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High pressure synthesis, structure and thermoelectric properties of BiCuChO (Ch = S, Se, Te)
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High pressure synthesis, structure and thermoelectric properties of BiCuChO (Ch = S, Se, Te) Hongyu Zhu a,b , Taichao Su b,∗ , Hongtao Li c , Chunying Pu d , Dawei Zhou d , Pinwen Zhu a,∗ , Xin Wang a,∗ a
State Key Lab of Superhard Materials, Jilin University, Changchun 130012, China School of Physics and Electronic Information Engineering, Henan Polytechnic University, Jiaozuo 454000, China c Shanghai Entry-Exit Inspection & Quarantine Bureau, Shanghai 200135, China d College of Physics and Electronic Engineering, Nanyang Normal University, Nanyang 473061, China b
a r t i c l e
i n f o
Article history: Received 11 June 2016 Received in revised form 15 October 2016 Accepted 16 October 2016 Available online xxx Keywords: Thermoelectric High pressure BiCuChO
a b s t r a c t Bismuth copper oxychalcogenides, BiCuChO (Ch = S, Se, Te), are facile, and rapidly synthesized by high pressure method. The Rietveld refinement of powder X-ray diffractions shows that BiCuChO compounds have a layered crystal structure with a space group of P4/nmm. All the high pressure synthesised samples show semiconductor characteristics, while BiCuTeO prepared by the conventional method displays metal conducting behavior. The conducting behavior of BiCuTeO obtained in this study originates from the low crystal defect concentrations under the effects of high pressure; evidenced by density functional theory calculations. Large Seebeck coefficient ∼600 V/K was obtained for BiCuSO, due to its high carrier effect mass. BiCuChO exhibits extremely low thermal conductivity (<1 Wm−1 K−1 ), which decreases with an increase in the Ch2− ion radius. The maximum figure of merit reaches 0.03, 0.31 and 0.65 for BiCuSO, BiCuSeO and BiCuTeO, respectively, values which are comparable to those for samples prepared by the conventional, complex method. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Solid-state thermoelectric (TE) devices are generally made from heavily doped semiconductors. They can be used both as generators that directly convert heat to electricity from a heat source, and refrigeration devices that use electricity to pump heat from a cold side to a hot side without any moving parts or bulk fluids. The efficiency of TE materials and TE devices is determined by the dimensionless figure of merit (ZT), defined as ZT = S2 /, where S is the Seebeck coefficient, is the electrical conductivity and is the thermal conductivity [1]. Evidently, that the best performance of TE materials can be obtained by maximizing the Seebeck coefficient and electrical conductivity, while simultaneously minimizing the thermal conductivity. Bismuth copper oxychalcogenides, BiCuSeO, are some of the best TE materials which can be used for TE generation [2–7]. They comprise a (Cu2 Se2 )2− layer alternately stacked with (Bi2 O2 )2+ along the c axis of the tetragonal cell, wherein the (Bi2 O2 )2+
∗ Corresponding authors. E-mail addresses: [email protected] (T. Su), [email protected] (P. Zhu), xin [email protected] (X. Wang).
layer acts as a charge reservoir and the conductive (Cu2 Se2 )2− layer provides a conduction pathway for carrier transport. Because of the weak bonding between layers, low Young’s modulus and high Grüneisen parameter, an extremely low thermal conductivity (∼0.9 Wm−1 K−1 @300 K) is exhibited [2]. High ZT values of 0.7 ∼ 1.5 have been achieved by further optimizing electrical transport properties through doping [2–4], optimizing Cu vacancies [5], ball milling methods [6] and band gap tuning [7], etc. By now, most experimental studies on BiCuSeO are carried out by solid state reaction (SSR). SSR is a very simple and useful method with which to prepare oxides. However, the conventional method for BiCuSeO synthesis requires the use of high temperature annealing in sealed silica tubes under a vacuum, or argon protection, over a long period of time. This is cumbersome and difficult to carry out on an industrial scale. Besides, there are few reports of any research on other bismuth copper oxychalcogenides BiCuSO [8–10] and BiCuTeO [11,12]. Encouraged by the excellent TE performance of BiCuSeO, other novel synthesis techniques including a hydrothermal method [10], mechanical alloying [13] and self-propagating high-temperature synthesis [32] have also been used to prepare BiCuChO (Ch = S, Se, Te). The two formal methods are very facile and the last one can rapidly synthesize a sample. Apart from these methods, high
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Please cite this article in press as: H. Zhu, et al., High pressure synthesis, structure and thermoelectric properties of BiCuChO (Ch = S, Se, Te), J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.10.021
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Fig. 1. (a) Powder X-ray diffraction patterns of BiCuChO prepared by high pressure, (b) X-ray diffraction patterns of bulk BiCuChO prepared by high pressure.
pressure method can also easily synthesize TE materials. Compared with other preparation methods, the high pressure method has many advantages, including the ability to tune rapidly and cleanly, typically without introducing phase separation, or any other complicating factors [14]. It has been reported that many TE alloys including PbTe [15], AgSbTe2 [16] and CoSb3 [17] could be synthesized rapidly by the high pressure method. As far as we know, there is no report on high pressure synthesis of TE oxides. In this study, the high pressure method was employed to synthesise BiCuChO (Ch = S, Se, Te). The crystal structure, electrical transport and TE properties were then investigated.
to 650 K, were measured simultaneously by a LSR-3 apparatus (Linseis). The thermal conductivity was calculated from =DCP , where D is the thermal diffusivity measured on a LFA457 laser flash apparatus (Netzsch), is the sample density measured by the Archimedes method, and Cp is the specific heat capacity estimated by the Dulong Petit law. The densities are 98.5%, 96.3% and 93.6% of the theoretical values for BiCuSO BiCuSeO and BiCuTeO respectively. The Seebeck coefficient, resistivity and thermal diffusivity were measured under the radial direction of the samples.
2. Experimental
The density functional theory (DFT) calculations used the frozen-core projector augmented-wave (PAW) method [20,21], encoded in the Vienna ab initio simulation package (VASP) [22]. The generalized-gradient approximation (GGA) [23] of Perdew, Burke and Ernzerhof (PBE) was used for the exchange-correlation functional with the plane wave-cut-off energy of 400 eV. The crystal structure optimization for the BiCuTeO compound was carried out using 8 × 8 × 3 Monkhorst k-points for the irreducible Brillouin zone. The convergence thresholds for energy were chosen to be 10−4 eV and 10−3 eV/atom, respectively. The calculations of Bi and Cu vacancy formation energy for BiCuTeO as a function of pressure were performed in a supercell with 72 atoms according to the formation energy equation
2.1. Sample synthesis Bismuth copper oxychalcogenides, BiCuChO (Ch = S, Se, Te), were prepared with high purity (>99.99%) elements of Bi, Cu, Ch (S, Se,Te), and Bi2 O3 as sources, which were weighed according to their respective stoichiometry. After being uniformly mixed under argon protection, the mixtures were pressed into cylindrical samples (10.5 × 8.5 mm). The pole shaped sample was put into a hexagonal BN capsule, which was used as insulation material, to prevent the starting material reacting with the carbon heater placed in a pyrophyllite cube. The high pressure synthesis experiments were carried out with a cubic anvil high-pressure apparatus (XYK–6 × 12 MN, China). The high pressure apparatus and assembling diagrams are shown in Ref. [18] and Ref. [19] respectively. In the high pressure apparatus six cubic tungsten carbide anvils, each with a square anvil tip, are actuated by three pairs of hydraulic rams, forming a cubic cell, within which the sample assembly is compressed uniformly. All the samples were heated at 573 K for 20 min, then up to higher temperatures (BiCuSO ∼ 1023 K, BiCuSeO ∼ 973 K, BiCuTeO ∼ 923 K) for another 20 min.
