Piezoresistive behavior and multi-directional strain sensing ability of carbon

nanotube–graphene nanoplatelet hybrid sheets

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Smart Mater. Struct. 22 (2013) 015013 (9pp) doi:10.1088/0964-1726/22/1/015013

Piezoresistive behavior andmulti-directional strain sensing ability ofcarbon nanotube–graphene nanoplatelethybrid sheets

Sang-Ha Hwang, Hyung Wook Park and Young-Bin Park

School of Mechanical and Advanced Materials Engineering and Two-Dimensional Carbon MaterialsCenter, Ulsan National Institute of Science and Technology, UNIST-gil 50, Eonyang-eup, Ulju-gun,Ulsan 689-798, Korea

E-mail: ypark@unist.ac.kr

Received 12 August 2012, in final form 19 November 2012Published 10 December 2012Online at stacks.iop.org/SMS/22/015013

AbstractFree-standing carbon nanomaterial hybrid sheets, consisting of multi-walled carbon nanotubes(MWCNTs), exfoliated graphite nanoplatelets (xGnPs) and nanographene platelets (NGPs) ofdifferent lengths and lateral dimensions, have been prepared using various materialcombinations and compositions. When subjected to tensile strains, the carbon nanomaterialsheets showed piezoresistive behavior, characterized by a change in electrical resistance withapplied strain. Simultaneous measurement of resistance changes among multiple electrodesplaced on the periphery of the hybrid sheets showed the dependence of resistance changes onstrain direction, which potentially allows multi-directional strain sensing. Variouscombinations of MWCNT length, xGnP size and MWCNT-to-xGnP/NGP ratio result indifferent specific surface areas and nanoparticle interactions, which serve as critical factors forcontrolling the sensitivity of hybrid sheets. The smaller the nanoplatelet size and the higher thecontent as compared to MWCNT, the higher the sensitivity. Buckypapers, which arefree-standing sheets composed of CNTs, are used as the control materials, and the uniquecharacteristics of hybrid sheets are discussed.

(Some figures may appear in colour only in the online journal)

1. Introduction

There has been much research on composite materialscontaining nanoscale carbon allotropes, including carbonnanofibers [1], exfoliated graphite nanoplatelets (xGnPs) [2],fullerenes [3], carbon nanotubes (CNTs) [4] and graphene [5].These carbon-nanomaterial-filled composites have beenintensively studied and reviewed as multifunctional ma-terial platforms, owing to their remarkable mechanical,electrical/electronic and thermal properties [6, 7]. Amongvarious smart applications, significant advances have beenachieved from CNT-filled polymeric materials used as strainsensors [8–13]. There are two main working principlesof strain sensing using carbon-nanomaterial-based polymer

composites. The first mechanism derives from the relationshipbetween the Raman spectrum shift and mechanical strain,as observed by Frogley et al [14] in CNT composites.The second and more generally applied mechanism isstrain-induced change in electrical resistance, known aspiezoresistivity, which is observed in both buckypapers, thatis, free-standing sheets composed of densely entangled CNTnetwork [14, 15], and CNT-filled polymer composites [8, 9].In the case of CNT-filled polymer composites, the sensitivityand applicable strain range can be relatively easily controlledby filler content, filler orientation and matrix type. However,it is difficult to fabricate buckypaper-based strain sensorswith tailored sensitivity and mechanical properties due to therobust nature of the conductive network formed by densely

10964-1726/13/015013+09$33.00 c© 2013 IOP Publishing Ltd Printed in the UK & the USA

Smart Mater. Struct. 22 (2013) 015013 S-H Hwang et al

entangled CNT bundles. Despite several disadvantages, adensely entangled CNT bundle is expected to achievehigher sensitivity in a local area and direction-dependentpiezoresistivity due to its isotropic structure [14]. In recentyears, free-standing graphene and graphene oxide filmsor papers have been processed and characterized [16–18].However, the primary focus has been placed on uniformdispersion of graphene or graphene oxide nanosheets in anaqueous or organic solution and controlled assembly intohigh-strength, high-stiffness free-standing films or papers.Their electromechanical properties are yet to be investigated.

