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Chemical Physics Letters 402 (2005) 340–345

Meso- and nano-scale investigation of carbon fibers coated bynano-crystalline diamond

M. Rossi a, M.L. Terranova b,*, S. Piccirillo b, V. Sessa b, D. Manno c

a Dipartimento di Energetica, Universita �La Sapienza� and Unita INFM-RM1, Via A.Scarpa 14, 00161 Roma, Italyb Dipartimento di Scienze e Tecnologie Chimiche and INSTM, Universita �Tor Vergata�, Via della Ricerca Scientifica, 00133 Roma, Italy

c Dipartimento di Scienza dei Materiali and Unita INFM-Lecce, Universita di Lecce, Via Arnesano, 00133 Lecce, Italy

Received 20 October 2004; in final form 13 December 2004

Available online 4 January 2005

Abstract

A morphological–structural investigation regarding the coating of vapour grown carbon fibers (VGCF) by polycrystalline dia-

mond layers, produced by CVD activation of CH4/H2 mixtures is reported. Transmission electron microscopy (TEM) and electron

diffraction (ED) techniques allowed detailing, for the first time at nanoscale, the morphology and structure of different carbon layers

across the 3-D geometry of the diamond coated fibers. Our results indicate an isotropic distribution of the various carbon phases

along the direction transversal to the fiber axis, and the occurrence of turbostratic graphite layers at the fiber/diamond interface

region. The diamond coating is formed by nano-grains with sizes ranging between 5 and 50 nm and substantially free of extended

defects.

� 2004 Elsevier B.V. All rights reserved.

1. Introduction

Carbon fibers represent an extremely important tech-

nological material [1] and extensive efforts have been

dedicated over the past decade to the research and devel-

opment of new types of meso- and micro-structured Cfibers [2,3].

Exploiting potential applications in various strategic

fields, such as energy saving, thermal management and

space technology, caused researchers to develop strate-

gies apt to produce meso- and micro-sized fibers with

enhanced properties in terms of hardness, chemical

inertness, high thermal conductivity, high sputter-resis-

tance [4,5].The coupling of carbon fibers with diamond, indeed,

may offer significant advantages. However, one major

concern in producing diamond-coated micro-sized car-

0009-2614/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2004.12.052

* Corresponding author.

E-mail addresses: �marialetizia.terranova@roma2.infn.it (M.L.

Terranova).

bon fibers is the strong control of the structural features

of the outermost diamond phase in terms of size, texture

and crystalline quality of grains, but also of the inner

fiber at the end of the coating process. Diamond-coated

carbon-fibers can be considered an innovative diamond-

based composite material for which many possible tech-nological applications can be envisaged [6–10].

In the present Letter, the coating of vapour grown car-

bon fibers by layers of nanocrystalline diamond is

reported. In particular, we investigated and identified

the stacking sequence of the different carbon-based mate-

rial when diamond coating of a carbon fiber is carried out.

VGCF fibers were chosen in view of their outstanding

properties (in primis thermal and electrical conductivity)which make them a widely used functional material [11].

2. Experimental

The objective of our research was to monitor the sta-

bility of the fibers under the conditions of the CVD

M. Rossi et al. / Chemical Physics Letters 402 (2005) 340–345 341

process, to investigate the crystallographic features of

the deposited diamond phase and to detail the micro-

structure of the fiber/diamond systems.

The nanocrystalline diamond deposits have been pro-

duced by CVD technique on isolate fibers, as well as on

textures formed by woven filaments. The deposition runswere performed in a hot filament CVD reactor using

CH4/H2 mixtures. A detailed description of our deposi-

tion system can be found in [12]. The fibers were

like-amorphous VGCFs; we used fibers from different

production stocks with diameters ranging between

7.5 and 15 lm, preliminarily submitted to a seeding

pre-treatment by placing them in an ultrasonic cleaner

with a suspension of fine-grained diamond powders (witha mean size d < 0.25 lm) in hexane. We used ad hoc

designed fiber-holders, screwed at the standard DC-

heated sample holder, to keep the fibers at controlled dis-

tance and position with respect to the heated Ta filament.

