Electronic and Thermal Transport Properties of Complex Structured Cu-BiSe Thermoelectric Compound with Low Lattice Thermal Conductivity

1 Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Daejeon 305-701, Republic of Korea 2Department of Energy Science, Department of Physics, Sungkyunkwan University, Suwon 440-746, Republic of Korea 3Materials R&D Center, Samsung Advanced Institute of Technology, Samsung Electronics, Yongin 446-712, Republic of Korea 4 Powder and Ceramics Division, Powder Technology Department, Korea Institute of Materials Science, Changwon 642-831, Republic of Korea


Introduction
In thermoelectric (TE) materials, thermal energy is directly converted into electrical energy and vice versa through the flow of charge carriers (electrons or holes) in solid state without any moving parts [1,2].The performance of TE device depends on the temperature gradient (Δ) and the dimensionless figure of merit (ZT) in TE material.The conversion efficiency of TE material is governed by ZT, which is defined as  =  2 ⋅  ⋅ /, where  is the thermopower (Seebeck coefficient),  is the electrical conductivity,  is the thermal conductivity, and  is the absolute temperature.Intuitively, good TE materials should have sufficiently large S, high , and low .Based on this relation, there are two different kinds of straightforward strategies for achieving high ZT.The first one is maximizing power factor (PF =  2 ⋅) by the modification of the electronic states through the optimization of doping, introduction of nanoscale materials, and so forth.The other one is minimizing  by the perturbation of structural arrangements for enhancing phonon scattering through solid-solution alloying [3][4][5][6], nanostructuring [7,8], and the development of new materials with intrinsically low .However, since , S, and  are the functions of charge carriers as well as these physical parameters are intercorrelated to each other, it is very difficult to control these parameters individually.Recent TE researches have been mainly focused on breaking the trade-off relationship between  and  by reducing the lattice thermal conductivity ( latt ) due to the relative easiness of reducing  latt without considerably affecting other electronic transport parameters (S and ).Thus, extensive researches have been focused on reducing  latt by utilizing nanostructures and compositional inhomogeneities of state-of-the-art TE materials such as Bi 2 Te 3 , PbTe, and SiGe [3][4][5][6][7][8].It has been reported that dramatically reduced  latt in TE materials is due to one of the following mechanisms: alloy scattering [3][4][5][6], resonant scattering [9][10][11][12][13], anharmonic scattering [14], and interface scattering of phonons [15][16][17] or their combination.
Recently, much effort has been focused on discovering new semiconducting materials with intrinsically low  latt .Such materials have a solid potential compared to conventional TE materials because their  latt can be further reduced by compositional and microstructural engineering without sacrificing the other electronic properties.The common structural characteristics of low  latt materials are complex structures, which have relatively large unit cells, sublattice disorders, low crystal symmetry, and/or a variety of atom types combined with random distributions.These result in the effective phonon scatterings, which make these materials promising candidates for TE applications.
By understanding the origin of low  in complex structured system, it is possible to find a clue for novel TE materials with an appropriate structure and to manipulate structural arrangement for the achievement of low  in conventional TE materials.In a previous article [18], we reported new TE materials, which are the pavonite homologues Cu + Bi 5− Se 8 (1.2 ≤ x ≤ 1.5, 0.1 ≤ y ≤ 0.4) with low  latt (0.41-0.55 W⋅m −1 ⋅K −1 at 300 K) owing to the structural complexity.In the present study, we investigated the electronic and thermal transport properties in the view point of the correlation with crystal structure in order to clarify the origin of TE properties in the homologous Cu + Bi 5− Se 8 compounds.

