1 IntroductionWhat are smart materials? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Smart CompositesComposite materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 Shape Memory MaterialsShape memory alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Shape memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4 Smart AdditivesMotion control gel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Dilatent compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Ferrofluid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Expansel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Chromatic indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5 Smart ConductorsQTC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Piezoelectric materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

6 Smart ImagingThermochromic materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Photochromic materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Phosphorescent materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Light pipes, optical fibres and printed lenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26Light scattering particles and films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

» Appendix 1 Smart wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

» Appendix 2QTC: some electronic applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

» Appendix 3Fibres, composites and smart textiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

» Appendix 4Polymorph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

» Appendix 5Smart structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

» Appendix 6Using Smart colours . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

n Supply of materials

n Further reading

»Contents

» IntroductionWhat are smart materials?

The term ‘smart’ is used to describe a material thatexhibits some kind of useful or beneficial response toexternal changes of temperature, light level or othercondition. These materials are becoming increasinglyimportant in engineering because they open up newdesign opportunities ranging from more efficientloudspeakers to miniaturised mechanisms in cameras.Apart from the increased use of smart materials withinengineered products, it also has to be remembered thatengineering solutions are essential for utilising smartmaterials in applications such as holographic printing,optical systems and consumer products generally.

Most of the materials discussed in this book originatedduring the latter part of the 20th century and reflect theenormous advances in materials science made during thatperiod. But smart materials, as well as many ‘intelligent’products, have a much longer history and it is worthmentioning some earlier examples:

n porous ceramics, used in the ancient world for self-regulating evaporative cooling

n metal springs, also used in the ancient world and thereafter as a one-bit ‘memory’

n metals in bi-metallic form used from the 17th century for temperature indication, compensation and regulation

Similarly, we can identify components that pre-date muchof what we now consider smart. Century-old light bulbs,for example, are arguably ‘smarter’ than LEDs. Asfilament temperature increases, resistance rises toprevent excess current flow: bulbs are self-regulating.

Some materials are clearly smarter than others. Theplastic body of a kettle that changes colour as it heats upis clearly smarter than a stone in a mass of aggregate. Butin between these two extremes we see a vast range ofmodern materials – many with specifically engineeredproperties.

Many textbooks divide the materials ‘kingdom’ into fivebroad groups:

n Mineralsn Ceramics n Organics n Metalsn Plastics

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This kind of classification now has to be viewed withsome caution because the boundaries are becomingincreasingly blurred by the development of new hybridmaterials. Examples include ferro-fluid (metal + organic),QTC (metal + polymer).

As a result of progress during the last sixty years, we nowhave available countless numbers of ‘designer’ metals,plastics and composites etc., and the knowledge base tocreate many more. If history is a reliable guide, thenumber of materials with novel properties is likely togrow exponentially and designers will be faced with anunexpected problem: the challenge of choice.

The materials described here are available now andinclude some that are ‘ahead of the game’ waiting tomake a real commercial impact. We have unparalleledaccess to smart and modern materials, and what followsis both a guide to their properties and a pointer towardscreative applications.

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1. Smart materials2. Smart fabrics

3. Everyday objectsUtilising smart material

4. LEDs and bulbs

5. Colour changing kettle6. Materials ‘Kingdom’7. ‘Mutant Materials’

Smart materials created fromcross-category combinations.Ferrofluid and QTC shown.

S Introduction

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»Composite Materials

A composite comprises two or more differentmaterials combined to give special properties –usually exceptional strength and stiffness.Increasingly, composites are displacing moreconventional materials in the construction ofaircraft and vehicles and smaller productsincluding a wide range of consumer goods and sports equipment.

Perhaps the most familiar composite is glass fibreembedded in a matrix material such as polyester resin orconcrete. This combination exploits the high tensilestrength of the glass fibres – a modest quantity of whichcan have the same effect in concrete as a steel reinforcingmesh. Important trends in composite technology includethe use of more exotic combinations of materials and thedevelopment of new forming methods. Composites nowtypically incorporate carbon fibre, Kevlar, metal oxides,alumina fibres and carbon nanotubes - and productionmethods include injection moulding.

Carbon fibre reinforcedcompositesCarbon fibre, in chopped or woven form, confersenormous strength and stiffness when combined with apolyester resin matrix. Modern kites typically use carbonfibre rods made by a process called pultrusion - hence thename pultruded rod. This involves coating the fibre withresin and continuously pulling the mix through a die tocreate solid rod or tubing. To create flat or curvedsurfaces carbon fibre composite material is oftenmanufactured (and supplied) as a soft pre-impregnatedsheet (‘pre-preg.’) which is shaped and then heated forfinal curing. One of the hallmarks of products usingwoven carbon fibre cloth reinforcement is the surfacedetailing showing the cloth weave.

Kevlar fibre reinforcementKevlar is a patented polymer fibre used particularly whereextreme impact and cutting resistance is important in acomposite. This makes it ideal for applications such asbody armour – and even industrial gloves.

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1. Carbon fibre2. Carbon fibre rods in kites

3. Kevlar polymer fabric

4. Carbon nanotubes

5. Hybrix™6. Laminated composites7. Foamed metals

Aluminium foam,Compressed Aluminium foil.

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Carbon nanotubecompositesCarbon nanotubes are regular single-atom thick tubes ofcarbon atoms with extraordinary properties includingextreme strength and exceptional electrical and thermalconductivity. (see smart additives). Carbon nanotubes,used as micro-fibres, can potentially make ultra-strongcomposites but also many other confer other propertiessuch as high conductivity. Emerging products includeconductive polymer films and high tech. sportsequipment.

Laminated compositesThis class of materials is very wide and typically includeslayers of different materials strongly bonded at theinterface between each type. A tradition example is theuse of thin rubber strips bonded between layers of woodto make handles for cricket bats. This combinationabsorbs shock.

ALU composite is a relatively modern compositecomprising a core of polythene sandwiched between twovery thin surface layers of coloured aluminium. This sheetis extremely stiff but has a surprisingly low weight. It isused extensively for interior panelling, signage etc. and insome engineering applications

Hybrix™

Hybrix™ is an entirely new patented material consisting oftwo outer stainless steel skins held together by a uniformmatrix of steel whiskers bonded to the inside surface ofthe skins. Hybrix™ is manufactured as a very thin sheetmaterial, and because of its makeup has very low weightand high stiffness. Uses include aircraft components andconsumer goods such as computer cases.

Foamed metalsAlthough not strictly composites, foamed metals – theequivalent of foamed plastics – have similarcharacteristics. Aluminium foam, for example, is producedby blowing nitrogen into a molten mix of metal toproduce a sheet with a continuous outer skin andfoamed core. This material is used extensively inarchitecture because of its high strength, low weight andexcellent acoustic properties.

[This material can be replicated on a small scale bycompressing aluminium foil in a special tool. Please seeteaching resources.]

S Shape memory materials

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Most materials exhibit limited ‘memory’ dueto elasticity. A typical material deforms whenstressed and then returns to its original shapewhen the stress is removed. Metal, plastic(and even concrete) springs exploit thisproperty to give functions ranging fromspring-return mechanisms (e.g. switches andpens) to energy storage (e.g. clockworkradios). Shape memory materials, on theother hand, are specially formulated to changeshape at specific temperatures and providesignificant movement and forces.

Shape memory alloy (SMA)Several different alloys exhibit memory properties, butthe most widely used is a mixture of nickel and titaniumcalled Nitinol - normally available in the form of wire.This material has a crystalline structure that can be‘switched’ between two or more states by heating to aspecific temperature called the transition temperature.These different states result in a change of externaldimensions – e.g. a wire that contracts at a settemperature.

SMAs are used in wide ranging applications including:

n Robotics - actuators/artificial muscles.n Orthodontics - teeth braces.n Surgery - wire mesh ‘balloons’

(stents) used to open blocked arteries.

n Communications - precision joining of optical fibres.

n Control - temperature sensing/actuation.

Manufacture of Nitinol wireEqual amounts of nickel and titanium are combined andmelted in an electric furnace at 1300°C. The process iscarried out in a vacuum and any contamination by othermaterials is strictly avoided. The melted metal alloy isthen cast into small ingots.

The ingots are rolled into rod, bar or sheet forms. If wireis required, the bar stock is further drawn down throughdies of decreasing diameter.

Finally, the wire is given its memory by carefullycontrolled heat treatment and conditioning (sometimescalled ‘training’). This process is carried out continuously.The memory behaviour of the SMA depends on heattreatment.