2.3. Computational details
Ef = Ej − E0 +
ni i
(1)
where Ej and E0 are the DFT total energies of a supercell with and without the defect, respectively. The number of type i atoms removed to the host is ni and i is the atomic chemical potential. 3. Results and discussions 3.1. Crystal structure
2.2. Physical measurements X-ray diffraction (XRD) measurements with Cu-K␣ radiation were measured on an X-ray diffractometer (D/MAX-RA). Hall coefficients were measured by the Van der Pauw method using a HMS-5300 apparatus (Corpia). The Seebeck coefficient and electrical resistivity at different temperatures, from room temperature
The results of X-ray diffraction patterns for bulk and powder samples, shown in Fig. 1a and b, confirm that BiCuSO, BiCuSeO and BiCuTeO crystallize in the ZrCuSiAs structure type, with a P4/nmm space group. As seen from Fig. 1, both BiCuSO and BiCuSeO are single phase and well crystallized. However, there are minor secondary phases observed in BiCuTeO. This is similar to that of the results
Please cite this article in press as: H. Zhu, et al., High pressure synthesis, structure and thermoelectric properties of BiCuChO (Ch = S, Se, Te), J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.10.021
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Table 2 The electrical transport properties of BiCuChO (Ch = S, Se, Te) at room temperature.
Table 1 The refined crystal structure parameters of BiCuChO. Sample
BiCuSO
BiCuSeO
BiCuTeO
Cell parameter (Å)
a = b = 3.8707(1) c = 8.5482(4)
a = b = 3.9303(1) c = 8.9336(7)
a = b = 4.0428(1) c = 9.5208(8)
Bi-O (Å) Bi-Ch (Å) Cu-Ch (Å)
2.3127(1) 3.1596(5) 2.4061(3)
2.3238(8) 3.2442(2) 2.4817(6)
2.3562(7) 3.7978(5) 2.6496(3)
Cu1-Ch-Cu2 (deg) Cu1-Ch-Cu3 (deg) O1-Bi-O2 (deg) O1-Bi-O3 (deg)
107.64(4) 110.39(2) 113.61(3) 72.55(9)
103.84(6) 112.35(5) 114.64(3) 73.05(1)
99.44(2) 114.70(7) 118.16(0) 74.69(1)
Sample
Carrier concentration n (1018 cm−3 )
Hall mobility (cm2 V−1 S−1 )
Hall coefficient RH (cm−3 )
BiCuSO BiCuSeO BiCuTeO
4.00(3) 12.65(7) 116.25(0)
0.13(9) 2.20(1) 4.22(5)
1.55(9) 0.49(3) 0.05(4)
for BiCuTeO prepared by the conventional SSR method [11,12]. The impurities included in BiCuTeO are Bi2 O3 . BiCuSeO crystals with layered structures might exhibit preferential orientation. However, the XRD patterns along the radial and thickness direction for all the samples did not show any preferential orientation of the crystallites in this study. For example, the ab orientation degree for the (00l) crystal planes of the bulk sample termed as F(00l) was calculated, using the Lotgering method, by the following equations: F=
P − P0 1 − P0
P0 = P=
I0(00l)
I0(hkl)
I(00l)
I(hkl)
(2) (3)
(4)
The F(00l) value of the bulk sample is approximately 0.03, 0.01 and 0.02 for BiCuTeO, BiCuSeO and BiCuSO respectively; this small F(00l) factor indicating that the high pressure synthesized BiCuChO is not preferentially oriented in the ab-plane. Therefore, the TE transport properties for BiCuChO polycrystals prepared by the high pressure method can be assumed to be isotropic. Rietveld refinement using the General Structure Analysis System (GSAS) program package [24] was also employed to perform in-depth structural analysis of BiCuChO. The refined crystal structure parameters are summarized in Table 1. As seen from Table 1, the lengths of both the a and c axes grow linearly with the covalent radii of the Ch atoms (S ∼ 1.02 Å, Se ∼ 1.16 Å and Te ∼ 1.36 Å) in the BiCuOCh materials, which obey Vegard’s law [25]. The Bi-Ch bond lengths are longer than those of Bi-O and Cu-Ch, suggesting that the Bi-Ch bonds are much weaker. This supports the conclusion that BiCuChO compounds have a layered crystal structure. The Ch-Cu-Ch angles (␣ and ) in BiCuSO have values close to those for a regular tetrahedron (109.5◦ ). The ␣ value decreases and the  value increases as the Ch ions change from S to Se to Te. This indicates that a larger Ch ion makes the distortion larger and the tetrahedral vertex more stereoscopic. The different bond angles in the polyhedra in BiCuChO show the different bonding characters, reflecting different electronic configurations. This can further affect the TE performance of BiCuChO. 3.2. Electrical transport and thermoelectric properties The electrical transport properties, including Hall coefficients, carrier concentrations and Hall mobilities of BiCuChO, are shown in Table 2. The Hall coefficients of all the three samples are positive, indicating that electrical transport properties of BiCuChO are dominated by holes, which probably originate from a small number of Cu vacancies. The carrier concentration and Hall mobility of BiCuChO increases along with the change of Ch from S to Te. The different electrical transport properties of BiCuChO can be explained by their different electronic structures. The band gap is a major
Fig. 2. Temperature dependent electrical resistivities of BiCuChO.
factor determining electrical transport properties, reflecting how the difficulty for the carrier (electrons or holes) requires a particular minimum amount of energy for the transition. The band gap is equivalent to the energy required to jump to the conduction band by absorbing energy. So a narrow band gap semiconductor usually corresponds to large carrier concentration. According to the optical absorption measurement [26], the band gap is about 1.1 eV and 0.8 eV for BiCuSO and BiCuSeO, respectively, and the value of BiCuTeO is just 0.4–0.5 eV. The different Hall mobility with Ch in BiCuChO can be attributed to the greater hybridization of the valence-band maximum (VBM) with spatially diffuse Ch p orbitals. According to the electronic structure calculation based on DFT [27], the Ch np orbitals become less localized along with the increase of the Ch2− ion radius, and the Cu-Ch antibonding states become more hybridized. The temperature dependent electrical resistivities of BiCuChO are shown in Fig. 2. All the three samples obtained in this study show typical semiconductor conducting behavior; electrical resistivity decreases with increasing temperature. It is obvious that the values of BiCuSO are much higher than those of BiCuSeO and BiCuTeO, which is consistent with the results of carrier concentration and Hall mobility. It should be mentioned that BiCuTeO prepared by other conventional methods at ambient pressure usually shows metal conduction behaviour [11,12]. The semiconductor behavior of BiCuTeO obtained in this study probably originates from the low amount of crystal defects. In order to understand the detailed crystal defect formation mechanism, the formation energies of possible point defects were calculated based on DFT calculations. The formation energies of Cu and Bi vacancies were calculated to be 0.219 and 0.238 meV per formula unit under ambient pressure, respectively. Both types of vacancies could together contribute to the hole generation, due to their very low formation energies. As the pressure increases in BiCuTeO, both of the vacancy formation energies in Bi and Cu sites increase at a rate of ∼0.12 meV/GPa.
