Journal of ELECTRONIC MATERIALS, Vol. 41, No. 6, 2012
DOI: 10.1007/s11664-012-1998-5 2012 TMS
Effect of Sintering on the Thermoelectric Transport Properties of Bulk Nanostructured Bi0.5Sb1.5Te3 Pellets Prepared by Chemical Synthesis JEFFREY S. DYCK,1,3 BAODONG MAO,2 JUNWEI WANG,2 STEVEN DORROH,1 and CLEMENS BURDA2 1.—Department of Physics, John Carroll University, University Heights, OH 44118, USA. 2.—Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106, USA. 3.—e-mail:
[email protected]
Considerable research effort has gone into improving the performance of traditional thermoelectric materials such as Bi2 xSbxTe3 through a variety of nanostructuring approaches. Bottom-up, chemical approaches have the potential to produce very small nanoparticles (>100 nm) with narrow size distribution and controlled shape. For this study, nanocrystalline powder of Bi0.5Sb1.5Te3 was synthesized using a ligand-assisted chemical method, and consolidated into pellets with cold pressing followed by sintering in Ar atmosphere. The thermoelectric transport properties were measured from 7 K to 300 K as a function of sintering temperature. Sintering is found to increase ZT and to move the maximum in ZT to lower temperatures due to a reduction in the free charge concentration. Hall mobility studies indicate that sintering increases the electron mean free path more than it increases the phonon mean free path up to sintering temperature of 598 K. A maximum ZT of 0.42 was measured at temperature of 275 K. Key words: Chemical synthesis, thermoelectric properties, Bi0.5Sb1.5Te3, nanocrystalline
INTRODUCTION Bismuth telluride-based materials are known to have the best thermoelectric (TE) efficiency near room temperature.1 Recently, improvements in the dimensionless figure of merit (ZT) of Bi2Te3-Sb2Te3 alloys and other compounds have been achieved through nanostructuring;2,3 for example, roomtemperature ZT values for Bi2 xSbxTe3 bulk nanocrystalline (NC) alloys reaching 1.2 were reported for samples prepared by ball milling followed by hot pressing,4 and 1.56 for material prepared by melt spinning followed by spark plasma sintering.5 Another route for preparation of nanostructured TE material is through bottom-up methods whereby nanoparticles are synthesized by wet chemical approaches and then consolidated into (Received July 17, 2011; accepted February 9, 2012; published online March 13, 2012)
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nanostructured bulk material.2,6,7 Ligand-assisted chemical methods have shown the ability to tailor the particle size, shape, and crystallinity in NC thermoelectric materials. Full thermoelectric transport characterizations have recently been conducted on Bi2 xSbxTe3 bulk structures composed entirely of nanocrystals prepared by ligand-assisted methods,8–15 to begin to investigate the effect of this control of NC properties on the actual thermoelectric performance. One of the challenges for chemical synthesis is the removal of the insulating organic capping ligands from the NCs before consolidation into bulk pellets. Annealing of the NC powder to remove the ligands before consolidation has been shown to improve the TE properties by orders of magnitude without increasing the NC size too much.7,10 Another challenge is to obtain sufficiently high pellet density and intergranular connectivity during compaction without further increase of the grain size. In this report, we explore the effect of
Effect of Sintering on the Thermoelectric Transport Properties of Bulk Nanostructured Bi0.5Sb1.5Te3 Pellets Prepared by Chemical Synthesis
RESULTS AND DISCUSSION
(2 1 10)
(0 1 23)
(1 1 21)
(1 2 5)
(0 2 16)
(1 0 19) (0 0 21) (0 1 20)
(0 2 10)
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TS = 623 K (0 0 18)
(1 0 10) (0 1 11)
(0 1 8)
(1 1 0) (0 0 15) (1 1 6) (0 1 14)
(0 1 5)
Figure 1 shows the XRD data of the Bi0.5Sb1.5Te sample pellets for different sintering temperatures. The peaks of all the samples index well to the Bi0.5Sb1.5Te3 structure, and no significant secondary phase or oxide peaks were detected. Before sintering, the average grain size as estimated by the width of the two strongest peaks [(0 1 5) and (1 0 10)
(0 0 9)
