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Structural and electrical properties of β-FeSi2 bulk materials for thermoelectric applications
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Physics Procedia
Physics Procedia 0 (2011) 000–000
Physics Procedia 23 (2012) 13 – 16
www.elsevier.com/locate/procedia
Asian School-Conference on Physics and Technology of Nanostructured Materials (ASCO-NANOMAT 2011)
Structural and electrical properties of ȕ-FeSi2 bulk materials for thermoelectric applications H. Yamadaa,*, H. Katsumataa, D. Yuasaa, S. Uekusaa, M. Ishiyamab, H. Soumab, I. Azumayab a Department of Electronics and Bioinformatics, School of Science and Technology, Meiji University, Kawasaki 214-8571, Japan b
ELENIX, Inc., 2-20-4 Komatsubara Zama-shi 252-0002, Japan
Abstract ȕ-FeSi2 bulk materials for thermoelectric applications were prepared from high-purity (99.99 %) ȕ-FeSi2 powders or mixed Si and Fe powders. First, FeSi2 powders were milled to reduce the particle size, which can improve the thermoelectric figure of merit by reducing lattice thermal conductivity. Second, they were cold pressed and then sintered by two different sintering methods: a conventional electrical furnace and a Plasma Activated Sintering (PAS) system. The bulk densities and grain sizes were decreased by ball milling of the powders starting materials. The comparison of the two sintering methods showed that, the sample density of the PAS samples was 11%- 23 % higher than that of the samples sintered in the conventional electrical furnace. Moreover, the comparison of the two types of powders, E-FeSi2 and the mixture of Fe and Si, indicated that, E-FeSi2 bulk crystals were formed only when E-FeSi2 powders were used.
© B.V. Selection and/or peer-review under responsibility of Publication © 2011 2011Published PublishedbybyElsevier Elsevier Ltd. Selection and/or peer-review under responsibility of GuestCommittee Editors of of ASCO-NANOMAT 2011 and Far Eastern Federal University (FEFU)2011 Physics Procedia, Publication Committee of ASCO-NANOMAT Keywords: thermoelectric; FeSi2
1. Introduction The advancing threat of global warming and the global rise in energy demand have led to searching for alternative sources of energy other than fossil fuels. Semiconducting ȕ-FeSi2 is composed of nontoxic * Corresponding author. Tel.: +8-144-934-7301; fax: +8-144-934-7909. E-mail address: [email protected], [email protected].
1875-3892 © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of Publication Committee of ASCO-NANOMAT 2011 and Far Eastern Federal University (FEFU) doi:10.1016/j.phpro.2012.01.004
14 2
H. Yamada et al. / Physics Procedia 23 (2012) 13 – 16 H. Yamada et al./ Physics Procedia 00 (2011) 000–000
elements, which exist in great abundance on earth. ȕ-FeSi2 is a potentially useful material in thermoelectric [1]. The potential of thermoelectric materials is de¿ned by the dimensionless ¿gure of merit, ZT = S2VT/N, where S, V, N, and T are the Seebeck coefficient, electrical conductivity, total thermal conductivity, and temperature, respectively. The lattice thermal conductivity of fine-grained materials with an average grain size of several hundred nanometers is strongly reduced in comparison with coarse-grained materials such as polycrystalline materials, which can lead to an increase in the ZT value. The purpose of this study is to investigate the relation between ZT value and grain size of FeSi 2 bulk materials as a function of the particle size of FeSi 2 powders. 2. Experimental For the preparation of semiconducting E-FeSi2 bulk materials, high-purity (99.999 %) FeSi 2 powders, which were specially produced for us by Toshima MFG Co. Ltd., were used as starting materials. Mixed Fe and Si powders (atomic ratio of Fe/Si = 1/2) were also used for comparison. These powders were milled using ZrO2 balls with a mass ratio of ZrO2-balls/FeSi2-powders = 9/1. The orbital speed was maintained at 1260 rpm, the powder millings were carried out sequentially for 10 min followed by cooling intervals of 5 min, and the actual milling time was set at 30 min. After milling, the FeSi2 powders, were cold pressed and then sintered using either a conventional electrical furnace or Plasma Activated Sintering (PAS, ELENIX, Inc.) system. After sintering, the E-FeSi2 bulk samples were characterized by X-ray diffraction (XRD, Rigaku Co.) using CuKD (D = 1.541781 Å) radiation with glancing angle in the range of 20-60° to identify the phases in the bulk samples. The morphology of the powder particles and the structure of the bulk samples were analyzed by secondary electron microscopy (SEM, JEOL Canny Scope JCM-5700). Thermoelectric properties such as, the Seebeck coefficient and electrical conductivity were measured using a ULVAC-RIKO ZEM-2 equipment [2] and Hall Effect (HL 5500 PC), respectively. 3. Results and discussions Fig. 1 shows the temperature dependence of the power factor, D2V for the bulk E-FeSi2, especially produced by Toshima MFG Co. Ltd., which was finally processed to powder. It was found that power factor increased by more than 10 times by annealing at 900 °C for 20 h. XRD intensity (arb.unit)
-1
1E-7
1E-8
1E-9
20h annealed as-sintered
1E-10 300
1h milled as-sintered
䖃
-2
Power factor (Wm K )
1E-6
400
500
600
700
800
900
Temperature (K) Fig. 1. Temperature dependence of the power factor, D2Vfor the bulk E-FeSi2, especially produced by Toshima MFG Co. Ltd.
