Ceramic fibers fiber curing

The first useful organosilicon preceramic polymer, a silicon carbide fiber precursor, was developed by S. Yajima and his coworkers at Tohoku University in Japan [5]. As might be expected on the basis of the 2 C/l Si ratio of the (CH3)2SiCl2 starting material used in this process, the ceramic fibers contain free carbon as well as silicon carbide. A typical analysis [5] showed a composition 1 SiC/0.78 C/0.22 Si02- (The latter is introduced in the oxidative cure step of the polycarbosilane fiber). 

A ceramic fiber with Si-C-N-0 composition can be prepared by melt-spinning, cure and pyrolysis of a polymethyldisilylazane polymer precursor (14, 15), which is the reaction product of a mixture of 50 mol % 1,1,2,2- tetrachloro-1,2-dimethyldisilane (la), 40 mol % 1,1,2-trichloro- 1,2,2-trimethyldisilane (lb) and 10 mol %... 

Summary A brief review of the preparation of silicon containing preceramic polymers to prepare silicon carbide and silicon carbonitride fibers is given. Methylchlorodisilanes are converted to polysilanes and polysilazanes which yield ceramic fibers after meltspinning, curing, and pyrolysis. 

The spun polysilane fibers are cured by oxidation in air at a temperature of 160 - 200°C. The curing process is necessary to permit the conversion of the green fiber to the ceramic fiber without softening during pyrolysis. It is presumed that the oxidation mechanism results in the formation of Si-O-C and Si-O-Si bonds by the reaction of Si-CH3 and Si-Si in the polysilane. 

Polysilazane fibers are rendered infusible by humidity or in the absence of oxygen by ammonia. The final step of producing ceramic fibers is the pyrolysis. The cured fibers are heated at 1200 -1300°C in argon, nitrogen, or in vacuo, and SiC- or SiC/SijN fibers with a diameter of around 15 /xm are obtained. Heating up silicon-polymers, whether polysilanes or polysilazanes, results in the evolution of CH4 and H2. 

High temperature stability of these nonoxide fibers in air is another critical problem. Thermal stability of ceramic fibers derived from polymeric precursors is of special concern mainly because, as mentioned above, they frequently have undesirable phases present in them. Polycarbosilane-derived SiC fibers, such as Nicalon or Tyranno, involve a thermal oxidation curing process as described above and can contain as much as 10 wt % oxygen (Okamura and Seguchi, 1992). Such fibers decompose at temperatures above 1200 C in a nitrogen or argon atmosphere with SiO and CO gas evolution ... 

Fibers could be drawn from concentrated syrups of these products in toluene. The drawn fibers could not directly be converted to ceramic fibers, because they melted on being heated, and a cure step was required to render them infusible. The required cure could be accomplished by UV irradiation of the fibers. Following this treatment, pyrolysis of the fibers in a stream of argon gave black ceramic fibers, and pyrolysis in a stream of ammonia gave white ceramic fibers. 

The spun fibers were cross-linked (cured) by air oxidation and pyrolyzed to give silicon-carbide-type fibers. Yajima et al. (7) reported further that heteroatoms such as titanium could be incorporated into the polymers and ceramic fibers to enhance their stability (equation 3). 

H. Ichikawa, K. Okamura, and T. Seguchi, Oxygen-fiee Ceramic Fibers From Organo-silicon Precursors and E-beam Curing, Proc. Second International Conference on High-temperature Ceramic Matrix Composites, Santa Barbara, CA, USA, 65-74 (1995). 

Several other studies on Nicalon-based ceramic fibers have also been conducted in addition to the investigation of oxidation curing of PCS fibers and effect of oxygen in tensile strength of SiC fibers [51]. Similarly, studies dealing with the chemistry, characterization, modification, use, and applications of polysilanes and polycarbosUane are also available [52]. 

Figure 3. Gas evolution during the vacuum pyrolysis under of PCS fibers cured in heiium by E-beam irradiation [28] reproduced with permission from the American Ceramic Society, PO Box 6136, Westervlile, Ohio 43086-6136.

M. Sugimoto, T. ShinxK), K. Okamura and T. Seguchi, Reaction mechanisms of silicon carbide fiber synthesis by heat treatment of polycarbosilane fibers cured by radiation I Evolved gas analysis, J. Amer. Ceram. Soc., 78 [4], 1013-17(1995). 

Si-C-N-0 and Si-C-N(O) fibers exhibit linear elastic tensile behavior up to failure. At room temperature HPZ based fibers have tensile strengths ranging from 1.9 GPa [22] to 3.1 GPa [21], and elastic moduli ranging from 200 GPa [22] to 260 GPa [21]. PCSZ based fibers have similar mechanical properties. Oxygen cured Si-C-N-0 ceramic fibers and radiation cured Si-C-N(O) fibers have moduli of 175 GPa and 215 GPa, respectively, and tensile strengths of 1.8 GPa and 2.4 GPa, respectively [10-11]. 

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