Investigation On The Multicomponent Copper-based Thermoelectric Materials

Due to the ever-growing concern on energy crisis and environmental pollution,more and more attention has been paid on thermoelectric materials by worldwide scientists.Unfortunately,most of TE materials with high ZT value more or less contain some expensive or toxic elements.Therefore,it is of great significance to develop environmental-friendly and cost-effective new TE materials for large-scale application.In this scenario,Cu-based compounds,i.e.,Cu Sb Se2,Cu3 Sb Se4 and Cu2 Co Sn S4 are selected and focused on in this thesis,and their electrical and thermal properties are analyzed and regulated by both the First-Principles calculations and detailed experimental studies,the main contents and results are as follows:1.By the route of mechanical alloying,single phase Cu Sb Se2 compound had been facilely synthesized for the first time.Cu Sb Se2 compound is a promising mid-temperature TE material with large Seebeck coefficient and extremely low thermal conductivity.However,the poor electrical properties hinder the efficiency of Cu Sb Se2.Theoretically,the large Seebeck coefficient and poor electric conductivity in this compound are mainly ascribed to the existing Cu-Se ionic bonding and heavy band at the VBM.The active lone-pair s2 electrons in Sb3+ ions are likely to be responsible for the experimentally observed low thermal conductivity.a.Introducing narrow band-gap Cu3 Sb Se4 into Cu Sb Se2 matrix during the MA process can improve its power factor remarkably and resultantly the enhanced ZT value.b.The TE properties of Cu Sb1-xAgx Se2 compounds had been studied,the ratio of Cu Sb Se2:Cu3Sb Se4:Ag Sb Se2 could be regulated by changing Ag content.Due to the enhancement in electrical properties,a maximum ZT value of 0.5 was achieved at 598 K for the sample of Cu Sb0.7Ag0.3Se2.2.Cu3 Sb Se4 is a potential material for mid-temperature thermoelectric application.However,the poor electrical conductivity deteriorates its intrinsic ZT value.To improve its TE performance,element doping has been conducted on Cu3SbSe4 in the 4 chapter.a.First-Principles calculations indicated that Al,Ga and In were efficient p-type dopants and the substitution of Te for Se could increase the density of states(DOS)near the Fermi level in Cu3 Sb Se4.b.The TE properties of Ga-doped Cu3 Sb Se4 had been studied,and the results showed the electrical properties could be enhanced obviously and the large Seebeck coefficient could be kept because of the maintenance of high degeneracy at a tiny Ga doping content.Thus,a highest power factor of 1312.7μWm-1K-2 and an enhanced ZT value of 0.90 were achieved at 623 K in Cu3Sb0.995Ga0.005Se4 sample,and the ZT value increases by 80% in compared with that of pristine Cu3 Sb Se4.c.The order of the solid solubility of M(M=Al,Ga,In)in Cu3Sb1-x Mx Se4 is Ga,In to Al.Due to the great improvement in electrical properties,high ZT values of 0.83 and 0.88 at 623 K were obtained in Cu3Sb0.995Al0.005Se4 and Cu3Sb0.995In0.005Se4,respectively.d.Due to the substitution effect of Te for Se in Cu3 Sb Se4,the band gap was narrowed while the DOS of VBM increases.Consequently,a maximum ZT value of 0.71 at 623 K was achieved.3.By means of the First-Principles calculations,the electronic structure and lattice dynamics properties of Cu2 XSn S4(X= Mn、Fe、Co)were analyzed.It reveals that Cu2 XSn S4 has low Debye temperatures and sound velocity,and thereby the low thermal conductivity.It is found that Cu2 Co Sn S4(CCTS)compound presents multiple converged valence bands,which is related to the short ionic radius of Co2+ with large crystal field stabilization energy in the tetrahedral field and therefore the strong hybridization between Cu and S.4.Single phase Cu2 Co Sn S4(CCTS)compound had been successfully prepared by mechanical alloying and the TE properties of this compound had been regulated by combination of band engineering and bonds weakening.a.Our calculations predict Cu-doping at Co-sites could effectively tune the electrical properties and enhance the Seebeck coefficient via increasing DOS of VBM near the Fermi level.Meanwhile,Mn/Fe/Zn substitution for Co had tiny effect on DOS near Fermi level,and replacing Co with Mn was more effective in depressing the lattice thermal conductivity in CCTS as compared with Fe/Zn doping.b.Guided by the theoretical calculations,we tuned the electrical transport properties of CCTS with Cu doping and brought multiple effects.Improving the electrical conductivity by increasing carrier concentration;enhancing Seebeck coefficient via increasing effective mass and lowering lattice thermal conductivity by point defects scattering.Ultimately,both the power factor and ZT value had been improved,and a highest ZT value of 0.63 was achieved for Cu2.15Co0.85 Sn S4 sample at 773 K,which is 2.4 times higher than that of un-doped matrix.c.On the basis of Cu substitution,transition metal M(M = Mn,Fe,Zn)was further doped into the compound,and resulted in enhancement of TE performance,which was mainly related to the improved electrical properties from the metal-like Cu Co2S4 phase.As a consequence,a peak ZT value close to 0.8 at 800 K was achieved in the CCTS compound simply by Cu/Mn dual-doping due to the improved electrical properties and suppressed lattice thermal conductivity by weakening of covalent bonding.