ContentslistsavailableatScienceDirect
Journal of Materials Science & Technology
jou rn a l h o m e p a g e :w w w . j m s t . o r g
Research Article
A flexible and high temperature tolerant strain sensor of La 0.7 Sr 0.3 MnO 3 /Mica
Min Guo
a, Cheng Yang
a, Dong Gao
a, Qiang Li
a, Aihua Zhang
a, Jiajun Feng
c, Hui Yang
a, Ruiqiang Tao
a, Zhen Fan
a, Min Zeng
a, Guofu Zhou
b,c, Xubing Lu
a,∗, J.- M. Liu
a,daInstituteforAdvancedMaterialsandGuangdongProvincialKeyLaboratoryofOpticalInformationMaterialsandTechnology,SouthChinaAcademyof AdvancedOptoelectronics,SouthChinaNormalUniversity,Guangzhou510006,China
bGuangdongProvincialKeyLaboratoryofOpticalInformationMaterialsandInstituteofElectronicPaperDisplays,SouthChinaAcademyofAdvanced Optoelectronics,SouthChinaNormalUniversity,Guangzhou510006,China
cNationalCenterforInternationalResearchonGreenOptoelectronics,SouthChinaNormalUniversity,Guangzhou510006,China
dLaboratoryofSolidStateMicrostructuresandInnovationCenterofAdvancedMicrostructures,NanjingUniversity,Nanjing210093,China
a r t i c l e i n f o
Articlehistory:
Received14August2019 Receivedinrevisedform 13September2019 Accepted8October2019 Availableonline8January2020
Keywords:
Flexiblesensor Mica La0.7Sr0.3MnO3
Hightemperature Multimodalsensing
a b s t r a c t
Flexiblesensorshavebeenwidelyinvestigatedduetotheirbroadapplicationprospectsinvariousflexible electronics.However,mostofthepresentlystudiedflexiblesensorsareonlysuitableforworkingat roomtemperature,andtheirapplicationsathighorlowtemperaturesarestillabigchallenge.Inthis work,wepresentamultimodalflexiblesensorbasedonfunctionaloxideLa0.7Sr0.3MnO3(LSMO)thin filmdepositedonmicasubstrate.Asastrainsensor,itshowsexcellentsensitivitytomechanicalbending andhighbendingdurability(upto3600cycles).Moreover,theLSMO/Micasensoralsoshowsasensitive responsetothemagneticfield,implyingitsmultimodalsensingability.Mostimportantly,itcanworkin awidetemperaturerangefromextremelowtemperaturedownto20Ktohightemperatureupto773K.
TheflexiblesensorbasedontheflexibleLSMO/micahetero-structureshowsgreatpotentialapplications forflexibleelectronicsusingatextremetemperatureenvironmentinthefuture.
©2020PublishedbyElsevierLtdonbehalfofTheeditorialofficeofJournalofMaterialsScience&
Technology.
1. Introduction
Flexibleandwearableelectronicproductshaveattractedenor- mousattentions due to theirpotential applicationsin artificial intelligencerobot,wearablehealthcaretechnologiesandportable personal electronics compared with conventional rigid silicon based electrons. The core components of building flexible and wearableelectronicdevicesarevariouskindsofflexiblesensors, suchasstresssensors[1–3],photoelectricsensors[4,5],magnetic fieldsensors[6,7]andtemperaturesensors[8,9].Flexiblestrain sensorhasbeenthemostwidelystudiedflexiblesensor, which canconvertvarious mechanicalsignals intoelectricalsignals in thesurroundingenvironment,indicatingpotentialapplicationin motiondetection[10,11],lifeactivitymonitoring[12,13],andarti- ficialintelligence[14,15].Zhongetal.reportedamultifunctional electronicskinspreparedbyalignedcarbonnanotubesinflexible polymercomposites[16].Thisstrainsensorexhibitshighaccuracy
∗Correspondingauthor.