Synthesis of Bi0.5Sb1.5Te3 NC powder was carried out via a ligand-assisted chemical method as described elsewhere with minor revision.10 Briefly, 0.5 mmol bismuth acetate (>99%), 1.5 mmol antimony chloride (>99%), and 10 mL dodecanethiol were dissolved in 60 mL benzyl ether and heated up to 333 K for 20 min under Ar flow. After that, the solution was quickly heated up to 413 K, and 3 mL freshly made 1 M trioctylphosphine Te (Top-Te) solution was injected. The temperature was further raised to 423 K and maintained for 30 min. The resulting black NC precipitate was cooled down, centrifuged, and washed with toluene, chloroform, and ethanol, respectively. The vacuum-dried NC powder was annealed at 653 K in flowing Ar gas to remove the capping ligands. The powder was then consolidated by room-temperature pressing at 0.9 GPa, achieving a 7-mm-diameter, 1.5-mm-thick pellet. Consolidated pellets were then sintered in flowing Ar gas at temperatures of TS = 573 K, 598 K, and 623 K for 5 h. The structure of the NC powder and the pressed pellets was examined with powder x-ray diffraction (XRD) with a Philips X’Pert powder diffractometer using Cu Ka radiation. The morphology was investigated with a Hitachi S4500 field-emission scanning electron microscope (SEM), and microstructure was examined with a Philips Tecnai TF30 transmission electron microscope (TEM). Before thermoelectric transport measurements were performed, Hall effect and electrical resistivity (q) measurements were performed in the van der Pauw configuration on the pellet disks in magnetic fields up to 1.5 T. Samples for thermoelectric transport measurements were cut from the center of the disks to typical dimensions of 1.5 mm 9 2.5 mm 9 5.5 mm using a diamond wheel saw. Densities of the transport specimens, estimated by ordinary
(0 0 6)
EXPERIMENTAL PROCEDURES
dimension and weight measurement, are provided in Table I. Temperature dependence of the transport measurements was determined from 7 K to 300 K in a cryostat equipped with a radiation shield. A longitudinal steady-state technique was used for Seebeck (S) and thermal conductivity (j) measurements, and the standard four-probe method for resistivity measurements. All transport measurements were performed on the same sample and in the same direction (perpendicular to the pressing direction). Estimated errors in these measurements are 2% for S and carrier concentration p, and 5% for q and j, due primarily to sample geometry uncertainty. Losses due to lead wires and radiation were measured experimentally and subtracted from the thermal conductivity data.16 This correction is roughly 20% at room temperature and negligible below 150 K.
Intensity (a.u.)
sintering of cold-pressed pellets on the thermoelectric transport properties for Bi0.5Sb1.5Te3 nanocrystals grown by chemical synthesis. Systematic investigation of the grain growth, carrier concentration and mobility, and lattice thermal conductivity is performed in an effort to understand the sintering process and the role that nanostructuring plays in the TE efficiency.
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TS = 598 K
TS = 573 K
unsintered 20
30
40
50
60
70
80
2θ (deg.) Fig. 1. Powder XRD patterns of Bi0.5Sb1.5Te3 pellets sintered at different temperatures TS.
Table I. Room-temperature values of the density d (as a percentage of theoretical density), carrier concentration p, Hall mobility lH, lattice thermal conductivity jL, ratio of Hall mobility to lattice thermal conductivity lH/jL, and dimensionless figure of merit FS for NC Bi0.5Sb1.5Te3 sintered at different sintering temperatures TS TS (K) Unsintered 573 598 623
d (%) 92 97 97 95
p (cm23) 1.4 5.3 5.7 4.7
9 9 9 9
1019 1018 1018 1018
lH (cm2 V21 s21)
jL (W m21 K21)
lH/jL (100 cm3 K V22 C)
FS
60 160 190 200
0.50 0.70 0.79 1.37
120 230 240 150
0.33 0.38 0.41 0.20
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Fig. 2. SEM images of: (a) as-grown Bi0.5Sb1.5Te3 NC powder (inset shows TEM image of large platelets of NC powder; scale bar = 200 nm); freshly fractured cross-section of Bi0.5Sb1.5Te3 pellet (b) before sintering and after sintering at (c) 573 K, (d) 598 K, and (e) 623 K; and (f) TEM image of the pellet sintered at 598 K.
