Study On Band Structure Modification And Performance Optimization Of SnTe-based Thermoelectric Materials

With the prosperous development of global economy,the increasing energy crunch and the environmental pollution are the significant challenges around the world.It is an urgent problem for us to develop new,green and environmentally friendly renewable energy conversion materials and technologies.Thermoelectric material as one of the emerging energy conversion materials has great potential in power generation and solid-state refrigeration due to its direct inter-conversion between heat and electricity.In many high thermoelectric performance materials,SnTe as a lead-free analog of PbTe has same crystal structure and similar two-valley band structure,which is an excellent candidate for replacing PbTe to achieve high thermoelectric performance.However,the high intrinsic hole concentration,the large energy offset between L and ∑ band and the high lattice thermal conductivity of SnTe have resulted in relatively low thermoelectric performance.Hence,improving the thermoelectric performance of SnTe has become a hot research topic.Based on the above problem,several SnTe-based thermoelectric materials with different dopants and additives are prepared by vacuum melting combining with spark plasma sintering and high-pressure sintering.A series of strategies containing band structure modification and defect engineering are used to optimize the thermoelectric performance.The results are as follows:(1)Aiming at the failure of the resonance effect at elevated temperature and the scarcity of effective resonance dopants,the trigger condition of achieving the resonance effect in SnTe is investigated in depth.It is found that the achievement of the resonance effect is intensively related to the relative energy positions among the impurity state level,host state level and Fermi level.Only adjacent energy positions among them can trigger the resonance effect and thus distort density-ofstates(DOS)to improve the Seebeck coefficient.This trigger condition combining with the Cotton’s diagram is conductive to fleetly select a series of resonance dopants suitable for thermoelectric materials by means of the atomic energy levels.For example,the resonance dopants suitable for SnTe are selected as Y,Ru,In,Sb,La,Gd,Lu,Os,T1,and Bi.In order to check the selected resonance dopants,the resonance effect introduced by Bi in SnTe is investigated.The results show that Bi doping induces the impurity levels beneficial to achieving the resonance effects at high temperature,which is ascribed to the Fermi level at high temperature activates the resonance levels.Additionally,Bi doping reduces the lattice thermal conductivity by means of introducing point defects.As a result,a maximum ZT value of～1.23 is obtained at 873 K in the Sn0.94Bi0.04Te sample,which is～120%higher than the pristine SnTe.This work helps fleetly select the effective resonance dopants of thermoelectric materials and achieve the widespread resonance effect in semiconductors.(2)Based on the composition of Sn0.94Bi0.04Te,the effect of both Mn doping and the CdTe/Cu2Te additions on the thermoelectric performance is studied.The result reveals that the effect of dopants and additions are dependent.The subsequent Mn doping not only weakens the resonance effect but also introduces an obvious bipolar effect,which is harmful to improve the electrical and thermal transport properties.As a result,only an unimpressive ZT value of～0.76 is obtained in the Sn0.86Bi0.04Mn0.08Te sample at 873 K.Furthermore,it is found both CdTe and Cu2Te introduce precipitates(Cu4Mn2Te4 and Cd0.36Mn0.64Te)that are conducive not only to enhancing the phonon scattering and suppressing the bipolar effect,but also to realizing a fine regulation of carrier concentration and carrier mobility.As a result,a high ZT value of～1.21 is achieved at 873 K in the Sn0.86Bi0.04Mn0.08Te-3 at%Cu2Te sample,which is～116%higher than the pristine SnTe.This work demonstrates that although the effect of dopants and additives is coupled each other,it is still a useful route to optimize the thermoelectric performance of SnTe.(3)The halogen Cl doping SnTe activates the band sharpening effect,which helps alleviate the contradictory interrelationship between the Seebeck coefficient and the carrier mobility,and improve the thermoelectric performance.Benefitting from the band sharpening effect,Cl doping sharpens the light hole band and then reduces the effective mass from 0.168 me to 0.06 me,which promotes ultra-high carrier mobilities(over 2000 cm2V-1s-1)and thus improves electrical conductivities effectively.Based on the temperature-driven improves the Seebeck coefficient and the power factor of Cl doping samples is enhanced.Besides band sharpening,Cl doping also introduces several nano-precipitates which are regarded as the phonon scattering centers to obviously reduce the lattice thermal conductivity.As a result,an ultra-low lattice thermal conductivity of～0.31 Wm-1K-1 is obtained at 523 K in the SnTe0.88Cl0.12 sample,which is close to the Born-von Kaman periodic conditions for the minimum limit.However,Cl doping unfortunately introduces an obvious bipolar effect limiting the enhancement of ZT value.Nevertheless,a maximum ZT value of～0.78 is realized at 873 K in the SnTe0.88Cl0.12 sample,which is still higher than the pristine SnTe(～0.4).This work provides a new route for coordinating the relationship between the Seebeck coefficient and the carrier mobility to enhance the thermoelectric performance.(4)To further balance the relationship between the Seebck coefficient and the carrier mobility,the band convergence combining with band sharpening is adopted in BiBr3 doping Sn0.93Mn0.1Te samples.It is found that BiBr3 doping induces the band sharpening optimizing the carrier mobility and the electrical conductivity,and Mn doping introduces the band convergence improving the Seebeck coefficient.The two dopants promote a maximum PF of～23.85 μWcm-1K-2 at 873 K in the Sn0.93Mn0.1Te-3 at%BiBr3 sample,which is higher than the value of～19.75μWcm-1K-2 for the Sn0.93Mn0.1Te sample.Furthermore,BiBr3 doping can also suppress the bipolar effect at elevated temperature and introduce large mass and stress field fluctuations in the lattices,which effectively reduce the lattice thermal conductivity and the bipolar thermal conductivity.Ultimately,a maximum ZT value of～1.38 is obtained at 873 K in the Sn0.93Mn0.1Te-3 at%BiBr3 sample,which is～145%higher than the pristine SnTe.This work indicates the band convergence combining with band sharpening is an excellent route to optimize the thermoelectric performance of SnTe.(5)The vacuum melting combined with high-pressure sintering is adopted to prepare the binary pristine SnTe samples and the effects of high-pressure sintering on the microstructures and thermoelectric performance are investigated.The results show that there is a micron-nanometer scale microstructure defects network in highpressured SnTe,which contains the point defects(substitutional and vacancy defects),the line defects(dislocations)and the planar defects(domain and grain boundaries).Furthermore,we quantify the microstructure defects,and calculate the effect of these defects on the phonon transport based on the Debye-Callaway model.It is found that the multi-scale microstructure defects network is conducive to establishing a full frequency phonon scattering network to effectually reduce the lattice thermal conductivity.Besides,the lattice softening is introduced in the highpressured SnTe,which further reduces the lattice thermal conductivity.As a result,the sample with sintering temperature of 993 K exhibits an ultra-low lattice thermal conductivity of～0.78 Wm-1K-1 at 873 K,which is～150%lower than SPSed SnTe.This work demonstrates the high-pressure sintering is a useful sintering technique beneficial to optimizing the thermal transport properties.