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DOI: 10.1002/adem.200900222MM
UNI
Synthesis and Characterization of Electrospun MulliteNanofibers**
CATIO
By Jiang Wu, Hong Lin,* Jianbao Li, Xiaobo Zhan and Junfeng LiN
In this study, continuous mullite (3Al2O3�2SiO2) nanofibers were fabricated by sol–gel and electro-spinning techniques and calcined at high temperatures to obtain pure ceramic mullite nanofibers.Detail structural characterizations suggested that the productive rate of electrospun mullite nanofiberswere very high and the sintered nanofibers were made of single crystalline grains with size of�100 nm.
[*] Prof. H. Lin, Dr. J. Wu, Prof. J. B. Li, Dr. X. B. Zhan, Dr. J. F. LiState Key Laboratory of New Ceramics and Fine ProcessingDepartment of Materials Science and Engineering TsinghuaUniversity Beijing, 100084, ChinaE-mail: [email protected]
[**] The authors would like to express their gratitude to thesupport provided by the Ministry of Science and Technologyof China (973 Program, 2007CB607504; 863 Program,2007AA03Z524), the National Natural Science Foundationof China (NSFC, 50672041).
ADVANCED ENGINEERING MATERIALS 2010, 12, No. 1--2 � 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 71
Mullite (3Al2O3�2SiO2) is well-known as superior engineer-
ing ceramic materials owing to its relatively high chemical
stability, good refractory properties, high temperature
mechanical strength, and low thermal expansion coeffi-
cient.[1,2] Fabrication of continuous mullite nanofibers is
highly desired because it is an important candidate used as
reinforcement of metals, ceramics, and resins.[3,4] Several
traditional methods such as sol–gel, melt spinning, and
solution spinningmethods have been used to fabricate mullite
fibers.[5–7] Electrospinning is a promising technique that
utilizes electric forces along to drive the spinning process and
to produce fibers. Unlike sol–gel fabricated nanofibers, most
of which contain foreign cations as catalysts and require
further expensive purification, electrospun nanofibers are
more pure, inexpensive, continuous, and relatively easy to
align, assemble, and process into applications.[8–11] Unlike
conventional spinning techniques (e.g., melt spinning and
solution spinning methods) that are capable of producing
fibers with diameters in the micrometer range (�5–15mm),
electrospinning is capable of producing fibers with diameters
in the nanometer range (�50–1000 nm) with less cracks. In the
recent decade, numerous electrospun ceramic nanofibers
including silica, titania, and zirconia, have been fabricated and
studied throughout the world.[12–14] However, very little
literatures have been reported about electrospinning
approach fabricating mullite nanofibers due to its more
complicated stoichiometry/microstructure and relatively
higher synthesis temperature. Recnetly, Dharmaraj et al. have
reported the fabrication of electrospun mullite nanofibers,
which focused on the sintering process, studying the effect of
calcination temperature on the morphology evolution of
mullite nanofibers.[15] However, much less attention has been
paid to the electrospinning process.
In the present work, electrospinning method is introduced
to fabricate mullite nanofibers. Three different solvent
systems including de-ionized water (DI) (solution A), DI/
absolute alcohol (EtOH) (solution B), and DI/N, N-dimethyl-
formamide (DMF) (solution C) were used to prepare precur-
sor spinning solutions. The effect of different solvents on the
micromorphological evolution, crystallization sequence, and
structural properties of both the as-electrospun fibers and the
final sintered mullite nanofibers will be primarily focused and
systematically investigated.
Results and Discussion
The representative morphologies of as-electrospun nano-
fibers prepared from different solutions are presented in
Figure 1(a–f). Although no obvious beads and/or beaded
fibers were identified at low resolution, as shown in
Figure 1(a–c), for the fibers made from solution A, some
small beaded fibers with diameter ranging from 800nm to
2mm were observed with in the scope of this investigation at
high resolution, as shown in Figure 1(d). On the contrary, the
fibers made from solutions B and C had common cylindrical
morphologies with diameters ranging from 300 to 600 nm, as
shown in Figure 1(e) and (f).
