1. Cezary Czosnek*, Mariusz Drygaś, Bartłomiej Szmyd#, Jerzy F. Janik
C/SiC carbon/silicon carbide foams obtained from polycarbosilane by a direct self-blowing process
Cezary Czosnek*, Mariusz Drygaś, Bartłomiej Szmyd#, Jerzy F. Janik
AGH University of Science and Technology, Faculty of Energy and Fuels; Al. Mickiewicza 30, Kraków, Poland, # student
Composite carbon/silicon carbide foams were synthesized from a precursor system made of the mixtures of polycarbosilanes of different molecular weights using a direct foaming phenomenon – a self-blowing process resulting from an on-going precursor pyrolysis with evolution of volatiles. In such a mixture, the polycarbosilanes with the lowest molecular weights play the role of a blowing agent. During the thermal decomposition, the blowing agent produces large quantities of volatiles which, eventually, take part in the formation of macropores in the decomposed high molecular weight polycarbosilanes that progressively increase viscosity with temperature and time turning at the end to a foamy solid product. The selected mixtures of the precursors were pyrolyzed with heating rate of 7 C/min up to 1200 C under an argon atmosphere.
The black foamy products were examined using SEM, XRD, and FT-IR spectroscopy.
Substrates
neat polycarbosilane, {CH2  SiH(CH3)}n, MW  800 (low molecular weight polycarbosilane  LMW PCS)
neat polycarbosilane, {CH2  SiH(CH3)}n, MW  2000 (high molecular weight polycarbosilane  HMW PCS)
LMW/HMW PCS system (1:1)
Procedures
preparing two-component precursor system
bulk pyrolysis of neat- and mixed precursors system at 1200 C, argon atmosphere
Results
FT-IR spectroscopic data: A) neat LMW PCS as received, B) LMW PCS after pyrolysis at 1200 oC (Ar), C) HMW PCS after pyrolysis at 1200 oC (Ar), D) LMW/HMW PCS system after pyrolysis at 1200 oC (Ar).
The broad absorption bands at ca. 800 cm –1 suggest the presence of Si–C bonds in the pyrolyzed samples.
XRD diffraction patterns of pyrolysis products at 1200 oC, Ar: A) LMW/HMW PCS system, B) LMW PCS.
The broad diffraction peaks confirm poor crystallinity of silicon carbide formed under such circumstances.
SEM images of the foams obtained at 1200 oC under an argon atmosphere: (A) from LMW PCS, (B) from HMW PCS, (C) from LMW/HMW PCS system. A) B) C)
Conclusions
Macrocellular open cell foams were obtained from pyrolysis under argon of polycarbosilanes of different molecular weight due to a direct self-blowing process.
The results showed that the prevailing crystalline phase in the foams was the cubic polytype of silicon carbide, β-SiC, accompanied by an excess of some free carbon C, supporting the feasibility of this simple one-step process to prepare the composite C/SiC foams.
The plausible removal of free carbon from the system by mild oxidation/combustion in air can, in principle, lead to the foams of pure silicon carbide SiC.
Acknowledgement: This work was supported by AGH University of Science and Technology Grant No