SMART MATERIALS – ME 417

INTRODUCTION Over the past few decades materials have had to meet increasing

requirements. Reason for this is the continuous desire of mankind to

raise the performance of all types of structures; buildings have to be built

higher and unconventional shapes are being used, cars have to go faster

and be safer, while being lighter at the same time, and so on. To meet

these increasing demands a large variety of high performance materials

have been developed. Typically this ongoing development has led to the

production of materials, of which one or several important properties

have been improved in comparison to previously used materials, like an

improvement in the specific stiffness, strength or toughness. However, in

most everyday materials these physical properties are more or less fixed.

Oil, for example, has a viscosity that depends on the temperature. With a

change in the temperature also the viscosity will change, but this is only

a slight change. In contrast to these conventional materials, the ongoing

(re)search for improved performance has also produced multifunctional

materials, which do have properties that can be altered significantly

during use and, moreover, in a fast and reversible manner. These types

of materials are called ‘smart’ materials and are defined as materials that

possess adaptive capabilities to external stimuli, such as load or

environment, with an inherent ‘intelligence’. This ‘intelligence’ or the

adaptive capability of the material can be ‘programmed’ by material

composition, processing, defects, microstructure etc. Each type of ‘smart’

material has a different physical property that can be altered, like its

viscosity, volume or conductivity. The nature of the property that can be

altered has a direct influence on the type of applications the material can

be used for. The possibilities for utilization of such materials are almost

endless. Some everyday items already make use of ‘smart’ materials,

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such as bimetals in thermostatic faucet to control the temperature of the

water, photochromic lenses that react to light and so on.

They can either be used to react to a change in their surrounding

environment (like a change in temperature) or to change their properties

on demand of either electronic or human control. Very important is that

the reaction of ‘smart’ materials to a given stimulus is controllable and

predictable.

Nowadays a variety of ‘smart’ materials already exists and these

materials are being researched extensively. The most materials that

belong to this class are:

• (Ferromagnetic) Shape-memory alloys

• Piezoelectric materials

• Electrostrictive and magnetostrictive materials

• Field responsive composites

Electrorheological fluids and solids

Magnetorheological fluids and solids

Ferrofluids

Sensing and Actuating Properties of ‘Smart’ Materials

‘Smart’ materials are not only capable of sensing changes in their

environment; they are also able to take action correspondingly. Therefore

it can be said that they possess both sensory, as well as actuatory

properties (See Fig.1).

A sensor is defined as a device that can detect changes in its

environment, like changes in the temperature, electric field or magnetic

field. A sensor can therefore be seen as an artificial nerve. The detected

changes can then be used to adapt certain elements of a structure or

device to these changes to improve its performance. Damage control,

vibration damping and intelligent processing all require accurate

information describing the state of the material, system or structure,

which is provided by these sensors. Sensing capabilities can be given to

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structures by externally attaching sensors or by incorporating them into

the structure during manufacture. The use of ‘smart’ materials as

sensors, as stated above, has the benefit that these sensors can function

as actuators at the same time, thereby reducing system complexity.

An actuator is defined as a device that can perform a task, like exerting a

force or moment, as a reaction to a stimulus. This stimulus can either be

one that is given on demand (like an electric current) or it can be a

certain change in the environment (temperature, pressure etc.) upon

which a reaction will take place. An actuator can be seen as an artificial

muscle, which has the ability to change shape, stiffness, position,

natural frequency, damping, friction, fluid flow rate and other

mechanical characteristics. It can do this either on demand or in

response to changes in its environment (temperature, electric or

magnetic field, etc.). An actuator can therefore be seen as the

manifestation of motion, mobility and change.

Automobiles, trains and aircraft nowadays already contain a large

number of sensors and actuators. All actuation, however, is either under

human or electronic control. ‘Smart’ systems available today are mainly

structures where sensors and actuators have been added.

Fig.1 Smart Materials are able to function as both an actuator and a

sensor

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PIEZO ELECTRIC MATERIALS In 1880, Jacques and Pierre Curie discovered an unusual characteristic

of certain crystalline minerals: when subjected to a mechanical force, the

crystals became electrically polarized. Tension and compression

generated voltages of opposite polarity, and in proportion to the applied

force. Subsequently, the converse of this relationship was confirmed: if

one of these voltage-generating crystals was exposed to an electric field it

lengthened or shortened according to the polarity of the field, and in

proportion to the strength of the field. These behaviors were labeled the

piezoelectric effect and the inverse piezoelectric effect, respectively, from

the Greek word piezein, meaning to press or squeeze. Although the

magnitudes of piezoelectric voltages, movements, or forces are small, and

often require amplification (a typical disc of piezoelectric ceramic will

increase or decrease in thickness by only a small fraction of a millimeter,

for example) piezoelectric materials have been adapted to an impressive

range of applications. The piezoelectric effect is used in sensing

applications, such as in force or displacement sensors. The inverse

piezoelectric effect is used in actuation applications, such as in motors

and devices that precisely control positioning, and in generating sonic

and ultrasonic signals.

