1054 Chem. Mater. 2010, 22, 1054–1058 DOI:10.1021/cm902009t
High Thermoelectric Figure of Merit and Nanostructuring in Bulk p-type Gex(SnyPb1-y)1-xTe Alloys Following a Spinodal Decomposition Reaction† Yaniv Gelbstein,* Boaz Dado, Ohad Ben-Yehuda, Yatir Sadia, Zinovy Dashevsky, and Moshe P. Dariel Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel Received July 6, 2009. Revised Manuscript Received August 6, 2009
The use of thermoelectricity for direct energy conversion from thermal to electrical energy has a long-standing history. We report that the complex p-type Ge(Sn,Pb,Te)Te alloys upon suitable alloying with SnTe can be designed to obtain very high ZT values of up to 1.2 because of both optimal electronic transport properties and minimal lattice thermal conductivity. Such alloys follow a spinodal decomposition reaction, leading to an ordered periodic nanostructure that is effective in the scattering of phonons and reduction of the lattice thermal conductivity. Introduction Thermoelectricity is concerned with the interaction between thermal and electrical phenomena. The most common applications are concerned with the conversion of thermal energy (or heat) into electrical power and with the use of electrical current for cooling. The dimensionless thermoelectric figure of merit (ZT), expressed in eq 1, is widely used as a measure of the thermoelectric efficiency with respect to the material’s properties. R2 σ T ð1Þ ZT ¼ K where Z is the thermoelectric figure of merit, R the Seebeck coefficient, σ the electrical conductivity, κ the thermal conductivity, and T the absolute temperature. Materials with high ZT values are known as thermoelectric materials. Today’s commercial materials have a ZT = 1, leading to conversion efficiency on the order of several percent (∼6%). Increasing ZT can be achieved by either increasing the numerator of ZT, P = R2σ, known as the power factor, or decreasing the thermal conductivity, κ (the denominator of ZT in eq 1). κ is a sum of two major contributions, the lattice thermal conductivity κL (due to phonons flow) and the electronic thermal conductivity κe (due to electronic flow), as described in eq 2. ð2Þ K ¼ KL þKe Currently, PbTe-based thermocouples, with an n-type leg (doped with PbI2 for achieving maximal ZT values of about † Accepted as part of the 2010 “Materials Chemistry of Energy Conversion Special Issue”. *Corresponding author. E-mail:
[email protected].
pubs.acs.org/cm
1.1 in the temperature range of 400-600 °C for carrier concentrations of 2-6 1019 cm-3, respectively) and a p-type leg (alloyed with SnTe) have been employed in real applications.1,2 The latter, in the form of Pb1-xSnxTe, unfortunately display much lower ZT values (e0.5) than those for the n-type PbTe component. Higher ZT values (∼0.8 at 450 °C) for p-type legs were found in Na-doped PbTe;3 however, because of the brittle nature of this material and the correspondent poor mechanical properties, its usage in practical applications was eliminated.3,4 Therefore, further improvement of the ZT values of the p-type legs is a significant challenge that carries substantial benefit. Recently, optimally doped pseudobinary Pb1-xGexTe compounds were proposed as potential candidates to improve the thermoelectric performance of p-type legs.5,6 However, the ZT values of these pseudobinary alloys can be further increased by decreasing the lattice thermal conductivity due to the presence of additional structural defects. Recently, very low thermal conductivity values were observed in several nanostructured PbTe-based materials.7-12 One mechanism for nanopattern generation is based on the (1) Gelbstein, Y.; Dashevsky, Z.; Dariel, M. P. Physica B 2005, 363, 196–205. (2) Gelbstein, Y.; Dashevsky, Z.; Dariel, M. P. Physica B 2007, 391, 256–265. (3) Gelbstein, Y.; Gotesman, G.; Lishzinker, Y.; Dashevsky, Z.; Dariel, M. P. Scr. Mater. 2008, 58, 251–254. (4) Gelbstein, Y.; Dashevsky, Z.; Dariel, M. P. J. Appl. Phys. 2008, 104, 033702. (5) Gelbstein, Yaniv; Dashevsky, Zinovi; Dariel, Moshe P. Phys. Status Solidi 2007, 1(No. 6), 232–234. (6) Gelbstein, Y.; Ben-Yehuda, O.; Pinhas, E.; Edrei, T.; Sadia, Y.; Dashevsky, Z.; Dariel, M. P. J. Electron. Mater. 2009, 38(7), 1478. (7) Poudeu, P. F. P.; D’Angelo, J.; Downey, A. D.; Short, J. L.; Hogan, T. P.; Kanatzidis, M. G. Angew. Chem., Int. Ed. 2006, 45, 1–5. (8) Sootsman, J. R.; Pcionek, R. J.; Kong, H.; Uher, C.; Kanatzidis, M.G. Chem. Mater. 2006, 18, 4993–4995.
