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Thermoelectric properties of (Fe1−xCox)2VAl Heusler-type compounds
Journal of Alloys and Compounds 484 (2009) 812–815
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Thermoelectric properties of (Fe1−x Cox )2 VAl Heusler-type compounds Wenjia Lu a,b , Wenqing Zhang a , Lidong Chen a,∗ a State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China b Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 2 February 2009 Received in revised form 9 May 2009 Accepted 11 May 2009 Available online 18 May 2009 Keywords: Heusler-type Electrical conductivity Seebeck coefficient ZT
a b s t r a c t Polycrystalline (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5) alloys were synthesized through a combined process of arc-melting, annealing and sintering. Thermoelectric properties were investigated from 300 to 850 K. Stoichiometric Fe2 VAl exhibited p-type conduction, however, it shifted to n-type with slight Co substitution for Fe site. After Co substitution, the electrical conductivity and absolute value of Seebeck coefficient at room temperature increased from 1.2 × 105 to 5 × 105 Sm−1 and 30 to 112 V/K, respectively. Meanwhile, the lattice thermal conductivity dropped from 23 to 6 W/m K because of the introduction of extra phonon scattering due to alloying effect. The highest dimensionless figure of merit ZT value for (Fe0.7 Co0.3 )2 VAl is 0.09 at 650 K. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Heusler-type alloy is a kind of intermetallic compounds with a formula of X2 YZ, where X and Y are transition metals and Z is the III or IV element in the periodic table. Among these compounds, Fe2 VAl has attracted intense attention because there is a deep, well-developed pseudogap about 0.1–0.2 eV near the Fermi level, which is highly unusual for intermetallic compounds [1–4]. This characteristic makes Fe2 VAl show semiconductor-like behavior in electrical transport [3,4]. In recent years, Fe2 VAl-based alloys have been investigated widely as a promising thermoelectric compound because semiconductors with a narrow band-gap are inclined to have good thermoelectric performance [5,6]. Investigations on the low temperature transport between 10 and 300 K of Fe2−x V1+x Al (0 ≤ x ≤ 0.11) revealed that the thermoelectric property is greatly affected by Fe and V contents [7]. The Seebeck coefficient of Fe2 VAl at room temperature is about 40 V/K. When Fe was replaced by V, the conduction shifted from p-type to ntype, while the Seebeck coefficient and low-temperature electrical conductivity increased sharply [7]. Other research showed that the Seebeck coefficient of Fe2 VAl was also sensitive to Al content [8]. The Seebeck coefficient shifted from −130 to 75 V/K with the deviation of the Al content in (Fe2/3 V1/3 )100−y Aly (23.8 ≤ y ≤ 25.8) alloys [8]. Besides, the effects of Ge substitution and Si substitution for Al site were also reported by Nishino et al. [9] and Lue et al. [10], respectively. The Fe2 VAl1−x Gex (0 ≤ x ≤ 0.20) and
∗ Corresponding author. Tel.: +86 21 52412500. E-mail address: [email protected] (L. Chen). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.032
Fe2 VAl1−x Six (0 ≤ x ≤ 0.50) alloys were synthesized by arc-melting and the results showed that Ge or Si substituting for Al site could dramatically enhance the Seebeck coefficient and electrical conductivity, leading to a rather large power factor at room temperature. Recently, the Fe2 VAl alloy with fine microstructure was synthesized by mechanical alloying and pulse-current sintering, exhibiting an improved ZT value [11,12]. Furthermore, the thermoelectric properties of (Fe1−x Cox )2 TiAl and p-type (Fe2−x Cox )(V1−y Tiy )Al alloys were also studied, which showed that the Co is a promising substituting element for Fe2 VAl system [13,14]. However, the systematical effect of Co substitution for Fe2 VAl remains a key issue for the thermoelectric property of Fe2 VAl. For Fe2 VAl-based alloys, tuning the electrical transport is a key approach to improve the thermoelectric property and it is also crucial to further reduce the thermal conductivity in order to achieve higher ZT value. In this paper, Fe2 VAl alloy with Co substitution was synthesized and the effect of Co substitution on the electrical and thermal properties of Fe2 VAl was systematically studied. The electrical conductivity and Seebeck coefficient was largely increased by Co substitution while the thermal conductivity was remarkably reduced. 2. Experimental details Ingots of (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5) alloys were prepared by arc-melting the mixture of pure elements Fe (99.99%, pieces), V (99.99, pieces), Co (99.99%, grains) and Al (99.999%, wire) under argon atmosphere. To improve homogeneity, the alloys were arc-melted for more than five times. Then, the ingots were sealed in evacuated quartz tubes and annealed at 1073 K for 72 h in vacuum. After that, the samples were milled into powder and consolidated by spark plasma sintering (SPS, SUMITOMO SPS2040) technique.