Recently, the graphitic nanostructured particles were‘hybridized’ with CNTs and were mainly studied asfunctional materials for particular applications, such assupercapacitors [19], transparent electrodes [20], catalystsupports [21], field emission devices [22], gas sensors [23]and photonic applications [24]. Knowledge about thesynergistic effect was obtained from a recent electro-chemical study of ‘hybrid films’ while their effects onelectromechanical properties are still elusive.

We report the fabrication and characterization of multi-walled carbon nanotube (MWCNT) buckypapers hybridizedwith xGnPs and nanographene platelets (NGPs). Variouscombinations of MWCNT length, xGnP size, NGP sizeand MWCNT:xGnP or MWCNT:NGP content ratio wereused to process carbon nanomaterial ‘hybrid sheets’ throughfiltration, and their mechanical, electrical, piezoresistive andmicrostructural properties were characterized. xGnPs andNGPs consist of sp2 carbon atoms, which allow them to havestrong π–π interactions as well as van der Waals interactions.Thus, CNTs combined with xGnPs or NGPs in sheet form areexpected to have a whole new range of interesting properties,especially a change in electrical behavior when subjected to anexternal loading, due to nanofiber–nanoplatelet interactions.Only a few research groups have reported their work onhybrid sheets consisting of CNTs and graphitically structurednanomaterials, and their electromechanical properties are yetto be studied. In order to capture the unique propertiesof MWCNT–xGnP and MWCNT–NGP hybrid sheets, thestudy was focused on two aspects: (1) investigating theeffects of the hybridized nanostructure on piezoresistivityand understanding the underlying physics; and (2) exploringthe potential for wide-area strain sensing by demonstratingproof-of-concept of a two-dimensional strain sensing abilityunder flexure.

2. Experimental details

2.1. Materials

MWCNTs, grown by chemical vapor deposition, with a purityrating of >95%, 5–10 nm inner diameter, 60–100 nm outerdiameter, and two different lengths—100 µm (CM-100) and250µm (CM-250)—were purchased from Hanwha Nanotech.Two types of xGnP with average lateral dimensions of5 µm (M-5) and 15 µm (M-15) were purchased from XGSciences. NGP with a documented average lateral dimensionof ≤10 µm, average thickness of <1 nm, oxygen content of

≤2.1%, and surface area of 400–800 m2 g−1 was purchasedfrom Angstron Materials. The carbon nanomaterials wereused without any further chemical modification.

2.2. Preparation of MWCNT–xGnP and MWCNT–NGPhybrid sheets

Surfactants, sodium lauryl sulfate (SLS) and Triton X-100,were purchased from Sigma-Aldrich and Daejung Chemicals,respectively, and solutions were prepared with a concentrationof 0.05 wt% in methyl alcohol. 120 mg of MWCNT–xGnPor MWCNT–NGP mixture with various proportions of xGnPor NGP versus MWCNTs and various MWCNT and xGnPtypes was dispersed in 2 l of SLS/Triton X-100 solution. After1 h of stirring and 3 h of sonication in a bath-type sonicator,the solutions were further treated for 2 h using a horn-typesonicator. A nylon filtering membrane (0.45 µm pore size,Millipore) was used for making hybrid sheets by vacuumfiltration. Two liters of suspension was filtered to yield one78-mm diameter paper. The paper was then washed usingmethanol and acetone to remove residual surfactants and thenplaced in an oven and dried at 60 ◦C overnight before beingpeeled off the membrane.