reflections] using the Scherrer equation is 30 nm. The XRD patterns of the sintered samples show systematically intensified and narrowed peaks, which is consistent with increasing grain size. The as-grown NC powder before annealing, shown in Fig. 2a, is made up primarily of thin platelets with largest crystal dimension in the range of 50 nm to 500 nm, although some crystals are larger. The inset to Fig. 2a displays a TEM image of several large platelets on top of one another, suggesting that thicknesses are >50 nm. Before pressing, the residual capping ligands were removed by annealing the powder, which causes agglomerated NCs to ripen to some degree. An SEM image of the cross-section of the fractured surface of the cold-pressed, unsintered pellet is shown in Fig. 2b. While grain boundaries are not particularly clear, the morphology suggests a range of crystallite sizes from sub 50 nm to greater than 500 nm. Figure 2c–e shows SEM images of the fractured surface from each sintered pellet. Up to sintering temperature of 598 K, the morphology is not significantly changed aside from a coarsening of grains overall and fewer of the smallest grains, consistent with the XRD data. For TS = 623 K, much larger grains begin to form. The TEM image of the sample
sintered at 598 K shown in Fig. 2f reveals a range of crystallite sizes with very few in the sub-50-nm range, mostly being 200 nm. Figure 3a shows a plot of the electrical resistivity, q, versus temperature. The unsintered sample shows very little temperature dependence, with a modest increase in resistance upon cooling indicating thermally activated transport. Hall data at room temperature reveal hole concentration of 1.4 9 1019 cm 3 and mobility of 60 cm2 V 1 s 1 for this sample. We note that optimized bulk p-type Bi0.5Sb1.5Te3 material normally has a somewhat higher carrier concentration and higher mobility, and correspondingly lower q.1 We find that chemically synthesized p-type Bi2 xSbxTe3 generally has lower hole concentration than the bulk grown counterpart with the same value of x,9,10 and several researchers have found that pure Bi2Te3 prepared by similar solution chemical methods is in fact n-type.7,8 This has been attributed to sulfur doping from the thiol ligands. The Seebeck coefficient, S, of the unsintered sample, as shown in Fig. 3b has a room-temperature value of +218 lV K 1, very close to that of optimized bulk material. The effect of sintering on q versus T is to change the temperature coefficient from negative to positive.
Effect of Sintering on the Thermoelectric Transport Properties of Bulk Nanostructured Bi0.5Sb1.5Te3 Pellets Prepared by Chemical Synthesis 10
300
(a)
unsintered
200
S (μV/K)
ρ (10-5 Ωm)
(b)
250
8
6
TS = 573 K 4
2
150 100
TS = 598 K TS = 623 K
50
0
0
0
50
100
150
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50
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T (K)
150
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T (K) 0.5
2.0
(d)
(c) 0.4
1.5
unsintered TS = 573 K
0.3
ZT
κ (Wm-1K-1)
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TS = 598 K TS = 623 K
0.2 0.5
0.1
0.0
0.0 0
50
100
150
200
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T (K)
0
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T (K)
Fig. 3. Thermoelectric transport properties versus temperature: (a) electrical resistivity, (b) Seebeck coefficient, (c) thermal conductivity, and (d) dimensionless figure of merit. Symbol definitions are all the same and are given in Fig. 3d.
The value of q decreases significantly at low temperature, but remains nearly the same at room temperature. This indicates a clear change in the primary mechanism of charge carrier scattering. Hall data at room temperature on the three sintered samples yielded similar values of hole concentration, p, of around 4 9 1018 cm 3 to 6 9 1018 cm 3, roughly three times lower than before sintering (Table I). We do not fully understand this decrease in carrier concentration with sintering, but postulate that it is connected to an increase in Te vacancies in the structure, or a possible incorporation of additional sulfur from the capping ligands if there was not complete removal during the annealing of the NC powder before pressing. The calculated Hall mobilities, lH, systematically increased with sintering temperature up to a value of 200 cm2 V 1 s 1 for TS = 623 K. The combination of increasing lH and suppressed p with sintering can explain the nonmonotonic behavior of resistivity at room temperature. The value of S for sintered samples is increased relative to the unsintered sample with values near +270 lV K 1 at room temperature, consistent with the lower carrier concentration. Of particular note is that the maximum in S is reached near room temperature, indicating the onset of bipolar conduction at lower temperature than for optimized material due to the low carrier concentration.
