Encyclopedia of Aerospace Engineering. Part 2

2072 Pages·2016·207.97 MB·English

by  Blockley R.

Electroactive Polymers (EAP) as Actuators
for Aerospace Engineering
Yoseph Bar-Cohen
JetPropulsionLaboratory,CaliforniaInstituteofTechnology,CA,USA
activatable polymers. In addition, some of the EAP mate-
1 Introduction 1 rialshavethereverseeffectofconvertingmechanicalstrain
2 HistoricalPerspective 1 toelectricalsignalallowingtheiruseforsensorsandenergy
harvesting mechanisms. Applications of EAP materials for
3 TheElectroactivePolymers(EAP)Groups 2
biologicallyinspiredmechanisms,afieldwhichisalsoknown
4 TheArmWrestlingChallengeforEAPActuators 5
as biomimetics (Bar-Cohen, 2005; Bar-Cohen and Hanson,
5 ApplicationsofEAP 6
2009),enableuniquepossibilitiesthatwerepreviouslyimag-
6 Summary 7 inableonlyinsciencefiction.Mostlysincetheearly1990s,
Acknowledgment 7 therehasbeenemergenceofthemanyEAPmaterialtypesand
References 7 theyarethesubjectofextensivestudies(Bar-Cohen,2004).
Since these materials are still relatively new it is neces-
sarytoestablishtheirscientificandengineeringfoundations,
which includes improving the understanding of the basic
1 INTRODUCTION principlesthatdrivethem(Bar-Cohen,2004).Theserequire
developingeffectivecomputationalchemistrymodels,com-
Polymershavemanyattractivepropertiesfornumerousappli- prehensive material science, electro-mechanic analytical
cationsandtheyincludemechanicalflexibility,lightweight, tools, and material processing techniques. In order to max-
andareeasytoprocess.Inaddition,somepolymerschange imize the actuation capability and operation durability,
theirpropertiesinresponsetoelectric,chemical,pneumatic, effectiveprocessingtechniquesarebeingdevelopedfortheir
optical,ormagneticstimulationmakingthemusefulforac- fabrication, shaping, and electroding. Methods of reliably
tuation applications with unique capabilities. The polymers characterizingtheresponseofEAPmaterialsarebeingdevel-
thatcanbeexcitedbyelectricalstimulationarethemostcon- opedandeffortsareunderwaytocreatedatabaseswithdoc-
venient to use and easy to implement and they are known umented material properties (http://www.actuatorweb.org/).
as electroactive polymers (EAP). The electrical stimulation Bringing these materials to the level of making daily used
of mechanical deformation enables important possibilities products will necessitate finding a niche application that
where the increasing improvements of their response make addressescriticalneeds.
these materials as one of the most attractive among the
2 HISTORICAL PERSPECTIVE
Roentgen(1880)conductedthefirstdocumentedexperiment
with EAP materials – he subjected electric field across a
EncyclopediaofAerospaceEngineering,Online©2010JohnWiley&Sons,Ltd.
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DOI: 10.1002/9780470686652.eae234
2 ActiveMaterials
rubber-band with fixed end and a mass attached to the free available,theauthorestablishedawebsitethatprovidesfab-
end.Eguchi(1925)isattributedwiththenextimportantmile- rication procedures for the leading types of EAP materials
stone contribution to the field. He discovered a piezoelec- and also a website about sources for materials and samples
tric polymer called electret when carnauba wax, rosin, and (http://eap.jpl.nasa.gov).
beeswaxweresolidifiedbycoolingwhilesubjectedtoaDC
biasfield.Electretsgeneratevoltagewhensubjectedtostress
andhavethereversebehaviorofbeingdeformedunderelec-
3 THE ELECTROACTIVE POLYMERS
tricfield.Theyareinsulationmaterialsthatcanholdelectric
(EAP) GROUPS
charges after being polarized in an electric field. Since the
electricallygeneratedstraininelectretsisverylowforoperat-
There are many EAP material types that are known today.
ingasactuatorstheirapplicationhasbeenlimitedtosensors.
Depending on their activation mechanism, these types of
ThefollowingmilestoneinthefieldofEAPwasreportedby
polymers were divided by the author into two groups:
Kawai(1969)withhisobservationofsignificantpiezoelec-
electronic (also known as the field activated) and ionic
tricactivityinPolyvinylideneFluoride(PVDF).Thisbreak-
(Bar-Cohen,2004).
through was preceded by Fukada’s work on piezoelectric
biopolymers (Fukada, 1955). Still the generated strain has
(cid:1)
been limited to fraction of percent. Investigators continued Electronic (also known as field-activated) EAP: These
toexaminepolymersystemsforsignificantelectroactivere- materials are activated by Coulomb force that causes
sponseandmanyoftheeffectivematerialsthatemergedwere either direct thickness reduction (e.g., dielectric elas-
reported mostly in the 1990s (Bar-Cohen, 2004). Among tomers) or dimension change due to internal polariza-
these materials the dielectric elastomer has reached the tionwithmolecularalignment(ChengandZhang,2008).
highest strain exceeding 100% strain with a relatively fast Generally, a high voltage (>10-V(cid:1)m−1) is required,
responsespeed(<0.1s). whichmaybeclosetotheelectricbreakdownlevel,and
Starting his research in the field of EAP in 1995, the itistheresultofthelowdielectricconstantinpolymers.