Peizo electric effect

The piezoelectric effect is based on the elastic deformation and

orientation of electric dipoles in a crystal structure when subjected to an

electric field. One fundamental of this effect is the non-symmetrical

structure of the crystal, in which the centers of electrical charge do not

coincide and therefore form dipoles. A bundle of several of these

elementary dipoles form a dipole region, known as a domain. In the

absence of external strain, the charge distribution within the crystal is

symmetric and the net electric dipole moment is zero. However, the

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application of an external mechanical force deforms and displaces the

dipoles and the charge distribution is no longer symmetric. In this way a

charge is generated at the surface of the crystal: the direct piezoelectric

or sensory effect (see figure). This property is used in converting

mechanical quantities such as pressure and acceleration into electrical

quantities.

The direct piezoelectric or sensory effect Conversely, the application of a high electric field causes deformation

and forces the randomly oriented micro-dipoles into alignment. This

alignment is called poling and leads to a constant volume strain of the

crystal: the inverse piezoelectric or actuator effect (see figure). It is this

property that is used for turning electrical signals into mechanical

movement or oscillations. Because of these two special properties

piezoelectric materials possess, they can be used as natural sensor

and/or actuator materials.

The inverse piezoelectric or actuatory effect

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Advantages and Disadvantages of Peizo electric materials (PZT) The advantages are:

• Low cost

• Low power requirement during static operation

• High stiffness

• Very high frequencies attainable, thus very fast actuation

• Compact and light

• High position accuracy

• High generation of force per unit of volume

• No maintenance is required, because they are solid-state effects

The disadvantages are:

• Brittleness in tension

• Power consumption increases linearly with frequency and actuator

capacitance

• High driving voltage required

• Limited strain

• The possible health risks of lead in PZT piezoelectric ceramics.

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SHAPE MEMORY ALLOYS A group of metallic materials that can return to some previously defined

shape or size when subjected to the appropriate thermal procedure. That

is, shape memory alloys can be plastically deformed at some relatively

low temperature and, upon exposure to some higher temperature, will

return to their original shape.

Shape memory effect is the result of a thermo-elastic martensitic

transformation. Above the transformation temperature, A shape memory

alloy (Ni-Ti alloy) is austenitc. The crystalline structure of the austenite is

a cubic B2 or Calsium chloride structure. Cooling below the

transformation temperature transforms the B2 structure into a twinned

monoclinic structure, called martensite. No macroscopic shape change

occurs with this transformation. However, the twinned martensite can be

easily deformed up to approximately 8% strain by an unconventional de-

twinning mechanism. This deformation can be recovered by heating the

material to temperatures above the transformation temperature,

completing the shape memory cycle.

As mentioned above, significant changes of material properties

accompany this phase transformation. A typical plot of property changes

with temperature is shown in Figure, depicting the characteristic

hysteresis curve with the associated defining temperatures As (Austenite

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Start), Af (Austenite Finish), Ms (Martensite Start) and Mf (Martensite

Finish) (See figure). At temperatures above Af, martensite can be stress

induced, i.e. subjecting the material to a deforming stress yields

recoverable strains of up 8% with little stress increase by transforming

the austenite into martensite and immediately deforming it by

detwinning. As austenite is the stable phase at this temperature under

no-load conditions, the material springs back into its original shape

when the stress is no longer applied. This is an isothermal event called

super elasticity or elastic memory.

One-way shape memory effect

When a shape memory alloy is in its cold state (below As), the metal can

be bent or stretched and will hold those shapes until heated above the

transition temperature.

One-way shape memory effect

Upon heating, the shape changes to its original. When the metal cools

again it will remain in the hot shape, until deformed again. With the one-

way effect, cooling from high temperatures does not cause a macroscopic

shape change. A deformation is necessary to create the low-temperature

shape. On heating, transformation starts at As and is completed

at Af (typically 2 to 20 °C or hotter, depending on the alloy or the loading

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conditions). As is determined by the alloy type and composition. It can be

varied between −150 °C and 200 °C.

Two way shape memory effect

The two-way shape memory effect is the effect that the material

remembers two different shapes: one at low temperatures, and one at the

high temperature shape. A material that shows a shape memory effect

during both heating and cooling is called two-way shape memory. This

can also be obtained without the application of an external force

(intrinsic two-way effect).

Two way shape memory effect

The reason the material behaves so differently in these situations lies in

training. Training implies that a shape memory can "learn" to behave in a

certain way. Under normal circumstances, a shape memory alloy

"remembers" its high-temperature shape, but upon heating to recover the

high-temperature shape, immediately "forgets" the low-temperature

shape. However, it can be "trained" to "remember" to leave some

reminders of the deformed low-temperature condition in the high-

temperature phases. There are several ways of doing this. A shaped,

trained object heated beyond a certain point will lose the two way

memory effect, this is known as "amnesia".

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Properties of NITINOL and Copper-Zinc alloy

NITINOL:

The properties of Nitinol alloys are dictated by their composition. The

equiatomic composition forms the basis of many Nitinol alloys. Adding an

additional nickel up to an extra 1% is the most common modification.

