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Combustion synthesis of Cu2SnSe3 thermoelectric materials
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Combustion synthesis of Cu2 SnSe3 thermoelectric materials Guanghua Liu a,∗ , Kexin Chen b,∗ , Jiangtao Li a,∗ , Yuyang Li a , Min Zhou a , Laifeng Li a a b
Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China State Key Laboratory of New Ceramics & Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
a r t i c l e
i n f o
Article history: Received 3 November 2015 Received in revised form 19 December 2015 Accepted 22 December 2015 Available online xxx Keywords: Combustion synthesis Thermoelectric materials Solidification Gas pressure High-gravity
a b s t r a c t Thermoelectric materials are attractive for solar thermal energy conversion and waste heat recovery. The existing methods for fabricating thermoelectric materials involve multi-step processes with considerable time and energy consumption. Here we report a fast and one-step way to prepare thermoelectric materials by gas-pressure or high-gravity assisted combustion synthesis. Dense Cu2 SnSe3 samples with a porosity below 2% were prepared from self-sustained combustion reaction of element powders. The electrical conductivity of the Cu2 SnSe3 samples was greatly enhanced and the thermal conductivity was reduced by partial substitution of Sn with In. The ZT values of the un-doped and In-doped Cu2 SnSe3 samples reached 0.51 and 0.62 at 773 K, respectively, which are comparable to the best results reported for Cu2 SnSe3 produced by other methods. Combustion synthesis offers an efficient way to prepare thermoelectric materials with reduced time and energy consumption, which may open up new possibilities for synthesis of thermoelectric materials. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Thermoelectric materials enable direct and reversible conversion between thermal and electrical energy, and have aroused interest in various areas such as solar thermal energy conversion, waste heat recovery, and solid state cooling [1–3]. The efficiency of thermoelectric materials is determined by the dimensionless figure of merit (ZT), defined as ZT = (S2 /)T, where S, , and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity and the absolute temperature, respectively. To achieve high efficiency, a large ZT is desired. In order to improve ZT, a large Seebeck coefficient, high electrical conductivity and low thermal conductivity are required [4]. In the past decade, several approaches to increase ZT have been reported by enhancing the Seebeck coefficient and reducing the lattice thermal conductivity, such as modification of electron states or band engineering [5,6], nanostructuring [7–10], all-scale hierarchical architecturing [11], and exploring materials with intrinsically low thermal conductivity [12]. Besides performance (ZT and efficiency), the cost is also important for industrial application and massive use of thermoelectric materials. At present, most high-ZT thermoelectric materials con-
∗ Corresponding authors. E-mail addresses: [email protected] (G. Liu), [email protected] (K. Chen), [email protected] (J. Li).
tain scarce, expensive, or toxic elements (e.g., Te, Pb) [13]. In this case, the discovery of new thermoelectric materials composed of earth-abundant, less expensive, and nontoxic elements becomes the focus of attention, in which an example is the copper-based selenides (such as Cu2 Se and Cu2 SnSe3 ) showing low lattice thermal conductivity and relatively high ZT values [14,15]. Another approach to address the cost issue is to develop more efficient methods for the fabrication of thermoelectric materials. In general, bulk thermoelectric materials are prepared by two ways, viz. growth from the melt and powder sintering [12,14]. Both the ways involve prolonged heat treatment by furnaces with impressive time and energy consumption. An alternative route to prepare thermoelectric materials is combustion synthesis (also known as self-propagating hightemperature synthesis or briefly SHS). Combustion synthesis is a technique to synthesize inorganic compounds from self-sustained combustion reactions in a furnace-free way, and has been used for producing a large variety of refractory materials [16–18]. Recently, it has also been applied to synthesize thermoelectric materials [19–22]. For example, Cu2 Se prepared by combustion synthesis plus spark plasma sintering showed a high ZT of 1.8 at 1000 K [21]. A major drawback of combustion synthesis is that, the synthesized products are usually porous bodies with poor strength, which need to be pulverized into powders and then densified by sintering to form usable bulk materials [19,21,22]. If the densification can be finished simultaneously with the synthesis in one step, combus-
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tion synthesis will be more promising for producing thermoelectric materials. Herein, we report a new method for directly producing dense thermoelectric materials by combustion synthesis, in which a gas pressure or high gravity is applied to realize synchronous densification. By this method, nearly full-dense Cu2 SnSe3 samples have been prepared in one step requiring no further sintering, showing ZT values comparable to the best results reported before. 2. Experimental Materials synthesis High-purity element powders of Cu (3N), Sn (4N), Se (4N), and In (4N) were weighed and mixed in an agate mortar according to the chemical formula of Cu2 SnSe3 for un-doped and Cu2 Sn0.95 In0.05 Se3 for In-doped samples. The powder mixture was cold pressed into a compact for combustion synthesis, which was carried out in three ways, as illustrated in Fig. 1. (1) Combustion synthesis in air. The reactant compact was loaded in a quartz crucible on a graphite substrate. A tungsten coil was fixed above the top surface of the compact. The compact was ignited by passing an electric current in the tungsten coil, and then continued burning in a self-sustained way in open air, finally burning off in a few seconds. (2) Combustion synthesis in vacuum or Ar atmosphere. The reactant compact was loaded in a quartz crucible on a graphite substrate and placed into a reaction chamber, and a tungsten coil was fixed above the top surface of the compact. The reaction chamber, which was made in stainless steel, was a cylindrical vessel with a door at one end. By opening the door, the crucible with sample could be placed into or moved out of the chamber,
and by closing and mechanically locking the door the chamber was sealed. After the chamber was evacuated, the compact was ignited by passing an electric current in the tungsten coil. For combustion synthesis in Ar atmosphere, the reaction chamber was evacuated at first and subsequently filled with high-purity Ar gas up to a pressure of 2 MPa, and then the reactant compact was ignited. (3) Combustion synthesis in a high-gravity field. The reactant compact was loaded in a quartz crucible, which was then wrapped with carbon felt and put into a larger graphite crucible. A graphite cap was used to close the quartz and graphite crucibles. The graphite crucible was wrapped with carbon felt and placed in a steel cup, and the cup was horizontally mounted at one side of a rotator in the reaction chamber. A counterweight was mounted at the other side of the rotator to keep balance. After the chamber was evacuated, the rotator was started, and an equivalent high gravity (G) was induced by high-speed rotation and expressed as G = ω2 L, where ω is the angle velocity and L is the distance from the axis of rotation to the point of interest. In high-gravity combustion synthesis, the magnitude of high gravity had a strong influence on the synthesized samples. Generally speaking, a smaller G was not adequate to produce dense samples but too large G often resulted in very inhomogeneous samples. In this work, the parameters of L = 0.254 m, ω = 1680 rpm, and thus G = 800 g (g = 9.8 m/s2 ) were selected for preparing dense and homogeneous samples. When the rotation speed and high gravity reached ω = 1680 rpm and G = 800 g, the reactant compact was ignited. After ignition, the high-gravity filed was further kept for 10 min until the combustion reaction was complete and the sample was cooled, and then the rotator was switched off.
