Materials Transactions, Vol. 45, No. 4 (2004) pp. 1375 to 1378
#2004 The Japan Institute of Metals
EXPRESS REGULAR ARTICLE
A Study of Magnetic Field Effect on Nanofluid Stability of CuO
Ho Chang1; * , Tsing-Tshih Tsung1 , Chii-Ruey Lin2 , Hong-Ming Lin3 ,
Chung-Kwei Lin4 , Chih-Hung Lo1 and Hung-Ting Su1
1
Department of Mechanical Engineering, National Taipei University of Technology, Taipei, 10608, R.O. China
Graduate Institute of Mechatronic Engineering, National Taipei University of Technology, Taipei, 10608, R.O. China
3
Department of Materials Engineering, Tatung University, Taipei,10452, R.O. China
4
Department of Material Science, Feng Chia University, Taizhong,40724, R.O. China
2
This study investigates the effect of additional magnetic field on the stability of CuO nanofluid. Experiments are conducted by imposing an
additional magnetic field to the CuO nanofluid prepared by the self-developed Arc-Submerged Nanoparticle Synthesis System (ASNSS), so as to
investigate the aggregation phenomenon and the stability of the nanoparticle suspension. It is subsequently known that the permeance strength,
time and frequency of the additional magnetic field can affect the CuO nanofluid. Under the influence a strong magnetic field, the longer the
permeance time, the more apparent the sedimentation phenomenon will be owing to the aggregation of the nanoparticles. However, the
permeance frequency has a relatively slight effect on the CuO nanofluid.
(Received January 20, 2004; Accepted February 27, 2004)
Keywords: arc-submerged nanoparticle synthesis system (ASNSS), nanofluid, magnetic field effect, surface potential
1.
Introduction
In recent years, application and technological development
of nanomaterials have been growing rapidly worldwide in
engineering industries and academic fields. Nanomaterials
are usually defined as the materials with their sizes ranged
from 1 to 100 nanometers. It has the characteristics of size,
surface, quantum and quantum tunneling effects. The
physical properties of nanomaterials are reflected in the
areas of heat transfer, electricity, magnetism and mechanics,
as well as its chemical properties which are obviously
different from those of bulks. These phenomena motivate
researchers to further investigate the physical and chemical
properties of the surface structure of nanomaterials. Moreover, the thermal conductive efficiency of the nanofluid is
inversely proportional to the size of the particles.1) A
maximum increase in thermal conductivity of approximately
20% was observed in the study for 4 vol% of CuO nanoparticles with average diameter of 35 nm dispersed in
ethylene glycol.2) Furthermore, the effective thermal conductivity has shown to increase up to 40% for the nanofluid
consisting of ethylene glycol with approximately 0.3 vol%
Cu nanoparticles of mean diameter <10 nm.3) In addition,
when the nanofluids are in high electric potential, their
thermal conductivity would be increased.4) Carrying an
excellent thermal conductivity, CuO nanofluid can be used in
machine tools as a highly effective circulation fluid. When
the machine tool is in motion, the magnetic field created by
the power source and dynamic systems would affect the
material properties of the circulation fluid. Thus, it is very
important to investigate the effect of magnetic field on the
nanoparticle suspension.
This study develops a magnetic environment system to
simulate an environment having additional magnetic field
imposed onto the fluid. It further investigates the stability of
the CuO nanofluid that is prepared by Arc-Submerged
*Corresponding
author, E-mial: [email protected]
Table 1
Process variables of preparing nanofluid by means of ASNSS.
Working condition
Description
Peak current (A)
2.5
Breakdown voltage (V)
Pulse duration (ms)
150
25
Off time (ms)
Temperature of
dielectric fluid ( C)
25
5
Tool polarity
positive
Dielectric fluid
deionized water
Nanoparticle Synthesis System (ASNSS)5) under the influence of both weak and strong magnetic fields.
2.
Experimental
The nanofluid used in the experiments is prepared by the
ASNSS, and the process variables are shown in Table 1. By
remaining in static state, the prepared nanofluid formed
stable suspension particles. Figure 1 is the TEM image of the
prepared CuO nanoparticle of acicular structure, and having
an average length of 60 nm and width of 25 nm.