This increases the yield strength of the austenitic phase while at the

same time depressing the transformation temperature.Other common

additions are made to alter the phase transformation temperature, such

as iron and chromium which lower the temperature. Copper can also be

added to lower the stress required to deform the martensitic phase and

decreases hysteresis.The properties of Nitinol alloys are dictated by their

composition. heat treatments can also play a part in affecting properties

of Nitinol alloys. However, for most Nitonol alloys, their density is

approximately 6.5 g/cm3 and will have a melting temperature in the

range 1240 to 1310°C. The transformation temperature can be modified

from less than –100°C to over 100°C.

For most Nitonol alloys, their density is approximately 6.5 g/cm3 and will

have a melting temperature in the range 1240 to 1310°C. The

transformation temperature can be modified from less than –100°C to

over 100°C.

Key properties of Nitinol alloys include:

• Large forces that can be generated due to the shape memory effect

• Excellent damping properties below the transition temperature

• Excellent corrosion resistance

• Nonmagnetic

• High fatigue strength

• Moderate impact resistance

• Moderate heat resistance

• Biocompatible

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Applications

• Aerospace and naval applications - Nitonol fluid fittings or

coupling have are being used in military aircraft and naval craft.

Medical Applications - Tweezers for removing foreign objects via

small incisions, anchors for tendon fixation and stents for

cardiovascular applications

• Dentistry - Orthodontic wires, which no not need to be retightened

and adjusted

• Safety devices - Safety valves/actuators to control water

temperature and fire sprinklers

• Other uses include:

• Spectacle frames

• Household appliances and deep fryers

• Clothing including underwire brassieres

• Vibration control in the form of engine mounts and actuators for

buildings

• Fasteners, seals, connectors and clamps

• Mobile telephone antennaes

Copper-Zinc alloy

CuZnAl SMA’s usually contain 15-30% zinc and 3-7% aluminium, with

the balance being copper. The addition of small quantities (usually less

than 1%) of boron, cerium, cobalt, iron, titanium, vanadium and

zirconium are commonly added to control grain size. Use of grain growth

control additives keeps grain size down and overcomes brittleness issues.

However, additions should be made carefully as they can upset the

stability of the structure, thus affecting the shape memory

characteristics.

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• Density 7.60-7.65 g/cm3

• Values for Young’s modulus and yield strength are higher for the

high temperature phase of the alloy.

• Recoverable strain is approximately 4-5%

• Poor corrosion resistance

• CuZnAl SMA’s have the advantage that they are made from

relatively cheap metals using conventional metallurgical processes.

Pseudoelastic (or Superelastic) Effect In elastic deformation of typical materials a load will result in

deformation that will disappear again upon removal of this load. The

difference between this typical elastic deformation and the pseudoelastic

(or superelastic) deformation, which occurs in SMAs, is that these can be

stretched or compressed elastically 5-10 times the amount of

conventional materials. This is called the Pseudoelastic Effect (PE). This

PE can be observed at temperatures above the temperature necessary to

transform the SMA from martensite to austenite (Af). By selecting an

alloy on basis of the fact that this temperature corresponds with ambient

temperatures in which the SMA is intended to be used, it is able to

demonstrate the PE. Unlike the Shape Memory Effect, the PE occurs

without the need for a change in temperature. This is because the solid-

to-solid phase change, which is the cause of the special properties of

SMAs, can also be induced by the application of a load.

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Advantages and Disadvantages of Shape Memory Alloys The advantages are:

• High work output per unit of volume, thus compact powerful

actuators

• Large actuation displacement

• Large recovery force

• Few mechanical parts, reducing overall system complexity

• Variable shapes

• Low voltage operation (below 40 V)

• Usable in clean room environment

• Intrinsic sensory capabilities (in case of thermal actuation)

• Large energy absorption and damping capacity (because of

hysteresis, which can also be undesirable)

• Noiseless operation

• Reliability: a SMA active element ‘has to’ transform to its original

parent

• phase/shape, even when the actuator was not in use for some

decades

The Disadvantages are:

• Limited range of transformation temperatures (which, however, can

be controlled by a change in the composition of the alloy)

• Hysteresis (which can also be beneficiary)

• Duration and stability of the SME uncertain, because research in

this area is relative young.

• Relatively low velocity

• Low efficiency when applying resistive heating

• Actuation requires heating and cooling (which is cause to the low

velocity)

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ELECTRORHEOLOGICAL FLUIDS

ER fluids experience a significant change in their magnetic, electric,

thermal, acoustical and optical properties upon exposure to an electric

field. The most important change, however, takes place in their

microstructure and with that in their rheological behavior: the resistance

to flow increases with increasing electric field. This is caused by an

increase of the viscosity of the fluid, because of which the consistency of

ER fluids can change from a thick fluid (similar to motor oil) when no

field is applied, to a nearly solid substance when subjected to an electric

field. This change in the structure and also in the rheological properties

of a liquid or another type of dispersed system under the application of

an external electric field is called the ER effect. The liquid or the

dispersed system is generally called an ER fluid. This change can take

place within the span of a few milliseconds from the moment the field is

applied. Furthermore, it is a fully reversible process. ER fluids that have

solidified under subjection to an electric field can start to flow again by

removing the electric field or by applying a shear stress that exceeds a

certain critical value. This critical shear stress that makes ER fluids flow

is also known as the yield stress, which is one of the most important

factors in evaluating the performance of the fluid.

