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Enhanced thermoelectric performance in SnSe based composites with PbTe nanoinclusions
Energy 116 (2016) 861e866
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Enhanced thermoelectric performance in SnSe based composites with PbTe nanoinclusions D. Li*, J.C. Li, X.Y. Qin**, J. Zhang***, H.X. Xin, C.J. Song, L. Wang Key Laboratory of Materials Physics, Institute of Solid State Physics Chinese Academy of Sciences, 230031 Hefei, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 June 2016 Received in revised form 12 September 2016 Accepted 7 October 2016
Although single crystalline SnSe has high thermoelectric figure of merit ZT, the highest ZT reported for polycrystalline SnSe-based material is not larger than 1. Here we show that a record value of ZT ¼ 1.26 at 880 K is achieved for polycrystalline SnSe based composites with PbTe nanoinclusions. Our results indicate that the high performance arises from simultaneous increase in power factor and thermal resistance. The increased power factor mainly originates from enhanced electrical conductivity due to increased carrier concentration; while the large (~25%) reduction of thermal conductivity can be ascribed to both the interface scattering of phonons and inhibition of bipolar effect. Present result demonstrates that the introduction of proper nanoinclusions can effectively elevate the thermoelectric performance of polycrystalline SnSe compound. © 2016 Published by Elsevier Ltd.
Keywords: Thermoelectric properties Nanocomposite SnSe
1. Introduction Because of concerns about pollution and limited oil sources, an environment-friendly and reliable technology based on thermoelectric materials is appealing because it can realize direct conversion of waste heat to electrical power [1]. The efficiency of a thermoelectric material is determined by its figure of merit ZT ¼ S2sT/k, where T, S, s, and k are the absolute temperature, Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively. A good thermoelectric material should have a high power factor PF ¼ S2s combined with low k. Recently, SnSe is shown to have high thermoelectric performance. A surprising record ZT z 2.6 at 923 K is reported for p-type single-crystalline SnSe along b-axis, attributed to a favorable combination of a high Seebeck coefficient and ultralow thermal conductivity (~0.25 Wm1 K2 at 973 K)) due to high anharmonicity of the chemical bonds [2]. However, considering low productivity and high-cost as well as the poor mechanical properties of layered single-crystalline SnSe, researchers focus recently on polycrystalline SnSe that is low-cost and easy to massively prepare for large-scale applications [3e6]. Polycrystalline SnSe is reported
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (D. Li), [email protected] (X.Y. Qin), zhangjian@ issp.ac.cn (J. Zhang). http://dx.doi.org/10.1016/j.energy.2016.10.023 0360-5442/© 2016 Published by Elsevier Ltd.
to have much low thermoelectric performance as compared to SnSe single crystals. For instance, the maximum ZT of 0.5 is obtained due to poor electrical conductivity and high thermal conductivity [3]. ZT ¼ 1.0 for p-type polycrystalline SnSe is achieved owing to texture modulation [4]. Peak ZT of 0.6 at 750 K is observed in p-type Ag-doped polycrystalline SnSe [5]. The maximum ZT z 1.0 at about 773 K is reported for n-type polycrystalline SnS0.1Se0.87I0.03 [6]. Hence, the highest ZT, reported so far, for polycrystalline SnSe is not larger than 1. In order to enhance the thermoelectric performance of polycrystalline SnSe, we introduce nanocrystals of PbTe into the matrix of SnSe as a secondary phase. Here PbTe nanocrystals are chosen as the nanoinclusions due to its good thermoelectric performance in the medium-temperature range [13]. Our results indicate that by embedding PbTe nanocrystals in bulk SnSe matrix, the increased s at elevated temperatures due to increased carrier concentration and reduced thermal conductivity ascribed to both the interface scattering of phonons and inhibition of bipolar effect results in the enhancement of thermoelectric performance for nano-composite samples f PbTe/SnSe (f ¼ 0.5, 1.5 and 2.5 vol%). The highest value of ZT ¼ 1.26 is achieved for f ¼ 1.5 vol% at 880 K, which outperforms any reported polycrystalline SnSe-based materials. 1.1. Experimental procedure Elemental Sn (99.99%, Alfa Aesar) and Se (99.999%, Alfa Aesar) powders are weighed according to the atomic ratio of SnSe. The
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powder mixture is loaded into quartz ampoule sealed under vacuum at 103 Pa, and then heated to 1223 K over 10 h, soaking at that temperature for 24 h, followed by furnace-cooling to room temperature. The obtained ingot is ground into powders. To prepare lead telluride nanocrystals, 15 g NaOH and 5 g NaBH4 are put into a 1000 ml glass beaker containing 600 ml deionized water, to which 200 ml anhydrous ethylenediamine is added. After mixing uniformly, 50 mmol Te and 50 mmol PbCl2 are added into the beaker. The solution is stirred by a magnetic stirrer at a speed of 1700 r/S, maintaining a temperature of 150 C during the synthesis, after about 1 h, a large quantity of powder precipitates from the solution. Then the precipitates are collected, filtered and washed with anhydrous ethanol and distilled water until pH value is close to 7, then dried in vacuum at 80 C for 10 h. The nanometer-sized PbTe and SnSe powders are mixed in a planetary mill for 2 h in the percent by volume of 0.5%, 1.5% and 2.5% for PbTe. The powders are compacted by hot-pressing at a pressure of 600 MPa in a 15 mm diameter tungsten carbide die in vacuum for 1 h. The sintering temperature and heating rate are 673 K and 7 K/ min, respectively. The product is characterized by X-ray diffraction (XRD) using a Philips X'Pert PRO X-ray diffractometer equipped with a graphite monochromatic Cu Ka radiation source (l ¼ 1.54056 Å). The operation voltage and current are kept at 40 kV and 40 mA, respectively. The microstructure is characterized by field emission scanning electron microscopy (FESEM; SU8020). TEM (transmission electron microscope) observations are performed on a JEOL-F2010 instrument with an acceleration voltage of 200 kV. Bar samples of about 2 mm 3 mm 10 mm are measured for the electrical resistivity and Seebeck coefficient using a commercial four-probe apparatus (ULVAC-RIKO ZEM-3). The thermal diffusion D is obtained by a laser flash method (Netzsch LFA-457) performed on square shaped samples with length of about 11 mm and thicknesses of about 1.1 mm. The specific heat, Cp is determined by differential scanning calorimetry (DSC) (Netzsch DSC-404C). The thermal conductivity is calculated from k ¼ DCpd, where d is the density of the sample determined by Archimedes' method. The hole concentration is measured using the van der Pauw technique at 300 Ke850 K under a reversible magnetic field of 1.5 T. 2. Results and discussion Fig. 1 shows XRD patterns of SnSe, PbTe and f(PbTe)/SnSe
Fig. 1. (a) XRD patterns of (a) SnSe, (b) PbTe and f PbTe/SnSe ((c) f ¼ 0.5, (d) 1.5 and (e) 2.5 vol%).
