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Letter
Enhanced thermoelectric properties of Ag-modified Bi Sb Te composites by a facile electroless plating method 0.5
1.5
3
Shen Cao, Zhong-Yue Huang, Fang-Qiu Zu, Ju Xu, Lei Yang, and Zhi-Gang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11989 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017
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Enhanced thermoelectric properties of Agmodified Bi0.5Sb1.5Te3 composites by a facile electroless plating method Shen Cao †, ‡, Zhong-Yue Huang†,*, Fang-Qiu Zu†, Ju Xu‡,*, Lei Yang§, Zhi-Gang Chen#,⊥* †
Liquid/Solid Metal Processing Institute, School of Materials Science & Engineering, Hefei
University of Technology, Hefei 230009, PR China ‡
Micro-nano Fabrication Technology Department, Institute of Electrical Engineering, Chinese
Academy of Sciences, Beijing 100190, PR China §
College of Materials Science and Engineering, Sichuan University, Chengdu, 610064, PR China
#
Centre for Future Materials, University of Southern Queensland, Springfield, QLD 4300,
Australia ⊥
Material Engineering, University of Queensland, QLD 4072, Australia
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ABSTRACT A large-scale and facile electroless plating Ag method has been developed to fabricate highperforming Ag/Bi0.5Sb1.5Te3 (Ag/BST) composites. Ag can be doped into BST and also forms Ag2Te secondary phase in BST, leading to a low lattice thermal conductivity of 0.34 Wm-1K-1. Consequently, a peak zT of 1.07 and average zT of 1.02 are achieved in 0.03wt% Ag/BST. The average zT value is enhanced by 100% in the temperature interval from 300 to 500 K compared with that of Ag-free BST. This work provides a facile and large-scale method to fabricate the high performance Bi2Te3-based alloy for applying in the low-temperature power generation.
KEYWORDS: thermoelectric materials, bismuth telluride, point defect, electroless plating, Ag doping, Ag2Te phase
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Thermoelectric materials have attracted increasing attention in the past decades due to their huge potential in the field of energy harvesting and semiconductor cooling via Seebeck effect and Peltier effect.1-3 The properties of thermoelectric materials are qualified by the dimensionless figure of merit, zT, defined as 𝑧𝑇 = 𝑆 2 𝜎𝑇/𝜅, where S, σ, κ and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively.4 Obviously, a high S2σ and low κ are needed for an ideal thermoelectric material. However, these parameters are correlated and conflicted, which is a key challenge for the enhancement of zT.5 The ternary Bi0.5Sb1.5Te3 (BST) alloy is currently the most commercially available p-type thermoelectric materials for electronic refrigeration.6 Although its zT can reach 1 approximately at room temperature via zone melting,7 zT value is degraded drastically8 due to the enlarged κ and quickly deteriorated S caused by the bipolar conduction from 300 K. Therefore, in order to harvest the energy in temperature range of 300 ~ 500 K, the peak zT of p-type bismuth telluride-based alloys be shifted to higher temperatures. Theoretically, two strategies, including broadening band gap9 and increasing the majority carrier concentration (holes),10 can be applied to suppress the bipolar conduction. Particularly, doping is an effective way to tune the level of carrier concentration. Among few known p-type dopants, Ag is one of the most widely used elements. Lošt’á et al11 reported that Ag doping can increase the hole concentration due to the formation of ′′ ′ negatively charged defects such as 𝐴𝑔𝑆𝑏 or 𝐴𝑔𝑆𝑏 . Recently, various methods, including melting
constituent elements,12 and ball-milling with subsequent thermal decomposition,13 have been used to dope a small amount of Ag into BST. Enhanced thermoelectric properties have been observed in such p-type Ag-doped BST. However, to our knowledge, most of the Ag/BST composites are fabricated by the time-consuming and energy-intensive methods such as ball milling and melting.
