MEMORYMetals

SMART MATERIALS
This booklet has been produced as the 2003 resource for the Institute of Materials,
Minerals and Mining Schools Affiliate Scheme and was written by Dr Diane Talbot.
Cover photographs appear courtesy of:
Tony Anson of Anson Medical Limited (top left and bottom left)
Sandy Hill of the University of Rochester (top centre).

Thanks are also due to:
Jackel International, the producers of the Tommee Tippee © baby spoons for their information and kind donation of spoons to support this resource.
Tony Anson of Anson Medical for the use of the photographs, useful information and kind donation of the shape memory metal wire to support this resource.
Memory Metals Limited for the kind donation of the shape memory metal springs to support this resource.
The Institute of Materials, Minerals and Mining Smart Materials Division, in particular Laura
Walker, for their information.

CONTENTS
Introduction

1

Shape memory alloys

2

Piezoelectric materials

6

Magnetostrictive materials

8

Magneto- and electro-rheological materials

10

Chromic materials

12

Bibliography

16

How to use the accompanying resources

Shape memory alloy spring
Shape memory alloy wire
Thermochromic spoon
Thermochromic pen

17

SMART MATERIALS
Smart materials have been around for many years and they have found a large number of applications. The use of the terms 'smart' and 'intelligent' to describe materials and

systems came from the US and started in the 1980‟s despite the fact that some of these so-called smart materials had been around for decades. Many of the smart materials were developed by government agencies working on military and aerospace projects but in recent years their use has transferred into the civil sector for applications in the construction, transport, medical, leisure and domestic areas.
The first problem encountered with these unusual materials is defining what the word
„smart‟ actually means. One dictionary definition of smart describes something which is astute or 'operating as if by human intelligence' and this is what smart materials are. A smart material is one which reacts to its environment all by itself. The change is inherent to the material and not a result of some electronics. The reaction may exhibit itself as a change in volume, a change in colour or a change in viscosity and this may occur in response to a change in temperature, stress, electrical current, or magnetic field. In many cases this reaction is reversible, a common example being the coating on spectacles which reacts to the level of UV light, turning your ordinary glasses into sunglasses when you go outside and back again when you return inside. This coating is made from a smart material which is described as being photochromic.
There are many groups of smart materials, each exhibiting particular properties which can be harnessed in a variety of high-tech and everyday applications. These include shape memory alloys, piezoelectric materials, magneto-rheological and electro-rheological materials, magnetostrictive materials and chromic materials which change their colour in reaction to various stimuli.
The distinction between a smart material and a smart structure should be emphasised. A smart structure incorporates some form of actuator and sensor (which may be made from smart materials) with control hardware and software to form a system which reacts to its environment. Such a structure might be an aircraft wing which continuously alters its profile during flight to give the optimum shape for the operating conditions at the time.
The aim of this booklet is to describe the different types of smart materials in terms of how they work, what types of materials are used and where they are used.

1

SHAPE MEMORY ALLOYS

memory effect once are known as oneway SMAs. However some alloys can be trained to show a two-way effect in which

Shape memory alloys (SMAs) are one of

they remember two shapes, one below

the most well known types of smart

and one above the memory temperature.

material and they have found extensive

At the memory temperature the alloy

uses in the 70 years since their discovery.

undergoes

a

solid

transformation.

What are SMAs?

state

phase

That is, the crystal

structure of the material changes resulting
A shape memory transformation was first

in a volume or shape change and this

observed in 1932 in an alloy of gold and

change

cadmium, and then later in brass in 1938.

„thermoelastic martensitic transformation‟.