The ‘‘bending instability’’ is a widely acceptable explana-
tion for electrospinning process.[8,9,16] During electrospinning,
when the electrostatic field reaches a critical value and the
electric force overcomes the surface tension and viscoelastic
force, a jet ejects and starts to bend, forming long fibers. The
properties of the solutions, such as dielectric constant,
viscosity, and volatility, have great influence on the micro-
morphology of as-electrospun fibers.[16] In the present work,
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Fig. 1. Representative morphologies of as-electrospun nanofibers from (a, d) solution A; (b, e) solution B; and (c, f) solution C.
all the solution systems are conductive due to the addition of
aluminum salt, the micromorphology of as-electrospun fibers
are mainly dependent on the viscosity and the solvent
evaporation. For solution A, the concentration of polyvi-
nylprrolidone (PVP) is as low as 4.4wt%, surface tension is
still the dominant factor and small beaded fibers are formed
during the electrospinning process.[8,9,12] Due to the less
volatilization of de-ionized water, the diameters of obtained
as-electrospun fibers are in the range of 1–2mm, which are
much larger than those of the other two fibers. On the
contrary, for solutions B and C, the concentration of PVP are 6
and 8wt%, respectively. The viscosity of solutions are high
enough to form continuous fibrous structure. The
as-electrospun fibers are thinner and have regular circular
cross-section, which are created by quick evaporation of the
Fig. 2. TG/DSC curves of all three as-electrospun fibers obtained from (a) solution A,(b) solution B, and (c) solution C.
72 http://www.aem-journal.com � 2010 WILEY-VCH Verlag GmbH & Co.
solvents through the drying surface of the jet.[9,16] Although
the small beaded fibers derived from solution A can be
avoided if the PVP consent exceeds 4.4wt%, the diameters of
the final sintered fibers are observed in the range of
micrometers, which will not be discussed in this paper.
Figure 2 shows the thermogravimetry differential scanning
calorimetry (TG/DSC) analysis of all as-electrospun nano-
fibers prepared from three different solutions. TG curves
suggested that themass losses of all three as-electrospun fibers
were finished before 800 8C, indicating the complete volati-
lization and decomposition of solvents and PVP. The
small endothermic peaks occurred at 120 8C were considered
as the dehydration of absorbed water. The broad exothermic
peaks from 300 to 600 8Cwere attributed to the decomposition
and volatilization of PVP. It is noticed that there were
two distinct exothermic peaks at 973 and 1113 8C for the
as-electrospun fibers prepared from solution A, which were
attributed to the formation of g-Al2O3 and mullite. While
only one obvious exothermic peak was observed for the
as-electrospun fibersmade from the other two solutions, at 985
and 983 8C, respectively, indicating the direct formation of
mullite.
It has been reported by Schneider et al. that the crystal-
lization sequence of mullite gel is related to the degree of
mixing of the constituents.[17] If the mixing scale is at atomic
level, the gel is single phase and mullitization can be achieved
directly at 980 8C. If themixing scale is higher than that, the gel
is diphsic and transforms into Al2O3 before the crystallization
of mullite. Typically, ‘‘bending instability’’ causes the length
of an electrospinning jet to elongate up to 50 000 times in a very
short time period.[12] For solutions B and C, such a huge draw
rate and the associated extremely intensive stretching and/or
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Fig. 3. Representative morphologies of all sintered mullite nanofibers obtained from (a) solution A, (b) solution B, and (c) solution C.
Fig. 4. Representative TEM and high-resolution images of all the sintered mullite nanofibers obtained from (a) solution A, (b) solution B, and (c) solution C.
Fig. 5. XRD patterns of sintered nanofibers obtained from (a) solution A, (b) solution B,and (c) solution C.
shearing could result in the meta-stable and almost
atom-uniform mixing of resource components, leading to
the direct formation of mullite.[12] While for solution A, the
high water content induced the fast hydrolyzation and
condensation reactions of aluminum isopropoxide (AIP)
and tetraethylor orthosilicate (TEOS), resulting in less
homogeneity of the as-electrospun fibers and later the
formation of Al2O3 as the secondary phase.
In order to obtain pure mullite nanofibers, all the
as-electrospun fibers were sintered at 1200 8C for 2 h according
to DSC results. The representative morphologies of sintered
nanofibers are shown in Figure 3(a–c). All the sintered
nanofibers were thinner than the as-electrospun ones. The
diameters were 800–1000 nm for the ones made from solution
A, and the sizes/widths were 100–200 nm for the ones made
from solutions B and C. The contraction in fiber diameter/size
was due to the removal and decomposition of PVP.
