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Thermoelectric Properties and Nanostructuring in the p-Type Materials NaPb18-xSnxMTe20 (M ) Sb, Bi) Aure´lie Gue´guen,†,‡ Pierre F. P. Poudeu,‡ Chang-Peng Li,§ Steven Moses,§ Ctirad Uher,§ Jiaqing He,| Vinayak Dravid,| Konstantinos M. Paraskevopoulos,⊥ and Mercouri G. Kanatzidis*,‡ Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48824, Department of Chemistry and Department of Materials Science, NUANCE Center, Northwestern UniVersity, EVanston, Illinois 60208, Department of Physics, UniVersity of Michigan, Ann Arbor, Michigan 48109, and Department of Physics, Aristotle UniVersity of Thessaloniki, 54124 Thessaloniki, Greece ReceiVed December 31, 2008. ReVised Manuscript ReceiVed February 1, 2009
The thermoelectric properties of materials with nominal compositions NaPb18-xSnxMTe20 (M ) Sb, Bi) were investigated in the temperature range 300-650 K. All the members of NaPb18-xSnxMTe20 have a cubic rock-salt (NaCl) type structure and exhibit p-type charge transport behavior between 300-650 K. The relative fraction of Sn strongly affects the physical, structural, and transport properties of the materials. Independent of the nature of the pnicogen atom (M), the electrical conductivity increases with decreasing Pb:Sn ratio, whereas the thermopower decreases. Hall effect data for selected samples, e.g., NaPb15Sn3BiTe20 and NaPb13Sn5SbTe20, show high carrier concentrations of ∼1 × 1020 cm-3 at room temperature. Comparing corresponding members from the antimony and bismuth series, we observed that the Sn-free compositions (x ) 0) exhibit the highest power factors, and as a consequence, the highest ZT, with NaPb18BiTe20 reaching a ZT ≈ 1.3 at 670 K. The NaPb18-xSnxMTe20 series exhibit increasing total thermal conductivity with increasing fraction of Sn with room temperature values between 1.37 W/(m K) for x ) 3 and 3.9 W/(m K) for x ) 16 for NaPb18-xSnxSbTe20. The lowest lattice thermal conductivity, ∼0.4 W/(m K), was observed for the composition NaPb2Sn16BiTe20 at 650 K. High-resolution transmission electron microscopy on several members of the NaPb18-xSnxMTe20 series reveal that they are inhomogeneous on the nanoscale with widely dispersed nanocrystals embedded in a Pb1-ySnyTe matrix. Also observed are lamellar features in these materials associated with compositional fluctuations and significant strain at the nanocrystal/matrix interface.
Introduction Thermoelectric-based cooling systems and power generators have limited applications because of their relatively low efficiency.1 As a consequence, a worldwide research for systems with higher performance is currently underway.2-7 The performance of a thermoelectric material is defined by its figure of merit ZT ) (σS2)T/κ, where σ ) electrical conductivity, S ) Seebeck coefficient (also called thermopower), and κ ) thermal conductivity. The numerator * Corresponding author. Tel: (847) 467-1541. Fax: (847) 491-5937. E-mail:
[email protected]. † Michigan State University. ‡ Department of Chemistry, Northwestern University. § University of Michigan. | Department of Materials Science, NUANCE Center, Northwestern University. ⊥ Aristotle University of Thessaloniki.
(1) (2) (3) (4)
Di Salvo, F. J. Science 1999, 285, 703. Tritt, T. M.; Subramanian, M. A. MRS Bull. 2006, 31, 188. Snyder, G. J.; Toberer, E. S. Nat. Mater. 2008, 7, 105. Heremans, J. P.; Jovovic, V.; Toberer, E. S.; Saramat, A.; Kurosaki, K.; Charoenphakdee, A.; Yamanaka, S.; Snyder, G. J. Science 2008, 321 (5888), 554–557. (5) Nolas, G. S.; Cohn, J. L.; Slack, G. A.; Schujman, S. B. Appl. Phys. Lett. 1998, 73 (2), 178–180. (6) Nolas, G. S.; Poon, J.; Kanatzidis, M. MRS Bull. 2006, 31 (3), 199– 205. (7) Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y. C.; Minnich, A.; Yu, B.; Yan, X.; Wang, D. Z.; Muto, A.; Vashaee, D.; Chen, X. Y.; Liu, J. M.; Dresselhaus, M. S.; Chen, G.; Ren, Z. Science 2008, 320 (5876), 634– 638.
