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Electrospinning of beta silicon carbide nanofibers

Materials Letters 63 (2009) 2361–2364
Contents lists available at ScienceDirect
Materials Letters
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Electrospinning of beta silicon carbide nanofibers
Harvey A. Liu, Kenneth J. Balkus Jr. ⁎
The University of Texas at Dallas, Department of Chemistry and the Alan G. MacDiarmid NanoTech Institute, Richardson, TX 75083-0688, USA
a r t i c l e
i n f o
Article history:
Received 13 May 2009
Accepted 4 August 2009
Available online 9 August 2009
Silicon carbide
a b s t r a c t
Silicon carbide exhibits many unique properties such as its mechanical robustness, chemical inertness, and
thermal stability, which make the material appealing for many applications. Some of these applications
include its use as a support for nanocomposites or as a high temperature filter material. The ability to
fabricate nanofibers of SiC could enhance its utility in these applications. In the current study, nanofibers of
β-SiC have been fabricated through the technique of concentric electrospinning. This method demonstrates
the ability to fabricate uniform SiC nanofibers with a diameter ranging from 1 to 2 nm, the smallest to date.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Silicon carbide has been acclaimed for many of its characteristics
such as its mechanical robustness, chemical inertness, and thermal
stability. Due to the many unique properties of silicon carbide,
different morphologies of this material have been fabricated, including: thin films [1,2], rods [3,4], and fibers [5,6]. For example, fibers with
a diameter of 1–3 μm were fabricated through wet-type electrospinning [6]. A more recent study in the electrospinning of silicon carbide
by Eick et al. demonstrated the ability to fabricate composite fibers
with diameters as small as 20 nm, though the majority of the fibers
observed were much larger, some of which exceeded 100 nm in
diameter. In addition to the wide variation in the diameter of these
fibers, a silica shell formed with a SiC core caused by the atmospheric
oxygen and impurities in the furnace [5]. A fibrous morphology
provides many unique favorable properties; the most prevalent is the
increase in surface to volume ratio, which expands the utility of a
material to various applications. Nanofibers of silicon carbide in
particular offer potential applications as a support material for
nanocomposites [7], substrates for catalysis [8–10], and as well as
high temperature filters [6].
Electrospinning of the polycarbosilane followed by curing and
pyrolysis has proven to be a successful approach to fabricating SiC
fibers [5,6]. However, the resulting SiC fibers exhibited a wide range of
fiber diameters as well as nonuniform and impure structures. In the
present study, we utilized a polycarbosilane as a precursor, but employ
the technique of concentric, or core–shell, electrospinning to fabricate
uniform nanofibers. By employing concentric electrospinning we
demonstrated the first fabrication method to attain consistently
uniform silicon carbide fibers with diameters in the range of 1–2 nm.
There has been growing interest in the method of electrospinning,
especially in the area of ceramic materials. This method has been
utilized in the fabrication of submicron to nanosized fibers composed
of various materials including polymers [11], ceramics [12–18], and
composites[19], which have found application in areas such as tissue
engineering [20,21], catalysis [22], composite materials [19], supercapacitors [23,24], drug delivery [25], as well as self-healing materials
[26]. In the past few years, a variation of conventional electrospinning
has been developed, known as coaxial or concentric electrospinning,
and has resulted in the fabrication of fibrous materials that previously
could not have been achieved [27–29]. A schematic of concentric
electrospinning is shown in Fig. 1. Similar to conventional electrospinning, concentric electrospinning utilizes an external electrostatic
force, which is applied to a polymer melt [27]. Upon excess
electrostatic force, the polymer melt forms a Taylor cone and
undergoes a whipping action, stretching the polymer solution and
forming a nonwoven mat [10]. The technique of coaxial electrospinning allows for the formation of fibers for materials that cannot be
electrospun through conventional spinning. Typically, a polymer that
is easily electrospun is fed through the outer sheath and a material that
is difficult to spin is fed through the core. The outer sheath material
serves to contain the inner core and form the fiber morphology. After
the fabrication of these coaxial fibers, post-fabrication methods can be
performed on the inner material which may include either annealing
and/or washing away of the outer sheath to reveal the inner fibrous
2. Experimental
⁎ Corresponding author. Tel.: +1 972 883 2659; fax: +1 972 883 2925.
E-mail address: [email protected] (K.J. Balkus).
0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
In the present study, high molecular weight polystyrene was
used for the fabrication of the outer sheath while polycarbosilane
H.A. Liu, K.J. Balkus Jr. / Materials Letters 63 (2009) 2361–2364
Fig. 1. Schematic of concentric electrospinning setup.
(SMP-10, Starfire Systems Inc.) (Fig. 2a) was fed through the core as the
silicon carbide precursor. The concentric electrospinning setup was
assembled through the insertion of a 22 gauge needle through an 18
gauge needle. The flow rate through the outer and inner needle was
independently regulated with a syringe pump (7802CO, KD Scientific Inc.,
USA). In a typical synthesis, a 15 wt.% polystyrene in chloroform melt
was fed through the outer sheath at a rate of 7× 10− 2–1×10− 1 mL/min
and the silicon carbide precursor, SMP-10, was used as received and fed
through the inner core at a rate of 8× 10− 3 mL/min. A 10 kV charge
was applied to the needle tip, which was adjusted to be 25 cm from
the grounded rotating drum wrapped with foil as the collecting surface.
