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Lattice distortion and anisotropic thermoelectric properties in hot-deformed CuI-doped Bi2Te2·7Se0.3
Journal of Alloys and Compounds 815 (2020) 152649
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Lattice distortion and anisotropic thermoelectric properties in hotdeformed CuI-doped Bi2Te2$7Se0.3 Jin Hee Kim a, 1, Hyunyong Cho a, 1, Song Yi Back a, Jae Hyun Yun a, Ho Seong Lee b, Jong-Soo Rhyee a, * a b
Department of Applied Physics and Institute of Natural Sciences, Kyung Hee University, Yong-in, Gyeong-gi, 17104, South Korea School of Materials Science and Engineering, Kyungpook National University, Daegu, 41566, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2019 Received in revised form 9 October 2019 Accepted 10 October 2019 Available online 11 October 2019
We investigated anisotropic thermoelectric properties of (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds, synthesized by the hot-press and hot-deformation process. In spite of polycrystalline compound, the hot-deformed compounds exhibit preferred orientation along the c-axis, parallel with the applied press direction. The sample of x ¼ 0.3 mol.% shows the maximum power factor (3.8 mW m1 K2 at 300 K) and ZT value (0.97 at 423 K), which is relatively high thermoelectric performance in n-type thermoelectric materials as a mild-temperature operation. Notably, the in-plane lattice thermal conductivity of the x ¼ 0.3% compound with covalent bonding layer has lower value than the one of out-of-plane lattice thermal conductivity with van der Waals bonding layer. From the high resolution transmission electron microscopy and electron diffraction measurements, we observe the lattice distortion of the x ¼ 0.3% compound. Therefore, the unconventional anisotropic lattice thermal conductivity can be associated with the lattice distortion along the in-plane on the compound driven by the CuI doping. © 2019 Elsevier B.V. All rights reserved.
Keywords: Thermoelectric High ZT Lattice distortion Hot deformation
1. Introduction Thermoelectric devices can be used for waste heat recovery and solid-state refrigeration, so that much attention has been increased steadily, due to demands for renewable energy and eco-friendly system. The thermoelectric generator directly converts heat into electric energy by a temperature gradient between the ends of a device. Also, a thermoelectric refrigerator can transport heat from the end of the device to the opposite end by electric bias. The thermoelectric device has the advantage of solid-state operation, no mechanical moving parts with no vibration, no release of greenhouse gases, and extended operating lifetime [1]. High thermoelectric performance is needed for efficient wasteheat recovery and to widen the application fields near room temperature. The performance of the thermoelectric devices mainly depends on the thermoelectric figure of merit (ZT) defined by ZT ¼ S2sT/k, where S, s, T, and k are the Seebeck coefficient,
* Corresponding author. E-mail address: [email protected] (J.-S. Rhyee). 1 Two authors (J.H.K. and H.C.) are equally contributed on this work. https://doi.org/10.1016/j.jallcom.2019.152649 0925-8388/© 2019 Elsevier B.V. All rights reserved.
electrical conductivity, absolute temperature, and thermal conductivity, respectively. The high ZT materials which have maximum performance at room temperature can be used for a wide range of applications, including not only the conventional thermoelectric generator or refrigerator but also wearable and flexible thermoelectric devices [2,3]. Furthermore, the thermoelectric materials that have high performance near room temperature can be applied as low-grade waste heat recovery (below 150 C) [4], which is abundant in solar-thermal, body heat, and many mechanical systems. The bismuth telluride based compounds are well known high ZT materials operating near room temperature [5]. The p-type polycrystalline bismuth tellurides were reported as high ZT values by Hot-deformation (ZT ¼ 1.3 at 380 K) [6], hot-press sintering with nanoparticles (ZT ¼ 1.4 at 373 K) [7] and melt-spinning and spark plasma sintering (SPS) (ZT ¼ 1.56 at 300 K [8], ZT ¼ 1.86 at 320 K) [9]. On the other hand, n-type bismuth telluride based compounds also reported high ZT values in the compounds such as hotdeformed Bi2Te2$3Se0.7 (ZT ¼ 1.2 at 445 K and ZT ¼ 1.04 at 398 K) [6,10], I-doped polycrystalline Bi2Te2$7Se0.3 (ZT ¼ 1.13 at 423 K) [11], Cu-doped polycrystalline Bi2Te2$7Se0.3 (ZT ¼ 1.10 at 373 K) [12] and Cu-doped single-crystalline Bi2Te3 (ZT ¼ 1.15 near 300 K) [13].
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The thermoelectric performance of the n-type materials should be increased as high as one of the p-type bismuth tellurides. Because the average values of the p- and n-type thermoelectric properties determine the performance of the thermoelectric device, the relatively low ZT values of the n-type materials decrease the performance of the thermoelectric device [14]. There were many studies on thermoelectric properties of the sintered n-type bismuth telluride such as Se-substituted Bi2Te3-xSex [15,16], Cudoping [12,17], Ga-doping [18], I-doping [11,17], controlling the hot-press condition [19,20], optimizing the hot-deformation condition [10,12,21], etc. Despite those various efforts, the thermoelectric performances of the n-type bismuth tellurides are not compatible with the p-type thermoelectric properties. Here, we investigate the CuI co-doping effect on the thermoelectric performances of polycrystalline Bi2Te2$7Se0.3 with hot deformation. The Cu and I co-doping on bismuth telluride are efficient to increase the carrier concentration and decrease thermal conductivity [11e13,22]. Also, the CuI co-doping on single-crystal Bi2Te2$7Se0.3 is the ideal carrier dopants by 1.8 electrons/CuI, indicating that the CuI doping can precisely control the carrier concentration of bismuth telluride [23]. Because the bismuth telluride has relatively weak mechanical strength due to the van der Waals bonding layers, it needs to enhance mechanical properties by optimizing powder metallurgical synthesis and hot-press sintering method [24]. It has been reported that the enhancement of anisotropic properties leads to the increase of thermoelectric performance [10,12,21]. Therefore, we used the hot-deformation method to promote anisotropic properties on the compounds.