(σS2) is called the power factor. Many recent experimental and theoretical studies on low-dimensional nanostructured materials8-10 and mixed-phase nanocomposites11-14 point to encouraging routes for achieving low lattice thermal conductivity while maintaining high power factors. We are interested in developing bulk materials with large figures of merit. Recently, we have reported on the bulk n-type systems AgPbmSbTem+2 (LAST),15,16 Pb1-xSnxTe-PbS,17 and PbTe: (Pb,Sb)18 Pb9.6Sb0.2Te10-xSex19 and the p-type systems (8) Harman, T. C.; Taylor, P. J.; Walsh, M. P.; LaForge, B. E. Science 2002, 297, 2229. (9) Harman, T. C.; Taylor, P. J.; Walsh, M. P.; LaForge, B. E. J. Electron. Mater. 2005, 34, L19. (10) Venkatasubramanian, R.; Siivola, E.; Colpitts, T.; O’Quinn, B. Nature 2001, 413, 597. (11) Caylor, J. C.; Coonley, K.; Stuart, J.; Colpitts, T.; Venkatasubramanian, R. Appl. Phys. Lett. 2005, 87 (2), 023105. (12) Hicks, L. D.; Dresselhaus, G. Phys. ReV. B 1993, 47, 16631. (13) Hicks, L. D.; Dresselhaus, M. S. Phys. ReV. B 1993, 47 (19), 12727. (14) Kim, W.; Singer, K. L.; Majumdar, A.; Vashaee, D.; Bian, Z.; Shakouri, A.; G., Z.; Bowers, E. J.; Zide, J. M. O.; Gossard, C. Appl. Phys. Lett. 2006, 88, 242107. (15) Hsu, K. F.; Loo, S.; Guo, F.; Chen, W.; Dyck, J. S.; Uher, C.; Hogan, T.; Polychroniadis, E. K.; Kanatzidis, M. G. Science 2004, 303, 818. (16) Quarez, E.; Hsu, K. F.; Pcionek, R.; Frangis, N.; Polychroniadis, E. K.; Kanatzidis, M. G. J. Am. Chem. Soc. 2005, 127, 9177. (17) Androulakis, J.; Lin, C. H.; Kong, H. J.; Uher, C.; Wu, C. I.; T., H.; Cook, B. A.; T., C.; Paraskevopoulos, M.; Kanatzidis, M. G. J. Am. Chem. Soc. 2007, 129, 9780.
