4.1 Fundamental characteristicsTh i s chapte r fi r s t i dent i fies charac te r i s t i c s tha t d i s t i ngu i sh smart materials from other materials, and then systematically rev iews many o f t he more w ide ly used ones . We beg in byno t ing tha t the five fundamenta l charac te r i s t i c s t ha t weredefined as d i s t i ngu i sh ing a smar t mate r i a l f rom the moret rad i t iona l mater i a l s used i n a rch i t ec tu re were t rans iency ,s e lec t iv i t y , immed iacy , se l f - ac tua t i on and d i rec tness . I f wea p p l y t h e s e c h a r a c t e r i s t i c s t o t h e o r g a n i z a t i o n o f t h e s e materials then we can group them into:

1.Property change capability2Energy exchange capability3Discrete size/location4Reversibility

T h e s e f e a t u r e s c a n p o t e n t i a l l y b e e x p l o i t e d t o e i t h e r optimize a material property to better match transient inputconditions or to optimize certain behaviors to maintain steadystate conditions in the environment. As we begin to explore these distinguishing characteristics,we will see that the reasons why smart materials exhibit theseand o ther t ra i t s i s no t easy to exp la in w i thout recourse t o th ink ing about bo th the mate r i a l s c i ence p recept s no ted i n the last chapter and the specific conditions surrounding theplacement and use of the material. Of particular importance isthe concept of the surrounding energy or stimulus field thatwas discussed in Chapter 3. We recall that energy fields can beconstructed of many types of energy – potential, electrical,thermal, mechanical, chemical, nuclear, kinetic – all of whichcan be exchanged or converted according to the First Law of Thermodynamics (the law of the conservation of energy).The phys ica l charac te r i s t i c s o f smar t mater i a l s a re dete r -m ined by these ene rgy fie lds and the mechan i sm th rough w h i c h t h i s e n e r g y i n p u t t o a m a t e r i a l i s c o n v e r t e d . I f t h e m e c h a n i s m a ff e c t s t h e i n t e r n a l e n e r g y o f t h e m a t e r i a l b y a l te r ing e i t her t he mate r ia l ’ s mo lecu la r s t r uc tu re o r m ic ro -s t ruc tu re then the i nput resu l t s i n aproperty change o f t h e mater i a l . I f the mechan i sm changes the energy s ta te o f theTypes and characteristics of smart materials794Types and characteristics of smart materials

material composition, but does not alter the material, then thei n p u t r e s u l t s i n a nexchangeo fenergy f r o m o n e f o r m t o a n o t h e r . A simple way of differentiating between the two mechan-isms is that for property change type, the material absorbs theinput energy and undergoes a change, whereas for the energyexchange t ype , the mate r ia l s tays the same bu t the energyu n d e r g o e s a c h a n g e . W e c o n s i d e r b o t h o f t h e s e m e c h a n i s m s t o operate at the micro-scale, as none will affect anything larger than the mo lecu le , and fu r the rmore , many o f t he ene rgy - exchanges take place at the atomic level. As such, we cannot‘see’ this physical behavior at the scale at which it occurs.Property change T h e c l a s s o f s m a r t m a t e r i a l s w i t h t h e g r e a t e s t n u m b e r o f p o t e n t i a l a p p l i c a t i o n s t o t h e fi e l d o f a r c h i t e c t u r e i s t h e property-changing class. These materials undergo a changein a property or properties – chemical, thermal, mechanical,magnetic, optical or electrical – in response to a change in theconditions of the environment of the material. The conditionsof the environment may be ambient or may be produced via ad i r e c t e n e r g y i n p u t . I n c l u d e d i n t h i s c l a s s a r e a l l c o l o r - chang ing mater i a l s , such as t he rmoch romics , e l ec t rochro -mics , pho tochromics , e t c . , i n wh i ch the in t r i ns i c su r face o r mo lecu la r spec t ra l abso rp t i v i t y o f v i s i b l e e lec t romagnet i c r a d i a t i o n i s m o d i fi e d t h r o u g h a n e n v i r o n m e n t a l c h a n g e ( inc ident so la r rad ia t i on , su r face tempera tu re ) o r a d i rec t energy input to the material (current, voltage).Energy exchange T h e n e x t c l a s s o f m a t e r i a l s t h a t i s e x p e c t e d t o h a v e l a r g e p e n e t r a t i o n i n t o t h e fi e l d o f a r c h i t e c t u r e i s t h e e n e r g y -exchanging class. These materials, which can also be called ‘ F i r s t Law ’ mater i a l s , change an i nput energy i n to ano ther fo rm to p roduce an outpu t ene rgy i n acco rdance w i th the First Law of Thermodynamics. Although the energy conver-sion efficiency for smart materials such as photovoltaics andthermoelectrics is typically much less than for more conven-tional technologies, the potential utility of the energy is muchgreater. For example, the direct relationship between inpute n e r g y a n d o u t p u t e n e r g y r e n d e r s m a n y o f t h e e n e r g y - exchanging smart materials, including piezoelectrics, pyro-electrics and photovoltaics, as excellent environmental sen-so rs . The f o rm o f the ou tpu t energy can fu r the r add

d i rec tactuation capabilities such as those currently demonstratedby e lec t ros t r i c t i ves , chemo luminescents and conduc t ing polymers.Smart Materials and New Technologies80Types and characteristics of smart materials

Reversibility/directionality Many of the materials in the two above classes also exhibit thecharac te r i s t i c e i t he r o f revers ib i l i t y o r o f b i -d i rec t i ona l i t y .Severa l o f t he e lec t r i c i t y conver t i ng mater i a l s can reve rse t h e i r i n p u t a n d o u t p u t e n e r g y f o r m s . F o r e x a m p l e , s o m e piezoelectric materials can produce a current with an appliedstrain or can deform with an applied current. Materials withbi-directional property change or energy exchange behaviorscan often allow further exploitation of their transient changer a t h e r t h a n o n l y o f t h e i n p u t a n d o u t p u t e n e r g i e s a n d / o r properties. The energy absorption characteristics of phase-c h a n g i n g m a t e r i a l s c a n b e u s e d e i t h e r t o s t a b i l i z e a n e n v i r o n m e n t o r t o r e l e a s e e n e r g y t o t h e e n v i r o n m e n t depending on in which direction the phase change is takingplace. The bi-directional nature of shape memory alloys canb e e x p l o i t e d t o p r o d u c e m u l t i p l e o r s w i t c h a b l e o u t p u t s , a l l ow ing the mater i a l to rep lace componen ts compr i sed o f many parts.Size/locationRega rd less o f t he c l ass o f smar t mate r ia l , one o f the mos t fundamenta l charac te r i s t i c s t ha t d iffe ren t i a te them f romtrad i t iona l mater i a l s i s t he d i sc re te s i ze and d i rec t ac t ion o f t h e m a t e r i a l . T h e e l i m i n a t i o n o r r e d u c t i o n i n s e c o n d a r y t r a n s d u c t i o n n e t w o r k s , a d d i t i o n a l c o m p o n e n t s , a n d , i n some cases, even packaging and power connections allowsthe min im iza t i on i n s i ze o f the ac t i ve par t o f t he mate r i a l . A component or element composed of a smart material will notonly be much smaller than a similar construction using moretraditional materials but will also require less infrastructuralsupport. The resulting component can then be deployed inthe most efficacious location. The smaller size coupled withthe d i rec tness o f the p roper ty change o r ene rgy exchange renders these materials to be particularly effective as sensors:they are less likely to interfere with the environment that theyare measuring, and they are less likely to require calibrationadjustments.Type characterizations Fo r t h i s d i scuss ion , we w i l l d i s t ingu is h be tween these two primary classes of smart materials discussed above by callingthem Type 1 and Type 2 materials:*T y p e 1 – a m a t e r i a l t h a t c h a n g e s o n e o f i t s p r o p e r t i e s (chemica l , mechan i ca l , op t i ca l , e l ec t r i ca l , magnet i c o r the rma l ) i n response to a change i n the cond i t i ons o f i t sSmart Materials and New TechnologiesTypes and characteristics of smart materials81

env i ronment and does so w i thout t he need o f ex te rna l control.*Type 2 – a material or device that transforms energy fromone form to another to effect a desired final state.The note in Chapter 1 on the confusion of meanings of thete rm ‘mate r i a l ’ i s pa r t i cu la r l y re l evan t here . Severa l o f thedescriptions given below for these smart material types edge in to wha t a re bet te r desc r i bed as p roduc ts o r dev i ces s ince they either consist of multiple types of individual materials or they assume a product form. For example, electrochromism isa phenomenon , bu t e lec t roch romic ‘mater i a l s ’ i nvar i ab l y involve multiple layers of different materials serving

