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Colloid Polym Sci (2010) 288:79–84
DOI 10.1007/s00396-009-2138-5
SHORT COMMUNICATION
Fabrication of multiwalled carbon nanotube-wrapped
magnetic carbonyl iron microspheres
and their magnetorheology
Fei Fei Fang & Hyoung Jin Choi
Received: 17 August 2009 / Revised: 21 September 2009 / Accepted: 9 October 2009 / Published online: 27 October 2009
# Springer-Verlag 2009
Abstract Magnetorheological (MR) properties and dispersion stability of magnetic carbonyl iron (CI) microspheres
were examined and found to be enhanced by fabricating a
dense nest composed of multiwalled carbon nanotubes
(MWCNTs) on the surface of CI particles in this study. The
coating process is achieved by using 4-aminobenzoic acid
as a grafting agent via self-assembly mechanism under
sonication in which the MWCNTs were adopted as the
coating material because MWCNTs possess similar density
with polymer but better magnetic properties due to the iron
catalyst originally included within the walls. The coating
thickness and morphology of the MWCNTs nest were
found to be related with the sonication duration. The
influence of the coating layers on the magnetic properties
and MR performance (yield stress behavior, shear viscosity)
were examined using a vibrating sample magnetometer and
rotational rheometer. Sedimentation rates of the fabricated
MWCNT/CI suspension and pure CI suspension were also
investigated.
Keywords Magnetorheological fluid . Carbonyl iron .
Multiwalled carbon nanotube . Sedimentation
Introduction
A magnetorheological (MR) fluid is a fascinatingly smart
and intelligent suspension which is capable of alternating
the state between liquid-like and solid-like with the aid of
an external magnetic field [1–4]. Usually, the MR fluids are
F. F. Fang : H. J. Choi (*)
Department of Polymer Science and Engineering, Inha University,
Incheon 402-751, South Korea
e-mail: [email protected]
composed of nonmagnetic continuous medium and magnetic particles which respond to a magnetic field. The
medium of MR fluid can be various hydrocarbon oils or
even ionic liquid, while the dispersed magnetic particles are
generally micrometer-scaled ferromagnetic or paramagnetic
particles. Without an external magnetic field, all particles
float freely, causing the materials to behave like any
colloidal mixture. When the magnetic field is applied, the
magnetic particles immediately become active and attract
each other to form a chain-like structure and thus turn to be
a viscoelastic solid. During this state-changing process
from a liquid-like state to a solid-like state, the rheological
properties (yield stress, apparent viscosity, storage modulus,
etc.) get changed obviously. Therefore, MR fluids, along
with their electrically analogous electrorheological fluids
[5–9], have attracted considerable attention for a variety of
engineering applications, such as dampers, torque transducers, or polishing devices [10, 11].
In order to meet the advanced engineering applications,
plenty of magnetic species or magnetic alloys have been
explored [12–16]. So far, huge attention is focused on a
kind of soft magnetic material: carbonyl iron (CI)
microbeads, which are considered to be excellent
candidates for MR fluids due to their superior magnetic
properties and suitable size [17–20]. Nevertheless,
CI-based MR fluid has one serious defect in sustaining
dispersion stability because the density of CI particles
(7.91 g/cm3) is much larger than that of continuous
medium (0.84 g/cm3) in general. Therefore, considerable
effort (introducing submicron additives or polymer coating technology) has been made to prevent CI particles
settling or decrease the density of CI composite particles,
which would improve the sedimentation rate indirectly.
Various submicron fillers (carbon nanotube, graphite
nanotube, organoclay, and fumed silica) have been intro-
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duced to the CI-based MR suspensions, by which way the
interspaces of the CI particles are occupied, consequently
preventing the direct contact of CI particles and improve
the stability of a MR fluid [21–24]. Additionally, the
employed nanofillers can also play a role on repairing the
deformed chain structure under an external shear field.
However, when we evaluate the role of nanofillers in
improving the stability for MR fluids, we have to consider
the effects of size dimension and morphology of the
additives, as well as the affinity with CI particles. Besides
this system, polymer coating technology has also played an
important role in improving the dispersion stability. There
are two ways to fabricate polymer/CI composite particles.
One is to embed CI particles within polymeric matrix via a
solvent-casting method; the other is to wrap CI particles
with polymeric shell via dispersion polymerization. The
advantage of the former method is that the sedimentation
problem can be improved due to the decreased density by
introducing polymeric matrix; however, the MR behavior is
relatively low because the polymer is nonmagnetic [25, 26].
As for the latter method which successfully provides
individual composite particles with a favored core–shell
structure, the experimental condition has to be carefully
adjusted to obtain the expected coating [27, 28]. Properly
selected grafting reagent, the mole ratio of the monomer,
initiator, or stabilizer as well as the reaction temperature has
been proved to play a critical role in affecting the final shell
product. Considering these mentioned complicated procedures, a new strategy based on the coating technology has
to be developed.
