Abstract Presented are results of a study on specific technological properties of affordable C/SiC composite nanomaterials obtained via pyrolysis of several pitch/silicon-bearing precursor systems (elemental Si, silica SiO 2 , poly(carbomethylsilane) [-CH 2 -Si(H)CH 3 -] n , commercial SiC). For pyrolysis at 1300°C, the formation of nanosized SiC is detected in the systems with elemental Si and poly(carbomethylsilane) while 1650°C pyrolysis is required for silica to achieve such conversion. In situ formed nano-SiC is homogeneously dispersed in the simultaneously evolving graphitic carbon matrix of the composites. Reactivities vs. CO 2 , electrical resistivities, and surface properties of the nanocomposites are determined. Significant differences and patterns in the properties among the materials obtained from these precursor systems and at the selected pyrolysis temperatures are clearly established. Among others, the data suggest potential for carbon removal from the most reactive nanocomposites via reactions with CO 2 to yield unique nano-SiC powder products for further processing towards electronic and ceramic applications

Cezary Czosnek

Jerzy F Janik

Mariusz Drygaś

CiteSeerX

Microelectronic Engineering 81 (2005) 353–357
www.elsevier.com/locate/meeThe effect of precursor system on the resistivity and
oxidation susceptibility of C/SiC nanocomposites en route
to electronic grade nanomaterials
Cezary Czosnek, Mariusz Drygaś, Jerzy F. Janik *
AGH University of Science and Technology, Faculty of Fuels and Energy, Al. Mickiewicza 30, 30-059 Kraków, Poland
Available online 7 April 2005
Dedicated to honor Professor Aleksander Karcz on the occasion of his 65th birthday. Sto lat!Abstract
Presented are results of a study on specific technological properties of affordable C/SiC composite nanomaterials
obtained via pyrolysis of several pitch/silicon-bearing precursor systems (elemental Si, silica SiO2, poly(carbomethylsi-
lane) [–CH2–Si(H)CH3–]n, commercial SiC). For pyrolysis at 1300 C, the formation of nanosized SiC is detected in the
systems with elemental Si and poly(carbomethylsilane) while 1650 C pyrolysis is required for silica to achieve such con-
version. In situ formed nano-SiC is homogeneously dispersed in the simultaneously evolving graphitic carbon matrix of
the composites. Reactivities vs. CO2, electrical resistivities, and surface properties of the nanocomposites are deter-
mined. Significant differences and patterns in the properties among the materials obtained from these precursor systems
and at the selected pyrolysis temperatures are clearly established. Among others, the data suggest potential for carbon
removal from the most reactive nanocomposites via reactions with CO2 to yield unique nano-SiC powder products for
further processing towards electronic and ceramic applications.
 2005 Elsevier B.V. All rights reserved.
Keywords: Coal tar pitch; Silicon carbide; SiC; Nanomaterials; Nanocrystalline; Composites1. Introduction
Silicon carbide SiC thanks to its advantageous
mechanical, thermal, and electrical properties has0167-9317/$ - see front matter  2005 Elsevier B.V. All rights reserv
doi:10.1016/j.mee.2005.03.031
* Corresponding author. Tel.: +48 12 617 2577; fax: +48 12
617 2066.
E-mail address: janikj@uci.agh.edu.pl (J.F. Janik).found numerous applications. It has been exten-
sively used in modern electronics to make arti-
facts designed to work at increased temperatures
and/or under high voltage and high AC frequen-
cies where it often supercedes elemental silicon Si
[1]. Both silicon and silicon carbide are known to
get oxidized under suitable conditions to ther-
mally stable silicon dioxide SiO2 (silica) and theired.
354 C. Czosnek et al. / Microelectronic Engineering 81 (2005) 353–357grains are always covered by thin compact films
made of Si–O moieties. This is especially true
with nanocrystalline forms of SiC that under oxi-
dizing conditions become easily coated with the
oxide layer, compared with somewhat more
robust monocrystalline SiC, resulting in electri-
cally negative character of the oxidized surface
layer that may improve the electric field distribu-