author realized that in order to accelerate development of To reduce the required high voltage two alternative ap-
EAPmaterialsandleadtoeffectiveactuatorsitiscriticalto proaches are used: (i) stacking thin multilayers to reach
have worldwide cooperation in this field. For this purpose, therequiredthickness;and(ii)formingacompositema-
heinitiatedvariousformsofinformationexchange.Thisin- terialusingfillermaterialwithahighdielectricconstant.
cludesthefirstEAPActuatorsandDevices(EAPAD)Con- Sincethematerialisactivatedbytheelectricfield,there-
ferencethatwasheldfrom1March1999to2March1999, sponseisquitefastandcanreachmilliseconds.Examples
as part of the SPIE smart structures and materials sympo- ofthesematerialsaredielectricelastomers,piezoelectric,
sium. This conference marked an important milestone for and ferroelectric. The advantages of the electronic EAP
thefieldbringingthesematerialstotheattentionofscientist materialsincludeholdingthegenerateddisplacementun-
andengineersworldwide.Inrecentyears,agrowingnumber deraDCvoltagemakingthemattractiveforroboticappli-
ofworkshops,meetings,andconferenceshavebeeninitiated cations.Further,theseEAPmaterialshaveagreaterme-
with sessions on EAP. For information archive and links to chanicalenergydensitythantheionicEAPandtheycan
EAPwebsitesworldwidetheauthorcreatedtheWorldWide be activated in air with no major constraints. However,
EAP(WW-EAP)Webhub(http://eap.jpl.nasa.gov).Also,in their main disadvantage is the required high activation
June 1999 he started publishing the semi-annual WW-EAP fieldthatcanbeclosetotheelectricbreakdownlevel.
(cid:1)
newsletter that includes short inputs from authors world- IonicEAP:Asactuationmaterialtheyconsistofapolymer
wideprovidingasnapshotoftheadvancesinthefield.This filmwithtwoelectrodesandelectrolyteandtheiractiva-
newsletter is published electronically and its issues are ac- tion mechanism involves mobility of ions under electri-
cessible from the above-mentioned WW-EAP webhub. For calexcitation(Parketal.,2008).ExamplesofionicEAP
comprehensiveinformationaboutthefield,theauthoredited materials include ionomeric polymer-metal composites,
andco-authoredareferencebookonEAP(Bar-Cohen,2004). conductivepolymers,carbonnanotubes,andionicpoly-
This text-book covers the capabilities, potentials, and chal- mer gels. These polymers have the advantages of need-
lengestothefieldincludingdescriptionofthevarioustypes ing low activation voltage (1–2V) and generating large
ofEAPmaterials,analyticalmodels,processingtechniques, bendingdisplacement.Theirdisadvantagesaretheneedto
characterizationmethodsandapplicationsincludingcurrent maintainelectrolyteswetness,lowconversionefficiency
andexpected.EAPmaterialsarestillmostlycustommadeby (∼1%),anddifficultiessustainingconstantdisplacement
researchers.Inordertohelpinmakingthesematerialswidely under activation of a DC voltage (except for conductive
EncyclopediaofAerospaceEngineering,Online©2010JohnWiley&Sons,Ltd.
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DOI: 10.1002/9780470686652.eae234
ElectroactivePolymers(EAP)asActuatorsforAerospaceEngineering 3
polymers).TheslowresponseoftheseEAPgroupinthe purpose.Applicationofanelectricfieldresultsinastrainthat
rangeoftenstoafractionofasecondisattributedtothe is proportional to the square of the electric field and to the
relatedmacroscopicmotionofions. dielectricconstantwhileinverselyproportionaltotheelastic
modulus. Practically, to apply the required electric field at
ThetypesofEAPineachofthetwomaterialgroupsare thehighvoltagelevels(100V(cid:1)m−1andabove)necessitates
brieflydescribedinthenextsection. usingthinfilmsanditwasshowntogeneratestainlevelsas
highas380%(Pelrineetal.,2000).
Theappliedelectricfieldcausesthicknesscontractionand
3.1 Electronic/Field-activatedEAP lateralexpansion.Inordertoproduceanactuator,scientists
atSRIInternationalrolledtwoelastomerlayerswithcarbon
Dielectricelectroactivepolymers(DEAP). Polymerswith based electrodes on both sides of one of the layers form-
low elastic stiffness and high dielectric breakdown strength ingacylindricalactuator.Furthermodificationsoftheirac-
that are subjected to an electrostatic field generate a large tuator design led to the development of the multifunctional
actuationstrain.ThistypeofEAPisincreasinglybecoming electroelastomer roll (MER). For this actuator, highly pre-
the actuator of choice for engineers and EAP producers. In strained dielectric elastomer films are rolled around a com-
termsoftechnologyreadinessthesematerialshavetheleast pression spring (Pei et al., 2004). One of the concerns in
issuesassociatedwiththeunderstandingoftheiroperationor using dielectric elastomers is the required high voltages for
theproductionprocedures. its activation, which is close to the breakdown strength of
Thefirstobservationofthefactthatdielectricelastomers thematerial.Forthisreasonasafetyfactorisrequiredlower-
sustain large strain (23% in silicone films) when subjected ingtheactuationcapabilitythatcouldbeobtained.Another
to high electric field was reported in a 1992–1993 study by concernistherequiredprestrainingthatisreleasedovertime
Pelrine and his co-investigators (Pelrine et al., 2000). The duetocreepthatdegradestheactuatorperformance.Recent
resultsofthisstudyledtosuggestingtheiruseforEAPactu- research (Ha et al., 2006) have shown potential for elimi-
ationmechanisms.Independently,in1994,itwasreportedby nating the requirement for prestrain by using an interpen-
Zhenyiandhisco-investigatorsthattheymeasured3%strain etrating polymer network (IPN) where tension in the net-
inpolyurethanewhensubjectedafilmtoa20V(cid:1)m−1electric workisbalancedbycompression.ToformtheIPNthermally
field(Zhenyietal.,1994).Significantlevelsofstrainstarted crosslinkableliquidadditiveswereusedincludingadifunc-
tobeobservedintheyearsthatfollowedandin1998alevel tionalacrylate(e.g.,HDDA)andatrifunctionalacrylate(e.g.,
ofstrainof30%wasreportedbeingmeasuredusingsilicone TMPTMA).Alternatively,designsoftheactuatorconfigura-
(Pelrineetal.,2000).Amajormilestoneinthedevelopment tion have shown promise too and these include the use of
wasdocumentedin2000when,usingAcrylicasadielectric folded film structure as reported by scientist from the Uni-
elastomer and preload, Pelrine and his co-investigators re- versityofPisa,Italy(Figure2)(CarpiandDeRossi,2007).
