The AMPTIAC Quarterly, Volume 7, Number 2 9

MaterialE A S E

MATERIALS THAT SENSE AND RESPOND:

AN INTRODUCTION TO SMART MATERIALS

Richard Lane

and

Benjamin CraigAMPTIAC Technical Staff, Rome, NY

INTRODUCTION

Each of us reacts to the world around and within us by sensing and actuating. When our hand is in contact with a hot object, we sense the heat,our brain sends a command, and our arm muscles actuate our hand awayfrom the object. Similarly, because of internal sensing, we will tend to favorthe burned hand until it has healed. As technology progresses, it becomesreasonable to ask, “Can we design analogous mechanisms that can intel-ligently interact with their environment, and structures that assess theirown health?” Such smart structures could have a tremendous impact inadvancing many fields including medicine, microelectronics, and robotics,among others.

Often, simple devices made from a single sensing or actuating materialare used in certain applications. However, systems that involve both sensingand actuating materials can be used to build more sophisticated applica-tions. Such systems are referred to as smart structures, which incorporatesensors and actuators with processing/control units connecting them. To getan idea of how smart structures can be implemented, it is necessary tounderstand the fundamental components of these structures: sensor andactuator materials.

For centuries, materials have been known to react to the surrounding

environment producing some form of response. For instance, in 1824,Rochelle salt was discovered to become electrically polarized by the applica-tion of heat. That was the first discovery of the effect known as pyroelectric-ity. Since that time, numerous additional materials have been discoveredhaving the inherent capability to convert one form of energy into another.Sensors are materials that respond to a physical stimulus, such as a changein temperature, pressure, or illumination, and transmit a resulting signalfor monitoring or operating a control. Actuators are materials that respondto a stimulus in the form of a mechanical property change such as a dimen-sional or a viscosity change. Table 1 lists the sensor/actuator material classes that will be discussed in this MaterialEASE along with their associated energy stimulus and response forms.

SMART MATERIAL TYPESPyroelectrics

Pyroelectrics, as previously mentioned, are materials that become electri-cally polarized upon an applied temperature change. Materials used aspyroelectrics include barium strontium titanate (Ba1-xSrxTiO3), lead zirconate titanate (PbZr1-x TixO3), barium strontium niobate (Ba1-xSrxNb2O6), triglycine sulfates (TGS), lithium tantalate (LiTaO3),

22AMPTIAC

Sensors and actuators have become an increasingly important technology as more systems become automated, requiring less human input or operation.This MaterialEASE characterizes the leading sensor and actuator materials currently in use along with some new materials under research. For moreinformation on this exciting topic, consider purchasing the more complete AMPTIAC Report: Materials for Sensors and Actuators.

Table 1. Sensor and Actuator Material Classes.

Material Class Stimulus ResponsePyroelectrics Temperature Change Electric PolarizationPiezoelectrics Mechanical Strain Electric PolarizationElectrostrictors Mechanical Strain Electric Polarization

Magnetostrictors Mechanical Strain Change in Magnetic FieldElectroactive Polymers Mechanical Strain Electric Polarization

Electroluminescent Materials Electric Field Light EmissionPhotoluminescent Materials Incident Light Light EmissionElectrochromic Materials Electric Field Color Change

Piezoelectrics Electric Current Mechanical StrainElectrostrictors Electric Current Mechanical Strain

Magnetostrictors Magnetic/Electric Field Mechanical StrainShape Memory Alloys Temperature Change Mechanical StrainElectroactive Polymers Electric Field/pH change Mechanical Strain

Electrorheological Fluids Electric Field Viscosity ChangeMagnetorheological Fluids Magnetic Field Viscosity Change

Act

uato

rs

S

enso

rs

MACROSCOPIC MAGNETOSTRICTION MECHANISM

Magnetic materials have internal areascalled domains, within which all magneticdipoles are oriented in the same direction.Domains with different orientations are sep-arated by domain walls or boundaries. Withthe application of an external magnetic field,the boundaries move and domains rotateand align themselves in the same direction,ultimately causing a slight length/shapechange in the bulk material. The magne-tostriction effect is illustrated in the figure.

