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Scattering characteristics of grain boundaries in electrically sintered Bi0.4Sb1.6Te3 compounds
Accepted Manuscript Scattering characteristics of grain boundaries in electrically sintered Bi0.4Sb1.6Te3 compounds Yao-Hsiang Chen, Chien-Neng Liao PII: DOI: Reference:
S0167-577X(17)30480-9 http://dx.doi.org/10.1016/j.matlet.2017.03.136 MLBLUE 22375
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
7 February 2017 21 March 2017 25 March 2017
Please cite this article as: Y-H. Chen, C-N. Liao, Scattering characteristics of grain boundaries in electrically sintered Bi0.4Sb1.6Te3 compounds, Materials Letters (2017), doi: http://dx.doi.org/10.1016/j.matlet.2017.03.136
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Scattering characteristics of grain boundaries in electrically sintered Bi0.4Sb1.6Te3 compounds
Yao-Hsiang Chen and Chien-Neng Liao * Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan Corresponding author: Chien-Neng Liao, e-mail: [email protected] Address: 101 Sec. 2 Kuang-Fu Road, Hsinchu 30013, Taiwan Tel: +886-3-5715131 EXT 33843; FAX: +886-3-5722366 Abstract: Electrical transport properties of grain boundaries in hot-pressed (HP) and electrically-sintered (ES) Bi0.4Sb1.6Te3 compounds are examined and interpreted based on the Mayadas-Shatzkes model. Both the HP and ES samples were pressed at 300 °C under a pressure of 110 MPa except an electric current of 260 A/cm2 was applied concurrently through the ES sample. The ES sample exhibits enhanced Hall mobility and increased carrier concentration while maintaining low lattice thermal conductivity. The improved carrier mobility is ascribed to the reduced reflection coefficient of grain boundaries in the slightly textured ES sample. Keywords: electrical properties; thermoelectric; grain boundaries; sintering; texture 1. Introduction Bismuth telluride based compounds are famous low-temperature thermoelectric materials. Their brittle nature usually leads to a severe yield loss when producing tiny pellets for miniature thermoelectric devices. Although polycrystalline bismuth telluride possesses improved fracture toughness [1], they may suffer strong degradation of electrical or thermal conductivity due to electron/phonon scattering at grain boundaries (GBs). The overall performance of
1
thermoelectrics can be enhanced by adjusting the electrical and thermal transport properties non-proportionally through GB engineering [2]. Lan et al. [3] pointed out that nanometer-scaled Bi-rich precipitates at GBs are effective electron barriers for Bi0.5Sb1.5Te3. Kim et al. [4] proposed that dense dislocation arrays at GBs are efficient scatter centers for middle-frequency phonons, leading to greatly reduced thermal conductivity for Bi0.5Sb1.5Te3. Among various powder sintering techniques, spark plasma sintering or current-assisted sintering methods have been widely used to prepare bulk thermoelectric materials [5]. Although the interaction between electric current and crystal defects in bismuth telluride based compounds was reported previously [6], the effect of electrical stressing on GB transport properties has not been explored extensively. In this letter, we examine the reflection characteristics of GBs in the hot-pressed (HP) and electrically-sintered (ES) Bi0.4Sb1.6Te3 based on the Mayadas-Shatzkes model. The influence of electrical sintering on GB transport properties is also investigated. 2. Materials and methods A Bi0.4Sb1.6Te3 ingot was prepared by zone-melting process. The solidified ingot was hand ground into powders and sieved with a mesh size of 150 µm. A modified hot-press system that allows for the simultaneous introduction of electric current through the pressed sample was used to sinter the compacted Bi0.4Sb1.6Te3 powders. The HP samples were prepared at 300 °C for 35 min under a pressure of 110 MPa. The ES samples were prepared at the same pressure and temperature except an electric current was introduced after the temperature was stabilized at 300 °C for 10 min. To prevent temperature overshooting, the current was gradually increased to 260 A/cm2 in 15 min and maintained for another 10 min. Electrical and thermal transport properties were measured along the direction of electric current and pressure applied on the samples. A temperature gradient technique and a four-probe method were used to measure the Seebeck
