- Email: [email protected]
Bulk Mg2Si Based n-type Thermoelectric Material Produced by Gas Atomization and Hot Extrusion
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 2 (2015) 523 – 531
12th European Conference on Thermoelectrics
Bulk Mg2Si based n-type thermoelectric material produced by gas atomization and hot extrusion D. Vasilevskiya, M.K. Keshavarza*, J. Dufourcqb, H. Ihou-Moukob, C. Navonnec, R.A. Masuta, S. Turennea a
Polytechnique Montréal, C.P. 6079, Succ. Centre-Ville, Montréal, H3C 3A7, Canada b
HotBlock Onboard, 7 parvis Louis Néel, 38000 Grenoble, France
c
Commissariat à l’Energie Atomique et aux Energies Alternatives DRT/LITEN/DTNM/SERE/LTE 17, rue des Martyrs 38054 Grenoble Cedex 9, France
Abstract Bulk thermoelectric (TE) materials for large scale energy harvesting applications need to demonstrate not only high TE performance, but should also be composed from elements which are nontoxic, largely available in nature, and preferably be as light as possible for applications such as automobiles. Magnesium silicide based materials possess a combination of these properties and are among the best candidates for these applications. For the successful implementation of Mg2Si based alloys, all material manufacturing steps have to be compatible with the requirements of mass production. We report two processes to manufacture these alloys which can be easily scaled up: (i) a gas atomization method for powder production, followed by (ii) hot extrusion to obtain bulk specimens. The Mg2Si1-xSnx (0.3 ≤ x≤ 0.7) powders were prepared by gas atomization using a stoichiometric mixture of the commercially available high purity elements. Particle size distribution analysis shows that 90% of particles thus obtained have sizes ranging from 10 to 100 μm with a mean value of 46.5 μm. Hot extrusion was carried out for Mg2Si0.4Sn0.6 composition at 450-480qC and 500-550qC applying pressures up to 1 GPa. Addition of 4 vol% of nanometre sized MoS2 particles in the powder facilitates material densification. Specimens extruded at temperature range of 500-550qC demonstrate 99.7% of the theoretical density. The TE properties were studied by the Harman method between 300 K and 700 K.
* Corresponding author. Fax: +1 (514) 340-4468. E-mail address: [email protected]
2214-7853 © 2015 Published by Elsevier Ltd. Selection and peer-review under responsibility of Conference Committee members of the 12th European Conference on Thermoelectrics. doi:10.1016/j.matpr.2015.05.072
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D. Vasilevskiy et al. / Materials Today: Proceedings 2 (2015) 523 – 531
The Seebeck coefficient varies from -150 to -220 μV/K, passing through a maximum value at 450 K. The electrical conductivity of 88 S/cm at 300 K increases with temperature showing a typical behavior for under-doped TE material. The ZT value reaches a maximum of 0.08 at 620 K. Our work shows that implementation of a gas atomization process for powder production combined with hot extrusion, as a densification and sintering method, can be successfully implemented for bulk Mg2Si1-xSnx manufacturing. This approach to obtain Mg2Si based alloys, once optimized, has a great potential for large scale waste heat recovery applications. © 2014 Elsevier Ltd. All rights reserved. © 2015 Published by Elsevierunder Ltd. responsibility of Conference Committee members of the 12th European Conference on Selection and peer-review Selection and peer-review under responsibility of Conference Committee members of the 12th European Conference on Thermoelectrics. Thermoelectrics. Keywords: Thermoelectric, Magnesium Silicide, Gas atomization, Hot Extrusion,Nano-particle, MoS2.
