Thermoelectric Properties Research Based On The Relationship Between Real Space And Reciprocal Space

Although far from satisfying the needs of commercial applications because of the poor performance,thermoelectric materials for solid-state thermoelectric generators and Peltier coolers still captures considerable attention.Understanding and manipulate-ing the electrical and thermal transport behavior play significant roles in tailoring the performance of thermoelectric materials.The transport properties of electron and phonon are closely related to the crystal structure and chemical composition,which are defined in real space.On the other hand,the electrical and thermal transport behavior also connect with the band structure and phonon dispersion,both of which are represented in the reciprocal-space first Brillouin zone.The combination of real-space and reciprocal-space properties could provide more possibilities for regulating the transport characteristics.Based on the relationship between real space and reciprocal space,this work investigates the electrical and thermal transport properties of some thermoelectric materials.The main results are as follow:(1)This work presents a promising strategy for optimizing the thermoelectric properties in layered BiCuSeO by means of achieving efficient interlayer charge release and thereby activating multiple Fermi pockets.Specifically,the Bi/Cu dual vacancies constructs the interlayer charge diffusion channels,and the Pb doping introduces the interlayer charge concentration gradient.As expected,the interlayer charge concentra-tion gradient drives the release of the confined charges trapped in[Bi2O2]2+sublayers,enabling these charges to almost completely diffuse into[Cu2Se2]-sublayers along the interlayer charge diffusion channels and thus become conduction carriers.As a result,the concentration of conduction holes is remarkably increased in vacancies/Pb codoped Bi1-x-yPbyCu1-xSeO,reaching the theoretical limiting value.This efficient interlayer charge release in Bi1-x-yPbyCu1-xSeO results in the significant enhancement in the carrier concentration and thus electrical conductivity.Meanwhile,the substantial increase in carrier concentration pushes the Fermi level into the valence band,activating multiple converged valence bands,which enables a relatively high Seebeck coefficient and yields an increased power factor for Bi1-x-yPbyCui-xSeO.Consequently,benefitting from the combination of improved power factor and low thermal conductivity,a significant enhanced ZT value of?1.4 is achieved in Bi0.90Pb0.06Cu0.96SeO at 823 K.This present strategy opens up a promising avenue for regulating the transport properties in thermoelectric materials with layered structure.(2)This work demonstrates the(Bi2)m(Bi2Te3)n(m:n=3:9,2:7 and 1:5)natural superlattice series as potential candidates for thermoelectric applications above room temperature,and reveals the intrinsically low lattice thermal conductivity driven by multiple mechanisms in Bi8Te9,Bi6Te7,and Bi4Tes.Low-temperature heat capacity measurements provide compelling evidence for the existence of multiple low-frequency optical phonons in(Bi2)m(Bi2Te3)n{m/n=3:9,2:7,and 1:5)compounds,suggestive of the coupling between heat-carrying acoustic phonons and low-frequency optical phonons.This endows(Bi2)m(Bi2Te3)n(m/n=3:9,2:7,and 1:5)compounds with strong phonon resonance scattering and,thus,intrinsically low lattice thermal conductivity.Moreover,phonon velocity measurements demonstrate that both the chemical bond softening and lattice anharmonicity contribute to the low lattice thermal conductivity in(Bi2)m(Bi2Te3)n(m/n?3:9,2:7,and 1:5)compounds.Additionally,the small volume of the Brillouin zone gives rise to low cutoff frequency of acoustic phonon modes in the(Bi2)m(Bi2Te3)n(m/n=3:9,2:7,and 1:5)natural superlattice series,which are also favorable for the realization of low lattice thermal conductivity.The present result provides evidence of coexisting multiple lattice dynamics mechanisms governing thermal transport behavior of(Bi2)m(Bi2Te3)n(m/n=3:9,2:7,and 1:5)compounds.(3)This work shows the optimization of thermoelectric performance for BiTe compound through tailoring the carrier concentration by defect engineering.Sb alloying could effectively reduce the electron concentration in n-type Bi1-xSbxTe.The reduced carrier concentration contributes to the remarkable increase in Seebeck coefficient,as well as the simultaneous reduction in electrical conductivity and electronic thermal conductivity for Bii1-x,SbxTe over a broad temperature range.Moreover,the low-temperature heat capacity data gives unequivocal evidence of low-lying Einstein modes in all the Bi1-xSbxTe compounds,indicative of strong coupling between the heat carrying acoustical phonons and low frequency optical phonons.This conclusion is in agreement with the observed low lattice thermal conductivity of Bi1-xSbxTe compounds.All of the above features contribute to the enhanced thermoelectric figure of merit for Bi1-xSbxTe compounds in the 300-500 K range.In addition,the simultaneous optimization of the electrical and thermal transport properties leads the temperature corresponding to the maximum thermoelectric figure of merit to shift toward room temperature with the increase of Sb content in Bi1-xSbxTe.Consequently,the maximum ZT of?0.35 is achieved in Bi0.7Sb0.3Te at?373 K along the direction parallel to the pressure axis,thus making Bi1-xSbxTe a potential competitor to bismuth telluride for near-room-temperature thermoelectric applications.