Environmental Engineering Reference
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and subsequently cross-linked in air at 190
°
C. After pyrolysis of the cured
bers at
1,200
°
C in an inert gas atmosphere (Ar or N 2 ), ceramic SiC
fibers having a
diameter of
14
µ
m and an oxygen content of
12 wt% are obtained. Another
*
*
line of SiC
fibers were commercialized by Ube Industries under the trade name
Tyranno SiC
fibers are derived from a novel polytitanocarbosilane
(PTC), also developed by Yajima and coworkers. Similar to Nicalon
bers. These
fibers, PTC is
processed into
fibers, which are cross-linked in air and further pyrolyzed leading to
ceramic
fibers with oxygen contents up to about 20 wt%. Tyranno S and LOX-M
(Si
Ti
C
O) represent the
first generation of ceramic SiC
bers produced by Ube
-
-
-
Industries.
With the understanding of the oxygen in
fl
uence, amorphous state, and C/Si
relation in the microstructure of these
bers
with reduced oxygen but high amounts of carbon content was produced. A typical
way used to avoid the oxygen is curing the polymeric
fibers, the second generation of SiC
-ray or electron
beam irradiation in inert atmosphere. The almost complete removal of oxygen in the
second generation of SiC
fibers with
γ
fibers resulted in a microstructure with almost no
amorphous phase. Larger SiC grains are formed at a higher pyrolysis temperature,
and a Young
'
s modulus up to 270 GPa was achieved. However, the excess free
carbon in the microstructure of the
fibers limited the growth of the SiC grains.
C. Hi-Nicalon TM
°
These
bers represent the
second generation of SiC fibers in the Nicalon family, characterized by a reduced
oxygen content of <1 wt%. These
bers are processed at about 1,300
fibers are produced by using the same manu-
facturing process as the classic Nicalon
fibers, except for the curing method. Fibers
are cured by using electron beam irradiation doses between 10 and 15 MGy in an
inert atmosphere to avoid oxygen contamination. Second-generation Tyranno ZMI
ceramic
bers
have reduced oxygen content (9 wt%) and improved strength retention at high
temperatures compared to the titanium-containing Tyranno S
fibers based on a Si
Zr
C
O structure are produced by Ube. These
-
-
-
bers. They are
produced from a modi
ed preceramic polymer based on a Si
Zr
C
O structure
-
-
-
(instead Si
Ti
C
O), but the polymer
fiber is still cross-linked in air.
-
-
-
fibers did not meet
the high-temperature requirements of advanced applications such as high-ef
CMCs reinforced with the
first and second generations of SiC
ciency
turbines. Therefore, further developments led to the production of third generation of
SiC
fibers based on highly crystalline SiC materials. The strength of these
fibers is
>2 GPa and the modulus >300 GPa, and the
fibers are thermally stable up to tem-
C. Third-generation Hi-Nicalon TM Type S
peratures of 1,400
fibers, with the
highest modulus of about 420 GPa and improved thermal properties in comparison
with the other
°
fibers from the Nicalon family from Nippon Carbon, are manufactured
taking advantage of the curing method via electron beam irradiation used for the
processing of low-oxygen-content SiC
fibers. The enhanced properties of these third-
generation SiC
fibers were achieved by controlling the pyrolysis using hydrogen as
process gas to reduce the carbon content of the
fibers from C/Si = 1.39 (second
generation) to near stoichiometric ratios (C/Si = 1.05) and further sintering at higher
temperatures to achieve polycrystalline ceramic
fibers with SiC grain sizes of 50
-
100 nm. As a result, Hi-Nicalon Type S
fibers exhibit an improved creep and
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