E-mailaddress:[email protected](X.Lu).
inthemeasurementofbendingscale,lowenergyconsumption(<10
W)andexcellentbendingstability, demonstratingacapability forportablemotiondetectors.Parketal.developedhighlyflexi- ble,scalableandsensitivestrainsensorsbasedonsilvernanowire networkcompositesandpolydimethylsiloxane(PDMS)elastomer sandwichstructures[17].Theydemonstratedapracticalapplica- tionofthestrainsensorsbymakingglovesfixedwithfivestrain sensorsforfingermovementdetectionandvirtualcharactercontrol invirtualenvironments.Wangetal.presentedaflexiblevessel-like sensorconsistingofbraidedcottonhosesubstrate,carbonnano- tubes(SWCNTs)/ZnO@polyvinylidenefluoride(PVDF)arraysand flexiblePVDFfibrousmembrane[18]. Itcandetectphysicalsig- nalssuchastemperature,strainandfrequency,implyingpotential applicationsinwearableorportablephysicalsensing.
Theabovementionedflexiblestrainsensorsaremainlycom- posed of two parts. One is the active layer and the other is theflexiblesubstrate.Theactivelayermainlyconsistsofcarbon nanotubes [19,20], conductive polymers [21,22], nanomaterials [23,24],etc.TheflexiblesubstratemainlyconsistsofPDMS[25,26], polyethyleneterephthalate[13,27],PVDF[2,28]andotherpolymer materials[20,24].Amongthem,flexiblesubstratesaremainlypoly- https://doi.org/10.1016/j.jmst.2019.10.019
1005-0302/©2020PublishedbyElsevierLtdonbehalfofTheeditorialofficeofJournalofMaterialsScience&Technology.
M.Guoetal./JournalofMaterialsScience&Technology44(2020)42–47 43
merswithlowmeltingpoint(generallylessthan300◦C)whichcan notwithstandhightemperatureandbecomebrittleatlowtemper- ature.Theflexibleelectronicsreportedatpresentarenotsuitable forhightemperatureenvironment,whichlimitsitsapplicationin aerospace,metallurgicalindustry,undergroundenergyexploration andotherextremehightemperatureenvironments.
Inorder tofabricateflexible strainsensor forhightempera- tureapplications,our ideais to finda flexible substrate which canwithstandhightemperature.Mica(KAl2(Si3Al)O10(OH)2)isan inorganicmaterialwithlayeredstructure.Athinmicasheet(about 100m)exhibitsexcellentflexibilityandhightransmittanceinthe ultraviolet-visible-infraredlightrange[29].Itshighmeltingpoint (1150−1300K)enablesittowithstand700◦C hightemperature environment.Therefore itisbelieved tobeanexcellentflexible substratematerialforhightemperatureenvironment.
Inthiswork,weadoptfunctionaloxideLSMOthinfilmandhigh temperatureresistantflexiblesubstratemicatoconstructaflexible strainsensorwhichcanworkathightemperature.Althoughseveral functionoxideshavebeendepositedonMicasubstratewithhigh quality,suchasCoFe2O4[30],La0.67Sr0.33MnO3[31],VO2[32]and MoO2 [33].Noworkhasbeendoneforstrainsensorapplication especially workingathightemperature. In ourwork,wefabri- catedLSMO/Micahetero-structurebypulsedlaserdepositionfor flexiblestrainsensorapplication.Thisstrainsensorexhibitsgood flexibleperformancewithexcellentbendingproperties,outstand- ingresistanceenduranceatroomtemperature.Moreover,itcan workwellinawidetemperaturerangebetween20Kand773K.
Theresponse ofthesensor tothemagnetic fieldhasalsobeen studied,witharelativelylargemagnetoresistancechangeof32%
at20Kunder2Tmagneticfield,implyingitsmultimodalsensing ability.
2. Experimental 2.1. Preparation
Fig. 1(a) shows the detailed preparation processes of La0.7Sr0.3MnO3/Micahetero-structure. The polycrystallineLSMO filmsweredepositedonmica(001)substratethroughpulsedlaser depositionusingacommercialLa0.7Sr0.3MnO3 ceramictarget.A KrFexcimerlaser(=248nm),operatedat2Hzrepetitionrateand afluenceof∼2.91J/cm2wasusedtoablatethetarget.Agrowth temperatureof650◦Candanoxygenpartialpressureof10Pawere adoptedduringthegrowthoftheLSMOthinfilm.Afterdeposition, thefabricated sampleswere subsequently cooleddownwith a temperature decreasing rate of 1◦C/min in 1mbar of pure O2 ambient.Ptelectrodeswith100nmthicknessweregrownat10−4 Pa oxygenpressure on4corner oftheLSMOfilm bya shadow maskforelectricalmeasurement.
