Silicon carbide poly pyrolysis

The first type of polycarbosilane synthesized by using ADMET methodology was a poly[carbo(dimethyl)silane].14c Linear poly(carbosilanes) are an important class of silicon-containing polymers due to their thermal, electronic, and optical properties.41 They are also ceramic precursors to silicon carbide after pyrolysis. ADMET opens up a new route to synthesize poly(carbosilanes), one that avoids many of the limitations found in earlier synthetic methods.41... 

Turning now to other types of ceramic fibre, the most important material made by pyrolysis of organic polymer precursors is silicon carbide fibre. This is commonly made from a poly(diorgano)silane precursor, as described in detail by Riedel (1996) and more concisely by Chawla (1998). Silicon nitride fibres are also made by this sort of approach. Much of this work originates in Japan, where Yajima (1976) was a notable pioneer. 

On heating in air at 10°C per min, poly(m-carborane-siloxane) shows typically only 4% mass loss at 450°C and 7% mass loss at 600°C (see Fig. 4). In comparison, siloxanes without carborane units, show an approximate 50% mass loss at 450°C. As a consequence of the relatively high boron and carbon content of these materials, pyrolysis is expected to generate ceramic residues of boron carbide/silicon carbide. 

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. 

Specihcally with regard to the pyrolysis of plastics, new patents have been filed recently containing variable degrees of process description and equipment detail. For example, a process is described for the microwave pyrolysis of polymers to their constituent monomers with particular emphasis on the decomposition of poly (methylmethacrylate) (PMMA). A comprehensive list is presented of possible microwave-absorbents, including carbon black, silicon carbide, ferrites, barium titanate and sodium oxide. Furthermore, detailed descriptions of apparatus to perform the process at different scales are presented [120]. Similarly, Patent US 6,184,427 presents a process for the microwave cracking of plastics with detailed descriptions of equipment. However, as with some earlier patents, this document claims that the process is initiated by the direct action of microwaves initiating free-radical reactions on the surface of catalysts or sensitizers (i.e. microwave-absorbents) [121]. Even though the catalytic pyrolysis of plastics does involve free-radical chain reaction on the surface of catalysts, it is unlikely that the microwaves on their own are responsible for their initiation. 

Spinning of PCS, often with the use of an organic-polymer spinning aid such as poly (ethylene oxide), followed by curing and high-temperature pyrolysis gives black silicon-carbide-like fibers. 

Baney and co-workers (iO) used the known Si-Si and Si-Cl bond redistribution of methylchlorodisilanes to prepare poly(methylchlorosilane)s. Pyrolysis of these branched polysilane polymers gives nearly stoichiometric amounts of silicon carbide (equation 6). 

Poly(methylchlorosilanes) which have been prepared by Lewis-base catalyzed disproportionation of 1,1,2,2-tetrachlorodimethyldisilane give rise to a complex pyrolytic conversion into silicon carbide. In the temperature region 180-450 °C under dry argon, the polymer samples are cross-linked. Characterization of the pyrolysis intermediates and final products by and Si MAS in both CP and IRCP techniques as well as TG/MS studies strongly suggest the loss of methylchlorosilanes as well as oligosilanes during pyrolysis (c.f. Scheme 21). 

Figure 13.20. A. Poly[acrylonitrile]-crossliiiked silica decorated with Si-AIBN (pt = 0.47 gcm", 60%, w/w, polymer, 228 g 60%, v/v, empty space). B. Pure silicon carbide aerogel by pyrolysis of the poly[acrylonitrile]-crosslinked silica shown in A. (pi, = 0.97 gcm, skeletal density Ps = 3.12 gcm , 39 g" , 69%, v/v, empty...

As an alternative to thermal decomposition, CVD, and laser-assisted H elimination, the flash pyrolysis of soluble alkylpolysilyne in a vacuum is reported recently. Unless they are under vacuum conditions, polysilanes and polysilyne may convert to silicon carbide (SiC). The flash process, which is a rapid removal of volatile organic substances and H2 gas, enables the control of the dimension of Si structure from 2D to 3D. Flash pyrolysis of alkylpolysilyne in a vacuum above 500 °C leads to the formation of poly- Si with a minimal amount of SiH termini, although the size of the crystals is limited to the range between several pm and several nm. 

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