# The Study of Magnetorheology of Iron Oxide Nanowires

Authors: S. P. Rwei; L. Y. Wang; M. J. Chen
Journal: Journal of Nanomaterials
 (2013)

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## Abstract

In this work, circular Fe3O4 particles with a diameter of
24 nm and quasicircular Fe3O4 particles with a diameter of
10 nm were synthesized using peptization and coprecipitation methods, respectively.
The coprecipitated particles were further formed into Fe3O4
nanowires at high temperature and high pressure in a strongly alkaline environment. The
optimal environment for forming nanowire was 15 m (molality) NaOH at 120°C for
three days; the resulting proportional relationship between its width and its height, the
aspect ratio, was 50.5/1. In the second part of this study, the nanoparticles and nanowire
were dispersed in silicon oil and formed into magnetorheological (MR) fluids of different
concentrations, before undergoing various MR tests—a shear test, a compression
test, and a creep recovery test. The results revealed that the MR performance of nanowire
fluid was better than that of circular particle fluid, in terms of yield stress
(35 Pa versus 60 Pa), compression displacement (Δd)
(0.19 mm versus 0.44 mm), and creep recovery ratio (82% versus
48%). The experimental results conclude that the nanowire network is more robust than
the nanoparticle network. The test of the storage shelf time revealed that
12 wt% nanowire fluid retained more than 80% of its original yield stress
after three months, indicating slight precipitation in the nanowire fluid. In summary, the
nanowire MR fluid had a stronger MR effect than traditional MR fluid that was prepared
with spherical MR particles.


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## Body

## 1. Introduction

A magnetorheological (MR) suspension can be promptly transformed from a fluid-like
structure to a solid-like structure with a viscoelastic to plastic yielding by subjecting it
into a magnetic field [1–7]. Rabinow [8] discovered this
phenomenon in 1948. Since then, various metal or ceramic powders with magnetic
characteristics have been dispersed in nonmagnetic carrier liquids (such as oil or aqueous
liquids) in the preparation of MR suspensions. Smart damper [9] is an important application of magnetorheological (MR) fluid. The releasing of
its internal energy depends mainly on the viscosity of the dashpot. Therefore, a method for
preparing MR fluid with a wide range of viscosities, ensuring a strong response of the yield
stress or creep recovery from low to high magnetic strength, is crucial. Notably,
Fe3O4, having a net ferromagnetic moment arising from the incomplete
cancellation of spin materials of Fe2+ ions, can respond very sensitively to the
magnetic fields. More importantly, the permanent magnetization of
Fe3O4 nanoparticles is still weak enough that it cannot cause any
interparticle agglomeration under zero magnetic field strength; Fe3O4
nanoparticle thus becomes a good candidate for the preparation of MR fluids. This work
employs various chemical methods to synthesize Fe3O4 magnetic
particles of various sizes and Fe3O4 magnetic nanowire and to form
them into MR fluids, whose MR properties, including shear yield stress, compression
resistance, creep recovery, and yield strength decay, are then observed. The goal is to find
the formulation of MR fluids with optimal performance.


## 2. Experimental

### 2.1. Peptization Synthesis of NanoparticleI[10–12]

Add iron chloride hexahydrate (10.8 g, 80 mmol), sodium oleate
(36.5 g, 240 mmol) to a mixed solution of ethanol (80 mL), deionized
water (60 mL), and hexane (140 mL). Stir the mixture well in a 500 mL
three-neck flask that is mounted on a condensation system. Heat to 70°C and maintain
this temperature for 4.5 h; then purify three times using deionized water
(30 mL) and finally store the iron-oleate complex product in hexane. Add
iron-oleate complex 9 g, oleic acid 1.7 g to 30 g tri-n-octylamine
and stir well in a 500 mL three-neck flask. Heat at 3.3°C/min to 366°C or
300°C and maintain this temperature for 30 min. Wash the excessive oleic acid
from the surface of Fe3O4 nanoparticles using ethanol or hexane,
respectively.