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»Shape memory materials

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Memory wireMemory wire is supplied as straight wire and trained toremember this default condition. If, for example, alength of the wire is folded into a paperclip shape it willremain like this until heated to its transitiontemperature of approximately 70°C. At thistemperature the wire straightens out very rapidly andexerts a mechanical force

The wire can be trained to remember any other shapesby forming it as required and then heating to a hightemperature for a few minutes. If, for example, we wantthe wire to remember the shape of a closedcompression-type spring, the wire is wound and held ona metal or ceramic mandrel. It is then heated forapproximately 15 minutes in a temperature-controlledoven at a temperature of 500°C – and left to cooldown.

If the trained spring is now stretched out, it will remainextended until re-heated to the transition temperatureof 70°C – e.g. by immersing it in very water. The springwill then close very rapidly with a useful pulling force. Asmall spring of 5mm diameter wound from wire of0.8mm cross sectional area is capable of lifting a mass of1 kg when it ‘remembers’ to close.

Memory wire, in helical spring or other shapes, has anumber of applications including:

n coffee machines – to open a valve so that hot water falls onto the coffee

n air conditioning units – to move louvers or flaps to direct air movement

n shower units – to control hot water valves

n fire alarm systems - to trigger water sprinklers

Shape memory metal in the form of tubing also hasimportant applications – e.g.

n seals for hydraulic tubing – which contract down over flexible pipes

n electrical connectors – which contract to join wires

n optical fibre joiners – which contract to align and join optical fibres.

Smart wireA common form of nitinol wire, referred to as eithersmart wire or muscle wire, is trained to ‘remember’ achange in length. Unlike most metals that expand onheating, this wire contracts in length by about 5% whenheated to its transition temperature of approximately70°C. It retains contracted until pulled back to itsoriginal length using a spring or other means.

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1. Wind up radio2. Clockwork mechanism

3. Medical stents Used to open blocked arteries.

4. Trained memory wire

5. Memory wire in coffee machines6. Example smart wire project

S Shape memory materials

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Plastics with memoryMost polymers exhibit some shape ‘memory’, and a goodexample is acrylic. If a section of this material is heated,compressed and then cooled, it retains its new shape. Butif it is re-heated, it reverts partly to its original shape. Thisproperty has been exploited to create relief features insheet materials using the following technique:

A typical thermoplastic polymer such as acrylic consists ofbillions of long molecules intertwined in a randompattern. Heating the material enables these molecules toslip causing it to become soft and capable of moulding orshaping. Most thermoplastics plastics carry a significant‘internal history’ of heat treatment - for example, plasticbags consisting of a thin film blown from a continuouslyextruded tube. If these films are re-heated, they contract– some much more than others. Because some printedbags contract by a very high percentage if re-heated, onetrick is to reheat them in an oven and watch inamazement as the printed image also reduces –sometimes to the size of a postage stamp.

High shrinkage on re-heating is a very useful property andis the raison d’etre of many commercial plastics includingheat-shrink wrapping film and heat-shrink tubing. A typicalshrink-wrapped product is first sealed into a bag and thenpassed through a heated tunnel which causes the film tocontract by up to 50% and form a tight outer wrapping.

Heat-shrinkable tubing is a polyolefin polymer thatexhibits a shrinkage ratio of something like 2:1 at itstransition temperature. It is used in a wide-rangingapplications from simple cable strain relief to pre-formedmouldings for wiring harnesses. The material is very easyto use and can be heated, for example, with a hot air gunor even by holding it in close proximity to a solderingiron.

1. Heat the acrylic to a softcondition and press a wire profileinto the surface.

2. Let the acrylic cool andremove the wire.

3. Machine or abrade the acrylicsurface away until flat again and(optionally) polish.

4. Re-heat and soften the acrylic. The profile of the wire will now pop up in relief as thecompressed material expands to its original condition.

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7-10. Acrylic thermoplastic polymer11. Plastic Memory

When heated the printed image on this yogurt pot reduces with the plastic.

12-13. Heat shrinkable tubing

14. Shape memory polymer (SMP)

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Shape memory polymers(SMP)Like shape memory alloys (SMA), shape memorypolymers are specially formulated to ‘remember’ one ormore shapes when heated to a specific transitiontemperature. The several types in commercial use carrydifferent trade names such as Smart Polymer. Thisparticular example is a composite comprising a wovenfibre mat embedded in a shape memory polymer thatsoftens at just 70°C. The sheet is heated and formed andretains its new shape indefinitely until re-heated again to70°C. It then ‘remembers’ to return to its original flatcondition. Smart Polymer is used, for example, to createcomplex shapes for formers and tooling that cannotnormally be removed after a manufacturing operation –e.g.0 creating a length of circular section fibreglass ductingwith reduced diameter ends. The material has also beenused experimentally to create deployable aerospacestructures – e.g. a strut that extends in length whendeployed in space.

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Motion control gel (smart grease)Hidden away and taken for granted within manysuccessful products is an almost invisible material: motioncontrol gel. This unusual class of material is one thatcombines the properties of a lubricant and an adhesive tocreate a ‘sticky’ grease. In smaller scale engineeredproducts, motion control gel is used to give the smoothmotion and damping needed for focusing opticalinstruments such as microscopes and binoculars. It is alsothe active retarding agent in miniature rotary dampers –the spring-loaded mechanisms that imparts the slowconstant-speed travel to CD trays, car cup holders.Motion control gel is a widely used and inexpensive wayof enhancing product performance and value. Used inproducts ranging from pens to volume controls, itprovides the ‘feel’ of quality when, in fact, engineeringtolerances are set relatively low.

Motion control gel is used to improve the performance ofproducts and their prototypes –often in the form of‘lubricant’ on bearing surfaces. It can also be used in novelapplications such as stored energy actuators that yield auniform (slow) output instead of fast run-down. Theelastic band motor illustrated consisting of a cylindricalcontainer, elastic band and wheel. Under normalcircumstances, if the wheel is wound up relative to thecontainer, it will spin rapidly when released. If a smallamount of motion control gel is applied to the top edgeof the container, the wheel now turns very slowly and ata uniform speed.

Shear-thickeningcompoundsAs its name suggests, this class of material becomesincreasingly viscous when agitated or stirred in some way.There are several commercial compounds that exhibitthis behaviour – but the phenomenon can bedemonstrated with a familiar kitchen ingredient:cornflower. If this is mixed with water into theconsistency of thick cream it becomes almost solid whenstirred but quickly reverts to a liquid when static.

Shear thickening compounds can thus be used in dynamiccontrol applications where, for example, a safe speedlimit has to be built into a rotary transmission system or‘smart’ damping is needed.

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»Smart additives

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1-2. Motion control gel Used in many successful everyday products.

3. Elastic band motorexperiment

4. Cornflower Shear-thickening demonstration.

5. ‘Silly Putty’

6. Ferrofluid magnetic fluidAs used in audio speakers.

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Dilatent compounds‘Silly putty’ or ‘bouncy putty’ has long been available as anamusing toy. In fact, this class of silicon-based material –properly known as a dilatent compound – has seriousengineering applications. It normally behaves as a softmouldable material but, if suddenly impacted, it changesinto a rubber-like material and bounces.

The unique rheological (flow) properties of this materialmake it ideal for use in difficult ‘machining’ operations.For example, it is used for polishing the small borecooling channels cast into turbine blades. The compoundis loaded with grit and forced under high pressure intothe channels.

Increasingly, the material is finding new applications insideas well as outside engineering. It can provide more or lessinstant protection against impact damage when placedaround vulnerable components. In a form used in safetyclothing, it is embedded into the cells of a plastic foamsheet. This can normally flex but becomes stiff if impacted – e.g. elbow and knee patches in ski clothing.

FerrofluidFerrofluid is a magnetic liquid consisting of a mineral oilthat contains billions of nano-sized magnetic particlescoated with a surfactant to prevent them clumpingtogether. When a magnet is brought close to ferrofluid,the fluid takes up a distinctive three dimensional shape –analagous to the field pattern formed by iron filings near amagnet.

Ferrofluid is a little known but highly important material.It is used, for example, as a dynamic seal around bearings(kept in place by a magnetic ring) and as a key part ofdamper systems that are ‘hardened’ by the action ofelectromagnets. Ferrofluid is commonly used inloudspeakers between the permanent magnet and movingvoice coil. This improves magnetic permeability(effectively efficiency) and heat dissipation in higherpower units.

S Smart additives

ExpanselExpansel is the commercial name of an unusual materialthat has the appearance of a fine powder. In fact, eachpowder grain is a micron-size hollow polymer spherecontaining a minute drop of hydrocarbon liquid. When amass of expansel is heated, the shell of each micro-spheresoftens and expands as the liquid inside turns into a gas.This process creates micro-balloons that can be added toother materials such as concrete and some polymers toreduce their density. Expansel has found several specificuses in engineering and manufacturing – e.g. theunexpanded material is incorporated into a polymer thatis part-moulded into shape at a temperature less than the‘activation’ point of the spheres. The part-formed objectis then heated in a finishing mould above the activationpoint, and the spheres expand the mass of polymer.

