Vol. 137 (2020)

ACTA PHYSICA POLONICA A

No. 5

Proceedings of the 17th Czech and Slovak Conference on Magnetism, Košice, Slovakia, June 3–7, 2019

Shaped Crystal Growth of Fe–Al Alloy Platesby Micro Pulling down Method
K. Kamadaa,b,∗ , R. Murakamib , M. Arakawac , T. Minamitanid , T. Uenod ,Y. Shojib,e , A. Yamajie , M. Yoshinob,e , S. Kurosawaa,e , Y. Yokotaa ,Y. Ohashie , V.V. Kochurikhinf and A. Yoshikawaa,b,ea

New Industry Creation Hatchery Center, Tohoku University, Sendai, 980-8577, JapanbC & A Corporation, Sendai, 980-8579, Japan
cGraduate School of Biomedical Engineering, Tohoku University, Sendai, 980-8577, JapandKanazawa University, Kanazawa, 920-1192, Japane
Institute for Materials Research, Tohoku University, Sendai, 980-8577, JapanfGeneral Physics InstituteRussian Academy of Sciences, 38 Vavilov Str., 119991 Moscow, Russia
Plate shaped Fe–Al alloy crystals were successfully grown using the µ-PD method. The orientation andgrain size could be controlled by using the well oriented Fe0.95 Al0.05 seed crystal. The as-grown Fe0.82 Al0.18 andFe0.80 Al0.20 showed the magnetostriction 3/2λ = 121 and 149 ppm, respectively, which is comparable to thevalues for single crystal samples grown by the Bridgman-Stockbarger method. Distributions of Fe and Al were
homogenous along the growth direction within standard deviations of 0.35% and 1.3%, respectively. A prototype
vibration energy harvester was demonstrated using the grown Fe–Al alloys.
DOI: 10.12693/APhysPolA.137.1009
PACS/topics: µ-PD method, crystal growth, Fe–Al alloy, magnetostriction, vibration energy harvester

1. Introduction
Conversion technique of mechanical vibration energy
into electrical energy using magnetostrictive materials have attracted to attention in energy harvesting
applications. Among the magnetostrictive materials
the Fe–Ga [1–5] and Fe–Al alloys [3, 6, 7] are found attractive mainly due to advantages in terms of price, mechanical strength, and high magnetostriction constant.
Up to now, bulk crystals of Fe–Ga alloy have been produced by the Bridgman-Stockbarger (BS) method [8–10],or the Czochralski (Cz) method [4, 11]. Using thesemethods, a big bulk crystal up to 2 ∼ 3 inch diameter canbe grown. Especially Fe–Al has lower magnetostriction
3/2λ than Fe–Ga, but does not contain rare metal. Fe–Al
has a high advantage in terms of price of raw materials
for its mass production.
However, uniformity of chemical composition along
the crystal growth direction can not be avoided due tothe segregation in these alloys by the BS and Cz method.
This results in uniformity of magnetostriction constantand reduction of the production efficiency. The micro
pulling down µ-PD method has been developed asa shaped crystal growth technique. Our group have reported shaped crystal growth of oxide and fluoride single crystals with different shape such as rod, plate tube,
thin fiber, etc. Advantages of this method is low segregation due to high growth rate and small diffusion of melt

∗ corresponding author

at the solid-liquid interface, and small kerf loss due tonear net shape crystal [5, 12, 13]. In this study we report the shaped long plate crystal growth of Fe–Al alloys
using the µ-PD method. Chemical composition analysis
and magnetostrictive properties are evaluated comparing with a commercially available Fe–Ga alloy grown by
the Cz method. A prototype vibration energy harvester
using the grown Fe–Al alloy was demonstrated.
2. Experimental procedure
Plate shaped Fe–Ga alloy crystals were grown by
the µ-PD method using high-frequency induction heating
under Ar + 1% H2 atmosphere. High purity Fe and Alpowders (4N) were used as a starting material and they
were set into an alumina crucible with a hole of 1×7 mm2
size opening at the bottom. The ingots were directly
heated by the HF induction coil up to the melting pointof each Fe–Al alloys. After the raw materials were completely melted, a seed plate was immersed into the opening and brought into physical contact with the melt in
the crucible. The pulling down rate were 0.3 mm/min.
Details of crystal growth procedure was described inprevious reports [5, 14]. Scanning electron microscope(SEM) observation with an energy dispersive X-ray spectrometry (EDX) detector was performed using a fieldemission SEM (FE-SEM, JSM7800-F, JEOL) to observechemical composition along the growth direction. Electron back scatter diffraction (EBSD) measurements wereperformed to investigate the crystal orientations witha step size of 1 to 2 µm. INCA software was used for(1009)