Working Principle Most Electrorheological (ER) fluids are dispersions of small dielectric

particles, which can be solid or liquid, with sizes in the order of a few

microns, suspended in a non-conducting carrier liquid. In the absence of

an electric field the ER fluid is in the ‘off’ state. The particles are then

randomly dispersed throughout the carrier liquid. In this state, the

consistency and also the value for the viscosity of ER fluids can be

compared to motor oil (±0.1 Pa · s at low shear rates). When subjected to

an electric field, the viscosity of ER fluids increases. Physically, the ER

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effect originates from the polarization of particles in carrier liquid. The

field induces dipoles in the dielectric particles and these particles then

will start to aggregate. They will start to form fibrous structures (chains

or column-like structures) between the electrodes, aligning along the

direction of this applied field. They do so to minimize the dipole-dipole

interaction energy, because minimization of the potential energy leads to

a stable position. These chain-like structures restrict the motion of the

fluid, thereby increasing the viscous characteristics of the suspension,

changing the rheology of the MR fluid to a near solid state. Figure. shows

this change of the microstructure of an ER fluid between before and after

an electric field is applied.

Schematic illustration of the structural change in (a) an ER fluid and (b) a non-ER fluid upon subjection to an electric field

This change is fully reversible. By breaking the chain of particles, which

can be achieved either by removing the electric field or by applying a

shear stress that exceeds a certain critical value, the fluid will return to

its original configuration. The critical shear stress, that makes ER fluids

flow, is also known as the yield stress. The value of this yield stress is

field dependent. The stronger the electric field ( E ), the higher the yield

stress of the ER fluid gets (up to a certain maximum). Reason for this is

that the stronger the applied field is, the stronger the bond between the

particles is and the harder it will be to break this bond. Electric field

strengths up to 4 kV / mm (limited by the breakdown of the ER fluid) are

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required, which make the use of expensive high voltage power supplies

necessary. Typical values for the yield strength of ER fluids range from 2

to 5 kPa , depending on the strength of the electric field and the

composition of the fluid. Higher values of around 10 kPa are achievable

at this moment, but not yet commercially applicable.

The change in the microstructure determines the response time of the ER

effect. This is typically within a time span of few milliseconds. The

density of the whole suspension ranges from 1 to 2 gr / cm3 . Some

factors can affect the ER effect substantially, such as the frequency of

electric field, the particle conductivity, the particle dielectric properties,

the particle volume fraction and the temperature. Also the water content

is of great influence on the ER effect, as will be explained later on. Finally

it has to be noted that ER fluids are very sensitive to contaminants and

impurities. The functioning of all ER fluids depends on the movement of

ions or electric charge, which can be obstructed by these impurities.

Therefore they can have a negative influence on the polarization

mechanism, which can lead to a significant decrease in the ER effect.

The components of an ER fluid

1. Continuous phase (carrier liquid)

The primary function of the continuous phase or carrier liquid is to

provide a matrix material in which the particles (dispersed phase) can

remain suspended. The continuous phase consists of an insulating oil of

some type. An ideal carrier liquid material should meet the requirements

stated below. The boiling point and the solidification point have to be

high and low enough, respectively. This is because otherwise the liquid

will evaporate to easy when used at elevated temperatures, as this will

decrease the ER effect. Secondly, the carrier liquid should also have a

low viscosity in order to keep the base viscosity of the whole suspension

at a low level. This is necessary to avoid friction or flow losses in

hydraulic circuits. The third requirements is that the density should be

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relatively high, preferably larger than 1.2 g / cm2 , so that the density

mismatch between the carrier liquid and the particles is minimized,

thereby diminishing the risk of sedimentation. The fourth demand is

that, in order to withstand the high electric fields the ER fluid is

subjected to, the carrier liquid should have a high resistance and high

breakdown strength. Furthermore it has to have a high chemical stability

and a low toxicity, so the fluid will not react with other materials, like the

particulate material or the materials the device is made of. It should also

have an obvious hydrophobicity, so that it will not adsorb too much

moisture from the environment as this will lessen the MR effect. Finally,

in order to keep the cost of the entire suspension at a moderate level, the

carrier liquid should have low cost. Currently used materials are mostly

different types of oils, such as silicone oil, vegetable oil, mineral oil,

paraffin, kerosene, chlorinated hydrocarbon, transformer oil. Typical

volume ratios of the continuous phase range from 0.5 to 0.95.

