Precursors polymer-ceramic transformations

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. 

One potential solution to these problems, suggested some 20 years ago by Chantrell and Popper (1), involves the use of inorganic or organo-metallic polymers as precursors to the desired ceramic material. The concept (2) centers on the use of a tractable (soluble, meltable or malleable) inorganic precursor polymer that can be shaped at low temperature (as one shapes organic polymers) into a coating, a fiber or as a matrix (binder) for a ceramic powder. Once the final shape is obtained, the precursor polymer can be pyrolytically transformed into the desired ceramic material. With careful control of the pyrolysis conditions, the final piece will have the appropriate physical and/or electronic properties. 

Since the development of the first useful organosilicon compound, a silicon carbide precursor, by Yajima [11], there has been extensive research on the design of polymers which transform to silicon—containing ceramics upon pyrolysis. In recent years, progress has been made in the synthesis of polymeric precursors of silicon nitride. Seyferth and coworkers [12] developed a methylsilazane compound, basically an ammonolysis product of methyldichlorosilane, and reported the... 

Polymer-derived ceramics (PDCs) represent a rather novel class of ceramics which can be synthesized via cross-linking and pyrolysis of suitable polymeric precursors. In the last decades, PDCs have been attaining increased attention due to their outstanding ultrahigh-temperature properties, such as stability with respect to decomposition and crystallization processes as well as resistance in oxidative and corrosive environments. Moreover, their creep resistance is excellent at temperatures far beyond 1000 °C. The properties of PDCs were shown to be strongly related to their microstructure (network topology) and phase composition, which are determined by the chemistry and molecular structure of the polymeric precursor used and by the conditions of the polymer-to-ceramic transformation. 

The cyclotrisilazane (R = Me) produced in reaction (14) is recycled at 650°C [by reaction with MeNHo) the reverse of reaction (14)] to increase the yield of processible polymer. Physicochemical characterization of this material shows it to have a softening point at 190°C and a C Si ratio of 1 1.18. Filaments 5-18 pm in diameter can be spun at 315°C. The precursor fiber is then rendered infusible by exposure to air and transformed into a ceramic fiber by heating to 1200°C under N2- The ceramic yield is on the order of 54% although, the composition of the resulting amorphous product is not reported. The approach used by Verbeek is quite similar to that employed by Yajima et al. (13) in the pyrolytic preparation of polycarbosilane and its transformation into SiC fibers. 

The other process is the transformation of an organic precursor into a continuous thin ceramic fiber. In the spinning process, polycarbosilane, a high molecular weight polymer containing Si and C, is obtained by thermal decomposition and polymerization of polydimethylsilane. The fiber thus produced consists of a mixture of P-SiC, carbon crystallite and SiO. The presence of carbon crystallite suppresses the growth of SiC crystals. Yajima and coworkers (Yajima et al., 1976, 1978, 1979) were the first to produce fine (10-30 pm in diameter), continuous and flexible fibers, which are commercialized with the trade name of Nicalon (Nippon Carbon Co.). 

For some uses it is important to form bonds that link different polysilane chains, to transform soluble, meltable polysilanes into insoluble resins. This process is vital if the polysilanes are to be used as precursors to silicon carbide ceramics, since, if cross-linking is not carried out, most of the polymer is volatilized before thermolysis to silicon carbide can take place. Several methods have therefore been developed to bring about cross-linking of polysilanes.109 110... 

Metallic nanopartides were deposited on ceramic and polymeric partides using ultrasound radiation. A few papers report also on the deposition of nanomaterials produced sonochemically on flat surfaces. Our attention will be devoted to spheres. In a typical reaction, commerdally available spheres of ceramic materials or polymers were introduced into a sonication bath and sonicated with the precursor of the metallic nanopartides. In the first report Ramesh et al. [43] employed the Sto-ber method [44] for the preparation of 250 nm silica spheres. These spheres were introduced into a sonication bath containing a decalin solution of Ni(CO)4. The as-deposited amorphous clusters transform to polyciystalline, nanophasic, fee nickel on heating in an inert atmosphere of argon at a temperature of 400 °C. Nitrogen adsorption measurements showed that the amorphous nickel with a high surface area undergoes a loss in surface area on crystallization. 

Using similar procedures polycarbosilanes could be obtained which were transformed into nearly stoichiometric silicon carbide [98]. In order to obtain higher ceramic yields the precursors were thermolyzed at 300 °C for 3 h imder argon prior to pyrolysis. Crosslinking of the polymer chains occurred during the thermolysis under elimination of hydrogen and methylsilanes (Scheme 8). 

Polysilazanes have been shown to be excellent polymeric precursors to amorphous silicon carbonitride (SiCN), silicon nitride, silicon carbide (SiC) and their composites. The actual chemical and phase compositions of the ceramic products depend on the polymer composition and pyrolysis conditions, such as temperature, time and atmosphere. Polymeric silazanes consist of amorphous networks, which transform to amorphous SiCN ceramics by pyrolysis under inert atmosphere at around 1000 C. These ceramic products remain amorphous up to 1400 °C in an inert atmosphere [a.322]. However, at higher temperatures the non-stoichiometric SiCN matrix decomposes, with nitrogen loss, giving the thermodynamically stable phases, namely Si3N4 and SiC. Polysilanes, polycarbosilanes and polysilazanes are commonly used for the preparation of high-performance ceramics such as silicon carbide, silicon nitride and silicon carbonitride. 

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