Please cite this article in press as: H. Zhu, et al., High pressure synthesis, structure and thermoelectric properties of BiCuChO (Ch = S, Se, Te), J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.10.021
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according to classical theories. To understand the origin of the different Seebeck coefficient for BiCuChO compounds, the effective masses were calculated by using observed carrier concentration (n) and Seebeck coefficient (S) values according to the following equation in the single band assumption: m∗ =
h2 2kB T
S=±
kB e
0
Fig. 4. Temperature dependence of the Seebeck coefficients for BiCuChO.
These calculated results indicate that high pressure can restrain the formation of crystal defects. Above 400 K, the temperature dependent resistivity behavior follows Arrhenius plots approximately, i.e., = 0 exp (En /kB T), where is the electrical resistivity, kB is the Boltzmann constant, and En is the activation energy. En was calculated from the slope of the graph of ln () vs 1/T, which is listed in the inset of Fig. 3. It can be seen that En of BiCuChO decreases with increasing Ch2− ionic radius, which agrees with the changes to their electrical resistivity. The lower the En value, the easier the motion of electrical carriers, which results in a low electrical resistivity. Fig. 4 shows the temperature dependence of the Seebeck coefficients for BiCuChO. The positive values indicate that all the samples are a p-type semiconductor, which is consistent with the result of Hall coefficient measurement. The Seebeck coefficient for BiCuSO is very large; the values are higher than 500 V/K when the temperature is higher than 400 K. Similar to the situation for resistivity, the Seebeck coefficients of BiCuSeO and BiCuTeO are much lower than that of BiCuSO. The Seebeck coefficient for a semiconductor is mainly determined by its carrier concentration and effective mass
n
2/3 (1)
4F1/2 () (r + 5/2)Fr+3/2 () (r + 3/2)Fr+1/2 ()
Fn () =
Fig. 3. Ln() vs 1/T for BiCuChO.
∞
n d 1 + e−
−
(2)
(3)
where Fn () is the n-th order Fermi integral, kB is the Boltzmann constant, e is the electron charge, h is the Planck constant and r is the scattering factor. The scattering factor r is ½, since the acoustic phonon scattering is independent of the grain size, and is generally assumed to be the main scattering mechanism at room temperature. The reduced Fermi energy = Ef /kB T was derived from the measured Seebeck coefficients by using Eq. (2). The effective mass m∗ can be obtained by substituting the calculated Fermi integral F1/2 () and carrier concentration n into Eq. (1). The estimated effective mass of the BiCuSeO is about 2.36 me ,which is slightly higher than that (∼1.74 me ) of BiCuSeO prepared by conventional solid state reaction (SSR) following the spark plasma sintering (SPS) method [5]. The effective mass of the BiCuSO is about 6.83 me , which is much higher than that of BiCuSeO and BiCuTeO (0.13 me ). So the different Seebeck coefficient of BiCuChO can be mainly attributed to the difference of the effective mass. The Ch2− radiusdependent effective mass obtained in this study is consistent with that of the DFT calculation results reported by Zou et al. [21]. Fig. 5 shows the temperature dependence of thermal conductivities for BiCuChO prepared by high pressure. As is well known, the total thermal conductivity has two main components, the lattice part ph and the electronic part e . The value of e can be simply estimated through the Wiedemann-Franz law e = LT, where L is the Lorenz number. The Lorenz number is estimated by the equation L = 1.5 + exp[-|S|/116], which was proposed by Kim et al. [28]. As seen from Figs. 5a and b, the values of both and ph, apart from that of BiCuSO at room temperature, are lower than 1Wm−1 K−1 and decrease with increasing temperature. These values are much lower than those of many well-known TE materials such as PbTe [15] and CoSb3 [17]. According to first-principal calculations and an in-situ neutron diffraction experiment [30], the presence of a localised low-energy vibrational mode on the copper is the fundamental mechanism leading to low thermal conductivity in these oxychalcogenides. As seen from Figs. 5a and b, both and ph values decrease with the increase of the Ch2− ionic radius. The low phonon thermal conductivity of BiCuTeO originates mainly from the weak Cu Te bonding. As seen from Table 1, the long bond of BiCuChO increases with an increase of Ch2− ionic radius. The weak Cu Te bond is related to low frequency copper vibrations [29] and thus results in anharmonicity and a large Grüneisen parameter in these oxychalcogenides. In addition, the high atomic mass of Te in BiCuChO is likely to produce slower sound velocities and stronger optical-acoustic scattering. Recently, Kumar and Schwingenschlögl [30] and Shao et al. [31] found that high-frequency optical phonons contribute considerably to the total thermal conductivity. The scattering of acoustical into optical phonons of BiCuChO is strongly enhanced, which dramatically reduces lattice thermal conductivity [31]. As seen from Fig. 6, the dimensionless figure of merit (ZT) values for all of the BiCuChO increase with increases in temperature. The maximum ZT value reaches 0.07, 0.31, and 0.65 for BiCuSO,
Please cite this article in press as: H. Zhu, et al., High pressure synthesis, structure and thermoelectric properties of BiCuChO (Ch = S, Se, Te), J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.10.021
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Fig. 5. (a) Temperature dependence of total thermal conductivities for BiCuChO, (b) Temperature dependence of phonon thermal conductivities for BiCuChO.
BiCuSO, BiCuSeO and BiCuTeO, respectively. All of the values are comparable to that of BiCuChO prepared using conventional methods. However, the facile and fast synthesis process shows that the high pressure method is more competitive than the conventional method. Acknowledgments This work was supported by the Fundamental Research Funds for the Universities of Henan Province (No. NSFRF140202), the Foundation for Distinguished Young Scientists of Henan Polytechnic University, (No. J2016-5) the Scientific and Technological Project under Shanghai Entry-Exit Inspection and Quarantine Bureau (No. HK021-2014), the Scientific and Technological Projects under General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China (No. 2012IK049) References Fig. 6. The dimensionless figure of merit values for the BiCuChO.