2
coefficients (S) and electrical resistivity (ρ), respectively. The carrier concentration (p) was measured using a Hall measurement system (HMS-3000, ECOPIA). The Hall mobility (µ) was determined by the relation µ = 1/epρ. The thermal diffusivity (D) and density (ρD) were measured using a laser flash technique (LFA457, NETZSCH) and an Archimedes method, respectively. The thermal conductivity was then determined according to the relation κ = D⋅ρD⋅cp with the reported heat capacity (cp~ 0.196 J/g⋅K) [7]. The morphology and crystal structure were examined by scanning electron microscopy (SEM, SU-8010, Hitachi) and x-ray diffractometer (XRD, XRD-6000, Shimadzu). 3. Results and discussion By breaking the HP and ES samples, the former reveals a rough fracture morphology and the latter shows many sharp and flat cleavage planes (Fig. 1). The rough morphology is associated with inter-granular fracture along the poorly sintered powder interfaces. Alternatively, the flat surface results from intra-granular fracture along the van der Waals gap between two neighboring Te layers in bismuth telluride [8]. The preferential intra-granular fracture also suggests that the mechanical strength of GBs or powder interfaces is improved because atomic diffusion and GB restructuring wre enhanced by local Joule heating. Now, the next question would be whether the electrically-sintered GBs exhibit different carrier scattering characteristics. Table 1 lists the Seebeck coefficient, resistivity, carrier concentration and Hall mobility of HP and ES samples at room temperature. The reduced Seebeck coefficient and electrical resistivity are attributed to the increased carrier concentration and Hall mobility for the ES sample. Note that the carrier concentration of bismuth antimony telluride depends on acceptor-like SbTe (BiTe) defects and donor-like VTe vacancies. The local heating at GBs and powder interfaces would facilitate the escape of volatile Te elements, leading to excess Sb (Bi)
3
and VTe vacancies in crystal lattice. Current-driven Sb elements may precipitate at GBs or surface [6], or recombine with VTe by forming SbTe defects. The elimination of donor-like VTe defects and the increase of acceptor-like SbTe defects are responsible for the increased carrier concentration in the ES sample. Noting that bismuth telluride exhibits anisotropic electrical conductivity [9], the effect of crystallographic orientation on the transport properties of HP and ES samples must be considered. Fig. 2 shows the XRD patterns of the HP and ES samples measured in the direction parallel and perpendicular to the press direction, respectively. All the reflections agree with the XRD pattern of Bi0.4Sb1.6Te3 (JCPDS 72-1836) for both samples. To assess the crystallographic texture of the samples studied, five (00l) reflections including (006), (009), (0012), (0015), and (0018) were used to calculate the (00l) texture coefficient (݂ ) according to the Lotgering method [10]. A high ݂ reflects more and larger crystal grains aligned with the basal plane parallel to the sample surface. Here, the samples measured with the surface normal perpendicular to the press direction exhibit a ݂ value around 0 (Fig. 2(a)), while those with the surface normal parallel to the press direction reveal a ݂ value around 0.13 – 0.18 (Fig. 2(b)). The texture effect is considered in the following calculation of GB reflection coefficient. Following the Mayadas-Shatzkes analysis [11], a reflection coefficient (R) is used to evaluate the scattering strength of a specific boundary, which is given by ఙ ఙబ
= 1−
ଷఈ ଶ
ଵ
+ 3ߙ ଶ − 3ߙ ଷ ln ቀ1 + ఈ ቁ, α =
బ ோ
ௗ ଵିோ
(1)
where σg and σ0 are electrical conductivity of polycrystalline and GB-free solids, respectively, l0 is mean free path of charge carrier, and d is average boundary spacing. Theoretically, a large R value indicates a severe GB scattering and hence a low carrier mobility. To take the texture-dependent transport properties into account, we estimate the σ0 for the randomly oriented 4