1. Introduction Harvesting waste heat energy, such as in automobiles, by using a thermoelectric (TE) generator that directly converts heat to electricity has gained increasing attention during the last decade [1-4]. Currently, the TE materials most commonly used in commercial applications are (i) bismuth telluride based alloys for near room temperature applications and (ii) lead telluride for medium range temperature applications (up to 800 K) [5, 6]. However, besides the need of relatively high performance for TE materials, other considerations such as lower cost and weight, abundance, and nontoxicity are also important. These advantages make magnesium silicide based TE materials far more favourable candidates [7]. Although the very first efforts to synthesize magnesium silicide based TE materials goes back to the 1960’s [8], until the first decade of the 21st century these materials had not attracted much attention [9-11]. Magnesium silicide TE materials have been mostly produced from the melt or by mechanical alloying approaches [10, 12, 13]. In the powder metallurgy approach, sintering and consolidation techniques such as hot-press, pulse-current sintering, or arc plasma sintering have been reported [9, 11, 14]. However, it is evident that more studies on approaches that may lead to mass production are needed for commercialization. In this work we present a study of a novel synthesis of magnesium silicide based TE alloy which is produced by means of gas atomization and hot extrusion. Gas atomization has been used to synthesize some other TE materials such as bismuth telluride, bismuth selenide, and Si-Ge alloy [14-17]. Hot extrusion is a known approach to sinter TE materials such as lead telluride [6], bismuth telluride based alloys and composites with anisotropic properties when texture becomes essential [18-20]. This technique, which is advantageous for mass production of TE materials, has now been demonstrated in Mg2Si based alloys. 2. Experimental Procedure Mg2Si1-xSnx (0.3≤x≤0.7) powders were supplied by CEA (French alternative Energies and Atomic Energy Commission) where they used gas atomization method to prepare the powders. Hot extrusion was implemented to consolidate one of the powders with nominal composition of Mg2Si0.4Sn0.6 sintering the powders to bulk rods. The extrusion process was carried out under an argon atmosphere in temperature ranges from 450 to 480⁰C, and 500 to 550 ⁰C when pressure was progressively increased up to 1GPa during the extrusion process. Based on our finding during former studies (See Ref. [19, 21]), 4 vol.% of MoS2 nanoparticles were added to the powders in order to facilitate the material densification during the hot extrusion process. The extruded samples were cylindrical rods with 11 mm in diameter. The phase identification of the powder and extruded samples was completed by an X-ray diffractometer (XRD, Philips X’Pert, Using Cu Kα radiation). A JEOL JSM-7600 TFE high resolution scanning electron microscope (SEM) was used to assess the morphology of powders and the microstructure of extruded samples. Particle size distribution analysis was carried out using a Beckman Coulter LS 200 instrument where the measurement parameters were based on ASTM B822-10.
D. Vasilevskiy et al. / Materials Today: Proceedings 2 (2015) 523 – 531
Thermoelectric properties such as thermal conductivity λ, electrical conductivity σ, Seebeck coefficient α, and figure of merit Z= σα2/λ,of extruded samples were investigated in the temperature range from 300 to 725 K using a ZT-Scanner 2.3 (TEMTE Inc.), which is based on a Harman setup. 3. Results and Discussion The SEM image (Figure 1) of the produced powders shows a large majority of particles whose shape is almost spherical. It also illustrates a wide particle size range from a few microns to few tens of microns. The inset shows that there are submicron satellite particles agglomerated on the surface of larger particles. The cumulative and differential volume-based particle size distribution of those Mg2Si1-xSnx powders is illustrated in Figure 2. It confirms that particle diameters are mostly between 10 to 100 microns with a mean value of 46.5 μm. The maximum volume fraction of 5.8% is obtained by particles in the range between 59-62 μm diameters. These powders were stored under protective argon atmosphere and have been used for the extrusion process without sieving or any other phase separation procedure.
Fig.1. SEM images of the Mg2Si1-xSnx (0.3≤x≤0.7) powders obtained by gas atomization. The inset shows submicron satellite particles attached to the surface of larger particles.
Fig.2. Volume-based particle size distribution of the Mg2Si1-xSnx (0.3≤x≤0.7) powders. Left vertical axis indicates the volume percentage of differential particle size distribution and right vertical axis shows volume percentage of cumulative particle size distribution.
