Nanostructured Fe-Pd thin films for thermoelastic shape memory alloys-electrochemical preparation and characterization

Article abstract of DOI:10.1016/j.electacta.2005.11.044

Nanostructured Fe-Pd thin films with about 30 at.% Pd have been successfully synthesized on the substrates of Pt buffer layer/Cr seed layer/Si by an electrodeposition process from a plating bath containing ammonium tartrate, citric acid and ammonia solution as complexing agents. Results clearly show that the as-deposited films with body-centered cubic structure were transformed into face-centered cubic structure by heating at 900 °C for 45 min followed by quenching into iced water. The in situ X-ray diffraction analysis results indicate that the quenched film with 29.8 at.% Pd undergoes a reversible thermoelastic austenite-to-martensite transformation with a narrow temperature hysteresis and a martensite transformation temperature of about -30 °C. The present study demonstrates the effectiveness of electrodeposition for synthesizing nanostructured Fe-Pd thin films for the application of low-temperature-type thermoelastic shape memory alloys.

Full text of DOI:10.1016/j.electacta.2005.11.044

Electrochimica Acta 51 (2006) 4250–4254
Nanostructured Fe–Pd thin films for thermoelastic shape memory
alloys—electrochemical preparation and characterization
a,∗
a
a
a
Feng Wang , Sayaka Doi , Kaori Hosoiri , Hirohisa Yoshida ,
b
b
a
Toshio Kuzushima , Masao Sasadaira , Torhu Watanabe
Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan University,
-1 Minami-ohsawa, Hachioji-shi, Tokyo 192-0397, Japan
Electroplating Engineers of Japan Ltd., 5-50 Shinmachi, Hiratsuka, Kanagawa 254-0076, Japan
a
1
b
Received 4 August 2005; received in revised form 28 November 2005; accepted 30 November 2005
Available online 26 January 2006
Abstract
Nanostructured Fe–Pd thin films with about 30 at.% Pd have been successfully synthesized on the substrates of Pt buffer layer/Cr seed layer/Si
by an electrodeposition process from a plating bath containing ammonium tartrate, citric acid and ammonia solution as complexing agents. Results
◦
clearly show that the as-deposited films with body-centered cubic structure were transformed into face-centered cubic structure by heating at 900 C
for 45 min followed by quenching into iced water. The in situ X-ray diffraction analysis results indicate that the quenched film with 29.8 at.% Pd
undergoes a reversible thermoelastic austenite-to-martensite transformation with a narrow temperature hysteresis and a martensite transformation
◦
temperature of about −30 C. The present study demonstrates the effectiveness of electrodeposition for synthesizing nanostructured Fe–Pd thin
films for the application of low-temperature-type thermoelastic shape memory alloys.
©
2005 Elsevier Ltd. All rights reserved.
Keywords: Iron–palladium; Thin films; Electrodeposition; Martensitic phase transformation
1
. Introduction
can be improved from bulk form by two orders of magnitudes
3].
Fe–Pd alloy films with about 30 at.% Pd are considered to
[
Shape memory alloys (SMAs) as the intelligent/smart mate-
rials have attracted considerable attention for their applications
of sensors and microactuators, which can be controlled either by
alternating temperature in a certain range or by alternating inten-
sity of an external magnetic field at constant temperature [1]. The
shape memory effect in the thermoelastic SMAs is related to the
austenite-to-martensite reversible thermoelastic transformation
that is driven by a thermal field. The bulk thermoelastic SMAs,
however, are not suitable for the rapid actuation because the
response speed of actuators is significantly limited by the heat
conduction of materials [2]. One of way to get rid of this dis-
advantage is to fabricate the SMAs nanostructured thin films. It
has been proven that the actuation response speed in thin films
be highly promising SMAs due to their strain reversibility with
a narrow hysteresis and high ductility of the alloy films [4].