specificfunctions that enable the phenomenon to be manifest. Nonethe le ss , common usage by eng inee rs and des igne rs wou ld typ i ca l l y b road ly re fe r to an a r t i f ac t o f t h i s type as a smar t ‘material’, largely because of the way it is used in practice.Figure 4–1 and the following two sections briefly describethe basic characteristics of a number of common Type 1 andType 2 smar t mater i a l s . T here a re , o f cou rse , many o thers .Smart Materials and New Technologies82Types and characteristics of smart materialsTYPE OF SMART MATERIALINPUTOUTPUTType 1 Property-changingThermomochromicsPhotochromicsMechanochromicsChemochromicsElectrochromicsLiquid crystalsSuspended particleElectrorheologicalMagnetorheologicalType 2 Energy-exchangingElectroluminescentsPhotoluminescentsChemoluminescentsThermoluminescentsLight-emitting diodesPhotovoltaicsType 2 Energy-exchanging (reversible)PiezoelectricPyroelectricThermoelectricElectrorestrictiveMagnetorestrictiveTemperature differenceRadiation (Light)DeformationChemical concentrationElectric potential differenceElectric potential differenceElectric potential differenceElectric potential differenceElectric potential differenceElectric potential differenceRadiationChemical concentrationTemperature differenceElectric potential differenceRadiation (Light)DeformationTemperature differenceTemperature differenceElectric potential differenceMagnetic fieldColor changeColor changeColor changeColor changeColor changeColor changeColor changeStiffness/viscosity changeStiffness/viscosity changeLightLightLightLightLightElectric potential differenceElectric potential differenceElectric potential differenceElectric potential differenceDeformationDeformationsFigure 4-1Sampling of different Type 1 and Type 2 smart materials in relation to input and output stimuli

Specific applications in design for these and other materialswill be discussed in subsequent chapters.4.2 Type 1 smart mater ia ls – property-changingCHROMICS OR ‘COLOR-CHANGING’ SMARTMATERIALSFundamental characteristics of chromics A class of smart materials that are invariably fascinating to anydes igner i s t he so -ca l l ed ‘ co lo r - chang ing ’ ma ter ia l g roup which includes the following:*Photochromics – materials that change color when exposedto light*Thermochromics – m a t e r i a l s t h a t c h a n g e c o l o r d u e t o temperature changes.*Mechanochromics – m a t e r i a l s t h a t c h a n g e c o l o r d u e t o imposed stresses and/or deformations.*Chemochromics – m a t e r i a l s t h a t c h a n g e c o l o r w h e n exposed to specific chemical environments.*Electrochromics – m a t e r i a l s t h a t c h a n g e c o l o r w h e n a vo l tage i s app l ied . Re la ted techno log ies i nc ludeliquid crystals andsuspended particle dev i ces tha t change co lo r or transparencies when electrically activated.These constitute a class of materials in which a change inan external energy source produces a property change in theoptical properties of a material – its absorptance, reflectance,or scattering. So-called ‘color-changing’ materials thus do notrea l l y change co lo r . They change the i r op t i ca l p rope r t i es under different external stimuli (e.g., heat, light or a chemicalenvironment), which we often perceive as a color change. Aswas discussed previously, our perception of color depends onboth external factors (light and the nature of the human eye)and in te rna l f ac to rs such as t hose no ted above . An unde r - s tand ing o f these mate r i a l s i s thus more comp l i ca ted than simply saying that they ‘change colors’.Recall that the external factors that affect our perception of color are many.C olor is fundamentally a property of light. Allincident light can be characterized by its spectral distributiono f e l ec t romagnet i c wave leng ths . Su r faces can on ly reflec t , absorb or transmit the available wavelengths – as such theyare a lways subt rac t i ve . The human eye i s a l so a sub t rac t ive surface, but does so comparatively. As a result, depending onSmart Materials and New TechnologiesTypes and characteristics of smart materials83

the spectral and intensity distributions within the field of view,color is alsorelative within the context of the human eye.O f d i rec t i n te res t he re in i s tha t the obs erved co lo r o f an o b j e c t a l s o d e p e n d s o n t h e i n t r i n s i c o p t i c a l q u a l i t i e s o f a material. In our discussion of fundamental material properties,we noted that atomic structures include negatively chargede l e c t r o n s . S i n c e l i g h t c o n s i s t s f u n d a m e n t a l l y o f e n e r g y impulses, it reacts with the negatively charged electrons in amaterial. Depending on the crystalline or molecular structureof the material, the light that attempts to pass through

maybe delayed, redirected, absorbed or converted to some other type of energy. The precise crystalline or molecular structureof the material present will determine which of these possibleb e h a v i o r s w i l l t a k e p l a c e , a n d i n t u r n d e t e r m i n e w h a t wavelengths of light are in some way altered (which in turnaffects the perceived color of the material). Interestingly, it isthe mo lecu la r s t ruc tu re fi rs t encoun tered on a mater i a l ’ ssurface that determines the resultant behavior. As such, thinfilms, coatings and paints will predominantly determine theresponse to light, more so than the substrate.I n t h e c a s e o f a s m a r t m a t e r i a l w i t h a p p a r e n t c o l o r - changing properties, the intrinsic optical properties – absorp- tance, reflectance, scattering – of the material are designed tochange with the input of external energy. Fundamentally, thei n p u t e n e r g y p r o d u c e s a n a l t e r e d m o l e c u l a r s t r u c t u r e o r o r i en ta t i on on the su r f ace o f the mater ia l on wh i ch l i gh t i s incident. The structure depends on chemical composition asw e l l a s o r g a n i z a t i o n o f t h e c r y s t a l o r t h e m o l e c u l e . T h i s external energy can be in several forms (e.g., heat or radiantenergy associated with light), but in each case it induces somechange in the i n te rna l su r f ace s t ruc tu res o f the mater i a l by reacting with the negatively charged electrons present. Thesechanges in tu rn affec t the mate r ia l ’ s absorp tance o r reflec - tance charac te r i s t i c s and hence i t s pe rce ived co lo r . These c h a n g e s c a n b e o v e r t h e e n t i r e s p e c t r u m o r b e s p e c t r a l l y selective. Interestingly, these changes are reversible. Whenthe external energy stimulus disappears, an altered structurereverts back to its original state.The ma in c l asses o f co lo r - chang ing smar t mater i a l s a redesc r ibed by the na tu re o f the i npu t energy tha t causes theproperty change, and includephotochromics, electrochromics,thermochromics, mechanochromics,andchemochromics.Photochromic materials Photochromic materials absorb radiant energy which causes areversible change of a single chemical species between twoSmart Materials and New Technologies84Types and characteristics of smart materials

different energy states, both of which have different absorp-tion spectra. Photochromic materials absorb electromagnetice n e r g y i n t h e u l t r a v i o l e t r e g i o n t o p r o d u c e a n i n t r i n s i c p rope r ty change . Depend ing on the i nc ident ene rgy , the material switches between the reflectively and absorptivelyselective parts of the visible spectrum. The molecule used for photochromic dyes appears colorless in its unactivated form. When exposed to photons o f a pa r t i cu la r wave leng th , the molecular structure is altered into an excited state, and thus itb e g i n s t o r e fl e c t a t l o n g e r w a v e l e n g t h s i n t h e v i s i b l e spec t rum. On remova l o f the u l t rav io le t (UV) sou rce , the m o l e c u l e w i l l r e v e r t t o i t s o r i g i n a l s t a t e . A t y p i c a l p h o t o - chromic film, for example, can be essentially transparent andcolorless until it is exposed to sunlight, when the film beginsselectively to reflect or transmit certain wavelengths (such as atransparent blue). Its intensity depends upon the directness of exposu re . I t rever t s to i t s o r i g ina l co lo r l ess s ta te i n the dark when there is no sunlight.P h o t o c h r o m i c m a t e r i a l s a r e u s e d i n a w i d e r a n g e o f applications. Certainly we see them used in a wide range of consumer p roduc t s , such as sung lasses tha t change the i r color. In architecture, they have been used in various windowor fac¸ade treatments, albeit with varying amounts of success,Smart Materials and New TechnologiesTypes and characteristics of smart materials

