碳纤维和碳纳米管的一篇综述

Elastomeric transparent capacitive sensors based on an interpenetrating composite of silver nanowires and polyurethaneWeili Hu, Xiaofan Niu, Ran Zhao, and Qibing PeiCitation: Appl. Phys. Lett. 102, 083303 (2013); doi: 10.1063/1.4794143View online: /10.1063/1.4794143View Table of Contents: /resource/1/APPLAB/v102/i8Published by the American Institute of Physics.Additional information on Appl. Phys. Lett.Journal Homepage: /Journal Information: /about/about_the_journalTop downloads: /features/most_downloadedInformation for Authors: /authorsDownloaded 09 Apr 2013 to 210.34.5.240. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: /about/rights_and_permissionsElastomeric transparent capacitive sensors based on an interpenetrating composite of silver nanowires and polyurethaneWeili Hu,1,2Xiaofan Niu,1Ran Zhao,1and Qibing Pei 1,a)1Department of Materials Science and Engineering,Henry Samueli School of Engineering and Applied Science,University of California,Los Angeles,California 90095,USA 2State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,Department of Materials Science and Engineering,Donghua University,Shanghai 201620,China(Received 12January 2013;accepted 18February 2013;published online 28February 2013)Highly flexible transparent capacitive sensors have been demonstrated for the detection of deformation and pressure.The elastomeric sensors employ a pair of compliant electrodes comprising silver nanowire networks embedded in the surface layer of polyurethane matrix,and a highly compliant dielectric spacer sandwiched between the electrodes.The capacitance of the sensor sheets increases linearly with strains up to 60%during uniaxial stretching,and linearly with externally applied transverse pressure from 1MPa down to 1kPa.Stretchable sensor arrays consisting of 10Â10pixels have also been fabricated by patterning the composite electrodes into X-Y addressablepassive matrix.VC 2013American Institute of Physics .[/10.1063/1.4794143]Future electronics are expected to be flexible,conforma-ble,and can take various form factors in order to realize aplethora of user-friendly applications such as expandable dis-plays,conformable photovoltaic sheets,and artificial skins.1–4Pressure sensors strategically placed on a flexible synthetic skin working in the pressure range of 1kPa to 1MPa would be able to detect fingertip texture,hand grip,and in-shoe pressures.5Future robots wearing such a flexible pressure-sensitive skin can “feel”a touch or pressure.An electronic skin could also be used for advanced prosthetic limbs or applied as surgical gloves to perform feel-real remote opera-tions.6Being stretchable and able to detect the pressure or de-formation at the same time would allow for feedback control.Patients with prosthetic hand or limbs equipped with such arti-ficial skins could completely regain dexterity and maneuver-ability.In consumer electronics,capacitive touch sensors found on most current smartphones and tablet computers do not differentiate pressure differences.The addition of pressure sensing could make the touch screen more intuitive for users.Replacing the touch screen with an electronic skin would sig-nificantly increase the amount of data input or the “livelihood”in each finger touch via pressure sensitivity.7High visual transparency is required for such applications.In these device applications,stretchable transparent elec-trode is an indispensable element.A variable capacitor using a pair of compliant electrodes sandwiching an elastomeric spacer can detect pressure or deformation in the change of ca-pacitance.8–12The Bao’s group recently reported a skin-like capacitive sensor based on single-walled carbon nanotubes (SWNTs)with detectable pressure of 50kPa and strain up to 50%.13This sensor,though not as sensitive to low pressures as their previously reported microstructured sensors,which could detect pressures generated by a fly weighing about 20mg,is stretchable and transparent.The