Bi-Sb-Te alloy,but it even deteriorates the performance of N-type Bi-Te-Se alloys.As well known,a TE device is consist of P-and N-type materials,its efficiency is governed by both of the TE performance of P-and N-type materials.For Bi2Te3-based alloys,the performance of P-type Bi-Sb-Te alloys is much higher than the N-type one(Bi-Te-Se alloys),which seriously limits the application of TE devices.Therefore,it is important to impvove the TE properties of N-type Bi-Te-Se alloys.The main reason of the low property for N-type Bi-Te-Se alloys is the carrier and phonon transport are much sensitive to the preferred orientation,which could simutanelously increase electrical and thermal conductivity.If we can retain preferred orientation but decrease thermal conductivity,a huge enhancement of ZT for N-type Bi-Te-Se alloys is expected.Our previous works about the relevance of the liquid state and solidification structures suggest that the transition of liquid states would result in the change solidification structructure and thus alter thermal conductivity.Based on this,this work explored the behavior of liquid state change in N-type Bi-Te-Se based alloys and then designed the experiments to optimize the thermoelectric performance based on the liquid state manipulation and other post-treatment process,such as direction-soilification and ball milling combined with spark plasma sintering.Whereby we have found:(1)The best KI dopant content for free solidified N-type Bi-Te-Se alloys is 0.3w.t%.Based on this,increasing Se content could suppress cationic anti-defects and enhance anion vacancy,which leads an increased electron concentration.As a result,the improved ? and decreased Seebeck is obtained with the increasing Se content.Meanwhile,the ? is also decreased with the increasing Se content due to the enhanced phonon scattering.Finially,Bi2Te2.7Se0.3 and Bi2Te2 4Se0.6 alloy has the highest ZT,which demonstrate the best composition of N-type Bi-Te-Se alloys has changed to Bi2Te2.7Se0.3 and Bi2Te2.4Se0.6.(2)The curves of temperature-dependent liquid resistivity of Bi2Te2.7Se0.3 and Bi2Te2.4Se0.6 alloys show abnormal behaviors during the first heating process,i.e.there is a "hump" within 1075-1201K and1103-1185K for i2Te2.7Se0.3 and Bi2Te2.4Se0.6,respectively.These abnormal behaviors suggest a t emperature-induced liquid-liquid structural transition(TI-LLST).(3)Based on the investigated abnormal temperature regime,the raw materials were smelted at temperatures below and above TI-LLST,and then the melts were cooled down at the same temperature.Subsequently,the solidification curves were recorded and analyzed,the microstructure was observed and TE performances were tested.The results show that,compared with the alloy melted at temperatures below TI-LLST(without liquid state manipulation(LSM)),the alloys with LSM exhibits a lower solidification temperature and a larger nucleation rate,which result in a much refined grains and more Te-rich phases.Moreover,hierarchical crystal defects are obviously increased,with tinier nanoprecipitates,more subgrains and boundaries,and much more densified lattice distortions in the alloy with LSM.Such microstructures would scatter more phonons and result in a low ?.Meanwhile the electrical properties were also improved due to the increased Te-rich phases and the change of carrier scattering mechanism.All these alterations contribute to a synergistic enhancement of ZT,which is 100%higher than the alloy without LSM and comparable to the value of commercialized ingot prepared by the time-consuming zone melting method.(4)We have proved that the LSM could improve the ZT of free solidified Bi2Te2.7Se0.3 and Bi2Te2.4Se0.6 alloys to the comparable value of commercialized zone melting ingot.But to further enhance ZT,we combined LSM with large temperature gradient direction solidification techniques to fabricate Bi2Te2.7Se0.3 and Bi2Te2.4Se0.6 polycrastalline alloys.The results show that decreasing growth rate could easily enhance preferred orientation and thus increasing electrical properties.LSM could retain preferred orientation and high electrical properties but induce a high density of lattice defects,such as dislocations and lattice distorsions,which would decrease ?.As a result,the Bi2Te2.4Se0.6 alloy with LSM has a high peak ZT of 1.22 and average ZT of 1.09,which reaches the highest level of N-type Bi-Te-Se alloys.(5)We also investigated the influence of LSM and BM+SPS on TE performance for Bi2Te2.4Se0.6 alloys.The results show that a)the increased BM speed could not significantly influence the preferred orientation of Bi2Te2.4Se0.6 polycrastalline alloys,but the "donor like effect"is enhanced,results in an increased ? and decreased S.Meanwhile,the ? is also decreased due to the strengthed phonon scattering with the increasing BM speed.As a result,the sample with BM speed of 500rpm has the largest ZT of 0.85,after LSM the ZT is further enhanced to 0.96.b)With increasing SPS temperature,the grains grow along certain directions,which lead to an enhanced preferred orientation,but phonon scattering is also weakened.Moreover,the increased SPS temperature could recover point defects and suppress "donor like effect" thus optimizing carrier concentration.Finally,with increasing SPS temperature,the electrical property and thermal conductivity are simutanelously enhanced,and the sample with SPS temperature of 450? has the largest ZT of 0.85.However,based on the retained or even little increased electrical property,LSM could significantly decreased ?,results in an improved peak ZT of 1.12 and average ZT of 1.02,This work implies the existence of temperature-induce liquid-liquid structural transition in N-type Bi-Te-Se based alloys well above their liquidus.The solidification behaviors and solidified microstructures are strongly related to their parent liquid states.The corresponding thermoelectric properties can be also improved by the liquid state manipulation method or in conjunction with direction solidification or ball milling combined with spark plasma sintering techniques.Our work provides an alternative approach to tailor the thermoelectric properties and our method is also expected to be applied in other anisotropic TE materials,such as P-type Bi-Sb-Te alloys or SnSe polycrastalline alloys.