Magnetic alignment of nickel-coated carbon fibers

Article abstract of DOI:10.1016/j.materresbull.2011.06.032

Magnetic composites of nickel-coated carbon nanofibers have been successfully fabricated by employing a simple microwave-assisted procedure. The scanning electron microscopy images show that a complete and uniform nickel coating with mean size of 25 nm could be deposited on carbon fibers. Magnetization curves demonstrate that the prepared composites are ferromagnetic and that the coercivity is 96 Oe. The magnetic carbon nanofibers can be aligned as a long-chain structure in an external magnetic field.

Full text of DOI:10.1016/j.materresbull.2011.06.032

Materials Research Bulletin 46 (2011) 2090–2093
Contents lists available at ScienceDirect
Materials Research Bulletin
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Magnetic alignment of nickel-coated carbon fibers
a,b,
a
a
Chuncheng Hao *, Xiaojiao Li , Guizhen Wang
a
College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China
State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an, 710049, China
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Magnetic composites of nickel-coated carbon nanofibers have been successfully fabricated by employing
a simple microwave-assisted procedure. The scanning electron microscopy images show that a complete
and uniform nickel coating with mean size of 25 nm could be deposited on carbon fibers. Magnetization
curves demonstrate that the prepared composites are ferromagnetic and that the coercivity is 96 Oe. The
magnetic carbon nanofibers can be aligned as a long-chain structure in an external magnetic field.
ß 2011 Elsevier Ltd. All rights reserved.
Received 10 November 2010
Received in revised form 17 March 2011
Accepted 24 June 2011
Available online 2 July 2011
Keywords:
A. Composites
A. Nanostructures
D. Ferroelectricity
D. Magnetic properties
1
. Introduction
[12] produced fullerene-like Ni–C (graphitic) core–shell nanos-
tructures with a large negative magnetoresistance, of the order of
In recent years, carbon nanofibers (CNFs) and carbon nanotubes
CNTs) have attracted much attention because of their fine
10%. Xiaosi Qi [13] observed a relatively high magnetization value
over carbon nanocoils due to the entrapment of Ni nanoparticles
inside the carbon flakes. However, all these methods were
complicated and difficult to be processed. There is a need to
develop an easier fabrication of magnetic CNFs-embedded
composites with better properties.
Herein, we report a simple microwave-assisted procedure for
the assembly of a compact layer of magnetic nickel nanoparticles
onto CNFs in a water/ethylene glycol system. The CNFs based on
the assembly of nickel nanoparticles show an excellent magnetic
property. The alignment of CNFs in relatively low magnetic fields
could be obtained.
(
electromagnetic wave-absorbing properties, high conductivity,
corrosion resistance and oxidation resistance [1–4]. CNFs are ideal
fillers of structural and functional composites. However, when
introduced into a polymeric matrix, CNFs disperse randomly,
losing their orientations. The randomly embedded CNFs in bulk
samples limited some potential applications of CNFs and exhibited
substantially lower electrical and thermal conductivities [5,6] than
expected.
Coatings, prepared by the attachment of metals (nickel, iron and
cobalt or their alloys) onto CNFs, can overcome these limitations.
The fabrication of magnetic CNFs-embedded composites has been
extensively investigated. Huang [7] prepared CNFs coated with
barium ferrite by sol–gel method and theoretically analyzed their
electromagnetic properties. Xu [8] obtained the similar coated
2. Experimental
The CNFs used in this study were synthesized by copper
3 4
CNFs with complete and uniform Fe O nanoparticles by annealing
nanoparticles using catalytic chemical vapor deposition at low
temperature (300 8C) [14,15]. All of the chemicals used in the
experiments were analytical grade without any further purifica-
tion.
in vacuum in the temperature range of 300–550 8C. Yang [9]
reported an electrochemical method to fabricate iron-coated CNFs.
Recently, Xie [10] employed an electroless deposition route to
deposit continuous and uniform Ni–Fe–Co–P coatings on CNFs. The
coated CNFs exhibited fine electromagnetic wave-absorbing
property in the frequency region of 8–18 GHz. Guzeliya Korneva
2.1. Preparation of composites
[
11] investigated the magnetic CNTs based on the phenomenon of
The starting CNFs were subjected to an oxidation treatment in
spontaneous penetration of wetting fluids into capillaries. S.V. Pol
the nitric acid to modify the surface in order to achieve better
dispersion. A two-step pretreatment, sensitization and activation,
was used to catalyze the CNFs. The sensitizer and activator were
*
Corresponding author. Tel.: +86 532 84022975; fax: +86 532 84022975.
stannous chloride/hydrochloric acid (15 g/L SnCl
HCl) and palladium chloride/hydrochloric acid (0.5 g/L PdCl
2
Á2H
2
O and 41 g/L
and
E-mail address: clx@qust.edu.cn (C. Hao).
2
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.06.032