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the orientation measurement and analysis. EBSD wasconducted at 15 kV. The magnetostriction was measured
by attaching a strain gauge, and applying magnetic fieldof up to ± 2000 Oe parallel and perpendicular to the crystal growth direction.
3. Results and discussion
At the beginning of this study, we tried to grow oriented Fe0.95 Al0.05 alloy crystals by using a polycrystalline
iron plate because Fe–Al alloy crystals are not marketed. Fe0.95 Al0.05 can be used as a seed for growth of
Fe0.82 Al0.18 , as their melting points are close, without
the low temperature phase transition.
Figure 1 (top) shows the result of the second trialof Fe0.95 Al0.05 alloy crystal growth by the shape controlled µ-PD method. The growth rate was 0.3 mm/min.
The grown plate was shaped with a cross section of
1 × 5 mm2 size and 62 mm length. Figure 1 (bottom)
show inverse pole figure (IPF) images of the first and second trials of Fe0.95 Al0.05 in parallel direction of crystalgrowth. The images focus near 30 mm growth positionof a plate-shaped crystal. The grown crystals showedsingle domain within the field of measurement even in
the first trial. Optimized growth condition identical towhat was reported previously for Fe–Ga alloy was applied
for this Fe0.95 Al0.05 alloy crystal growth. It is thoughtthat the optimized growth condition promoted the stable
crystal growth and expansion of grain size. The measured
disorientations from h100i of body centered cubic structure for the 1st trial was around less than around 23.7◦ .
The grown crystal of first trial was used as a seed forthe 2nd trial. The grown crystal of first trial was cut andpolished along h100i orientation. The measured misorientations from h100i for the second trial was around 10.5◦ .
Finally, we could prepare several plates of Fe0.95 Al0.05
alloy crystals as seeds for Fe0.82 Al0.18 crystal growth.

Fig. 1. As-grown Fe0.95 Al0.05 alloy crystal grown by
the µ-PD method (top), and IPF images of perpendicular plane to the pulling down direction of µ-PD(bottom).

Fig. 2. As-grown Fe0.82 Al0.18 and Fe0.80 Al0.20 alloy
crystals grown by the µ-PD method using
the Fe0.95 Al0.05 seeds (left), and the composition distribution in along the growth direction in the Fe0.80 Al0.20
alloy crystal (right).

Fig. 3. The IPF image of perpendicular plane tothe pulling down direction of the Fe0.82 Al0.18 alloy
crystal.

Figure 2 (left) shows the as-grown Fe0.82 Al0.18 andFe0.80 Al0.20 alloy crystals grown by the µ-PD method
using the Fe0.95 Al0.05 seed.
The growth rate was
0.3 mm/min. The growth plates were shaped witha cross section of 1 × 5 mm2 size, 160 mm and 125 mm
length for Fe0.82 Al0.18 and Fe0.80 Al0.20 alloy, respectively.
The composition distribution along the growth direction of the Fe0.80 Al0.20 alloy crystal is shown in Fig. 2(right). The solidification fraction (g) was about 0.16.
The effective segregation coefficient of Fe and Al showedkeff = 1.02 and 0.942, respectively. Actual average chemical composition of the grown Fe0.80 Al0.20 alloy wasaround 81.2 at.% for Fe and 18.8 at.% for Al. Distributions of Fe and Al were homogeneous along the growthdirection within standard deviations of 0.35% and 1.3%,respectively. Observed homogeneity is better than thatof Fe–Ga alloy grown by the Cz [4] and µ-PD method [5].
In the case of Fe–Al, the linearity between solid line andliquid line in the phase diagram is much closer compared to Fe–Ga. This is one of the reason of the homogeneity. Figure 3 shows inverse pole figure images of
the Fe0.82 Al0.18 in parallel direction of crystal growth.
The image focuses near the 80 mm growth position ofa plate-shaped crystal. The measured misorientation ofthe main grain from h100i was around less than 3.5◦ . The
plate shaped Fe0.82 Al0.18 and Fe0.80 Al0.20 alloy crystals,oriented much close to h100i direction, were successfullygrown by using the oriented Fe0.95 Al0.05 seeds.