3. Result and discussion Fig. 1(a) shows the X-ray diffraction patterns of the hotdeformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds with the parallel (Pa) and perpendicular (Pe) directions to the press direction. The XRD peaks are indexed by Bi2Te3 (Rhombohedral, space grope No. 166) structure with peak shifts, caused by the Se substitution at Te site. The peak intensities are normalized by the intensity ratio between the peaks and the strongest (015) peak. The preferred orientation is defined by the relative intensity (00l) peaks between the Pa-direction and the Pe-direction. In terms of the (00l) peaks such as (006) and (00 15) peaks, the intensity of the Pa-direction is much more enhanced than those of Pa-direction, as shown in Fig. 1(a). The high intensity of the (00l) peaks in the Padirection indicates that the layered structure of the bismuth telluride is well stacked along the Pa-direction. The Pa-and Pe-direction are comparable with the out-of plane and in-plane of the layered single crystal bismuth telluride, respectively. Therefore, the hotdeformed bismuth tellurides may have the anisotropic thermoelectric properties [6,10,12,21]. Meanwhile, the lattice parameter c is increased with increasing CuI doping concentration comparing with the change of the a-axis lattice parameter, as shown in Fig. 1(b). The increase of c-axis lattice parameter implies that Cu atoms intercalate between van der Waals bonding layers [12,13]. However, the increase of c-axis lattice parameter of the hotdeformed (CuI)xBi2Te2$7Se0.3 samples (red circle in Fig. 2(a)) shows a different behavior with the CuI-doped Bi2Te2$7Se0.3 single
2. Experimental section The (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) samples were prepared by melting, hot-press, and hot-deformation method. The stoichiometric elements of Bi (99.999%), Te (99.999%), Se (99.999%) and CuI(99.99%) were sealed in evacuated quartz tubes under high vacuum. The quartz tubes were heated at 1073 K for 24 h and slowly cooled down to room temperature. The ingot samples were pulverized in an agate mortar under argon atmosphere and sintered by hot-press method at 773 K for 1 h under uniaxial pressure of 50 MPa using a graphite die with an inner diameter of 12 mm. The sintered samples with a diameter of 12 mm were re-pressed at the same condition using a graphite die with an inner diameter of 15 mm. The relative densities of the hotdeformed samples were above 98% (7.79e7.83 g/cm3) comparing with the calculated densities (7.83e7.84 g/cm3). The X-ray diffraction (XRD) patterns of the sintered samples were obtained using the Cu ka radiation (D8 advance, Bruker). Temperature-dependent electrical conductivities and Seebeck coefficients were measured under the helium atmosphere by a four-probe method using a thermoelectric properties measure system (ZEM-3, ULVAC-RIKO). The Hall carrier concentrations were obtained by the relation of nH ¼ 1/(RHe), RH ¼ rxy/H, where RH, e, rxy, and H are Hall coefficient, electronic charge, Hall resistivity, and applied magnetic fields, respectively. The Hall resistivity was measured by the five-probe contact method under various magnetic fields from -5 T to 5 T using physical property measurement system (PPMS Dynacool 14 T, Quantum Design, U.S.A). The total thermal conductivity k was obtained from the relation of k ¼ lrsCp, where l, rs, and Cp are thermal diffusivity, sample density, and specific heat, respectively. The thermal diffusivity l and specific heat Cp were measured by a laser flash method (LFA-457, NETZSCH) and by high temperature fitting from the physical property measurement system measurements, respectively.
Fig. 1. X-ray diffraction (XRD) patterns for the parallel (Pa) and perpendicular (Pe) directions to the press direction (a) and lattice parameters with CuI concentration (b) in hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds.
J.H. Kim et al. / Journal of Alloys and Compounds 815 (2020) 152649
Fig. 2. The a- and c-axis lattice parameters (a) and the Hall carrier density with CuIdoping concentration (b) of the CuI-doped (CuI)xBi2Te2$7Se0.3 single crystal and the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds. Black square and red circle represents single-crystalline and poly-crystalline compounds of (CuI)xBi2Te2$7Se0.3, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
crystal (black square in Fig. 2(a)). Generally, the anti-site and vacancy defects are generated in sintered bismuth telluride during the powder metallurgical and sintering process at high temperature [6]. Therefore, the difference of the lattice parameters is mainly caused by various defects. The transport properties of Hall carrier density nH, electrical conductivity s, Hall mobility mH, Seebeck coefficient S, and effective mass of carrier m* on the hot-deformed compounds of (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) are listed in Table 1. The carrier density of the hot-deformed sample (4.89 1019 cm3 for x ¼ 0.0 mol.%) is much higher than those of
Table 1 Hall carrier density nH, electrical conductivity s, Hall mobility mH, Seebeck coefficient S, and effective mass of carrier m* of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds at 300 K.
19
nH (10 cm s (104 S/m)
3
)
mH (cm2 V1 s1) - S (mV/K) m* (me)
Pe Pa Pe Pa Pe Pa Pe Pa
0.0%
0.3%
0.6%
0.9%
4.89 16.0 9.1 204 116 144 141 0.96 0.94
4.79 19.4 11.3 253 147 141 135 0.93 0.89
6.63 22.9 14.8 216 139 114 110 0.93 0.90
8.94 23.1 16.0 161 112 100 97 0.99 0.96
3
the single crystal sample (0.56 1019 cm3 for x ¼ 0.0 mol.%), as shown in Fig. 2(b). It implies that the powder metallurgical process of the hot-deformed Bi2Te2$7Se0.3 enhances the carrier concentration by the increase of defects, such as the Te-vacancy and anti-site defects. The Hall carrier concentration of the hot-deformed (CuI)xBi2Te2$7Se0.3 samples is increased with increasing CuIdoping concentration similar to the one of (CuI)xBi2Te2$7Se0.3 single crystalline compound. The CuI doping in the hot-deformed samples generates l.1 electrons per CuI substitution, while the single-crystalline compound of (CuI)xBi2Te2$7Se0.3 gives 1.8 electrons per CuI substitution. The lower contribution of electron donor in hot deformed samples than those of the single crystalline one may come from the complex defect structure during the hot deformation process. The anisotropic temperature-dependent electrical conductivities s(T) of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) samples are shown in Fig. 3(a). Open (closed) symbols with solid lines (dashed line) represent parallel (perpendicular) direction to the applied pressure direction. The measured electrical conductivities show a metallic or degenerated semiconducting behavior. Moreover, the electrical conductivities are increased with increasing CuI-doping concentration in the measured temperature range, as shown in Fig. 3(a). Fig. 3(b) represents room-temperature electrical conductivity (left axis) and absolute Seebeck coefficient (right axis) with CuI-doping concentration. For increasing CuI-doping concentration, the electrical conductivity is decreased, while the Seebeck coefficients are increased, which is generally observed in conventional materials by trade-off relationship. In addition, because of the anisotropic texture arrangement on the samples by the hot-deformation, the electrical conductivities and Seebeck coefficients along the perpendicular direction are higher than the ones of parallel direction. It is consistent with the preferred orientation of in-plane (outof-plane) direction for perpendicular (parallel) to the press direction. The low doped compound of (CuI)xBi2Te2$7Se0.3 (x ¼ 0.3 mol.%) shows the increase of the Hall mobility comparing with the pristine compound owing to the decrease of Hall carrier density for CuI 0.3% doped compound. The mobility is decreased with increasing carrier concentration for high doping of CuI (x ¼ 0.6 and 0.9 mol.%) which is mainly affected by the carrier scattering due to the increase of Hall carrier concentration. The Hall mobilities along the perpendicular direction for the applying pressure (in-plane) are higher than those of parallel direction (out-of-plane) which is also consistent with the anisotropic transport properties in layered van der Waals structure. The anisotropic temperature-dependent Seebeck coefficients of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) samples are presented in Fig. 3(c). The absolute values of Seebeck coefficients are linearly increased with increasing temperature for CuI-doped compounds, indicating a metallic behavior according to the Sommerfeld theory of metal [25]. The linearly increase of Seebeck coefficients for highly CuI-doped compounds (0.6 and 0.9 mol.%) comparing with the pristine and low CuI-doped compound (0.3 mol.%) indicates that the CuI-doping depress the bipolar effect of the bismuth telluride. The decrease of the Seebeck coefficient with increasing CuI doping concentration is affected by the increase of Hall carrier concentration. From the Pisarenko relation, we obtain the effective mass as presented in Table 1. The carrier concentration versus Seebeck coefficients follows the Pisarenko relation within 5% errors (not shown). The effective masses are not changed significantly and systematically with respect to CuI-doping concentration. The lower effective mass along the parallel direction (out-of-plane) than those of perpendicular (in-plane) direction come from the low Seebeck coefficient
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J.H. Kim et al. / Journal of Alloys and Compounds 815 (2020) 152649
Fig. 3. Temperature-dependent electrical conductivity s(T) (a), electrical conductivity (left axis) and Seebeck coefficient (right axis) as a function of CuI concentration at 300 K (b), temperature-dependent Seebeck coefficient S(T) (c), and temperature-dependent power factor S2s(T) (d) of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds. Open (closed) symbols with solid lines (dashed line) represent parallel (perpendicular) direction to the applied pressure direction.