Fig. 1. An illustration of combustion synthesis of Cu2 SnSe3 : (a) combustion synthesis in air; (b) combustion synthesis in vacuum or Ar atmosphere; (c) combustion synthesis in a high-gravity field.
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Fig. 2. Photos showing the reaction process during combustion synthesis of Cu2 SnSe3 in air. Table 1 Chemical composition, porosity, and electrical properties of synthesized Cu2 SnSe3 samples. Sample
2 MPa Ar 800G 800G-In
Chemical composition
Porosity
Nominal
By EDS
Cu2 SnSe3 Cu2 SnSe3 Cu2 Sn0.95 In0.05 Se3
Cu2.21 Sn0.96 Se2.83 Cu2.24 Sn0.97 Se2.79 Cu2.26 Sn0.91 In0.04 Se2.79
H
pH 4
−1
(10 Sm 2% 1% 1%
)
0.37 2.02 4.94
H 20
(10
−3
cm
)
7.4 2.5 4.3
(cm2 V−1 s−1 ) 0.3 5.0 7.2
2 MPa Ar: synthesized in Ar atmosphere with a pressure of 2 MPa. 800G: synthesized in a high-gravity field of 800 g. 800G-In: synthesized in a high-gravity field of 800 g and doped with 5 mol% In. EDS data are normalized to 6 atoms per unit cell. The electrical properties ( H : electrical conductivity, pH : carrier concentration, H : carrier mobility) are obtained from Hall measurements at 300 K.
2.1. Characterization and measurements The bulk densities (D) of the samples were measured according to the Archimedes principle at room temperature, the theoretical densities (TD) of the samples were calculated from their lattice parameters and chemical compositions, and the porosities (P) of the samples were calculated from the relationship of P = 1-D/TD. A small piece of each sample was pulverized into fine powder (−325 mesh) for identification of phase assemblage by X-ray diffraction (XRD; D8 Focus, Bruker, Germany), using Cu-K␣ radiation ( = 1.5418 Å) and with a scanning step of 0.02◦ and scanning rate of 4◦ /min. The microstructure was examined by scanning electron microscopy (SEM; S-4800, Hitachi, Japan) for both fracture surface and polished surface of samples, and energy dispersive spectroscopy (EDS; INCA, Oxford Instrument, UK) was used for chemical composition analysis on polished surface of samples. Xray photoelectron spectra (XPS) were recorded with an ESCALAB 250Xi (ThermoFisher Scientific, USA) system using pressed powder compacts of samples and the monochromated Al K␣ line as the X-ray source. The electrical conductivity () and the Seebeck coefficient (S) were measured using LSR-3 (Linseis, Germany) in a static He atmosphere using sample bars of 20 × 2 × 2 mm3 . The electrical
conductivity was measured by the standard DC four-probe method, and the Seebeck coefficient was determined from the slope of the thermoelectromotive force (V) versus temperature gradient (T, 0 < T< 5 K). The Hall coefficient (RH ) and electrical conductivity ( H ) at 300 K were measured with a magnetic field strength of 0.5 T, and the carrier concentration (pH ) and the hole mobility (H ) were calculated from the equations of pH = 1/eRH and H = H /pH e, where e is the elementary charge. The thermal diffusivity () was measured by a laser flash method (LFA 457, Netzsch, Germany) using sample discs with a diameter of 10 mm and thickness of 1.0 mm, where the surface of the samples was coated with carbon before measurement. The thermal conductivity () was calculated according to the relationship of = Cp , where Cp is the specific heat capacity referred to the Dulong–Petit approximation and is the bulk density measured by the Archimedes method. The lattice thermal conductivity (L ) is calculated based on the Wiedemann–Franz law = L + c = L + LT, where L is the Lorentz factor (we used 2.0 × 10−8 V2 K−2 ). 3. Results and discussion Fig. 2 shows the reaction process during combustion synthesis of Cu2 SnSe3 in air. Once the reactant compact is ignited, it continues
Fig. 3. Photos of the Cu2 SnSe3 sample prepared by high-gravity combustion synthesis: (a) an overview of the sample with a diameter of 40 mm; (b) fracture surface showing three different parts of the sample, viz. top dark layer, intermediate grey layer, and main body with metallic luster.
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Fig. 4. XRD patterns of synthesized Cu2 SnSe3 samples: (a) and (b) for samples synthesized under different conditions, Vacuum: synthesized in vacuum, 2 MPa Ar: synthesized in 2 MPa Ar atmosphere, 800G: synthesized in a high-gravity field of 800 g, 800G-In: synthesized in a high-gravity field of 800 g and with doping of 5 mol% In (Cu2 Sn0.95 In0.05 Se3 ). (c) and (d) for different parts of the sample prepared in a high-gravity field.
to burn in a self-sustained way and burns out in a few seconds. The reaction of 2Cu + Sn + 3Se = Cu2 SnSe3 is exothermic, and by estimation from the relevant thermodynamic data [23–25], the maximum
reaction temperature under adiabatic condition reaches the melting point of Cu2 SnSe3 (695 ◦ C) [24] and most of the synthesized Cu2 SnSe3 exists in a molten state. The temperature of the sam-
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Fig. 5. SEM images (for fracture surface) of different parts of the Cu2 SnSe3 sample prepared by high-gravity combustion synthesis: (a) and (b) top dark layer; (c) and (d) intermediate grey layer; (e) center; (f) edge.
ple during combustion reaction was also experimentally measured by thermocouples, showing an apex of 688 ◦ C in the temperature profile. Considering the factors such as ultrafast reaction and high cooling rate, response time of the thermocouple (about 0.1 s), and contact between the thermocouple and the sample, the real maximum temperature of the sample should be more or less higher than that recorded by thermocouple. In this case, it is expected that the maximum reaction temperature should reach the melting point of Cu2 SnSe3 (695 ◦ C). For combustion synthesis in air, the molten product overflows out of the quartz crucible and finally porous samples with irregular shape are obtained (Fig. 2). Similar phenomenon with the overflow of product is observed for combustion synthesis in vacuum. The overflow phenomenon may be related with the evaporation of reactant powders. For example, the maximum reaction temperature (695 ◦ C) exceeds the boiling point of Se (685 ◦ C) [24], and hence strong evaporation of Se is expected. According to the thermodynamics [26], the boiling point of a substance depends on the ambient pressure, and increases with increasing pressure. Under a gas pressure of 2 MPa, the boiling point of Se is 1004 ◦ C, which is enhanced by more than 300 ◦ C compared with that at 1 atm. In this way, by combustion synthesis in 2 MPa Ar atmosphere, bulk sample with regular cylindrical shape was obtained. The sample consists of two different parts, where the upper part is porous and the lower part is dense with a porosity of 2% (Table 1), and the volume fraction of the dense part in the whole sample is about 50%.