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MaterialE A S E

and polyvinylidene fluoride (PVDF). Ceramics are widely used due totheir lower cost including availability and ease of processing, and goodstability. Their weakness comes from their inherent brittleness.Polymers, PVDF and its copolymer with trifluoroethylene (PVDF-TrFE), provide a non-brittle alternative but do not obtain the level ofperformance of ceramics.

Pyroelectrics are widely used for infrared detection in surveillanceand targeting applications. A pyroelectric infrared detector generallyemploys a thin film of pyroelectric material oriented with the elec-troded surfaces normal to its polarization direction. When infraredradiation is absorbed by the detector, the temperature of the pyroelec-tric material rises. This change in temperature alters the material’spolarization causing a change in surface charges across the materialwhich produces an electrical signal via the electrodes. The resultingsignal is proportional to the incoming radiation.

Piezoelectrics

Piezoelectrics are materials that exhibit an electrical polarization withan applied mechanical stress (direct effect), or a dimensional changewith an applied electric field (converse effect). They are used for bothsensing and actuating devices. Lead zirconate titanate (PbZr1-xTixO3)is the premier piezoelectric material as it may be doped to produce ann-type or p-type material with a range of dielectric constants to meetthe requirements of numerous applications. Other piezoelectric mate-rials that may be used are barium titanate (BaTiO3), lead titanate(PbTiO3), lead metaniobate (PbNb2O6), and PVDF. Polymers are gen-erally favored for sensing applications while ceramics are favored foractuating. Figure 1 depicts the piezoelectric effect observed in lead zir-conate titanate upon the application of compressive forces relative tothe crystal structure.

Piezoelectrics is a mature technology with numerous applicationsthroughout the military and commercial sectors. Devices using piezo-

electrics include: adaptive optics, hydrophones and sonobuoys, fusedevices, depth sounders, thickness gauging, flaw detection, level indi-cators, alarm systems, strain gauges, airplane beacon locators, fetalheart detectors, and tire pressure indicators among many others.

Electrostrictors

Electrostrictors, like piezoelectrics, exhibit a dimensional change uponan applied electric field or an electric polarization upon an appliedmechanical stress. Electrostrictors however, exhibit a non-linearresponse as opposed to the linear response of piezoelectrics. The advan-tages of electrostrictive materials over piezoelectrics are that they havealmost no hysteresis, a quick response time, and higher displacementswith good reproducibility. These properties are desired for microposi-

Figure 1. The Piezoelectric Effect

Lead

Oxygen

Titanium or Zirconium

ForceForce

Force

Force

Polarization UpPolarization Down

No External Magnetic Field

Apply External Magnetic Field

Arrows indicate magnetic dipolein each domain External Magnetic Field Direction

THE BASIC ONE-WAY SHAPE MEMORY EFFECT

A D VA N C E D M AT E R I A L S A N D P R O C E S S E S T E C H N O L O G Y

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AMPTIAC

tioner and adaptive optic actuation applications. A disadvantage of availableelectrostrictors is their higher cost compared with piezoelectrics. Ceramicmaterials that are used in electrostrictive devices are in a class called “relax-or ferroelectrics.” The term “relaxor” refers to the significant decrease indielectric constant with increasing frequency. Lead magnesium niobate –lead titanate (0.9PbMg1/3Nb2/3O3 – 0.1PbTiO3) is the most popular elec-trostrictive material. Research on polymeric electrostrictors has taken place,but none have been developed thus far for practical use.

Magnetostrictors

Magnetostrictive materials (used in actuators) exhibit a change in shape(strain or elongation) when an external magnetic field is applied. Thereverse effect is called piezomagnetism (used in sensors), where a magnet-ic field is produced/altered upon application of a mechanical strain.

Although usually minimal, ferromagnetic materials (e.g. Fe, Ni, Co,etc.) exhibit magnetostriction to some extent. Ferromagnetic elements,such as cobalt, are often alloyed with iron. Rare earth elements have exhib-ited significantly higher magnetostriction but only at temperatures lowerthan room temperature. Other common magnetostrictive materials includenickel-based alloys and particulate composites containing magnetostrictiveparticles. Magnetostrictive sensors and actuators are used for transducers,transformers, MEMS, vibration and noise control, linear motors, adaptiveoptics, ultrasonics, speakers, drills, pumps, and position and mechanicaltorque sensors.