The temperature dependence of the thermal conductivity, j, is plotted in Fig. 3c. The unsintered sample has a very low value of j at all temperatures, and does not display the typical crystalline peak at low T. This temperature dependence is similar to that seen in other chemically synthesized Bi2Te3-based material with grain size much less than 100 nm.4,5 As sintering temperature is increased, a low-temperature crystalline peak develops, and j is enhanced at all temperatures. At TS = 623 K, j begins to increase with increasing temperature for T > 150 K, unlike the other samples. We estimate the lattice portion of the thermal conductivity, jL, by subtracting the electronic contribution, ke, from the total using the expression je = LT/q. Here we have used the value L = 2.0 9 10 8 V2 K 2 for the Lorenz number, appropriate for a heavily doped semiconductor system.5 This analysis shows that je is £0.1 W m 1 K 1. The calculated values of jL at room temperature are provided in Table I. We find that jL is suppressed relative to typical bulk values for the unsintered sample, but already approaching bulk values for TS = 598 K. Due to the unusual upturn in j for the sample sintered at 623 K, the estimated value of jL is higher than optimized bulk Bi0.5Sb1.5Te3 without any nanostructuring.1 While the origin of this upturn is so far unknown and is clearly not related to
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electronic thermal conductivity, bipolar thermal diffusion could give rise to this behavior.17,18 The onset of this effect normally occurs near the temperature of the peak in S; however, it has been observed to become apparent at temperatures lower than the peak in S in both single crystals17 and NC samples of Bi2 x SbxTe3.19,20 More detailed analysis of both charge carrier and phonon transport and scattering mechanisms in these samples is currently underway. The dimensionless figure of merit, ZT = S2/(qj)T, was computed and is plotted in Fig. 3d. An improvement in ZT is found up to sintering temperature of 598 K, followed by a decrease for TS = 623 K due primarily to the significantly larger thermal conductivity. The highest value of ZT is 0.42 for TS = 598 K and occurs at T = 275 K. Although the unsintered sample has a somewhat lower ZT at room temperature, it will reach a maximum above room temperature based on the temperature dependence of the transport coefficients. Given the fact that these Bi2 xSbxTe3 samples are not fully dense (Table I), it is worth discussing the effect that the porosity may have on the thermoelectric transport properties. In particular, it is important to distinguish grain size and porosity effects. It is understood that an increase in porosity decreases the electrical and thermal conductivity, while the overall effect on the Seebeck coefficient, carrier concentration, and ZT is very minimal.19,21–23 Following the approach taken by Scheele et al.19 employing a Maxwell–Eucken expression,21,22 we estimate that the pore-corrected (fully dense) values of j, 1/q, and lH would be roughly 25% larger for the porosity of 8% measured for the unsintered sample. Meanwhile, the same parameters for the sintered samples would be larger by roughly 8% for TS = 573 K and 598 K and roughly 17% for TS = 623 K. With these corrections, the overall trends in the transport parameters with sintering are still preserved and our conclusions remain the same. Of critical interest is to assess the role that nanostructuring has on this system. We examine the attempt to improve the material with sintering by monitoring whether carrier mobility is increased faster than the lattice thermal conductivity at room temperature. Table I shows that the ratio lH/jL increases with sintering to TS = 598 K, which indicates that the electron mean free path is increasing faster than the phonon mean free path (note that this ratio is unaffected by porosity under our assumptions described above). On the other hand, the grain size is clearly increased as evidenced by XRD, SEM, and lattice thermal conductivity to the point that the beneficial effect of nanostructuring is minimized under these sintering conditions. CONCLUSIONS In this work, the effect of sintering on cold-pressed Bi0.5Sb1.5Te3 bulk, NC thermoelectric materials