(f ¼ 0.5, 1.5 and 2.5 vol%) composite samples. The main diffraction peaks correspond well to the standard JCPDS cards (SnSe: No. 890233, PbTe: No. 08-0028), as shown in Fig. 1(curve (a) and curve (b)). Moreover, apart from the peaks of SnSe, there is an additional small peak at 2 theta ¼ 45.31 in patterns from (c) to (e), corresponding to peak of PbTe, indicating that there are two phases SnSe and PbTe in the composite samples. Weak extra line can be seen in the XRD pattern (Fig. 1(bee)) around 2theta ¼ 28 , which corresponds to the impurity of SnO with JCPDS No.13-0111. TEM images provide insight into the microstructural details of assynthesized PbTe powder (Fig. 2(aeb)) by chemical method. It can be clearly seen from Fig. 2(a) that the PbTe nanocrystals are composed of particles with sizes ranging from 20 to 150 nm. A high-resolution image looking down onto the surface of a PbTe nanocrystal is given in Fig. 2(b). The lattice spacing of about 3.23 Å corresponds to (2 0 0) planes of cubic PbTe. A Fourier transform diffraction (FTD) pattern of a square within the nanocrystal is provided in the inset of Fig. 2(b), which confirms that the nanocrystal is PbTe. The Scanning Electron Microscopy is employed to study the surface morphology of fractured surface of composite bulk sample, as shown in Fig. 3(a). It is observed that the presence of white spots, size ranges from 10 to 50 nm, is surrounded by the large grey grains. Energy Dispersive X-ray (EDX) spectroscopy has shown the successful incorporation of PbTe into SnSe matrix to form SnSe/PbTe bulk nanocomposite, as contained area indicates the presence of all elements Sn, Se, Pb and Te in composite sample (Fig. 3(b)). The atomic ratio of Pb: Te: Sn: Te is 0.93:0.81:52.37:45.88. Detailed microstructure studies by TEM were carried out on 1.5 vol% PbTe/SnSe bulk sample. Fig. 4(a) shows a typical lowmagnification bright-field TEM image of the 1.5 vol% PbTe/SnSe sample, depicting some inclusions, with a size distribution of 5e20 nm in the matrix. A higher-resolution image, of the area within the rectangle, looking down onto the surface of the matrix is given in Fig. 4(b). The lattice spacing of about 3.0Å corresponds to the (0 1 1) planes of orthorhombic SnSe. The selected-area electron diffraction (SAED) image taken of the rectangular area, inset in Fig. 4(b), indicates that the particle is a highly crystalline single crystal. Typical HRTEM images (Fig. 4(c)) of individual nanoparticles reveal that the products are well crystallized and the lattice fringes can be observed clearly. Regular parallel fringe spacing of about 3.46 Åand 2.88Å are found, as shown in Fig. 4(c), which can be assigned to the inter-planar spacings of the (2 0 1) and (4 0 0) lattice planes of orthorhombic SnSe. Fig. 4(d) provides a HRTEM image of a PbTe nanophase. The inter-planar distance is 3.23Å, corresponding to the (2 0 0) lattice planes of cubic PbTe. A Fourier transform diffraction pattern of a square within the nanoparticle is provided in the inset of Fig. 4(d), which confirms that the nanoparticle is PbTe. In addition, under high-resolution TEM observation, these grains consist of 5e15 nm-sized nanodots with clear boundaries. The electrical conductivity (s) of the composite samples f PbTe/ SnSe (f ¼ 0, 0.5, 1.5 and 2.5 vol%) are shown in Fig. 5(a) s for all the composite samples show the same temperature-dependent trend: as the temperature increases from 300 to 700 K, the electrical conductivity increases slowly; then increases rapidly to the maximum at a certain temperature; and above that, decreasing s is observed with increasing temperature. A good linear relationship between lns and 1/T for f PbTe/SnSe (f ¼ 0, 0.5, 1.5 and 2.5 vol%) exists in the high temperature range of 600e775 K, as shown in Fig. 5(b). By using a thermally activated expression in the corresponding temperature regime, written as [7]:
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Fig. 2. (a) TEM and (b) HRTEM patterns of PbTe particles; inset of (b): FTD pattern.
