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Enhanced thermoelectric properties of Cu1.8Se1−xSx alloys prepared by mechanical alloying and spark plasma sintering
Journal of Alloys and Compounds 680 (2016) 273e277
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Enhanced thermoelectric properties of Cu1.8Se1xSx alloys prepared by mechanical alloying and spark plasma sintering Yi-Hong Ji, Zhen-Hua Ge*, Zhidong Li, Jing Feng Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming, 650093, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 January 2016 Received in revised form 6 April 2016 Accepted 14 April 2016 Available online 18 April 2016
Cu1.8Se1xSx (0 x 1) thermoelectric alloys were prepared by mechanical alloying (MA) combined with spark plasma sintering (SPS) technology. The phase structure and morphologies of all the samples were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The electrical conductivity, Seebeck coefficient, thermal conductivity were investigated for all the Cu1.8Se1xSx alloys with a special emphasis on the influence of the S doping. With the increasing of S contents, a phase transition of Cu1.8Se1xSx was occurred from cubic to hexagonal. The electrical and thermal transport properties of the samples changed accordingly. The Cu1.8Se0.7S0.3 alloy achieves the highest ZT of 0.78 at 773 K due to both optimized electrical transport properties and thermal transport properties, which is 44% higher than that of pristine Cu1.8S (0.54 at 773 K) and 95% higher than that of pristine Cu1.8Se (0.4 at 773 K). © 2016 Elsevier B.V. All rights reserved.
Keywords: Thermoelectric Cu1.8Se Cu1.8S Mechanical alloying
1. Introduction Thermoelectric (TE) material is a kind of functional materiS100al which can fulfill the direct conversion between thermal energy and electrical energy. TE devices are silent, reliable and scalable, making them ideal for small, distributed power generation [1e4]. The efficiency of TE devices is determined by the dimensionless figure of merit (ZT), defined as ZT ¼ S2sT/k, where S, s, T, k, are Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively [4]. Therefore, highperformance thermoelectric materials require a high power factor (ɑ2s) and a low thermal conductivity k at the same time. The main purposes of the research on thermoelectric materials is, therefore, to simultaneously increase the electrical conductivity and decrease the total thermal conductivity [5,6]. Sulfides materials have been reported as promising thermoelectric materials [7e9]. Copper sulfides and copper selenides both are important semiconductor sulfides. The series of Cu2xS (0 x 1) compounds were widely used in thin film solar cells, transparent conductive films and conductive fibers [10e12]. The Cu1.8Se and Cu1.8S are both reported as superionic conductors [13,14]. Superionic conductors are solid materials that have ion
* Corresponding author. E-mail address: [email protected] (Z.-H. Ge). http://dx.doi.org/10.1016/j.jallcom.2016.04.140 0925-8388/© 2016 Elsevier B.V. All rights reserved.
conductivities as high as that in molten salts or liquid water at room temperature [15,16]. In superionic conductor, the ions are a kind of change carrier and contribute to the electrical conductivity. The similar superionic conductors CuCrS2, Ag2S also have been reported [17,18]. Although the high carrier concentration leads a very high electrical conductivity (~7000 S/cm) for Cu1.8Se bulk, the low Seebeck coefficient (~10 mV/K) restricts its application as a good thermoelectric material [19e21]. Cu2Se has been reported as a kind of promising thermoelectric material [22,23]. However, there are rare reports about Cu1.8Se for thermoelectric application. In this work, because of S doping, the Seebeck coefficients of Cu1.8Se alloys were enhanced significantly and the thermal conductivity was reduced resulting in an enhanced ZT value. The effect of the S contents in Cu1.8Se1xSx was investigated. The stoichiometric of Cu1.8Se0.7S0.3 sample achieves the highest ZT of 0.78 at 773 K, which is 44% higher than that of pristine Cu1.8S (0.54 at 773 K) and 95% higher than that of pure Cu1.8Se (0.4 at 773 K). 2. Experimental section The commercially available high-purity powders of Cu(99.99%), S(99.99%), Se(99.99%) were used as raw materials, which are all purchased from Alfa Aesar company without any further purification. Cu1.8Se1xSx compounds were synthesized according to the stoichiometric ratios, of which x ¼ 0, 0.3, 0.5, 0.7,