䖃 E -FeSi2
䖃
䕧 D-Fe2Si5 䕕 H-FeSi 䕕
䖃
䖃 䖃
䖃 䖃 䕧 䖃
20
30
䖃
䖃
䖃
40
50
60
2T (degree)
Fig. 2. XRD pattern of the FeSi2 powders before milling (a) and after milling for 1h (b)
15 3
H. Yamada et al. / Physics Procedia 23 (2012) 13 – 16 H. Yamada et al./ Physics Procedia 00 (2011) 000–000
(a)
(b)
5 Pm
20 Pm Fig. 3. SEM micrographs of the FeSi2 powders before milling (a) and after milling for 1 h (b).
Density (%)
XRD intensity (arb.unit)
90 䖃 E-FeSi2 , 䕧D-Fe2Si5 , 䕕 H-FeSi
䕧
䕧 䕧
o
䕕
950 C
䕧 䕧
䕧
o
900 C o
850 C
䖃 䖃
o
800 C 䖃 䖃䖃 䖃 䖃 䕕
o
780 C
䖃
80 75 70 65
䖃
740
o
750 C
20
85
30
40
2T (degree)
50
60
Fig. 4. Relation between PAS temperature and XRD spectra.
790
840
890
PAS temperature
940
(oC)
Fig. 5. Relation between density and PAS temperature.
Fig. 2 shows the XRD patterns of FeSi2 powders before milling (a) and after milling for 1 h (b), which indicates that all peaks are E-FeSi2 and the peak intensity decreased with milling. Fig. 3 shows the SEM micrographs of the FeSi2 powder before milling (a) and after milling for 1 h (b), which gives a clear indication of the reduction in particle size after milling. Thus, we can say that the decrease in the XRD intensity of the E-FeSi2 peaks observed in Fig. 2 results from the reduction in grain size. Using ball milling, the bulk densities of samples prepared from E-FeSi2 powders and the mixed Fe and Si powders decreased from 53 % to 41 % and 49 % to 42 %, respectively. The density of the samples sintered with a conventional electrical furnace was lower than that sintered by PAS. Moreover, the mixed Fe and Si powders exhibited lower densities in both sintering methods. Fig. 4 and 5, respectively show the relation between XRD patterns and PAS temperature and that between PAS temperature and density of the FeSi 2 bulk samples prepared from E-FeSi2 powders. The PAS time was 5 min in all samples. The major and minor phases of the samples sintered below 780 °C were E-FeSi2 and H-FeSi, respectively. Although the density increases with increasing PAS temperature, we determined the optimal PAS temperature to be 780 °C, because PAS at 800 °C caused phase transformation to D-Fe2Si5. Fig. 6 shows the relation between sintering time and XRD spectra in the PAS samples prepared from E-FeSi2 powders. The FWHM of E-FeSi2 (2 2 0) observed at 29° was found to decrease with increasing PAS time, as shown in Fig. 7, which indicates that the grain size may increase with increasing PAS time. Furthermore, the FWHM of E-FeSi2 (2 2 0) increased after ball milling, which could be due to the decrease in grain size. Table .1 summarizes our experimental results. First, the comparison of between the two sintering methods, show that the density of the PAS samples is significantly higher than that of the samples sintered in the conventional electrical furnace. Second, the comparison of the samples before and after milling shows
16 4
H. Yamada et al. / Physics Procedia 23 (2012) 13 – 16 H. Yamada et al./ Physics Procedia 00 (2011) 000–000
XRD intensity (arb.unit)
䖃
mil_ 300s
FWHM (degree)
that the density decreased after ball milling. Finally, when comparing the two types of powders as starting material, we find that the E-FeSi2 bulk crystals form only when E-FeSi2 powders are used. 䖃 E-FeSi2 , 䕧D-Fe 2Si5 , 䕕 H-FeSi 䖃 䖃 䖃 䕕 䖃 䖃 䖃 䖃
䕧
䖃
3600s 1800s 900s
0.23 0.21
䕕 Before milling 䕔 After milling
0.19 0.17 0.15 0.13 0
1000
300s 20
2000
3000
4000
PAS time (S) 30
40
50
60
2T (degree)
Fig. 6. Relation between PAS time and XRD spectra.