transparency could expand the application scope,such as touch screens and elec-tronic skins with various options of skin color.Cohen et al.reported elastic strain gauges based on percolation SWNTcoatings.10The high sheet resistance of the SWNT coatings could limit the data speed and active area of these devices.The challenge still remains unresolved in developing an elas-tomeric electrode with low surface resistance and high visual transparency.Several methods have been reported in the literature for the preparation of stretchable electrodes.1,3,14–19However,none has simultaneously achieved high stretchability,high conductivity,high transparency,and a low-cost large-area fabrication.Here,we report the fabrication of a high-performance transparent electrode by embedding an ultrathin silver nanowire (AgNW)network in the surface layer of an elastomeric polyurethane (PU)matrix.The composite elec-trodes retain high surface conductance at tensile strains up to 60%,and can be stretched repeatedly with minimal loss of electrical conductivity under both slow and fast strain rates.High-performance stretchable transparent capacitors can be readily fabricated by sandwiching an acrylic elastomer layer between two AgNW-PU composite electrodes.The resulting variable capacitors can detect pressure and deformation in a wide range of stretching and pressure conditions.The preparation of the elastomeric transparent electro-des started with the formation of a conductive AgNW coat-ing on glass.19AgNWs with an average diameter of 60nm and an average length of 10l m were employed.Its disper-sion obtained from Seashell Technologies was diluted with methanol to a concentration of 2mg/ml,drop-cast on a pre-cleaned glass substrate,dried and annealed on a hotplate for 30min at 190 C to form a conductive AgNW coating.AgNWs in the network are uniformly distributed and ran-domly oriented over the entire coating area as shown in Figure 1(a).For the fabrication of capacitive sensor arrays,patterned AgNW coatings were deposited by spray-coating through a contact mask.The patterned AgNW coatings were annealed under the same condition as the un-patterned AgNW coatings.Urethane liquid rubber compounds (Clear Flex V R95,Smooth-On USA LLC)were mixed,degassed,and then drop-cast over the AgNW coatings on glass substrate.A seconda)Electronic mail:qpei@.0003-6951/2013/102(8)/083303/5/$30.00VC 2013American Institute of Physics 102,083303-1APPLIED PHYSICS LETTERS 102,083303(2013)sheet of glass was pressed onto the liquid coating.An adhe-sive tape with 340l m thickness was used as the spacer between the two sheets of glass to control the thickness of the sandwiched liquid layer.The urethane compounds were then cured at room temperature for 24h.The resulting AgNW-PU composite sheet was peeled off and cut into various sample sizes.The PU monomer deposited on the AgNW coating can infiltrate the pores in the network,but should not disrupt the conductive network.After curing,the resulting composite is a transparent sheet with a thickness of 340l m.The thin AgNW network is embedded in the surface of the PU sheet and imparts the surface with high conductivity.Figure 1(b)shows the SEM image of the conductive surface of the composite electrodes with a sheet resistance of 16X /sq.The AgNW net-work is clearly seen to be buried in the surface of the compo-sites and no voids are left on the composite surface,which implies a complete transfer of the nanowires from the original glass substrate.19The PU liquid monomer and polymer both contain -N(H)-groups,which bond to the nanowire surfaces.20The strong bonding between the PU matrix and the AgNW network plays an important role in the transfer of the nano-wires,and prevents the formation of voids between the nano-wires and the matrix.Figure 1(c)shows the transmittance spectra and photo-graph of AgNW-PU composite electrodes with various sheet resistances.The transmittance,inclusive of the PU substrate,approaches 82.7%at 550nm wavelength