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Production of iron-disilicide thermoelectric devices and thermoelectric module by the slip casting method
Materials Science and Engineering A307 (2001) 129– 133 www.elsevier.com/locate/msea
Production of iron-disilicide thermoelectric devices and thermoelectric module by the slip casting method Kiyoshi Nogi a, Takuji Kita b,*, Xiang-Qun Yan c,1 b
a Joining and Welding Research Institute, Osaka Uni6ersity, 11 -1 Mihogaoka, Ibaraki, Osaka 567 -0047, Japan Graduate student, Department of Materials Science and Processing, Faculty of Engineering, Osaka Uni6ersity, Osaka, Japan c Graduate student of Osaka Uni6ersity, Osaka, Japan
Received 18 October 2000
Abstract FeSi2 thermoelectric devices and thermoelectric modules were produced by the slip casting method, and their thermoelectric properties were evaluated. The starting materials were Fe0.91Mn0.09Si2 (p-type) and Fe0.98Co0.02Si2 (n-type). The powder concentration was 77 mass%, and the hydrogen exponents of p- and n-type were 8.5 and 7, respectively. The p- and n-type slips were simultaneously poured into a mold. The green body was demolded in approximately 10 min, then dried at 353 K for about 3 h in air. It was then sintered at 1443 K for 3 h, and annealed at 1123 K for 14 h. Both the sintering and annealing were performed in a vacuum of 10 − 2 Pa. The p–n junctions were formed within the range of 10 mm from the edge of the hot junction. The electrode was formed by silver brazing. Reliable electrodes can be obtained by adding m-phase powder or nickel powder. They formed ohmic contacts with b-FeSi2. The open circuit voltage, the internal resistance and the maximum power output of the thermoelectric module consisting of 20 devices were 5.25 V, 15.2 V and 0.45 W, respectively, for the maximum temperature difference ( =572 K). © 2001 Elsevier Science B.V. All rights reserved. Keywords: Thermoelectric device; Thermoelectric module; FeSi2; Slip casting; Ohmic contact; Stress-relaxation layer
1. Introduction The b-FeSi2 thermoelectric material has not been yet in the actual use, although it is one of the most attractive materials. The main conventional production methods of b-FeSi2 are: (1) press forming and pressureless sintering; and (2) hot-pressing. The slip casting method is seldom applied to the iron-disilicide system [1], though it has been applied to the CrSi2 and CoSi systems, which are also known as thermoelectric materials [2,3]. Uniaxial pressing requires expensive pressing equipment. Because a thermoelectric device requires a p– n junction, the powders granulated to achieve good fluidity should be avoided from mixing with each other during pouring into the die. * Corresponding author. Tel.: + 81-6-68798663; fax: +81-668798653. E-mail address: [email protected] (T. Kita). 1 Present address: Hosokawa Micron Co., 1-9 Shoudai, Tajika, Hirakata, Osaka 573-1132, Japan.
The authors reported the production of b-FeSi2 by the slip casting method [4,5]. The density and thermoelectric properties of the specimens formed by slip casting are almost the same as those of the specimens formed by cold isostatic pressing (CIP). In view of the complicated process and high cost of the equipment for CIP, it is concluded that slip casting is superior to CIP in productivity. The purpose of this paper is to produce thermoelectric devices and thermoelectric modules by the slip casting method, and to evaluate the thermoelectric properties of the produced modules.