Chromatic indicatorsMany materials now incorporate special chemicals thatchange colour as the material undergoes change. A goodexample is chromatic alginate – a precision flexiblemould-making material that can exhibit up to fourdistinctive colour changes to indicate completion of thevarious stages of mixing and hardening. This material iswidely used in dentistry for producing casts of teeth andfor precision prototyping.

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7. Expansel

8. Chromatic Alginate

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Conductive polymers Many polymers have been developed that exhibit lowerthan normal electrical resistance. These fall into twocategories: polymers with inherently lower electricalresistance and composites which contain conductivematerials such as carbon or metal particles. The secondcategory conducts via the particles by a process calledpercolation and is pressure sensitive. It is used, forexample, as conductive foam to combat static build-upwhen storing or transporting sensitive electroniccomponents. Another example of this type of material is acarbon-loaded plastic ribbon used for low temperatureheating. Electrical current passing through the ribbonproduces sufficient heat to protect pipe work fromfreezing and paths from icing over. It is also self regulatingsince expansion of the hot ribbon causes the carbonparticles to move apart and reduce current flow.

QTC (Quantum tunnelling composite)

Quantum tunnelling composite is a flexible polymer thatexhibits extraordinary electrical properties. In its normalstate it is a perfect insulator, but when compressed itbecomes a more or less perfect conductor and able topass very high currents. Polymers loaded with carbon are,at best, only partially conductive. In QTC, the changefrom non conductor to conductor is dramatic, and a tinypiece measuring 4mm square and 1.5mm thick can pass acurrent of up to 10 amps when squeezed!

Instead of carbon, QTC contains tiny metal particles, but it does NOT work by percolation. Instead, electrons‘pass’ through the insulation by a process called quantumtunnelling – hence the name of the material. To explainthis effect, we have to appeal to quantum theory andthink of the electrons as waves. In classical physics, theelectrons cannot pass through an insulation barrier, butaccording to quantum theory a wave can – and this iswhat happens in QTC. To some extent we have tosuspend belief, because the world seen through quantumtheory appears so much at odds with its common sensecounterpart. (Another way of describing the quantumtunnelling effect is to say that a probability exists ofelectrons at point A - one side of the insulation barrier -appearing at point B - the other side. This is all very weird – but demonstrably true.)

»Smart conductors

1. Conductive foam

2. QTCQuantum Tunnelling composite,QTC cable, pills and sheet.

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Since its recent discovery, QTC has passed quickly from alaboratory curiosity into a commercial product poised torevolutionise aspects of both engineering and productdesign. It has already been used to make smart garmentsthat can be wired directly to electronic products such asan iPod. It is also being retro-fitted, for example, withinconventional switches to eliminate arcing and electricalnoise. However, the material is so new that thecommercial world is only just waking up to the vastnumber of new possibilities and applications. Theseinclude advanced membrane panel switches, speedcontrollers and sensors.

When QTC is heated or comes into contact with reactivematerials, its dimensions change slightly to bring about ameasurable change in resistance. Because of this property,it can even sense small concentrations of organicmolecules in liquid or gas form.

QTC is currently available in three forms: pills, thin sheetor cable. Each form of QTC offers different potential forpractical use, but they all share a striking resistancechange when deformed by squeezing, pulling or twisting.In practice, the different forms of QTC can be connectedin quite simple ways to create anything from switches toforce sensors.

Using QTC pillsThe basic use of a QTC pill can be demonstrated simplywith an electric motor. If the pill is placed on a conductor– say a ruler – and touched with a probe to complete thecircuit, the motor will not run. When the pill iscompressed, the motor starts up. Squeeze very hard andit will run at top speed. If you apply varying pressure themotor speeds up or slows down since the QTC functionsas an infinitely variable resistor.

It will be obvious from this experiment that the pill canbe used to bridge across or between conducting tracks. Itwill also become clear that conductivity is proportional topressure in the initial phase of squeezing – opening up allkinds of light and motor speed control applications.

Examples of pillapplicationsA. A QTC pill bridges the gap between two pieces ofself-adhesive aluminium foil laid near the ‘hinge’ of a pieceof folded card or plastic sheet. When the card above thepill is pressed, it acts either as an on/off press switch or avariable resistor. Many other card/pill configurations arepossible including warning switches and alarms.

B. Controlled and sustained pressure on the QTC pill viaa screw or lever system turns it into a potentially usefulvariable resistor for heavier currents. In this context, thealternative is an expensive wire-wound resistor with arating above 1 watt.

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3. Smart garments

4. Basic QTC pill demonstration

5A-E. QTC pill applications

6. QTC coaxial cable

7-8. QTC cable application examples

C. Three QTC pills placed between a pair of metal platesprovide the basis for a weighing machine. The greater theload, the smaller the measured resistance. The resistancechange can be read directly from a meter or fed to a PCwith interpretative software.

D. If three pills are placed on separate self-adhesive foilstrips on a card or plastic disk and overlaid with a metalplate, the weighing machine becomes a 2D joystick. Applypressure evenly and all three pills ‘turn on’. Apply morepressure to one side and the resistances changeaccordingly. The example shown will control themovement and direction of a two-motor buggy or robot– with a third ‘channel’ for a light, buzzer etc.

E. Movement or force sensors become possible whenone or more QTC pills are placed between a relativelylarge mass and a ‘reference’ surface. The example shownis a simply braking indicator consisting of a metal massable to slide within a plastic tube. If the tube is movingand suddenly slowed down, the mass will exert pressureon a QTC pill and change its resistance.

Using QTC cableIn cable form, the QTC material is placed between aninner core and outer woven braid in a form of coaxialcable. It is therefore omnidirectional as the switch orsensor in a simple circuit since it will conduct if squeezedor bent tightly in any direction. In commercialapplications, the cable can be laid in long lengths toprovide a continuous switching sensor.

Examples of cableapplications1. The cable can be used as a pressure sensor fortriggering an alarm circuit or for monitoring (e.g.counting) events. The QTC can be connected directly tothe input of most counter modules if it is set up correctly.

2. Like the pills, QTC cable can be used as a transducerelement in weighing applications but for much heavierloads – e.g. personal scales.

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8

QTC sheetQTC sheet consists of a very thin deposit of (shown inlight grey) active material on a conductor substrate. If two probes are pressed onto the sheet, the QTCchanges resistance between probe tip and substrate andthe sheet then passes current along the underlyingconductor substrate from one probe to another. Anyconductivity changes are thus measured across thesurface of the sheet and not from one side to the other.It is important to note that the resistance drop is not asdramatic as the pill: it will typically drop from insulatorvalue to approximately 20k. This means that in alarm andswitching applications, additional circuitry will be needed.

So far, the main commercial application of QTC sheet isin membrane panel switches. In one type of conventionalmembrane switch, there are three layers: one with pairsof conductor tracks, a middle insulating layer with one ormore windows and a top ‘bridging’ conductor. When thetop layer is pressed, its underside dips through a windowand bridges across a pair of tracks. This construction cannow be simplified with QTC since the sheet will functionas both the middle insulating layer (no window holesrequired) and the bridging conductor.

Applications of QTC sheetQTC sheet is capable of most sensing and switchingapplications in membrane panel form. For example, itfunctions as an alarm matt switch if folded over betweentwo conductive films – e.g. pieces of card with an appliedaluminium film surface. A suitable latching circuit isshown.

Ω

paper membrane panel

QTC equivalent membrane panel

20

foldedQTC

conductivefilm surfaces

6V

9

11

12

13

10

21

Shape memory ceramicsInterest is growing in a number of glassy ceramicmaterials that exhibit useful shape-changing properties.These include naturally occurring examples such asquartz. This is a piezoelectric material – one thatproduces a momentary electrical voltage if rapidlycompressed or, conversely, a minute change in shapewhen a voltage is applied across a crystal. Other materials have been found to exhibit piezoelectricbehaviour and they deserve mention here because oftheir increasing use in ‘smart’ engineering – notably in anew generation of electric motors referred to as sonicwave or piezo-motors.

Piezoelectric materials have many uses including:

n sensors (e.g. for alarm systems)n microphonesn loudspeakersn electrical generatorsn electric motors/actuatorsn regulation of electronic circuits

The piezoelectric effect is exhibited in an inexpensiveelectronics component used as the flat ‘loudspeaker’element found in talking greetings cards and ultra-flatproducts such as calculators. A battery touched acrossone of these sounders produces an audible ‘clicking’sound. Conversely, the element will produce a voltage iftapped.

