29-08-2013, 04:43 PM
Magnetorheological (MR) fluids
Magnetorheological .pdf (Size: 212.53 KB / Downloads: 24)
ABSTRACT
Magnetorheological (MR) fluids are dispersions of fine (~0.05-10
m)
magnetically soft, multi domain particles. The apparent yield strength of these fluids
can be changed significantly within milliseconds by the application of an external
magnetic field. MR fluid devices are being used and developed for shock absorbers,
clutches, brakes, and seismic dampers. The major goal of any research in this is to
advance the science of MR fluids. More specifically, the goals were: (a) influence
of interparticle forces on stability and redispersibility of MR fluids and (b) factors
affecting the “on” and “off” state rheological properties of MR fluids.
Rheological experiments were conducted on MR fluids based on two different
grades of carbonyl iron powder. Grade A (average size 7-9microm) and Grade B
(average size ~2microm). The yield stresses of 33 and 40 vol% Grade A were 100 ±
3 and 124 ± 3 kPa, respectively at 0.8 ± 0.1T. The yield stress values of MR fluids
were based on finer particles (Grade B) were consistently smaller. For example, the
yield stresses for 33 and 40 vol% Grade B based MR fluid were 80 ± 8 and 102 ± 2
kPa respectively at 0.8 ± 0.1T. These experimental results were in good agreement
with the analytical models developed by Ginder and co-workers. The decrease in the
apparent yield strength of MR fluids based on smaller particles was attributed to the
smaller saturation magnetization of these particles.
Magnetorheological Fluids
Magnetorheology is a branch of rheology that deals with the flow and deformation of
the materials under an applied magnetic field. The discovery of MR fluids is credited
to Jacob Rabinow in 1949. Magnetorheological (MR) fluids are suspensions of non-
colloidal (~0.05-10 m), multi-domain, and magnetically soft particles in organic or
aqueous liquids. Many different ceramic metals and alloys have been described and can
be used to prepare MR fluids as long as the particles are magnetically multi-domain
and exhibit low levels of magnetic coercivity. Particle size, shape, density, particle size
distribution, saturation magnetization and coercive field are important characteristics of
the magnetically active dispersed phase. Other than magnetic particles, the base fluids,
surfactants, anticorrosion additives are important factors that affect the rheological
properties, stability and redispersibility of the MR fluid.
Electrorheological (ER) Fluids
ER fluids are suspensions of electrically polarizable particles dispersed in an electrically
insulating oil . The ER fluid is typically composed of 0.5 to 100 m particles of
cornstarch, silica, barium titanate or semiconductors . For particles such as silica,
polyelectrolyte need to be added to cause the adsorption of water onto the particulate
material to enhance the ER effect, thus increasing the electrostatic force of attraction
between the particles. The water also creates a conductive layer on the surface of the
particles in which the ions in the water can drift in response to an electric field . These
materials are called extrinsically polarizable materials in which the ER effect results
from interfacial polarization. The ER effect decreases as the amount of water absorbed
decreases. Therefore, at temperatures of ~50 0C, the ER activity decreases significantly
and thus the temperature instability limits the potential use of the ER fluids. Materials
such as ferroelectrics, inorganic, semiconductor polymers, metals, coated conductors
Comparison of Field Responsive Fluids
More recently MR fluids have gained considerably more attention than their electric
analogue Electrorheological (ER) fluids which where discovered by Winslow in 1948.
One of the advantages of MR fluids is the higher yield stress value than ER fluids. The
reason for having higher yield stress for MR fluids is the higher magneto static energy
density, of MR fluids compared to electrostatic energy density, of ER fluids. Low voltage
power supplies for MR fluids and relative temperature stability between –40 and +150
0C
make them more attractive materials than ER fluids. Ferro fluids do not exhibit yield
stress, but show an increase in the viscosity. The viscosity under an applied magnetic
field increases almost twice as much as the viscosity when there is no magnetic field
applied. Since Ferro fluids are synthesized by colloidal magnetic particles, these fluids
are more stable than MR fluids which are based on non-colloidal magnetic particles. The
comparison of MR, ER fluids, and Ferro fluids is summarized in Table 1-1.
Applications
In the marketplace today state-of-the-art MR fluids are becoming increasingly important
in applications concerning,
1. Active control of vibrations or torque transfer. Shock absorbers, Vibration dampers,
seismic vibration dampers, clutches and seals are the most exciting applications of MR
fluid.
For these applications, rheological properties of fluids, working mode of the device,
design of the magnetic circuit, flux guide and coil configuration are crucial parameters
for the operation of the actuators and devices. One of the most important and recent
development in MR fluid applications has been developed by Delphi Automotive
Systems. Delphi and Cadillac developed MagneRideTM Semi active Suspension System
which adjusts damping levels with the combination of MR fluid based struts and shock
absorbers.
Heat Treatment of MR Fluids
Silicone oil and synthetic oil based MR fluids were heated at 175 °C for 24 hours.
MR fluids were stored in tin containers during heat treatment. The samples were
weighed before and after heat treatment in order to control whether there was any
evaporation. Some of the samples were vacuumed under -30 mm-Hg in order to
vacuum the air trapped in the fluid. NPC-ST samples could not be heat treated due to
low flash point of the liquid.
