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Composite fabrication for improvement of thermoelectric properties in AlSb
Materials Science in Semiconductor Processing 110 (2020) 104974
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Composite fabrication for improvement of thermoelectric properties in AlSb A.K.M. Ashiquzzaman Shawon , Il-Ho Kim , Soon-Chul Ur * Dept. of Material Sci. and Eng., Research Center for Sustainable Eco-Devices and Materials (ReSEM), Korea National University of Transportation, Chungbuk, Chungju, 27469, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Composite Intermetallic AlSb Zn4Sb3 Thermoelectric
The III-V semiconductor AlSb has an indirect band gap and a relatively moderate thermal conductivity. Intrinsic AlSb has a promising thermopower, but a very high resistivity and the lattice thermal conductivity keeps the ZT value significantly low. This work shows a process of reducing the thermal conductivity by formation of binary composite with Zn4Sb3. β-Zn4Sb3 powders were synthesized by sintering of cold compacted pellets and AlSb was fabricated using controlled casting in tapped graphite crucible. Inclusion and dispersion of 10 wt % β-Zn4Sb3 within the matrix of AlSb has reduced the thermal conductivity by more than 5 times. In addition, the composite shows improved electrical conductivity without a significant decrease in thermopower. Overall, the thermo electric figure of merit shows proportionality with the increasing amount of β-Zn4Sb3, where the ZT increases significantly.
1. Introduction Wastage of heat is a common phenomenon in all industrial and household operations. Thermoelectricity is the study of effective con version of such waste heat into electricity and vice versa. Thermoelectric (TE) efficiency of a material system is given by the thermoelectric figure of merit, ZT, which is defined by equation (1), where S denotes Seebeck coefficient, σ gives electrical conductivity, κ represents thermal con ductivity and T gives absolute temperature [1]. So far, κ is considered to be the sum of κlattice and κelectron, heat conduction by phonons and electrons respectively. The significant challenge in thermoelectric study arises from the interrelationship of the factors. S and σ are both dependent on carrier concentration in the opposite way to one another [2]. In addition, κelectron and σ share a proportional relationship estab lished by the Wiedemann-Franz law [3]. Consequently, increasing the ZT value is also extremely challenging. However, as Eq. (1) shows, the main objective is to increase the power factor (S2σ) and decrease the κlattice in a material system. ZT ¼
S2 σ T κlattice þ κelectron
(1)
III-V semiconductors are of significant interest because of their properties being similar to that of silicon and the possibility of replacing
silicon with tunable semiconductors are already real in different aspects of materials engineering. Although significant contributions have been made in thin-film synthesis for such materials [4], bulk single phase AlSb and its properties have been one of the least studied sections. Being a highly stoichiometric compound, synthesis of single phase bulk AlSb is an experimental challenge [5]. Previous reports have, however, shown synthetic routes for such fabrication [6]. Antimonide semiconductors have dominated the field of thermo electric study for quite some time now. β-Zn4Sb3 [7], InSb [8], GaSb [9], Mg3 þ xSb2 [10] are some of the antimonide compounds that have been established as thermoelectric materials of both n and p type. AlSb is a similar compound and the thermoelectric properties of this system have been very recently reported. Although the intrinsic intermetallic com pound shows a low ZT, the Seebeck coefficient has shown promising values. However, due to a moderate indirect band gap of 1.69 eV [11] and a relatively large thermal conductivity, the improvement of ZT is puzzling. One of the methods for improvement of ZT could be to decrease the thermal conductivity without decreasing the Seebeck co efficient. Tuning the carrier concentration can increase the electrical conductivity too, but that would decrease the Seebeck coefficient, which may decrease the power factor significantly. In addition, the increase in electrical conductivity will also increase the κelectron. Thus, an attempt to decrease the lattice thermal conductivity of the zinc blende structure
* Corresponding author. E-mail address: [email protected] (S.-C. Ur). https://doi.org/10.1016/j.mssp.2020.104974 Received 17 September 2019; Received in revised form 15 January 2020; Accepted 29 January 2020 Available online 5 February 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.
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Materials Science in Semiconductor Processing 110 (2020) 104974
would essentially increase the thermoelectric figure of merit of AlSb. Bergman et al showed by theoretical predictions that a composite synthesized using mixing and compacting two dissimilar materials can have an enhanced figure of merit [12]. Katsuyama et al later showed this effect in CoSb3–FeSb2 composite, enhancing the thermoelectric figure of merit by decreasing the thermal conductivity [13]. This decrease in the lattice thermal conductivity was attributed to the increased phonon scattering, which occurred due to the existence of FeSb2 within the matrix. Recently, Zou et al studied the mechanism in details between β-Zn4Sb3 and Cu3SbSe4 and proved adequately that the dispersion of nano to micro-sized secondary phase can enhance phonon scattering and increase the thermoelectric properties in the material system [14]. This work, based on the above hypothesis, intends to increase the ZT in AlSb by dispersion of small amount of β-Zn4Sb3, sometimes referred to as β-Zn13Sb10 [15], secondary phases. This could decrease the thermal conductivity without significantly hampering the Seebeck coefficient, in turn increasing the thermoelectric efficiency. Mechanical attrition milling has been used to mix powders before compaction of composites in the recent studies [16]. This is a very effective method to disperse the phases homogenously before hot pressing into pellets. Ur et al showed that β-Zn4Sb3 powders can be synthesized by sintering cold compacted elemental powders, using an excess of Zn [17]. The high vapor pressure of zinc can result in a Zn-deficiency, leaving some α-ZnSb phases to be found. AlSb has been previously synthesized as single phase by controlled melting method using Al and Sb shots and its thermoelectric properties have been re ported [6]. Milling the as-casted AlSb and as-sintered β-Zn4Sb3 may produce a homogenous composite. According to the Zn–Sb phase diagram, the phase β-Zn4Sb3 is stable up to 767 K, after which the phase changes to α-ZnSb [18]. Recent Al–Zn–Sb ternary phase diagram studies have proven this system to be rather complex, with the existence of a possible ternary eutectic region at 647 K [19]. However, the compositions being used in this study fall outside of the proposed region. The possible phases within the ternary phase diagram used in this study are AlSb, ZnSb and β-Zn4Sb3 [20]. Previous reports have shown that the thermoelectric performance in the β-Zn4Sb3 is the best among the other compositions [21]. Thus, fabrica tion and measurement of TE properties needed careful consideration of temperature limits.
