JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

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Investigation of Nano Porous SiC Based Fibers
Synthesized by Precursor Method
M. Sugimoto a) , M. Yoshikawa a) , K. Kita b) , M. Narisawa b) and A. Nakahira b)
a) Environment and Industrial Materials Research Division, QuBS, JAEA,
b) Graduate School of Engineering, Osaka Prefecture University
Silicon carbide (SiC) based porous ceramics is one of
promising thermoelectric materials, thermal generators and
gas separators in high temperature. In recent years, there
are many studies about Si-C base porous ceramics
synthesized by polymer precursor method 1, 2) . There are,
however, rare reports about porous ceramic fiber production
available in a mass scale in spite of its potential application.
Recently, we have paid attention to various polysiloxanes
for supplemental agents to polycarbosilane (PCS) for Si-C-O
fibers 3) . The fiber derived from such polymer blend often
yields characteristic porous structure. In this study, we
investigate the effect of additive polymers to polycarbosilane
(PCS) for controlling pore structures in the pyrolyzed fiber.
Commercialized polymethylphenylsiloxane (KF-54,
Shin-Etsu Chemicals) and polymethylhydrosiloxanes (KF-99,
Shin-etsu Chemicals) were blended with PCS (NIPUSI-Type
A, Nippon Carbon). The blend ratios were 15 mass%.
These polymer blends were melt-spun into polymer fibers at
523-578 K. The fibers including PMPhS and PMHS were
identified as PS15 and HS15 respectively. The fibers were
irradiated for curing by -ray in air or electron beam (EB) in
He. The dose rate of -ray was 0.0057 kGy/s and the dose
was 2 MGy, as for the EB, the dose rate was 1.6 kGy/s and
the dose was 8 MGy. After the curing, the fibers were
pyrolyzed at 1,273 K for 3.6 ks in Ar. After the pyrolysis,
the pyrolyzed fibers were heated up again at 1,573-1,773 K
for 1.8 ks in Ar. After the above process, the obtained
ceramic fibers were observed by FE-SEM. Tensile strength
and specific surface area of the fibers were also measured.
Figure 1 shows the FE-SEM images of the cross-sections
of PS15 pyrolyzed at 1,573, 1,673, 1,773 K. The
cross-section pyrolyzed at 1,573 K could not be observed,
that of pyrolyzed at 1,673 K showed pores whose average
diameter was less than 100 nm and that of PS15 pyrolyzed at
1,773 K shows pores whose average diameter was among
100 nm. The PS15 pyrolyzed at 1,773 K seems to be
sintered. On the other hand, the cross-sections of PS15
cured by EB in He do not show any indication of pore
formation.
The fibers cured by thermal or γ-ray oxidation maintained
inner pores whose diameter was around 100 nm. Moreover,
the surface area of the HS15 fiber was 12.1 g/m 2 and was
larger than that of PS15 fiber. On the other hand, the fibers
cured by electron beam without oxidation maintained only
surface pores, while inner pores were collapsed. It was
considered that the method of curing and the kind of additive
polysiloxane for PCS determined the pore size in the fibers.
In the case of PS15 -ray oxidation curing and pyrolysis
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at 1,673 K was the best condition to achieve large surface
area and the polymer blend with more PMPhS was
appropriate to increase the specific surface area. As for the
HS15, the surface area was less than that of PS15; however
the tensile strength of the ceramic fiber pyrolyzed by the
HS15 at 1,723 K was over 0.35 GPa. It seems that the
component of polysiloxane has two effects of a deterrent to
SiC crystal growth and a binder among the crystals in the
porous ceramic fiber. The synthesized nano porous
ceramics are promising as parts in catalyst or adsorbent for
high temperature because of expected large specific surface
area with high strength.
References
1) P. Colombo, and M. Modesti, J. Am. Ceram. Soc. 82,
(1999) 573-78.
2) Y. Kim et al., J. Ceram. Soc. Jpn. 11, (2003) 863-4.
3) K. Kita et al., J. Am. Ceram. Soc. 92, (2009) 1192-7.
(a)
(b)
(c) (d)
Fig. 1 SEM images of the cross sections of PS15 cured by
-ray oxidation pyrolyzed at 1,573(a), 1,673(b),
1,773 K (c) and PS15 cured by EB without oxygen
pyrolyzed at 1,673 K (d).