Synthesis And Thermoelectric Properties Of P-Type SnTe-based Compounds

Nowadays,the increasing energy consumption and environmental problems are becoming increasingly prominent,which stimulates the widely concerned development and utilization of diversified and clean energy.Thermoelectric material as an environment-friendly material can realize reversible conversion between thermal and electric energy.The thermoelectric devices are capable of converting waste heat into electric energy or enabling solid state cooling by employing electricity.This type of devices has some advantages such as small size,simple structure,safety and so on,which leads to a wide range of potential applications in the fields of medical care,space exploration,common life and so on.The IV-VI thermoelectric materials with Na Cl structure have shown great thermoelectric potential in the middle temperature region because of their high symmetry crystal structure and excellent thermoelectric performance after optimization.SnTe,as one member of IV-VI thermoelectric compounds with similar crystal structure and energy band structure as Pb Te,has also received intensive attentions in recent years.However,this compound has a rather high hole concentration,a large energy offset between the light and heavy valence bands and a high lattice thermal conductivity,which result in a relative poor thermoelectric performance compared to Pb Te.It is expected that the thermoelectric properties of SnTe based compounds will be further improved by synergistically reducing the carrier concentration,modifying the band structure and suppressing the lattice thermal conductivity.Also,considering the complexity of previous synthesis method,the development of rapid synthesis method to producing these compounds with lowered lattice thermal conductivity in large quantities is desirable.In this thesis,three types of P-SnTe based compounds with Na Cl structure(SnTe)_1-x(NaBiTe₂)_x?Sn_0.97M_0.03Te and SnTe·xCu₂Te have been successfully prepared by the traditional solid-state method or the melt spinning technology and their electrical and thermal properties have been investigated,which provides a helpful reference for further study of similar compounds.The main contents of this study are as follows:(1)Traditional solid state reaction method associated by vacuum hot pressing is employed for the synthesis of(SnTe)_1-x(NaBiTe₂)_x(x=0,0.01,0.02,0.03,0.04,0.05)solid solution alloys and their thermoelectric properties are investigated.Pristine SnTe exhibits a high carrier concentration and NaBiTe₂-alloying effectively reduces the carrier concentration,which leads to enlarged Seebeck coefficient.Although the electrical conductivity of the sample decreases with the increase of the alloying content,the final power factor is still improved by tuning the carrier concentration.At the same time,the point defects induced by alloying serve to reduce the thermal conductivity.With enhanced power factor and reduced thermal conductivity,the sample with the nominal composition of(SnTe)_0.96(NaBiTe₂)_0.04 achieves the maximum z T value of 1.1at 834 K,which is 88%higher than that of the pure sample.(2)Sn_0.97M_0.03Te(M=In,Bi,Mg)samples with fine grain and uniform composition is successfully and rapidly prepared via melt spinning and the synthesis temperature and other parameters are explored.The powder is densified by vacuum hot pressing sintering,and its micro morphology and thermoelectric properties are investigated.Although the electrical conductivity of the doped sample decreases,the Seebeck coefficient increases and the final power factor remains high.In principle,the melt spinning process can refine the grain size and thus enhance the grain boundary scattering for phonons.Meanwhile,the point defect also plays a important role in phonon scattering,and ultimately reduces the lattice thermal conductivity of the samples.The maximum z T value of 0.91 at 834 K is achieved for the sample with the nominal composition of Sn_0.97Bi_0.03Te,which is 65%higher than that of the undoped samples.(3)SnTe·xCu₂Te(x=0,0.01,0.03,0.07)solid solutions are prepared by melt spinning,and the powders are consolidated into compact bulk samples via vacuum hot pressing.Subsequently,the micro morphology and thermoelectric properties of these samples are investigated.Due to the increase of electrical conductivity and Seebeck coefficient at high temperature,the power factor of the alloyed sample is higher than that of the pure sample at high temperature,while power factor remains relatively stable in the low temperature region.The grain size is refined by melt spinning and the grain boundary scattering for phonons is strengthened.At the same time,the point defect and interstitial atoms introduced by alloying further strengthen the phonon scattering,which significantly reduces the lattice thermal conductivity of these samples.However,the alloying with Cu₂Te in this study leads to a further increase of carrier concentration and resultant increase in electronic thermal conductivity.As a result,the total thermal conductivity is unexpectedly increased.Finally,the sample with the nominal compositions of SnTe·0.01Cu₂Te and SnTe·0.03Cu₂Te achieve the maximum z T value of 0.88 at 834 K,which is 60%higher than that of the undoped sample.