sample (݂ = 0) to be σ0,random = 2074 Ω-1cm-1 from (ߪଵଵ /4) ∙ (1 + ඥ1 + 8ߪଷଷ /ߪଵଵ ) [12], with σ11 = 2720 Ω-1cm-1 and σ33 = 1088 Ω-1cm-1 [9]. Alternatively, the σ0 for the fully textured sample (݂ = 1) is σ33 = 1088 Ω-1cm-1. By knowing the carrier concentration of Bi0.4Sb1.6Te3 [9], we can estimate the texture-dependent mobility in the GB-free sample to be ߤ = ߤ,ௗ ∙ (1 − ݂ ) + ߤଷଷ ∙ ݂ according to the Vegard’s law. Once the σ0 is obtained from ߤ and the measured carrier concentration, we can further calculate the α parameter against the texture-dependent µ0 for specific µg values, as shown in Fig. 3. According to the measured µg and ݂ , we found that the α values are 0.45 for HP and 0.12 for ES, respectively. Using l0 = 4 nm [12] and the averaged grain size (dHP = 0.38 µm and dES = 0.57 µm), the R values were found to be 0.98 for HP and 0.94 for ES, respectively. The result suggests that the electrical sintering would lead to enhanced GB transport property for bismuth antimony telluride. Although electronic transport through GBs is improved, phonon transport is not significantly changed for the ES sample. The thermal conductivities of the HP and ES samples were measured to be 1.0 ± 0.1 and 1.2 ± 0.1 W/mK, respectively. By subtracting the electronic contribution of thermal conduction, both the HP and ES samples show the same lattice thermal conductivity (κL) ~ 0.80 W/mK. The results suggest that the electrical sintering is beneficial for improving electronic conduction while still blocking phonon transport through GBs in Bi0.4Sb1.6Te3 compounds. The increased µ/κL ratio indeed is highly desired for developing high-performance thermoelectric materials [13]. 4. Conclusions The hot-pressed Bi0.4Sb1.6Te3 with simultaneous passage of a high-density electric current shows increased carrier concentration, improved Hall mobility and unchanged lattice thermal conductivity. The increased carrier concentration is associated with the current-induced SbTe 5
defect formation. Carrier transport is subjected to a reduced reflection coefficient at GBs in Bi0.4Sb1.6Te3. Acknowledgement The work was supported by Ministry of Science and Technology, Taiwan through Grant No. MOST 104-2221-E-007-028-MY3. References 1.
X. A. Fan, J. Y. Yang, R. G. Chen, H. S. Yun, W. Zhu, S. Q. Bao, X. K. Duan, Characterization and thermoelectric properties of p-type 25%Bi2Te3–75%Sb2Te3 prepared via mechanical alloying and plasma activated sintering, J. Phys. D: Appl. Phys. 39 (2006) 740–745.
2.
D. H. Kim, T. Mitani, Thermoelectric properties of fine-grained Bi2Te3 alloys, J. Alloys and Compd. 399 (2005) 14–19.
3.
Y. Lan, B. Poudel, Y. Ma, D. Wang, M. S. Dresselhaus, G. Chen, Z. Ren, Structure study of bulk nanograined thermoelectric bismuth antimony telluride, Nano Lett. 9 (2009) 1419–1422.
4.
S. I. Kim, K. H. Lee, H. A. Mun, H. S. Kim, S. W. Hwang, J. W. Roh, D. J. Yang, W. H. Shin, X. S. Li, Y. H. Lee, G. J. Snyder, S. W. Kim, Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics, Science 348 (2015) 109–114.
5.
J. Jiang, L. Chen, S. Bai, Q. Yao, Q. Wang, Thermoelectric properties of textured p-type (Bi,Sb)2Te3 fabricated by spark plasma sintering, Scripta Mater. 52 (2005) 347–351.
6.
C. N. Liao, K. M. Liou, H. S. Chu, Enhancement of thermoelectric properties of sputtered Bi–Sb–Te thin films by electric current stressing, Appl. Phys. Lett. 93 (2008) 042103.
7.
L. Han, S. H. Spangsdorf, N. V. Nong, L. T. Hung, H. N. Pham, Y. Z. Chen, A. Roch, L. Stepien, N. Pryds, Effects of spark plasma sintering conditions on the anisotropic 6
thermoelectric properties of bismuth antimony telluride, RSC Adv. 6 (2016) 59565–59573. 8.
H. Scherrer, S. Scherrer, Thermoelectric properties of bismuth antimony telluride solid solutions , in: D. M. Rowe (Ed.), Thermoelectrics handbook: macro to nano, CRC Press, Boca Raton, 2006, Chap. 27.
9.
M. Stordeur, M. Stölzer, H. Sobotta, V. Riede, Investigation of the valence band structure of thermoelectric (Bi1-xSbx)2Te3 single crystals, Phys. Stat. Sol. (b) 150 (1988) 165–176.