85UVNaphthopyransThe molecular structure changes (a twisting inthis case) due to exposure to the input of radiant energy from light sFigure 4-2Photochromic materials changecolor when exposed to light (a change in themolecular structure of a photochromic mate-rial causes a change in its optical properties)sFigure 4-3Design Experiment: In the proposed ‘Coolhouse’, interior panels are covered with photochromic cloth that changes from a basecolor of white to blue upon exposure to sunlight. The panel shapes aredesigned for a particular solar angle for a specified time and placeduring the summer. At this time, the interior becomes a cool blue. Inthe winter, the cloth is not exposed and the interior remains white. (Teran and Teman Evans)

C. Differentcolors indicate different temperature levelsin the film. Blue is the highest temperaturelevel and black is the lowestsF i g u r e 4 - 5D e s i g n e x p e r i m e n t : i n t h i s s i m p l e s e t u p , h e a t e d w i r e s a r e u s e d t o generate a specific color change pattern ona thermochromic material. (Antonio GarciaOrozco)

consumer goods p ieces , f o r examp le , a re sens i t i ve to body heat and show a colored ‘imprint’ of a person who just sat onthe furniture. The image fades with time.T h e n o t i o n o f u s i n g t h e r m o c h r o m i c m a t e r i a l s o n t h e ex ter i o r o f a bu i l d ing has s im i l a r l y a lways a roused i n te res t .Un fo r tuna te l y , a ma jo r p rob lem w i th the use o f cu r rent ly available thermochromic paints on the exterior is that expo-sure to ultraviolet wavelengths in the sun’s light may cause thematerial to degrade and lose its color-changing capabilities.Mechanochromic and chemochromic materials Mechanochromics have altered optical properties when thematerial is subjected to stresses and deformations associatedwith external forces. Many polymers have been designed toexhibit these kinds of properties. The old household device for imprinting raised text onto plastic strips utilizes a plastic of thistype. The raised text that results from a mechanical deforma-tion shows through as a different color.Chemochromics include a wide range of materials whoseproperties are sensitive to different chemical environments. You might perhaps recall the ancient litmus paper in a basicchemistry class.Electrochromic materials E l e c t r o c h r o m i s m i s b r o a d l y d e fi n e d a s a r e v e r s i b l e c o l o r c h a n g e o f a m a t e r i a l c a u s e d b y a p p l i c a t i o n o f a n e l e c t r i c current or potential. An electrochromic window, for example,darkens or lightens electronically. A small voltage causes theglazing material to darken, and reversing the voltage causes itto lighten.There are three main classes of materials that change color when electrically activated: electrochromics, liquid crystalsand suspended par t i c l es . These techno log ies a re no t one -Smart Materials and New Technologies

Types and characteristics of smart materials87sF i g u r e 4 - 6Memor ies o f t ouch v ia t hermoch romic mater i a l s . (Courtesy Juergen Mayer H)

constituent materials, but consist of multi-layer assemblies of different materials working together.Fundamentally, color change in an electrochromic materialresu l t s f rom a chemica l l y i nduced mo lecu la r change a t thesur faceo f themater i a l th roughox ida t ion -reduc t ion . Ino rde r to achievethis result, layersof differentmaterials serving differente n d s a r e u s e d . B r i e fl y , h y d r o g e n o r l i t h i u m i o n s a r e t r a n s p o r t e d f r o ma n i o n s t o r a g e l a y e r t h r o u g h a n i o n c o n d u c t i n g l a y e r , a n d injected into an electrochromic layer. In glass assemblies, thee lec t rochromic laye r i so f ten tungs tenox ide (WO3) . A p p l y i n g a voltage drives the hydrogen or lithium ions from the storagelayer through the conducting layer, and into the electrochro-mic layer, thus changing the optical properties of the electro-chromic l ayer and caus ing i t to absorb ce r ta in v i s i b l e l i gh twave leng ths . I n th i s case , the g lass da rkens . Revers ing the v o l t a g e d r i v e s i o n s o u t o f t h e e l e c t r o c h r o m i c l a y e r i n t h e oppos i te d i rec t ion ( th rough the conduc t ing l aye r i n to the storage layer), thus causing the glass to lighten. The process isrelatively slow and requires a constant current.The layers forming the electrochromic component can bequite thin and readily sandwiched between traditional glazingmaterials. Many companies have been developing productsthat incorporate these features in systems from as small as aresidential window to as large as the curtain wall of a building.In a t yp i ca l app l i ca t i on , the re l a t ive t ranspa rency and co lo r tint of electrochromic windows can be electrically controlled.Note, however, that it is necessary for the voltage to remainon for the window to remain in a darkened state. This can bed i s a d v a n t a g e o u s f o r m a n y r e a s o n s . I n C h a p t e r s 6 a n d 7 we w i l l r e tu rn to a d i s cuss ion o f t he app l i ca t i ons o f e l ec t ro - chromic technologies.PHASE-CHANGING MATERIALSA s d i s c u s s e d i n t h e e a r l i e r s e c t i o n o n p h a s e c h a n g e s i n materials, many materials can exist in several different physicals t a t e s – g a s , l i q u i d o r s o l i d – t h a t a r e k n o w n a s p h a s e s . A change in the temperature or pressure on a material can causeit to change from one state to another, thereby undergoingwhat i s te rmed a ‘ phase change ’ . Phase change p rocesses invariably involve the absorbing, storing or releasing of largeamounts of energy in the form of latent heat. A phase changef r o m a s o l i d t o a l i q u i d , o r l i q u i d t o a g a s , a n d v i c e v e r s a , o cc u r s at precise temperatures. Thus, where energy is absorbed or re l eased

can be p red ic ted based on the compos i t i on o f the material. Phase-changing materials deliberately seek to takeadvantage of these absorption/release actions.Smart Materials and New Technologies88Types and characteristics of smart materials+ + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - -+++++++++++++++++++++++-----------------------DARKClear conductinglayer Clear conductinglayer Electro-chromiclayer Ionsource/sinkIonconductinglayer +++----TRANSPARENT+++----Light blocked Light passessFigure 4-7Electrochromic glass

Whi le mos t mate r i a l s undergo phase changes , there a re several particular compositions, such as inorganic hydratedsalts, that absorb and release large amounts of heat energy. Asthe material changes from a solid to a liquid state, and thensubsequent l y to a gaseous s ta te , l a rge amount s o f energymust be absorbed. When the material reverts from a gaseousto a l i qu id s t a te , and then t o a s o l i d s ta te , l a rge amoun ts o f energy w i l l be re l eased . These p rocesses a re revers ib le and phase-changing materials can undergo an unlimited number of cycles without degradation.Since phase-changing materials can be designed to absorbor re l ease energy a t p red ic tab le t empera tu res , they havena tura l l y been exp lo red fo r use in a rch i tec tu re as a way o f helping deal with the thermal environment in a building. Oneea r ly app l i ca t ion was the deve lopment o f so -ca l led ‘ phase c h a n g e w a l l b o a r d ’ w h i c h r e l i e d o n d i ff e r e n t e m b e d d e d materials to impart phase change capabilities. Salt hydrates,paraffins and fatty acids were commonly used. The paraffinand fatty acids were incorporated into the wallboard initiallyby direct immersion. Subsequently, filled plastic pellets wereused . T rans i t ion tempera tu res were des igned to be a round 65–728

F for heating dominated climates with primary heatingneeds and 72–798F for climates with primary cooling needs.P roduc t s based on d i rec t immers ion techno log ies neve r worked well and proved to have problems of their own thatwere assoc ia ted w i th the more o r le ss exposed paraffin and f a t t y a c i d s ( i n c l u d i n g p r o b l e m s w i t h a n i m a l s e a t i n g t h e wallboard products). Technologies based on sealing phase-chang ing mater i a l s i n to sma l l pe l l e t s worked be t te r . Pe l l e t technologies have achieved widespread use, for example, inc o n n e c t i o n w i t h r a d i a n t fl o o r h e a t i n g s y s t e m s . I n m a n y climates, radiant floor systems installed in concrete slabs canp r o v i d e q u i t e c o m f o r t a b l e h e a t i n g , b u t a r e s u b j e c t e d t o undes i r ed cyc l i ng and tempera tu re sw ings because o f then e e d t o k e e p t h e t e m p e r a t u r e o f t h e s l a b a t t h e d e s i r e d leve l , wh i ch typ i ca l l y requ i res a h igh i n i t i a l tempera tu re . Embedding phase-changing materials in the form of encasedp e l l e t s c a n h e l p l e v e l o u t t h e s e u n d e s i r a b l e t e m p e r a t u r e swings.Phase-changing materials have also successfully found their way in to ou tdoor c lo th ing . Pa tented techno log ies ex i s t f o r embedding microencapsulated phase-changing materials in atex t i l e . T hes e encaps u la t i ons a re m ic roscop ic i n s i ze . The phase-changing materials within these capsules are designedt o b e i n a h a l f - s o l i d , h a l f - l i q u i d s t a t e n e a r n o r m a l s k i n temperature. As a person exercises and generates heat, themater i a l s unde rgo a phase change and absorb excess heat ,Smart Materials and New TechnologiesTypes and characteristics of smart materials89CrystalineIntermediateAmorphoussFigure 4-8Phase change transformation