BiCuSeO and BiCuTeO, respectively. These results are comparable to that of the pristine BiCuChO samples prepared by conventional SSR, followed by the SPS method at the same temperature [3,8,11]. In particular, the high pressure method used in this work is quicker and the technique is very easy to carry out, compared to conventional methods. 4. Conclusion TE oxides BiCuChO (Ch = S, Se, Te), were facile and rapidly synthesized (less than 1 h) by the high pressure method. Structural data from powder X-ray diffraction confirms that BiCuChO compounds have a layered crystal structure. All the crystal parameters, including that of the bond lengths and bond angles are in good agreement with published data for samples prepared by conventional methods. DFT calculation results show that the high pressure method can inhibit the formation of crystal defects, so all the BiCuChO samples show typical semiconductor behavior. BiCuSO has the largest Seebeck coefficient due to its high carrier effect mass. Extremely low thermal conductivity (<1 W/mK) was obtained for all of the BiCuChO samples, and this figure decreases with the increase of Ch2− ion radius. The maximum ZT reaches 0.03, 0.31 and 0.65 for
[1] L.E. Bell, Cooling, heating, generating power, and recovering waste heat with thermoelectric systems, Science 321 (2008) 1457–1461. [2] Y.L. Pei, J. He, J. Li, F. Li, Q. Liu, W. Pan, C. Barreteau, D. Berardan, N. Dragoe, L.D. Zhao, High thermoelectric performance of oxyselenides: intrinsically low thermal conductivity of Ca-doped BiCuSeO, NPG Asia Mater. 53 (2013) 425–434. [3] L.D. Zhao, D. Berardan, Y.L. Pei, C. Byl, L. Pinsard-Gaudart, N. Dragoe, Bi1-xSrxCuSeO oxyselenides as promising thermoelectric materials, Appl. Phys. Lett. 97 (2010) 092118. [4] J.L. Lan, Y.C. Liu, B. Zhan, Y.H. Lin, B.P. Zhang, X. Yuan, W. Zhang, W. Xu, C.W. Nan, Enhanced thermoelectric properties of Pb-Doped BiCuSeO ceramics, Adv. Mater. 44 (2013) 5086–5090. [5] Y. Liu, L.D. Zhao, Y. Liu, W. Xu, F. Li, B.P. Zhang, D. Berardan, N. Dragoe, Y.H. Lin, C.W. Nan, J.F. Li, H. Zhu, Remarkable enhancement in thermoelectric performance of BiCuSeO by Cu deficiencies, J. Am. Chem. Soc. 133 (2011) 20112–20115. [6] F. Li, J.F. Li, L.D. Zhao, K. Xiang, Y. Liu, B.P. Zhang, Y.H. Lin, C.W. Nan, H.M. Zhu, Polycrystalline BiCuSeO oxide as a potential thermoelectric material, Energy Environ. Sci. 5 (2012) 7188–7195. [7] Y. Liu, J. Lan, W. Xu, Y. Liu, Y.L. Pei, B. Cheng, D.B. Liu, Y.H. Lin, L.D. Zhao, Enhanced thermoelectric performance of a BiCuSeO system via band gap tuning, Chem. Commun. 249 (2013) 8075–8077. [8] D. Berardan, J. Li, E. Amzallag, S. Mitra, J. Sui, W. Cai, N. Dragoe, Structure and transport properties of the biCuSeO-BiCuSO solid solution, Materials 8 (2015) 1043–1058. [9] D. Berthebaud, E. Guilmeau, O.I. Lebedev, A. Maignana, J. Gamonb, P. Barbouxb, The BiCu1-xOS oxysulfide: copper deficiency and electronic properties, J. Solid State Chem. 237 (2016) 292–299. [10] W.C. Sheets, E.S. Stampler, H. Kabbour, M.I. Bertoni, L. Cario, T.O. Mason, T.J. Marks, K.R. Poeppelmeier, Facile synthesis of BiCuOS by hydrothermal methods, Inorg. Chem. 25 (2007) 10741–10748. [11] T.H. An, Y.S. Lim, H.S. Choi, W.S. Seo, C.H. Park, G.R. Kim, C. Park, C.H. Lee, J.H. Shim, Point defect-assisted doping mechanism and related thermoelectric
Please cite this article in press as: H. Zhu, et al., High pressure synthesis, structure and thermoelectric properties of BiCuChO (Ch = S, Se, Te), J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.10.021
G Model JECS-10885; No. of Pages 6
ARTICLE IN PRESS H. Zhu et al. / Journal of the European Ceramic Society xxx (2016) xxx–xxx
6
[12]
[13]
[14] [15]
[16]
[17]
[18]
[19]
[20] [21]
transport properties in Pb-doped BiCuOTe, J. Mater. Chem. A 2 (2014) 19759–19764. P. Vaqueiro, G. Gu´ıelou, M. Stec, E. Guilmeau, A.V. Powell, A copper-containing oxytelluride as a promising thermoelectric material for waste heat recovery, J. Mater. Chem. A 1 (2013) 520–523. V. Pele, C. Barreteau, D. Berardan, L. Zhao, N. Dragoe, Direct synthesis of BiCuChO-type oxychalcogenides by mechanical alloying, J. Solid State Chem. 203 (2013) 187–191. J.V. Badding, High-pressure synthesis characterization, and tuning of solid-state materials, Annu. Rev. Mater. Sci. 28 (1998) 631–658. H. Fan, T. Su, H. Li, S. Li, M. Hu, B. Liu, B. Du, H. Ma, X. Jia, Enhanced low temperature thermoelectric performance and weakly temperature-dependent figure-of-merit values of PbTe-PbSe solid solutions, J. Alloy Compd. 658 (2015) 885–890. T. Su, X. Jia, H. Ma, F. Yu, Y. Tian, G. Zuo, Y. Zheng, Y. Jiang, D. Dong, L. Deng, B. Qin, S. Zheng, Enhanced thermoelectric performance of AgSbTe2 synthesized by high pressure and high temperature, J. Appl. Phys. 105 (2009) 073713. T. Su, C. He, H. Li, X. Guo, S. Li, H. Ma, X. Jia, Fast preparation and low-temperature thermoelectric properties of CoSb3, J. Electron. Mater. 42 (2013) 109–113. Q. Han, B. Liu, M. Hu, Z. Li, X. Jia, M. Li, H. Ma, S. Li, H. Xiao, Y. Li, Design an effective solution for commercial production and scientific research on gem-quality, large single-crystal diamond by high pressure and high temperature, Cryst. Growth Des. 11 (2011) 1000–1005. H. Sun, X. Jia, L. Deng, P. Lv, X. Guo, Y. Zhang, B. Sun, B. Liu, H. Ma, Effect of HPHT processing on the structure, and thermoelectric properties of Co4Sb12 co-doped with Te and Sn, J. Mater. Chem. A 3 (2015) 4637–4641. P.E. Blöchl, Projector augmented-wave method, Phys. Rev. B 50 (1994) 17953–17979. G. Kresse, D. Joubert, From ultrasoft pseudopotentials to the projector augmented-wave method, Phys. Rev. B 59 (1999) 1758–1775.
[22] G. Kresse, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54 (1996) 11169–11186. [23] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865–3868. [24] A.C. Larson, R.B. Von, Dreele general structure analysis system LANSCE, in: MS-H805, 1994 (Los Alamos, New Mexico). [25] A.R. Denton, N.W. Ashcroft, Vegard’s law, Phys. Rev. A 43 (1991) 3161–3164. [26] H. Hiramatsu, H. Yanagi, T. Kamiya, K. Ueda, M. Hirano, H. Hosono, Crystal structures, optoelectronic properties, and electronic structures of layered oxychalcogenides MCuOCh (M = Bi, La; Ch = S, Se, Te): effects of electronic configurations of M3+ ions, Chem. Mater. 20 (2007) 326–334. [27] D.F. Zou, S.H. Xie, Y.Y. Liu, J.G. Lin, J.Y. Li, Electronic structures and thermoelectric properties of layered BiCuOCh oxychalcogenides (Ch = S, Se and Te): first-principles calculations, J. Mater. Chem. A 1 (2013) 8888–8896. [28] H.S. Kim, Z.M. Gibbs, Y. Tang, H. Wang, G.J. Snyder, Characterization of Lorenz number with Seebeck coefficient measurement, APL Mater. 3 (2015) 041506. [29] P. Vaqueiro, R.A.R. Al Orabi, S.D.N. Luu, G. Gue lou, A.V. Powell, R.I. Smith, J.-P. Song, D. Wee, M. Fornari, The role of copper in the thermal conductivity of thermoelectric oxychalcogenides: do lone pairs matter? Phys. Chem. Chem. Phys. 17 (2015) 31735–31740. [30] S. Kumar, U. Schwingenschlögl, Lattice thermal conductivity in layered BiCuSeO, Phys. Chem. Chem. Phys. 18 (2016) 19158–19164. [31] H. Shao, X. Tan, G.Q. Liu, J. Jiang, H. Jiang, A first-principles study on the phonon transport in layered BiCuOSe, Sci. Rep. 6 (2016) 21035. [32] G.K. Ren, J. Lan, S. Butt, K.J. Ventura, Y.H. Lin, C.W. Nan, Enhanced thermoelectric properties in Pb-doped BiCuSeO oxyselenides prepared by ultrafast synthesis, RSC Adv. 5 (2015) 69878–69885.