10. F. K. Lotgering, Topotactical reactions with ferrimagnetic oxides having hexagonal crystal structures–I, J. Inorg. Nucl. Chem. 9 (1959) 113–123. 11. A. F. Mayadas, M. Shatzkes, Electrical-resistivity model for polycrystalline films: the case of arbitrary reflection at external surfaces, Phys. Rev. B 1 (1970) 1382–1389. 12. L. P. Bulat, I. A. Drabkin, V. V. Karataev, V. B. Osvenskiĭ, D. A. Pshenaĭ-Severin, Effect of boundary scattering on the thermal conductivity of a nanostructured semiconductor material based on the BixSb2–xTe3 solid solution, Phys. Solid State 52 (2010) 1836–1841. 13. H. J. Goldsmid, Introduction to thermoelectricity, 2nd ed., Springer, Berlin, 2016. Table Table 1 Seebeck coefficient, electrical resistivity, carrier concentration, and Hall mobility of the hot-pressed and electrically-sintered Bi0.4Sb1.6 Te3 samples. Sample
S (µV/K)
ρ (× 10-3Ω cm)
p (× 1019 cm-3)
µ (cm2/Vs)
Hot-pressed
192 ± 1
2.1 ± 0.2
2.2 ± 0.1
132 ± 16
Electrically-sintered
183 ± 1
1.2 ± 0.1
2.8 ± 0.2
189 ± 19
7
Figure captions Fig. 1
Cross-sectional SEM images of fracture morphologies for (a) hot-pressed and (b) electrically-sintered Bi0.4Sb1.6Te3 samples.
Fig. 2
XRD patterns of the hot-pressed and electrically-sintered Bi0.4Sb1.6Te3 samples with the surface normal (a) perpendicular and (b) parallel to the press direction (P).
Fig. 3
Variation of α parameter as a function of texture-dependent µ0 for Bi0.4Sb1.6Te3 samples with different µg values.
Figure 2
Figure 1
Figure 3 8
Highlights
Electrical/thermal conductions in electrically-sintered Bi0.4Sb1.6Te3 were studied. Texture-dependent carrier mobility for polycrystalline Bi0.4Sb1.6Te3 was evaluated. Carrier transport behavior at grain boundaries in Bi0.4Sb1.6Te3 was analyzed. Reflection coefficient of grain boundaries in Bi0.4Sb1.6Te3 was determined.
9
S0167-577X(17)30480-9 http://dx.doi.org/10.1016/j.matlet.2017.03.136 MLBLUE 22375
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
7 February 2017 21 March 2017 25 March 2017
Please cite this article as: Y-H. Chen, C-N. Liao, Scattering characteristics of grain boundaries in electrically sintered Bi0.4Sb1.6Te3 compounds, Materials Letters (2017), doi: http://dx.doi.org/10.1016/j.matlet.2017.03.136
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Scattering characteristics of grain boundaries in electrically sintered Bi0.4Sb1.6Te3 compounds
Yao-Hsiang Chen and Chien-Neng Liao * Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan Corresponding author: Chien-Neng Liao, e-mail: [email protected] Address: 101 Sec. 2 Kuang-Fu Road, Hsinchu 30013, Taiwan Tel: +886-3-5715131 EXT 33843; FAX: +886-3-5722366 Abstract: Electrical transport properties of grain boundaries in hot-pressed (HP) and electrically-sintered (ES) Bi0.4Sb1.6Te3 compounds are examined and interpreted based on the Mayadas-Shatzkes model. Both the HP and ES samples were pressed at 300 °C under a pressure of 110 MPa except an electric current of 260 A/cm2 was applied concurrently through the ES sample. The ES sample exhibits enhanced Hall mobility and increased carrier concentration while maintaining low lattice thermal conductivity. The improved carrier mobility is ascribed to the reduced reflection coefficient of grain boundaries in the slightly textured ES sample. Keywords: electrical properties; thermoelectric; grain boundaries; sintering; texture 1. Introduction Bismuth telluride based compounds are famous low-temperature thermoelectric materials. Their brittle nature usually leads to a severe yield loss when producing tiny pellets for miniature thermoelectric devices. Although polycrystalline bismuth telluride possesses improved fracture toughness [1], they may suffer strong degradation of electrical or thermal conductivity due to electron/phonon scattering at grain boundaries (GBs). The overall performance of
1
