Microstructural Regulation And Thermoelectric Performance Optimization Of P-Type Cu₃SbSe₄-Based Materials

Thermoelectric(TE)material can directly convert heat into electricity or vice versa based on the Seebeck and Peltier effect,respectively.As result,it is promising in power generation and solid state refrigeration,contributing significantly to solving the energy and environmental crsis that human faces at present.Many TE materials are widely investigated in recent years.Cu₃Sb Se₄ is an narrow band gap semiconductor consisting of earth-abundant,cheap,nontoxic and environmentally friendly constituent elements.However,pristine Cu₃Sb Se₄ shows very low carrier mobiliy and high thermal conductivity,which derived from its complex and rigid crystal structure.In this dissertation,we synthesized Cu₃Sb Se₄-based materials mainly employing solid-state reaction.We show that defects including poin defect,dislocation,precipitate can be introduced into Cu₃Sb Se₄matrix by doping,alloying and secondary phase engineering.These defects markedly scatter heat-carrying phonons across the entire spectrum,drastically depressing lattice thermal conductivity.First principle calculations reveal the dopants optimized the density of states in the vicinity of Fermi level,resulting in the enhanced power factor.The synergistically optimized power factor and lattice thermal conductivity give rise to a greatly improve ZT values.The main research contents and results are as follows:1.The thermoelectric performance of Bi doped and Ag/Bi co-doped Cu₃Sb Se₄ were investigated.Density functional theory(DFT)calculations show that the density of states near Fermi level increases after Bi doping or Ag/Bi co-doping,which is beneficial to obtain large Seebeck coefficient.The experimental results indicate that impurity phases are generated in addition to the main phase of Cu₃Sb Se₄ in the doping samples.Ag-Bi co-doping can decouple the inherently interrelated relationship between the conductivity and Seebeck coefficient,which not only increases the electrical conductivity due to the increased carrier concentration but also enhances the Seebeck coefficient,and therefore achieving high power factor in Cu₃Sb Se₄.Moreover,the multi-scale defects(including point defects,dislocation,nanophase and grain boundary)introduced by co-doping can reduce the lattice thermal conductivity.Consequently,a maximum ZT value?0.95(90%higher than that of pristine Cu₃Sb Se₄)at 673 K is obtained for the isovalent heavy-element Bi doped sample of Cu₃Sb_0.985Bi_0.015Se₄.A maximum ZT of?1.18(136%higher than that of pristine Cu₃Sb Se₄)at 673 K and an averaged ZT of?0.51 at 300–673 K are achieved for Cu_2.85Ag_0.15Sb_0.985Bi_0.015Se₄sample due to a low?and a high S²?.2.The thermoelectric properties of sulfur alloying at Se site were investigated.Cu₃Sb Se₄-Cu₃Sb S₄ solid solution achieves the larger Seebeck coefficient compared with pristine Cu₃Sb Se₄due to the increasing the density of states near the Fermi level and the effective carrier mass.Additionally,the lattice thermal conductivity is significantly reduced in account of the lattice distortion(stress and strain fluctuations of mass differences between Se and S atoms)and defects(dislocation,grain boundaries),all of which can enhance phonon scattering.As a result,a maximum ZT value of?0.60 at 673 K is obtained for the Cu₃Sb Se_2.8S_1.2 sample,which is?1.2times more than that of pristine Cu₃Sb Se₄.S alloying at Se site can significantly reduce the lattice thermal conductivity,but it has little effect on increasing the carrier concentration and electrical conductivity.Therefore,based on the S alloying(Solid solution),the effects of magnetic cation Fe doping on the thermoelectric properties of Cu₃Sb Se₄ were investigated.After Fe doping,the band gap of Cu₃Sb Se_2.8S_1.2 solid solution is narrowed,and the electrical conductivity is significantly increased.The increased density of states near the Fermi level increases the effective carrier mass,maintaining the large Seebeck coefficient.In addition to the defect of point defect and grain boundary,Fe doping can induce the impurity phases such as Cu Se and Cu Fe Se₂ in Cu₃Sb Se_2.8S_1.2solid solution to further reduce the lattice thermal conductivity.As a consequence,a maximum ZT value of 0.86 at 673 K is obtained for Cu₃Sb_(1-x)Fe_xSe_2.8S_1.2(x=0.05)sample,which is 72%higher than that of pristine Cu₃Sb Se₄ and 40%higher than that of S alloyed sample.3.The effects of Sb₂Se₃ nanophase on the thermoelectric properties of Cu₃Sb Se₄-based materials were studied.The results show that Cu₃Sb Se₄+x mol%Sb₂Se₃(x=0–3)composite contributes to the high carrier concentration and large carrier effective mass.Thus,the electrical conductivity and Seebeck coefficient are synergistically optimized for obtaining a high power factor.Meanwhile,Sb₂Se₃ nanophase can induce high-density phase boundaries,which can reduce lattice thermal conductivity of Cu₃Sb Se₄.Hence,an outstanding ZT value of 0.85 at 673 K is realized for Cu₃Sb Se₄+1.5 mol%Sb₂Se₃sample,which is 70%higher than that of pristine Cu₃Sb Se₄ sample.4.The Sn Se precipitates were embedded into Cu₃Sb Se₄ matrix to improve the thermoelectric properties.The experimental results show that the addition of Sn and Se elements(Cu₃Sb Se₄+x mol%Sn Se(x=0–6)can decouple the electrical and thermal transport to improve the thermoelectric performance of Cu₃Sb Se₄.The increased density of states near Fermi level is beneficial to obtain a larger Seebeck coefficient.Sn and Se elements can also adjust the carrier concentration and improve the electrical conductivity of Cu₃Sb Se₄-based materials.Therefore,the maximum power factor value of?1645?Wm^-1K^-2at 673 K is obtained for x=1 sample,which is the highest value in the Cu₃Sb Se₄-based materials as far as we know.On the other hand,the introduced Sn and Se point defects(can produce large mass and stress fluctuations),and heterogeneous interfaces of in-situ Sn Se with Cu₃Sb Se₄ can effectively scatter phonons,reducing the lattice thermal conductivity.Finally,a peck ZT value of 1.08(116%higher than pristine Cu₃Sb Se₄ sample)at 673 K and an averaged ZT of?0.46 at 300–673 K are realized for Cu₃Sb Se₄+1 mol%Sn Se sample.