W. Lu et al. / Journal of Alloys and Compounds 484 (2009) 812–815
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X-ray diffraction (XRD, Rigaku RINT 2000 V) with Cu K␣ radiation and scanning electron microscopy (SEM, JSM-6700F) with electron probe micro-analyzer (EPMA, Shimadzu EPMA-8705QH2 ) were used to examine the phase composition and microstructure. The thermal diffusivity of samples was measured by the laser flash technique under argon flow (NETZSCH LFA427) and the thermal conductivity was calculated based on the values of thermal diffusivity, density and heat capacity. The total heat capacity data of compound were estimated according to the heat capacity of respective component and the uncertainty is about 10%. Samples were cut into rectangular bars with the approximate dimensions of 1.8 mm × 1.8 mm × 10 mm for electrical conductivity and Seebeck coefficient measurements, which were tested by ZEM-3 (ULVAC-RIKO) in Helium atmosphere. All the measurements were carried out from 300 to 850 K. The uncertainties of the measurements including the thermal conductivity, Seebeck coefficient and electrical conductivity are 5, 5 and 7%, respectively.
3. Results and discussion The XRD patterns for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5) samples are shown in Fig. 1, where all the samples are assigned to L21 phase (Heusler-type) structure without impurities. Further SEM and EPMA analyses also indicate that the samples are pure phase and no secondary phase signals can be tracked, as shown in Fig. 2. The temperature dependence of electrical conductivity () and Seebeck coefficient for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5) are plotted in Figs. 3 and 4, respectively. The Co content (x) dependence of electrical conductivity and Seebeck coefficient for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5) at room temperature is displayed in Fig. 5. The electrical conductivity of stoichiometric Fe2 VAl (x = 0) exhibits semiconductor behavior at the measurement temperature range. However, it transits into semimetal gradually when Fe is replaced by Co. With increasing temperature, the electrical conductivity increases monotonically at the whole measured temperature range when x ≤ 0.08 while it decreases when x ≥ 0.3. The electrical conductivity increases with the increase of x because the substitution of Co for Fe sites can lead to the increase of carrier concentration and cause the improvement in electrical conductivity, which is similar to some previous reports [9,10,12]. For x = 0.5, the electrical conductivity reaches the maximum value of 5 × 105 S m−1 at room temperature. The Seebeck coefficient of stoichiometric Fe2 VAl (x = 0) compound is positive, showing the p-type conduction. When Fe is slightly replaced by Co, the Seebeck coefficient becomes negative rapidly. The intrinsic Fe2 VAl shows p-type semiconductor behavior and when Fe is partly replaced by Co, extra electrons are introduced since cobalt supplies one more electron than iron does, resulting in the main carrier changing from hole to electron. Usually, the Seebeck coefficient in a double charge system can be demonstrated by
Fig. 1. XRD patterns of (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5).
Fig. 2. SEM images of the (a) Fe2 VAl (x = 0) and (b) FeCoVAl (x = 0.5).
the formula below [15]: S=
p Sp + n Sn , p + n
(1)
where Sp , Sn , p and n represents the Seebeck coefficient and electrical conductivity for the p-type and n-type carriers, respectively. Because the Sp is positive and Sn is negative, the sign of S might be reversed when electron is donated in p-type conduction or hole is added in an n-type one. With increasing x composition, the |S| value of room temperature increases when x ≤ 0.05 and it decreases grad-
Fig. 3. Temperature dependence of electrical conductivity for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5).