Advance D-8, Germany) using Cu-Kα radiation. Scanning electron mi croscope (SEM; Quanta-400, Netherlands) was used for energy disper sive spectroscopy (EDS) and microstructural elucidation. Seebeck coefficient and electrical conductivity were measured using ZEM-III (ULVAC-RIKO, Japan) by the four-probe method from a 3 � 3 � 10 mm3 cuboidal sample. Thermal diffusivity, d, was measured by laser flash method integrated into TC-9000H (ULVAC-RIKO, Japan). Thermal conductivity was calculated using the formula κ ¼ d � Cp � ρ, where Cp is the specific heat capacity and ρ is the density measured by Archime dean principle. 4. Result and discussion Fig. 1 shows the x-ray diffraction data from XRD. In Fig. 1(a), it can be clearly seen that single phase β-Zn4Sb3 has been fabricated using cold compaction followed by sintering. AlSb phases are also seen as single phase. The major peak for β-Zn4Sb3 overlaps with the major peak for AlSb and makes it difficult to confirm the synthesis of composite by XRD. Similar phenomenon has been observed regarding secondary phases in previous reports [22–24]. Smaller amounts of β-Zn4Sb3 crystals within the AlSb matrix are randomly dispersed, due to which significant scat tering may not be visible within the relative intensity scale. However, the phases may be visible in the logarithmic scale, as shown in Fig. 1(b). Logarithmic scale reduces the gap between large and very small peaks, due to which β-Zn4Sb3 peaks are clearly identifiable [6]. It can also be noted that the peaks for α-ZnSb phases are not seen after hot pressing, as confirmed by Fig. 1(b). However, when the composite was synthesized using 30 wt % β-Zn4Sb3, the phase changed to α-ZnSb during hot press at 773 K. Fig. 1(c) shows the Rietveld measurement of phases in (AlSb)0.7(Zn4Sb3)0.3 composite. The measurements have revealed that 32.2 wt % ZnSb is present within the AlSb matrix. The microscopic structure of the pulverized and mixed powders is shown in Fig. 2(a), while the inner surface of hot pressed pellet is shown in Fig. 2(b). Powder samples are near round shaped. The particle size was confirmed to be within the range of 300 nm to 5 μm after pulveri zation. Backscattering mode of the SEM has revealed that the zinc an timonide phases (light grey) are homogeneously dispersed within AlSb matrix (dark grey). To confirm composite formation further, line map ping using EDS integrated into SEM was used to check the position of individual elements within the hot pressed sample. Fig. 3 confirms that AlSb is the primary matrix, while zinc is evenly distributed within the matrix. The thermopower (S), in μV/K, has been shown as a function of temperature in Fig. 4(a), while Fig. 4(b) shows the variation of electrical conductivity with temperature in the binary composite (AlSb)1x(Zn4Sb3)x. Previous reports have shown the thermoelectric properties in near single phase AlSb, and the literature value has been used as x ¼ 0 [6]. Adding 10 wt % Zn4Sb3 increases the Seebeck coefficient even further, up to 400 μV/K, while still showing a low electrical conduc tivity. However, as temperature increases above 470 K, effective carrier concentration in the conduction band increases, increasing the electrical conductivity and dramatically reducing the Seebeck coefficient to just over 100 μV/K. The Seebeck coefficient curve and electrical conduc tivity curve show reciprocal behavior to one another for x ¼ 0.1, which shows how carrier concentration changes as a function of temperature. Increasing fractions of Zn4Sb3 increases the carrier concentration in the composite. Both x ¼ 0.2 and 0.3 composites show increased absolute value of Seebeck coefficient and electrical conductivity than their former compound at temperatures higher than 470 K. For x ¼ 0.3, the Seebeck coefficient is close to that of intrinsic AlSb, but with an elec trical conductivity significantly higher. This change can be accounted for the higher net carrier concentration in the composite and the inherently high thermopower in AlSb. As shown in Fig. 5, the power factor, S2σ, keeps increasing with increasing Zn4Sb3, and finally reaches a peak of ~450 μV/mK2. Since the thermoelectric properties of Zn4Sb3 is considerably higher than that of AlSb, the trends observed so far have