Figure 3 (a) demonstrates the very first hot extruded rod of magnesium silicide based composition and the micrograph of its fracture surface. The density of this specimen which was processed in the 450-480qC temperature range is below 80% of the theoretical density. The fractured surface can be described as compacted particles of the initial powder. On the other hand, for the specimen extruded within the 500-550qC
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range, the SEM image of the fractured surface (Figure 3 (b)) shows advanced recrystallization with 99.7% of the crystalline density. Compounds of Mg2B when B is an element of group IV, typically, crystallize in the cubic system. The lattice constants of Mg2Si and Mg2Sn compounds are 6.33 and 6.76 Å, respectively [22]. According to the equilibrium phase diagram of Mg-Si-Sn system, there is an immiscibility gap for Mg2Si1-xSnx when x is in the range of 0.4 to 0.6 [23]. Consequently there is coexistence of Si- and Sn-rich phases in this composition range. The XRD patterns of our samples, illustrated in Figure 4, indicate that the main phase in the powder and bulk samples is an Sn-rich phase. However, as it is shown in panel (b) of Figure 4, several intermetallic compounds of magnesium silicide and magnesium tin, whose crystal structures differ from that of the known structure for these compounds, are also present in the powder and extruded samples. The synthesis parameters, specially temperature and pressure, may cause a phase transition in Mg2Si and Mg2Sn compounds leading to intermetallic compounds with crystal structures other than cubic [24-26]. The XRD results show that the extruded material is more homogeneous than its powder. This is due to the recrystallization process during the extrusion. Although the XRD results indicate that powder composition is not yet optimized, the phase analysis of the extruded sample shows that the hot extrusion process partially eliminates the undesirable phases making the material more homogeneous. The electrical resistivity and thermal conductivity of the extruded samples as a function of temperature are shown in Figure 5. The measurements indicated with green data points in all graphs were made on a sample with contacts of PbSn soldering alloy, which limits measurements up to a temperature near 450⁰C due to the low melting point of the contacts. Using silver paste as contacts we could measure all thermoelectric properties up to 750⁰C for the rest of the samples. The electrical resistivity depends on the temperature of the extrusion process, but is generally high surpassing 1000 μΩm at 20⁰C for the sample extruded at 480 ⁰C (blue data in Figure 5(a)). For the samples with a recrystallized microstructure and higher density (green and red) the electrical resistivity is of the order of 100 μΩm at room temperature. In all cases the resistivity decreases with temperature showing a typical behavior for an under-doped TE material. The observed difference in electrical resistivity and thermal conductivity is related to the difference in density of the samples without and with recrystallization of initial particles.
D. Vasilevskiy et al. / Materials Today: Proceedings 2 (2015) 523 – 531
Fig.3. (a) Right: rod of Mg2Si0.4Sn0.6 extruded with temperatures below 480 ⁰C, Left: fractured surface of this sample revealing particles of the initial powder, (b) SEM image of the fractured surface of Mg2Si0.4Sn0.6 extruded at a temperature higher than 500⁰C.
Figure 6 illustrates the variation with temperature of the Seebeck coefficient and ZT values of the extruded samples. The Seebeck coefficient varies from -150 to -220 μV/K, passing through a maximum value at 450 K. Since the powders used for this study have not yet been optimized the ZT values reach a maximum of only 0.08 at 620 K.
4. Conclusion Despite the low values obtained for ZT, our work shows that the implementation of a gas atomization process for powder production combined with hot extrusion, as a densification and sintering method, can be successfully implemented for bulk Mg2Si1-xSnx manufacturing. The electrical conductivity of these materials need to be improved thus doping level is due to be optimized. This approach to obtain Mg2Si based alloys, once optimized, has a promising potential for large scale waste heat recovery applications particularly for the automobile industry. This human oriented application requires materials without potentially harmful components.
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Fig.4 XRD patterns of powder and bulk (extruded) samples (a) compared to the reference patterns of Mg2Si0.4Sn0.6, Mg2Si and Mg2Sn with (indicated by “C”) cubic crystal structure.(b) compared to Mg2Si with hexagonal (indicated by “H”) crystal structure and Mg2Sn with orthorhombic (indicated by “O”) crystal structure, in this panel 2θ form 20 to 50 degrees is presented to magnify different peaks in this region.
D. Vasilevskiy et al. / Materials Today: Proceedings 2 (2015) 523 – 531
Fig.5. Variation of (a) electrical resistivity and (b) thermal conductivity with temperature for samples extruded at different temperatures.
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Fig.6. (a) Seebeck coefficient and (b) dimensionless figure of merit versus temperature for samples extruded at different temperatures.