when the sheet resistance is 44.7X /sq,79.2%at 24.2X /sq,and 74.6%at 8.0X /sq.These sheet resistance values are almost the same as the original AgNW coatings on glass,which further proves the complete transfer of AgNWs.The values are 1–2orders of magnitude lower than typical SWNT percolation networks,thanks to the ability of silver to fuse at the nanowire intersec-tions.In the composite,the AgNWs on the surface form a con-ductive pathway and interconnect with the embedded AgNWs.In repeated Scotch tape test,the surface resistance remained unchanged,indicative of no nanowires being removed from the composite.The PU employed exhibits rubbery elasticity,and the composite retains this property.The composite sheets could be stretched reversibly up to 170%strain,and the stretched sheets relaxed back fairly rapidly to the original shape when the external load was removed.Changes of the surface resist-ance of the AgNW-PU composites under various tensile strains were measured at room temperature.The normalized resistance (R norm ),defined as the ratio of the instantaneous resistance at a specific tensile strain to the initial resistance at zero strain,is shown in Figure 2(a).The elongation was accompanied by an increase in the surface resistance R.At relatively small strains,the normalized resistance increases somewhat linearly with strain for samples with various initial sheet resistances.As strain is further increased,the R norm rises superlinearly.At 60%strain,the resistance has increased to roughly 5.4,8.0,11.9,14.3,and 20.1times the initial resistance for the films with initial sheet resistance values of 8.0,16.1,24.2,32.0,and 44.7X /sq,respectively.For conductive materials,whose bulk conductivities do not change with stretching,the R norm increases with strain due to geometrical changes during a uniaxial stretching fol-lowing the equation:21R norm ¼ð1þstrain Þ2:(1)As Figure 2(a)inset shows,R norm of the electrode with 8.0X /sq initial sheet resistance follows this equation fairly well until about 20%strain.This observation indicates that during small-strain stretching,the loss of nanowire interconnections is insig-nifiposite sheets with higher sheet resistance tend to exhibit lower stretchability.The relative motion or sliding dur-ing stretching could induce more significant loss ofnanowireFIG.1.(a)SEM image of a AgNW coating on a glass substrate.(b)SEM image of the conductive surface of AgNW-PU composite electrode with the sheet resistance of 16X /sq.(c)Transmittance spectra of AgNW-PU com-posite electrodes with sheet resistance specified.Inset:photographs of the composites with specified sheetresistance.FIG.2.(a)Normalized resistance of the composite electrodes as a function of tensile strain with specified initial sheet resistances.(b)Normalized resistance of the composite electrodes with the initial sheet resistances of 8X /sq during one stretching/releasing cycle at different stretching speeds.interconnections for the thinner AgNW networks,which thus causes larger increase of macroscopic resistance.Figure2(b)shows resistance changes during a full cycle of strain from0%!60%!0%at various strain rates.It can be seen that the sheet resistance of the composite electro-des increases with stretching speed.The resistance shows20.0times increase at the60%peak strain when stretched ata speed of1mm/s.At0.01mm/s,the resistance increase at the same peak strain is5.4times.It is unclear what causes such a strain rate effect.However,the resistance change is fully reversible:it recovers the initial value after being relaxed to0%strain.In practical applications,strain can be encountered at var-ious rates,and electronics should operate under all strain or stress conditions.The stability of the composite electrodes at different strain rates was thus performed.The transient R norm during cyclic stretching at different strain rates are shown in Figure S1.22The peaks of the profiles are the resistance at the strain