portedstrainsmuchhigherthan100%(Pelrineetal.,2000). Stackingofthousandsofthinlayersofadielectricelastomer
Today, there are many researchers who are using dielectric atEMPA,Switzerland,formedaneffectiveactuatorthatgen-
elastomers as EAP materials and significant progress has eratescontractionanddoesnotrequirepreload(Kovacsand
been made towards making practical actuators (Cheng and Du¨ring,2009).Further,developmentatPolyPower,Danfoss,
Zhang,2008). Denmark,whereadielectricelastomerfilmwasdesignedin
AnactuatorthatismadeofdielectricelastomerEAPcan a wave configuration led to a process that is easy to mass
berepresentedbyaparallelplatecapacitorasshownschemat- produce(Figure3)(KiilandBenslimane,2009).
icallyinFigure1(Pelrineetal.,2000).Theelectrodeshaveto
behighlycompliantinordertoavoidimpedingthegenerated Ferroelectric polymers. The phenomenon of piezoelec-
strainandconductivecarbongreasearewidelyusedforthis tricity is found in noncentro-symmetric materials and it
is called ferroelectricity when a non-conducting crystal or
dielectricmaterialexhibitsspontaneouselectricpolarization.
Piezoelectricitywasdiscoveredin1880byPierreandPaul-
JacquesCurieobservingvoltagebeingproducedonthesur-
faceofsuchcrystalmaterialswhentheyarecompressedalong
certainaxes.Polymersalsoexhibitferroelectricbehaviorand
Figure1. Underelectricfieldadielectricelastomerwithcompliant
themostwidelyknownmaterialsarethePolyvinylideneflu-
electrodesonbothsurfacesbecomesthinnerandexpandslaterally
allowingvariousactuatordesigns.ReproducedfromPelrineetal. oride and its copolymers (Chang and Zhang, 2008). These
(2000)(cid:1)c AmericanAssociationfortheAdvancementofScience. polymers,whicharealsoknownasPVDForPVF2,arepartly
EncyclopediaofAerospaceEngineering,Online©2010JohnWiley&Sons,Ltd.
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DOI: 10.1002/9780470686652.eae234
4 ActiveMaterials
Figure 3. A dielectric elastomer with wave shape film made by
PolyPower, Danfoss, Denmark, is shown lifting 10kg (the red
cylinders).ReproducedwithpermissionfromKiilandBenslimane
(2009)(cid:1)c SPIE.
subjected to a large electric-field with strain levels of
∼4% (Su et al., 1999). Grafted-elastomer EAP was devel-
oped at NASA Langley Research Center by Su and his
co-investigators (Su et al., 1999). This type of EAP ma-
Figure2. ContractileactuatorusingdielectricelastomerEAPwith terials produces a relatively high electromechanical power
foldedfilmstructure.ReproducedwithpermissionfromCarpiand density and they are relatively easier to process compared
DeRossi(2007)(cid:1)c SPIE. to the ferroelectric EAP. It consists of a flexible backbone
macromolecule and a grafted polymer that forms a crys-
talline structure. The grafted crystalline polar phase pro-
crystallinewithaninactiveamorphousphase.Theirrelatively
vides moieties in response to an applied electric field and
highYoung’smodulus(1–10GPa)providesarelativelyhigh
cross-linkingsitesfortheelastomersystem.Severalcompo-
mechanical energy density at stain levels that are mostly a
sitions of ferroelectric-electrostrictive molecular composite
fractionofapercent.In1998,Zhangandhisco-investigators
systems were produced by combining an electrostrictive-
introduceddefectsintothecrystallinestructureusingelectron
grafted elastomer and a piezoelectric polyvinylidene
radiationtoincreasethedielectricconstantofthecopolymer
fluoride-trifluoroethylene copolymer. Careful selection of
PVDF-TrFE(ChengandZhang,2008).Theresultingmate-
thepolymercompositionallowsforoptimizationwithrespect
rialgeneratesstrainsaslargeas5%undervoltagesofabout
to its electrical, mechanical, and electromechanical proper-
150V(cid:1)m−1.Thedrawbacktotheirradiationistheintroduc-
ties. The produced combinations can be operated both as
tionofmanyundesirabledefectsincludingtheformationof
piezoelectricsensorandelectrostrictiveactuator.
