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The effect of the precursor nanopowder size on the thermoelectric properties of nanostructured Bi–Sb–Te bulk materials
Physica B 405 (2010) 4931–4936
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Physica B journal homepage: www.elsevier.com/locate/physb
The effect of the precursor nanopowder size on the thermoelectric properties of nanostructured Bi–Sb–Te bulk materials Weili Ren n, Chunxia Cheng, Zhongming Ren, Yunbo Zhong Laboratory of Metallurgy and Materials Processing, College of Materials Science and Engineering, Shanghai University, Shanghai 200072, PR China
a r t i c l e in fo
abstract
Article history: Received 30 August 2010 Accepted 26 September 2010
This paper presents the effect of precursor powder size on the thermoelectric properties of sintered nanostructured bulk materials. The transport properties of the nanostructured bulk show a dramatic size effect. There are a lower thermal and electrical conductivity for the bulk with smaller nanopowders. The dimensionless figure-of merit values (ZT) of almost all the samples are much lower than those of the list reported data in the paper because the decrease in the thermal conductivity is counteracted by the reduction in the electrical conductivity and the Seebeck coefficient. The combination route of hydro/solvothermal synthesis and spark-plasma-sintering method provide a well controlled way to significantly reduce the thermal conductivity. & 2010 Elsevier B.V. All rights reserved.
Keywords: Thermoelectric properties Nanostructure Bi–Sb–Te bulk materials Hydro/solvothermal synthesis Spark plasma sintering
1. Introduction Thermoelectric materials have been widely used in solid-state electronic cooling and power generation with many advantages such as long life, no moving parts, no emissions of toxic gases, low maintenance and high reliability [1]. The thermoelectric efficiency is determined by a dimensionless figure-of-merit ZT¼ TS2s/k, where Z is the figure-of-merit, T is the absolute temperature, S is the Seebeck coefficient, s is the electrical conductivity and k is the total thermal conductivity. The low ZT value ( r1) of commercially available thermoelectric materials limits the large-scale application of thermoelectric devices. In recent years, ZT is significantly improved by constructing nanostructured bulk for the most thermoelectric materials system, such as Bi2Te3 [2–7], PbTe [8], SiGe [9], etc. Bi2Te3-based compounds are known as the best thermoelectric materials currently available for application near room temperature. The construction of the nanostructured bulk for the materials system has aroused great interest [2–7]. Poudel et al. [2] and Ma et al. [3] have achieved a peak ZT of 1.4 and 1.3 in p-type BiSbTe alloy by ball milling the alloyed crystalline ingot and the elemental chunks followed by hot pressing, respectively. Cao et al. [4,5] used the hot-pressing method to construct the nanostructured Bi–Sb–Te bulk with the different ratio of Bi2Te3/Sb2Te3 powders by the hydrothermal route. A maximal ZT value of about 1.47 is obtained at about 180 1C in the bulk constituted by 1:1 Bi2Te3/Sb2Te3 powder. They [4] also fabricated the bulk by a combined process of melting/ milling/hot-pressing according to different ratios (1:1 and 1:3) of
n
Corresponding author. Tel.: + 86 21 56335470; fax: + 86 21 56331102. E-mail address: [email protected] (W. Ren).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.09.034
powder, while the ZT is about 0.2 in the temperature range from 300 to 500 K. Tang et al. [6,7] employed melting–spinning/hand-milling/ spark-plasma-sintering to synthesize the Bi0.52Sb1.48Te3 bulk alloy. A maximum value of 1.5 at room temperature is obtained. The reported papers [1,9] show that the nanostructure presence within the bulk matrix leads to a significant enhancement in the ZT value, primarily due to a significant reduction in the lattice thermal conductivity. The nanostructure in bulk materials is optimized by tuning the composition [4] and preparation route [3,4,6]. To our knowledge, however, there is no report about the effect of precursor powder size on the thermoelectric properties of nanostructured bulk materials. In this study, the nanostructured bulk of the ternary Bi0.5Sb1.5Te3 compound was obtained by the spark-plasma-sintering (SPS) from the precursor nanopowders with different sizes. The nanopowders of ternary Bi0.5Sb1.5Te3 compounds were directly synthesized with the hydro/solvo thermal route assisted by surfactant addition. This paper presents the effect of precursor powder size on the thermoelectric properties of nanostructured bulk materials. The transport properties of bulk sample show the dramatic nanopowder size effect. Although the ZT value of the bulk is not as high as expected, this study provides a meaningful revelation for the future research to improve the thermoelectric properties of the nanostructured bulk.