Numerous methods, such as magnetron sputtering [5], electro-
magnetic melt-spinning [6], and arc-melting methods [7–9] have
been employed to prepare Fe–Pd thin films. Although an elec-
trodeposition process has been proven to be a feasible and
effective electrochemical means for synthesizing various func-
tional nanostructured materials [10], however, there is only a
few research works on Fe–Pd thin films [11] by the electrode-
position method because of the great difficulty in preparing
stable and effective deposition electrolytes. In particular, to the
best of our knowledge, there has no report on the thermoelastic
behavior of Fe–Pd thin films prepared by the electrodeposition
technique.
In the present study, we describe an effective and reliable
method based on the electrodeposition method that is suit-
able for the preparation of nanostructured Fe–Pd thin films.
Firstly, we have developed a novel plating bath and appropriate
∗
Corresponding author at: Department of Chemistry and Material Engineer-
ing, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano-shi,
Nagano 380-8553, Japan. Tel.: +81 26 269 5670; fax: +81 26 269 5667.
E-mail address: wangf@gipwc.shinshu-u.ac.jp (F. Wang).
0
013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2005.11.044

F. Wang et al. / Electrochimica Acta 51 (2006) 4250–4254
4251
operating parameters for fabricating the Fe–Pd thin films with
about 30 at.% Pd. Subsequently, by X-ray diffractometer (XRD),
scanning electron microscope (SEM) and high-resolution trans-
mission microscope (HR-TEM), the microstructures and the
thermoelastic behaviors of the films have been investigated in
detail.
2
. Experimental
The plating bath consisted of 0.1875 mol l FeSO4·7H2O
−
1
−
1
and 0.0125 mol l PdCl2·6H2O as the source of metal ions
2
+
2+
−1
−1
(
(
Fe /Pd = 15:1). In addition to the metal salts, 0.375 mol l
NH4)2C4H4O , 0.0625 mol l C H8O7·H2O and 0.30 mol l
−
1
6
6
−
1
NH3·H2Owereemployedasacomplexagents, while0.6 mol l
NH4Cl was added as the supporting electrolyte. The pH of plat-
ing bath was finally adjusted to 8.0 with dilute NH3·H2O. The
Fe–Pd thin films were electrodeposited on Pt buffer layers with
3
0 nm thickness, which were grown on Cr-seeded Si substrates.
The buffer/seed layers were deposited by the sputtering method.
The anode material was a titanium grid coated with platinum.
−1
Fig. 1. Voltammetric curves for deposition of Fe and Pd: (a) 0.1875 mol l
−
2
2
+
−1
2+
−1
The current density was ranged from 8.0 to 30 mA cm , and all
Fe ; (b) 0.1875 mol l
(c) 0.0125 mol l
Fe + 0.375 mol l
(NH ) C H O + NH ·H O;
4
2
4
4
6
3
2
◦
−1
Pd2+
−1
2+
−1
the deposition processes were performed at 25 C under the total
;
(d) 0.0125 mol l
Pd + 0.0625 mol l
−
1
−
2
C H O ·H O + NH ·H O; (e) (b) + (d). Supporting electrolyte: 0.6 mol l
6
8
7
2
3
2
electric charge of 10 C cm . The plating cell was a beaker of
00 cm with magnetic stirrer agitation. Finally, the as-deposited
films were sealed in a quartz tube, and heated at 900 C for
−
1
NH4Cl. Scan rate: 2 mV s
.
3
2
◦
4
5 min followed by quenching into iced water.
Polarization behaviors of the plating baths were measured
voltammetric curves for Fe–Pd electrodeposition at the temper-
◦
2+
2+
ature of 25 C. The deposition potentials of Fe and Pd were
compared with those of the solutions without complexing agent.
using an automatic polarization system (Hz-3000, Hokuto
Denko) with an Ag/AgCl electrode as the reference electrode.
All curves were measured at the scanning rate of 2 mV s
The chemical compositions of thin films were measured by
the energy dispersive X-ray analysis (EDX) (JED-2001, JEOL)
performed in a JOEL JEM-6100 scanning electron microscope.