Terfenol-D is used almost exclusively as the choice material for magne-tostrictive applications because of its exceptional magnetostrictive proper-ties. Terfenol is an intermetallic compound, TbFe2 (TER – Terbium; FE –Iron; NOL – Naval Ordnance Laboratory, now the Naval Surface WarfareCenter), and D denotes the presence of the rare earth metal dysprosium.

SMAs

Shape memory alloys (SMAs) are actuators that, upon proper thermal andmechanical treatment, have the ability to remember up to two shapes whichthey had previously occupied. When subjected to a mechanical load belowa certain temperature, these special materials can be plastically deformedbeyond their elastic limit, but then are capable of regaining their originalshape if they are then heated above a certain temperature.

Nickel-titanium is the most common SMA and has some of the bestshape memory properties, but has relatively low transformation tempera-tures. Although they cannot match the outstanding shape memory capabil-ities of Ni-Ti alloys, copper-based alloys are a less expensive alternative. Themost common ternary Cu-based systems are Cu-Zn-Al and Cu-Al-Ni whichcan generally achieve a shape memory strain of 4 to 5% (compared to about8% for Ni-Ti) and have a broader range of transformation temperatures.[1]Other alloys, such as iron-based alloys (e.g. Fe-Mn, Fe-Mn-Si, Fe-Pt, Fe-Ni,Fe-Ni-Co and Fe-Pd), exhibit shape memory but are not capable of regain-ing their shape to the same extent as nickel-titanium and copper-basedalloys. Other forms of SMAs include composites and ferromagnetic alloys.Some of the current SMA applications include use in spacecraft, aircraft,automobiles, electronics, medicine, process systems, robotics, and domesticappliances.

Wet EAPs

Wet electroactive polymers (EAPs) (also known as ionic EAPs) are poly-mers immersed in an electrolytic fluid that upon the application of anexternal stimulus (such as an electric field,) can exhibit a significantchange in dimension or geometry. EAPs can act as sensors and/or actuators and are typically of the following compositions.

Polymer gels can take on water and expand, or conversely expel water

The actual mechanism of the shape memory effect can be described simply as areversible, thermoelastic, phase transformation between a parent austenitic phaseand a martensitic phase. The phase transformation occurs when a material in theaustenitic phase is cooled below the martensite start temperature (Ms), where thetwo phases coexist. The material must then accommodate the two phases withoutchanging shape through a mechanism called twinning. This is where mirror-imagelattices form adjacent to the parent lattices. The phase transformation is completedupon reaching the martensite finish temperature (Mf). The material can then beplastically deformed into another shape. During this deformation the twinnedmartensite is converted to a deformed martensite. The material remains deformeduntil it is heated to the austenite start temperature (As), and at this point themartensite begins to transform back into austenite. Heating above the austenite fin-ish temperature (Af) allows the material to regain its original shape. (The extent towhich the shape is regained usually depends on the type of SMA, amount of defor-mation, and the material’s thermomechanical history.) When cooled again thematerial does not automatically revert to the deformed shape. This is called the one-way shape memory effect. The entire shape memory process can be repeated.

Austenite (a) Cooled to Form Twinned Martensite (b) without a Shape Change, then Deformed by MovingTwin Boundaries (c). Heating State (b) or (c) will Returnthe Original Austenitic Structure and Shape.[1]

(b) Twinned Martensite

DeformationCoolin

g

Heating

(a) Austenite (c) Deformed Martensite

THE ELECTRO- AND MAGNETO-RHEOLOGICAL EFFECTS

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MaterialE A S E

and contract in response to a change in the surrounding chemicalenvironment. The movement of water can be induced by a change inpH, electric field, temperature, light or other environmental condi-tions. Polyacrylonitrile (PAN) has been used in polymer gel systems,as it is a polyelectrolyte.[2] Another polymer gel activated by achange in pH includes a polyvinyl alcohol-polyacrylic acid (PVA-PAA) component.