The shape memory effect (SME) was

This effect occurs as the material has a

seen in the gold-cadmium alloy in 1951,

martensitic

but this was of little use. Some ten years

transformation

later in 1962 an equiatomic alloy of

characterised by a zig-zag arrangement of

titanium and nickel was found to exhibit a

the

significant SME and Nitinol (so named

martensitic structure is relatively soft and

because it is made from nickel and

is easily deformed by removing the

titanium

twinned structure.

and

its

properties

at

the

Naval

discovered

were

in

atoms,

structure

is

microstructure

below

temperature,

known

as

called

which

twins.

a

the is The

The material has an

Ordinance

austenitic structure above the memory

the

most

temperature, which is much stronger. To

common SMA. Other SMAs include those

change from the martensitic or deformed

based on copper (in particular CuZnAl),

structure to the austenitic shape the

NiAl and FeMnSi, though it should be

material is simply heated through the

noted that the NiTi alloy has by far the

memory temperature.

most superior properties.

again reverts the alloy to the martensitic

Laboratories)

has

become

state as shown in Figure 1.

How do SMAs work?
The SME describes the process of a material changing shape or remembering a particular

shape

at

a

specific

temperature (i.e. its transformation or memory temperature).

Materials which

can only exhibit the shape change or
2

Cooling down

The shape change may exhibit itself as either an expansion or contraction. The transformation temperature can be tuned to within a couple of degrees by changing the alloy composition.

Nitinol can be

made with a transformation temperature

anywhere between –100ºC and +100ºC which makes it very versatile.

Low temperature twinned martensite

Where are SMAs used?
Shape memory alloys have found a large number of uses in aerospace, medicine

Heat or cool through transformation temperature

and the leisure industry. A few of these applications are described below.
High temperature austenite Medical applications
Quite fortunately Nitinol is biocompatible, that is, it can be used in the body without

Figure 1 – Change in structure associated with the shape memory effect.

an adverse reaction, so it has found a number of medical uses. These include stents in which rings of SMA wire hold open a polymer tube to open up a blocked vein (Figure 2), blood filters, and bone plates which contract upon transformation to pull the two ends of the broken bone in to closer contact and encourage more rapid healing (Figure 3).

Figure 2 – This reinforced vascular graft contains rings of
SMA wire which open out the polyester tube on warming with warm saline solution once in-situ. (Courtesy of
Tony Anson, Anson
Medical Ltd)

It is possible that SMAs could also find use in dentistry for orthodontic braces which straighten teeth.

The memory

shape of the material is made to be the desired shape of the teeth. This is then deformed to fit the teeth as they are and the memory

is

activated

temperature of the mouth.

by

the

The SMA

exerts enough force as it contracts to move the teeth slowly and gradually
(Figure 4).

Surgical tools, particularly

those used in key hole surgery may also be made from SMAs.

These tools are

Figure 3 – This NiTi bone plated has been heat treated such that the central part changes from its deformed shape (top) to its memory shape
(bottom) when warmed with saline solution, thus drawing the two ends of the fracture closer together. The modulus of this material has also been closely matched to that of human bone.
(Courtesy of Tony Anson, Anson Medical Ltd)

3

often bent to fit the geometry of a

change, and this can be repeated over

particular patient, however, in order for

many thousands of cycles.

them to be used again they return to a

include springs which are incorporated in

default shape upon sterilisation in an

to greenhouse windows such that they

autoclave.

open and close themselves at a given temperature. Applications

Along a similar theme are

pan lids which incorporate an SMA spring in the steam vent.

When the spring is

heated by the boiling water in the pan it
Figure 4 – SMA wire has been used here to close the gap between two teeth. Two parallelograms of NiTi wire are attached to the teeth using stainless steel brackets which are glued to the teeth (left). After six months the gap between the teeth has decreased noticeably (right). (Courtesy of Tony Anson,
Anson Medical Ltd)

changes shape and opens the vent, thus preventing the pan from boiling over and maintaining efficient cooking. The springs are similar to those shown in Figure 5.

Still many years away is the use of SMAs as artificial muscles, i.e. simulating the expansion and contraction of human muscles. This process will utilise a piece of SMA wire in place of a muscle on the

(a)

finger of a robotic hand.

Figure 5 – Showing the two memory shapes of a memory metal wire coil or 'spring'. In (a) the spring is at room temperature and in (b) the higher temperature state has been activated by pouring on boiling water.

