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X. Hou et al. / Journal of Alloys and Compounds 649 (2015) 135e141
literature [34e40], the difference and advantage of our work are
following: (1) In view of the synthesis condition, the reaction
temperature and the ratio of gangue and carbon black are
selected at a reasonable range in this work. This can avoid both
the removal of the residual carbon [34,35] and too high reaction
temperature [36]. (2) As for SiC fibers produced in this work, the
synthesized SiC fibers are well-developed cubic structure with
high purity and uniform morphology. The stacking defect (SF) of
SiC fibers is relatively less than the reported result in the liter-
ature [37e40]. What's more, the synthesized product is self-
assembled into membrane structure, showing its potential
application as filter. (3) As for the properties of SiC fibers, less
work about the thermal property of SiC fibers is reported. In this
work, the properties of SiC fibers including oxidation resistance
and thermal stability are investigated. These fundamental data
may pave the way for the application of SiC fibers at high
temperature.
the experimental temperature T, respectively. Ak(T) (mm) is the
correction value of the instrument at the experimental temperature
T.
The oxidation behavior of SiC fibers was investigated under both
non-isothermal and isothermal conditions using a thermoanalyzer
(Netzsch STA 449C, Netzsch, Germany). The TG microbalance had
the sensitivity of 1 mg. Before isothermal oxidation experiments,
non-isothermal oxidation from 600 to 1500 ꢀC at the heating rate of
10 ꢀC/min in air was investigated to have knowledge of the
oxidation behavior of SiC fibrous membrane. In the isothermal
experiments, SiC fibers (about 7.1 mg) were placed in an alumina
crucible and the temperature was rapidly raised to the required
level in a flowing purified argon gas. After the thermal equilibrium
was established, argon was stopped and air was then introduced.
The mass change due to oxidation was then monitored continu-
ously for 2 h at the rate of 2 point/min. In all the experiments, the
flow rate of air was kept constant, i.e. 40 ml/min. To ensure the data
to be as accurate as possible, every experiment was repeated at
least three times and got the average value.
2. Material and methods
2.1. Material preparation
3. Results and discussion
Gangue (SiO2 > 99%) and carbon black were used as silicon and
carbon sources respectively. The powder with the mole ratio of 1:3
(SiO2/C) was ball mixed. Preparation of SiC fibers was mainly taken
place in a high temperature furnace using carbon as the inner lin-
ing, which is described in detail in our recent work [41]. During the
whole process, argon with the purity of 99.9% at the rate of 0.4 L/
min was adopted as protecting atmosphere and the total pressure
was kept at 0.1 MPa. When the furnace was cooled naturally to
room temperature, argon was stopped and grayewhite self-
standing membrane in large scale was obtained mainly on the
top of the furnace. The resulting fibers were washed with 10% hy-
drofluoric acid (HF) for 1 h to remove the remained silica.
3.1. Phase and microstructure characterization
Fig. 1 is the optical image of the synthesized product. It can be
seen that it is self-assembled into membrane structure. The cross
section of the membrane is consisted of SiC fibers as shown in the
inset of Fig. 1. The XRD pattern as shown in Fig. 2 is indexed from
left to right as (111), (200), (220) and (311) corresponding to 3CeSiC
(Card No.104 01-073-1665), indicating that SiC with high purity is
obtained. Traditionally, SF are frequently found in 3CeSiC [44],
whose density can be measured by SF peak (about 33.6ꢀ) be divided
by (200) peak (41.4ꢀ). In this work the SF peak almost disappeared
and (200) peak was also very weak, indicating that the density of SF
in the synthesized SiC fibers was relatively less.
Fig. 3a and b shows SEM images of the overall look of SiC at low
magnification. It can be seen SiC fibers are uniform. At high
magnification (Fig. 3c), SiC fibers are long and straight filaments
with diameter between 100 and 500 nm and length up to several
millimeters. In addition, the straight fibers possess a smooth sur-
face. There are also some SiC fibers with the bamboo-like
morphology (Fig. 3d). A large number of SEM examinations have
been carried out. The bamboo-like whiskers account for 5% or so of
the total fibers using statistical method.
2.2. Phase and microstructure characterization
The phase was characterized using a 21 kW extra-power powder
X-ray diffractometer (XRD) (M21XVHF22, Mac Science Co. Ltd.,
Yokohama, Japan) with Cu K
a
(
l
¼ 1.54056 Å) radiation over a 2
q
range from 10 to 90ꢀ. The morphology and microstructure of the
synthesized fibers were observed using a scanning electron mi-
croscopy (SEM: Model JSM-840A, JEOL, Tokyo, Japan) and trans-
mission electron microscopy with SAED (TEM: Model Tecnai G2 F30
S-TWIN, FEI, America). The pore size of the fibrous membrane was
investigated by
a capillary flow porometer (IB-FT Germany,
POROLUX 1000) based on bubble-point method [42,43].
2.3. High properties of SiC fibers
The thermal expansion coefficient of SiC fibers was determined
using an advanced analyzer (SETARAM Setsys Evo TMA). The ex-
periments were carried out in an accurate dilatometric cycle
running measurements at the heating rate of 10 ꢀC/min from room
temperature to 750 ꢀC and then cooled to room temperature as a
cycle at nitrogen atmosphere. In the experiment, the sample was
pressed into the cylinder with 10 mm in diameter and 3 mm in
height. The linear thermal expansion coefficient was calculated
from the following equation:
LT ꢂ L0 þ AkðTÞ
r ¼
(1)
L0
where
r
is the linear thermal expansion coefficient (/ꢀC). L0 (mm)
Fig. 1. Optical image of the synthesized SiC fibers and the inset of cross section at high
magnification.
and LT (mm) are the length of the sample at room temperature and