The ternary system uranium-boron-carbon

Abstract

Phase equilibria in the ternary system U-B-C have been established by means of X-ray, metallographic and melting point analyses in the temperature range from 1000 ° C to melting. Three ternary compounds were found to exist: besides the well known monoboroncarbide UBC two new uranium boroncarbides, UB 2C and “U 5B 2C 7”. Ternary phase equilibria are characterized by the incompatibility of uranium metal with boroncarbide B 4C and by the incompatibility of elemental boron and uranium carbides; an isothermal section of the system U-B-C at 1600° C is presented. At high temperatures the crystal structure of UB 2C was found to be isotypic with the homologous compound ThB 2C; at temperatures below (1675 ± 25)°C h-UB 2C transforms into a low temperature modification with a new (unknown) structure type. The crystal structure of “U 5B 2C 7” is closely related to the structure type of Ho 5B 2C 6−7 as a derivative of La 5 2C 6 Employing the Pirani-technique, congruent melting was revealed for UBC and UB 2C at (2144 ± 25)°C and (2282 ± 30)°C respectively. Using the clear-cross principle in studying possible phase reactions, the thermodynamic stabilities of UBC, UB 2C and U 5B 2C 7 were estimated.