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W. Lu et al. / Journal of Alloys and Compounds 484 (2009) 812–815
Fig. 4. Temperature dependence of Seebeck coefficient for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5).
ually when x ≥ 0.08. When x ≥ 0.3, the electron becomes dominant and the compound turns to be a single charge system. Therefore, the measured S values are less dependent with Co contents. For (Fe0.95 Co0.05 )2 VAl sample, the Seebeck coefficient reaches the maximum value of −112 V/K at room temperature. In addition, it has also been reported that adjusting the Fe and Al compositions can result in a large Seebeck coefficient for Fe2 VAl system [7,8]. Consequently, in the present experimental condition, the enhancement of Seebeck coefficient should be attributed to the combination effect of Co substitution for Fe and the small deviation from stoichiometry of Fe and Al. The power factor, S2 , as a function of temperature is shown in Fig. 6. For undoped Fe2 VAl, the power factor is relative small and it increases greatly after Co substitution. When x ≤ 0.08, the room temperature power factor increases dramatically due to the enhanced Seebeck coefficient and electrical conductivity at room temperature. Furthermore, the power factor decreases monotonically with increasing temperature when x ≤ 0.15 and the maximum value of 27.5 W/K2 cm is achieved at room temperature for x = 0.08. In addition, it is interesting that the power factor remain a relative high value in the whole measured temperature range when x ≥ 0.3, which might be important for further applications. The temperature dependence of thermal conductivity () for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5) alloys is shown in Fig. 7. It can be observed that the thermal conductivity is significantly decreased after substituting Co for Fe in Fe2 VAl. For stoichiometric Fe2 VAl, thermal conductivity at room temperature is 24 W/m K while it decreases to about 10 W/m K after 50% Co substitution for Fe. The total thermal conductivity consists of electronic and phonon terms,
Fig. 5. The electrical conductivity and Seebeck coefficient as a function of Co content (x) for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5) at 300 K.
Fig. 6. Temperature dependence of power factor for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5).
which can be described as = L + e , where L is the lattice thermal conductivity and e is the electronic thermal conductivity. e can be calculated by the Wiedemann–Franz law e = L0 T, where L0 (=2.45 × 10−8 W K−2 ) is the Lorentz number and is the electrical conductivity. The L values, which are obtained by subtracting e from , and e values as functions of Co composition at 300 K is plotted in the inset of Fig. 7. The lattice thermal conductivity drops from 23 to 6 W/m K after Co substitution at room temperature, which is a larger reduction in lattice thermal conductivity than doped quaternary Fe2 VAl systems previously reported [9,10]. Meanwhile, it is obvious that e is smaller than L in the whole measured composition range at 300 K. In another word, the total value is primarily based on L especially when x ≤ 0.2. But the values of e are getting close to those of L with increasing Co content, which indicates that the lattice thermal conductivity is greatly reduced by substitution and it well conforms to the point defect phonon scattering mechanism. The figure of merit, ZT, for all samples measured from room temperature to 850 K is demonstrated in Fig. 8. The ZT value is significantly improved by the reduction in thermal conductivity and enhancement in S2 . When x = 0.3, the highest ZT value about 0.09 is achieved at 650 K. The ZT value is still one order of magnitude smaller than state-of-the-art thermoelectric materials such as filled skutterudites and Be2 Te3 -based alloys. Generally, the Fe2 VAl-based alloys obtain the maximum ZT values at around room temperature [9–12], but in our work, it is interesting that the highest ZT value was achieved at 650 K. It can be attributed to the higher electron concentration due to the higher Co-substitution content, leading to
Fig. 7. Temperature dependence of thermal conductivity for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5). Inset: the lattice thermal conductivity and electronic thermal conductivity as a function of Co content (x) for (Fe1−x Cox )2 VAl at 300 K.
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815
conductivity was significantly reduced due to point defect phonon scattering introduced by the Co substitution. The lattice thermal conductivity dropped from 23 to 6 W/m K at 300 K, which is relative low among Fe2 VAl-based alloys. The highest ZT value for (Fe0.7 Co0.3 )2 VAl reached 0.09 at 650 K. Acknowledgements This work was partly supported by National Basic Research Program of China (No. 2007CB607503) and National Science Foundation of China (No. 50821004). References
Fig. 8. Temperature dependence of ZT for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5).