thermoelectrics can be enhanced by adjusting the electrical and thermal transport properties non-proportionally through GB engineering [2]. Lan et al. [3] pointed out that nanometer-scaled Bi-rich precipitates at GBs are effective electron barriers for Bi0.5Sb1.5Te3. Kim et al. [4] proposed that dense dislocation arrays at GBs are efficient scatter centers for middle-frequency phonons, leading to greatly reduced thermal conductivity for Bi0.5Sb1.5Te3. Among various powder sintering techniques, spark plasma sintering or current-assisted sintering methods have been widely used to prepare bulk thermoelectric materials [5]. Although the interaction between electric current and crystal defects in bismuth telluride based compounds was reported previously [6], the effect of electrical stressing on GB transport properties has not been explored extensively. In this letter, we examine the reflection characteristics of GBs in the hot-pressed (HP) and electrically-sintered (ES) Bi0.4Sb1.6Te3 based on the Mayadas-Shatzkes model. The influence of electrical sintering on GB transport properties is also investigated. 2. Materials and methods A Bi0.4Sb1.6Te3 ingot was prepared by zone-melting process. The solidified ingot was hand ground into powders and sieved with a mesh size of 150 µm. A modified hot-press system that allows for the simultaneous introduction of electric current through the pressed sample was used to sinter the compacted Bi0.4Sb1.6Te3 powders. The HP samples were prepared at 300 °C for 35 min under a pressure of 110 MPa. The ES samples were prepared at the same pressure and temperature except an electric current was introduced after the temperature was stabilized at 300 °C for 10 min. To prevent temperature overshooting, the current was gradually increased to 260 A/cm2 in 15 min and maintained for another 10 min. Electrical and thermal transport properties were measured along the direction of electric current and pressure applied on the samples. A temperature gradient technique and a four-probe method were used to measure the Seebeck
2
coefficients (S) and electrical resistivity (ρ), respectively. The carrier concentration (p) was measured using a Hall measurement system (HMS-3000, ECOPIA). The Hall mobility (µ) was determined by the relation µ = 1/epρ. The thermal diffusivity (D) and density (ρD) were measured using a laser flash technique (LFA457, NETZSCH) and an Archimedes method, respectively. The thermal conductivity was then determined according to the relation κ = D⋅ρD⋅cp with the reported heat capacity (cp~ 0.196 J/g⋅K) [7]. The morphology and crystal structure were examined by scanning electron microscopy (SEM, SU-8010, Hitachi) and x-ray diffractometer (XRD, XRD-6000, Shimadzu). 3. Results and discussion By breaking the HP and ES samples, the former reveals a rough fracture morphology and the latter shows many sharp and flat cleavage planes (Fig. 1). The rough morphology is associated with inter-granular fracture along the poorly sintered powder interfaces. Alternatively, the flat surface results from intra-granular fracture along the van der Waals gap between two neighboring Te layers in bismuth telluride [8]. The preferential intra-granular fracture also suggests that the mechanical strength of GBs or powder interfaces is improved because atomic diffusion and GB restructuring wre enhanced by local Joule heating. Now, the next question would be whether the electrically-sintered GBs exhibit different carrier scattering characteristics. Table 1 lists the Seebeck coefficient, resistivity, carrier concentration and Hall mobility of HP and ES samples at room temperature. The reduced Seebeck coefficient and electrical resistivity are attributed to the increased carrier concentration and Hall mobility for the ES sample. Note that the carrier concentration of bismuth antimony telluride depends on acceptor-like SbTe (BiTe) defects and donor-like VTe vacancies. The local heating at GBs and powder interfaces would facilitate the escape of volatile Te elements, leading to excess Sb (Bi)
3
