| Thermoelectric materials and devices,enabling the direct conversion of electricity from heat or vice versa,have the advantages of no mechanical rotating parts,no vibration and no noise during operation.Great efforts have been devoted to improving ZT of traditional thermoelectric semiconductors,such as Bi2Te3 and PbTe.At present,however,Te-free compounds become one of the hot issues for the researchers.Recently,eco-friendly thermoelectric material Cu2SnSe3 has attracted much attention due to its constituents being abundant and free of toxic elements.However,its low electrical conductivity and thermopower result in the small power factor and the low ZT(～0.3).Therefore,in this study,we try to enhance the thermoelectric properties of Cu2SnSe3 through micro-and electronic structure regulation and defect engineering.The main results achieved in this thesis are summarized as bellow:(1)To improve the small electrical conductivity σ caused by the intrinsic low hole concentration p,we regulated the micro-and electronic structure,and thermoelectric transport properties of Cu2SnSe3 through high energy ball milling.Our results show that,through high energy mechanical milling,14-follder rises in σ and-2 times increase in thermopower S(at a given p)can be simultaneously achieved,which leads to the enhancement of PF from 4.2 to 10.1μW·cm-1·K-2(at-800K).First-principles calculation indicates that the increased σ should be attributed to the significantly increased p contributed by the Sn vacancy and Se dangling bonds;the enhanced S caused by the elevated density of states effective mass md*originates from the flattened valance bands due to Sn vacancy.Besides,the intensified phonon scattering from dense grain boundaries,Sn vacancy,and SnO2 nanoprecipitates leads to a sharp reduction of κL(40%).As a result,a large ZT=0.9 is achieved at 848K.(2)To simultaneously elevate the small σ and S,the band structure of Cu2SnSe3 was comprehensively regulated through S and In co-doping.Our results show that alloying S in Cu2SnSe3 can increase the md*and enlarge the band gap,leading to the rise of S from 170 to 277μV/K(at 773K);besides,doping In at Sn site leads to both multi-valance bands transport and the further enhancement of md*from 1.3 to 3.1 me.Moreover,heavily doping In into Cu2SnSe3 induces formation of multidimensional defects,such as CuInSe2 nano-precipitates,Sn vacancies,dislocations and phase boundaries.The intensified phonon scattering from the above defects leads to the reduction of from 2.35 to 1.32W·K-1·m-1(300K).Consequently,ahigh ZT=1.51(858K)was obtained for Cu2Sn0.82In0.18Se2.7S0.3 sample.(3)To reduce the κL of Cu2SnSe3,we created high-dense stacking faults(SFs)and needle shaped nano-inclusions(nanoneedles)to block the mild-and low-frequency phonons.Our results show that the generation of SFs can be achieved through reducing SFs energy by doping Fe and Ag;the AgInSnSe4 nanoneedles can be created through complex decomposition reaction of Ag2Se with Cu2Sn0.88Fe0.06In0.06Se3.The intrinsic cause of AgInSnSe4 nanoneedle growth is found to be its large surface energy of(1 12)plane.Besides,our analyses,based on Debye-Callaway model,reveal that the aspect ratio ξ of nanoneedles works as an extra degree of freedom that tunes which frequency phonons to be scattered and strong scattering mild-and low-frequency phonons occurs as ξ=10.Moreover,doping Fe,In and Ag can lead to the rise of md*and create extra hole conduction path between Fesn-Se bonds,resulting in a high PF=12μW·cm-1K-2 at～800K.Consequently,a record high ZT=1.61 was achieved for Cu2Sn0.88Fe0.06In0.06Se35wt.%Ag2Se sample.(4)The dominate intrinsic point defects,defects concentration and potential extrinsic dopants in Cu2SnSe3 were investigated by using the density functional theory based first-principles calculation.Our results revel that the intrinsic P-type characteristic of Cu2SnSe3 is originated from the dominated intrinsic acceptor defects CuSn and Cu vacancy with shallow energy-transition levels.Moreover,CuSn is beneficial to the S and μ and Cu vacancy is detrimental to the μ of Cu2SnSe3.On the other hand,it is found that the formation of Cu vacancy can be inhibited through doping Fe,Mn,Al,Ga,Cd,In and Zn at Sn site;doping Fe/Mn at Sn site and substitution of Ni for Cu can lead to the increase of md*due to the contribution of their 3d orbitals.Therefore,increasing Cu content and/or introducing the above beneficial dopants appropriately are expected to cause enhancement of μ and/or S of Cu2SnSe3.Furthermore,substitution Ag for Cu or substation Al,Zn,Ge and Mn for Sn can induce large mass and strain field fluctuations in Cu2SnSe3,leading to a sharp reduction of κL.Present results not only deepen one’s insight into point defects in Cu2SnSe3,but also could provide one with a guide to improve its thermoelectric properties. |
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