crosslinkings, and chain scission. This issue was addressed
byproducingterpolymersviamoleculardesignthatenhances
thedegreeofconformationalchangesatthemolecularlevel 3.2 IonicEAP
in the polymer. The resulting terpolymer generates higher
electromechanicalresponsethanthehigh-energyelectronir- Ionomeric polymer-metal composites (IPMC). IPMC is
radiatedcopolymer(ChengandZhang,2008). anEAPthatbendsinresponsetoanelectricalactivationre-
sultingfrommobilityofcationsinthepolymernetwork(Park
Electrostrictive graft elastomers. Graft-elastomer EAP et al., 2008; Oguro, Kawami and Takenaka, 1992). A rela-
is a polymer that exhibits electrostriction behavior when tively low voltage at the level of 1V stimulates bending of
EncyclopediaofAerospaceEngineering,Online©2010JohnWiley&Sons,Ltd.
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DOI: 10.1002/9780470686652.eae234
ElectroactivePolymers(EAP)asActuatorsforAerospaceEngineering 5
IPMC, where the base polymer provides channels for mo- injection of large charges that affect the ionic charge bal-
bilityofpositiveionsinafixednetworkofnegativeionson ancebetweentheNTandtheelectrolyte.Themorecharges
interconnectedclusters.Electrodesonthesurfaceareusedto areinjectedintothebondthelargerthedimensionchanges.
supplytherequiredelectricalfieldandthepolaritydetermines Removalofelectronscausesthenanotubestocarryanetpos-
thedirectionofbendingandevenfractionofavoltleadsto itivecharge,whichisspreadacrossallthecarbonnucleicaus-
response.Twotypesofbasepolymersarewidelyusedtoform ingrepulsionbetweenadjacentcarbonnucleiandincreasing
IPMC:perfluorosulfonate,alsoknownasNafion® (DuPont, the C C bond length. Injection of electrons into the bond
USA) and perfluorocarboxylate, also known as Flemion® alsocauseslengtheningofthebondresultinginanincrease
(AsahiGlass,Japan).Inordertoelectrodethepolymerfilms, in nanotube diameter and length. These dimension changes
metalions(platinum,goldorothers)aredispersedthrough- aretranslatedintomacroscopicmovementinthenetworkel-
out the hydrophilic regions of the polymer surface and are ementofentanglednanotubesandthenetresultisupto1%
subsequently reduced to zero-valence metal atoms. Gener- extension of the CNT. Such actuators have the potential of
ally, the response of IPMC is relatively slow (<10Hz) be- producingaveryhighworkpercyclecomparedtoothertype
cause of the fact that ions need to physically travel though ofactuators.However,suchenergydensitiesandforceshave
thematerial.TheioniccontentofIPMCdeterminesthespeed notbeenrealizedyetinmacro-scaledevices.
oftheelectromechanicalresponse(Bar-Cohen,2004),where
Li+cationsthataresmallexhibithighermobilityandfaster Ionicpolymergels(IPG). IPG,suchaspoly-acrylonitrile,
bending response. The displacement of IPMC was further aregenerallyactivatedbyachemicalreactionwherechang-
increased using gold-metallization as a result of the higher ing from an acid to an alkaline environment causes the gel
electricconductivity. tobecomedenseorswollen,respectively.Thisreactionwas
stimulated electrically by researchers at the University of
Conductive polymers (CP). Operating conductive poly- Arizona, USA (Calvert, 2008). When activated, these gels
mers as EAP is done using reversible counter-ion inser- bend as the cathode side becomes more alkalinic and the
tionandexpulsionthatoccursduringredoxcycling(Smela, anode side more acidic. However, because of the need to
2008).Oxidationandreductiontakesplaceattheelectrodes migrate ions through the gel, the response of this multilay-
causingvolumechangeduemainlytoexchangeofionswith eredgelstructureisrelativelyslow.Asignificantamountof
an electrolyte. An actuator is produced by creating a sand- researchanddevelopmenteffortsandapplicationconsidera-
wichoftwoconductivepolymerelectrodes(e.g.,polypyrrole) tionsusingionicgelpolymerswereexploredattheHokkaido
with an electrolyte between them. Voltage applied between University,Japan(OsadaandKishi,1989).
theelectrodescausesoxidationattheanodeandreductionat
thecathodeandtheelectricchargeisbalancedbymigration
4 THE ARM WRESTLING CHALLENGE
ofionsbetweentheelectrolyteandtheelectrodes.Theadded
ions cause swelling of the polymer while their removal re- FOR EAP ACTUATORS
sultsinshrinkageandthusbendingthesandwich.Thelayers’
thicknessdeterminesthespeedoftheresponseandthethinner The field of EAP is still in emerging state but in recent
the layers the faster the reaction allowing to reach levels of yearsenormousadvanceshavebeenmadewithrespecttothe
40Hzbutgeneratinglowerforce.Conductivepolymeractu- understanding of the various operation mechanisms, meth-
atorsrequirevoltagesintherangeof1–5Vandthespeedin- ods of producing and testing as well as potential applica-
creaseswiththevoltage.Conductivepolymeractuatorsoffer tions.Inanefforttopromoteworldwidedevelopmenttowards
relativelyhighmechanicalenergydensitiesofover20Jcm−3 therealizationofthepotentialofEAPmaterials,theauthor
butwithlowefficiencies(∼1%)ifnoelectricalenergyisre- posed in 1999 an armwrestling challenge (http://ndeaa.jpl.