2.2. Characterization
CrystalstructuresoftheLSMOthinfilmswerecharacterizedby X-raydiffraction(XRD)usingaPANalyticalX’PertProdiffractome- terwithCu K␣radiation. Surfacemorphologieswere measured byatomicforcemicroscope(AFM)(CypherAsylumResearchLtd.).
Cross-sectionalimageofMicawerecharacterizedbyfieldemis- sionscanningelectronmicroscopy(FE-SEM,ZEISS-Ultra55).The electricaltransportandmagnetoresistancepropertywereinves- tigatedbyVanderPauwmethodatatemperaturerangefrom20K to300Kusingaphysicalpropertymeasurementsystem(PPMS9, QuantumDesign).TheresistancevaluemeasuredbyVanderPauw methodwasdefinedbythesensedvoltagedividedbythesensing current.
Themechanicalbendingpropertieswerecharacterizedbyusing aself-madetest systemcomposedof aKeysightB2902A source meter,astepping-motorandcontrolsoftware.Thesampleresis- tance was measured by monitoring its voltage by applying a constantsensingcurrentwhen bendingthesensorbystepping- motor.
3. Resultanddiscussion
It is wellknown that mica hasa layeredstructure and the adjacentlayersareattractedbyVanderWaalsforcesinsteadof chemical bonds,soitis convenienttogain athin andflatmica sheet.Fig.1(b)showsatypicalSEMimageforacleavedmicasheet, indicatingalayeredstructureanda4.47mthickness.Thesurface morphologyofLSMOthinfilmonmicawascharacterizedbyAFM.
AsshowninFig.1(c),theLSMOthinfilmshowsasmoothsurface, andtherootofmeansquareisaround1.62nmovera5m×5m scanningarea.Fig.1(d)showstheXRD-2scanningpatternsof theLSMO/Mica hetero-structure,whichrevealsapolycrystalline structureoftheLSMOthinfilm.
Theresistancechangesunderdifferentbendingstateareinves- tigatedbyVanderPauwfour-pointmethod.Duringthetest,one endofthedeviceisfixedandtheotherendisdriventomovebya steppingmotor.Consequently,differentbendingstateswerereal- ized.Resistancesofthedeviceinbendingandunbendingstatesare definedasRbandR0,respectively.Thereal-timeresistancechange isdefinedasR=Rb-R0,thenthereal-timeresistancechangerate canbedefinedasR/R0=(Rb-R0)/R0.Fig.2(a)showstheresistance changerateofthedeviceasafunctionofbendingradius.Theinsetof Fig.2(a)showsaschematicdiagramoftheresistancemeasurement setupduringbendingstate,thesensingcurrentdirectionissameto thebendingdirection.TheR/R0showsanincreasemonotonically withthedecreaseofthebendingradius.Atthebendingradiusof 3mm,aR/R0of5.0%canbeobserved.Fig.2(b)showstheinstanta- neousresistancechangeratewithcontinuousbending/unbending cycles. Theinsetof Fig.2(b)shows a singlebending/unbending cycle,inwhichthebendingradiuschangeswiththesequenceof flat→10mm→8mm→5mm→8mm→10mm→flat.After6bend- ing/unbendingcycles, theresistancestateat eachflatstateand differentbendingstatesnearlyremainconstant,indicatinggood repeatabilityandstabilityofthedeviceduringthevariousbend- ingstates.Fig.2(c)depictsalongtimeresistancefatiguetestof thedeviceundervariousbending/unbendingcycles.Thetimefrom bendingtounbendingfor onecycleis 8s.It wasclearlyshown thattherewasnoperformancedeteriorationafter3600bending- unbendingcycles.After8hoflong-termbending/unbending,the changeinR/R0ofthedeviceonlyshowsareductionbylessthan 0.5%.TheR/R0valuesinthebeginning,intermediate,andending stagesareveryconsistentandrepeatable,asshownintheinsetof Fig.2(c),implyinggoodstabilityandreliabilityofthepresentstrain sensor.
Fig.3(a)showsthetemperaturedependenceofresistancefor the LSMO thin filmsat differentbending radius. As the bend- ingradiusdecreased,theresistanceofLSMOthinfilmincreased.