Sharply tapping a sounder with a pencil will produceenough output to momentarily light up an LED. In fact, anew generation of piezo-generators are now emergingthat capture and store the output when a piezoelectricelement is repeatedly stressed.

Modern cameras and similar products requiring physicallens movement now use piezoelectric motors. These arehardly recognisable as motors because they exploit thealmost invisible dimensional movement of piezoelectricmaterial when a voltage is applied. Several forms ofpiezo-motor exist, but the general principle can besummed up by describing just one. This consists of acarbon rotor that fits tightly within a ring of piezoelectricsegments. These segments are each connected to acircuit that supplies a momentary voltage to each segmentin turn. The resulting peristaltic action or ripple of thesegment ring causes the rotor to move. Each completecircular ripple might produce a rotor movement of justone micron, but because the segments can be energisedsequentially at something like 80 kHz, the rotor can turnat very high speed.

Footnote: Quartz crystal is a naturally occurringpiezoelectric material and gives the eponymous clockmovement its name. In these clocks an electronic circuitgenerates a high-speed pulse signal which is then ‘geareddown’ electronically to drive a miniature stepper motor.The high-speed signal is regulated by a quartz crystalwhich resonates at a specific frequency - in effect,functioning as an electronic pendulum.

14

15

9. QTC sheet

10. QTC sheet resistance probe test

11-13. QTC sheet applications

14. Piezoelectric segments and motors

15. Quartz clock movement

The most easily recognised class of smartmaterials are those that exhibit striking opticalchanges in response to heating, illuminationand angular changes. These materials havealready transformed the look of things such ascars and packaging, re-written the rule bookfor the creation of products such asthermometers and generally made the world asafer place with temperature-indicatingpolymers.

Thermochromic materialsThermochromic materials change colour at specifictemperatures. Typically they are incorporated into aspecial ink or carrier and printed onto plastic films tocreate products such as thermometers or visualindicators. The battery test strip is a good example of thelatter. When a battery is in good condition, current willflow through a printed resistor under a thermochromicfilm and heat it to change colour.

Most thermochromic materials are based on liquidcrystals. At specific temperatures the liquid crystals re-orientate and scatter light to produce an apparentchange of colour. The liquid crystal material itself ismicro-encapsulated – i.e. contained within microscopicspherical capsules typically just 10 microns in diameter.Billions of these capsules are mixed with a suitable carrierto create printing inks or thermochromic colouringcompounds for injection moulding plastics.

»Smart imaging

22

1 2

3

4

1. Water temperature indicator

2. Thermochromic mug

3. Celcius temperature indicator

4. Battery power level indicator

5. Thermocolour sheet

6-8. Thermochromic pigments

Thermocolour sheetThis material is a black plastic self-adhesive film coatedwith thermochromic ink. When heated above itstransition temperature (e.g. 27°C) the sheet turns blue –providing a striking indication of temperature change. Thistype of sheet can be cut into any size and applied toproducts such as electronic circuitry or engineeredcomponents to give visual over-temperature warning. It isalso capable of simple thermal imaging – for example, awarm hand placed on the sheet will leave a clear thermalimpression.

The material can also be used in scientific investigations toreveal thermal conduction pathways and the localisedheating effects of radiation. (A small curved section of thematerial held near a radiating heat source can even beused to demonstrate why the earth is warmer at theequator!)

Coloured thermochromic pigmentsMicroencapsulated liquid crystals can be formulated sothey change from a distinct colour to transparent at thetransition temperature. A range of thermochromicpigments with the trade name Smart Colours is availableas a paste compatible with water-based media such as theacrylics used by artists. The pigments can be mixed withvirtually any acrylic media to make a thermochromic inksor paints. At normal room temperature, the paint iscoloured, but this disappears at the transitiontemperature (e.g. 27°C) and re-appears when thetemperature drops.

In a typical engineering context a thermochromic pigmentis mixed with a suitable media and applied to a surfacethe temperature of which requires constant monitoring.For example, black thermochromic paint can be appliedover a word such as OVERHEATING. This word will beobscured until the transition temperature is reached. Theblack thermochromic paint then turns clear to reveal thewarning underneath.

Coloured thermochromic pigments can also be mixedwith coloured acrylic media to provide different colour-changing effects. For example, if blue pigment is mixedwith yellow acrylic paint, the resulting colour is green.However, at the transition temperature, the bluedisappears and the overall green colour changes toyellow.

23

5

6

7 8

24

S Smart imaging

Photochromic materialsPhotochromic materials change colour according todifferent lighting conditions and are used commercially forproducts ranging from nail varnish to precision opticalequipment. Since most photochromic dyes areparticularly reactive to ultraviolet light – a potentiallydangerous component of sunlight – they can beincorporated into materials such as window glass andspectacle lenses to reduce light transmission and generallyprotect against UV radiation.

PhotoluminescentmaterialsPhotoluminescent materials are those that can absorb andstore energy from light from the UV end of the visiblespectrum and re-emit it as white light. Such materials arepopularly described as ‘glow in the dark’.

A new generation of ceramic-based photoluminescentpigments can provide a useful afterglow lasting up to tentimes longer than previously used materials. Thesepigments, available in powder form, were originallydeveloped as a substitute for radio-active materials andthe less efficient zinc sulphide compounds used to coatwatch and clock hands.

9

10 11

Photoluminescent filmA number of types of photoluminescent products arecurrently manufactured, e.g. a torch, the body of whichcontains a photoluminescent additive that makes it easyto locate in a dark place.

For a major group of applications, the photoluminescentpigments are applied to flexible films that are typicallyused for emergency lighting in buildings and aircraft. ThePSPA (Photoluminescent Safety Products Association) setsexacting commercial standards for such films and placesthem in different classes according to level and time oflight emission.

The following table gives examples:

Photochromic imagingAdvanced manufacturing and imaging techniques havegiven rise to a whole new generation of materials thatprovide a basis for holograms, 3D effects, precisionmeasurement, diagnostic methods – and even movingpictures. Many of these materials are available in ‘raw’sheet form and can be used for a host of applications inproduct design or engineering.

25

12

9. Photochromic paint

10-11. Photoluminescent material

12. Light emissions table

S Smart colours

Optical materialsClear materials such as glass and optical grade polymerscan be formed to channel light by both reflection andrefraction. The classic example is optical fibre based oneither glass or a suitable polymer. A typical optical fibreconsists of a core, boundary layer and (sometimes) anouter protective sheath. The core and boundary layer are of a different refractive index and so light entering the fibre from one end is internally reflected within thefibre – and ultimately emerges from the other end.Optical fibres are used extensively in control andcommunications so large amounts of information can betransmitted through them using laser light. Optical fibrescan also be used as sensors, as, applied pressure at apoint along the fibre can produce a measurable change intransmitted light.

Light can be transmitted other than by optical fibres. Theterm Light pipe is commonly given to optically clearmouldings (usually made from acrylic) that transmit lightfrom a single source, such as an LED, through branchesto different points. In these components, light is internallyreflected from the polymer/air boundary of the moulding.

Lenticular sheet (often polypropylene) has an embossedpattern of micro-lenses that creates illusions of depth orapparent movement. One type of sheet, although paperthin, appears to be 5mm thick and any object placed on itseems to sink below the surface. Because the effect is sostriking, this material is used widely, for example, in CDpackaging, book covers and for small products such asaccess cards.

Moulded lenticular lens material, incorporating parallelmicro-lenses, is also used to create sequences of movingimages when a card or screen is turned through a smallangle. In combination with printed patterns, lenticularmaterials can be used for optical amplification of smallmovements.

26

13 14

15 16

13. Acrylic light pipes

14-15. Lenticular sheet

16. Moving image lenticular sheet

17. Highly reflective metallic films

18. Hologram on credit cards

19. Holgraphic card

20-21. Smart images film

Reflective films and hologramsSheet plastic can be coated with highly reflective ultra-thin metallic films. This provides the basis for productssuch as mirrors, including two-way mirrors and beamsplitters for scientific equipment and hologramproduction. If the surface of the plastic sheet is embossedon a micro scale then light reflected from the surfacecoating can produce thin film interference – the samephenomenon that causes colours to arise from thin oilfilms on water. This effect is seen on CDs, commercialpackaging and security markings.

One type of film (‘smart images’ film) is manufactured sothat the micro-embossed metal foil can be transferredfrom its polyester carrier onto other substrates such asplastic card or paper. The foil carries a deposit of heat-activated adhesive and, when heated, the foil partscompany with the polyester carrier and will stick toanother surface.