Fig. 7. Relation between PAS time and FWHM
Table 1. Summary of experimental results E-FeSi2
Powders as starting material Characterization results
Fe and Si mixture
Density
XRD
Electrical conductivity
Density
XRD
Electrical conductivity
53 %
E-FeSi2>>D-Fe2Si5
-
49 %
Fe , Si
-
Electrical furnace
Before milling After milling
41 %
H-FeSi>>E-FeSi2
-
42 %
Fe , Si
-
Ed-PAS
Before milling
71 %
E-FeSi2>>D-Fe2Si5
0.03
60 %
Fe, Si>>İ-FeSi
-
After milling
64 %
E-FeSi2>>D-Fe2Si5
-
56 %
Fe, Si>>İ-FeSi
-
4. Conclusions By comparing the two sintering methods, we find that the sample density of the PAS samples was 11%-23 % higher than that of the samples sintered in the conventional electrical furnace. The bulk density percentage decrease was several points after ball milling the powder starting materials. Furthermore, the FWHM of the E-FeSi2 (2 2 0) peak increased after ball milling, which could be due to the decrease in grain size. Furthermore, when comparing the two types of powders (i.e., E-FeSi2 and the mixture of Fe and Si), it was found that the E-FeSi2 bulk crystals are formed only when E-FeSi2 powders are used. Further investigations are needed to understand the effect on thermal conductivity and how to grow bulk E-FeSi2 from mixed powders consisting of Fe and Si. References [1] Tritt TM. Subramanian MA. Thermoelectric Materials, Phenomena, and Applications: A Bird's Eye View. MRS Bull 2006; 31: 188-7. [2] Sakamoto T, Iida T. Thermoelectric Characteristics of a Commercialized Mg 2Si Source Doped with Al, Bi, Ag, and Cu. J. Electron. Mater 2010; 39: 1708-6.
Physics Procedia
Physics Procedia 0 (2011) 000–000
Physics Procedia 23 (2012) 13 – 16
www.elsevier.com/locate/procedia
Asian School-Conference on Physics and Technology of Nanostructured Materials (ASCO-NANOMAT 2011)
Structural and electrical properties of ȕ-FeSi2 bulk materials for thermoelectric applications H. Yamadaa,*, H. Katsumataa, D. Yuasaa, S. Uekusaa, M. Ishiyamab, H. Soumab, I. Azumayab a Department of Electronics and Bioinformatics, School of Science and Technology, Meiji University, Kawasaki 214-8571, Japan b
ELENIX, Inc., 2-20-4 Komatsubara Zama-shi 252-0002, Japan
Abstract ȕ-FeSi2 bulk materials for thermoelectric applications were prepared from high-purity (99.99 %) ȕ-FeSi2 powders or mixed Si and Fe powders. First, FeSi2 powders were milled to reduce the particle size, which can improve the thermoelectric figure of merit by reducing lattice thermal conductivity. Second, they were cold pressed and then sintered by two different sintering methods: a conventional electrical furnace and a Plasma Activated Sintering (PAS) system. The bulk densities and grain sizes were decreased by ball milling of the powders starting materials. The comparison of the two sintering methods showed that, the sample density of the PAS samples was 11%- 23 % higher than that of the samples sintered in the conventional electrical furnace. Moreover, the comparison of the two types of powders, E-FeSi2 and the mixture of Fe and Si, indicated that, E-FeSi2 bulk crystals were formed only when E-FeSi2 powders were used.
© B.V. Selection and/or peer-review under responsibility of Publication © 2011 2011Published PublishedbybyElsevier Elsevier Ltd. Selection and/or peer-review under responsibility of GuestCommittee Editors of of ASCO-NANOMAT 2011 and Far Eastern Federal University (FEFU)2011 Physics Procedia, Publication Committee of ASCO-NANOMAT Keywords: thermoelectric; FeSi2
1. Introduction The advancing threat of global warming and the global rise in energy demand have led to searching for alternative sources of energy other than fossil fuels. Semiconducting ȕ-FeSi2 is composed of nontoxic * Corresponding author. Tel.: +8-144-934-7301; fax: +8-144-934-7909. E-mail address: [email protected], [email protected].