Bulk Cu2 SnSe3 samples with even higher density are produced by combustion synthesis in a high-gravity field. Thus-prepared sample is composed of three parts, viz. top dark part, intermediate grey part, and main body with metallic luster, as shown in Fig. 3. The top and intermediate parts are porous, and the main body is dense with a porosity of 1% (Table 1). The volume fraction of the main body in the whole sample is about 80%. It is likely that the high-gravity field is more effective than the gas pressure in reducing the porosity and increasing the volume fraction of the dense part. As discussed before, most of the synthesized Cu2 SnSe3 is molten, in which some gas bubbles are usually entrapped and rise upwards because of the buoyancy with a rising velocity proportional to the gravity [27]. As a result, the lower part of the sample has a higher density while the upper part shows a larger porosity. In a high-gravity field with an acceleration of 800 g, the rising velocity of bubbles in the Cu2 SnSe3 melt is 800 times of that under normal gravity of g = 9.8 m/s2 . In other words, the removal of gas bubbles from the Cu2 SnSe3 melt can be accelerated by nearly three magnitude in a high-gravity field, thus resulting in smaller porosity and larger fraction of dense part in the sample. After combustion synthesis, Cu2 SnSe3 is synthesized as the major product in all the samples (Fig. 4). In the un-doped samples, the trace of SnSe is detected as the secondary phase. Compared with the sample synthesized in 2 MPa Ar atmosphere, the sample synthesized in vacuum contains more SnSe, which possibly indicates larger compositional inhomogeneity and may be attributed
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Fig. 6. SEM images (in back-scattered mode) and EDS mapping for polished surface of samples synthesized in a high-gravity field: (a) un-doped Cu2 SnSe3 ; (b) In-doped Cu2 SnSe3 (Cu2 Sn0.95 In0.05 Se3 ).
to stronger vaporization of Se. In the In-doped sample (800G-In, Cu2 Sn0.95 In0.05 Se3 ), no impurity phase is found, and the sample appears to be a single-phase Cu2 SnSe3 . In the Cu2 SnSe3 samples synthesized in a high-gravity field, a small difference in phase assemblage is observed for different parts of the sample. The center part consists of major Cu2 SnSe3 , minor SnSe, and the trace of other impurity phases that cannot be identified. The edge part is almost pure Cu2 SnSe3 with no impurity phases. In the top and intermediate parts, however, Cu-rich Cu7 Se4 instead of Sn-rich SnSe is found as the impurity phase. The difference in the phase assemblages of different parts suggests an inhomogeneity in the sample.
Fig. 5 shows SEM images of different parts in the Cu2 SnSe3 sample synthesized in a high-gravity field. The top and intermediate parts are porous, while the center and edge parts are relatively dense. The microstructure of the top part is characterized with weakly-connected particulate agglomerates, and no strong continuous network forms. In the intermediate part, strong bonding between crystalline grains contributes to a continuous network despite of the presence of pores. The fracture behavior is dominated by intragranular fracture, where fine pores inside grains are clearly visible, in addition to the larger pores mostly distributed at grain boundaries. The center part shows a relatively dense appearance with very few macro pores. The edge part is a thin layer with a
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Fig. 7. High-resolution XPS spectra of the sample 800G-In (Cu2 Sn0.95 In0.05 Se3 ).
thickness of about 0.6 mm, and shows a finer microstructure compared with the center part. In combustion synthesis, the sample is self-heated from the exothermic combustion reaction, and its temperature is higher than the surroundings. In this case, during reaction the heat is transferred from the sample to its surroundings like quartz crucible, graphite crucible, and steel cup (see Fig. 1(c)). The heat loss at the edge of the sample is thought to be more severe than that in the center part, because the edge is in an immediate contact with the crucible. Therefore, a higher cooling rate is expected at the edge of the sample, which is a possible reason for the observed finer microstructure. Fig. 6 shows SEM images (in back-scattered mode) and EDS mapping of polished surface of the samples synthesized in a high-gravity field. In the un-doped sample, the presence of SnSe secondary phase at the boundary of Cu2 SnSe3 grains is clearly verified. The In-doped sample shows a homogeneous microstructure with no secondary phase or element segregation, which agrees well with its single phase assemblage as revealed by XRD (Fig. 4). In both the samples, very few pores are observed, corresponding to the relatively low porosity of 1% in them (Table 1). Similar microstructure was observed in the sample synthesized in 2 MPa Ar atmosphere and thus not shown here. The chemical compositions of the synthesized samples were determined by EDS (for each sample five different areas of 500 × 500 m2 were measured) and listed in
Table 1. In comparison with the nominal compositions, the samples show a higher Cu concentration and lower Se concentration. The compositional deviation from the nominal one can be caused by various factors, in which a possible reason is the difference in evaporation loss of the reactants (e.g., Cu and Se) with very different melting and boiling points. XPS analysis was performed to examine the chemical electronic state of the elements in the synthesized Cu2 SnSe3 samples. As an example, the XPS spectra for the In-doped sample synthesized in a high-gravity field are shown in Fig. 7. The Cu 2 p spectrum shows two narrow symmetric peaks at 932.6 eV (2p3/2 ) and 952.4 eV (2p1/2 ) with a peak splitting of 19.8 eV, which is characteristic of Cu (I). The Sn 3 d peaks appear at 486.7 eV (3d5/2 ) and 495.2 eV (3d3/2 ) with a peak splitting of 8.5 eV, which can be assigned to Sn (IV). The In 3 d peaks appearing at 444.9 eV (3d5/2 ) and 452.4 eV (3d3/2 ) with a peak splitting of 7.5 eV correspond to In (III). The Se 3d5/2 peak located at 55.0 eV indicates Se with a valence of −2. Similar XPS spectra were observed for the other samples, confirming the chemical valence of +1, +4, and −2 for Cu, Sn, and Se, respectively. The electrical and thermal transport properties of the synthesized Cu2 SnSe3 samples were measured in the temperature range of 323–773 K (Fig. 8). For most samples, the electrical conductivity decreases with increasing temperature, indicating a heavily-doped semiconductor behavior. The Seebeck coefficient increases with
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Fig. 8. Thermoelectric properties of the synthesized Cu2 SnSe3 samples: (a) temperature dependence of electrical conductivity (), Seebeck coefficient (S), and power factor (PF = S2 ), (b) temperature dependence of thermal conductivity () and ZT, the lattice conductivity (L ) shown in the inset is calculated by L = -LT (L = 2.0 × 10−8 V2 K−2 ). The uncertainty of electrical transport properties is about 5%, the uncertainty of thermal conductivity is about 10%, and the uncertainty of ZT is estimated to be within 20%. 2 MPa Ar: synthesized in 2 MPa Ar atmosphere, 800G: synthesized in a high-gravity field of 800 g, 800G-In: synthesized in a high-gravity field of 800 g and with doping of 5 mol% In (Cu2 Sn0.95 In0.05 Se3 ).
