Silicon nitride fibers

Seeding technique, procedure 130, 131 Sequential addition of monomers 164, 167 Silicon-carbide fibers 8 Silicon-nitride fibers 8 Silicone rubber, crosslinked 4, 7-9, 31, 67 Siloxane, definition of 5 Siloxane-acrylate copolymers 27, 29, 56, 57, 64, 70, 71, 73, 74... 

In the area of preceramic polysilazanes, sufficient progress has been made to produce precursors for silicon nitride fibers, coatings and as binders for silicon nitride powder. However, particular problems still remain to be solved particularly with regard to reducing impurity levels and improving densification during pyrolysis. 

The polysilazanes were also melt spun, cured, and pyrolyzed to give silicon carbonitride fibers (Eq. 7). The carbon content of these fibers depends on the molecular composition of the polysilazane and the pyrolysis gas. When ammonia is used as reactive gas pure silicon nitride fibers will be obtained (Eq. 8) [14]. 

Polymeric Routes to Silicon Carbide and Silicon Nitride Fibers... 

Okamura and co-workers (18) have taken air-cured PCS polymer and, through pyrolysis in the presence of ammonia, prepared essentially carbon-free silicon oxynitride fibers (equation 13). However, if the PCS polymer fiber is cured by electron beam radiation (to prevent oxygen addition), the same ammonia pyrolysis conditions provide nearly stoichiometric quantities of silicon nitride fibers (equation 14). 

This route is useful for only silicon nitride powder and can not be used for silicon nitride fibers or coatings (2-3). 

The drawing of these precursor polysilazane polymers to form fibers and their subsequent pyrolysis to silicon nitride fibers is a complex process that will be reported separately. 

For the preparation of technically important metal carbide and metal nitride materials the application of organosilicon compounds as preceramic precursors is advantageous under certain conditions [1-5]. Compared with the conventional metallurgical powder process, one benefit is the utilization of very low process temperatures for the preparation of individual ceramic materials. Another improvement is the high purity of the ceramics obtained from tailor-made preceramic precursors. Usually, after pyrolysis organosilicon compounds afford silicon-containing ceramic powders Likewise, they can also be used under certain conditions for the production of silicon carbide or silicon nitride fibers. 

SiC fibers were produced using polycarbosilanes by Yajima et al. in 1975 [1,2]. Besides SiC fibers, Si-Ti-C-O fibers prepared from a polytitanocarbosi-lane have been obtained by adding a titanium tetrabutoxide to polycarbosilane or polysilane [3]. SiC fibers (Nicalon) and Si-Ti-C-O fibers (Tyranno) are manufactured on an industrial scale. Colorless silicon oxynitride fibers and silicon nitride fibers [4] have been obtained by the nitridation of polycarbosilanes in the author s laboratory. Polymers used for ceramic precursor and the resulting ceramic fibers are listed in Table 1. 

D. Synthesis of Silicon Nitride Fibers from Polysilazanes... 

Okamura, K., M. Sato, and Y. Hasegawa. 1987. Silicon nitride fibers and silicon oxynitride fibers obtained by the nitridation of polycarbosilane. Ceramic International 13(1) 55-61. ... 

Metal particle catalyzed CVI Short silicon nitride fibers, coils 2.2 [24]... 

Metal particle catalyzed chemical vapor infiltration Liquid metal and/or liquid metal alloy droplets 0 Silicon nitride fibers with high strength, 0 Uniform SiN microcoils... 

The decomposition of ammonia increases above 850°C, a temperature at virhich nitridation begins to increase. As a result active hydrogen breaks the Si-0 bonds while active nitrogen reacts with unbonded silicon to form silicon nitride fibers (Equation 8b). The process does not only yield straight fibers but also well defined, coiled fibers. 

The structure property relationships observed in the Si-N system [17], are reminiscent of those observed for silicon nitride whiskers grown by metal catalyzed chemical vapor deposition (Chapter 2.2). The difficulty in obtaining binary (i.e., boron or silicon nitride) fibers with exactly required stoichiometry seems to have so far precluded the production of single crystal fibers by this route. 

F. T. Wallenberger and P. C. Nordine, Silicon nitride fibers produced by high pressure LCVD, Journal of Materials Research, 9 [3], 527-530 (1994). 

W. H. Atwell, Polymeric routes to silicon carbide and silicon nitride fibers, in Advances in Chemistry Series 224 Silicon-Based Polymer Science, A Comprehensive Resource, J. M. Zeigler, F. W. Gordon and M. Fearon, eds.. The Amer. Chem. Soc., Wash., 593-606 (1990). 

The ammonolysis of dimethyidichlorosilane offers another route to PSZ precursor fibers and ultimately silicon nitride fibers. In a first step, Me2SiCl2 and MeHSiCl2 are mixed in nearly a 1 1 molar ratio in benzene. The mixture is treated with NHs, NH4CI precipitates, and the solvent is removed. The ammonolysis product, a viscous polymer, is converted in a second step at 400°C into melt spinnable polymer, and is melt spun. The resulting PSZ precursor fiber has a (Si-N) backbone. Carbon is present in pendent methyl groups, and the empirical formula of the fiber is SiCi.7No.9oH57 [23]. 

T. Isoda, Surface of high purity silicon nitride fiber made from perhydropolysilazane, in Controlled Interphases in Composite Materials, H. Ishida. ed., Elsevier Science Publishing, Amsterdam, 255-265 (1990). 

Funayama, M. Arai, Y. Tashiro, H. Aoki, T. Suzuki, K. Tamura, H. Kaya, H. Nishli and T. Isoda, Tensile strength of silicon nitride fibers produced from perhydropolysilazane, Nippon Seramikkusu Kyokai Gakujutsu Ronbushi. 98 [1], 104-107 (1990). 

H. Matsuo, 0. Funayama, T. Kato, H. Kaya and T. Isoda, Crystallization behavior of high purity amorphous silicon nitride fiber, J. Ceram. Soc. Japan, 102 [5], 409-413 (1994). 

S. Kamimura, K. Watanabe, N. Kasai, T. Seguchl and K. Okamura, Silicon nitride fiber synthesis from polycarbosilane fiber by radiation curing and pyrolysis under ammonia, Ceram. Transactions, 58, 281-286 (1995). 

The principle of sol-gel processing is summarized in Sec. 2.6. Sol-gel is used on a large scale in the production of alumina-based fibers Nextel by the 3M Co.) and more recently of silicon-carbide and silicon-nitride fibers. 

Figure 14.5 Tensile strength of silicon-carbide and silicon-nitride fibers before and after exposure to high temperature. I ...

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