10.1021/cm803519p CCC: $40.75 2009 American Chemical Society Published on Web 03/24/2009
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Na1-xPbmSbyTem+2 (SALT)20 and Ag(Pb1-ySny)mSbTe2+m (LASTT),21 all bulk materials that exhibit high thermoelectric figure of merit. High-resolution transmission electron microscopy images of these materials revealed the presence of nanoparticles coherently embedded in what is essentially a PbTe matrix. This feature is believed to be at the origin of the very low thermal conductivity, which is a common property of all these systems. For example, the p-type composition Na0.95Pb20SbTe22 (SALT-20) reached ZT of ∼1.6 at 650 K, which is nearly twice that of p-type PbTe, and arises predominantly from a very low total thermal conductivity, namely 0.85 W/(m K) at 700 K for Na0.95Pb20SbTe22. In this work, we examine the partial substitution of Pb by Sn in the high ZT material NaPb18SbTe20 to produce the solid solutions NaPb18-xSnxSbTe20 and study their physical and thermoelectric properties. To better understand the role of the pnicogen element on the properties of the system, corresponding Bi analogs with general formula NaPb18-xSnxBiTe20 were prepared as well. In a recent study, we showed that the role of the trivalent element in AgPb18MTe20 (M ) Sb, Bi) in the power factor and lattice thermal conductivity is significant with the Sb producing more favorable thermoelectric properties than Bi.22 We also report here that partial substitution of Pb by Sn in the NaPb18MTe20 materials impacts the carrier concentration and mobility. The electrical conductivity of NaPb18-xSnxMTe20 (M ) Sb, Bi) increases and the thermopower decreases with increasing fraction of Sn. All the compositions examined showed p-type behavior and exhibited very low lattice thermal conductivity. This is attributed to a combination of point defect scattering associated with solid-solution behavior and the presence of nanosized inclusions in the crystalline matrix. These inclusions are readily observable by highresolution transmission electron microscopy and they appear to be qualitatively different from those reported in other systems. The lowest lattice thermal conductivity at high temperature of 0.54 and 0.64 W/(m K) were observed respectively for NaPb13Sn5SbTe20 and NaPb9Sn9SbTe20, however, the highest power factors were observed in the Snfree (x ) 0) materials. The latter also exhibit the highest ZT values of up to 1.3 at 670 K for NaPb18BiTe20. Experimental Section Synthesis. All samples were prepared as polycrystalline ingots in silica tubes by mixing high-purity Na (Aldrich, 99.95%), Pb and Sn (Rotometals, 99.99%), Sb (Tellurex, 99.999%), and Te (Plasmaterials, 99.999%) in the appropriate stoichiometric ratio. To (18) Sootsman, J. R.; Kong, H.; Uher, C.; D’Angelo, J. J.; Wu, C. I.; Hogan, T. P.; Caillat, T.; Kanatzidis, M. G. Angew. Chem., Int. Ed. 2008, 47 (45), 8618–8622. (19) Poudeu, P. F. P.; D’Angelo, J.; Kong, H. J.; Short, J. L.; Pcionek, R.; Hogan, T.; Uher, C.; Kanatzidis, M. G. J. Am. Chem. Soc. 2006, 128, 14347. (20) Poudeu, P. F. P.; D’Angelo, J.; Downey, A. D.; Short, J. L.; Hogan, T.; Kanatzidis, M. G. Angew. Chem., Int. Ed. 2006, 45, 3835. (21) Androulakis, J.; Hsu, K. F.; Pcionek, R.; Kong, H. J.; Uher, C.; D’Angelo, J.; Downey, A. D.; Hogan, T.; Kanatzidis, M. G. AdV. Mater. 2006, 18, 1170. (22) Han, M.; Hoang, K.; Kong, H. J.; Pcionek, R.; Uher, C.; Paraskevopoulos, M.; Mahanti, S. D.; Kanatzidis, M. G. Chem. Mater. 2008, 20, 3512.