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0.9 and 1. Cu1.8Se1xSx (0 x 1) bulk samples were fabricated by combining mechanical alloying (MA) and spark plasma sintering (SPS) methods. Specific procedures are as follows: The powders with a chemical composition of Cu1.8Se1xSx (0 x 1) were ballmilled under 425 rpm for 2 h in a mixture gas atmosphere of argon (95%) and hydrogen (5%) using a planetary ball mill (QM1SP2,China) and then black blue powders were obtained. Stainless steel vessel and balls were used, and the weight ratio of ball to powders was kept at 20:1. The MAed powders were sintered at 873 K for 5 min in a ɸ20 mm graphite mould under axial compressive stress 40 MPa in vacuum using a spark plasma sintering (SPS) system (Sumitomo SPS1050, Japan). The sintered specimens were disk-shaped with a dimension ɸ20 mm 4 mm. Then the bulk materials with a dimension ɸ10 mm 1.5 mm were fabricated after cutting and polishing. The phase structure was analyzed by X-ray diffraction (XRD, Brucker D8, Germany) with a Cu Kɑ radiation. The morphologies of powders and the fractographs of bulk samples were observed by field emission scanning electron microscopy (FESEM, SUPRA™ 55, Japan). The electrical transport properties were evaluated along the sample section perpendicular to the pressing direction of SPS. The Seebeck coefficient and electrical resistivity were measured in a thin helium atmosphere using a Seebeck coefficient/electric resistance measuring system (ZEM-3 Ulvac-Riko, Japan). The density (r) of the samples was measured by the Archimedes method. In addition, the thermal conductivity of the samples was measured using the laser flash method (NETZSCH, LFA457, Germany). 3. Results and discussions Fig. 1 shows the XRD patterns of Cu1.8Se1xSx (x ¼ 0, 0.3, 0.5, 0.7, 0.9, 1.) powders after MA. As shown in Fig. 1a, all the powder samples are pure phase except Cu1.8Se0.3S0.7 which includes the impurity phase of Cu1.5Se. This result indicates that the solubility of Cu1.8S in Cu1.8Se is below 70% during the MA process. In Fig. 1b, within 0 < x < 1, all the diffraction peaks for the samples shift to high angle. That’s because the ion radius of S2 is smaller than that of Se2. The increase of S2 contents results in the decreased unit cell of Cu1.8Se, when x ¼ 0.7, the XRD patterns shows that the
Fig. 1. XRD patterns of the Cu1.8Se1xSx (x ¼ 0, 0.3, 0.5, 0.7, 0.9, and 1.) powders prepared by MA (a) and XRD patterns of 2q range from 44 to 48 (b).
coexistence of Cu1.8Se and Cu1.8S with an impurity phase Cu1.5Se was generated in matrix. With the increasing S contents, the pure Cu1.8S was obtained. Fig. 2a shows the X-ray diffraction (XRD) patterns of Cu1.8Se1xSx bulk samples after SPS. Compared with powder samples, the diffraction peaks of the bulks are sharper than those of the powder samples, indicating that the bulk samples crystallized better after sintering. There is no impurity detected in all the bulk samples, indicating the solubility of Cu1.8S in Cu1.8Se is almost unlimited after SPS processing. The peaks of (0 0 7) and (0 1 17) crystal faces of Cu1.8S were disappeared, which can be explained as follows: when x ranged from 0.9 to 1, the matrix phase structure is lowtemperature phase hexagonal crystal system of Cu1.8S, and when x 0.7, the (0 0 7) and (0 1 17) crystal planes disappeared completely, that’s because Se incorporated into the matrix and that makes the low-temperature phase Cu1.8S (hexagonal structure) transformed into high temperature phase Cu1.8S (cubic structure) according to standard cards PDF#47-1748 and PDF#24-0061. Fig. 2b shows that the diffraction peaks shift to the high angle gradually, that’s because the radius of S2 is smaller than that of Se2. The increase of S2 in lattice leads to a decreased lattice parameter. Cu1.8S sample is hexagonal digenite according to the standard card PDF#47-1748 at low temperature (under 353 K [24]), but when x ¼ 0.9, the crystal structure changed to cubic corresponding with the standard card PDF#24-0061. This indicates that selenium added into Cu1.8S leads the Cu1.8S has a high temperature phase at the low temperature (298 K) and the phase diagram reveals the phase transformation temperature of Cu1.8S is 353 K [19], and the phase transformation leads a turning point in the electrical conductivity curves. Now because of the decrease of phase transition temperature, the turning point would disappear. Fig. 3 shows the FESEM micrographs of the fractured surfaces for Cu1.8Se1xSx bulks after SPS at 873 K for 5 min by using MAed powders. As shown in Fig. 3a, the Cu1.8Se bulk has a porous morphology. However, with the increasing sulfur contents, pores in bulks became less, which is attributed to the decrease of the selenium volatilization which leads to the decrease of the sample density. We measured the density and calculated the theoretical density of all the bulk samples. The results were shown in Fig. 4. As displayed in the density curves, the theoretical density decreases
Fig. 2. XRD patterns of Cu1.8Se1xSx (x ¼ 0, 0.3, 0.5, 0.7, 0.9, and 1.) bulk samples sintered by SPS (a) and XRD patterns of 2q range from 44 to 48 (b).