the relative large Seebeck coefficient for Fe2 VAl at high temperature range. 4. Conclusions The Heusler-type alloys (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5) were fabricated by arc-melting. The compound shifts to n-type conduction rapidly by a slight Co substitution for Fe site. The electrical conductivity and Seebeck coefficient were enhanced by Co substitution. Therefore, a large power factor of 27.5 W/K2 cm was achieved at room temperature for (Fe0.92 Co0.08 )2 VAl. Furthermore, the thermal
[1] M. Weinert, R.E. Watson, Phys. Rev. B 58 (15) (1998) 9732–9740. [2] H. Okamura, J. Kawahara, T. Nanba, I. Shimoyama, et al., Phys. Rev. Lett. 84 (2000) 3674–3677. [3] Y. Nishino, M. Kato, S. Asano, K. Soda, M. Hayasaki, U. Mizutani, Phys. Rev. Lett. 79 (1997) 1909–1912. [4] Y. Nishino, Intermetallics 8 (2000) 1233–1241. [5] D.M. Rowe, CRC Handbook of Thermoelectrics, CRC Press, New York, USA, 1995. [6] F.J. Disalvo, Science 285 (1999) 703–706. [7] C.S. Lue, Y.-K. Kuo, Phys. Rev. B 66 (2002) 085121. [8] K. Soda, H. Murayama, S. Yagi, et al., Phys. Rev. B 71 (2005) 235112. [9] Y. Nishino, S. Deguchi, U. Mizutani, Phys. Rev. B 74 (2006) 115115. [10] C.S. Lue, C.F. Chen, K. Kuo, Phys. Rev. B 75 (2007) 064204. [11] M. Mikami, A. Matsumoto, K. Kobayashi, J. Alloys Compd. 461 (2008) 423–426. [12] M. Mikami, K. Kobayashi, J. Alloys Compd. 466 (2008) 530–534. [13] K. Iwase, Y. Nishino, H. Miyazaki, K. Soda, J. Jpn. Inst. Met. 72 (2008) 464–469. [14] H. Nakayama, N. Ide, Y. Nishino, Mater. Trans. 49 (2008) 1858–1862. [15] J. Yang, D.T. Morelli, G.P. Meisner, W. Chen, J.S. Dyck, C. Uher, Phys. Rev. B 67 (2003) 165207.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Thermoelectric properties of (Fe1−x Cox )2 VAl Heusler-type compounds Wenjia Lu a,b , Wenqing Zhang a , Lidong Chen a,∗ a State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China b Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 2 February 2009 Received in revised form 9 May 2009 Accepted 11 May 2009 Available online 18 May 2009 Keywords: Heusler-type Electrical conductivity Seebeck coefficient ZT
a b s t r a c t Polycrystalline (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5) alloys were synthesized through a combined process of arc-melting, annealing and sintering. Thermoelectric properties were investigated from 300 to 850 K. Stoichiometric Fe2 VAl exhibited p-type conduction, however, it shifted to n-type with slight Co substitution for Fe site. After Co substitution, the electrical conductivity and absolute value of Seebeck coefficient at room temperature increased from 1.2 × 105 to 5 × 105 Sm−1 and 30 to 112 V/K, respectively. Meanwhile, the lattice thermal conductivity dropped from 23 to 6 W/m K because of the introduction of extra phonon scattering due to alloying effect. The highest dimensionless figure of merit ZT value for (Fe0.7 Co0.3 )2 VAl is 0.09 at 650 K. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Heusler-type alloy is a kind of intermetallic compounds with a formula of X2 YZ, where X and Y are transition metals and Z is the III or IV element in the periodic table. Among these compounds, Fe2 VAl has attracted intense attention because there is a deep, well-developed pseudogap about 0.1–0.2 eV near the Fermi level, which is highly unusual for intermetallic compounds [1–4]. This characteristic makes Fe2 VAl show semiconductor-like behavior in electrical transport [3,4]. In recent years, Fe2 VAl-based alloys have been investigated widely as a promising thermoelectric compound because semiconductors with a narrow band-gap are inclined to have good thermoelectric performance [5,6]. Investigations on the low temperature transport between 10 and 300 K of Fe2−x V1+x Al (0 ≤ x ≤ 0.11) revealed that the thermoelectric property is greatly affected by Fe and V contents [7]. The Seebeck coefficient