temperature, leading to a larger power factor at higher temperatures. In-doping greatly improves the electrical conductivity, which is enhanced from 1.97 × 104 to 5.33 × 104 S m−1 at 323 K by doping with 5 mol% In. As a result, the power factor is almost doubled (changing from 0.15 to 0.30 mW m−1 K−2 ) by doping with 5 mol% In despite the slight reduction in Seebeck coefficient. Cu2 SnSe3 is a ptype semiconductor material, in which partial substitution of Sn by other elements with smaller valence electron numbers will create more holes [15]. In this way, by doping with 5 mol% In, the hole concentration is enhanced from 2.5 × 1020 to 4.3 × 1020 cm−3 (Table 1). In-doping also promotes the hole mobility, which changes from 5.0 to 7.2 cm2 V−1 s−1 by doping with 5 mol% In. Besides the doping effect, the higher phase purity (Fig. 4(b)) and more homogeneous microstructure as well as element distribution (Fig. 6(b)) of the Indoped sample may also contribute to its enhanced hole mobility. As a result, the In-doped sample showed a much higher electrical conductivity than the un-doped one. In comparison with the sample synthesized in a high-gravity field, the sample synthesized in 2 MPa Ar atmosphere exhibits lower electrical conductivity but larger Seebeck coefficient, and at higher temperatures (673–773 K) the two samples showed similar power factor values. Hall measurement reveals that the sample synthesized in 2 MPa Ar atmosphere has a higher concentration but lower mobility of holes. The exact reason for the difference in electrical transport properties of the two samples is not very clear and requires further investigations, where the higher hole mobility in the sample synthesized in a high-gravity field may be connected with its lower porosity (Table 1). The thermal conductivity of the samples decreases rapidly with increasing temperature and is below 1.0 W m−1 K−1 at 773 K. The low thermal conductivity is connected with the lattice structure of Cu2 SnSe3 . In Cu2 SnSe3 the Cu Se bond stabilizes the structure and forms an electrically conductive framework, and Sn atoms reside in the framework and donate electrons to balance the structure [15]. Such structure is similar to the phonon glass electron crystal (PGEC) compounds consisting of an open-structure framework
and filler atoms [28], and contributes to the intrinsic low thermal conductivity. The thermal conductivity can be further reduced by partial substitution of Sn with In, which will create atomic mass fluctuations and enhance phonon scattering. The lattice thermal conductivity of the sample with 5 mol% In-doping is clearly lower than the un-doped one, and reaches 0.57 W m−1 K−1 at 773 K, which is close to the minimum lattice thermal conductivity (around 0.4 W m−1 K−1 ) calculated for Cu2 SnSe3 at high temperatures [15]. The sample synthesized in 2 MPa Ar atmosphere shows a higher thermal conductivity than that synthesized in a high-gravity field, but the difference in thermal conductivities of the two samples becomes small at higher temperatures. The ZT of the synthesized Cu2 SnSe3 samples increases with temperature and reaches the maximum at 773 K. For un-doped and 5 mol% In-doped samples, the maximum ZT values (at 773 K) were 0.51 and 0.62 respectively, which are comparable to the best results reported for Cu2 SnSe3 materials prepared by other methods [15,29–32]. 4. Conclusion Bulk Cu2 SnSe3 samples with a porosity below 2% were prepared by gas-pressure or high-gravity assisted combustion synthesis from self-sustained combustion reaction of element powders of Cu, Sn, and Se. In all the samples, Cu2 SnSe3 was synthesized as the predominant phase, except for minor impurity phases such as SnSe and Cu7 Se4 , and no impurity phase was found in the In-doped sample (Cu2 Sn0.95 In0.05 Se3 ). The electrical conductivity of the Cu2 SnSe3 samples was greatly improved and the thermal conductivity was reduced by partial substitution of Sn with In. Hall measurement revealed that both the carrier concentration and the hole mobility were enhanced by In-doping. Compared with the sample synthesized in a high-gravity field, the sample synthesized in 2 MPa Ar atmosphere showed smaller electrical conductivity, larger Seebeck coefficient, and higher thermal conductivity. For the un-doped and In-doped Cu2 SnSe3 samples, the maximum ZT values reached 0.51
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and 0.62 at 773 K, respectively, which are comparable to the best results reported for Cu2 SnSe3 produced by other methods. Compared with the conventional melt growth and powder sintering methods, combustion synthesis is very fast and usually completes in a few seconds and does not require furnaces, which may offer a more efficient way to prepare Cu2 SnSe3 thermoelectric materials with reduced time and energy consumption. Acknowledgements Financial supports from National Natural Science Foundation of China (Grant No. 51422211), National Magnetic Confinement Fusion Science Program of China (Grant No. 2014GB125000 and 2014GB125005), Instrument Developing Project of the Chinese Academy of Sciences (Grant No. YZ201322), and Beijing Nova Program (Grant No. Z131103000413053) are acknowledged. References [1] CRC Handbook of Thermoelectrics, in: D.M. Rowe (Ed.), CRC, Boca Raton, USA, 1995. [2] L.E. Bell, Cooling, heating, generating power, and recovering waste heat with thermoelectric systems, Science 321 (2008) 1457–1461. [3] T.M. Tritt, H. Bottner, L.D. Chen, Thermoelectrics: direct solar thermal energy conversion, MRS Bull. 33 (2008) 366–368. [4] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Nat. Mater. 7 (2008) 105–114. [5] J.P. Heremans, V. Jovovic, E.S. Toberer, A. Saramat, K. Kurosaki, A. Charoenphakdee, S. Yamanaka, G.J. Snyder, Enhancement of thermoelectric efficiency in PbTe by distortion of the electronic density of states, Science 321 (2008) 554–557. [6] Y. Pei, X. Shi, A.D. LaLonde, H. Wang, L. Chen, G.J. Snyder, Convergence of electronic bands for high performance bulk thermoelectrics, Nature 473 (2011) 66–69. [7] K.F. Hsu, S. Loo, F. Guo, W. Chen, J.S. Dyck, C. Uher, T. Hogan, E.K. Polychroniadis, M.G. Kanatzidis, Cubic AgPbmSbTe2 + m: bulk thermoelectric materials with high figure of merit, Science 303 (2004) 818–821. [8] J.F. Li, W.S. Liu, L.D. Zhao, M. Zhou, High-performance nanostructured thermoelectric materials, NPG Asia Mater. 2 (2010) 152–158. [9] K. Biswas, J. He, Q. Zhang, G. Wang, C. Uher, V.P. Dravid, M.G. Kanatzidis, Strained endotaxial nanostructures with high thermoelectric figure of merit, Nat. Chem. 3 (2011) 160–166. [10] J.P. Heremans, M.S. Dresselhaus, L.E. Bell, D.T. Morelli, When thermoelectrics reached the nanoscale, Nat. Nanotechnol. 8 (2013) 471–473. [11] K. Biswas, J. He, I.D. Blum, C.I. Wu, T.P. Hogan, D.N. Seidman, V.P. Dravid, M.G. Kanatzidis, High-performance bulk thermoelectrics with all-scale hierarchical architectures, Nature 489 (2012) 414–418. [12] L. Zhao, S. Lo, Y. Zhang, H. Sun, G. Tan, C. Uher, C. Wolverton, V.P. Dravid, M.G. Kanatzidis, Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals, Nature 508 (2014) 373–377.