Ionic polymer metal composites (IPMCs) are made up of a per-fluorinated membrane (meaning all of the hydrogen atoms attachedto carbon atoms are replaced with fluorine atoms) with noble metalplating (typically gold or platinum) for electrodes.[3] The ionic poly-mers used are typically either Flemion® (a perfluorocarboxylate) orNafion® (a perfluorosulfonate) which swell in water due to their ionicand hydrophilic nature. Anions are located on the polymer mem-brane, while cations are present and freely moving in the fluid. Whenan electric field is applied to the IPMC, the cations diffuse toward thenegative electrode, which causes the composite to deform.

Conductive polymers (CP) experience a volume change when theyare subjected to an electrochemical oxidation or reduction processcalled doping. Polyacetylene, polyaniline (PANi), nylon, polypyrrole(PPy) and polythiophene (PT) are examples of conductive polymers.For these to function, an electrolyte must be present in order to supplythe ions. The application of a small electric field will prompt the elec-trochemical reactions causing the polymer to become conductive orinsulative, thus causing the ions and solvent to migrate either into orout from the polymer matrix resulting in the volume change.

Although carbon nanotubes are not polymers, they are consideredto be the most recent addition to this list of electroactive actuators.[3, 4] Carbon nanotubes, known to exhibit sensing and actuatingcapabilities, are sheets of graphite one atom thick that are rolled intocylinders forming a structure that has a very small diameter (on theorder of a nanometer) but a length greater than a micrometer.Applying a voltage to a carbon nanotube structure residing in an elec-trolyte will cause it to bend achieving strains up to 0.2%.[3] Ions fromthe electrolyte build up around the carbon nanotube surface and thecharge injection causes the bond lengths between carbon atoms tochange, thus causing the dimensions of the nanotube to change. Withdimensional changes in individual nanotubes being coherent theeffect is seen macroscopically also.

Electrorheological and Magnetorheological Fluids

Electrorheological (ER) and magnetorheological (MR) fluids experi-ence a nearly instantaneous change in their rheological propertiesupon the application of an electric or magnetic field, respectively. (Seeaccompanying article in this issue.) This change is reversible andoccurs also nearly instantaneously upon the removal of the appliedfield. The physical changes can be quite substantial, turning a low viscosity fluid into a much more viscous, almost-solid substance.

These special fluids typically consist of a dispersed, polarizableparticulate phase suspended in a carrier fluid. While ER and MR fluids can use the same type of carrier fluid (e.g. high electrical resis-tivity, low-viscosity substance such as a hydrocarbon or silicone oil),

The electrorheological effect occurs in an ER fluid when anelectric field is applied causing the uniformly dispersedsolid particles to become polarized. Once polarized, theybegin to interact with each other, and form chain-likestructures, parallel to the electric field direction, connectingthe two electrodes (see figure below). Upon further intensi-fication of the electric field, the chains begin to form thick-er columns. A dramatic change in the suspension’s rheo-logical properties is associated with this change in its struc-ture. The columnar particle structures give the fluid agreater yield stress. Upon removing the electric field, theparticles lose their polarization and return to their freelyroaming state. The period of time over which these eventsoccur is on the order of milliseconds. The magnetorheo-logical effect is similar to the ER effect, but obviously,instead of an electric field, a magnetic field is applied topolarize the particles.

(a) No Electric Field (b) Applied Electric Field

Field direction

the particulate phase is different. For ER fluids the dispersed particles arecommonly metal oxides, aluminosilicates, silica, organics, or polymers.[5]MR fluids use ferromagnetic or paramagnetic solid particles. Activators, sur-factants, and other additives are also commonly included in both types offluids to improve their properties.

ER and MR fluids are mostly considered for use in damping applica-tions. Specific applications include exercise equipment, valve, braking andclutch systems, as well as in vibration control and shock absorbing systems.

Electroluminescent and Photoluminescent Materials

Luminescence is the property of emitting energy in the form of light (having a wavelength in the visible spectrum). The process of luminescenceoccurs when a material’s valence electrons are excited to higher energy levels by some stimulus, then fall back to lower energy levels giving off pho-tons, or light. Luminescent properties can be classified by the type of inputenergy used to excite electrons to higher energy levels. Thermoluminescent,electroluminescent and photoluminescent materials’ electrons are excitedby heat, applied electric field, and incident photons, respectively.

