J. Ind. Eng. Chem., Vol. 12, No. 5, (2006) 806-810
SHORT COMMUNICATION
Electrical Conductivity of Magnetorheological Suspensions
Based on Iron Microparticles and Mineral Oil in Alternative
Magnetic Field
†
Ioan Bica
Department of Physics, West University of Timisoara, Bd. V. Parvan, no. 4, 300223, Timisoara, Romania
Received March 30, 2006; Accepted July 3, 2006
Abstract: A magnetorheological suspension (MRS) obtained by thermal decomposition of Fe2(CO)9 in
mineral oil with stearic acid was prepared. The mean diameter of the microparticles was 2.10 µm at a standard
deviation of 0.40 µm. For volume fractions of 0.06 and 0.30, the MRS is conductive only for alternative
magnetic fields with a minimal effective intensity of 60 kA/m. The experimental results are discussed.
Keywords: magnetoresistance, magnetorheological suspension, magnetic field, iron microparticles, electrical
conductivity, alternative current
Introduction
Sample Preparation
Magnetorheological suspensions (MRSs) are known
under the generic name of intelligent fluids [1,2]. They
are obtained from mixtures consisting of a liquid phase
media (e.g., mineral oil, silicon oil), tensioactive substances, and ferro- or ferrimagnetic microparticles [3-15].
When mixed with graphite microparticles [16,17], MRSs
become electroconductive under the action of an external
magnetic field, similar to the behavior of electroconductive polymers subjected to an electric field [18].
Recent research [19] has shown that the MRSs obtained
through thermal decomposition of Fe2(CO)9 in mineral
oil with stearic acid become electroconductive in a timeconstant magnetic field. The electrical conductivity of an
MRS depends on the intensity and direction of the
external magnetic field [17]. In this paper, we show that
the MRSs obtained by the procedure described in Ref.
[5] are conductive in an alternative magnetic field.
The electrical conductivity characterizing MRS magnetoresistances occurs in narrow stripes [20]. In addition,
we are in the position to prepare sensors for the detection
of fringe fields in magnetic heads [21] as well as warfare
agents [22].
A mixture consisting of 0.082 kg ±5 % Fe2(CO)9
powder with granulation ranging between 4.5 and 5.2
µm, 0.025 kg ± 5 % mineral oil (Aneron/Merck type),
and 0.002 kg ± 2 % stearic acid was treated thermally
[5] for 1.800 s at the temperature of 510 K ±10 %. An
MRS was obtained, having iron microparticles dispersed
in mineral oil (Figure 1).
The possible losses of mineral oil with stearic acid can be
compensated using the supplier, as described in Ref. [5].
1)
†
To whom all correspondence should be addressed.
(e-mail: [email protected])
Experimental Device
The experimental device used for the study of the
electrical conductivity of MRSs in an alternative magnetic
field is described in Ref. [17]. The overall configuration
of the device is shown in Figure 2.
The device consists of a coil 1, placed on a core 2.
Between the polar parts 3 and 4 there is a magnetoresistance (electric resistance with MRS). The magnetoresistance is built by means of the glass tube 5 and
the electrodes 6. Between the electrodes there is the MRS
(position 7 in Figure 2). The body, consisting of the MRS,
has its length and diameter equal to 0.005 m± 10 %.
The electrodes of the magnetoresistance are provided
Electrical Conductivity of Magnetorheological Suspensions Based on Iron Microparticles and Mineral oil in Alternative Magnetic Field 807
(b)
(a)
Figure 1. (a) Iron microparticles in mineral oil. (b) Cummulative frequency function of the diameter, d of the iron particles: dm,
mean diameter; σ, standard deviation.
Figure 3. Magnetization M function of the intensity Hef for the
MRS with different volume fractions ø.
Figure 2. Experimental device: 1-coil; 2-magnetic core; 3 and
4-polar parts; 5-glass tube; 6-non-magnetic electrodes; 7-MRS;
8-digital ohmmeter (type DT9208 A); OX-coordinate axis; ↑ H direction of the magnetic field intensity vector.
with tightness fittings (not specified in Figure 2). They
are connected to the digital ohmmeter 8 in Figure 2.
The magnetoresistance is fixed and positioned between
the polar parts 3 and 4 by means of a positioning fixing
subassembly. It is not presented in Figure 2.
Results and Discussion
The MRS used for building the magnetoresistance was
prepared as shown under 2. By adding mineral oil, MRSs
were obtained with volume fractions of ø1 = 0.30 and ø2
= 0.06. The MRS magnetization curves obtained by
means of the magnetometer VSM 880 are shown in
Figure 3.