thus keeping the body cooler. As the body cools down, andheat is needed, the phase-changing materials begin to releaseheat to warm the body.Of particular interest in the applications discussed is thatsuccessful applications of phase-changing materials occurredwhen they were encapsu la ted i n one fo rm o r

ano the r . I t i s easy to imagine how encapsulated phase-changing materialsc o u l d b e u s e d i n m a n y o t h e r p r o d u c t s , f r o m l a m p s t o furniture, as a way of mitigating temperature swings.CONDUCTING POLYMERS AND OTHER SMARTCONDUCTORSIn this day and age of electronic devices, it is no wonder that al o t o f a t t e n t i o n h a s b e e n p a i d t o m a t e r i a l s t h a t c o n d u c t electricity. Any reader of scientific news has heard about thestrong interest in materials such as superconductors that offer little or no resistance to the flow of electricity. In this section,h o w e v e r , w e w i l l l o o k a t a b r o a d e r r a n g e o f c o n d u c t i n g materials, including those that offer great potential in differentdesign applications.I n g e n e r a l , t h e r e i s a b r o a d s p e c t r u m a s s o c i a t e d w i t h electrical conductivity through terms like ‘insulators’, ‘con-duc to rs ’ , ‘ sem i - conduc to rs ’ and ‘ super - conduc to rs ’ – w i th insulators being the least conductive of all materials. Many of the p roduc t s t ha t a rch i tec tu ra l and indus t r i a l des igne rs a re most familiar with are simple conductors. Obviously, manymeta l s a re i nherent ly e l ec t r i ca l l y conduc t ive due to the i r atomic bonding structures with their loosely bound electronsallowing easy electron flow through the material. As discussedin more detail in Chapter 6, many traditional products that arenot intrinsically conductive, e.g., glasses or many polymers,can be made so by va r i ous means . Po l ymers can be madeconduc t i ve by the d i rec t add i t i on o f conduc t ive mater i a l s ( e . g . , g r a p h i t e , m e t a l o x i d e p a r t i c l e s ) i n t o t h e m a t e r i a l . Glasses, normally highly insulating, can be made conductivea n d s t i l l b e t r a n s p a r e n t v i a t h i n fi l m m e t a l d e p o s i t i o n processes on their surfaces.There are other polymers whose electrical conductivity isintrinsic. Electroactive polymers change their electrical con-d u c t i v i t y i n r e s p o n s e t o a c h a n g e i n t h e s t r e n g t h o f a n electrical field applied to the material. A molecular rearrange-ment occurs, which aligns molecules in a particular way andf rees e lec t rons t o se rve as e l ec t r i c i t y conduc to rs . Examp les i n c l u d e p o l y a n i l i n e a n d p o l y p y r r o l e . T h e s e a r e n o r m a l l y c o n j u g a t e d p o l y m e r s b a s e d o n o r g a n i c c o m p o u n d s t h a t have i n te rna l s t ruc tu res i n wh i ch e lec t rons can move moreSmart Materials and New Technologies90Types and characteristics of smart materials

freely. Some polymers exhibit semiconductor behavior andcan be l igh t -em i t t i ng ( seeSemiconductors be low andLight-emitting polymers i n Chap ter 6 ) . E lec t rochemica l po l ymers exhibit a change in response to the strength of the chemicalenvironment present. A number o f app l i ca t i ons have been p roposed fo r con -duc t i ng po lymers . A r t i fic i a l musc les have been deve loped u s i n g p o l y p y r r o l e a n d p o l y a n i l i n e fi l m s . T h e s e fi l m s a r e laminated around an ion-conducting film to form a sandwichconstruction. When subjected to a current, a transfer of ionsoccurs. The current flow tends to reduce one side and oxidizethe o ther . One s ide expands and the o ther con t rac ts . S incethe fi lms a re separa ted , bend ing occu rs . Th i s bend ing can then be utilized to create mechanical forces and actions.Despite the dream of many designers to cover a buildingwith conducting polymers, and to have computer-generated images appear ing anywhere one des i res , i t i s necessary to remember t ha t these mate r i a l s a re es sen t i a l l y conduc to rs only. In the same way it would not be easy to make an imageappear on a sheet of copper, it is similarly difficult to make animage appear on a conduc t ing po lymer . S ince fi lms can be manipulated (cut, patterned, laminated, etc.), possibilities inthis realm do exist, but remain elusive.O t h e r s m a r t c o n d u c t o r s i n c l u d ephotoconductors

andphotoresistors that exhibit changes in their electrical conduc-t i v i t y w h e n e x p o s e d t o a l i g h t s o u r c e .Pyroconductors arematerials whose conductivities are temperature-dependent,and can have min ima l conduc t i v i t y near ce r ta in c r i t i ca l l ow temperatures.Magnetoconductors have conductivities respon-s i ve to the s t r eng th o f an app l i ed magnet i c fie ld . Many o f these s pec ia l i zed conduc t ing mate r i a l s find app l i ca t i ons as sensors of one type or another. Many small devices, includingmotion sensors, already employ various kinds of photocon-ductors or photoresistors (see Chapter 7). Others, includingpyroconductors, are used for thermal sensing.RHEOLOGICAL PROPERTY-CHANGING MATERIALSThe te rm ‘ rheo log i ca l ’ genera l l y re f e rs to the p rope r t i es o f flowing matter, notably fluids and viscous materials. While notamong the more obvious materials that the typical designer would seek to use, there are many interesting properties, inparticular viscosity, that might well be worth exploring.Many o f these mate r ia l s a re te rmed ‘fie ld -dependen t ’ . S p e c i fi c a l l y , t h e y c h a n g e t h e i r p r o p e r t i e s i n r e s p o n s e t o electric or magnetic fields. Most of these fluids are so-called‘ s t ruc tu red flu ids ’ ’ w i t h co l lo ida l d i spe rs i ons t ha t changeSmart Materials and New TechnologiesTypes and characteristics of smart materials91

p h a s e w h e n s u b j e c t e d t o a n e l e c t r i c o r m a g n e t i c fi e l d . Accompanying the phase change is a change in the propertiesof the fluid.Electrorheological (ER) fluids are particularly interesting. When an external electric field is applied to an electrorheo-logical fluid, the viscosity of the fluid increases remarkably. When the e lec t r i c fie ld i s removed , the v i scos i ty o f the flu id reverts to its original state. Magnetorheological fluids behavesimilarly in response to a magnetic field.T h e c h a n g e s i n v i s c o s i t y w h e n e l e c t r o r h e o l o g i c a l o r magnetorheological fluids are exposed to electric or magneticfie lds , re spec t i ve l y , can be s ta r t l ing . A l i qu id i s seeming ly transformed into a solid, and back again to a liquid as the fieldis turned off and on.These phenomena are beginning to be utilized in a number o f p r o d u c t s . A n e l e c t r o r h e o l o g i c a l fl u i d e m b e d d e d i n a n automobile tire, for example, can cause the stiffness of the tireto change upon demand ; thus mak ing i t poss ib le to ‘ tune ’ tires for better cornering or more comfortable straight riding.Some dev i ces t ha t t yp i ca l l y requ i re mechan i ca l i n te r faces , e.g., clutches, might conceivably use smart rheological fluidsas replacements for mechanical parts. In a rch i tec tu re and indus t r i a l des ign , l i t t l e use has been made of smart rheological fluids. One can imagine, however,cha i r s w i th smar t rheo log i ca l flu ids embedded i n sea ts anda r m s s o t h a t t h e r e l a t i v e h a r d n e s s o r s o f t n e s s o f t h e s e a t could be electrically adjusted. The same is obviously true for beds.LIQUID CRYSTAL TECHNOLOGIESLiquid crystal displays are now ubiquitously used in a host of products. It would be hard to find someone in today’s modernsociety that has not seen or used one. This widespread usage,however, does not mean that liquid crystal technologies areunsophisticated. Quite the contrary; they are a great successstory in technological progress.Liquid crystals are an intermediate phase between crystal- l i n e s o l i d s a n d i s o t r o p i c l i q u i d s . T h e y a r e o r i e n t a t i o n a l l y ordered liquids with anisotropic properties that are sensitiveto electrical fields, and therefore are particularly applicable for optical displays. Liquid crystal displays utilize two sheets of po la r i z i ng mate r ia l w i t h a l i qu id c rys ta l s o lu t i on between them. An electric current passed through

the liquid causes thecrystals to align so that light cannot pass through them. Eachcrystal is like a shutter, either allowing light to pass through or blocking the light.Smart Materials and New Technologies92Types and characteristics of smart materials