prepared by ligand-assisted chemical synthesis is investigated. Sintering under these conditions increases the sample density and electrical conductivity and decreases the carrier concentration, resulting in an enhanced Seebeck coefficient that peaks near room temperature. This study shows that higher sintering temperature increases the lattice thermal conductivity from 0.5 W m 1 K 1 for unsintered material to values close to what is expected for optimized bulk microcrystalline Bi0.5Sb1.5Te3. At the same time, sintering increases the charge carrier mobility more than it increases the lattice thermal conductivity, resulting in an overall increase in thermoelectric performance. A maximum ZT of 0.42 at temperature of 275 K was found for pressed pellets sintered at 598 K. Investigation of ways to enable an order-of-magnitude larger carrier concentration after sintering is warranted, as this would likely lead to higher ZT with a maximum above room temperature. ACKNOWLEDGEMENTS C.B. is grateful for the support from NSF Career (CHE-0239688) and NIRT (CTS-0608896). REFERENCES 1. G.S. Nolas, J. Sharp, and H.J. Goldsmid, Thermoelectrics: Basic Principles and New Materials Developments (Berlin: Springer, 2001). 2. S.K. Bux, J.-P. Fleurial, and R.B. Kaner, Chem. Commun. 46, 8311 (2010). 3. M.G. Kanatzidis, Chem. Mater. 22, 648 (2010). 4. B. Poudel, Q. Hao, Y. Ma, Y. Lan, A. Minnich, B. Yu, X.A. Yan, D.Z. Wang, A. Muto, D. Vashaee, X.Y. Chen, J.M. Liu, M.S. Dresselhaus, G. Chen, and Z.F. Ren, Science 320, 634 (2008). 5. W. Xie, X. Tang, Y. Yan, Q. Zhang, and T.M. Tritt, J. Appl. Phys. 105, 113713 (2009). 6. D.V. Talapin, J.S. Lee, M.V. Kovalenko, and E.V. Shevchenko, Chem. Rev. 110, 389 (2010). 7. Y.X. Zhao, J.S. Dyck, and C. Burda, J. Mater. Chem. 21, 17049 (2011). 8. M.R. Dirmyer, J. Martin, G.S. Nolas, A. Sen, and J.V. Badding, Small 5, 933 (2009). 9. M. Scheele, N. Oeschler, K. Meier, A. Kornowski, C. Klinke, and H. Weller, Adv. Funct. Mater. 19, 3476 (2009). 10. Y.X. Zhao, J.S. Dyck, B.M. Hernandez, and C. Burda, J. Am. Chem. Soc. 132, 4982 (2010). 11. Y.X. Zhao, J.S. Dyck, B.M. Hernandez, and C. Burda, J. Phys. Chem. C 114, 11607 (2010). 12. M.V. Kovalenko, B. Spokoyny, J.-S. Lee, M. Scheele, A. Weber, S. Perera, D. Landry, and D.V. Talapin, J. Am. Chem. Soc. 132, 6686 (2010). 13. A. Purkayastha, A. Jain, C. Hapenciuc, R. Buckley, B. Singh, C. Karthik, R.J. Mehta, T. Borca-Tasciuc, and G. Ramanath, Chem. Mater. 23, 3029 (2011). 14. X.B. Zhao, X.H. Ji, Y.H. Zhang, T.J. Zhu, J.P. Tu, and X.B. Zhang, Appl. Phys. Lett. 86, 062111 (2005). 15. C.Q. Cao, X.B. Zhao, T.J. Zhu, X.B. Zhang, and J.P. Tu, Appl. Phys. Lett. 92, 143106 (2008). 16. J.S. Dyck, W. Chen, C. Uher, L.D. Chen, X.F. Tang, and T. Hirai, J. Appl. Phys. 91, 3698 (2002). 17. L.D. Ivanova and Y.V. Granatkina, Inorg. Mater. 36, 672 (2000). 18. F.R. Yu, J.J. Zhang, D.L. Yu, J.L. He, Z.Y. Liu, B. Xu, and Y.J. Tian, J. Appl. Phys. 105, 094303 (2009).
Effect of Sintering on the Thermoelectric Transport Properties of Bulk Nanostructured Bi0.5Sb1.5Te3 Pellets Prepared by Chemical Synthesis 19. M. Scheele, N. Oeschler, I. Veremchuk, K.-G. Reinsberg, A.-M. Kreuziger, A. Kornowski, J. Broekaert, C. Klinke, and H. Weller, ACS Nano 4, 4283 (2010). 20. W. Xie, J. He, H.J. Kang, X. Tang, S. Zhu, M. Laver, S. Wang, J.R.D. Copley, C.M. Brown, Q. Zhang, and T.M. Tritt, Nano Lett. 10, 3283 (2010).
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21. Y.Q. Cao, T.J. Zhu, and X.B. Zhao, J. Phys. D 42, 015406 (2007). 22. J. Adachi, K. Kurosaki, M. Uno, and S. Yamanaka, J. Alloys Compd. 432, 7 (2007). 23. I. Sumirat, Y. Ando, and S. Shimamura, J. Porous. Mater. 13, 439 (2006).
