Journal of ELECTRONIC MATERIALS, Vol. 43, No. 6, 2014
DOI: 10.1007/s11664-014-3072-y Ó 2014 TMS
Phase Stability and Thermoelectric Properties of CuFeS2-Based Magnetic Semiconductor NAOHITO TSUJII,1,2 TAKAO MORI,1 and YUKIHIRO ISODA1 1.—National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan. 2.—e-mail:
[email protected]
Recently, based on measurement results below 400 K, we suggested that chalcopyrite CuFeS2-based alloys hold promise as thermoelectric materials. In this study, we have investigated the phase stability of such compounds and measured their thermoelectric properties at temperatures above 400 K. Thermogravimetric data indicate that the samples synthesized by a spark plasma sintering method were stable up to 700 K, above which sulfur deficiency becomes prominent. The electrical resistivity of the electron-doped samples showed metallic behavior up to 700 K. The Seebeck coefficients show large negative values of about 300 lV/K above 400 K. As a result, the power factor of Cu0.97Fe1.03S2 is 1 mW/K2m in the temperature range of 400 K to 600 K. Key words: Thermoelectric property, power factor, magnetic semiconductor, chalcopyrite
INTRODUCTION Thermoelectric generation is expected to play an important role in meeting the increasing demand for higher energy efficiency. The performance of a thermoelectric material is generally measured by the dimensionless figure of merit, ZT = S2T/qj, where T, S, q, and j represent the absolute temperature, Seebeck coefficient, electrical resistivity, and thermal conductivity, respectively. Based on extensive experimental and theoretical studies, values of ZT have increased dramatically in the last decade. This has mainly been made possible by discoveries of very low-j materials.1–5 In addition to ZT, the power factor S2/q is also an important parameter when it comes to power generation using a bulk heat source such as a power plant or geothermal heat. However, increasing the power factor is still a challenge, since it depends on the precise electronic structure around the Fermi level, preventing a general material-design strategy. This situation motivated us to focus on CuFeS2based compounds as promising thermoelectric materials. We expected that doped carriers in (Received June 30, 2013; accepted February 10, 2014; published online March 4, 2014)
magnetic semiconductors would interact strongly with the magnetic moments, potentially yielding a large Seebeck coefficient through an enhanced electron mass while preserving good carrier conduction. We indeed observed a high power factor of 1 mW/K2m at 400 K in electron-doped CuFeS2 samples.6,7 In addition, several semiconductors with chalcopyrite-type structure were recently found to show distinct thermoelectric properties.8–10 This calls for more detailed research on this class of materials. In this study, we investigated the thermal stability of CuFeS2-based alloys and measured their thermoelectric properties above 400 K. We prepared sintered samples of Cu1 xZnxFeS2 and Cu1 xFe1+xS2. In the former case, a Cu+ ion is replaced by a Zn2+ ion. Thus, one 4s electron per Zn atom is doped into CuFeS2. In the latter case, the extra Fe ion should be in either the Fe2+ or Fe3+ state, resulting in one-electron or two-electron doping. In both cases, these substitutions are expected to lead to n-type conductivity. EXPERIMENTAL PROCEDURES Samples were synthesized by solid-state reaction, followed by spark plasma sintering (SPS) under 2371
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40 MPa at 770 K in argon atmosphere. The details have been reported elsewhere.6,7 The SPSed samples were subsequently annealed in an evacuated quartz tube at 650 K for 1 day, then slowly cooled to room temperature. The starting compositions were Cu0.95Fe1.05S2 and Zn0.05Cu0.95FeS2. However, electron probe microanalysis (EPMA) indicated that the actual chemical compositions of the chalcopyrite phase were better described as Cu0.97Fe1.03S2 and Zn0.03Cu0.97FeS2, respectively. Thus, in this paper, we use the chemical compositions obtained by EPMA. The crystal structure of the compounds was checked by powder x-ray diffraction (XRD) analysis. The XRD data were collected using Cu Ka radiation with a RINT TTR-3 diffractometer (Rigaku Co., Akishima, Tokyo, Japan), operated with an accelerating voltage of 40 kV and a cathode current of 150 mA. The thermal stability of the samples was studied by thermogravimetric (TG) and differential thermal