along the parallel direction. The temperature-dependent anisotropic power factors of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds are presented in Fig. 3(d). The power factor of the pristine sample is about 3.3 mW m1 K2 at 300 K, which is comparable with other hot-deformed Bi2Te2$7Se0.3 (3.5 mW m1 K2) [10]. The power factors along the perpendicular- (in-plane) direction of the hot-deformed samples are higher than the ones along the parallel- (out-of-plane) direction due to high electrical conductivity along the in-plane direction. Even though power factor is reduced for high CuI-doped compounds (x ¼ 0.6 and 0.9 mol.%), the maximum power factor is reached to 3.8 mW m1 K2 at 300 K for x ¼ 0.3 mol.% doped compound. The temperature-dependent anisotropic thermal conductivities of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) samples are presented in Fig. 4(a). The thermal conductivity of the perpendicular (in-plane) direction (closed symbols) is higher than those of parallel (out-of-plane) direction (open symbols). The anisotropic thermal conductivity of the perpendicular and parallel directions is also associated with the anisotropic atomic structure of the bismuth telluride. The thermal conductivity k is composed of electronic and lattice thermal conductivities k ¼ kel þ kL . The electronic thermal conductivity kel is governed by the Wiedemann-Franz law kel ¼ L0 sT, where L0, s, and T are the Lorenz number, electrical conductivity, and absolute temperature, respectively. In simple metals, the Lorenz number is written as:
L0 ¼
p2 kB 3
2
e
¼ 2:45 108 W UK 2
However, the Lorenz number is incorrect in correlated metal, and many degenerated semiconductors. In order to get a more reliable Lorenz number, we calculated the Lorenz number by using the following equation [26]:
1 0 1 0 1 0 1 32 1 2 B C B C B C B C 7 F 5 F r þ r þ h h 5 3 @ A @ A @ A @ A B rþ2 rþ2 2 2 7 C 6 2 B 7 C 6 kB B 7 C 6 B0 0 1 0 1 1 0 1 L¼ 7 C 6 B 7 C 6 e B C 4 @B B C B C C B C5 A 3 3 @r þ 2AFrþ12 @hA @r þ 2AFrþ12 @hA 0
0
where r is the scattering parameter, h ¼ EF/kBT is the reduced Fermi energy, and Fn(h) is the n-th order Fermi integral given by ∞ ð
Fn ðhÞ ¼ 0
xn dx 1 þ exh
For most cases, the scattering parameter for acoustic phonon scattering is r ¼ 1/2. The reduced Fermi energy h can be obtained from fitting of the Seebeck coefficient to the following equation:
J.H. Kim et al. / Journal of Alloys and Compounds 815 (2020) 152649
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Fig. 4. Temperature-dependent total thermal conductivity k (a), Lorenz number L (b), lattice thermal conductivity kL (c), and thermal conductivity as a function of CuI concentration at 300 K (d) of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds.
9 8 > > > > > > > > > > r þ 52 Frþ3 ðhÞ = < 2 kB h S¼± > e > > > > > r þ 32 Frþ1 ðhÞ > > > > 2 ; : The calculated temperature-dependent Lorenz numbers of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) samples are increased with increasing CuI doping concentration as shown in Fig. 4(b). The lattice thermal conductivities are calculated by subtracting the electronic thermal conductivity from the total thermal conductivity as presented in Fig. 4(c). In general, the lattice thermal conductivities of the Cu-doped Bismuth Telluride [27] and I-doped Bismuth Telluride [11,22] are decreased with increasing doping concentrations by the phonon scattering which is caused by point defect. On the other hand, the lattice thermal conductivities of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) samples are not changed monotonically with the increasing CuI-doping. The lattice thermal conductivity of the highly doped sample (x ¼ 0.9 mol.%) shows much higher values than the ones of the low doped samples (x ¼ 0.3, 0.6 mol.%). Notably, the lattice thermal conductivity of the x ¼ 0.3 mol.% compound shows unusual behavior in that the kL along the perpendicular (in-plane) direction (closed symbols) has lower value than the one of parallel (out-of-plane) direction (open symbols). Fig. 4(d) clearly shows that only the lattice thermal
conductivity of x ¼ 0.3 mol.% has higher kL value along the parallel direction (open red circle) than those of along the perpendicular direction (closed red circle). The bismuth telluride consist of the quintuple layers along the caxis which are composed of the five individual atomic layers in the sequence Te (1)eBieTe (2)eBieTe (1), where Te (1) and Te (2) denote the two different types of tellurium atoms in the crystal structure. It is the covalenteionic bonding within a quintuple layer, while the van der Waals bonding along the c-axis [28,29]. Because of the weak bonding along the c-axis as compared with the abplane, the in-plane thermal conductivity (perpendicular direction) should be higher than the out-of-plane thermal conductivity (parallel direction). The anisotropic thermal conductivity can be found not only the bismuth telluride single crystal [13] but also the sintered bismuth tellurides [10,12,30]. In the case of the sintered bismuth tellurides, the thermal conductivity of the perpendicular direction (in-plane) to the press direction is higher than the parallel direction (out-of-plane) caused by a preferred orientation of the layered structure. Also, the anisotropic behavior of the thermal conductivity of the sintered samples is enhanced by the hotdeformation method [10,12,30]. The unusual anisotropic behavior of thermal conductivity was also found in the In4Se3-x single crystal [31] and the misfit-layer composite of (BiSe)1$09TaSe2/TaSe2 [32]. The In4Se3 consists of the covalent bonding IneSe layers which are stacked along the a-axis direction by van der Waals bonding. The in-plane (the bc-plane)
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J.H. Kim et al. / Journal of Alloys and Compounds 815 (2020) 152649
thermal conductivity of the In4Se3-x single crystal is lower than the out-of-plane (the ab-plane) thermal conductivity by the charge density wave (CDW) formation [31,33]. The theoretical band structure and charge density calculation of In4Se3-x show that the Se vacancy strongly suppress phonon propagation along the chain direction (strong covalent bonding layer; c-direction) comparing with the other a- and b-direction [33]. In addition, the misfitlayered (BiSe)1$09TaSe2 has the alternating stacks of the BiSe layers and the TaSe2 layers along the c-axis (out-of-plane) with van der Waals bonding [32,34]. Because of the weak van der Waals bonding, the lattice thermal conductivity of the out-of-plane direction of the misfit-layer should be lower than the in-plane lattice thermal conductivity [35]. On the other hand, the in-plane lattice thermal conductivity of (BiSe)1$09TaSe2/TaSe2 also has lower value than the out-of-plane lattice thermal conductivity. The CDW formation of TaSe2 can explain the unusual anisotropic behavior of lattice thermal conductivity [32]. The charge density wave accompany lattice modulation, resulting in the super-structural lattice distortion. We perform the high-resolution transmission electron microscopy (HR-TEM) and electron diffraction measurements, as presented in Fig. 5. Fig. 5 (a) and (b) show the HR-TEM image of x ¼ 0.0 and 0.3 mol.% compounds, respectively. The electron diffraction (ED) pattern of x ¼ 0.3 mol.% sample (Fig. 5(d)) presents the weak superstructure peaks, indicated by arrow direction, while there is no superstructure peak on the pristine compound (Fig. 5(c)), implying that there is a periodic lattice modulation in the matrix. The presence of periodic lattice modulation in the matrix can cause the decrease of lattice thermal conductivity along the plane by Cu/I co-doping. The lowering lattice thermal conductivity by lattice distortion is well known, for example in a case of charge density wave formation due to phonon softening along the plane [32,32,36]. Therefore, we believe that the unusual thermal conductivity of x ¼ 0.3 mol.% is closely related to the lattice modulation. We do not argue that the
unusual behavior of lattice thermal conductivity is presumably due to the formation of CDW state, but the origin of the lattice distortion should be remained as a further research. The temperature-dependent anisotropic ZT values of the hotdeformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds are shown in Fig. 6(a). The ZT value of the x ¼ 0.3 mol.% compound along the perpendicular-direction is enhanced (0.97 at 423 K) comparing with pristine compound, which is relatively high value as an n-type bismuth telluride based compounds, while higher CuI-doped compounds of x ¼ 0.6% and 0.9% deteriorate thermoelectric performance in both parallel- and perpendicular direction to the applied pressure direction. The enhancement of ZT value for x ¼ 0.3 mol.% as presented in Fig. 6(b) with CuI-doping concentration is attributed from the unusually low lattice thermal conductivity and high power factor. Therefore, the enhancement of thermoelectric performance in the x ¼ 0.3 mol.% compound is associated with the lattice modulation and phonon scattering along the in-plane direction.