2. Experiment The powders of different size were synthesized by the hydro/ solvothermal route. All the regents were of analytical grade and used without further purification. In the typical synthesis processes, 0.05 g surfactant hexadecyltrimethylammonium
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bromide (CTAB) and trioctylphosphine oxide (TOPO) was dissolved into a 45 ml water and ethylene glycol (EG), respectively, followed by the addition of BiCl3 (0.5 mol/l), SbCl3 (1.5 mol/l), Te (3 mol/l), NaOH (0.27 g) and NaBH4 (8 mol/l). The resulting precursor suspension was stirred vigorously for 0.5 h and then sealed into a Teflon-lined autoclave of 50 ml capacity. The autoclave was heated and maintained at 210 1C for 8 h. The products were filtered off, washed several times with distilled water and absolute ethanol, and then finally dried in a vacuum at 60 1C for 4 h. The nanopowder samples by hydrothermal and solvothermal routes without and with surfactant are named as NPH1, NPH2, NPS1 and NPS2, respectively. Each nanopowder synthesis experiment was repeated ten times in order to meet the quantities for preparing the bulk nanocomposites. The powders were sintered by spark plasma sintering at 623 K for 8 min using a pressure of 60 MPa in vacuum. The size of sintered disks is |(10–15) mm (2–3) mm. Disks (|10 2 mm) and bars (4 2 13 mm3) for k and s, S measurement were cut from the axial and disk plane directions, respectively. Then they were also polished before measurement. The bulk samples from the powders NPH1, NPH2, NPS1 and NPS2 are named as NBH1, NBH2, NBS1 and NBS2, respectively. The phase structures were investigated by X-ray diffraction (XRD) with a Rigaku D/MAX-2550P diffractometer using CuKa radiation (l ¼0.154056 nm). The nanopowder morphologies and bulk microstructure were observed by scanning electron microscopy (SEM) and transmission electron microscope (TEM). The composition analysis was performed on SEM with an attached energy dispersed X-ray spectrometer (EDS). The Seebeck coefficient S and electrical conductivity s were measured on a computer-assisted device. A temperature difference of about 5 1C across both ends of the sample was used for Seebeck coefficient measurements. The convectional four-probe method was adopted for the electrical conductivity measurement. The thermal diffusivity k was calculated by using k¼ rca, where r is the sample density estimated by an ordinary dimension and weight measurement. The thermal diffusivity a and specific heat c were measured by a laser flash method on a Netzsch LFA427 NanoflashTM. The dimensionless figures of merit were calculated by ZT ¼TS2s/k.
3. Results and discussion The nanopowders of the ternary Bi0.5Sb1.5Te3 compound were synthesized by the hydro/solvo thermal route assisted by the addition of the surfactant. The XRD patterns of the nanopowder are shown in Fig. 1 together with the corresponding standard JCPDS 49-1713. All peaks in this product pattern can be indexed according to the standard card. No remarkable diffractions of other phases such as tellurium, bismuth or their other compounds can be found in the pattern, indicating that a pure Bi0.5Sb1.5Te3 phase has been obtained under all the synthetic conditions. The product morphology was observed by SEM and TEM. Figs. 2 and 3 show the SEM and TEM images of the hydro/solvothermally synthesized Bi0.5Sb1.5Te3 nanopowders, respectively. It can be observed that the surfactant addition dramatically reduces the nanopowder size by the magnitude of 1–2 orders. It can be determined from the TEM images in Fig. 3 that the size of the products assisted with the surfactant addition is about 10–20 nm and that of the ones without the surfactant is in the range 100–300 nm. The element proportions of Bi, Sb and Te for samples NPH2 and NPS2 determined by EDS is 11.6:28.25:60.15 and 10.06:28.37:61.57 (at%), respectively. The values are in agreement with the nominal composition. Owing to the steric effect arising
Fig. 1. XRD patterns of the hydro/solvo-thermally synthesized Bi0.5Sb1.5Te3 nanopowders compared with the standard data of JCPDS 49-1713 (NPH1, NPH2, NPS1 and NPS2 denote the nanopowder sample by the hydrothermal and solvothermal routes without surfactant and with the surfactant, respectively).