2.3. Microstructure characterization

The morphologies and microstructures of as-received MWC-NTs, xGnPs and NGPs were observed using a scanningelectron microscope (SEM, FEI Nanonova 230) and atransmission electron microscope (TEM, JEOL JEM-2100).A confocal Raman microscope (WITec alpha300 R) was usedto assess the nature of carbon atom bonding in the basal planeand the degree of defects in xGnPs and NGP.

Hybrid sheets with various carbon nanomaterial com-positions were analyzed at the micro- and nanoscale usingthe SEM operated at 15 kV to observe the interactionbetween MWCNTs and xGnPs/NGPs as well as the sheetmorphologies. For selected sheet samples, SEM images weretaken after stretching the sheet in tension to observe theMWCNT–xGnP/NGP interaction after undergoing tensiledeformation.

Buckypapers and hybrid sheets are inherent porousmedia, as they are formed by entangled, inter-connectedMWCNTs, xGnPs and GNPs. In order to understand theeffects of various material combinations on the porosityof the hybrid sheets, their surface areas were measuredusing a Brunauer–Emmett–Teller (BET) surface area analyzer(Micromeritics ASAP 2020) using N2 adsorption at 77 K.

2.4. Mechanical property characterization

Specimens of size 5 mm × 15 mm were cut from each hybridsheet using a 25 W laser cutter (Universal Laser SystemsVersaLaser 2.30) for tension tests. A dynamic mechanicalanalyzer (DMA, TA Instruments Q800) was used in staticmode to measure the tensile properties. After being installedin the DMA, the specimens were subjected to quasi-statictension to failure at a constant strain rate of 1 min−1. A totalof seven specimens were tested for each sample, and fiveintermediate measurement data were selected to obtain theaverage tensile modulus, tensile strength and % elongation tofailure.

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Figure 1. Inter-probe resistance and piezoresistivity measurement setup. (a) Multi-electrode measurement scheme. (b) Piezoresistivitymeasurement setup. (c) Loading configuration during piezoresistivity measurement. (d) Bottom view showing the quasi-uniaxial strain fieldin the longitudinal direction.

2.5. Electrical property and piezoresistivity characterization

The surface and volume resistivities of hybrid sheets weremeasured using a four-point probe measurement system(Advanced Instrument Technology CMT-SR1000N). Fivemeasurements were made at different locations per sample,and the average values were recorded. In addition, resistancemeasurements were made to indirectly assess the degree ofcarbon nanomaterial dispersion. Hybrid sheets were cut into50 mm × 50 mm squares and were bonded to 10 mm-thickpolymethylmethacrylate (PMMA) plates using an epoxy glue,as shown in figure 1(a). Eight electrodes were attached onthe periphery of the square film using a silver paste, whichwere connected to the electrical signal acquisition systemconsisting of a Keithley 2002 multimeter and 7001/7012-Sswitching system, operated by the manufacturer-programmeddata acquisition software. A set of inter-probe resistancesper unit length from all electrode pairs was obtained, andthe averages and standard deviations were compared amongvarious inter-probe distance groups.

The hybrid-sheet-bonded PMMA plates were subjectedto three-point bending using a universal materials testingsystem (Instron 5982), as shown in figures 1(b)–(d), andpiezoresistivity was measured. The upper flexural strain limitsto ensure linear, elastic deformations of hybrid sheets weredetermined through preliminary experiments, and accordingly2 and 1% of flexural strains were applied at a strain rate of4% min−1 for MWCNT–xGnP and MWCNT–NGP hybridsheets, respectively.

3. Results and discussion

3.1. Microstructure

Typical TEM and SEM images of both surfaces andcross-sections of xGnPs (M-5, M-15) and NGP particles areshown in figures 2(a)–(c), respectively. The high aspect ratioof xGnP is evidenced by the large lateral dimension of theparticle as compared to the thickness, which is ∼18 and∼9 nm for M-5 and M-15, respectively. Figure 2(c) shows

the lateral dimension and thickness of NGP are <2 µm and∼5 nm, respectively, which would result in much greatersurface area than xGnP.

