Silicon foam ceramic

P. Colombo and M. Modesti, Silicon oxycarbide ceramic foams from a preceramic polymer, J. Am. Ceram. Soc. 1999, 82, 573-578. 

Filters for Molten Metal Inclusions in molten metals, such as oxide skins, mold sand particles, inoculation reaction byproducts or furnace slags, can be efficiently removed by filtering through pressed cellular, extruded cellular, or foam ceramic filters manufactured from high-temperature-resistant, chemically inert alumina, and also mullite, silicon carbide, and stabilized zirconia. Alumina is particularly useful when filtering liquid aluminum alloys in the range of 750-850 °C, and copper-based alloys at 1000-1200°C (Matthews, 1996). These ceramic filters are designed for use in either batch or in-mold filtration ... 

Syntactic foamed plastics (from the Greek ovvxa C, to put together) or spheroplastics are a special kind of gas filled polymeric material. They consist of a polymer matrix, called the binder, and a filler of hollow spherical particles, called microspheres, microcapsules, or microballoons, distributed within the binder. Expoxy and phenolic resins, polyesters, silicones, polyurethanes, and several other polymers and oligomers are used as binders, while the fillers have been made of glass, carbon, metal, ceramics, polymers, and resins. The foamed plastic is formed by the microcapsular method, i.e. the gas-filled particles are inserted into the polymer binder1,2). 

Syntactic foams have been made from organosilicone polymers and glass, ceramic, or polymer microspheres 23,100). They are used mainly for heat insulation and ablation coating101). For the latter, the two components are sprayed together onto the exterior of rockets, and cold setting silicone glues are used to improve ablation102). Syntactic materials with carbon microspheres and silicones have also been proposed 39>. 

A mixture of ammonium chloride and borax was one of the treatments of cellulosic fabrics reported by Gay-Lussac in 1821. Due to its low dehydration temperature and water solubility, sodium borates are only used as flame retardants in cellulose insulation (ground-up newspaper— see Sections 9.2.1.2 and 9.2.2.1), wood timber, textiles, urethane foam, and coatings. For example, a mixture of urethane (100 parts), borax (100 phr), and perlite (30phr) was claimed to provide flame-retardant urethane foam.8 Borax in conjunction with boric oxide, silica, ammonium chloride, and APB as ceramizing additives and volume builders, are claimed in a fire-protection coating based on polybutadiene and silicone microemulsion.9 Using a modified DIN 4102 test, the chipboard with the coating showed a loss of mass less than 1% and there was no pyrolysis of the wood sample. 

Precursors must have different properties " " " (1) a high content of the final elements (mostly aluminum, silicon, zirconium, titanium, phosphorus), (2) a low content of health hazardous elements and elements that corrode the equipment (e.g., chlorine, sulfur), (3) a viscosity adapted to the process low viscosity for preform infiltration, medium viscosity for spinning and coating, (4) a controlled precursor-ceramic transformation (bubbling is researched for foams but not for dense parts), (5) the ability to be mixed with other precursors or to be processed ( good hydrolysis rate), and (6) low cost. 

Infiltration combines a melt with a porous free-standing solid (the preform ). In the main and defining step of the process, the melt flows into open pores of the preform after solidification a new material results. Composites of all classes (polymer, ceramic and metal) are produced by this process, as are compounds such as reaction bonded silicon carbide. The process can also be adapted to make open-pored foams of carbon, ceramic, polymer or metal. 

Foam monoliths can be produced from a broad variety of materials. Commercially available are ceramic foams made of alumina, mullite, cordierite, silicon carbide, or zirconia, but numerous other oxides, carbides, nitrides, and... 

Recent research in packed beds [38], ceramic monoliths [20] and microreactors [30] also revealed an excellent performance of a Ce-Zr support mixture, which was explained by an increase in surface area by the addition of zirconium oxide [38]. A zirconium oxide-supported rhodium catalyst also revealed similar ignition performance to a mixed Ce-Zr oxide supported rhodium catalyst on the surface of Fecralloy monoliths with trapezoidal channels [9]. A novel route to a support, which might be useful for CPO and OSR, is the synthesis of silicon carbide foam, recently used for steam reforming [44] (Figure 25.2). This support would also be less acidic and is suitable for building a compact foam catalyst... 

These foams can be defined as composites consisting of hollow microspheres and a polymeric matrix. This one is made of a thermosetting (PU, PIR, PF, EP, silicone or unsaturated polyester) or of a thermoplastic (PE, PP, PVC, PS, polyimide) [56]. The microspheres can be made of silica, glass, carbon, ceramics or polymers such as PS, PE, PP, polyamide (PA), polymethyl methacrylate (PMMA), divinyl benzene (DVB)-maleic anhydride, and so on [56-58]. The diameter of the tiny hollow spheres is 300 mm or less [35]. They contain an inert gas such as nitrogen or a CFC. The properties of these syntactic foams depend on matrix type, microsphere type (and the contained gas), ratio matrix to microspheres, curing process, production technology. Syntactic foams can be made in combination with the conventional ones. Such a complex composite can be formnlated into a mouldable mass then shaped or pressed into cavities. 

The TCON material is known for its impressive mechanical properties compared to its pure metallic and ceramic parts ". The TCON process allows the modifications of the ceramic precursor and molten metal to optimize the final properties of the composites for specific applications. The TCON foams studied here had a general composition based on aluminum, silicon carbide (SiC), aluminum oxide and silicon (Si). Two samples labeled TCON A and TCON B were created using pure aluminum or aluminum alloy baths. TCON A had a precursor material consisted of Si02, SIC, and hollow AI2O3 spheres. In contrast, TCON B s precursor consisted of a fiber material, which is a proprietary reinforcement developed by Fireline, Inc., and hollow AI2O3 spheres. Here, the hollow sphoical particles were added in an attempt lower the density of the foam system. 

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