2. Materials and methods Zinc powders (99.99%, Kojundo), Sb powders (99.9%, Kojundo), Al shots (99.9%, Aldrich) and Sb shots (99.999%, Kojundo) were used to separately synthesize AlSb and β-Zn4Sb3. Melting and sintering were both done in a vacuum furnace using a tapped graphite crucible. High energy vibratory mill (HEVM; KMTech TMM-70, Korea) was used to mix the compounds homogenously in zirconia vial with zirconia balls. Adequate care was taken to prevent oxidation as the chemicals were handled in a glove box under Ar-atmosphere. 3. Experimental Zn and Sb powders were weighed in a glove box and cold pressed using a stainless steel die of diameter 10 mm. The cold pressed pellet was put in a tapped graphite crucible and sealed in argon-atmosphere before sintering it at 673 K for 24 h. Al and Sb shots were separately weighed and melted in a tapped graphite crucible at 1273 K for 1 h. To prevent contamination of carbon, boron nitride was sprayed on the inner walls of the graphite crucibles. β-Zn4Sb3 and AlSb were mixed according to weight percent ratio and put in a zirconia vial in argon atmosphere. Zirconia balls were added at 10:1 ratio by weight and the powders were mixed for 10 min in the HEVM (KMTech TMM-70, Korea) used as an attrition mill. As-milled powders were sieved by a 325 mesh sieve and compacted in graphite die for 6 h at 773 K and 80 MPa pressure. Phase analysis was carried out by X-ray diffraction (XRD; BRUKER AXS 2
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Materials Science in Semiconductor Processing 110 (2020) 104974
Fig. 1. X-ray diffraction peaks for the casted AlSb pellet, sintered Zn4Sb3 powders, and hot pressed composite (a), while (b) shows the phases of (AlSb)0.8(Zn4Sb3)0.2 in the logarithmic scale. The amount of phases have been calculated in x ¼ 0.3 sample using Rietveld refinement, shown in (c).
Fig. 2. Scanning Electron Microscopic images from x ¼ 0.3 composite after (a) mixing and (b) hot pressing (Backscattering mode).
been well in accordance with what can be expected in the binary com posite. The thermoelectric power factor has risen considerably from x ¼ 0 to x ¼ 0.3. This increase can be attributed to the higher carrier con centration in Zn4Sb3 than in AlSb. The thermal conductivity data, as illustrated by Fig. 6(a), shows that with 10 wt % inclusion of secondary β-Zn4Sb3 phase, the thermal con ductivity drops immensely from 8.6 W/mK to 2.8 W/mK. Lattice ther mal conductivity is directly proportional to the phonon mean free path available for propagation of acoustic phonons [25]. The existence of a
secondary phase in a very small amount largely increases the phonon scattering, which decreases the phonon mean free path required for heat conduction through the crystal structure [26]. Thus, the lattice thermal conductivity, as shown in Fig. 6(b), decreases dramatically. This phe nomenon is also well compliant to literature. The total thermal conductivity in AlSb is quite large for thermo electric compounds. In contrast, β-Zn4Sb3, being categorized as a Zintl phase [27], has a very low thermal conductivity. According to the Eucken derivation of Maxwell equation, which was later confirmed for 3
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Materials Science in Semiconductor Processing 110 (2020) 104974
Fig. 3. EDS line images showing distribution of elemental atoms in hot pressed composite.
Fig. 4. Variation of (a) Seebeck coefficient and (b) electrical conductivity as a function of temperature and β-Zn4Sb3 weight fraction.
discrepancies [14]. One possibility might be that due to the difference in crystal size and shape between the two components, small amount of β-Zn4Sb3 act as defects within the matrix of AlSb [26], which reduces the phonon mean free path and increases acoustic phonon scattering. However, with an increase in β-Zn4Sb3 above 10 wt %, the interface boundaries might increase in coherency, possibly moving from completely incoherent to near coherent phase boundaries. Conse quently, the phonon mean free path may have increased again, in accordance with the percolation theory [25], and an increase in lattice thermal conductivity has been seen. There are very limited resources available in this regard to conclusively assert any hypothesis. More study regarding the phenomena can elucidate underlying principles better and help explain future results. The increase in thermal conduc tivity may also be a result of increasing relative density of the pellets. Table 1 shows the relative densities in the composite. The theoretical density has been calculated using a ratio of the theoretical densities of AlSb (4.26 g/cm3) and β-Zn4Sb3 (6.20 g/cm3). The relatively lower density even after 24 h of hot press for x ¼ 0.1 composite may be due the significant difference in crystal structures of AlSb and β-Zn4Sb3. The high thermal conductivity for x ¼ 0.3 composite may be because of the phase change from β-Zn4Sb3 to α-ZnSb, which has a higher lattice thermal conductivity than the former. The thermoelectric properties of the resultant composite elucidated the dimensionless figure of merit using Eq. (1). Fig. 7 shows the variation of ZT with temperature. Due to the considerable drop in thermal con ductivity, the ZT increased after the addition of 10 wt % β-Zn4Sb3 threefold. However, the peak ZT of 0.065 was achieved with x ¼ 0.3. This increase can be attributed to the escalated power factor due to effective tuning of carrier concentration. Although the increase in
Fig. 5. The relationship between Power factor (S2σ), fraction of Zn4Sb3 and temperature.