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High pressure synthesis, structure and thermoelectric properties of BiCuChO (Ch = S, Se, Te) Hongyu Zhu a,b , Taichao Su b,∗ , Hongtao Li c , Chunying Pu d , Dawei Zhou d , Pinwen Zhu a,∗ , Xin Wang a,∗ a
State Key Lab of Superhard Materials, Jilin University, Changchun 130012, China School of Physics and Electronic Information Engineering, Henan Polytechnic University, Jiaozuo 454000, China c Shanghai Entry-Exit Inspection & Quarantine Bureau, Shanghai 200135, China d College of Physics and Electronic Engineering, Nanyang Normal University, Nanyang 473061, China b
a r t i c l e
i n f o
Article history: Received 11 June 2016 Received in revised form 15 October 2016 Accepted 16 October 2016 Available online xxx Keywords: Thermoelectric High pressure BiCuChO
a b s t r a c t Bismuth copper oxychalcogenides, BiCuChO (Ch = S, Se, Te), are facile, and rapidly synthesized by high pressure method. The Rietveld refinement of powder X-ray diffractions shows that BiCuChO compounds have a layered crystal structure with a space group of P4/nmm. All the high pressure synthesised samples show semiconductor characteristics, while BiCuTeO prepared by the conventional method displays metal conducting behavior. The conducting behavior of BiCuTeO obtained in this study originates from the low crystal defect concentrations under the effects of high pressure; evidenced by density functional theory calculations. Large Seebeck coefficient ∼600 V/K was obtained for BiCuSO, due to its high carrier effect mass. BiCuChO exhibits extremely low thermal conductivity (<1 Wm−1 K−1 ), which decreases with an increase in the Ch2− ion radius. The maximum figure of merit reaches 0.03, 0.31 and 0.65 for BiCuSO, BiCuSeO and BiCuTeO, respectively, values which are comparable to those for samples prepared by the conventional, complex method. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Solid-state thermoelectric (TE) devices are generally made from heavily doped semiconductors. They can be used both as generators that directly convert heat to electricity from a heat source, and refrigeration devices that use electricity to pump heat from a cold side to a hot side without any moving parts or bulk fluids. The efficiency of TE materials and TE devices is determined by the dimensionless figure of merit (ZT), defined as ZT = S2 /, where S is the Seebeck coefficient, is the electrical conductivity and is the thermal conductivity [1]. Evidently, that the best performance of TE materials can be obtained by maximizing the Seebeck coefficient and electrical conductivity, while simultaneously minimizing the thermal conductivity. Bismuth copper oxychalcogenides, BiCuSeO, are some of the best TE materials which can be used for TE generation [2–7]. They comprise a (Cu2 Se2 )2− layer alternately stacked with (Bi2 O2 )2+ along the c axis of the tetragonal cell, wherein the (Bi2 O2 )2+
∗ Corresponding authors. E-mail addresses: [email protected] (T. Su), [email protected] (P. Zhu), xin [email protected] (X. Wang).
layer acts as a charge reservoir and the conductive (Cu2 Se2 )2− layer provides a conduction pathway for carrier transport. Because of the weak bonding between layers, low Young’s modulus and high Grüneisen parameter, an extremely low thermal conductivity (∼0.9 Wm−1 K−1 @300 K) is exhibited [2]. High ZT values of 0.7 ∼ 1.5 have been achieved by further optimizing electrical transport properties through doping [2–4], optimizing Cu vacancies [5], ball milling methods [6] and band gap tuning [7], etc. By now, most experimental studies on BiCuSeO are carried out by solid state reaction (SSR). SSR is a very simple and useful method with which to prepare oxides. However, the conventional method for BiCuSeO synthesis requires the use of high temperature annealing in sealed silica tubes under a vacuum, or argon protection, over a long period of time. This is cumbersome and difficult to carry out on an industrial scale. Besides, there are few reports of any research on other bismuth copper oxychalcogenides BiCuSO [8–10] and BiCuTeO [11,12]. Encouraged by the excellent TE performance of BiCuSeO, other novel synthesis techniques including a hydrothermal method [10], mechanical alloying [13] and self-propagating high-temperature synthesis [32] have also been used to prepare BiCuChO (Ch = S, Se, Te). The two formal methods are very facile and the last one can rapidly synthesize a sample. Apart from these methods, high
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Fig. 1. (a) Powder X-ray diffraction patterns of BiCuChO prepared by high pressure, (b) X-ray diffraction patterns of bulk BiCuChO prepared by high pressure.
pressure method can also easily synthesize TE materials. Compared with other preparation methods, the high pressure method has many advantages, including the ability to tune rapidly and cleanly, typically without introducing phase separation, or any other complicating factors [14]. It has been reported that many TE alloys including PbTe [15], AgSbTe2 [16] and CoSb3 [17] could be synthesized rapidly by the high pressure method. As far as we know, there is no report on high pressure synthesis of TE oxides. In this study, the high pressure method was employed to synthesise BiCuChO (Ch = S, Se, Te). The crystal structure, electrical transport and TE properties were then investigated.
to 650 K, were measured simultaneously by a LSR-3 apparatus (Linseis). The thermal conductivity was calculated from =DCP , where D is the thermal diffusivity measured on a LFA457 laser flash apparatus (Netzsch), is the sample density measured by the Archimedes method, and Cp is the specific heat capacity estimated by the Dulong Petit law. The densities are 98.5%, 96.3% and 93.6% of the theoretical values for BiCuSO BiCuSeO and BiCuTeO respectively. The Seebeck coefficient, resistivity and thermal diffusivity were measured under the radial direction of the samples.
2. Experimental
The density functional theory (DFT) calculations used the frozen-core projector augmented-wave (PAW) method [20,21], encoded in the Vienna ab initio simulation package (VASP) [22]. The generalized-gradient approximation (GGA) [23] of Perdew, Burke and Ernzerhof (PBE) was used for the exchange-correlation functional with the plane wave-cut-off energy of 400 eV. The crystal structure optimization for the BiCuTeO compound was carried out using 8 × 8 × 3 Monkhorst k-points for the irreducible Brillouin zone. The convergence thresholds for energy were chosen to be 10−4 eV and 10−3 eV/atom, respectively. The calculations of Bi and Cu vacancy formation energy for BiCuTeO as a function of pressure were performed in a supercell with 72 atoms according to the formation energy equation
2.1. Sample synthesis Bismuth copper oxychalcogenides, BiCuChO (Ch = S, Se, Te), were prepared with high purity (>99.99%) elements of Bi, Cu, Ch (S, Se,Te), and Bi2 O3 as sources, which were weighed according to their respective stoichiometry. After being uniformly mixed under argon protection, the mixtures were pressed into cylindrical samples (10.5 × 8.5 mm). The pole shaped sample was put into a hexagonal BN capsule, which was used as insulation material, to prevent the starting material reacting with the carbon heater placed in a pyrophyllite cube. The high pressure synthesis experiments were carried out with a cubic anvil high-pressure apparatus (XYK–6 × 12 MN, China). The high pressure apparatus and assembling diagrams are shown in Ref. [18] and Ref. [19] respectively. In the high pressure apparatus six cubic tungsten carbide anvils, each with a square anvil tip, are actuated by three pairs of hydraulic rams, forming a cubic cell, within which the sample assembly is compressed uniformly. All the samples were heated at 573 K for 20 min, then up to higher temperatures (BiCuSO ∼ 1023 K, BiCuSeO ∼ 973 K, BiCuTeO ∼ 923 K) for another 20 min.