References [1] G. Chen, M.S. Dresselhaus, G. Dresselhaus, J.P. Fleurial, T. Caillat, Int Mater Rev, 48 (2003) 45-66. [2] M.S. Dresselhaus, G. Chen, M.Y. Tang, R.G. Yang, H. Lee, D.Z. Wang, Z.F. Ren, J.P. Fleurial, P. Gogna, Adv Mater, 19 (2007) 1043-1053. [3] G.J. Snyder, E.S. Toberer, Nat Mater, 7 (2008) 105-114. [4] J.H. Yang, F.R. Stabler, J Electron Mater, 38 (2009) 1245-1251. [5] H.J. Goldsmid, Materials, 7 (2014) 2577-2592. [6] D. Vasilevskiy, R.A. Masut, S. Turenne, J Electron Mater, 41 (2012) 1057-1061. [7] M.B.A. Bashir, S.M. Said, M.F.M. Sabri, D.A. Shnawah, M.H. Elsheikh, Renew Sust Energ Rev, 37 (2014) 569584.
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[8] E.N. Nikitin, V.G. Bazanov, V.I. Tarasov, Soviet Physics Solid State, 3 (1961) 2648-7846. [9] M. Umemoto, Y. Shirai, K. Tsuchiya, in: S. Hanada, Z. Zhong, S.W. Nam, R.N. Wright (Eds.) Pricm 4: Forth Pacific Rim International Conference on Advanced Materials and Processing, Vols I and Ii, Japan Inst Metals, Sendai, 2001, pp. 2145-2148. [10] A. Matsumoto, K. Kobayashi, K. Ozaki, T. Nishio, in: S. Hanada, Z. Zhong, S.W. Nam, R.N. Wright (Eds.) Pricm 4: Forth Pacific Rim International Conference on Advanced Materials and Processing, Vols I and Ii, Japan Inst Metals, Sendai, 2001, pp. 2177-2180. [11] T. Kajikawa, T. Sugiyama, M. Serizawa, K. Kamio, M. Koike, T. Ohta, M. Omori, S. China Industrial Association Of Power, in: Twentieth International Conference on Thermoelectrics, Proceedings, Ieee, New York, 2001, pp. 236-239. [12] D. Tamura, R. Nagai, K. Sugimoto, H. Udono, I. Kikuma, H. Tajima, I.J. Ohsugi, Thin Solid Films, 515 (2007) 8272-8276. [13] S.W. You, I.H. Kim, S.M. Choi, W.S. Seo, J Nanomater, (2013). [14] M. Otake, K. Sato, O. Sugiyama, S. Kaneko, Solid State Ionics, 172 (2004) 523-526. [15] H.-S. Kim, S.-J. Hong, J Alloy Compd, 586, Supplement 1 (2014) S428-S431. [16] S.J. Hong, S.H. Lee, B.S. Chun, Mat Sci Eng B-Solid, 98 (2003) 232-238. [17] M.H. Bhuiyan, T.-S. Kim, J.M. Koo, S.-J. Hong, J Alloy Compd, 509 (2011) 1722-1728. [18] M.K. Keshavarz, D. Vasilevskiy, R.A. Masut, S. Turenne, J Electron Mater, 42 (2013) 1429-1435. [19] M.K. Keshavarz, D. Vasilevskiy, R.A. Masut, S. Turenne, Materials Characterization, 95 (2014) 44-49. [20] C. Andre, D. Vasilevskiy, S. Turenne, R.A. Masut, J Electron Mater, 38 (2009) 1061-1067. [21] M.K. Keshavarz, D. Vasilevskiy, R.A. Masut, S. Turenne, J Electron Mater, 43 (2014) 2239-2246. [22] V.K. Zaitsev, Thermoelectrics on the Base of Solid Solutions of Mg2BIV Compounds (BIV = Si, Ge, Sn), in: Thermoelectrics Handbook, CRC Press, 2005, pp. 29-21-29-12. [23] S. Wang, N. Mingo, Appl Phys Lett, 94 (2009). [24] K.J. Range, G.H. Grosch, M. Andratschke, J Alloy Compd, 244 (1996) 170-174. [25] P. Cannon, E.T. Conlin, Science, 145 (1964) 487-488. [26] T.I. Dyuzheva, S.S. Kabalkina, L.F. Vereshchagin, Soviet Physics Doklady, 21 (1976) 342-344.