Gue´guen et al. prevent reaction between the sodium metal and silica, the tubes were carbon-coated prior to use. For example, sodium (0.050 g), lead (5.8583 g), tin (1.2909 g), antimony (0.2648 g), and tellurium (5.5503 g) were used to prepare NaPb13Sn5SbTe20; for NaPb9Sn9SbTe20, sodium (0.060 g), lead (4.8669 g), tin (2.7883 g), antimony (0.3178 g), and tellurium (6.6604 g) were used. All components (except Na) were loaded into silica tubes under ambient atmosphere and the corresponding amount of Na was later added under inert atmosphere in a dry glovebox. The silica tubes were then flame-sealed under a residual pressure of ∼1 × 10-4 Torr, placed into a tube furnace (mounted on a rocking table), and heated at 1250 K for 4 h to allow complete melting of all components. While molten, the furnace was allowed to rock for 2 h to facilitate complete mixing and homogeneity of the liquid phase. The furnace was finally immobilized at the vertical position and was cooled from 1250 to 820 K over 43 h followed by a faster cool (6-8 h) to room temperature. The resulting ingots generally were silverymetallic in color with a smooth surface. Powder X-ray Diffraction. Powder X-ray patterns of the grinded materials were recorded using Cu KR radiation (λ ) 1.54056 Å) in reflection geometry on a CPS-120 Inel X-ray powder diffractometer operating at 40 kV and 20 mA equipped with a positionsensitive detector. The lattice parameters were refined from the X-ray powder diffraction patterns using the appropriate software. Infrared spectroscopy. Room-temperature optical diffuse reflectance measurements were performed on finely ground powder to probe the optical band gap of the materials. The spectra were monitored in the mid-IR region (6000-400 cm-1) using a Nicolet 6700 FTIR spectrometer. Absorption (R/S) data were calculated from reflectance data using the Kubelka-Munk function.23-25 The optical band gaps were derived from R/S versus E (eV) plots as described elsewhere.26-28 Specular IR reflectivity measurements were carried out on polished specimens in the spectral range 100-2500 cm-1, at room temperature, with nonpolarized light, using a Bruker IFS 113V Fourier transform interferometer working under a vacuum and equipped with the special reflectance unit. The angle of incidence was less than 10°. DTA Analysis. Differential thermal analysis (DTA) data were collected with a Shimadzu DTA-50 thermal analyzer as described elsewhere.29-31 Approximately 35 mg of finely ground powder of material was sealed in a carbon-coated quartz ampule under residual pressure of ∼1 × 10-4 Torr. Another ampule containing similar amount of alumina and prepared the same way was used as a reference. The samples were heated to 1273 K at a rate of 10 K/min, held at 1273 K for 2 min, and cooled to 323 K at a rate of -10 K/min. Electrical Transport Properties. Thermopower and electrical conductivity properties were measured simultaneously under helium atmosphere using a ZEM-3 Seebeck coefficient/electrical resistivity measurement system (ULVAC-RIKO, Japan). Samples for transport (23) Wendlandt, W.; Hecht, H. G. Reflectance Spectroscopy; Interscience: New York, 1966. (24) Kotuem, G. Reflectance Spectroscopy; Interscience: New York, 1969. (25) Tandon, S. P.; Gupta, J. P. Phys. Status Solidi 1970, 38, 363. (26) Chung, D. Y.; Jobic, S.; Hogan, T.; Kannewurf, C. R.; Brec, R.; Rouxel, J.; Kanatzidis, M. G. J. Am. Chem. Soc. 1997, 119 (10), 2505– 2515. (27) McCarthy, T. J.; Tanzer, T. A.; Kanatzidis, M. G. J. Am. Chem. Soc. 1995, 117 (4), 1294–1301. (28) Trikalitis, P. N.; Rangan, K. K.; Kanatzidis, M. G. J. Am. Chem. Soc. 2002, 124 (11), 2604–2613. (29) Chondroudis, K.; Hanko, J. A.; Kanatzidis, M. G. Inorg. Chem. 1997, 36 (12), 2623–2632. (30) Chondroudis, K.; Kanatzidis, M. G. Inorg. Chem. 1995, 34 (22), 5401– &. (31) Chondroudis, K.; McCarthy, T. J.; Kanatzidis, M. G. Inorg. Chem. 1996, 35 (4), 840–844.
Properties and Nanostructuring in NaPb18-xSnxMTe20
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Figure 1. X-ray powder diffraction patterns of NaPb18-xSnxMTe20 with x ) 5, 9, 13, and 16 for (a) M ) Sb and (b) M ) Bi; variation of the unitcell parameter as a function of x for (c) M ) Sb and (d) M ) Bi.