of Fe2 VAl at room temperature is about 40 V/K. When Fe was replaced by V, the conduction shifted from p-type to ntype, while the Seebeck coefficient and low-temperature electrical conductivity increased sharply [7]. Other research showed that the Seebeck coefficient of Fe2 VAl was also sensitive to Al content [8]. The Seebeck coefficient shifted from −130 to 75 V/K with the deviation of the Al content in (Fe2/3 V1/3 )100−y Aly (23.8 ≤ y ≤ 25.8) alloys [8]. Besides, the effects of Ge substitution and Si substitution for Al site were also reported by Nishino et al. [9] and Lue et al. [10], respectively. The Fe2 VAl1−x Gex (0 ≤ x ≤ 0.20) and
∗ Corresponding author. Tel.: +86 21 52412500. E-mail address: [email protected] (L. Chen). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.032
Fe2 VAl1−x Six (0 ≤ x ≤ 0.50) alloys were synthesized by arc-melting and the results showed that Ge or Si substituting for Al site could dramatically enhance the Seebeck coefficient and electrical conductivity, leading to a rather large power factor at room temperature. Recently, the Fe2 VAl alloy with fine microstructure was synthesized by mechanical alloying and pulse-current sintering, exhibiting an improved ZT value [11,12]. Furthermore, the thermoelectric properties of (Fe1−x Cox )2 TiAl and p-type (Fe2−x Cox )(V1−y Tiy )Al alloys were also studied, which showed that the Co is a promising substituting element for Fe2 VAl system [13,14]. However, the systematical effect of Co substitution for Fe2 VAl remains a key issue for the thermoelectric property of Fe2 VAl. For Fe2 VAl-based alloys, tuning the electrical transport is a key approach to improve the thermoelectric property and it is also crucial to further reduce the thermal conductivity in order to achieve higher ZT value. In this paper, Fe2 VAl alloy with Co substitution was synthesized and the effect of Co substitution on the electrical and thermal properties of Fe2 VAl was systematically studied. The electrical conductivity and Seebeck coefficient was largely increased by Co substitution while the thermal conductivity was remarkably reduced. 2. Experimental details Ingots of (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5) alloys were prepared by arc-melting the mixture of pure elements Fe (99.99%, pieces), V (99.99, pieces), Co (99.99%, grains) and Al (99.999%, wire) under argon atmosphere. To improve homogeneity, the alloys were arc-melted for more than five times. Then, the ingots were sealed in evacuated quartz tubes and annealed at 1073 K for 72 h in vacuum. After that, the samples were milled into powder and consolidated by spark plasma sintering (SPS, SUMITOMO SPS2040) technique.
W. Lu et al. / Journal of Alloys and Compounds 484 (2009) 812–815
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X-ray diffraction (XRD, Rigaku RINT 2000 V) with Cu K␣ radiation and scanning electron microscopy (SEM, JSM-6700F) with electron probe micro-analyzer (EPMA, Shimadzu EPMA-8705QH2 ) were used to examine the phase composition and microstructure. The thermal diffusivity of samples was measured by the laser flash technique under argon flow (NETZSCH LFA427) and the thermal conductivity was calculated based on the values of thermal diffusivity, density and heat capacity. The total heat capacity data of compound were estimated according to the heat capacity of respective component and the uncertainty is about 10%. Samples were cut into rectangular bars with the approximate dimensions of 1.8 mm × 1.8 mm × 10 mm for electrical conductivity and Seebeck coefficient measurements, which were tested by ZEM-3 (ULVAC-RIKO) in Helium atmosphere. All the measurements were carried out from 300 to 850 K. The uncertainties of the measurements including the thermal conductivity, Seebeck coefficient and electrical conductivity are 5, 5 and 7%, respectively.