from the long alkyl chain of the surfactant, the rate of chemical reaction can be increased without surfactant addition. Therefore, it can be considered that the element composition of the sample NPH1 and NPS1 is also in agreement with the nominal constitution. The bulk is fabricated by the spark plasma sintering in order to investigate the thermoelectric properties. The XRD patterns of sintered bulk Bi0.5Sb1.5Te3 samples are shown in Fig. 4. All peaks can be indexed according to JCPDS 49-1713 for Bi0.5Sb1.5Te3. The sharp (0 0 9) peak shows the grain preferred orientation during the sintering process. Fig. 5 shows the SEM images of the microstructure for sintered bulk Bi0.5Sb1.5Te3 samples. No obvious cracks and defects appear in the microstructure, which indicates a good sintering quality for the bulks. The transport properties of the bulk sample were measured and are plotted in Fig. 6 together with the reported data for comparison. The dependence of electrical conductivity of sintered bulk Bi0.5Sb1.5Te3 sample on temperature is shown in Fig. 6(a). The electrical conductivity of sample NBS2 is lower by about 50% than that of NBS1, and the one of sample NBH2 is lower by about 60% than that of NBH1. This is attributed to a dramatic decrease in precursor nanopowder size from the surfactant use during the hydro/solvo-thermal synthesis. The decrease in grain size introduces the high density of grain boundary interfaces and an increase in interfacial scattering. The electrical conductivity of sample NBH1 is almost same as that of the sample in Ref. [4], in which the composition of the sample is BiSbTe3 and is also synthesized by the hydrothermal route. However, the value is the same as that of the sample by a combined melting/milling/hot pressing process in Ref. [4]. The electrical conductivity of all the samples, except for sample NBH1, is much lower than that of the sample in Ref. [2]. This may be because of the different compositions and preparation processes. The dependence of the Seebeck coefficient of the sintered bulk Bi0.5Sb1.5Te3 sample on the temperature is shown in Fig. 6(b). The Seebeck coefficients of all the samples does not show a dramatic size effect. They are the same as or lower than that of the sample in Ref. [4] in the temperature range 290–373 and 373–473 K,
W. Ren et al. / Physica B 405 (2010) 4931–4936
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Fig. 2. SEM images of the hydro/solvo-thermally synthesized Bi0.5Sb1.5Te3 nanopowders sample NPH1 (b) sample NPH2, (c) sample NPS1 and (d) sample NPS2 (NPH1, NPH2, NPS1 and NPS2 denote the nanopowder sample by the hydrothermal and solvothermal routes without and with the surfactant, respectively).
Fig. 3. TEM images of the hydro/solvo-thermally synthesized Bi0.5Sb1.5Te3 nanopowder (a) sample NPH1 (b) sample NPH2 (c) sample NPS1 (d) sample NPS2 (NPH1, NPH2, NPS1 and NPS2 denote the nanopowder sample by the hydrothermal and solvothermal routes without and with the surfactant, respectively).
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respectively. And it is lower than that of the sample in Refs. [2,7], which may be attributed to the much smaller size of the precursor nanopowders, the difference compositions and preparation processes. The decrease in grain size introduces the high density of grain boundary interfaces. This results in an increase interfacial scattering to the carrier mobility and a decrease in electrical potential between the unit temperature. The dependence of thermal conductivity of the sintered bulk Bi0.5Sb1.5Te3 sample on the temperature is shown in Fig. 6(c). The thermal conductivity of all the samples shows an obvious size
0018
0015
1010
015
Intensity /a.u.
009
Sample NBS2 Sample NBS1 Sample NBH2 Sample NBH1
10
20
30 2
40 / degree
50
60
70
Fig. 4. XRD patterns of the sintered bulk Bi0.5Sb1.5Te3 samples (NBH1, NBH2, NBS1 and NBS2 denote the bulk sample from the nanopowders by the hydrothermal and solvothermal routes without and with the surfactant, respectively).
effect. The smaller original nanopowder size, the lower the thermal conductivity. The thermal conductivity of sample NBH2 is lower by about 30% than that of NBH1. The thermal conductivity of sample NBS2 with the smallest powder size is the lowest, which is almost close to that of the nanocomposites with the amorphous structure in Ref. [6]. A dramatic decrease in the thermal conductivity is attributed to the increase in phonon scattering from the high density interface and imperfections in the nanostructured composites. The dependence of dimensionless figure of merit (ZT) of the sintered bulk Bi0.5Sb1.5Te3 sample on temperature is shown in Fig. 6(d). The ZT of all the samples is much lower than that of the list reported data [2–7], except for that of sample NBH1 at the temperature of 323 K. The reason is the decrease in thermal conductivity is counteracted by the electrical conductivity and the Seebeck coefficient from the synthesis route. The electrical and thermal conductivity of all the samples shows a dramatic size effect of nanopowder. But the electrical conductivity in this study is much lower than that of reported data [2–7], except for that of the NBH1 sample. Besides the much smaller nanostructure, the different compositions and preparation processes, it is perhaps due to the introduction of large quantities of impurity in nanopowder synthesis. The organic solvent and surfactant are used during hydro/solvo-thermal synthesis. The impurity content of the product is analyzed in Table 1. It can be observed that the impurity content of the samples is more than that of the commercial ingot by two orders of magnitude. ZT should be only improved on the condition that the thermal conductivity is significantly reduced while the electrical conductivity and the Seebeck coefficient are preserved or changed slightly. The synthesis route in this study provides a well controlled way to significantly reduce the thermal
Fig. 5. The SEM images of the microstructure for sintered bulk Bi0.5Sb1.5Te3 samples (a) sample NBH1 (b) sample NBH2 (c) sample NBS1 (d) sample NBS2 (NBH1, NBH2, NBS1 and NBS2 denote the bulk sample from the nanopowders by the hydrothermal and solvothermal routes without and with the surfactant, respectively).