analysis (DTA) measurements using a Thermo-Plus EVO (Rigaku Co.). A sample specimen of about 20 mg was cut from the pellet, put on an alumina cell, and heated under argon flow from room temperature to 1300 K with a ramping rate of 10 K/min. An empty alumina cell was measured simultaneously as a reference. Thermoelectric properties below 400 K were measured using a physical property measurement system (Quantum Design Inc., San Diego, CA), under a vacuum better than 0.02 Pa. The electrical resistivity q and Seebeck coefficient S above room temperature were measured by a ZEM3 (ULVACRIKO Inc., Yokohama, Japan) under helium atmosphere. The q data measured by the PPMS and ZEM3 between 300 K and 400 K differed from each other by up to 10%, probably because of the error in the estimation of the lead distance and the sample cross-section. The q values from the ZEM3 were then normalized to those from the PPMS. On the other hand, the Seebeck coefficient data from the PPMS and ZEM3 were used as obtained. RESULTS Figure 1 shows the powder XRD patterns of the CuFeS2, Cu0.97Fe1.03S2, and Zn0.03Cu0.97FeS2 samples prepared by SPS and subsequent annealing. The data indicate that these compounds have the tetragonal chalcopyrite-type structure. For Zn0.03 Cu0.97FeS2, weak extra peaks are also observed, being assigned to cubic ZnS. This indicates that the chemical composition of the chalcopyrite phase differs from the nominal composition. The composition was therefore determined by EPMA. In this paper, we describe the samples by the chemical compositions obtained by EPMA. Figure 2 shows the TG–DTA results for the Cu0.97Fe1.03S2 and Zn0.03Cu0.97FeS2 samples. Figure 2a shows the DTA data of the samples prepared by SPS and annealing. An endothermic peak
Fig. 1. Powder XRD patterns of samples prepared by the SPS and annealing procedure.
is seen at 820 K for Zn0.03Cu0.97FeS2 and at 830 K for Cu0.97Fe1.03S2. This corresponds to decomposition of the chalcopyrite phase into an isometric (Cu,Fe)S phase and pyrite FeS2.11 Figure 2b and c show the TG curves of Zn0.03Cu0.97FeS2 and Cu0.97Fe1.03S2 samples synthesized with two different procedures: a conventional solid-state reaction (SSR), and the SPS and subsequent annealing (SPS+A) method. For the SSR samples, a weight loss is seen above 590 K. This corresponds to sulfur deficiency becoming prominent at elevated temperatures. On the other hand, the SPS+A samples appear to be more stable, because the weight loss starts above 700 K. It is observed that the density of the SSR samples is about 80% of the ideal density, whereas the SPS+A samples have a higher density of 97% to 99%. Thus, employing SPS is effective in improving the hightemperature stability. Based on these results, we measured the thermoelectric properties of the SPS+A samples below 700 K. Figure 3a shows the electrical resistivity q of Cu0.97Fe1.03S2 and Zn0.03Cu0.97FeS2 as a function of temperature. The q of CuFeS2 is also shown for comparison. It is seen that the q values of Cu0.97Fe1.03S2 and Zn0.03Cu0.97FeS2 are reduced by one to two orders of magnitude from that of CuFeS2. The q values of Cu0.97Fe1.03S2 and Zn0.03Cu0.97FeS2 increase monotonically with temperature. Notably, the q values of Cu0.97Fe1.03S2 in Fig. 3a are reduced by about 50% compared with those reported in Ref. 6, where the samples were synthesized by solidstate reaction.*
*Here, the sample of Cu0.97Fe1.03S2 prepared by the solid-state reaction method is the same as the one reported in Ref. 6 as Cu0.95Fe1.05S2. In Ref. 6, the chemical composition was described as the nominal one. However, the EPMA results indicated that the actual chemical composition is better described as Cu0.97Fe1.03S2. Thus, in this paper, the latter description is used.
Phase Stability and Thermoelectric Properties of CuFeS2-Based Magnetic Semiconductor
(a)
(a)
(b)
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(c) (b)
Fig. 2. Results of thermal analysis measurements as functions of temperature: (a) DTA data of Cu0.97Fe1.03S2 and Zn0.03Cu0.97FeS2 samples prepared by SPS and annealing. The temperature at which chalcopyrite decomposes into (Cu,Fe)S and pyrite is indicated by arrows. (b, c) Thermogravimetry (TG) data for Zn0.03Cu0.97FeS2 and Cu0.97Fe1.03S2, respectively. Here, comparisons are made between samples prepared by conventional solid-state reaction (SSR) and those prepared by SPS and subsequent annealing (SPS+A). Only the heating process is shown in the TG plot.
