Thermoelectric properties of Fe2TiAl Heusler alloys

Journal of Alloys and Compounds 377 (2004) 38–42

Thermoelectric properties of Fe2 TiAl Heusler alloys Ryosuke O. Suzuki a,∗ , Takashi Kyono a,b a

Department of Energy Science and Technology, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan b Sumitomo Electric Industries Ltd., Itami, Hyogo 664-0016, Japan Received 11 December 2003; received in revised form 16 January 2004; accepted 16 January 2004

Abstract The Fe2 VAl Heusler alloy was reported to have good thermoelectric properties. Because the natural resource of vanadium is insufficient, Fe2 TiAl based alloys were studied. Ni, Co, or Cr was substituted to stabilize the Heusler structure and to control the valence electrons. The Seebeck coefficients decreased in the Heusler single-phase region when these fourth elements were added, probably because the Fermi level was lying above the minimum in the density of state. The maximum and minimum Seebeck coefficients were +50 ␮V/K in Fe2 (Ti0.6 Cr0.4 )Al and −34 ␮V/K in (Fe0.2 Co0.8 )2 TiAl, respectively. Their electric conductivities were metallic judged by their temperature dependency. The characteristic semiconducting behavior of the Fe2 VAl Heusler alloy due to a pseudo gap in the density of state was not observed in the substituted Fe2 TiAl alloys. © 2004 Elsevier B.V. All rights reserved. Keywords: Intermetallics; Thermoelectrics; X-ray diffraction; Electronic band structure

1. Introduction Thermoelectric power generation has been studied as environmental friendly technology. However, some representing thermoelectric materials contain rare and harmful elements. Because iron is an abundant and cheap industrial material and because its Seebeck coefficient is larger than that of the other common elements, the iron based alloys have been extensively studied [1–8]. Systematically, Fe3 Si and Fe3 Al with D03 crystal structure were studied, and the (Fe, M1 )3 M2 alloys (M1 = Mn or V, and M2 = Al or Si) were reported to have good Seebeck coefficients in the metallic alloys [3–8]. Especially, Hanada et al. found excellent thermoelectric properties in the Fe2 VAl compound [7–9]. Concurrently three other groups confirmed that the off-stoichiometric Fe2 VAl showed an anomalously large Seebeck coefficient at a level of 150–200 ␮V/K [7,9–13]. Because its Seebeck coefficient could change from p to n types by a small compositional change or doping, the Fe2 VAl Heusler alloy has the potential for a new thermoelectric material. However, vanadium is one of the rare ele-

∗ Corresponding author. Tel.: +81-75-753-5453; fax: +81-75-753-5453. E-mail address: [email protected] (R.O. Suzuki).

0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.01.035

ments and there is lack of specified ores, and a replacement of V by other elements is desired. The purpose of this work is to design a new thermoelectric Heusler alloy by replacing V by Ti, which is naturally abundant.

2. Alloy design The crystal structure type of Fe2 VAl is L21 , so-called Heusler alloy, as shown in Fig. 1. Its cubic structure is based on the D03 type crystal, but it is a more ordered phase than D03 . In case of the D03 structure, the atomic sites of X and Y in Fig. 1 are occupied by the same element, and the Z site by the other non-metallic element. In the replacement of Fe in the D03 structure by the transition metals, regarding the preferential occupation it was reported that the elements located at the right and left of Fe in the periodic table could replace X and Y, respectively [14]. In case of the L21 Heusler alloy, the X and Y sites are thus occupied by Fe and V, respectively. A narrow pseudo energy-gap in the density of state is characteristic for the Fe2 VAl compound, as shown in Fig. 2 [15–17]. The Fermi level is located in this narrow pseudo gap and the electric resistivity shows semiconductive behavior. Because the electron density near this Fermi level changes sharply, the Seebeck coefficient is large [7,10].

