Boron carbide-carbon composites and composites for cryogenic applications

Because of its neutronic properties, high hardness, and high melting temperature, boron carbide (B4C) is widely used at the Los Alamos Scientific Laboratory. However because of its hardness and mode of manufacture, it is expensive to machine finish to tight dimensional specifictions. For some neutronic applications, a density considerably below the theoretical 2.52 Mg/m3 was acceptable, and this relaxation in density specification permitted addition of carbon as a second phase to reduce machining costs. We conducted an experimental program to prepare 50.8-mm-diam by 34.8-mm-thick cylinders of B4C and B4C-C composites with concentrations of carbon varying from 5.5 to 30 volume percent. Additionally we used three forms of carbon, natural flake graphite, synthetic graphite flour, and a fine furnace black as the source of the second phase. We determined the sound velocity, compressive strength, coefficient of thermal expansion, electrical resistivity, and microstructure as functions of composition. Additionally, an enriched boron (10B)-carbon composite was studied as an alternate material