3. Results and discussion
Scanning electron micrographs (SEM) of the as-spun core shell fibers
are shown in Fig. 2b–c. The average diameter of the core–shell fibers
typically ranged from 10 to 20 μm (Fig. 2b), however, fibers as small as
2 μm were observed as well (Fig. 2c). The fibers were spun into a freestanding sheet and cured in an oven overnight at 100 °C for 12 h
followed by heating to 200 °C for 24 h to cure the polycarbosilane. The
as-spun core–shell fibers as a free-standing sheet exhibited a slight
yellow tint from the polycarbosilane precursor embedded in the core
(Fig. 2d). It can be seen in the SEM micrograph (Fig. 2c) that some fibers
Fig. 2. (a) Molecular structure of polycarbosilane (b,c) SEM micrographs of the as-spun core–shell fibers (d) digital image of free-standing sheet of fibers.
H.A. Liu, K.J. Balkus Jr. / Materials Letters 63 (2009) 2361–2364
Fig. 3. XRD of crystallized β-SiC nanofibers showing miller indices.
Fig. 5. TEM of crystallized β-SiC.
were fractured, due to the brittle nature of polystyrene, though the
fibers were robust enough to handle (Fig. 2d). SEM imaging of the cured
fibers revealed no changes in the morphology of the fibers from the
curing process. The film was then washed in chloroform and centrifuged
repeatedly to wash away the polystyrene outer sheath. The resulting
cured fibers were then transferred to a graphite crucible and subjected
to pyrolysis in a high temperature vacuum furnace under an inert gas
atmosphere (Red Devil™, R. D. Webb Company, Inc.).
The pyrolysis procedure was executed under argon gas according
to published processing procedures [30]. The fibers were heated from
room temperature to 650 °C at a rate of 1 °C/min, then from 650 °C to
850 °C at 3 °C/min. The oven was then held at 850 °C for 1 h whereupon heating was stopped and cooled to below 100 °C before the
sample was recovered. The pyrolysis treatment results in the recovery
of amorphous (glassy) silicon carbide, which was confirmed by X-ray
diffraction. Further heat treatment was used in order to convert the
glassy silicon carbide to polycrystalline β-SiC.
The crystallization was conducted in a high temperature vacuum
oven under argon where the sample was heated from room
temperature to 1600–1650 °C at an approximate rate of 2–3 °C/min.
The sample was held at this temperature for 6 h to crystallize the fibers.
The X-ray diffraction pattern of the crystallized SiC nanofibers is
shown in Fig. 3. Trace amounts of the hexagonal α-SiC are seen
accompanied by a strong diffraction pattern for β-SiC (Fig. 3) In
addition to SiC, trace amounts of graphite were also seen, represented
by a peak appearing at approximately 26°. The appearance of graphite
is likely from the graphite crucible and/or the graphitic insulation of
the furnace. SEM micrographs of the crystalline β-SiC reveal uniform
nanofibers with diameters approximately 10 nm in size (Fig. 4).
Comparisons of the fibers before the washing and pyrolysis of the
electrospun composite fibers show a large difference in the diameter
size. This is expected since the polystyrene was fed through the needle
at a much higher rate than the silicon carbide precursor, and thus a
smaller volume of the precursor was actually present in the core of the
composite fibers. Transmission electron microscopy (TEM) of the
crystallized silicon carbide is shown in Fig. 5, and is in good agreement
with SEM micrographs. Sample preparation was carried out by
physically grinding the large crystals and sonication followed by
fishing with a lacey carbon TEM grids. The TEM images revealed similar
structures as the SEM micrographs. Bundles were seen throughout the
lacey carbon grid consisting of entangled nanofibers less than 1–2 nm
in diameter. The discrepancy between the fiber diameter observed in
the SEM and TEM was attributed to the Pd/Au coating for SEM imaging.
In a typical Pd/Au coating, approximately 10 nm of the conductive
metal was sputtered, accounting for the increased diameter observed
in the SEM micrographs.
In addition to the electrospinning of β-SiC with the SMP-10
precursor, we have also attempted to electrospin the hexagonal α-SiC.
This was done by seeding the polycarbosilane precursor polymer with
crystals of the hexagonal α-SiC during the electrospinning process. The
curing, pyrolysis, and crystallization were performed in the same
manner as the non-seeded precursor. However, there were no
significant change in ratio of the α-SiC to the β-SiC nanofibers.
4. Conclusion
The ability to fabricate uniform nanofibers of β-SiC 1–2 nm in
diameter has been demonstrated via the technique of concentric
Fig. 4. SEM micrographs of crystallized β-SiC.
H.A. Liu, K.J. Balkus Jr. / Materials Letters 63 (2009) 2361–2364
electrospinning. These represent the smallest SiC nanofibers produced
to date. The strategy demonstrated here to form nanofibers of silicon
carbide can be applied to the preparation of other carbide fibers.
Future work is focused on electrospinning the hexagonal α-SiC fibers,
which may have more versatile applications.
Financial support for this work was provided by the Robert A.
Welch Foundation and SPRING.
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