peak(20%,40%,60%,or80%).Apparently,high stretching speed leads to high transient resistance,however, resistance change is quite reversible.The low resistance at the 0%strain state is retained even after200stretching cycles.Transparent variable capacitive sensors were fabricated by laminating two AgNW-PU composite electrodes,with the conducting surface facing inward,to sandwich a clear acrylic elastomeric dielectric spacer(3M TM VHB TM Tape4905or 3M TM Scotch924ATG Tape).Electrical contacts were made using a conductive copper tape onto each of the pat-terned AgNW lines.Capacitances were measured at1kHz frequency with a1V ac signal.The resulting capacitor is elastomeric and useful for strain and pressure sensing.The strain was applied as a uni-axial elongation using a stretching setup with precise length control.The(transverse)pressure was applied by a vertical load on the surface of the device.For cyclic stretching/ releasing tests,the composite strips were mounted onto a pair of tensile grips in a dynamic mechanical analyzer.The variable capacitor sensor samples had a15Â8mm2overlap-ping area between the opposite electrodes,and the dielectric spacer was0.25mm thick.The capacitance of a parallel plate capacitor is given byC¼e0e r Ad;(2)where e o is the permittivity of free space,e r is the permittivity of the dielectric,A is the overlapping electrode area,and d is the thickness of the spacer.The calculated capacitance of the samples at0%strain is25.84pF.Longitudinal stretching and transverse stress applied both cause an increase in capacitor area and reduction of the dielectric spacer,which in turn increase the capacitance.When the capacitor is stretched,it shrinks in width and thickness due to Poisson’s effect.With the assumption that the elastomers in capacitor are isotropic, its capacitance varies with the strain according to10D C¼aÃe lÃC0;(3) where e l is the longitudinal strain and a is the capacitive gauge factor.The gauge factor has a theoretical value of1 for a pure Poisson deformation,but is generally less than1 in experimental measurements.13,23As shown in Figures3(a)and3(b),the changes in capaci-tance exhibit a linear response to strains up to60%and pres-sure to1MPa.The capacitive gauge factor is0.5over the range of0%to60%strain for the stretchable devices.Varying the stretching speed has no discernible effect on the change of capacitance.The smallest change in capacitance that can be distinguished from instrument noise is$20kPa,which corre-sponds to humanfingertip texture and shape sensing.5 In order to study the stability of the devices,a capacitor was stretched to60%strain or pressed at1MPa and then released,and the process was repeated for10cycles.Figure 3(c)shows the transient capacitance change during the cyclic test.In each stretching cycle,the capacitance increases to1.3 FIG. 3.(a)Change of capacitance D C=C0versus strain.(b)Change of ca-pacitance D C=C0versus pressure.(c) Change in capacitance versus time over 10stretching/releasing cycles with60% stretching strain.(d)Change in capaci-tance versus time over10pressing/releas-ing cycles with1MPa pressure.times its original value and reverses back to the initial num-ber after released to0%strain.Figure3(d)shows a similar plot during cyclic testing under1MPa transverse pressure. The capacitance increase is51%of its original value.The re-versibility during both stretching and compressing tests is high in these cycles.For a typical measurement,a sample had a measured capacitance of25.84pF when unstretched, which increased to33.94pF when stretched to60%,and returned to25.85pF when released to0%strain.Capacitive sensor arrays were also fabricated to emu-late the spatial resolution of natural skin.A highly porous dielectricfilm(3M Scotch924ATG Tape)was used as the dielectric spacer in the variable capacitor arrays to increase sensitivity toward small pressures.The fabrica-tion process is illustrated in Figure4(a).The AgNW-PU composite electrodes were patterned into parallel