covered. The shear forces that act on the electrolyte layer nasa.gov/nasa-nde/lommas/eap/EAP-armwrestling.htm).The
causeprematurefailureafterasshortasseveraltensofcycles. challengeistohaveanEAPactivatedroboticarmwinagainst
human in a wrestling match and a graphic rendering of the
Carbonnanotubes(CNT). Theuseofcarbonnanotubesas challenge icon is illustrated in Figure 4. The emphasis on
EAPwasfirstreportedin1999(Quetal.,2008).Theactua- armwrestlingwithhumanwaschoseninordertousetheintu-
tionmechanismisattributedtothechangeinlengthcarbon- itivecomparisonofourmusclesasabaselineforgaugingthe
carbon bond in nanotubes (NT), which are suspended in an performanceadvances.SuccesswillallowapplyingEAPma-
electrolyte, when subjected to electric field. A network of terialstoimprovemanyaspectsofourlifewheresomeofthe
conjugated bonds connects all carbons and provides a path possibilitiesincludeeffectiveimplantsandprosthetics,active
for the flow of electrons along the bonds. The electrolyte clothing,realisticbiologicallyinspiredrobotsaswellasfab-
formsanelectricdoublelayerwiththenanotubesandallows ricatingproductswithunmatchedcapabilitiesanddexterity.
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DOI: 10.1002/9780470686652.eae234
6 ActiveMaterials
Figure4. Theiconofthegrandchallengeforthedevelopmentof
EAPactuatedrobotics.
The first armwrestling match was held in San Diego,
California, USA, on 7 March 2005 as part of the EAP-in-
Action Session of the SPIE’s EAPAD Conference. Three
roboticarmsparticipatedinthecontestagainsta17-yearold
Figure6. 4-fingerEAPgripperliftingarock.
high school female student and the student won against all
thesearms(seeFigure5).
The second artificial muscles Armwrestling Contest was theactionliftedviathecablea0.5kgweight.Todetermine
heldon27February2006,whereratherthanwrestlingwith thewrestlingspeed,thetimetolifttheweightfromthebottom
a human opponent the contest consisted of measuring the to the top section of the fixture was measured. To establish
arm’s performance and comparing the results. The measur- abaselineforperformancecomparison,thecapabilityofthe
ingfixture(developedjointlybyindividualsfromUCLAand studentwhowasthehumanwrestlerin2005wasmeasured
theauthor’sgroupatJPL)wasstrappedtothecontesttable firstandthenthethreeparticipatingroboticarmsweretested.
andtheEAPactuatedarmsweretestedforspeedandpulling Theresultshaveshowntwoordersofmagnitudelowerper-
force.Eachcompetingarmpulledonthefixturecablethathas formance of the arms compared to the student. Due to the
aforcegaugeonitsotherend.Tosimulatewrestlingaction, significant weakness of the current state of the art there are
nocurrentnearfutureplanstoholdanotherwrestlingmatch
againsthuman.However,ifimprovementswillbringalevel
that would allow potential winning, a professional wrestler
willbeinvitedforthenexthuman/machinewrestlingmatch.
5 APPLICATIONS OF EAP
SignificanteffortsareunderwaytodeveloppracticalEAPac-
tuatorsandcommercialproductsarestartingtoemerge.The
mechanismsanddevicesthatarebeingconsideredordevel-
oped are applicable to many fields. Eamax, Japan, is cred-
ited for developing the first EAP related commercial prod-
uct,andattheendof2002itannouncedthemarketingofa
biomimetic device in the form of a fish robot (Bar-Cohen,
2004). Today, various prototypes of potential commercial
products are being developed including zoom lens of cel-
Figure5. AnEAPdrivenarmmadebystudentsfromVirginiaTech lular phones, valves, energy harvesting, pumps, and many
andthehumanopponent,17-yearoldstudent. others. EAP actuators offer many important capabilities for
EncyclopediaofAerospaceEngineering,Online©2010JohnWiley&Sons,Ltd.
Thisarticleis©2010USGovernmentintheUSand©2010JohnWiley&Sons,Ltdintherestoftheworld.
ThisarticlewaspublishedintheEncyclopediaofAerospaceEngineeringin2010byJohnWiley&Sons,Ltd.
DOI: 10.1002/9780470686652.eae234
ElectroactivePolymers(EAP)asActuatorsforAerospaceEngineering 7
of percents and this development brought great attention to
these materials. EAP materials offer attractive characteris-
tics of operation similarity to biological muscles, including
resilience,damagetolerance,andabilitytoinducelargeac-
tuationstrains.EventhoughEAPactuatorsstillhavelimited
capability there has already been a series of reported suc-
cessesindrivingnovelmechanisms.Mostoftheconsidered
applications are still far from being practical for daily ap-
plications. If EAP materials are developed to operate inter-
Figure 7. Blimp with fins steering (made by EMPA) demon- nal organs inside a human body, this technology can make
stratedattheEAP-in-ActionSessionoftheSPIE’sEAPAD2008in
tremendouslypositiveimpactonmanyhumanlives.