W. Ren et al. / Physica B 405 (2010) 4931–4936
Sample NBH1 Sample NBS1 Ref. [2] Ren Ref. [6] Tang
120 100
Sample NBH2 Sample NBS2 Ref.[4] Zhao
80 60 40
270 Seebeck coefficient, µ VK-1
Electric conductivity, 103S.m-1
140
4935
240 210 180 150 120 Sample NBH1 Sample NBS1 Ref.[2] Ren Ref.[4] Zhao Ref.[6] Tang
90 60
20
Sample NBH2 Sample NBS2
30
1.7
390 420 450 Temperature, K
Sample NBH1 Sample NBS1 Ref.[2] Ren Ref.[6] Tang
1.6 Thermal conductivity, W.m-1K-1
360
1.5 1.4
480
Sample NBH2 Sample NBS2 Ref.[4] Zhao
1.2 1.1 1.0 0.9 0.8 0.7 0.6 330
360 390 420 Temperature,K
330
360 390 420 Temperature, K
450
480
1.8
1.3
300
300
270
510
Dimensionless figure of merit, ZT
330
300
450
480
1.6 1.4 1.2
Sample NBH1 Sample NBH2 Sample NBS1 Sample NBS2 Ref.[2] Ren Ref.[4] Zhao Ref.[6] Tang
1.0 0.8 0.6 0.4 0.2 0.0
300
350
400 450 Temperature, K
500
550
Fig. 6. Dependence of electrical conductivity (a), Seebeck coefficient (b), thermal conductivity (c) and dimensionless figure of merit (ZT) (d) of the sintered bulk Bi0.5Sb1.5Te3 samples on temperature (NBH1, NBH2, NBS1 and NBS2 denote the bulk sample from the nanopowders by the hydrothermal and solvothermal routes without and with the surfactant, respectively).
Table 1 The impurity content of sample NBS2 and commercial ingot. Alloy
C (wt%)
N (wt%)
O (wt%)
p-Type Bi0.5Sb1.5Te3 commercial ingot Sample NBS2
0.0031
0.0004
0.0070
1.09
0.0176
1.244
conductivity, which reaches the level of the nanostructured composites with the amorphous crystal. Therefore, much effort will be made to avoid impurity introduction by the hydro/solvothermal route in future work.
4. Conclusions The effect of precursor powder size on the thermoelectric properties of nanostructured bulk materials is investigated in this study. The proper amount of surfactant can dramatically reduce the nanopowder size by the magnitude of two orders using the surfactant-assisted hydro/solvo thermal route. The transport properties of the nanostructured bulk show a dramatic size effect. The smaller the original nanopowders, the lower the conductivity. The ZT of almost all the samples is much lower than that of the list
reported data in the paper because the decrease in thermal conductivity is counteracted by a reduction in the electrical conductivity and the Seebeck coefficient. The combination route of hydro/solvothermal synthesis and SPS provides a well controlled way to significantly reduce the thermal conductivity, which reaches the level of the nanostructured composites with the amorphous crystal. Much effort will be made to avoid impurity introduction by the hydro/solvo-thermal route in future work.
Acknowledgement This work was supported by Program for Changjiang Scholars and Innovative Research Team in University (no. IRT0739). The authors would like to thank Prof. Junyou Yang, Huazhong University of Science and Technology, for the electrical conductivity and the Seebeck coefficient measurement. References [1] Y.C. Lan, A.J. Minnich, G. Chen, Z.F. Ren, Adv. Funct. Mater. 20 (2010) 357. [2] B. Poudel, Q. Yao, Y. Ma, Y.C. Lan, A. Minnich, B. Yu, X. Yan, D.Z. Wang, A. Muto, D. Vashaee, X.Y. Chen, J.M. Liu, M.S. Dresselhaus, G. Chen, Z.F. Ren, Science 320 (2008) 634. [3] Y. Ma, Q. Hao, B. Poudel, Y.C. Lan, B. Yu, D.Z. Wang, G. Chen, Z.F. Ren, Nano Lett. 8 (2008) 2580.
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[4] Y.Q. Cao, X.B. Zhao, T.J. Zhu, X.B. Zhang, J.P. Tu, Appl. Phys. Lett. 92 (2008) 143106. [5] Y.Q. Cao, T.J. Zhu, X.B. Zhao, X.B. Zhang, J.P. Tu, Appl. Phys. A 92 (2008) 321. [6] W.J. Xie, X.F. Tang, Y.G. Yan, Q.J. Zhang, T.M. Tritt, Appl. Phys. Lett. 94 (2009) 102111. [7] W.J. Xie, X.F. Tang, Y.G. Yan, Q.J. Zhang, T.M. Tritt, J. Appl. Phys. 105 (2009) 113713.
