Advances in densification, microtubes, and tailoring surface tension have been made.

Some notable innovations in the design and manufacture of carbonbased materials have been made in a continuing program of basic research on carbon- based materials for use in propulsion systems of aircraft and spacecraft. The research has ranged over diverse topics that have included fabrication of carboncarbon composite-material components, protection of carbon against oxidation, microelectromechanical devices, and surface- tension phenomena.

Carbon-carbon composites are the materials of choice in many high-temperature thermostructural applications, including rocket nozzles and exit cones, missile nose tips, and leading edges of hypersonic vehicles. Although these materials are stronger and stiffer than steel, less dense than aluminum, and resistant to thermal shock, they are susceptible to oxidation at temperatures above 450 °C. In addition, they are very costly, principally because fabrication of a reinforced carbon-carbon component includes a densification process in which a carbon matrix is placed among carbon fibers in a preform that has been constructed to have certain mechanical properties. The present innovations, which have emerged from efforts to address some of the issues mentioned above, are summarized as follows:

• An in situ densification process has been developed. In this process, a carbon- fiber preform is impregnated with molten naphthalene, with some AlCl3 mixed into the naphthalene as a polymerization catalyst. Following in situ polymerization, the polymerimpregnated preform is heated to convert the polymer matrix material to carbon. This process yields a matrix of very high quality at a fraction of the cost and in a fraction of the time required for prior commercial densification processes. The electrical and thermal conductivities of a material produced by this process exceed those of the best commercial composites. In addition, by virtue of the complete penetration of the liquid precursor material into the preform (a key advantage of this process over prior densification processes), the density of a composite made by this process is more nearly uniform than are densities of commercial carbon- carbon composites.

• Microtube technology — which overlaps with the disciplines of microelectromechanical systems (MEMS) and microfluidics — has emerged as a byproduct of efforts to address the issue of protecting carbon-carbon composites against oxidation. Briefly, microtube technology provides for the fabrication tubes having features as small as fractions of a micron by use of micromachining techniques to form mandrels, coating the mandrels with metallic or nonmetallic materials destined to become the tube walls, then removal (e.g., by etching) of the mandrel materials. The finished tubes can be free-standing or embedded and can have almost any complex threedimensional shapes, including noncircular cross sections and cross sections that vary with axial position (e.g., bellows-like tubes).

• A method of tailoring the wetting properties of microtubes and other devices having microscopic features has been developed out of recognition that surface tension is a dominant force at microscopic dimensions. This method is an important innovation because surface tension and wetting behavior are major considerations in potential applications for microtubes, which include cooling, separation of materials, venting, and sensing. This method involves understanding the relationship between contact angle and surface tension and tailoring microscopic surface features to exploit this relationship to obtain or prevent wetting or to obtain different degrees of wetting at different locations. For example, through geometric tailoring of surfaces, one could control entrance of a liquid into a capillary, cause multiple fluids to flow in distinct streams through a capillary device, make the surface of an ordinarily non-wetting material surface wetting, or make the surface of an ordinarily wetting material non-wetting.

This work was done by Wesley P. Hoffman of the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Materials category. AFRL-0003

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