ceramics by Kingery et al [28], the thermal conductivity in the com posite is supposed to decrease with an increase in weight fraction of β-Zn4Sb3. However, from the experimentally determined results, it can be seen that the lowest thermal conductivity is seen with only 10 wt % β-Zn4Sb3, while with 30 wt % β-Zn4Sb3 the total thermal conductivity is close to the intrinsic thermal conductivity of AlSb. The reason for such deviation is unknown, but previous accounts have also shown such 4
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Materials Science in Semiconductor Processing 110 (2020) 104974
Fig. 6. The variation of (a) thermal conductivity and (b) lattice thermal conductivity as a function of temperature for different weight fractions of β-Zn4Sb3.
the tuning of carrier concentration, the power factor also enhanced greatly to provide a significant improvement in thermoelectric properties.
Table 1 Relative density of composite samples as a function of hot press duration and x from (AlSb)1-x(Zn4Sb3)x. Nominal Composition
Hot pressing time
Relative Density
x ¼ 0.1 x ¼ 0.1 x ¼ 0.2 x ¼ 0.3
6h 24 h 6h 6h
70.7% 71.2% 76.2% 88.8%
Declaration of competing interest The authors declare no conflict of interest. CRediT authorship contribution statement A.K.M. Ashiquzzaman Shawon: Conceptualization, Data curation, Formal analysis, Investigation, Software, Visualization, Writing - orig inal draft. Il-Ho Kim: Conceptualization, Funding acquisition, Valida tion, Resources, Writing - review & editing. Soon-Chul Ur: Conceptualization, Funding acquisition, Methodology, Project admin istration, Supervision, Validation, Resources, Writing - review & editing. Acknowledgement This research was supported by the Korea Basic Science Institute grant funded by the Ministry of Education (grant no. 2019R1A6C1010047). References [1] H.S. Kim, W. Liu, G. Chen, C.-W. Chu, Z. Ren, Relationship between thermoelectric figure of merit and energy conversion efficiency, Proc. Natl. Acad. Sci. Unit. States Am. 112 (2015) 8205, https://doi.org/10.1073/pnas.1510231112. [2] Z.G. Chen, G. Hana, L. Yanga, L. Cheng, J. Zou, Nanostructured thermoelectric materials: current research and future challenge, Prog. Nat. Sci. Mater. Int. 22 (2012) 535–549, https://doi.org/10.1016/j.pnsc.2012.11.011. [3] A. Jaoui, B. Fauqu� e, C.W. Rischau, A. Subedi, C. Fu, J. Gooth, N. Kumar, V. Süß, D. L. Maslov, C. Felser, K. Behnia, Departure from the Wiedemann–Franz law in WP2 driven by mismatch in T-square resistivity prefactors, Npj Quantum Mater 3 (2018) 64, https://doi.org/10.1038/s41535-018-0136-x. [4] R.H. Athab, B.H. Hussein, S.A. Makki, Effect of in on the Properties of AlSb Thin Film Solar Cell, 2019, 020030, https://doi.org/10.1063/1.5116957. [5] D. Strauch, AlSb: crystal structures, phase transitions, transition pressure, equation of state, in: U. R€ ossler (Ed.), New Data Updat. IV-IV, III-V, II-VI I-VII Compd. Their Mix. Cryst. Diluted Magn. Semicond, Springer Berlin Heidelberg, Berlin, Heidelberg, 2011, pp. 134–135, https://doi.org/10.1007/978-3-642-14148-5_78. [6] A.K.M.A. Shawon, S.-C. Ur, Mechanical and thermoelectric properties of bulk AlSb synthesized by controlled melting, pulverizing and subsequent vacuum hot pressing, Appl. Sci. 9 (2019), https://doi.org/10.3390/app9081609. [7] T. Caillat, J.-P. Fleurial, A. Borshchevsky, Preparation and thermoelectric properties of semiconducting Zn4Sb3, J. Phys. Chem. Solid. 58 (1997) 1119–1125, https://doi.org/10.1016/S0022-3697(96)00228-4. [8] Y. Cheng, J. Yang, Q. Jiang, D. He, J. He, L. Yubo, D. Zhang, Z. Zhou, Y. Ren, X. Jw, New insight to InSb-based thermoelectric materials: from the divorced eutectic design to remarkable high thermoelectric performance. https://doi.org/10.1039/C 6TA10827J, 2017.