[8] X.W. Wang, H. Lee, Y.C. Lan, G.H. Zhu, G. Joshi, D.Z. Wang, J. Yang, A.J. Muto, M.Y. Tang, J. Klatsky, S. Song, M.S. Dresselhaus, G. Chen, Z.F. Ren, Appl. Phys. Lett. 93 (2008) 193121. [9] K.F. Hsu, S. Loo, F. Guo, W. Chen, J.S. Dyck, C. Uher, T. Hogan, E.K. Polychroniadis, M.G. Kanatzidis, Science 303 (2004) 818.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
The effect of the precursor nanopowder size on the thermoelectric properties of nanostructured Bi–Sb–Te bulk materials Weili Ren n, Chunxia Cheng, Zhongming Ren, Yunbo Zhong Laboratory of Metallurgy and Materials Processing, College of Materials Science and Engineering, Shanghai University, Shanghai 200072, PR China
a r t i c l e in fo
abstract
Article history: Received 30 August 2010 Accepted 26 September 2010
This paper presents the effect of precursor powder size on the thermoelectric properties of sintered nanostructured bulk materials. The transport properties of the nanostructured bulk show a dramatic size effect. There are a lower thermal and electrical conductivity for the bulk with smaller nanopowders. The dimensionless figure-of merit values (ZT) of almost all the samples are much lower than those of the list reported data in the paper because the decrease in the thermal conductivity is counteracted by the reduction in the electrical conductivity and the Seebeck coefficient. The combination route of hydro/solvothermal synthesis and spark-plasma-sintering method provide a well controlled way to significantly reduce the thermal conductivity. & 2010 Elsevier B.V. All rights reserved.
Keywords: Thermoelectric properties Nanostructure Bi–Sb–Te bulk materials Hydro/solvothermal synthesis Spark plasma sintering
1. Introduction Thermoelectric materials have been widely used in solid-state electronic cooling and power generation with many advantages such as long life, no moving parts, no emissions of toxic gases, low maintenance and high reliability [1]. The thermoelectric efficiency is determined by a dimensionless figure-of-merit ZT¼ TS2s/k, where Z is the figure-of-merit, T is the absolute temperature, S is the Seebeck coefficient, s is the electrical conductivity and k is the total thermal conductivity. The low ZT value ( r1) of commercially available thermoelectric materials limits the large-scale application of thermoelectric devices. In recent years, ZT is significantly improved by constructing nanostructured bulk for the most thermoelectric materials system, such as Bi2Te3 [2–7], PbTe [8], SiGe [9], etc. Bi2Te3-based compounds are known as the best thermoelectric materials currently available for application near room temperature. The construction of the nanostructured bulk for the materials system has aroused great interest [2–7]. Poudel et al. [2] and Ma et al. [3] have achieved a peak ZT of 1.4 and 1.3 in p-type BiSbTe alloy by ball milling the alloyed crystalline ingot and the elemental chunks followed by hot pressing, respectively. Cao et al. [4,5] used the hot-pressing method to construct the nanostructured Bi–Sb–Te bulk with the different ratio of Bi2Te3/Sb2Te3 powders by the hydrothermal route. A maximal ZT value of about 1.47 is obtained at about 180 1C in the bulk constituted by 1:1 Bi2Te3/Sb2Te3 powder. They [4] also fabricated the bulk by a combined process of melting/ milling/hot-pressing according to different ratios (1:1 and 1:3) of
n
Corresponding author. Tel.: + 86 21 56335470; fax: + 86 21 56331102. E-mail address: [email protected] (W. Ren).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.09.034
powder, while the ZT is about 0.2 in the temperature range from 300 to 500 K. Tang et al. [6,7] employed melting–spinning/hand-milling/ spark-plasma-sintering to synthesize the Bi0.52Sb1.48Te3 bulk alloy. A maximum value of 1.5 at room temperature is obtained. The reported papers [1,9] show that the nanostructure presence within the bulk matrix leads to a significant enhancement in the ZT value, primarily due to a significant reduction in the lattice thermal conductivity. The nanostructure in bulk materials is optimized by tuning the composition [4] and preparation route [3,4,6]. To our knowledge, however, there is no report about the effect of precursor powder size on the thermoelectric properties of nanostructured bulk materials. In this study, the nanostructured bulk of the ternary Bi0.5Sb1.5Te3 compound was obtained by the spark-plasma-sintering (SPS) from the precursor nanopowders with different sizes. The nanopowders of ternary Bi0.5Sb1.5Te3 compounds were directly synthesized with the hydro/solvo thermal route assisted by surfactant addition. This paper presents the effect of precursor powder size on the thermoelectric properties of nanostructured bulk materials. The transport properties of bulk sample show the dramatic nanopowder size effect. Although the ZT value of the bulk is not as high as expected, this study provides a meaningful revelation for the future research to improve the thermoelectric properties of the nanostructured bulk.