Recently, researches have focused on multiwalled carbon
nanotubes (MWCNTs) used as filler materials in fabricating
polymer/MWCNTs nanocomposites due to its synergistic
mechanical, electrical, and thermal properties. Chemical
functionalization of the MWCNTs with carboxyl/amino
groups (COOH/NH2) or physical adsorption of dispersants
onto the walls of MWCNTs has been developed to enhance
the dispersibility of MWCNTs in various solvents [29]. In
addition, MWCNTs exhibit a unique self-assembling ability
[30]. Therefore, in this study, a dense nest of MWCNTs
was constructed on the surface of CI particles using
4-aminobenzoic acid (PABA) as a grafting agent. There
are two other principal reasons for choosing MWCNTs as a
shell material for coating CI particles. On one hand, the
density of MWCNTs is similar to that of the polymer. On
the other hand, the existence of a magnetic iron catalyst
within the CNT wall may improve the magnetic properties,
which does not occur in the polymer coating systems. The
thickness and morphology of the MWCNTs nest were
examined by alternating the sonication duration. The
magnetic properties and MR performance (yield stress
behavior, viscosity) of the MWCNT/CI particle-based MR
fluid were studied using controlled shear rate and controlled
Colloid Polym Sci (2010) 288:79–84
shear stress methods. Finally, the sedimentation rate of the
particles produced was checked.
Experimental
Sample preparation
In this experiment, conventional core–shell morphology
for the magnetic microbeads was used based on adopting
CI particles (standard CD grade, BASF, Germany) as the
core and MWCNTs nests as the shell. The average particle
size and density of the CI particles was approximately
4.25 μm and 7.91 g/cm3, respectively.
Initially, the grafting agent (3 g), PABA, was dispersed
in distilled water at 60 °С for 2 h. Raw MWCNTs were
chemically pretreated in a strong acid bath to obtain
COOH-MWCNTs (in short, MWCNTs) [26]. A certain
amount of CI particles (8 g) was dispersed in this solution
and sonicated for 15 min to modify the surface. After
removing the excess PABA by washing it with distilled (di)
water, the PABA-modified CI particles were added to the
reactor in which a dispersion of MWCNTs (0.1 wt.% of CI
particles) in di water had been prepared. The reaction was
maintained under sonication at room temperature with
vigorous stirring (450 rpm) to avoid sedimentation due to
the large density of the CI particles. Initially, the color of
the reaction dispersion was absolute black due to the
homogeneous dispersion of MWCNT in di water. With
time, the liquid turned light brown and later became
transparent brown. On the other hand, the CI particles
changed color from initially silver gray to complete
black, which suggests the successful wrapping of
COOH-MWCNT. Finally, the fabricated MWCNT/CI
particles were obtained by washing with di water and
drying in a vacuum oven at 60 °С for 24 h.
Characterization
In order to obtain densely coated MWCNT nests and a
perfect spherical profile along the surface of the CI
particles, the coating process was repeated for different
sonication durations (12 and 24 h) with the other reaction
condition left unchanged. The surface morphology of the
pure CI particles and MWCNT/CI particles was observed
by scanning electron microscopy (SEM; S-4200, Hitachi
Japan). X-ray energy dispersive spectroscopy was also
performed using an attached EDAX (couple with Hitachi
S-4200) spectrometer. The section view of the MWCNT/CI
particles was examined by transmission electron microscopy (TEM; Philips CM200). Sampling for TEM was
performed by an initial molding of particles dispersed in an
epoxy bath followed by nanoscaled cutting by using an
Colloid Polym Sci (2010) 288:79–84
ultramicrotome as well as the final dropping onto a copper
grid. Both the density and magnetic properties were
examined using a pycnometer and vibration sample
microscopy, respectively. Finally, the MR behavior of the
MR fluid was investigated using a rotational rheometer
(Physica MC 300, Stuttgart) equipped with a magnetic field
generator. In order to prepare the MR fluid, the synthesized
MWCNT/CI and pure CI particles were dispersed in
lubricant oil (Yubase 8, SK Corp. Korea) with a particle
concentration of 20 vol.%. The MR characterizations were
performed at 20 °С at a controlled shear rate sweep and
shear stress sweep, respectively.
Results and discussion
The unique morphology was examined after the coating. In
Fig. 1a, pristine CI particles exhibited a very smooth
surface and with a polydispersed size distribution. The
chemically treated COOH-MWCNT (the inset picture) used
in this experiment with a mean diameter of 15 nm showed
serious aggregation. After being modified with the
PABA/di water solution under sonication, the morphology
of the CI particles was preserved, as shown in Fig. 1b.
