1. A novel method for the processing of carbon foam containing in situ
grown nano-materials and silicon nanowires
Shameel Farhan n
, Rumin Wang, Hao Jiang
Department of Applied Chemistry School of Science, NWPU, Xi’an 710072, China
a r t i c l e i n f o
Article history:
Received 21 April 2015
Received in revised form
1 July 2015
Accepted 12 July 2015
Available online 14 July 2015
Keywords:
Carbon materials
Porous materials
Microstructure
Nanocomposites
a b s t r a c t
A novel method for the processing of carbon foam has been developed by using a powder mixture
containing polyurethane foam and novolac resin hereafter called PN. Various additives like Si, Al, FeCl3,
activated carbon (AC), short-carbon fibers (SCF) were mixed individually or in combination, carbonized
under coal and analyzed for density, microstructure and compressive strength. The pore morphology
changed significantly by using different kinds of additives. SCFs were effectively mixed and bonded
within the pore walls. Si converted in situ into silicon carbide and a jungle of kinked and twisted na-
nowires all around the pores. CNTs with amorphous carbon beads were observed throughout the pore
surfaces when 10 wt% FeCl3 was added. Al increased the compressive strength when used upto 6 wt%.
& 2015 Elsevier B.V. All rights reserved.
1. Introduction
Carbon foams are rigid and porous materials, which exhibit a
unique characteristic of tailorability in physical, thermal and
functional properties [1]. There are several types, depending on
the raw materials, process parameters and the production method
[2,3]. The most important categories are non-graphitic and gra-
phite foam apart from the reticulated vitreous carbon and the
most recently developed biomass-based carbon foams [4]. Despite
first development in 1960s by W. Ford [5], the scientists at AFRL
Dayton OH [6] started the major break-through in 1990s after the
development of pitch-based foam. The processing of bulk carbon
foam is not an easy task and major challenges are encountered
when various additives/fillers are mixed during production [7].
This letter focuses on the development of a novel method for
carbon foam production with added features of mixing various
additives/fillers and in situ grown nano-reinforcements. The car-
bon foam with added features of nano-reinforcements and fillers
can be utilized as thermal insulating material, anti-oxidative and
anti-ablative compound, radar absorbing structure, catalyst sup-
port, reinforcement material and next-generation high-perfor-
mance electronic and energy storage component. No reports are
available in which carbon foam has been prepared with in situ
grown nanowires simultaneously. Recently, carbon foam has been
prepared using graphitic and non-graphitic carbon precursors
with a soft template [8]. Phenolic resin containing pitch and alu-
minum (Al) powders was used to impregnate an open cell poly-
urethane foam. In an attempt to increase the amount of Al, this
powder method has been emerged. To further validate this pro-
cess, various other fillers/additives like silicon (Si), silver (Ag),
activated carbon, (AC) and short-carbon fibers (SCFs) were used as
a test case and integrated into the foam structure in the form of
bonded or in situ compounds. Compressive strength was also
found as a complementary study.
2. Experimental
Semi-rigid PU foam, novolac resin, Al powder, Si powder, SCFs
and AC were purchased from Sinopharm Chemicals, Beijing China.
Ag paper, as traditionally used on sweets in Pakistan, was pur-
chased from the market. In Ref. [8], resole resin was used as carbon
precursor and in an attempt to add more quantity of Al, many
trials were made to get homogenized and stabilized green foam.
The problem became more severe after curing due to the differ-
ence in mass mobility and wettability in the PU foam. Novolac
solved this issue when it was used instead of resole, as it is an
amorphous solid. Dissolved novolac was poured on a piece of PU
foam and kept at 25 °C for 24 h after which it became hard and
easily crushed into powdered form. It was named foam novolac
(FN) instant precursor and further ground with other fillers/ad-
ditives individually or in a combination. A FN containing 40 wt%
PU and 60 wt% novolac, denoted as F4N6, was ball-milled with
additives/fillers using a QM-1SP2 planetary-type machine. The
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/matlet
Materials Letters
http://dx.doi.org/10.1016/j.matlet.2015.07.060
0167-577X/& 2015 Elsevier B.V. All rights reserved.
n
Corresponding author. Fax: þ86 29 88492943.
E-mail addresses: shameelfarhan@yahoo.com (S. Farhan),
rmwang@nwpu.edu.cn (R. Wang).
Materials Letters 159 (2015) 439–442