Fig. 3. (a) SEM images of the fractured surface of the bulk sample for 1.5 vol% PbTe/ SnSe and (b) EDS spectrum of 1.5 vol% PbTe/SnSe.
lns ¼ C þ
Eg 2kB T
(1)
where C, kB and Eg is a constant, Boltzmann constant and band gap, respectively. From the best fit of the experimental data with formula (1), one can obtain Eg ¼ 0.89 eV, 0.82eV, 0.88 eV and 0.84eV for f PbTe/SnSe with f ¼ 0, 0.5 vol%, 1.5 vol% and 2.5 vol%, respectively, which are almost equal to the energy gap Eg ¼ 0.86 eV for SnSe [2]. This indicates intrinsic excitation is occurred in the temperature range of 600e775 K and the introduction of PbTe does not significantly affect the structure of the energy band of SnSe. Moreover, the electrical conductivity of the composite samples is influenced by the addition of PbTe. For instance, s for the composite samples f PbTe/SnSe (f ¼ 0.5, 1.5 and 2.5 vol%) is larger than for pure SnSe in the temperature range of 700e850 K. In order to examine the effect of PbTe on the electrical conductivity of SnSe, the carrier concentration p as a function of temperature is given in Fig. 5(c). Seen from Fig. 5(c), p for pure SnSe increases from 5.78 1016 cm3 at 296 K to 1.43 1019 cm3 at 824 K; however, p
decreases to 1.25 1019cm3 as the temperature is further increased to 852 K. In the case of the composite samples f PbTe/ SnSe (f ¼ 0.5, 1.5 and 2.5 vol%), p increases over the measured temperature range. The increase in carrier concentration with temperature is caused by the thermal excitation of majority carriers; the decrease of p at 850 K is mostly likely to be caused by phase change, according to the DSC results in the present work. One can see that the increased carrier concentration for nanocomposite bulk samples is observed in the whole temperature investigated. The larger carrier concentration for composite sample as compare to SnSe originates from more excited majority. Combined with the data of electrical conductivity and carrier concentration, mobility is calculated as shown in the inset of Fig. 5(c). One can see that m for composite sample in the temperature range of 500e700 K increases as compare to that of SnSe due to the coulomb screening effect [8]. Above 800 K, although m for f ¼ 2.5 vol% decreases as compare to that of SnSe, m for f PbTe/SnSe (f ¼ 0.5, 1.5 vol%) keeps non-decreased. The S values for the composite samples f PbTe/SnSe (f ¼ 0, 0.5, 1.5 and 2.5 vol%) as shown in Fig. 5(d) are positive, indicating that they are p-type semiconductors. The temperature behaviors of S for all the samples are similar: with increasing temperature S increases in the temperature ranges of 300e400 K and 490e600 K; it decreases in the temperature ranges of 400e490 K and 600e800 K, and above 800 K, S increases with increasing temperature. The decrease of the Seebeck coefficient in the temperature range of 600e800 K for the samples originates from the thermal excitation of the carriers. A pit is observed in the temperature range from 400 to 600 K for the pure SnSe, while this phenomenon is not found for single crystal samples [2]. The Seebeck coefficient has shown minimum value about 500 K in Ref. [3], which has been recognized as a secondary phase due to melting of element Sn (Sn: melting point ¼ 505 K). A good chemical homogeneity for SnSe has shown by X-ray mappings results and no element Sn exists at grain boundaries. Hence, the pit in our sample may be ascribed to the increasing carrier concentration that is related to the defects in polycrystals [9]. The observed anomaly in the S-T curve above 800 K is associated with a phase transition [10]. DSC analysis (see Figure S1 in the supplementary material) indicates that an endothermic peak P is located in the temperature range 768e811 K and the peak heat-flow temperature TS is at around 795 K. This result is quite consistent with the results reported by H. Wiedemeier [10] where with increasing temperature, the structure of SnSe was observed to evolve continuously from a layered orthorhombic crystal structure (GeS structure type; space group Pnma) into a higher-symmetry variant (TlI structure type; space group Cmcm, No. 63), with a critical temperature of 807 K, indicating that the corresponding anomaly appeared in the SeT plot is caused by phase transition. Moreover, although S for SnSe keeps nearly unchanged at T > 850 K due to excitation of minorities,
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Fig. 4. TEM images for 1.5 vol% PbTe/SnSe: (a) low magnification TEM image for PbTe/SnSe; (b) HRTEM image of SnSe matrix (inset of (b): SAED pattern); (c) HRTEM image for nanostructured SnSe; (d) HRTEM image for nanostructured PbTe (inset of (d): FTD pattern).
Fig. 5. Temperature dependence for composite samples f PbTe/SnSe (f ¼ 0, 0.5, 1.5 and 2.5 vol%) (a) s; (b) logse1/T; (c) carrier concentration p; (d) S.
S for the composite samples PbTe/SnSe (f ¼ 0.5, 1.5 and 2.5 vol%) increases up to 923 K. This result suggests that the introduced interfaces potentials lead to strongly scattering of electrons. Since in the case of mixing conduction S can be written as follows [11]:
ST ¼
sp Sp þ sn Sn sp þ sn
(2)
where the subscript T, p and n represent total (hole and electronic conduction), hole conduction and electronic conduction, respectively. If electrons are strongly scattered by the introduced interface potentials, sn (¼numn) will become negligibly small, and the term
jsnSnj (here Sn < 0) in formula (2) can be neglected as compared to spSp. Then formula (2) becomes ST z Sp, which explains why S of composite samples monotonically increases with increasing temperature at T > 850 K. Power factor (see Fig. 6(a)) for f PbTe/SnSe increases with increasing temperature from 300 to 800 K. Above 800 K, the composite samples preserve a high power factor. The maximum power factor for SnSe is 5.7 mWcm1 K2 at 915 K. Although the power factor of the heavily incorporated sample (2.5 vol% PbTe/ SnSe) is nearly the same as that of pure SnSe for the whole temperature range investigated, power factor of the lightly incorporated samples with f ¼ 0.5 vol% and 1.5 vol% are obviously enhanced
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Fig. 6. Temperature dependent (a) power factor PF, (b) thermal conductivity k, (c) lattice thermal conductivity kL, and (c) figure of merit ZT for f PbTe/SnSe (f ¼ 0, 0.5, 1.5 and 2.5 vol %).