The value Hef. of the intensity of the alternative
magnetic field was measured by means of a Hall sonde
[23]. Magnetoresistances with MRSs were built having
two values of ø(ø1 = 0.30 and ø2 = 0.06). The resistance R of the magnetoresistances was registered at the
808
Ioan Bica
(a)
(a)
(b)
Figure 4. Variation of R by time t and Hef. : a)ø1=0.3; b) ø2 =
0.06.
(b)
Figure 5. Variation of the conductivity σ of MRS by time t
and different values of Hef., for: a) ø1 = 0.3; b) ø2 = 0.06.
moment of application of H and at intervals of 1 s,
respectively, up to 45 s to determine effective intensities
(Hef.) of the magnetic field as a parameter.
For longitudinal magnetic fields with Heff. ≥60 kA/m,
Rø1 →∞and Rø2 →∞.
For transversal magnetic fields ( H⊥ Ox - Figure 2),
namely from Hef = 60 kA/m, however, the phenomenon
of electrical conductivity in the MRS occurs. The
variation of R with respect to t for various values of the
transversal magnetic field (Hef.) is shown in Figure 4.
The electrical conductivity of the MRS body is
calculated using the relation [17]
time after the application of H , of
∙ 9 s, for Hef. = 60 kA/m;
∙ 4 s, for Hef. = 70 kA/m;
∙ instantaneously, for 80 ≤ Hef. (kA/m) ≤ 90.
For ø2 = 0.06, however, conductivity in the MRS occurs
upon application of transversal H , from Hef. = 70 kA/m.
For Hef. = 60 kA/m, conductivity in the MRS occurs
with a 4-s delay from the moment of application of H .
The variation velocity of σ is higher during of the first 5 s
after the application of H and then it exhibits a slow
variation with time.
The electrical conductivity σ (Figure 5) is considerably
influenced by ø for fixed Hef.. If we denote as σø1 and
σø2 the electrical conductivities for the volume fractions
ø1 and ø2 , respectively, of the MRS, then the relation
σ=
250
R( M Ω )
(1)
With the values of R(t) in Figure 4 inserted into Eq. (1),
σ(t) is obtained in the form of the graphs in Figure 5. It
results from Figure 5 that the electrical conductivity of
the MRS depends on ø and Hef. Thus, for ø1 = 0.30, the
electrical conductivity in the MRS arises at intervals of
α=
σ ø2
σ ø1
describes the time variation shown in Figure 6.
(2)
Electrical Conductivity of Magnetorheological Suspensions Based on Iron Microparticles and Mineral oil in Alternative Magnetic Field 809
∙ The electrical conductivity in MRS occurs only in
transversal magnetic field;
∙ The moment of onset of electrical conductivity in the
MRS is determined by Hef. for a fixed value of ø
(Figure 4);
∙ The electrical conductivity in the MRS is considerably
influenced by the value ofø for fixed values of Hef.
and t (Figures 5 and 6).
Acknowledgment
Figure 6. Plots of α as function of the time t from the
application of the alternative magnetic field, for various values
of Hef.
It is observed from Figure 6 that α was greater than 1 for
0 ≤ t(s) ≤ 45 and 60≤ Hef. (kA/m) ≤ 80.
At Hef. = 90 kA/m, however, we observe
∙ α = 1.44 for t = 0 s;
∙ α≈ 2.21 for t = 2.5 s;
∙ α ≤ 1 for 2.5 < t(s) ≤ 33;
∙ α = 1.44 for t = 45 s.
In an alternative magnetic field, the magnetic particles
perform oscillatory movements along the directions
identical and parallel to the direction of H . Permanently,
the iron particles are placed at high distances, for which
R(t)→∞ [19]. Thus, along directions parallel to H
(longitudinal magnetic field), no electrical conductivity
occurs in the MRS. The vibrations of the chains of
magnetic dipoles propagate along directions perpendicular on the direction of H . In MRSs, fields of
mechanical stresses occur [24]. Under the actions of
mechanical stresses, the iron microparticles become
positioned closer together. The initial thickness of the oil
layer is inversely proportional to Hef. and it diminishes
with time [19]. As a result [19], the value of R is
dependent on Hef. and on the time t for a fixed value of ø.
The passage from ø1 to ø2 (ø2 < ø1) has as its effect
the diminution of the interactions between the iron
microparticles during the migration in alternative
magnetic field ( H⊥ Ox, Figure 2). The result is that, for
fixed Hef. and t, at ø2<ø1, the electrical conductivity of the
MRS increases (Figure 6).
Conclusions
∙ The MRSs obtained by thermal decomposition of
Fe2(CO)9 in mineral oil with stearic acid exhibit
electrical conductivity in an alternative magnetic field;
I am thankful to E. Papp, C. T. Cheveresan, and D.
Baltateanu of the West University of Timisoara for interesting discussions. I also thank F. Stoian and O. Marinica
of the Politehnica University of Timisoara (Complex
Fluids Department) for assistance in performing the
magnetorheological measurements.
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