Smart Materials and New TechnologiesTypes and characteristics of smart materials93sFigure 4 -9Progressive phase change of n e m a t i c l i q u i d c r y s t a l fi l m s ( t h e t y p i c a l thermotropic liquid crystal similar to what is used in LCDs). (Images courtesy of Oleg D.Lavrentovich of the Liquid Crystal Institute,Kent State University)

SUSPENDED PARTICLE DISPLAYS

Newly developed suspended particle displays are attracting alot of attention for both display systems and for more generaluses. These displays are electrically activated and can changef rom an opaque to a c l ea r co lo r i ns tan t l y and v i ce -ve rsa . A typical suspended particle device consists of multiple layers of d ifferent mate r i a l s . The ac t ive layer as soc ia ted w i th co lo r change has need le - shaped par t i c l es suspended in a l i qu id . (fi lms have a l so been used ) . T h i s ac t i ve l aye r i s sandwiched between two parallel conducting sheets. When no voltage isapp l i ed , the pa r t i c l es a re r andomly pos i t ioned and abso rb light. An applied voltage causes the particles to align with thefie ld . When a l i gned , l i gh t t ransmiss i on i s g rea t l y i nc reased through the composite layers.Interestingly, the color or transparency level remains at thelast setting when voltage was applied or turned off. A constantvoltage need not be applied for the state to remain.Smart Materials and New Technologies94Types and characteristics of smart materialssFigure 4-10A liquid crystal display (LCD) uses two sheets of polarizing material and aliquid crystal solution sandwiched in between them

always composite materials – exceptions include magneto-strictive iron and naturally occurring piezoelectric quartz.The fo l l ow ing sec t ions desc r i be a number o f common ly used Type 2 energy-exchanging materials.Smart Materials and New Technologies96Types and characteristics of smart materialssFigure 4 -13Three types o f fluoresc ing calcite crystals (middle image also has fluor-i t e m i x e d i n ) ( I m a g e s c o u r t e s y o f T e m a Hecht and Maureen Verbeek)

LIGHT-EMITTING MATERIALSLuminescence, fluorescence and phosphorescence A definition of luminescence can be backed into by saying thatit is emitted light that is not caused by incandescence,

1butrather by some other means, such as chemical action. Morep r e c i s e l y , t h e t e r m l u m i n e s c e n c e g e n e r a l l y r e f e r s t o t h e emission of light due to incident energy. The light is causedby the re -em is s ion o f energy in wave leng ths i n the v i s i b l es pec t rum and i s assoc ia ted w i th t he revers i on o f e l ec t rons f r o m a h i g h e r e n e r g y s t a t e t o a l o w e r e n e r g y s t a t e . T h e phenomenon can be caused by a variety of excitation sources,i nc lud ing e lec t r i ca l , chemica l reac t i ons , o r even f r i c t i on . Ac lass i c examp le o f a mater i a l tha t i s l um inescen t due to a c h e m i c a l a c t i o n i s t h e w e l l - k n o w n ‘ l i g h t s t i c k ’ u s e d f o r emergency lighting or by children during Halloween.Luminescence is the general term used to describe differentphenomena based on emi t ted l igh t . I f t he emiss i on occu rsmore o r l ess i ns tantaneous ly , the te rmfluorescent i s u sed .F luo rescents g low par t i cu la r l y b r igh t ly when ba thed i n a ‘ b l a c k l i g h t ’ ( a l i g h t i n t h e u l t r a v i o l e t s p e c t r u m ) . I f t h eSmart Materials and New TechnologiesTypes and characteristics of smart materials97sFigure 4 -14Diagram show ing gene ra l phenomenon of luminescence

emis s ion i s s l ower o r de layed t o s eve ra l m ic roseconds o r milliseconds, the termphosphorescence i s u sed . Many com- pounds are either naturally phosphorescent or designed to beso. The amount of delay time depends on the particular kindo f phosphor used . Common phosphors i nc lude d iffe ren tmeta l su lfides (e .g . , ZnS ) . Common te lev i s i on sc reens r e l y o n t h e u s e o f Z n S . S t r o n t i u m a l u m i n a t e i s a l s o s t r o n g l y phosphorescen t . I n some s i tua t i ons , the l igh t emiss i on can c o n t i n u e l o n g a f t e r t h e s o u r c e o f e x c i t a t i o n i s r e m o v e d because the electrons become temporarily trapped becauseof material characteristics. Here the termafterglow is used.Most materials that are luminescent are solids that contains m a l l i m p u r i t i e s , e . g . , z i n c s u l f a t e s w i t h t i n y a m o u n t s o f copper. When these materials are exposed to incident energyi n a n y o f s e v e r a l f o r m s , t h e e n e r g y a s s o c i a t e d w i t h t h e impinging electrons or photons is absorbed by the material,which in turn causes electrons within the material to rise to ahigher level. Following the descriptive model suggested byFlinn and Trojan, these electrons subsequently may fall intow h a t a r e c o m m o n l y c a l l e d ‘ t r a p s ’ a s s o c i a t e d w i t h t h e impurities.2A f te r a wh i l e , a t rapped e lec t ron ga ins enough e n e r g y t o l e a v e i t s t r a p a n d i n d o i n g s o p r o d u c e s a l i g h t photon i n a wave length in the v i s ib l e spec t rum. I t s wave - length is dependent on the ion (e.g., copper) producing thetrap. Thus, the nature of the emitted light and its speed andd u r a t i o n o f e m i s s i o n d e p e n d u p o n t h e t y p e o f i m p u r i t y present.D iffe rent p rope r t i es , i nc lud ing the co lo r o f the em i t ted light, can be engineered by varying different compounds andimpur i ty i nc lus i ons t o y i e ld spec ific k inds o f l i gh t -em i t t i ng materials. In particular, there is a constant quest to improvethe duration of the phosphorescent effect once the excitations o u r c e h a s b e e n r e m o v e d . M a t e r i a l s s u c h a s s t r o n t i u m aluminate, for example, have been exploited for use becauseof their long afterglow duration once the excitation source hasbeen removed.Photoluminescence gene ra l l y r e fe rs to a k ind o f l um ines -cence tha t occurs when i nc ident energy assoc ia ted w i th an external light source acts upon a material that then re-emitslight at a lower energy level. A process of electronic excitationby photon absorption is involved. As a consequence of energyconse rva t ion , the wave length o f the em i t ted l i gh t i s l onger (i.e., ‘redder’ and involves less energy) than the wavelength of the incident light. Several kinds of phosphors photoluminescebrightly, particularly when exposed to ultraviolet light.Typical fluorescent lamps are also based on photolumines-cen t effec ts . T he i ns ide o f a l amp i s coa ted w i th a phosphor Smart Materials and New Technologies98Types and characteristics of smart materials

tha t i s exc i ted by u l t rav io l e t mercu ry rad ia t ion f rom a g low discharge.Inchemoluminescence , the excitation comes from a chemi-ca l ac t i on o f one type o r ano ther . The l i gh ts t i ck ment ioned e a r l i e r s t i l l p r o v i d e s t h e b e s t c o m m o n e x a m p l e o f t h i s phenomenon. Particularly interesting here is that chemolu-minescence p roduces l i gh t w i thout a co r respond ing heat o u t p u t , w h i c h i s s u r p r i s i n g s i n c e a c h e m i c a l r e a c t i o n i s involved. If the temperature of the surrounding heat environ-ment i s i nc reased , however , t he re w i l l be an i nc rease i n the r e a c t i o n t i m e , h e n c e l i g h t o u t p u t , a n d a r e d u c t i o n i n temperature will