The Raman spectra of both xGnP and NGP particlesare shown in figure 2(d). The low D band intensity andsharp G band confirm the sp2-type bonding of the carbonatoms in the basal plane. It is thus believed that the acidintercalation and subsequent pulverization involved during thexGnP manufacturing process did not oxidize the surface ofthe xGnP, which may degrade the electrical and mechanicalproperties of an individual platelet. However, a much largerID/IG ratio was observed from NGP, which may be dueto more destructive intercalation and pulverization processescompared to xGnP.

It is noted in figures 3(a) and (d) that both xGnP and NGPhybrid sheets are macroscopically robust under flexural stress,as they can be almost folded without failure. Figures 3(b),(c), (e) and (f) show the basal plane morphologies of theCM-250/M-5 and CM-250/NGP hybrid sheets before andafter stretching. In figures 3(c) and (f), the MWCNT bridgingsuggests that the surface adhesion between MWCNT andxGnP or NGP is very strong because of the van der Waalsinteractions and π–π interactions of sp2 carbon atoms.

Table 1 summarizes the eight different sets of buck-ypapers and hybrid sheets prepared for this study. Thesample sets and names are based on the combination ofMWCNT and xGnP or NGP types and the MWCNT:xGnPor MWCNT:NGP ratio in hybrid sheets. The specific surfaceareas of buckypapers and selected hybrid sheets, denoted asABET, are also shown in table 1. The reported specific surfacearea of MWCNT buckypaper is around 200 m2 g−1, andour measurement showed similar values for the buckypapersof both CNT lengths. From the BET measurement, itis evident that buckypapers have much higher surfaceareas than MWCNT–xGnP hybrid sheets because of theone-dimensional structure, unique morphology and nanoscalesize of CNTs. The measured ABET of xGnP M-5 and M-15was around 90 and 70 m2 g−1, respectively. The measuredABET of NGP was around 400 m2 g−1, and therefore, the ABET

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Figure 2. Typical SEM and TEM micrographs of the surface and the cross sectional view of xGnPs and NGP ((a) M-5, (b) M-15, (c) NGP,respectively). (d) Raman spectra of as-received carbon nanomaterials.

Figure 3. Digital photographs and SEM images of CM-250/M-5 (5:5) hybrid sheet ((a)–(c)) and CM-250/NGP (5:5) hybrid sheet((d)–(f)). (a), (c) Digital photographs upon folding. (b), (e) SEM images of hybrid sheet surface. (c), (f) SEM images of crack morphologyin hybrid sheets with in-plane tension.

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Table 1. Specific surface areas (ABET’s) of buckypapers and hybrid sheets.

Material type Sample namexGnP or NGPcontent (wt%) ABET (m2 g−1)

CM-250 CM-250 0 210.0

CM-250 and M-5a CM-250/M-5 (8:2) 20 197.2CM-250/M-5 (5:5) 50 157.2CM-250/M-5 (3:7) 70 121.3

CM-250 and M-15a CM-250/M-15 (8:2) 20 —CM-250/M-15 (5:5) 50 109.4b

CM-250/M-15 (3:7) 70 —CM-250 and NGPc CM-250/NGP (8:2) 20 273.8

CM-250/NGP (7:3) 30 324.2CM-250/NGP (6:4) 40 349.8CM-250/NGP (5:5) 50 375.3

CM-100 CM-100 0 197.2CM-100 and M-5a CM-100/M-5 (8:2) 20 —

CM-100/M-5 (5:5) 50 147.8b

CM-100/M-5 (3:7) 70 —CM-100 and M-15a CM-100/M-15 (8:2) 20 —

CM-100/M-15 (5:5) 50 101.6b

CM-100/M-15 (3:7) 70 —CM-100 and NGPc CM-100/NGP (8:2) 20 —

CM-100/NGP (7:3) 30 —CM-100/NGP (6:4) 40 —CM-100/NGP (5:5) 50 381.3b

a 70 wt% was the highest xGnP content achievable.b Selected hybrid sheets (MWCNT:xGnP/NGP = 5:5) for ABET comparisonpurposes.c 50 wt% was the highest NGP content achievable.

