Oxynitride 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). 

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. 

C. Synthesis of Silicon Nitride and Silicon Oxynitride Fibers from Polycarbosiianes... 

The nitridation of polycarbosilane (with ammonia) begins at about 500°C and terminates almost completely at about 800°C. There is scarcely any carbon in the nitride obtained at 1400°C. Chemical analysis indicates its composition to be Si3N3.72, almost that of pure silicon nitride, Si3N4 [39]. Up to 1300°C, the X-ray diffraction pattern of nitride fiber is broad and characteristic of the amorphous state, whereas those at 1400°C indicate crystalline a-SisN4 (Fig. 11). Both silicon nitride and silicon oxynitride fibers have been obtained by the heat... 

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. ... 

Fibers Tvpical E-glass variants E-CR glass HS-glass HM-glass Oxynitride fibers ... 

Oxynitride fibers are formed by an adaptation of the conventional bushing process. Since nitrides would corrode precious metals in an oxidative environment, the bushings with up to 200 tips are therefore made from boron nitride-coated carbon or from molybdenum. The melts are formed under nitrogen at 1600-1750 °C and refined at lower temperature. Fibers with diameters ranging from 12 to 20 pm are continuously drawn from the melt 200°C below the melt temperature, and mechanically wound at 1000-2000 m/min [24]. 

Figure 6. Modulus and microhardness of Sialon or oxynitride fibers versus nitrogen content. Redrawn from J. Kobayashi, M. Oota, K. Kada and H. Minakuchi, Oxynitride Glass and the Fiber Thereof, US Patent 4,957,883, September 18,1990.

H. Kaplan-Diedrich and G. H. Frischat, Properties of some oxynitride fibers. Journal of Non-crystalline Solids, 184,1343-136 (1995). 

The different types of boron nitride composites cited can be reinforced with fibrous materials such as titanium alloy fibers [287], Si/Zr oxynitride fibers [288], SiOg/TiOg/ZrOg fibers [289], and carbon fibers [290 to 292, 313] (see also Section 4.1.1.10.1, p. 58). BN-containing oxide and carbide ceramics are used to protect graphite from being attacked in metallurgical processes [293 to 295]. Porous ceramics and ceramic foams which can be infiltrated either with metals or lubricants may contain a-BN or are produced in boron nitride ceramic molds [296 to 299]. 

Practical interest in nitrides is mainly focussed in their use as tough ceramics such as sialons and interstitial nitride coatings on metals [368,369]. Silicon oxynitride glasses are of importance in fiber optics as their refractive indices depend on the nitrogen content. There are surely other interesting properties awaiting exploration. 

But CMCs will be commercially successful only when they are produced cost-effectively. Polymer-derived ceramic (PDC) technology is one of the most promising low cost fabrication methods for ceramic matrix composites, particularly for large, complex shapes. In PDC technology, a silicon-based polymer (siloxane, carbosilane, silazane, etc) with fiber or particle reinforcement is shaped and cured in the polymer condition and then pyrolyzed in a controlled atmosphere to form a stable silicon-based ceramic, such as silicon carbide, sihcon nitride, silicon oxycarbide, or silicon oxynitride. 

HM oxynitride glass ceramic fiber (reference) Melt spun through bushing [26]... 

K. Suganuma, H. Minakuchi, K. Kada, H. Osafune and H. Fujii, Properties and micro-structure of continuous oxynitride glass fiber and its application to aluminum matrix composite, J. Mater. Res., 8 [1], 178-186 (1993). 

V. Budov, P. Sarkisov, K. Bormotunov, N. Trofimov, V. Khazanov, and Z. Shaina, Oxynitride glasses- the material for prospective glass fibers, Bolet. Socied. Espan. Ceram. Vidr., 31-C, 2,391-396 (1992). 

Si-C-N(O) fibers derived from HPZ precursor fibers are nanoporous and heterogeneous with a skin/core structure. The composition changes from SiOxCy in the external porous surface to SiNxC, in the core. The molecuiar formuia of this fiber is close to 4 mol. >4 SiOa, 81 mol.% SiNxCy (x = 1.02, y = 0.23) and 15 mol.% free C [22]. The presence of complex tetrahedral units is supported by the Si NMR spectrum which shows a broad signal covering the chemical shift region expected for silicon oxycarbide, siiicon oxynitride and silicon carbonitride units [21]. The occurrence of free carbon, expected from the nature of the precursor, is supported by the C Is XPS pattern [22]. 

In this process, the substrate is placed inside a reactor supplied by different gases [21], The principle of the process is that a chemical reaction takes place between the source gases producing a solid material which condenses on all surfaces inside the reactor. CVD is widely used in the semiconductor industry to deposit various materials such as polycrystalline, amorphous, and epitaxial silicon, carbon fiber, filaments, carbon nanotubes, Si02, silicon-germanium, tungsten, silicon nitride, silicon oxynitride, titanium nitride, and various high-k dielectrics. 

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