Figure 3b displays the temperature dependence of the Seebeck coefficient S of CuFeS2, Cu0.97 Fe1.03S2, and Zn0.03Cu0.97FeS2. For all the compounds, S is negative, indicating n-type conduction. The absolute values of S of Cu0.97Fe1.03S2 and Zn0.03Cu0.97FeS2 increase monotonically with temperature. This is a typical behavior of a degenerate semiconductor. The absolute values of S of Zn0.03 Cu0.97FeS2 are greater than those of Cu0.97Fe1.03S2, suggesting a lower carrier concentration in the former sample. This is consistent with the higher electrical resistivity of Zn0.03Cu0.97FeS2 compared with Cu0.97Fe1.03S2. It is noteworthy that the S values in Fig. 3b for Cu0.97Fe1.03S2 are in good agreement with those of the sample made by solidstate reaction reported in Ref. 6. This confirms that the carrier concentrations are not altered by the synthesis process. Electrical resistivity is expressed as q = 1/nel. Since the q of the SPS-synthesized Cu0.97Fe1.03S2 sample is about half that of the solidstate reaction sample, the carrier mobility l has been almost doubled by the SPS process. This is most likely due to the high density of the SPS sample.
Fig. 3. Temperature dependence of the electrical resistivity q (a) and the Seebeck coefficient S (b) of CuFeS2-based samples prepared by the SPS and annealing method.
In Fig. 4, the temperature dependence of the power factor S2/q is plotted for CuFeS2, Cu0.97Fe1.03S2, and Zn0.03Cu0.97FeS2. The S2/q values of Cu0.97Fe1.03S2 and Zn0.03Cu0.97FeS2 are greatly improved compared with that of CuFeS2. In particular, the S2/q value of Cu0.97Fe1.03S2 reaches 1 mW/K2m at 400 K, and stays at that value almost up to the highest temperature measured. The thermal conductivity of these samples has not yet been measured above 400 K. We thus estimated the dimensionless figure of merit ZT using the thermal conductivity data from the PPMS.7 The thermal conductivity of Cu0.97Fe1.03S2 exhibits a maximum at 20 K, above which j decreases monotonically with temperature. The minimum value observed is j = 6 W/Km at 400 K. By combining this value and the power factor S2/q = 1 mW/K2m above 400 K, the dimensionless figure of merit of Cu0.97Fe1.03S2 is estimated to be ZT 0.12 at 700 K. DISCUSSION CuFeS2 is widely known as chalcopyrite. Using such natural minerals for thermoelectric
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Fig. 4. Power factor S2/q of CuFeS2-based samples as a function of temperature.
applications should be a useful approach because they are environmentally friendly, air stable, and abundant. It is notable that good thermoelectric properties have also been reported for the natural tetrahedrite minerals (Cu,M)12Sb4S13, with M being 3d transition metals.12–15 Another noteworthy point about CuFeS2 is that this compound is a magnetic semiconductor where the Fe3+ spins show antiferromagnetic ordering. Although magnetic semiconductors have been extensively investigated for spintronic applications, reports on their thermoelectric properties are rare. Some compounds show good thermoelectric performance, such as FexCr3 xSe416 and Yb14MnSb11.3,17 In Yb14MnSb11, a strong coupling between the carriers and the Mn2+ moments is suggested. The effective carrier mass estimated from the optical conductivity is as large as 20m0 at low temperatures.18,19 Even above room temperature, the Seebeck coefficient suggests a moderately enhanced mass of 3m0.3 Here, the enhanced carrier mass is not attributed to the strong correlation of the 4f electron of Yb, because x-ray absorption spectroscopy demonstrates that the Yb ion in Yb14MnSb11 is purely divalent with a closed 4f shell.20 This suggests that the enhanced mass observed in Yb14MnSb11 is due to the coupling between carriers and Mn2+ moments. If this enhanced mass can be used to increase the Seebeck coefficient, magnetic semiconductors could be good candidate thermoelectric materials. It is notable that the magnetic ordered state itself can contribute to increase the Seebeck coefficient via the spin Seebeck effect in ferromagnetic or ferrimagnetic materials.21,22 In the present case, CuFeS2 is an antiferromagnet, and the spin Seebeck effect will be canceled.23 The properties of ferromagnetic semiconductors would hence be quite interesting. In the present results, we observed a high power factor of 1 mW/K2m for Cu0.97Fe1.03S2 above
Tsujii, Mori, and Isoda