2. Experimental procedure The chemical compositions of the starting material were Fe0.91Mn0.09Si2 (p-type) and Fe0.98Co0.02Si2 (ntype). The mean particle diameters of both powders were approximately 6 mm. A slip was prepared by adding distilled water to the powder, and then the
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 9 4 9 - 3
K. Nogi et al. / Materials Science and Engineering A307 (2001) 129–133
130
hydrogen exponent (pH) of the slip was controlled by adding 3 mass% aqueous ammonia solution. The powder concentration was 77 mass%, and the pHs of p- and n-type were 8.5 and 7, respectively [4], in order to achieve good fluidity. The p-type and n-type slips were simultaneously poured into a mold, as shown in Fig. 1. The green body was demolded in approximately 10 min, then dried at 353 K for about 3 h in air. It was then sintered at 1443 K for 3 h, and annealed at 1123 K for 14 h. Both the sintering and annealing were performed in a vacuum of 10 − 2 Pa. The microstructures of the sintered compacts were observed using a scanning electron microscope (SEM) and analyzed using an electron probe X-ray microanalyzer (EPMA). The electromotive force (Uo) and the internal resistance (Ri) were simultaneously measured in the temperature range from 300 to 1073 K in a vacuum of 10 − 2 Pa. The maximum power output was calculated from the equation Pmax = (Uo)2/4Ri. The coefficient of the linear expansion was measured in the temperature range from 300 to 1000 K. The chemical compositions of the silver solder and the flux for the silver solder are shown in Table 1 and Table 2, respectively. The relationship between the current and voltage on the electrode was measured using the constant direct current. 3. Results and discussion
3.1. Forming and sintering Fig. 1. Construction of mold for thermoelectric device. Table 1 Chemical composition of silver solder Composition/at% Ag 45
Cu 15
Zn 16
Cd 24
Table 2 Chemical composition of the flux Composition/mass% H3BO4 30
KF 35
KBF4 15
H 2O 18
Other additives 2
Fig. 2. Photograph of the thermoelectric device.
The green body with a p–n junction is easily obtained. When the p-type slip and the n-type slip are not simultaneously poured into a mold, the green body or sintered compact is cracked at the p–n junction. When it is dehydrated for over 1 h in the mold, the green body is easily broken. Fig. 2 shows a photograph of a sintered specimen. All the p–n junctions of the fourty specimens were formed in the range of 10 mm from the edge of the hot junction. Thus, the position of the p– n junction does not significantly affect the module properties, because the temperature is homogeneous in the range of 10 mm from the edge of the hot junction. The sintered specimens do not have serious cracks and strains. The shrinkage of the specimen during sintering is 109 1%. Fig. 3 shows the relationship between the thermoelectric properties of the device and the temperature difference between the hot junction and the cold junction. Table 3 shows the properties of the thermoelectric device for the maximum temperature difference. The calculated internal resistance and electromotive force based on the properties of the specimen of 5×5×10 mm [5] are 0.61 V and 290 mV, respectively. Thus, it is concluded that the slipcasting method allows thermoelectric devices to be produced extremely rapidly and inexpensively.
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Fig. 3. Thermoelectric properties of the thermoelectric device.
3.2. Formation of electrode Investigation of the electrode formation is indispensable for constructing a module with devices. The electrode was formed by silver brazing in this study. Although the surface of iron-silicide is covered with a stable oxide film [6], the surfaces can be joined with flux which removes the oxide films. The electrical properties of the electrode are shown in Fig. 4. Silver solder can be used as the electrode for the FeSi2 thermoelectric device, because it is clear from Fig. 4 that silver solder has a low resistance ohmic contact to both the p- and n-types of b-FeSi2. However, some brazed specimens cracked, as shown in Fig. 5, due to the difference in the coefficients of thermal expansion between the matrix and silver solder. Fig. 6 shows the relationship between the temperature and the coefficients of linear expansion of the silver solder and b-Fe0.98Co0.02Si2. It is reported that the coefficient of the linear expansion of o-FeSi is varied from 15.75 to 18.2×10 − 6 K − 1 in the temperature range from 293 to 1273 K [7], and is between the coefficients of the silver solder and b-Fe0.98Co0.02Si2. Although the formation of an m-phase layer was attempted by adding iron powder on the surface of the sample, only a slight reaction occurred between the iron and the sample, and iron and the sample could not be joined. When the o-phase powder was added, the mphase and the sample were joined without forming other phases. Nickel powder rapidly reacted with the sample to form diffusion layers. The samples brazed with the m-phase powder or nickel powder did not crack. Fig. 7 shows reflection electron images of the cross section of the electrode of Fe0.91Mn0.09Si2. These electrodes form ohmic contacts with b-FeSi2. If the silver solder layer is thinner, the specimen without
additives does not crack, because the thermal stress depends on the thickness of the silver solder. However, a more reliable electrode can be obtained by forming a Table 3 The properties of the thermoelectric device at the maximum temperature difference Maximum temperature difference Hot junction temperature Cold junction temperature Electromotive force Internal resistance Maximum power output
647 K 1073 K 426 K 287 mV 659 V 31.3 mW
Fig. 4. Relationship between the current and the voltage on the electrode of silver solder.
