【作者】 朱晨；

【导师】 秦晓英； 张建；

【作者基本信息】 中国科学技术大学 ， 材料物理与化学， 2022， 博士

【摘要】 热电材料是一种能够直接实现电能和热能相互转换的新型清洁能源材料。当前热电领域的研究热点为进一步提升传统热电材料的热电优值及探索开发新型热电材料。立方硫族化合物因储量丰富,容易合成、便于大规模生产而且性能优异得到了广泛关注。因此,本文以具有立方结构的传统硫族化合物PbTe及新型硫族化合物Cu12Sb4S13为目标展开研究,旨在通过多种手段优化其电、热输运性能以大幅提高其热电优值。论文取得的主要研究结果及创新性如下:(1)研究了 Bi元素掺杂和Cu1.75Te纳米相复合对PbTe热电性能的影响。结果表明,Bi掺杂可以优化PbTe的载流子浓度n、增大其电子态密度DOS,将Pb0.995Bi0.005Te的功率因子PF提高至16.5 μW cm-1 K-2。在此基础上复合Cu1.75Te纳米相进一步增大了 PF,同时增强声子散射降低了晶格热导率κL。最终,Pb0.995Bi0.005Te/0.86 wt.%Cu1.75Te的最大热电优值ZTmax达到了 1.4,平均热电优值 ZTave 达到了 0.9。(2)研究了复合纳米MgO对n型PbTe热电性能的影响。结果表明,加入适量的MgO纳米颗粒可以显著增大PbTe的PF同时降低κL。分析表明PF的增强来自因电子浓度优化而降低的电阻率ρ及因界面势产生的能量过滤效应而增强的热电势S。与此同时,MgO第二相和Pb0.995Bi0.005Te基体之间形成的界面增强了声子散射,降低了 κL。最终,Pb0.995Bi0.005Te/0.3 wt.%MgO 在 773 K 获得了最大热电优值ZTmax=1.4。(3)研究了 Zn和Se双掺杂对新型硫族化合物Cu12Sb4S13热电性能的影响。结果表明,S位掺Se和Cu位掺Zn可以优化Cu12Sb4S13的载流子浓度,将PF提高33%。并且掺杂原子Se和Zn的存在增强了声子散射,使得κL降低约30%(723K)。最终,Cu11.975Zn0.025Sb4S12.8Se0.2 和 Cu11.95Zn0.05Sb4S12.8Se0.2 的 ZT在 723 K达到了 0.9,相比未掺杂的Cu12Sb4S13提高41%。(4)研究了 In和Se双掺杂对Cu12Sb4S13电、热输运性质及热电性能的影响。结果表明,Cu12-xInxSb4S12.8Se0.2在300-623 K温度范围内为小极化子导电机制,在623-723K温度范围内为由声学声子散射占主导的能带传导机制。此外,我们发现用In代替Cu及Se代替S,可以有效地散射载热声子从而显著降低其晶格热导率。最终,Cu11.95In0.05Sb4S12.8Se0.2在723 K时获得了最大热电优值ZTmax=1.0,相比未掺杂的Cu12Sb4S13提高～56%。(5)研究了 Cu位稀土元素Gd掺杂对Cu12Sb4S13微观结构、电子结构及热电性能的影响。结果表明,Gd掺杂后杂质相Cu3SbS4含量逐渐减少,Cu空位数量急剧上升(x≤0.3),使得空穴浓度及电导率增大;同时点缺陷声子散射加强,κL显著降低。此外,Cu位掺Gd可以增大费米能级处的电子态密度DOS,第一性原理计算显示DOS的增加来自Gd 4f轨道的贡献。最终,Cu11.7Gd0.3Sb4S13的ZTmax达到了 0.94,相比未掺杂的Cu12Sb4S13提高约41%。更多还原