and VTe vacancies in crystal lattice. Current-driven Sb elements may precipitate at GBs or surface [6], or recombine with VTe by forming SbTe defects. The elimination of donor-like VTe defects and the increase of acceptor-like SbTe defects are responsible for the increased carrier concentration in the ES sample. Noting that bismuth telluride exhibits anisotropic electrical conductivity [9], the effect of crystallographic orientation on the transport properties of HP and ES samples must be considered. Fig. 2 shows the XRD patterns of the HP and ES samples measured in the direction parallel and perpendicular to the press direction, respectively. All the reflections agree with the XRD pattern of Bi0.4Sb1.6Te3 (JCPDS 72-1836) for both samples. To assess the crystallographic texture of the samples studied, five (00l) reflections including (006), (009), (0012), (0015), and (0018) were used to calculate the (00l) texture coefficient (݂ ) according to the Lotgering method [10]. A high ݂ reflects more and larger crystal grains aligned with the basal plane parallel to the sample surface. Here, the samples measured with the surface normal perpendicular to the press direction exhibit a ݂ value around 0 (Fig. 2(a)), while those with the surface normal parallel to the press direction reveal a ݂ value around 0.13 – 0.18 (Fig. 2(b)). The texture effect is considered in the following calculation of GB reflection coefficient. Following the Mayadas-Shatzkes analysis [11], a reflection coefficient (R) is used to evaluate the scattering strength of a specific boundary, which is given by ఙ ఙబ
= 1−
ଷఈ ଶ
ଵ
+ 3ߙ ଶ − 3ߙ ଷ ln ቀ1 + ఈ ቁ, α =
బ ோ
ௗ ଵିோ
(1)
where σg and σ0 are electrical conductivity of polycrystalline and GB-free solids, respectively, l0 is mean free path of charge carrier, and d is average boundary spacing. Theoretically, a large R value indicates a severe GB scattering and hence a low carrier mobility. To take the texture-dependent transport properties into account, we estimate the σ0 for the randomly oriented 4
sample (݂ = 0) to be σ0,random = 2074 Ω-1cm-1 from (ߪଵଵ /4) ∙ (1 + ඥ1 + 8ߪଷଷ /ߪଵଵ ) [12], with σ11 = 2720 Ω-1cm-1 and σ33 = 1088 Ω-1cm-1 [9]. Alternatively, the σ0 for the fully textured sample (݂ = 1) is σ33 = 1088 Ω-1cm-1. By knowing the carrier concentration of Bi0.4Sb1.6Te3 [9], we can estimate the texture-dependent mobility in the GB-free sample to be ߤ = ߤ,ௗ ∙ (1 − ݂ ) + ߤଷଷ ∙ ݂ according to the Vegard’s law. Once the σ0 is obtained from ߤ and the measured carrier concentration, we can further calculate the α parameter against the texture-dependent µ0 for specific µg values, as shown in Fig. 3. According to the measured µg and ݂ , we found that the α values are 0.45 for HP and 0.12 for ES, respectively. Using l0 = 4 nm [12] and the averaged grain size (dHP = 0.38 µm and dES = 0.57 µm), the R values were found to be 0.98 for HP and 0.94 for ES, respectively. The result suggests that the electrical sintering would lead to enhanced GB transport property for bismuth antimony telluride. Although electronic transport through GBs is improved, phonon transport is not significantly changed for the ES sample. The thermal conductivities of the HP and ES samples were measured to be 1.0 ± 0.1 and 1.2 ± 0.1 W/mK, respectively. By subtracting the electronic contribution of thermal conduction, both the HP and ES samples show the same lattice thermal conductivity (κL) ~ 0.80 W/mK. The results suggest that the electrical sintering is beneficial for improving electronic conduction while still blocking phonon transport through GBs in Bi0.4Sb1.6Te3 compounds. The increased µ/κL ratio indeed is highly desired for developing high-performance thermoelectric materials [13]. 4. Conclusions The hot-pressed Bi0.4Sb1.6Te3 with simultaneous passage of a high-density electric current shows increased carrier concentration, improved Hall mobility and unchanged lattice thermal conductivity. The increased carrier concentration is associated with the current-induced SbTe 5
defect formation. Carrier transport is subjected to a reduced reflection coefficient at GBs in Bi0.4Sb1.6Te3. Acknowledgement The work was supported by Ministry of Science and Technology, Taiwan through Grant No. MOST 104-2221-E-007-028-MY3. References 1.
X. A. Fan, J. Y. Yang, R. G. Chen, H. S. Yun, W. Zhu, S. Q. Bao, X. K. Duan, Characterization and thermoelectric properties of p-type 25%Bi2Te3–75%Sb2Te3 prepared via mechanical alloying and plasma activated sintering, J. Phys. D: Appl. Phys. 39 (2006) 740–745.
2.
D. H. Kim, T. Mitani, Thermoelectric properties of fine-grained Bi2Te3 alloys, J. Alloys and Compd. 399 (2005) 14–19.
3.