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Searching low-energy-intensity and scalable methods will expand the research of Ag/BST composites. In this work, we applied a simple electroless plating Ag method for the preparation of Ag/BST composites. This method introduced here has two advantages compared to other techniques. Firstly, it is easy to control the amount of Ag (generally 0.01 ~ 0.1 wt %) compared with the traditional methods, such as ball-milling. Secondly, more homogeneous Ag coating on BST powders with various shapes such as nanowires,14 nanoplates,15 and nanobelts,16 can be further achieved by facile electroless plating method. The typical synthesis fabrication method can be seen in the supporting information. Figure 1(a) shows the X-ray diffraction (XRD) patterns of the 0.03wt% Ag/BST, 0.05wt%Ag/BST, and commercial BST samples. The diffraction peaks cited from the database of the BST (PDF#491713) are also given in the bottom of Figure 1(a) by the vertical lines. All the peaks of Ag/BST composites are well matched with the standard patterns of BST and no peaks of Ag or Ag-related alloys can be detected because their weight fractions are lower than the detection limit. Nevertheless, the (015) peak (as shown in Figure 1(b)) shifts to a higher angle with increase of Ag contents, indicating that Ag was indeed doped into the BST matrix, which is in good agreement with the EDS results in the inset of the Figure 1(c). It should be noted that the atomic radius of Ag is a smaller than that of Sb or Bi (rAg = 0.144 nm, rSb = 0.159 nm, and rBi = 0.170 nm). Thus, it can be concluded that Ag can diffuse into BST matrix during the SPS process and Ag can substitute for Sb11 or Bi17 considering if the fine Ag particles can be molten (Tm ≈ 623 K18) at 673 K.
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Figure 1. (a) XRD patterns of all Ag/BST bulk samples; (b) Enlarged XRD of (015) peaks; (c) FE-SEM image of 0.05wt% Ag/BST (inset: the EDS spectrums of different spots).
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Table 1 Carrier concentration and mobility of all bulk samples at 300K. Carrier concentration (n, cm−3)
Carrier mobility (μ, cm2Vs-1)
BST
1.38×1019
252.11
0.03wt% Ag
4.29×1019
192.79
0.05wt% Ag
13.4×1019
90.70
Figure 2 shows the temperature dependence of thermoelectric properties of Ag/BST bulks. As shown in Figure 2(a), the σ of all samples decreased with temperature, which indicates semiconductor characteristic. Obviously, introducing a tiny amount of Ag can enhance the σ drastically from 482.85 Scm-1 (BST) to 1179.24 Scm-1 (0.03wt%Ag) and 1754.38 Scm-1 (0.05wt%Ag) at 323 K. The σ boosts linearly with the increase of Ag content. Table 1 shows the carrier concentration (n) and carrier mobility (μ) of all bulk samples at 300 K. n is increased from 1.38×1019 to 4.29×1019 and 13.4×1019 with the increase of Ag content to 0.03 and 0.05 wt %, respectively. The increase of the n can be ascribed to the existence of Ag and the formation of substitutional defects.11, 17 Figure 2(b) shows the temperature dependent S of Ag/BST bulks. The S of commercial BST increases with increasing temperature before 350 K, suggesting that the electrical transports are mainly dominated by holes. After 350 K, The S of commercial BST decreases sharply due to the intrinsic excitation. With the intrinsic conduction region of commercial BST, the S can be expressed as:19 𝑆=
𝑆𝑒 𝜎𝑒 +𝑆ℎ 𝜎ℎ
(1)
𝜎𝑒 +𝜎ℎ
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Where e and h denote electrons and holes, respectively. Especially in Eq. (1), 𝑆𝑒 or 𝑆ℎ is the contribution of electrons or holes and they play the opposite role. Due to intrinsic conduction, more electrons are excited into the conduction band and the increased concentration minority carriers (electrons) decreases the total S. Ag can suppress the intrinsic excitation and shift its occurring temperature (Tmax, corresponding to the Smax) to high temperatures. For example, Tmax for 0.05wt%Ag/BST sample is 500 K, which is about 150 K higher than that for BST sample. All the S2σ values decrease with temperature, as shown in Figure 2(c). The maximal S2σ values of Ag/BST increased about by 50 % than that of commercial BST, from 21.71 μWcm-1K-2 to 31.62 μWcm-1K2
(0.03wt%Ag) and 29.84 μWcm-1K-2 (0.05wt%Ag). Evidently, compared with commercial BST,
σ is largely enhanced and S is improved at the high temperatures. As a result, the enhanced S2σ of Ag/BST should be attributed to the improved σ and S in high temperature range.