When it is

heated, by passing an electrical current through it, the material expands and

(b)

straightens the joint, on cooling the wire contracts again bending the finger again.

SMAs can be used to replace bimetallic

In reality this is incredibly difficult to

strips in many domestic applications.

achieve since complex software and

SMAs offer the advantage of giving a

surrounding systems are also required.

larger deflection and exerting a stronger

NASA have been researching the use of

force for a given change in temperature.

SMA muscles in robots which walk, fly and

They can be used in cut out switches for

swim!

kettles and other devices, security door locks, fire protection devices such as

Domestic applications
SMAs can be used as actuators which exert a force associated with the shape

4

smoke

alarms

and

cooking

safety

indicators (for example for checking the temperature of a roast joint).

Aerospace applications

bras.

The kink resistance of the wires

makes them useful in surgical tools which
A more high tech application is the use of
SMA wire to control the flaps on the trailing edge of aircraft wings. The flaps are currently controlled

by extensive

hydraulic systems but these could be

need to remain straight as they are passed through the body. Nitinol can be bent significantly further than stainless steel without

suffering

permanent

deformation.

replaced by wires which are resistance heated, by passing a current along them,

Another rather novel application of SMAs

to produce the desired shape change.

which combines both the thermal memory

Such a system would be considerably

and

simpler than the conventional hydraulics,

materials is in intelligent fabrics. Very fine

thus reducing maintenance and it would

wires are woven in to ordinary polyester /

also decrease the weight of the system.

cotton fabric.

superelastic

properties

of

these

Since the material is

superelastic the wires spring back to being
Manufacturing applications

straight even if the fabric is screwed up in

SMA tubes can be used as couplings for

a heap at the bottom of the washing

connecting two tubes.

basket! So creases fall out of the fabric,

The coupling

diameter is made slightly smaller than the tubes it is to join.

The coupling is

deformed such that it slips over the tube ends and the temperature changed to activate the memory. The coupling tube shrinks to hold the two ends together but can never fully transform so it exerts a

giving you a true non-iron garment!
In addition the wires in the sleeves have a memory which is activated at a given temperature (for example 38 C) causing the sleeves to roll themselves up and keeping the wearer cool.

constant force on the joined tubes.

Summary

Why are SMAs so flexible?

These are just a few of the possible uses

In addition to the shape memory effect,
SMAs are also known to be very flexible or superelastic, which arises from the structure of the martensite. This property

for shape memory alloys. However it is likely that as more research is carried out and these materials are better understood, more applications will be developed.

of SMAs has also been exploited for example in

mobile

phone

aerials,

spectacle frames and the underwires in

5

The main advantage of these materials is

PIEZOELECTRIC MATERIALS

the almost instantaneous change in the shape of the material or the generation of
The piezoelectric effect was discovered in
1880 by Jaques and Pierre Curie who

an electrical field.
What materials exhibit this effect?

conducted a number of experiments using
This probably makes

The piezoelectric effect was first observed

piezoelectric materials the oldest type of

in quartz and various other crystals such

smart material.

as tourmaline.

quartz crystals.

These materials, which

Barium titanate and

are mainly ceramics, have since found a

cadmium sulphate have also been shown

number of uses.

to demonstrate the effect but by far the most What is the piezoelectric effect?
The

piezoelectric

effect

and

and both relate a shape change with voltage. As with SMAs the shape change is associated with a change in the crystal structure of the material and piezoelectric also exhibit

two

crystalline

forms.

One form is ordered and this

relates

to

molecules.

the

polarisation

used

piezoelectric

ceramic today is lead zirconium titanate

electrostriction are opposite phenomena

materials

commonly

of

(PZT). The physical properties of PZT can be controlled by changing the chemistry of the material and how it is processed.
There are limitations associated with PZT; like all ceramics it is brittle giving rise to mechanical durability issues and there are also problems associated with joining it with other components in a system.

the

Where

The second state is non-

used?

are

piezoelectric

materials

polarised and this is disordered.
The main use of piezoelectric ceramics is
If a voltage is applied to the non-polarised

in

material a shape change occurs as the

described as a component or material

molecules reorganise to align in the

which converts energy (in this case

electrical

electrical) in to mechanical form. When a

field.

