Thermoelectric Properties of Fe2VAl Based Thin Films Deposited at High Temperature

Thermoelectric Properties of Fe

VAl-Based Thin-Films Deposited at High

Temperature

Satoshi Hiroi

_{, Masashi Mikami}

_{and Tsunehiro Takeuchi}

1_{Toyota Technological Institute, Nagoya 468–8511, Japan}

2_{National Institute of Advanced Industrial Science and Technology, Nagoya 463–8560, Japan}

3_{Precursory Research for Embryonic Science and Technology, Japanese Science and Technology Agency, Tokyo 102–0076, Japan} 4_{Green Mobility Collaborative Research Center, Nagoya University, Nagoya 464–8603, Japan}

Fe2VAl-based thin-films were prepared using radio frequency magnetron sputtering technique at various substrate temperatures up to 1073 K. At low substrate temperature below 773 K, we did not observe any evidences of L21 Heusler phase but the epitaxially grown B2-phase, which is considered as a chemically disordered structure of Heusler-phase. At high substrate temperatures above 773 K, L21 ordering became observable and its volume fraction was increased with increasing the substrate temperature. The sample deposited at 1073 K, that was consid-ered as the highly ordconsid-ered L21-phase, possessed S ≈  −120 μV K−1 at around 340 K, and this value is almost the same with that previously re-ported for bulk samples. The power factor indicated large values exceeding 2.0 mWm−1_K−2 _{at the room temperature. The thermal conductivity} of Fe2VAl thin-film was reduced to a half value of the bulk. As a result, the maximum figure of merit was almost doubled to 0.07 at 400 K from 0.04 of bulk samples. [doi:10.2320/matertrans.E-M2016824]

(Received January 6, 2016; Accepted June 11, 2016; Published August 25, 2016)

Keywords:  heusler-type structure, iron vanadium aluminum alloy, thermoelectric material, thin-film, radio frequency magnetron sputtering technique

1.  Introduction

Obtaining high-performance thermoelectric devices is a fascinating and challenging subject because of their capabili-ty in recovering energy from waste heat. Since most of the waste heat is emitted from industry, automobile, etc. to envi-ronment at low temperatures below 500 K, the development of thermoelectric devices working at this particular tempera-ture range is of great importance. Besides, thermoelectric de-vices working at low temperature below 350 K are applicable for the power supply of wearable devices with generating electricity from human body. Development of such high per-formance thermoelectric devices, therefore, is highly desired to contribute both to Green Innovation and Life Innovation.

Efficiency of energy conversion in thermoelectric devices is an increasing function of dimensionless figure of merit cal-culated from the thermoelectric properties of involved mate-rials,

ZT = S

2

ρκT, (1)

where S, ρ and κ correspond to Seebeck coefficient, electrical resistivity, and thermal conductivity, respectively. Develop-ment of thermoelectric materials possessing a large value of ZT is an absolute necessity for practical applications of ther-moelectric devices. Bi2Te3-based materials possess large

val-ues of ZT near room temperature, and have been widely used in practical thermoelectric devices working at low tempera-ture below 500 K1)_{. Unfortunately, however, one of the}

con-stitute elements, tellurium, is toxic, rare, and expensive, and has prevented us from utilizing thermoelectric devices for variety of applications.

L21-type Heusler phase stabilizes at around Fe2VAl is

widely considered as one of the potential thermoelectric ma-terials working at low temperature below 500 K. Its power

factor reaches 5.5 mW m−1 _K−2 _{at the optimal carrier}

concen-tration2–5)_{. Notably this value is larger than that of Bi} 2Te3

-based thermoelectric materials1)_{. Besides, all the constituent}

elements of Fe2VAl-based Heusler alloys are cheap,

non-tox-ic, and abundant. The surpassing resistance for oxidation and corrosion together with the very strong mechanical strength let us further believe that Fe2VAl-based Heusler phase is one

of the most plausible, potential, practical thermoelectric ma-terials6) _{that can replace of Bi}

2Te3-based materials.

We have to mention, however, that the lattice thermal con-ductivity of Fe2VAl-based alloy is larger than 20 Wm−1K−1,

and the large magnitude of lattice thermal conductivity pre-vents the value of ZT from increasing to large values exceed-ing 0.1. Recently, several different groups employed small amount of heavy element substitution and succeeded in de-creasing the lattice thermal conductivity to 5 Wm−1_K−1

without greatly altering the electron transport properties7–9)_.

The consequently obtained ZT, however, was still smaller than 0.3. We need to find new strategy to further reduce the lattice thermal conductivity of Fe2VAl-based thermoelectric

materials.

