Silicon-based ceramics from polymer precursors
L. V. Interrante1,*, K. Moraes1,
Q. Liu1, N. Lu1, A. Puerta2, and L.
G. Sneddon2
1 Departments of Chemistry, Rensselaer
Polytechnic Institute, Troy, NY 12180-3590, USA;
2 University of Pennsylvania, Philadelphia, PA 19104, USA
Abstract: A hyperbranched polycarbosilane of the type [R3SiCH2]x[SiR2CH2]y[SiR(CH2)1.5]z[Si(CH2)2]l
(R = H, CH2CH=CH2, or OR) has been prepared,
which was used as a source of inorganic/organic hybrid materials and,
through pyrolysis, as a precursor to inorganic solids with unusual microstructures
and properties. A partially allyl-substituted derivative AHPCS,
nominally ['Si(allyl)0.1H0.9CH2']n,
has been extensively studied as a precursor to silicon carbide (SiC)
and is now used commercially as a SiC matrix source for C- and SiC-fiber-reinforced
composites and binder for particulate ceramics. The alkoxy derivatives,
['Si(OR)2CH2'], (R = Me, Et) yield, after hydrolysis
and condensation, carbosilane/siloxane gels with unusually high surface
areas (700900 m2/g) and microporosity that is retained
in the resultant SiOxCy ceramics formed after
pyrolysis to 1000 °C. The fully condensed ['Si(O)CH2']
gel in the latter case was obtained as thin, adherent films on Si surfaces
by spin coating and was found to exhibit dielectric constants as low
as 2.0 after heating to 400 °C. The SiC precursor, AHPCS, has also
been used recently, along with other polymeric precursors, to make two-phase
(SiC/C and SiC/BN) amorphous ceramics that exhibit unusual microstructures
and thermal/mechanical properties. These microstructures are formed
during the mixing and thermosetting of the constituent polymers, which
undergo phase separation due to their immiscibility. Certain of the
SiC/C composites, which have the C phase uniformly distributed as ca.1-µm
droplets in a SiCx matrix, exhibit high oxidation resistance,
and microindentation tests on the SiC/BN system suggest unusual toughness.
* Lecture presented at the 5th Conference on Solid
State Chemistry (SSC 2002), Bratislava, Slovakia, 7-12 July 2002. Other
presentations are published in this issue, pp.
2083-2168.