C A R B O N 5 0 ( 2 0 1 2 ) 4 1 3 0 – 4 1 4 3

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Electrophoretic deposition of carbon nanotubes ontocarbon-fiber fabric for production of carbon/epoxycomposites with improved mechanical properties

Qi An a,c, Andrew N. Rider b, Erik T. Thostenson a,c,d,*

a Center for Composite Materials, University of Delaware, Newark, DE 19716, USAb Air Vehicles Division, Defence Science and Technology Organization, Fisherman’s Bend, Victoria 3207, Australiac Department of Materials Science and Engineering, University of Delaware, Newark, DE 19716, USAd Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, USA

A R T I C L E I N F O

Article history:

Received 16 January 2012

Accepted 23 April 2012

Available online 1 May 2012

0008-6223/$ - see front matter � 2012 Elsevihttp://dx.doi.org/10.1016/j.carbon.2012.04.061

* Corresponding author at: Center for CompoE-mail address: thosten@udel.edu (E.T. Th

A B S T R A C T

Carbon nanotubes (CNTs) have been deposited onto carbon-fiber fabric using electropho-

retic deposition (EPD) prior to the infusion of epoxy resin for the production of carbon/

epoxy composites. The carbon-fiber fabric employed for EPD was used in the as-received

condition, in which the proprietary epoxy sizing-agent was present. CNTs were functional-

ized prior to EPD using ozone treatment for oxidation, followed by chemical reaction with

polyethyleneimine. The CNT oxidation used a novel recirculating system which enabled

ozonolysis to be conducted on large-volume solutions of CNTs in the presence of high-

powered sonication, facilitating preparation of stable dispersions suitable for EPD. Signifi-

cant increases in the shear strength and fracture toughness of the carbon/epoxy compos-

ites with the CNT treatment have been measured relative to composites without the CNT

treatment. Analysis of fracture surfaces revealed interlaminar regions with high levels of

CNTs and evidence of good adhesion between the carbon nanotubes and sized carbon-

fiber, which is believed to have contributed to the measured improvement in mechanical

properties.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Carbon-fiber reinforced polymer (CFRP) composites offer good

in-plane tensile properties for their equivalent weight in com-

parison with traditional metallic materials, however, they

may exhibit poor through-thickness strength and toughness

properties. Efforts to improve the through-thickness proper-

ties of CFRP composites have examined the addition of car-

bon nanotubes (CNTs). CNTs offer high strength and

stiffness on a sub-micron scale and, therefore, are potential

candidates to be used to modify the interstitial regions be-

tween the carbon-fibers, where the polymer matrix domi-

nates the composite strength and toughness properties. An

er Ltd. All rights reserved

site Materials, Universityostenson).

elegant approach to incorporating CNTs into CFRPs has in-

volved the growth of CNTs directly onto the reinforcing fiber

using chemical vapor deposition (CVD) prior to resin infusion.

The direct growth approach enables perpendicularly-aligned

CNTs to be grown at high coverage, leading to high-effective

volume fractions of the CNTs in the matrix. Processing the

composite with the high CNT volumes in the resin would be

difficult to achieve due to factors such as viscosity increases,

fabric filtering effects and adequate dispersion. The CVD

approach leads to composite properties with improved

mechanical and electrical properties [1,2]. The CVD approach,

however, can cause a reduction in the strength of the carbon-

fibers and, therefore, compromise the tensile properties [2,3],

.

of Delaware, Newark, DE 19716, USA. Fax: +1 302 831 3619.

C A R B O N 5 0 ( 2 0 1 2 ) 4 1 3 0 – 4 1 4 3 4131

although recent work using fiber oxidation may be able to

minimize this strength loss [3]. While the process may be

scalable, the high temperatures typically employed for CVD,

between 600 and 1000 �C, also make it energy intensive. The

CVD process may also be less amenable to the control of

CNT purity and manipulation of surface chemistry and adhe-

sion of the CNT to the fiber surface. On this basis, some re-

search has been dedicated in recent years to alternative

methods to prepare these hierarchically-scaled composite

materials.

Electrophoretic deposition (EPD) is a widely used indus-

trial-coating process employed in areas ranging from auto-

motive to electronics production with assets which include

low-energy use and the ability to homogenously coat complex

shapes with well adhered films of controlled thickness and

density. A considerable research effort in developing reliable

methods to prepare nanostructured-ceramic materials using

EPD [4] has occurred in recent years. Detailed reviews of the

methods used to prepare carbon nanotube and ceramic com-

posites in a wide range of applications have indicated the

important processing parameters for preparing coatings and

films with tailored properties [5,6]. A variety of methods to

create stable-CNT dispersions in aqueous [7] solutions have

been used, with the aim of creating charged CNTs which will

migrate under the influence of an external electric field to the

coating surface [8]. The choice of solvent, CNT-functionaliza-

tion method and voltage can all influence the film formation,

with electrolysis of water affecting film-porosity levels [8]. A

recent review has also detailed some new techniques utilizing

aperiodic AC and pulsed DC EPD methods to provide more

uniform, low porosity coatings by minimizing the water elec-

trolysis reactions [9].

EPD has also been applied to preparing hierarchical-com-

posite structures with carbon-fiber fabrics [10,11] using aque-

ous-CNT solutions. The carbon/epoxy-composite laminates

have exhibited only modest improvements in mechanical

properties. However, electrical conductivity has shown con-

siderable increase, particularly in cases where copper has

been incorporated in the CNT coating during deposition [11].

The approach offers a similar outcome to CVD coating by

offering the potential to have high-effective CNT volume frac-

tions in the composite, although EPD work to date has used

relatively low-volume concentrations around 1%, where the

thin CNT coating is employed like a traditional sizing-agent

[11].

