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Evolution of thermoelectric properties and anisotropic features of Bi2Te3 prepared by high pressure and high temperature
Journal of Alloys and Compounds 632 (2015) 514–519
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Evolution of thermoelectric properties and anisotropic features of Bi2Te3 prepared by high pressure and high temperature Yuewen Zhang a, Xiaopeng Jia a, Le Deng b, Xin Guo a, Hairui Sun a, Bing Sun a, Binwu Liu a, Hongan Ma a,⇑ a b
National Key Lab of Superhard Materials, Jilin University, Changchun 130012, China Department of Material Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China
a r t i c l e
i n f o
Article history: Received 21 November 2014 Received in revised form 23 January 2015 Accepted 29 January 2015 Available online 7 February 2015 Keywords: Thermoelectric materials Anisotropy Crystal defects Bi2Te3 High pressure
a b s t r a c t Bi2Te3 bulks were synthesized by a simple high pressure and high temperature method to investigate the texture evolution and anisotropic thermoelectric properties. The multiple microstructures, defects and strains were revealed by HRTEM images. Pressure tuning resulted in increasing of electrical conductivity and thermopower, yielding a 10-fold improvement of power factor at 4 GPa, with respect to that at 0.5 GPa. The abnormally weak anisotropy of electrical resistivity was attributed to the inhibition of carrier mobility by roughness and strains along the layer surface. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectric materials have aroused extensive attention in power generation to alleviate energy shortage dilemma. Good thermoelectric performance requires a high figure of merit, ZT = S2rT/j, where S, r, T and j are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. However, it is the relatively low conversion efficiency that limits the widespread applications. As the strong coupling of the three parameters (S, r and j) involves in the complicated details of band structure, it is challenging to optimize those simultaneously. Bi2Te3 has a tetradymite crystal structure: along the c axis, ionic and covalent bonds joint the five hexagonal closed-packed layers, with Van der Waals force connecting the quintuple layers. Undoubtedly, the anisotropic structure will result in the anisotropy of thermoelectric properties. Diverse concepts and strategies have been employed to improve ZT effectively, especially low dimensional nanotechnology, such as quantum-dot superlattices [1] and nanowires [2]. Nevertheless, that advantage is partially offset owing to the challenge for highvolume applications, because those procedures appear complicated and time consuming. On the contrary, pressure tuning proves a potentially effective approach to enhance thermoelectric perfor⇑ Corresponding author. Tel./fax: +86 431 85168858. E-mail address: [email protected] (H. Ma). http://dx.doi.org/10.1016/j.jallcom.2015.01.271 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
mance of Bi2Te3 based solids [3,4]; large volume multi-anvil high pressure technique allows the synthesis of macroscopic amount of materials [5]. The available literature about pressure is mainly limited to structural phase transition and pressure tuning on as-synthesized samples. Recently, pressure merely on order of 1 GPa has been proved effective to yield ultra low thermal conductivity [6]. Consequently, we report a rapid, scalable, and simple high pressure and high temperature method, which is a combination of synthesis and pressure tuning concurrently [7,8]. The synthetic procedure was as rapid as 30 min. In this paper the role of pressure in tuning thermoelectric properties and preferential grain orientation is discussed. Furthermore, the nearly isotropic Seebeck coefficient and weak anisotropic electrical resistivity were measured at ambient conditions. 2. Experimental procedure Appropriate amounts of Bi (5 N) and Te (5 N) were weighed according to the stoichiometry of Bi2Te3, grinded under Ar atmosphere, and then consolidated into a graphite mold to cold press. The as-pressed bulk disk (10.5 mm in diameter and 4 mm in thickness) was encased with molybdenum foil to prevent contamination and then subjected to a high temperature and high pressure (HPHT) condition for 30 min, which was achieved by a China-type large volume cubic high-pressure apparatus (CHPA) (SPD-6x1200). Then the chamber was quenched to room temperature at a high cooling rate of at least 400 °C min1. The temperature was measured with type Pt RH/Pt Rh6 thermocouple junction, which was embedded at surface of the sample. The pressure was calibrated by the change in resistance of standard materials.
Y. Zhang et al. / Journal of Alloys and Compounds 632 (2015) 514–519 The X-ray diffraction (XRD) patterns were obtained using a Cu Ka (k = 1.5418 Å) radiation (D/MAX-RA). A bar of 3 3 10 mm was cut from the disk to measure thermoelectric properties along two directions. The Seebeck coefficient was derived from the slope of the voltage versus temperature-difference curves (obtained by VI-Logger program from NI-company), and the uncertainty was within 5% compared with ZEM-3 (ULVAC-RIKO) at room temperature. The electrical resistivity was measured by a typical dc four-point technique. The fractured surfaces were investigated with field emission scanning electron microscopy (FESEM, JEOL JSM-6700F), and the microstructures were analyzed by high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2200FS).