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Figure 2 Temperature-dependent (a) σ, (b) S, (c) S2σ (d) κtotal, (e) κtotal – κe, and (f) κb of BST, 0.03wt%Ag/BST, and 0.05wt%Ag/BST, respectively. According to the classical theory of κ, κ can be divided into three parts: κtotal = κe + κl + κb, where carrier (κe) and lattice (κl) thermal conductivity are conducting heat by electrons/holes and phonons, and κb is bipolar component. κe can be estimated by using Weidman-Franz law, κe = LσT.20 The Lorenz number L can be estimated by the following equations:
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𝐿=
(𝑟+ )𝐹 5 (𝜉) 2 𝑟+ 𝑘𝐵 2 2 (𝑒) { 3 (𝑟+ )𝐹 1 (𝜉) 2
5 2
−[
(𝑟+ )𝐹
𝑟+ 2
3 2
3 (𝜉) 𝑟+ 2
(𝑟+ )𝐹
1 (𝜉) 𝑟+ 2
]2
(2)
where 𝑟 is the scattering parameter, and Fm (ξ) is the Fermi integral, given by:21-22 𝜒𝑚
∞
𝐹𝑚 (𝜉) = ∫0
1+𝑒 𝜒−𝜉
𝑑𝜒
(3)
𝐸
𝜉 = 𝑘 𝑓𝑇
(4)
𝐵
where ξ is the reduced Fermi energy and ξ can be calculated from the S as well as r, which is given by:
𝑆=
𝑘𝐵 𝑒
5 2
[
3 (𝜉) 𝑟+ 2
(𝑟+ )𝐹 3 2
1 (𝜉) 𝑟+ 2
(𝑟+ )𝐹
− 𝜉]
(5)
In principle, r = -1/2 is assumed for acoustic phonon scattering parameter.23 As shown in Figure 2(d), for BST, κtotal is linearly increased from 1.15 Wm-1K-1 with increasing temperature, while for 0.03wt%Ag/BST, and 0.05wt%Ag/BST, both of the κtotal increased from ~ 423 K after the initial decrease. The lowest κtotal of 0.03wt%Ag/BST, and 0.05wt%Ag/BST are 0.97 and 1.04 Wm-1K-1, respectively. Figure 2(e) shows the calculated κtotal - κe of BST, 0.03wt%Ag/BST, and 0.05wt%Ag/BST. As can be seen, Ag modification can significantly decrease κtotal - κe. Moreover, a value of 0.345 Wm-1K-1 at 400 K is observed for 0.05wt%Ag/BST, which almost approaches the theoretical limit of the κl for Bi0.5Sb1.5Te3.24 To understand the mechanism of such a low κtotal - κe of Ag/BST, we performed transmission electron microscopy (TEM) investigations of the 0.05wt%Ag/BST and the results are shown in Figure 3. Figure 3(a) is a typical low-magnification TEM image of the 0.05wt%Ag/BST. As can be seen, there are many nanoscale distorted regions,
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implying that Ag doping causes the increasing of stress. The defect is one of the main reason to form stress. As mentioned above, Ag can substitute Bi or Sb to form substitutional defects. Such defects can effectively scatter phonons as scattering cores. The corresponding high-resolution TEM (HRTEM) image is shown in Figure 3(b), indicating a good crystallinity of Ag/BST. Figure 3(c) shows an HRTEM image of a typical nanoparticle. The interplanar spacing of the nanoparticle can be measured as 0.67 nm (yellow circle), which is well matched with the d spacing of the (1̅01) plane of Ag2Te. Therefore, such nanoparticle can be indexed as Ag2Te precipitates. Within the intrinsic conduction region for Bi2Te3-based alloys, there exists an additional component to the κtotal arising from the diffusion of electron-hole pairs, called the bipolar component κb. In order to estimate roughly the scale of κb, we further analyse the κtotal - κe. The relationship between the κl and the reciprocal of temperature (1/T) can be linearly plotted using the ratio of 𝜅𝑙 ∝ 1/T,10, 25-26 as shown in dotted line of Figure 2(e). As can be seen, the observed κl is approximately equal to the κtotal - κe before intrinsic excitation. Compared with commercial BST, the κl of Ag/BST significantly decreases, which can be attributed to the effective Ag doping, fine Ag2Te nanoparticles, stress, and increased boundaries. Figure 2(f) shows the calculated κb by subtracting the calculated κl from the experimental κtotal - κe. As can be seen, the calculated κb at 423 K for commercial BST is 0.225 Wm-1K-1, while the κb of 0.03wt%Ag/BST is only 0.04 Wm-1K-1, contributing to the low κtotal in the high-temperature range. In summary, the κtotal of Ag/BST is reduced due to the slight increase in κe compensated by a significant decrease in κl and κb.