Fig. 5. Microstructure and electron diffraction spots of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0 and 0.003) samples: HR-TEM images of x ¼ 0.0% (a) and x ¼ 0.3 mol.% (b). Electron diffraction patterns of x ¼ 0.0 (c) and x ¼ 0.3 mol.% (d).
Fig. 6. Temperature-dependent anisotropic ZT values (a) and the ZT value with CuI concentration at 300 K (b) of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds.
4. Conclusion We investigated anisotropic thermoelectric properties on the
J.H. Kim et al. / Journal of Alloys and Compounds 815 (2020) 152649
polycrystalline compounds of (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%), synthesized by hot press and hot deformation process. CuI-doping gives rise to the electron doping by 1.1 electrons per CuI in formula unit. In spite of polycrystalline compound, the x-ray diffraction patterns for different faces of parallel- and perpendicular-direction to the applied pressure direction shows anisotropic texture with enhanced (00l) peaks along the paralleldirection to the pressure direction, indicating the preferred orientation along the c-axis (van der Waals bonding layer) parallel with the pressure direction. The temperature-dependent electrical conductivity exhibited anisotropic behavior such that s(T) along the perpendicular direction (in-plane) shows higher values than those of parallel (out-ofplane) direction, which is consistent with the preferred crystal orientation by hot deformation process. The temperaturedependent Seebeck coefficient S(T) presented less sensitive anisotropic properties than the ones of s(T). The compound of x ¼ 0.3 mol.% shows the maximum power factor (3.8 mW m1 K2 at 300 K) due to enhancement of electrical conductivity with preserving relatively high Seebeck coefficient. The lattice thermal conductivity of the x ¼ 0.3% doped compound along the perpendicular direction (in-plane) is lower than those of the kL along the parallel direction (out-of-plane) while other compounds exhibited higher kL along the perpendicular direction than those of parallel direction. It is very noteworthy because the lattice thermal conductivity along the covalent bonding layer (perpendicular, i.e. in-plane direction) should be higher than the one along the weak van der Waals bonding layer (parallel, i.e. out-of-plane direction). We observed the lattice modulation from the superstructure spots in electron diffraction for the x ¼ 0.3 mol.% compound. We believe that the unusual behavior of lattice thermal conductivity is associated with the lattice distortion along the in-plane direction. Owing to the enhancement of power factor and reduction of lattice thermal conductivity, we observed the maximum ZT value of the x ¼ 0.3 mol.% compound (0.97 at 423 K) along the perpendicular (in-plane) direction. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This is supported by the Materials and Components Technology Development Program of MOTIE/KEIT (10063286) and by the National Research Foundation of Korea (NRF2016R1A6A3A11936385). References [1] Q.H. Zhang, X.Y. Huang, S.Q. Bai, X. Shi, C. Uher, L.D. Chen, Thermoelectric devices for power generation: recent progress and future challenges, Adv. Eng. Mater. 18 (2016) 194. [2] A.R.M. Siddique, S. Mahmud, B.V. Heyst, A review of the state of the science on wearable thermoelectric power generators (TEGs) and their existing challenges, Renew. Sustain. Energy Rev. 73 (2017) 730. [3] D. Beretta, M. Massetti, G. Lanzani, M. Caironi, Thermoelectric characterization of flexible micro-thermoelectric generators, Rev. Sci. Instrum. 88 (2017), 015103. [4] N. Toshima, Recent progress of organic and hybrid thermoelectric materials, Synth. Met. 225 (2017) 3e21. [5] C. Gayner, K.K. Kar, Recent advances in thermoelectric materials, Prog. Mater. Sci. 83 (2016) 330e416. [6] L. Hu, T. Zhu, X. Liu, X. Zhao, Point defect engineering of high-performance
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Li, Improved thermoelectric properties of Bi2Te3xSex alloys by melt spinning and resistance pressing sintering, J. Phys. D Appl. Phys. 47 (2014) 115101. G.eE. Lee, I.eH. Kim, Y.S. Lim, W.eS. Seo, B.eJ. Choi, C.eW. Hwang, Preparation and thermoelectric properties of doped Bi2Te3-Bi2Se3 solid solutions, J. Electron. Mater. 43 (2013) 1650e1655. X. Duan, K. Hu, S. Ding, D. Man, H. Jin, Influence of Ga-doping on the thermoelectric properties of Bi(2x)GaxTe2.7Se0.3 alloy, Prog. Nat. Sci.: Met. Mater. Int. 25 (2015) 29e33. J.Y. Yang, X.A. Fan, R.G. Chen, W. Zhu, S.Q. Bao, X.K. Duan, Consolidation and thermoelectric properties of n-type bismuth telluride based materials by mechanical alloying and hot pressing, J. Alloy. Comp. 416 (2006) 270e273. M. Fusa, N. Sumida, K. Hasezaki, Preparation of n-type Bi2Te2.85Se0.15 thermoelectric semiconductor without harmful dopants, Mater. Trans. 53 (2012) 597e600. L. Hu, H. Gao, X. Liu, H. Xie, J. shen, T. Zhu, X. Zhao, Enhancement in thermoelectric performance of bismuth telluride based alloys by multi-scale microstructural effects, J. Mater. Chem. 22 (2012) 16484e16490. S. Wang, G. Tan, W. Xie, G. Zheng, H. Li, J. Yang, X. Tang, Enhanced thermoelectric properties of Bi2(Te1xSex)3-based compounds as n-type legs for lowtemperature power generation, J. Mater. Chem. 22 (2012) 20943e20951. J.H. Kim, H. Cho, J.H. Yun, S.Y. Back, C.eH. Lee, J. Shim, J.eS. Rhyee, Possible Rashba band splitting and thermoelectric properties in CuI-doped Bi2Te2.7Se0.3 bulk crystals, J. Alloy. Comp. 806 (2019) 636e642. J. Jiang, L. Chen, S. Bai, Q. Yao, Q. Wang, Thermoelectric properties of textured p-type (Bi,Sb)2Te3 fabricated by spark plasma sintering, Scr. Mater. 52 (2005) 347e351. N.W. Ashcroft, N.D. Mermin, Solid State Physics, Harcourt Brace, Jovanovitch, San Diego, 1976, p. 52. J.H. Kim, M.J. Kim, S. Oh, J.