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Therefore, modification with PABA does not affect the
morphology of CI particles but introduces organic chemical
functional groups to the CI particles.
After sonicating the PABA-modified CI particles in a
COOH-MWCNT bath for 12 h, a change in surface
morphology was observed, as shown in Fig. 1c. All particles
show a rough rather than smooth spherical profile. The CI
particles are wrapped with dense MWCNT nest, leading to a
considerably rougher surface than that of the pure CI
particles. Innumerable MWCNT pile together, thus forming
a firm network covering the entire surface of the CI particles.
In addition, the length of MWCNT shown in the nest appears
to be shorter than that of the pure COOH-MWCNTs, which
is attributed to the applied sonication.
An additional experiment without PABA was performed
to determine the role of PABA as a grafting agent. There
was no apparent MWCNT layer around the CI particles,
confirming that the PABA plays a key role in joining the
hydrophilic CI particles and MWCNT networks. Knowing
that the PABA has a carboxyl group (COOH) and amino
group (NH2), it was assumed that an amide functional
group might form between the PABA-modified CI and
COOH-MWCNTs. In addition, a noncovalent bond appearing between the adjacent –COOH functionalized MWCNTs
Fig. 1 SEM images of pure CI particles (a) and used COOH-MWCNT (inset; the scale bar is the same), PABA-modified CI particles (b),
MWCNT-wrapped CI particles sonicated for 12 h (c), and MWCNT-wrapped CI particles sonicated for 24 h (d)
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may also help to form the network of MWCNTs [31, 32]. A
further study using either Fourier transform infrared or
X-ray photoelectron spectroscopy will be necessary to
confirm this prediction.
The coating process was repeated twice in order to make
the MWCNT coating on the CI particle surface as dense as
possible, and the effect of the sonication duration was
examined. When the COOH-MWCNTs are dispersed in di
water without a dispersant, sonication will help break the
MWCNT agglomerates leading to a homogeneous
MWCNT dispersion and chemical bonding between the
carboxyl and amino groups [33, 34]. Therefore, sonication
duration was set to 12 or 24 h to determine the difference.
Figure 1d shows a SEM image of the MWCNT/CI particles
sonicated for 24 h. Unfortunately, the expected denser
MWCNT nest is not observed compared to the apparent
nest structure formed after sonication for 12 h. Although
the surface is rougher than the pure CI particle, which is
possibly caused by the initial coating of MWCNT, many
short MWCNT segments are observed rather than long
ones. This might be due to the vigorous long-term
sonication, which may cut the long MWCNT. The
interspace of the adjacent CI particles is filled with short
MWCNT segments. Therefore, the MWCNTs within the
interspaces may be less affected by sonication than those
loaded on the surface of the CI particles. In this experiment,
the chemical interaction produced between the PABAmodified CI and COOH-MWCNTs as well as the noncovalent bond within the –COOH functional groups of the
adjacent MWCNTs is believed to be the driving force for
constructing the MWCNT network [30]. Although the
optimal sonication duration could not be determined
exactly, excess sonication does have an adverse effect on
the coating quality.
The cross-sectional view of an individual MWCNT/CI
particle was examined by TEM to observe the internal
structure within the MWCNT nest and determine the coating
thickness, as indicated in Fig. 2. The internal black center
represents the CI core particle, while the outside fibril part is
the MWCNT nest. Compared to pure CI particles (the inset
image), which have a fairly smooth surface, the MWCNTcoated CI particles exhibited rough surface profile as well as
the expected core–shell structure. The MWCNT nest is
composed of huge aggregated MWCNTs, probably due to
the overlap of multilayer MWCNT shell. Interestingly, the
MWCNT is short and straight, besides, some leakages are
also observable. The short segments resulted from the
sonication which is a general method to cut MWCNT with
long chains. The orientation of MWCNT as well as leakages
can be attributed to the cutting process which produces
alignment along the direction of cutting and may destroy the
network. The coating thickness of the constructed MWCNT
nest was approximately 80 nm.
Colloid Polym Sci (2010) 288:79–84
Fig. 2 TEM image of MWCNT-wrapped CI particles; the inset image
is the TEM view of pure CI particles
Figure 3 shows the magnetic hysteresis loops measured
in the powder state for the fabricated MWCNT/CI particles
and pure CI particles. There was a small difference in
saturation magnetization between the pure CI particles and
MWCNT-coated CI particles in which the former and latter
was approximately 217 and 209 A∙m2/kg, respectively. This
little difference can be attributed to the built MWCNT nest
which possesses weak magnetic property due to the small
amount of residue iron catalyst within the walls. In
addition, the intrinsic hysteresis behavior of the CI particles
was well sustained in MWCNT/CI composite particles.