### 2.2. Coprecipitation Synthesis of Nanoparticle II

To 6 g iron chloride tetrahydrate and 12 g iron chloride hexahydrate add
25 mL deionized water and mix to form an ionic solution. Add 50 mL ammonium
hydroxide rapidly while agitating mechanically; keep stirring for 6 min and
coprecipitate [13–16] to obtain Fe3O4 nanoparticles. Use deionized
water to wash off excessive ammonium hydroxide. Remove remaining deionized water in
Fe3O4 nanoparticles by freeze-drying.


### 2.3. Hydrothermal Synthesis of Nanowire III

Put 0.5 g Fe3O4 nanoparticles in a pressurized reaction
flask and add sodium hydroxide solution to a concentration of 15 M. Place the flask
in an oven and allow the hydrothermal reaction [17–19] to proceed at 120°C
(2 atm) for three days to yield Fe3O4 nanowires. Use
deionized water to remove the excess sodium hydroxide solution. Remove the remaining
deionized water from the Fe3O4 nanowires by freeze-drying.


### 2.4. Preparation of Magnetic Fluid

Add materialsI, or II, or III to
silicon oil with viscosities of 10 cp and 100 cp (polydimethylsiloxane
PDMS). Stir well to obtain MR fluid. Pour the MR fluid into the groove on the lower plate
of the rheometer (Anton Paar Physica MCR301 MRD). Then, lower the upper plate until the
clearance is 0.1 cm. The temperature of the rheometer is held constant, 25°C.
Apply various currents and measure the yield stress, the compression displacement, and the
creep recovery rate of the specimen. The electronic micrographs were taken using
transmission electron microscope at 300 kV (TEM Hitachi: H-7100). The X-ray powder
diffractometer (XRD PANalytical X’Pert PRO MPD), operated at 45 kV and
20 mA under CuKα radiation, was applied to analyze the
crystal phase. The detector was scanned over a range of 4θ angles
from 0° to 70° at a speed of 4° per minute.


## 2.1. Peptization Synthesis of NanoparticleI[10–12]

Add iron chloride hexahydrate (10.8 g, 80 mmol), sodium oleate
(36.5 g, 240 mmol) to a mixed solution of ethanol (80 mL), deionized
water (60 mL), and hexane (140 mL). Stir the mixture well in a 500 mL
three-neck flask that is mounted on a condensation system. Heat to 70°C and maintain
this temperature for 4.5 h; then purify three times using deionized water
(30 mL) and finally store the iron-oleate complex product in hexane. Add
iron-oleate complex 9 g, oleic acid 1.7 g to 30 g tri-n-octylamine
and stir well in a 500 mL three-neck flask. Heat at 3.3°C/min to 366°C or
300°C and maintain this temperature for 30 min. Wash the excessive oleic acid
from the surface of Fe3O4 nanoparticles using ethanol or hexane,
respectively.


## 2.2. Coprecipitation Synthesis of Nanoparticle II

To 6 g iron chloride tetrahydrate and 12 g iron chloride hexahydrate add
25 mL deionized water and mix to form an ionic solution. Add 50 mL ammonium
hydroxide rapidly while agitating mechanically; keep stirring for 6 min and
coprecipitate [13–16] to obtain Fe3O4 nanoparticles. Use deionized
water to wash off excessive ammonium hydroxide. Remove remaining deionized water in
Fe3O4 nanoparticles by freeze-drying.


## 2.3. Hydrothermal Synthesis of Nanowire III

Put 0.5 g Fe3O4 nanoparticles in a pressurized reaction
flask and add sodium hydroxide solution to a concentration of 15 M. Place the flask
in an oven and allow the hydrothermal reaction [17–19] to proceed at 120°C
(2 atm) for three days to yield Fe3O4 nanowires. Use
deionized water to remove the excess sodium hydroxide solution. Remove the remaining
deionized water from the Fe3O4 nanowires by freeze-drying.


## 2.4. Preparation of Magnetic Fluid

Add materialsI, or II, or III to
silicon oil with viscosities of 10 cp and 100 cp (polydimethylsiloxane
PDMS). Stir well to obtain MR fluid. Pour the MR fluid into the groove on the lower plate
of the rheometer (Anton Paar Physica MCR301 MRD). Then, lower the upper plate until the
clearance is 0.1 cm. The temperature of the rheometer is held constant, 25°C.
Apply various currents and measure the yield stress, the compression displacement, and the
creep recovery rate of the specimen. The electronic micrographs were taken using
transmission electron microscope at 300 kV (TEM Hitachi: H-7100). The X-ray powder
diffractometer (XRD PANalytical X’Pert PRO MPD), operated at 45 kV and
20 mA under CuKα radiation, was applied to analyze the
crystal phase. The detector was scanned over a range of 4θ angles
from 0° to 70° at a speed of 4° per minute.