correspondingly reduce the light output. A subset of chemoluminescence normally calledbiolumi-nescence i s pa r t i cu la r ly f asc ina t ing because i t p rov ides the glow associated with various light-emitting insects, such asfireflies, or fish such as the Malacosteus, which navigates thed e p t h s o f t h e s e a v i a i t s o w n n i g h t l i g h t . C o n s i d e r , f o r example, the squid that can alter its luminescence to matcheither moonlight or sunlight.Electroluminescence Withelectroluminescent materials the source of excitation is anapplied voltage or an electric field. The voltage provides theenergy requ i red . The re a re ac tua l l y two d iffe ren t ways tha telectroluminescence can occur. The first and typical conditionoccurs when there are impurities scattered through the basicp h o s p h o r . A h i g h e l e c t r i c fi e l d c a u s e s e l e c t r o n s t o m o v e through the phosphor and hit the impurities. Jumps occurringin connection with the ionized impurity cause luminescencet o o c c u r . T h e c o l o r e m i t t e d i s d e p e n d e n t o n t h e t y p e o f impur i ty ma ter i a l tha t fo rms the ac t i ve i ons . A second andmore complex behavior occurs in special materials, such assemiconductors, because of a general movement of electronsand holes (seeSemiconductors ,Lasers andLEDS (light-emitting diodes)below).Electroluminescent materials are widely used for light stripsa n d p a n e l s o f a l l d e s c r i p t i o n s . T h e b r i g h t b a c k l i g h t s i n inexpensive watches are invariably electroluminescent panels. A s n o t e d a b o v e , c o l o r s a r e d e p e n d e n t o n t h e a c t i v e i o n s selected for use. In very inexpensive systems, however, simplecolored filters are used to give variety. Strips or panels can bedesigned to work off of different applied voltages. They canbe battery operated. On the other hand, larger panels can bemade to respond to household voltages.Since the luminescent effect depends on phosphors and anelectric field, electroluminescent strips or panels can be madeSmart Materials and New TechnologiesTypes and characteristics of smart materials99Phosphorescent materialTranslucent coveringVoltagesource towires+-sFigure 4-15Electroluminescent wire

us ing a va r i e t y o f d ifferent neu t ra l subs t ra tes . Ve ry s imp le strips can be made in which a phosphorous material is appliedevenly to a polymeric strip, and covered by another transpar-en t s t r i p fo r p ro tec t i on . A sma l l w i re to p rov ide the e lec t r i c field is applied to the strip. Voltage sources can be batteries.Larger panels can be made using polymeric materials as well. A n e l e c t r i c s t r i p w o u l d e s s e n t i a l l y s u r r o u n d t h e p a n e l . Interestingly, these polymeric panels can literally be cut intod i ff e r e n t s h a p e s a s l o n g a s t h e e l e c t r i c a l fi e l d c a n b e maintained. Other materials that are often used as substratesinclude glass, ceramics and plastics.Electroluminescent lamps are becoming widely used. Theyd r a w l i t t l e p o w e r a n d g e n e r a t e n o h e a t . T h e y p r o v i d e a uniformly illuminated surface that appears equally bright fromall angles. Since they do not have moving or delicate parts,they do not break easily. Chapter 6 discusses applications inmore detail.BASIC SEMICONDUCTOR PHENOMENAFew people have not heard of semiconductors – the materialsthat have helped usher in an age of high-powered microelec-t ron ic dev i ces . The bas i c phenomenon unde r l y ing a semi - conduc to r fo rms the bas i s fo r o ther techno log ies a s we l l , including transistors and, of special interest in this book, thephotovoltaic effect associated with solar power. Few people,o f cour se , have any i dea o f wha t th i s phenomenon

ac tua l l y involves and what semiconducting devices actually do. Herewe w i l l on l y touch on the sa l i en t fea tu res o f these comp lex materials.Basic semiconductor materials, such as silicon, are neither good conductors nor good insulators, but, with the additiono f sma l l impur i t i es ca l l eddopants , t h e y c a n b e m a d e t o possess many fascinating electrical properties. The addition of these dopants or impurities allows electron movements to beprecisely controlled. Exploitation of the resultant propertieshas allowed a semiconductor to serve the same functions ascomplicated multipart electronic circuitries.Silicon is the most widely used semiconducting material,a l though o ther ma ter i a l t ypes a re pos s ib l e . Bas ic semicon -duc t ing mater ia l s exh ib i t i n te res t ing p rope r t i es when su r - r o u n d i n g t e m p e r a t u r e s a r e v a r i e d . U n l i k e m o s t m e t a l s where in i nc reases i n t empera tu res caus e i nc reases i n res i s - tance, the conductivity of semiconducting materials increasesw i th inc reas ing tempera tu res . Th i s p roper t y a l ready makes i t q u i t e a t t r a c t i v e f o r m a n y a p p l i c a t i o n s . I t r e s u l t s f r o m a pa r t i cu la r type o f e lec t ron band s t ruc tu re i n the in te rna lSmart Materials and New Technologies100Types and characteristics of smart materialsVoltagesource+-sFigure 4-16Electroluminescent strips

s t r u c t u r e o f t h e m a t e r i a l s . A g a p e x i s t s b e t w e e n b a n d s through which thermally excited electrons cross in particular conditions.T h e a d d i t i o n o f d o p a n t s o r i m p u r i t i e s c r e a t e s o t h e r c o n d i t i o n s . T h e r o l e o f i m p u r i t i e s w i t h r e s p e c t t o l i g h t -em i t t i ng mate r i a l s was p rev ious l y no ted . O f impor tance in this discussion is the role of the impurities in affecting the flowo f e l e c t r o n s t h r o u g h a m a t e r i a l . H e r e a g a i n t h e fl o w i s affected, but in this case in a controllable way. Silicon matrixmaterials are alloyed with specific concentrations of a dopant,such as boron, via a complex deposition layering procedure toform a semiconductive device. Multiple dopants of differenttypes may be used . The spec ific na tu re o f these assembl i es determines their useful electronic properties.F igure 4–17 i l l us t ra tes a typ ica l makeup o f a dev i ce tha tcons i s t s o f a junc t i on o f so -ca l l edpandnsemiconductor m a t e r i a l s ( m a d e b y u s i n g d i ff e r e n t d o p a n t s o n s i l i c o n subs t ra tes ) . I n t he fi rs t t ype o f mater i a l ,n, e l ec t rons w i th anega t i ve cha rge a re p redominant l y p resent . I n the second type,p

, ho les ( l oca t i ons o f m i ss ing e lec t rons ) a re p r imar i l y p r e s e n t r e s u l t i n g i n a p o s i t i v e c h a r g e . A p p l i c a t i o n o f a n e g a t i v e c h a r g e t o t h eps i d e c a u s e s t h e c h a r g e s t o b e electrostatically attracted away from each other, creating azone tha t i s f ree o f e l ec t rons . No cur rent flows th rough th i s region. Application of a positive charge to thepside causesthe reverse situation. Electrons flow through the barrier zonecreating a current.PHOTOVOLTAICS, LEDS, TRANSISTORS,THERMOELECTRICSMany w ide ly used dev i ces have the i r fundamenta l bas i s i nsemiconductor technology.Photovoltaic techno log ies a re d i s c u s s e d i n d e t a i l e l s e w h e r e ( s e e C h a p t e r 7 ) . H e r e i t i s impo r tan t to no te tha t the bas ic unde r l y ing phenomenon i s r e l a t e d t o t h e s e m i c o n d u c t o r b e h a v i o r n o t e d a b o v e . A photovoltaic device consists primarily of apandnjunction.Instead of there being an applied voltage as described above,however, there is an incident energy (typically solar) that actson the j unc t i on and p rov ides t he ex te rna l energy i npu t . I n typical solar cells, thenlayer is formed on top of theplayer.Incident energy impinges on thenlayer. This incident energycauses a change in electron levels that in turn causes adjacentelectrons to move because of electrostatic forces. This move-ment of electrons produces a current flow. Phototransistorsare similar in that they convert radiant energy from light into acurrent.Smart Materials and New TechnologiesTypes and characteristics of smart materials101Barrier region- - - -+ + + +- - - -+ + + +p- typen- typeInstrinsic negativecharge - electronsdominateIntrinsic positivecharge - holesdominateSemiconductor materials with different addedselected impurities (dopants) - conductivitiesincrease with temperatures in semiconductor materials- - - -++++- - - -+++++--+In thereverse biasmode, there is no flow of current across the barrier region+ + + ++--+Energy source (applied voltage)+ + + +

- - - -- - - -Energy source (applied voltage)In theforward biasmode, the current increasesexponentially with the applied voltagesFigure 4-17Basic semiconductor behavior