Figure 2 is a schematic diagram of the experimental setup
for the magnetic field. Two pairs of magnets, which are
capable of creating the same magnetic field, are installed on
the platform. They are then set at weak (600–1000 Gauss)
and strong magnetic fields (1850–3000 Gauss) for comparative studies. The magnetization frequency is controlled by the
rotative speed that is pre-set by the motor. Moreover, the
installation platform has to be made of diamagnetic material,
which has lesser influence on the magnetic field, while its
base has to be made of shock-absorbent material to avoid the
interference of vibration. Then, 15 cm3 of CuO nanofluid,
which is prepared by the process variables as shown in
Table 1, are extracted and poured into a cylindrical test tube
made of glass with a diameter of 10 mm and length of
1376
H. Chang et al.
The total magnetic flux density B produced by additional
magnetic field can be deduced by eqs. (1) and (2):6)
B ¼ 0 H þ 0 M
or
B ¼ H
ð1Þ
0 in eq. (1) denotes the permeability in a vacuum, which is
4 107 ; H is the magnetic field strength, M is the
magnetization, is the permeability of the material and M
can be represented by the following equation:
M ¼ n m H
ð2Þ
where n is the magnetic dipoles of the unit volume, and m is
the magnetization coefficient of the material. The nanofluid
employed in this experiment is CuO, and its permeability and
magnetization coefficients are both 0:9999 ; 1. When measuring the additional weak and strong magnetic fields by
gauss meter, the range of average magnetic field on top of the
magnet and center of test tube lies within 650–1000 Gauss
and 1850–3000 Gauss, respectively. Therefore, the range
of magnetic flux density produced towards the nanofluid
lies within the range of 0.065–0.1 T and 0.185–0.3 T, respectively.
Fig. 1
TEM image of CuO nanoparticles.
3.
S
M
Motor
Magnet
Nanofluid
Fig. 2 Schematic diagram of the experimental setup.
100 mm. The test tube is then placed on a fixed position at the
center of a turntable. After the tests, a particle size
distribution analyzer and TEM are used to perform inspection
of particle size distribution and morphological observation.
The following tests are conducted.
(1) Use permeance time as a variable: To investigate the
effect of additional magnetic field on the changes of
nanoparticles inside the nanofluid under different time
periods, namely 10, 20, 30, 40, 50 and 60 minutes.
Moreover, in order to learn the influence of magnetic
field on CuO nanofluid under a long time period, an
additional test of 8-hour permeance time is conducted.
(2) Use intermittent permeance frequency as a variable:
Power supply is used to control the rotative speed of the
variable speed motor so as to acquire different
permeance frequencies. When the voltage is set at
10 V, 15 V and 20 V, the rotative speed of motor would
be 58 rpm, 154 rpm and 214 rpm, respectively, and the
permeance frequency would be 1.933 Hz, 5.133 Hz and
7.133 Hz, respectively.
(3) Use the strength of magnet flux density as a variable:
The magnetic flux density is set at strong and weak
magnetic fields for comparison.
The experimental results indicate that under an additional
magnetic field action, the CuO nanoparticle suspension
would aggregate after being permeated for 10 minutes,
causing their mean particle size to increase as shown in
Figs. 3 and 4 according to the measurement of HORIBA LB500 particle size distribution analyzer. Also revealed in the
figures is the fact that particles created by strong magnetic
field are apparently coarser than those by weak magnetic
field. It is further seen by comparing Figs. 3 and 4 that,
whether the magnetic field action is strong or weak, different
permeance frequencies would not impose an apparent change
to the particle size. The aggregation of nanoparticle suspension can be attributed to the activation of the surface atoms of
the nanoparticles by the additional magnetic field. These
atoms, which make up a relatively large proportion of the
nanoparticles, begin to spin when activated. At the same
time, additional magnetic field causes the electron energy
180
Mean Particle Size, D /nm
N
Results and Discussion
160
140
120
1.933Hz
100
5.133Hz
80
7.133Hz
60
40
20
0
0
10
20
30
40
50
60
Time, t /min
Fig. 3 Relationship between permeance time and particle size (weak
magnetic field).
A Study of Magnetic Field Effect on Nanofluid Stability of CuO
1377
Mean particle size, D /nm
250
200
1.933Hz
5.133Hz
150
7.133Hz
100
50
0
0
10
20
30
40
50
60
Time, t /min
Fig. 4 Relationship between permeance time and particle size (strong
magnetic field).
Fig. 6
TEM images of nanofluid after weak magnetic field action.
Table 2 Time needed for nanoparticles to fully precipitate because of
aggregation.
Fig. 5
TEM images of nanofluid after strong magnetic field action.
levels to change, thus creating surface and interface effects.7)
These changes would further lead to the difference between
the crystal field environment and bonding energy of the
surface atoms of particles and internal atoms. Plenty of
dangling bonds would then occur on the particle surface.
