Oxygen-free SiC fibers

The behaviors of the unsaturated poly(diorganylsilylene-co-ethynylene)s XII-XXIV are dominated by the polymerization tendency, induced thermally or catalytically. These compounds are partially excellent precursors for the fabrication of oxygen-free SiC-fibers. Furthermore, subsequent reactions of the side groups lead to a number of new and interesting derivatives. 

The first approach to nearly-oxygen-free SiC fibers is to melt spin green fibers from Yajima type PCSs (m = 1600-2000 Da) and to rendered them infusible by anaerobic curing with y-... 

Similarly, an experimental, quasi-stoichiometric, oxygen-free SiC fiber has been produced from a Si-AI-C-0 precursor. In this process, aluminum is introduced into the polymer as aluminum (III) acetylacetonate, and acts like boron as a SiC sintering aid during heat treatment at 1800°C [40]. 

Summary The heterogeneous catalytic redistribution reaction of methylchlorodisilanes provides spinnable poly(methylchlorosilanes/-carbosilanes). Especially copolymers like poly(methylchlorosilanes-co-styrenes) are suitable polymers for melt spinning. The high reactivity caused by Si-Cl bonds enables oxygen free curing methods of the melt spun polymer filaments with ammonia. The synthesis is achieved without the employment of highly reactive metals and any solvents. The thus produced SiC fibers exhibit oxygen contents lower than 1 wt. %. 

As fiber is a primary component in continuous fiber reinforced ceramic matrix composites, its characteristic is an important factor that confines the thermal conductivity of the composites. The ideal SiC fiber should be highly crystalline, oxygen-free, and stoichiometric. As shows in Table I,... 

Table 1 lists some key production and compositional details for a variety of SiC-based fiber types of current interest and availability as CMC reinforcement. The polymer-derived types range from first-generation fibers with very high percentages of oxygen and excess carbon, such as Nicalon and Tyranno Lox M, to the more recent near-stoichiometric (atomic C/Si 1) fibers, such as Tyraimo SA and Sylramic. For the CVD-derived types, such as the SCS family with carbon cores, the only compositional variables in the SiC sheaths are slight excesses of free sihcon or free carbon. Table 2 lists some of the key physical and mechanical properties of the SiC fiber types in their as-produced condition, as well as estimated commercial cost per kilogram, all properties important to fiber application as CMC reinforcement. These SiC fiber properties in Tables 1 and 2 are in most part those published by the indicated commercial vendors. It should be noted that the Sylramic fiber... 

This mechanism accounts for the evolution of a gaseous SiO CO mixture, the growth of the SiC crystals and the deaease in the amounts of free cartxin and silicon oxycarbide. The relative amounts of CO and SiO which are formed and the composition of the final residue (i.e., SiC or SiC + C) depend upon the relative amounts of free caitton and silicon oxycarbide in the fiber. For the Nicalon fiber (NL 200), the main species in the gas phase is CO and the solid residue is SiC. There is therefore enough SiO formed by decomposition of the silicon oxycarbide phase (Equation 9) to consume all the free carbon by Equation 10 [73]. This mechanism also accounts for one of the processes used to produce oxygen-free nearly-stoichiomefric SiC fibers [38] [54]. Since CO and SiO diffuse and escape from the fiber, the decomposition starts near its surface and the decomposition front moves radially towards the fiber axis, yielding a skin/core microstructure [16]. 

The modulus of SiC based fibers strongly depends upon the occurrence of intergranular phases. It Is of the order of 580 GPa for SIC whiskers and 400-450 GPa for the nearly stoichiometric, oxygen-free polycrystalline SiC fibers [54] [65]. Conversely, it is much lower... 

Si-C-0 fibers (Nicalon NLM-200) creep at low temperatures (1000-1200"C), whereas the sintered a-SiC fibers (Carborundum) creep at much higher temperatures (1300-1600 0). The other fibers, which are derived from PCS precursor fibers, i.e., oxygen-free Si-C fibers (Hi-Nicalon) and the quasi-stoichiometric fibers (Hi-Nicalon S or Dow Coming fiber) fail between these two limits. The fibers which have been heat treated at 1400-1600 0 and display a larger SIC grain size (e.g., 30-50 nm) exhibit a creep resistance similar to that of the a-SiC sintered fiber. 

An electron beam curing process can reduce the oxygen content by an oxygen free cross-linking process [32]. The fiber, composed of fine fi-SiC crystallites... 

The multifilament fiber (10-20 xm diameter) as commercially produced consists of a mixture of /3-SiC, free carbon and SiOj. The properties of this fiber are summarized in Table 6.5. The properties of Nicalon start to degrade at temperatures above about 600°C because of the thermodynamic instability of composition and microstructure. A ceramic grade of Nicalon, called Hi Nicalon, having low oxygen content is also available Yet another version of a multifilament silicon carbide fiber is Tyranno, produced by Ube Industries, Japan. This is made by pyrolysis of poly (titano carbosilanes) and contains between 1.5 and 4wt% titanium. 

Thus, the fibers resulting from the pyrolysis at 1200-1300°C of oxygen cured Yajima type PCS precursor fibers in an inert atmosphere are far from consisting of pure SiC since the molar fraction of SiC is about 50%. They also contain significant amounts of partly hydrogenated free carbon and silicon oxycarbide. It Is noteworthy that the value, or perhaps mean value [14] [64], of the term 1-x/2 = 0.447 is dose to 0.5 in Nicalon NL-200 (Figure 7) and that the main tetrahedral units are therefore close to SiOiCz [15]. 

The atomic distribution of silicon, carbon and oxygen in the three phases (SiC, SiOxCi. s and free carbon) can be calculated as a function of x for a given overall atomic composition [59]. Other approaches have been used to calculate the molar composition of PCS based Si-C-0 fibers from the overall atomic percentages [16] [60]. 

The Nicalon fibers have variable composition which includes a substantial amount of oxygen and free carbon and consist of microcrystalline SiC in Si-C-0 glassy matrix together with thin plates of turbostratic carbon. Their properties vary with the composition.I" They generally have high electrical conductivity. 

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