Yajima process

Although these polysilane by-products were not noticed at that time, they are now important main products. In the so-called "Yajima process" [10] (the usual reductive dehalogenating coupling of dimethyldichlorosilane with sodium) polysilanes were obtained and fibres could be formed from these which were then pyrolyzed to form silicon carbide fibres. 

SCHEME 2. Original Yajima process for producing SiC1.45O036H0.03 fibers... 

In addition to not offering properties expected for phase pure SiC, the original Yajima process suffers from other drawbacks that include a multistep precursor synthesis and the inability to self-cure. Finally, the presence of oxygen limits the upper use temperature for both Nicalon and Tyranno fibers to ca 1200 °C because above this temperature CO and SiO gases evolve, generating defects (large crystallites, pores and voids) that contribute to substantial decreases in mechanical properties. 

As a result of these disadvantages, tremendous efforts over the past 20 years have focused on developing chemistries/processes to improve or replace the Yajima process. At this point, as discussed below, several precursors can be synthesized and/or processed to produce phase pure SiC shapes with controlled microstructures that offer the exceptional properties expected of SiC. Thus, this area of precursor chemistry can now be considered to be mature. Consequently, only engineering and/or cost considerations now dictate which precursor systems are useful for a given application. Furthermore, because the general area of SiC precursors has been reviewed in detail6,7, we focus here only on those precursors and processing methods that provide phase pure SiC. 

Figure 6.21 Yajima process of making Nicalon (Si-C-O) fiber.

Specifically, the Yajima process of making SiC involves the following step shown schematically in Fig. 6.21. Polycarbosilane, a high molecular weight polymer containing Si and C, is synthesized. The starting material is commercially... 

After about 20 years of intensive investigations for accessing to silicon carbide-based materials, the Yajima process [1] remains the reference, especially in the field of industrial obtaining of fibers (Eq.l). 

Technically the most important access to polycarbosilanes is the Yajima process, in which polydimethylsilane [SiMe2] (compare previous section) is... 

SiC fiber was produced from polycarbosilane (PCS) by Yajima et al. " in 1975, which is the earliest case of organosilicon polymer utilization for an industrial structural material. In the Yajima process, PCS was mainly synthesized from polydimethylsilane (PDS) by a thermal conversion process using an autoclave or an open reflux system. This is the commonly available PCS. Its melt spinability, solubility in various orgaiuc solvents, and stability for storing at room temperature are critically important for industrial uses. 

The polyorganosilanes serve as precursors for the fabrication of ceramic fibers made of p silicon carbide. In a first step, low-molecular-weight (around 8000) carbosilanes are thermolyzed, then ceramization is achieved (Yajima process) ... 

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