Successful EPD relies on the functionalization of the CNTs,

which enables a surface charge to develop. The surface

charge or zeta-potential is dependent on the solution pH

and helps repulse adjacent CNTs to aid dispersion and mobil-

ity under applied electric fields. While acid treatment has

been used traditionally to prepare stable dispersions [7],

ozone and ozone-uv oxidation has also been employed [12–

14]. While ozone treatment appears to produce fewer carbox-

ylic acid groups compared to aggressive acid treatments [15],

there is some evidence that the treatment is less harmful to

the CNT structure [16]. Depending on ozone treatment time

and CNT structure, zeta-potentials approach �40 mV in water

at neutral pH [12]. At neutral pH this high zeta-potential leads

to rapid and efficient anodic EPD [8]. Ozone has been used for

treating carbon materials for many years [17] and recent work

has also characterized the ozone oxidation of polyacryloni-

trile (PAN) based carbon-fibers [18]. Alkaline titration of the

ozone-treated carbon [16] is consistent with XPS analysis of

the ozone-uv treated CNTs [15] and carbon-fibers [18], indicat-

ing oxygen groups are a combination of mildly-acidic carbox-

ylic acid, alcohol and carbonyl-functional groups. Changes

observed in the graphitic structure of the PAN fibers were

not seen in Raman spectra, suggesting ozonolysis was limited

to the outer fiber surface, which is consistent with XPS [15]

and Raman analysis of ozone-treated multi-walled CNTs [16].

An alternative functionalization technique for CNTs in-

volves the use of polyelectrolytes such as polyethyleneimine

(PEI) [19]. PEI has a high-natural pH in aqueous solution, but

with addition of a mild acid, the amine groups protonate

and a +50 mV zeta-potential can be established below a pH

of 8 [20], enabling cathodic deposition of the PEI-functional-

ized CNTs. It is hypothesized that the high-pH gradients at

the cathode lead to precipitation of the CNTs and coating for-

mation [19]. The concentration of PEI used can influence the

adsorption efficiency onto the functionalized CNT [21], with

optimal coverage of acid-functionalized CNTs being achieved

with 30% by weight of PEI [22]. An extensive review of func-

tionalization methods for CNTs suggests that chemical treat-

ments leading to covalent bonding between CNTs and the

polymer matrix, such as oxidation, are suited to mechanical

applications but tend to compromise the electrical properties

due to damage to the CNT structure [23]. The adsorption of

PEI onto untreated CNTs is through weaker-van der Waals

interactions, which does not affect the graphitic-CNT struc-

ture, but may limit mechanical applications. Methods to

establish chemical bonding between the PEI and CNTs have

been developed using a benzoic-acid treatment, which has

facilitated deposition of composite films, while minimizing

damage to the CNTs [22].

The following work details an investigation into EPD of

ozone-oxidized CNTs, with and without additional PEI func-

tionalization, onto carbon-fiber fabric. A novel system which

uses temperature controlled, recirculating, high-energy soni-

cation and an ozone generator has enabled the preparation of

large volumes of stable-CNT dispersions suitable for EPD. For

the first time, the effect of chemically bonding PEI to CNTs for

modifying composite mechanical properties has been exam-

ined using significantly higher CNT concentrations than pre-

viously. Carbon-fiber fabric with EPD coatings of CNT have

been infused with epoxy resin to prepare composite lami-

nates. The shear strength and fracture toughness properties

of the composites have been measured and fracture surfaces

characterized.

2. Experimental

2.1. Materials and processing

Multi-walled carbon nanotubes (MWCNT) (CM-95, Hanwha

Nanotech, Korea) were oxidized using either ultrasonicated

ozonolysis or traditional-acid treatment. Ultrasonic ozonoly-

sis used the arrangement shown in Fig. 1. Moisture-free oxy-

gen with a flow rate of 500 mL/min passed through an ozone

generator (1000BT-12 from Taoture International) and treated

the aqueous-MWCNT solution cooled at 5 �C. Ozone levels

Recirculating water bath

MWCNT solution

Peristaltic Pump

Ozone Generator

Moisture trap

Flow-meter

Sonicator Cell

O2

Fig. 1 – Instrumental configuration used to functionalize the

MWCNTs using ozone and ultrasonication.

4132 C A R B O N 5 0 ( 2 0 1 2 ) 4 1 3 0 – 4 1 4 3

reached 20 mg/L after 2 h of operation as determined by iodo-

metric titration [24]. A peristaltic pump (Model MU-D01 from

Major Science, USA) circulated the MWCNT solution into the

sonicator cell (800B Flocell, Qsonica, USA), also maintained at

5 �C. High-powered sonication (Sonicator 3000 from Misonix,

USA) used a 12.7 mm diameter horn operating at 60 W. Total

sonication time during ozonolysis was 16 h. When the

ozone-treated MWCNTs were also mixed with polyethylene-

imine (H(NHCH2CH2)58NH2, Mw: 25,000 Sigma–Aldrich, USA)

at equal concentration to the MWCNTs, they were sonicated

for a further 4 h. The pH of the PEI and ozone-treated

MWCNTs was adjusted with glacial-acetic acid (Sigma–Al-

drich) to a pH around 6. Acid treatment involved 30 min

refluxing of CNTs in a 1:3 volume ratio of HNO3 and H2SO4

solution (Sigma Aldrich, USA) [7]. CNTs were then washed

in distilled water and ultrasonicated for 16 h prior to use.

When c-glycidoxypropyltrimethoxysilane (GPS), (Sigma–Al-

drich) was used it was added to the functionalized MWCNTs

at 0.5% by weight and sonicated for 4 h prior to use.

2.2. Electrophoretic deposition (EPD)

EPD of the ozone and acid-functionalized MWCNTs used a

field strength of 43 V/cm, whereas, deposition of the ozone

and PEI-functionalized MWCNTs used 28 V/cm field strength.

The lower-field strength used for the PEI-MWCNT deposition

was used to reduce Joule heating of the higher-conductivity

solution. Deposition times up to 40 min were used to coat

stainless steel (316 SS McMaster-Carr, USA) and unidirectional

(UD) carbon-fiber (T700S, Soller Composites, USA). EPD treat-

ment of UD carbon-fiber fabric used four electrodes each with

a coating area of 75 cm2 with opposing stainless-steel elec-

trodes placed either side of the fabric. Deposition used a 1 L

rectangular-glass cell and film-deposition rates were deter-

mined by weight change of the fabric electrode after set-treat-

ment times.

2.3. Material characterization

X-ray photoelectron spectroscopy (XPS) of functionalized

MWCNTs used an Omicron EA125 electron analyzer using

100 W Mg Ka 1,2 X-rays at constant-analyzer energy (CAE)

mode with a pass energy of 50 eV (survey spectra) or 20 eV (re-

gion spectra) and an analysis area of 6.75 mm2. Survey spec-

tra were quantified using Scofield cross-sections [25] after

correction of signal intensity for the transmission function

of the spectrometer, kinetic energy dependence of the photo-

electron escape depth and source to analyzer angle [26]. The

transmission function was determined by normalizing the

gold spectrum to the National Physical Laboratories reference

spectrum [27]. Shirley-background subtraction was used for

quantification and peak-fitting.