Magnetorheological .pdf (Size: 212.53 KB / Downloads: 24)
ABSTRACT
Magnetorheological (MR) fluids are dispersions of fine (~0.05-10
m)
magnetically soft, multi domain particles. The apparent yield strength of these fluids
can be changed significantly within milliseconds by the application of an external
magnetic field. MR fluid devices are being used and developed for shock absorbers,
clutches, brakes, and seismic dampers. The major goal of any research in this is to
advance the science of MR fluids. More specifically, the goals were: (a) influence
of interparticle forces on stability and redispersibility of MR fluids and (b) factors
affecting the “on” and “off” state rheological properties of MR fluids.
Rheological experiments were conducted on MR fluids based on two different
grades of carbonyl iron powder. Grade A (average size 7-9microm) and Grade B
(average size ~2microm). The yield stresses of 33 and 40 vol% Grade A were 100 ±
3 and 124 ± 3 kPa, respectively at 0.8 ± 0.1T. The yield stress values of MR fluids
were based on finer particles (Grade B) were consistently smaller. For example, the
yield stresses for 33 and 40 vol% Grade B based MR fluid were 80 ± 8 and 102 ± 2
kPa respectively at 0.8 ± 0.1T. These experimental results were in good agreement
with the analytical models developed by Ginder and co-workers. The decrease in the
apparent yield strength of MR fluids based on smaller particles was attributed to the
smaller saturation magnetization of these particles.
Magnetorheological Fluids
Magnetorheology is a branch of rheology that deals with the flow and deformation of
the materials under an applied magnetic field. The discovery of MR fluids is credited
to Jacob Rabinow in 1949. Magnetorheological (MR) fluids are suspensions of non-
colloidal (~0.05-10 m), multi-domain, and magnetically soft particles in organic or
aqueous liquids. Many different ceramic metals and alloys have been described and can
be used to prepare MR fluids as long as the particles are magnetically multi-domain
and exhibit low levels of magnetic coercivity. Particle size, shape, density, particle size
distribution, saturation magnetization and coercive field are important characteristics of
the magnetically active dispersed phase. Other than magnetic particles, the base fluids,
surfactants, anticorrosion additives are important factors that affect the rheological
properties, stability and redispersibility of the MR fluid.
Electrorheological (ER) Fluids
ER fluids are suspensions of electrically polarizable particles dispersed in an electrically
insulating oil . The ER fluid is typically composed of 0.5 to 100 m particles of
cornstarch, silica, barium titanate or semiconductors . For particles such as silica,
polyelectrolyte need to be added to cause the adsorption of water onto the particulate
material to enhance the ER effect, thus increasing the electrostatic force of attraction
between the particles. The water also creates a conductive layer on the surface of the
particles in which the ions in the water can drift in response to an electric field . These
materials are called extrinsically polarizable materials in which the ER effect results
from interfacial polarization. The ER effect decreases as the amount of water absorbed
decreases. Therefore, at temperatures of ~50 0C, the ER activity decreases significantly
and thus the temperature instability limits the potential use of the ER fluids. Materials
such as ferroelectrics, inorganic, semiconductor polymers, metals, coated conductors
Comparison of Field Responsive Fluids
More recently MR fluids have gained considerably more attention than their electric
analogue Electrorheological (ER) fluids which where discovered by Winslow in 1948.
One of the advantages of MR fluids is the higher yield stress value than ER fluids. The
reason for having higher yield stress for MR fluids is the higher magneto static energy
density, of MR fluids compared to electrostatic energy density, of ER fluids. Low voltage
power supplies for MR fluids and relative temperature stability between –40 and +150
0C
make them more attractive materials than ER fluids. Ferro fluids do not exhibit yield
stress, but show an increase in the viscosity. The viscosity under an applied magnetic
field increases almost twice as much as the viscosity when there is no magnetic field
applied. Since Ferro fluids are synthesized by colloidal magnetic particles, these fluids
are more stable than MR fluids which are based on non-colloidal magnetic particles. The
comparison of MR, ER fluids, and Ferro fluids is summarized in Table 1-1.
Applications
In the marketplace today state-of-the-art MR fluids are becoming increasingly important
in applications concerning,
1. Active control of vibrations or torque transfer. Shock absorbers, Vibration dampers,
seismic vibration dampers, clutches and seals are the most exciting applications of MR
fluid.
For these applications, rheological properties of fluids, working mode of the device,
design of the magnetic circuit, flux guide and coil configuration are crucial parameters
for the operation of the actuators and devices. One of the most important and recent
development in MR fluid applications has been developed by Delphi Automotive
Systems. Delphi and Cadillac developed MagneRideTM Semi active Suspension System
which adjusts damping levels with the combination of MR fluid based struts and shock
absorbers.
Heat Treatment of MR Fluids
Silicone oil and synthetic oil based MR fluids were heated at 175 °C for 24 hours.
MR fluids were stored in tin containers during heat treatment. The samples were
weighed before and after heat treatment in order to control whether there was any
evaporation. Some of the samples were vacuumed under -30 mm-Hg in order to
vacuum the air trapped in the fluid. NPC-ST samples could not be heat treated due to
low flash point of the liquid.