Preceramic polymers composition

Preceramic polymer precursors (45,68) can be used to make ceramic composites from polymer ceramic mixtures that transform to the desired material when heated. Preceramic polymers have been used to produce oxide ceramics and are of considerable interest in nonoxide ceramic powder processing. Low ceramic yields and incomplete burnout currently limit the use of preceramic polymers in ceramics processing. 

In the design of preceramic polymers, achievement of the desired elemental composition in the ceramic obtained from them (SiC and Si3N4 in the present cases) is a major problem. For instance, in the case of polymers aimed at the production of SiC on pyrolysis, it is more usual than not to obtain solid residues after pyrolysis which, in addition to SiC, contain an excess either of free carbon or free silicon. In order to get close to the desired elemental composition, two approaches have been found useful in our research (1) The use of two comonomers in the appropriate ratio in preparation of the polymer, and (2) the use of chemical or physical combinations of two different polymers in the appropriate ratio. 

The first useful organosilicon preceramic polymer, a silicon carbide fiber precursor, was developed by S. Yajima and his coworkers at Tohoku University in Japan [5]. As might be expected on the basis of the 2 C/l Si ratio of the (CH3)2SiCl2 starting material used in this process, the ceramic fibers contain free carbon as well as silicon carbide. A typical analysis [5] showed a composition 1 SiC/0.78 C/0.22 Si02- (The latter is introduced in the oxidative cure step of the polycarbosilane fiber). 

In the design of preceramic polymers, achievement of the desired elemental composition in the resulting ceramics (SiC and Si3N4 in the present cases) is a major problem. For instance, in the case of polymers aimed at the production of SiC on pyrolysis, solid residues usually are obtained after... 

Silicon-containing preceramic polymers are useful precursors for the preparation of ceramic powders and fibers and for ceramic binder applications (i). Ceramic fibers are increasingly important for the reinforcement of ceramic, plastic, and metal matrix composites (2, 3). This chapter will emphasize those polymer systems that have been used to prepare ceramic fibers. An overview of polymer and fiber processing, as well as polymer and fiber characterization, will be described to illustrate the current status of this field. Finally, some key issues will be presented that must be addressed if this area is to continue to advance. 

The excellent high-temperature properties of the ceramic materials strongly depend on the molecular structure and composition of the polymeric precursors. This chapter reviews the fundamentals of synthetic approaches to silicon-based nonoxide preceramic polymers and briefly discusses their processing. 

The above examples show that preparative organometallic chemistry allows for the production of a wide variety of silicon-based molecular precursors for high-temperature ceramics. The desired physical chemical properties and appropriate thermolysis chemistry can be realized by an intelligent precnrsor design. Nevertheless, there is still a need for further development for example, investigations into the synthesis of precursors that release phase-pure ceramics or composites with tunable composition and properties. The focns will also be on designing preceramic polymers, which release functional materials. In this field, very little investigation has been performed so far. 

By preceramic polymers, several joints were obtained through PIP, RS, and new NITE processes The NITE SiC/SiC composites were produced to demonstrate high-performance hot-pressed joining and its tensile strengths is > 200 MPa however, the joining process requires... 

Another area in which preceramic polymers can be utilized effectively is as binders for ceramic powders in near net shaping fabrication processes, such as compression or injection molding with subsequent sintering. Alternatively, an active filler and a polymer [67,68], as reported by Greil and Seibold, can be used in such fabrication. Other potential applications of preceramic polymers is in the general area of coatings, especially for carbon-carbon composites [69], and in the synthesis of nanostructured ceramic particles and composites [70-73]. 

In principle, the simplest way to produce preceramic polymers for ternary silicon boron nitrides is to coammonolize mixtures of silicon and boron chloride. Dietz has applied for a patent for such a process, with Si/B ratios ranging from 9 1 to 1 9. There are two major disadvantages of this approach (1) the polymer is only accessible as a mixture with the by-product, ammonium chloride, and (2) the ceramics obtained are composites constituted of the binaries BN and Si3N4 [49]. 

DAVID WILSON is a research specialist with 3M and has more than 10 years of research and development experience in the synthesis of new ceramic fibers using sol-gel techniques and is the inventor of the Nextel 610 and Nextel 720 ceramic fibers. His experience includes the development of novel fiber precursor formulations, continuous fiber processing, and characterization of ceramic fibers at room and elevated temperatures. Current research activities include preceramic polymer-derived SiC fibers and MOCVD coating and composite fabrication for the Mullite Matrix Composite Consortium Program. Mr. Wilson holds four patents and is the author of several publications. 

R. M. Laine, F. Babonneau, K. Y. Blohowiak, R. A. Kennish, J. A. Rahn, and G. J. Exharos, The evolutionary process during pyrolytic transformation of poly(n-methylsilazane) from a preceramic polymer into an amorphous silicon nitride/carbon composite, J. Am. Ceram. Soc. 1995, 78, 137-145. 

The purpose of this chapter is to provide an overview of the chemistry, processing and application of boron-containing preceramic polymers in the BN system. The nonoxide precursor route, also called the Polymer Derived Ceramics (PDCs) route, represents a chemical approach based on the use of air- and/or moisture-sensitive (molecular or polymeric) precursors by means of standard Schlenk techniques and vacuum/argon lines. This precursor route allows the chemistry (e.g., elemental composition, compositional homogeneity and atomic architecture) of molecular precursors to be controlled and tailored in order to provide the ensuing preceramic polymers... 

Silicon nitride has been obtained by the pyrolysis (in a stream of NHj) of the perhydropolysilazane prepared by the ammonialysis of the H SiClj-pyridine adduct [23, 24]. The gas stream employed during the pyrolysis of the preceramic polymer plays a crucial role [25]. Pyrolysis of [B, H,j diamine] polymers in an NHj stream gives BN [26]. TiN is similarly obtained by the pyrolysis of an amine precursor [27]. TiN has been prepared from titanazane [28]. Pyrolysis of Nicalon in NHj is reported to give SijN [24]. Besides single-phase ceramics, multiphase ceramics (e.g. composites of SiC and TiC, BiN and SijN ) have been prepared from precursors [29, 30]. Group 13 metal nitrides (GaN, AIN, hiN) have been prepared by the decomposition of urea complexes [31]. This method has been extended for the synthesis of BN, TiN and NbN [32]. 

Since the oxidation resistance of SiC is much better than that of carbon, SiC/SiC composites have been developed for aerospace application such as propulsion and high velocity systems. Similar to carbon/carbon composites, the SiC/SiC continuous fiber composites consist of a fiber architecture made of silicon car-hide fibers in a matrix of silicon carbide. The matrix is usually produced by CVl or preceramic polymer impregnation and pyrolysis. 

Sato, K., Morizumi, H., Tezuka, A., Furuyama, O., andlsoda, T. (1993). Interface and mechanical properties of ceramic fiber reinforced silicon nitride composites prepared by a preceramic polymer impregnation method, High-Temperature Ceramic-Matrix Composite II Manufacturing and Materials Development, pp. 199-203, Evans, A. G. and Naslain, R. eds., Westerville, OH The American Ceramic Society. 

The nonstoichiometric composition coupled with the microporosity and amorphous phase in the microstructure reduces the thermal stability of the fibers. Depending on the magnimde and duration of the applied stress, the use of the fibers may be limited to temperatures lower than 1000°C. To overcome this deficiency, recent efforts have been devoted to producing dense, polycrystalline, stoichiometric SiC through modification of the synthesis and processing conditions or through the synthesis of more useful preceramic polymers (63,67,68). 

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