Hpht Synthesis And Optimization Of Performance Of PbTe And PbSe-based Thermoelectric Materials

Thermoelectric?TE?material is a kind of functional semiconductor materials which could make a direct conversion between thermal and electric energy as it is applied within the temperature or electric potential gradient.Compared with the conventional energy technique,thermoelectric conversion technology has unique advantages such as the convenient mobility,working with no moving parts,releasing of no noise or harmful gas.In some special application environment such as space exploration,recycle using for the waste heat of industry,the advantages of thermoelectric technology will play a more important role.PbTe is one of the best thermoelectric materials during the moderate and high temperature interval?400⁸00K?which have been researched deeply and applied in some commercial area.As the same group compound of PbTe,PbSe has a similar lattice and band structure with it.As an emerging TE material,PbSe have also its own advantages over PbSe,such as the less cost,the higher melting point which make an important basement for the extent application in commercial.In comparison with the typical synthesis method of bulk materials such as fusion casting,hydrothermal synthesis,powder metallurgic method,spark plasma sintering etal.,high pressure and high temperature?HPHT?synthesis could maintain partial high pressure TE performance of samples and lower the activation energy effectively.With the quick quenching process under high,HPHT synthesis could modulate the lattice and electric structure under high pressure effectively and capture partial TE performance to the ambient pressure.In this paper,we synthesized the PbTe and PbSe-based bulk materials via HPHT and made a deep research for the influence mechanism of synthesis pressure,solid solution,element doping for the TE properties of samples.The main results are listed as follows:1.Research on the modulation mechanism of the synthesis pressure on the crystal and electric performances of PbTe synthesized by HPHT.N-type PbTe was synthesized via HPHT from 3.0 GPa to 5.0 GPa.XRD characterization on samples show that the XRD patterns of all samples contain the diffraction peak of Pb corresponding with the?111?lattice plane.Intensity of the diffraction peak at 3.0 GPa is stronger than 4.0 GPa and 5.0 GPa.With the rising synthesis pressure,the average grain size gradually decreases and the grain boundaries grow more refining.This demonstrate the higher synthesis pressure will contribute to the nucleating and restrict the growing of grains in some degree.The refining grain boundaries will enhance the scattering effect on the carries with low energy hence makes contribution for the Seebeck coefficient.In this way,it could induce an effective modulation on the electrical properties of synthesis samples.Rich microstructures in different scale will be beneficial for the scattering of phonons in different frequencies and cause an overall constrain on the lattice thermal conductivity within different temperature interval.Both the Seebeck coefficient and the resistivity of synthesis samples rise with the rising measurement temperature.PbTe synthesized at 3.0 GPa attained a higher power factor than samples synthesized at 4.0 GPa and 5.0 GPa from 353 K to553 K.The highest value of the power factor of PbTe synthesized at 3.0 GPa is 18.7mWcm^-1K^-1@353 K.2.Exploration for the influence mechanism of synthesis pressure for the TE properties of PbSe synthesized by HPHT.P-type PbSe were successfully synthesized under different pressure.From 1.0 GPa to 4.0 GPa,with the rising of the synthesis pressure,the Seebeck coeffieient and resistivity of PbSe at the ambient pressure firstly decrease then rise.Both the energy gap and lattice constant of PbTe exhibit a decreasing trend with the rising synthesis pressure from 1.5 GPa to 4.0 GPa.In the ambient,the power factor of PbSe synthesized at 1.5 GPa attained the highest value.By a series comparisons and analyses,we find that the varying trend of the thermoelectrical properties of PbSe synthesized via HPHT is consistent with the high-pressure in-situ measurement.These characterizations demonstrate that HPHT synthesis could capture the partial thermoelectrical properties of PbSe to the ambient pressure.3.Exploration for the influence mechanism of S doping for the thermoelectric properties of PbSe1-x Sx?x=0.06,0.12,0.15?within the moderate temperature?423⁴53 K?synthesized via HPHT.In the present,the average zT values of the P-type PbSe-based is higher than the N-type.PbS synthesized at 2.0 GPa,903 K exhibit a N-type semiconductor of which the Seebeck value at the ambient temperature and pressure is-173?V/K.With the doping of S,we expect to convert the conduction type of PbSe.Characterizations on the Seebeck coefficient of synthesis sample demonstrate that PbSe_1-xS_x?x=0.06⁰.15?synthesized with the high-temperature and high-pressure method is the N-type semiconductor with the whole measurement inerval?293⁵73 K?.The lattice constant of PbSe_1-xS_x gradually deceases with the rising x.Resistivity of PbSe start a sharp rise after 423 K and hence induce a corresponding decrease of the power factor and zT values.Characterizations on the electrical conductivity of PbSe_1-x-x S_x shows that solid solution with S restricts the sharp rise of the resistivity of PbSe within the moderate temperature?423K⁵73K?interval and finally cause the obvious growth of the electrical conductivity and zT values of PbSe_1-xS_x?x=0.12,0.15?.As x=0.12,PbSe_0.88S_0.12.12 attained the highest zT=0.46@573 K.4.The investigation of Al doping on the optimization mechanism of the thermoelectric performances of Al_x Pb_1-xSe_0.88S_0.12?x=0.008,0.016,0.024?synthesized by HPHT.Al_x Pb_1-xSe_0.88S_0.12 synthesized at 2.0 GPa,903 K exhibits the PbSe-type lattice structure at ambient.With the rising of x,the carries concentrations of Al_0.008Pb_0.992Se_0.88S_0.12.12 rise gradually but the Seebeck coefficient do not exhibit an obvious decrease.This might be result from the influence of the resonance level induced by the Al doping.In comparison to PbSe_0.88S_0.12,the power factor of Al_0.008Pb_0.992Se_0.88S_0.12.12 exhibit a whole rise during the measurement interval.As x=0.008,the power factor of Al_0.008Pb_0.992Se_0.88S_0.12.12 achieves the highest value,28.5mWcm^-1K^-2,at 333K.With the rapid cooling under high pressure,Al doping induce rich microstructures into the synthesized samples,such as nanoscale polycrystal,lattice deformation,stacking faults,fringes,twin boundaries.Coupled with the rich grain boundaries in micron and nano scale,these micro-structures makes up of the multi-scale scattering mechanism for phonons in different frequency and promote the whole decrease of the thermal conductivity of Al_x Pb_1-xSe_0.88S_0.12 in comparison to PbSe_0.88S_0.12.Finally,since the simultaneous optimization of the thermal and electric performances,compared with PbSe_0.88S_0.12,the TE properties of doping samples are significantly improved.Al_0.008Pb_0.992Se_0.88S_0.12.12 achieved the highest zT=1.14 values of all contents at 453 K.In summary,HPHT could synthesize the bulk materials of PbSe and PbTe efficiently.With the further optimization of synthesis pressure,solid solution,and element doping,it will induce an effective modulation on the electric and thermal transport properties of synthesis samples and eventually make a well improvement on the figure of merit.