A Mettler Toledo thermal-gravimetric analyzer (TGA),

which comprised a TGA/DSC 1 STARe system with a GC200

gas controller and STARe software, was used to analyse

MWCNT films. Samples weighing 2–3 mg were placed in

150 lL-alumina crucibles and heated from 30 to 1000 �C at a

heating rate of 2 �C min�1 under a nitrogen flow rate of

80 mL min�1. SEM (JEOL JSM-7400F) characterized the CNT

film morphology after functionalization and electrophoresis,

as well as the composite fracture surfaces, using a 3 kV accel-

erating voltage. All samples were sputter coated with a 5 nm

Pt/Au layer prior to imaging.

2.4. Composite manufacture, testing and analysis

The composite laminates were prepared by the adhesive

bonding of three parts prepared separately, comprising an in-

ner four-ply laminate with the EPD-MWCNT coating and two

outer-laminates of untreated carbon-fiber fabric (T700S fibers,

Soller Composites, USA). The four-layers of MWCNT- coated

fabric used unidirectional (UD) carbon fiber with 139 g/cm2

areal density, whereas, each outer-laminate was comprised

of 305 g/cm2 UD fabric between four and eight-plies thick.

The outer surface of each laminate was lightly sanded, fol-

lowed by cleaning with distilled water and a final treatment

with an AtomfloTM plasma unit (Surfx Technologies, USA), be-

fore bonding with EA9309.3 NA paste adhesive (Henkel, USA).

The dry, MWCNT coated and uncoated UD-fabric layers were

infused separately prior to bonding using vacuum assisted re-

sin transfer molding (VARTM) with EPON 862 and Epi-Kure W

(Hexion Specialty Chemicals) at a ratio of 100/26.4. Infusion of

the vacuum-degassed resin occurred under full vacuum

(�100 kPa) and 55 �C using a flow media to aid through-thick-

ness resin diffusion. Infusion took place over several hours

before final cure under full vacuum at 130 �C for 6 h. The lam-

inate compositions were established from weight changes in

the fabric after MWCNT deposition and from density mea-

surements and nitric-acid digestion of the laminate after fab-

rication [28,29].

The in-plane shear strength and mode I fracture-energy

(GIc) of laminates were measured using the methods detailed

in ASTM D3846-02 [30] and ASTM D5528 [31], respectively.

Mode I analysis used the modified-beam theory to account

for adherend rotation of the loading blocks during testing

and GIc was determined from the fracture initiation. All test-

ing used an Instron 5565 load frame in displacement control

mode. Analysis of the failed in-plane shear and mode I frac-

ture-energy specimens used SEM conditions detailed above.

The configuration and dimensions of both specimen types

are shown in the Supplementary information Figs. S1 and S2

for the in-plane shear and mode I specimens, respectively.

Table 1 – Atomic concentrations for MWCNTs after ultraso-nicated ozone, ozone plus PEI, acid and acid plus GPStreatments.

Treatment Atomic Concentration (%)

C 1s O 1s N 1s Si 2p F 1s

As received 97.8 2.2 0.0 0.0 0.016 h ozone 83.5 12.9 3.7 0.0 0.016 h ozone + PEI 68.3 15.0 12.6 1.1 3.0Acid treated 77.6 17.1 5.4 0.0 0.0Acid treated + GPS 74.4 20.7 0.0 5.0 0.0

C A R B O N 5 0 ( 2 0 1 2 ) 4 1 3 0 – 4 1 4 3 4133

The in-plane shear specimens were 79.5 mm long by 12.5 mm

wide with an overlap distance of 6.5 mm. The mode I speci-

mens were 150 mm long and 25 mm wide and used a

50 mm crack starter. Both specimens typically had a 4-ply

CNT-treated laminate of approximately 0.6 mm and outer

adherend laminates of 2.5 mm of thickness.

3. Results and discussion

3.1. Chemical characterization

Fig. 2 shows the oxygen to carbon ratio measured using XPS

for the ultrasonicated and ozone-treated MWCNTs. Run 1

and run 2 used 0.5 and 1.0 g/L of MWCNTs, respectively, but

indicate similar rates of oxidation. The O/C ratio approached

0.15 after 16 h of treatment and was relatively similar for the

two MWCNT concentrations used and is slightly higher than

oxidation rates for single-walled CNTs sonicated under low

power and at similar ozone concentrations [12]. The oxidation

rate is considerably lower than for an ozone/water vapor

treatment [14]. However, if the rates in the current work and

references [12] and [14] are compared at 2 h there is almost

a direct correlation with the different ozone concentrations

used. As the concentrations and types of CNTs varied signif-

icantly, this would suggest ozone concentration is the most

significant parameter affecting ozonolysis, with sonication

primarily aimed at dispersing the MWCNT agglomerates.

The acid-treated MWCNTs exhibited higher oxygen con-

tent than the ozone-treated MWCNTs. Changes in the O/C ra-

tio after further functionalization with PEI and GPS indicated

increased nitrogen levels for the PEI and oxygen levels for the

GPS, which is also confirmed in Table 1 where the atomic

compositions of the functionalization treatments are shown.

Almost 5% silicon and 13% nitrogen suggest that both GPS

and PEI have effectively adsorbed onto the acid and ozone-

treated MWCNTs, respectively. The source of low level silicon

and fluorine contaminant present on PEI-functionalized

MWCNTs is unknown but is likely to have been introduced

during handling and transfer of the sample prior to analysis.

CNT treatment time (h)

0

0.05

0.10

0.15

0.20

0.25

0.30

0 2 4 6 8 10 12 14 16 18 20

O/C

Rat

io

Ozone: Run 1

Ozone: Run 2

Acid Ozone and PEI

Acid and GPS

Fig. 2 – Oxygen to carbon ratio measured with XPS for

MWCNTs treated with ultrasonicated ozone, ozone and PEI,

acid oxidation and acid oxidation and GPS.