Y.-H. Ji et al. / Journal of Alloys and Compounds 680 (2016) 273e277
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Fig. 3. FESEM micrographs of Cu1.8Se1xSx (x ¼ 0, 0.3, 0.5, 0.7, 0.9, and 1.) bulk samples sintered by SPS. (a) x ¼ 0, (b) x ¼ 0.3, (c) x ¼ 0.5, (d) x ¼ 0.7, (e) x ¼ 0.9 and (f) x ¼ 1.
Fig. 4. Measured density (black square), relative density (blue blank circle) and theoretical density (red circle) dependence of S contents of the Cu1.8Se1xSx (x ¼ 0, 0.3, 0.5, 0.7, 0.9, and 1.) bulk samples sintered by SPS. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
with the increasing of sulfur contents while the actual density is on the opposite. And the relative density shows the similar trend, which increased with the increasing sulfur contents. And when x ¼ 0, the minimum relative density of 79% is obtained. The reason is the volatility of element selenium during SPS processing. The increase of Se volatilization results in more pores and the low relative density for bulk samples. This result is agreement with the FESEM observations. Fig. 5 shows the temperature dependence of electrical transport properties for Cu1.8Se1xSx (x ¼ 0, 0.3, 0.5, 0.7, 0.9, and 1.) bulk samples after SPS. In Fig. 5a, the electrical conductivity for all Cu1.8Se1xSx bulks decreased with the increasing measuring temperature showing a metal conducting behavior. Among the whole measuring temperature range, as the increase of x, the electrical conductivity for Cu1.8Se1xSx bulk samples first decreased and then increased. when x increased from 0 to 0.7, the electrical conductivity decreased gradually. The reason is that Se has one more electronic shell than that of S, so the selenium’s ability to constrain electron is weaker than that of sulfur, the CueSe bond is also weaker than CueS bond. There is no turning point when x ranges
from 0 to 0.7. Cu1.8Se possesses the highest electrical conductivity of about 7000 Scm1 at room temperature. The Cu1.8Se0.3S0.7 bulk sample achieves the lowest electrical conductivity. Because of the increased relative density of Cu1.8Se0.1S0.9 and pure Cu1.8S, they got the higher electrical conductivities. There are turning points in the electrical conductivity curves for pure Cu1.8S and Cu1.8Se0.1S0.9 bulk samples at 373 K, which is agreement with our previous report [19]. That’s because Cu1.8S has a superionic transition at that temperature and the copper ion turned from order state into disorder state. Fig. 5b is the variation of Seebeck coefficient as a function of temperature. The Seebeck coefficients of all Cu1.8Se1xSx bulk samples are positive being indicative of a p-type conducting behavior and the major carrier is hole. When x ranged from 0 to 0.9, the Seebeck coefficient of these samples increased. That’s because the charge carrier concentration decreased and the complexity of the system increased. Pure Cu1.8Se0.1S0.9 sample has the highest Seebeck coefficient of 110 mV/K at 750 K. Cu1.8Se has the lowest Seebeck coefficient, which ranges from 17 to 65 mV/K through the whole measuring temperature. According to the results of electrical conductivity and Seebeck coefficient, the power factors of all the bulk samples were calculated by the formula PF ¼ a2s. The power factor of all the bulk samples were shown in Fig. 5c, on account of high Seebeck coefficient, the samples of which x ¼ 0.9 and x ¼ 1 get the high power factors of 1200 W/mK2 at 773 K. Although pure Cu1.8Se has a low Seebeck coefficient, the power factor still reached 1200 W/mK2 at 773 K due to its very high electrical conductivity. Fig. 6a shows temperature dependence of thermal conductivity(a) and ZT value(b) for Cu1.8Se1xSx bulk samples of which x ¼ 0, 0.3, 0.5, 0.7, 0.9, 1. Generally speaking, the thermal conductivity for Cu1.8Se1xSx (0 x 1) bulk samples decreased with the increase of measuring temperature due to the enhanced thermal vibration of lattice. One exception is Cu1.8S bulk, which is attributed to the phase transition that leads to the appearance of turning point. With the decrease of x, the thermal conductivity of the samples increased firstly when x ranged from 1 to 0.7. Then because more and more pores generated in samples, the thermal conductivity of decreased gradually and reached the lowest value of 0.98 Wm1K1 for Cu1.8Se0.7S0.3 bulk sample. Although Cu1.8Se bulk has the lowest relative density, which still got the highest thermal conductivity from 2.3 to 4.8 Wm1K1 in the all temperature range due to its very high electrical conductivity. According to the measuring results of electrical conductivity,
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Fig. 6. Temperature dependence of thermal conductivity (a) and ZT value (b) for Cu1.8Se1xSx (x ¼ 0, 0.3, 0.5, 0.7, 0.9, and 1.) bulk samples sintered by SPS.