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K. Nogi et al. / Materials Science and Engineering A307 (2001) 129–133
properties of the thermoelectric module at the maximum temperature difference. The open circuit voltage, the internal resistance and the maximum power output per device were 263 mV, 0.76 V and 23 mW, respectively. These values are not much different to the values of the specimen shown in Table 3.
4. Conclusions
Fig. 5. Reflection electron image of the cross section of the silver solder electrode.
Thermoelectric devices and a thermoelectric module were produced by the slip casting method, and their thermoelectric properties were measured. 1. The green body with a p–n junction is easily obtained, when the p-type slip and the n-type slip are simultaneously poured into a mold. The p–n junction is obtained within the range of 10 mm from the edge of the hot junction. 2. The electromotive force, the internal resistance and the maximum power output of the thermoelectric device for the maximum temperature difference (= 647 K) were 287 mV, 0.659 V and 31.3 mW, respectively. 3. The electrode was formed by silver brazing. Reliable electrodes can be obtained by adding m-phase powder or nickel powder. They form ohmic contacts with b-FeSi2. 4. The open circuit voltage, the internal resistance and the maximum power output of the thermoelectric module consisting of 20 devices were 5.25 V, 15.2 V and 0.45 W, respectively, for the maximum temperature difference (= 572 K).
Acknowledgements The authors greatly appreciate the support of the Hosokawa Powder Technology Foundation. Fig. 6. Temperature dependence of coefficient of linear expansion.
stress-relaxation layer. Although an ohmic contact is achieved between b-FeSi2 and the solder consisting of lead and tin, most solder forms a liquid phase even at less than 600 K. Because the temperature of the cold junction can be reached around 600 K, solder consisting of lead and tin is unsuitable for reliable electrodes.
3.3. Production of thermoelectric module The thermoelectric module consists of 20 thermoelectric devices connected in series. The schematic diagram and the photograph of the thermoelectric module are shown in Fig. 8 and Fig. 9, respectively. The copper plate electrically connects the devices, and also acts as a cooling fin for the cold junction. Table 4 shows the
Fig. 7. Reflection electron images of the cross section of the electrode of Fe0.91Mn0.09Si2.
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Fig. 8. Schematic diagram of the thermoelectric module. Table 4 Properties of the thermoelectric module with 20 thermoelectric devices at the maximum temperature difference Maximum temperature difference Hot junction temperature Cold junction temperature Open circuit voltage Internal resistance Maximum power output
572 K 1099 K 527 K 5.25 V 263 mV/device 15.2 V 760 mV/device 0.45 W
References
Fig. 9. Photograph of the thermoelectric module. .
[1] L.A. Salam, R.D. Matthews, H. Robertson, Mat. Design 20 (1999) 223. [2] I. Nishida, T. Sakata, Kinzokuzairyou Gijutsu Kenkyusyo Kenkyu Houkokusyu 8 (4) (1965) 35 – 54 In Japanese. [3] I. Nishida, M. Okamoto, T. Masumoto, T. Ogoe, Y. Isoda, Kinzokuzairyou Gijutsu Kenkyusyo Kenkyu Houkokusyu 9 (1988) 115– 125 In Japanese. [4] K. Nogi, T. Kita, X.Q. Yan, J. Ceram. Soc. Jpn. 109(1) (2001) 71. [5] K. Nogi, T. Kita, X.Q. Yan, J. Ceram. Soc. Jpn. 109(3) (2001) 265. [6] I. Nishida, J. Mat. Sci. Soc. Japan 15 (1978) 72. [7] G.V. Samsonov, I.M. Vinitskii, Handbook of Refractory Compounds, Plenum Press, New York, 1980, p. 206.
