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Mechanical properties of vapor grown carbon nanofibers

Tanil Ozkan a, Mohammad Naraghi b, Ioannis Chasiotis b,*

a Mechanical Science and Engineering, University of Illinois at Urbana, Champaign, Urbana, IL 61801, USAb Aerospace Engineering, University of Illinois at Urbana, Champaign, Urbana, IL 61801, USA

A R T I C L E I N F O

Article history:

Received 14 December 2008

Accepted 3 September 2009

Available online 11 September 2009

0008-6223/$ - see front matter � 2009 Elsevidoi:10.1016/j.carbon.2009.09.011

* Corresponding author: Fax: +1 217 244 0720E-mail address: chasioti@illinois.edu (I. C

A B S T R A C T

Individual as-fabricated, high temperature heat-treated and graphitized/surface oxidized

vapor grown carbon nanofibers (VGCNFs), with average diameter of 150 nm were tested

for their elastic modulus and their tensile strength by a MEMS-based mechanical testing

platform. The elastic modulus increased from 180 GPa for as-fabricated, to 245 GPa for high

temperature heat-treated nanofibers. The nominal fiber strengths followed Weibull distri-

butions with characteristic strengths between 2.74 and 3.34 GPa, which correlated well with

the expected effects of heat treatment and oxidative post-processing. As-fabricated

VGCNFs had small Weibull modulus indicating a broad flaw population, which was con-

densed upon heat treatment. For all VGCNF grades, the nanofiber fracture surface included

the stacked truncated cup structure of the oblique graphene layers comprising its backbone

and cleavage of the outer turbostratic or thermally graphitized layer.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

In the last two decades [1], significant research has been de-

voted to vapor grown carbon nanofibers (VGCNFs). They are

produced by catalytic exposure of gaseous hydrocarbons to

high temperatures often resulting in highly ordered oblique

graphene layers arranged as stacked truncated cones sur-

rounded by turbostratic carbon, which is less ordered and,

therefore, more defect prone compared to the graphitic inner

phase [2]. Although the fabrication and structural aspects of

this class of nanofibers have received sufficient attention,

very few studies have directly and indirectly addressed their

mechanical behavior [3,4] as unequivocal mechanical prop-

erty measurement methods are still lacking at this scale. In

this paper, the effects of heat treatment and surface function-

alization on the mechanical strength of three grades of

VGCNFs were investigated by nanoscale tension experiments

on individual nanofibers with the aid of Microelectromechan-

ical Systems (MEMS) tools. These first direct tensile strength

measurements from single nanofibers at the scale of 150–

200 nm are compared to previous experiments with submi-

er Ltd. All rights reserved

.hasiotis).

cron and micron sized fibers [5,6] since no strength data exist

for this class of nanofibers. The measured mechanical proper-

ties are discussed in the context of material post-processing,

the resulting nanofiber structure and the mode of failure as

observed in SEM images.

2. Materials and experimental methods

Three grades of highly graphitic, Pyrograf-III, carbon nanofi-

bers were obtained from Applied Sciences Inc. (Dayton, OH).

The first grade, PR-24-XT-PS, was fabricated from as-grown

VGCNFs through pyrolytic stripping (PS) to remove polyaro-

matic hydrocarbon residues of the synthesis process from

the nanofiber surface. This surface stripping takes place at

around 600 �C without altering the existing carbon nanofiber

microstructure. The second grade, PR-24-XT-HHT-LD, was

the high temperature heat-treated (2800 �C) form of low den-

sity VGCNFs with significantly different structure than the PR-

24-XT-PS fibers. The heat-treated VGCNFs have improved

electrical and thermal conductivities by reducing the struc-

tural disorder of the turbostratic layer and by increasing the

.

240 C A R B O N 4 8 ( 2 0 1 0 ) 2 3 9 – 2 4 4

graphitic content of the fiber [2]. The last grade, PR-24-XT-

HHT-LD-OX, was the surface functionalized derivative of PR-

24-XT-HHT-LD produced by oxidative surface treatment of

the PR-24-XT-HHT-LD VGCNFs to improve their bonded inter-

actions with organic materials. The VGCNFs were received in

a highly entangled form, from which, individual nanofibers

were isolated.