The filament temperature, kept at 2200 ± 10 �C, was

monitored by a two-colour optical pyrometer. We sub-

mitted to the coating process single and textured fibers.

In particular, the single carbon fibers have beenmechanically separated by the textured material and

manipulated by using a suitable microtweezers system

under a optical microscope.

In case of single fiber coating, the fiber has been

clamped at the extremities, kept parallel to the filament

at a distance of about 6 mm and heated combining

Joule-effect and heating from the standard sample

holder at a distance of about 1 mm. The relatively lim-ited diameter of the fibers helps to achieve a uniform dis-

tribution of the coating that is carried out under the

Fig. 1. SEM images of: (a) and (b) typical textured array of as-received VGC

single fiber, mechanically detached from its textured array, before (c) and af

typical conditions of a chemical vapour deposition. In

case of textured fibers, the rectangular shaped samples,

with sizes of 2 · 15 mm, were always clamped at the

extremities and kept aligned (along the larger side) to

the filament and vertically positioned (in other words,

sample and filament were in the same vertical plane dur-ing the deposition process).

Several experiments have been performed by varying

the fibers temperature between 650 and 750 �C and

using active gas phases containing CH4/H2 ratios be-

tween 0.5% and 2.0%. The flow rate of the CH4/H2 mix-

tures was fixed at 150 sccm (standard cubic centimeters

per minute), and the pressure in the deposition chamber

was 30 Torr. The experimental set-up allowed a com-plete and uniform coating of the fibers during a single

deposition run, lasting 1 h.

The general shape of the fibers before diamond coat-

ing was observed by SEM (scanning electron

microscopy).

A JEOL 2010 electron microscope operating at

160 kV was used to carry out HRTEM (high resolution

transmission electron microscopy) investigations inbright field mode and TED (transmission electron dif-

fraction) analysis. In particular we used a TEM espe-

cially configured for convergent beam electron

diffraction (CBED) observation with a double tilting

system, allowing a sample tilting of ±60�, particularlysuitable to check the homogeneity of the deposits along

the radial direction.

The obtained HRTEM micrographs have been suc-cessively digitized and filtered in the spatial frequency

field in order to improve the signal-to-noise ratio. The

Fs before (a) and after (b) the diamond deposition process; (c) and (d)

ter (d) the diamond deposition process.

342 M. Rossi et al. / Chemical Physics Letters 402 (2005) 340–345

TED (transmission electron diffraction) patterns were

obtained in selected area electron diffraction (SAED)

conditions.

The RHEED (reflection high energy electron diffrac-

tion) analysis has been performed at variable angles

using a TEM-EM6G apparatus operating at 60 kVand equipped with a high resolution diffraction goniom-

eter stage.

Fig. 2. (a) and (b) Diffraction analysis: experimental SAED pattern (a)

taken from the edge of a single-coated carbon fiber. The comparison (b)

of experimental and [13] interplanar distances and indexing puts in

evidence that all the electron diffraction signals belong to the diamond

phase (s.g. Fd3m). (c)–(f) HRTEM analysis: images (c) and (e), at

increasing magnification, from the outermost polycrystalline diamond

film covering the VGC fiber after CVD processing. Bragg-filtered image

(f) of a single diamond nanograin, from the region evidenced in (e).

3. Results and discussion

The results reported in this Section refer to sample

produced using a 0.5% CH4 in H2 and at substrate tem-perature of 650 �C. These were, indeed, the suitable

experimental conditions for the formation of nanosized

diamond.

The SEM images of Fig. 1 shows the typical morpho-

logies of as-received textured (a and b) and single (c and

d) VGCFs, before (a and c) and after (b and d) diamond

deposition. The fibers were originally assembled as a

cross-texture and the structural analysis by electron dif-fraction did not reveal any presence of crystalline order,

within the sensitivity limit of the used experimental

techniques.

