Synthesis and characterization of polysilane precursors for silicon carbide fibers
- PDF / 911,797 Bytes
- 9 Pages / 576 x 792 pts Page_size
- 12 Downloads / 225 Views
A series of polysilanes was prepared by the alkali metal dechlorination of chlorosilane monomers, in which the overall functionality, F, of the reaction was varied. Starting monomers of functionality / = 2.0 and 3.0 were reacted together in various proportions to achieve values of F of 2.2, 2.35, and 2.5. In addition to varying the functionality of the reaction, three different difunctional monomers, dimethyldichlorosilane (DMDCS), diphenyldichlorosilane (DPDCS), and methylphenyldichlorosilane (MPDCS), and two trifunctional monomers, phenyltrichlorosilane (PTCS) and ethyltrichlorosilane (ETCS), were used. The effect of these changes on the yields of the polysilanes was determined, and the products were investigated by the use of thermogravimetry (TG), gel permeation chromatography (GPC), and thermomechanical analysis (TMA). The ability to spin a polysilane fiber was also assessed.
I. INTRODUCTION Extensive research has been carried out in recent years into the production of ceramic fibers via a polymeric precursor route. This process requires a polymer that can be readily formed into a fiber and which will pyrolyze to a ceramic material after suitable curing and heat treatment. Silicon carbide fibers have been successfully produced in this way and are used in wide-ranging applications in metal, ceramic, glass, and polymer matrix composites. There are several recent articles reviewing the polymer precursor route to ceramic fibers, for example, those by Miller and Michl,1 Emsley et al.,2 Seyferth et al.,3 and Laine and Babonneau,4 so only brief details of relevant literature will be given here. The major breakthrough in obtaining a polymeric precursor route to silicon carbide fibers was made by Yajima5 and eventually led to the production of Nicalon fiber. DMDCS was reacted with lithium metal to produce a polysilane that was subsequently converted into a polycarbosilane that could be melt spun into fibers. However, this process was expensive, difficult, and time consuming. Improvements to the synthesis were therefore made, including the replacement of the lithium by sodium, the addition of a small amount of diphenyldichlorosilane to aid the spinning, and the use of "Python" (polyborophenylsiloxane) to remove the autoclave step previously required. West et al.6 extended Yajima's work by synthesizing a polysilane, referred to as polysilastyrene, from a mixture of dimethyldichlorosilane (DMDCS) and ''Author to whom correspondence should be addressed. J. Mater. Res., Vol. 10, No. 10, Oct 1995 http://journals.cambridge.org
Downloaded: 15 Mar 2015
methylphenyldichlorosilane (MPDCS). However, on pyrolysis, the ceramic yield of the product was only about 30%, too low for a potential fiber precursor. In an attempt to improve the potential ceramic yield, Schilling et al.1 introduced monomers of higher functionalities. By mixing monomers of different functionalities, it is possible to produce either linear, branched, or network copolymers and therefore influence the properties of the polymeric product. These authors conclude
Data Loading...