The level of silicon determined for the EPD-MWCNTs with

GPS is higher than the level reported for GPS functionalization

of uv/ozone-treated MWCNTs [32]. The difference may be due

to the increased functionalization achieved by acid treatment

and the efficiency of the EPD process, where the excess or

unreacted cationic GPS present in the EPD solution preferen-

tially adsorbed onto the cathode, as reported for traditional

electrodeposition of silane solutions [33]. The EPD method re-

duces the need for traditional cleaning or extraction tech-

niques to remove excess silane, which can also lead to

removal of chemically-adsorbed coupling agent. In the EPD

process the silane adsorbed on the oxidized MWCNTs depos-

its anodically.

Deconvolution of the C 1s peak for the ozone-PEI and acid-

treated MWCNTs is shown in Fig. 3. The peak positions for

each of the components was determined from reference [34]

and used very similar positions to those detailed in reference

[35], where a comprehensive survey of peak positions for

CNTs was undertaken. The percentage of oxygen determined

from the C 1s peak-fitting was also cross-referenced with the

total oxygen measured from the O 1s peak to ensure consis-

tency in the overall analysis. The positions used for the peak

fitting are detailed in Table 2, together with the atomic con-

centrations calculated from the quantification of the individ-

ual components. The major changes observed with

ultrasonicated ozonolysis (Fig. 3c) were an increase in alcohol,

carbonyl and carboxylic-acid components relative to the un-

treated MWCNTs (Fig. 3a). The treatment also led to a de-

crease in the graphite peak and the energy-loss peak

associated with the graphite, which is expected due to the

combined effects of high-powered sonication and ozonolysis.

Ozone treatment in the gas phase for MWCNTs [14,15],

graphite-fibers [18] and activated carbon [17] have all indi-

cated that the ratio of O–C@O to C@O groups is similar,

whereas the addition of water vapor to the ozone tends to

see an increase in the proportion of O–C@O groups [14]. In

the current work, there is also a higher ratio of carboxylic-

acid groups to carbonyl groups, suggesting that the presence

of water facilitates O–C@O production.

The current work contrasts with low-powered ultrasonic

ozonolysis of SWCNTs [12], where a very high ratio of car-

bonyl bonding was reported, possibly indicating differences

in the mechanisms of oxidation between SWCNTs and

MWCNTs and the effect of sonication power. The proportion

of C–O bonding reported in the current work is also signifi-

cantly higher than that reported for ozone treatment of

MWCNTs [14,15] and SWCNTs [12], but is similar to the rela-

280 285 290 295 280 285 290 295

280 285 290 295280 285 290 295

525 530 535 540

Cou

nts

(arb

.)

Binding Energy (eV)

Cou

nts

(arb

.)

Binding energy (eV)

Cou

nts

(arb

.)

Binding energy (eV)

Cou

nts

(arb

.)

Binding energy (eV)

Cou

nts

(arb

.)

Binding (eV)

395 400 405

Cou

nts

(arb

.)

Binding energy (eV)

a b

c d

e f

Fig. 3 – C 1s peak-fitting components for the MWCNTs (a) before functionalization and after (b) acid treatment, (c) ozone (d)

ozone plus PEI, and (e) O 1s and (f) N 1s peak-fitting components for the ozone plus PEI MWCNTs.

Table 2 – XPS peak fitting results for the untreated, acid oxidized, ozone and ozone plus PEI functionalized MWCNTs.

Treatment Atomic concentration (%)

C 1s Peak fitting component

Graphite C–C C–O/C–N C@O/C@N O–C@O/N–C@O p! p*

284.6 eV 285.0 eV 286.5 eV 288.2 eV 289.3 eV 291.4 eV

Untreated 72 10 2 1 0 15Acid 46 11 7 3 5 6Ozonolysis 57 9 6 2 3 7Ozone-PEI 10 18 25 9 4 1

Treatment N 1s Peak fitting component O 1s Peak fitting component

Pyridinic N–C@O Oxidized-Pyridinic N–C@O O–C/O@C398.4 eV 400.6 eV 402.1 eV 531.4 eV 533.0 eV

Ozone-PEI 1 10 2 4 10

4134 C A R B O N 5 0 ( 2 0 1 2 ) 4 1 3 0 – 4 1 4 3

Temperature (°C)

Mas

s (%

)

PEI Ozone CNT + PEI

Untreated CNT

Ozone CNT

0

20

40

60

80

100

250 350 450 550 650 750 150

Fig. 4 – Thermal-gravimetric analysis for MWCNTs after

ozonolysis and PEI treatment with PEI-reference compound.

C A R B O N 5 0 ( 2 0 1 2 ) 4 1 3 0 – 4 1 4 3 4135

tive amounts reported for ozone treatment of carbon-fibers

[18] and activated carbon [17]. Possibly, the relative water con-

centration and high-sonication power also influences the pro-

portion of C–OH groups that form during ozonolysis.

A comparison of the ozone (Fig. 3c) and acid-treated

(Fig. 3b) MWCNTs shows that the acid treatment produces a

higher proportion of carboxylic-acid groups compared to the

ozone treatment, consistent with previous characterizations

[7,15]. However, the main differences between the acid and

ozone-treated MWCNTs are simply the higher levels of oxida-

tion achieved with hot-acid treatment, as shown in Table 1.

The ozone and PEI-functionalized MWCNTs (Fig. 3d) show

significant changes in the C 1s spectrum due to the adsorp-

tion of the nitrogen-rich polyelectrolyte, with more than

30% of the carbon species attributable to amine or amide

bonding, as shown in Fig. 3. Some graphitic character is still

detected, which may suggest that the PEI-layer thickness is

less than the escape depth of C 1s photoelectrons produced

from the MWCNTs, estimated to be 3 to 5 nm. Similarly, the

ratio of C to N in the PEI is around 2 to 1, suggesting around

25% of the carbon signal is attributable to PEI, based on

Table 1.

The O 1s and N 1s-photoelectron peaks in Fig. 3e and

Fig. 3f, respectively, for the ozone and PEI-treated MWCNTs

may suggest the presence of amide bonding. The O 1s peak

shows a distinct shoulder and the two fitted components at

531.4 and 533.0 eV are likely to include amide and carbon in

oxygen bonding environments, respectively, based on previ-

ously reported measurements [34,36–38]. The major peak in

the N 1s spectrum at 400.6 eV is approximately 1.1 eV higher

than the position quoted for amine bonding in linear PEI

[34], electrochemically-deposited PEI [39] and PEI films on me-

tal where chemical bonding was absent [40]. The binding en-

ergy shift may be associated with chemical bonding between

the PEI and ozone-treated MWCNTs. However, amide and

imide bonding environments have been measured at values

between 399.8 to 400.6 eV [38,41] and the formation of pyrid-

inic-ring structures [36] in the same range can make accurate

assignment difficult. The peaks at 398.4 and 402.1 eV have

been associated with pyridinic and oxidized-pyridine struc-

tures [36], but several different nitrogen bonding environ-

ments also occur in these positions. Nevertheless, the

combined analysis of the C 1s, O 1s and N 1s peaks suggests

that amide bonds are likely to be present, indicative of a

chemical reaction between the PEI and ozone-treated

MWCNTs.

