Ceramic polymer precursor

In the sol-gel process, ceramic polymer precursors are formed in solution at ambient temperature shaped by casting, film formation, or fiber drawing and then consolidated to furnish dense glasses or polycrystalline ceramics. The most common sol-gel procedures involve alkoxides of silicon, boron, titanium, and aluminum. In alcohol water solution, the alkoxide groups are removed stepwise by hydrolysis under acidic or basic catalysis and... 

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. 

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. 

Chan, VZH Hoffman, J Lee, VY latrou, H Avgeropoulos, A Hadjichristidis, N Miller, RD Thomas, EL, Ordered Bicontinuous Nanoporous and Nanorelief Ceramic Pihns from Self-Assembling Polymer Precursors, Science 286, 1716, 1999. 

Organometallic polymer precursors offer the potential to manufacture shaped forms of advanced ceramic materials using low temperature processing. Polysilazanes, compounds containing Si-N bonds in the polymer backbone, can be used as precursors to silicon nitride containing ceramic materials. This chapter provides an overview of the general synthetic approaches to polysilazanes with particular emphasis on the synthesis of preceramic polysilazanes. 

A ceramic fiber with Si-C-N-0 composition can be prepared by melt-spinning, cure and pyrolysis of a polymethyldisilylazane polymer precursor (14, 15), which is the reaction product of a mixture of 50 mol % 1,1,2,2- tetrachloro-1,2-dimethyldisilane (la), 40 mol % 1,1,2-trichloro- 1,2,2-trimethyldisilane (lb) and 10 mol %... 

Significant recent advances have occured with phosphazenes (J ) and to a lesser extent silizanes (6) in contrast, polymers based on phosphorus(III) and nitrogen are virtually unknown. Because such systems offer opportunities for metal coordination, and in their oxidized forms could be valuable polymer precursors to PON ceramics (2), we have begun systematic studies of the structural and reactivity factors necessary for their formation. 

Other organosilicon polymer precursors for ceramics have either been prepared or improved by means of transition metal complex-catalyzed chemistry. For instance, the Nicalon silicon carbide-based ceramic fibers are fabricated from a polycarbosilane that is produced by thermal rearrangement of poly(dimethylsilylene) [18]. The CH3(H)SiCH2 group is the major constituent of this polycarbosilane. 

A less explored area of transition metal catalysis involves bond formation between Group 14 elements and nitrogen. In direct analogy to previously discussed areas of research, silicon-nitrogen bonds can be formed by dehydrocoupling, hydrosilylation, and dehydrogenative silylation. The compounds produced are valuable for use in organic synthesis or as polymer precursors to silicon nitride ceramics. 

As observed by D. Johnson and J. Stiegler, "Polymer-precursor routes lor fabricating ceramics offer one potential means or producing reliable, cost-effective ceramics. Pyrolysis of polymeric metalloorganic compounds can be used to produce a wide variety of ceramic materials." Silicon carbide and silicon oxycarbide fibers have been produced and sol gel methods have been used In prepare line oxide ceramic powders, such as spherical alumina, as well as porous and fully dense monolithic forms. 

Hence, for most applications, high ceramic yield precursors are essential. Consequently, it is important to formulate a preceramic polymer that contains minimal amounts of extraneous ligands that allow it to meet the processability criterion and yet provide high weight percent conversions to ceramic product. Thus, in many of the precursors discussed... 

This polymer precursor (1) requires relatively inexpensive starting materials, (2) is quite stable in air, (3) offers good processability for polymer infiltration processing of composites, (4) provides excellent SiC ceramic yields and (5) high purity with controllable microstructures. However, one important drawback is the use of costly LiAlHzt. This polymer is now available commercially (Starfire Inc., NY). 

Few fibers have the excellent mechanical properties reported for the UF-HM fibers however, if the synthesis parallels the original one (see above), the expected drawbacks for the process include (1) the use of an autoclave, not desirable for large-scale production and (2) low ceramic yields of SiC, given that the fiber is derived from a polymer precursor containing a 1 2 Si C ratio. 

Interrante, L. V., Moraes, K.,Liu, Q., Lu, N., Puerta, A. Sneddon, L. G. Sihcon-based ceramics from polymer precursors. Pure Appl. Chem. 74, 2111-2117 (2002). 

The focus here on self-organizing polymers, electrical and optical properties, and biosynthesis is not even close to comprehensive in the enumeration of avenues and opportunities in new polymeric materials. Degradable plastics that do not remain in our environment forever [24] and polymer precursors to ceramics [19] and inorganic fibers are two of many more areas in which new polymeric materials will provide new challenges to engineers in production and processing. 

K. Okamura, Ceramic Fibers From Polymer Precursors, Composites, 18[2] 107 (1987). 

X HE USE OF CHEMICAL APPROACHES to improve the processing, properties, and performance of advanced ceramic materials is a rapidly growing area of research and development. One approach involves the preparation of organometallic polymer precursors and their controlled pyrolysis to ceramic materials. This chapter will review the preparation and application of silicon-, carbon-, and nitrogen-containing polymer systems. However, the discussion is not exhaustive the focus is on systems with historical significance or that demonstrate key technological advances. 

The development of processing ceramics from polymer precursors has attracted great attention. In particular, inorganic polymers containing silicon are important for SiC-based ceramic synthesis. SiC ceramics have the advantage of high-temperature stability in an oxidation atmosphere. SiC is not readily sintered, and thus is difficult to obtain in either fiber or film form by traditional inorganic processes. 

Hydrosilylation of 1 with trichlorosilane has been performed with Pt on charcoal (1% by weight) in quantitative yields (Scheme 1). The B-tris(trichlorosilylvinyl)borazine (2) was obtained with a high regioselectivity of proximately 80% trans hydrosUylation product [4], Pure 2 can be obtained by fiactional crystallization of the synthesized product fiom hexane. For further synthesis, both a- and P-hydrosilylation products can be used. No further hydrosilylation was observed in this case. In order to interconnect the single source precursor molecule 2 to a pre-ceramic polymer, methylamine was added to the solution of 2 in hexane, and a high viscosity, colorless oil was formed. By changing the reaction parameters (excess of CH3NH2, temperature), the viscosity of the polymer can be varied [5]. The obtained polymer (3P) is pure after evaporation of the solvent, which is checked by NMR. Other solvents like thf or toluene are also possible for the reaction, as well as for dissolution of the polymer. Furthermore, ethylamine leads to similar results in the formation of the polymer. 

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