Ligament Structure of Open-Cell Carbon Foams and the Construction of Models Based on That Structure
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LIGAMENT STRUCTURE OF OPEN-CELL CARBON FOAMS AND THE CONSTRUCTION OF MODELS BASED ON THAT STRUCTURE
DAVID P. ANDERSON*, KATIE E. GUNNISON*, AND JOSEPH W. HAGER**
*University of Dayton Research Institute, 300 College Park Avenue, Dayton, OH 45469-0168 **Wright Laboratory, WI/MLBC Wright-Patterson AFB, OH 45433-6533
ABSTRACT The ligament structure of several open-cell carbon foams was examined by optical and electron microscopy. The arrangement, sizes, and shapes of the ligaments were measured and analyzed according to the cell sizes. The ligament lengths and cross-sections vary with the cell sizes in a simply scaled fashion. A models based on the observed dodecahedral-like arrangement of ligaments was constructed consisting of 12-, 14-, and 15-faced polyhedra with five-edged faces dominating. INTRODUCTION The ability to manufacture fully reticulated graphitic foams offers the opportunity to obtain an interconnected three-dimensional network with properties similar to high modulus carbon fibers [1]. While we are attempting to make these materials [2-5], the structure of reticulated amorphous carbon foams was viewed as the most promising goal structure for maximum isotropic 3-D properties. The shape, arrangement, and juncture of ligaments must. be known to fully understand the mechanics of these materials. This lead to our microscopic examination of commercial foams. Current modeling and mechanic analysis of foams tends to focus on regular solids that fill space as described by Gibson and Ashby [6] or only the tetrahedral ligament junctures and not the 3-D space-filling aspects observed in foams [7]. Our modeling work was an attempt to get a repeating space-filling structure of irregular polyhedra from which we could extract geometric parameters and use for mechanical analysis. FOAM CHARACTERIZATION ERG Duocel vitreous carbon foams with open cell sizes ranging from 10 to 100 pores per inch (ppi) were examined by optical and scanning electron microscopy. Optical microscopy was performed on a Nikon FXL Optiphot equipped with fluorescent illumination on foam samples which were vacuum impregnated with a fluorescently tagged epoxy resin and polished by ordinary metallographic techniques. A low magnification reflective micrograph is shown in Figure 1. The foams can be viewed very easily under ordinary brightfield illumination, but the fluorescent light used for these micrographs greatly enhances the ligament contrast. The quantitative measurements of ligament sizes and cross-sections were achieved using Image- 1 image analysis software of higher magnification video images taken directly from the microscope. Fractal analysis of the cell volumes, ligaments, and crosssections indicated the cell components are simply scaled. SEM micrographs of AuPd coated foams were obtained using a JOEL-840 instrument (see Figure 2). The higher magnification available by SEM was not as useful as the longer depth-of-field (compare Figure 1 to Figure 2). Lengths and areas cannot be easily measured in these SEM micrographs but the number of li
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