This

is

known

as

electrostriction.
Conversely, an electrical field is generated if a mechanical force is applied to the material to change its shape. This is the piezoelectric effect.

6

actuators.

An

actuator

can

be

electric field is applied to the piezoelectric material it changes its shape very rapidly and very precisely in accordance with the magnitude of the field.

Applications exploiting the electrostrictive

By passing an alternating voltage across

effect of piezoelectric materials include

these materials a vibration is produced.

actuators in the semiconductor industry in

This process is very efficient and almost

the systems used for handling silicon

all of the electrical energy is converted

wafers,

in

into motion. Possible uses of this property

microscopic cell handling systems, in fibre

are silent alarms for pagers which fit into a

optics and acoustics, in ink-jet printers

wristwatch. The vibration is silent at low

where fine movement control is necessary

frequencies but at high frequencies an

and for vibration damping.

audible sound is also produced.

in

the

microbiology

field

This

leads to the concept of solid state
The piezoelectric effect can also be used in sensors which generate an electrical

speakers based on piezoelectric materials which could also be miniaturised.

field in response to a mechanical force.
This is useful in damping systems and

Do polymers exhibit these effects?

earthquake detection systems in buildings.
But the most well known application is in the sensors which deploy car airbags.
The material changes in shape with the impact thus generating a field which

Ionic polymers work in a similar way to piezoelectric ceramics, however they need to be wet to function. An electrical current is passed through the polymer when it is wet to produce a change in its crystal

deploys the airbag.

structure and thus its shape.
A novel use of these materials, which

fibres

exploits

and

operate in a similar way, so research in

electrostrictive effects, is in smart skis

this field has focussed on potential uses in

which have been designed to perform well

medicine.

both

the

piezoelectric

are

essentially

Muscle

polymeric

and

on both soft and hard snow. Piezoelectric sensors detect vibrations (i.e. the shape of

Summary

the ceramic detector is changed resulting

It is clear the scope for applications of

in the generation of a field) and the

piezoelectric materials is more limited than

electrostrictive property of the material is

that of shape memory alloys.

then exploited by generating an opposing

those applications where they are used

shape change to cancel out the vibration.

rely on the very precise and controllable

The

piezoelectric

nature of the piezoelectric effect making

elements which detect and cancel out

them invaluable for the niche applications

large vibrations in real time since the

which they occupy.

system

uses

three

However,

reaction time of the ceramics is very small.
7

The

MAGNETOSTRICTIVE

original

observation

of

the

magnetostrictive effect became known as

MATERIALS

the Joule effect, but other effects have also been observed. The Villari effect is
Magnetostrictive materials are similar to piezoelectric and electrostrictive materials except the change in shape is related to a

the opposite of the Joule effect, that is applying a stress to the material causes a change in its magnetization.

magnetic field rather than an electrical

Applying

field.

magnetostrictive

What are magnetostrictive materials?

a

torsional

force

material

to

generates

a a helical magnetic field and this is known as the Matteuci effect.

Its inverse is the

convert

Wiedemann effect in which the material

magnetic to mechanical energy or vice

twists in the presence of a helical magnet

versa.

field.

Magnetostrictive

materials

The magnetostrictive effect was

first observed in 1842 by James Joule who noticed that a sample of nickel exhibited a change in length when it was magnetised. How

do

magnetostrictive

materials

work?

The other ferromagnetic

Magnetic materials contain domains which

elements (cobalt and iron) were also

can be likened to tiny magnets within the

found to demonstrate the effect as were

material. When an external magnetic field

alloys of these materials.

