Preparation of pyrolytic carbon coating from acetylene by cold wall chemical vapour deposition process

Article abstract of DOI:10.14233/ajchem.2013.12974

Pyrolytic carbon coatings from acetylene were deposited on carbon monofilaments at different temperatures by cold wall chemical vapour deposition equipment. The morphology and microstructure of coatings were investigated by scanning electron microscopy an

Full text of DOI:10.14233/ajchem.2013.12974

Asian Journal of Chemistry; Vol. 25, No. 2 (2013), 871-873
http://dx.doi.org/10.14233/ajchem.2013.12974
Preparation of Pyrolytic Carbon Coating from Acetylene by
Cold Wall Chemical Vapour Deposition Process
1
,2,*
1
1
1
1
1
SONG ZHAO , JINGLI SHI , XINGHAI WEI , XI YAN , QUANGUI GUO and LANG LIU
1
2
Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Science, Taiyuan 030001, P.R. China
Graduate University of the Chinese Academy of Sciences, Beijing 100039, P.R. China
*
Corresponding author: Fax: +86 351 4083952; Tel: +86 351 4083952; E-mail: zsbird1223@yahoo.com.cn
Received: 31 October 2011; Accepted: 24 August 2012)
(
AJC-11999
Pyrolytic carbon coatings from acetylene were deposited on carbon monofilaments at different temperatures by cold wall chemical vapour
deposition equipment. The morphology and microstructure of coatings were investigated by scanning electron microscopy and Raman
spectroscopy. The results indicate that the increase of temperature from 1273 to 1773 K promotes the crystallization of pyrolytic carbon.
Following the "particle-filler" model of carbon formation, it can be explained that the increase of temperature is famous for the elimination
of amorphous carbon and crystallization of pyrolytic coating through the inhibition of formation of small linear hydrocarbons in the gas
phase.
Key Words: Pyrolytic carbon, Monofilaments, Chemical vapour deposition.
using acetylene as the precursor in a cold wall reactor. The
effect of deposition temperature on the microstructure of the
pyrolytic carbon coating was discussed.
INTRODUCTION
Compared with tungsten wire, the carbon filament has
lower density and can avoid detrimental reactions between
1,2
the substrate and coating . Thus, carbon monofilaments are
often used as the substrate to fabricate advanced SiC fibers by
EXPERIMENTAL
3
-5
The laboratory-scale reactor for the chemical vapour
deposition on a static monofilament is shown in Fig. 1. The
pitch based carbon filament was fed into the reactor from
upper end and it was resistance heated by applying direct
current through mercury contacts. The coating was carried
out in a cold-wall reactor consisting of a vertical glass tube (L
chemical vapour deposition (CVD) . The CVD-SiC fibers
with carbon core have outstanding properties at high tempe-
ratures and have been eagerly sought for use in the aerospace
6-10
field
.
It has been reported that before the SiC was deposited,
the carbon monofilament was usually coated with a thin layer
of pyrolytic carbon (PyC) providing a capacity to absorb the
effect of the thermal expansion mismatch between the carbon
=
540 mm, d = 30 mm) connected to gas inlet/outlet flanges at
both ends (top and bottom).Acetylene was used as the precursor
for depositing pyrolytic carbon coating. The temperature of
the filament was adjusted by the power output.
1
1-14
core and the SiC coating . However, the investigation about
of pyrolytic carbon coating deposited on the carbon mono-
filament was hardly found.
For a better understanding of the CVD process and
accordingly of the improvement of the filament properties,
research of the deposition mechanism and the interface
between the carbon core and pyrolytic carbon coating is nece-
ssary. Though the deposition mechanism of pyrolytic carbon
15-19
coating in a hot wall reactor has been studied greatly , the
study of pyrolytic carbon deposited on carbon monofilaments
by cold wall chemical vapour deposition in atmospheric
pressure has not yet been reported. In the present experiment,
the pyrolytic carbon coating on carbon fibers was prepared by
Fig. 1. Schematic diagram of the typical CVD apparatus used for PC
coating on carbon monofilament.