3. Results and discussion 3.1. XRD and FESEM The X-ray diffraction analysis was performed on the sections perpendicular to c axis (Fig. 1a). All characteristic peaks are indexed to the rhombohedral structure of Bi2Te3 (JCPDS 15-0863) without any noticeable impurity phase. The peak positions shift to higher angle evince the shrinkage of lattice constants, manifesting that high pressure has a distinct impact on interatomic parameters. Furthermore, it indirectly illustrates that the high pressure effect is trapped and quenchable to atmospheric conditions. Apparently, a considerable preferential grain orientation of (0 0 l) planes is observed compared with standard spectrum. Specifically, the integrated intensity ratio of the (0 0 15) to (0 1 5) peak reaches 22 for the sample synthesized at 1 GPa, far above the standard one (I(0 0 1 5)/I(0 1 5) = 0.06). However, the sample of 4.5 GPa reveals no (0 0 l) orientation, resulting from decreased grain size and nonbasal slips by heavy plastic deformation. FESEM images are presented to survey the morphology change. Large grain sheets with an average thick of 0.5 lm are orderly arranged to form stacking of basal planes (Fig. 1b and c). The sheets show relative slippage along the cleavage planes, owing to the two-dimensional nucleation mechanism and shear force by high pressure. Besides, it reveals that each sheet consists of ultrathin slices with several nanometers in thickness (indicated by the circles). However, no typical laminar structure is shown in higher pressure (Fig. 1d). It can be inferred that more cracks appear with increasing pressure.
515
3.2. HRTEM Three aspects of the microstructures are discussed by HRTEM analysis (Fig. 2). First, high-crystallinity grain with clear coherent boundaries incorporates an amorphous region (Fig. 2a); while other image reveals that a nanocrystal is embedded in amorphous matrix (Fig. 2b) (marked by the enclosed area). Although it remains unclear how these nanostructures emerge, intriguingly, the microstructures manufactured by high pressure are comparatively analogous to those in nanoengineering, for instance, ball milling [9] and melt spinning [10]. Second, Fig. 2c shows a great quantity of lattice distortions, including lattice curvatures and dislocations (indicated by lines and circles). Distinct lattice bending appears as dislocation is present. The occurrence of the distortions can be ascribed to pressure-induced strain and shear force. The inverse fast Fourier transform (IFFT) reconstructed image (Fig. 2d) yields the distribution of dislocations (pointed by circles) and lattice curvatures. In addition, shear strain map profiles by Biswas confirmed the existence of strain distribution around dislocations [11]. Third, fringes caused by strain fields with high dark and white contrast occur (Fig. 2e). It seems that several stacking planes are folded by compressive stress and afterward turned into wrinkles. Similarly, the fringe features conceptually resemble to those in previous works [12,13], which is called structural modulations. In contrast, the most obvious distinction is that the stripe periodicity of about 2 nm by pressure is relatively shorter. After all, stripes have been proved general features for Bi2Te3 [13,14]. In addition, the region with good crystallinity displays an interplanar distance of 0.238 nm (Fig. 2f), which is approximately consistent with that of (1 0 10) planes. 3.3. Thermoelectric properties and corresponding anisotropic features Absolute value of n-type conduction Seebeck coefficient exhibits two peaks about 140 lV/K at 1 and 4 GPa in both directions (Fig. 3a). The non-monotonic tendency of thermopower is coincident with that by pressure application in Ref. [15]. However, the notable distinction is that the investigation in Ref. [15] was the
Fig. 1. (a) XRD patterns of Bi2Te3 bulks by various pressures from the sections perpendicular to c axis. FESEM images of fractured surfaces of Bi2Te3 prepared at (b) 1 GPa, (c) 3 GPa, (d) 4.5 GPa.
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Fig. 2. Representative HRTEM images of Bi2Te3 synthesized at 1 GPa. (a) Amorphous region within crystalline matrix. (b) Nanocrystal in amorphous matrix. (c) Dislocations and lattice curvatures. (d) The IFFT image corresponding to (c). (e) Stripes. (f) Crystalline grain lattices.