2.3. Computational details
Ef = Ej − E0 +
ni i
(1)
where Ej and E0 are the DFT total energies of a supercell with and without the defect, respectively. The number of type i atoms removed to the host is ni and i is the atomic chemical potential. 3. Results and discussions 3.1. Crystal structure
2.2. Physical measurements X-ray diffraction (XRD) measurements with Cu-K␣ radiation were measured on an X-ray diffractometer (D/MAX-RA). Hall coefficients were measured by the Van der Pauw method using a HMS-5300 apparatus (Corpia). The Seebeck coefficient and electrical resistivity at different temperatures, from room temperature
The results of X-ray diffraction patterns for bulk and powder samples, shown in Fig. 1a and b, confirm that BiCuSO, BiCuSeO and BiCuTeO crystallize in the ZrCuSiAs structure type, with a P4/nmm space group. As seen from Fig. 1, both BiCuSO and BiCuSeO are single phase and well crystallized. However, there are minor secondary phases observed in BiCuTeO. This is similar to that of the results
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Table 2 The electrical transport properties of BiCuChO (Ch = S, Se, Te) at room temperature.
Table 1 The refined crystal structure parameters of BiCuChO. Sample
BiCuSO
BiCuSeO
BiCuTeO
Cell parameter (Å)
a = b = 3.8707(1) c = 8.5482(4)
a = b = 3.9303(1) c = 8.9336(7)
a = b = 4.0428(1) c = 9.5208(8)
Bi-O (Å) Bi-Ch (Å) Cu-Ch (Å)
2.3127(1) 3.1596(5) 2.4061(3)
2.3238(8) 3.2442(2) 2.4817(6)
2.3562(7) 3.7978(5) 2.6496(3)
Cu1-Ch-Cu2 (deg) Cu1-Ch-Cu3 (deg) O1-Bi-O2 (deg) O1-Bi-O3 (deg)
107.64(4) 110.39(2) 113.61(3) 72.55(9)
103.84(6) 112.35(5) 114.64(3) 73.05(1)
99.44(2) 114.70(7) 118.16(0) 74.69(1)
Sample
Carrier concentration n (1018 cm−3 )
Hall mobility (cm2 V−1 S−1 )
Hall coefficient RH (cm−3 )
BiCuSO BiCuSeO BiCuTeO
4.00(3) 12.65(7) 116.25(0)
0.13(9) 2.20(1) 4.22(5)
1.55(9) 0.49(3) 0.05(4)
for BiCuTeO prepared by the conventional SSR method [11,12]. The impurities included in BiCuTeO are Bi2 O3 . BiCuSeO crystals with layered structures might exhibit preferential orientation. However, the XRD patterns along the radial and thickness direction for all the samples did not show any preferential orientation of the crystallites in this study. For example, the ab orientation degree for the (00l) crystal planes of the bulk sample termed as F(00l) was calculated, using the Lotgering method, by the following equations: F=
P − P0 1 − P0
P0 = P=
I0(00l)
I0(hkl)
I(00l)
I(hkl)
(2) (3)
(4)
The F(00l) value of the bulk sample is approximately 0.03, 0.01 and 0.02 for BiCuTeO, BiCuSeO and BiCuSO respectively; this small F(00l) factor indicating that the high pressure synthesized BiCuChO is not preferentially oriented in the ab-plane. Therefore, the TE transport properties for BiCuChO polycrystals prepared by the high pressure method can be assumed to be isotropic. Rietveld refinement using the General Structure Analysis System (GSAS) program package [24] was also employed to perform in-depth structural analysis of BiCuChO. The refined crystal structure parameters are summarized in Table 1. As seen from Table 1, the lengths of both the a and c axes grow linearly with the covalent radii of the Ch atoms (S ∼ 1.02 Å, Se ∼ 1.16 Å and Te ∼ 1.36 Å) in the BiCuOCh materials, which obey Vegard’s law [25]. The Bi-Ch bond lengths are longer than those of Bi-O and Cu-Ch, suggesting that the Bi-Ch bonds are much weaker. This supports the conclusion that BiCuChO compounds have a layered crystal structure. The Ch-Cu-Ch angles (␣ and ) in BiCuSO have values close to those for a regular tetrahedron (109.5◦ ). The ␣ value decreases and the  value increases as the Ch ions change from S to Se to Te. This indicates that a larger Ch ion makes the distortion larger and the tetrahedral vertex more stereoscopic. The different bond angles in the polyhedra in BiCuChO show the different bonding characters, reflecting different electronic configurations. This can further affect the TE performance of BiCuChO. 3.2. Electrical transport and thermoelectric properties The electrical transport properties, including Hall coefficients, carrier concentrations and Hall mobilities of BiCuChO, are shown in Table 2. The Hall coefficients of all the three samples are positive, indicating that electrical transport properties of BiCuChO are dominated by holes, which probably originate from a small number of Cu vacancies. The carrier concentration and Hall mobility of BiCuChO increases along with the change of Ch from S to Te. The different electrical transport properties of BiCuChO can be explained by their different electronic structures. The band gap is a major
Fig. 2. Temperature dependent electrical resistivities of BiCuChO.
factor determining electrical transport properties, reflecting how the difficulty for the carrier (electrons or holes) requires a particular minimum amount of energy for the transition. The band gap is equivalent to the energy required to jump to the conduction band by absorbing energy. So a narrow band gap semiconductor usually corresponds to large carrier concentration. According to the optical absorption measurement [26], the band gap is about 1.1 eV and 0.8 eV for BiCuSO and BiCuSeO, respectively, and the value of BiCuTeO is just 0.4–0.5 eV. The different Hall mobility with Ch in BiCuChO can be attributed to the greater hybridization of the valence-band maximum (VBM) with spatially diffuse Ch p orbitals. According to the electronic structure calculation based on DFT [27], the Ch np orbitals become less localized along with the increase of the Ch2− ion radius, and the Cu-Ch antibonding states become more hybridized. The temperature dependent electrical resistivities of BiCuChO are shown in Fig. 2. All the three samples obtained in this study show typical semiconductor conducting behavior; electrical resistivity decreases with increasing temperature. It is obvious that the values of BiCuSO are much higher than those of BiCuSeO and BiCuTeO, which is consistent with the results of carrier concentration and Hall mobility. It should be mentioned that BiCuTeO prepared by other conventional methods at ambient pressure usually shows metal conduction behaviour [11,12]. The semiconductor behavior of BiCuTeO obtained in this study probably originates from the low amount of crystal defects. In order to understand the detailed crystal defect formation mechanism, the formation energies of possible point defects were calculated based on DFT calculations. The formation energies of Cu and Bi vacancies were calculated to be 0.219 and 0.238 meV per formula unit under ambient pressure, respectively. Both types of vacancies could together contribute to the hole generation, due to their very low formation energies. As the pressure increases in BiCuTeO, both of the vacancy formation energies in Bi and Cu sites increase at a rate of ∼0.12 meV/GPa.