2. final mixtures denoted by A, B, C, D, E, F and G were molded under
a slight pressure, cured and carbonized while buried under coal
and N2 flow. Further details can be found in [8]. Fig. 1 shows the
various stages of processing prototype samples. Density was
measured by ratio of mass to the total apparent volume. The SEM
images were carried out using a JEOL (model JSM-6610LV) scan-
ning electron microscope equipped by an energy dispersive X-ray
analyzer Compressive strength of 10 mm  10 mm  10 mm sam-
ples was measured by the Instron-3369 UTM.
3. Results and discussion
Physical properties and surface morphology of various proto-
type samples are shown in Table 1 and Figs. 2–4 respectively.
Additional information regarding porosity, pore volume and in-
cremental pore size distribution is given in S1. Open-cell structure
is seen in all the pictures with irregular, uneven and rough mor-
phology. Due to the complexity and randomness in the pore shape,
pore sizes fall between a range upto 500 μm, although some pores
are seen broken due to cutting and grinding The carbon foam
possesses well-developed pore structures with a bulk density of
0.55–0.69 g/cm3
, and there are no obvious micro-cracks on the
walls. Fig. 2(a and b) shows the sample A where the pore wall
thickness is also not the same and varies between 10 and 100 μm.
This was due to the largest shrinkage of PU and novolac in the
carbon foam containing no additive/fillers. It is quite evident that
AC increased the roughness of pores and the original cellular-
structure of the biomass was largely maintained albeit in the
sample B. Some part of 200–400 μm long carbon fiber existed in
the ligament and the remaining was exposed outside in the
sample C with 5% SCFs. The SCFs are seen well dispersed and well
bonded with the walls of carbon foam without cracks and de-
bondings. In Fig. 3, the resulting spectra show that Al metal along
with carbon, nitrogen and oxygen was detected in the inspection
field. Ag and Al were well dispersed in the samples and some
agglomerated particles with outer surface either smooth or cov-
ered with some nanoparticles, most probably some oxy-carbides
and nitrides (EDX results), were also visible (Figs. S2–S5). The
nano-fibers and particles of about 100 nm diameter fully covered
the Al particles with random orientations. Due to addition of FeCl3,
CNTs with amorphous beads were grown homogeneously in the
pores of sample F as in Fig. 4(a and b). The appearance of these
beads was suggestive of solidification of super-cooled liquid dro-
plets. Small beads were aligned to form fishbone-shaped tubes
with knots and necks approximately at equal distances between
them. Some CNTs were 45 nm in diameter and others were grown
upto huge diameters, upto 0.8 μm, with bending and sharp elbow
like morphology.
Fig. 4(c and d) shows the sample G containing Si which was
converted in situ into nanowires. The clump that looks tangled up
is actually many Si nanowires that are difficult to distinguish and
some of them fuse together into thicker structures. Among the
jungle of kinked and curly wires, several very straight wires with
smaller diameter were formed. The nanowires with 20–40 nm
diameters have been observed but there are also some large wires
with 400–600 nm diameters. X-ray diffraction scan also confirm
the formation of SiC along with some residual Si (Fig. S6). In this
work, the gases (CO, CO2, H2, N2 and CH4), high-temperature
(1000 °C), close-space (coal), and the slow-cooling rate are key
factors for Si nanowires formation. The protective gases and high
temperature met the condition for formation of SiO and CO; the
closed space increased the concentration of SiO and CO mixed
gases; and slow-cooling rate provided an interval for the genera-
tion of nanowires. Furthermore, the cellular template has enough
flow space for the mass transport of the gaseous species (SiO and
CO). Compressive strength of carbon foam is closely related to
microstructure, amount of additives/fillers and in situ compounds.
The sample D containing 6 wt% Al showed the highest value of
specific strength as shown in Table 1. The fact indicates that the
Fig. 1. Processing of carbon foam; (a) powdered-precursor, (b) molding, (c) curing, (d) green foam and (e) carbonized foam.
Table 1
Physical properties of carbon foam: composition, density, porosity, open pore volume and compressive strength.
Sample ID Description Fillers
/additive
Bulk density (g/cm3
) True density (g/cm3
) Open por-
osity (%)
Open pore
volume
(cm3
/g)
Compressive
strength (MPa)
Specific strength (MPa-cm3
/g)
A (F4N6) No filler 0.55 1.65 66.67 1.21 18 32.7
B (F4N6)80(AC)20 20%AC 0.56 1.77 68.36 1.22 15 26.8
C (F4N6)95(SCF)05 05%SCF 0.59 1.82 67.58 1.15 25 42.4
D (F4N6)94(Al)06 06%Al 0.59 1.87 68.44 1.16 32 54.0
E (F4N6)97(Ag)03 03%Ag 0.55 1.67 67.06 1.22 28 51.0
F (F4N6)90(FeCl3)10 10% FeCl3 0.59 1.91 69.10 1.17 25 42.4
G (F4N6)80(Si)20 20%Si 0.69 2.06 66.50 0.96 17 24.6
S. Farhan et al. / Materials Letters 159 (2015) 439–442440

3. Fig. 2. Morphology of carbon foam; (a) low-density, (b) high-density, (c) with 10%AC and (d) with 5%SCFs.
Fig. 3. Surface morphology of carbon foam; (a, b) with Ag and (c, d) with Al.
S. Farhan et al. / Materials Letters 159 (2015) 439–442 441

4. carbon foam was toughened due to the addition of Al, formation of
Al4C3 and to some extent AlN by substitution reaction [9]. Inter
diffusion between Al and carbon-matrix led to the formation of an
interlayer that contributed in strengthening the ligaments. The
sample F containing Si showed the lowest strength due to some
residual Si in the carbon foam. A high temperature treatment
above 1450 °C can convert the residual Si in nitrides and carbides,
which can improve the strength manifold. High temperature
treatment at 1500 °C and detailed physical, thermal and functional
characterization will be done in future.
4. Conclusions
A simple powder method for making carbon foam has been
developed which can incorporate various kinds of fillers/additives
in various proportions and combinations. During carbonization,
these were bonded or converted into in situ compounds, nano-
particles, nanotubes and nanowires along with the pyrolyzing
carbon precursors simultaneously. For the current prototype
samples, the focus was mainly on the process development with a
general characterization. Microstructure and compressive strength
values further suggested the potential application of this material
in various multifunctional, commercial and aerospace applications.
Appendix A. Supplementary information
Supplementary data associated with this article can be found in
the online version at http://dx.doi.org/10.1016/j.matlet.2015.07.
060.
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Fig. 4. Surface morphology of carbon foam; (a, b) with 10%FeCl3 Á 6H2O, (c, d) with 20%Si.
S. Farhan et al. / Materials Letters 159 (2015) 439–442442