1875-3892 © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of Publication Committee of ASCO-NANOMAT 2011 and Far Eastern Federal University (FEFU) doi:10.1016/j.phpro.2012.01.004
14 2
H. Yamada et al. / Physics Procedia 23 (2012) 13 – 16 H. Yamada et al./ Physics Procedia 00 (2011) 000–000
elements, which exist in great abundance on earth. ȕ-FeSi2 is a potentially useful material in thermoelectric [1]. The potential of thermoelectric materials is de¿ned by the dimensionless ¿gure of merit, ZT = S2VT/N, where S, V, N, and T are the Seebeck coefficient, electrical conductivity, total thermal conductivity, and temperature, respectively. The lattice thermal conductivity of fine-grained materials with an average grain size of several hundred nanometers is strongly reduced in comparison with coarse-grained materials such as polycrystalline materials, which can lead to an increase in the ZT value. The purpose of this study is to investigate the relation between ZT value and grain size of FeSi 2 bulk materials as a function of the particle size of FeSi 2 powders. 2. Experimental For the preparation of semiconducting E-FeSi2 bulk materials, high-purity (99.999 %) FeSi 2 powders, which were specially produced for us by Toshima MFG Co. Ltd., were used as starting materials. Mixed Fe and Si powders (atomic ratio of Fe/Si = 1/2) were also used for comparison. These powders were milled using ZrO2 balls with a mass ratio of ZrO2-balls/FeSi2-powders = 9/1. The orbital speed was maintained at 1260 rpm, the powder millings were carried out sequentially for 10 min followed by cooling intervals of 5 min, and the actual milling time was set at 30 min. After milling, the FeSi2 powders, were cold pressed and then sintered using either a conventional electrical furnace or Plasma Activated Sintering (PAS, ELENIX, Inc.) system. After sintering, the E-FeSi2 bulk samples were characterized by X-ray diffraction (XRD, Rigaku Co.) using CuKD (D = 1.541781 Å) radiation with glancing angle in the range of 20-60° to identify the phases in the bulk samples. The morphology of the powder particles and the structure of the bulk samples were analyzed by secondary electron microscopy (SEM, JEOL Canny Scope JCM-5700). Thermoelectric properties such as, the Seebeck coefficient and electrical conductivity were measured using a ULVAC-RIKO ZEM-2 equipment [2] and Hall Effect (HL 5500 PC), respectively. 3. Results and discussions Fig. 1 shows the temperature dependence of the power factor, D2V for the bulk E-FeSi2, especially produced by Toshima MFG Co. Ltd., which was finally processed to powder. It was found that power factor increased by more than 10 times by annealing at 900 °C for 20 h. XRD intensity (arb.unit)
-1
1E-7
1E-8
1E-9
20h annealed as-sintered
1E-10 300
1h milled as-sintered
䖃
-2
Power factor (Wm K )
1E-6
400
500
600
700
800
900
Temperature (K) Fig. 1. Temperature dependence of the power factor, D2Vfor the bulk E-FeSi2, especially produced by Toshima MFG Co. Ltd.
䖃 E -FeSi2
䖃
䕧 D-Fe2Si5 䕕 H-FeSi 䕕
䖃
䖃 䖃
䖃 䖃 䕧 䖃
20
30
䖃
䖃
䖃
40
50
60
2T (degree)
Fig. 2. XRD pattern of the FeSi2 powders before milling (a) and after milling for 1h (b)
15 3
H. Yamada et al. / Physics Procedia 23 (2012) 13 – 16 H. Yamada et al./ Physics Procedia 00 (2011) 000–000
(a)
(b)
5 Pm
20 Pm Fig. 3. SEM micrographs of the FeSi2 powders before milling (a) and after milling for 1 h (b).
Density (%)
XRD intensity (arb.unit)
90 䖃 E-FeSi2 , 䕧D-Fe2Si5 , 䕕 H-FeSi
䕧
䕧 䕧
o
䕕
950 C
䕧 䕧
䕧
o
900 C o
850 C
䖃 䖃
o
800 C 䖃 䖃䖃 䖃 䖃 䕕
o
780 C
䖃
80 75 70 65
䖃
740
o
750 C
20
85
30
40
2T (degree)
50
60
Fig. 4. Relation between PAS temperature and XRD spectra.
790
840
890
PAS temperature
940
(oC)
Fig. 5. Relation between density and PAS temperature.