It was also reported that thermal conductivity was effec-tively reduced by making thin-film presumably due to the re-duction of mean free paths of the phonons10,11)_{. More}

recent-ly, Furuta et al. reported that the lattice thermal conductivity was effectively reduced to small values less than 3 Wm−1_K−1

in thin-film samples, but the Seebeck coefficient was kept small and, hence, a sufficient value of power factor was not obtained12)_{. They attributed the reason for the small Seebeck}

coefficient to the chemical disordering between vanadium and aluminum caused during deposition. These results sug-gest that we may have large values of ZT in the Fe2VAl-based

thin-films, provided that the samples of well-ordered L21

structure are carefully prepared.

In this study, therefore, we tried to prepare the Fe2VAl-based

thin-films of L21 structure by means of radio-frequency (RF)

*

magnetron sputtering technique under various conditions, and succeeded in preparing the samples possessing the objec-tive structure. We investigated the thermoelectric properties of these samples in detail, and consequently found that the Fe2VAl-based thin-films of disordering-free L21 structure

possess almost the same behaviors of electron transport prop-erties observed for the bulk samples.

2.  Experimental Procedure

2.1  Sample preparation

We prepared Fe2VAl-based thin-film samples using radio

frequency magnetron sputtering technique in a commercially available apparatus, VTR-150M/SRF purchased by ULVAC kiko Inc., in which three disks with 2 inches in diameter were used as the sputtering target. We selected Fe2VAl0.95Si0.05 as

the composition of target because a high power factor with n-type behavior was reported for the bulk samples at this composition2–5)_{. Despite that the composition of films form}

the Fe2VAl0.95Si0.05 target had an excess of Fe due to the

dif-ference of sputtering probability, we controlled the composi-tion by using an Al-V alloy chip placed at the center of target disk. Consequently, the difference of composition between thin-film and the target became less than 1.5 at%. MgO single crystal with (100) surface was used as the substrate for the epitaxial growth of Heusler phase because its lattice constant is almost the same value with that of Fe2VAl Heusler phase.

The distance between the target disk and the substrate was fixed at 60 mm. The background pressure in the deposition chamber and the working pressure of argon gas was set at less than 2.0 ×  10−4 _{Pa and 2.0 Pa, respectively. The substrate}

temperatures during deposition were controlled over a wide range from 373 K to 1073 K. The film samples were deposit-ed under 20 W RF power for 180 minutes. As a result, the thin-films with thickness of 500 900 nm were obtained.

2.2  Analysis

Crystallographic structural analysis for the samples was carried out using x-ray diffraction measurement apparatus, Bruker D8 ADVANCE, with Cu Kα x-ray source. We em-ployed symmetric and asymmetric θ-2θ scans together with φ scan around 202 peak. Scanning electron microscope, SU6600 developed by Hitachi High-Technologies Corpora-tion, at the accelerating voltage of 20 kV and atomic force microscope, Nano Navi Real/Nano Cute of Seiko Instru-ments Inc., were used for investigating the surface structure and the roughness. The thickness of the films was determined from the observed SEM image of the cross section of cut samples. For the determination of the sample composition, we employed energy dispersive x-ray spectroscopy, INCA Energy of Oxford Instruments, with the acceleration voltage of 10 kV.

Electrical resistivity ρ was measured for the thin-films at the temperature range from 300 to 700 K under vacuum at-mosphere of 10−3 _{Pa by using conventional four-probes DC}

method. Seebeck coefficient S was measured using SB-2000 made by MMR Technologies. Thermal diffusivity α of the samples was measured at room temperature by means of ther-mo-reflectance method using NanoTR developed by Pico-Therm Corporation. Pico-Thermal conductivity κ was deduced

from thermal diffusivity α, density d, and heat capacity C us-ing the equation of κ =  αdC. The density and heat capacity of the Fe2VAl alloys reported by Kawaharada et al.13) were used

as the values of the thin-films.

3.  Results and Discussion

Figure 1(a) shows the deposition temperature dependence of symmetric θ-2θ XRD patterns observed for the Fe2VAl0.95Si0.05 thin-films. Absence of peaks from the XRD

patterns indicated that the thin-films made at low deposition temperature below 473 K were composed solely of amor-phous phase. In this temperature range, the atomic diffusion is presumably slow and the thickness of films increases be-fore the construction of getting well ordered structure.