CommonLEDs (light-emitting diodes)are based essentiallyo n t h e c o n v e r s e o f p h o t o v o l t a i c e ff e c t s . A n L E D i s a s e m i c o n d u c t o r t h a t l u m i n e s c e s w h e n a c u r r e n t p a s s e s through it. It is basically the opposite of a photovoltaic cell.L E D s a r e d i s c u s s e d i n d e t a i l i n C h a p t e r 7 . T r a n s i s t o r s a r e s i m i l a r l y b a s e d o n s e m i c o n d u c t o r t e c h n o l o g i e s . F u n d a - menta l l y , a t rans i s to r can be used as a s igna l amp l ifica t i on device, or as a switching device.Thermoelectrics or Peltier devices are an electronic form of hea t pump . A t yp i ca l Pe l t ie r dev i ce uses a vo l tage i nput toc rea te ho t and co ld j unc t i ons , hence they can be used fo r heating or cooling. They are found in computers as coolingdevices, and in common automotive and household goods assma l l hea te rs o r coo le r s . When i n use , t here mus t be a way provided to carry the heat generated away from the unit. Inlarger units, fans are commonly used.Lasers a re one o f t he ub iqu i tous workhorses o f t oday ’s technological society. Laser light occurs via stimulated emis-sion. In a laser, an electron can be caused to move from oneenergy state to another because of an energy input, and, as aconsequence, emit a light photon. This emitted photon can inturn stimulate another electron to change energy levels andemit another photon that vibrates in phase with the first. Thecha in bu i l ds up qu ick l y w i th i nc reas ing i n tens i t y . Emi t ted photons vibrate in phase with one another. Hence the light isphase-coherent. The term ‘coherent light’ is often used. Thel igh t i s monochromat ic , wh i ch i n tu rn a l lows i t to be h igh ly focused. Since the light occurs via stimulated emissions, thea c r o n y m

L a s e r w a s a d o p t e d ( i . e . , l i g h t a m p l i fi c a t i o n b y stimulated emission of radiation).Many types of lasers exist that rely on different methods of excitation and use different materials. There are ruby lasers,Smart Materials and New Technologies102Types and characteristics of smart materials+ + + ++ + + +- - - -- - - -Energy source (applied voltage)+-Light emitted In alight-emitting diode (LED), energy inputinto the junction creates a voltage outputMetalliccontact T y p i c a l L E D s ' F l e x i b l e ' L E D ssFigure 4-18Light emitting diodes (LED)are based on semiconductor technologiessFigure 4-19Photovoltaic (PV) devices arebased on semiconductor technologiesApplied voltageHotCool+-I pn p- typesemiconductor n- typesemiconductor Insulator sFigure 4-20In a Peltier device an input electrical current causes oneface to heat up and the other to cool down. Ceramic plates are used inthe device shown. It is necessary to transfer heat away from the hotsurface via fans or heat spreaders. Peltier devices are used in manyproducts, including drink coolers and in computers to cool microchips

gas l ase rs and so fo r th . Powers can va ry . Gas l ase rs can be quite powerful and cut many materials. The most ubiquitouskind of lasers used in printers, pointers, construction levels,su rvey ing in s t ruments e tc . a re typ ica l l y based on semicon - ductor technologies (see Figure 4–21).PIEZOELECTRIC EFFECTS AND MATERIALSIn this section we enter into the world of thepiezoelectric effect tha t fo rms the under l y ing bas i s fo r p roduc ts as d i ve rse a s some types of microphones and speakers, charcoal grill fires ta r te rs , v i b ra t i on reduc ing sk i s , doorbe l l pushers and an endless number of position sensors and small actuators. All of these devices involve use of a piezoelectric material in whichan applied mechanical force produces a deformation that inturn produces an electric voltage, or, conversely, an appliedvoltage that causes a mechanical deformation in the materialtha t can be used to p roduce a fo rce . Th i s genera l phenom- enon is called the piezoelectric effect.Thepiezoelectric phenomenon(p iezo means p res sure i n Greek) was observed by the brothers Pierre and Jacques Curiewhen they were 21 and 24 years old in 1880. They observedtha t when a p ressure i s app l i ed to a po la r i zed c rys ta l , themechan i ca l de fo rmat ion i nduced resu l ted i n an e lec t r i ca lSmart Materials and New TechnologiesTypes and characteristics of smart materials103sFigure 4-21Lasers – basic principles and types

charge. The phenomenon is based upon a reversible energyconvers i on be tween e lec t r i ca l and mechan i ca l f o rms tha t occurs naturally in permanently polarized materials in whichparts of molecules are positively charged and other parts aren e g a t i v e l y c h a r g e d . M a n y n a t u r a l l y f o u n d c r y s t a l s ( e . g . , quartz) possess this property, as do many newly developedpo lymers and ce ramics . The p roper t y i s cu r ious l y s im i la r to that found in magnets where permanent magnetic polariza-t i o n o c c u r s , e x c e p t h e r e w e a r e d e a l i n g w i t h e l e c t r i c a l charges.In piezoelectric materials, each cell or molecule is a dipolewith a positive and negative charges onto either end. Therei s an a l i gnment o f the i n te rna l e l ec t r i c d ipo les . Th i s a l i gn - m e n t c a n r e s u l t i n a s u r f a c e c h a r g e , b u t t h i s c h a r g e i s n e u t r a l i z e d b y f r e e c h a r g e s p r e s e n t i n t h e s u r r o u n d i n g atmosphere. A force is applied to the piezoelectric materialtha t causes de fo rmat ions t o t ake p lace , wh i ch in tu rn a l te r s t h e n e u t r a l i z e d s t a t e o f t h e s u r f a c e b y c h a n g i n g t h e orientation of the dipoles. The reverse can also be achieved. A p p l y i n g a v o l t a g e c a u s e s p o l a r i z e d m o l e c u l e s t o a l i g n themse lves w i th the e lec t r i c fie ld , wh i ch , in tu rn , causes adeformation to develop.The piezoelectric effect has long been exploited in manydifferent devices. The obviously desirable property wherein apressure produces a voltage is used in many different ways. In the common doorbe l l pushe r , an app l ied fo rce p roduces a voltage, which in turn is used to control an electrical circuitc a u s i n g t h e i r r i t a t i n g c h i m e o r d e l i g h t f u l b u z z . I n t h e previously mentioned charcoal lighter, application of a forceto a piezoelectric device causes an ignition spark. Less obviousto mos t peop le , bu t more w ide ly used , a re a who le hos t o f piezo-based devices that serve as small electrically controlleda c t u a t o r s u s e d i n a v a r i e t y o f m e c h a n i c a l a n d i n d u s t r i a l s i tua t i ons where in a sma l l vo l tage causes a par t movement that controls something else, such as a valve.The piezoelectric effect is literally instantaneous and piezo-e lec t r i c dev ices can be qu i te sens i t i ve t o sma l l p ressures o r voltages. Many microphones based on piezoelectric materialstransform an acoustical pressure into a voltage. Alternatively,in piezoelectric speakers, application of an electrical

chargecauses a mechanical deformation, which can in turn create anacoustical pressure.Uses can be surprising. Piezoelectric materials have beenused in skis to damp out undesirable vibrations that can occur u n d e r c e r t a i n c o n d i t i o n s . H e r e , t h e p i e z o e l e c t r i c e ff e c t d a m p e n s v i b r a t i o n s b y d i s s i p a t i n g t h e e l e c t r i c a l e n e r g y d e v e l o p e d a c r o s s a s h u n t i n g . O t h e r s i t u a t i o n s i n v o l v i n gSmart Materials and New Technologies104Types and characteristics of smart materialsNormal stateWhen deformedby a force, avoltage output isgeneratedWhen a voltageis applied, adeformationresultsDeformationsgenerating avoltage outputmay also becaused bybending (seeFig. 5-5)+-+-+-CompressedpiezoelectricPiezoelectricmaterial expandsPiezoelectricmaterialElongationof piezoelectricmaterialContractionsFigure 4-22Piezoelectric behavior

v ib ra to ry movements in many p roduc ts can be s e lec t ive l y damped out using similar technologies.SHAPE MEMORY ALLOYSPerhaps su rp r i s ing l y , eyeg las s f rames tha t a re amaz ing ly b e n d a b l e , m e d i c a l s t e n t s f o r o p e n i n g a r t e r i e s t h a t a r e i m p l a n t e d i n a c o m p r e s s e d f o r m a n d t h e n e x p a n d t o t h e right size and shape when