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Figure 3 Low (a) & high (b) magnified TEM images of matrix BST; (c) Secondary phase Ag2Te of 0.05wt%Ag/BST. Figure 4(a) is the calculated zT of the commercial BST, 0.03wt%Ag/BST, and 0.05wt%Ag/BST. The zT values of BST drastically decreased with the increase in temperature while zT values of Ag/BST samples can be maintained at a high value in the whole temperature range. Thus, by virtue of the enhanced S2σ and reduced κtotal, the peak zT of 0.05wt%Ag/BST can reach 1.07 at 373 K, which can be competitive to the reported modified BST, fabricated by traditional casting27 or zone melting.7 Besides, the zT values of Ag/BST are much superior to those from other methods at high temperature. Average zT is more competitive data in practical applications which represents the thermoelectric performance of materials in the whole temperature range. Figure 4(b) shows the peak zT and average zTs from 300 to 500 K. As can be seen, our best Ag/BST samples have the average zT of 1.02 and 0.97, which are better than most of other fabricated BST,28-30 suggesting the electroless plating Ag/BST can be working in a broader temperature range with more practical applications.
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Figure 4 (a) Temperature-dependent zT of BST, 0.03wt%Ag/BST, and 0.05wt%Ag/BST, respectively, and (b) Comparisons of average zTs with the reported values for BixSb2-xTe328-30 from 323 to 473 K.
In summary, the Ag/BST composites have been successfully fabricated by a facile electroless plating Ag method. An enhanced peak zT of 1.07 at 373 K and average ZT of 1.02 at the temperature interval 323~500 K have been observed in the 0.03wt%Ag/BST. Detail structure analysis and performance evaluations illustrate that doped Ag can form Ag2Te nanoprecipitates and point defects to lower kl up to 0.34 Wm-1K-1, and simultaneously increase the n and σ, which not only suppresses the intrinsic conduction but also enhances the power factor. Our simple, fast, and effective method can be widely used to enhance the thermoelectric performance of Bi2Te3based alloy for power generation in the temperature range from 300 to 500 K.
ASSOCIATED CONTENT
*S Supporting Information The Supporting Information is available free of charge on the ACS Publications website
AUTHOR INFORMATION
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Corresponding Author *Email: [email protected]; *Email: [email protected]; *Email: [email protected]
Notes The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
This work is financially supported by the National Natural Science Foundation of China under Grant No. 51371073, Chinese Academy of Sciences “Hundred Talents” project, and the Australian Research Council. ZGC thanks the USQ start-up grant and strategic research funds. The Australian Microscopy & Microanalysis Research Facility is acknowledged for providing characterization facilities.
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(27) Yu, Y.; Zhu, B.; Wu, Z.; Huang, Z. Y.; Wang, X. Y.; Zu, F. Q. Enhancing the Thermoelectric Performance of Free Solidified p-type Bi0.5Sb1.5Te3 Alloy by Manipulating its Parent Liquid State. Intermetallics 2015, 66, 40-47. (28) Hong, M.; Chen, Z. G.; Yang, L.; Zou, J. BixSb2−xTe3 Nanoplates with Enhanced Thermoelectric Performance due to Sufficiently Decoupled Electronic Transport Properties and Strong Wide-frequency Phonon Scatterings. Nano Energy 2016, 20, 144-155. (29) Madavali, B.; Kim, H. S.; Lee, K. H.; Hong, S. J. Enhanced Seebeck Coefficient by Energy Filtering in Bi-Sb-Te Based Composites with Dispersed Y2O3 Nanoparticles. Intermetallics 2017, 82, 68-75. (30) Lee, M. H.; Rhyee, J.-S.; Kim, S.; Choa, Y.-H. Thermoelectric Properties of Bi0.5Sb1.5Te3 /Ag2Te Bulk Composites with Size- and Shape-controlled Ag2Te Nano-particles Dispersion. J. Alloy. Compd. 2016, 657, 639-645.