2. Experiment The powders of different size were synthesized by the hydro/ solvothermal route. All the regents were of analytical grade and used without further purification. In the typical synthesis processes, 0.05 g surfactant hexadecyltrimethylammonium
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W. Ren et al. / Physica B 405 (2010) 4931–4936
bromide (CTAB) and trioctylphosphine oxide (TOPO) was dissolved into a 45 ml water and ethylene glycol (EG), respectively, followed by the addition of BiCl3 (0.5 mol/l), SbCl3 (1.5 mol/l), Te (3 mol/l), NaOH (0.27 g) and NaBH4 (8 mol/l). The resulting precursor suspension was stirred vigorously for 0.5 h and then sealed into a Teflon-lined autoclave of 50 ml capacity. The autoclave was heated and maintained at 210 1C for 8 h. The products were filtered off, washed several times with distilled water and absolute ethanol, and then finally dried in a vacuum at 60 1C for 4 h. The nanopowder samples by hydrothermal and solvothermal routes without and with surfactant are named as NPH1, NPH2, NPS1 and NPS2, respectively. Each nanopowder synthesis experiment was repeated ten times in order to meet the quantities for preparing the bulk nanocomposites. The powders were sintered by spark plasma sintering at 623 K for 8 min using a pressure of 60 MPa in vacuum. The size of sintered disks is |(10–15) mm (2–3) mm. Disks (|10 2 mm) and bars (4 2 13 mm3) for k and s, S measurement were cut from the axial and disk plane directions, respectively. Then they were also polished before measurement. The bulk samples from the powders NPH1, NPH2, NPS1 and NPS2 are named as NBH1, NBH2, NBS1 and NBS2, respectively. The phase structures were investigated by X-ray diffraction (XRD) with a Rigaku D/MAX-2550P diffractometer using CuKa radiation (l ¼0.154056 nm). The nanopowder morphologies and bulk microstructure were observed by scanning electron microscopy (SEM) and transmission electron microscope (TEM). The composition analysis was performed on SEM with an attached energy dispersed X-ray spectrometer (EDS). The Seebeck coefficient S and electrical conductivity s were measured on a computer-assisted device. A temperature difference of about 5 1C across both ends of the sample was used for Seebeck coefficient measurements. The convectional four-probe method was adopted for the electrical conductivity measurement. The thermal diffusivity k was calculated by using k¼ rca, where r is the sample density estimated by an ordinary dimension and weight measurement. The thermal diffusivity a and specific heat c were measured by a laser flash method on a Netzsch LFA427 NanoflashTM. The dimensionless figures of merit were calculated by ZT ¼TS2s/k.
3. Results and discussion The nanopowders of the ternary Bi0.5Sb1.5Te3 compound were synthesized by the hydro/solvo thermal route assisted by the addition of the surfactant. The XRD patterns of the nanopowder are shown in Fig. 1 together with the corresponding standard JCPDS 49-1713. All peaks in this product pattern can be indexed according to the standard card. No remarkable diffractions of other phases such as tellurium, bismuth or their other compounds can be found in the pattern, indicating that a pure Bi0.5Sb1.5Te3 phase has been obtained under all the synthetic conditions. The product morphology was observed by SEM and TEM. Figs. 2 and 3 show the SEM and TEM images of the hydro/solvothermally synthesized Bi0.5Sb1.5Te3 nanopowders, respectively. It can be observed that the surfactant addition dramatically reduces the nanopowder size by the magnitude of 1–2 orders. It can be determined from the TEM images in Fig. 3 that the size of the products assisted with the surfactant addition is about 10–20 nm and that of the ones without the surfactant is in the range 100–300 nm. The element proportions of Bi, Sb and Te for samples NPH2 and NPS2 determined by EDS is 11.6:28.25:60.15 and 10.06:28.37:61.57 (at%), respectively. The values are in agreement with the nominal composition. Owing to the steric effect arising
Fig. 1. XRD patterns of the hydro/solvo-thermally synthesized Bi0.5Sb1.5Te3 nanopowders compared with the standard data of JCPDS 49-1713 (NPH1, NPH2, NPS1 and NPS2 denote the nanopowder sample by the hydrothermal and solvothermal routes without surfactant and with the surfactant, respectively).