There are many applications for ‘smart images’ materialranging from the purely functional to the purelydecorative: Examples include:

n reflective patches on safety garments n reflective warning patches on posts etc.n jewelleryn graphics –

e.g. personal organiser and book coversn small product prototypes –

e.g. security hologram simulations

27

17 18

20 21

19

S Smart colours

Chameleon coloursSeveral materials can now mimic the colour-changingtricks of the chameleon. Early attempts to do this usedground fish scales mixed into a varnish. Like the originalfish, the varnish changed colour when viewed fromslightly different angles. Modern smart pigments can nowaccomplish similar effects – for example, by using billionsof light-refracting nano-sized particles in a surface coating.‘Chameleon’ particles are now routinely incorporated intocar paints and printing inks whose colour depends onviewing angle.

The chameleon effect can be seen on many products –e.g. mobile phone cases, sunglasses, window tinting,speciality packaging and precision lenses. Many of theseexamples use multi-layered ultra- thin polymer films thatswitch colour dramatically if turned through just 5’degrees. These films work by selectively scattering lightor thin film interference (the same phenomenonresponsible for the colours seen in soap bubbles). Onerecent example, called smart film, comprises some 250layers rolled down to approximately 10 microns inthickness. Viewed from one angle, the surface appears tobe metallic gold; from other angles it changes through aspectrum of colours.

Footnote:3D images can be created in several more ‘conventional’ways – e.g. by viewing two photographs of an objecttaken at about 100mm apart. This is very easy to do usinga digital camera and printer. Low-cost stereo viewers,developed for commercial 3D photo-souvenirs, are idealfor looking at the pairs of images.

Animation plastic sheet(multi-lens polycarbonate)This material provides the optical basis for the animationor image switching effects seen on advertising displays(and smaller hand-held cards) as the viewing angle of theobserver changes. It can also demonstrate opticalamplification of movement – a principle often exploitedin an engineering context.

Animation plastic consists of optically clear polycarbonatesheet with parallel lenses moulded on one side. The waythe system works can be explained with reference to avery simple example. A series of thin black lines areprinted on paper so that each line falls exactly under eachof the lenses. When the plastic sheet is viewed from thetop, the black lines appear through the lenses and thewhole area appears black. If the viewing angle is alteredto look slightly sideways at the lenses, they each showclear paper and so the whole image looks white. If you want to switch between two colours – say, red andgreen - then each of parallel lines lying under the lensesis printed as a red and green stripe. At one viewing angleonly the red side of the stripe is seen and the whole arealooks red; when the angle is changed, only the green isseen.

28

22 25

23

24

To create the illusion of picture switching, two separatepictorial images are divided into narrow stripes andinterleaved. Viewed from one angle, only the side of thestripes comprising one of the pictures is seen. From analternative angle, you see the side of the stripescomprising the other picture. If more than two imagesare divided and interleaved, it is possible to createmultiple switching and simple animation effects – e.g. a butterfly flapping its wings.

If the image is turned at a slight angle to the lens, anymovement of either lens or image will cause the passageof moiré fringes – rapidly moving dark and light (orcoloured) bands that run at an angle to the lenses.Because the moiré fringes exceed the speed ofdisplacement of the lens and image (or two translucentimages), the principle is used for optical amplification andwas employed, for example, as a sensing device in earlyelectronic micrometers.

Using animation plasticThe material supplied by TR is designed for dual switching – but this is not an absolute limit. Howevermany switches are intended, the crucial thing is to alignthe image stripes so that each lies exactly under thelenses – i.e, in phase with the lenses. Most graphicssoftware will enable you to create regularly spacedstripes, Photoshop or similar software will enable two ormore pictorial images to be sliced up and interleaved. The spacing between the lenses is 1.7mm – as measuredbetween the crests.

Positioning the image stripes are slightly out of phase withthe lenses can give rise to spectacular optical effects –especially when the printed graphics are moved under thelenses. This phenomenon offers endless unexploredgraphic design opportunities.

Three effect are illustrated here

1. Parallel black lines – giving a white/black/white switch

2. Red and green interleaved stripes – giving a red/green switch

3. Red and green interleaved stripes out of phase with the lenses – giving rises to optical ‘pyrotechnics’ when the sheet is moved at different angles under the sheet.

Animation plastic sheet (150mm x 150mm x 3.5mm)

Stock code: 234-111A Price: £3.00

29

22. Chameleon colours

23-24. 3D stereo imaging

25-28. Animation plastic sheet

26 27

28

Note: All the figures given below, assume the use of 100micron smart wire.

Stages in manufacturingn Equal amounts of nickel and titanium

are combined and melted in an electric furnace at 1300ºC. The process is carried out in a vacuum and any contamination by other material is strictly avoided. The melt is then cast into small ingots.

n The ingots are rolled into rod, bar or sheet form. Ifwire is required, the bar stock is further drawn down todiameter through very hard dies of decreasing diameter.

n Finally, the wire is given its memory by carefullycontrolled heat treatment and ‘conditioning’. This processis carried out continuously.

S Appendix 1 Shape memory alloys data

Casting

»Smart WireAppendix 1

30

Rolling

Wiredrawing

1

2

3

Smart wire data

Melting point: 1300ºC

Ultimate Tensile Strength (UTS): 1100 MN/m2

[Note: will undergo deformation of 15%-30% beforefailure]

Bias force 0.3 NPulling force 1.5 N

Resistance 150 ohms per metreMax. current 180 milliampsMax. power 5 Watts per metre

Shortening time 0.1 secondRelaxation time 1.0 second

Recommended extension 5% Minimum bend radius 5 mm

Effective transitiontemperature 70º Centigrade

Pulling starts at 68ºCPulling finishes at 78ºCRelaxation starts at 52ºCRelaxation finishes at 42ºC

Smart wire behaviour andtemperature hysteresisNitinol wire exists in two ‘states’ (phases) dependent ontemperature:

n Low temperature state in which the material has a martensite crystal structure (‘relaxed’, extended condition).

n High temperature state in which the material has an austenite crystal structure (‘remembered’, shortened condition).

The change from one state or phase to the other, causedby temperature change, is responsible for the changes inlength of the wire. The behaviour of the wire can berepresented on a graph showing length againsttemperature change.

When the wire is heated or cooled, the changes of stateor phase are not immediate. At 68ºC the hightemperature phase begins and ends at 82ºC. A figure of70ºC is normally quoted for convenience as the effectivetransition temperature. On cooling down, the lowtemperature phase begins at 52ºC and ends at 42ºC. Thistemperature range differs considerably from the first andshows up on the graph as a hysteresis curve. (Hysteresisis defined as the retardation or lagging of an effect behindthe cause of the effect.)

In practice, it is usually only important to know aboutthese hysteresis characteristics if very rapid responses arecalled for. For example, the wire can be heated rapidlywith a large current which may then be reduced simply to'hold' it in the shortened condition. Rapid relaxation,however, might necessitate an active method of coolingsuch as moving air – or even immersion in a liquid.(The difference between the higher transitiontemperature and the relaxation temperature is calledhysteresis.)

31

< Hysterisis >

Cool

and

rela

x

Heat and contract

Wire

leng

th

Temperature

Longer

Shorter

Transitiontemperature range

4

1-3. Casting, rolling and wire drawing

4. Temperature hysteresis table

Using SMA dataThe table (on the previous page) tells us that at normalroom temperature the wire needs to be stretched with abias force of 0.3 Newtons – which is roughly equivalentto hanging a weight of approximately 30 grams on theend of it. When heated to the transition temperature ofbetween 70º to 80ºC, the wire shortens about 5% inlength and will exert a pulling force of 1.5 Newtons –roughly equivalent to lifting a weight of 150 grams.

The speed at which the wire shortens when it reachesthe transition temperature is about 0.1 seconds. It takeslonger to relax or stretch back to its longer length –about 1 second. The table also tells us that when heated,the wire actually starts changing length at 68ºC andfinishes at 78ºC. When it cools, however, the stretchingor relaxation does not take place until it has reached52ºC.

The figures given in the table are the recommended onesfor 100 micron smart wire; if they are exceeded, theuseful life of the wire will be reduced.

The supply needed to heat the wire can be determinedusing Ohm’s Law. This states the relationship betweenvoltage (V), current (I) and resistance (R). Ohm’s Lawstates that:

V = I x RI = V ÷ RR = V ÷ I

The table gives us the resistance of the wire and alsostates the maximum current. Using Ohm’s Law, we cantherefore work out the voltage needed.

For example, what is the voltage needed to pass themaximum safe current through the 10 cm length of 100micron wire?

Step 1The resistance of the wire is 150Ω per metre. Divide by 100 = 1.5Ω per cm.10 cm of wire = 1.5Ω x 10 = 15Ω.