3. Results and discussion The XRD patterns for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5) samples are shown in Fig. 1, where all the samples are assigned to L21 phase (Heusler-type) structure without impurities. Further SEM and EPMA analyses also indicate that the samples are pure phase and no secondary phase signals can be tracked, as shown in Fig. 2. The temperature dependence of electrical conductivity () and Seebeck coefficient for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5) are plotted in Figs. 3 and 4, respectively. The Co content (x) dependence of electrical conductivity and Seebeck coefficient for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5) at room temperature is displayed in Fig. 5. The electrical conductivity of stoichiometric Fe2 VAl (x = 0) exhibits semiconductor behavior at the measurement temperature range. However, it transits into semimetal gradually when Fe is replaced by Co. With increasing temperature, the electrical conductivity increases monotonically at the whole measured temperature range when x ≤ 0.08 while it decreases when x ≥ 0.3. The electrical conductivity increases with the increase of x because the substitution of Co for Fe sites can lead to the increase of carrier concentration and cause the improvement in electrical conductivity, which is similar to some previous reports [9,10,12]. For x = 0.5, the electrical conductivity reaches the maximum value of 5 × 105 S m−1 at room temperature. The Seebeck coefficient of stoichiometric Fe2 VAl (x = 0) compound is positive, showing the p-type conduction. When Fe is slightly replaced by Co, the Seebeck coefficient becomes negative rapidly. The intrinsic Fe2 VAl shows p-type semiconductor behavior and when Fe is partly replaced by Co, extra electrons are introduced since cobalt supplies one more electron than iron does, resulting in the main carrier changing from hole to electron. Usually, the Seebeck coefficient in a double charge system can be demonstrated by
Fig. 1. XRD patterns of (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5).
Fig. 2. SEM images of the (a) Fe2 VAl (x = 0) and (b) FeCoVAl (x = 0.5).
the formula below [15]: S=
p Sp + n Sn , p + n
(1)
where Sp , Sn , p and n represents the Seebeck coefficient and electrical conductivity for the p-type and n-type carriers, respectively. Because the Sp is positive and Sn is negative, the sign of S might be reversed when electron is donated in p-type conduction or hole is added in an n-type one. With increasing x composition, the |S| value of room temperature increases when x ≤ 0.05 and it decreases grad-
Fig. 3. Temperature dependence of electrical conductivity for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5).
814
W. Lu et al. / Journal of Alloys and Compounds 484 (2009) 812–815
Fig. 4. Temperature dependence of Seebeck coefficient for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5).
ually when x ≥ 0.08. When x ≥ 0.3, the electron becomes dominant and the compound turns to be a single charge system. Therefore, the measured S values are less dependent with Co contents. For (Fe0.95 Co0.05 )2 VAl sample, the Seebeck coefficient reaches the maximum value of −112 V/K at room temperature. In addition, it has also been reported that adjusting the Fe and Al compositions can result in a large Seebeck coefficient for Fe2 VAl system [7,8]. Consequently, in the present experimental condition, the enhancement of Seebeck coefficient should be attributed to the combination effect of Co substitution for Fe and the small deviation from stoichiometry of Fe and Al. The power factor, S2 , as a function of temperature is shown in Fig. 6. For undoped Fe2 VAl, the power factor is relative small and it increases greatly after Co substitution. When x ≤ 0.08, the room temperature power factor increases dramatically due to the enhanced Seebeck coefficient and electrical conductivity at room temperature. Furthermore, the power factor decreases monotonically with increasing temperature when x ≤ 0.15 and the maximum value of 27.5 W/K2 cm is achieved at room temperature for x = 0.08. In addition, it is interesting that the power factor remain a relative high value in the whole measured temperature range when x ≥ 0.3, which might be important for further applications. The temperature dependence of thermal conductivity () for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5) alloys is shown in Fig. 7. It can be observed that the thermal conductivity is significantly decreased after substituting Co for Fe in Fe2 VAl. For stoichiometric Fe2 VAl, thermal conductivity at room temperature is 24 W/m K while it decreases to about 10 W/m K after 50% Co substitution for Fe. The total thermal conductivity consists of electronic and phonon terms,
Fig. 5. The electrical conductivity and Seebeck coefficient as a function of Co content (x) for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5) at 300 K.