Published on Web 08/21/2009
r 2009 American Chemical Society
Article
Chem. Mater., Vol. 22, No. 3, 2010
spinodal decomposition. Spinodal decomposition is a mechanism for phase separation that leads to a characteristic modulated structure that can be exploited to control the microstructure at the nanometer scale.13 Nanostructures for spinodal decomposed systems were reported for TiO2-SnO2,13 Mn-Cu,14 and Cu-Ni-Sn 15 alloys. Recently, a reduction of the lattice thermal conductivity and a corresponding ZT enhancement in Pb1-xSnxTe-PbS system was attributed to nano structures resulting from the spinodal decomposition.12 Alloying of (GeTe)x(PbTe)1-x compounds with SnTe can result in a spinodal decomposition with changed periodicity that depends on the heat treatment.16 To the best of our knowledge, the nanostructure and the transport properties of these alloys have not yet been investigated. The present communication is concerned with development of highly efficient nanostructured p-type Gex(SnyPb1-y)1-xTe alloys for thermoelectric applications, using spark plasma sintering (SPS). The thermoelectric figure of merit was optimized by means of both alloying and doping methods. Two alloys in this family, Ge0.5Sn0.25Pb0.25Te (undoped, Ag-doped and Bi2Te3doped) and Ge0.6Sn0.1Pb0.3Te, were prepared and investigated. The results were compared to a GeTe reference sample. Experimental Details Synthesis. GeTe, Ge0.5Sn0.25Pb0.25Te (undoped, 3 mol % Agdoped, and 3 mol % Bi2Te3-doped), and Ge0.6Sn0.1Pb0.3Te alloys were prepared according to the following procedure: (a) sealing the source materials (purity of 5N) at appropriate concentrations in a quartz ampule under a vacuum of 1 10-5 Torr, (b) melting the alloys in a rocking furnace at 800 °C for 1 h followed by water quenching, (c) milling the compound to a maximal particle size of 60 mesh powder, and (d) Spark Plasma Sintering (SPS) (type HP D 5/1 FCT Systeme GmbH) at 450 °C for 30 min under a mechanical pressure of 32 MPa for GeTe and at 550 °C for 60 min under a mechanical pressure of 25 MPa for the Gex(SnyPb1-y)1-xTe alloys. The different SPS parameters were optimized in order to obtain dense samples (>96%) without any lateral cracks for the various investigated alloys. Electrical Properties. Seebeck coefficient and the electrical conductivity measurements were performed in a self-constructed apparatus under an argon atmosphere up to ∼450 °C at a heating rate of 3 °C/min. For Seebeck coefficient measurements, an auxiliary heater was used to maintain a temperature (9) Poudeu, P. F. P.; D’Angelo, J.; Kong, H.; Downey, A. D.; Short, J. L.; Pcionek, R.; Hogan, T. P.; Uher, C.; Kanatzidis, M. G. J. Am. Chem. Soc. 2006, 126, 14347–14355. (10) Androulakis, J.; Hsu, K. F.; Pcionek, R. J.; Kong, H.; Uher, C.; D’Angelo, J.; Downey, A. D.; Hogan, T. P.; Kanatzidis, M. G. Adv. Mater. 2006, 18, 1170–1173. (11) Hsu, K. F.; Loo, S.; Guo, F.; Chen, W.; Dyck, J. S.; Uher, C.; Hogan, Tim; Polychroniadis, E.K.; Kanatzidis, M. G. Science 2004, 303, 818–820. (12) Androulakis, J.; Lin, C.-H.; Kong, H.; Uher, C.; Wu, C.-I.; Hogan, T.; Cook, B. A.; Caillat, T.; Paraskevopoulos, K. M.; Kanatzidis, M. G. J. Am. Chem. Soc. 2007, 129, 9780–9788. (13) Chaisan, W.; Yimnirun, R.; Ananta, S.; Cann, D. P. J. Solid State Chem. 2005, 178, 613–620. (14) Yin, F.; Ohsawa, Y.; Sato, A.; Kawahara, K. Acta Mater. 2000, 48, 1273–1282. (15) Zhao, J.-C.; Notis, M. R. Acta Mater. 1998, 46, 4203–4218. (16) Yashina, L. V.; Leute, V.; Shtanov, V. I.; Schmidtke, H. M.; Neudachina, V. S. J. Alloys Compd. 2006, 413, 133–143.