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Table of Contents
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Letter
Enhanced thermoelectric properties of Ag-modified Bi Sb Te composites by a facile electroless plating method 0.5
1.5
3
Shen Cao, Zhong-Yue Huang, Fang-Qiu Zu, Ju Xu, Lei Yang, and Zhi-Gang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11989 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017
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Enhanced thermoelectric properties of Agmodified Bi0.5Sb1.5Te3 composites by a facile electroless plating method Shen Cao †, ‡, Zhong-Yue Huang†,*, Fang-Qiu Zu†, Ju Xu‡,*, Lei Yang§, Zhi-Gang Chen#,⊥* †
Liquid/Solid Metal Processing Institute, School of Materials Science & Engineering, Hefei
University of Technology, Hefei 230009, PR China ‡
Micro-nano Fabrication Technology Department, Institute of Electrical Engineering, Chinese
Academy of Sciences, Beijing 100190, PR China §
College of Materials Science and Engineering, Sichuan University, Chengdu, 610064, PR China
#
Centre for Future Materials, University of Southern Queensland, Springfield, QLD 4300,
Australia ⊥
Material Engineering, University of Queensland, QLD 4072, Australia
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ABSTRACT A large-scale and facile electroless plating Ag method has been developed to fabricate highperforming Ag/Bi0.5Sb1.5Te3 (Ag/BST) composites. Ag can be doped into BST and also forms Ag2Te secondary phase in BST, leading to a low lattice thermal conductivity of 0.34 Wm-1K-1. Consequently, a peak zT of 1.07 and average zT of 1.02 are achieved in 0.03wt% Ag/BST. The average zT value is enhanced by 100% in the temperature interval from 300 to 500 K compared with that of Ag-free BST. This work provides a facile and large-scale method to fabricate the high performance Bi2Te3-based alloy for applying in the low-temperature power generation.
KEYWORDS: thermoelectric materials, bismuth telluride, point defect, electroless plating, Ag doping, Ag2Te phase
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Thermoelectric materials have attracted increasing attention in the past decades due to their huge potential in the field of energy harvesting and semiconductor cooling via Seebeck effect and Peltier effect.1-3 The properties of thermoelectric materials are qualified by the dimensionless figure of merit, zT, defined as 𝑧𝑇 = 𝑆 2 𝜎𝑇/𝜅, where S, σ, κ and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively.4 Obviously, a high S2σ and low κ are needed for an ideal thermoelectric material. However, these parameters are correlated and conflicted, which is a key challenge for the enhancement of zT.5 The ternary Bi0.5Sb1.5Te3 (BST) alloy is currently the most commercially available p-type thermoelectric materials for electronic refrigeration.6 Although its zT can reach 1 approximately at room temperature via zone melting,7 zT value is degraded drastically8 due to the enlarged κ and quickly deteriorated S caused by the bipolar conduction from 300 K. Therefore, in order to harvest the energy in temperature range of 300 ~ 500 K, the peak zT of p-type bismuth telluride-based alloys be shifted to higher temperatures. Theoretically, two strategies, including broadening band gap9 and increasing the majority carrier concentration (holes),10 can be applied to suppress the bipolar conduction. Particularly, doping is an effective way to tune the level of carrier concentration. Among few known p-type dopants, Ag is one of the most widely used elements. Lošt’á et al11 reported that Ag doping can increase the hole concentration due to the formation of ′′ ′ negatively charged defects such as 𝐴𝑔𝑆𝑏 or 𝐴𝑔𝑆𝑏 . Recently, various methods, including melting
constituent elements,12 and ball-milling with subsequent thermal decomposition,13 have been used to dope a small amount of Ag into BST. Enhanced thermoelectric properties have been observed in such p-type Ag-doped BST. However, to our knowledge, most of the Ag/BST composites are fabricated by the time-consuming and energy-intensive methods such as ball milling and melting.
