Syntactic Closed-cell Foams Based on Silicon Carbide
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Syntactic Closed-cell Foams Based on Silicon Carbide Engin Ozcivici and Raman P. Singh Mechanics of Advanced Materials Laboratory Department of Mechanical Engineering, Stony Brook University Stony Brook, NY 11794-2300, USA ABSTRACT Closed cell foams were fabricated by incorporating two different grades of hollow aluminosilicate spheres (cenospheres) into a silicon carbide matrix. The silicon carbide matrix was formed by the pyrolysis of a preceramic polymer, and multiple polymer infiltration and pyrolysis (PIP) cycles were employed to minimize the open voids in the material. The physical, mechanical and thermal properties of the fabricated foams were characterized as functions of the number of reinfiltration cycles. The open- and closed-void volume fractions were determined by measurements of bulk and skeletal densities. Mechanical properties, including strength and modulus, were evaluated using four-point bend and compression tests. Finally, thermophysical (thermal conductivity) values of the material were determined using laser-flash technique. This processing technique results in closed-cell syntactic foams with low density (≤ 1.8g/cm3), reasonable mechanical strength (~ 30 MPa) and very low thermal conductivity (≤ 1 W/m-K). In this manner, this process can be used for the low-cost and net-shape fabrication of closed-cell silicon carbide syntactic foams for high temperature applications. INTRODUCTION The most efficient structures in nature, such as wood, coral and bone, occur in cellular forms, which have inspired and driven significant research in the development and application of the syntactic foams [1,2]. Syntactic foams (synthetic cellular structures) are the imitation of natural foams, fabricated to serve engineering applications while allowing the simultaneous optimization of engineering parameters such as density, strength, stiffness, conductivity, permeability, porosity and toughness. The characteristics of such foams depend on the properties of the base material, the relative density of the foam and the spatial distribution of material within the cellular solid or foam. These dependencies are also conventionally used to classify syntactic foams either on the basis of the physical formation of their cells (as open- and closed-cell structure), or the underlying material (as polymer, metallic or ceramic matrix). The main goal of this research is the fabrication and characterization of syntactic closed-cell ceramic foams. A closed-cell architecture was selected because the existence of load bearing cell walls, in such foams, provides advantages such as gas impermeability, higher specific strength and stiffness [3]. However, closed-cell foams are more difficult to fabricate and have higher densities as compared to open-cell counterparts. Nevertheless, closed-cell foams continue to receive considerable attention for a wide variety of applications that benefit from the closed cellular structure such as high temperature insulation for furnaces, fire protection and thermomechanical aerospace structures
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