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Combustion synthesis of Cu2 SnSe3 thermoelectric materials Guanghua Liu a,∗ , Kexin Chen b,∗ , Jiangtao Li a,∗ , Yuyang Li a , Min Zhou a , Laifeng Li a a b
Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China State Key Laboratory of New Ceramics & Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
a r t i c l e
i n f o
Article history: Received 3 November 2015 Received in revised form 19 December 2015 Accepted 22 December 2015 Available online xxx Keywords: Combustion synthesis Thermoelectric materials Solidification Gas pressure High-gravity
a b s t r a c t Thermoelectric materials are attractive for solar thermal energy conversion and waste heat recovery. The existing methods for fabricating thermoelectric materials involve multi-step processes with considerable time and energy consumption. Here we report a fast and one-step way to prepare thermoelectric materials by gas-pressure or high-gravity assisted combustion synthesis. Dense Cu2 SnSe3 samples with a porosity below 2% were prepared from self-sustained combustion reaction of element powders. The electrical conductivity of the Cu2 SnSe3 samples was greatly enhanced and the thermal conductivity was reduced by partial substitution of Sn with In. The ZT values of the un-doped and In-doped Cu2 SnSe3 samples reached 0.51 and 0.62 at 773 K, respectively, which are comparable to the best results reported for Cu2 SnSe3 produced by other methods. Combustion synthesis offers an efficient way to prepare thermoelectric materials with reduced time and energy consumption, which may open up new possibilities for synthesis of thermoelectric materials. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Thermoelectric materials enable direct and reversible conversion between thermal and electrical energy, and have aroused interest in various areas such as solar thermal energy conversion, waste heat recovery, and solid state cooling [1–3]. The efficiency of thermoelectric materials is determined by the dimensionless figure of merit (ZT), defined as ZT = (S2 /)T, where S, , and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity and the absolute temperature, respectively. To achieve high efficiency, a large ZT is desired. In order to improve ZT, a large Seebeck coefficient, high electrical conductivity and low thermal conductivity are required [4]. In the past decade, several approaches to increase ZT have been reported by enhancing the Seebeck coefficient and reducing the lattice thermal conductivity, such as modification of electron states or band engineering [5,6], nanostructuring [7–10], all-scale hierarchical architecturing [11], and exploring materials with intrinsically low thermal conductivity [12]. Besides performance (ZT and efficiency), the cost is also important for industrial application and massive use of thermoelectric materials. At present, most high-ZT thermoelectric materials con-
∗ Corresponding authors. E-mail addresses: [email protected] (G. Liu), [email protected] (K. Chen), [email protected] (J. Li).
tain scarce, expensive, or toxic elements (e.g., Te, Pb) [13]. In this case, the discovery of new thermoelectric materials composed of earth-abundant, less expensive, and nontoxic elements becomes the focus of attention, in which an example is the copper-based selenides (such as Cu2 Se and Cu2 SnSe3 ) showing low lattice thermal conductivity and relatively high ZT values [14,15]. Another approach to address the cost issue is to develop more efficient methods for the fabrication of thermoelectric materials. In general, bulk thermoelectric materials are prepared by two ways, viz. growth from the melt and powder sintering [12,14]. Both the ways involve prolonged heat treatment by furnaces with impressive time and energy consumption. An alternative route to prepare thermoelectric materials is combustion synthesis (also known as self-propagating hightemperature synthesis or briefly SHS). Combustion synthesis is a technique to synthesize inorganic compounds from self-sustained combustion reactions in a furnace-free way, and has been used for producing a large variety of refractory materials [16–18]. Recently, it has also been applied to synthesize thermoelectric materials [19–22]. For example, Cu2 Se prepared by combustion synthesis plus spark plasma sintering showed a high ZT of 1.8 at 1000 K [21]. A major drawback of combustion synthesis is that, the synthesized products are usually porous bodies with poor strength, which need to be pulverized into powders and then densified by sintering to form usable bulk materials [19,21,22]. If the densification can be finished simultaneously with the synthesis in one step, combus-
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tion synthesis will be more promising for producing thermoelectric materials. Herein, we report a new method for directly producing dense thermoelectric materials by combustion synthesis, in which a gas pressure or high gravity is applied to realize synchronous densification. By this method, nearly full-dense Cu2 SnSe3 samples have been prepared in one step requiring no further sintering, showing ZT values comparable to the best results reported before. 2. Experimental Materials synthesis High-purity element powders of Cu (3N), Sn (4N), Se (4N), and In (4N) were weighed and mixed in an agate mortar according to the chemical formula of Cu2 SnSe3 for un-doped and Cu2 Sn0.95 In0.05 Se3 for In-doped samples. The powder mixture was cold pressed into a compact for combustion synthesis, which was carried out in three ways, as illustrated in Fig. 1. (1) Combustion synthesis in air. The reactant compact was loaded in a quartz crucible on a graphite substrate. A tungsten coil was fixed above the top surface of the compact. The compact was ignited by passing an electric current in the tungsten coil, and then continued burning in a self-sustained way in open air, finally burning off in a few seconds. (2) Combustion synthesis in vacuum or Ar atmosphere. The reactant compact was loaded in a quartz crucible on a graphite substrate and placed into a reaction chamber, and a tungsten coil was fixed above the top surface of the compact. The reaction chamber, which was made in stainless steel, was a cylindrical vessel with a door at one end. By opening the door, the crucible with sample could be placed into or moved out of the chamber,
and by closing and mechanically locking the door the chamber was sealed. After the chamber was evacuated, the compact was ignited by passing an electric current in the tungsten coil. For combustion synthesis in Ar atmosphere, the reaction chamber was evacuated at first and subsequently filled with high-purity Ar gas up to a pressure of 2 MPa, and then the reactant compact was ignited. (3) Combustion synthesis in a high-gravity field. The reactant compact was loaded in a quartz crucible, which was then wrapped with carbon felt and put into a larger graphite crucible. A graphite cap was used to close the quartz and graphite crucibles. The graphite crucible was wrapped with carbon felt and placed in a steel cup, and the cup was horizontally mounted at one side of a rotator in the reaction chamber. A counterweight was mounted at the other side of the rotator to keep balance. After the chamber was evacuated, the rotator was started, and an equivalent high gravity (G) was induced by high-speed rotation and expressed as G = ω2 L, where ω is the angle velocity and L is the distance from the axis of rotation to the point of interest. In high-gravity combustion synthesis, the magnitude of high gravity had a strong influence on the synthesized samples. Generally speaking, a smaller G was not adequate to produce dense samples but too large G often resulted in very inhomogeneous samples. In this work, the parameters of L = 0.254 m, ω = 1680 rpm, and thus G = 800 g (g = 9.8 m/s2 ) were selected for preparing dense and homogeneous samples. When the rotation speed and high gravity reached ω = 1680 rpm and G = 800 g, the reactant compact was ignited. After ignition, the high-gravity filed was further kept for 10 min until the combustion reaction was complete and the sample was cooled, and then the rotator was switched off.