A series of detailed morphological and structural

investigations have been carried out by HRTEM and

TED on deposits obtained on single fibers. Isolated fi-

bers, mechanically detached from the textured substrate,

were also analysed, without revealing detectabledifferences.

The studies by electron diffraction (in both transmis-

sion and reflection conditions) have been performed on

the longitudinal edge of the diamond-coated fibers in or-

der to deduce microstructure and relative location of the

various carbon phases.

The analysis of the SAED patterns produced by the

outermost layers of the material (Fig. 2a and b) indicatesthe presence of the set of Debye�s rings characteristic ofthe diamond phase (s.g. Fd3m); the experimental inter-

planar spacings are found to match with the diamond

reference values [13] within 1% uncertainty. The

HRTEM images of Fig. 2c–f display, at increasing mag-

nification, the morphological features of the outermost

polycrystalline deposits. The SAED and HRTEM anal-

ysis reveal that the outermost layers are formed by dia-mond crystallites with size ranging between about 5 and

50 nm. The digital image processing allows an easy

evaluation of the high crystalline quality of the single

nano-grains. Fig. 2f shows a typical single grain after

digital filtering, evidencing a crystal lattice substantially

lacking of extended defects.

Going on through the outer periphery of the coated

fibers towards the inner core fibers, the ED pattern re-veals the presence of additional diffraction signals, evi-

denced by arrows in the ED pattern of Fig. 3a, that

can be rationalized taking into account the contempo-

rary presence of two different phases in the region inter-

acting with the e-beam. The set of diffraction rings

belongs to diamond phase, while the diffraction arcs

indicate the presence of a different and partially oriented

crystalline phase with an interplanar distance of about

3.43 A, near to the 0002 d-spacing of graphite [13]. By

considering that in the experimental approach thee-beam is driven to a direction normal to the fiber axis,

from the outermost region towards the fiber core, from

the transmission electron diffraction pattern analysis we

Fig. 4. HRTEM images of the intermediate turbostratic-graphite

layers between carbon fiber and diamond layer coating, after its

mechanical detachment. The HRTEM images (a) and (b) have been

taken from the longitudinal edge of single fibers isolated by the sample

used for RHEED analysis after the delamination of the deposited film

from its textured substrate. The arrows in (a) point out some diamond

nanocrystallites still remaining attached on the ordered layer covering

the carbon fiber. The presence of a stacking order in the direction

normal to the fiber axis is enhanced by the digital filtering of the area

reported in the inset ABCD in (b).

Fig. 3. (a) Experimental SAED pattern taken on going from the outer

periphery of the coated fibers towards the inner core fibers. The arrows

point out the diffraction signals that do not originate from the

diamond phase. (b) and (c) Experimental RHEED pattern (b) from

delaminated surface film and calculated diffraction pattern for

turbostratic graphite (c). In (c) the expected planes are also indicated

in case of diffraction signals belonging to regular graphite phase.

M. Rossi et al. / Chemical Physics Letters 402 (2005) 340–345 343

deduced that a possible stacking sequence of the two ob-

served phases exists. Previous experiments regarding the

diamond growth onto different substrates [14–16], sug-gest the presence of an intermediate phase between the

outermost diamond layers and the inner carbon fiber.

To verify this hypothesis on a mesoscopic scale, after

CVD processing we delaminated a portion of the depos-

ited material from its textured substrate, in order to

make possible the analysis of the intermediate layers

by RHEED. The observation results are shown in

Fig. 3b and c. Confirming the TED analysis, the largest

interplanar distance (corresponding to smallest ring) is

d = 3.43 ± 0.03 A, slightly larger than the characteristic

value (d = 3.38 A) reported for the graphite-2H [13].

Moreover, the experimental patterns taken from

this intermediate phase show a reduced number of De-

bye rings in comparison with those expected from reg-ular graphite. The absence of diffraction signals from

the h,k, l (l 6¼ 0) planes and of the 00.4 diffraction, to-

gether with the broadening of the 00.2 Debye ring,

indicates that the observed patterns can be generated

by a disturbed graphitic-like hexagonal lattice with

the basal planes rotated and/or translated in a random

way and not well defined interplanar distances be-

tween the graphene sheets. These features are consis-tent with the presence of the so-called turbostratic

graphite [17–20].