During the

is applied the domains rotate to align with

1960s terbium and dysprosium were also

this field and this results in a shape

found to be magnetostrictive but only at

change as shown in Figure 6. Conversely

low temperatures which limited their use,

if the material is squashed or stretched by

despite the fact that the size change was

means of an external force the domains

many times greater than that of nickel.

are forced to move and this causes a

The

most

common

magnetostrictive

change in the magnetisation.

material today is called TERFENOL-D

Where are magnetostrictive materials

(terbium

used?

Ordanance

(TER),

iron

Laboratory

dysprosium (D)).

(FE),
(NOL)

Naval and This alloy of terbium,

iron and dysprosium shows a large magnetostrictive effect and is used in transducers and actuators.
8

Magnetostrictive materials can be used as both actuators (where a magnetic field is applied to cause a shape change) and

have been used in ultrasonic cleaners and surgical tools. Other applications include hearing aids,

linear

motors,

razorblade damping sharpeners, systems, positioning equipment, and sonar.
Summary
Although

are

very

specialised

materials they have found niche markets

Base state of material
Bar contracts if field applied

these

in which their properties are ideal. More
Bar expands if field applied

Figure 6 – Magnetostrictive materials are made up from domains (represented by red and blue bar magnets). If a magnetic field is applied the bar either decreases in length from its base state (centre to left) or increases in length
(centre to right) depending on the polarisation of the applied field.

applications are likely to come along as the materials are developed and their performance improved.

sensors (which convert a movement into a magnetic field).
In actuators the magnetic field is usually generated by passing an electrical current along a wire.

Likewise the electrical

current generated by the magnetic field arising from a shape change is usually measured in sensors.
Early

applications

of

magnetostrictive

materials included telephone receivers, hydrophones, oscillators and scanning sonar. The development of alloys with better properties led

to the

use

of

these

materials in a wide variety of applications.
Ultrasonic magnetostrictive transducers

9

MAGNETO- AND ELECTRO

fluid align and are attracted to each other

RHEOLOGICAL MATERIALS

resulting in a dramatic change in viscosity as shown in Figure 7. The effect takes milliseconds to occur and is completely

All of the groups of smart materials discussed so far have been based on solids. However, there are also smart

reversible by the removal of the field.
Particle
Suspension fluid
Electrodes

fluids which change their rheological properties in

accordance

with

their

environment.
What are smart fluids?
There are two types of smart fluids which were both discovered in the 1940s.
Electro-rheological (ER) materials change their properties with the application of an

Figure 7 – Schematic diagram showing the structure of a electrorheological fluid between two electrodes. The top figure shows the structure in a low field strength where the particles are randomly distributed. When a higher field strength is applied, as in the bottom diagram, the particles align causing a change in the viscosity of the fluid.

electrical field and consist of an insulating

Figure 8 clearly shows the effect of a

oil such as mineral oil containing a

magnet on such an MR fluid.

dispersion

(early

fluids a field strength of up to 6kV/mm is

experiments used starch, stone, carbon,

needed and for MR fluids a magnetic field

silica, gypsum and lime).

of less than 1Tesla is needed.

of

solid

particles

Magneto-

rheological materials (MR) are again based on a mineral or silicone oil carrier

With ER

Where are smart fluids used?

but this time the solid dispersed within the

Uses of these unusual materials in civil

fluid is a magnetically soft material (such

engineering, robotics and manufacturing

as iron) and the properties of the fluid are altered by applying a magnetic field.

In

both cases the dispersed particles are of the order of microns in size.
How do smart fluids work?
In both cases the smart fluid changes from a fluid to a solid with the application of the relevant field. The small particles in the

10

Figure 8 - A puddle of magnetorheological fluid stiffens in the presence of a magnetic field.
(courtesy of Sandy Hill / University of
Rochester)

are being explored. But the first industries to identify uses were the automotive and aerospace industries where the fluids are used in vibration damping and variable torque transmission.