872 Zhao et al.
Asian J. Chem.
General procedure: The pyrolytic carbon coating was
deposited on the carbon monofilament at temperatures from
lline graphite structure with the D band (for disorder) at 1365
-1
-1
cm and the G band (for graphite) at 1580 cm (Fig. 3). The
intensity ratio I /I and the full widths at half maximum
1
273 to 1773 K using acetylene as a source material and H
2
as
D
G
a carrier gas. The gases were kept contant: H 1 L/min, C
2
2
H
2
(FWHM) of the D and the G bands have been widely used to
characterize the degree of in-plane crystallinity and the crysta-
llite size of primary graphitic domains in carbons.
2
00 mL/min. The deposition time of every sample was 1 min.
Detection method: The surface temperature was measured
with a high-resolution bichromatic pyrometer (Ircon Mirage).
The morphology and the microstructure of the filaments were
characterized with a field emission gun scanning electron
microscope (FEG-SEM) (NOVA NANOSEM430, FEI). RMS
analyses were carried-out at room temperature in a back-
scattering geometry using the 532 nm line of an Ar laser as
the excitation source with a power of 3 mW focused to a 1 µm
diam spot size through an optical microscope (100X). Spectra
were obtained at the longitudinal surface of filaments.
The I
D
/I ratio (R) and the FWHM of the G and the D
G
bands of PyC coatings were determined and are presented in
Table-1. When the deposited temperature increased from 1273
to 1673 K, the value of R was decreased from 1.24-0.90 and
the FWHM of the G band became narrower. The FWHM
intensity of the G band has been correlated to the level of
graphitization of carbon samples . Additionally, it has been
previously found empirically that the ratio I /I is inversely
proportional to the in-plane "crystal size" (La) which would
be better described as the diameter of a coherent domain for
20
D
G
RESULTS AND DISCUSSION
21
lattice fringes on carbon samples . Thus, the results indicate
that larger in-plane crystallites and a higher degree of order of
pyrolytic carbon are deposited when the deposition temperature
increases.
The morphology of pyrolytic carbon coatings deposited
on carbon monofilaments were observed by SEM. Fig. 1a-f
are the typical SEM images of transverse section of pyrolytic
carbon coatings deposited at different temperatures, respec-
tively. It can be seen from Fig. 2a-b that the coating deposited
at low temperature is smooth and dense. When the deposition
temperature increases (Fig. 2c-f), the morphology of coating
becomes rough and several layers appear. It can be seen obvi-
ously that the pyrolytic carbon grows laminar by laminar in
the pattern of the curved surface. From the microscopy of
Fig. 2e, it can be concluded that the laminar microstructure
has great effect on the crack deflection.
Based on the analysis above, it can be indicated that the
microstructure of pyrolytic carbon coating from acetylene is
22,23
a function of the temperature of deposition. Lieberman et al.
suggested that the value of the C /C ratio in the gas phase
H
2 2
H
6 6
TABLE-1
RMS PARAMETERS FOR THE SURFACE
OF THE PyC COATINGS
T (K)
273
1473
1673
I /I
FWHMG
83.7
67.1
FWHMD
76.9
65.8
D
G
1
1.24
1.13
0.90
The Raman spectrum of the pyrolytic carbon coatings
deposited at different temperature show a typical polycrysta-
58.8
87.3
Fig. 2. SEM micrographs of the pyrolytic carbon coating deposited for 1 min on carbon monofilaments at different temperatures. (a) 1273 K,
b) 1373 K, (c) 1473 K, (d) 1573 K, (e) 1673 K, (f) 1773 K
(

Vol. 25, No. 2 (2013)
Preparation of Pyrolytic Carbon Coating from Acetylene 873
G band
1
linear hydrocarbons are used to form amorphous carbon. Thus,
the increase of temperature is for the elimination of amorphous
carbon and crystallization of pyrolytic coating through the
inhibition of formation of small linear hydrocarbons in the
gas phase.
42000
36000
30000
24000
D band
673K
Conclusion
The increase of deposition temperature from 1273 to 1773 K
promotes the crystallization of pyrolytic coating from acetylene
by cold wall chemical vapour deposition process. Following
the "particle-filler" model of carbon formation, it can be explai-
ned that the increase of temperature is famous for the elimination
of amorphous carbon and crystallization of pyrolytic coating
through the inhibition of formation of small linear hydrocarbons
in the gas phase.
3
2
1
000
000
000
D band
G band
1
473K
D band
1
2000
G band
8
000
000
1273K
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