in-situ measurement on prefabricated ingot, while in this letter the samples prepared directly from elements via high pressure were measured at ambient conditions. Besides, the thermopower values in our work are significantly larger than those by high pressure in Ref. [16]. The thermopower has an intimate association with band structure, particularly near the Fermi surface. A high degeneracy is considered to bring about a large thermopower, since thermopower increases with reduced carrier concentration in each pocket for constant carrier summation [17]. Recently, a theoretical simulation manifested that as the pressure up to 1.5 GPa, the degeneracy factor of Sb2Te3 was made from 6 to 12 by the occurrence of another additional sixfold peak, and then it reverted to a single sixfold degenerate band [18]. Therefore, the first peak can be attributed to electronic topological transition of Fermi surface as the pressure up to 1 GPa, and the enlarged pressure-induced band degeneracy is responsible for the first peak. Besides, the increasing effective mass with pressure is also beneficial for an enhanced ZT. Consequently, all parameters
simultaneously result in a better performance for the flowing relationship [18]
ZT c
3=2 2 1 2kB T k Tl ðmx my mz Þ1=2 B x 2 2 3p ekL h
ð1Þ
where mi is the effective mass of the carriers (holes or electrons) in the i direction, c is the band degeneracy. With regard to the second maximum of thermopower, no highpressure distorted phase [15] is detected from the XRD patterns under ambient condition. Although it is not exactly distinct what induces the second augment, some plausible speculations are proposed to be verified: (a) A sharp diminution in forbidden gap at 3– 5 GPa [19] might imply a drastic band structure alteration; (b) Pressure dependence of the Raman linewidths [20] and Hall coefficient [21] suggested a possibility of an electronic topological transition at 4 GPa. The Seebeck coefficient seems basically isotropic in the two measured directions within the measurement accuracy, as do other
Y. Zhang et al. / Journal of Alloys and Compounds 632 (2015) 514–519
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Fig. 3. Pressure dependence of thermoelectric properties of n-type Bi2Te3: (a) the absolute value of thermopower, (b) electrical resistivity, (c) power factor, and (d) the relative change in power factor. The solid lines are guides to the eye to indicate the general tendency.
preferred orientation materials. It means that the Seebeck coefficient is intense insensitive to the grain texture. Furthermore, theoretical study also indicated the isotropy in the Seebeck coefficient based on the assumption of relaxation time approximation in a six-valley model [22]. The electrical resistivity curve drops down with increasing pressure, at least a 30% decrease (Fig. 3b). The decrease of electrical resistivity originates from the two competing factors: carrier concentration and mobility. Though the higher pressure impedes the carrier mobility for the decreased grain size and increased defect amount, the increased carrier concentration seems enough to compensate for the loss in mobility. More defects are introduced because the formation energies of antisite defects and vacancies become lower with increasing pressure [23]. Besides, excess donor-like defects are generated from non-basal slips produced by heavy plastic deformation [24]. The additional carrier concentration should be responsible for the decrease of electrical resistivity. The electrical resistivity perpendicular to c axis is smaller than that in parallel direction. Since the carrier concentration is isotropic, the anisotropic feature originates from the anisotropic carrier mobility, because of the layered structure and different bonds along different crystallographic directions. The lower mobility along c axis can be implied by intense scattering from the layered interface and microcracks along cleavage surface. According to the two peaks in the Seebeck coefficient, the curve of power factor reveals two enhanced segments (Fig. 3c). The maxima reach 14.80 and 15.01 lW cm1 K2 at 4 GPa in parallel and perpendicular to c axis respectively. From the relative change curve (Fig. 3d), power factor at 4 GPa shows 10 times improvement
compared with 0.5 GPa in parallel directions, which demonstrates the beneficial impact of pressure on thermoelectric materials. As to anisotropic properties, power factors along the direction perpendicular to c axis are somewhat larger, originating from the relatively higher electrical conductivity. In order to investigate the degree of anisotropy quantitatively, orientation factor by Lotgering method and anisotropy factor of electrical resistivity (the ratio of parallel to perpendicular c-axis direction) are shown (Fig. 4a). As the samples pressurized over 1 GPa, the orientation factors vary in a range of 0.7–0.9, which are much higher than other synthesis methods [25,26]. Anomalously, the anisotropy factor is distinctly lower compared with the corresponding high orientation factor. The maximum of anisotropy factor is 1.41 at 1 GPa corresponding to highest orientation factor. The room temperature (300 K) electrical resistivities of Bi2Te3-based samples are summarized in Table 1. Compare to the samples [6,22,27–30] by other methods, the samples in our work show abnormally lower anisotropy factors. The anisotropy factors, which should have been more pronounced, might be partially counteracted by the obstructive impact of roughness (microstructures and defects) and strains on mobility along layer surface in perpendicular c-axis direction. The carrier mobility is expressed by
l ¼ es =m
ð2Þ
where e, s⁄, and m⁄ are the unit charge, relaxation time tensor, and effective mass respectively. The roughness and strains in the layer interface confirmed by HRTEM can notably enhance carrier scattering, shorten the electron mean free path, and decrease the s⁄. Meanwhile, the Seebeck coefficient has a small dependence on
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Fig. 4. (a) Orientation factor and anisotropy factor, (b) comparison of XRD patterns obtained from two surfaces. FESEM graphs of Bi2Te3 on the section perpendicular (c) and parallel (d) to c axis.