400 K. It is observed that the present samples have a high carrier density of 1020 cm 3, whereas the estimated carrier mobilities are low, l = 5 cm2/V/s to 7 cm2/V/s.7 Thus, the high power factor of the present samples is mainly attributed to the large electron mass m* = 3.5m0 to 5.6m0. These results support our attempt to achieve a high power factor by using magnetic semiconductors. However, it is still unclear whether the large electron mass is due to the strong coupling between magnetic ions and carriers, or is explained by the density of the conduction-band electron states. An earlier theoretical study of CuFeS2 suggested that the doped electrons go into the Fe 3d band with spin direction antiparallel to the ordered moment.24 Electrons in the 3d-based conduction band are known to show a strongly correlated effect with enhanced mass because of the spatially constrained 3d orbitals and the corresponding strong Coulomb repulsion between electrons. Indeed, large Seebeck effects with good electrical conduction are observed in several transition-metal systems, such as Co oxides25,26 and Ti sulfides.27–29 These transitionmetal oxides and sulfides do not show magnetic ordering. The high power factors in these materials are therefore attributed to the strong electron– electron correlation within the 3d-band. In our case, the effect of a strongly correlated electron band may also be important. To understand the origin of the high power factor of CuFeS2-based materials, detailed theoretical studies are necessary. CONCLUSIONS We have investigated the thermal stability and thermoelectric properties of CuFeS2-based materials. The TG data indicated that the samples prepared by the SPS method have better thermal stability than those synthesized by solid-state reaction. Moreover, the electrical resistivity of the SPS samples decreased to about 50% of the solidstate reaction sample values. Meanwhile, the Seebeck coefficients are unchanged by the different sample preparation procedures. As a result, the power factor increased in the SPS samples to 1 mW/K2m above 400 K. The large negative Seebeck coefficients in the present compounds are attributed to the enhanced carrier mass, m* = 3.5m0 to 5.6m0. There are several examples of magnetic and semiconductors, such as FexCr3 xSe4 Yb14MnSb11, in which the strong coupling between the magnetic moments and carriers seems to be responsible for the large Seebeck coefficients. Theoretical studies are necessary to understand the thermoelectric properties of magnetic semiconductors. One problem yet to be overcome for CuFeS2-based compounds is their high thermal conductivity of j = 6 W/Km at 400 K. This yields a maximum ZT value of about 0.12 at 700 K. Possible approaches to reduce j are alloying, replacing constituents by
Phase Stability and Thermoelectric Properties of CuFeS2-Based Magnetic Semiconductor
heavier elements, and nanostructuring. There is plenty of room to improve the thermoelectric performance in this system. ACKNOWLEDGEMENTS This work was supported by Grant-in-Aid for Scientific Research (C) No. 24550168 from the Japan Society for the Promotion of Science. T.M. was partly supported by AOARD. We thank M. Nishio of Materials Analysis Station, NIMS, for EPMA measurements. N.T. is grateful to R. Nakamura for help with sample synthesis and XRD measurements. REFERENCES 1. G.J. Snyder and E.S. Toberer, Nat. Mater. 7, 105 (2008). 2. G.S. Nolas, D.T. Morelli, and T.M. Tritt, Annu. Rev. Mater. Sci. 29, 89 (1999). 3. S.M. Kauzlarich, S.R. Brown, and G.J. Snyder, Dalton Trans. 2099 (2007). 4. E.S. Toberer, A.F. May, and G.J. Snyder, Chem. Mater. 22, 624 (2010). 5. M. G. Kanatzidis, Chem. Mater. 22, 648 (2010). 6. N. Tsujii, J. Electron. Mater. 42, 1974 (2013). 7. N. Tsujii and T. Mori, Appl. Phys. Express 6, 043001 (2013). 8. R. Liu, L. Xi, H. Liu, X. Shi, W. Zhang, and L. Chen, Chem. Commun. 48, 3818 (2012). 9. A. Yusufu, K. Kurosaki, A. Kosuga, T. Sugahara, Y. Ohishi, H. Muta, and S. Yamanaka, Appl. Phys. Lett. 99, 061902 (2011). 10. A. Kosuga, T. Plirdpring, R. Higashine, M. Matsuzawa, K. Kurosaki, and S. Yamanaka, Appl. Phys. Lett. 100, 042108 (2012). 11. R.A. Yund and G. Kullerud, J. Petrol. 7, 454 (1966). 12. K. Suekuni, K. Tsuruta, T. Ariga, and M. Koyano, Appl. Phys. Express 5, 051201 (2012). 13. X. Lu, D.T. Morelli, Y. Xia, F. Zhou, V. Ozolins, H. Chi, X.Y. Zhou, and C. Uher, Adv. Energy Mater. 3, 342 (2013).
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