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This paper describes the use of contact microradiography to define and characterize the microstructure of graphite. Contact microradiography uses a thin (20 to 500 μm) specimen in contact with a finegrained photographic emulsion. The emulsion has a resolution limit of 1500 to 2000 lines/mm and can be magnified ×500 without excessive loss of detail. Because the source-to-film distance is large (about 10 in.) in comparison with the maximum specimen-to-film distance, ordinary X-ray sources can be used. Thus, most of the problems of point projection X-ray microscopy are avoided. Long wavelength X-rays were used to examine microcracks and other structural features of graphite specimens 100 μm thick. The microcrack structure and coke particle orientation were clearly visible. Direct observation of the surface had not fully defined these microcracks. The technique was also used to locate and characterize very small amounts of metal carbide in graphite-metal systems. For example, 10 μm particles of uranium dicarbide were located in graphite specimens where photomicrography had shown no carbide. Vacuum impregnation with liquids opaque to X-rays has proven useful in defining graphite pore structure. If tetrabromoethane, for example, is used, short wavelength X-rays provide an image of only the filled pores. Longer wavelengths then are used to show graphite structure and unfilled pores. Mixing resins with the tetrabromoethane will immobilize it in the specimen. Impregnating bulk samples with this mixture before thin sections are taken permits the investigation of possible access routes for chemical attack.
radiography, X-ray analysis, microradiography, graphite structure, graphite
Bunnell, L. R.
Research engineer, Pacific Northwest Laboratory, Battelle Memorial Institute, Richland, Wash.