Fig. 1. An illustration of combustion synthesis of Cu2 SnSe3 : (a) combustion synthesis in air; (b) combustion synthesis in vacuum or Ar atmosphere; (c) combustion synthesis in a high-gravity field.
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Fig. 2. Photos showing the reaction process during combustion synthesis of Cu2 SnSe3 in air. Table 1 Chemical composition, porosity, and electrical properties of synthesized Cu2 SnSe3 samples. Sample
2 MPa Ar 800G 800G-In
Chemical composition
Porosity
Nominal
By EDS
Cu2 SnSe3 Cu2 SnSe3 Cu2 Sn0.95 In0.05 Se3
Cu2.21 Sn0.96 Se2.83 Cu2.24 Sn0.97 Se2.79 Cu2.26 Sn0.91 In0.04 Se2.79
H
pH 4
−1
(10 Sm 2% 1% 1%
)
0.37 2.02 4.94
H 20
(10
−3
cm
)
7.4 2.5 4.3
(cm2 V−1 s−1 ) 0.3 5.0 7.2
2 MPa Ar: synthesized in Ar atmosphere with a pressure of 2 MPa. 800G: synthesized in a high-gravity field of 800 g. 800G-In: synthesized in a high-gravity field of 800 g and doped with 5 mol% In. EDS data are normalized to 6 atoms per unit cell. The electrical properties ( H : electrical conductivity, pH : carrier concentration, H : carrier mobility) are obtained from Hall measurements at 300 K.
2.1. Characterization and measurements The bulk densities (D) of the samples were measured according to the Archimedes principle at room temperature, the theoretical densities (TD) of the samples were calculated from their lattice parameters and chemical compositions, and the porosities (P) of the samples were calculated from the relationship of P = 1-D/TD. A small piece of each sample was pulverized into fine powder (−325 mesh) for identification of phase assemblage by X-ray diffraction (XRD; D8 Focus, Bruker, Germany), using Cu-K␣ radiation ( = 1.5418 Å) and with a scanning step of 0.02◦ and scanning rate of 4◦ /min. The microstructure was examined by scanning electron microscopy (SEM; S-4800, Hitachi, Japan) for both fracture surface and polished surface of samples, and energy dispersive spectroscopy (EDS; INCA, Oxford Instrument, UK) was used for chemical composition analysis on polished surface of samples. Xray photoelectron spectra (XPS) were recorded with an ESCALAB 250Xi (ThermoFisher Scientific, USA) system using pressed powder compacts of samples and the monochromated Al K␣ line as the X-ray source. The electrical conductivity () and the Seebeck coefficient (S) were measured using LSR-3 (Linseis, Germany) in a static He atmosphere using sample bars of 20 × 2 × 2 mm3 . The electrical
conductivity was measured by the standard DC four-probe method, and the Seebeck coefficient was determined from the slope of the thermoelectromotive force (V) versus temperature gradient (T, 0 < T< 5 K). The Hall coefficient (RH ) and electrical conductivity ( H ) at 300 K were measured with a magnetic field strength of 0.5 T, and the carrier concentration (pH ) and the hole mobility (H ) were calculated from the equations of pH = 1/eRH and H = H /pH e, where e is the elementary charge. The thermal diffusivity () was measured by a laser flash method (LFA 457, Netzsch, Germany) using sample discs with a diameter of 10 mm and thickness of 1.0 mm, where the surface of the samples was coated with carbon before measurement. The thermal conductivity () was calculated according to the relationship of = Cp , where Cp is the specific heat capacity referred to the Dulong–Petit approximation and is the bulk density measured by the Archimedes method. The lattice thermal conductivity (L ) is calculated based on the Wiedemann–Franz law = L + c = L + LT, where L is the Lorentz factor (we used 2.0 × 10−8 V2 K−2 ). 3. Results and discussion Fig. 2 shows the reaction process during combustion synthesis of Cu2 SnSe3 in air. Once the reactant compact is ignited, it continues
Fig. 3. Photos of the Cu2 SnSe3 sample prepared by high-gravity combustion synthesis: (a) an overview of the sample with a diameter of 40 mm; (b) fracture surface showing three different parts of the sample, viz. top dark layer, intermediate grey layer, and main body with metallic luster.
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Fig. 4. XRD patterns of synthesized Cu2 SnSe3 samples: (a) and (b) for samples synthesized under different conditions, Vacuum: synthesized in vacuum, 2 MPa Ar: synthesized in 2 MPa Ar atmosphere, 800G: synthesized in a high-gravity field of 800 g, 800G-In: synthesized in a high-gravity field of 800 g and with doping of 5 mol% In (Cu2 Sn0.95 In0.05 Se3 ). (c) and (d) for different parts of the sample prepared in a high-gravity field.
to burn in a self-sustained way and burns out in a few seconds. The reaction of 2Cu + Sn + 3Se = Cu2 SnSe3 is exothermic, and by estimation from the relevant thermodynamic data [23–25], the maximum
reaction temperature under adiabatic condition reaches the melting point of Cu2 SnSe3 (695 ◦ C) [24] and most of the synthesized Cu2 SnSe3 exists in a molten state. The temperature of the sam-
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Fig. 5. SEM images (for fracture surface) of different parts of the Cu2 SnSe3 sample prepared by high-gravity combustion synthesis: (a) and (b) top dark layer; (c) and (d) intermediate grey layer; (e) center; (f) edge.
ple during combustion reaction was also experimentally measured by thermocouples, showing an apex of 688 ◦ C in the temperature profile. Considering the factors such as ultrafast reaction and high cooling rate, response time of the thermocouple (about 0.1 s), and contact between the thermocouple and the sample, the real maximum temperature of the sample should be more or less higher than that recorded by thermocouple. In this case, it is expected that the maximum reaction temperature should reach the melting point of Cu2 SnSe3 (695 ◦ C). For combustion synthesis in air, the molten product overflows out of the quartz crucible and finally porous samples with irregular shape are obtained (Fig. 2). Similar phenomenon with the overflow of product is observed for combustion synthesis in vacuum. The overflow phenomenon may be related with the evaporation of reactant powders. For example, the maximum reaction temperature (695 ◦ C) exceeds the boiling point of Se (685 ◦ C) [24], and hence strong evaporation of Se is expected. According to the thermodynamics [26], the boiling point of a substance depends on the ambient pressure, and increases with increasing pressure. Under a gas pressure of 2 MPa, the boiling point of Se is 1004 ◦ C, which is enhanced by more than 300 ◦ C compared with that at 1 atm. In this way, by combustion synthesis in 2 MPa Ar atmosphere, bulk sample with regular cylindrical shape was obtained. The sample consists of two different parts, where the upper part is porous and the lower part is dense with a porosity of 2% (Table 1), and the volume fraction of the dense part in the whole sample is about 50%.