Electroluminescence (EL), in basic terms, is the emission of lightinduced by an applied electric current. It is not the same phenomenon asincandescence used in standard light bulbs, which generates heat. EL,rather, is a much more efficient process since it is the direct conversion ofenergy from an electric current into light. Electroluminescent and photo-luminescent (PL) materials include inorganic as well as organic materi-als. Often the inorganic materials are made up of a combination of ele-ments from the II, III, V, and VI groups on the periodic table. Some of theinorganic base materials that have been studied include GaAs, CdS, InP,and GaN.

Electroluminescence in polymers has received much attention recent-ly. They can be fabricated to emit red, yellow or blue light, with blue lightbeing the most difficult to achieve. Some examples of conjugated (alsoknown as conducting) electroluminescent polymers include poly(alkyl-fluorenes), polyparaphenylene (PPP), and poly(p-pyridine) (PPy).[6]Other electroluminescent polymers include polythiophenes (PT), poly(p-phenylene vinylene) (PPV), poly(p-phenylene), and polyfluorenes.[7]Polythiophenes and PPVs can exhibit electroluminescence from blue tothe near-infrared depending on the side group. Copolymers with an elec-troactive segment of poly(p-phenylene benzobisthiazole) (PBZT), exhibit-ed photoluminescence with wavelengths representing blue to red visiblelight.[8, 9] In general, conjugated polyquinolines can be modified to emitlight having a wavelength anywhere in the visible spectrum.[7]

The most common application of an electroluminescent material is ina light emitting diode/device (LED). LEDs consist of an electroluminescentmaterial sandwiched between two electrodes, one of which is transparentallowing the transmittance of the emitted light.

Electrochromic

When an electric current is passed through an electrochromic material its optical properties change such that the material has a significantly different appearance. That is, the absorptive, reflective, or transmissiveproperties are altered enough to cause a change in its color, reflectivity or transparency.[10]

The mechanism relies on electrochemical reactions (oxidation/reduc-tion) involving a transfer of electrons and ions between electrodes and theelectrochromic material. When an electric current is applied and electronsand ions are being exchanged from one electrode to the other, there is anamount of incident energy, with wavelengths in the visible spectrum, whichcan be absorbed, therefore causing the material to change color and ineffect become less transmissive. The degree to which the color is changeddepends on the extent of the reaction, which is basically controlled by theapplied electric field. Oxides of the following metals exhibit electrochromicproperties: tungsten, molybdenum, titanium, manganese, vanadium,ruthenium, iridium, rhodium, and niobium.[11]

WO3, which turns from clear to blue, is the most studied electrochromicmaterial. [11] The reaction is rather slow, but the coloration remains for asignificant amount of time after the electric current is removed.[12] V2O5changes from favoring the red end of the spectrum to the blue end. Lithiumnickel oxide, LixNi1-xO (x is typically 0.3-0.5), turns from being clear into aneutral gray color.[13]

The electrochemical oxidation of polyaniline can change it frombeing transparent to green, then blue, and then purple. Upon electro-chemical reduction this color scheme is reversed, and thus it is an electrochromic material. Polydiphenylamine (PDPA) is another elec-trochromic polymer.[10]

Electrochromic materials can be used to create a smart windows capa-ble of reducing their transmittance by at least 65% in less than a minutewhen an electric field is applied . Once the electric field is removed the effectremains, but it is reversible when the current is reversed.[14]

SUMMARY

Major sensor and actuator technologies as well as current state-of-the-artmaterials for those technologies have been presented in this MaterialEASE.The use of sensor and actuator materials is continuing to grow in anincreasing effort to optimize operating systems. Pertinent examples include:• Pyroelectric sensors offer infrared detection capabilities used for night

vision and motion detectors.• Piezoelectric materials offer both sensing and actuating applications for

pressure detection and micropositioning applications respectively.• Electrostrictors and magnetostrictors offer non-linear micropositioning

capabilities.• Shape memory alloys offer temperature sensing and a high strain

response (but with a slow response time.)Smart structures built with these materials will allow for more flexibilityand greater functionality over conventional materials systems in variousapplications.

There are numerous unexplored avenues for the application of sensingand actuating materials, but their development is still far from complete.Continued research on the materials themselves will spawn new as-yet-unknown applications, and will also provide improved performance inexisting applications.