1055
difference of 10 °C between the extremities of the samples. Electrical conductivity was measured by the “four-probe” method using an alternating power source of 1 V/50 Hz. Thermal Conductivity. The thermal conductivity (κ) was determined as a function of temperature from room temperature to 500 °C using the flash diffusivity method (LFA 457, Netzsch). The front face of a disk-shaped sample (L = 12 mm, thickness ≈ 1-2 mm) is irradiated by a short laser burst and the resulting rear face temperature is recorded and analyzed. Thermal conductivity values were calculated using the equation κ=R 3 CP 3 F, where R is the thermal diffusivity, CP is the specific heat (measured using differential scanning calorimetry, STA 449, Netzsch), and F is the bulk density of the sample (calculated from the sample’s geometry and mass). Microscopy. The microstructure of Ge0.5Sn0.25Pb0.25Te (undoped and 3 mol % Bi2Te3-doped) was investigated by optical microscopy (Zeiss Axiovert 25. Etching prior to optical microscopy by immersion for few seconds in a 60 mL solution composed of 10 mL of alcohol, 10 g of potassium hydroxide, 17.5 mL of glycerol, and 22.5 mL of distilled water), highresolution scanning electron microscopy (Jeol-7400F HRSEM), transmission electron microscopy (TEM, FEI TECNAI, G2), and high-resolution transmission electron microscopy (Jeol2010 HRTEM). Specimens used for the TEM and HRTEM were prepared as follows. Small square shape pieces with approximate sizes of 5 mm 5 mm 2 mm were first cut from the spark plasma sintered disk using a low speed diamond wheel saw. The samples were then mounted inside a copper tube (L = 3 mm) and handpolished with subsequently increasing grit (1000-1500) sand paper to about 200-300 μm in thickness. Samples were then thinned using a Gatan precision dimple grinder and ion milled to electron transparency using a Gatan precision ion polishing system (PIPS).
Results and Discussion All the investigated Gex(SnyPb1-y)1-xTe alloys displayed a microscale quasi-ordered periodic fishbone structure, the orientation of which depended on the original high temperature cubic phase grain in which the demixing process took place. A typical structure of these alloys as observed in Ge0.5Sn0.25Pb0.25Te is shown in Figure 1. From the optical microscope (a) and HRSEM (b) micrographs of this figure, the wavelength of the dissociation as determined from the side branches of the fishbone was about 10 μm. High-magnification TEM micrograph (c) revealed a finer periodic nanostructure with a wavelength of about 100 nm. In addition, electrondispersive spectroscopy (EDS) analysis showed that the fishbone structure is Pb-rich, whereas the matrix is Gerich. The bright aspect of the Pb-rich regions (Pb is heavier than Ge) is obtained in the backscattered electron (BSE) examination mode of the HRSEM (b). Line scan EDS analysis crossing the fishbone patterns and the surrounding matrix revealed continuous and complementary compositional modulations of Pb and Ge when scanning across the side-branches of the fishbone pattern (Figure 1). It is noteworthy that the Sn composition remained constant while crossing the Pb-rich fishbone lamellas through the surrounding Ge-rich matrix. Higher magnification of HRTEM revealed that the Pb-rich
1056
Chem. Mater., Vol. 22, No. 3, 2010
Gelbstein et al.
Figure 3. HRSEM of scattered Bi-rich nanoprecipitates in 3 mol % Bi2Te3-doped Ge0.5Sn0.25Pb0.25Te.
Figure 1. (a) Optical microscopy, (b) backscattered electrons HRSEM, and (c) TEM micrographs of Ge0.5Sn0.25Pb0.25Te.
Figure 4. Temperature dependence of the Seebeck coefficient for (1) GeTe, (2) Ge0.5Sn0.25Pb0.25Te þ 3 mol % Ag, (3) Ge0.5Sn0.25Pb0.25Te, (4) Ge0.6Sn0.1Pb0.3Te, and (5) Ge0.5Sn0.25Pb0.25Te þ 3 mol % Bi2Te3.