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according to classical theories. To understand the origin of the different Seebeck coefficient for BiCuChO compounds, the effective masses were calculated by using observed carrier concentration (n) and Seebeck coefficient (S) values according to the following equation in the single band assumption: m∗ =
h2 2kB T
S=±
kB e
0
Fig. 4. Temperature dependence of the Seebeck coefficients for BiCuChO.
These calculated results indicate that high pressure can restrain the formation of crystal defects. Above 400 K, the temperature dependent resistivity behavior follows Arrhenius plots approximately, i.e., = 0 exp (En /kB T), where is the electrical resistivity, kB is the Boltzmann constant, and En is the activation energy. En was calculated from the slope of the graph of ln () vs 1/T, which is listed in the inset of Fig. 3. It can be seen that En of BiCuChO decreases with increasing Ch2− ionic radius, which agrees with the changes to their electrical resistivity. The lower the En value, the easier the motion of electrical carriers, which results in a low electrical resistivity. Fig. 4 shows the temperature dependence of the Seebeck coefficients for BiCuChO. The positive values indicate that all the samples are a p-type semiconductor, which is consistent with the result of Hall coefficient measurement. The Seebeck coefficient for BiCuSO is very large; the values are higher than 500 V/K when the temperature is higher than 400 K. Similar to the situation for resistivity, the Seebeck coefficients of BiCuSeO and BiCuTeO are much lower than that of BiCuSO. The Seebeck coefficient for a semiconductor is mainly determined by its carrier concentration and effective mass
n
2/3 (1)
4F1/2 () (r + 5/2)Fr+3/2 () (r + 3/2)Fr+1/2 ()
Fn () =
Fig. 3. Ln() vs 1/T for BiCuChO.
∞
n d 1 + e−
−
(2)
(3)
where Fn () is the n-th order Fermi integral, kB is the Boltzmann constant, e is the electron charge, h is the Planck constant and r is the scattering factor. The scattering factor r is ½, since the acoustic phonon scattering is independent of the grain size, and is generally assumed to be the main scattering mechanism at room temperature. The reduced Fermi energy = Ef /kB T was derived from the measured Seebeck coefficients by using Eq. (2). The effective mass m∗ can be obtained by substituting the calculated Fermi integral F1/2 () and carrier concentration n into Eq. (1). The estimated effective mass of the BiCuSeO is about 2.36 me ,which is slightly higher than that (∼1.74 me ) of BiCuSeO prepared by conventional solid state reaction (SSR) following the spark plasma sintering (SPS) method [5]. The effective mass of the BiCuSO is about 6.83 me , which is much higher than that of BiCuSeO and BiCuTeO (0.13 me ). So the different Seebeck coefficient of BiCuChO can be mainly attributed to the difference of the effective mass. The Ch2− radiusdependent effective mass obtained in this study is consistent with that of the DFT calculation results reported by Zou et al. [21]. Fig. 5 shows the temperature dependence of thermal conductivities for BiCuChO prepared by high pressure. As is well known, the total thermal conductivity has two main components, the lattice part ph and the electronic part e . The value of e can be simply estimated through the Wiedemann-Franz law e = LT, where L is the Lorenz number. The Lorenz number is estimated by the equation L = 1.5 + exp[-|S|/116], which was proposed by Kim et al. [28]. As seen from Figs. 5a and b, the values of both and ph, apart from that of BiCuSO at room temperature, are lower than 1Wm−1 K−1 and decrease with increasing temperature. These values are much lower than those of many well-known TE materials such as PbTe [15] and CoSb3 [17]. According to first-principal calculations and an in-situ neutron diffraction experiment [30], the presence of a localised low-energy vibrational mode on the copper is the fundamental mechanism leading to low thermal conductivity in these oxychalcogenides. As seen from Figs. 5a and b, both and ph values decrease with the increase of the Ch2− ionic radius. The low phonon thermal conductivity of BiCuTeO originates mainly from the weak Cu Te bonding. As seen from Table 1, the long bond of BiCuChO increases with an increase of Ch2− ionic radius. The weak Cu Te bond is related to low frequency copper vibrations [29] and thus results in anharmonicity and a large Grüneisen parameter in these oxychalcogenides. In addition, the high atomic mass of Te in BiCuChO is likely to produce slower sound velocities and stronger optical-acoustic scattering. Recently, Kumar and Schwingenschlögl [30] and Shao et al. [31] found that high-frequency optical phonons contribute considerably to the total thermal conductivity. The scattering of acoustical into optical phonons of BiCuChO is strongly enhanced, which dramatically reduces lattice thermal conductivity [31]. As seen from Fig. 6, the dimensionless figure of merit (ZT) values for all of the BiCuChO increase with increases in temperature. The maximum ZT value reaches 0.07, 0.31, and 0.65 for BiCuSO,
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Fig. 5. (a) Temperature dependence of total thermal conductivities for BiCuChO, (b) Temperature dependence of phonon thermal conductivities for BiCuChO.
BiCuSO, BiCuSeO and BiCuTeO, respectively. All of the values are comparable to that of BiCuChO prepared using conventional methods. However, the facile and fast synthesis process shows that the high pressure method is more competitive than the conventional method. Acknowledgments This work was supported by the Fundamental Research Funds for the Universities of Henan Province (No. NSFRF140202), the Foundation for Distinguished Young Scientists of Henan Polytechnic University, (No. J2016-5) the Scientific and Technological Project under Shanghai Entry-Exit Inspection and Quarantine Bureau (No. HK021-2014), the Scientific and Technological Projects under General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China (No. 2012IK049) References Fig. 6. The dimensionless figure of merit values for the BiCuChO.