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ScienceDirect Materials Today: Proceedings 2 (2015) 523 – 531
12th European Conference on Thermoelectrics
Bulk Mg2Si based n-type thermoelectric material produced by gas atomization and hot extrusion D. Vasilevskiya, M.K. Keshavarza*, J. Dufourcqb, H. Ihou-Moukob, C. Navonnec, R.A. Masuta, S. Turennea a
Polytechnique Montréal, C.P. 6079, Succ. Centre-Ville, Montréal, H3C 3A7, Canada b
HotBlock Onboard, 7 parvis Louis Néel, 38000 Grenoble, France
c
Commissariat à l’Energie Atomique et aux Energies Alternatives DRT/LITEN/DTNM/SERE/LTE 17, rue des Martyrs 38054 Grenoble Cedex 9, France
Abstract Bulk thermoelectric (TE) materials for large scale energy harvesting applications need to demonstrate not only high TE performance, but should also be composed from elements which are nontoxic, largely available in nature, and preferably be as light as possible for applications such as automobiles. Magnesium silicide based materials possess a combination of these properties and are among the best candidates for these applications. For the successful implementation of Mg2Si based alloys, all material manufacturing steps have to be compatible with the requirements of mass production. We report two processes to manufacture these alloys which can be easily scaled up: (i) a gas atomization method for powder production, followed by (ii) hot extrusion to obtain bulk specimens. The Mg2Si1-xSnx (0.3 ≤ x≤ 0.7) powders were prepared by gas atomization using a stoichiometric mixture of the commercially available high purity elements. Particle size distribution analysis shows that 90% of particles thus obtained have sizes ranging from 10 to 100 μm with a mean value of 46.5 μm. Hot extrusion was carried out for Mg2Si0.4Sn0.6 composition at 450-480qC and 500-550qC applying pressures up to 1 GPa. Addition of 4 vol% of nanometre sized MoS2 particles in the powder facilitates material densification. Specimens extruded at temperature range of 500-550qC demonstrate 99.7% of the theoretical density. The TE properties were studied by the Harman method between 300 K and 700 K.
* Corresponding author. Fax: +1 (514) 340-4468. E-mail address: [email protected]
2214-7853 © 2015 Published by Elsevier Ltd. Selection and peer-review under responsibility of Conference Committee members of the 12th European Conference on Thermoelectrics. doi:10.1016/j.matpr.2015.05.072
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D. Vasilevskiy et al. / Materials Today: Proceedings 2 (2015) 523 – 531
The Seebeck coefficient varies from -150 to -220 μV/K, passing through a maximum value at 450 K. The electrical conductivity of 88 S/cm at 300 K increases with temperature showing a typical behavior for under-doped TE material. The ZT value reaches a maximum of 0.08 at 620 K. Our work shows that implementation of a gas atomization process for powder production combined with hot extrusion, as a densification and sintering method, can be successfully implemented for bulk Mg2Si1-xSnx manufacturing. This approach to obtain Mg2Si based alloys, once optimized, has a great potential for large scale waste heat recovery applications. © 2014 Elsevier Ltd. All rights reserved. © 2015 Published by Elsevierunder Ltd. responsibility of Conference Committee members of the 12th European Conference on Selection and peer-review Selection and peer-review under responsibility of Conference Committee members of the 12th European Conference on Thermoelectrics. Thermoelectrics. Keywords: Thermoelectric, Magnesium Silicide, Gas atomization, Hot Extrusion,Nano-particle, MoS2.
1. Introduction Harvesting waste heat energy, such as in automobiles, by using a thermoelectric (TE) generator that directly converts heat to electricity has gained increasing attention during the last decade [1-4]. Currently, the TE materials most commonly used in commercial applications are (i) bismuth telluride based alloys for near room temperature applications and (ii) lead telluride for medium range temperature applications (up to 800 K) [5, 6]. However, besides the need of relatively high performance for TE materials, other considerations such as lower cost and weight, abundance, and nontoxicity are also important. These advantages make magnesium silicide based TE materials far more favourable candidates [7]. Although the very first efforts to synthesize magnesium silicide based TE materials goes back to the 1960’s [8], until the first decade of the 21st century these materials had not attracted much attention [9-11]. Magnesium silicide TE materials have been mostly produced from the melt or by mechanical alloying approaches [10, 12, 13]. In the powder metallurgy approach, sintering and consolidation techniques such as hot-press, pulse-current sintering, or arc plasma sintering have been reported [9, 11, 14]. However, it is evident that more studies on approaches that may lead to mass production are needed for commercialization. In this work we present a study of a novel synthesis of magnesium silicide based TE alloy which is produced by means of gas atomization and hot extrusion. Gas atomization has been used to synthesize some other TE materials such as bismuth telluride, bismuth selenide, and Si-Ge alloy [14-17]. Hot extrusion is a known approach to sinter TE materials such as lead telluride [6], bismuth telluride based alloys and composites with anisotropic properties when texture becomes essential [18-20]. This technique, which is advantageous for mass production of TE materials, has now been demonstrated in Mg2Si based alloys. 