2. Dispersed phase (Particles)

For heterogeneous ER fluids, the particles typically range in size from 0.1

to 100 μm. The particle volume fraction is between 0.05 and 0.50. The

materials of which these solid particles are made of can be:

• Inorganic oxide materials

Some metallic oxides or ceramic materials sintered from several oxides

were found to give a good ER effect. This was the first material used for

the particles. Most oxide ER fluids however contain water, which is a big

drawback for this type of fluid.

• Non-oxide inorganic

Non-oxide inorganic ER fluids were mainly developed in the late 1980’s

and early 1990’s. They could give an extremely strong ER effect without

any amount of water, but water could also enhance the ER effect

substantially. The development of these kinds of ER fluids was very

inspiring at that time. Among them especially Zeolite family materials,

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received a great deal of attention. Although these non-oxide inorganic ER

fluids have a strong ER effect, their conductivity is too high, especially in

a high temperature environment. Also, the density mismatch between the

dispersed phase and the continuous phase is quite large, which usually

leads to unstable suspension (sedimentation). Finally, the particles are

too hard and abrasive to the ER device.

Organic and polymeric materials

The organic and polymeric materials were believed to be better than

inorganic materials and were investigated thoroughly. However, the ER

effect of the organic and polymeric ER fluids is weaker compared with

that of the non-oxide inorganic material.

Homogeneous ER fluids consist of a liquid material, instead of a solid,

dispersed into the continuous phase. Low-molecular liquid crystals and

polymer liquid crystals constitute a large portion of the homogeneous ER

fluids. These ER fluids have some benefits over the heterogeneous, such

as the fact that they don’t have the particle sedimentation problem, in

contrast to heterogeneous ER fluids. However, such an ER system hasn’t

been able to give a strong ER effect and it also segregate into two phases

fairly easy. Another drawback is its large viscosity at zero electric field,

which leads to large friction/flow losses. This, off course, is unwanted in

practical applications.

3. Additives

Additives form the third component of an ER fluid. Water, alkali, salt,

and surfactants are the most common. They are firstly used to hydrolyze

ER fluids. This is necessary because it’s usually not possible to activate

an ER system without water. Besides water, other polar liquids, such as

alcohol, dimethylamine, acetamide, diethylamine and glycerol, can be

used as they have also proven to enhance the ER effect substantially.

Possible reason for this is that small amounts of polar liquid can

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dramatically increase the dielectric constant of the dispersed particle,

which activates the ER effect. Other commonly used additives are

surfactants, which can either be an-ionic, cationic or non-ionic. These

surfactant help to stabilize the ER suspension and in some cases it also

enhances the ER effect. Final note is that the amount of additive is very

important. Too little might not give any enhancement, but too much can

lead to a large electric current.

Advantages and disadvantages of ER fluids The advantages are:

• ER fluids a reversible and controllable change in their rheological

properties when subjected to an electric field.

• Electric fields are fairly easy to supply.

• The response time of ER fluid based devices is estimated to be

fewer than 10ms. Therefore ER fluids are very suitable for dynamic

applications

• The particles in ER fluids typically have a density that is close to

that of the carrier liquid. This means that the density mismatch is

relatively low, leading to a lower tendency of sedimentation of the

particles.

• The low density of the particles also helps to keep the density of

the entire ER fluids at a moderate level, ranging from 1 to 2 gr /

cm3. Finally, the lower density of the particles also results in

relatively low base viscosity (below 100 mPa.s ) of the ER fluid as a

whole. This results in low friction or flow losses, which can be

important in hydraulic circuits.

• ER fluids show very low abrasiveness. Together with the fact that

they can make several moving parts redundant, this can lead to an

increase in reliability and a reduction of maintenance

requirements.

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The disadvantages are:

• The relatively low attainable yield stress, which result in the need a

relatively large amount of active ER fluid. This can lead to large

device sizes and weights to achieve a certain performance level.

• ER fluids are voltage driven. They require large voltages (some kV )

at a low current (few mA), which means that they require relatively

expensive high voltage power supply.

• ER fluids are very sensitive to impurities or contaminants, as these

can negatively affect the polarization mechanism. This can lead to

a significant decrease of the ER effect or even a complete

malfunctioning of the device.

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MAGNETORHEOLOGICAL FLUIDS

Magnetorheological (MR) fluids react to an external stimulus in a similar

way Electrorheological (ER) fluids, which are discussed in the previous

paragraph, do. They too undergo a dramatic change in their rheological

properties when subjected to a specific field. The difference is that MR

fluids respond to a magnetic field, instead of an electric field. When

subjected to a magnetic field, MR fluids undergo a change in their

viscosity and can change from a liquid state with a relatively low

viscosity, like motor oils, to an almost solid state. The time span in which

this change occurs lies within a few milliseconds and the effect is

completely reversible. Upon removal of the applied field, the fluid returns

to its original configuration.

Working principle

Magnetorheological (MR) fluids are typically colloidal suspensions

consisting of highly polarizable magnetic particles, with sizes in the order

of a few microns, in a low permeability, non-magnetic base fluid. In the

absence of a magnetic field the MR fluid is in the ‘off’ state. It is then not

yet magnetized and in this state the particles exhibit a random pattern.