from the long alkyl chain of the surfactant, the rate of chemical reaction can be increased without surfactant addition. Therefore, it can be considered that the element composition of the sample NPH1 and NPS1 is also in agreement with the nominal constitution. The bulk is fabricated by the spark plasma sintering in order to investigate the thermoelectric properties. The XRD patterns of sintered bulk Bi0.5Sb1.5Te3 samples are shown in Fig. 4. All peaks can be indexed according to JCPDS 49-1713 for Bi0.5Sb1.5Te3. The sharp (0 0 9) peak shows the grain preferred orientation during the sintering process. Fig. 5 shows the SEM images of the microstructure for sintered bulk Bi0.5Sb1.5Te3 samples. No obvious cracks and defects appear in the microstructure, which indicates a good sintering quality for the bulks. The transport properties of the bulk sample were measured and are plotted in Fig. 6 together with the reported data for comparison. The dependence of electrical conductivity of sintered bulk Bi0.5Sb1.5Te3 sample on temperature is shown in Fig. 6(a). The electrical conductivity of sample NBS2 is lower by about 50% than that of NBS1, and the one of sample NBH2 is lower by about 60% than that of NBH1. This is attributed to a dramatic decrease in precursor nanopowder size from the surfactant use during the hydro/solvo-thermal synthesis. The decrease in grain size introduces the high density of grain boundary interfaces and an increase in interfacial scattering. The electrical conductivity of sample NBH1 is almost same as that of the sample in Ref. [4], in which the composition of the sample is BiSbTe3 and is also synthesized by the hydrothermal route. However, the value is the same as that of the sample by a combined melting/milling/hot pressing process in Ref. [4]. The electrical conductivity of all the samples, except for sample NBH1, is much lower than that of the sample in Ref. [2]. This may be because of the different compositions and preparation processes. The dependence of the Seebeck coefficient of the sintered bulk Bi0.5Sb1.5Te3 sample on the temperature is shown in Fig. 6(b). The Seebeck coefficients of all the samples does not show a dramatic size effect. They are the same as or lower than that of the sample in Ref. [4] in the temperature range 290–373 and 373–473 K,
W. Ren et al. / Physica B 405 (2010) 4931–4936
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Fig. 2. SEM images of the hydro/solvo-thermally synthesized Bi0.5Sb1.5Te3 nanopowders sample NPH1 (b) sample NPH2, (c) sample NPS1 and (d) sample NPS2 (NPH1, NPH2, NPS1 and NPS2 denote the nanopowder sample by the hydrothermal and solvothermal routes without and with the surfactant, respectively).
Fig. 3. TEM images of the hydro/solvo-thermally synthesized Bi0.5Sb1.5Te3 nanopowder (a) sample NPH1 (b) sample NPH2 (c) sample NPS1 (d) sample NPS2 (NPH1, NPH2, NPS1 and NPS2 denote the nanopowder sample by the hydrothermal and solvothermal routes without and with the surfactant, respectively).
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respectively. And it is lower than that of the sample in Refs. [2,7], which may be attributed to the much smaller size of the precursor nanopowders, the difference compositions and preparation processes. The decrease in grain size introduces the high density of grain boundary interfaces. This results in an increase interfacial scattering to the carrier mobility and a decrease in electrical potential between the unit temperature. The dependence of thermal conductivity of the sintered bulk Bi0.5Sb1.5Te3 sample on the temperature is shown in Fig. 6(c). The thermal conductivity of all the samples shows an obvious size
0018
0015
1010
015
Intensity /a.u.
009
Sample NBS2 Sample NBS1 Sample NBH2 Sample NBH1
10
20
30 2
40 / degree
50
60
70
Fig. 4. XRD patterns of the sintered bulk Bi0.5Sb1.5Te3 samples (NBH1, NBH2, NBS1 and NBS2 denote the bulk sample from the nanopowders by the hydrothermal and solvothermal routes without and with the surfactant, respectively).
effect. The smaller original nanopowder size, the lower the thermal conductivity. The thermal conductivity of sample NBH2 is lower by about 30% than that of NBH1. The thermal conductivity of sample NBS2 with the smallest powder size is the lowest, which is almost close to that of the nanocomposites with the amorphous structure in Ref. [6]. A dramatic decrease in the thermal conductivity is attributed to the increase in phonon scattering from the high density interface and imperfections in the nanostructured composites. The dependence of dimensionless figure of merit (ZT) of the sintered bulk Bi0.5Sb1.5Te3 sample on temperature is shown in Fig. 6(d). The ZT of all the samples is much lower than that of the list reported data [2–7], except for that of sample NBH1 at the temperature of 323 K. The reason is the decrease in thermal conductivity is counteracted by the electrical conductivity and the Seebeck coefficient from the synthesis route. The electrical and thermal conductivity of all the samples shows a dramatic size effect of nanopowder. But the electrical conductivity in this study is much lower than that of reported data [2–7], except for that of the NBH1 sample. Besides the much smaller nanostructure, the different compositions and preparation processes, it is perhaps due to the introduction of large quantities of impurity in nanopowder synthesis. The organic solvent and surfactant are used during hydro/solvo-thermal synthesis. The impurity content of the product is analyzed in Table 1. It can be observed that the impurity content of the samples is more than that of the commercial ingot by two orders of magnitude. ZT should be only improved on the condition that the thermal conductivity is significantly reduced while the electrical conductivity and the Seebeck coefficient are preserved or changed slightly. The synthesis route in this study provides a well controlled way to significantly reduce the thermal
Fig. 5. The SEM images of the microstructure for sintered bulk Bi0.5Sb1.5Te3 samples (a) sample NBH1 (b) sample NBH2 (c) sample NBS1 (d) sample NBS2 (NBH1, NBH2, NBS1 and NBS2 denote the bulk sample from the nanopowders by the hydrothermal and solvothermal routes without and with the surfactant, respectively).