-S. Rhyee, Thermoelectric properties of Se-deficient and Pb-/Sn-codoped In4Pb0.01Sn0.03Se3x polycrystalline compounds, J. Alloy. Comp. 615 (2014) 933e936. S. Wang, H. Li, R. Lu, G. Zheng, X. Tang, Metal nanoparticle decorated n-type Bi2Te3-based materials with enhanced thermoelectric performances, Nanotechnology 24 (2013) 285702. S.K. Mishra, S. Satpathyyz, O. Jepsen, Electronic structure and thermoelectric properties of bismuth telluride and bismuth selenide, J. Phys. Condens. Matter 9 (1997) 461e470. H.-J. Noh, H. Koh, S.-J. Oh, J.eH. Park, H.-D. Kim, J.D. Rameau, T. Valla, T.E. Kidd, P.D. Johnson, Y. Hu, Q. Li, Spin-orbit interaction effect in the electronic structure of Bi2Te3 observed by angle-resolved photoemission spectroscopy, Europhys. Lett. 81 (2008) 57006. L. Hu, H. Wu, T. Zhu, C. Fu, J. He, P. Ying, X. Zhao, Tuning multiscale microstructures to enhance thermoelectric performance of n-type bismuth-telluride-based solid solutions, Adv. Energy Mater. 5 (2015) 1500411. J.-S. Rhyee, K.H. Lee, S.M. Lee, E. Cho, S.I. Kim, E. Lee, Y.S. Kwon, J.H. Shim, G. Kotliar, Peierls distortion as a route to high thermoelectric performance in In4Se3-d crystals, Nature 459 (2009) 965e968. J.H. Kim, Y.J. Song, J.-S. Rhyee, B.-S. Kim, S.-D. Park, H.J. Lee, J.-W. Shin, Smallpolaron transport and thermoelectric properties of the misfit-layer composite (BiSe)1.09TaSe2/TaSe2, Phys. Rev. B 87 (2013) 224305. H.S. Ji, H. Kim, C. Lee, J.eS. Rhyee, M.H. Kim, M. Kaviany, J.H. Shim, Vacancy-
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suppressed lattice conductivity of high-ZT In4Se3x, Phys. Rev. B 87 (2013) 125111. [34] Y. Oosawa, J. Akimoto, M. Sohma, T. Tsunoda, H. Hayakawa, M. Onoda, Preparation, characterization and intercalation of ternary chalcogenides with layered composite crystal structures formed in the BiTaS and BiTaSe systems, Solid State Ion. 101 (1997) 9e16.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Lattice distortion and anisotropic thermoelectric properties in hotdeformed CuI-doped Bi2Te2$7Se0.3 Jin Hee Kim a, 1, Hyunyong Cho a, 1, Song Yi Back a, Jae Hyun Yun a, Ho Seong Lee b, Jong-Soo Rhyee a, * a b
Department of Applied Physics and Institute of Natural Sciences, Kyung Hee University, Yong-in, Gyeong-gi, 17104, South Korea School of Materials Science and Engineering, Kyungpook National University, Daegu, 41566, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2019 Received in revised form 9 October 2019 Accepted 10 October 2019 Available online 11 October 2019
We investigated anisotropic thermoelectric properties of (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds, synthesized by the hot-press and hot-deformation process. In spite of polycrystalline compound, the hot-deformed compounds exhibit preferred orientation along the c-axis, parallel with the applied press direction. The sample of x ¼ 0.3 mol.% shows the maximum power factor (3.8 mW m1 K2 at 300 K) and ZT value (0.97 at 423 K), which is relatively high thermoelectric performance in n-type thermoelectric materials as a mild-temperature operation. Notably, the in-plane lattice thermal conductivity of the x ¼ 0.3% compound with covalent bonding layer has lower value than the one of out-of-plane lattice thermal conductivity with van der Waals bonding layer. From the high resolution transmission electron microscopy and electron diffraction measurements, we observe the lattice distortion of the x ¼ 0.3% compound. Therefore, the unconventional anisotropic lattice thermal conductivity can be associated with the lattice distortion along the in-plane on the compound driven by the CuI doping. © 2019 Elsevier B.V. All rights reserved.
Keywords: Thermoelectric High ZT Lattice distortion Hot deformation
1. Introduction Thermoelectric devices can be used for waste heat recovery and solid-state refrigeration, so that much attention has been increased steadily, due to demands for renewable energy and eco-friendly system. The thermoelectric generator directly converts heat into electric energy by a temperature gradient between the ends of a device. Also, a thermoelectric refrigerator can transport heat from the end of the device to the opposite end by electric bias. The thermoelectric device has the advantage of solid-state operation, no mechanical moving parts with no vibration, no release of greenhouse gases, and extended operating lifetime [1]. High thermoelectric performance is needed for efficient wasteheat recovery and to widen the application fields near room temperature. The performance of the thermoelectric devices mainly depends on the thermoelectric figure of merit (ZT) defined by ZT ¼ S2sT/k, where S, s, T, and k are the Seebeck coefficient,
* Corresponding author. E-mail address: [email protected] (J.-S. Rhyee). 1 Two authors (J.H.K. and H.C.) are equally contributed on this work. https://doi.org/10.1016/j.jallcom.2019.152649 0925-8388/© 2019 Elsevier B.V. All rights reserved.
electrical conductivity, absolute temperature, and thermal conductivity, respectively. The high ZT materials which have maximum performance at room temperature can be used for a wide range of applications, including not only the conventional thermoelectric generator or refrigerator but also wearable and flexible thermoelectric devices [2,3]. Furthermore, the thermoelectric materials that have high performance near room temperature can be applied as low-grade waste heat recovery (below 150 C) [4], which is abundant in solar-thermal, body heat, and many mechanical systems. The bismuth telluride based compounds are well known high ZT materials operating near room temperature [5]. The p-type polycrystalline bismuth tellurides were reported as high ZT values by Hot-deformation (ZT ¼ 1.3 at 380 K) [6], hot-press sintering with nanoparticles (ZT ¼ 1.4 at 373 K) [7] and melt-spinning and spark plasma sintering (SPS) (ZT ¼ 1.56 at 300 K [8], ZT ¼ 1.86 at 320 K) [9]. On the other hand, n-type bismuth telluride based compounds also reported high ZT values in the compounds such as hotdeformed Bi2Te2$3Se0.7 (ZT ¼ 1.2 at 445 K and ZT ¼ 1.04 at 398 K) [6,10], I-doped polycrystalline Bi2Te2$7Se0.3 (ZT ¼ 1.13 at 423 K) [11], Cu-doped polycrystalline Bi2Te2$7Se0.3 (ZT ¼ 1.10 at 373 K) [12] and Cu-doped single-crystalline Bi2Te3 (ZT ¼ 1.15 near 300 K) [13].