at T > 750 K. The highest value reaches 7.8 mWcm1K2 and 9.2 mWcm1K2 for f ¼ 0.5 vol% and 1.5 vol%, respectively, which are 37% and 61% larger than that of pure polycrystalline SnSe. The total thermal conductivity k as a function of temperature for f PbTe/SnSe (f ¼ 0, 0.5, 1.5 and 2.5 vol%) is given in Fig. 6(b). One can see that k for pure SnSe first decreases, then rapidly increases and further decreases with increasing temperature. In comparison, a relatively slow increase in k as a function of temperature for the composite samples (f > 0) is observed. k for SnSe in our work has a value of 0.55e1.55 WK1 m1 over the whole temperature range investigated. Subtracting the carrier thermal conductivity (kC), expressed as kC ¼ L0Ts (here L0 is the Lorenz number (1.5 108 V2 K2), from k, k-LsT can be obtained as shown in Fig. 6(c). It can be seen that k-LsT for SnSe first decreases with increasing temperature because of the phononephonon Umklapp scattering (U-scattering), but then quickly increases above 773 K. For semiconductors with U-scattering as the dominant phonon scattering mechanism, it is expected that above the Debye temperature (qDz220 K for SnSe) kL should change with 1/T [10]. The portion of the plot around 773 K that deviates from this temperature dependence as shown in the inset of Fig. 6(c) can be ascribed to the phase change and contribution from bipolar effect [12] to the conductivity by thermally generated electrons in polycrystalline SnSe material. The bipolar contribution to the lattice thermal conductivity decreases rapidly with the addition of PbTe. Clearly, this reduced bipolar effect can be attributed to strong scattering of thermally excited minorities (i.e. electrons) at interface potentials. It can be seen from Fig. 6(c) that k-LsT decreases monotonically with the increasing PbTe content (except for the sample f ¼ 2.5 vol %), which is due to the strong phonon scattering by enhanced number of interfaces in our sample. However, we notice that when f is increased from 1.5 to 2.5 vol%, k-LsT rises, which could be caused by the inherent large lattice thermal conductivity (0.5 WK1 m1) of PbTe [13]. Fig. 6(d) shows the ZT for all the samples. The ZT for the pure
SnSe reaches a maximum of 0.78 at 910 K, which is significant larger than that (ZTmax ¼ 0.5) of polycrystalline SnSe reported by S. Sassi [2]. The larger ZT for polycrystalline SnSe in this work can be attributed to the reduction of thermal conductivity by the enhanced scattering of phonons by so many nanocrystals of SnSe. Moreover, one can see that ZT for f ¼ 0.5 vol% and 1.5 vol% are larger than that of pure SnSe above 700 K, reaching 1.15 and 1.26 at 880 K, respectively, which are 47% and 61% larger than of the value for pure SnSe. The reduced thermal conductivity and improved power factor primarily contribute to increase ZT (¼1.26) at 880 K for composite sample of f ¼ 1.5 vol %. 3. Conclusions In this work, thermoelectric properties of nanocomposite fPbTe/ SnSe (f ¼ 0, 0.5, 1.5 and 2.5 vol %) bulk samples have been studied in the temperature range from 300 K to 900 K. It is observed from the experimental results that the inclusion of PbTe nanoparticles into SnSe matrix shows increase in power factor mainly from increased electrical conductivity and reduction in thermal conductivity attributed to the additional phonon scattering by nano-inclusions and enhanced scattering by minorities. The maximum value of ZT ¼ 1.26 is reached at 880 K in the incorporated nanocomposite 1.5 vol % PbTe/SnSe. The current research study has shown that the incorporation with suitable quantity of nanophase PbTe particles into SnSe matrix can efficiently enhance the thermoelectric performance of SnSe material. Acknowledgements We acknowledge the funding support from the Natural Science Foundation of China under grant no. 51672278, 11674322 and 11374306, Anhui Provincial Natural Science Foundation (No. 1608085MA17 and 1408085QB45) and Institute of Solid State Physics (2016DFS01, 2016DFY11).
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Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.energy.2016.10.023 References [1] Heremans JP, Dresselhaus MS, Bell LE, Morelli DT. When thermoelectrics reached the nanoscale. Nat Nanotechnol 2013;8:471e3. [2] Zhao LD, Lo SH, Zhang YS, Sun H, Tan GJ, Uher C, et al. Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals. Nature 2014;508:373e7. [3] Sassi S, Candolfi C, Vaney JB, Ohorodniichuk V, Masschelein P, Dauscher A, et al. Assessment of the thermoelectric performance of polycrystalline p-type SnSe. Appl Phys Lett 2014;104. 212105 (1-4). [4] Fu YJ, Xu JT, Liu GQ, Yang JK, Tan XJ, Liu Z, et al. Enhanced thermoelectric performance in p-type polycrystalline SnSe benefiting from texture modulation. J Mater Chem C 2016;4:1201e7. [5] Chen CL, Wang H, Chen YY, Daya T, Snyder GJ. Thermoelectric properties of ptype polycrystalline SnSe doped with Ag. J Mater Chem A 2014;2:11171e6.