warmed by the body, tiny actuatorsthat eject disks from laptop computers, small microvalves anda h o s t o f o t h e r d e v i c e s , a l l s h a r e a c o m m o n m a t e r i a l technology. The interesting behavior of each of these devicesrelies upon a phenomenon called the ‘shape memory effect’that refers to the ability of a particular kind of alloy material to revert, or remember, a previously memorized or preset shape.The charac te r i s t i c de r i ves f r om the phase - t rans fo rmat ioncharac te r i s t i c s o f the mater i a l . A so l i d s ta te phase change –a mo lecu la r rea r rangement – occu rs i n t he shape memory a l l o y t h a t i s t e m p e r a t u r e - d e p e n d e n t a n d r e v e r s i b l e . F o r example, the material can be shaped into one configurationat a high temperature, deformed dramatically while at a lowtemperature, and then revert back to its original shape uponthe application of heat in any form, including by an electricalcurrent. The phenomenon of superelasticity – the ability of amaterial to undergo enormous elastic or reversible deforma-tions – is also related to the shape memory effect.Nickel–titanium (NiTi) alloys are commonly used in shapememory app l i ca t ions , a l t hough many o the r k inds o f a l l oys also exhibit shape memory effects. These alloys can exist infinal product form in two different temperature-dependentcrystalline states or phases. The primary and higher tempera-ture phase is called the austenite state. The lower temperaturephase is called the martensite state. The physical properties of the material in the austenite and martensite phases are quited ifferent . The mate r i a l in t he aus ten i te s ta te i s s t rong and hard, while it is soft and ductile in the martensite phase. Theaus ten i t e c rys ta l s t ruc tu re i s a s imp le body -cen te red cub i cs t ruc tu re , wh i le mar tens i t e has a more comp lex rhomb ic structure. With respect to its stress–strain curve, the higher tempera-ture austenite behaves similarly to most metals. The stress–strain curve of the lower temperature martensitic structure,however, almost looks like that of an elastomer in that it has‘plateau’ stress-deformation characteristics where large defor-m a t i o n s c a n e a s i l y o c c u r w i t h l i t t l e f o r c e . I n t h i s s t a t e , i t behaves like pure tin, which can (within limits) be bent backand forth repeatedly without strain hardening that can lead toSmart Materials and New TechnologiesTypes and characteristics of smart materials105Material is given a shape while inthe higher temperature austenitephaseWhile in the lower temperature martensitephase, the material can beeasily deformed into another shapeUpon the application of heat,the material returns to its higher temperature austenite phaseand to its original shape Shape memory effectDesired shape(austenite)Deformedshape(martensite)Originalstate(austenite)sFigure 4-23In a thermally induced shapememory effect, a material can be deformed,bu t ‘ remembers ’ i t s o r i g ina l shape a f t e r heating. Shape memory effects may also beinduced in other materials by magnetic fields

failure. The material in the lower temperature martensite statehas a ‘twinned’ crystalline structure, which involves a mirror symmet ry d i sp lacement o f a toms ac ross a pa r t i cu la r p lane .Tw in boundar i e s a re fo rmed tha t can be moved eas i l y and without the formation of microdefects such as dislocations.U n l i k e m o s t m e t a l s t h a t u n d e r g o d e f o r m a t i o n s b y s l i p o r d i s l oca t ion movement , de fo rmat ion i n a tw inned s t ruc tu re o c c u r s b y l a r g e c h a n g e s i n t h e o r i e n t a t i o n o f i t s w h o l e c rys ta l l i ne s t ruc tu re a ssoc ia ted w i th movements o f i t s tw inboundaries.The thermally induced shape memory effect is associatedwith these different phases. In the primary high temperatureenv i ronment , t he mater i a l i s i n t he aus ten i te phase . Upon cooling the material becomes martensitic. No obvious shapechange occurs upon coo l i ng , bu t now the mater i a l can bemechan i ca l l y de fo rmed. I t w i l l r ema in de fo rmed wh i l e i t i s cool. Upon heating, the austenitic structure again appears andthe material returns to its initial shape. A related mechanically induced phenomenon calledsuper-elasticity can a l so take p lace . The app l i ca t i on o f a s t res s t o a s h a p e m e m o r y a l l o y b e i n g d e f o r m e d i n d u c e s a p h a s e t rans fo rmat ion f r om the aus ten i t e phase t o the mar tens i teSmart Materials and New Technologies106Types and characteristics of smart materialssFigure 4-24Shape memory alloys (e.g., Nitinol) that exhibit thermally induced shape memoryeffects

p h a s e ( w h i c h i s h i g h l y d e f o r m a b l e ) . T h e s t r e s s c a u s e s mar tens i te to fo rm a t tempera tu res h igher than p rev ious l y and there is high ductility associated with the martensite. Theassoc ia ted s t ra ins o r de fo rmat ions a re revers ib l e when the applied stress level is removed and the material reverts back toaustenite. High deformations, on the order of 5–8%, can beachieved. Changes in the external temperature environmenta r e n o t n e c e s s a r y f o r t h e s u p e r e l a s t i c i t y p h e n o m e n o n t o occur. Why these phenomena occur is fundamentally a result of the need for a crystal lattice structure to accommodate to themin imum ene rgy s ta te fo r a g i ven t empera tu re . The re a re many different configurations that a crystal lattice structurecan as sume in the mar tens i te phase , bu t there i s on l y one possible configuration or orientation in the austenite state,and all martensitic configurations must ultimately revert totha t s i ng le s hape and s t ruc tu re upon heat ing pas t a c r i t i ca l p h a s e t r a n s i t i o n t e m p e r a t u r e . T h e p r o c e s s d e s c r i b e d i s repea tab le as long as l im i t s ass oc ia ted w i th the t rans i t ionphases a re ma in ta ined . Unde r h igh s t ress o r de fo rmat ion levels, a form of fatigue failure can occur after repeated cycles.Both of the two primary phenomena associated with shapememory effects – thermally induced effects and mechanicallyi n d u c e d e ff e c t s – h a v e d i r e c t a p p l i c a t i o n s . I n t h e s h a p e memory effec t as soc ia ted w i th the the rma l env i ronment , amaterial having an initial shape while in its high temperatureaus ten i t e phase can subsequent l y be de fo rmed wh i l e i n a lower temperature phase martensite phase. When reheated tothe high temperature austenite phase by a heat stimulus, suchas an electric current (but any heat source will work), the alloyrever t s back to i t s i n i t i a l shape . Du r ing th i s p rocess , a h igh f o r c e i s g e n e r a t e d b y t h e p h a s e - c h a n g i n g m a t e r i a l . T h e mater i a l can thus be used as an ac tua to r in many d ifferent applications. Usually the material provides the primary forceor ac tua t ing movement as par t o f a l a rge r dev i ce . S ince the force and movement occur within the material itself, devicesusing it are often very simple as compared to more traditionalmechanical actuators. Heat in the form of electrical current iseasy

to app ly and e lec t ron i ca l l y cont ro l . Hence , t he w ide - spread use o f shape memory a l l oys in re l ease l a tches and a host of other devices.In the shape memory effect associated with the mechanicalenvironment, or superelasticity, the material can undergo anelastic deformation (caused by an external force) that can beas h igh as twen ty o r more t imes the e las t i c s t ra in o f no rma l steel. Superelastic materials thus exhibit incredible abilities todeform and still ‘spring back’ to their original shape. An initialSmart Materials and New TechnologiesTypes and characteristics of smart materials107The shape memory alloy changes from anaustenite phase to a martensite phase duringdeformation.sFigure 4-25Superelasticity – a mechani-cally induced shape memory effect

consumer app l i ca t i on o f supe re las t i c ma ter i a l s was i n eye - glass frames that could seemingly be tied in knots, but whichreverted to their original shape upon release.SHAPE MEMORY POLYMERSA l l oys a re no t the on ly mate r ia l s t o exh ib i t shape memory e ff e c t s . A m a j o r e ff o r t h a s b e e n r e c e n t l y d i r e c t e d w i t h cons iderab le success to eng ineer ing po lymers t o have the same effects. Applications are enormous, since polymers canbe easily fabricated in a number of different forms. Medicalapplications, for example, include the development of shapememory polymeric strands to be used in surgical operations asself-tying knots. The strands are used to tie off blood vessels.The strands are given an initial

shape, looped around a vesseland, as the body heat operates on the polymer, the strand tiesitself into a knot (its remembered shape).Notes and references1Incandescent light is generated by the glowing of a materialdue to high temperatures, i.e., it is emitted visible radiationassociated with a hot body.2Flinn, Richard and Trojan, Paul (1986)Engineering Materials and Their Applications . Boston, MA: Houghton Mifflin.