Subsequently, individual VGCNFs were attached to the

grips of a MEMS-based experimental setup with a 2-lm thick

Pt layer deposited with the aid of a Focused Ion Beam (FIB) to

achieve perfect bonding of the nanofibers to the grips. The

mechanical strength experiments were conducted according

to the works by Naraghi et al. [7,8] with the MEMS devices fab-

ricated at the Sandia National Laboratories using the SUM-

MITTM process. They incorporated a double column

polycrystalline silicon loadcell, as shown in Fig. 1a. The load-

cell deflection and the opening of the grips in each experiment

were recorded by a CCD camera at 500· optical magnification.

A Digital Image Correlation (DIC) calculation of the loadcell

deflection and the nanofiber extension was performed with

displacement resolution of �25 nm [7,8]. This method elimi-

nates the need for high resolution Scanning Electron Micros-

copy (SEM). In order to calculate the axial force on the fiber,

the spring constants of the loadcells were determined experi-

mentally by a new traceable force calibration method de-

scribed in [9]. After each experiment, both ends of the

ruptured carbon nanofibers were imaged by an SEM at

·200 K magnification to measure the outer fiber diameter as

well as to identify the mode of failure. Although the outer ra-

Fiber ends attached with Pt blocks

Fiber gage section

60 µm

Loadcell(a)

Fig. 1 – (a) A VGCNF tested by the MEMS-based mechanical prop

VGCNF starting with a loose fiber. The slope of the linear fit of

gradual change in the fiber stiffness is due to the fiber wavines

Table 1 – Weibull parameters, elastic moduli and mean strength

Nanofiber grade Weibullmodulus, m

Characteristrength, r

PR-24-XT PS 2.4 3.35PR-24-XT-HHT-LD 7.3 2.85PR-24-XT-HHT-LD-OX 4.4 2.75

dius was determined accurately, the inner radius was not al-

ways possible to measure. Thus, the analysis in this paper

employs the nominal nanofiber strength using solely the outer

fiber diameter.

The stress–strain curves were similar to that shown in

Fig. 1b. The elastic modulus was obtained after significant

force was applied to the fibers to straighten them completely

as they were wavy in their unloaded state. Therefore, mea-

surements taken at small applied forces do not provide a reli-

able value for the nanofiber modulus. The tensile strength

was calculated by a probabilistic analysis using a two param-

eter Weibull cumulative probability density function [10]:

Pf ðrÞ ¼ 1� exp � rr0

� �m� �; ð1Þ

where r, is the applied stress resulting in a probability of fail-

ure, Pf(r), m is the Weibull modulus, which provides a mea-

sure of the scatter in the strength data, and r0 is the

material stress parameter. The Weibull parameters were com-

puted by the maximum likelihood method where a system of

coupled nonlinear equations was solved numerically accord-

ing to [11].

3. Results and discussion

Table 1 lists the Weibull cumulative probability density func-

tion parameters computed for the three data sets in Fig. 2a–c

by using the maximum likelihood method. The maximum

likelihood analysis provided marginally better fitting than

E = 302.5 GPa

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.01 0.02 0.03 0.04Strain

Stre

ss (G

Pa)

(b)

erty measurement platform. (b) Stress–strain curve of a

the data in filled circles is the elastic modulus. The initial

s in its natural state.

for three VGCNF grades.

stic

C (GPa)Elastic modulus,E (GPa)

Average strength(GPa)

180 ± 60 2.90 ± 1.4245 ± 52 2.35 ± 0.4– 2.40 ± 0.6

Fig. 2 – Probability of failure vs. tensile strength for (a) pyrolytically stripped, (b) high temperature heat-treated, and (c)

oxidatively processed high temperature heat-treated carbon nanofibers. The maximum likelihood function provided in

general better fitting to the experimental data compared to the linear regression analysis due to the limitations in the

probability estimators that can be used in the latter to describe small sets of experimental data.