Additionally, the degree of surface roughness for the sintered
nanofibers was significantly higher than that for the
as-electrospun nanofibers, which was due to the nucleation
and growth of mullite grains during sintering process.
Figure 4(a–c) show the representative transmission elec-
tron microscopy (TEM) and high-resolution images of all the
sinteredmullite nanofibers. The particular nanofiber as shown
in Figure 4(a) had diameters of�500 nm, which was relatively
thin among the nanofibers made from solution A. Even
though, it was still very hard to observe the detailed surface
condition of the fiber clearly due to its large thickness. While
the high-resolution observation was very easy for the
ADVANCED ENGINEERING MATERIALS 2010, 12, No. 1--2 � 2010 WILEY-VCH Ve
particular nanofibers obtained from solutions B and C due
to their ralative thin size. Results indicated that both of the
sintered nanofibers consisted of single crystalline grains with
size of �100 nm.
Figure 5 shows the X-ray diffractometer (XRD) patterns of
all the sintered mullite nanofibers. It is clear that mullite is the
primary phase with Al2O3 as secondary phase. For nanofibers
derived from solution A, the amount of Al2O3 phase was
much higher, which was attributed to the less homogeneity of
its precursor and the different crystallization sequence. While
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Wu et al./Synthesis and Characterization of Electrospun . . .
for the other two sintered nanofibers, the track amount of
Al2O3 is likely introduced from theAl2O3 crucible used during
the sintering process. It is noted worthy that the XRD result is
obtained from large amount of nanofibers mat, confirming a
high productive rate of electrospun mullite nanofibers.
Conclusions
In summary, nano-scale mullite nanofibers could be easily
synthesized by electrospinning method followed by sintering
process at 1200 8C for 2 h. Results indicated that the properties
of solvents had great influence on the microphology
development and crystallization sequence of as-electrospun
fibers. Due to the less PVP content and low volatilization of
de-ionized water, the diameters of the sintered nanofibers
derived from solution A are in the range of 800–1000 nm. Plus
with the less homogeneity of constituents, large amount of
Al2O3 was formed as secondary phase. While for the sintered
nanofibers made from solutions B and C, the diameters were
in the range of 100–200 nm. XRD and TEM results indicated
that the productive rate of electrospun mullite nanofibers
were very high and the sintered nanofibers were made of
single crystalline grains with size of �100 nm. Due to their
dramatic good performance of mechanical property and
stability, plus with the nano-scale diameters and relatively
high surface area to volume ratio, electrospun mullite
nanofibers are highly expected to be used as structural
materials, such as filtrating membrane and aircraft space
accessories, especially in high temperature oxygen environ-
ment.
Experimental
Commercially available AIP (C9H21O3Al), hydrous alumi-
num nitrate [AN, Al(NO3)3�9H2O) and PVP (Mw¼ 1 000 000)
were used as the raw materials. Three different spinning
solutions (DI, DI/EtOH, and DI/DMF), were prepared. The
source solutions were mixed and stirred at 80 8C for 0.5 h,
resulting viscous semilucent solutions for electrospinning.
The following three spinning solutions were suggested as the
optimal and representative systems:
Solution A: AIP/AN/TEOS/DI¼ 2:1:1:50 with 4.4wt% PVP
Solution B: AIP/AN/TEOS/DI/EtOH¼ 2:1:1:25:25 with
6wt% PVP
Solution C: AIP/AN/TEOS/DI/DMF¼ 2:1:1:25:25 with
8wt% PVP
The viscous solution was contained in a glass capillary and
ejected with a voltage of 18 kV. A dense web of fibers was
collected on the aluminum foil which was served as the
counter electrode. The obtained web of fibers was calcined at
74 http://www.aem-journal.com � 2010 WILEY-VCH Verlag GmbH & Co.
1200 8C for 2 h with a heating rate of 10 8Cmin�1. The thermal
analysis of the as-electrospun precursor nanofibers was
carried out up to 1200 8C in N2 by TG/DSC (NETZSCH
STA 409PC/PG, Germany). The phase transformation and
microstructure of mullite nanofibers were studied by XRD
(D/max-III, Rigaku, Japan), scanning electron microscopy
(SEM, LEO 1530, Germany) and high resolution TEM (JEOL
JEM-2010F, Japan).
Received: July 28, 2009
Final Version: August 19, 2009
Published online: November 27, 2009
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