[6] Zhang Q, Chere EK, Sun JY, Cao F, Dahal K, Chen S, et al. Studies on Thermoelectric properties of n-type polycrystalline SnSe1-xSx by iodine doping. Adv Energy Mater 2015. 1500360 (1-8). [7] Li JC, Li D, Qin XY, Zhang J. Enhanced thermoelectric performance of p-type SnSe doped with Zn. Scr Mater 2017;126:6e10. [8] Wang Y, Miao WD, Zhai LX. An effective screened Coulomb interaction in a quasi-one-dimensional system. Phys Lett A 2014;378:442e5. [9] Wei TR, Tan G, Zhang X, Wu CF, Li JF, Dravid VP, et al. Distinct impact of alkaliion doping on electrical transport properties of thermoelectric p-type polycrystalline SnSe. J Am Chem Soc 2016;138:8875e82. [10] Wiedemeier H, von Schnering HG. Refinement of the structures of GeS, GeSe, SnS and SnSe. Z Phys Chem 1978;148:295e303. [11] Li D, Qin XY. Thermoelectric properties of CuSbSe2 and its doped compounds by Ti and Pb at low temperatures from 5 to 310 K. J Appl Phys 2006:100. 023713(1-5). [12] Xiong Z, Chen XH, Huang XY, Bai SQ, Chen LD. High thermoelectric performance of Yb0.26Co4Sb12/yGaSb nanocomposites originating from scattering electrons of low energy. Acta Mater 2010;58:3995e4002. [13] Heremans JP, Jovovic V, Toberer ES, Saramat A, Kurosaki K, Charoenphakdee A, et al. Enhancement of thermoelectric efficiency in PbTe by distortion of the electronic density of states. Science 2008;321:554e7.
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Enhanced thermoelectric performance in SnSe based composites with PbTe nanoinclusions D. Li*, J.C. Li, X.Y. Qin**, J. Zhang***, H.X. Xin, C.J. Song, L. Wang Key Laboratory of Materials Physics, Institute of Solid State Physics Chinese Academy of Sciences, 230031 Hefei, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 June 2016 Received in revised form 12 September 2016 Accepted 7 October 2016
Although single crystalline SnSe has high thermoelectric figure of merit ZT, the highest ZT reported for polycrystalline SnSe-based material is not larger than 1. Here we show that a record value of ZT ¼ 1.26 at 880 K is achieved for polycrystalline SnSe based composites with PbTe nanoinclusions. Our results indicate that the high performance arises from simultaneous increase in power factor and thermal resistance. The increased power factor mainly originates from enhanced electrical conductivity due to increased carrier concentration; while the large (~25%) reduction of thermal conductivity can be ascribed to both the interface scattering of phonons and inhibition of bipolar effect. Present result demonstrates that the introduction of proper nanoinclusions can effectively elevate the thermoelectric performance of polycrystalline SnSe compound. © 2016 Published by Elsevier Ltd.
Keywords: Thermoelectric properties Nanocomposite SnSe
1. Introduction Because of concerns about pollution and limited oil sources, an environment-friendly and reliable technology based on thermoelectric materials is appealing because it can realize direct conversion of waste heat to electrical power [1]. The efficiency of a thermoelectric material is determined by its figure of merit ZT ¼ S2sT/k, where T, S, s, and k are the absolute temperature, Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively. A good thermoelectric material should have a high power factor PF ¼ S2s combined with low k. Recently, SnSe is shown to have high thermoelectric performance. A surprising record ZT z 2.6 at 923 K is reported for p-type single-crystalline SnSe along b-axis, attributed to a favorable combination of a high Seebeck coefficient and ultralow thermal conductivity (~0.25 Wm1 K2 at 973 K)) due to high anharmonicity of the chemical bonds [2]. However, considering low productivity and high-cost as well as the poor mechanical properties of layered single-crystalline SnSe, researchers focus recently on polycrystalline SnSe that is low-cost and easy to massively prepare for large-scale applications [3e6]. Polycrystalline SnSe is reported
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (D. Li), [email protected] (X.Y. Qin), zhangjian@ issp.ac.cn (J. Zhang). http://dx.doi.org/10.1016/j.energy.2016.10.023 0360-5442/© 2016 Published by Elsevier Ltd.
to have much low thermoelectric performance as compared to SnSe single crystals. For instance, the maximum ZT of 0.5 is obtained due to poor electrical conductivity and high thermal conductivity [3]. ZT ¼ 1.0 for p-type polycrystalline SnSe is achieved owing to texture modulation [4]. Peak ZT of 0.6 at 750 K is observed in p-type Ag-doped polycrystalline SnSe [5]. The maximum ZT z 1.0 at about 773 K is reported for n-type polycrystalline SnS0.1Se0.87I0.03 [6]. Hence, the highest ZT, reported so far, for polycrystalline SnSe is not larger than 1. In order to enhance the thermoelectric performance of polycrystalline SnSe, we introduce nanocrystals of PbTe into the matrix of SnSe as a secondary phase. Here PbTe nanocrystals are chosen as the nanoinclusions due to its good thermoelectric performance in the medium-temperature range [13]. Our results indicate that by embedding PbTe nanocrystals in bulk SnSe matrix, the increased s at elevated temperatures due to increased carrier concentration and reduced thermal conductivity ascribed to both the interface scattering of phonons and inhibition of bipolar effect results in the enhancement of thermoelectric performance for nano-composite samples f PbTe/SnSe (f ¼ 0.5, 1.5 and 2.5 vol%). The highest value of ZT ¼ 1.26 is achieved for f ¼ 1.5 vol% at 880 K, which outperforms any reported polycrystalline SnSe-based materials. 1.1. Experimental procedure Elemental Sn (99.99%, Alfa Aesar) and Se (99.999%, Alfa Aesar) powders are weighed according to the atomic ratio of SnSe. The