C A R B O N 4 8 ( 2 0 1 0 ) 2 3 9 – 2 4 4 241

linear regression, due to the relatively small number of data

points that could not be described as precisely by a probability

estimator [11]. However, linear regression provided virtually

the same values for the Weibull parameters. For two of the

three fiber grades, the calculated Weibull moduli were rela-

tively small indicating the presence of a broad spectrum of

flaws [12]. The characteristic strength of pyrolytically stripped

(PS) nanofibers was 3.34 GPa compared to 2.84 GPa for the high

temperature heat-treated carbon nanofibers (HHT), see Table

1. The effect of further oxidative surface treatment on the

average fiber strength was less significant than that of the high

temperature heat treatment, as the characteristic strength de-

creased slightly to 2.74 GPa. The positive effect of post-fabrica-

tion heat treatment on the spread of the tensile strength data

was reflected in the higher Weibull moduli of HHT and HHT-

OX grades compared to that of the PS grade, as shown in Table

1. A comparison between the characteristic strength of the

HHT and the HHT-OX fibers implies a slight or no increase in

the average size of the catastrophic flaws. On the other hand,

it appears that the oxidative surface treatment process in-

creases flaw size randomness as evidenced by the increase

of the standard deviation of the strength data and the associ-

ated decrease in the Weibull modulus listed in Table 1.

Our strength analysis considered only the nanofiber outer

diameter. Although this analysis does not provide the true

strength of the nanofibers, it results in nominal strengths

which can be directly used in nanocomposite materials prop-

erty calculations, as it is highly unlikely that the nanofiber in-

ner hollow core interacts with polymer matrix. For a few

carbon nanofibers we obtained both their outer and inner

diameters via post-mortem imaging by TEM or SEM as shown

in Figs. 3 and 4. In general, the diameter of the hollow core

was about ½ that of the outer fiber diameter. Consequently,

the true fiber strength is �33% higher than the nominal,

which is similar to that of high strength microscale carbon fi-

bers [13,14].

The tensile strength results in Table 1 are in agreement

with existing literature, where reductions in the fiber strength

were observed with increasing graphitization temperature.

According to [2], this is due to the reduction of the turbostratic

layer and the increase of the order in the graphene planes.

These structural transformations occur at the expense of

the vapor deposited turbostratic carbon layer, as also shown

in the high resolution TEM pictures of Fig. 3a and b. Similarly

to microscale carbon fibers, the outermost turbostratic carbon

layer in pyrolytically stripped carbon nanofibers resists crack

Fig. 3 – TEM images of (a) pyrolytically stripped, and (b, c and d) high temperature heat-treated VGCNFs. The turbostratic

carbon layer is significantly reduced after heat treatment as shown in (b) with two collinear side arrows. (c) The originally

turbostratic layer is graphitized after high temperature heat treatment, and connectivity loops form between this

transformed annular layer and the oblique graphene planes of the inner layer, while wedge discontinuities arise at the

interface of the two layers, as pointed by the single sided arrows in (b) and (c). (d) Loops also form at the interior end of the

stacked graphene inner layers during heat treatment. Notice the significantly larger surface roughness of the pyrolytically

stripped nanofiber compared to the high temperature heat-treated nanofiber.

Fig. 4 – Rupture surfaces of (a) PS and (b) HHT-LD VGCNFs. The fractured fiber profiles show cleavage fracture of the outer

turbostratic and sliding of the inner graphene planes of the stacked truncated cone structure of the nanofibers as shown by

the remnant protruding cones.

242 C A R B O N 4 8 ( 2 0 1 0 ) 2 3 9 – 2 4 4

initiation, while the inner oblique graphene layer structure is

more prone to sliding. The mechanical benefit of the turbost-

ratic carbon layer is owed to its co-axial orientation with re-

spect to the nanofiber, although the catalytically grown

inner graphene layers are oriented obliquely with respect to

the nanofiber axis, see Fig. 3a and c.

C A R B O N 4 8 ( 2 0 1 0 ) 2 3 9 – 2 4 4 243

SEM fractography showed the fracture of the outer tur-

bostratic layer in the as-fabricated nanofibers, Fig. 4a, and

of the graphitized turbostratic (co-axial) layer in high temper-

ature treated nanofibers, Fig. 4b, and protruding inner graph-

ene layers originating in the original stacked truncated cone

structure of VGCNFs. The cone angle in the protruding seg-

ment of the fracture cross-sections in Fig. 4a and b is close

to that shown in the TEM images in Fig. 3b–d pointing out

to mutual sliding of the graphene planes of the stacked cup

inner fiber structure.