These bonds possess nonsaturated property and enable the
particles to integrate with other particles, so as to form larger
particles.8) Figures 5 and 6 are the TEM images of nanofluid
under strong and weak magnetic field action, respectively. As
seen in Fig. 5, the particles appear in the shape of bamboo
leaves, and their length and width are larger than those not
under magnetic field action, which are in the shape of
acicular as shown in Fig. 1. In addition, comparing Figs. 5
and 6 reveals that under strong magnetic field, the particles
are also larger in size than those under weak magnetic field.
Furthermore, under strong magnetic field action, nanofluid of
different permeance time is extracted and maintained in static
state to observe the time required for nanoparticles to
Permeance
Time needed for the
time (min)
sedimentation of nanofluid (h)
10
20
48
45
30
36
40
24
50
12
60
4
488
2
aggregate and result in a total sedimentation phenomenon. As
indicated in Table 2, the longer the permeance time, the
shorter the time required for the particles to precipitate under
static state.
After the nanoparticle suspension has been permeated, the
nanoparticle suspension shows a drop in electric potential
because the additional magnetic field can reduce the
repulsion force of the static electricity between the particles,
thus enabling the nanoparticles to aggregate. The Zeta
Potential Analyzer (Zeta Plus) of Brookhaven Instruments
Corporation is used to measure surface potentials in the
experiment. Figures 7 and 8 illustrate the Zeta potential
changes of nanoparticle suspension under two different
additional magnetic fields. Comparing these two figures
shows that after being permeated for 10 minutes, the Zeta
potential of nanofluid would drop immediately. Moreover,
under strong magnetic field, Zeta potential of nanofluid is
lower than that influenced by weak magnetic field action.
However, whether the magnetic field action is strong or
weak, different permeance frequencies does not impose an
apparent effect on the Zeta potential.
A relative movement has occurred on the intersecting
surface of the suspension particles and the fluid, which
H. Chang et al.
30
25
20
15
1.933 Hz
10
5.133 Hz
7.133 Hz
5
0
0
10
20
30
40
50
60
Time, t /min
further led to the interface electromotive effect, thus changing the surface potential of the suspension. The surface
molecules in deionized water and particles inside the
suspension formed a fixed layer, and when the particles and
fluid start to move relatively, this fixed layer would move
together with the particles, and the variation in electric
potential of the particle movement equals that of the fixed
layer and the inner part of fluid. The relationship between the
repulsion force of the static electricity and the surface
potential is indicated in eq. (3):9)
ð3Þ
From eq. (3), Vrep is the repulsion force of static electricity, "
and "0 are the permittivity of a vacuum and the dielectric
fluid, respectively, r is the radius of particles, d is the surface
potential, and k is ðz2i Ci F 2 =""0 RTÞ1=2 . From eq. (3), it is
known that the strength of magnetic field H can lower Vrep ,
and according to the Principles of Schulze-Hardy, the lower
the electric potential, the larger the aggregation force
between the particles will be.
The experimental results also reveal the average particle
size upon 8 hours of continuous permeance is similar to that
upon 60 minutes. However, it is known from Fig. 2 that after
8 hours of permeance, nanofluid would start to precipitate
within 2 hours. This indicates that after long hours of
permeance, the particles will remain in an unstable state, and
can easily be precipitated within short hours owing to the
aggregation of particles.
4.
25
20
1.933Hz
15
5.133Hz
7.133Hz
10
5
0
Fig. 7 Relationship between surface potential and permeance time (weak
magnetic field).
Vrep ¼ 2 " "0 r 2d ekH
Zeta Potential, E /mV
Zeta Potential, E /mV
1378
Conclusions
This study investigates the effect of additional magnetic
field on the nanofluid applied on machine tool as circulative
fluid. From the experimental results and discussion, the
following conclusions are made.
(1) Affected by the additional magnetic field, the particles
0
10
20
30
40
50
60
Time, t /min
Fig. 8 Relationship between surface potential and permeance time (strong
magnetic field).
inside the CuO nanofluid would become unstable within
a short period of time. The repulsion force of the static
electric charge between the suspension particles would
be reduced, and thereby enabling them to aggregate. It
is also known from the experiments that strong
magnetic field contributes to coarser particles than
those formed under weak magnetic field.
(2) Under magnetic field action, the longer the permeance
time, the shorter the time required for the nanoparticles
to aggregate and completely precipitate. However, the
level of permeance frequency has no apparent effect on
nanoparticle suspension.
(3) Affected by additional magnetic field, the repulsion
force between the particles decreases, causing the
electric potential to drop and allowing the nanoparticles
to aggregate.
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