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The thermoelectric performance of the n-type materials should be increased as high as one of the p-type bismuth tellurides. Because the average values of the p- and n-type thermoelectric properties determine the performance of the thermoelectric device, the relatively low ZT values of the n-type materials decrease the performance of the thermoelectric device [14]. There were many studies on thermoelectric properties of the sintered n-type bismuth telluride such as Se-substituted Bi2Te3-xSex [15,16], Cudoping [12,17], Ga-doping [18], I-doping [11,17], controlling the hot-press condition [19,20], optimizing the hot-deformation condition [10,12,21], etc. Despite those various efforts, the thermoelectric performances of the n-type bismuth tellurides are not compatible with the p-type thermoelectric properties. Here, we investigate the CuI co-doping effect on the thermoelectric performances of polycrystalline Bi2Te2$7Se0.3 with hot deformation. The Cu and I co-doping on bismuth telluride are efficient to increase the carrier concentration and decrease thermal conductivity [11e13,22]. Also, the CuI co-doping on single-crystal Bi2Te2$7Se0.3 is the ideal carrier dopants by 1.8 electrons/CuI, indicating that the CuI doping can precisely control the carrier concentration of bismuth telluride [23]. Because the bismuth telluride has relatively weak mechanical strength due to the van der Waals bonding layers, it needs to enhance mechanical properties by optimizing powder metallurgical synthesis and hot-press sintering method [24]. It has been reported that the enhancement of anisotropic properties leads to the increase of thermoelectric performance [10,12,21]. Therefore, we used the hot-deformation method to promote anisotropic properties on the compounds.
3. Result and discussion Fig. 1(a) shows the X-ray diffraction patterns of the hotdeformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds with the parallel (Pa) and perpendicular (Pe) directions to the press direction. The XRD peaks are indexed by Bi2Te3 (Rhombohedral, space grope No. 166) structure with peak shifts, caused by the Se substitution at Te site. The peak intensities are normalized by the intensity ratio between the peaks and the strongest (015) peak. The preferred orientation is defined by the relative intensity (00l) peaks between the Pa-direction and the Pe-direction. In terms of the (00l) peaks such as (006) and (00 15) peaks, the intensity of the Pa-direction is much more enhanced than those of Pa-direction, as shown in Fig. 1(a). The high intensity of the (00l) peaks in the Padirection indicates that the layered structure of the bismuth telluride is well stacked along the Pa-direction. The Pa-and Pe-direction are comparable with the out-of plane and in-plane of the layered single crystal bismuth telluride, respectively. Therefore, the hotdeformed bismuth tellurides may have the anisotropic thermoelectric properties [6,10,12,21]. Meanwhile, the lattice parameter c is increased with increasing CuI doping concentration comparing with the change of the a-axis lattice parameter, as shown in Fig. 1(b). The increase of c-axis lattice parameter implies that Cu atoms intercalate between van der Waals bonding layers [12,13]. However, the increase of c-axis lattice parameter of the hotdeformed (CuI)xBi2Te2$7Se0.3 samples (red circle in Fig. 2(a)) shows a different behavior with the CuI-doped Bi2Te2$7Se0.3 single
2. Experimental section The (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) samples were prepared by melting, hot-press, and hot-deformation method. The stoichiometric elements of Bi (99.999%), Te (99.999%), Se (99.999%) and CuI(99.99%) were sealed in evacuated quartz tubes under high vacuum. The quartz tubes were heated at 1073 K for 24 h and slowly cooled down to room temperature. The ingot samples were pulverized in an agate mortar under argon atmosphere and sintered by hot-press method at 773 K for 1 h under uniaxial pressure of 50 MPa using a graphite die with an inner diameter of 12 mm. The sintered samples with a diameter of 12 mm were re-pressed at the same condition using a graphite die with an inner diameter of 15 mm. The relative densities of the hotdeformed samples were above 98% (7.79e7.83 g/cm3) comparing with the calculated densities (7.83e7.84 g/cm3). The X-ray diffraction (XRD) patterns of the sintered samples were obtained using the Cu ka radiation (D8 advance, Bruker). Temperature-dependent electrical conductivities and Seebeck coefficients were measured under the helium atmosphere by a four-probe method using a thermoelectric properties measure system (ZEM-3, ULVAC-RIKO). The Hall carrier concentrations were obtained by the relation of nH ¼ 1/(RHe), RH ¼ rxy/H, where RH, e, rxy, and H are Hall coefficient, electronic charge, Hall resistivity, and applied magnetic fields, respectively. The Hall resistivity was measured by the five-probe contact method under various magnetic fields from -5 T to 5 T using physical property measurement system (PPMS Dynacool 14 T, Quantum Design, U.S.A). The total thermal conductivity k was obtained from the relation of k ¼ lrsCp, where l, rs, and Cp are thermal diffusivity, sample density, and specific heat, respectively. The thermal diffusivity l and specific heat Cp were measured by a laser flash method (LFA-457, NETZSCH) and by high temperature fitting from the physical property measurement system measurements, respectively.
Fig. 1. X-ray diffraction (XRD) patterns for the parallel (Pa) and perpendicular (Pe) directions to the press direction (a) and lattice parameters with CuI concentration (b) in hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds.
J.H. Kim et al. / Journal of Alloys and Compounds 815 (2020) 152649
Fig. 2. The a- and c-axis lattice parameters (a) and the Hall carrier density with CuIdoping concentration (b) of the CuI-doped (CuI)xBi2Te2$7Se0.3 single crystal and the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds. Black square and red circle represents single-crystalline and poly-crystalline compounds of (CuI)xBi2Te2$7Se0.3, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
crystal (black square in Fig. 2(a)). Generally, the anti-site and vacancy defects are generated in sintered bismuth telluride during the powder metallurgical and sintering process at high temperature [6]. Therefore, the difference of the lattice parameters is mainly caused by various defects. The transport properties of Hall carrier density nH, electrical conductivity s, Hall mobility mH, Seebeck coefficient S, and effective mass of carrier m* on the hot-deformed compounds of (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) are listed in Table 1. The carrier density of the hot-deformed sample (4.89 1019 cm3 for x ¼ 0.0 mol.%) is much higher than those of
Table 1 Hall carrier density nH, electrical conductivity s, Hall mobility mH, Seebeck coefficient S, and effective mass of carrier m* of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds at 300 K.