The high temperature heat treatment reduced the average

and the characteristic strengths of as-fabricated VGCNFs by

15–20%, but also reduced the outer surface roughness of the

VGCNFs, as shown in a comparison of Fig. 3a and c, which,

in turn, significantly reduced the scatter in the catastrophic

flaw population, as evidenced by the increase in the Weibull

modulus and the reduction in the standard deviation of the

strength values of the high temperature heat-treated VGCNFs.

Finally, the marginal additional reduction in the fiber strength

after surface oxidation implies that while the average detri-

mental flaw size remains the same, the variability in the flaw

size did increase as pointed out by the clear reduction in the

Weibull modulus.

In comparison to the few literature data, the VGCNF

strengths measured here were obtained from the thinnest fi-

bers tested to date; their diameter was on average 50% smaller

than that of the smallest fibers tested in [5]. The majority of fi-

ber strength values reported in [5] were in the 500–1000 MPa

range for nanofiber diameters between 300 and 1000 nm.

The authors showed a significant diameter size effect on the

mechanical strength of their nanofibers and a rapidly increas-

ing strength for diameters �300 nm. If extrapolated to smaller

diameters, their reported trends could predict the consider-

ably higher nanofiber strengths reported here. The only other

literature report on vapor grown carbon fiber strength [6] pro-

vided a strength value of 2.92 GPa, which is very similar to the

nanofiber strengths measured in this paper.

It is of interest to put the experimental strength and the

elastic modulus measurements in perspective of the struc-

ture of the three VGCNF grades. In microscale carbon fibers,

high temperature heat treatment above 1600–1800 �C does de-

crease the carbon fiber strength and increases its Young’s

modulus [13,14]. This increase in the Young’s modulus is

due to the graphitization of the turbostratic layer. These

trends are also reflected in the experimentally measured val-

ues reported in Table 1. High temperature heat treatment in-

creased the nanofiber elastic modulus by 35% compared to

the modulus of as-fabricated nanofibers. The elastic moduli

that were obtained in this work (180 ± 60 GPa for pyrolytically

stripped and 245 ± 52 GPa for high temperature heat-treated)

are higher than those obtained by AFM-based simple bending

experiments [3]. The much lower values and the large scatter

in the moduli reported in [3] could be explained by the small

slope of the initial segment of the stress–strain curve in

Fig. 1b where until the fiber is completely stretched, which re-

quires several tens of lN of force, the effective fiber stiffness

is significantly smaller than the fiber elastic modulus. This

implies that, even when fully aligned, unless significant stress

is applied to VGCNFs to remove their initial tortuosity, these

fibers do not contribute to the composite stiffness with their

true stiffness. The experiments reported in [3] were con-

ducted via bending of VGCNFs with AFM probes that provide

limited force capacity which is not enough to perfectly

straighten the nanofibers.

4. Conclusions

The mechanical strength and the elastic modulus of individ-

ual nanofibers of pyrolytically stripped, high temperature

heat-treated and oxidized VGCNF were measured by a

MEMS-based nanomechanical property characterization

method. Heat-treatment increased the nanofiber elastic mod-

ulus from 180 to 245 GPa while the mechanical nanofiber

strength was reduced by 15–20%. Therefore, an improvement

in the thermal and electrical properties is achieved at the ex-

pense of the mechanical strength of VGCNFs. On the other

hand, the strength of heat-treated nanofibers exhibited re-

duced standard deviation compared to the pyrolytically

stripped grade, implying a broad flaw distribution in the tur-

bostratic layer, which is annealed with heat treatment. These

experimental trends agree with the randomness and the size

of the surface flaws in pyrolytically stripped and high temper-

ature heat-treated VGCNFs. The narrow flaw distribution in

the graphitized VGCNFs was slightly broadened by the oxida-

tive treatment that affects only the fiber surface. Post-mortem

SEM/TEM imaging showed a cleaved turbostratic and graphite

converted turbostratic layer for the as-fabricated and the

heat-treated nanofibers, respectively, and a protruding cone

at the center of the fracture surface of all grades of nanofibers

that originated in the stacked truncated graphene cones in

the nanofiber interior.

Acknowledgements

The authors acknowledge the support by the Air Force Office

of Scientific Research (AFOSR) through Grant FA9550-06-1-

0140 with Dr. B.L. Lee as the program manager. The authors

also thank Drs. M. Marshall and W. Swiech at the Frederick

Seitz Materials Research Laboratory of UIUC for their help

with SEM and TEM imaging.

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