## 3. Results and Discussion

Figure1 presents a TEM image of
Fe3O4 nanoparticles that were synthesized by oleic acid peptization.
The diameters of the particles are uniformly distributed with an average of 24 nm.
X-ray diffraction proves that the nanoparticles are pure Fe3O4 (Figure
2 lower curve). The peaks are consistent with the
previous study [20]. Figure 3 presents a TEM image of Fe3O4 nanoparticles that
were formed by coprecipitation. As is seen, the distribution of particle sizes is very
broad—much broader than achieved by peptization. The particles are not spherical but
appear quasicircular or even polygonal. However, X-ray diffraction pattern (Figure 2-upper curve) proves that they are still pure
Fe3O4 particles.Figure 1
The TEM image of Fe3O4 nanoparticles synthesized by oleic acid
peptization.Figure 2
The X-ray diffraction spectrum of Fe3O4 nanoparticle and
nanowire.Figure 3
The TEM image of Fe3O4 nanoparticles formed by
coprecipitation.Figures4(a)–4(c) present the influence of NaOH concentration on the lengths of
Fe3O4 nanowires formed at high temperature (110°C) and high
pressure in a hydrothermal environment. As is seen, the nanowires were longest when the NaOH
concentration was 15 m. As the NaOH concentration was further increased, less
deionized water can be vaporized, resulting in the pressure in the flask at high temperature
being reduced. Consequently, the Fe3O4 nanoparticles are not driven to
combine to form nanowire. The length of the rod shapes is therefore reduced. In short, the
optimal environment in which to form nanowire is 15 m NaOH, which was utilized
hereafter.(a)~(c) The TEM image of Fe3O4 nanowires formed at high
temperature, 110°C, and under various NaOH concentrations, (a) 5 m, (b)
15 m, and (c) 20 m.
(a)
(b)
(c)Figures5(a)–5(d) present a TEM photograph of Fe3O4 nanowires
that were synthesized at various temperatures or reaction durations. The conditions of
Figure 5(a), including a temperature of 120°C,
in which the fewest particles were unreacted, were obviously optimal. Analysis of the image
reveals that the aspect ratio of 50.5 : 1 obtained at 120°C is also the
maximum value. The optimal conditions for forming Fe3O4 nanowire
formation are thus determined to be 15 m NaOH, 120°C, high pressure, and a
strongly alkaline ambient.(a)~(d) The TEM photographs of Fe3O4 nanowires synthesized
under constant NaOH concentration, 15 m, and at various temperatures or reaction
durations, (a) 120°C, 3 days, (b) 130°C, 3 days, (c) 140°C, 3 days, and
(d) 120°C, 9 days.
(a)
(b)
(c)
(d)As shown in Figure6, the yield stress of
Fe3O4 nanowire fluid, 60 Pa, is nearly double that of
Fe3O4 nanoparticle fluid, 35 Pa, at the same concentration
(6 wt%). This result indicates that the asymmetry of nanowire can enhance the MR
effect. Figure 7 plots the dependence of shear stress
on shear rate for various concentrations of Fe3O4 nanowires that were
suspended in silicon oil with various viscosities (10 cp, 100 cp). The MR
fluid exhibits typical Bingham character [21], with a
yield stress threshold at which fluidity in a magnetic field is first exhibited. Figure
7 demonstrates that the yield stress increases with
the nanowire concentration. Moreover, when the solid loading is 12 wt%, the
yield stress of the 100 cp suspension is 1.5 times that of the 10 cp
suspension (1100 Pa versus 720 Pa). This result demonstrates that an increase
of PDMS viscosity results in a higher yield stress at a given Fe3O4
loading. Figure 8 plots the yield stress against
Fe3O4 solid concentrations when 0–36 wt% of
Fe3O4 nanowires are suspended in 100 cp silicon oil. In the
plot, as the solid loading increases, the yield stress increases as approximately a
second-order polynomial (slope
=
2.35) function. Even though the increase of
Fe3O4 loading positively affects the MR effect, the nanowire
agglomeration that arises from high loading may occur seriously. This work demonstrates that
the solid loading of Fe3O4 nanowires can be increased to
36 wt% without causing serious agglomeration. The authors’ ultimate goal
is to formulate a highly effective MR fluid at a sufficiently high concentration so that it
can flow freely in a nonmagnetic field without solid-liquid separation but has a yield
stress that can increase to 104 Pa in a magnetic field.Figure 6
The plot of shear stress against shear rate for MR fluids containing 6 wt%
Fe3O4 nanoparticles and nanowires.Figure 7
The plot of shear stress against shear rate for MR fluids containing nanowires at
various solid loadings and dispersed at different viscosities of PDMS.Figure 8
The yield stress is a function of solid loading of Fe3O4 MR
fluids.Along with the shear test, the compression test can also be employed to characterize MR
fluids. When a magnetic fluid is placed in a magnetic field, MR particles are temporarily in
a network state, as shown in Figure9(a). The purpose
of shear test, as stated previously, is to determine the critical shear stress, also known
as the yield stress that disrupts the network state. Similarly, the action of compression
stress on the network can alter the network structure. Figure 9(b) demonstrates that under a given compression stress, a smaller
change in volume, or a smaller Δd, corresponds to a greater magnetic
attractive force among the magnetic particles within the network. When subjected to an
external force, a more robust network is associated with a greater displacement resistance,
and the displacement in the direction of the force is smaller. Figure 10 reveals that the compression displacements of the
Fe3O4 nanoparticle are 0.39 mm and 0.44 mm in