In this ‘off’-state, MR fluids appear to have a consistency similar to liquid

paint and also exhibit comparable levels of apparent viscosity (±0.3 Pa · s

at low shear rates). In the absence of an applied field, the controllable

fluids exhibit Newtonian-like behavior. When the MR fluid is subjected to

a magnetic field, the particles become magnetized (induced dipoles) and

they start to behave like tiny magnets. This is the ‘on’ or magnetized

state of the MR fluid. The magnetic interaction, and with that the total

potential energy, between these particles can be minimized if the

particles line-up along the direction of the magnetic field lines. With the

potential energy minimized the particles are in a stable position. The

interaction between the resulting induced dipoles causes the particles to

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aggregate and form fibrous structures within the carrier liquid (chains or

column-like structures, see figure), changing the rheology of the MR fluid

to a near solid state.

Illustration of the structural change in an ER fluid

These chain-like structures restrict the flow of the fluid, thereby

increasing the viscous characteristics of the suspension. The mechanical

energy needed to yield these chain-like structures increases non-linearly

with increasing applied magnetic field (H ), resulting in a field dependent

yield stress. This non-linearity is explained by the non-uniform

magnetization of different parts of the particles.

Magnetic fields intensities up to 160kAm−1 are required, which are easy

to obtain using a standard 12 V or 24 V power supplier, such as the

battery in a car. Depending on the flux density ( B ) and the composition

of the fluid, an apparent yield stress ranging from 10 to 100 kPa can be

achieved. Materials with higher magnetization saturation can be used to

increase this even further, but these are often less available and thus

more costly. The process is fully variable and also reversible. By

controlling the strength of the magnetic field, the shear strength of the

MR fluid can be altered and with this the resistance to fluid flow can be

varied. The fine-tuning of the magnetic current allows for any state

between the low forces of ‘off’ to the high forces of ‘on’ to be achieved.

Reversible means that when the magnetic field is removed, the chains are

broken and the particles return to their original statistical distribution.

Both the activation and the deactivation of the MR fluid are completed

within a few milliseconds after the introduction or removal, respectively,

of the magnetic field. The response time of a complete system using MR

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device(s) is estimated to be around between 15-25 ms , strongly

depending on the nature of the device. The density of the whole

suspension ranges from 3 to 4 gr / cm3 A final remark about MR fluids is

that they, unlike ER fluids, are not highly sensitive to contaminants or

impurities, which can be commonly encountered during manufacture

and also during usage.

Material Composition

Magnetorheological (MR) fluids are built up of 3 components: the

dispersed phase (the magnetizable particles), the continuous phase (the

carrier liquid), and small amounts of additives and stabilizers. Each of

these components is described separately below.

1. Dispersed Phase

The requirements set on the choice of particle material are that the

particles have to be magnetically multi-domain and that they exhibit low

levels of magnetic coercivity. In addition, maximizing the inter-particle

forces and thus maximizing the MR effect can be achieved by choosing

the particle material on basis the saturation magnetization s J [Tesla].

The higher s J , the higher the inter-particle forces and the higher the MR

effect is. The material most used today is high purity carbonyl iron ( Fe )

powder, made by chemical vapor deposition (CVD) of iron penta-

carbonyl( Fe(CO)5 ). The reasons for this are:

• The high chemical purity (>99.9%), which leads to less domain pinning

• The mesoscale dimensions, which have many magnetic domains

• The spherical shape, which minimizes the magnetical shape anisotropy

• It’s high magnetization saturation ( s J =2.1 [Tesla])

• The particles are magnetically soft, and thus non-abrasive

There are alloys of iron and cobalt, which are known to have slightly

higher saturation magnetization (up to s J =2.4 [Tesla]) and these have

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also been used in MR fluids, but these are often less available and thus

more costly. The particles usually have a diameter ranging from 1-10 μm

and a density of approximately 7 gr / cm3 . They usually make up 40-

60% of the suspension’s weight and between 10 and 50 % of the

suspension’s volume.

2. Continuous Phase

The primary function of the carrier liquid is to provide a low permeability,

nonmagnetic base liquid in which the magnetically active phase particles

can remain suspended. The liquid has to be low permeable to allow the

particles to polarize with the utmost effectiveness, thus enhancing the

MR effect. Furthermore, the carrier liquid is chosen based upon its

rheological and tribological properties, as well as on its temperature

stability. Important aspects to consider are the boiling temperature and

the vapor pressure at elevated temperatures and at the freezing point.

Also, the carrier liquid has to be largely non-reactive towards the

magnetic particles and to the materials used in the device construction.

Finally, the off-state viscosity of the MR fluid depends largely on the

selection of the base fluid. To avoid large friction losses the ‘off’-state

viscosity should be as low as possible. Typical materials used as carrier

liquid are water or other polar organic liquids (such as glycol), silicone

oils, (semi-)synthetic oils, mineral oils, petroleum based oils and

combinations of several types of oil. Also polyesters and polyethers are

used. The volume ratio of the carrier liquid ranges from 0.5 to 0.9.