Table 1 Room temperature electrical resistivity of Bi2Te3-based samples compared with the sample in present paper. The symbols (parallel and perpendicular) refer to c-axis preferred orientation. Electrical resistivity (105 X cm)
Samples
Bi0.5Sb1.5Te3 Bi2Te2.85Se0.15 Bi0.5Sb1.5Te3 Bi2Te3 Bi0.4Sb1.6Te3 Bi2Te2.79Se0.21 Bi2Te3(1 GPa)
Single crystal Single crystal Polycrystal Polycrystal Polycrystal Polycrystal Polycrystal
Para c-axis
Perp c-axis
2.91 4.40 7.69 4.84 2.28 1.47 2.25
0.89 0.67 1.73 0.79 1.07 0.65 1.6
carrier relaxation time at room temperature [31], which results in a larger electrical resistivity in perpendicular c-axis direction. Consequently, the roughness and strains generate a weak anisotropy in electrical resistivity. The fractured sections in both directions are examined by XRD and FESEM to acquire detailed texture features. The difference (Fig. 4b), obtained by subtracting perpendicular curve from parallel normalized by (0 1 5) peak intensity, clearly reveals the (0 0 l) preferred orientation. This XRD patterns are coincident with grain texture. The plate-like grains rearrange and stack together regularly with normal direction of the laminate orienting to the c axis (Fig. 4c and d). The textured construction is the interaction of the intrinsic lamellar structure bonding by van der Waals force and basal plane slip induced by shear force.
Anisotropy factor
References
3.27 6.56 4.44 6.13 2.13 2.26 1.41
Ref. [27] Ref. [28] Ref. [6] Ref. [29] Ref. [22] Ref. [30] Present work
4. Conclusions In summary, a feasible high temperature and high pressure method was utilized to synthesis Bi2Te3 bulks. By contrast, the high pressure method has an advantage over conventional synthesized approaches with synthesis time of 30 min. Nanocrystals, dislocations, and structural modulations have been characterized by HRTEM images. The enhancement in the Seebeck coefficient is caused by the variation of the band structure under pressure. Over the whole investigated pressure range, the electrical resistivity shows downward trend. Large orientation factor in grain texture and isotropic feature in thermopower were investigated. In particular, the relatively weak anisotropy in electrical resistivity can be attributed to the roughness and strains in layer surface.
Y. Zhang et al. / Journal of Alloys and Compounds 632 (2015) 514–519
Furthermore, the HPHT method can be applicable for large-scale commercial application combined with optimal chemical doping. Acknowledgements This work was financially supported by National Natural Science Foundation of China (51171070, 51071074 and 51301024), and Project No. 2014007 supported by Graduate Innovation Fund of Jilin University. References [1] R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O’quinn, Nature 413 (2001) 597–602. [2] G. Zhang, B. Kirk, L.A. Jauregui, H. Yang, X. Xu, Y.P. Chen, Y. Wu, Nano Lett. 12 (2011) 56–60. [3] X. Guo, X.P. Jia, K.K. Jie, H.R. Sun, Y.W. Zhang, B. Sun, H.A. Ma, CrystEngComm 15 (2013) 7236–7242. [4] D. Polvani, J. Meng, N. Chandra Shekar, J. Sharp, J. Badding, Chem. Mater. 13 (2001) 2068–2071. [5] J. Badding, Annu. Rev. Mater. Sci. 28 (1998) 631–658. [6] S. Grasso, N. Tsujii, Q. Jiang, J. Khaliq, S. Maruyama, M. Miranda, K. Simpson, T. Mori, M.J. Reece, J. Mater. Chem. C 1 (2013) 2362–2367. [7] H.R. Sun, X.P. Jia, L. Deng, P. Lv, X. Guo, B. Sun, Y.W. Zhang, B.W. Liu, H.A. Ma, J. Alloys Comp. 615 (2014) 1056–1059. [8] T.C. Su, X.P. Jia, H.A. Ma, J.G. Guo, Y.P. Jiang, N. Dong, L. Deng, X.B. Zhao, T.J. Zhu, C. Wei, J. Alloys Comp. 468 (2009) 410–413. [9] B. Poudel, Q. Hao, Y. Ma, Y. Lan, A. Minnich, B. Yu, X. Yan, D. Wang, A. Muto, D. Vashaee, Science 320 (2008) 634–638. [10] S.Y. Wang, W.J. Xie, H. Li, X.F. Tang, Q.J. Zhang, J. Electron. Mater. 40 (2011) 1150–1157. [11] K. Biswas, J. He, Q. Zhang, G. Wang, C. Uher, V.P. Dravid, M.G. Kanatzidis, Nat. Chem. 3 (2011) 160–166.