transport properties, and which is 44% higher than that of pristine Cu1.8S (0.54 at 773 K) and 95% higher than that of pristine Cu1.8Se (0.4 at 773 K). Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51501086). Fig. 5. Temperature dependence of electrical conductivity (a), Seebeck coefficient (b) and power factor (c) for Cu1.8Se1xSx (x ¼ 0, 0.3, 0.5, 0.7, 0.9, and 1.) bulk samples sintered by SPS.
Seebeck coefficient and thermal conductivity, the ZT value was obtained through the formula ZT ¼ a2sT/k. As is shown in Fig. 6b, the ZT value for all samples increased with the increasing temperature. The Cu1.8Se0.7S0.3 bulk sample achieves the highest ZT value of 0.78 at 773 K, which is 44% higher than that of pristine Cu1.8S (0.54 at 773 K) and 95% higher than that of Cu1.8Se (0.4 at 773 K). For the Cu1.8Se based alloys, the S doping increased the complexity of the system, the Seebeck coefficient was increased and the thermal conductivity was decreased resulting in an enhanced ZT value. 4. Conclusions Cu1.8Se1xSx (x ¼ 0, 0.3, 0.5, 0.7, 0.9, and 1.) thermoelectric alloys were synthesized by MA combined with SPS techniques. All Cu1.8Se1xSx (0 < x < 1) bulk samples after SPS are pure phase, indicating the unlimited solubility of Cu1.8Se and Cu1.8S. The Cu1.8Se0.7S0.3 alloy achieves the highest ZT of 0.78 at 773 K due to both optimized electrical transport properties and thermal
References [1] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Nat. Mater. 7 (2008) 105e114. [2] J.R. Sootsman, D.Y. Chung, M.G. Kanatzidis, New and old concepts in thermoelectric materials, Angew. Chem. Int. Ed. 48 (2009) 8616e8639. [3] M. Stordeur, D.M. Row, Handbook of Thermoelectrics, CRC Press, Boca Raton, 1995. [4] G.S. Nolas, Basic Principles and New Materials Developments, Springer, New York, 2001. [5] J.F. Li, W.S. Liu, L.D. Zhao, M. Zhou, High-performance nanostructured thermoelectric materials, NPG Asia Mater. 2 (2010) 152e158. [6] G.L. Sun, X.Y. Qin, D. Li, J. Zhang, B.J. Ren, T.H. Zou, H.X. Xin, S.B. Paschen, X.L. Yan, Enhanced thermoelectric performance of n-type Bi2Se3 doped with Cu, J. Alloy. Compd. 639 (2015) 9e14. [7] Q. Tan, W. Sun, Z.L. Li, J.F. Li, Enhanced thermoelectric properties of earthabundant Cu2SnS3 via in doping effect, J. Alloy. Compd. 672 (2016) 558e563. [8] J.H. Li, Q. Tan, J.F. Li, Synthesis and property evaluation of CuFeS2x as earthabundant and environmentally-friendly thermoelectric materials, J. Alloy. Compd. 551 (2013) 143e149. [9] A.P. Goncalves, E.B. Lopes, J. Monnier, J. Bourgon, J.B. Vaney, A. Piarristeguy, A. Pradel, B. Lenoir, G. Delaizir, M.F.C. Pereira, E. Alleno, C. Godart, Fast and scalable preparation of tetrahedrite for thermoelectrics via glass crystallization, J. Alloy. Compd. 664 (2016) 209e217. [10] L.D. Partain, P.S. Mcleod, J.A. Duisman, T.M. Peterson, D.E. Sawyer, C.S. Dean, Degradation of a CuxS/CdS solar cell in hot, moist air and recovery in hydrogen and air, J. Appl. Phys. 54 (1983) 6708. [11] Z.H. Ge, L.D. Zhao, D. Wu, X.Y. Liu, B.P. Zhang, J.F. Li, J.Q. He, Low-cost, abundant binary sulfides as promising thermoelectric materials, Mater. Today (2015), http://dx.doi.org/10.1016/j.mattod.2015.10.004.
