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XX3.03 - Ceramic-Carbon Nanostructured Material for High-Temperature Structures 
December 1, 2014   2:30pm - 2:45pm

A new ceramic-carbon nanostructured hybrid (86 vol.% ceramics, 14 vol.% carbon) formed from organoclay during pyrolysis is reported. It functions as a reinforcing filler and a binder for carbon/carbon (C/C) composites, which are important as high-temperature lightweight structural materials, as used for reentry vehicles, missiles and aircraft brakes. However, they suffer from high processing cost, which is due to the need to use multiple impregnation-carbonization cycles and methods such as chemical vapor infiltration (CVI) in order to achieve sufficient densification and hence adequate mechanical properties. Alone, the cerami-carbon hybrid can also serve as a high-temperature structural monolith. During pyrolysis, the ordered montmorillonite clay (d001 31.5 Å) is transformed to mullite, cristobalite and disordered clay, and probably amourphous silica as well, allowing the clay part of the organoclay to serve as both binder and reinforcement. The organic part serves as a binder. Although organclay has been investigated extensively for use as a filler in composite materials, particularly polymer-matrix composites, it has not been previously explored for use as both a filler and a binder. The dual functions allow the organoclay to be highly effective for strengthening C/C composites. Thus, a unidirectional C/C composite (50 vol.% fibers, 33 vol.% carbon matrix, 5 vol.% ceramic-carbon hybrid and 12% porosity) exhibiting flexural strength 290 MPa, modulus 55 GPa and toughness 2.9 MPa is obtained by 1000°C 21-MPa hot-press pyrolysis in the presence of mesophase pitch powder, which serves as an additional binder, without densification after the pyrolysis. With the ceramic-carbon hybrid incorporation, the fiber content decreases from 53 to 50 vol.%, but the flexural strength and modulus are increased by 46% and 14% respectively, relative to the composite without the hybrid but with densification. Furthermore, this work has shown that the hot pressing of organoclay in the absence of any other ingredient results in a monolithic material, due to the binding ability of the organoclay. Hot pressing the organoclay alone forms a black monolithic sheet with high thermal stability, electrical resistivity 6 x 106 Ω.cm, flexural strength 180 MPa, modulus 69 GPa, but low ductility. This work is primarily aimed at (i) investigating the mechanical properties of C/C composites containing organoclay through filler incorporation and elucidating the structure of this multi-scale composite, (ii) investigating the feasibility and effect of conversion of the organic component of organoclay to carbon and the associated feasibility of forming a ceramic-carbon hybrid from organoclay in the absence of any other ingredient, (iii) investigating the feasibility of organoclay serving as both filler and binder, and (iv) investigating the feasibility of using organoclay in the absence of any other ingredient to form a low-cost monolithic high-temperature structural material.

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