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[12] W.S. Liu, Q. Zhang, Y. Lan, S. Chen, X. Yan, Q. Zhang, H. Wang, D. Wang, G. Chen, Z. Ren, Adv. Energy Mater. 1 (2011) 577–578. [13] N. Peranio, O. Eibl, J. Appl. Phys. 103 (2008) 024314. [14] Z.C. Chen, K. Suzuki, S. Miura, K. Nishimura, K. Ikeda, Mater. Sci. Eng. A 500 (2009) 70–78. [15] S.V. Ovsyannikov, V.V. Shchennikov, G.V. Vorontsov, A.Y. Manakov, A.Y. Likhacheva, V.A. Kulbachinskii, J. Appl. Phys. 104 (2008) 053713–053715. [16] M.K. Jacobsen, S.V. Sinogeikin, R.S. Kumar, A.L. Cornelius, J. Phys. Chem. Solids 73 (2012) 1154–1158. [17] P. Larson, S. Mahanti, M. Kanatzidis, Phys. Rev. B 61 (2000) 8162–8171. [18] P. Larson, Phys. Rev. B 74 (2006) 205113. [19] S.V. Ovsyannikov, V.V. Shchennikov, Chem. Mater. 22 (2010) 635–647. [20] F.J. Manjón, R. Vilaplana, O. Gomis, E. Pérez-González, D. Santamaría-Pérez, V. Marín-Borrás, A. Segura, J. González, P. Rodríguez-Hernández, A. Muñoz, C. Drasar, V. Kucek, V. Muñoz-Sanjosé, Phys. Stat. Sol. B 250 (2013) 669–676. [21] J.K. Zhang, C.L. Liu, X. Zhang, F. Ke, Y.H. Han, G. Peng, Y.Z. Ma, C.X. Gao, Appl. Phys. Lett. 103 (2013) 052102. [22] O. Ben-Yehuda, R. Shuker, Y. Gelbstein, Z. Dashevsky, M.P. Dariel, J. Appl. Phys. 101 (2007) 113707. [23] T. Thonhauser, G.S. Jeon, G. Mahan, J. Sofo, Phys. Rev. B 68 (2003) 205207. [24] J.M. Schultz, J.P. McHugh, W.A. Tiller, J. Appl. Phys. 33 (1962) 2443–2450. [25] L.P. Hu, T.J. Zhu, Y.G. Wang, H.H. Xie, Z.J. Xu, X.B. Zhao, NPG Asia Mater. 6 (2014) e88. [26] J. Jiang, L.D. Chen, S.Q. Bai, Q. Yao, Q. Wang, Scripta Mater. 52 (2005) 347–351. [27] T. Caillat, M. Carle, P. Pierrat, H. Scherrer, S. Scherrer, J. Phys. Chem. Solids 53 (1992) 1121–1129. [28] M. Carle, P. Pierrat, C. Lahalle-Gravier, S. Scherrer, H. Scherrer, J. Phys. Chem. Solids 56 (1995) 201–209. [29] Y. Zhang, T. Day, M.L. Snedaker, H. Wang, S. Krämer, C.S. Birkel, X. Ji, D. Liu, G.J. Snyder, G.D. Stucky, Adv. Mater. 24 (2012) 5065–5070. [30] J. Jiang, L.D. Chen, S.Q. Bai, Q. Yao, Q. Wang, Mater. Sci. Eng.: B 117 (2005) 334– 338. [31] T. Kyratsi, J.S. Dyck, W. Chen, D.Y. Chung, C. Uher, K.M. Paraskevopoulos, M.G. Kanatzidis, J. Appl. Phys. 92 (2002) 965–975.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Evolution of thermoelectric properties and anisotropic features of Bi2Te3 prepared by high pressure and high temperature Yuewen Zhang a, Xiaopeng Jia a, Le Deng b, Xin Guo a, Hairui Sun a, Bing Sun a, Binwu Liu a, Hongan Ma a,⇑ a b
National Key Lab of Superhard Materials, Jilin University, Changchun 130012, China Department of Material Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China
a r t i c l e
i n f o
Article history: Received 21 November 2014 Received in revised form 23 January 2015 Accepted 29 January 2015 Available online 7 February 2015 Keywords: Thermoelectric materials Anisotropy Crystal defects Bi2Te3 High pressure
a b s t r a c t Bi2Te3 bulks were synthesized by a simple high pressure and high temperature method to investigate the texture evolution and anisotropic thermoelectric properties. The multiple microstructures, defects and strains were revealed by HRTEM images. Pressure tuning resulted in increasing of electrical conductivity and thermopower, yielding a 10-fold improvement of power factor at 4 GPa, with respect to that at 0.5 GPa. The abnormally weak anisotropy of electrical resistivity was attributed to the inhibition of carrier mobility by roughness and strains along the layer surface. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectric materials have aroused extensive attention in power generation to alleviate energy shortage dilemma. Good thermoelectric performance requires a high figure of merit, ZT = S2rT/j, where S, r, T and j are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. However, it is the relatively low conversion efficiency that limits the widespread applications. As the strong coupling of the three parameters (S, r and j) involves in the complicated details of band structure, it is challenging to optimize those simultaneously. Bi2Te3 has a tetradymite crystal structure: along the c axis, ionic and covalent bonds joint the five hexagonal closed-packed layers, with Van der Waals force connecting the quintuple layers. Undoubtedly, the anisotropic structure will result in the anisotropy of thermoelectric properties. Diverse concepts and strategies have been employed to improve ZT effectively, especially low dimensional nanotechnology, such as quantum-dot superlattices [1] and nanowires [2]. Nevertheless, that advantage is partially offset owing to the challenge for highvolume applications, because those procedures appear complicated and time consuming. On the contrary, pressure tuning proves a potentially effective approach to enhance thermoelectric perfor⇑ Corresponding author. Tel./fax: +86 431 85168858. E-mail address: [email protected] (H. Ma). http://dx.doi.org/10.1016/j.jallcom.2015.01.271 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