The structures of the deposited films were determined by X-ray
diffractometer (XRD) (M21X TXJ-FO88, MAC Science) and
a high-resolution transmission microscope (HR-TEM) (JEM-
As can be seen, the deposition potentials of Fe2 and Pd are
shifted toward more negative potential by the complex formation
with ammonium tartrate, citric acid monohydrate and ammonia
+
2+
−
1
.
2+
solution, and the potential shift of Pd by complex formation is
2
+
2+
significantly larger than that of Fe . Deposition of the Pd is
2
+
markedly inhibited by the formation of [Pd(NH3)4] complex
ions between Pd2 and NH3·H2O since the complex ions cannot
+
2
+
be reduced as easilyas the free Pd . It is evidentthatthe addition
of complexing agents in the plating baths causes the deposition
2
000FE, JEOL). To characterize the thermoelastic behaviors
of the quenched Fe–Pd films, in situ observation of crystal
structures was performed on the thermal cycling between room
potentials of Fe2 and Pd shift to close with each other, thus
+
2+
2
+
2+
◦
promoting the co-deposition of Fe and Pd . Moreover, it is
observed that the deposition of Pd2 in plating bath contain-
ing complexing agents starts at about −0.65 V versus Ag/AgCl,
while that of Fe2 starts at about −1.1 V versus Ag/AgCl. The
temperature (RT) and −160 C using the X-ray diffraction tech-
+
nique. Magnetization measurements were performed with a
vibrating sample magneto-meter (VSM-5-18, Toei).
+
co-deposition of Fe2 and Pd starts at about −1.1 V versus
+
2+
3
. Results and discussion
2
+
Ag/AgCl. This indicates that for the co-deposition of Fe and
2+
Pd under galvanostatic condition, the cathodic current density
3
.1. Potentiodynamic studies of Fe–Pd electrodeposition
−2
should be controlled to be larger than 5.0 mA cm . The voltam-
metric measurement results demonstrate the effectiveness of the
addition of complexing agents in the plating baths for achiev-
ing the co-deposition of iron and palladium from the aqueous
solutions.
In Fe–Pd, electrodeposition system, it is difficult to obtain
Fe–Pd thin films from the plating baths only containing sim-
ple metal ions since the standard electrode potentials of iron
and palladium differ by approximately 1.4 V. In order to bring
the potentials of iron and palladium close enough to achieve
the co-deposition, ammonium tartrate, citric acid monohydrate
and ammonia solution were used to form complexes with metal
ions in the plating baths. All the plating baths prepared in the
present study showed superior electrochemical stability, thereby
allowing the preparation of Fe–Pd thin films. Fig. 1 shows
3.2. Preparation and characterization of Fe–Pd thin films
Fig. 2 shows the current density-dependence of Pd content
in the Fe–Pd films. A decrease of the Pd content in deposit was
achieved by an increase of cathodic current density. This sug-

4
252
F. Wang et al. / Electrochimica Acta 51 (2006) 4250–4254
Fig. 2. Influence of cathodic current density on the Fe and Pd contents in the
Fe–Pd thin films.
gests that the composition of Fe–Pd thin films can be controlled
by adjusting the cathodic current density, showing a controllable
electrochemical feature.
Fig. 3 shows the representative HR-TEM image of as-
deposited Fe–31.3 at.% Pd thin film and the corresponding
selected area diffraction pattern (SADP). The TEM image shows
a fine-grained structure in the film, and the average grain size is
Fig. 4. X-ray diffraction patterns and SEM images of (a) as-deposited and (b)
quenched Fe–29.8 at.% Pd film on the Pt/Cr/Si substrate. The as-deposited films
◦
are heat-treated at 900 C for 45 min followed by quenching into iced water.
estimated to be around 15 nm, showing a nanostructured feature.
The SADP clearly reveals that the as-deposited film consists of
randomly oriented fine grains with typical body-centered cubic
structure (bcc).
According to their thermal equilibrium phase diagram [12],
the Fe–Pd thin films with about 30 at.% Pd exist as face-centered
◦
cubic structure (fcc, austenite) above approximately 770 C.