Y.-H. Ji et al. / Journal of Alloys and Compounds 680 (2016) 273e277 [12] Y.X. Zhao, H.C. Pan, Y.B. Lou, X.F. Qiu, J.J. Zhu, C. Burda, Plasmonic Cu2xS nanocrystals: optical and structural properties of copper-deficient copper (I) sulfides, J. Am. Chem. Soc. 131 (2009) 4253e4261. [13] M.T.S. Nair, L. Guerrero, P.K. Nair, Conversion of chemically deposited CuS thin films to and by annealing, Semicond. Sci. Technol. 13 (1998) 1164. [14] C. Nascu, I. Pop, V. Ionescu, E. Indrea, I. Bratu, Spray pyrolysis deposition of CuS thin films, Mater. Lett. 32 (1997) 73. [15] J.B. Boyce, B.A. Hberman, Superionic conductors: transitions, structures, dynamics, Phys. Rep. 54 (1979) 189. [16] M.B. Salamon, Springer-Verlag, Belrin, Heidelberg, New York, 1979. [17] Y.X. Chen, B.P. Zhang, Z.H. Ge, P.P. Shang, Y.Q. Yu, Preparation and thermoelectric properties of ternary superionic conductor CuCrS2, J. Solid State Chem. 186 (2012) 109e115. [18] S. Hull, D.A. Keen, D.S. Sivia, P.A. Madden, M. Wilson, The high-temperature superionic behaviour of Ag2S, J. Phys. Condens. Matter 14 (2002) L9eL17. [19] Z.H. Ge, B.P. Zhang, Z.X. Yu, Y. Liu, J.F. Li, Synthesis and transport property of
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Enhanced thermoelectric properties of Cu1.8Se1xSx alloys prepared by mechanical alloying and spark plasma sintering Yi-Hong Ji, Zhen-Hua Ge*, Zhidong Li, Jing Feng Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming, 650093, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 January 2016 Received in revised form 6 April 2016 Accepted 14 April 2016 Available online 18 April 2016
Cu1.8Se1xSx (0 x 1) thermoelectric alloys were prepared by mechanical alloying (MA) combined with spark plasma sintering (SPS) technology. The phase structure and morphologies of all the samples were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The electrical conductivity, Seebeck coefficient, thermal conductivity were investigated for all the Cu1.8Se1xSx alloys with a special emphasis on the influence of the S doping. With the increasing of S contents, a phase transition of Cu1.8Se1xSx was occurred from cubic to hexagonal. The electrical and thermal transport properties of the samples changed accordingly. The Cu1.8Se0.7S0.3 alloy achieves the highest ZT of 0.78 at 773 K due to both optimized electrical transport properties and thermal transport properties, which is 44% higher than that of pristine Cu1.8S (0.54 at 773 K) and 95% higher than that of pristine Cu1.8Se (0.4 at 773 K). © 2016 Elsevier B.V. All rights reserved.
Keywords: Thermoelectric Cu1.8Se Cu1.8S Mechanical alloying
1. Introduction Thermoelectric (TE) material is a kind of functional materiS100al which can fulfill the direct conversion between thermal energy and electrical energy. TE devices are silent, reliable and scalable, making them ideal for small, distributed power generation [1e4]. The efficiency of TE devices is determined by the dimensionless figure of merit (ZT), defined as ZT ¼ S2sT/k, where S, s, T, k, are Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively [4]. Therefore, highperformance thermoelectric materials require a high power factor (ɑ2s) and a low thermal conductivity k at the same time. The main purposes of the research on thermoelectric materials is, therefore, to simultaneously increase the electrical conductivity and decrease the total thermal conductivity [5,6]. Sulfides materials have been reported as promising thermoelectric materials [7e9]. Copper sulfides and copper selenides both are important semiconductor sulfides. The series of Cu2xS (0 x 1) compounds were widely used in thin film solar cells, transparent conductive films and conductive fibers [10e12]. The Cu1.8Se and Cu1.8S are both reported as superionic conductors [13,14]. Superionic conductors are solid materials that have ion
* Corresponding author. E-mail address: [email protected] (Z.-H. Ge). http://dx.doi.org/10.1016/j.jallcom.2016.04.140 0925-8388/© 2016 Elsevier B.V. All rights reserved.