C. Hao et al. / Materials Research Bulletin 46 (2011) 2090–2093
2091
Fig. 1. SEM images of nickel-coated coiled CNFs.
1
2 g/L HCl) solutions, respectively. Ultrasonic vibration was
applied during the catalyzation to facilitate uniform activation
of the entire surface of the CNFs.
In a typical experiment to prepare the magnetic composites
of nickel-coated CNFs, CNFs and 0.15 g NiCl
dispersed in 40 mL ethylene glycol by ultrasonic vibration for
.5 h. 0.12 g N O (80 wt%) solution was added dropwise to
ÁH
2
Á6H
2
O were
0
2
H
4
2
the solution, followed by the addition of 2 ml 0.25 M NaOH
aqueous solution. 60% (425 W) of the output power of the
microwave was used to irradiate the mixture for 5 min. Then,
the microwave heating was stopped and the products were
collected by centrifugation, washed with distilled water and
absolute ethanol repeatedly for three times, and dried at 45 8C
under vacuum.
2.2. Characterization
The microwave oven (2.45 GHz, maximum power of 750 W)
used for sample preparation was a focused single-mode micro-
wave synthesis system which was equipped with mechanical
stirring and a water-cooled condenser. The temperature was
controlled by the automatic adjustment of the microwave power.
X-ray powder diffraction (XRD) measurements were performed on
Fig. 2. XRD pattern of nickel-coated coiled CNFs.
a Rigaku D/max-
˚
(l = 1.54178 A) using a 40 kV operation voltage and 70 mA current.
g
A X-ray diffractometer with Cu K
a
radiation
diffraction peaks can be found, indicating that the coating consists
only of nickel nanoparticles.
Scanning electron microscopy (SEM) images were recorded by
JEOL JSM-6700F equipment. Magnetic properties were recorded in
a quantum design vibrating sample magnetometer (VSM) at room
temperature.
The magnetic properties of nickel nanoparticles and the nickel-
coated CNFs were determined using a vibrating sample magne-
tometer. Fig. 3 shows the magnetization curves and the enlarged
hysteresis loops of the products. It is clear to be seen from Fig. 3a
that the nickel nanoparticles and the nickel-coated CNFs are all
ferromagnetic. The saturation magnetization (Ms) of nickel
nanoparticles and magnetic composites are 29.0 emu/g and
3
. Results and discussion
6.0 emu/g, respectively, considerably smaller than that of bulk
Fig. 1 shows SEM images of nickel-coated CNFs. The nickel
nickel (Ms = 54.39 emu/g) [16]. The curve of the composites, as
assumed, mainly displays the magnetic contribution of nickel
nanoparticles deposited on the CNFs. Thus, the nickel-coated CNFs
are ferromagnetic with sizes beyond the super paramagnetic limit,
which is 15 nm for spherical nickel nanoparticles [17]. As can be
seen in Fig. 3b, the nickel nanoparticles and magnetic composites
have different coercivities. After the nickel nanoparticles coated
CNFs, the coercivity decreases from 209 Oe to 96 Oe. The magnetic
composite tends to transform to paramagnetism.
Fig. 4 shows illustration of the process and the SEM images of
nickel-coated CNFs in the absence (c) and presence (d, e) of an
external magnetic field. The specimens were obtained when
ethanol solution containing magnetic carbon nanofibers was
drying. Fig. 4b shows the histogram of the chain length. It is clear
nanoparticles were synthesized by redox in microwave. CNFs were
coated directly by the prepared nickel nanoparticles in one-step. As
the low magnification SEM image shows in Fig. 1a, the CNFs have
been coated by a uniform layer of nickel particles. The CNFs are 50–
2
00 nm in diameter. The dense layer of nickel homogeneously
covers the CNFs. Fig. 1b shows part of the CNFs which are coated by
nickel nanoparticles. It can be observed that the nickel nanopar-
ticles are spheres and 25 nm in diameter. No aggregation exists
around the CNFs.
The XRD pattern of as-synthesized samples is shown in Fig. 2.
The diffraction peaks at 2u = 44.498 and 51.838 can be assigned to
(
1 1 1) and (2 0 0) planes of nickel, respectively. The diameter of
nanoparticles characterized from Scherrer formula is 21.5 nm,
which is slightly lower than that observed in Fig. 1. A scattered
peak at 20–408 is considered to be the carbon peak. No other
that the nanofibers are mostly 50–80
mm long. As no magnetic
field was applied in Fig. 4c, the CNFs randomly deposit on a silicon

2
092
C. Hao et al. / Materials Research Bulletin 46 (2011) 2090–2093
Fig. 3. Magnetization curves (a) and the enlarged hysteresis loops (b) of nickel nanoparticles and the nickel-coated CNFs.
Fig. 4. (a) Illustration of the process; (b) histogram of the chain length; (c) SEM image of nickel-coated CNFs in the absence of an external magnetic field; (d and e) SEM images
of nickel-coated CNFs with an external magnetic field (0.5 T).
substrate which agrees with reported observations [18]. In order to
visualize the magnetic response of these magnetic CNFs, a small
parallel magnetic field (magnetic flux density = 0.5 T) was applied
when the CNFs dried on a silicon wafer. It is interesting that the
coated CNFs were aligned as a long-chain structure under magnetic
field (see Fig. 4d and e). The arrows indicated the direction of the
magnetic field.
In zero field, the magnetic moments of the nickel nanopar-
ticles randomly point in different directions, resulting in a
vanishing net magnetization. However, after a sufficiently large
homogeneous magnetic field was applied, the magnetic
moments of the nanoparticles aligned in parallel. And the
resulting dipolar interactions are sufficiently large to overcome
thermal motion and to reorient the magnetic CNFs favoring the

C. Hao et al. / Materials Research Bulletin 46 (2011) 2090–2093
2093
formation of chains of CNFs [19]. As shown in Fig. 4d and e, these
chain-like structures are formed by connecting the magnetic
CNFs in line, touching each other in a head-to-tail fashion. This
can be explained by capillary and Van der Waals forces. The
magnetic chains form double or triple structures which are
parallel and close to each other but are offset to each other along
the long axis.
Youth (2007BS04003) and State Key Laboratory of Electrical
Insulation and Power Equipment (EIPE10207).
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Acknowledgements
The authors would like to acknowledge the support by National
Natural Science Foundation of China (51077075), Science and
Technology Program of Shandong Provincial Education Depart-
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