Thus it can be considered that the as-deposited films with bcc
solid solution structures can be transformed into fcc austenite
by rapid quenching to avoid the separation of ␣-Fe and ordered
FePd. Fig. 4 illustrates the typical X-ray diffraction patterns and
SEM images of (a) as-deposited and (b) quenched Fe–29.8 at.%
◦
Pd thin films on the Pt/Cr/Si substrates (heating at 900 C for
4
5 min followed by quenching into iced water). It should be
◦
◦
◦
mentioned that three peaks at 2θ = 33 , 62 and 69 are due to
the Si substrate, while one peak at 2θ = 39 in the as-deposited
◦
film is due to the Pt buffer layer. Two peaks corresponding to
the (1 1 0) and (2 1 1) planes of bcc (␣-Fe, Pd) solid solution are
clearly observed in the diffraction pattern of the as-deposited
film, while four peaks corresponding to the (1 1 1), (2 0 0), (2 2 0)
and (3 1 1) of fcc (␥-Fe, Pd) solid solution are observed in the
quenched film. These suggest that the rapid quenching converts
the as-deposited films from bcc structure to fcc structure. More-
Fig. 3. HR-TEM image of as-deposited Fe–31.3 at.% Pd film and the corre-
sponding selected area diffraction pattern (SADP).

F. Wang et al. / Electrochimica Acta 51 (2006) 4250–4254
4253
over, it is worthy to note that the diffraction peak of Pt buffer
layer occurring in the as-deposited film completely disappears
in the quenched films. This can be ascribed to the formation
of Fe–Pd–Pt solid solution through the atoms diffusion of Pt
ment between cooling and heating shows a good reversibility of
the process.
From the XRD results (Fig. 5), the variations of the lattice
constants of Fe–Pd film are calculated according to Bragg’s for-
mula. Fig. 6 shows the changes of (1 0 0)fcc and (1 0 0)fct lattice
constants of Fe–Pd films as a function of the temperature of
◦
toward the Fe–Pd films under heating at 900 C for 45 min. The
addition of Pt lowers the martensite transformation temperature
and strengthens the films due to the solution hardening [13].
The SEM images present that the rapid quenching roughens the
surface morphology of thin films probably associated with the
change of phase structure.
◦
thermal field. At the temperature from RT to −20 C in the
cooling process, the lattice constant of (1 0 0) keeps as a con-
fcc
stant of about 0.370 nm, showing a independence correlation
to the cooling temperature. As the temperature changes from
◦
−
40 to −160 C, a new fct phase appears instead of fcc phase.
The lattice constant of (1 0 0) gradually increases reaching
3
.3. Thermoelastic behavior of Fe–Pd thin films
fct
as about 0.384 nm with lowering the temperature. The reverse
phenomenon is observed in the heating process, that is, the lat-
tice constant of (1 0 0)fct gradually decreases with increasing
In order to observe the thermoelastic behavior of quenched
Fe–Pd films, in situ observation of XRD spectral changes with
the change of temperature was performed. The measurement
◦
the temperature from −160 to −40 C and completely disap-
◦
◦
◦
pears at the temperature between −20 C to RT, meanwhile,
was carried out in a diffraction angle from 46 to 52 while
◦
the fcc phase appear as the new phase, and the lattice con-
stant of (1 0 0)fcc keeps as a constant independent of the heating
temperature. The changes of lattice constant as a function of
temperature both in the cooling and heating process indicate a
narrow temperature hysteresis of austenite-to-martensite trans-
formation. Accordingly, the hysteresis curves are approximately
fitted as the broken lines in Fig. 6. Based the fitted curves, we can
approximately estimate a narrow temperature hysteresis of the
keeping the sample temperature constant at an interval of 20 C
◦
in thermal cycling between RT and −160 C. Fig. 5 shows the
changes of (2 0 0) and (2 0 0) (fct: face-centered tetragonal,
fcc
fct
martensite) peaks of quenched Fe–29.8 at.% Pd film at various
temperatures during (a) cooling and (b) heating process. As pre-
◦
sented in Fig. 5(a), the peak of (2 0 0)fcc is observed at 2θ = 49.1
at RT. This peak decreases with the decrease of temperature and
◦
◦
disappears at −40 C, while (2 0 0) peak at 2θ = 47.3 appear
fct
◦
◦
fcc–fct transformation of about 10 C and the fcc-to-fct trans-
as second phase and gradually increases by cooling to −160 C.