mance of Bi2Te3 based solids [3,4]; large volume multi-anvil high pressure technique allows the synthesis of macroscopic amount of materials [5]. The available literature about pressure is mainly limited to structural phase transition and pressure tuning on as-synthesized samples. Recently, pressure merely on order of 1 GPa has been proved effective to yield ultra low thermal conductivity [6]. Consequently, we report a rapid, scalable, and simple high pressure and high temperature method, which is a combination of synthesis and pressure tuning concurrently [7,8]. The synthetic procedure was as rapid as 30 min. In this paper the role of pressure in tuning thermoelectric properties and preferential grain orientation is discussed. Furthermore, the nearly isotropic Seebeck coefficient and weak anisotropic electrical resistivity were measured at ambient conditions. 2. Experimental procedure Appropriate amounts of Bi (5 N) and Te (5 N) were weighed according to the stoichiometry of Bi2Te3, grinded under Ar atmosphere, and then consolidated into a graphite mold to cold press. The as-pressed bulk disk (10.5 mm in diameter and 4 mm in thickness) was encased with molybdenum foil to prevent contamination and then subjected to a high temperature and high pressure (HPHT) condition for 30 min, which was achieved by a China-type large volume cubic high-pressure apparatus (CHPA) (SPD-6x1200). Then the chamber was quenched to room temperature at a high cooling rate of at least 400 °C min1. The temperature was measured with type Pt RH/Pt Rh6 thermocouple junction, which was embedded at surface of the sample. The pressure was calibrated by the change in resistance of standard materials.
Y. Zhang et al. / Journal of Alloys and Compounds 632 (2015) 514–519 The X-ray diffraction (XRD) patterns were obtained using a Cu Ka (k = 1.5418 Å) radiation (D/MAX-RA). A bar of 3 3 10 mm was cut from the disk to measure thermoelectric properties along two directions. The Seebeck coefficient was derived from the slope of the voltage versus temperature-difference curves (obtained by VI-Logger program from NI-company), and the uncertainty was within 5% compared with ZEM-3 (ULVAC-RIKO) at room temperature. The electrical resistivity was measured by a typical dc four-point technique. The fractured surfaces were investigated with field emission scanning electron microscopy (FESEM, JEOL JSM-6700F), and the microstructures were analyzed by high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2200FS).
3. Results and discussion 3.1. XRD and FESEM The X-ray diffraction analysis was performed on the sections perpendicular to c axis (Fig. 1a). All characteristic peaks are indexed to the rhombohedral structure of Bi2Te3 (JCPDS 15-0863) without any noticeable impurity phase. The peak positions shift to higher angle evince the shrinkage of lattice constants, manifesting that high pressure has a distinct impact on interatomic parameters. Furthermore, it indirectly illustrates that the high pressure effect is trapped and quenchable to atmospheric conditions. Apparently, a considerable preferential grain orientation of (0 0 l) planes is observed compared with standard spectrum. Specifically, the integrated intensity ratio of the (0 0 15) to (0 1 5) peak reaches 22 for the sample synthesized at 1 GPa, far above the standard one (I(0 0 1 5)/I(0 1 5) = 0.06). However, the sample of 4.5 GPa reveals no (0 0 l) orientation, resulting from decreased grain size and nonbasal slips by heavy plastic deformation. FESEM images are presented to survey the morphology change. Large grain sheets with an average thick of 0.5 lm are orderly arranged to form stacking of basal planes (Fig. 1b and c). The sheets show relative slippage along the cleavage planes, owing to the two-dimensional nucleation mechanism and shear force by high pressure. Besides, it reveals that each sheet consists of ultrathin slices with several nanometers in thickness (indicated by the circles). However, no typical laminar structure is shown in higher pressure (Fig. 1d). It can be inferred that more cracks appear with increasing pressure.