3. Additives

Additives form the third part of a MR fluid. Because the magnetic

polarization mechanism, the working principle of MR fluids, is not

affected by the surface chemistry of surfactants, it is relatively

straightforward to use additives in MR fluids for all kinds of purposes,

such as:

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• Prevention or minimization of sedimentation

• Prevention or minimization of coagulating of the particles

• To Maintain a coating on the particles in order to enhance

redispersibility

• Provide additional lubricating properties to improve anti-wear

• To enhance anti-oxidation

• In water-based carrier liquids additives are used to control the pH-

value

The prevention of sedimentation is one of the most important aspects. If

this is not prevented, MR fluids will alter their properties significantly

over time. Some examples of additives are given which help to prevent

this. Sedimentation is typically controlled by the use of thixotropic

agents and surfactants such as xantham gum, silica gel, stearates and

carboxylic acids. The thixotropic networks disrupt the flow at ultra low

shear rates (the viscosity becomes nearly infinite) but thins as the shear

rate is increased. The stearates form a network of swollen strands when

used in conjunction with mineral oil and synthetic esters that serve to

entrap particles and immobilize them. Fine carbon fibers have also been

used for this purpose. The fibers increase the viscosity through physical

entanglement but exhibit shear thinning due to shear-induced

alignment. In this way they all contribute to keep the particles

suspended in the carrier liquid and in this way the MR fluid will not alter

its properties much over time.

An important conclusion that was derived from the literature is that for

each application and for each device, a specially formulated MR fluid

should be developed. This because each MR fluid application or device

has its own distinct working conditions, such as the environment in

which it has to operate and the forces it is subjected to.

The advantages are:

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• MR fluids show a reversible and controllable change in their

rheological properties upon subjection to a magnetic field

• MR fluids are current driven. For the control of the field coil

voltages below 10 V and currents below 2 A can be sufficient to

operate the device properly.

• These can be obtained using a standard 12 V or 24 V power

supplier, such as the battery in a car.

• The response time of MR based devices is estimated to be around

15-25 ms , which is still very fast, but slower than their electrically

activated cousins, the Electrorheological (ER) fluids.

• MR fluids are able to attain high shear stresses in the order of

magnitude of 50-100 kPa , which results in the fact that only a

small amount of active fluid is required to achieve a certain

performance level. This can positively affect the size and weight of

a MR based device.

• MR fluids are not very sensitive to contaminants and impurities,

which can be commonly encountered during manufacture and also

during usage.

• This insensitivity of the working principle of MR fluids towards

contaminants also holds for the surface chemistry of surfactants

and additives. That is why it is relatively straightforward to

stabilize MR fluids against particle-liquid separation

(sedimentation), in spite of the large density mismatch.

• Fail-safe operation of MR based devices can be achieved through

the use of permanent magnets MR fluids. In this way MR fluids

can be energized without the need for any steady-state power.

• MR fluids are able to operate over a wide range of temperatures.

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The disadvantages are:

• Magnetic fields are not easy to supply and use.

• The density mismatch between that of the particles and the carrier

liquid is typically fairly high, which can lead to a higher risk of

sedimentation

• This high density of the particles also leads to a relatively high

density of the entire MR fluid, ranging from 3 to 4 gr / cm3 .

• The typical ‘off’-state viscosity of MR fluids is relatively high. This

can lead to relatively high friction or flow losses in devices, such as

rotary fluid brakes, when no activation of the fluid is required.

Electrostrictive materials The electrostrictive effect is, as with piezoelectricity, a means by which

an electrical pulse can be converted into a mechanical output.

Electrostriction is defined as a dimensional change of a material under

the influence of applied electric field and having a quadratic dependence

on it. This electrostrictive effect is caused by electric polarization.

However, the main difference between electrostrictive and piezoelectric

materials is that the first doesn’t show spontaneous polarization. The

lack of a spontaneous polarization means that electrostrictive materials

display little or no hysteresis, even at very high frequencies.

Electrostriction occurs in virtually all materials, but the induced strain is

usually too small to be utilized practically (strains ca.10−5 −10−7 %).

However, some electrostrictive ceramics, based on a class of materials

known as relaxor ferroelectrics, show strains comparable to those of

piezoelectric materials (strain ca.10−1 %) and have already found

application in many commercial systems. Although sometimes advertised

as a recent discovery, these materials have been around for many years.

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Working principle: The Lead Magnesium Niobate (PMN) electrostrictive ceramic is non-poled

and the unit cells of PMN are centro-symmetric at zero volts (neutral).

Upon subjection to an electric field the positively and negatively charged

ions separate, thereby changing the dimensions of the cell and resulting

in an expansion. The unit cell experiences an elongation proportional to

the square of the applied voltage. This means that, in contrast to

piezoelectric materials, the polarity of field has no affect on strain

direction and the strain will always be positive. On a macroscopic scale

the material is therefore always elongated. Main disadvantage of

electrostrictive materials like PMN is that they suffer from a very low

temperature stability, which makes their use in practical applications

difficult.