MR characterization was performed at magnetic field
strengths ranging from 0 to 343 kA/m. The yield stress was
Fig. 3 Magnetization (M) curves of pure CI and MWCNT/CI
particles as a function of magnetic field strength (H)
Colloid Polym Sci (2010) 288:79–84
examined via two modes: controlled shear rate (CSR) and
controlled shear stress (CSS). The CSR mode is the most
common way of obtaining a yield stress by shearing the
sample over a range of shear rates, then plotting the shear
stress as a function of the shear rate under different
magnetic field strength [35–37]. Figure 4 represents the
changes of shear stress and shear viscosity for MWCNT/CI
suspension. A characteristically typical shear-thinning
behavior of a concentrated suspension is observed. The
continuous increase of shear viscosity and shear stress with
the strength of the magnetic field corresponds to the general
feature of MR fluids due to the induced robust columns.
When the magnetic field is present, all shear stress curves
shifted upward, representing a wide plateau range over the
entire region of the applied shear rate, which may be caused
by the robustly formed columns via the strong dipole–
dipole interaction between the adjacent magnetic particles.
The intersection on the stress axis is then taken as the
dynamic yield stress demonstrating that any stress below
this is insufficient to cause the sample to flow [38]. In this
system, the dynamic yield stress at magnetic field strengths
of 86, 171, and 343 kA/m was extrapolated to be
approximately 0.76, 3.3, and 11.3 kPa, respectively.
The static yield stress was also examined using the CSS
approach, which begins testing in the rest state and
increases the stress to a critical value at which the sample
begins to flow [39]. Figure 5 shows the change in viscosity
as a function of the shear stress in which the viscosity
initially holds its value at a low shear rate region then
decreases abruptly, finally approaching a constant value at
high shear stress. A static yield stress developed below
which there was no real macroscopic flow. From the point
marked with the arrowhead, all curves indicated an
approximately four-decade drop in viscosity. Therefore,
the value of this point can be considered to be the static
Fig. 4 Flow curves (CSR mode) of MWCNT/CI particles-based MR
fluid under various magnetic field strength: shear viscosity (left axis);
shear stress (right axis)
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Fig. 5 Shear stress curves (CSS mode) for MWCNT/CI particles-based
MR fluid under various magnetic field strength
yield stress, which is about 1.02, 5.31, and 13.9 kPa.
Indeed, the sample underwent a creep behavior below this
stress, but it was assumed to be static for simplicity.
Finally, the sedimentation problem was examined. The
density of the synthesized MWCNT/CI particles is
6.42 g/cm3, which is much lower than that of the pure CI
particles (7.91 g/cm3). Obviously, the density mismatch
between the dispersed particles and medium oil (0.84 g/cm3)
is reduced, which should lead to an improved sedimentation
rate [17, 27, 28]. Figure 6 shows the sedimentation ratio of
the MWCNT/CI suspension and pure CI suspension as a
function of time. In this method, settling of the macroscopic
phase boundary between the concentrated suspension and
supernatant liquid was observed. It should be noted that,
initially, the MWCNT-coated CI particles settled down more
slowly than that of the pure CI particles for the same time
Fig. 6 Sedimentation ratio as a function of time for CI (red symbols)
and MWCNT/CI (black symbols) suspensions; the inset image is a
snapshot taken for CI and MWCNT/CI suspensions
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Colloid Polym Sci (2010) 288:79–84
duration. With time, after most particles had settled down
completely, the MWCNT-coated CI suspension contained
many MWCNT/CI particles still suspended in the medium
oil as shown in the inset image of Fig. 6. In contrast, the pure
CI suspension showed an almost transparent supernatant
which suggested its rapid sedimentation. Actually, when the
MR fluid was prepared and a MR test was performed, the
MWCNT/CI particles dispersed more easily in the medium,
and a homogeneous redispersion of the MR fluid could be
achieved by mild shaking. The improved sedimentation and
easy redispersion can be attributed to the reduced density and
rough surface. Compared with the smooth surface of the pure
CI particles, the rough surface of MWCNT/CI particles
produces a friction force with the medium oil, which may
help postpone settling and reduce the sedimentation rate.
Conclusions
Unique spherical MWCNT/CI particles with a dense
MWCNT coating were fabricated using PABA as the
grafting agent based on the self-assembling principle of
MWCNTs. The perfect core–shell morphology and coating
thickness was confirmed by SEM and TEM. Long-term
sonication was not favored to construct a much denser
MWCNT nest on the CI particle surface. Although the
possible mechanism of forming a MWCNT nest was
assumed, further physical characterization should be continued. In the case of MR characterization, both the
dynamic yield stress and static yield stress were examined.
Finally, sedimentation was observed to be improved by the
reduced density and easy redispersion. Work on coating the
surface of CI particles more densely with a MWCNT shell
to improve sedimentation is currently underway.
Acknowledgement This study was supported by a grant from Inha
University (2009) via the GU8 program.
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