515
3.2. HRTEM Three aspects of the microstructures are discussed by HRTEM analysis (Fig. 2). First, high-crystallinity grain with clear coherent boundaries incorporates an amorphous region (Fig. 2a); while other image reveals that a nanocrystal is embedded in amorphous matrix (Fig. 2b) (marked by the enclosed area). Although it remains unclear how these nanostructures emerge, intriguingly, the microstructures manufactured by high pressure are comparatively analogous to those in nanoengineering, for instance, ball milling [9] and melt spinning [10]. Second, Fig. 2c shows a great quantity of lattice distortions, including lattice curvatures and dislocations (indicated by lines and circles). Distinct lattice bending appears as dislocation is present. The occurrence of the distortions can be ascribed to pressure-induced strain and shear force. The inverse fast Fourier transform (IFFT) reconstructed image (Fig. 2d) yields the distribution of dislocations (pointed by circles) and lattice curvatures. In addition, shear strain map profiles by Biswas confirmed the existence of strain distribution around dislocations [11]. Third, fringes caused by strain fields with high dark and white contrast occur (Fig. 2e). It seems that several stacking planes are folded by compressive stress and afterward turned into wrinkles. Similarly, the fringe features conceptually resemble to those in previous works [12,13], which is called structural modulations. In contrast, the most obvious distinction is that the stripe periodicity of about 2 nm by pressure is relatively shorter. After all, stripes have been proved general features for Bi2Te3 [13,14]. In addition, the region with good crystallinity displays an interplanar distance of 0.238 nm (Fig. 2f), which is approximately consistent with that of (1 0 10) planes. 3.3. Thermoelectric properties and corresponding anisotropic features Absolute value of n-type conduction Seebeck coefficient exhibits two peaks about 140 lV/K at 1 and 4 GPa in both directions (Fig. 3a). The non-monotonic tendency of thermopower is coincident with that by pressure application in Ref. [15]. However, the notable distinction is that the investigation in Ref. [15] was the
Fig. 1. (a) XRD patterns of Bi2Te3 bulks by various pressures from the sections perpendicular to c axis. FESEM images of fractured surfaces of Bi2Te3 prepared at (b) 1 GPa, (c) 3 GPa, (d) 4.5 GPa.
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Fig. 2. Representative HRTEM images of Bi2Te3 synthesized at 1 GPa. (a) Amorphous region within crystalline matrix. (b) Nanocrystal in amorphous matrix. (c) Dislocations and lattice curvatures. (d) The IFFT image corresponding to (c). (e) Stripes. (f) Crystalline grain lattices.
in-situ measurement on prefabricated ingot, while in this letter the samples prepared directly from elements via high pressure were measured at ambient conditions. Besides, the thermopower values in our work are significantly larger than those by high pressure in Ref. [16]. The thermopower has an intimate association with band structure, particularly near the Fermi surface. A high degeneracy is considered to bring about a large thermopower, since thermopower increases with reduced carrier concentration in each pocket for constant carrier summation [17]. Recently, a theoretical simulation manifested that as the pressure up to 1.5 GPa, the degeneracy factor of Sb2Te3 was made from 6 to 12 by the occurrence of another additional sixfold peak, and then it reverted to a single sixfold degenerate band [18]. Therefore, the first peak can be attributed to electronic topological transition of Fermi surface as the pressure up to 1 GPa, and the enlarged pressure-induced band degeneracy is responsible for the first peak. Besides, the increasing effective mass with pressure is also beneficial for an enhanced ZT. Consequently, all parameters
simultaneously result in a better performance for the flowing relationship [18]
ZT c
3=2 2 1 2kB T k Tl ðmx my mz Þ1=2 B x 2 2 3p ekL h
ð1Þ
where mi is the effective mass of the carriers (holes or electrons) in the i direction, c is the band degeneracy. With regard to the second maximum of thermopower, no highpressure distorted phase [15] is detected from the XRD patterns under ambient condition. Although it is not exactly distinct what induces the second augment, some plausible speculations are proposed to be verified: (a) A sharp diminution in forbidden gap at 3– 5 GPa [19] might imply a drastic band structure alteration; (b) Pressure dependence of the Raman linewidths [20] and Hall coefficient [21] suggested a possibility of an electronic topological transition at 4 GPa. The Seebeck coefficient seems basically isotropic in the two measured directions within the measurement accuracy, as do other
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Fig. 3. Pressure dependence of thermoelectric properties of n-type Bi2Te3: (a) the absolute value of thermopower, (b) electrical resistivity, (c) power factor, and (d) the relative change in power factor. The solid lines are guides to the eye to indicate the general tendency.