tion at the edge of power devices. Also, SiC has
been considered in composite materials for con-
struction of nuclear reactors and various reactors
linings [2,3].
Traditional ways to synthesize bulk SiC rely on
thermochemical reactions between elemental Si or
SiO2 and solid sources of carbon C. Plausible car-
bon sources also include coal tar pitches with low
softening points which are complex mixtures of
hydrocarbons with a range of molecular make-up
that are usually much more reactive toward silicon
precursors than solid carbons. Other available
routes to bulk SiC depend on pyrolytic decompo-
sition of single-source precursors such as carbosi-
lane and carbosiloxane polymers or even biomass
containing silicon-bearing components. For exam-
ple, the latter source enables at relatively low tem-
peratures the preparation of nano-SiC powders
that may, subsequently, be used in sintering/grain
growth processes to yield ceramic composites with
tailored microstructures [4,5] and advantageous
mechanical features due to high grain boundary
strength [6].
Herein, presented is a study aimed at appraising
several ‘‘practical’’ properties of nanocomposites
C/SiC prepared via pyrolysis of mixtures made of
various silicon-bearing precursors and a coal tar
pitch (reactive and affordable source of C), mainly,
the composites suitability for making pure nano-
powders of SiC by controlled oxidation/removal
of the carbon component.2. Experimental
The final composites C/SiC were prepared by
pyrolysis under an argon atmosphere of the initial
solid carbonizates obtained at 500 C that were,
subsequently, treated for 1 h at 1300 or 1650 C.
Each initial carbonizate was made from a thor-oughly homogenized mixture of a coal tar pitch
and a selected silicon-bearing additive (15 wt%).
Reference carbonizates were also made at the
two final temperatures from a non-modified (pure)
pitch. The silicon-bearing precursors included ele-
mental Si, silica SiO2, poly(carbomethylsilane)
(PCS), and commercial SiC as yet another refer-
ence system. The starting mixtures were first
appropriately homogenized as powdered solids
followed by stirring in a liquid medium of a molten
pitch at 160 C. The detailed descriptions of the
procedures and characterization data for these sys-
tems have already been published [7–9].
Herein, the nanocomposites C/SiC that con-
tained free carbon were subjected to investigations
of their suitability to oxidation under a CO2 atmo-
sphere by determinations of the reactivity indices
R characterizing a propensity of the carbon com-
ponent toward mild oxidation with CO2 [10]. In
this method, two grams of a 0.2–0.4 mm grain size
fraction of a material are placed in a quartz tube
and heated to 1000 C while being purged by a
3 cm3/s stream of CO2 at a constant pressure of
101.325 kPa. The carbon dioxide gas passes
through the materials bed of which temperature
is continuously measured by a thermocouple.
The content/concentration of CO in the off-gas is
recorded by an analyzer and used to calculate
the reactivity index R (cm3/g Æ s]. In general, the
bigger is the index the higher is the materials reac-
tivity. Electrical resistivities of the powder materi-
als were determined with a home-made apparatus
based on the methodology described in Polish
Standard BN 77/0511-30. The apparatus consists
of a cylindrical volume bored in an insulating body
and equipped with two fitted copper pistons. One
of the pistons is able to move inside the cylinder
and, therefore, used to make compacts of the pow-
ders. Both pistons are connected by electrical
cables to an electronic set-up and also serve as
‘‘points’’ for measurements of electric voltage gra-
dients. The resistivities are derived from measure-
ments of the current and voltage gradient, the
latter resulting from the resistance and geometrical
constraints of the sample in the cylinder according
to: r = U Æ S/I Æ L, where r is the resistivity, U the
voltage gradient, S the cross-sectional area of the
sample, I the current, and L the distance between
C. Czosnek et al. / Microelectronic Engineering 81 (2005) 353–357 355the pistons (length of sample). All determinations
were carried out under identical conditions includ-