Fig. 7. Thermoelectric figure of merit of (AlSb)1-x(Zn4Sb3)x up-to 773 K
thermoelectric efficiency is considerably large, a decrease in the com posite thermal conductivity for x ¼ 0.3 would have increased the ZT even further. While this can be seen as a low thermopower for effective applications in real devices, there is still further scope for improvement by optimizing phase interfaces and use of appropriate dopants. 5. Conclusion AlSb, a recently reported thermoelectric material, has a low intrinsic thermoelectric property despite its high thermopower. In order to in crease the ZT, a binary composite of (AlSb)1-x(Zn4Sb3)x was successfully synthesized. Controlled melting and sintering of cold compacts were used to synthesize the individual semiconductors, which were then mixed thoroughly and hot pressed to give final composite. The com posite shows considerably low lattice thermal conductivity, possibly due to effective phonon scattering and a decreased mean free path. Due to 5
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6
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Composite fabrication for improvement of thermoelectric properties in AlSb A.K.M. Ashiquzzaman Shawon , Il-Ho Kim , Soon-Chul Ur * Dept. of Material Sci. and Eng., Research Center for Sustainable Eco-Devices and Materials (ReSEM), Korea National University of Transportation, Chungbuk, Chungju, 27469, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Composite Intermetallic AlSb Zn4Sb3 Thermoelectric
The III-V semiconductor AlSb has an indirect band gap and a relatively moderate thermal conductivity. Intrinsic AlSb has a promising thermopower, but a very high resistivity and the lattice thermal conductivity keeps the ZT value significantly low. This work shows a process of reducing the thermal conductivity by formation of binary composite with Zn4Sb3. β-Zn4Sb3 powders were synthesized by sintering of cold compacted pellets and AlSb was fabricated using controlled casting in tapped graphite crucible. Inclusion and dispersion of 10 wt % β-Zn4Sb3 within the matrix of AlSb has reduced the thermal conductivity by more than 5 times. In addition, the composite shows improved electrical conductivity without a significant decrease in thermopower. Overall, the thermo electric figure of merit shows proportionality with the increasing amount of β-Zn4Sb3, where the ZT increases significantly.
1. Introduction Wastage of heat is a common phenomenon in all industrial and household operations. Thermoelectricity is the study of effective con version of such waste heat into electricity and vice versa. Thermoelectric (TE) efficiency of a material system is given by the thermoelectric figure of merit, ZT, which is defined by equation (1), where S denotes Seebeck coefficient, σ gives electrical conductivity, κ represents thermal con ductivity and T gives absolute temperature [1]. So far, κ is considered to be the sum of κlattice and κelectron, heat conduction by phonons and electrons respectively. The significant challenge in thermoelectric study arises from the interrelationship of the factors. S and σ are both dependent on carrier concentration in the opposite way to one another [2]. In addition, κelectron and σ share a proportional relationship estab lished by the Wiedemann-Franz law [3]. Consequently, increasing the ZT value is also extremely challenging. However, as Eq. (1) shows, the main objective is to increase the power factor (S2σ) and decrease the κlattice in a material system. ZT ¼
S2 σ T κlattice þ κelectron
(1)
III-V semiconductors are of significant interest because of their properties being similar to that of silicon and the possibility of replacing
silicon with tunable semiconductors are already real in different aspects of materials engineering. Although significant contributions have been made in thin-film synthesis for such materials [4], bulk single phase AlSb and its properties have been one of the least studied sections. Being a highly stoichiometric compound, synthesis of single phase bulk AlSb is an experimental challenge [5]. Previous reports have, however, shown synthetic routes for such fabrication [6]. Antimonide semiconductors have dominated the field of thermo electric study for quite some time now. β-Zn4Sb3 [7], InSb [8], GaSb [9], Mg3 þ xSb2 [10] are some of the antimonide compounds that have been established as thermoelectric materials of both n and p type. AlSb is a similar compound and the thermoelectric properties of this system have been very recently reported. Although the intrinsic intermetallic com pound shows a low ZT, the Seebeck coefficient has shown promising values. However, due to a moderate indirect band gap of 1.69 eV [11] and a relatively large thermal conductivity, the improvement of ZT is puzzling. One of the methods for improvement of ZT could be to decrease the thermal conductivity without decreasing the Seebeck co efficient. Tuning the carrier concentration can increase the electrical conductivity too, but that would decrease the Seebeck coefficient, which may decrease the power factor significantly. In addition, the increase in electrical conductivity will also increase the κelectron. Thus, an attempt to decrease the lattice thermal conductivity of the zinc blende structure
* Corresponding author. E-mail address: [email protected] (S.-C. Ur). https://doi.org/10.1016/j.mssp.2020.104974 Received 17 September 2019; Received in revised form 15 January 2020; Accepted 29 January 2020 Available online 5 February 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.