Bulk Cu2 SnSe3 samples with even higher density are produced by combustion synthesis in a high-gravity field. Thus-prepared sample is composed of three parts, viz. top dark part, intermediate grey part, and main body with metallic luster, as shown in Fig. 3. The top and intermediate parts are porous, and the main body is dense with a porosity of 1% (Table 1). The volume fraction of the main body in the whole sample is about 80%. It is likely that the high-gravity field is more effective than the gas pressure in reducing the porosity and increasing the volume fraction of the dense part. As discussed before, most of the synthesized Cu2 SnSe3 is molten, in which some gas bubbles are usually entrapped and rise upwards because of the buoyancy with a rising velocity proportional to the gravity [27]. As a result, the lower part of the sample has a higher density while the upper part shows a larger porosity. In a high-gravity field with an acceleration of 800 g, the rising velocity of bubbles in the Cu2 SnSe3 melt is 800 times of that under normal gravity of g = 9.8 m/s2 . In other words, the removal of gas bubbles from the Cu2 SnSe3 melt can be accelerated by nearly three magnitude in a high-gravity field, thus resulting in smaller porosity and larger fraction of dense part in the sample. After combustion synthesis, Cu2 SnSe3 is synthesized as the major product in all the samples (Fig. 4). In the un-doped samples, the trace of SnSe is detected as the secondary phase. Compared with the sample synthesized in 2 MPa Ar atmosphere, the sample synthesized in vacuum contains more SnSe, which possibly indicates larger compositional inhomogeneity and may be attributed
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Fig. 6. SEM images (in back-scattered mode) and EDS mapping for polished surface of samples synthesized in a high-gravity field: (a) un-doped Cu2 SnSe3 ; (b) In-doped Cu2 SnSe3 (Cu2 Sn0.95 In0.05 Se3 ).
to stronger vaporization of Se. In the In-doped sample (800G-In, Cu2 Sn0.95 In0.05 Se3 ), no impurity phase is found, and the sample appears to be a single-phase Cu2 SnSe3 . In the Cu2 SnSe3 samples synthesized in a high-gravity field, a small difference in phase assemblage is observed for different parts of the sample. The center part consists of major Cu2 SnSe3 , minor SnSe, and the trace of other impurity phases that cannot be identified. The edge part is almost pure Cu2 SnSe3 with no impurity phases. In the top and intermediate parts, however, Cu-rich Cu7 Se4 instead of Sn-rich SnSe is found as the impurity phase. The difference in the phase assemblages of different parts suggests an inhomogeneity in the sample.
Fig. 5 shows SEM images of different parts in the Cu2 SnSe3 sample synthesized in a high-gravity field. The top and intermediate parts are porous, while the center and edge parts are relatively dense. The microstructure of the top part is characterized with weakly-connected particulate agglomerates, and no strong continuous network forms. In the intermediate part, strong bonding between crystalline grains contributes to a continuous network despite of the presence of pores. The fracture behavior is dominated by intragranular fracture, where fine pores inside grains are clearly visible, in addition to the larger pores mostly distributed at grain boundaries. The center part shows a relatively dense appearance with very few macro pores. The edge part is a thin layer with a
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Fig. 7. High-resolution XPS spectra of the sample 800G-In (Cu2 Sn0.95 In0.05 Se3 ).
thickness of about 0.6 mm, and shows a finer microstructure compared with the center part. In combustion synthesis, the sample is self-heated from the exothermic combustion reaction, and its temperature is higher than the surroundings. In this case, during reaction the heat is transferred from the sample to its surroundings like quartz crucible, graphite crucible, and steel cup (see Fig. 1(c)). The heat loss at the edge of the sample is thought to be more severe than that in the center part, because the edge is in an immediate contact with the crucible. Therefore, a higher cooling rate is expected at the edge of the sample, which is a possible reason for the observed finer microstructure. Fig. 6 shows SEM images (in back-scattered mode) and EDS mapping of polished surface of the samples synthesized in a high-gravity field. In the un-doped sample, the presence of SnSe secondary phase at the boundary of Cu2 SnSe3 grains is clearly verified. The In-doped sample shows a homogeneous microstructure with no secondary phase or element segregation, which agrees well with its single phase assemblage as revealed by XRD (Fig. 4). In both the samples, very few pores are observed, corresponding to the relatively low porosity of 1% in them (Table 1). Similar microstructure was observed in the sample synthesized in 2 MPa Ar atmosphere and thus not shown here. The chemical compositions of the synthesized samples were determined by EDS (for each sample five different areas of 500 × 500 m2 were measured) and listed in
Table 1. In comparison with the nominal compositions, the samples show a higher Cu concentration and lower Se concentration. The compositional deviation from the nominal one can be caused by various factors, in which a possible reason is the difference in evaporation loss of the reactants (e.g., Cu and Se) with very different melting and boiling points. XPS analysis was performed to examine the chemical electronic state of the elements in the synthesized Cu2 SnSe3 samples. As an example, the XPS spectra for the In-doped sample synthesized in a high-gravity field are shown in Fig. 7. The Cu 2 p spectrum shows two narrow symmetric peaks at 932.6 eV (2p3/2 ) and 952.4 eV (2p1/2 ) with a peak splitting of 19.8 eV, which is characteristic of Cu (I). The Sn 3 d peaks appear at 486.7 eV (3d5/2 ) and 495.2 eV (3d3/2 ) with a peak splitting of 8.5 eV, which can be assigned to Sn (IV). The In 3 d peaks appearing at 444.9 eV (3d5/2 ) and 452.4 eV (3d3/2 ) with a peak splitting of 7.5 eV correspond to In (III). The Se 3d5/2 peak located at 55.0 eV indicates Se with a valence of −2. Similar XPS spectra were observed for the other samples, confirming the chemical valence of +1, +4, and −2 for Cu, Sn, and Se, respectively. The electrical and thermal transport properties of the synthesized Cu2 SnSe3 samples were measured in the temperature range of 323–773 K (Fig. 8). For most samples, the electrical conductivity decreases with increasing temperature, indicating a heavily-doped semiconductor behavior. The Seebeck coefficient increases with
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Fig. 8. Thermoelectric properties of the synthesized Cu2 SnSe3 samples: (a) temperature dependence of electrical conductivity (), Seebeck coefficient (S), and power factor (PF = S2 ), (b) temperature dependence of thermal conductivity () and ZT, the lattice conductivity (L ) shown in the inset is calculated by L = -LT (L = 2.0 × 10−8 V2 K−2 ). The uncertainty of electrical transport properties is about 5%, the uncertainty of thermal conductivity is about 10%, and the uncertainty of ZT is estimated to be within 20%. 2 MPa Ar: synthesized in 2 MPa Ar atmosphere, 800G: synthesized in a high-gravity field of 800 g, 800G-In: synthesized in a high-gravity field of 800 g and with doping of 5 mol% In (Cu2 Sn0.95 In0.05 Se3 ).