ing applications of the same compacting force.3. Results and discussion
The reactivity and electrical resistivity indices
along with the data on helium densities and
BET surface areas determined for the composites
and reference materials are presented in Table 1.
Two types of reference materials were prepared
including the non-modified carbonizates derived
from a pure pitch and composites from the sys-
tem pitch/commercial SiC both after the 1300
and 1650 C treatments. At this point, it is useful
to recall that our previous studies on the forma-
tion of SiC in these systems point out to its evo-
lution already at 1300 C in the systems with
elemental Si and PCS while treatments at
1650 C were necessary in the system pitch/SiO2
and with other related Si–O group containing
precursors [7–9,11].
The current data for the 1300 C carbonizates
indicate that the highest reactivity indices are
found for the non-modified carbonizate and for
the composite from the system pitch/PCS. A sig-
nificantly lower value is obtained for the compara-
ble composite in the system pitch/elemental Si. On
the other hand, the composites from the two
remaining systems in which the silicon precursors
are mostly chemically unchanged after this treat-
ment, i.e., pitch/SiO2 and pitch/commercial SiC,
are characterized by comparable indices that are
about half of the reactivity index value for the ref-Table 1
Characteristics of the composites C/SiC and reference carbonizates
Sample dHe (g/cm
3) SBET (m
2/g)
1300 C 1650 C 1300 C 1650 C
A 2.15 2.43 1.4 2.0
B 2.18 2.44 4.0 8.3
C 2.28 2.37 0.4 1.2
D 1.83 1.76 1.3 1.9
E 2.04 2.09 1.4 0.7
Note: samples (carbonizates) obtained from different precursor syste
(silica); C, pitch/commercial SiC; D, pitch/poly(carbomethylsilane) (Perence non-modified carbonizate. This can be
interpreted in terms of the propensity of materials
for free carbon oxidation/removal. Therefore, the
composite from the system pitch/elemental Si is
the most robust and less reactive than the non-
modified carbonizate while the one from the sys-
tem pitch/PCS is the least resistant to oxidation
under applied conditions. In this regard, it is worth
mentioning that CO2 interacts with free carbon on
accessible pore surfaces according to the Boudou-
ards reaction
CO2ðgÞ þ CðsÞ $ 2COðgÞ ð1Þ
while silicon carbide of which presence was con-
firmed by XRD in the 1300 C composites from
the systems with elemental Si and PCS but not
with SiO2 [7] is not expected to appreciably react
with CO2 at 1000 C, the temperature applied in
the reactivity determinations [12].
The relatively low reactivity indices for the
1300 C carbonizates from the individual systems
pitch/elemental Si, SiO2, and commercial SiC
may likely result from the formation of surface
species SiOxCy via chemical interactions of the
precursors with the evolving carbon matrix. In this
regard, on the one hand, both elemental Si and
commercial SiC grain surfaces are covered with
protective layers of SiO2 and, on the other hand,
there are reactive oxygen-bearing pitch compo-
nents that all could be the source of oxygen toward
the formation of such unreactive/protective spe-
cies. The SiOxCy linkages are known to resist oxi-
dation up to 1400 C [13,14] and, therefore, could
contribute to enhanced oxidation resistance in the
composite system with carbon [15].Reactivity Index
RCO2 (cm
3/g Æ s)
Resistivity (X Æ cm)
1300 C 1650 C 1300 C 1650 C
0.08 0.33 0.22 0.18
0.18 0.76 0.16 0.63
0.17 0.30 0.10 0.08
0.42 0.27 0.29 0.02
0.30 0.16 0.05 0.05
ms are labeled as follows: A, pitch/elemental Si; B, pitch/SiO2
CS); E, non-modified pitch.
356 C. Czosnek et al. / Microelectronic Engineering 81 (2005) 353–357Among the 1650 C carbonizates, the one from
the system pitch/SiO2 shows an outstandingly high
reactivity index that can be linked to the compos-
ites highest BET surface area as a decisive factor
influencing its reactivity. All the remaining com-
posites show similar alas higher reactivities than
the reference non-modified carbonizate. The gen-
erally higher and rather unexpected reactivities of
the 1650 C composites compared to the relevant
non-modified carbonizate might be explained by