conductivities as high as that in molten salts or liquid water at room temperature [15,16]. In superionic conductor, the ions are a kind of change carrier and contribute to the electrical conductivity. The similar superionic conductors CuCrS2, Ag2S also have been reported [17,18]. Although the high carrier concentration leads a very high electrical conductivity (~7000 S/cm) for Cu1.8Se bulk, the low Seebeck coefficient (~10 mV/K) restricts its application as a good thermoelectric material [19e21]. Cu2Se has been reported as a kind of promising thermoelectric material [22,23]. However, there are rare reports about Cu1.8Se for thermoelectric application. In this work, because of S doping, the Seebeck coefficients of Cu1.8Se alloys were enhanced significantly and the thermal conductivity was reduced resulting in an enhanced ZT value. The effect of the S contents in Cu1.8Se1xSx was investigated. The stoichiometric of Cu1.8Se0.7S0.3 sample achieves the highest ZT of 0.78 at 773 K, which is 44% higher than that of pristine Cu1.8S (0.54 at 773 K) and 95% higher than that of pure Cu1.8Se (0.4 at 773 K). 2. Experimental section The commercially available high-purity powders of Cu(99.99%), S(99.99%), Se(99.99%) were used as raw materials, which are all purchased from Alfa Aesar company without any further purification. Cu1.8Se1xSx compounds were synthesized according to the stoichiometric ratios, of which x ¼ 0, 0.3, 0.5, 0.7,
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0.9 and 1. Cu1.8Se1xSx (0 x 1) bulk samples were fabricated by combining mechanical alloying (MA) and spark plasma sintering (SPS) methods. Specific procedures are as follows: The powders with a chemical composition of Cu1.8Se1xSx (0 x 1) were ballmilled under 425 rpm for 2 h in a mixture gas atmosphere of argon (95%) and hydrogen (5%) using a planetary ball mill (QM1SP2,China) and then black blue powders were obtained. Stainless steel vessel and balls were used, and the weight ratio of ball to powders was kept at 20:1. The MAed powders were sintered at 873 K for 5 min in a ɸ20 mm graphite mould under axial compressive stress 40 MPa in vacuum using a spark plasma sintering (SPS) system (Sumitomo SPS1050, Japan). The sintered specimens were disk-shaped with a dimension ɸ20 mm 4 mm. Then the bulk materials with a dimension ɸ10 mm 1.5 mm were fabricated after cutting and polishing. The phase structure was analyzed by X-ray diffraction (XRD, Brucker D8, Germany) with a Cu Kɑ radiation. The morphologies of powders and the fractographs of bulk samples were observed by field emission scanning electron microscopy (FESEM, SUPRA™ 55, Japan). The electrical transport properties were evaluated along the sample section perpendicular to the pressing direction of SPS. The Seebeck coefficient and electrical resistivity were measured in a thin helium atmosphere using a Seebeck coefficient/electric resistance measuring system (ZEM-3 Ulvac-Riko, Japan). The density (r) of the samples was measured by the Archimedes method. In addition, the thermal conductivity of the samples was measured using the laser flash method (NETZSCH, LFA457, Germany). 3. Results and discussions Fig. 1 shows the XRD patterns of Cu1.8Se1xSx (x ¼ 0, 0.3, 0.5, 0.7, 0.9, 1.) powders after MA. As shown in Fig. 1a, all the powder samples are pure phase except Cu1.8Se0.3S0.7 which includes the impurity phase of Cu1.5Se. This result indicates that the solubility of Cu1.8S in Cu1.8Se is below 70% during the MA process. In Fig. 1b, within 0 < x < 1, all the diffraction peaks for the samples shift to high angle. That’s because the ion radius of S2 is smaller than that of Se2. The increase of S2 contents results in the decreased unit cell of Cu1.8Se, when x ¼ 0.7, the XRD patterns shows that the
Fig. 1. XRD patterns of the Cu1.8Se1xSx (x ¼ 0, 0.3, 0.5, 0.7, 0.9, and 1.) powders prepared by MA (a) and XRD patterns of 2q range from 44 to 48 (b).
