College, London, England). Thin carbon films, prepared by arc evaporation in vaeuo on to glass substrates, were heat treated at temperatures in the range lOOO-3OOO"C, and the crystal dimensions L, and L, measured, using electron diffraction and electron microscopy. The influence of variations in specimen thickness, evaporation pressure and heat treatment atmosphere on crystal size and orientation was examined. The behaviour during graphitization of films prepared by evaporation on heated graphite substrates was also investigated.
III. PYROLYTIC GRAPHITE
- The structure of pyrolytic carbon deposited in a fluidized bed* J. C. Bokros (General Atomic Division of General Dynamics Corporation, San Diego, California). A variety of pyrolytic-carbon structures deposited in fluidized beds have been categorized according to their structure. For a given coater configuration and flow rate, the structure of the deposits was found to vary systematically with bed temperature, gas composition, and bed surface area. The structures were divided into three groups according to their structure, i.e. laminar, isotropic, or granular. The structural data, together with kinetic measurements and published results on related processes, suggest that the low-temperature laminar structures are built up from large planar molecules that form in the gas phase, dehydrogenate, and, when deposited, align themselves parallel to each other and to the substrate. The isotropic low-density structures are formed at conditions where general gas-phase nucleation of droplets and soot occurs in the bed. The high-density granular structures form when there is orderly crystal growth over reiativefy large distances. *This work was supported by the U.S. Atomic Energy Commission under Contract AY(O4-3)-167. 54. Deformation and fracture of pyrolytic carbons deposited in a fluidized bed* J. C. Bokros and R. J. Price (General Atomic Division of General Dynamics Corporation, San Diego, CuZifornia). Pyrolytic carbon formed by the pyrolysis of hydrocarbon gas in a ffuidized bed is used as a coating for nuclear fuel particles. The present investigation was undertaken to relate the mechanical properties of such deposits to their structure, which in turn depends on the deposition conditions and subsequent heat treatment. Carbons deposited from methane between 1350ยฐC and 1925ยฐC were characterized according to their density, degree of preferred orientation, apparent crystallite size, and micrographic appearance (laminar, isotropic, or granular). Stress-strain curves to fracture were obtained from room temperature three-point bend tests on planar samples and the mode of fracture was investigated by optical microscopy and electron micro-fractography.
Both the elastic modulus and the fracture stress decreased in progressing from laminar to isotropic to granular structures, whereas the fracture strain was highest in the isotropic deposits. Annealing at temperatures between 3700ยฐC and 2800ยฐC was found to change greatly the properties of only the high density laminar structures deposited from high methane concentrations, where the elastic modulus and fracture stress were reduced by annealing. The mechanical properties of the various carbons are discussed in terms of their structure.