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Materials Science in Semiconductor Processing 110 (2020) 104974
would essentially increase the thermoelectric figure of merit of AlSb. Bergman et al showed by theoretical predictions that a composite synthesized using mixing and compacting two dissimilar materials can have an enhanced figure of merit [12]. Katsuyama et al later showed this effect in CoSb3–FeSb2 composite, enhancing the thermoelectric figure of merit by decreasing the thermal conductivity [13]. This decrease in the lattice thermal conductivity was attributed to the increased phonon scattering, which occurred due to the existence of FeSb2 within the matrix. Recently, Zou et al studied the mechanism in details between β-Zn4Sb3 and Cu3SbSe4 and proved adequately that the dispersion of nano to micro-sized secondary phase can enhance phonon scattering and increase the thermoelectric properties in the material system [14]. This work, based on the above hypothesis, intends to increase the ZT in AlSb by dispersion of small amount of β-Zn4Sb3, sometimes referred to as β-Zn13Sb10 [15], secondary phases. This could decrease the thermal conductivity without significantly hampering the Seebeck coefficient, in turn increasing the thermoelectric efficiency. Mechanical attrition milling has been used to mix powders before compaction of composites in the recent studies [16]. This is a very effective method to disperse the phases homogenously before hot pressing into pellets. Ur et al showed that β-Zn4Sb3 powders can be synthesized by sintering cold compacted elemental powders, using an excess of Zn [17]. The high vapor pressure of zinc can result in a Zn-deficiency, leaving some α-ZnSb phases to be found. AlSb has been previously synthesized as single phase by controlled melting method using Al and Sb shots and its thermoelectric properties have been re ported [6]. Milling the as-casted AlSb and as-sintered β-Zn4Sb3 may produce a homogenous composite. According to the Zn–Sb phase diagram, the phase β-Zn4Sb3 is stable up to 767 K, after which the phase changes to α-ZnSb [18]. Recent Al–Zn–Sb ternary phase diagram studies have proven this system to be rather complex, with the existence of a possible ternary eutectic region at 647 K [19]. However, the compositions being used in this study fall outside of the proposed region. The possible phases within the ternary phase diagram used in this study are AlSb, ZnSb and β-Zn4Sb3 [20]. Previous reports have shown that the thermoelectric performance in the β-Zn4Sb3 is the best among the other compositions [21]. Thus, fabrica tion and measurement of TE properties needed careful consideration of temperature limits.
Advance D-8, Germany) using Cu-Kα radiation. Scanning electron mi croscope (SEM; Quanta-400, Netherlands) was used for energy disper sive spectroscopy (EDS) and microstructural elucidation. Seebeck coefficient and electrical conductivity were measured using ZEM-III (ULVAC-RIKO, Japan) by the four-probe method from a 3 � 3 � 10 mm3 cuboidal sample. Thermal diffusivity, d, was measured by laser flash method integrated into TC-9000H (ULVAC-RIKO, Japan). Thermal conductivity was calculated using the formula κ ¼ d � Cp � ρ, where Cp is the specific heat capacity and ρ is the density measured by Archime dean principle. 4. Result and discussion Fig. 1 shows the x-ray diffraction data from XRD. In Fig. 1(a), it can be clearly seen that single phase β-Zn4Sb3 has been fabricated using cold compaction followed by sintering. AlSb phases are also seen as single phase. The major peak for β-Zn4Sb3 overlaps with the major peak for AlSb and makes it difficult to confirm the synthesis of composite by XRD. Similar phenomenon has been observed regarding secondary phases in previous reports [22–24]. Smaller amounts of β-Zn4Sb3 crystals within the AlSb matrix are randomly dispersed, due to which significant scat tering may not be visible within the relative intensity scale. However, the phases may be visible in the logarithmic scale, as shown in Fig. 1(b). Logarithmic scale reduces the gap between large and very small peaks, due to which β-Zn4Sb3 peaks are clearly identifiable [6]. It can also be noted that the peaks for α-ZnSb phases are not seen after hot pressing, as confirmed by Fig. 1(b). However, when the composite was synthesized using 30 wt % β-Zn4Sb3, the phase changed to α-ZnSb during hot press at 773 K. Fig. 1(c) shows the Rietveld measurement of phases in (AlSb)0.7(Zn4Sb3)0.3 composite. The measurements have revealed that 32.2 wt % ZnSb is present within the AlSb matrix. The microscopic structure of the pulverized and mixed powders is shown in Fig. 2(a), while the inner surface of hot pressed pellet is shown in Fig. 2(b). Powder samples are near round shaped. The particle size was confirmed to be within the range of 300 nm to 5 μm after pulveri zation. Backscattering mode of the SEM has revealed that the zinc an timonide phases (light grey) are homogeneously dispersed within AlSb matrix (dark grey). To confirm composite formation further, line map ping using EDS integrated into SEM was used to check the position of individual elements within the hot pressed sample. Fig. 3 confirms that AlSb is the primary matrix, while zinc is evenly distributed within the matrix. The thermopower (S), in μV/K, has been shown as a function of temperature in Fig. 4(a), while Fig. 4(b) shows the variation of electrical conductivity with temperature in the binary composite (AlSb)1x(Zn4Sb3)x. Previous reports have shown the thermoelectric properties in near single phase AlSb, and the literature value has been used as x ¼ 0 [6]. Adding 10 wt % Zn4Sb3 increases the Seebeck coefficient even further, up to 400 μV/K, while still showing a low electrical conduc tivity. However, as temperature increases above 470 K, effective carrier concentration in the conduction band increases, increasing the electrical conductivity and dramatically reducing the Seebeck coefficient to just over 100 μV/K. The Seebeck coefficient curve and electrical conduc tivity curve show reciprocal behavior to one another for x ¼ 0.1, which shows how carrier concentration changes as a function of temperature. Increasing fractions of Zn4Sb3 increases the carrier concentration in the composite. Both x ¼ 0.2 and 0.3 composites show increased absolute value of Seebeck coefficient and electrical conductivity than their former compound at temperatures higher than 470 K. For x ¼ 0.3, the Seebeck coefficient is close to that of intrinsic AlSb, but with an elec trical conductivity significantly higher. This change can be accounted for the higher net carrier concentration in the composite and the inherently high thermopower in AlSb. As shown in Fig. 5, the power factor, S2σ, keeps increasing with increasing Zn4Sb3, and finally reaches a peak of ~450 μV/mK2. Since the thermoelectric properties of Zn4Sb3 is considerably higher than that of AlSb, the trends observed so far have