of MWCNT–NGP hybrid sheets was much higher than that ofbuckypapers. The ABET of xGnP and NGP hybrid sheets asfunctions of xGnP and NGP contents were in good agreementwith the values calculated using the rule-of-mixtures. At50 wt% xGnP or NGP, which is the highest achievable NGPloading from the point of view of hybrid sheet manufacture,regardless of the type of MWCNT, CM-100/NGP showedthe highest ABET while CM-100/M-15 and CM-250/M-15showed the lowest. This suggests that the length of CNTs doesnot have a significant effect on the specific surface areas ofhybrid sheets, while the lateral dimension of xGnP or NGPhas a significant influence; that is, the smaller the lateraldimension, the larger the specific surface area.

3.2. Mechanical properties

The tensile properties of MWCNT/xGnP andMWCNT/NGP hybrid sheets at 5:5 content ratio are shownin figure 4. The tensile modulus of the CM-250 buckypaperwas increased by 21% from 513 to 609 MPa with the additionof xGnP-M-5, while its tensile strength and elongation weredecreased from 21.3 MPa and 6.3% to 14.3 MPa and3.5%, respectively. Despite the fact that the aspect ratio ofxGnP-M-15 is much higher than xGnP-M-5, the stiffeningefficiency of xGnP in CM-250/M-15 hybrid sheet (only 2.5%improvement in tensile modulus as compared to CM-250buckypaper) was lower than that in the CM-250/M-5 hybridsheet. A similar trend is observed in CM-100/xGnP hybridsheets. This suggests that the size effect, which can be

Figure 4. Tensile properties of MWCNT/xGnP andMWCNT/NGP hybrid sheets with various material types at 50 wt%of xGnP or NGP content.

translated into the specific surface area, is more dominant thanthe aspect ratio, for enhancement of tensile modulus, as thesmaller size increases the volume of the interphase betweenxGnPs and the domain.

The tensile moduli of the porous sheets studied areindicative of the packing densities of the carbon nanosheets,and the sheet strengths and elongations are governed bythe interactions of carbon nanoparticles or domains. Wecan infer from the SEM images and BET results that

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Figure 5. Effect of M-5 and NGP on the normalized inter-electroderesistance of hybrid sheets with each distance group at 50 wt% ofxGnP and NGP loading.

the MWCNT bundles exhibit not only strong van derWaals and π–π interactions but also mechanical interlockingthrough entanglements. This results in the higher strengthand elongation of buckypapers as compared to hybridsheets. Although it is clear that xGnPs interfere with theinteractions between MWCNTs, the small xGnP particlesfrom sonication-induced damage fill the buckypaper pores,thus increasing the occupied volume and modulus.

Comparison of MWCNT/xGnP and MWCNT/NGPhybrid sheets at a 5:5 ratio revealed that inclusion ofNGPs in buckypaper significantly decreases its modulusby 57.4% from 513.1 to 218.3 MPa, its ultimate tensilestrength by 71.9% from 21.3 to 6.0 MPa, and its maximumelongation by 95% from 6.3 to 0.3%. A similar trend isobserved from the CM-100/NGP (5:5) hybrid sheet. Thisis attributed not only to weak interactions between CNTsand NGP particles, but also to the low packing density ofthe MWCNT/NGP hybrid sheet. It is elaborated by thecombined results from the microstructure and degree ofdefects of as-received NGPs (figures 2(c) and (d)) and themicrostructure of the mechanically stretched MWCNT/NGPhybrid sheet (figure 3(f)). Few-atom-thin NGP nanosheets arethermodynamically unstable, and their high specific surfacearea maximizes the van der Waals interactions, which resultsin re-aggregation of MWCNTs and NGPs in suspension,leading to a wrinkled morphology of the hybrid sheet.These pre-agglomerated domains are barely inter-connectedfor a robust free-standing hybrid sheet, as can be observedfrom figure 3(f), which leads to deteriorated mechanicalproperties. In addition, we can easily imagine that theinteractions between CNT and NGP particles are weakerthan those between CNT and xGnP particles, due to the lowcrystallinity of NGPs, as confirmed from the Raman spectrumin figure 2(d).