Production of iron-disilicide thermoelectric devices and thermoelectric module by the slip casting method Kiyoshi Nogi a, Takuji Kita b,*, Xiang-Qun Yan c,1 b
a Joining and Welding Research Institute, Osaka Uni6ersity, 11 -1 Mihogaoka, Ibaraki, Osaka 567 -0047, Japan Graduate student, Department of Materials Science and Processing, Faculty of Engineering, Osaka Uni6ersity, Osaka, Japan c Graduate student of Osaka Uni6ersity, Osaka, Japan
Received 18 October 2000
Abstract FeSi2 thermoelectric devices and thermoelectric modules were produced by the slip casting method, and their thermoelectric properties were evaluated. The starting materials were Fe0.91Mn0.09Si2 (p-type) and Fe0.98Co0.02Si2 (n-type). The powder concentration was 77 mass%, and the hydrogen exponents of p- and n-type were 8.5 and 7, respectively. The p- and n-type slips were simultaneously poured into a mold. The green body was demolded in approximately 10 min, then dried at 353 K for about 3 h in air. It was then sintered at 1443 K for 3 h, and annealed at 1123 K for 14 h. Both the sintering and annealing were performed in a vacuum of 10 − 2 Pa. The p–n junctions were formed within the range of 10 mm from the edge of the hot junction. The electrode was formed by silver brazing. Reliable electrodes can be obtained by adding m-phase powder or nickel powder. They formed ohmic contacts with b-FeSi2. The open circuit voltage, the internal resistance and the maximum power output of the thermoelectric module consisting of 20 devices were 5.25 V, 15.2 V and 0.45 W, respectively, for the maximum temperature difference ( =572 K). © 2001 Elsevier Science B.V. All rights reserved. Keywords: Thermoelectric device; Thermoelectric module; FeSi2; Slip casting; Ohmic contact; Stress-relaxation layer
1. Introduction The b-FeSi2 thermoelectric material has not been yet in the actual use, although it is one of the most attractive materials. The main conventional production methods of b-FeSi2 are: (1) press forming and pressureless sintering; and (2) hot-pressing. The slip casting method is seldom applied to the iron-disilicide system [1], though it has been applied to the CrSi2 and CoSi systems, which are also known as thermoelectric materials [2,3]. Uniaxial pressing requires expensive pressing equipment. Because a thermoelectric device requires a p– n junction, the powders granulated to achieve good fluidity should be avoided from mixing with each other during pouring into the die. * Corresponding author. Tel.: + 81-6-68798663; fax: +81-668798653. E-mail address: [email protected] (T. Kita). 1 Present address: Hosokawa Micron Co., 1-9 Shoudai, Tajika, Hirakata, Osaka 573-1132, Japan.
The authors reported the production of b-FeSi2 by the slip casting method [4,5]. The density and thermoelectric properties of the specimens formed by slip casting are almost the same as those of the specimens formed by cold isostatic pressing (CIP). In view of the complicated process and high cost of the equipment for CIP, it is concluded that slip casting is superior to CIP in productivity. The purpose of this paper is to produce thermoelectric devices and thermoelectric modules by the slip casting method, and to evaluate the thermoelectric properties of the produced modules.