◦
formation temperature TM of about −30 C. The increase of the
Then it can be considered that an austenite-to-martensite trans-
◦
lattice constant of (1 0 0) during the cooling process may be
formation takes place at −20 to −40 C during the cooling
fct
caused not only by the quantitative increase of martensite phase
in the films, but also by the expansion of a-axis and the shrink-
age of c-axis, which are schematically illustrated in the inset of
Fig. 4. Martensite in Fe–Pd thin films has a tetragonal structure
with the c/a ratio of 0.94–0.96 [14]. Further cooling may cause a
process. The reverse behavior of the phase transformation is
also observed during the heating process in Fig. 5(b), in which
the fct structure returns back to the fcc structure during heating
◦
process from −160 C to RT. Moreover, it is apparent that the
X-ray diffraction patterns of cooling process are almost similar
to those of heating process. This suggests that the cyclic treat-
Fig. 6. Variations of the lattice constants of (1 0 0)fct (cooling and heating) and
(
1 0 0)fcc (cooling and heating): ꢀ, (1 0 0)fct (cooling); ♦, (1 0 0)fct (heating); ꢁ,
Fig. 5. Changes of (2 0 0)fcc and (2 0 0)fct peaks of quenched Fe–29.8 at.% Pd
film at different temperature during (a) cooling and (b) heating.
(1 0 0) (cooling); ꢂ, (1 0 0) (heating). Inset shows the schematic illustration
of austenite and martensite structures.
fcc
fcc

4
254
F. Wang et al. / Electrochimica Acta 51 (2006) 4250–4254
the Fe–Pd thin films may exhibit magnetic field-induced actu-
ation in the martensite phase [17]. This implies the possibility
that the Fe–Pd thin films prepared by an electrodeposition pro-
cess also can be used as the ferromagnetic shape memory alloys
(FSMAs).
4. Conclusions
We have successfully synthesized the nanostructured
Fe–29.8 at.%Pd thin films by an electrodeposition process.
The in situ X-ray diffraction analysis results indicate that the
quenched film with 29.8 at.% Pd undergoes a thermoelastic
austenite-to-martensite transformation with a narrow tempera-
◦
◦
ture hysteresis of about 10 C, and the martensite transformation
temperature is about −30 C. The present study has proven that
the electrodeposition technique is available to the fabrication
of Fe–Pd thin films for the application of low-temperature-type
shape memory alloys.
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Fig. 7. Magnetization curves of quenched Fe–29.8 at.% Pd film measured at RT
◦
and −160 C.
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(
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isobservedintheXRDpatternsinthepresentstudyduringwhole
cooling process, indicating that bct martensite phase would not
[
3
[
[
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3
.4. Magnetic properties of Fe–Pd thin films
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◦
and −160 C (fct) are shown in Fig. 7. The slopes of initial
parts of the magnetization curves are similar for both fcc and fct
phases, showing a similar initial susceptibility. The same sus-
ceptibility in the martensite state reflects a possibility that the
magnetic domain wall movement is as easy as in the austenite
[
[
[
−
1
[
state [16]. The coercivity of fct phase is 7.38 kA m , which
−1
is larger than that of fcc phase (4.61 kA m ), showing that
both austenite and martensite phases of Fe–29.8 at.% Pd films
are ferromagnetic. Considering that martensite phase is more
strongly magnetized under a magnetic field than austenite phase,
[
[
17] H. Morito, A. Fujita, K. Fukamichi, R. Kainuma, K. Ishida, Appl. Phys.
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