SanDiego,CA.
Turning EAP materials to actuators-of-choice requires
well-establishedscientificandengineeringfoundations,that
devices with biomimetic characteristics (Bar-Cohen, 2004;
is,infrastructure.Thisinvolvesimprovingtheunderstanding
Bar-Cohen, 2005). One may produce such devices as arti-
ofthebasicprinciplesthatdrivethevariousEAPmaterials.
ficial bugs that may walk, swim, hop, crawl, and dig while
Also,itisnecessarytodevelopacomprehensivematerialsci-
reconfigurethemselvesasneeded.Mimickingnaturewould
ence,aswellaseffectiveelectro-mechanicsanalyticaltools
immenselyexpandthecollectionandfunctionalityofrobots
andmaterialprocessingtechniques.Researchersarecontinu-
allowingperformanceoftasksthatareimpossiblewithexist-
allymakingimprovementsinthevariousrelatedfieldsinclud-
ingcapabilities(Bar-CohenandBreazeal,2003;Bar-Cohen,
ingthebetterunderstandingoftheoperationmechanismsof
2005;Bar-CohenandHanson,2009).Forexample,theauthor
thevariousEAPmaterialtypes.Theprocessesofsynthesiz-
andhisco-investigatorsconstructedaminiaturegripperwith
ing,fabricating,electroding,shaping,andhandlingarebeing
four IPMC-based fingers as bending actuators (Bar-Cohen,
refinedtomaximizetheactuationcapabilityandrobustness.
2004).Thisgrippergrabbedrocksverysimilartothehuman
Methodsofreliablycharacterizingtheresponseofthesema-
handusinghooksatthebottom-emulatedfingernails.Other
terials are being developed and efforts are being made to
recently reported robotic applications include a blimp with
establish database with documented material properties in
steeringfinsthatwasmadebyEMPA(Figure7).
order to support design engineers that are considering the
Space applications are among the most demanding in
useofthesematerials.
termsoftheharshnessoftheoperatingconditions(extreme
The application of these materials as actuators involves
temperatures,highpressureorvacuum)andtheyrequirevery
many fields of science and engineering including materials
highreliabilityanddurability.Today’savailablematerialsare
science, chemistry, electromechanics, computers, and elec-
notapplicabletohandletherequiredeffectiveactuationma-
tronics. Using EAP to replace existing actuators may be a
terials that can operate down to as low as −200◦C as on
difficultchallengeandthereforeitishighlydesirabletoiden-
Europa and Titan or +460◦C as on Venus. Another chal-
tifyanicheapplicationwhereEAPmaterialswouldnotneed
lenge is the need for large scale EAP in the form of films,
tocompetewithexistingtechnologies.
fibersandothers.Therequireddimensionscanbeaslargeas
severalmetersorkilometerstoproducelargegossamerstruc-
turessuchasantennas,solarsails,andvariouslargeoptical
components.MakingbiomimeticcapabilityusingEAPma- ACKNOWLEDGMENT
terialwillpotentiallyallowNASAandotherspaceagencies
toconductmissionsinotherplanetsusinghumanlikerobots Some of the research reported in this manuscript was con-
thatemulatehumanoperationbeforesendingrealhumans. ducted at the Jet Propulsion Laboratory (JPL), California
InstituteofTechnology,underacontractwithNationalAero-
nauticsandSpaceAdministration(NASA).
6 SUMMARY
For many years, EAP materials received relatively little REFERENCES
attentionduetotheirlimitedactuationcapabilityatthelevel
offractionofpercentstrainandthesmallnumberofavailable
Bar-Cohen,Y.andBreazeal,C.(eds.)(2003)Biologically-Inspired
materials.Sincetheearly1990s,newmaterialshaveemerged Intelligent Robots, SPIE Press, Bellingham, Washington, vol.
that generate large strain ranging from several to hundreds PM122,ISBN:0-8194-4872-9,pp.1–393.
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DOI: 10.1002/9780470686652.eae234
8 ActiveMaterials
Bar-Cohen,Y.(ed.)(2004)ElectroactivePolymer(EAP)Actuators Oguro,K.,Kawami,Y.andTakenaka,H.(1992)BendingofanIon-
asArtificialMuscles–Reality,PotentialandChallenges,2ndedn, Conducting Polymer Film-Electrode Composite by an Electric
ISBN:0-8194-5297-1,SPIEPress,Bellingham,Washington,vol. StimulusatLowVoltage.Trans.J.MicromachineSoc.,5,27–30.
PM136,pp.1–765.
Osada, Y. and Kishi, R. (1989) Reversible volume change of mi-
Bar-Cohen, Y. (ed.) (2005) Biomimetics – Biologically Inspired croparticlesinanelectricfield.J.Chem.Soc.,85,665–662.
Technologies,CRCPress,BocaRaton,FL,ISBN:0849331633,
Park,I.-S.,Jung,K.,Kim,D.,S.-M.andKim,K.J.(2008)Physical
pp.1–527.
PrinciplesofIonicPolymer-MetalCompositesasElectroactive
Bar-Cohen,Y.andHanson,D.(2009)TheComingRobotRevolution ActuatorsandSensors,SpecialIssuededicatedtoEAP,Materials
–ExpectationsandFearsAboutEmergingIntelligent,Humanlike ResearchSociety(MRS)Bulletin,vol.33,no.3,pp.190–195.