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Searching low-energy-intensity and scalable methods will expand the research of Ag/BST composites. In this work, we applied a simple electroless plating Ag method for the preparation of Ag/BST composites. This method introduced here has two advantages compared to other techniques. Firstly, it is easy to control the amount of Ag (generally 0.01 ~ 0.1 wt %) compared with the traditional methods, such as ball-milling. Secondly, more homogeneous Ag coating on BST powders with various shapes such as nanowires,14 nanoplates,15 and nanobelts,16 can be further achieved by facile electroless plating method. The typical synthesis fabrication method can be seen in the supporting information. Figure 1(a) shows the X-ray diffraction (XRD) patterns of the 0.03wt% Ag/BST, 0.05wt%Ag/BST, and commercial BST samples. The diffraction peaks cited from the database of the BST (PDF#491713) are also given in the bottom of Figure 1(a) by the vertical lines. All the peaks of Ag/BST composites are well matched with the standard patterns of BST and no peaks of Ag or Ag-related alloys can be detected because their weight fractions are lower than the detection limit. Nevertheless, the (015) peak (as shown in Figure 1(b)) shifts to a higher angle with increase of Ag contents, indicating that Ag was indeed doped into the BST matrix, which is in good agreement with the EDS results in the inset of the Figure 1(c). It should be noted that the atomic radius of Ag is a smaller than that of Sb or Bi (rAg = 0.144 nm, rSb = 0.159 nm, and rBi = 0.170 nm). Thus, it can be concluded that Ag can diffuse into BST matrix during the SPS process and Ag can substitute for Sb11 or Bi17 considering if the fine Ag particles can be molten (Tm ≈ 623 K18) at 673 K.
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Figure 1. (a) XRD patterns of all Ag/BST bulk samples; (b) Enlarged XRD of (015) peaks; (c) FE-SEM image of 0.05wt% Ag/BST (inset: the EDS spectrums of different spots).
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Table 1 Carrier concentration and mobility of all bulk samples at 300K. Carrier concentration (n, cm−3)
Carrier mobility (μ, cm2Vs-1)
BST
1.38×1019
252.11
0.03wt% Ag
4.29×1019
192.79
0.05wt% Ag
13.4×1019
90.70
Figure 2 shows the temperature dependence of thermoelectric properties of Ag/BST bulks. As shown in Figure 2(a), the σ of all samples decreased with temperature, which indicates semiconductor characteristic. Obviously, introducing a tiny amount of Ag can enhance the σ drastically from 482.85 Scm-1 (BST) to 1179.24 Scm-1 (0.03wt%Ag) and 1754.38 Scm-1 (0.05wt%Ag) at 323 K. The σ boosts linearly with the increase of Ag content. Table 1 shows the carrier concentration (n) and carrier mobility (μ) of all bulk samples at 300 K. n is increased from 1.38×1019 to 4.29×1019 and 13.4×1019 with the increase of Ag content to 0.03 and 0.05 wt %, respectively. The increase of the n can be ascribed to the existence of Ag and the formation of substitutional defects.11, 17 Figure 2(b) shows the temperature dependent S of Ag/BST bulks. The S of commercial BST increases with increasing temperature before 350 K, suggesting that the electrical transports are mainly dominated by holes. After 350 K, The S of commercial BST decreases sharply due to the intrinsic excitation. With the intrinsic conduction region of commercial BST, the S can be expressed as:19 𝑆=
𝑆𝑒 𝜎𝑒 +𝑆ℎ 𝜎ℎ
(1)
𝜎𝑒 +𝜎ℎ
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Where e and h denote electrons and holes, respectively. Especially in Eq. (1), 𝑆𝑒 or 𝑆ℎ is the contribution of electrons or holes and they play the opposite role. Due to intrinsic conduction, more electrons are excited into the conduction band and the increased concentration minority carriers (electrons) decreases the total S. Ag can suppress the intrinsic excitation and shift its occurring temperature (Tmax, corresponding to the Smax) to high temperatures. For example, Tmax for 0.05wt%Ag/BST sample is 500 K, which is about 150 K higher than that for BST sample. All the S2σ values decrease with temperature, as shown in Figure 2(c). The maximal S2σ values of Ag/BST increased about by 50 % than that of commercial BST, from 21.71 μWcm-1K-2 to 31.62 μWcm-1K2
(0.03wt%Ag) and 29.84 μWcm-1K-2 (0.05wt%Ag). Evidently, compared with commercial BST,
σ is largely enhanced and S is improved at the high temperatures. As a result, the enhanced S2σ of Ag/BST should be attributed to the improved σ and S in high temperature range.