3.2. Physical characterization and film growth

Fig. 4 shows the TGA results for the MWCNTs after ultrasonic

ozonolysis and PEI functionalization. The results suggest that

the EPD coating contains around 25% PEI, which is within the

range estimated from the XPS measurements. The decompo-

sition temperatures are also quite similar to those reported

for benzoic acid and PEI-functionalized CNTs [22].

Fig. 5 shows the morphology of the ultrasonicated and

ozone-treated MWCNTs at 20 min (Fig. 5a) and 16 h (Fig. 5b)

treatment time. The images show a clear reduction in the

number of small agglomerates around the 1–2 lm range, sug-

gesting that the treatment has assisted in reducing the aver-

age size of MWCNT agglomerates present in the original

material. Despite the agglomerates present in the 20 min-

treated MWCNTs, the dispersion has remained stable after

several months. Fig. 5 also shows the EPD films deposited

on stainless steel produced from the acid treated (Fig. 5c)

and ozone-PEI MWCNTs (Fig. 5d). The two films both show

sub-micron-scale porosity resulting from evolved gas pro-

duced by water electrolysis, but indicate clear differences in

local alignment and size. The acid-treated MWCNT exhibit

some local alignment that has previously been observed for

films formed by filtration of dispersions with concentrations

greater than 5 g/L [7], whereas the ozone-PEI MWCNTs show

more random entanglement where the average length ap-

pears greater. Deposition of the acid (Fig. 5e) and ozone-PEI

MWCNTs (Fig. 5f) onto the T700S carbon-fibers for short treat-

ment times shows very uniform coverage and similar porosity

to the films deposited on the planar stainless-steel surfaces,

although the acid-treated MWCNTs do not exhibit the local

quasi-alignment.

Fig. 6 plots the mass increase in ozone-PEI MWCNTs with

deposition time on the carbon-fiber fabric. The peak in weight

occurs rapidly, corresponding to almost full depletion of the

MWCNT solution, which had an original concentration of

0.5 g/L. However, with time the mass decreases before re-

establishing the original peak weight at extended deposition

time. SEM images of the deposited films at 1 min (Fig. 7a),

4 min (Fig. 7b) and 20 min (Fig. 7c and d) suggest that a film

around 0.5 lm accumulates on the outer fibers quickly, but

at extended periods this film thins to less than 100 nm and

larger pores develop (Fig. 7c), particularly, in the inter-fiber re-

gions. Sub-micron porosity of the film also increases with

deposition time due to vigorous gas evolution. The deposition

cell also showed the gas-evolution reactions increased as the

EPD-cell temperature increased from Joule heating, displacing

the outer-coating, which redeposited the redispersed MWCNT

at extended times.

Modeling of EPD indicates processes such as Joule heating,

electrothermal flow and field-induced electro-osmotic fluid

flow affect film formation [42]. The electrokinetic factors lead

to particle aggregation and film formation, whereas Brownian

Fig. 5 – SEM images showing (a) and (b) the influence of the combined ozonolysis and ultrasonication on the MWCNT film

morphology, a comparison of EPD films of (c) acid and (d) ozone-PEI treated MWCNTs deposited on stainless-steel and (e) acid

and (f) ozone-PEI treated MWCNTs deposited on carbon-fiber.

4136 C A R B O N 5 0 ( 2 0 1 2 ) 4 1 3 0 – 4 1 4 3

diffusion acts to randomize the particle distribution in solu-

tion [43]. Potentially, at increased deposition times, Brownian

diffusion assists the redistribution of the MWCNTs into the

inner-fabric regions. A previously proposed mechanism for

the initial deposition of the PEI-functionalized MWCNTs in-

volves the hydroxyl ions, generated at the cathode through

water electrolysis, neutralizing the protonated amino groups

and aggregating at the electrode [44]. In the case of increased

heating and vigorous gas evolution displacing the coating, it

may be possible that a combination of electro-osmosis and

Brownian diffusion assist the movement of MWCNTs with

different zeta-potentials to the inner-fabric regions. Previous

work on EPD of ceramic particles suggests that if they possess

similar charges to the fiber then a repulsive force is estab-

lished, facilitating good penetration of the coating into the

fabric [45]. The high concentration of hydroxyl groups at the

cathode may assist in creating repulsive charge between the

solution and negatively-charged fibers, aiding diffusion of

the neutralized MWCNTs.

3.3. Laminate characterization and mechanical properties

Table 3 shows the composition of the carbon/epoxy laminates

with the different functionalized MWCNTs deposited on the

0

0.1

0.2

0.3

0.4

0.5

0.6

0

3

6

9

12

15

0 5 10 15 20 25

MWCNT weight %

Deposition Rate

Deposition time (min)

MW

CN

T w

eigh

t (%

)

Deposition rate (m

g.cm-2.m

in-1)

Fig. 6 – Weight increase and deposition rate for

electrophoretic coating of carbon-fiber fabric with ozone and

PEI-functionalized MWCNTs.