Experimental
Crystal ingots of Cu + Bi 5− Se 8 compounds were fabricated by conventional melting technique by the use of highpurity elemental Cu (99.999%,CERAC), Bi (99.999%, 5N Plus), and Se (99.9%, 5N Plus) as starting materials.The stoichiometric mixtures of the elements were loaded into a quartz tube of 14 mm in diameter.The tube was vacuumsealed and the mixed contents were melted in a furnace for 10 hrs at 1273 K; then, they were water quenched.The ingots were ground using ball mill, and compacted bulk samples of 10 mm in diameter and 13 mm in thickness were prepared using spark plasma sintering (SPS) technique under dynamic vacuum and with the application of 50 MPa of uniaxial pressure at 663 K.The relative densities of the resulting consolidated samples were found to range from 7.22 to 7.48 g⋅cm −3 , which are more than 95% of the theoretical value.We used Cu 1.7 Bi 4.7 Se 8 (Cu + Bi 5− Se 8 , x = 1.4,y = 0.3) as a reference material, and small amount (w and z = 0.0085 and 0.025 for the interstitial and substitutional M sites, resp.) of Zn or In was introduced in host Cu 1.7− M  Bi 4.7 Se 8 or Cu 1.7 Bi 4.7− M  Se 8 matrix to elucidate the electronic and thermal properties according to the different carrier concentrations.The electronic transport properties including  and S were measured from 300 K to 560 K using an ULVAC ZEM-3 system.The  values ( =   ⋅   ⋅ ) were calculated from measurements taken separately: sample density (  ), heat capacity (  ), and thermal diffusivity () measured under vacuum by laser-flash method (TC-9000, ULVAC, Japan), in which   was used as the constant value  of 0.225 J⋅g −1 ⋅K −1 estimated from the Dulong-Petit fitting using low temperature   data.All measured data, which were acquired at the same dimension and configuration, are obtained within the experimental error of about 5%.with effective mass of m * (∼0.21 0 ) .These electronic transport properties might be related to the changes in the Hall mobility (  ) and m * of doped compounds.In order to clarify the origin for affecting  in doped compounds, m * values were calculated using the measured  and the carrier concentration (  ) for the whole samples.The m * is one of the critical factors determining  and estimated with the following equation:

Results and Discussions
where h,   ,   , and  are the Planck constant, Boltzmann constant, th order Fermi integral, and the reduced Fermi energy, respectively.As shown in Figure 3, m * was decreased by the doping compared to that of undoped compound.It should be noted that m * values of the interstitial Cu site modified ).According to the theoretical calculation, interstitial Cu generates electron carriers in Cu + Bi 5− Se 8 system [18].Therefore, more electrons can be generated by the elemental doping at the interstitial Cu site.Indeed, higher   ,   , and the smaller m * were observed in Cu 1.7− M  Bi 4.7 Se 8 compared to those of Cu 1.7 Bi 4.7− M  Se 8 .In Figures 4(a) and 4(b), with increasing the doping level, the region of intrinsic conduction also shifts to higher temperatures.This is a typical behavior of a degenerate semiconductor, since the increased extrinsic majority carriers suppress the contribution of the minority carriers and hence increase the onset temperature of intrinsic conduction.As shown in Figures 4(c) and 4(d), negative  values for all samples were observed over the entire temperature range, suggesting that the electrical conduction was dominated by n-type carriers.In Figure 5, it was confirmed that the change in  at 560 K was due to the combined effect of decreased m * and increased   .Regardless of doping level and sites, carrier relaxation time () was nearly the same (a few femtoseconds) for all samples.Figure 6 shows  latt as a function of temperature.The value of  latt was calculated from the relation of  =  latt +  elec , in which  elec is the electronic contribution.

Figure 1 showsSubstitutional 1 )Cu 1 . 7 Figure 2 :
Figure1shows TE properties for the homologous Cu + Bi 5− Se 8 (1.2 ≤ x ≤ 1.5, 0.1 ≤ y ≤ 0.4) system at 560 K with varying compositional ratio between Cu and Bi.The absolute value of  decreases linearly with increasing , indicating that the semiconducting transport properties do not significantly change with varying the Cu/Bi ratio.Based on theoretical calculations of the previous report, interstitial Cu is regarded as the main source of electron carriers in the Cu + Bi 5− Se 8 system[18].Because the electron distributions around interstitial Cu ions, which are shown in the crystal structure of Cu-Bi-Se pavonite compound (Figure2(a)), overlapped each other along b-axis, the degree of overlapping depends on the content of the interstitial Cu.This might be a possible reason for the changes in electrical properties by perturbing Cu sites, since the interstitial Cu sites locate along b-axis and work as an electrical conduction path, leading to an improvement of electron transfer across the basal plane.On the other hand,  decreases from 0.65 to 0.51 W⋅m −1 ⋅K −1 by increasing Cu content.The highest ZT value of 0.27 at 560 K was obtained in the Cu 1.9 Bi 4.6 Se 8 composition, which has the highest Cu concentration among samples, mainly due to its low .Thus, it is suggested that statistically distributed interstitial Cu along b-axis may be the main factor affecting the electronic (by generating electron carriers) and thermal transport (by enhancing phonon scattering) properties in this system.In order to understand the TE transport behavior based on the roles of the interstitial Cu and substitutional Bi sites, we individually modified those sites by impurity doping.Cu 1.7 Bi 4.7 Se 8 was chosen as a base composition for doping, since this composition showed stable structure