10 cp and 100 cp silicon oil, respectively. In contrast, the compression
displacements (Δd) of the nanowires dispersed in 10 cp and
100 cp silicon oil are 0.19 mm and 0.07 mm, respectively. The
experimental result demonstrates that nanowire can exhibit favorable MR behavior.
Additionally, the nanowire network has a higher compact density and toughness than the
nanoparticle network. In fact, the particles are orientated and line up along the magnetic
line of force with fewer joints. However, the line derived from the circular nanoparticles
has more junctions than that derived from a nanowire. These junctions are the weakest points
of the line. They usually collapse under a shear force or compression stress (Figures 11(a) and 11(b)). The resistance of the nanowire network to an external force is better
because it has fewer junctions. The results also reveal that 10 cp silicon oil is
associated with a shorter compression displacement than 100 cp silicon oil, perhaps
because the former lubricates the joints less well than the latter. Better lubrication is
generally associated with longer compression displacement.Figure 9
(a), (b) The illustration of structure change for an MR fluid under the compression
test.Figure 10
The plot of normal force versus gap distance for various MR fluids subjecting to the
compression test.The illustration of Fe3O4 nanowires lining up along the magnetic
line of force for the MR fluid (a) without shear and (b) under shear.
(a)
(b)The creep and recovery test [22, 23] focuses on investigating the recovery behavior of a
magnetic network following deformation by shearing, which has rarely been studied as part of
MR investigations. In this work, the elasticity of the magnetic network structure is
determined by applying a constant shear stress less than the yield stress to the MR fluid.
The transient displacement of the MR structure reaches a constant, and then the stress is
removed to observe the recovery of the MR fluid. A high recovery ratio demonstrates
excellent elasticity of the magnetic network. It also indicates that the arrangement of
magnetic particles inside the network is in a very stable state. Notably, the stress applied
herein is less than the yield stress, and none of the magnetic fluids flows; rather, it
exhibits various degrees of displacement. If obvious damage or permanent displacement occurs
in the creep test, then the stress has permanently and detrimentally affected the relative
positions of the network particles, such that the magnetic network structure fails to
recover after shearing. This situation reveals that the solid character of the magnetic
network is not strong. Figure 12 plots the result of
the creep and recovery test for nanoparticles and nanowire. When the shear
stress (10 Pa or 20 Pa) is much less than the yield stresses of the MR fluids
(35 Pa for nanoparticles and 60 Pa for nanowire), the recovery ratio of
elasticity is nearly 100%. However, at a shear stress of 30 Pa, which is close
to the yield stress of nanoparticles, the recovery ratio is only 48%. In contrast,
30 Pa is far below the yield stress of the nanowire, for which the recovery ratio is
82%. This result confirms that a nanowire network is more robust than a nanoparticle
network. Additionally, Figure 12 shows that a
network formed by the magnetic field suffers serious damage when the applied stress is only
slightly less than the yield stress. Figure 13 plots
the creep and recovery of MR fluids when the applied stress exceeds the yield stress. In the
creep stage in Figure 13, the creep strain is not
constant but increases with time. When the creep recovery stage is triggered 100 seconds
later, the recovery is unobservable because the previous magnetic network structure had
completely disintegrated. Even if the applied stress were terminated, a new network that is
oriented with the magnetic field would gradually form, but it would have no relation to the
previously damaged network in terms of structure or relative position of particles.
Accordingly, recovery cannot occur when the applied stress exceeds the yield stress.Figure 12
The plot of creep and recovery test for MR fluids containing 6 wt% of
Fe3O4 nanoparticles and nanowires under various shear stresses
(10, 20, and 30 Pa).Figure 13
The plot of creep and recovery test for MR fluids containing 6 wt% of
Fe3O4 nanoparticles and nanowires under shear stress of
70 Pa, which is far above the yield stresses of both MR fluids. The experiment
was repeated 3 times and a good reproducibility is shown.To determine whether the nanowire fluid meets the operating requirements,
12 wt% nanowire suspensions were stored for one, two, and three months,
restirred, and then individually subjected to the MR shear test. Figure14 shows that the yield stress decays from an initial value of
400 Pa at a rate from 20 Pa–30 Pa per month to 325 Pa
after three months. This result shows that, because the size of the suspended nanowires is
of the order of microns, their Brownian motion cannot overcome gravity, and precipitation
eventually occurs. However, after restirring, this suspended matter is again dispersed,
reducing the agglomeration. Figures 15(a) to 15(c) are TEM images of nanowire fluid with various
storage periods. A comparison with Figure 5(b)
reveals slight wire agglomeration, which, while not severe, should be reduced.Figure 14
The plot of yield stress versus the shelf time for an MR fluid containing
12 wt% of Fe3O4 nanowires.(a)~(c) The TEM images of a MR fluid containing 12 wt%
Fe3O4 nanowires, which were stored at various periods of time:
(a) 1 month, (b) 2 months, and (c) 3 months.
(a)
(b)
(c)