19
nH (10 cm s (104 S/m)
3
)
mH (cm2 V1 s1) - S (mV/K) m* (me)
Pe Pa Pe Pa Pe Pa Pe Pa
0.0%
0.3%
0.6%
0.9%
4.89 16.0 9.1 204 116 144 141 0.96 0.94
4.79 19.4 11.3 253 147 141 135 0.93 0.89
6.63 22.9 14.8 216 139 114 110 0.93 0.90
8.94 23.1 16.0 161 112 100 97 0.99 0.96
3
the single crystal sample (0.56 1019 cm3 for x ¼ 0.0 mol.%), as shown in Fig. 2(b). It implies that the powder metallurgical process of the hot-deformed Bi2Te2$7Se0.3 enhances the carrier concentration by the increase of defects, such as the Te-vacancy and anti-site defects. The Hall carrier concentration of the hot-deformed (CuI)xBi2Te2$7Se0.3 samples is increased with increasing CuIdoping concentration similar to the one of (CuI)xBi2Te2$7Se0.3 single crystalline compound. The CuI doping in the hot-deformed samples generates l.1 electrons per CuI substitution, while the single-crystalline compound of (CuI)xBi2Te2$7Se0.3 gives 1.8 electrons per CuI substitution. The lower contribution of electron donor in hot deformed samples than those of the single crystalline one may come from the complex defect structure during the hot deformation process. The anisotropic temperature-dependent electrical conductivities s(T) of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) samples are shown in Fig. 3(a). Open (closed) symbols with solid lines (dashed line) represent parallel (perpendicular) direction to the applied pressure direction. The measured electrical conductivities show a metallic or degenerated semiconducting behavior. Moreover, the electrical conductivities are increased with increasing CuI-doping concentration in the measured temperature range, as shown in Fig. 3(a). Fig. 3(b) represents room-temperature electrical conductivity (left axis) and absolute Seebeck coefficient (right axis) with CuI-doping concentration. For increasing CuI-doping concentration, the electrical conductivity is decreased, while the Seebeck coefficients are increased, which is generally observed in conventional materials by trade-off relationship. In addition, because of the anisotropic texture arrangement on the samples by the hot-deformation, the electrical conductivities and Seebeck coefficients along the perpendicular direction are higher than the ones of parallel direction. It is consistent with the preferred orientation of in-plane (outof-plane) direction for perpendicular (parallel) to the press direction. The low doped compound of (CuI)xBi2Te2$7Se0.3 (x ¼ 0.3 mol.%) shows the increase of the Hall mobility comparing with the pristine compound owing to the decrease of Hall carrier density for CuI 0.3% doped compound. The mobility is decreased with increasing carrier concentration for high doping of CuI (x ¼ 0.6 and 0.9 mol.%) which is mainly affected by the carrier scattering due to the increase of Hall carrier concentration. The Hall mobilities along the perpendicular direction for the applying pressure (in-plane) are higher than those of parallel direction (out-of-plane) which is also consistent with the anisotropic transport properties in layered van der Waals structure. The anisotropic temperature-dependent Seebeck coefficients of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) samples are presented in Fig. 3(c). The absolute values of Seebeck coefficients are linearly increased with increasing temperature for CuI-doped compounds, indicating a metallic behavior according to the Sommerfeld theory of metal [25]. The linearly increase of Seebeck coefficients for highly CuI-doped compounds (0.6 and 0.9 mol.%) comparing with the pristine and low CuI-doped compound (0.3 mol.%) indicates that the CuI-doping depress the bipolar effect of the bismuth telluride. The decrease of the Seebeck coefficient with increasing CuI doping concentration is affected by the increase of Hall carrier concentration. From the Pisarenko relation, we obtain the effective mass as presented in Table 1. The carrier concentration versus Seebeck coefficients follows the Pisarenko relation within 5% errors (not shown). The effective masses are not changed significantly and systematically with respect to CuI-doping concentration. The lower effective mass along the parallel direction (out-of-plane) than those of perpendicular (in-plane) direction come from the low Seebeck coefficient
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Fig. 3. Temperature-dependent electrical conductivity s(T) (a), electrical conductivity (left axis) and Seebeck coefficient (right axis) as a function of CuI concentration at 300 K (b), temperature-dependent Seebeck coefficient S(T) (c), and temperature-dependent power factor S2s(T) (d) of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds. Open (closed) symbols with solid lines (dashed line) represent parallel (perpendicular) direction to the applied pressure direction.
along the parallel direction. The temperature-dependent anisotropic power factors of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds are presented in Fig. 3(d). The power factor of the pristine sample is about 3.3 mW m1 K2 at 300 K, which is comparable with other hot-deformed Bi2Te2$7Se0.3 (3.5 mW m1 K2) [10]. The power factors along the perpendicular- (in-plane) direction of the hot-deformed samples are higher than the ones along the parallel- (out-of-plane) direction due to high electrical conductivity along the in-plane direction. Even though power factor is reduced for high CuI-doped compounds (x ¼ 0.6 and 0.9 mol.%), the maximum power factor is reached to 3.8 mW m1 K2 at 300 K for x ¼ 0.3 mol.% doped compound. The temperature-dependent anisotropic thermal conductivities of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) samples are presented in Fig. 4(a). The thermal conductivity of the perpendicular (in-plane) direction (closed symbols) is higher than those of parallel (out-of-plane) direction (open symbols). The anisotropic thermal conductivity of the perpendicular and parallel directions is also associated with the anisotropic atomic structure of the bismuth telluride. The thermal conductivity k is composed of electronic and lattice thermal conductivities k ¼ kel þ kL . The electronic thermal conductivity kel is governed by the Wiedemann-Franz law kel ¼ L0 sT, where L0, s, and T are the Lorenz number, electrical conductivity, and absolute temperature, respectively. In simple metals, the Lorenz number is written as:
L0 ¼
p2 kB 3
2
e
¼ 2:45 108 W UK 2
However, the Lorenz number is incorrect in correlated metal, and many degenerated semiconductors. In order to get a more reliable Lorenz number, we calculated the Lorenz number by using the following equation [26]:
1 0 1 0 1 0 1 32 1 2 B C B C B C B C 7 F 5 F r þ r þ h h 5 3 @ A @ A @ A @ A B rþ2 rþ2 2 2 7 C 6 2 B 7 C 6 kB B 7 C 6 B0 0 1 0 1 1 0 1 L¼ 7 C 6 B 7 C 6 e B C 4 @B B C B C C B C5 A 3 3 @r þ 2AFrþ12 @hA @r þ 2AFrþ12 @hA 0
0
where r is the scattering parameter, h ¼ EF/kBT is the reduced Fermi energy, and Fn(h) is the n-th order Fermi integral given by ∞ ð
Fn ðhÞ ¼ 0
xn dx 1 þ exh
For most cases, the scattering parameter for acoustic phonon scattering is r ¼ 1/2. The reduced Fermi energy h can be obtained from fitting of the Seebeck coefficient to the following equation:
J.H. Kim et al. / Journal of Alloys and Compounds 815 (2020) 152649
5
Fig. 4. Temperature-dependent total thermal conductivity k (a), Lorenz number L (b), lattice thermal conductivity kL (c), and thermal conductivity as a function of CuI concentration at 300 K (d) of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds.