3.3. Electrical properties

For the experimental study on piezoresistivity of hybridsheets, eight electrodes were attached to the periphery of ahybrid sheet bonded to a PMMA plate as previously shown infigure 1(a). The resistances were measured from all possible

Table 2. Average normalized inter-electrode resistances of hybridsheets and their standard deviations.

Average inter-proberesistance (� cm−1)

Standard deviation(� cm−1)

CM-250 2.24 0.17CM-250/M-5 (5:5) 2.63 0.20CM-250/M-15(5:5)

2.85 0.22

CM-250/NGP(5:5)

3.37 0.51

CM-100 2.34 0.15CM-100/M-5 (5:5) 2.63 0.24

electrode pairs (a total of C28 = 28 pairs) in sequence. In order

to eliminate the inter-electrode distance dependence of theraw resistances measured, each resistance was divided by thedistance between the corresponding electrodes as follows:

r̄i,j =Ri,j

Li,j(1)

where r̄i,j denotes the normalized resistance measuredbetween electrodes i and j (i, j = 1, 2, 3, . . . , 8), Ri,j is thecorresponding raw resistance measured and Li,j is the distancebetween electrodes i and j.

The normalized inter-electrode resistances were catego-rized into five distance groups, and selected values are plottedin figure 5. The averages and standard deviations of thenormalized resistances of various material combinations aresummarized in table 2. Among all carbon nanomaterial sheettypes, the CM-250 buckypaper shows the lowest averagenormalized inter-electrode resistance of 2.24 � cm−1 andstandard deviation of 0.17 � cm−1, indicative of isotropicand homogeneous electrical properties. It can also be inferredthat both xGnP-M-5 and M-15 were well dispersed inCM-250/M-5 and CM-250/M-15 hybrid sheets, suggestedby low standard deviations of 0.20 and 0.22 � cm−1,respectively. However, the CM-250/NGP hybrid sheet showsa relatively high standard deviation of 0.51 � cm−1,suggesting less uniform dispersion of NGPs in the hybridsheet.

3.4. Electromechanical behavior of buckypapers and hybridsheets

Electromechanical tests were performed with buckypapersand hybrid sheets with various material combinations andxGnP or NGP contents under cyclic flexural loading. Foreach electrode pair, the change in resistance was computedby comparing the resistances obtained from two consecutivemeasurement rounds, and was normalized by dividing it bythe initial resistance and the physical distance between theelectrodes as follows:

1ri,j =1

Li,j

1Ri,j

(R0)i,j(2)

where 1ri,j denotes the normalized change in resistancemeasured between electrodes i and j (i, j = 1, 2, 3, . . . , 8),1Ri,j is the corresponding change in resistance, (R0)i,j is the

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Figure 6. Resistance change in: (a) CM-250 buckypaper; (b)CM-250/M-5 (5:5) hybrid sheet; (c) CM-250/NGP (5:5) hybridsheet under cyclic flexural strain. (‘R45’ denotes resistance betweenelectrodes 4 and 5, and so forth.)

initial resistance and Li,j is the distance between electrodes iand j.

