| As a result of the increasing and unscientific use of energy in the world,non-renewable energy sources,such as fossil fuels,will become increasingly exhausted which will induce serious negative impacts on the global environment,and how to transform the waste heat and natural heat into electric energy has become a major concern in the developed countries.The key of thermoelectric conversion and practical application is the research and application of thermoelectric materials with excellent performance.Thermoelectric material is a kind of functional materials which can direct thermoelectric conversion.It has a wide application prospect,such as low temperature refrigeration,thermoelectric generation and so on.Among the numerous thermoelectric materials,SnSe with significant non-harmonic properties and "phonon liquid" materials(such as Cu2Se,Ag2Se and CuAgSe)have become research hotspots in recent years.This study focuses on the thermoelectric properties and mechanisms of selenium compounds with different compositions.The main results are as follows:(1)A series of elements and compounds were selected for doping SnSe,and the samples were prepared by high pressure sintering technology(HPS).The effects of high pressure sintering and various dopants on the thermoelectric properties of polycrystalline SnSe were studied.The results show that high pressure sintering induced lattice distortion and decreased the lattice constant of SnSe.After doping with Sm,Cu or Ag element,or co-doping with GeTe and SnTe.the carrier concentration of SnSe will increase obviously.Notably,the increases of carrier concentration by Ag doping and GeTe-SnTe co-doping are more significant,resulting in the high electrical conductivity.When the proportion of Cu is low,the Seebeck coefficient can be increased because of the increase of effective mass.The doping of Pb and Ge will decrease materials Fermi level,which leads to the decrease of carrier concentration and the increase of Seebeck coefficient.The largest enhancement of Seebeck coefficient is observed in Ge doped samples.For Sm,Cu,Pb and Ge elements doped SnSe or GeTe-SnTe co-doped SnSe,the thermal conductivity decreases are due to the influence of alloy scattering.The Seebeck coefficient increases slightly after doping with a small amount of Ga,but the existence of Ga2O3 deteriorates thennoelectric properties.Except for Ga doping,the doping of Sm,Cu,Pb,Ge and Ag elements,GeTe and SnTe co-doped SnSe all have higher ZT values than pure SnSe.Sn0.97Cu0.03Se sample has a ZTmax of 0.79 when Sn0.99Pb0.01Se is 0.73 at 823 K;(2)The carrier scattering mechanism of pure SnSe and SnSe doped with various elements and compounds were analyzed by electrical conductivity ratio method.It is found that pure SnSe exhibits the charged impurity scattering at low temperature range,and gradually experiences acoustical phonon scattering and ionizing impurity scattering as the temperature increases.Because of the lattice distortion of high pressure sintered samples,there are strong residual stresses within the grains,resulting in greater distortion energy.After the charged impurity scattering and acoustical phonon scattering at the low temperature stage,the active atoms on the grain boundary are ionized with the temperature further increasing,and a large number of the second ionization are introduced.At the same time,distorted crystal cell starts phase transition.As a result of the ions and atoms migration,there is a strong scattering effect on carriers,leading to the charged impurity scattering in the high temperature stage;(3)The samples of(Cu1-xAgx)2Se(x = 1,0,0.1,0.15,0.2,0.25)were prepaied by argon atmosphere synthesis and high pressure sintering technology,and the thermoelectric properties of different composition of the two preparation methods were studied.The results show that the pores in the samples were reduced and the density was improved with the increase of the Ag content in the argon atmosphere synthesis process.When raw materials are Cu,Ag and Se,the synthesized composite materials are made up of Cu2Se and CuAgSe.With the increase of Ag content,the Seebeck coefficient decreases at low temperature and becomes negative when x is greater than 0.2.The sample of x = 0.2 has a clear advantage over the others in the whole temperature range.For sample of x = 0.15,when the temperature is higher than 423 K,the effective mass increases with the temperature,which leads to the decrease of the carrier mobility and electrical conductivity,and the Seebeck coefficient increases correspondingly.The rapid enhancement of the Seebeck coefficient and the higher conductivities result in greater power factors.At the same time,due to the strong alloy scattering,the thermal conductivity is maintained in a lower level.Eventually this sample has a higher ZT(1.87)at 823 K.For high pressure sintering samples,the densities of samples are obviously increased,and the phase structure has changed in those samples of 0<x<1.Cu2Se consists of two orthorhombic phases and a cubic phase,and CuAgSe consists of two orthorhombic phases.It is found that the Seebeck coefficient of all the samples rises first and then decreases except for the sample of x = 0.2,while the conductivity shows the opposite variation trend.The seebeck coefficients are still negative for the samples of x>0.2,and for the sample x = 0.2,the conductivity is higher than that of the other samples before 723 K.The thermal conductivity of all samples gradually decreases after 423 K.Comparing the thermoelectric properties of all high-pressure sintered samples,the maximum ZT value reaches 1.05 at 823 K for the sample of x = 0.1;(4)The samples of(GeTe)0.98-x(SnSe)x(SnTe)0.02(x = 0.1,0.13,0.16,0.18,0.28)were prepared by high pressure sintering.The experimental results show that the power factors are obviously improved with the slight increase of SnSe content.The results of the calculated simplified fermi energy level show that the fermi level is in the deeper position of the valence band for the sample of x = 0.16,and its effective mass is close to 2.5me,thus it is a heavy doped semiconductor.It is found that thermal conductivity first increases and then decreases with the temperature increasing for all of the samples.The thermal conductivity is only 1.06 W m-1 K-1 at room temperature for the sample of x = 0.28.Due to the higher seebeck coefficients in the low temperature range and the lower thermal conductivities in the whole temperature range,the ZT value is relatively higher at high temperature and low temperature stage for the sample of x = 0.28,and the maximum ZT value reaches 0.56 at 773 K. |
PDF Full Text Request