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powder mixture is loaded into quartz ampoule sealed under vacuum at 103 Pa, and then heated to 1223 K over 10 h, soaking at that temperature for 24 h, followed by furnace-cooling to room temperature. The obtained ingot is ground into powders. To prepare lead telluride nanocrystals, 15 g NaOH and 5 g NaBH4 are put into a 1000 ml glass beaker containing 600 ml deionized water, to which 200 ml anhydrous ethylenediamine is added. After mixing uniformly, 50 mmol Te and 50 mmol PbCl2 are added into the beaker. The solution is stirred by a magnetic stirrer at a speed of 1700 r/S, maintaining a temperature of 150 C during the synthesis, after about 1 h, a large quantity of powder precipitates from the solution. Then the precipitates are collected, filtered and washed with anhydrous ethanol and distilled water until pH value is close to 7, then dried in vacuum at 80 C for 10 h. The nanometer-sized PbTe and SnSe powders are mixed in a planetary mill for 2 h in the percent by volume of 0.5%, 1.5% and 2.5% for PbTe. The powders are compacted by hot-pressing at a pressure of 600 MPa in a 15 mm diameter tungsten carbide die in vacuum for 1 h. The sintering temperature and heating rate are 673 K and 7 K/ min, respectively. The product is characterized by X-ray diffraction (XRD) using a Philips X'Pert PRO X-ray diffractometer equipped with a graphite monochromatic Cu Ka radiation source (l ¼ 1.54056 Å). The operation voltage and current are kept at 40 kV and 40 mA, respectively. The microstructure is characterized by field emission scanning electron microscopy (FESEM; SU8020). TEM (transmission electron microscope) observations are performed on a JEOL-F2010 instrument with an acceleration voltage of 200 kV. Bar samples of about 2 mm 3 mm 10 mm are measured for the electrical resistivity and Seebeck coefficient using a commercial four-probe apparatus (ULVAC-RIKO ZEM-3). The thermal diffusion D is obtained by a laser flash method (Netzsch LFA-457) performed on square shaped samples with length of about 11 mm and thicknesses of about 1.1 mm. The specific heat, Cp is determined by differential scanning calorimetry (DSC) (Netzsch DSC-404C). The thermal conductivity is calculated from k ¼ DCpd, where d is the density of the sample determined by Archimedes' method. The hole concentration is measured using the van der Pauw technique at 300 Ke850 K under a reversible magnetic field of 1.5 T. 2. Results and discussion Fig. 1 shows XRD patterns of SnSe, PbTe and f(PbTe)/SnSe
Fig. 1. (a) XRD patterns of (a) SnSe, (b) PbTe and f PbTe/SnSe ((c) f ¼ 0.5, (d) 1.5 and (e) 2.5 vol%).
(f ¼ 0.5, 1.5 and 2.5 vol%) composite samples. The main diffraction peaks correspond well to the standard JCPDS cards (SnSe: No. 890233, PbTe: No. 08-0028), as shown in Fig. 1(curve (a) and curve (b)). Moreover, apart from the peaks of SnSe, there is an additional small peak at 2 theta ¼ 45.31 in patterns from (c) to (e), corresponding to peak of PbTe, indicating that there are two phases SnSe and PbTe in the composite samples. Weak extra line can be seen in the XRD pattern (Fig. 1(bee)) around 2theta ¼ 28 , which corresponds to the impurity of SnO with JCPDS No.13-0111. TEM images provide insight into the microstructural details of assynthesized PbTe powder (Fig. 2(aeb)) by chemical method. It can be clearly seen from Fig. 2(a) that the PbTe nanocrystals are composed of particles with sizes ranging from 20 to 150 nm. A high-resolution image looking down onto the surface of a PbTe nanocrystal is given in Fig. 2(b). The lattice spacing of about 3.23 Å corresponds to (2 0 0) planes of cubic PbTe. A Fourier transform diffraction (FTD) pattern of a square within the nanocrystal is provided in the inset of Fig. 2(b), which confirms that the nanocrystal is PbTe. The Scanning Electron Microscopy is employed to study the surface morphology of fractured surface of composite bulk sample, as shown in Fig. 3(a). It is observed that the presence of white spots, size ranges from 10 to 50 nm, is surrounded by the large grey grains. Energy Dispersive X-ray (EDX) spectroscopy has shown the successful incorporation of PbTe into SnSe matrix to form SnSe/PbTe bulk nanocomposite, as contained area indicates the presence of all elements Sn, Se, Pb and Te in composite sample (Fig. 3(b)). The atomic ratio of Pb: Te: Sn: Te is 0.93:0.81:52.37:45.88. Detailed microstructure studies by TEM were carried out on 1.5 vol% PbTe/SnSe bulk sample. Fig. 4(a) shows a typical lowmagnification bright-field TEM image of the 1.5 vol% PbTe/SnSe sample, depicting some inclusions, with a size distribution of 5e20 nm in the matrix. A higher-resolution image, of the area within the rectangle, looking down onto the surface of the matrix is given in Fig. 4(b). The lattice spacing of about 3.0Å corresponds to the (0 1 1) planes of orthorhombic SnSe. The selected-area electron diffraction (SAED) image taken of the rectangular area, inset in Fig. 4(b), indicates that the particle is a highly crystalline single crystal. Typical HRTEM images (Fig. 4(c)) of individual nanoparticles reveal that the products are well crystallized and the lattice fringes can be observed clearly. Regular parallel fringe spacing of about 3.46 Åand 2.88Å are found, as shown in Fig. 4(c), which can be assigned to the inter-planar spacings of the (2 0 1) and (4 0 0) lattice planes of orthorhombic SnSe. Fig. 4(d) provides a HRTEM image of a PbTe nanophase. The inter-planar distance is 3.23Å, corresponding to the (2 0 0) lattice planes of cubic PbTe. A Fourier transform diffraction pattern of a square within the nanoparticle is provided in the inset of Fig. 4(d), which confirms that the nanoparticle is PbTe. In addition, under high-resolution TEM observation, these grains consist of 5e15 nm-sized nanodots with clear boundaries. The electrical conductivity (s) of the composite samples f PbTe/ SnSe (f ¼ 0, 0.5, 1.5 and 2.5 vol%) are shown in Fig. 5(a) s for all the composite samples show the same temperature-dependent trend: as the temperature increases from 300 to 700 K, the electrical conductivity increases slowly; then increases rapidly to the maximum at a certain temperature; and above that, decreasing s is observed with increasing temperature. A good linear relationship between lns and 1/T for f PbTe/SnSe (f ¼ 0, 0.5, 1.5 and 2.5 vol%) exists in the high temperature range of 600e775 K, as shown in Fig. 5(b). By using a thermally activated expression in the corresponding temperature regime, written as [7]:
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Fig. 2. (a) TEM and (b) HRTEM patterns of PbTe particles; inset of (b): FTD pattern.