As far as the relative location is concerned, this tur-

bostratic graphite is found as an intermediate layer

between the fiber and the uppermost diamond coverage.

The existence of the intermediate turbostratic-graphite

layers between carbon fiber and diamond deposit has

been fully confirmed by HRTEM observations (Fig. 4)carried out on the longitudinal edge of single fibers iso-

lated from the sample used for RHEED analysis and

prepared by delaminating the deposited film from its

Fig. 5. Schematic axial- and side-view of a single coated carbon fibre (T = 650 �C, 1% CH4/H2), summarizing the location of the different carbon

material, as deduced on the base of electron microscopy and diffraction observations. The sizes of the three different regions are not in scale; the true

sizes are: Region a (inner core): carbon fiber, with diameters ranging between 10 and 15 lm; Region b (intermediate layer): turbostratic graphite, with

an average thickness of about 7 nm; Region c (outermost coating): polycrystalline nanodiamond with an estimated thickness of about 200 nm.

344 M. Rossi et al. / Chemical Physics Letters 402 (2005) 340–345

textured substrate. The delamination of diamond coat-

ings can be easily achieved by strongly bending the sam-

ple, taking advantage of the different elastic propertiesof carbon fibers and diamond; moreover, after this first

mechanical detachment, the samples are ultrasonically

treated. This two steps process allows us achieving an

almost complete detachment of diamond coating.

On the edge of the delaminated fiber some diamond

nanocrystallites are still visible and grown on an ordered

layer covering the carbon fiber, as evidenced by arrows

in Fig. 4a.The presence of a stacking order in the direction nor-

mal to the fiber axis is strongly evidenced by the digital

filtering of the area reported in the inset ABCD of Fig.

4b. The analysis of HRTEM images reveals that the lat-

tice planes are characterized by an interplanar distance

of about 3.45 ± 0.05 A, in good agreement with electron

diffraction measurements. The average thickness of the

intermediate graphitic layer is of about 7 nm (with min-imum and maximum measured value of 5 and 10 nm,

respectively).

The results summarized in Figs. 2–4 remain substan-

tially unchanged tilting the fibers on a transversal plane

to the fiber-axis and containing the e-beam direction.

The homogeneous results obtained from observations

at different tilting angles of diametrically opposed exter-

nal areas of different fibers allow us deducing the pres-ence of an isotropic radial distribution of the different

carbon phases.

The overall fiber/diamond systems result in consisting

mainly of 3 materials, as indicated in the schematic view

sketched in Fig. 5:

(a) An inner core formed by the carbon fiber, which

still maintains the original structure.(b) An intermediate layer formed by turbostratic

graphite, with an average thickness of about 7 nm.

(c) The outermost coating formed by nano-sized dia-

mond crystals without any preferential crystallo-

graphic orientation; the estimated thickness of thepolycrystalline diamond layer is about 200 nm.

4. Conclusions

In the course of the present research proper proce-

dures have been selected for the coating of carbon fibersby homogeneous diamond layers formed by nano-grains

with sizes in the range 5–50 nm. The structure of the

external coating and of the inner carbon phases has been

detailed using high resolution electron microscopy and

electron diffraction in both transmission and reflection

conditions. The coating process was found to generate

an intermediate layer of turbostratic graphite at the

film/fiber interface, and to preserve in any case the struc-tural features of the fibers.

The present synthesis approach can produce isolated

self-supporting tubular structures as well as diamond-

coated textures and wire meshes, all of them

characterized by increased hardness, resistance to

radiation-induced damages and good tribological prop-

erties. Moreover, in view of the thermal hyper-

conductivity due to the coupling of nanodiamond[21,22] to VGCFs, this material is expected to contribute

exceedingly to the thermal management of composite

materials, especially when a reinforcement of the matrix

is also strongly required.

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