As the PMMA plate undergoes three-point bending, thecarbon nanomaterial sheet undergoes the same quasi-uniaxialtensile strain as the bottom surface of the plate, as shownin figure 1(d). The normalized resistance change in eachpair of electrodes shows orientation-dependent piezoresistivebehavior led by the flexural-strain-induced stretching anddisrupted conductive network (figure 3). Figure 6 shows the

orientation-dependent piezoresistive behavior of (a) CM-250buckypaper, (b) CM-250/M-5 (5:5) hybrid sheet and (c)CM-250/NGP (5:5) hybrid sheet. The buckypaper and hybridsheets exhibit different percentage changes in resistancein longitudinal (between electrodes 4 and 5), transverse(between electrodes 2 and 7) and diagonal (betweenelectrodes 1 and 8) directions. The percentage change inresistance is greatest in the longitudinal direction, as it isaligned in the direction of maximum tensile strain in thePMMA plate (figure 1(d)). On the contrary, negligible andintermediate percentage changes in resistance are observed inthe transverse and diagonal directions, respectively, which isin accordance with the relative in-plane strains generated inthe carbon nanomaterial sheets.

It can be observed that hybrid sheets show a higherpercentage change in resistance than that of buckypaperat the same strain level, which indicates the difference inthe strain sensitivity, as hybrid sheets have a relativelyloose inter-connected conductive network as compared tobuckypapers. In addition, at 1% strain, the CM-250/NGP(5:5) hybrid sheet shows 27% higher percentage change inresistance than the CM-250/M-5 (5:5) hybrid sheet. Theeffects of M-5 and NGP content on the piezoresistivityof CM-250-based hybrid sheets are shown in figure 7.Both CM-250/M-5 and CM-250/NGP hybrid sheets showincreasing percentage changes in resistance with increasingM-5 and NGP content. This is due to the fact that as thexGnP or NGP content increases, the conductive network inthe hybrid sheet becomes more loosely bound, thus reducingits robustness and therefore increasing the sensitivity.

The differences in the sensitivity of MWCNT buckypaperand MWCNT–xGnP and MWCNT–NGP hybrid sheets can befurther elucidated by the schematic shown in figure 8. At thesame MWCNT:xGnP/NGP ratio, the MWCNT–xGnP hybrid

Figure 7. Effects of M-5 and NGP content on piezoresistivity in the longitudinal direction (R45) of CM-250-based hybrid sheets: (a)CM-250/M-5 hybrid sheets; (b) CM-250/NGP hybrid sheets.

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Figure 8. Schematic representation of microstructural changes inbuckypaper and hybrid sheets subjected to mechanical strains.

sheet shows higher sensitivity than the MWCNT–NGP hybridsheet because of the large domains formed by pre-aggregatedNGP particles. The larger the particle or domain size, themore easily the conductive network gets disrupted by externalstrains. On the other hand, fiber-type MWCNTs form a muchmore robust conductive network, which results in lower strainsensitivity.

4. Conclusions

A series of MWCNT buckypapers and MWCNT–xGnP/NGPhybrid sheets with varying MWCNT:xGnP andMWCNT:NGP ratios were prepared through vacuum filtra-tion. Tensile loadings were applied to the carbon nanomaterialsheets, and their piezoresistive responses were analyzed.Inclusion of xGnP or NGP in buckypapers as well as theMWCNT:xGnP or MWCNT:NGP ratio is strongly correlatedwith the specific surface area of carbon nanomaterial sheetsand the interactions among their constituents, both ofwhich govern the sensitivity. It was revealed that at thesame content ratio, the MWCNT/NGP hybrid sheet showshigher sensitivity than the buckypaper or the MWCNT/xGnPhybrid sheet. Also, sensitivity increases as xGnP or NGPcontent increases. Another critical finding is that using amulti-electrode scheme, a single buckypaper or hybrid sheetallows multi-directional strain measurement. The breadthof parameter selection combined with multi-directionalsensing ability lays the groundwork for the development ofself-sensing, smart materials and structures enabled by carbonnanomaterial sheets with great design flexibility.

Acknowledgments

This work was supported by the National ResearchFoundation of Korea Grant funded by the Korean Government(project nos 2009-0070548, 2010-0023850).

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