superimposition of two opposite effects: first, an
intrinsically increased crystalline ordering of car-
bon matrix toward a graphite structure at higher
temperatures and, second, an increased pore sur-
face at grain boundaries due to efficient thermo-
chemical reactions between the carbon matrix
and the silicon-bearing additives often associated
with massive gas evolution.
As far as the first factor is concerned, increas-
ing ordering of graphitic domains of the carbon
matrix should result in an improved oxidation
resistance (lower oxidation reactivities) of the
materials. In this regard, our XRD study clearly
indicated that, other things being equal, the
1650 C composites showed inferior crystalline
ordering of the C-lattice relative to the reference
non-modified carbonizate [7]. Looking at the sec-
ond factor, one should expect a positive correla-
tion between the grain surface area/number of
active centers and reactivity toward oxidation
[16]. The BET surface areas of the 1650 C mate-
rials, SBET, show that the lowest value is found
for the non-modified carbonizate to be compared
with its lowest reactivity index. Apparently, in
this case both factors add synergistically causing
the carbonizate to be the least reactive toward
oxidation.
The highest reactivity among the 1650 C mate-
rials is exhibited by the composite from the system
pitch/SiO2 that, at the same time, has the highest
BET surface area. The latter observation is consis-
tent with the completion of carbothermal reduc-
tion of silica at this temperature according to the
overall reaction
SiO2ðsÞ þ 3CðsÞ $ SiCðsÞ þ 2COðgÞ ð2Þ
In the initial stage, SiO2 presumably reacts with C
to yield the following gaseous by-productsSiO2ðsÞ þ CðsÞ $ SiOðgÞ þ COðgÞ ð3Þ
while the resulting CO gas reacts further with
available silica
SiO2ðsÞ þ COðgÞ $ SiOðgÞ þ CO2ðgÞ ð4Þ
The carbon dioxide is then involved in the equilib-
rium with CO and C (reaction 1) and, subse-
quently, silicon carbide is mainly formed from
SiOðgÞ þ 2CðsÞ $ SiCðsÞ þ COðgÞ ð5Þ
It worth noting that the formation of solid SiC is
accompanied by the evolution of significant gas-
eous species CO/CO2 leaving the reaction system
with the purging argon gas. The net effect, there-
fore, is the enhancement/enlargement of pore sur-
face area both due to the reaction-promoted
evolution of inter-grain surfaces and, possibly,
opening of closed pores.
The results on electrical resistivity in Table 1
show that, generally, the resistivities of all the Si-
modified composites are higher than the resistivi-
ties of the reference non-modified carbonizates.
This can likely be linked to the fact that both the
pure silicon-bearing precursors and SiC are known
to have higher resistivities than the comparable
pure carbonizates obtained from the pitch. How-
ever, other factors such as quantities and charac-
teristics of inter-grain surface contacts and their
chemical nature (aspects not investigated in this
study) are anticipated to play an important role,
too. Among the 1300 C materials the composite
from the system pitch/PCS has the highest resistiv-
ity while after the 1650 C treatment the highest
resistivity is found for the composite from the sys-
tem pitch/SiO2.
Comparing the 1300 and 1650 C materials, it is
found that the increased structural ordering of the
graphitic carbon matrix appears to be a rather
insignificant factor in the materials resistivities
determined by the applied method. It is more the
specifics of chemical conversion and presence of
the silicon-bearing species dispersed in the carbon
matrix that are responsible for the observed trends
in the composites resistivities. For example, the
drastic almost threefold increase of the resistivity
for such a pair of composites in the system pitch/
SiO2 seems, likely, to result from the carbothermal
reduction of silica accompanied by massive gas
C. Czosnek et al. / Microelectronic Engineering 81 (2005) 353–357 357evolution, loss of carbon, and increased pore sur-
face area – the phenomena that significantly alter
amounts and nature of inter-grain connectivities.4. Conclusions
The composites C/SiC obtained from the se-
lected precursor systems are characterized by