Moreover,thestrainsenseofthedevicecanbeobservedwithin awidetemperaturerangefrom100Kto300K.Fig.3(b)depicts thetime dependentresistanceoftheLSMO thinfilmat 20Kat differentbending states. For theresistances observed atdiffer- entbendingradius,allofthemnearlyremainconstantafter10h measurementat 20K,suggestinga goodstabilityoftheflexible strainsensorevenatextremelowtemperature.Thehightemper- aturetoleranceoftheflexiblestrainsensorhasalsobeenstudied.
Fig.3(c)showsthebendingradiusdependentresistancemeasured atdifferentelevatedtemperatures.Forthedeviceunderthesame bendingstate,theresistancedecreaseswiththeincreaseof the
Fig.1. (a)SchematicdiagramofpreparationprocessesofLa0.7Sr0.3MnO3/Micahetero-structure;(b)cross-sectionalSEMimageofthemicasheet;(c)AFMsurfacemorphology oftheLa0.7Sr0.3MnO3filmonmica;(d)XRDpatternofLa0.7Sr0.3MnO3/micahetero-structure.
Fig.2. (a)Resistancechanges(R/R0(%))upondifferentbendingradius(theinsetshowsaschematicdiagramoftheresistancemeasurementsetupduringbendingstate);(b) theinstantaneouschangeoftheresistanceunderthreedifferentbendingradius;(c)repetitivemeasurementoftheresistancechangesover8hbendingtimeunderbending radiusof8mm.thethreetopfiguresshowsometypicalcyclesattheinitial,intermediate,andendingstagesofthetestingprocess,respectively.
M.Guoetal./JournalofMaterialsScience&Technology44(2020)42–47 45
Fig.3.(a)Temperaturedependentresistancesunderdifferentbendingradiusofcurvature;(b)time-dependentresistancesunderdifferentbendingstatesat20K;(c) resistanceasafunctionofbendingradiusofcurvatureatdifferenthightemperatures;(d)temperature-dependentresistancechangesunderdifferentbendingstates.
ambienttemperature,showingtypicalsemiconductorconducting characteristic.Nevertheless,forthestrainsensoratthesameele- vatedtemperature,theresistanceincreaseswiththedecreaseof curvatureradius,showingthesamechangetendencyasthatat roomtemperature.ThepresentresultsprovethattheLSMO/Mica strainsensorcanworknotonlyatroomtemperaturebutalsoat hightemperaturesuptoatleast773K.Fig.3(d)showsthequan- titativevaluesofresistancechangingrate(R/R0)asafunction ofambienttemperaturesatdifferentcurvatureradius.Compared withroom temperature,the magnitudeof resistancechange is moreobviousathightemperature,whichindicatesthatthedevice is more sensitive to bending action at high temperature. The present LSMO/Mica strain sensors show working temperatures of 773K and 100K, which are much higher/lowerthan that of previouslyreportedflexiblestrainsensors[9,34],indicatingtheir greatapplicationpotentialsinthefieldofharshelectronicsinthe future.
Multimodalsensingwithasinglesensorwashighlydemanded infutureflexibleelectronics.ConsideringthatLSMOisawellknown materialwithmagnetoresistance(MR)effect,wealsocarriedout thestudyonthemagneticresistanceandmagneticsensingability ofLSMOthinfilmsinourwork.Fig.4(a)showsthetemperature dependentresistanceforthefilmin0Tand1Tmagneticfields.
NoticeableMReffectcanbeclearlyobserved.Theresistancemea- suredwith1Tmagneticfieldisclearlysmallerthanthatmeasured withoutmagneticfield, especially atlow temperature.Fig. 4(b) showsthemagneticfielddependentMR%forthefilmatdiffer- enttemperatures.TheMRchangesaregenerallydefinedasMR%= {[R(H,T)−R(0,T)]}/R(0,T)×100%,whereR(H,T)andR(0,T)arethe temperaturedependentresistancevaluesunderappliedfieldand zerofield,respectively.TheMReffectincreasedobviouslyasthe temperaturereducesfromroomtemperaturetolowtemperature,
whichisconsistentwithwhathasbeenreportedresults[35,36].