coexistence of Cu1.8Se and Cu1.8S with an impurity phase Cu1.5Se was generated in matrix. With the increasing S contents, the pure Cu1.8S was obtained. Fig. 2a shows the X-ray diffraction (XRD) patterns of Cu1.8Se1xSx bulk samples after SPS. Compared with powder samples, the diffraction peaks of the bulks are sharper than those of the powder samples, indicating that the bulk samples crystallized better after sintering. There is no impurity detected in all the bulk samples, indicating the solubility of Cu1.8S in Cu1.8Se is almost unlimited after SPS processing. The peaks of (0 0 7) and (0 1 17) crystal faces of Cu1.8S were disappeared, which can be explained as follows: when x ranged from 0.9 to 1, the matrix phase structure is lowtemperature phase hexagonal crystal system of Cu1.8S, and when x 0.7, the (0 0 7) and (0 1 17) crystal planes disappeared completely, that’s because Se incorporated into the matrix and that makes the low-temperature phase Cu1.8S (hexagonal structure) transformed into high temperature phase Cu1.8S (cubic structure) according to standard cards PDF#47-1748 and PDF#24-0061. Fig. 2b shows that the diffraction peaks shift to the high angle gradually, that’s because the radius of S2 is smaller than that of Se2. The increase of S2 in lattice leads to a decreased lattice parameter. Cu1.8S sample is hexagonal digenite according to the standard card PDF#47-1748 at low temperature (under 353 K [24]), but when x ¼ 0.9, the crystal structure changed to cubic corresponding with the standard card PDF#24-0061. This indicates that selenium added into Cu1.8S leads the Cu1.8S has a high temperature phase at the low temperature (298 K) and the phase diagram reveals the phase transformation temperature of Cu1.8S is 353 K [19], and the phase transformation leads a turning point in the electrical conductivity curves. Now because of the decrease of phase transition temperature, the turning point would disappear. Fig. 3 shows the FESEM micrographs of the fractured surfaces for Cu1.8Se1xSx bulks after SPS at 873 K for 5 min by using MAed powders. As shown in Fig. 3a, the Cu1.8Se bulk has a porous morphology. However, with the increasing sulfur contents, pores in bulks became less, which is attributed to the decrease of the selenium volatilization which leads to the decrease of the sample density. We measured the density and calculated the theoretical density of all the bulk samples. The results were shown in Fig. 4. As displayed in the density curves, the theoretical density decreases
Fig. 2. XRD patterns of Cu1.8Se1xSx (x ¼ 0, 0.3, 0.5, 0.7, 0.9, and 1.) bulk samples sintered by SPS (a) and XRD patterns of 2q range from 44 to 48 (b).
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Fig. 3. FESEM micrographs of Cu1.8Se1xSx (x ¼ 0, 0.3, 0.5, 0.7, 0.9, and 1.) bulk samples sintered by SPS. (a) x ¼ 0, (b) x ¼ 0.3, (c) x ¼ 0.5, (d) x ¼ 0.7, (e) x ¼ 0.9 and (f) x ¼ 1.
Fig. 4. Measured density (black square), relative density (blue blank circle) and theoretical density (red circle) dependence of S contents of the Cu1.8Se1xSx (x ¼ 0, 0.3, 0.5, 0.7, 0.9, and 1.) bulk samples sintered by SPS. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
with the increasing of sulfur contents while the actual density is on the opposite. And the relative density shows the similar trend, which increased with the increasing sulfur contents. And when x ¼ 0, the minimum relative density of 79% is obtained. The reason is the volatility of element selenium during SPS processing. The increase of Se volatilization results in more pores and the low relative density for bulk samples. This result is agreement with the FESEM observations. Fig. 5 shows the temperature dependence of electrical transport properties for Cu1.8Se1xSx (x ¼ 0, 0.3, 0.5, 0.7, 0.9, and 1.) bulk samples after SPS. In Fig. 5a, the electrical conductivity for all Cu1.8Se1xSx bulks decreased with the increasing measuring temperature showing a metal conducting behavior. Among the whole measuring temperature range, as the increase of x, the electrical conductivity for Cu1.8Se1xSx bulk samples first decreased and then increased. when x increased from 0 to 0.7, the electrical conductivity decreased gradually. The reason is that Se has one more electronic shell than that of S, so the selenium’s ability to constrain electron is weaker than that of sulfur, the CueSe bond is also weaker than CueS bond. There is no turning point when x ranges