homogeneous distribution of SiC nano-grains in
the graphitic carbon matrix. The composites are
found to display the diversified electrical resistivi-
ties apparently resulting from the specifics of the
precursor routes. For example, the resistivity of
the 1650 C composite from the system pitch/
poly(carbomethylsilane) is comparable with the
resistivity for the reference non-modified carboniz-
ate which implies that, in this case, the presence of
well dispersed nanocrystalline grains of SiC does
not negatively impact on this property while mod-
ifying other materials features. The outstandingly
high resistivities observed in the system pitch/SiO2
that is characterized by high mass transport reac-
tion chemistry point out to the essential role of
inter-grain surface evolution/grain connectivities
evolved during composite preparation in electric
current conductivities of such powder materials.
The composite materials with a range of reactiv-
ities toward oxidation are obtained encompassing
products some of which are thermally and chemi-
cally robust and others that are prone to reactions
with CO2. The latter composites can in principle be
used, upon feasible removal of excess free carbon,
to make powders of pure nanocrystalline SiC.
Especially, well suited for this purpose appear to
be the low density 1300 C composite from the sys-
tem pitch/poly(carbomethylsilane) and 1650 C
composite from the system pitch/SiO2.
The composites prepared from the system pitch/
poly(carbomethylsilane) are characterized by the
outstandingly low helium densities that are even
lower than the densities for the reference non-
modified carbonizates. Such low density C/SiC
materials with improved chemical resistance can
be considered for applications as advantageousthermal insulation, catalytic support or filtration
modified carbons.
Further studies aimed at optimization of com-
posites make-up and controlled oxidation with
CO2 or air toward pure nanocrystalline SiC are
on the way and their results will be published
elsewhere.Acknowledgment
This work was supported by AGHUniversity of
Science and Technology Grant No. 11.11.210.62.References
[1] C. Raynaud, J. Non-Cryst. Solids 280 (2001) 1–31.
[2] Y. Zhang, W.J. Weber, C.M. Wang, Phys. Rev. B 69
(2004) 205201 (1–9).
[3] L. Giancarli, J.P. Bonal, A. Caso, G. Le Marois, N.B.
Morley, J.F. Salavy, Fusion Eng. Des. 41 (1998) 165–
171.
[4] Z. Shen, H. Peng, J. Liu, M. Nygren, J. Eur. Ceram. Soc.
24 (2004) 3447–3452.
[5] H.-J. Choi, Y.-W. Kim, M. Mitomo, T. Nishimura, J.-H.
Lee, D.-Y. Kim, Scripta Mater. 50 (2004) 1203–1207.
[6] A. Bellosi, D. Sciti, S. Guicciardi, J. Eur. Ceram. Soc. 24
(2004) 3295–3302.
[7] C. Czosnek, W. Ratuszek, J.F. Janik, Z. Olejniczak, Fuel
Process. Technol. 79 (2002) 199–206.
[8] C. Czosnek, J.F. Janik, Z. Olejniczak, J. Cluster Sci. 13
(2002) 487–502.
[9] C. Czosnek, Conversions of elemental silicon and its
compounds during copyrolysis with a pitch from the point
of view of product characteristics modifications, PhD
Dissertation, AGH University of Science and Technology,
Kraków, Poland, 2003 (in Polish).
[10] Polish Standard PN 90/C-04311 (in Polish).
[11] C. Czosnek, J. Wolszczak, M. Drygaś, M. Góra, J.F.
Janik, J. Phys. Chem. Solids 65 (2004) 647–653.
[12] M. Balat, R. Berjoan, G. Pichelin, D. Rochman, Appl.
Surf. Sci. 133 (1998) 115–123.
[13] Q. Wei, E. Pippel, J. Woltersdorf, M. Scheffler, P. Greil,
Mater. Chem. Phys. 73 (2002) 281–289.
[14] C.G. Pantano, A.K. Singh, H. Zhang, J. Sol–Gel Sci.
Technol. 14 (1999) 7–25.
[15] L.M. Manocha, S. Manocha, K.B. Patel, P. Glogar,
Carbon 38 (2000) 1481–1491.
[16] P. Ehrburger, F. Louys, J. Lahaye, Carbon 27 (1989) 389–
393.


The effect of precursor system on the resistivity and oxidation susceptibility of C/SiC nanocomposites en route to electronic grade nanomaterials

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=233615BC7882D8F5903C21E9A59791B1?doi=10.1.1.1069.8378&rep=rep1&type=pdf

The effect of precursor system on the resistivity and oxidation susceptibility of C/SiC nanocomposites en route to electronic grade nanomaterials

Abstract

Similar works

Full text

Available Versions

CiteSeerX