preferred orientation materials. It means that the Seebeck coefficient is intense insensitive to the grain texture. Furthermore, theoretical study also indicated the isotropy in the Seebeck coefficient based on the assumption of relaxation time approximation in a six-valley model [22]. The electrical resistivity curve drops down with increasing pressure, at least a 30% decrease (Fig. 3b). The decrease of electrical resistivity originates from the two competing factors: carrier concentration and mobility. Though the higher pressure impedes the carrier mobility for the decreased grain size and increased defect amount, the increased carrier concentration seems enough to compensate for the loss in mobility. More defects are introduced because the formation energies of antisite defects and vacancies become lower with increasing pressure [23]. Besides, excess donor-like defects are generated from non-basal slips produced by heavy plastic deformation [24]. The additional carrier concentration should be responsible for the decrease of electrical resistivity. The electrical resistivity perpendicular to c axis is smaller than that in parallel direction. Since the carrier concentration is isotropic, the anisotropic feature originates from the anisotropic carrier mobility, because of the layered structure and different bonds along different crystallographic directions. The lower mobility along c axis can be implied by intense scattering from the layered interface and microcracks along cleavage surface. According to the two peaks in the Seebeck coefficient, the curve of power factor reveals two enhanced segments (Fig. 3c). The maxima reach 14.80 and 15.01 lW cm1 K2 at 4 GPa in parallel and perpendicular to c axis respectively. From the relative change curve (Fig. 3d), power factor at 4 GPa shows 10 times improvement
compared with 0.5 GPa in parallel directions, which demonstrates the beneficial impact of pressure on thermoelectric materials. As to anisotropic properties, power factors along the direction perpendicular to c axis are somewhat larger, originating from the relatively higher electrical conductivity. In order to investigate the degree of anisotropy quantitatively, orientation factor by Lotgering method and anisotropy factor of electrical resistivity (the ratio of parallel to perpendicular c-axis direction) are shown (Fig. 4a). As the samples pressurized over 1 GPa, the orientation factors vary in a range of 0.7–0.9, which are much higher than other synthesis methods [25,26]. Anomalously, the anisotropy factor is distinctly lower compared with the corresponding high orientation factor. The maximum of anisotropy factor is 1.41 at 1 GPa corresponding to highest orientation factor. The room temperature (300 K) electrical resistivities of Bi2Te3-based samples are summarized in Table 1. Compare to the samples [6,22,27–30] by other methods, the samples in our work show abnormally lower anisotropy factors. The anisotropy factors, which should have been more pronounced, might be partially counteracted by the obstructive impact of roughness (microstructures and defects) and strains on mobility along layer surface in perpendicular c-axis direction. The carrier mobility is expressed by
l ¼ es =m
ð2Þ
where e, s⁄, and m⁄ are the unit charge, relaxation time tensor, and effective mass respectively. The roughness and strains in the layer interface confirmed by HRTEM can notably enhance carrier scattering, shorten the electron mean free path, and decrease the s⁄. Meanwhile, the Seebeck coefficient has a small dependence on
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Fig. 4. (a) Orientation factor and anisotropy factor, (b) comparison of XRD patterns obtained from two surfaces. FESEM graphs of Bi2Te3 on the section perpendicular (c) and parallel (d) to c axis.
Table 1 Room temperature electrical resistivity of Bi2Te3-based samples compared with the sample in present paper. The symbols (parallel and perpendicular) refer to c-axis preferred orientation. Electrical resistivity (105 X cm)
Samples
Bi0.5Sb1.5Te3 Bi2Te2.85Se0.15 Bi0.5Sb1.5Te3 Bi2Te3 Bi0.4Sb1.6Te3 Bi2Te2.79Se0.21 Bi2Te3(1 GPa)
Single crystal Single crystal Polycrystal Polycrystal Polycrystal Polycrystal Polycrystal
Para c-axis
Perp c-axis
2.91 4.40 7.69 4.84 2.28 1.47 2.25
0.89 0.67 1.73 0.79 1.07 0.65 1.6
carrier relaxation time at room temperature [31], which results in a larger electrical resistivity in perpendicular c-axis direction. Consequently, the roughness and strains generate a weak anisotropy in electrical resistivity. The fractured sections in both directions are examined by XRD and FESEM to acquire detailed texture features. The difference (Fig. 4b), obtained by subtracting perpendicular curve from parallel normalized by (0 1 5) peak intensity, clearly reveals the (0 0 l) preferred orientation. This XRD patterns are coincident with grain texture. The plate-like grains rearrange and stack together regularly with normal direction of the laminate orienting to the c axis (Fig. 4c and d). The textured construction is the interaction of the intrinsic lamellar structure bonding by van der Waals force and basal plane slip induced by shear force.
Anisotropy factor
References
3.27 6.56 4.44 6.13 2.13 2.26 1.41
Ref. [27] Ref. [28] Ref. [6] Ref. [29] Ref. [22] Ref. [30] Present work
4. Conclusions In summary, a feasible high temperature and high pressure method was utilized to synthesis Bi2Te3 bulks. By contrast, the high pressure method has an advantage over conventional synthesized approaches with synthesis time of 30 min. Nanocrystals, dislocations, and structural modulations have been characterized by HRTEM images. The enhancement in the Seebeck coefficient is caused by the variation of the band structure under pressure. Over the whole investigated pressure range, the electrical resistivity shows downward trend. Large orientation factor in grain texture and isotropic feature in thermopower were investigated. In particular, the relatively weak anisotropy in electrical resistivity can be attributed to the roughness and strains in layer surface.
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