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Figure 2 Temperature-dependent (a) σ, (b) S, (c) S2σ (d) κtotal, (e) κtotal – κe, and (f) κb of BST, 0.03wt%Ag/BST, and 0.05wt%Ag/BST, respectively. According to the classical theory of κ, κ can be divided into three parts: κtotal = κe + κl + κb, where carrier (κe) and lattice (κl) thermal conductivity are conducting heat by electrons/holes and phonons, and κb is bipolar component. κe can be estimated by using Weidman-Franz law, κe = LσT.20 The Lorenz number L can be estimated by the following equations:
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𝐿=
(𝑟+ )𝐹 5 (𝜉) 2 𝑟+ 𝑘𝐵 2 2 (𝑒) { 3 (𝑟+ )𝐹 1 (𝜉) 2
5 2
−[
(𝑟+ )𝐹
𝑟+ 2
3 2
3 (𝜉) 𝑟+ 2
(𝑟+ )𝐹
1 (𝜉) 𝑟+ 2
]2
(2)
where 𝑟 is the scattering parameter, and Fm (ξ) is the Fermi integral, given by:21-22 𝜒𝑚
∞
𝐹𝑚 (𝜉) = ∫0
1+𝑒 𝜒−𝜉
𝑑𝜒
(3)
𝐸
𝜉 = 𝑘 𝑓𝑇
(4)
𝐵
where ξ is the reduced Fermi energy and ξ can be calculated from the S as well as r, which is given by:
𝑆=
𝑘𝐵 𝑒
5 2
[
3 (𝜉) 𝑟+ 2
(𝑟+ )𝐹 3 2
1 (𝜉) 𝑟+ 2
(𝑟+ )𝐹
− 𝜉]
(5)
In principle, r = -1/2 is assumed for acoustic phonon scattering parameter.23 As shown in Figure 2(d), for BST, κtotal is linearly increased from 1.15 Wm-1K-1 with increasing temperature, while for 0.03wt%Ag/BST, and 0.05wt%Ag/BST, both of the κtotal increased from ~ 423 K after the initial decrease. The lowest κtotal of 0.03wt%Ag/BST, and 0.05wt%Ag/BST are 0.97 and 1.04 Wm-1K-1, respectively. Figure 2(e) shows the calculated κtotal - κe of BST, 0.03wt%Ag/BST, and 0.05wt%Ag/BST. As can be seen, Ag modification can significantly decrease κtotal - κe. Moreover, a value of 0.345 Wm-1K-1 at 400 K is observed for 0.05wt%Ag/BST, which almost approaches the theoretical limit of the κl for Bi0.5Sb1.5Te3.24 To understand the mechanism of such a low κtotal - κe of Ag/BST, we performed transmission electron microscopy (TEM) investigations of the 0.05wt%Ag/BST and the results are shown in Figure 3. Figure 3(a) is a typical low-magnification TEM image of the 0.05wt%Ag/BST. As can be seen, there are many nanoscale distorted regions,
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implying that Ag doping causes the increasing of stress. The defect is one of the main reason to form stress. As mentioned above, Ag can substitute Bi or Sb to form substitutional defects. Such defects can effectively scatter phonons as scattering cores. The corresponding high-resolution TEM (HRTEM) image is shown in Figure 3(b), indicating a good crystallinity of Ag/BST. Figure 3(c) shows an HRTEM image of a typical nanoparticle. The interplanar spacing of the nanoparticle can be measured as 0.67 nm (yellow circle), which is well matched with the d spacing of the (1̅01) plane of Ag2Te. Therefore, such nanoparticle can be indexed as Ag2Te precipitates. Within the intrinsic conduction region for Bi2Te3-based alloys, there exists an additional component to the κtotal arising from the diffusion of electron-hole pairs, called the bipolar component κb. In order to estimate roughly the scale of κb, we further analyse the κtotal - κe. The relationship between the κl and the reciprocal of temperature (1/T) can be linearly plotted using the ratio of 𝜅𝑙 ∝ 1/T,10, 25-26 as shown in dotted line of Figure 2(e). As can be seen, the observed κl is approximately equal to the κtotal - κe before intrinsic excitation. Compared with commercial BST, the κl of Ag/BST significantly decreases, which can be attributed to the effective Ag doping, fine Ag2Te nanoparticles, stress, and increased boundaries. Figure 2(f) shows the calculated κb by subtracting the calculated κl from the experimental κtotal - κe. As can be seen, the calculated κb at 423 K for commercial BST is 0.225 Wm-1K-1, while the κb of 0.03wt%Ag/BST is only 0.04 Wm-1K-1, contributing to the low κtotal in the high-temperature range. In summary, the κtotal of Ag/BST is reduced due to the slight increase in κe compensated by a significant decrease in κl and κb.