The 00l peaks appeared at high deposition temperatures above 473 K and their intensity were increased with increas-ing deposition temperature. We found that the 00l peaks were drastically sharpened at high temperatures above 873 K, and this fact indicates that the c-axis oriented structure were suc-cessfully grown in the present thin-film samples. Notably the 00l peaks of the thin-films slightly shifted toward large angles from that of bulk samples most likely due to the expansion along the sample surface in association with the lattice mis-match between MgO substrate and Fe2VAl0.95Si0.05. The

re-duction of lattice parameters for perpendicular to the surface was estimated to be 97% of bulk.

Figure 1(b) shows the results of XRD φ scans of 202 peak at 2θ =  44.4 . The thin-films samples deposited at low tem-peratures below 673 K showed no peaks in the φ scans at 2θ =  44.4 . Four-fold peaks were appeared at 773 K and increased with temperature because of large scale atomic diffusion along the surface of the sample.

Figure 1(c) shows asymmetric θ-2θ scan around 111 peak measured at 773 K, 873 K, 973 K, and 1073 K. The 111 peak is evidence of L21 structure, which was not observed for the

samples prepared at low deposition temperatures below 773 K. At the temperature range around 773 K, we did not find 111 peak but we observed the four-fold symmetry of 202 peaks. This fact means that epitaxially grown B2 phase rather than the objective L21 phase was formed at around 773 K.

Note here that B2 phase takes the place of L21 phase,

provid-ed that the heavy chemical disordering occurs between V- and Al-sites in L21 phase. It is definitely unfavorable for us to

have the chemical disorder between V and Al because the dis-ordered B2 structure possesses lower Seebeck coefficient than that of full-ordered L21 structure. Fortunately, we found

that the 111 peak appeared for the samples deposited at high temperatures above 873 K and its intensity became stronger with increase in deposition temperature. It is, therefore, ar-gued that 873 K is the minimum temperature for the epitaxial growth of L21 structure.

Figure 2 shows deposition temperature dependence of root-mean-square (RMS) of surface roughness Rq, which is

defined as

Rq= 1_n n

i

(Zi−Zave)2,

Here n, Zi, and Zave is the number of measuring point, the

surface roughness of thin-films sensitively varied with the substrate temperature during the deposition. The surface roughness was less than 7 nm at the deposition temperatures between 673 and 873 K, where B2-structure was developed. On the other hand, Rq suddenly increased with increasing

temperature above 873 K. The large values of Rq would be

closely related to the growth of L21 structure. AFM images

measured for the samples deposited at 1073 K are shown in Fig. 3 together with that at 873 K. It was clearly observed that, at high temperature above 873 K, large square blocks were formed with being inclined to 45 from the a- and b-ax-es of underlying L21 lattice, and these blocks led to the large

values of Rq.

Temperature dependence of electrical resistivity ρ is shown in Fig. 4. The electrical resistivity moderately decreased with increasing temperature regardless of the sample preparation condition. The same behavior was observed even for the bulk samples at high temperatures above 400 K5)_{. It is naturally}

attributed to the synergy effect of (1) the temperature inde-pendent mean free path of electrons in association with the strongest scattering limit known as Mott-Ioffe-Regel lim-it14,15)_{, (2) the fine electronic structure (pseudogap) near the}

chemical potential, and (3) the gradually increasing ener-gy-range where the electrons contribute to the electron trans-port properties with increasing temperature.

The weak reduction of electrical resistivity with decreasing Fig. 1 XRD patterns of (a) the symmetric θ-2θ scans along perpendicular

direction of the substrate surface, (b) the φ scans around 202 peak located 2θ =  44.4 and (c) the asymmetric θ-2θ scans near the 111 peak measured for the prepared Fe2VAl0.95Si0.05 thin-films.

Fig. 2 Deposition temperature dependence of the root-mean-square of sur-face roughness Rq for the Fe2VAl0.95Si0.05 thin-films.

temperature below 400 K was observed only for the sample deposited at 1073 K. This behavior is attributed to the in-crease of mean free path of electrons at low temperatures, and observable also for Si-substituted Fe2V1-xAlx bulk samples.

The sample deposited at 1073 K must possess the highly or-dered L21-structure, and this ordered structure presumably

allowed the mean free path of electrons to become longer for realizing the decrease of electrical resistivity. Other samples, on the other hand, structure disordering such as chemical dis-ordering between Al and V, defects, and/or dislocations pre-vented the mean free path to be elongated above the shortest limit.