ThemaximumvalueofMRisabout32%undertheappliedfieldof2T measuredat20K.Fig.4(c)showsthemagneticresistanceofLSMO thinfilmwithdifferentbendingradiusat20K,whichindicatesthat clearMReffectcanbeobservedevenatbendingstates.Further- more,whenthetemperatureincreasesto300K,theMReffectstill remainsforbothflatandbendingstates.Thepresentresultsshown inFig.4implythattheLSMO/Micahetero-structurecanworkfor amultimodalsensortomeasurestrainandmagneticsignalwitha singledevice.
Basedontheaboveshownresults,wecarriedoutexperiments tomeasurethestrainandmagneticsignalbyusingthesamedevice structureofLSMO/Mica.TheLSMO/Micastrainsensorswerefirst pastedonglovestomonitorthemovementoffingerjoint.Fig.5(a) showsthetimedependentresistancechange(R/R0)ofthestrain sensor.Ineachbendingmotion,thefingersarebenttothesame positionandreleasedafterholdingfor2s.Itcanbeseenthatthe sensorisverysensitivetothebendingactionofthefinger,andthe resistancecanbequicklyrestoredtoitsoriginalvalueafterreleas- ingduringtherepeatedbending/releasingcycles.Forthemagnetic signalmeasurement,weadoptedacommercialNd2Fe14Bperma- nent magnet asmagnetic field source.The measurement setup wasschematicallyshownintheinsetofFig.5(b).Thesensorwas arrangedtomovetowardthemagnetforsomedistance,andthen movebacktotheoriginalposition.Duringthemovement,theresis- tancewasin-situmonitored.Thesameprocesswasrepeatedfor severaltimes,and thedistancemoving forwarddecreaseswith theincreaseofthecyclenumber.Thetimedependentresistance changeduringthemovingforward/movingbackwardcycleswas shown inFig.5(b). Itcanbeseen thatthesmallerthedistance betweenthesensorandthemagnet,thehigherthemagneticfield intensity,andthemoreobvioustheresistancechanges.Whenthe
Fig.4.(a)Temperaturedependenceoftheresistanceat0Tand1Tmagneticfield;(b)magneticfielddependentMR%atdifferenttemperaturesfortheLSMOthinfilm;
resistance-magneticfieldcharacteristicsofLSMOthinfilmsunderdifferentbendingradiusofcurvatureat20K(c)and300K(d).
Fig.5.(a)Resistancechangeofthesensorduringthefingerbending/unbendingcycles(theinsetshowstheactualphotographoftheLSMO/Micasensorfixedonfingers duringthebendingandunbendingstates);(b)timedependentMRchangeupondifferentmagneticfieldintensityordistancebetweenmagnetandsensor(theinsetisa schematicdiagramofthemeasurementsetupforthemagneticsignal).
distancebetweenthesensorandthemagnetis1mm,theresistance changeratecan reach2.5%,indicating a clearmagneticsensing ability.
4. Conclusion
Insummary,wehavefabricatedaflexiblesensorcomposedof functionaloxideLSMOdepositedonflexiblemicasubstrate.Asin- gleLSMO/Micasensorshowsmultimodalsensingabilitytodetect strainaswellasmagneticsignals.Asastrainsensor,atthebend- ingradiusof3mm,aR/R0of5.0%canbeobserved,indicatinga goodsensitivity.After3600bending/unbendingcycles,thechange inR/R0ofthedeviceonlyshowsareductionbylessthan0.5%,
indicatingexcellent endurability.Asamagnetic sensor,amaxi- mumMRchangeof32%canbeobservedundertheappliedfield of2Tmeasuredat20K.Mostimportantly,theLSMO/Micaflexi- blesensorcanworkinawidetemperaturerangebetween20K and773K,demonstratingitsgreatapplicationpotentialinextreme harshenvironments.
Acknowledgements
This work was supported financially by the National Natu- ralScienceFoundation ofChina(No.51872099),theProjectfor GuangdongProvinceUniversitiesandCollegesPearlRiverScholar Funded Scheme (2016), the Guangdong Innovative Research
M.Guoetal./JournalofMaterialsScience&Technology44(2020)42–47 47
Team Program (No. 2013C102), the Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (No.2017B030301007)andScienceand TechnologyProgramof Guangzhou(No.2019050001).
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