2. Experimental Procedure Mg2Si1-xSnx (0.3≤x≤0.7) powders were supplied by CEA (French alternative Energies and Atomic Energy Commission) where they used gas atomization method to prepare the powders. Hot extrusion was implemented to consolidate one of the powders with nominal composition of Mg2Si0.4Sn0.6 sintering the powders to bulk rods. The extrusion process was carried out under an argon atmosphere in temperature ranges from 450 to 480⁰C, and 500 to 550 ⁰C when pressure was progressively increased up to 1GPa during the extrusion process. Based on our finding during former studies (See Ref. [19, 21]), 4 vol.% of MoS2 nanoparticles were added to the powders in order to facilitate the material densification during the hot extrusion process. The extruded samples were cylindrical rods with 11 mm in diameter. The phase identification of the powder and extruded samples was completed by an X-ray diffractometer (XRD, Philips X’Pert, Using Cu Kα radiation). A JEOL JSM-7600 TFE high resolution scanning electron microscope (SEM) was used to assess the morphology of powders and the microstructure of extruded samples. Particle size distribution analysis was carried out using a Beckman Coulter LS 200 instrument where the measurement parameters were based on ASTM B822-10.
D. Vasilevskiy et al. / Materials Today: Proceedings 2 (2015) 523 – 531
Thermoelectric properties such as thermal conductivity λ, electrical conductivity σ, Seebeck coefficient α, and figure of merit Z= σα2/λ,of extruded samples were investigated in the temperature range from 300 to 725 K using a ZT-Scanner 2.3 (TEMTE Inc.), which is based on a Harman setup. 3. Results and Discussion The SEM image (Figure 1) of the produced powders shows a large majority of particles whose shape is almost spherical. It also illustrates a wide particle size range from a few microns to few tens of microns. The inset shows that there are submicron satellite particles agglomerated on the surface of larger particles. The cumulative and differential volume-based particle size distribution of those Mg2Si1-xSnx powders is illustrated in Figure 2. It confirms that particle diameters are mostly between 10 to 100 microns with a mean value of 46.5 μm. The maximum volume fraction of 5.8% is obtained by particles in the range between 59-62 μm diameters. These powders were stored under protective argon atmosphere and have been used for the extrusion process without sieving or any other phase separation procedure.
Fig.1. SEM images of the Mg2Si1-xSnx (0.3≤x≤0.7) powders obtained by gas atomization. The inset shows submicron satellite particles attached to the surface of larger particles.
Fig.2. Volume-based particle size distribution of the Mg2Si1-xSnx (0.3≤x≤0.7) powders. Left vertical axis indicates the volume percentage of differential particle size distribution and right vertical axis shows volume percentage of cumulative particle size distribution.
Figure 3 (a) demonstrates the very first hot extruded rod of magnesium silicide based composition and the micrograph of its fracture surface. The density of this specimen which was processed in the 450-480qC temperature range is below 80% of the theoretical density. The fractured surface can be described as compacted particles of the initial powder. On the other hand, for the specimen extruded within the 500-550qC
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range, the SEM image of the fractured surface (Figure 3 (b)) shows advanced recrystallization with 99.7% of the crystalline density. Compounds of Mg2B when B is an element of group IV, typically, crystallize in the cubic system. The lattice constants of Mg2Si and Mg2Sn compounds are 6.33 and 6.76 Å, respectively [22]. According to the equilibrium phase diagram of Mg-Si-Sn system, there is an immiscibility gap for Mg2Si1-xSnx when x is in the range of 0.4 to 0.6 [23]. Consequently there is coexistence of Si- and Sn-rich phases in this composition range. The XRD patterns of our samples, illustrated in Figure 4, indicate that the main phase in the powder and bulk samples is an Sn-rich phase. However, as it is shown in panel (b) of Figure 4, several intermetallic compounds of magnesium silicide and magnesium tin, whose crystal structures differ from that of the known structure for these compounds, are also present in the powder and extruded samples. The synthesis parameters, specially temperature and pressure, may cause a phase transition in Mg2Si and Mg2Sn compounds leading to intermetallic compounds with crystal structures other than cubic [24-26]. The XRD results show that the extruded material is more homogeneous than its powder. This is due to the recrystallization process during the extrusion. Although the XRD results indicate that powder composition is not yet optimized, the phase analysis of the extruded sample shows that the hot extrusion process partially eliminates the undesirable phases making the material more homogeneous. The electrical resistivity and thermal conductivity of the extruded samples as a function of temperature are shown in Figure 5. The measurements indicated with green data points in all graphs were made on a sample with contacts of PbSn soldering alloy, which limits measurements up to a temperature near 450⁰C due to the low melting point of the contacts. Using silver paste as contacts we could measure all thermoelectric properties up to 750⁰C for the rest of the samples. The electrical resistivity depends on the temperature of the extrusion process, but is generally high surpassing 1000 μΩm at 20⁰C for the sample extruded at 480 ⁰C (blue data in Figure 5(a)). For the samples with a recrystallized microstructure and higher density (green and red) the electrical resistivity is of the order of 100 μΩm at room temperature. In all cases the resistivity decreases with temperature showing a typical behavior for an under-doped TE material. The observed difference in electrical resistivity and thermal conductivity is related to the difference in density of the samples without and with recrystallization of initial particles.