Fig. 7 – SEM micrographs indicating the change in film morpho

MWCNTs: (a) 1 min (7 wt.% MWCNTs), (b) 4 min (13 wt.% MWCN

Table 3 – Volume-fraction composition of carbon-fiber/epoxy lamMWCNTs prior to resin infusion, indicating fiber (f), coating, ma

Treatment Volume fraction composition (%)

Vf Vcoating

No treatment 63.2 0.0Std. dev. 1.3 0.0Ozone Treated 60.3 4.3Std dev. 0.3 0.1PEI ozone 48.5 6.6Std Dev. 0.2 0.1Acid treated 57.2 3.8Std dev. 3.1 0.2Acid treated + GPS 55.4 3.7Std dev. 1.7 0.1

C A R B O N 5 0 ( 2 0 1 2 ) 4 1 3 0 – 4 1 4 3 4137

fabric by EPD prior to resin infusion. The volume fraction of

MWCNTs in the epoxy matrix (VCNT/Vm) was around 10% for

each treatment, but there was a reduction in the fiber-volume

fraction (Vf) from the untreated laminate, particularly for the

ozone-PEI treated laminate. Aside from the acid-treated and

GPS-MWCNT treatment, all laminates exhibited similar void-

ing (Vv) between 1% and 3%. These results suggest that the

VARTM method has produced good-quality laminates. Addi-

tionally, for the purposes of examining the effect of MWCNTs

on matrix dominated properties such as shear strength, the

similar MWCNT concentrations should enable a reasonable

comparison of the deposition and functionalization methods

employed.

Initial tests examining the influence of the electrophoreti-

cally-deposited MWCNTs on the carbon/epoxy in-plane shear

strength are shown in Fig. 8. The ozone-treated MWCNTs do

not appear to make any significant difference to the baseline

shear strength, whereas the ozone-PEI treated MWCNTs show

almost 70% increase in baseline strength. The improvement is

logy and thickness with deposition times of ozone-PEI

Ts), (c) and (d) 20 min (13 wt.% MWCNTs).

inates in which the fiber was electrophoretically coated withtrix (m), void (v) and MWCNT (CNT) fractions.

Vm Vv VCNT/Vf VCNT/Vm

34.5 2.3 0.0 0.00.1 1.2 0.0 0.0

32.9 2.5 6.6 11.51.0 0.7 0.1 0.4

43.5 1.3 9.0 10.11.5 1.3 0.1 0.4

36.4 2.6 6.2 9.52.3 1.0 0.3 0.8

35.8 5.1 6.2 9.32.4 0.6 0.2 0.6

0

5

10

15

20

25

30

35

40

45

MWCNT treatment

No MWCNT Ozone Acid Acid+GPS

Ozone + PEI

Shea

r st

reng

th (

MP

a)

Fig. 8 – Shear strength measured for carbon/epoxy

composites where the unidirectional fiber was treated by

electrophoretic deposition of functionalized MWCNTs.

MWCNT deposition time (min)

0

10

20

30

40

50

60

0 0.5 10 10 40

Shea

r st

reng

th (

MP

a)

No MWCNT

Ozone + PEI

Ozone + PEI + GPS

MWCNT weight (%)0 4 12 12 13

Fig. 9 – Shear strength measured for carbon/epoxy

composites where the unidirectional carbon-fiber was

treated for increasing EPD times with ozone- PEI MWCNTs.

4138 C A R B O N 5 0 ( 2 0 1 2 ) 4 1 3 0 – 4 1 4 3

significantly greater than previously reported improvements

in shear properties for carbon/epoxy composites prepared

using EPD with PEI-functionalized CNTs [11]. The two lami-

nates prepared with the acid-treated MWCNTs showed signif-

icant variation in shear strength, which corresponded to

failure surfaces where the fracture plane propagated between

different plies. The variability in strength resulting from inho-

mogeneous-through-thickness coating of the fabric by the

MWCNTs, therefore, may have contributed to the experimen-

tal scatter. Due to the scatter in the results for the acid-trea-

ted MWCNTs, it is not possible to determine if the silane

functionalization led to improved strength.

The reasons for the lack of improvement observed for the

ozone-treated MWCNTs is not obvious, given previous work

suggests that ozone-treated MWCNTs can lead to increased

tensile properties of 20% for only 1% loading in an epoxy resin

[16]. In the present work, the XPS peak-fitting results (Fig. 3)

suggest that the ozone-functionalized MWCNTs had reduced

levels of oxidation and a lower concentration of carboxylic

acid groups compared to the acid-treated MWCNTs. Given

that some of the acid-treated MWCNT samples exhibited

good strength improvement, then the reduced number of O–

C@O groups in the ozone-treated MWCNTs may be a factor

affecting the shear-strength properties in the laminate. Previ-

ous studies on the adhesion strength of carbon-fibers to

epoxy resin suggest that the interfacial-shear strength in-

creases proportionally with oxygen content of the fiber [46].

The improvement in shear strength for the ozone-PEI

MWCNT treatment is quite significant relative to the baseline

and ozone only treatments and the variability is quite low

compared to the acid-functionalized MWCNT treatments.

The major difference in the deposited films is the addition

of the PEI polyelectrolyte to the matrix at around 5% by vol-

ume, resulting from reaction with the ozone-treated

MWCNTs. Previous work has used PEI as a graded-modulus

material for enhancing epoxy-resin toughness, while main-

taining stiffness [47]. At 2.5–5% addition of PEI to an epoxy re-

sin, the toughness was reported to have increased by 30–40%

with maintenance of stiffness, potentially facilitated by the

formation of localized, high cross-link density bonds between

the PEI-dendrimer branches and epoxy resin. In the present

work the 5% of PEI is also available where the PEI chemically

links between the MWCNT and the epoxy resin and this may

assist in toughening the film, without compromising the stiff-

ness increase provided by the MWCNTs. Further, the benefit

of the chemical link between the PEI and the ozone-treated

MWCNTs provides a strong bond via the PEI to the resin.

The additional toughening provided by the PEI links may pro-

vide some tolerance of the high-concentration MWCNT ma-

trix to voids, defects or localized agglomerates, which is not

present in the ozone or acid-treated MWCNT coatings, con-

tributing to the variability in those coatings measured

strength. Previous work has also shown small but measurable

increase in shear strength provided by EPD of PEI-functional-

ized MWCNTs [11], however, the MWCNTs were not oxygen

functionalized and the level of PEI and CNTs were at signifi-

cantly lower levels than the current work. It appears levels

above 2% PEI may be required to influence resin-toughness

properties [47]. The PEI may also offer improved bonding to

the as-received sized carbon-fiber.

Given the good initial results offered by the ozone-PEI

MWCNTs, further experiments were conducted to examine

the influence of deposition time on the in-plane shear

strength. Fig. 9 shows the shear strength for the carbon/epoxy

laminates as a function of deposition time for the ozone-PEI

functionalized MWCNTs. In one case, GPS was also added to

the ozone, MWCNT and PEI mixture to assess if a smaller

molecular-weight coupling agent may affect properties. The

results show that there is a clear increase in shear strength

with time and that the addition of GPS has no obvious effect.