Although these special materials will not simply replace conventionalhydraulic, pneumatic and electric motor actuators altogether, the new actuators may eventually be implemented more widely than their long-established counterparts, as they have a high energy density, large band-

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width, and simple packaging. Traditional actuators can be large andcomplex systems, adding weight and various additional costs to theapplication. Sensor and actuator materials can decrease manufactur-ing, operating and maintenance costs, reduce weight and improve sys-tem performance. Therefore, when designing new systems, these mate-rials should be given proper consideration as they present a very legit-imate alternative to conventional systems.

REFERENCES

[1] Metals Handbook, 10th Edition, Vol. 2: Properties and Selection:Nonferrous Alloys and Special-Purpose Materials, ASM International,1990; DTIC Doc. AD-D450 152[2] W.W. Gibbs, “Artificial Muscles,” Scientific American, 1996.Available at http://www.sciam.com/[3] Electroactive Polymer [EAP] Actuators as Artificial Muscles:Reality, Potential, and Challenges, edited by Y. Bar-Cohen, SPIE – TheInternational Society for Optical Engineering, The Society of Photo-Optical Instrumentation Engineers, 2001[4] Y. Bar-Cohen, “Nanotechnology Using Electroactive Polymers asArtificial Muscles”, MST news, International Newsletter onMicrosystems and MEMS, Vol. 3/01, 2001, pp. 45-46[5] S. Ramamurthy, M.V. Gandhi, and B.S. Thompson, SmartMaterials for Army Structures, Quantum Consultants, Inc.,Michigan 1992; DTIC Doc. AD-A300 215[6] D.D. Gebler, Y.Z. Wang, J.W. Blatchford, S.W. Jessen, L. –B. Lin, T.L.Gustafson, H.L. Wang, T.M. Swager, A.G. MacDiarmid, and A.J. Epstein,“Blue Electroluminescent Devices Based on Soluble Poly(p-pyri-dine),” Journal of Applied Physics, The American Institute of Physics,1995; DTIC Doc. AD-A297 041[7] X. Zhang, A.S. Shetty, and S.A. Jenekhe, “Electroluminescence

and Photophysical Properties of Polyquinolines,” University ofRochester, 1999; DTIC Doc. AD-A366 991[8] X.L. Chen and S.A. Jenekhe, “Electroactive and PhotoactiveNanostructured Materials from Self-Assembling Coil-Rod-CoilTriblock Copolymers,” Polymer Material Science and Engineering,Vol. 73, 1995, pp. 304-305; DTIC Doc. AD-A314 776[9] J.A. Osaheni, and S.A. Jenekhe, “Copolymers: Design, Synthesis,and Supramolecular Regulation of Photophysical Properties,”Journal of the American Chemical Society, 1995; DTIC Doc. AD-A294 825[10] L.H. Dao, M.T. Nguyen, J.W. Chevalier, T.N. Do, J.Y. Bergeron,and R. Paynter, “Processible, Stable, Conducting Polymers forElectrochromic Display and Window Applications,” ANTEC ‘91Conf. Proc., Society of Plastics Engineers and Plastics Engineering,1991, pp. 861-863[11] C.G. Granqvist, “Electrochromic Materials: Metal OxideNanocomposites with Variable Optical Properties,” Materials Scienceand Engineering, A168, Elsevier Sequoia, 1993, pp. 209-215; DTICDoc. AD-D258 149[12] P. Judeinstein and J. Livage, “Electrochromic Properties of Sol-Gel Derived WO3 Coatings,” Sol-Gel Optics, Vol. 1328, SPIE, 1990; pp.344-351[13] S.J. Wen, K. von Rottkay, and M. Rubin, “Electrochromic LithiumNickel Oxide Film by Pulsed Laser Deposition,” ElectrochemicalSociety Meeting Symposium on Electrochromic Materials and TheirApplication III, Lawrence Berkeley National Laboratory, 1996[14] J.C.L. Tonazzi, B. Valla, M.A. Macedo, P. Baudry, and M.A.Aegerter, “Characterization of an all Solid-State ElectrochromicWindow,” Sol-Gel Optics, Vol. 1328, SPIE, 1990, pp. 375-390; DTICDoc. AD-P006 442

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