D. Vasilevskiy et al. / Materials Today: Proceedings 2 (2015) 523 – 531
Fig.3. (a) Right: rod of Mg2Si0.4Sn0.6 extruded with temperatures below 480 ⁰C, Left: fractured surface of this sample revealing particles of the initial powder, (b) SEM image of the fractured surface of Mg2Si0.4Sn0.6 extruded at a temperature higher than 500⁰C.
Figure 6 illustrates the variation with temperature of the Seebeck coefficient and ZT values of the extruded samples. The Seebeck coefficient varies from -150 to -220 μV/K, passing through a maximum value at 450 K. Since the powders used for this study have not yet been optimized the ZT values reach a maximum of only 0.08 at 620 K.
4. Conclusion Despite the low values obtained for ZT, our work shows that the implementation of a gas atomization process for powder production combined with hot extrusion, as a densification and sintering method, can be successfully implemented for bulk Mg2Si1-xSnx manufacturing. The electrical conductivity of these materials need to be improved thus doping level is due to be optimized. This approach to obtain Mg2Si based alloys, once optimized, has a promising potential for large scale waste heat recovery applications particularly for the automobile industry. This human oriented application requires materials without potentially harmful components.
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Fig.4 XRD patterns of powder and bulk (extruded) samples (a) compared to the reference patterns of Mg2Si0.4Sn0.6, Mg2Si and Mg2Sn with (indicated by “C”) cubic crystal structure.(b) compared to Mg2Si with hexagonal (indicated by “H”) crystal structure and Mg2Sn with orthorhombic (indicated by “O”) crystal structure, in this panel 2θ form 20 to 50 degrees is presented to magnify different peaks in this region.
D. Vasilevskiy et al. / Materials Today: Proceedings 2 (2015) 523 – 531
Fig.5. Variation of (a) electrical resistivity and (b) thermal conductivity with temperature for samples extruded at different temperatures.
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Fig.6. (a) Seebeck coefficient and (b) dimensionless figure of merit versus temperature for samples extruded at different temperatures.
References [1] G. Chen, M.S. Dresselhaus, G. Dresselhaus, J.P. Fleurial, T. Caillat, Int Mater Rev, 48 (2003) 45-66. [2] M.S. Dresselhaus, G. Chen, M.Y. Tang, R.G. Yang, H. Lee, D.Z. Wang, Z.F. Ren, J.P. Fleurial, P. Gogna, Adv Mater, 19 (2007) 1043-1053. [3] G.J. Snyder, E.S. Toberer, Nat Mater, 7 (2008) 105-114. [4] J.H. Yang, F.R. Stabler, J Electron Mater, 38 (2009) 1245-1251. [5] H.J. Goldsmid, Materials, 7 (2014) 2577-2592. [6] D. Vasilevskiy, R.A. Masut, S. Turenne, J Electron Mater, 41 (2012) 1057-1061. [7] M.B.A. Bashir, S.M. Said, M.F.M. Sabri, D.A. Shnawah, M.H. Elsheikh, Renew Sust Energ Rev, 37 (2014) 569584.
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