MR dampers are

used to control the suspension in cars to allow the feel of the ride to be varied.
Dampers are also used in prosthetic limbs to allow the patient to adapt to various movements for example the change from running to walking.
Summary
Despite having rather unusual properties
MR and ER fluids are finding more uses as our understanding of them develops.
Further uses in damping systems are likely, as are applications where the fluids are used to polish other materials. In this case the materials flow to cover the surface evenly. This technique has been used to polish lenses for high precision optics systems.

11

rearrangement. In either case at a given

CHROMIC MATERIALS

temperature a change in the structure of the material occurs giving rise to an
This group of materials refers to those which change their colour in response to a change in their environment, leading to

apparent change in colour. The change is reversible so as the material cools down it changes colour back to its original state.

the suffix chromic. A variety of chromic

In liquid crystals the change from coloured

materials exist and they are described in

to transparent takes place over a small

terms of the stimuli which initiate a change, thus:

temperature range (around 1 C) and arises as the crystals in the material

Thermochromic materials change with

change

their

orientation

(Figure

9).

However, liquid crystals are relatively

temperature;

expensive and so where there is no need
Photochromic materials change with

for the colour change to take place in a

the light level;

very

Piezochromic materials change with applied pressure;
In

the

case

solvatechromic

of and narrow

employed.
Figure 9 – A liquid crystal contains needle shaped particles which are arranged randomly below the transformation temperature (top).
Above this temperature the particles are aligned, changing how the material reflects light, thus showing a change in colour
(bottom)

electrochromic, carsolchromic electrical potential, a liquid or an electron beam respectively. photochromic window

molecular rearrangement materials are

materials the stimulus is either an

Thermochromic,

temperature

and

piezochromic materials are the most popular with the first two groups finding everyday applications.
How

do

thermochromic

materials
Leucodyes change colour by molecular

work?

rearrangement and the colour and active
There are two types of thermochromic

temperature range of the dye can be

systems: those based on liquid crystals

controlled

and

groups on the corners and central site of

12

those

which

rely

on

molecular

by

changing

the

chemical

the molecule. Leucodyes have a broader temperature range than liquid crystals and will usually

become

colourless

over

approximately 5ºC.
In both cases the thermochromic material is encapsulated

inside

microscopic

spherical particles to protect it.

These

encapsulating molecules must themselves be transparent and able to withstand the thermal cycling which the thermochromic artefact will undergo.

Figure 10 – A thermochromic toothbrush at room temperature, being warmed by holding in the hand and when warm.

The pigments can be incorporated in to

The thermochromic material does not generally produce two or more colours itself. The encapsulated particles of the material are printed on to or mixed in to

dyes for fabric to produce clothing which changes colour

with

temperature.

Thermochromic inks can also be used for printing on to clothing and food packaging.

another material in the second colour. At

Thermochromic toothbrushes have been

room temperature the sample is the colour

produced that change colour as they are

of the thermochromic dye but when it is

warmed in the hand. It takes roughly two

heated above its transition temperature

minutes to warm the brush enough to see

the

becomes

a change in the colour and this is the

transparent thus showing the base colour

length of time dentists recommend teeth

underneath. For example if a layer of red

should be brushed. Such a toothbrush is

thermochromic pigment is applied to a

shown in Figure 10.

thermochromic

material

blue substrate at room temperature it will appear red but as if warms up it becomes

Thermochromic thermometers have been

blue. As the temperature decreases again

developed as they offer significant safety

the red colour will reappear.

advantages mercury over

traditional

thermometers.

The

glass

/

plastic

Where are thermochromic materials

substrate consists of stripes of different

used?

colours

Since thermochromic pigments can be used in a variety of ways they have found a varied range of applications.

representing

the

different

temperatures. This is then coated with a layer of thermochromic dye of varying thickness (it is thinner at the cool end of the thermometer than it is at the higher

13

temperature end). As the thermometer is warmed by placing it on the forehead the thin layer of dye warms up and becomes transparent first.