conduc-tive stripes with1.5mm width and spaced by1mm.Two of such patterned electrodes in perpendicular orientation were used to laminate the porous dielectricfilm.Figures 4(b)and4(c)show photographs of an as-prepared capaci-tor array,which exhibits high optical transparency and mechanicalflexibility.The transmittance of the sheet at 550nm is78%in areas without AgNWs and72%in areas where the perpendicular AgNW lines overlap each other.Figure4(d)shows SEM image of a surface area with the upper half comprising patterned AgNWs.The AgNW pat-terns have a well-defined edge,which is important for the ca-pacitor pixels to have the same base capacitance before any external load is applied.The measured capacitances of pixels varied in the narrow range of9.8to11.5pF,with a mean av-erage of10.60pF.Figure4(e)shows a characteristic response of capaci-tance of a sensor pixel to transverse pressures up to100KPa. The curve is linear with a standard deviation R2of0.995. Therefore,this sensor can meet the application requirement in tactile robotics.5The inset of Figure4(e)shows that the lowest detectable pressure is about1kPa,which corresponds tofingerprint sensing.The crosstalk between adjacent pixels for this10Â10pixel pressure sensor was investigated by applying a pressure of30KPa to one pixel,and measuring the capacitance change of this as well as its neighboring pix-els.As shown in Figure4(f),the change of capacitance of the addressed pixel is at least eight times higher than the neighboring pixels.Overall,theflexible transparent capaci-tive sensors have sufficient spatial resolution for robotic sen-sory skins and touchpanels,and can detect in the pressure range of1kPa to100KPa to sense gentle touch,finger press, andfirm hand grip.The capacitance change described above is caused by geometrical changes of the variable capacitors.In cases where afinger or object touches on the top surface of the sensor which does not cause any geometrical deformation, the capacitance could still change if the fringing electricfield above the sensor is disturbed.For instance,when the contact material is an earthed conducting medium such asfinger or pencil,the capacitance would decrease because the charges are(partially)grounded.This leads to the change of the fringing capacitance directly above the capacitor.11We investigated the change of capacitance of each pixel as a function of the applied uniaxial stretching with or without a delicatefinger touch that did not cause any additional trans-verse deformation.As shown in Figure5(a),the sensor with an initial capacitance of10.76pF exhibits a linearresponse FIG.4.(a)The schematic illustration of the fabrication of a transparent capacitive array comprising an acrylic elastomer layer as the dielectric spacer between two transparent AgNW-PU composite elec-trodes.(b)Photograph of a pressure sen-sor array(10Â10pixels),each pixel being a square area of1.5Â1.5mm2and separated by1mm from each other.(c) Photograph of the sensor array bent at 180 .(d)SEM image of a surface area half of which comprises patterned AgNW-PU electrode.(e)Change of ca-pacitance D C=C0of one pixel with trans-versely applied pressure.(f)Mapping of the measured capacitance changes of pixels in the area where a pressure of 30KPa was applied on the central pixel.to strains up to 60%with a sensitivity (capacitance change divided by applied strain in %)of $0.05pF/%.The gauge factor is 0.5,which is in consistency with the results described above.When the user delicately touches the sur-face of the capacitive arrays,the capacitance dropped to 10.02pF.However,the sensor retains the response linearity with strain and the same gauge factor when it is simultane-ously stretched and touched.In alternative experiments,transverse pressures were applied on the surface of sensor,while it was stretched to various strains.The change of capacitance as a function of the transverse pressure remains linear at various horizontal,independently applied