Evaluation of Activities of Carbons in Chemical Equilibrium with Uranium Carbonitride

Abstract

A mixture of uranium sesquinitride and carbon was prepared by the reaction of UC or UC₂ with N₂ in the temperature range from 700 to 1400°C. When the mixture of uranium sesquinitride and carbon is kept at temperatures above 1200°C in the atmosphere of N₂ at low pressure, the state where uranium carbonitride (UC_1-x, N_x) and carbon are present together in chemical equilibrium will be established. A thermodynamic analysis suggests that, in the equilibrium state, the composition of UC1-xNx is determined by the chemical activity of carbon, a_c, which is related to the chemical potential of the carbon, μE _c, by the equation, μ_c=μ_c°+RT 1n a_c. Here μ_c° refers to graphite, which is usually taken as the standard state of carbon (a_c=1). Mixtures of U₂N₃ and carbon with several degrees of graphitization were heat-treated at 1400°C, and the composition of UC_1-xN_x in the reaction product was determined. From these experimental results and the thermodynamic analysis, values of the activity of the carbon coexisting with UC_1-xN_x were estimated.