C A R B O N 5 0 ( 2 0 1 2 ) 4 1 3 0 – 4 1 4 3 4139

The change in shear strength is also shown as a function of

the weight percentage of MWCNTs on the fabric in the upper

axis in Fig. 9. The increasing trend in strength to 12 weight

percent of MWCNTs is understandable, but the further in-

crease in strength at 40 min deposition time, where the in-

crease in MWCNT weight is small must be related to

changes in the coating morphology and distribution observed

in Fig. 7. A better coating distribution through the fabric is im-

plied by the similar weights of the 10 and 40 min coatings but

the thinner outer-film present for the longer coating times

(Fig. 7). Additionally, the increased porosity, particularly in

the inter-fiber region, created by the extended gas-evolution

at higher temperatures for the 40-min films, leads to a 20%

thinner laminate than the 10-min treatment. It is envisaged

that the large pores developing in the coating aid in resin per-

meation throughout the fabric, providing better laminate

consolidation.

The through-thickness distribution of the ozone-PEI

MWCNTs for the 40 min EPD treated laminate was further

examined by taking optical micrographs of polished 0.2 mm

cross-sections of the composite laminate. Fig. 10 shows that

the transmitted light for the laminate containing the

MWCNTs is very low compared to the baseline laminate with-

out MWCNTs. The clear line at the top of the MWCNT con-

taining laminate is due to the clear potting resin used to

prepare the cross-section. The optical image suggests that

the extended treatment time has facilitated diffusion of the

ozone-PEI MWCNTs through the laminate thickness and has

contributed to the improved shear strength for the 40 min

EPD treatment.

Fracture surfaces corresponding to the different treatment

times are shown in Fig. 11. Progressively, two changes in the

interfacial fiber to matrix fracture can be observed. Firstly,

there appears to be an increase in the plasticity of the fracture

in the resin-rich interlaminar area, which corresponds to

higher MWCNT concentrations in the matrix. Secondly, the

percentage of failure within a thin, MWCNT-resin-rich layer

on the fiber surface appears to increase. Both these changes

in fracture appearance would suggest that there is a better

coverage of MWCNTs and improved bonding between the

MWCNT coating and the as-received sized fiber, which would

both be expected to contribute to the improved mechanical

strength. The improved bonding between the resin-impreg-

Fig. 10 – Optical micrographs acquired at 100· magnification us

composite laminates (a) without and (b) with MWCNT coating.

nated coating and fiber with treatment time may be related

to either increased coating porosity leading to better resin

wetting and/or extended exposure to the alkaline hydrolysis

conditions favorably affecting adhesion.

Fig. 12 shows a high-magnification SEM image in the resin-

rich interlaminar zone of a failed carbon/epoxy-shear sample,

where the fabric was treated with a 10 min-EPD coating of

ozone-PEI MWCNTs. The MWCNTs that are exposed after

fracture show some degree of alignment in the fiber direction,

which may be expected in order to facilitate their diffusion

into the fabric tows. MWCNT alignment could also improve

the shear strength and stiffness of the matrix region and

may contribute to the observed increase in measured

strength.

The results presented for the ozone-PEI treatments in

Fig. 9 suggest that the improvement in strength with increas-

ing deposition time is due to a combination of improved inter-

facial adhesion between the MWCNTs and the fiber surface

(Fig. 11) and some through-thickness distribution of

MWCNTs, which could enable better laminate consolidation

and higher fiber volume fraction. However, whilst these con-

clusions may be drawn for the influence of EPD time on

ozone-PEI treated laminates, the influence of fiber volume

fraction, Vf, should also be considered for the different treat-

ments examined.

The ozone-PEI treatments exhibit lower Vf than the lami-

nates with the ozone and acid treated MWCNTs (Table 3)

and the baseline laminate. Table 4 shows the Vf for the differ-

ent ozone-PEI laminates and the average in-plane shear

strength for the results shown in Fig. 9, compared to the base-

line case. Also shown is a separate treatment which used

ozone treated MWCNTs and a longer treatment time to pro-

vide a higher volume fraction of MWCNTs around 6%. The

overall results do not suggest there is a clear trend in Vf and

shear strength. The 40 min ozone-PEI laminate has a lower

Vf than the 10 min treatment, despite the higher MWCNT

fraction, and would be consistent with increased Vf leading

to better shear strength. The ozone treatment exhibits similar

Vf to the 40 min ozone-PEI treatment but has strength similar

to the baseline laminate, suggesting that a higher matrix vol-

ume does not necessarily provide higher shear strength. Gen-

erally, the fracture surfaces of the lower strength laminates

exhibited clean fracture at the fiber and matrix interface, sug-

ing transmitted light for cross-sections of carbon-epoxy

Fig. 12 – High-magnification SEM image in the resin-rich

interlaminar zone of a failed carbon/epoxy-shear sample

where the fabric was treated with a 10 min-EPD coating of

ozone-PEI MWCNTs.

Fig. 11 – SEM micrographs indicating the fracture surfaces of failed in-plane shear specimens for carbon/epoxy composite

with (a) no treatment and EPD-MWCNT treatment for (b) 30 s, (c) 10 min and (d) 40 min.

Table 4 – Fiber volume fraction (Vf) and in-plane shearstrength for carbon-epoxy laminates prepared using ozone-PEI (Fig. 9) and ozone functionalized MWCNTS.

Treatment None Ozone-PEI

Ozone-PEI

Ozone-PEI

Ozone

EPD time(min)

0 0.5 10 40 40

Vf (%) 60 55 47 50 49Shear strength(MPa)

32 38 44 55 33

4140 C A R B O N 5 0 ( 2 0 1 2 ) 4 1 3 0 – 4 1 4 3

gesting that adhesion between the MWCNTs and fiber surface

is a critical factor and that the interfacial properties make a

significant contribution to the shear strength (See Table 4).

The results and modeling presented by Barbero for E-glass

composite laminates also suggest that in-plane shear

strength increases with increasing Vf [48], which would be

consistent with the 40 min ozone-PEI treatment exhibiting

higher shear strength. In theory, the addition of MWCNTs

should increase the shear modulus and the good interfacial

adhesion afforded by the PEI functionalization should enable

higher shear strain to be achieved, resulting in higher

strength. The results suggest there is a motivation to further

increase the Vf of the ozone-PEI laminates as this may lead to

higher shear strength. The other motivation to achieve higher

Vf in the ozone-PEI MWCNT treated laminates would be to

replicate the in-plane tensile properties of the baseline lami-

nate, which would be higher than the MWCNT laminate due

to the higher Vf.