2. Materials and methods Zinc powders (99.99%, Kojundo), Sb powders (99.9%, Kojundo), Al shots (99.9%, Aldrich) and Sb shots (99.999%, Kojundo) were used to separately synthesize AlSb and β-Zn4Sb3. Melting and sintering were both done in a vacuum furnace using a tapped graphite crucible. High energy vibratory mill (HEVM; KMTech TMM-70, Korea) was used to mix the compounds homogenously in zirconia vial with zirconia balls. Adequate care was taken to prevent oxidation as the chemicals were handled in a glove box under Ar-atmosphere. 3. Experimental Zn and Sb powders were weighed in a glove box and cold pressed using a stainless steel die of diameter 10 mm. The cold pressed pellet was put in a tapped graphite crucible and sealed in argon-atmosphere before sintering it at 673 K for 24 h. Al and Sb shots were separately weighed and melted in a tapped graphite crucible at 1273 K for 1 h. To prevent contamination of carbon, boron nitride was sprayed on the inner walls of the graphite crucibles. β-Zn4Sb3 and AlSb were mixed according to weight percent ratio and put in a zirconia vial in argon atmosphere. Zirconia balls were added at 10:1 ratio by weight and the powders were mixed for 10 min in the HEVM (KMTech TMM-70, Korea) used as an attrition mill. As-milled powders were sieved by a 325 mesh sieve and compacted in graphite die for 6 h at 773 K and 80 MPa pressure. Phase analysis was carried out by X-ray diffraction (XRD; BRUKER AXS 2
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Fig. 1. X-ray diffraction peaks for the casted AlSb pellet, sintered Zn4Sb3 powders, and hot pressed composite (a), while (b) shows the phases of (AlSb)0.8(Zn4Sb3)0.2 in the logarithmic scale. The amount of phases have been calculated in x ¼ 0.3 sample using Rietveld refinement, shown in (c).
Fig. 2. Scanning Electron Microscopic images from x ¼ 0.3 composite after (a) mixing and (b) hot pressing (Backscattering mode).
been well in accordance with what can be expected in the binary com posite. The thermoelectric power factor has risen considerably from x ¼ 0 to x ¼ 0.3. This increase can be attributed to the higher carrier con centration in Zn4Sb3 than in AlSb. The thermal conductivity data, as illustrated by Fig. 6(a), shows that with 10 wt % inclusion of secondary β-Zn4Sb3 phase, the thermal con ductivity drops immensely from 8.6 W/mK to 2.8 W/mK. Lattice ther mal conductivity is directly proportional to the phonon mean free path available for propagation of acoustic phonons [25]. The existence of a
secondary phase in a very small amount largely increases the phonon scattering, which decreases the phonon mean free path required for heat conduction through the crystal structure [26]. Thus, the lattice thermal conductivity, as shown in Fig. 6(b), decreases dramatically. This phe nomenon is also well compliant to literature. The total thermal conductivity in AlSb is quite large for thermo electric compounds. In contrast, β-Zn4Sb3, being categorized as a Zintl phase [27], has a very low thermal conductivity. According to the Eucken derivation of Maxwell equation, which was later confirmed for 3
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Fig. 3. EDS line images showing distribution of elemental atoms in hot pressed composite.
Fig. 4. Variation of (a) Seebeck coefficient and (b) electrical conductivity as a function of temperature and β-Zn4Sb3 weight fraction.
discrepancies [14]. One possibility might be that due to the difference in crystal size and shape between the two components, small amount of β-Zn4Sb3 act as defects within the matrix of AlSb [26], which reduces the phonon mean free path and increases acoustic phonon scattering. However, with an increase in β-Zn4Sb3 above 10 wt %, the interface boundaries might increase in coherency, possibly moving from completely incoherent to near coherent phase boundaries. Conse quently, the phonon mean free path may have increased again, in accordance with the percolation theory [25], and an increase in lattice thermal conductivity has been seen. There are very limited resources available in this regard to conclusively assert any hypothesis. More study regarding the phenomena can elucidate underlying principles better and help explain future results. The increase in thermal conduc tivity may also be a result of increasing relative density of the pellets. Table 1 shows the relative densities in the composite. The theoretical density has been calculated using a ratio of the theoretical densities of AlSb (4.26 g/cm3) and β-Zn4Sb3 (6.20 g/cm3). The relatively lower density even after 24 h of hot press for x ¼ 0.1 composite may be due the significant difference in crystal structures of AlSb and β-Zn4Sb3. The high thermal conductivity for x ¼ 0.3 composite may be because of the phase change from β-Zn4Sb3 to α-ZnSb, which has a higher lattice thermal conductivity than the former. The thermoelectric properties of the resultant composite elucidated the dimensionless figure of merit using Eq. (1). Fig. 7 shows the variation of ZT with temperature. Due to the considerable drop in thermal con ductivity, the ZT increased after the addition of 10 wt % β-Zn4Sb3 threefold. However, the peak ZT of 0.065 was achieved with x ¼ 0.3. This increase can be attributed to the escalated power factor due to effective tuning of carrier concentration. Although the increase in
Fig. 5. The relationship between Power factor (S2σ), fraction of Zn4Sb3 and temperature.