## 4. Conclusions

This systematic study of the preparation of magnetic particles preparation and their MR
characterization supports the following conclusions.(1)
Circular Fe3O4 particles with a diameter of 24 nm and
quasicircular Fe3O4 particles with a diameter of 10 nm
were synthesized by peptization and coprecipitation. The peptized particles were
formed into Fe3O4 nanowire at high temperature and high pressure
in strongly alkaline environment. The optimal reaction conditions for forming
nanowires are 15 m NaOH at 120°C for three days. An aspect ratio as high
as 50.5 : 1 can thus be achieved.
(2)
The MR shear test revealed that the yield stress of Fe3O4
nanowire fluid is double that of the same concentration of Fe3O4
nanoparticle fluid. Furthermore, as the Fe3O4 particle
concentration increases, the yield stress increases following a second-order
polynomial (slope
=
2.35) curve.
(3)
The compression displacement (Δd) of the
Fe3O4 nanowire MR fluid is far less than that of the
Fe3O4 nanoparticle MR fluid (0.19 mm versus
0.44 mm). The creep recovery capacity of Fe3O4 nanowire
also exceeds that of Fe3O4 nanoparticle.
(4)
The creep and recovery test showed that a network formed by the magnetic field had
serious damage once the applied stress was close to the yield stress.
(5)
In the storage test, 12 wt% nanowire fluid retained more than 80% of
its original yield stress after three months.In summary, nanowire magnetic particles can exhibit a better MR effect than traditional
magnetic particles that were prepared with spherical MR particles.



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*Source: 745746-2013-06-15.xml*