Additional testing of the carbon/epoxy laminates with

MWCNT treatments examined the mode I fracture-energy.

Fig. 13 shows the mode I fracture-energy measured for car-

bon/epoxy composites where the unidirectional fiber was

treated for increasing EPD times with ozone-PEI MWCNTs.

There is approximately 80% increase in the fracture-energy

between the untreated laminate and the 40-min EPD treat-

ment. The scatter in the results make it difficult to identify

a definitive trend, but there is a clear increase in fracture en-

ergy which corresponds to an increase in the level of plastic

fracture between no-treatment and 40-min EPD treated sam-

ples, as shown in Fig. 14. As was observed with the shear-fail-

ure surfaces in Fig. 11, there appears to be an increase in the

fracture in the coating on the fiber surfaces, although, not to

0

50

100

150

200

250

MWCNT deposition time (min)0 0.5 10 40

GIc

(J/

m2 )

No MWCNT

Ozone + PEI

Fig. 13 – Mode-I fracture energy measured for carbon/epoxy

composites where the unidirectional fiber was treated for

increasing EPD times with ozone-PEI MWCNTs.

C A R B O N 5 0 ( 2 0 1 2 ) 4 1 3 0 – 4 1 4 3 4141

the same level as the shear-failure surfaces. The fracture en-

ergy values are modest compared to some examples where

the CNTs are grown by CVD and are oriented in a perpendic-

ular direction with respect to the fiber [49]. However, in cases

where MWCNTs have been added directly to epoxy resin, sim-

ilar levels of fracture energy were observed at 0.5% by weight

for well-dispersed systems [50]. The fact that the MWCNTs

are aligned more in a two-dimensional plane parallel with

the fiber may reduce their mode I-toughening capacity, com-

pared to perpendicular alignment or random-3D orientation.

Nevertheless, the increase in fracture energy is also associ-

ated with good increases in shear strength. Often with tradi-

tional rubber-toughening systems the increase in toughness

is at the expense of strength [47]. In relative terms, the frac-

ture energy levels measured for the MWCNT treated samples

are comparable to carbon/epoxy composites using toughened

resin systems [51].

Consideration should also be given to the influence of Vf

on the mode I fracture toughness. As for the in-plane shear

Fig. 14 – SEM micrographs indicating the fracture surfaces of fa

composite with no treatment (left) and 40-min EPD of ozone-PE

case, the 40 min EPD treatment had a higher Vf than the

10 min treatment, despite the higher MWCNT fraction. This

may also point towards higher Vf leading to improved fracture

toughness. The influence of resin volume on mode I fracture

toughness is also complicated by the fact that the introduc-

tion of the crack-starter film between the adjacent plies leads

to a localized increase in matrix volume in the region where

the crack initiation energy is measured. Given that the GIc va-

lue measured for the baseline laminate around 112 J/m2

(Fig. 13) is approaching the value of 130 J/m2 reported for the

neat resin [50] this may suggest that resin volume effects

are not making a significant contribution to the GIc value

measured for the laminate with the 40 min ozone-PEI

treatment.

Additional consideration should also be given to the po-

tential effect of resin-rich layers which may develop in the

interply regions as a result of the introduction of CNTs into

the carbon fabric, as identified in a recent review on factors

influencing the performance of hierarchical-composite mate-

rials [52]. Cross-sectional images of the ozone-PEI MWCNT

treated composites shown in the Supplemental information

(Fig. S3) for the different treatment times suggest that the re-

sin rich region in the CNT treated laminates are only margin-

ally thicker than the baseline case. The 10 min ozone-PEI

treated laminate has a resin rich region around 3 fiber diam-

eters or approximately 20 lm, whereas the 40 min treatment

is around 15 lm, compared to the baseline laminate which

is around 8 lm. If the resin-layer thickness was making a sig-

nificant contribution to the toughness then it would be ex-

pected the 10 min treatment should exhibit the highest

toughness, which is not the case, as shown in Fig. 13. Recent

work examining incorporation of resin-interply layers, using

the same resin system as the current work, suggested that

even quite thick resin-interleaf layers of the order of 50 lm

had minimal effect on initiation or propagation fracture

toughness [53]. Higher magnification images of the resin-rich

regions in the ozone-PEI treated CNT laminate (Fig. S4) also

show that the fracture propagates close to the fiber–matrix

interface, which suggests that improved bonding in this re-

gion makes a good contribution to the observed increases in

iled double-cantilever beam specimens for carbon/epoxy

I MCWNTs (right).

4142 C A R B O N 5 0 ( 2 0 1 2 ) 4 1 3 0 – 4 1 4 3

shear strength and fracture toughness. This conclusion is also

consistent with the observation that resin-interply layers

containing CNTs with different functionalization treatments

provided no toughness enhancement compared to a baseline

laminate [53].

4. Conclusions

The use of a recirculating, ultrasonicated, ozonolysis system

has provided an efficient and convenient method for prepar-

ing large volumes of stable-MWCNT dispersions that were

suitable for electrophoretic deposition onto carbon-fiber fab-

rics. The ozone-treated MWCNTs chemically reacted with

the PEI dendrimer and produced a functionalized-EPD coating

on carbon-fibers that enabled good adhesion with an epoxy

resin and the sized carbon-fiber surface. The hierarchical-

composite structures exhibited significant increases in shear

strength and fracture energy due to the relatively high levels

of MWCNTs that occupied the resin-rich interlaminar re-

gions. EPD offered a low-energy method to modify the

mechanical properties of carbon-fiber/epoxy composites in a

controlled manner.

Acknowledgements

This research is funded by the Air Force Office of Scientific Re-

search (AFOSR) Young Investigator Grant (FA9550-09-1-0218),

Dr. Byung-Lip Lee, Program Director and the US Army Pro-

gram Executive Office: Soldier and the Army Research Labora-

tory and was accomplished under Cooperative Agreement

Number W911NF-06-2-011. The authors would like to thank

Mr. Emre Yassitepe for acquisition of the XPS data and Dr.

Lin Gao for film deposition studies.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at http://dx.doi.org/10.1016/

j.carbon.2012.04.061.

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