9 8 > > > > > > > > > > r þ 52 Frþ3 ðhÞ = < 2 kB h S¼± > e > > > > > r þ 32 Frþ1 ðhÞ > > > > 2 ; : The calculated temperature-dependent Lorenz numbers of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) samples are increased with increasing CuI doping concentration as shown in Fig. 4(b). The lattice thermal conductivities are calculated by subtracting the electronic thermal conductivity from the total thermal conductivity as presented in Fig. 4(c). In general, the lattice thermal conductivities of the Cu-doped Bismuth Telluride [27] and I-doped Bismuth Telluride [11,22] are decreased with increasing doping concentrations by the phonon scattering which is caused by point defect. On the other hand, the lattice thermal conductivities of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) samples are not changed monotonically with the increasing CuI-doping. The lattice thermal conductivity of the highly doped sample (x ¼ 0.9 mol.%) shows much higher values than the ones of the low doped samples (x ¼ 0.3, 0.6 mol.%). Notably, the lattice thermal conductivity of the x ¼ 0.3 mol.% compound shows unusual behavior in that the kL along the perpendicular (in-plane) direction (closed symbols) has lower value than the one of parallel (out-of-plane) direction (open symbols). Fig. 4(d) clearly shows that only the lattice thermal
conductivity of x ¼ 0.3 mol.% has higher kL value along the parallel direction (open red circle) than those of along the perpendicular direction (closed red circle). The bismuth telluride consist of the quintuple layers along the caxis which are composed of the five individual atomic layers in the sequence Te (1)eBieTe (2)eBieTe (1), where Te (1) and Te (2) denote the two different types of tellurium atoms in the crystal structure. It is the covalenteionic bonding within a quintuple layer, while the van der Waals bonding along the c-axis [28,29]. Because of the weak bonding along the c-axis as compared with the abplane, the in-plane thermal conductivity (perpendicular direction) should be higher than the out-of-plane thermal conductivity (parallel direction). The anisotropic thermal conductivity can be found not only the bismuth telluride single crystal [13] but also the sintered bismuth tellurides [10,12,30]. In the case of the sintered bismuth tellurides, the thermal conductivity of the perpendicular direction (in-plane) to the press direction is higher than the parallel direction (out-of-plane) caused by a preferred orientation of the layered structure. Also, the anisotropic behavior of the thermal conductivity of the sintered samples is enhanced by the hotdeformation method [10,12,30]. The unusual anisotropic behavior of thermal conductivity was also found in the In4Se3-x single crystal [31] and the misfit-layer composite of (BiSe)1$09TaSe2/TaSe2 [32]. The In4Se3 consists of the covalent bonding IneSe layers which are stacked along the a-axis direction by van der Waals bonding. The in-plane (the bc-plane)
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thermal conductivity of the In4Se3-x single crystal is lower than the out-of-plane (the ab-plane) thermal conductivity by the charge density wave (CDW) formation [31,33]. The theoretical band structure and charge density calculation of In4Se3-x show that the Se vacancy strongly suppress phonon propagation along the chain direction (strong covalent bonding layer; c-direction) comparing with the other a- and b-direction [33]. In addition, the misfitlayered (BiSe)1$09TaSe2 has the alternating stacks of the BiSe layers and the TaSe2 layers along the c-axis (out-of-plane) with van der Waals bonding [32,34]. Because of the weak van der Waals bonding, the lattice thermal conductivity of the out-of-plane direction of the misfit-layer should be lower than the in-plane lattice thermal conductivity [35]. On the other hand, the in-plane lattice thermal conductivity of (BiSe)1$09TaSe2/TaSe2 also has lower value than the out-of-plane lattice thermal conductivity. The CDW formation of TaSe2 can explain the unusual anisotropic behavior of lattice thermal conductivity [32]. The charge density wave accompany lattice modulation, resulting in the super-structural lattice distortion. We perform the high-resolution transmission electron microscopy (HR-TEM) and electron diffraction measurements, as presented in Fig. 5. Fig. 5 (a) and (b) show the HR-TEM image of x ¼ 0.0 and 0.3 mol.% compounds, respectively. The electron diffraction (ED) pattern of x ¼ 0.3 mol.% sample (Fig. 5(d)) presents the weak superstructure peaks, indicated by arrow direction, while there is no superstructure peak on the pristine compound (Fig. 5(c)), implying that there is a periodic lattice modulation in the matrix. The presence of periodic lattice modulation in the matrix can cause the decrease of lattice thermal conductivity along the plane by Cu/I co-doping. The lowering lattice thermal conductivity by lattice distortion is well known, for example in a case of charge density wave formation due to phonon softening along the plane [32,32,36]. Therefore, we believe that the unusual thermal conductivity of x ¼ 0.3 mol.% is closely related to the lattice modulation. We do not argue that the
unusual behavior of lattice thermal conductivity is presumably due to the formation of CDW state, but the origin of the lattice distortion should be remained as a further research. The temperature-dependent anisotropic ZT values of the hotdeformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds are shown in Fig. 6(a). The ZT value of the x ¼ 0.3 mol.% compound along the perpendicular-direction is enhanced (0.97 at 423 K) comparing with pristine compound, which is relatively high value as an n-type bismuth telluride based compounds, while higher CuI-doped compounds of x ¼ 0.6% and 0.9% deteriorate thermoelectric performance in both parallel- and perpendicular direction to the applied pressure direction. The enhancement of ZT value for x ¼ 0.3 mol.% as presented in Fig. 6(b) with CuI-doping concentration is attributed from the unusually low lattice thermal conductivity and high power factor. Therefore, the enhancement of thermoelectric performance in the x ¼ 0.3 mol.% compound is associated with the lattice modulation and phonon scattering along the in-plane direction.
Fig. 5. Microstructure and electron diffraction spots of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0 and 0.003) samples: HR-TEM images of x ¼ 0.0% (a) and x ¼ 0.3 mol.% (b). Electron diffraction patterns of x ¼ 0.0 (c) and x ¼ 0.3 mol.% (d).
Fig. 6. Temperature-dependent anisotropic ZT values (a) and the ZT value with CuI concentration at 300 K (b) of the hot-deformed (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds.
4. Conclusion We investigated anisotropic thermoelectric properties on the
J.H. Kim et al. / Journal of Alloys and Compounds 815 (2020) 152649
polycrystalline compounds of (CuI)xBi2Te2$7Se0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%), synthesized by hot press and hot deformation process. CuI-doping gives rise to the electron doping by 1.1 electrons per CuI in formula unit. In spite of polycrystalline compound, the x-ray diffraction patterns for different faces of parallel- and perpendicular-direction to the applied pressure direction shows anisotropic texture with enhanced (00l) peaks along the paralleldirection to the pressure direction, indicating the preferred orientation along the c-axis (van der Waals bonding layer) parallel with the pressure direction. The temperature-dependent electrical conductivity exhibited anisotropic behavior such that s(T) along the perpendicular direction (in-plane) shows higher values than those of parallel (out-ofplane) direction, which is consistent with the preferred crystal orientation by hot deformation process. The temperaturedependent Seebeck coefficient S(T) presented less sensitive anisotropic properties than the ones of s(T). The compound of x ¼ 0.3 mol.% shows the maximum power factor (3.8 mW m1 K2 at 300 K) due to enhancement of electrical conductivity with preserving relatively high Seebeck coefficient. The lattice thermal conductivity of the x ¼ 0.3% doped compound along the perpendicular direction (in-plane) is lower than those of the kL along the parallel direction (out-of-plane) while other compounds exhibited higher kL along the perpendicular direction than those of parallel direction. It is very noteworthy because the lattice thermal conductivity along the covalent bonding layer (perpendicular, i.e. in-plane direction) should be higher than the one along the weak van der Waals bonding layer (parallel, i.e. out-of-plane direction). We observed the lattice modulation from the superstructure spots in electron diffraction for the x ¼ 0.3 mol.% compound. We believe that the unusual behavior of lattice thermal conductivity is associated with the lattice distortion along the in-plane direction. Owing to the enhancement of power factor and reduction of lattice thermal conductivity, we observed the maximum ZT value of the x ¼ 0.3 mol.% compound (0.97 at 423 K) along the perpendicular (in-plane) direction. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This is supported by the Materials and Components Technology Development Program of MOTIE/KEIT (10063286) and by the National Research Foundation of Korea (NRF2016R1A6A3A11936385). References [1] Q.H. Zhang, X.Y. Huang, S.Q. Bai, X. Shi, C. Uher, L.D. Chen, Thermoelectric devices for power generation: recent progress and future challenges, Adv. Eng. Mater. 18 (2016) 194. [2] A.R.M. Siddique, S. Mahmud, B.V. Heyst, A review of the state of the science on wearable thermoelectric power generators (TEGs) and their existing challenges, Renew. Sustain. Energy Rev. 73 (2017) 730. [3] D. Beretta, M. Massetti, G. Lanzani, M. Caironi, Thermoelectric characterization of flexible micro-thermoelectric generators, Rev. Sci. Instrum. 88 (2017), 015103. [4] N. Toshima, Recent progress of organic and hybrid thermoelectric materials, Synth. Met. 225 (2017) 3e21. [5] C. Gayner, K.K. Kar, Recent advances in thermoelectric materials, Prog. Mater. Sci. 83 (2016) 330e416. [6] L. Hu, T. Zhu, X. Liu, X. Zhao, Point defect engineering of high-performance
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