measurement were cut to size 10 × 3 × 3 mm using a diamond saw (Buehler Isomet 1000), a wire saw (South Bay Technology) and a polishing machine (Buehler Ecomet 3000). Rectangular shape samples with approximately 3 × 3 mm cross-section were sandwiched vertically by two nickel electrodes (current injection) with two Pt/PtRh thermocouples (for temperature difference and voltage measurements) attached on one side. The sample and measurement probes were covered by a nickel can to maintain a constant temperature during the measurement, and the base temperature was measured by a thermocouple attached to the outside of the can. The sample, electrodes, and nickel can were placed in a vacuum chamber and then evacuated and refilled with He gas (0.1 atm) to provide necessary heat transfer. Properties were measured from room temperature to 670 K under a helium atmosphere. Hall Measurements. Above 300 K, Hall measurements were carried out by an in-house high temperature/high magnetic field Hall apparatus. It consists of a nine Tesla air-bore superconducting magnet with a water-cooled oven inside the bore of the magnet, and a Linear Research AC bridge with 16 Hz excitation. Fourwire AC Hall measurements were performed on parallelepiped samples with the typical size of 1.5 × 3 × 10 mm3 to temperatures of at least 800 K with the protection of Argon gas. Thermal Conductivity. The thermal conductivity was determined as a function of temperature using the flash diffusivity method on a LFA 457/2/G Microflash NETZSCH. The front face of a small disk-shaped sample (diameter ≈ 8 mm; thickness ≈ 2 mm) coated with a thin layer of graphite is irradiated with a short laser burst, and the resulting rear face temperature rise is recorded and analyzed. The experiments were carried out under nitrogen atmosphere. Thermal conductivity values were calculated using the equation κ ) RCpd, where R is the thermal diffusivity, Cp the specific heat, and d the bulk density of the material calculated from the sample’s geometry and mass (accuracy is (10%). A pyroceram reference was used to determine the heat capacity of the sample. The measurements typically were over the temperature range 300-670 K. The electronic component of the thermal conductivity was quantified through the Wiedemann-Franz law according to which κel ) σTLo (Lo being the Lorenz number, Lo ) 2.45 × 10-8 W Ω K-1).32 The lattice contribution was then derived by subtracting the electronic component from the total thermal (32) Kittel, C. Introduction to Solid State Physics; Wiley: New York, 2005.
Figure 2. X-ray powder diffraction patterns of (a) Na0.8Pb13Sn5SbTe20, (b) Na0.8Pb13Sn5Sb0.4Te20. For each composition, powder from both top and bottom of the ingot was analyzed to check homogeneity along the ingot. The small arrows indicate diffraction peaks that do not belong to the NaCl structure-type and indicate the presence of Sb2Te3 as a minor phase.
Figure 3. (a) Typical DTA results of the composition NaPb5Sn13SbTe20, (b) variation of the melting and crystallization points of NaPb18-xSnxSbTe20 as a function of x.
conductivity. We provide a word of caution here. The exact value of the Lorenz number is difficult to get without extensive charge and thermal transport experiments involving the application of magnetic fields. It is known that this number can vary with carrier concentration and with temperature. So the lattice thermal conductivity values reported here have the Lorenz number uncertainty in them but we expect the trends observed between samples in this study to be valid. Transmission Electron Microscopy. The microstructures of several pieces cut from different locations of the ingots were examined by high resolution transmission electron microscopy (HRTEM). Specimens for the investigation were prepared by conventional standard methods which was described elsewhere.19 The HRTEM images were recorded at 200 kV using a JEOL
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Table 1. Summary of the Physical and Electronic Properties of the Materials NaPb18-xSnxSbTe20a σ (S/cm)
a
S (µV/K)
composition
a (Å)
Tm(K)
Eg (eV)
300 K
550 K
300 K
550 K
NaPb18SbTe20 NaPb15Sn3SbTe20 NaPb13Sn5SbTe20 NaPb9Sn9SbTe20 NaPb5Sn13SbTe20 NaPb2Sn16SbTe20 NaSn18SbTe20
6.510 (1) 6.441 (2) 6.422 (1) 6.395 (2) 6.371 (2) 6.378 (2) 6.325 (1)
1188 1150 1161 1137 1111 1093 1074
0.37 0.16 0.16