Figure 2. HRTEM micrograph and typical electron diffraction patterns of Ge0.5Sn0.25Pb0.25Te.
fishbone structure by itself composed from smaller scale modulations in the 10 nm range (Figure 2). These continuous modulations without any sharp borders, as in precipitates, are possibly consistent with the continuous nature of the spinodal decomposition generated structure. The electrons diffraction patterns in this figure show a doublet pattern (two nonconcentric periodic reflections), indicating the presence of two similar structures with closely related lattice parameters providing additional evidence to the decomposition of Gex(SnyPb1-y)1-xTe into Ge and Pb rich phases. In addition to the micro and nanoperiodic Pb-rich modulations, the 3 mol % Bi2Te3 doped Ge0.5Sn0.25Pb0.25Te sample showed Birich nano precipitates with a typical size of 50 nm (Figure 3), which were not observed at any of the other investigated compositions. The effect of these precipitates will be discussed later. The temperature dependence of the Seebeck coefficient, electrical conductivity and thermal conductivity of Ge0.5Sn0.25Pb0.25Te (undoped, 3 mol % Ag-doped, and 3 mol % Bi2Te3-doped) and Ge0.6Sn0.1Pb0.3Te alloys compared to GeTe are presented in Figures 4-6.
Figure 5. Temperature dependence of the electrical conductivity for the various prepared alloys. Notations are according to Figure 4.
Figure 6. Temperature dependence of the thermal conductivity for the various prepared alloys. Notations are according to Figure 4.
We discuss first the doping influence of Ge0.5Sn0.25Pb0.25Te by Ag and Bi2Te3 on the thermoelectric properties. By comparing curves 2, 3, and 5 for Ag doping, undoped, and Bi2Te3 doping, respectively, in Figures 4-6, a consistent trend of decreasing Seebeck coefficient and increasing electrical and thermal conductivity values for
Article
Chem. Mater., Vol. 22, No. 3, 2010
Figure 7. Temperature dependence of the lattice thermal conductivity for GeTe, Ge0.5Sn0.25Pb0.25Te (undoped and 3% Bi2Te3-doped), and Ge0.6Sn0.1Pb0.3Te.
Ag doping, as compared to the undoped condition, is apparent. This evidence indicates an Ag-acceptor behavior in the investigated alloys, leading to an increased carrier concentration. For Bi2Te3 doping (curve 5 in Figures 4-6), the opposite trend is observed, showing a donor behavior in the investigated alloys and leading to a decreased carrier concentration as compared to the undoped sample (curve 3). These trends show the potential of Bi and Ag to serve as active impurities (donors and acceptors, respectively) in Ge0.5Sn0.25Pb0.25Te for tuning the optimal carrier concentration for thermoelectric applications. By comparing of these curves to the GeTe curve (curve 1 in Figures 4-6), we observe that all the Ge0.5Sn0.25Pb0.25Te curves (2, 3, and 5) yield higher R and lower σ and κ values, as compared to the GeTe (curve 1), indicating a reduced carrier concentration at all the doping levels in Ge0.5Sn0.25Pb0.25Te, as compared to pure GeTe. Pure GeTe shows a very high holes concentration of p ≈ 6 1026 m-3. It is well-known that for thermoelectric applications, much lower carrier concentrations are needed for optimal performance. This stands behind Bi doping of GeTe (p ≈ 2 1026 m-3), for increasing the thermoelectric performance.6 Therefore, the reduced carrier concentrations obtained by using Ge0.5Sn0.25Pb0.25Te (in all of the investigated doping levels, curves 2, 3, and 5) as compared to GeTe (curve 1), is desirable. Variation of the alloying of Gex(SnyPb1-y)1-xTe from undoped Ge0.5Sn0.25Pb0.25Te (curve 3) to undoped Ge0.6Sn0.1Pb0.3Te (curve 4) does not result in any dramatic changes of the investigated transport properties. This behavior can be attributed to the carrier compensation obtained by increasing the GeTe amount (from 50 to 60 at %) and decreasing the SnTe amount (from 25 to 10 at %), both p-type compounds with very high hole concentrations. The temperature dependence of the lattice thermal conductivity, κL, for GeTe, Ge0.5Sn0.25Pb0.25Te (undoped and 3% Bi2Te3 doped), and Ge0.6Sn0.1Pb0.3Te is presented in Figure 7. κL values were calculated as followed: 1. Fermi Energy Calculation. Fermi energy (η) was calculated at low temperatures (