BiCuSeO and BiCuTeO, respectively. These results are comparable to that of the pristine BiCuChO samples prepared by conventional SSR, followed by the SPS method at the same temperature [3,8,11]. In particular, the high pressure method used in this work is quicker and the technique is very easy to carry out, compared to conventional methods. 4. Conclusion TE oxides BiCuChO (Ch = S, Se, Te), were facile and rapidly synthesized (less than 1 h) by the high pressure method. Structural data from powder X-ray diffraction confirms that BiCuChO compounds have a layered crystal structure. All the crystal parameters, including that of the bond lengths and bond angles are in good agreement with published data for samples prepared by conventional methods. DFT calculation results show that the high pressure method can inhibit the formation of crystal defects, so all the BiCuChO samples show typical semiconductor behavior. BiCuSO has the largest Seebeck coefficient due to its high carrier effect mass. Extremely low thermal conductivity (<1 W/mK) was obtained for all of the BiCuChO samples, and this figure decreases with the increase of Ch2− ion radius. The maximum ZT reaches 0.03, 0.31 and 0.65 for
[1] L.E. Bell, Cooling, heating, generating power, and recovering waste heat with thermoelectric systems, Science 321 (2008) 1457–1461. [2] Y.L. Pei, J. He, J. Li, F. Li, Q. Liu, W. Pan, C. Barreteau, D. Berardan, N. Dragoe, L.D. Zhao, High thermoelectric performance of oxyselenides: intrinsically low thermal conductivity of Ca-doped BiCuSeO, NPG Asia Mater. 53 (2013) 425–434. [3] L.D. Zhao, D. Berardan, Y.L. Pei, C. Byl, L. Pinsard-Gaudart, N. Dragoe, Bi1-xSrxCuSeO oxyselenides as promising thermoelectric materials, Appl. Phys. Lett. 97 (2010) 092118. [4] J.L. Lan, Y.C. Liu, B. Zhan, Y.H. Lin, B.P. Zhang, X. Yuan, W. Zhang, W. Xu, C.W. Nan, Enhanced thermoelectric properties of Pb-Doped BiCuSeO ceramics, Adv. Mater. 44 (2013) 5086–5090. [5] Y. Liu, L.D. Zhao, Y. Liu, W. Xu, F. Li, B.P. Zhang, D. Berardan, N. Dragoe, Y.H. Lin, C.W. Nan, J.F. Li, H. Zhu, Remarkable enhancement in thermoelectric performance of BiCuSeO by Cu deficiencies, J. Am. Chem. Soc. 133 (2011) 20112–20115. [6] F. Li, J.F. Li, L.D. Zhao, K. Xiang, Y. Liu, B.P. Zhang, Y.H. Lin, C.W. Nan, H.M. Zhu, Polycrystalline BiCuSeO oxide as a potential thermoelectric material, Energy Environ. Sci. 5 (2012) 7188–7195. [7] Y. Liu, J. Lan, W. Xu, Y. Liu, Y.L. Pei, B. Cheng, D.B. Liu, Y.H. Lin, L.D. Zhao, Enhanced thermoelectric performance of a BiCuSeO system via band gap tuning, Chem. Commun. 249 (2013) 8075–8077. [8] D. Berardan, J. Li, E. Amzallag, S. Mitra, J. Sui, W. Cai, N. Dragoe, Structure and transport properties of the biCuSeO-BiCuSO solid solution, Materials 8 (2015) 1043–1058. [9] D. Berthebaud, E. Guilmeau, O.I. Lebedev, A. Maignana, J. Gamonb, P. Barbouxb, The BiCu1-xOS oxysulfide: copper deficiency and electronic properties, J. Solid State Chem. 237 (2016) 292–299. [10] W.C. Sheets, E.S. Stampler, H. Kabbour, M.I. Bertoni, L. Cario, T.O. Mason, T.J. Marks, K.R. Poeppelmeier, Facile synthesis of BiCuOS by hydrothermal methods, Inorg. Chem. 25 (2007) 10741–10748. [11] T.H. An, Y.S. Lim, H.S. Choi, W.S. Seo, C.H. Park, G.R. Kim, C. Park, C.H. Lee, J.H. Shim, Point defect-assisted doping mechanism and related thermoelectric
Please cite this article in press as: H. Zhu, et al., High pressure synthesis, structure and thermoelectric properties of BiCuChO (Ch = S, Se, Te), J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.10.021
G Model JECS-10885; No. of Pages 6
ARTICLE IN PRESS H. Zhu et al. / Journal of the European Ceramic Society xxx (2016) xxx–xxx
6
[12]
[13]
[14] [15]
[16]
[17]
[18]
[19]
[20] [21]
transport properties in Pb-doped BiCuOTe, J. Mater. Chem. A 2 (2014) 19759–19764. P. Vaqueiro, G. Gu´ıelou, M. Stec, E. Guilmeau, A.V. Powell, A copper-containing oxytelluride as a promising thermoelectric material for waste heat recovery, J. Mater. Chem. A 1 (2013) 520–523. V. Pele, C. Barreteau, D. Berardan, L. Zhao, N. Dragoe, Direct synthesis of BiCuChO-type oxychalcogenides by mechanical alloying, J. Solid State Chem. 203 (2013) 187–191. J.V. Badding, High-pressure synthesis characterization, and tuning of solid-state materials, Annu. Rev. Mater. Sci. 28 (1998) 631–658. H. Fan, T. Su, H. Li, S. Li, M. Hu, B. Liu, B. Du, H. Ma, X. Jia, Enhanced low temperature thermoelectric performance and weakly temperature-dependent figure-of-merit values of PbTe-PbSe solid solutions, J. Alloy Compd. 658 (2015) 885–890. T. Su, X. Jia, H. Ma, F. Yu, Y. Tian, G. Zuo, Y. Zheng, Y. Jiang, D. Dong, L. Deng, B. Qin, S. Zheng, Enhanced thermoelectric performance of AgSbTe2 synthesized by high pressure and high temperature, J. Appl. Phys. 105 (2009) 073713. T. Su, C. He, H. Li, X. Guo, S. Li, H. Ma, X. Jia, Fast preparation and low-temperature thermoelectric properties of CoSb3, J. Electron. Mater. 42 (2013) 109–113. Q. Han, B. Liu, M. Hu, Z. Li, X. Jia, M. Li, H. Ma, S. Li, H. Xiao, Y. Li, Design an effective solution for commercial production and scientific research on gem-quality, large single-crystal diamond by high pressure and high temperature, Cryst. Growth Des. 11 (2011) 1000–1005. H. Sun, X. Jia, L. Deng, P. Lv, X. Guo, Y. Zhang, B. Sun, B. Liu, H. Ma, Effect of HPHT processing on the structure, and thermoelectric properties of Co4Sb12 co-doped with Te and Sn, J. Mater. Chem. A 3 (2015) 4637–4641. P.E. Blöchl, Projector augmented-wave method, Phys. Rev. B 50 (1994) 17953–17979. G. Kresse, D. Joubert, From ultrasoft pseudopotentials to the projector augmented-wave method, Phys. Rev. B 59 (1999) 1758–1775.
[22] G. Kresse, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54 (1996) 11169–11186. [23] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865–3868. [24] A.C. Larson, R.B. Von, Dreele general structure analysis system LANSCE, in: MS-H805, 1994 (Los Alamos, New Mexico). [25] A.R. Denton, N.W. Ashcroft, Vegard’s law, Phys. Rev. A 43 (1991) 3161–3164. [26] H. Hiramatsu, H. Yanagi, T. Kamiya, K. Ueda, M. Hirano, H. Hosono, Crystal structures, optoelectronic properties, and electronic structures of layered oxychalcogenides MCuOCh (M = Bi, La; Ch = S, Se, Te): effects of electronic configurations of M3+ ions, Chem. Mater. 20 (2007) 326–334. [27] D.F. Zou, S.H. Xie, Y.Y. Liu, J.G. Lin, J.Y. Li, Electronic structures and thermoelectric properties of layered BiCuOCh oxychalcogenides (Ch = S, Se and Te): first-principles calculations, J. Mater. Chem. A 1 (2013) 8888–8896. [28] H.S. Kim, Z.M. Gibbs, Y. Tang, H. Wang, G.J. Snyder, Characterization of Lorenz number with Seebeck coefficient measurement, APL Mater. 3 (2015) 041506. [29] P. Vaqueiro, R.A.R. Al Orabi, S.D.N. Luu, G. Gue lou, A.V. Powell, R.I. Smith, J.-P. Song, D. Wee, M. Fornari, The role of copper in the thermal conductivity of thermoelectric oxychalcogenides: do lone pairs matter? Phys. Chem. Chem. Phys. 17 (2015) 31735–31740. [30] S. Kumar, U. Schwingenschlögl, Lattice thermal conductivity in layered BiCuSeO, Phys. Chem. Chem. Phys. 18 (2016) 19158–19164. [31] H. Shao, X. Tan, G.Q. Liu, J. Jiang, H. Jiang, A first-principles study on the phonon transport in layered BiCuOSe, Sci. Rep. 6 (2016) 21035. [32] G.K. Ren, J. Lan, S. Butt, K.J. Ventura, Y.H. Lin, C.W. Nan, Enhanced thermoelectric properties in Pb-doped BiCuSeO oxyselenides prepared by ultrafast synthesis, RSC Adv. 5 (2015) 69878–69885.
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