Preparation And Properties Of Multi-scale P-type SnTe Based Thermoelectric Materials

Thermoelectric materials,which can convert heat into electricity directly,have been greatly developed under the background of sustainable energy development.SnTe is an ideal Pb-free alternative thermoelectric material for that it has similar physical and chemical features of Pb Te.The ultrahigh carrier concentration and large energy separation between light-hole band and heavy-hole band lead to a poor electrical performance,while the small band gap and large lattice thermal conductivity lead to a poor thermal properties,so the intrinsic thermoelectric performance of SnTe is not good as Pb Te.Under the interests of improving thermoelectric performance for p-SnTe alloys,many strategies,such as carrier concentration optimization,energy band structure adjustment,and multi-scale structural design have been adopted in this dissertation.Meanwhile,we revealed the deep reasons behind the changes of thermoelectric performance by various analytical methods such as XRD,SEM,energy band structure calculation and thermoelectric performance test.The main results can be drawn as following:Firstly,we achieving the co-optimization between electrical and thermal performance by AgCl cation-anion co-doped SnTe.Research suggests that double replacement affects the carrier concentration as well as modifies the band structure,which is benefit to electrical performance of SnTe alloys.Besides,the Multi-scale structure such as the point defects,grain boundaries and precipitated phase act as multi-scale phonon scattering centers and effectively suppress the transport of phonons in a wide temperature range,thus cut the thermal conductivity dramatically.This results in a z T_max as high as0.53 in(SnTe)_0.91(AgCl)_0.09 for 773 K,which enables a 52.9%enhancement as compared to SnTe.Secondly,Cu₂Se-SnTe solid solutions enable the independent optimization of thermal performance.It is found that the formation of Cu₂Se-SnTe solid solutions hardly affects the transport of carriers.At the same time,it promotes the formation of multi-scale phonon scattering center mainly composed with point defects and nano precipitates,which can effectively reduce the lattice thermal conductivity in the elevated temperature region.With the individual effect of multi-scale structure on thermal performance,this work realized a z T-enhancement at 773 K,z T increased from 0.34 of SnTe to 0.53 of(SnTe)0.99(Cu2Se)0.01.Finally,we optimize the overall thermoelectric properties of SnTe via compositing with CuGaTe₂,which have the higher Seebeck coefficient and the lower electrical conductivity than SnTe.The study demonstrates that the maximum solubility limit of CuGaTe₂ in SnTe is 1%,a trace of CuGaTe₂ as a second phase can be observed when exceed the limit.At low doping levels,carrier concentration optimization as well as point defect scattering caused by forming a solid solution improves the electrical properties at low temperature and thermal properties at elevated temperature respectively.Heavily doping SnTe with CuGaTe₂ lead to the electric properties at elevated temperature and the thermal properties at low temperature are optimized because the composite effect from CuGaTe₂ phase and phonon scattering from the phase interface respectively.When the doping levels are located between the above mentioned conditions,thermoelectric performance can be optimized in a wide temperature region.As a result,the most optimized properties can be obtained in(SnTe)_0.96(CuGaTe2)_0.04,and the z T_max reachs to0.78 at 773 K.