W. Ren et al. / Physica B 405 (2010) 4931–4936
Sample NBH1 Sample NBS1 Ref. [2] Ren Ref. [6] Tang
120 100
Sample NBH2 Sample NBS2 Ref.[4] Zhao
80 60 40
270 Seebeck coefficient, µ VK-1
Electric conductivity, 103S.m-1
140
4935
240 210 180 150 120 Sample NBH1 Sample NBS1 Ref.[2] Ren Ref.[4] Zhao Ref.[6] Tang
90 60
20
Sample NBH2 Sample NBS2
30
1.7
390 420 450 Temperature, K
Sample NBH1 Sample NBS1 Ref.[2] Ren Ref.[6] Tang
1.6 Thermal conductivity, W.m-1K-1
360
1.5 1.4
480
Sample NBH2 Sample NBS2 Ref.[4] Zhao
1.2 1.1 1.0 0.9 0.8 0.7 0.6 330
360 390 420 Temperature,K
330
360 390 420 Temperature, K
450
480
1.8
1.3
300
300
270
510
Dimensionless figure of merit, ZT
330
300
450
480
1.6 1.4 1.2
Sample NBH1 Sample NBH2 Sample NBS1 Sample NBS2 Ref.[2] Ren Ref.[4] Zhao Ref.[6] Tang
1.0 0.8 0.6 0.4 0.2 0.0
300
350
400 450 Temperature, K
500
550
Fig. 6. Dependence of electrical conductivity (a), Seebeck coefficient (b), thermal conductivity (c) and dimensionless figure of merit (ZT) (d) of the sintered bulk Bi0.5Sb1.5Te3 samples on temperature (NBH1, NBH2, NBS1 and NBS2 denote the bulk sample from the nanopowders by the hydrothermal and solvothermal routes without and with the surfactant, respectively).
Table 1 The impurity content of sample NBS2 and commercial ingot. Alloy
C (wt%)
N (wt%)
O (wt%)
p-Type Bi0.5Sb1.5Te3 commercial ingot Sample NBS2
0.0031
0.0004
0.0070
1.09
0.0176
1.244
conductivity, which reaches the level of the nanostructured composites with the amorphous crystal. Therefore, much effort will be made to avoid impurity introduction by the hydro/solvothermal route in future work.
4. Conclusions The effect of precursor powder size on the thermoelectric properties of nanostructured bulk materials is investigated in this study. The proper amount of surfactant can dramatically reduce the nanopowder size by the magnitude of two orders using the surfactant-assisted hydro/solvo thermal route. The transport properties of the nanostructured bulk show a dramatic size effect. The smaller the original nanopowders, the lower the conductivity. The ZT of almost all the samples is much lower than that of the list
reported data in the paper because the decrease in thermal conductivity is counteracted by a reduction in the electrical conductivity and the Seebeck coefficient. The combination route of hydro/solvothermal synthesis and SPS provides a well controlled way to significantly reduce the thermal conductivity, which reaches the level of the nanostructured composites with the amorphous crystal. Much effort will be made to avoid impurity introduction by the hydro/solvo-thermal route in future work.
Acknowledgement This work was supported by Program for Changjiang Scholars and Innovative Research Team in University (no. IRT0739). The authors would like to thank Prof. Junyou Yang, Huazhong University of Science and Technology, for the electrical conductivity and the Seebeck coefficient measurement. References [1] Y.C. Lan, A.J. Minnich, G. Chen, Z.F. Ren, Adv. Funct. Mater. 20 (2010) 357. [2] B. Poudel, Q. Yao, Y. Ma, Y.C. Lan, A. Minnich, B. Yu, X. Yan, D.Z. Wang, A. Muto, D. Vashaee, X.Y. Chen, J.M. Liu, M.S. Dresselhaus, G. Chen, Z.F. Ren, Science 320 (2008) 634. [3] Y. Ma, Q. Hao, B. Poudel, Y.C. Lan, B. Yu, D.Z. Wang, G. Chen, Z.F. Ren, Nano Lett. 8 (2008) 2580.
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[8] X.W. Wang, H. Lee, Y.C. Lan, G.H. Zhu, G. Joshi, D.Z. Wang, J. Yang, A.J. Muto, M.Y. Tang, J. Klatsky, S. Song, M.S. Dresselhaus, G. Chen, Z.F. Ren, Appl. Phys. Lett. 93 (2008) 193121. [9] K.F. Hsu, S. Loo, F. Guo, W. Chen, J.S. Dyck, C. Uher, T. Hogan, E.K. Polychroniadis, M.G. Kanatzidis, Science 303 (2004) 818.