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two-phase mixture. Here we tried to synthesize the Heusler single-phase by alloying, and to evaluate its thermoelectric properties. Taking account of the above-mentioned two criteria, Ni2 TiAl, Co2 TiAl, and Fe2 CrAl were combined with Fe2 TiAl as pseudo-binary Heusler alloys. They have more valence electrons than Fe2 TiAl. Ni and Co replace Fe, while Cr replaces Ti [20,21]. These alloys are here referred to as (Fe1−x Nix )2 TiAl, (Fe1−x Cox )2 TiAl, and Fe2 (Ti1−x Crx )Al.

3. Experimental procedures Fig. 1. Crystal structure and site occupancy of D03 and L21 structure.

Pure metals of high purity were mixed and arc-melted several times in Ar gas atmosphere. The samples of 10–15 g were annealed in vacuum at 1273 K for 86.4 ks. The phases were identified by X-ray diffraction (XRD) measurement, and examined by scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) analyzer. After the confirmation of the presence of a single Heusler phase in these compounds, about 100 g sample were newly melted in Ar gas atmosphere in a high frequency induction furnace, and cast into rectangular shapes (10 mm × 10 mm × 50 mm). After the same heat treatment at 1273 K, the Seebeck coefficient and the electric resistivity were measured for the ingots by the methods as reported previously [2–4,6]. Fig. 2. Density of state for Fe2 VAl and Fe2 TiAl [6,7].

4. Results The density of state for the L21 type Fe2 TiAl phase was calculated as in Fig. 2 [15,16], which is a little different from that for Fe2 VAl. The Fermi level is located at a lower energy than the pseudo energy-gap. Therefore, we need to introduce more valence electrons in Fe2 TiAl in order to shift the Fermi level to higher energy. Considering the preferential occupation sites in the crystal lattice, Ni, Co, and Cr were chosen as the additional elements for Fe2 TiAl. At the stoichiometric composition of Fe2 TiAl in the ternary phase diagram as shown in Fig. 3 [18,19], the Heusler phase cannot exist as a single-phase, but as a

Fig. 3. Phase diagram of Fe–Ti–Al system at 1073 K [18,19].

4.1. Compositions for single-phase Fig. 4 shows XRD patterns after annealing of the Ni substituted samples, and Table 1 lists the identified phases in the studied alloys. The XRD intensity due to impurity phase

Fig. 4. XRD patterns of (Fe1−x Nix )2 TiAl.

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R.O. Suzuki, T. Kyono / Journal of Alloys and Compounds 377 (2004) 38–42

Table 1 Phase identification by XRD x

Phases identified by XRD

(Fe1−x Nix )2 TiAl

0.1 0.15 0.2 0.25 0.3 0.4 0.5 0.6 0.7 0.8

L21 , L21 , L21 , L21 , L21 , L21 , L21 L21 L21 L21

Fe2 Ti Fe2 Ti, Al2 O3 ? Fe2 Ti Fe2 Ti, TiO2 ? Fe2 Ti Fe2 Ti?

(Fe1−x Cox )2 TiAl

0.1 0.2 0.3 0.5 0.6 0.7 0.8

L21 , L21 , L21 , L21 , L21 L21 L21 ,

Fe2 Ti Fe2 Ti Fe2 Ti Fe2 Ti?

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

L21 , Fe2 Ti L21 L21 L21 L21 L21 L21 L21 L21

Fe2 (Ti1−x Crx )Al

Fig. 5. Seebeck coefficients at room temperature.