【Abstract】 Thermoelectric(TE)materials,which can convert energy from heat to electricity or vice versa,hold promises in alleviating energy shortage and environmental pollution.Boosting the TE performance of traditional thermoelectric materials and exploring new thermoelectric materials are the research hotspots in the field of thermoelectricity.Cubic chalcogenides have attracted significant interests due to the features of wide source,being easy to be synthetic,convenience for large-scale production and excellent TE performance.Therefore,in this study,we focus on the optimization of thermoelectric properties for typical cubic chalcogenides:PbTe and Cu12Sb4S13.The main results achieved in this thesis are summarized as follows:(1)The thermoelectric properties of Pb1-xBixTe compounds and Pb0.995Bi0.0055Te/f wt.%Cu1.75Te composites were investigated in the temperature range of 300-773 K.The results indicate that Bi doping is useful to adjust the carrier concentration and increase the electronic density of states.The PF value of Pb0.995Bi0.005Te sample reaches～16.5 μW cm-1 K-2.In order to furtherly optimize the thermoelectric performance of ntype PbTe based materials,the compound Pb0.995Bi0.005Te is selected as the matrix to prepare composites,and coherent nanophase Cu1.75Te is in situ formed in the Pb0.995Bi0.005Te matrix,which can simultaneously optimize the thermal and electrical properties.As a result,the highest ZT value of 1.4 is obtained at 623 K for Pb0.99sBi0.005Te/0.86 wt.%Cu1.75Te sample and the ZT holds a higher value in broad temperature,which is beneficial to the average ZT value(ZTave～0.9).(2)The thermoelectric properties of Pb0.995Bi0.005Te(PBT)based composites incorporated with MgO nanoparticles were investigated in the temperature range from 300 to 773 K.The results show that the addition of appropriate amounts of MgO nanoparticles can give rise to obvious enhancement of PF and the certain reduction ofκL.The enhanced PF mainly comes from decreased electrical resistivity via increasing electron concentration.Meanwhile,the κL is reduced due to intensified phonon scattering by the interfaces between MgO inclusions and the PBT matrix.As a result,ZT～1.4 is achieved at 773 K in PBT/0.3 wt%MgO sample,indicating that incorporation of a proper amount of MgO in Pb0.995Bi0.005Te is an effective way to improve its thermoelectric performance.(3)Dually substituted tetrahedrite samples were prepared by fusion methods.The results indicate that through dual substitution of Se for S and Zn for Cu in the compound both the electrical conductivity and thermopower are enhanced,leading to increase of PF as high as～33%(at 723K).Furthermore,lattice thermal conductivity is reduced by as low as～30%(at 723K)upon dual substitution due to enhanced phonon scattering of the dopants Se and Zn.As a result,ZT=0.9(at 723 K)is achieved in Cu12yZnySb4S12.8Se0.2 samples with y=0.025 and 0.05,which is～41%higher than that of Cu12Sb4S13.(4)The electrical transport behavior and thermoelectric properties of Cu12xInxSb4S12.8Se0.2 samples were investigated.The electrical transport in Cu12xInxSb4S12.8Se0.2 samples is governed by small polaron hopping at T=300-623 K;at T>623 K the transport mechanism changes to band conduction dominated by acoustic phonon scattering.Dual substitution of In for Cu and Se for S can significantly reduce the lattice thermal conductivity κL of Cu12Sb4S13 as large as 58%(～300 K)through enhanced phonon scattering mainly by point defects.As a result,ZT～1.0 at 723 K is achieved in Cu11.95In0.05Sb4S12.8Se0.2 sample,which is～56%higher than that of Cu12Sb4S13,indicating that dual substitution is an effective way to improve the thermoelectric performance of tetrahedrite.(5)The effects of Gd substitution at the Cu sites on the microstructures,electronic structures and thermoelectric performance of Cu12Sb4S13 were investigated.Rietveld refinements of XRD results show that accompanying inhibition of second phase Cu3SbS4,the number of Cu vacancies rises dramatically with increasing x(x≤0.3),which leads to increased electrical conductivity σ and reduced κL.Our SRPES result shows a substantial increment of DOS near Fermi level upon Gd substitution;meanwhile our first-principle calculations reveals that the increased DOS comes from contribution of Gd 4f orbit,leading to enhanced S.Consequently,remarkable enhancement in thermoelectric performance is obtained with a highest ZT of～0.94 for the sample Cu11.7Gd0.3Sb4S13 at 749 K,which is～41%higher than that of undoped Cu12Sb4S13 sample.更多还原