Step 2The maximum current is 180 mA or 0.18 A. (1 milliamp = 1/1000 Amp.)

Step 3V = I x R Substituting the figures above gives:V = 0.18 A. x 15Ω = 2.7 volts.

A 3 volt battery (two AA cells in series) can be used topower this length of wire because as current is drawn, itsvoltage will reduce slightly.

To check that the power rating (the rate of doing work) isnot exceeded, we can use the power equation W (Watts) = I x V.

If we substitute the above figures W = 0.18 x 2.7 = 0.49Watts for a 10 cm length of wire and 10 x 0.49 = 4.9 fora metre length. This is the maximum figure given in thetable.

Increasing the pulling force of smart wireThe pulling force of smart wire cannot be increased bysupplying current beyond the recommended limit; thiswill damage it. However, two or more wires can be runin parallel. Two wires will give double the pulling forceand so on. You must remember, though, that if the wiresare connected in parallel, you also double the currentneeded to heat them up.

32

5

Power supplies for smart wireCurrent supplied to smart wire must be within therecommended limit to avoid any damage. There are several ways of doing this including:

n use of an appropriate number of 1.5 V batteries connected in series. An example is given in the calculation on page 30.

n use of an adjustable power supply unit (PSU).

n use of a series resistor to regulate the supply. It may not be possible to ‘fine tune’ a number of batteries accurately enough or you may have an unsuitable supply. In either case, current can be regulated by using a series resistor in the circuit. Ohm’s law can be used to work out the value of this resistor.

[Note: the resistor should be a higher wattage type. Thepower in the circuit can be worked out using W (watts) = I (current) x V (volts). If a variable resistor is used, it should be a wire-wound higher wattage type.]

n use of a voltage regulator.

LM 317 is a voltage regulator which is used in the circuitshown to limit current. The current limit of the regulator is given by:

Current (I) = 1.25÷R

To select the required resistor for a recommendedcurrent, this becomes:

Resistance (R) = 1.25÷I

For example, using a 12 volt supply, we require amaximum current of 180 milliamps (0.18 amps) for a length of 100 micron SMA wire.

R = 1.25÷0.18 amps = 6.9 Ω

The closest value resistor is 6.8Ω. Because of the highercurrent passing through the resistor, standard 1/4 wattresistors are unsuitable. A good choice would a 6.8Ω 2.5watt wirewound resistor.

33

LM317T

Case also Vout

AdjVout

Vin

6 7

8 9

5. Two wires run in parallel

6. 1.5 V batteries

7. Power supply unit (PSU)

8. Use of resistors

9. LM 317 voltage regulator

Some smart wire control circuits

1. Open loop controlA. In open loop control, there is no feedback. The supplyis simply switched on or off – for example, using a pressswitch or a timer circuit. Switches that can be usedinclude: reed switches operated by a magnet, microswitches or membrane panels.

B. Supply current can be 'switched' by a thyristor, bipolartransistor or FET (field effect transistor). The examplecircuits show how sensors can control the supplyswitching.

C. Bipolar transistors and FETs can also be used as theoutput stage of microelectronic control circuits – e.g. a555 timer.

Thyristor triggered by shock

FET switched on by placing fingeracross touch pads

Transistor switched on by waterbridging across probes

34

A

B

C

Reed switch and magnet Micro switch

Membrane panel

2. Closed loop controlClosed loop control involves something feeding back(feedback) from the output to the input of a system. Acentral heating system turns on and off at a temperatureset by a thermostat. A bi-metallic strip in the thermostatheats up and moves to switch off the heating boiler whenan appropriate temperature has been reached.

Because smart wire changes length when it is heated, themovement can be used as feedback – for example, toswitch the supply on and off. A very simple exampleinvolves connecting a length of smart wire to amicroswitch. When the wire is relaxed the switch is ‘on’and current flows through the wire. The wire thenshortens, depresses the switch contact and turns off thesupply. The wire then relaxes and the whole cycle beginsagain.

(Note: Ingenious heat engines have been built from SMAmaterials using a closed loop system. In one example, awire relaxes and dips into hot water. This causes it tochange shape and move out of the water to cool downand relax again. The same cycle repeats over and overagain and turns a small flywheel. Another engine uses awheel consisting of smart wire spokes attached to aflexible plastic rim. As each wire is energised, the wheel isdistorted and causes movement along a surface.)

Practical experiments withsmart wireA. Lifting weights

This experiment simply involves attaching a length of smart wire to a weight (e.g. ball bearings in a bag) and observing the contraction when the wire is heated by current. The bias force is automatically supplied by the weight.

B. Amplifying movement with leversA simple two dimensional lever system can be assembled on a baseboard using polystyrene or card strip for the lever and a drawing pin pivot. The ‘load’ on the lever can be supplied by weights or a spring (e.g. elastic band).

The distances from the pivot to (a) the wire attachment and (b) the weights can be expressed as a ratio. In the example shown the ratio is 5:1. For every millimetre moved by the wire end the weighted end will move through 5 millimetres.

C. Amplifying movement using geometryA weight is attached to the centre of a length of smart wire so that it forms two sides of an inverted triangle. Over a range of angles the vertical movement of the weight will be greater than the linear movement of the wire. This effect increases as the angle of x increases (i.e. as the wire becomes closer to horizontal). However, the forces required also increase. Try experimenting with smart wire at an angle of 140° and plot the movements of the weight on a piece of paper.

3235

Contacts

Thermostat

Bimetallic strip

Temperaturesetting

Microswitch

SMA

Power supply

Ball bearings

Ball bearings

10 cm 2 cm

A

C

B

The following examples illustrate ways inwhich QTC pills can be used in electroniccircuits both for control and special effects.

ProjectsThe following experimental circuits have all been built andtested. However there is ample opportunity for furtherexperimentation in the circuit design and particularly indevising methods for applying pressure to the QTCmaterial.

The method used for mounting and making connection tothe QTC material is shown in the picture above. A lengthof adhesive-backed strip of copper foil is stuck down to apiece of plain SRBP (Synthetic Resin Bonded Paper) sheet.A cut is made across the strip using a craft knife to makea break in the strip.

A small square of QTC material is placed across the breakin the strip and is held in place using a small piece ofmasking tape. The tape should just hold the QTC in placewith the minimum of pressure. A connecting wire isconnected to the end of each piece of copper foil and thedevice is ready for use. Various pieces of wood or plasticmay be used to ensure that the pressure is applieddirectly to the QTC and not to elsewhere on the SRBPbase.

Circuits1. Frequency control with QTC2. Volume control circuits with QTC3. QTC variable speed and reversing motor control4. QTC alarm system5. QTC tri-colour LED control6. QTC with bargraph display of pressure7. QTC impact measurement and charge pump

A. Frequency Control with QTCThe circuit consists of a conventional astable oscillatorusing a 555 timer IC. The oscillation frequency isdetermined by R1, RV1, R2 and C2. The output from the555 is connected to a Piezo Transducer (a ‘buzzer’ is notsuitable) or a 64 ohm loudspeaker, as shown.

The QTC device is connected to the slider of RV1 so thatits effect can be increased or reduced. When the QTC iscompressed, its normally high resistance changes to a lowvalue and causes the oscillation frequency to increase.RV1 may be set to provide a suitable change of oscillationfrequency.

This circuit has applications in remote pressure sensinge.g. measuring the weight (and contents) of a caravanwater container or a remote rain gauge. The audio tonecould be transmitted by wire, radio or fibre-optic

»QTC: some electronic applications

Appendix 2

36

1

A

B. Volume Control Circuits using QTCIn circuit (a) the QTC device is connected in series withthe Piezo Transducer. When the QTC is compressed itsresistance becomes lower and the volume of sound fromthe transducer increases. It is necessary to connect aresistor across the transducer for the circuit to workproperly. The value of the resistor may be changed to suitthe pressure being applied.

In circuit (b) the QTC device is connected across thetransducer and a resistor is connected in series. In thiscircuit pressure on the QTC will absorb some of thesignal and reduce the volume of sound. Again, the valueof the resistor may be changed to suit the pressure beingapplied.

Circuit (c) is intended for use with a 64 ohm loudspeakerand works in the same way as circuit (a). Circuit (d) isalso intended for use with a 64 ohm loudspeaker and issimilar in operation to circuit (b).

These circuits have applications for indicating a key pressand for muting audio signals.

C. QTC variable speed andreversing motor controlThe circuit consists of a Wheatstone bridge arrangementwith the motor connected across two arms of the bridge.With no pressure on the QTC devices there is no currentflowing and the motor is at rest.If QTC1 is pressed and becomes low resistance, currentflows through R2 and the motor (right to left), causing themotor to rotate. The speed depends on the pressure.Current also flows through R1 but this has no effect onthe motor. If QTC2 is pressed, then current flowsthrough R1 and the motor (left to right) causing rotationin the opposite direction. Current also flows through R2but this has no effect on the motor.