4.2. Seebeck coefficients FeAl3

Fe2 Ti decreased as the Ni content increased, as shown in Fig. 4. When x > 0.5, a single Heusler phase was obtained. The impurity phases disappeared also in (Fe1−x Cox )2 TiAl and Fe2 (Ti1−x Crx )Al alloys when some amount of the forth element was added. Heat treatment did not change the XRD patterns except for the Fe2 (Ti0.6 Cr0.4 )Al sample, where XRD of the as-cast sample showed the existence of Fe2 Ti and all the XRD peaks due to Fe2 Ti disappeared after annealing. Some samples were annealed for 86.4 and 345.6 ks, however, no difference was found in the XRD and SEM observations. Therefore, all the other samples were annealed only for 86.4 ks. The compositions of the impurity phases were locally analyzed by SEM–EDX. The average compositions were 42.9 mol% Fe–1.2 mol% Ni–32.5 mol% Ti–23.4 mol% Al, 42.5 mol% Fe–2.1 mol% Co–30.9 mol% Ti–24.5 mol% Al, and 8.6 mol% Fe–15.2 mol% Co–36.9 mol% Ti–39.3 mol% Al in the samples of (Fe0.8 Ni0.2 )2 TiAl, (Fe0.8 Co0.2 )2 TiAl, and (Fe0.2 Co0.8 )2 TiAl, respectively. This shows that both the impurity phases, Fe2 Ti and FeAl3 , formed the solid solutions consisting of four elements. This can be speculated from the ternary Fe–Ti–Al phase diagram (Fig. 2). In case of Cr addition, the XRD intensities of the ordered diffraction lines were weak even for the samples of the Fe2 (Ti1−x Crx )Al single-phase. When x increased, XRD patterns of Fe2 (Ti1−x Crx )Al became similar to that of the B2 structure, although they were referred to as “L21 ” in Table 1.

Fig. 5 shows Seebeck coefficients at room temperature for the annealed samples. The single-phase regions judged from Table 1 are shown in Fig. 5. No difference in Seebeck coefficient was found for the different melting methods. The Seebeck coefficients in the Heusler single-phase region decreased from positive to negative values, as the amount of the additional element, x increased. The absolute values of the Seebeck coefficients were low in the (Fe1−x Nix )2 TiAl alloys, while (Fe1−x Cox )2 TiAl and Fe2 (Ti1−x Crx )Al alloys showed larger values as n- and p-type thermoelectric materials, respectively. Fig. 6 shows the temperature dependency of the Seebeck coefficients in some of the (Fe1−x Cox )2 TiAl and Fe2 (Ti1−x Crx )Al alloys for which the Seebeck coefficients at room temperature was relatively large. The absolute value of the Seebeck coefficient in the (Fe1−x Cox )2 TiAl alloys increased as the temperature is raised. The temperature dependency of the Seebeck coefficients in the Fe2 (Ti1−x Crx )Al alloys changes for x = 0.4. The absolute values of the Seebeck coefficients in (Fe0.2 Co0.8 )2 TiAl and Fe2 (Ti0.6 Cr0.4 )Al are smaller than those of off-stoichiometric Fe2 VAl Heusler alloys [7–12]. However, the Seebeck coefficient of p-type

Fig. 6. Temperature dependency of Seebeck coefficients.

R.O. Suzuki, T. Kyono / Journal of Alloys and Compounds 377 (2004) 38–42

Fig. 7. Temperature dependency of the Seebeck coefficients for the annealed samples.

Fe2 (Ti0.6 Cr0.4 )Al is twice larger than that of Fe–Mn–Si (D03 type) [6], and that of n-type (Fe1−x Cox )2 TiAl exceeds that of Fe–Mn–Si above 350 K [6]. The values of the Fe2 TiAl systems are larger than the reported values for D03 type iron-based alloys [2–6,8]. Fig. 7 shows the change of the Seebeck coefficients by heat treatments. Because the samples are the Heusler phases, i.e., a highly ordered structure, the degree of order becomes generally higher at the lower temperature and may affect the Seebeck coefficient. However, the Seebeck coefficient did not vary significantly, as shown in Fig. 7.

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Fig. 9. Temperature dependency of resistivity for the samples that hold the better Seebeck coefficients.

dependency such as semiconducting behavior, while the resistivity of our Fe2 TiAl systems increased slightly as the temperature became higher. These resistivity measurements indicate typical metallic behavior of the Fe2 TiAl systems.