from 0 to 0.7. Cu1.8Se possesses the highest electrical conductivity of about 7000 Scm1 at room temperature. The Cu1.8Se0.3S0.7 bulk sample achieves the lowest electrical conductivity. Because of the increased relative density of Cu1.8Se0.1S0.9 and pure Cu1.8S, they got the higher electrical conductivities. There are turning points in the electrical conductivity curves for pure Cu1.8S and Cu1.8Se0.1S0.9 bulk samples at 373 K, which is agreement with our previous report [19]. That’s because Cu1.8S has a superionic transition at that temperature and the copper ion turned from order state into disorder state. Fig. 5b is the variation of Seebeck coefficient as a function of temperature. The Seebeck coefficients of all Cu1.8Se1xSx bulk samples are positive being indicative of a p-type conducting behavior and the major carrier is hole. When x ranged from 0 to 0.9, the Seebeck coefficient of these samples increased. That’s because the charge carrier concentration decreased and the complexity of the system increased. Pure Cu1.8Se0.1S0.9 sample has the highest Seebeck coefficient of 110 mV/K at 750 K. Cu1.8Se has the lowest Seebeck coefficient, which ranges from 17 to 65 mV/K through the whole measuring temperature. According to the results of electrical conductivity and Seebeck coefficient, the power factors of all the bulk samples were calculated by the formula PF ¼ a2s. The power factor of all the bulk samples were shown in Fig. 5c, on account of high Seebeck coefficient, the samples of which x ¼ 0.9 and x ¼ 1 get the high power factors of 1200 W/mK2 at 773 K. Although pure Cu1.8Se has a low Seebeck coefficient, the power factor still reached 1200 W/mK2 at 773 K due to its very high electrical conductivity. Fig. 6a shows temperature dependence of thermal conductivity(a) and ZT value(b) for Cu1.8Se1xSx bulk samples of which x ¼ 0, 0.3, 0.5, 0.7, 0.9, 1. Generally speaking, the thermal conductivity for Cu1.8Se1xSx (0 x 1) bulk samples decreased with the increase of measuring temperature due to the enhanced thermal vibration of lattice. One exception is Cu1.8S bulk, which is attributed to the phase transition that leads to the appearance of turning point. With the decrease of x, the thermal conductivity of the samples increased firstly when x ranged from 1 to 0.7. Then because more and more pores generated in samples, the thermal conductivity of decreased gradually and reached the lowest value of 0.98 Wm1K1 for Cu1.8Se0.7S0.3 bulk sample. Although Cu1.8Se bulk has the lowest relative density, which still got the highest thermal conductivity from 2.3 to 4.8 Wm1K1 in the all temperature range due to its very high electrical conductivity. According to the measuring results of electrical conductivity,
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Fig. 6. Temperature dependence of thermal conductivity (a) and ZT value (b) for Cu1.8Se1xSx (x ¼ 0, 0.3, 0.5, 0.7, 0.9, and 1.) bulk samples sintered by SPS.
transport properties, and which is 44% higher than that of pristine Cu1.8S (0.54 at 773 K) and 95% higher than that of pristine Cu1.8Se (0.4 at 773 K). Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51501086). Fig. 5. Temperature dependence of electrical conductivity (a), Seebeck coefficient (b) and power factor (c) for Cu1.8Se1xSx (x ¼ 0, 0.3, 0.5, 0.7, 0.9, and 1.) bulk samples sintered by SPS.
Seebeck coefficient and thermal conductivity, the ZT value was obtained through the formula ZT ¼ a2sT/k. As is shown in Fig. 6b, the ZT value for all samples increased with the increasing temperature. The Cu1.8Se0.7S0.3 bulk sample achieves the highest ZT value of 0.78 at 773 K, which is 44% higher than that of pristine Cu1.8S (0.54 at 773 K) and 95% higher than that of Cu1.8Se (0.4 at 773 K). For the Cu1.8Se based alloys, the S doping increased the complexity of the system, the Seebeck coefficient was increased and the thermal conductivity was decreased resulting in an enhanced ZT value. 4. Conclusions Cu1.8Se1xSx (x ¼ 0, 0.3, 0.5, 0.7, 0.9, and 1.) thermoelectric alloys were synthesized by MA combined with SPS techniques. All Cu1.8Se1xSx (0 < x < 1) bulk samples after SPS are pure phase, indicating the unlimited solubility of Cu1.8Se and Cu1.8S. The Cu1.8Se0.7S0.3 alloy achieves the highest ZT of 0.78 at 773 K due to both optimized electrical transport properties and thermal
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