The motor speed, in either direction, depends on therebeing a difference of pressure on the two QTC devices.Equal pressure, causes equal resistance, the bridge circuitis therefore ‘balanced’ and no current flows through themotor.

This circuit has applications in mechanical positioning.With the motor connected to a reduction gearbox itwould allow the precise setting of a greenhouseventilator, a car headlamp beam etc.

37

B

C

D. Pressure acoustic alarm systemThe circuit consists of two 555 timers, IC1 and IC2. Instand-by mode, IC1 produces negative pulses of about80ms duration at approximately one pulse per second.IC2 is arranged as an oscillator generating a 700 Hzsquare wave. IC2 is controlled by the output from IC1and so produces bursts of 700 Hz signal which is fed to aPiezo transducer or loudspeaker. The one second‘beeping’ gives assurance that the system is functioning.

The QTC device is connected to the slider of RV1 so thatits effect can be increased or reduced. When the QTC iscompressed the ‘beep’ repetition frequency increasesgiving a sense of urgent alarm.

One application could be its installation on the reversingbar of a lorry where it would sound an alarm whenmoving up to a loading bay. The resistor values could bechanged to provide different ‘beep’ frequencies andsensitivity.

E. QTC tri-colour LEDThis is a simple circuit using a red-green tri-colour LED.Two LEDs, one green and one red are housed in thesame casing, they have a common anode and separatecathodes, one for each LED. Either LED may be activatedto give a red or green illumination and if both LEDs areactivated then a yellow output will result.

By pressing on QTC1 and/or QTC2 the illumination canbe varied over the red-yellow-green part of thespectrum. In addition to the general interest in colourmixing, the device could be used in a lecture to indicateto the speaker when to finish his talk.

38

D

E

F. QTC with bargraphdisplay of pressureThe QTC device is connected in series with R3 and isused to sense pressure. Increasing pressure will result inthe QTC resistance decreasing and the voltage across R3increasing. This voltage is fed to the input of the bargraphdriver IC1. This particular device (LM3915N) has alogarithmic response which helps to compensate for thenon-linearity of the QTC device.

Because of the wide range of QTC resistance, RV1 isconnected across it to set a maximum value. In practice,RV1 is set so that the bottom bar of the display isilluminated when no pressure is being applied to thedevice.

The circuit gives an excellent indication of varyingpressure, but because the QTC material is not sufficientlystable and the results not sufficiently reproducible it couldonly be used in (say) a weighing machine of limitedaccuracy.

There is plenty of scope for the development of variousmechanical linkages and levers to produce a desired rangeof operation.

The system would be ideal where immediate visualfeedback of pressure is required e.g. in a hand exerciserfor an arthritic patient, to replace the usual soft rubberball. This circuit and a QTC based ‘squeezer’ would givean immediate indication of pressure to the arthriticpatient.

G. Impact measurementand Charge PumpThis circuit below uses the QTC device to move chargeinto a storage capacitor, C2 and the resultant storedcharge is displayed as a voltage level by the circuit.

When the QTC material (in the circuit) is subjected to ablow, the resistance drops dramatically and current flowsfrom C1 into C2, the quantity depending on the strengthand duration of the blow.

For example, if the blow lasts 10msec and the current is10mA then the charge transferred would result in the100µ capacitor being charged to 1.0V. A stronger orlonger blow would result in a higher voltage and a higherindication on the bargraph. RV1 is connected across C2 to provide a discharge pathand the bargraph will show the exponential decay of thevoltage. RV1 is adjusted so that the bargraph shows ‘fullscale’ when the QTC is pressed very hard.

This circuit might be used to compare energy, e.g. thedropping of a ball bearing from different heights onto theQTC.

By repeatedly tapping the QTC device, the effect of‘pumping’ charge into C2 may be observed. The voltagewaveform across C2 may also be displayed on anoscilloscope.

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G

F

New textiles are becoming increasingly‘smart’ and providing unprecedentedopportunities for garment design etc. Somebasic categories and applications can besummarised as follows:

Shape-changing textiles Pre-conditioned shape memory alloy wire can be woveninto into garments so that they change shape with bodyheat. This provides ‘smart’ responsive shaping andsupport.

Conductive textiles Garments have been designed and manufactured thatincorporate electrically conductive pathways so that thegarment can become an extension of a conventionalelectronic product. QTC (quantum channellingcomposite) has been used to create switches within thefabric of ski jackets for controlling MP3 players.Eventually, consumer products like these may becomeintegral parts of garments.

Colour-changing textiles Colour-changing images can be applied to textiles by anumber of methods. The special thermochromicpigments used for these images are now beingincorporated into the actual woven material of the textile.This means that specific colours over the whole or part ofthe garment can be ‘turned on’ by external temperaturechange or body heat.

Low-friction textiles In a competitive sport such as swimming, garments canmake all the different between winning and loosing. Mosttop athletes use body suits made from a patentedmaterial that mimics sharkskin. Like the real thing, thisprovides minimal drag when moving through water, andgives vital seconds advantage to those wearing it. Thescale-like surface structure of this material can be seenclearly under a low-power microscope.

Ultra-strong textiles A wide number of textiles have been developed thatexhibit extraordinary strength as well as resistance totearing. These might be woven from a single type of fibresuch as nylon or a combination of fibres such as Kevlarand nylon. Kite design has been revolutionised by the useof lightweight rip-stop nylon – a material that has alsotransformed the design and functionality of boat sails.

Protective clothing, ranging from industrial gloves toweapon-proof vests, is designed with textiles thatcombine the unique properties of different materials suchas carbon fibre and Kevlar. Both of which haveexceptional tensile strength which is why they are alsoused to reinforce fabrics intended for architecturalapplications.

40

»Fibres, composites andAppendix 3 smart textiles

1 2

3 4 5

4041

Carbon nanotubesDescribed as ‘one of the hottest topics in physics’, carbonnanotubes are lattice-type structures of carbon atomsthat possess properties including exceptional strength andelectrical and thermal conductivity. Nanotubes are madeby vapourising carbon in an arc or laser and then coolingit under tightly controlled conditions. In effect, theprocess creates atom-thick carbon sheets (fullerene)rolled up into tubes – either single wall tubes or multi-layered. The aspect ratio of these tubes (thickness tolength ratio) varies from one thousand to one million toone. (In an aggregate mass the tubes appear as a lumpyblack powder.)

The practical application of carbon nanotubes is alreadyimpacting on product design and the electronics industry,and the availability of this extraordinary material as itemerges from the laboratory, provides an excitingopportunity for pupils and students to witness some oftheir properties at first hand and to try out applications.Costing some £100 per gram just a couple of years ago,the nanotubes are now available at a fraction of this costfor prototype applications and experimentalinvestigations.

High-tech cycles, tennis rackets, skis are now fabricatedusing nanotube composites - in which context they can beregarded as a micro-fibre substitute for conventionalcarbon fibre. Because of their electrical conductivity,nanotubes are now also used to make conductivepolymers – especially anti-static materials. In the nearfuture, they will be used as a metal substitutes in wiring,copper substitute in circuit board fabrication etc.

Because of their unique size and structure, many novelproperties have emerged and are being explored. In thefuture we shall see many significant electronic andelectrical applications - e.g. nanotube transistors andcapacitors capable of powering cars.

Phase change materials (PCMs)

PCMs absorb heat during the process of melting andrelease it during solidification – hence the name ‘phasechange’. The best known example is probably water/icebut since this phase change transition takes place ataround 0°C it has limited practical applications. Materialsthat phase change at higher temperatures are now widelyexploited to create products that can be described asthermally ‘smart’. For example, a microencapsulated PCMadded to a fabric gives it a remarkably cool feel as bodyheat is absorbed during the phase change transition. Thesame method is used in smart heat sinks for electroniccomponents to absorb peaks of excessive heat.

As new phase change materials are developed, theirapplication is gradually being extended. On a large scalePCMs are incorporated into building blocks used forinterior wall construction. Although much thinner thantraditional building blocks, ‘smart blocks’ can maintainroom temperature at a more comfortable and even level.PCM technology is also used in vehicles that previouslyused mechanical refrigeration to keep the contents(usually food) cool over a period of time. A vehicle usingthis technology is well insulated as normal but has anadditional PCM lining selected to absorb heat at thecritical threshold for keeping food or medical suppliescool.