We should also mention that the absolute value of electrical resistivity becomes smallest at the samples deposited at 773 and 873 K, and that of highly ordered sample possess slightly larger values. This difference is mainly attributed to the for-mation of pseudogap near the chemical potential in the sam-ples of high temperature deposition. In the B2 structure, the pseudogap presumably became shallower, and hence the electrical resistivity was decreased. The formation of mosaic structure in association with the square blocks would also contribute to the increase of electrical resistivity.

Figure 5 shows the temperature dependence of Seebeck coefficients S for the Fe2VAl-based thin-films. All samples

indicated n-type behavior as we expected from the data re-ported for bulk samples of the same composition. Notably, maximum magnitude of Seebeck coefficient reached −120 μV K−1 _{at 340 K for the sample deposited at 1073 K.}

The temperature dependence and the magnitude of Seebeck coefficient were similar to the previously reported data of bulk sample obtained at Fe2VAl0.95Si0.054,5). In contrast, the

thin-films deposited at low temperatures below 973 K pos-sessed smaller magnitude of Seebeck coefficient than bulk material. Besides, its temperature dependence is certainly dif-ferent from the bulk samples. We speculate that the pseudogap, that leads to the large magnitude of Seebeck coefficient of Fe2VAl-based bulk samples, formed near the chemical

poten-tial only in the thin film sample deposited at 1073 K. In the remaining thin-film samples, the pseudogap was presumably

destructed due to the chemical disordering, and hence the magnitude and temperature dependence of Seebeck coeffi-cient became different from those of the bulk samples.

Figure 6 represents temperature dependence of power fac-tor P for the Fe2VAl-based thin-films. It reached

2.3 mWm−1_K−2 _{at around 340 K for the sample deposited at}

1073 K. This value corresponds to twice of maximum power factor for Ti-substituted Fe2VAl thin-film reported previously

by Mikami et al.10) _{The power factors of samples with}

depo-sition temperature of 773 K and 873 K indicated larger values than that of the sample deposited at 973 K. The larger magni-tude of power factor for the samples deposited at 773 K and 873 K than that of 973 K was mainly caused by the smaller electrical resistivity.

Thermal conductivity κ of the thin-film sputtered at 1073 K was observed by thermo-reflectance method with the rear heating & front detection configuration. The thermal conduc-tivity of 12.6 Wm−1_K−1 _{is half of bulk Fe}

2VAl alloy. This

re-Fig. 4 Temperature dependence of electrical resistivity measured for the Fe2VAl0.95Si0.05 thin-films deposited at various substrate temperatures. The substrate temperatures during deposition are superimposed in the fig-ure.

Fig. 5 Temperature dependence of Seebeck coefficient for the Fe2VAl0.95Si0.05 thin-films. The substrate temperatures during deposition are superimposed in the figure.

ducing of thermal conductivity was caused by the synergy effect of Si substitution, off-stoichiometry composition, sur-face roughness, and domain structure on the reduction of mean free path of phonons.

Figure 7 shows temperature dependence of figure of merit ZT for the thin-film deposited at 1073 K and Si-substituted Fe2VAl alloys estimated with assuming a constant thermal

conductivity. The measurement error was also estimated from the ratio of the RMS of roughness to the thickness because the thickness was used for calculation of the resistivity and thermal conductivity. Consequently, we obtained Fe2VAl

based thin-films with larger ZT value compared to the bulk alloy due to reduction of thermal conductivity. Although the maximum ZT value 0.07 obtained in this study is much small-er than unity, we have still chances to increase it because the electrical resistivity and the Seebeck coefficient could be im-proved by optimization of carrier concentration and thick-ness.

4.  Conclusion

We prepared Fe2VAl0.95Si0.05 thin-films by means of RF

magnetron sputtering technique with varying deposition tem-perature. It was clearly revealed that the substrate

tempera-ture during deposition was crucial parameter strongly affect-ing the orderaffect-ing of crystalline structure, surface morphology, and thermoelectric properties. The objective sample of highly ordered L21-structure was obtainable at the deposition

tem-perature of 1073 K. As a result, the thin-film sputtered at 1073 K possessed high Seebeck coefficient of |S|  ≈  120 μV K−1_{, and its power factor reached 2.3 mWm}−1 _K−2_.

The thermal conductivity of the thin-film was 12.6 Wm−1_K−1_,

which was nearly half of the bulk sample. Consequently, the figure of merit ZT of the thin-film reached 0.07 at room tem-perature.

Acknowledgements

This work was conducted under the financial support of JSPS KAKENHI Grant Numbers 26289236 and 26630332. One of the authors, T. Takeuchi, was also financially support-ed by JST PRESTO.

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