Fig. 3. (a) SEM images of the fractured surface of the bulk sample for 1.5 vol% PbTe/ SnSe and (b) EDS spectrum of 1.5 vol% PbTe/SnSe.
lns ¼ C þ
Eg 2kB T
(1)
where C, kB and Eg is a constant, Boltzmann constant and band gap, respectively. From the best fit of the experimental data with formula (1), one can obtain Eg ¼ 0.89 eV, 0.82eV, 0.88 eV and 0.84eV for f PbTe/SnSe with f ¼ 0, 0.5 vol%, 1.5 vol% and 2.5 vol%, respectively, which are almost equal to the energy gap Eg ¼ 0.86 eV for SnSe [2]. This indicates intrinsic excitation is occurred in the temperature range of 600e775 K and the introduction of PbTe does not significantly affect the structure of the energy band of SnSe. Moreover, the electrical conductivity of the composite samples is influenced by the addition of PbTe. For instance, s for the composite samples f PbTe/SnSe (f ¼ 0.5, 1.5 and 2.5 vol%) is larger than for pure SnSe in the temperature range of 700e850 K. In order to examine the effect of PbTe on the electrical conductivity of SnSe, the carrier concentration p as a function of temperature is given in Fig. 5(c). Seen from Fig. 5(c), p for pure SnSe increases from 5.78 1016 cm3 at 296 K to 1.43 1019 cm3 at 824 K; however, p
decreases to 1.25 1019cm3 as the temperature is further increased to 852 K. In the case of the composite samples f PbTe/ SnSe (f ¼ 0.5, 1.5 and 2.5 vol%), p increases over the measured temperature range. The increase in carrier concentration with temperature is caused by the thermal excitation of majority carriers; the decrease of p at 850 K is mostly likely to be caused by phase change, according to the DSC results in the present work. One can see that the increased carrier concentration for nanocomposite bulk samples is observed in the whole temperature investigated. The larger carrier concentration for composite sample as compare to SnSe originates from more excited majority. Combined with the data of electrical conductivity and carrier concentration, mobility is calculated as shown in the inset of Fig. 5(c). One can see that m for composite sample in the temperature range of 500e700 K increases as compare to that of SnSe due to the coulomb screening effect [8]. Above 800 K, although m for f ¼ 2.5 vol% decreases as compare to that of SnSe, m for f PbTe/SnSe (f ¼ 0.5, 1.5 vol%) keeps non-decreased. The S values for the composite samples f PbTe/SnSe (f ¼ 0, 0.5, 1.5 and 2.5 vol%) as shown in Fig. 5(d) are positive, indicating that they are p-type semiconductors. The temperature behaviors of S for all the samples are similar: with increasing temperature S increases in the temperature ranges of 300e400 K and 490e600 K; it decreases in the temperature ranges of 400e490 K and 600e800 K, and above 800 K, S increases with increasing temperature. The decrease of the Seebeck coefficient in the temperature range of 600e800 K for the samples originates from the thermal excitation of the carriers. A pit is observed in the temperature range from 400 to 600 K for the pure SnSe, while this phenomenon is not found for single crystal samples [2]. The Seebeck coefficient has shown minimum value about 500 K in Ref. [3], which has been recognized as a secondary phase due to melting of element Sn (Sn: melting point ¼ 505 K). A good chemical homogeneity for SnSe has shown by X-ray mappings results and no element Sn exists at grain boundaries. Hence, the pit in our sample may be ascribed to the increasing carrier concentration that is related to the defects in polycrystals [9]. The observed anomaly in the S-T curve above 800 K is associated with a phase transition [10]. DSC analysis (see Figure S1 in the supplementary material) indicates that an endothermic peak P is located in the temperature range 768e811 K and the peak heat-flow temperature TS is at around 795 K. This result is quite consistent with the results reported by H. Wiedemeier [10] where with increasing temperature, the structure of SnSe was observed to evolve continuously from a layered orthorhombic crystal structure (GeS structure type; space group Pnma) into a higher-symmetry variant (TlI structure type; space group Cmcm, No. 63), with a critical temperature of 807 K, indicating that the corresponding anomaly appeared in the SeT plot is caused by phase transition. Moreover, although S for SnSe keeps nearly unchanged at T > 850 K due to excitation of minorities,
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Fig. 4. TEM images for 1.5 vol% PbTe/SnSe: (a) low magnification TEM image for PbTe/SnSe; (b) HRTEM image of SnSe matrix (inset of (b): SAED pattern); (c) HRTEM image for nanostructured SnSe; (d) HRTEM image for nanostructured PbTe (inset of (d): FTD pattern).