Machines,Springer,NewYork,ISBN:978-0-387-85348-2
Pei,Q.,Pelrine,R.,Rosenthal,M.,Stanford,S.,Prahlad,H.andKo-
Calvert,P.(2008)GelSensorsandActuators,SpecialIssuededi- rnbluh,R.(2004)RecentProgressonElectroelastomerArtificial
catedtoEAP,MaterialsResearchSociety(MRS)Bulletin,vol. MusclesandtheirApplicationforBiomimeticRobots.Proceed-
33,no.3,pp.207–212. ingsofSPIEEAPADConference,vol.5385(ed.Y.Bar-Cohen),
Carpi, F. and De Rossi, D. (2007) Contractile folded dielectric pp.41–50.
elastomer actuators. Proceedings of the Electroactive Polymer
Pelrine,R.,Kornbluh,R.,Pei,Q.andJoseph,J.(2000)High-speed
ActuatorsandDevices(EAPAD)ConferenceIX,vol.6524(ed.
electrically actuated elastomers with strain greater than 100%.
Y.Bar-Cohen),SPIEElectronicLibrary.doi:10.1117/12.715594
Sci.,287(5454),836–839.
Cheng,Z.andZhang,Q.(2008)Field-ActivatedElectroactivePoly-
Qu,L.,Peng,Q.,Dai,L.,Spinks,G.M.,Wallace,G.G.andBaugh-
mers,SpecialIssuededicatedtoEAP,MaterialsResearchSociety
man, R.H. (2008) Carbon Nanotube Electroactive Polymers:
(MRS)Bulletin,vol.33,no.3,pp.190–195.
Opportunities and Challenges, Special Issue dedicated to EAP,
Eguchi,M.(1925)Onthepermanentelectret.Philos.Mag.,49,178. MaterialsResearchSociety(MRS)Bulletin,vol.33,no.3,pp.
Fukada,E.(1955)Piezoelectricityofwood.J.Phys.Soc.Japan,10, 215–234.
149–154. Roentgen, W.C. (1880) About the Changes in Shape and vol-
Ha, S.M., Yuan, W., Pei, Q., Pelrine, R. and Stanford, S. (2006) umeofDielectricsCausedbyElectricity,SectionIII,inAnnual
Interpenetrating polymer networks for high-performance elec- Physics and Chemistry Series, vol. 11 (ed. G. Wiedemann),
troelastomerartificialmuscles.Adv.Mater.,18,pp.887–891. JohnAmbrosiusBarthPublisher,Leipzig,Germanypp.771–786
(inGerman).
Kawai,H.(1969)Piezoelectricityofpoly(vinylidenefluoride).Jpn.
J.Appl.Phys.,8,975–977. Smela,E.(2008)ConjugatedPolymerActuators,SpecialIssueded-
icatedtoEAP,MaterialsResearchSociety(MRS)Bulletin,vol.
Kiil,H.-E.andBenslimane,M.(2009)ScalableIndustrialManu-
33,no.3,pp.197–204.
facturingofDEAP.ProceedingsofSPIEEAPADConferenceXI,
SmartStructuresSymposium,vol.7287,SPIEElectronicLibrary Su, J., Harrison, J.S., St. Clair, T., Bar-Cohen, Y. and Leary, S.
Paper7287-26. (1999) Electrostrictive graft elastomers and applications, MRS
Kovacs, G.M. and Du¨ring, L. (2009) Contractive Tension Force SymposiumProceedings,vol.600,Warrendale,PA,pp.131–136.
Stack Actuator Based on Soft Dielectric EAP. Proceedings of Zhenyi,M.,Scheinbeim,J.I.,Lee,J.W.andNewman,B.A.(1994)
SPIEEAPADConferenceXI,SmartStructuresSymposium,vol. Highfieldelectrostrictiveresponseofpolymers.J.PolymerSci.,
7287,SPIEElectronicLibraryPaper7287-8. PartB:PolymerPhys.,32(16),2721–2731.
EncyclopediaofAerospaceEngineering,Online©2010JohnWiley&Sons,Ltd.
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DOI: 10.1002/9780470686652.eae234
Materials in Space: An Introduction and Overview
Alan R. Chambers
SchoolofEngineeringSciences,UniversityofSouthampton,Southampton,UK
to be modeled. For example in space, radiation degrades
1 Introduction 1 materials and components and requires a materials/design
2 OperationalConsiderations 2 solution. To achieve this, the radiation environment in the
orbitofthespacecraftneedstobequantified,predictedand
3 TheSpaceEnvironment 2
understood.
4 SolarCells 6
The extreme environment of space has given birth to a
5 Conclusion 7
smallbutcrucialspacematerialsindustrywhichisconcerned
References 8 withdevelopingmaterialsforspacecraftoperations.Thissec-
tion aims to highlight the problems and the developments
whichhavebeenmadetosolvetheseproblems.
1 INTRODUCTION
Table1. Payloadmissiontypes.ReproducedfromFortescue,Stark
Itishardtoappreciatethatmanhasbeenoperatinginaspace
andSwinerd(2007)(cid:1)c JohnWiley&Sons,Ltd.
environmentforlittleover50years.SincethelaunchofSput-
nikin1957,theadvancesinspaceadventureandtechnology
Mission Trajectorytype
havebeenoutstanding.MissionstotheMoonandthedistant
planets attract public attention and capture the imagination Communications Geostationaryforlowlatitudes,
butmoresignificantlythelivesofallofushavebeenchanged MolniyaandTundraforhigh
byMan’sspaceactivitythroughthelaunchandoperationof latitudes(mainlyRussian),
ConstellationsofpolarLEO
satellites.Table1summarizessomeoftheimportantpayloads
satellitesforglobalcoverage
inspace,theirorbitsandfunction.