ceramics by Kingery et al [28], the thermal conductivity in the com posite is supposed to decrease with an increase in weight fraction of β-Zn4Sb3. However, from the experimentally determined results, it can be seen that the lowest thermal conductivity is seen with only 10 wt % β-Zn4Sb3, while with 30 wt % β-Zn4Sb3 the total thermal conductivity is close to the intrinsic thermal conductivity of AlSb. The reason for such deviation is unknown, but previous accounts have also shown such 4
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Fig. 6. The variation of (a) thermal conductivity and (b) lattice thermal conductivity as a function of temperature for different weight fractions of β-Zn4Sb3.
the tuning of carrier concentration, the power factor also enhanced greatly to provide a significant improvement in thermoelectric properties.
Table 1 Relative density of composite samples as a function of hot press duration and x from (AlSb)1-x(Zn4Sb3)x. Nominal Composition
Hot pressing time
Relative Density
x ¼ 0.1 x ¼ 0.1 x ¼ 0.2 x ¼ 0.3
6h 24 h 6h 6h
70.7% 71.2% 76.2% 88.8%
Declaration of competing interest The authors declare no conflict of interest. CRediT authorship contribution statement A.K.M. Ashiquzzaman Shawon: Conceptualization, Data curation, Formal analysis, Investigation, Software, Visualization, Writing - orig inal draft. Il-Ho Kim: Conceptualization, Funding acquisition, Valida tion, Resources, Writing - review & editing. Soon-Chul Ur: Conceptualization, Funding acquisition, Methodology, Project admin istration, Supervision, Validation, Resources, Writing - review & editing. Acknowledgement This research was supported by the Korea Basic Science Institute grant funded by the Ministry of Education (grant no. 2019R1A6C1010047). References [1] H.S. Kim, W. Liu, G. Chen, C.-W. Chu, Z. Ren, Relationship between thermoelectric figure of merit and energy conversion efficiency, Proc. Natl. Acad. Sci. Unit. States Am. 112 (2015) 8205, https://doi.org/10.1073/pnas.1510231112. [2] Z.G. Chen, G. Hana, L. Yanga, L. Cheng, J. Zou, Nanostructured thermoelectric materials: current research and future challenge, Prog. Nat. Sci. Mater. Int. 22 (2012) 535–549, https://doi.org/10.1016/j.pnsc.2012.11.011. [3] A. Jaoui, B. Fauqu� e, C.W. Rischau, A. Subedi, C. Fu, J. Gooth, N. Kumar, V. Süß, D. L. Maslov, C. Felser, K. Behnia, Departure from the Wiedemann–Franz law in WP2 driven by mismatch in T-square resistivity prefactors, Npj Quantum Mater 3 (2018) 64, https://doi.org/10.1038/s41535-018-0136-x. [4] R.H. Athab, B.H. Hussein, S.A. Makki, Effect of in on the Properties of AlSb Thin Film Solar Cell, 2019, 020030, https://doi.org/10.1063/1.5116957. [5] D. Strauch, AlSb: crystal structures, phase transitions, transition pressure, equation of state, in: U. R€ ossler (Ed.), New Data Updat. IV-IV, III-V, II-VI I-VII Compd. Their Mix. Cryst. Diluted Magn. Semicond, Springer Berlin Heidelberg, Berlin, Heidelberg, 2011, pp. 134–135, https://doi.org/10.1007/978-3-642-14148-5_78. [6] A.K.M.A. Shawon, S.-C. Ur, Mechanical and thermoelectric properties of bulk AlSb synthesized by controlled melting, pulverizing and subsequent vacuum hot pressing, Appl. Sci. 9 (2019), https://doi.org/10.3390/app9081609. [7] T. Caillat, J.-P. Fleurial, A. Borshchevsky, Preparation and thermoelectric properties of semiconducting Zn4Sb3, J. Phys. Chem. Solid. 58 (1997) 1119–1125, https://doi.org/10.1016/S0022-3697(96)00228-4. [8] Y. Cheng, J. Yang, Q. Jiang, D. He, J. He, L. Yubo, D. Zhang, Z. Zhou, Y. Ren, X. Jw, New insight to InSb-based thermoelectric materials: from the divorced eutectic design to remarkable high thermoelectric performance. https://doi.org/10.1039/C 6TA10827J, 2017.
Fig. 7. Thermoelectric figure of merit of (AlSb)1-x(Zn4Sb3)x up-to 773 K
thermoelectric efficiency is considerably large, a decrease in the com posite thermal conductivity for x ¼ 0.3 would have increased the ZT even further. While this can be seen as a low thermopower for effective applications in real devices, there is still further scope for improvement by optimizing phase interfaces and use of appropriate dopants. 5. Conclusion AlSb, a recently reported thermoelectric material, has a low intrinsic thermoelectric property despite its high thermopower. In order to in crease the ZT, a binary composite of (AlSb)1-x(Zn4Sb3)x was successfully synthesized. Controlled melting and sintering of cold compacts were used to synthesize the individual semiconductors, which were then mixed thoroughly and hot pressed to give final composite. The com posite shows considerably low lattice thermal conductivity, possibly due to effective phonon scattering and a decreased mean free path. Due to 5
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