Fig. 2 shows the XRD patterns of FeSi2 powders before milling (a) and after milling for 1 h (b), which indicates that all peaks are E-FeSi2 and the peak intensity decreased with milling. Fig. 3 shows the SEM micrographs of the FeSi2 powder before milling (a) and after milling for 1 h (b), which gives a clear indication of the reduction in particle size after milling. Thus, we can say that the decrease in the XRD intensity of the E-FeSi2 peaks observed in Fig. 2 results from the reduction in grain size. Using ball milling, the bulk densities of samples prepared from E-FeSi2 powders and the mixed Fe and Si powders decreased from 53 % to 41 % and 49 % to 42 %, respectively. The density of the samples sintered with a conventional electrical furnace was lower than that sintered by PAS. Moreover, the mixed Fe and Si powders exhibited lower densities in both sintering methods. Fig. 4 and 5, respectively show the relation between XRD patterns and PAS temperature and that between PAS temperature and density of the FeSi 2 bulk samples prepared from E-FeSi2 powders. The PAS time was 5 min in all samples. The major and minor phases of the samples sintered below 780 °C were E-FeSi2 and H-FeSi, respectively. Although the density increases with increasing PAS temperature, we determined the optimal PAS temperature to be 780 °C, because PAS at 800 °C caused phase transformation to D-Fe2Si5. Fig. 6 shows the relation between sintering time and XRD spectra in the PAS samples prepared from E-FeSi2 powders. The FWHM of E-FeSi2 (2 2 0) observed at 29° was found to decrease with increasing PAS time, as shown in Fig. 7, which indicates that the grain size may increase with increasing PAS time. Furthermore, the FWHM of E-FeSi2 (2 2 0) increased after ball milling, which could be due to the decrease in grain size. Table .1 summarizes our experimental results. First, the comparison of between the two sintering methods, show that the density of the PAS samples is significantly higher than that of the samples sintered in the conventional electrical furnace. Second, the comparison of the samples before and after milling shows
16 4
H. Yamada et al. / Physics Procedia 23 (2012) 13 – 16 H. Yamada et al./ Physics Procedia 00 (2011) 000–000
XRD intensity (arb.unit)
䖃
mil_ 300s
FWHM (degree)
that the density decreased after ball milling. Finally, when comparing the two types of powders as starting material, we find that the E-FeSi2 bulk crystals form only when E-FeSi2 powders are used. 䖃 E-FeSi2 , 䕧D-Fe 2Si5 , 䕕 H-FeSi 䖃 䖃 䖃 䕕 䖃 䖃 䖃 䖃
䕧
䖃
3600s 1800s 900s
0.23 0.21
䕕 Before milling 䕔 After milling
0.19 0.17 0.15 0.13 0
1000
300s 20
2000
3000
4000
PAS time (S) 30
40
50
60
2T (degree)
Fig. 6. Relation between PAS time and XRD spectra.
Fig. 7. Relation between PAS time and FWHM
Table 1. Summary of experimental results E-FeSi2
Powders as starting material Characterization results
Fe and Si mixture
Density
XRD
Electrical conductivity
Density
XRD
Electrical conductivity
53 %
E-FeSi2>>D-Fe2Si5
-
49 %
Fe , Si
-
Electrical furnace
Before milling After milling
41 %
H-FeSi>>E-FeSi2
-
42 %
Fe , Si
-
Ed-PAS
Before milling
71 %
E-FeSi2>>D-Fe2Si5
0.03
60 %
Fe, Si>>İ-FeSi
-
After milling
64 %
E-FeSi2>>D-Fe2Si5
-
56 %
Fe, Si>>İ-FeSi
-
4. Conclusions By comparing the two sintering methods, we find that the sample density of the PAS samples was 11%-23 % higher than that of the samples sintered in the conventional electrical furnace. The bulk density percentage decrease was several points after ball milling the powder starting materials. Furthermore, the FWHM of the E-FeSi2 (2 2 0) peak increased after ball milling, which could be due to the decrease in grain size. Furthermore, when comparing the two types of powders (i.e., E-FeSi2 and the mixture of Fe and Si), it was found that the E-FeSi2 bulk crystals are formed only when E-FeSi2 powders are used. Further investigations are needed to understand the effect on thermal conductivity and how to grow bulk E-FeSi2 from mixed powders consisting of Fe and Si. References [1] Tritt TM. Subramanian MA. Thermoelectric Materials, Phenomena, and Applications: A Bird's Eye View. MRS Bull 2006; 31: 188-7. [2] Sakamoto T, Iida T. Thermoelectric Characteristics of a Commercialized Mg 2Si Source Doped with Al, Bi, Ag, and Cu. J. Electron. Mater 2010; 39: 1708-6.