The

higher

the

temperature the thicker the layer of dye which can be warmed sufficiently to change colour.
This principle is also employed in the tester strips which appear on the sides of some batteries, but this time the heat is generated by the resistance heating effect

Figure 12 – Showing a thermochromic baby spoon at room temperature (top) and after immersion in boiling water (bottom)

the food the more rapidly the spoon changes from red to yellow.

yellow colour has been achieved by immersing the spoon in boiling water.

of a small electrical current flowing across

Thermochromic

the battery.

temperature

Thermochromic materials have also found safety applications in kettles and baby spoons. The body of the kettle is actually made from pink plastic, however this contains a

small

amount

of

a

thermochromic dye which is blue at room

The bright

dyes

with

resistance

a

and

higher higher transition temperature have also been produced and incorporated in to pans.
These pans have a small coloured circle in the bottom which changes colour when the pan

has

reached

the

optimum

temperature for cooking.

temperature and becomes transparent when warm, thus showing the pink colour.
A series of photographs showing the change in colour as the kettle is boiled are shown in Figure 11.
Thermochromic pigments have also been employed in baby spoons which change colour to warn if the food is too hot for feeding. Babies generally like to eat their food no hotter than one degree above body temperature, so the spoons are designed to change colour at 38 C. The room temperature and hot states of such a spoon are shown in Figure 12. The hotter

14

Figure 11 – Showing gradual change in colour as the thermochromic kettle boils.

These materials are already starting to

Figure 13 – An unground, photochromic spectacle lens sat on a sunny windowsill.

find a large number of uses in our everyday lives and there is no doubt that as thermochromic

dyes

are

further

understood more uses will be found.
Summary
What about photochromic materials?
Chromic materials are here to stay! From
Photochromic materials are those which change when they are exposed to light,
UV

light

in

particular.

thermochromic

materials,

As the with colour change arises from a change in the structure of the material allowing it to change from completely clear to dark yet still transparent.

the colour changing T-shirts of the 1980‟s to the new temperature sensing baby spoons, thermochromic

materials

are

finding more and more uses in our everyday lives.

Other forms of chromic

materials are also growing in popularity, photochromic spectacle lenses have come down in price and become far more

The main application of photochromic

popular over the past decade or so. The

materials is in spectacle lenses which

other types of chromic materials will also

change in to sunglasses outside. Figure

become

13 shows such a lens which has turned

properties are developed and the costs

brown after sitting on a sunny window sill.

come down. For example electrochromic

If you have a pair of these glasses you will

coatings may be used for modesty panels

know that they are always very slightly

on doors or to block out the sunshine in

tinted as there is always a small amount of

office

UV present. You may also have noticed

materials are already used to measure

that, rather annoyingly, they do not

pressure, however at the moment the

generally change colour when you are

colour change associated with this is not

driving as the windscreen contains a filter

reversible so the materials can only be

to cut out the UV portion of the light.

used once.

more

block

widespread

windows.

as

their

Piezochromic

These materials could also be used to coat large office windows so that they cut out some of the light on hot days.

15

BIBLIOGRAPHY
N D R Goddard, R M J Kemp and R Lane
“An overview of Smart Technology)
Packaging Technology and Science, Volume 10, pp 129 – 143 (1997)