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Polymorph is the familiar name for a material belonging tothe polycapralactone group of polymers. It has severalexceptional properties including low melting point, highstrength and biodegradability. Many regard it as a ‘smart’polymer by virtue of the fact that its low melting pointenables it to be moulded at room temperature. As acommercial material, its applications range from surgicalsplints (supplied in sheet form and softened for mouldingin water), to solving DIY problems.

Polymorph is a true thermoplastic and can therefore bere-heated and re-moulded. In its original granular form, ithas a large surface area and so melts very quickly –turning from opaque to clear – under hot water at 62ºC.To melt a solid mass will take a much longer immersiontime since, like other plastics, it is not a very goodthermal conductor.

Note: Polymorph should only be heated with hot water. Careshould be taken to avoid scalding from water that has beenoverheated.

Appendix 2

»PolymorphAppendix 4

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Like materials, some structures appear ‘smarter’ thanothers. The term ‘deployable structures’ is used todenote those that can exist in more than one state. Thewell known civil engineer and author Henry Petroskidescribes them as ‘having more than one appropriateconfiguration: open and closed, folded and unfolded,stored and deployed’. Structures that fall into thiscategory range from steel tape measures to thedeployable roofs of large high-tech buildings. Thecategory also encompasses telescopic aerials (and sometelescopes), plastic carrier bags, folding chairs, tents andso on. Not surprisingly, some deployable structuresinvolve the use of smart materials such as SMA.

In recent times huge investments have gone intodeployable structures for earth satellites and spaceexploration. A typical satellite, once in orbit, might berequired to deploy a wide area array of solar panels; aspacecraft might be required to unfold and unpack inseveral stages before useful surface exploration can takeplace. Interestingly, many of the basic principles used indeployable structures for aerospace applications are usedcloser to home. Exhibition screen systems that expandout of a small carrying case to occupy a large area are oneexample.

A recent book has attempted to define and classify suchstructures more generally and uses the complementaryterm ‘collapsible’ rather than deployable. The author, PerMollerup, describes collapsibles as “smart made objectswith the capacity to adjust in size and weight to meet apractical need. They grow and shrink according topractical need.” A range of examples are given anddiscussed.

»Smart structuresAppendix 5

It’s a good idea to begin by creating a test/demonstrationstrip on paper or plastic sheet (e.g. a thin walled plasticcup). Mix a small amount of pigment into the acrylic baseand dilute with water, if necessary, so it can be appliedwith a small paint brush. The painted surface should beleft to dry in a warm place. (If the samples are paintedonto a thin walled plastic cup which is filled alternatelywith hot and cold water, the colour changes areimmediate and dramatic even before the paint dries.)

Important points to remember

n

If painting or printing onto fabrics use only the acrylic medium supplied within the system. This is ideal for screen printing as well as hand painting or stencilling.

n

Use only the minimal amount of pigment for the desired colour.

n

Dilute the acrylic carrier with water if necessary – but keep mixing periodically to prevent separation of pigment and acrylic.

n

Take all normal precautions when using chemicals. Handle in accordance with good industrial hygiene and safety practice – e.g. use in a well ventilated area and wear disposable gloves.

Application examples ofthermochromic pigmentsThe Smart Colours range of pigments are formulated for acolour change at near body heat which makes them idealfor garments that change colour when worn or touched.In general, the pigments can be used for any applicationwhere a temperature warning is needed, e.g. drinkingcup, hot surface warning, food storage. The illustrationshows a visual display idea based on the use of resistancewire behind a sheet of purple coloured paper coveredwith black thermochromic pigment. Where the wireheats up the pigment, the purple starts to show through.This principle can be used to create a message display or,for example, a battery or fuse tester.

Warm water Cool water

»Using smart coloursAppendix 6

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Things that glow in the darkA new generation of ceramic-based phosphorescentpigments can absorb and store sufficient energy to give anafterglow lasting up to ten times longer than previouslyused materials. The pigments, available in powder form,were originally developed as a substitute for both theradioactive and the less efficient zinc sulphide compoundsused to coat clock and watch hands. After a few minutesexposure to natural or artificial light, the new material willglow practically all night. Like the active ingredient in many advanced washingpowders, the new phosphorescent pigments absorb UVradiation and re-emit it as white light. In darker places,the material glows continuously if lit with a UV source –e.g. shop displays of glow-in-dark stars etc.

Glow- in-the-dark filmThe new phosphorescent materials are available in severalforms including a flexible PVC film with self-adhesivebacking. This is typically used for emergency signs thatwill glow without power in the event of a lighting failure.The emergency message simply has to be printed ontothe surface of the material. The film is also used on safetyclothing where it can be seen at some distance in thedark.

The film is extremely easy to use and its high efficiencysuggests a number of novel applications apart from theestablished commercial ones. These include jewellery, andlocalized illumination for reading, glow-in-the dark toysand games. It can even be used as a contact ‘film’ ifsomething like a leaf or photographic negative is placedon the surface. After ‘exposure’ the after-image canclearly be seen in a dark place.

Glow-in-the-dark pigmentsThe raw ceramic-based pigment used in modern glow-in-the-dark products can be made into a paint by mixingwith ordinary acrylic media. The pigment degrades ifexposed to moisture for any length of time, but it can beused with acrylic if drying takes place right away. Whenthe acrylic hardens, it effectively locks the pigment into awater-resistant film.

The effectiveness of the pigment depends (a) on theproportion mixed into the acrylic and (b) the thickness offilm applied. These are both subject to trial and error, butit should be noted that the pigment is highlyconcentrated and only very small amounts areneeded to produce glow-in-the-dark effects.

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Using the pigmentIt is a good idea to create a test strip using paper orplastic sheet. Try mixing different quantities of pigmentinto either acrylic base or white acrylic paint – addingwater, as necessary, to give fluidity. The test strip shouldbe left to dry out as quickly as possible in a warmplace.

When dry, expose the strip to daylight or a bright artificiallight – e.g. a torch beam – and then examine the strip in adark place. If exposed to light for at least 15 minutes, thematerial should give a significant afterglow for up to 8hours. Because it is particularly responsive to UV, sunlightprovides the highest ‘charge’ rate.

Important points to remembern

Keep the pigment dry and stored away from moisture.

n

Use the minimum amount of the pigment to achieve the desired effect.

n

Dilute the acrylic media with water if necessary-but use immediately and then dry quickly.

n

Take all normal precautions when using chemicals. Handle in accordance with good industrial hygiene and safety practice – e.g. use a nose mask and wear disposable gloves.

Smart colours application methodsApplication methods for thermochromic, photochromic andphosphorescent pigments include:

n

Screen printing. n

Painting with a brush.n

Stippling/stencilling with a brush.n

Pad printing – e.g. with a lino-cut or rubber stamp.n

Rubber roller.

Example applications ofglow-in-the-dark pigmentAlthough the pigment was developed for clock and watchhands, it has rapidly found a host of other uses - asevidenced by the variety of glow-in-the dark objectsfound in shops. Current uses include:

n

Traffic signsn

Path markingn

Instruments and controlsn

Printing inksn

Emergency signsn

Fire fighting equipmentn

Home appliancesn

Novelty decorations

It is particularly effective for night-time illumination andprovides a distinctly visible light in conditions of absolutedarkness. Paper or other film coated with the pigmentwill act as a temporary photographic plate if exposed tolight through a mask or photographic negative. Thissuggests some recreational applications, but there areprobably many more serious uses waiting to beuncovered.

Stenciling Pad printing

K.Otsuka & C.M.Wayman, Shape Memory Materials, CUP, 1998

P. Mollerup, Collapsible Design, 2002, (available from MUTR)

R.Booham, Metals & Smart Alloys, SEP Publications, 2006 (available from MUTR)

R.Boohan, QTC: A New Material, SEP Publications, 2005 (available from MUTR)

C. Lefteri, Ingredients, MUTR Publications, 2007 (available from MUTR)

P. Hollamby, Go With the Flow: Investigating Bouncy Fluids and Strange Materials, 2004 (available from MUTR)

J. Day, More Applications for Smart Wire, TEP Publications, 1998 (available from MUTR)

R.G.Gilbertson, Muscle Wires Project Book, 1992, Mondo Tronics (available from MUTR)

»Further reading

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Middlesex University Teaching ResourcesUnit 10IO CentreLea Road Waltham CrossHertsEN9 1AS

Tel: 01992 716052Fax: 01992 719474Web: www.mutr.co.uk

Almost all of the materials and books described in this publication can be obtainedfrom Middlesex University Teaching Resources (MUTR).

Samples of each of the materials, are also available as a demonstration collection in astout aluminium flight case (stock no. 211-020).

Please visit the website at www.mutr.co.uk for prices and on-line purchasing.Alternatively, you can contact Teaching Resources for information and a catalogue atthe address and contact details below.

Appendix 2

»Supply of materials