5. Discussion 5.1. Thermoelectric carrier

Figs. 8 and 9 show the specific resistivity at room temperature and its temperature dependency, respectively. The resistivity increased in the single-phase region of (Fe1−x Nix )2 TiAl (x > 0.4), as x increased. The compositional dependencies of (Fe1−x Cox )2 TiAl and Fe2 (Ti1−x Crx )Al alloys are not clear because of the coexistence with impurity phases. These resistivities at room temperature are lower than that of Fe2 VAl [10,11]. The electrical resistivity of Fe2 VAl shows a negative temperature

The density of state in Fe2 TiAl based alloys is illustrated in Fig. 10. Because the polarity of the Seebeck coefficient changed from positive to negative when the amount of addition, x increased, the reason of the small Seebeck coefficient can be expected to arise from the fact that the Fermi level is shifted to higher energy across the pseudo energy-gap by electrons supplied from the replacing elements. The possible energy-gap might have crossed by too many electrons, when the sample became a Heusler single-phase in the Ni or Co doped samples. Electrons may become the main carriers for the thermoelectric property, because the obtained Heusler alloys showed metallic conduction. The second reason is that the pseudo energy-gap might disappear by addition of the fourth elements. Although the sharp energy-gap was essential for the large Seebeck coefficient in Fe2 VAl based alloys [7,10–12], the increased electronic charge due to the replacement may change the density of state.

Fig. 8. Specific resistivity at room temperature.

Fig. 10. Illustrations of density of state. (a) Before and (b) after alloying.

4.3. Resistivity

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these elements, several Fe2 TiAl based alloys became a single Heusler phase and their Seebeck coefficient shifted from positive to negative values. (Fe0.3 Co0.7 )2 TiAl and (Fe0.2 Co0.8 )2 TiAl show good n-type thermoelectric behavior, and Fe2 (Ti0.6 Cr0.4 )Al shows a p-type phenomenon. Although the conductivity of both alloys were metallic, the power factor of Fe2 (Ti0.7 Cr0.3 )Al and Fe2 (Ti0.6 Cr0.4 )Al at room temperature exceeded that of p-type Fe2 VAl alloy because of low resistivity.

Acknowledgements The authors acknowledge financial support from Japan Nuclear Cycle Development Institute (JNC), Yazaki Memorial Foundation for Science and Technology, and Sekisui Chemical Co. Ltd. Fig. 11. Power factor of some selected alloys.

References The third reason is the disturbance of the highly ordered structure by the fourth element. The diffraction lines from the super-superlattice and superlattice became weaker by Cr addition. It suggests that the crystal structure shifts to the disordered A2 or B2 type from the highly ordered L21 type. If disorder occurred, the sharp pseudo energy-gap would be smeared. When we assume the order parameter as 0.7, Rietvelt simulation shows that XRD analysis cannot distinguish between A2, B2, and L21 types clearly. For confirmation of ordering, therefore, other experimental techniques would be needed. 5.2. Power factor Fig. 11 shows the power factor of the Fe2 TiAl systems, evaluated from the Seebeck coefficient α and the resistivity ρ. The p-type Fe2 (Ti0.6 Cr0.4 )Al shows a higher power factor around room temperature than p-type Fe2 VAl. The n-type (Fe0.3 Co0.7 )2 TiAl and (Fe0.2 Co0.8 )2 TiAl did not match with the n-type Fe1.92 V1.08 Al [7]. Note that the power factor of the Bi2 Te3 semiconductor is 2–5 mWK2 m−1 , and for ␤-FeSi2 it is about 0.3–0.7 mW K2 m−1 at room temperature [22]. Because the resistivity of Fe2 TiAl systems is much better than those of these characteristic thermoelectric materials, the p-type Fe2 (Ti0.6 Cr0.4 )Al is expected to be developed further.

6. Conclusion The Heusler alloy Fe2 VAl was modified by replacing V by Ti. Cr, Ni, or Co was added to control the density of states at the Fermi level. By substituting some amount of

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