2. Experimental procedure The chemical compositions of the starting material were Fe0.91Mn0.09Si2 (p-type) and Fe0.98Co0.02Si2 (ntype). The mean particle diameters of both powders were approximately 6 mm. A slip was prepared by adding distilled water to the powder, and then the
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 9 4 9 - 3
K. Nogi et al. / Materials Science and Engineering A307 (2001) 129–133
130
hydrogen exponent (pH) of the slip was controlled by adding 3 mass% aqueous ammonia solution. The powder concentration was 77 mass%, and the pHs of p- and n-type were 8.5 and 7, respectively [4], in order to achieve good fluidity. The p-type and n-type slips were simultaneously poured into a mold, as shown in Fig. 1. The green body was demolded in approximately 10 min, then dried at 353 K for about 3 h in air. It was then sintered at 1443 K for 3 h, and annealed at 1123 K for 14 h. Both the sintering and annealing were performed in a vacuum of 10 − 2 Pa. The microstructures of the sintered compacts were observed using a scanning electron microscope (SEM) and analyzed using an electron probe X-ray microanalyzer (EPMA). The electromotive force (Uo) and the internal resistance (Ri) were simultaneously measured in the temperature range from 300 to 1073 K in a vacuum of 10 − 2 Pa. The maximum power output was calculated from the equation Pmax = (Uo)2/4Ri. The coefficient of the linear expansion was measured in the temperature range from 300 to 1000 K. The chemical compositions of the silver solder and the flux for the silver solder are shown in Table 1 and Table 2, respectively. The relationship between the current and voltage on the electrode was measured using the constant direct current. 3. Results and discussion
3.1. Forming and sintering Fig. 1. Construction of mold for thermoelectric device. Table 1 Chemical composition of silver solder Composition/at% Ag 45
Cu 15
Zn 16
Cd 24
Table 2 Chemical composition of the flux Composition/mass% H3BO4 30
KF 35
KBF4 15
H 2O 18
Other additives 2
Fig. 2. Photograph of the thermoelectric device.
The green body with a p–n junction is easily obtained. When the p-type slip and the n-type slip are not simultaneously poured into a mold, the green body or sintered compact is cracked at the p–n junction. When it is dehydrated for over 1 h in the mold, the green body is easily broken. Fig. 2 shows a photograph of a sintered specimen. All the p–n junctions of the fourty specimens were formed in the range of 10 mm from the edge of the hot junction. Thus, the position of the p– n junction does not significantly affect the module properties, because the temperature is homogeneous in the range of 10 mm from the edge of the hot junction. The sintered specimens do not have serious cracks and strains. The shrinkage of the specimen during sintering is 109 1%. Fig. 3 shows the relationship between the thermoelectric properties of the device and the temperature difference between the hot junction and the cold junction. Table 3 shows the properties of the thermoelectric device for the maximum temperature difference. The calculated internal resistance and electromotive force based on the properties of the specimen of 5×5×10 mm [5] are 0.61 V and 290 mV, respectively. Thus, it is concluded that the slipcasting method allows thermoelectric devices to be produced extremely rapidly and inexpensively.
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Fig. 3. Thermoelectric properties of the thermoelectric device.
3.2. Formation of electrode Investigation of the electrode formation is indispensable for constructing a module with devices. The electrode was formed by silver brazing in this study. Although the surface of iron-silicide is covered with a stable oxide film [6], the surfaces can be joined with flux which removes the oxide films. The electrical properties of the electrode are shown in Fig. 4. Silver solder can be used as the electrode for the FeSi2 thermoelectric device, because it is clear from Fig. 4 that silver solder has a low resistance ohmic contact to both the p- and n-types of b-FeSi2. However, some brazed specimens cracked, as shown in Fig. 5, due to the difference in the coefficients of thermal expansion between the matrix and silver solder. Fig. 6 shows the relationship between the temperature and the coefficients of linear expansion of the silver solder and b-Fe0.98Co0.02Si2. It is reported that the coefficient of the linear expansion of o-FeSi is varied from 15.75 to 18.2×10 − 6 K − 1 in the temperature range from 293 to 1273 K [7], and is between the coefficients of the silver solder and b-Fe0.98Co0.02Si2. Although the formation of an m-phase layer was attempted by adding iron powder on the surface of the sample, only a slight reaction occurred between the iron and the sample, and iron and the sample could not be joined. When the o-phase powder was added, the mphase and the sample were joined without forming other phases. Nickel powder rapidly reacted with the sample to form diffusion layers. The samples brazed with the m-phase powder or nickel powder did not crack. Fig. 7 shows reflection electron images of the cross section of the electrode of Fe0.91Mn0.09Si2. These electrodes form ohmic contacts with b-FeSi2. If the silver solder layer is thinner, the specimen without
additives does not crack, because the thermal stress depends on the thickness of the silver solder. However, a more reliable electrode can be obtained by forming a Table 3 The properties of the thermoelectric device at the maximum temperature difference Maximum temperature difference Hot junction temperature Cold junction temperature Electromotive force Internal resistance Maximum power output
647 K 1073 K 426 K 287 mV 659 V 31.3 mW
Fig. 4. Relationship between the current and the voltage on the electrode of silver solder.