Useful web-sites www.nitinol.com www.sma-inc.com www.cs.ualberta.ca/~database/MEMS/sma_mems/sma.html http://virtualskies.arc.nasa.gov/research/youdecide/Shapememalloys.html www.memorymetals.co.uk www.instmat.co.uk/iom/divisions/mst/smasc.htm http://virtualskies.arc.nasa.gov/research/youdecide/piezoelectmat.html www.npl.co.uk/npl/cmmt/functional/tutorial/html www.designinsite.dk/htmsider/k1307.htm www.memagazine.org/backissues/february98/features/alarms/alarms.html http://virtualskies.arc.nasa.gov/research/youdecide/ionicnconducpolym.html http://www.public.iastate.edu/~terfenol/ http://www.physics.hull.ac.uk/magnetics/Research/Facilities/Magnetostriction/magnetostriction.html http://www.etrema-usa.com/terfenol/index.asp http://virtualskies.arc.nasa.gov/research/youdecide/magnelectrholofluids.html www.cs.ualberta.ca/~database/MEMS/sma_mems/smrt.html www.instmat.co.uk/iom/divisions/mst/smasc/smartfluids.htm http://bilbo.chm.uri.edu/SST/thermochromic.html www.designinsite.dk/htmsider/m1317.htm www.tut.fi/units/ms/teva/project/intelligenttextiles/chromic.htm www.theprintdirectory.co.uk/hp/pressrelease/Aug2k/thermochromic.htm www.theprintdirectory.co.uk/hp/technical%20topics/thermochromic.htm www.ropnet.ru/niopik/TECH/tech_htm/te103411.htm Christina Astin, Diane Talbot and Peter Goodhew “Weird Materials”
Physics Education, volume 37, number 6, November 2002. Pp516 – 520

16

Useful articles from Materials World
Tony Anson "Shaping the body from memory"
Volume 7, number 12, December 1999. Pp745 – 747.
Donald Golini “A polished performance from MRF”
Volume 8, number 1, January 2000. Pp 23 – 24.
Derek Muckle “Cross linked PE – a shape memory solution for pipeline renovation”
Volume 8, number 4, April 2000. Pp 22 – 23.
Paul Butler “ Smart packaging goes back to nature”
Volume 9, number 3, March 2001. Pp 11 – 13.
Clifford Friend “Sense and sensibility of products around the home”
Volume 9, number 9, September 2001. Pp 12 – 14.
Roger Stanway “Smart fluids – the shock of the new”
Volume 10, number 2, February 2002. Pp 10 – 12.

17

HOW TO USE THE RESOURCES SUPPLIED WITH THIS PACK
A number of samples have been provided to help you demonstrate Smart Materials in the classroom. They can be used as follows.
Shape memory alloy spring
This spring is infact not a spring but a coiled piece of wire. As such UNDER NO
CIRCUMSTANCES should you try to extend or compress the sample by pulling or pushing the ends, as this will render it useless. This wire shows a two-way shape memory effect whereby at room temperature it is tightly coiled. To activate the high temperature of the alloy simply pour on very recently boiled water and the coil will expand to about double its length. This transformation occurs almost instantaneously.
The transformation temperature of this alloy is about 97ºC. To return the coil to its original size, simply remove it from the hot water and allow it to cool. You will be able to see this happen as it occurs much more slowly.
Shape memory metal wire
A small length of Nitinol wire has been supplied as a small coil and this material shows the one-way shape memory effect. The wire can be straightened or crumpled up at room temperature and then returned to its memory shape by immersing it in boiling water or passing it through a lighter flame (again the memory temperature is around 100ºC). If you would like to change the memory shape of the wire secure it to the desired shape at room temperature (by screwing it down to a metal plate or holding it in pliers) and heat to a temperature of about 450ºC, which can be achieved using a domestic blow torch. At this temperature the wire will take on a bright orange glow. To fix the new memory shape plunge the glowing wire into cold water. This quenches the material and changes its microstructure. It is essential that the wire is held securely as it is heated, as it will try to change shape to its original memory shape.
Thermochromic spoon
The red section at the end of this spoon contains a thermochromic dye. To change the colour of the spoon simply put it in some warm water. The transformation temperature of the dye is around 38ºC at which point it changes from being red to transparent, thus exposing the yellow plastic below. The colour change will occur more rapidly the higher the temperature of the water (i.e. the colour change is quicker in water at 100ºC than water at 40ºC). The original red colour returns when the spoon is allowed to cool.
Thermochromic pen
The thermochromic dye is in this case blue and the colour change can easily be activated by holding the barrel of the pen in a warm hand. The blue dye becomes transparent showing the white plastic underneath. The blue colour returns as the pen is allowed to cool back to room temperature.

Opening your eyes to the world of
Materials, Minerals and Mining
18