strains,while the sensitivity increases with the horizontal strain as shown in Figure 5(b).The pres-sure sensitivity increased from 0.8pF/100kPa at 0%strain (no horizontal stretching applied)to 1.2pF/100kPa at 60%strain.In conclusion,capacitive sensor sheets have been fabri-cated by employing highly compliant polymer composite elec-trodes and dielectric spacer.The sheets have a relatively simple,solid-state sandwich structure,exhibit good stretch-ability and transparency.The sheets can be stretched up to 60%strain and the corresponding capacitance change can be employed to detect the in-plane deformation.They can also detect transversely applied pressure in the range of 1–100KPa,which covers gentle finger touch,finger press,and firm hand gripping.The capacitive sensors can be operated reliably when they are pressed,bent,or stretched.The sheets can also be used for touch sensing which does not cause any physical de-formation,but disturbs the fringing electric field above the sen-sor.These multifunctionalities of stretch,pressure,and touch sensitivities,as well as the high mechanical compliancy of the sheets should find potential applications in prosthetic limbs,robotics,pressure-sensitive bandages,or touch screens for wearable electronics.The work reported here was supported by the Air Force Office of Scientific Research (FA9550-12-1-0074)and National Science Foundation (ECCS-1028412).Weili Hu thanks the “China Scholarship Council PostgraduateScholarship Program.”SEM was taken at the Scanning Probe Microscopy facility at the Nano and Pico Characterization Laboratory,California NanoSystems Institute,UCLA.1J.A.Rogers,T.Someya,and Y.G.Huang,Science 327,1603(2010).2D.H.Kim,N.S.Lu,R.Ma,Y.S.Kim,R.H.Kim,S.D.Wang,J.Wu,S.M.Won,H.Tao,A.Islam,K.J.Yu,T.I.Kim,R.Chowdhury,M.Ying,L.Z.Xu,M.Li,H.J.Chung,H.Keum,M.McCormick,P.Liu,Y.W.Zhang,F.G.Omenetto,Y.G.Huang,T.Coleman,and J.A.Rogers,Science 333,838(2011).3D.H.Kim and J.A.Rogers,Adv.Mater.20,4887(2008).4J.H.Ahn and J.H.Je,J.Phys.D Appl.Phys.45,103001(2012).5E.Pritchard,M.Mahfouz,B.Evans,S.Eliza,and M.Haider,in The 7th IEEE Conference on Sensors,Lecce,Italy,26-29October 2008,pp.1484–1487.6M.Ying,A.P.Bonifas,N.S.Lu,Y.W.Su,R.Li,H.Y.Cheng,A.Ameen,Y.G.Huang,and J.A.Rogers,Nanotechnology 23,344004(2012).7M.Ramuz,B.C.K.Tee,J.B.H.Tok,and Z.N.Bao,Adv.Mater.24,3223(2012).8T.Yamada,Y.Hayamizu,Y.Yamamoto,Y.Yomogida, A.Izadi-Najafabadi,D.N.Futaba,and K.Hata,Nat.Nanotechnol.6,296(2011).9C.Cochrane,V.Koncar,M.Lewandowski,and C.Dufour,Sensors 7,473(2007).10D.J.Cohen,D.Mitra,K.Peterson,and M.M.Maharbiz,Nano Lett.12,1821(2012).11D.P.J.Cotton,I.M.Graz,and cour,IEEE Sens.J.9,2008(2009).12S.C.B.Mannsfeld,B.C.K.Tee,R.M.Stoltenberg,C.V.H.H.Chen,S.Barman,B.V.O.Muir,A.N.Sokolov,C.Reese,and Z.N.Bao,Nature Mater.9,859(2010).13D.J.Lipomi,M.Vosgueritchian,B.C.K.Tee,S.L.Hellstrom,J.A.Lee,C.H.Fox,and Z.Bao,Nat.Nanotechnol.6,788(2011).14D.H.Kim,N.S.Lu,Y.G.Huang,and J.A.Rogers,MRS Bull.37,226(2012).15T.Akter and W.S.Kim,ACS Appl.Mater.Interfaces 4,1855(2012).16P.Lee,J.Lee,H.Lee,J.Yeo,S.Hong,K.H.Nam,D.Lee,S.S.Lee,and S.H.Ko,Adv.Mater.24,3326(2012).17Z.B.Yu,X.F.Niu,Z.T.Liu,and Q.B.Pei,Adv.Mater.23,3989(2011).18S.Yun,X.F.Niu,Z.B.Yu,W.L.Hu,P.Brochu,and Q.B.Pei,Adv.Mater.24,1321(2012).19W.Hu,X.Niu,L.Li,S.Yun,Z.Yu,and Q.Pei,Nanotechnology 23,344002(2012).20P.Jain and T.Pradeep,Biotechnol.Bioeng.90,59(2005).21N.S.Lu,X.Wang,Z.G.Suo,and J.Vlassak,Appl.Phys.Lett.91,221909(2007).22See supplementary material at /10.1063/1.4794143for the supplementary result of the stability of the composite electrodes at differ-ent strain rates.23Q.Cao and J.A.Rogers,Adv.Mater.21,29(2009).FIG.5.(a)Change in capacitance combined with figure touch versus uniaxial stretching strain.Inset illustrates how the two external operations are applied independ-ently.(b)Change in capacitance versus independently applied transverse pressure and uniaxial elongation.Inset illustrates how the transverse pressure and uniax-ial stretching are applied independently.。