Study On Manipulating Thermoelectric Properties Of SnTe Through Synergistic Tailoring Of The Band Structure And Introducing Cu₂Te Nanoprecipitates

The emerging worldwide energy appetite and global warming have built up interests in more functional means of energy production with environmentally benign sources.Thermoelectric materials can directly convert heat into electricity by imposing a temperature gradient between the hot and cold junction in the absence of any moving parts or liquid media.The heat can come from sunlight,from the combustion of fossil fuels,or as a by-product of various processes（i.e.,chemical reactions,combustion,and nuclear decay）,indicating thermoelectric materials can play an important role in both energy generation and conservation.Lead chalcogenides are the most studied thermoelectric materials with more than figure-of-merit（ZT）value of 2 through the synergy of the band engineering and all-scale hierarchical architectures.The concerns about Pb（i.e.,toxicity）,however,limit their wide-ranging applications.An alternative is tin chalcogenides including SnTe-and Sn Se-based semiconductors,which are lead-free environmentally favorable thermoelectric materials.SnTe presents the same rock-salt structure and the same multiple valence bands（light-and heavy-hole valence bands）as Pb Te,indicating all the strategies used for the enhancement of lead chalcogenides thermoelectric performance can be utilized to improve the thermoelectric properties of SnTe-based materials.In this doctoral study,we successfully fabricated SnTe with high thermoelectric performance by one-step method and spark plasma sintering.We did two main projects in this study.In the first project,we boosted the SnTe thermoelectric performance through the adjustment of the carrier concentration through additional 3 mol%Sn self-compensation in pristine SnTe.The comprehensive theoretical computational Density Function Theory（DFT）calculations and experimental work of Si-substituted SnTe for the improvement of thermoelectric（TE）properties has been established.In both（theoretical an experimental）works,it is revealed that Si doping to SnTe has encouraging results for the improvement of the band engineering and the power factor of SnTe thermoelectric material.Si has also produced the nano precipitates in the SnTe matrix,which helped to decrease the lattice thermal conductivity.We achieved ZT～0.63 at 900K for 8%Si-doped Sn_0.95Si_0.08Te sample.In the second project,we did band structure engineering of SnTe by the introduction of resonant levels and band convergence via co-doping with In&Ca,which leads to a notable enhancement of the power factor over a wide temperature range.We also supported band structure engineering by computational DFT calculations.Results showed that he resonant states effect weakens with the increase in temperature,especially above 500K.This indicates that resonance states effect work at relative low temperatures.On the other hand,band convergence of the valance band offset can enhance Seebeck coefficient in SnTe.Ca alloying converged the valance band offset as well as enlarged the valance-conduction band gap,leading to an enhanced Seebeck coefficient.In addition,the atomic scale point defects,the nanoscale elongated screw dislocations with random directions,and the microscale grain boundaries caused by the sintering efficiently scatter a wide spectrum of heat-carrying phonons,leading to a remarkable reduction in the lattice thermal conductivity.With the benefit of these factors,a high ZT of 1.43 at 823 K was obtained for Sn_0.91Ca_0.09In_0.03Te sample,being 249%higher than the thermoelectric value（0.41）of pure phase SnTe.The reduction of lattice thermal conductivity is another effective strategy to improve thermoelectric efficiency of SnTe.We demonstrated an ultralow lattice thermal conductivity in SnTe via introducing endotaxial Cu₂Te nanostructures.The endotaxial Cu₂Te nanostructures significantly reduces the lattice thermal conductivity to a value of 0.75 Wm^-1K^-1 at 823 K for Sn_0.95Ca_0.06In_0.02Te（Cu₂Te）_0.05 sample.Ultimately,the optimization of both electrical and thermal properties in SnTe leads to a record high ZT of～1.85 at 823 K,a high average ZT of 0.67,and a high conversion efficiency of 11.4%at 300-823 K in the composition of Sn_0.95Ca_0.06In_0.02Te（Cu₂Te）_0.05.The achieved high performance material without involving any toxic elements can be a competitive alternative to conventional PbTe thermoelectrics.