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Figure 3 Low (a) & high (b) magnified TEM images of matrix BST; (c) Secondary phase Ag2Te of 0.05wt%Ag/BST. Figure 4(a) is the calculated zT of the commercial BST, 0.03wt%Ag/BST, and 0.05wt%Ag/BST. The zT values of BST drastically decreased with the increase in temperature while zT values of Ag/BST samples can be maintained at a high value in the whole temperature range. Thus, by virtue of the enhanced S2σ and reduced κtotal, the peak zT of 0.05wt%Ag/BST can reach 1.07 at 373 K, which can be competitive to the reported modified BST, fabricated by traditional casting27 or zone melting.7 Besides, the zT values of Ag/BST are much superior to those from other methods at high temperature. Average zT is more competitive data in practical applications which represents the thermoelectric performance of materials in the whole temperature range. Figure 4(b) shows the peak zT and average zTs from 300 to 500 K. As can be seen, our best Ag/BST samples have the average zT of 1.02 and 0.97, which are better than most of other fabricated BST,28-30 suggesting the electroless plating Ag/BST can be working in a broader temperature range with more practical applications.
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Figure 4 (a) Temperature-dependent zT of BST, 0.03wt%Ag/BST, and 0.05wt%Ag/BST, respectively, and (b) Comparisons of average zTs with the reported values for BixSb2-xTe328-30 from 323 to 473 K.
In summary, the Ag/BST composites have been successfully fabricated by a facile electroless plating Ag method. An enhanced peak zT of 1.07 at 373 K and average ZT of 1.02 at the temperature interval 323~500 K have been observed in the 0.03wt%Ag/BST. Detail structure analysis and performance evaluations illustrate that doped Ag can form Ag2Te nanoprecipitates and point defects to lower kl up to 0.34 Wm-1K-1, and simultaneously increase the n and σ, which not only suppresses the intrinsic conduction but also enhances the power factor. Our simple, fast, and effective method can be widely used to enhance the thermoelectric performance of Bi2Te3based alloy for power generation in the temperature range from 300 to 500 K.
ASSOCIATED CONTENT
*S Supporting Information The Supporting Information is available free of charge on the ACS Publications website
AUTHOR INFORMATION
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Corresponding Author *Email: [email protected]; *Email: [email protected]; *Email: [email protected]
Notes The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
This work is financially supported by the National Natural Science Foundation of China under Grant No. 51371073, Chinese Academy of Sciences “Hundred Talents” project, and the Australian Research Council. ZGC thanks the USQ start-up grant and strategic research funds. The Australian Microscopy & Microanalysis Research Facility is acknowledged for providing characterization facilities.
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(27) Yu, Y.; Zhu, B.; Wu, Z.; Huang, Z. Y.; Wang, X. Y.; Zu, F. Q. Enhancing the Thermoelectric Performance of Free Solidified p-type Bi0.5Sb1.5Te3 Alloy by Manipulating its Parent Liquid State. Intermetallics 2015, 66, 40-47. (28) Hong, M.; Chen, Z. G.; Yang, L.; Zou, J. BixSb2−xTe3 Nanoplates with Enhanced Thermoelectric Performance due to Sufficiently Decoupled Electronic Transport Properties and Strong Wide-frequency Phonon Scatterings. Nano Energy 2016, 20, 144-155. (29) Madavali, B.; Kim, H. S.; Lee, K. H.; Hong, S. J. Enhanced Seebeck Coefficient by Energy Filtering in Bi-Sb-Te Based Composites with Dispersed Y2O3 Nanoparticles. Intermetallics 2017, 82, 68-75. (30) Lee, M. H.; Rhyee, J.-S.; Kim, S.; Choa, Y.-H. Thermoelectric Properties of Bi0.5Sb1.5Te3 /Ag2Te Bulk Composites with Size- and Shape-controlled Ag2Te Nano-particles Dispersion. J. Alloy. Compd. 2016, 657, 639-645.
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Table of Contents
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