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Fig. 5. Photograph of the prototype (top), and time
response of open-circuit voltage (Vop ) for the prototype
device using the plate-shaped Fe–Al alloys (bootom).

Fig. 4. Magnetic field dependence of magnetostrictionin the h100i crystal growth direction for the plateshaped Fe0.80 Al0.20 grown by the µ-PD method (a), andmagnetostriction 3/2λ of Fe–Al alloys as a function ofAl concentration (b). The µ-PD samples are plotted as
red circles on the graph in [7].

The magnetostrictive property was investigated by applying the magnetic field parallel (k) and perpendicular (⊥) to the measuring samples. Figure 4 shows thatmagnetostrictive property of the Fe0.80 Al0.20 sample in
the growth direction depends on the magnetic field. From
the difference between the saturation magnetostrictionof parallel and perpendicular measurements, the value of
3/2λ for the sample was evaluated to be ≈ 149 ppm,
(Fig. 5). The Fe0.82 Al0.18 sample yielded 121 ppm forthe same measurement set-up. This value is fully comparable to Fe–Al single crystal grown by the BS method [7].
It was evident that plate shaped Fe–Al crystals grown byµ-PD method can posses almost the same magnetostriction as single crystal alloy. To evaluate the potential for
vibration energy harvester, a prototype was made using
the Fe–Al alloy samples. An as-grown Fe–Ga alloy wasmachined to the size of 4 × 0.7 × 14.8 mm3 . A copper coil was wound with 3655 turns around the Fe–Alalloys. The prototype was set to a vibration generator and vibrated at an effective vibration accelerationof 0.1 Grms . A photograph of the prototype and time
response of open-circuit voltage (Vop ) is shown in Fig. 5(bottom). The maximum voltage of 0.046 and 0.096 V
was observed using the Fe0.82 Al0.18 and Fe0.80 Al0.20 alloy

samples, respectively. The same examination was performed on a device using commercially available Fe–Ga
alloys (Galfenol) grown by the CZ method. The Fe–Ga
standard has around 300 ppm of 3/2λ value. The maximum voltage was 0.135 V for the Galfenol device. Thecorrelation between magnetstriction of alloys and generated voltage in the energy harvester was confirmed. Fe–Al is highly advantageous in terms of not using rare metals and the cost of raw materials. Although, Fe0.80 Al0.20
showed almost half of magnetstriction and 70% of voltage
generation compared to the Galfenol standard.
4. Conclusion
Plate shaped Fe–Al alloy crystals were successfullygrown using the µ-PD method. At the beginning of
this work Fe0.95 Al0.05 seed crystals with good orientation were tried to be grown by using polycrystalline ironplate. The orientation and grain size could be controlled by using the well oriented Fe0.95 Al0.05 seed crystal. The as-grown Fe0.82 Al0.18 and Fe0.80 Al0.20 shown
the magnetostriction 3/2λ = 121 and 149 ppm, respectively, which is comparable value to single crystal samples grown by the BS method. Fe–Al is highly advantageous in terms of not using rare metals and the cost ofraw materials though Fe0.80 Al0.20 showed almost half ofmagnetstriction and 70% of voltage generation comparing to the Galfenol standard. Moreover, homogeneity ofchemical composition of Fe–Al alloy grown by the µ-PD
method is superior to Fe–Ga alloy grown by µ-PD andCz methods. Fe–Al have better mass productivity andcan be one of promising material for vibration energy
harvester.

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Acknowledgments[8]This work was supported by KAKENHI, Grants-inAid for Scientific Research 17H06159.

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