Preparation And Electrical Transport Properties Of Single Crystalline PbTe

Thermoelectric?TE?material as a kind of functional material,which can directly convert thermal energy to electrical energy with no noise,no moving parts and environmental friendly features,has become a hot spot of current research.PbTe is considered as one of the most promising thermoelectric materials at the medium temperature for its low thermal conductivity.In this paper,PbTe-based single crystalline thermoelectric materials are prepared by flux method combined with the melting and slow cooling process.The electrical transport properties of materials are optimized by adjusting the flux and doping amounts.Besides,the influence of Te vacancy(V_Te)and Pb vacancy(V_Pb)on the electronic structure of PbTe is explored by the first principle calculation.The main conclusions are as follows:PbTe samples containing V_Te are prepared by adjusting the initial content of Pb flux.The V_Te is easily formed in samples under Pb-rich environment for the lowest formation energy.EPMA shows that Pb content is slightly higher than that of Te.The single crystalline rocking curve result shows that the sample prepared by Pb flux method is single crystal.The increase of FHWM indicates that the samples contain vacancy defects.The weight loss of the sample is hardly detected below 1140 K,indicating that these samples possess good thermal stability.Adjusting the initial content of Pb flux to optimize the electrical transport properties,and the maximum power factor 2.07×10^-3 Wm^-1K^-22 is obtained for the sample with x=1.5 at 420 K.The first principle calculation shows that the V_Te modifies the band structure by introducing resonance level.The Pisarenko curve indicates that the carrier effective mass increases significantly,which further proves that resonance level has been introduced into the sample prepared by Pb flux method.PbTe samples containing V_Pb are prepared according to the stoichiometric ratio Pb:Te=1:x+1?x=2.0,2.5,3.0 and 3.5?.Te flux provides a Te-rich environment for crystal growth,which promotes the formation of V_Pb.EPMA test illustrates that Te content is higher than Pb mildly in these samples.At room temperature,the carrier concentration of all samples varies from 2.94×10¹⁸ to 8.01×10¹88 cm^-33 as the carrier mobility changes from 242 to 462 cm²V^-1s^-1.The maximum power factor of x=3.0sample is 3.17×10^-3 Wm^-1K^-2 at 450 K.The first principle calculation shows that the introduction of V_Pb can modify the band structure near the Fermi level,and reduce the band-gap compared with pristine PbTe.PbTe based thermoelectric materials are prepared by Bi doping combined with the melting and slow cooling process.The right shift of?400?diffraction peak reveals that the lattice parameter decreases in doped samples.HRTEM indicates that the samples have good mechanical property,and SAED diagram demonstrates that the tested sample is single crystal.TG-DSC curves illustrate that Bi doped samples have high thermal stability.The electrical properties of the samples are stable after two measurements.The highest power factor of 3.28×10^-3 Wm^-1K^-2 is achieved at 510 K for the sample with Bi initial content x=0.2,and the maximum ZT value is 0.84 at600 K.