132
K. Nogi et al. / Materials Science and Engineering A307 (2001) 129–133
properties of the thermoelectric module at the maximum temperature difference. The open circuit voltage, the internal resistance and the maximum power output per device were 263 mV, 0.76 V and 23 mW, respectively. These values are not much different to the values of the specimen shown in Table 3.
4. Conclusions
Fig. 5. Reflection electron image of the cross section of the silver solder electrode.
Thermoelectric devices and a thermoelectric module were produced by the slip casting method, and their thermoelectric properties were measured. 1. The green body with a p–n junction is easily obtained, when the p-type slip and the n-type slip are simultaneously poured into a mold. The p–n junction is obtained within the range of 10 mm from the edge of the hot junction. 2. The electromotive force, the internal resistance and the maximum power output of the thermoelectric device for the maximum temperature difference (= 647 K) were 287 mV, 0.659 V and 31.3 mW, respectively. 3. The electrode was formed by silver brazing. Reliable electrodes can be obtained by adding m-phase powder or nickel powder. They form ohmic contacts with b-FeSi2. 4. The open circuit voltage, the internal resistance and the maximum power output of the thermoelectric module consisting of 20 devices were 5.25 V, 15.2 V and 0.45 W, respectively, for the maximum temperature difference (= 572 K).
Acknowledgements The authors greatly appreciate the support of the Hosokawa Powder Technology Foundation. Fig. 6. Temperature dependence of coefficient of linear expansion.
stress-relaxation layer. Although an ohmic contact is achieved between b-FeSi2 and the solder consisting of lead and tin, most solder forms a liquid phase even at less than 600 K. Because the temperature of the cold junction can be reached around 600 K, solder consisting of lead and tin is unsuitable for reliable electrodes.
3.3. Production of thermoelectric module The thermoelectric module consists of 20 thermoelectric devices connected in series. The schematic diagram and the photograph of the thermoelectric module are shown in Fig. 8 and Fig. 9, respectively. The copper plate electrically connects the devices, and also acts as a cooling fin for the cold junction. Table 4 shows the
Fig. 7. Reflection electron images of the cross section of the electrode of Fe0.91Mn0.09Si2.
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Fig. 8. Schematic diagram of the thermoelectric module. Table 4 Properties of the thermoelectric module with 20 thermoelectric devices at the maximum temperature difference Maximum temperature difference Hot junction temperature Cold junction temperature Open circuit voltage Internal resistance Maximum power output
572 K 1099 K 527 K 5.25 V 263 mV/device 15.2 V 760 mV/device 0.45 W
References
Fig. 9. Photograph of the thermoelectric module. .
[1] L.A. Salam, R.D. Matthews, H. Robertson, Mat. Design 20 (1999) 223. [2] I. Nishida, T. Sakata, Kinzokuzairyou Gijutsu Kenkyusyo Kenkyu Houkokusyu 8 (4) (1965) 35 – 54 In Japanese. [3] I. Nishida, M. Okamoto, T. Masumoto, T. Ogoe, Y. Isoda, Kinzokuzairyou Gijutsu Kenkyusyo Kenkyu Houkokusyu 9 (1988) 115– 125 In Japanese. [4] K. Nogi, T. Kita, X.Q. Yan, J. Ceram. Soc. Jpn. 109(1) (2001) 71. [5] K. Nogi, T. Kita, X.Q. Yan, J. Ceram. Soc. Jpn. 109(3) (2001) 265. [6] I. Nishida, J. Mat. Sci. Soc. Japan 15 (1978) 72. [7] G.V. Samsonov, I.M. Vinitskii, Handbook of Refractory Compounds, Plenum Press, New York, 1980, p. 206.