Fig. 5. Temperature dependence for composite samples f PbTe/SnSe (f ¼ 0, 0.5, 1.5 and 2.5 vol%) (a) s; (b) logse1/T; (c) carrier concentration p; (d) S.
S for the composite samples PbTe/SnSe (f ¼ 0.5, 1.5 and 2.5 vol%) increases up to 923 K. This result suggests that the introduced interfaces potentials lead to strongly scattering of electrons. Since in the case of mixing conduction S can be written as follows [11]:
ST ¼
sp Sp þ sn Sn sp þ sn
(2)
where the subscript T, p and n represent total (hole and electronic conduction), hole conduction and electronic conduction, respectively. If electrons are strongly scattered by the introduced interface potentials, sn (¼numn) will become negligibly small, and the term
jsnSnj (here Sn < 0) in formula (2) can be neglected as compared to spSp. Then formula (2) becomes ST z Sp, which explains why S of composite samples monotonically increases with increasing temperature at T > 850 K. Power factor (see Fig. 6(a)) for f PbTe/SnSe increases with increasing temperature from 300 to 800 K. Above 800 K, the composite samples preserve a high power factor. The maximum power factor for SnSe is 5.7 mWcm1 K2 at 915 K. Although the power factor of the heavily incorporated sample (2.5 vol% PbTe/ SnSe) is nearly the same as that of pure SnSe for the whole temperature range investigated, power factor of the lightly incorporated samples with f ¼ 0.5 vol% and 1.5 vol% are obviously enhanced
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Fig. 6. Temperature dependent (a) power factor PF, (b) thermal conductivity k, (c) lattice thermal conductivity kL, and (c) figure of merit ZT for f PbTe/SnSe (f ¼ 0, 0.5, 1.5 and 2.5 vol %).
at T > 750 K. The highest value reaches 7.8 mWcm1K2 and 9.2 mWcm1K2 for f ¼ 0.5 vol% and 1.5 vol%, respectively, which are 37% and 61% larger than that of pure polycrystalline SnSe. The total thermal conductivity k as a function of temperature for f PbTe/SnSe (f ¼ 0, 0.5, 1.5 and 2.5 vol%) is given in Fig. 6(b). One can see that k for pure SnSe first decreases, then rapidly increases and further decreases with increasing temperature. In comparison, a relatively slow increase in k as a function of temperature for the composite samples (f > 0) is observed. k for SnSe in our work has a value of 0.55e1.55 WK1 m1 over the whole temperature range investigated. Subtracting the carrier thermal conductivity (kC), expressed as kC ¼ L0Ts (here L0 is the Lorenz number (1.5 108 V2 K2), from k, k-LsT can be obtained as shown in Fig. 6(c). It can be seen that k-LsT for SnSe first decreases with increasing temperature because of the phononephonon Umklapp scattering (U-scattering), but then quickly increases above 773 K. For semiconductors with U-scattering as the dominant phonon scattering mechanism, it is expected that above the Debye temperature (qDz220 K for SnSe) kL should change with 1/T [10]. The portion of the plot around 773 K that deviates from this temperature dependence as shown in the inset of Fig. 6(c) can be ascribed to the phase change and contribution from bipolar effect [12] to the conductivity by thermally generated electrons in polycrystalline SnSe material. The bipolar contribution to the lattice thermal conductivity decreases rapidly with the addition of PbTe. Clearly, this reduced bipolar effect can be attributed to strong scattering of thermally excited minorities (i.e. electrons) at interface potentials. It can be seen from Fig. 6(c) that k-LsT decreases monotonically with the increasing PbTe content (except for the sample f ¼ 2.5 vol %), which is due to the strong phonon scattering by enhanced number of interfaces in our sample. However, we notice that when f is increased from 1.5 to 2.5 vol%, k-LsT rises, which could be caused by the inherent large lattice thermal conductivity (0.5 WK1 m1) of PbTe [13]. Fig. 6(d) shows the ZT for all the samples. The ZT for the pure
SnSe reaches a maximum of 0.78 at 910 K, which is significant larger than that (ZTmax ¼ 0.5) of polycrystalline SnSe reported by S. Sassi [2]. The larger ZT for polycrystalline SnSe in this work can be attributed to the reduction of thermal conductivity by the enhanced scattering of phonons by so many nanocrystals of SnSe. Moreover, one can see that ZT for f ¼ 0.5 vol% and 1.5 vol% are larger than that of pure SnSe above 700 K, reaching 1.15 and 1.26 at 880 K, respectively, which are 47% and 61% larger than of the value for pure SnSe. The reduced thermal conductivity and improved power factor primarily contribute to increase ZT (¼1.26) at 880 K for composite sample of f ¼ 1.5 vol %. 3. Conclusions In this work, thermoelectric properties of nanocomposite fPbTe/ SnSe (f ¼ 0, 0.5, 1.5 and 2.5 vol %) bulk samples have been studied in the temperature range from 300 K to 900 K. It is observed from the experimental results that the inclusion of PbTe nanoparticles into SnSe matrix shows increase in power factor mainly from increased electrical conductivity and reduction in thermal conductivity attributed to the additional phonon scattering by nano-inclusions and enhanced scattering by minorities. The maximum value of ZT ¼ 1.26 is reached at 880 K in the incorporated nanocomposite 1.5 vol % PbTe/SnSe. The current research study has shown that the incorporation with suitable quantity of nanophase PbTe particles into SnSe matrix can efficiently enhance the thermoelectric performance of SnSe material. Acknowledgements We acknowledge the funding support from the Natural Science Foundation of China under grant no. 51672278, 11674322 and 11374306, Anhui Provincial Natural Science Foundation (No. 1608085MA17 and 1408085QB45) and Institute of Solid State Physics (2016DFS01, 2016DFY11).
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