Polysilazanes offer a convenient route for preparing a variety of ceramic materials.
Polysilazanes are polymers consisting of silicon, nitrogen, hydrogen and, in the case of organopolysilazanes, carbon. Such polymers can be considered either inorganic (perhydropolysilazanes) or organic (organopolysilazanes or polycarbosilazanes) in nature and have both silicon and nitrogen atoms in their backbone. Principally due to their high pyrolytic mass yields of silicon-based ceramic material (often > 80wt%), polysilazanes have been extensively used in the past as precursors to silicon dioxide,1 silicon nitride and silicon carbide ceramics.2-8 They offer a convenient route for preparing ceramic materials such as fibers, coatings or 3-D continuous fiber-reinforced ceramic matrix composites, which often cannot be made through traditional ceramic processing methods.3
Inorganic vs. Organic
Inorganic polysilazanes are currently used extensively in the electronics industry as precursors in the manufacture of dense or porous surface coatings of pure silica. In the manufacture of semiconductor devices, these coatings are produced on the surface of a substrate, such as a silicon wafer, by coating the substrate surface with a polysilazane-containing coating solution and then converting the coating layer of polysilazane into a silica-based coating film.9 Typically, such surfaces are prepared by spin-coating and then substantially converting the polysilazane layer to a silicon-oxide-containing ceramic, such as silica. The coatings are referred to by the generic term spin-on-glass (SOG).
Recently, a body of work has focused on the ability of various organic polysilazanes to form amorphous, non-glassy silicon carbonitride (SiCN) ceramics. These ceramics are resistant to chemical degradation,10 creep,11-13 oxidation,14 thermal shock, decomposition and softening.15 Thus, they are quite attractive for a variety of structural ceramic and electronic applications.16 A viable technique to fabricate ceramic devices for micro-electro mechanical systems (MEMS) by microcasting UV-curable organopolysilazanes, for example, has recently been developed.17-21 A microfabrication technique for constructing 3-D photonic crystals by deep X-ray lithography using organopolysilazanes has also been demonstrated.22
In the structural ceramics arena, both monolithic and composite ceramic structures have been fabricated using organopolysilazane ceramic precursors. Two methods have been developed to prepare crack-free monolithic ceramics from polysilazanes. The first method involves a warm pressing technique, in which the polymer is first crosslinked to an infusible state, compacted by cold isostatic pressing or warm pressing, and then pyrolyzed at a slow heating rate.23,24 The second method involves pre-pyrolysis of the polymer to a ceramic powder. The powder is subsequently compounded with additional polysilazane, and the composition is cured during a pressing step.25-27
The first approach results in ceramics having a high fraction of open porosity, while the second method results in much less gas generation and volume shrinkage in the resulting ceramic. Porous monolithic ceramic bodies can also be fabricated using organopolysilazanes. Representative of this work is the preparation of high-surface-area catalyst supports made of macroporous SiCN produced by a capillary micromolding technique.28
Coatings and Joints
Organopolysilazanes have also found utility in the joining of ceramics and as the continuous phase in formulated, high-temperature coatings. Coatings and joints from filled organopolysilazanes have been demonstrated for silicon nitride matrix composites,29 for the microwave joining of silicon carbide,30 in seals for solid oxide fuel cells,31 and as electronic adhesives for bonding integrated circuit chips to carriers or circuit boards.32 They have also been used for preform joining in the fabrication of both ceramic matrix composites and metal matrix composites.33
Several coatings companies are currently selling formulated ceramic coatings based on organopolysilazanes for under-the-hood automotive and truck applications (e.g., exhaust systems, pistons, etc.), as well as organopolysilazane-based coatings for firearms and high-temperature applications requiring substantial heat insulating characteristics. Organopolysilazanes have also been used effectively in such applications as anti-oxidation layers on carbon structures,34 carbon-silicon carbide composites,35 carbon- or silicon carbide-fiber,36 and the strength enhancement of carbon foams.37
Recent investigations have also shown that selective pyrolysis of organopolysilazanes in controlled atmospheres or in the presence of various organic polymers can result in ceramics with well-defined nanoscale structural features, including such novel structures as silicon nitride nanobelts.38 Controlled pyrolysis of organopolysilazanes has also resulted in the generation of silicon nitride/silicon carbide nanocomposite structures with ceramic grains in the 30 nm size domain.39
Mesoporous ceramic materials have also been produced based on an approach that uses an organic diblock copolymer structuring agent in combination with an organopolysilazane.40 The resulting ceramic has a lamellar nanostructure consisting of hexagonally packed cylinders with an average pore diameter of about 13 nm.
Fibers and Matrices
Organopolysilazanes are also useful for producing either fiber or matrices in the fabrication of ceramic matrix composites. Fibers, for example, can be produced by the melt-spinning and pyrolysis of pure polysilazanes41-43 or through the modification of polysilazanes with, for example, metal alkoxides (such as alkoxides of titanium or zirconium) followed by spinning and a subsequent pyrolysis step.44,45 In the former case, SiCN fibers are produced, while in the latter case, various oxycarbonitride compositions are obtained. Highly corrosion-resistant monolithic ceramics have also been prepared using this technique of metal alkoxide modification by the reaction of organopolysilazanes with aluminum isopropoxide.46,47
CMC matrices can also be generated from polysilazanes. Variations of the polymer infiltration pyrolysis (PIP) technique have been demonstrated over the years.48-50 Resin transfer molding (RTM) techniques into fibrous preforms with suitable organopolysilazane precursors (e.g., low viscosity, solvent-free), followed by subsequent thermally induced crosslinking and pyrolysis at temperatures greater than 1400°C, results in dense CMC materials when an iterative process of reinfiltration and pyrolysis is followed (four to six cycles).48 Organopolysilazanes also find use in the production of metal matrix composites (MMCs), where they can serve as extreme high-temperature binders for performs into which molten silicon or aluminum metals are introduced.33,36
Polysilazanes are the answer for a broad range of challenging applications that cannot be satisfied by conventional materials. These materials enable long-term, cost-effective and "user-friendly" solutions in applications where high temperature stability, corrosion resistance or long term durability are critical factors.
For more information about polysilazane precursors, contact KiON Corp. at firstname.lastname@example.org ; or Clariant Corp. at (49) 6196-757-7869; email@example.com . (215) 957-6100, e-mail firstname.lastname@example.org ; or visit http://www.kioncorp.com .
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