Y. Lan, B. Poudel, Y. Ma, D. Wang, M. S. Dresselhaus, G. Chen, Z. Ren, Structure study of bulk nanograined thermoelectric bismuth antimony telluride, Nano Lett. 9 (2009) 1419–1422.
4.
S. I. Kim, K. H. Lee, H. A. Mun, H. S. Kim, S. W. Hwang, J. W. Roh, D. J. Yang, W. H. Shin, X. S. Li, Y. H. Lee, G. J. Snyder, S. W. Kim, Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics, Science 348 (2015) 109–114.
5.
J. Jiang, L. Chen, S. Bai, Q. Yao, Q. Wang, Thermoelectric properties of textured p-type (Bi,Sb)2Te3 fabricated by spark plasma sintering, Scripta Mater. 52 (2005) 347–351.
6.
C. N. Liao, K. M. Liou, H. S. Chu, Enhancement of thermoelectric properties of sputtered Bi–Sb–Te thin films by electric current stressing, Appl. Phys. Lett. 93 (2008) 042103.
7.
L. Han, S. H. Spangsdorf, N. V. Nong, L. T. Hung, H. N. Pham, Y. Z. Chen, A. Roch, L. Stepien, N. Pryds, Effects of spark plasma sintering conditions on the anisotropic 6
thermoelectric properties of bismuth antimony telluride, RSC Adv. 6 (2016) 59565–59573. 8.
H. Scherrer, S. Scherrer, Thermoelectric properties of bismuth antimony telluride solid solutions , in: D. M. Rowe (Ed.), Thermoelectrics handbook: macro to nano, CRC Press, Boca Raton, 2006, Chap. 27.
9.
M. Stordeur, M. Stölzer, H. Sobotta, V. Riede, Investigation of the valence band structure of thermoelectric (Bi1-xSbx)2Te3 single crystals, Phys. Stat. Sol. (b) 150 (1988) 165–176.
10. F. K. Lotgering, Topotactical reactions with ferrimagnetic oxides having hexagonal crystal structures–I, J. Inorg. Nucl. Chem. 9 (1959) 113–123. 11. A. F. Mayadas, M. Shatzkes, Electrical-resistivity model for polycrystalline films: the case of arbitrary reflection at external surfaces, Phys. Rev. B 1 (1970) 1382–1389. 12. L. P. Bulat, I. A. Drabkin, V. V. Karataev, V. B. Osvenskiĭ, D. A. Pshenaĭ-Severin, Effect of boundary scattering on the thermal conductivity of a nanostructured semiconductor material based on the BixSb2–xTe3 solid solution, Phys. Solid State 52 (2010) 1836–1841. 13. H. J. Goldsmid, Introduction to thermoelectricity, 2nd ed., Springer, Berlin, 2016. Table Table 1 Seebeck coefficient, electrical resistivity, carrier concentration, and Hall mobility of the hot-pressed and electrically-sintered Bi0.4Sb1.6 Te3 samples. Sample
S (µV/K)
ρ (× 10-3Ω cm)
p (× 1019 cm-3)
µ (cm2/Vs)
Hot-pressed
192 ± 1
2.1 ± 0.2
2.2 ± 0.1
132 ± 16
Electrically-sintered
183 ± 1
1.2 ± 0.1
2.8 ± 0.2
189 ± 19
7
Figure captions Fig. 1
Cross-sectional SEM images of fracture morphologies for (a) hot-pressed and (b) electrically-sintered Bi0.4Sb1.6Te3 samples.
Fig. 2
XRD patterns of the hot-pressed and electrically-sintered Bi0.4Sb1.6Te3 samples with the surface normal (a) perpendicular and (b) parallel to the press direction (P).
Fig. 3
Variation of α parameter as a function of texture-dependent µ0 for Bi0.4Sb1.6Te3 samples with different µg values.
Figure 2
Figure 1
Figure 3 8
Highlights
Electrical/thermal conductions in electrically-sintered Bi0.4Sb1.6Te3 were studied. Texture-dependent carrier mobility for polycrystalline Bi0.4Sb1.6Te3 was evaluated. Carrier transport behavior at grain boundaries in Bi0.4Sb1.6Te3 was analyzed. Reflection coefficient of grain boundaries in Bi0.4Sb1.6Te3 was determined.
9