Applications

Electrostrictive materials can best be applied in situations where the

temperature can be stabilized to within a range of approximately 10 °C.

This may seem extremely limiting at first, but given that electrostrictors

excel at high frequencies and very low driving fields, then the

applications tend to be in specialized micro actuators. Temperature

stabilization of such small devices is relatively simple and often presents

only a minor problem in the overall design and development process.

However, at the moment electrostrictive materials have a niche

application for quasistatic positioning purposes under stable laboratory

conditions, like in the optical or semiconductor industry.

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Advantages • Very high sensitivity and accuracy (at specific temperatures)

• Very low hysteresis and creep (virtually loss free up to hundreds of

kHz.)

• More precise transfer ratio between the applied voltage and the

dilatation

• Their capacity to exert high pressures.

Disadvantages

• Very low temperature stability, therefore limiting application to a

very narrow temperature band around room temperature.

• The electrical capacitance is 4-5 times as high as piezoelectric

materials, thus requiring significantly higher driving currents for

dynamic applications

• The strain is quadratically proportional to the applied field,

therefore making it impossible to generate a negative strain and

this also provides a highly nonlinear motion.

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Magnetostrictive Materials Magnetostriction is a phenomenon that occurs in most ferromagnetic

materials, including Iron, Nickel, Cobalt and some of their alloys, upon

subjection to a magnetic field. This leads to many effects, but the most

useful is referred to as the ‘Joule’ effect, after its discoverer. This ‘Joule’

effect is responsible for the expansion (positive magnetostriction) or the

contraction (negative magnetostriction) of the material when a magnetic

field is applied. Both of these solid-state effects facilitate the generation

of high forces, strains and dynamics. There is also a reverse

phenomenon, in which the magnetic induction of the material changes

when the material is mechanically deformed. This is called the inverse

magnetostrictive effect. Magnetostriction can therefore be seen as the

magnetic counterpart of the indirect piezoelectric effect, with the main

differences being that magnetostrictive materials respond to magnetic,

rather that electric, fields. There are some other differences, which will be

discussed later on. Like the piezoelectric effects, the magnetostrictive

phenomena can be utilized for both the generation and detection of

mechanical stresses, deformations and oscillations.

Working Principle When a magnetic is applied to a magnetostrictive material, its magnetic

domains will rotate until they are aligned with the applied field. These

magnetic domains can be seen as many tiny permanent magnets which

are randomly arranged before application of the magnetic field. When the

magnetic field is applied, the poles of the magnetic domains align

themselves along the gradient of the flux lines of this field. This

alignment causes the material to change its shape, while its volume stays

approximately the same. This effect can be represented by the ellipses in

figure.

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Magnetostriction

The domains can be seen as ellipses, where the direction of the magnetic

field lines is parallel to the longest axis. These ellipses will rotate until

they have the same direction as the magnetic field and subsequently this

will result in a change in shape.

The magnetostrictive effect can be increased even further by applying a

preload to the material before applying the magnetic field. This is

depicted in figure.

Magnetostriction with preload

By rotating the ellipses until they are almost perpendicular to the

applied field before applying this field, the total magnetostrain is

increased in comparison to the non preloaded case. Most practical

applications therefore incorporate a preloading mechanism to benefit

from this effect.

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Applications

Like with piezoelectric materials, the magnetostrictive effects can be

utilized for both the generation and detection of mechanical stresses,

deformations, and oscillations. Thus, magnetostrictive materials can be

used as both an actuator and a sensor. When compared to piezoelectric

transducers, the efficiency of energy conversion of the magnetostrictive

method is generally lower, in particular when it is applied on ordinary

structural steel and in frequencies higher than a few hundred kHz .

However, magnetostrictive transducers are able to outperform the

piezoelectric types when used in relatively low frequencies (less than

several hundred kHz ). For sensory applications, this is mainly due to the

fact that there is no need for direct physical contact to the structural

surface, which means that there is no coupling medium is required.

Terfenol-D was initially developed to function as a sensor material in

sonar devices. Other, later developed sensory applications include

nondestructive evaluation, longrange inspection of steel pipes and tubes,

condition monitoring of machinery such as combustion engines. Another

possible application is the detection of vehicle crash events for operation

of onboard safety systems such as airbags. Here, the magnetostrictive

sensor detects transient stress signals, which are produced by the

mechanical impacts resulting from the crash event. Because of its non-

contact sensing ability, the relative insensitivity to the distance between

the sensor and impact locations together with its inherent ruggedness

and economy, magnetostrictive sensors have demonstrated their

potential as an onboard crash sensors. Actuatory applications include

rotating motors and hydraulic actuators. Recently also the application of

Terfenol-D in active vibration damping systems has been investigated, in

which it performs the function of both a sensor and an actuator.

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Advantages

• High power density (two to three orders higher than piezoelectric

materials)

• No need for direct physical contact to the structural surface

• Low non-linearity

Disadvantages

• The use of magnetic fields

• Lower efficiency of energy conversion of the magnetostrictive

method at frequencies higher than a few hundred kHz.

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