Fig. 6. Temperature dependence of power factor for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5).
which can be described as = L + e , where L is the lattice thermal conductivity and e is the electronic thermal conductivity. e can be calculated by the Wiedemann–Franz law e = L0 T, where L0 (=2.45 × 10−8 W K−2 ) is the Lorentz number and is the electrical conductivity. The L values, which are obtained by subtracting e from , and e values as functions of Co composition at 300 K is plotted in the inset of Fig. 7. The lattice thermal conductivity drops from 23 to 6 W/m K after Co substitution at room temperature, which is a larger reduction in lattice thermal conductivity than doped quaternary Fe2 VAl systems previously reported [9,10]. Meanwhile, it is obvious that e is smaller than L in the whole measured composition range at 300 K. In another word, the total value is primarily based on L especially when x ≤ 0.2. But the values of e are getting close to those of L with increasing Co content, which indicates that the lattice thermal conductivity is greatly reduced by substitution and it well conforms to the point defect phonon scattering mechanism. The figure of merit, ZT, for all samples measured from room temperature to 850 K is demonstrated in Fig. 8. The ZT value is significantly improved by the reduction in thermal conductivity and enhancement in S2 . When x = 0.3, the highest ZT value about 0.09 is achieved at 650 K. The ZT value is still one order of magnitude smaller than state-of-the-art thermoelectric materials such as filled skutterudites and Be2 Te3 -based alloys. Generally, the Fe2 VAl-based alloys obtain the maximum ZT values at around room temperature [9–12], but in our work, it is interesting that the highest ZT value was achieved at 650 K. It can be attributed to the higher electron concentration due to the higher Co-substitution content, leading to
Fig. 7. Temperature dependence of thermal conductivity for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5). Inset: the lattice thermal conductivity and electronic thermal conductivity as a function of Co content (x) for (Fe1−x Cox )2 VAl at 300 K.
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conductivity was significantly reduced due to point defect phonon scattering introduced by the Co substitution. The lattice thermal conductivity dropped from 23 to 6 W/m K at 300 K, which is relative low among Fe2 VAl-based alloys. The highest ZT value for (Fe0.7 Co0.3 )2 VAl reached 0.09 at 650 K. Acknowledgements This work was partly supported by National Basic Research Program of China (No. 2007CB607503) and National Science Foundation of China (No. 50821004). References
Fig. 8. Temperature dependence of ZT for (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5).
the relative large Seebeck coefficient for Fe2 VAl at high temperature range. 4. Conclusions The Heusler-type alloys (Fe1−x Cox )2 VAl (0 ≤ x ≤ 0.5) were fabricated by arc-melting. The compound shifts to n-type conduction rapidly by a slight Co substitution for Fe site. The electrical conductivity and Seebeck coefficient were enhanced by Co substitution. Therefore, a large power factor of 27.5 W/K2 cm was achieved at room temperature for (Fe0.92 Co0.08 )2 VAl. Furthermore, the thermal
[1] M. Weinert, R.E. Watson, Phys. Rev. B 58 (15) (1998) 9732–9740. [2] H. Okamura, J. Kawahara, T. Nanba, I. Shimoyama, et al., Phys. Rev. Lett. 84 (2000) 3674–3677. [3] Y. Nishino, M. Kato, S. Asano, K. Soda, M. Hayasaki, U. Mizutani, Phys. Rev. Lett. 79 (1997) 1909–1912. [4] Y. Nishino, Intermetallics 8 (2000) 1233–1241. [5] D.M. Rowe, CRC Handbook of Thermoelectrics, CRC Press, New York, USA, 1995. [6] F.J. Disalvo, Science 285 (1999) 703–706. [7] C.S. Lue, Y.-K. Kuo, Phys. Rev. B 66 (2002) 085121. [8] K. Soda, H. Murayama, S. Yagi, et al., Phys. Rev. B 71 (2005) 235112. [9] Y. Nishino, S. Deguchi, U. Mizutani, Phys. Rev. B 74 (2006) 115115. [10] C.S. Lue, C.F. Chen, K. Kuo, Phys. Rev. B 75 (2007) 064204. [11] M. Mikami, A. Matsumoto, K. Kobayashi, J. Alloys Compd. 461 (2008) 423–426. [12] M. Mikami, K. Kobayashi, J. Alloys Compd. 466 (2008) 530–534. [13] K. Iwase, Y. Nishino, H. Miyazaki, K. Soda, J. Jpn. Inst. Met. 72 (2008) 464–469. [14] H. Nakayama, N. Ide, Y. Nishino, Mater. Trans. 49 (2008) 1858–1862. [15] J. Yang, D.T. Morelli, G.P. Meisner, W. Chen, J.S. Dyck, C. Uher, Phys. Rev. B 67 (2003) 165207.