Earthresources PolarLEOforglobalcoverage
This section is concerned with the materials aspect
Weather PolarLEO,orgeostationary
of space missions. Engineering design and materials go Navigation InclinedMEOforglobal
together. Any component or structure is designed to a life coverage
and its interaction with the environment is key to achiev- Astronomy LEO,HEO,GEOand“orbits”
aroundLagrangepoints
ing this. Often the missing link in this algorithm is the
Spaceenvironment Various,includingsounding
measure of the environment which is needed if life is
rocketsandHEO
Military PolarLEOforglobalcoverage,
butvarious
Spacestations LEO
Technologydemonstration Various
Note:GEO,GeostationaryEarthorbit;HEO,highlyellipticalorbit;LEO,low
Earthorbit;MEO,mediumheightEarthorbit.
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DOI: 10.1002/9780470686652.eae235
2 MaterialsforSpaceApplications
2 OPERATIONAL CONSIDERATIONS 3 THE SPACE ENVIRONMENT
2.1 Launchandre-entry Theenvironmentaspacecraftexperiencesdependsuponthe
orbit. Some orbits are highly elliptical. The DERA STRV
Thefirstrequirementisthatthe“structure”surviveslaunch. satelliteslaunchedin1994operatedinGTOwithaperigeeof
The launch conditions are extreme involving high gravi- 200kmandanapogeeof30000km(Figure1).Thismeant
tational forces, vibrational loading and high temperatures. that in each orbit the satellite experienced a wide range of
Given the severity of the launch environment, the level of conditionsfromgaseoustovacuum,radiationfromVUVto
system/componenttestingishighinordertoensurethatpay- ionizingandpassedthroughdifferentdebrisenvironments.
load structural or component failure does not occur. This Some orbits such as geostationary are circular and at
is where design/materials interaction is so important. If a first sight it might be assumed that the orbital environment
structure survives the launch then it is unlikely to expe- is constant albeit hostile. This assumption would be incor-
rience any mechanical failure problem in the operational rect due to the fact that the environment is subject to space
phase. weathercausedbythechangingactivityofthesun.Increas-
Once launched safely into orbit, the requirement of the ingactivitywhichreleasesincreasedradiationisassociated
vehicleistomeetthemissiondesignlife.Forthepurposesof withincreasesinsunspotsandsolarflares.Thechangefrom
thissection,theveryrealissueofelectroniccomponentand maximum to minimum activity occurs in a regular 11-year
spacecraftsystemfailuresareignored.Instead,itisassumed cycle(Figure2).Fromthespacevehicles’perspectivedam-
thatthespacecraftwillsuffernoprematurefunctionfailures ageduetoenhancedradiationlevelsandenergiesisgreater
andthatlifewillbedeterminedbytheabilityofthespacecraft intheperiodsofhighactivity.
tosurvivethespaceenvironment. Inordertoselectmaterialsandtopredictlife,itisimpor-
In some instances such as the shuttle, the space vehicle tant to be able to measure the components of the space
isreturnedtoEarth.Thiscreatesyetanotherraftofmateri-
als and design issues; specifically in surviving the extreme
thermal conditions of re-entry into the Earth’s atmosphere.
Thedevelopmentoftheshuttleinsulationisamajormaterials
achievementwithoutwhichwewouldnothavetheInterna-
tionalSpaceStation.Spacecraftareoperatingattheirlimits
withlittleornomarginforerrorasdemonstratedbytheChal-
lengerdisaster.Re-entryisaspecialistareawhichisbeyond
thescopeofthiscontribution.
Forageneralintroductiontotheissuesinvolvedinspace-
craftdesign,operationandentryintotheatmosphereofEarth
andotherplanetsthereaderisreferredtoFortescue,Starkand
Swinerd(2007). Figure1. HighlyellipticalorbitofSTRV1.
2.2 Theoperationalphase
Vehiclesarelaunchedintospaceformanyreasons.Manyof
thesatellitesorbitingtheearthareforcommunicationspur-
posesandtheorbitstheyoperateindependontheirfunction.
For example, a position close to the Earth is attractive for
mobilecommunicationswherebothpowerandtimedelayare
importantconsiderations.Ontheotherhand,highorbitssuch
as geostationary have advantages regarding coverage. Thus
the choice of orbit and hence the environmental conditions
aredependentuponthefunctionofthemission.Examplesof
Figure2. Zurichsunspotactivityforthelasthundredyears.Repro-
thefunctionsofcommonspacemissionsandtheirorbitsare ducedfromFortescue,StarkandSwinerd(2007)(cid:1)c JohnWiley&
giveninTable1. Sons,Ltd.
EncyclopediaofAerospaceEngineering,Online©2010JohnWiley&Sons,Ltd.
Thisarticleis©2010JohnWiley&Sons,Ltd.
ThisarticlewaspublishedintheEncyclopediaofAerospaceEngineeringin2010byJohnWiley&Sons,Ltd.
DOI: 10.1002/9780470686652.eae235