temperature, leading to a larger power factor at higher temperatures. In-doping greatly improves the electrical conductivity, which is enhanced from 1.97 × 104 to 5.33 × 104 S m−1 at 323 K by doping with 5 mol% In. As a result, the power factor is almost doubled (changing from 0.15 to 0.30 mW m−1 K−2 ) by doping with 5 mol% In despite the slight reduction in Seebeck coefficient. Cu2 SnSe3 is a ptype semiconductor material, in which partial substitution of Sn by other elements with smaller valence electron numbers will create more holes [15]. In this way, by doping with 5 mol% In, the hole concentration is enhanced from 2.5 × 1020 to 4.3 × 1020 cm−3 (Table 1). In-doping also promotes the hole mobility, which changes from 5.0 to 7.2 cm2 V−1 s−1 by doping with 5 mol% In. Besides the doping effect, the higher phase purity (Fig. 4(b)) and more homogeneous microstructure as well as element distribution (Fig. 6(b)) of the Indoped sample may also contribute to its enhanced hole mobility. As a result, the In-doped sample showed a much higher electrical conductivity than the un-doped one. In comparison with the sample synthesized in a high-gravity field, the sample synthesized in 2 MPa Ar atmosphere exhibits lower electrical conductivity but larger Seebeck coefficient, and at higher temperatures (673–773 K) the two samples showed similar power factor values. Hall measurement reveals that the sample synthesized in 2 MPa Ar atmosphere has a higher concentration but lower mobility of holes. The exact reason for the difference in electrical transport properties of the two samples is not very clear and requires further investigations, where the higher hole mobility in the sample synthesized in a high-gravity field may be connected with its lower porosity (Table 1). The thermal conductivity of the samples decreases rapidly with increasing temperature and is below 1.0 W m−1 K−1 at 773 K. The low thermal conductivity is connected with the lattice structure of Cu2 SnSe3 . In Cu2 SnSe3 the Cu Se bond stabilizes the structure and forms an electrically conductive framework, and Sn atoms reside in the framework and donate electrons to balance the structure [15]. Such structure is similar to the phonon glass electron crystal (PGEC) compounds consisting of an open-structure framework
and filler atoms [28], and contributes to the intrinsic low thermal conductivity. The thermal conductivity can be further reduced by partial substitution of Sn with In, which will create atomic mass fluctuations and enhance phonon scattering. The lattice thermal conductivity of the sample with 5 mol% In-doping is clearly lower than the un-doped one, and reaches 0.57 W m−1 K−1 at 773 K, which is close to the minimum lattice thermal conductivity (around 0.4 W m−1 K−1 ) calculated for Cu2 SnSe3 at high temperatures [15]. The sample synthesized in 2 MPa Ar atmosphere shows a higher thermal conductivity than that synthesized in a high-gravity field, but the difference in thermal conductivities of the two samples becomes small at higher temperatures. The ZT of the synthesized Cu2 SnSe3 samples increases with temperature and reaches the maximum at 773 K. For un-doped and 5 mol% In-doped samples, the maximum ZT values (at 773 K) were 0.51 and 0.62 respectively, which are comparable to the best results reported for Cu2 SnSe3 materials prepared by other methods [15,29–32]. 4. Conclusion Bulk Cu2 SnSe3 samples with a porosity below 2% were prepared by gas-pressure or high-gravity assisted combustion synthesis from self-sustained combustion reaction of element powders of Cu, Sn, and Se. In all the samples, Cu2 SnSe3 was synthesized as the predominant phase, except for minor impurity phases such as SnSe and Cu7 Se4 , and no impurity phase was found in the In-doped sample (Cu2 Sn0.95 In0.05 Se3 ). The electrical conductivity of the Cu2 SnSe3 samples was greatly improved and the thermal conductivity was reduced by partial substitution of Sn with In. Hall measurement revealed that both the carrier concentration and the hole mobility were enhanced by In-doping. Compared with the sample synthesized in a high-gravity field, the sample synthesized in 2 MPa Ar atmosphere showed smaller electrical conductivity, larger Seebeck coefficient, and higher thermal conductivity. For the un-doped and In-doped Cu2 SnSe3 samples, the maximum ZT values reached 0.51
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and 0.62 at 773 K, respectively, which are comparable to the best results reported for Cu2 SnSe3 produced by other methods. Compared with the conventional melt growth and powder sintering methods, combustion synthesis is very fast and usually completes in a few seconds and does not require furnaces, which may offer a more efficient way to prepare Cu2 SnSe3 thermoelectric materials with reduced time and energy consumption. Acknowledgements Financial supports from National Natural Science Foundation of China (Grant No. 51422211), National Magnetic Confinement Fusion Science Program of China (Grant No. 2014GB125000 and 2014GB125005), Instrument Developing Project of the Chinese Academy of Sciences (Grant No. YZ201322), and Beijing Nova Program (Grant No. Z131103000413053) are acknowledged. References [1] CRC Handbook of Thermoelectrics, in: D.M. Rowe (Ed.), CRC, Boca Raton, USA, 1995. [2] L.E. Bell, Cooling, heating, generating power, and recovering waste heat with thermoelectric systems, Science 321 (2008) 1457–1461. [3] T.M. Tritt, H. Bottner, L.D. Chen, Thermoelectrics: direct solar thermal energy conversion, MRS Bull. 33 (2008) 366–368. [4] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Nat. Mater. 7 (2008) 105–114. [5] J.P. Heremans, V. Jovovic, E.S. Toberer, A. Saramat, K. Kurosaki, A. Charoenphakdee, S. Yamanaka, G.J. Snyder, Enhancement of thermoelectric efficiency in PbTe by distortion of the electronic density of states, Science 321 (2008) 554–557. [6] Y. Pei, X. Shi, A.D. LaLonde, H. Wang, L. Chen, G.J. Snyder, Convergence of electronic bands for high performance bulk thermoelectrics, Nature 473 (2011) 66–69. [7] K.F. Hsu, S. Loo, F. Guo, W. Chen, J.S. Dyck, C. Uher, T. Hogan, E.K. Polychroniadis, M.G. Kanatzidis, Cubic AgPbmSbTe2 + m: bulk thermoelectric materials with high figure of merit, Science 303 (2004) 818–821. [8] J.F. Li, W.S. Liu, L.D. Zhao, M. Zhou, High-performance nanostructured thermoelectric materials, NPG Asia Mater. 2 (2010) 152–158. [9] K. Biswas, J. He, Q. Zhang, G. Wang, C. Uher, V.P. Dravid, M.G. Kanatzidis, Strained endotaxial nanostructures with high thermoelectric figure of merit, Nat. Chem. 3 (2011) 160–166. [10] J.P. Heremans, M.S. Dresselhaus, L.E. Bell, D.T. Morelli, When thermoelectrics reached the nanoscale, Nat. Nanotechnol. 8 (2013) 471–473. [11] K. Biswas, J. He, I.D. Blum, C.I. Wu, T.P. Hogan, D.N. Seidman, V.P. Dravid, M.G. Kanatzidis, High-performance bulk thermoelectrics with all-scale hierarchical architectures, Nature 489 (2012) 414–418. [12] L. Zhao, S. Lo, Y. Zhang, H. Sun, G. Tan, C. Uher, C. Wolverton, V.P. Dravid, M.G. Kanatzidis, Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals, Nature 508 (2014) 373–377.
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Please cite this article in press as: G. Liu, et al., Combustion synthesis of Cu2 SnSe3 thermoelectric materials, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.12.034