CALPHAD modeling of uranium nitride (UN) fabrication routes enabled by first-principles calculations

The thermochemical details of fabricating uranium nitride (UN) by ammonolysis of uranium tetraflouride (UF4) were determined using density functional theory (DFT) and CALculation of PHAse Diagrams (CALPHAD) computational methods. The thermochemical data of all binary, ternary, and quaternary U–H–N–F phases were computed using DFT, and the data for the phases that have not been measured experimentally, including UN2 and NH4F(g), were combined with existing experimentally-determined data for CALPHAD modeling. The DFT data were benchmarked using experimental Gibbs energy of reaction and experimental thermochemical data for individual species. Phase diagrams relevant to the ammonolysis reaction are depicted, showing regions of stability for solid U–N, U–F and U–N–F phases. An unidentified phase produced in a previous experiment was identified as UN0.95F1.2 (UNF) by comparing its X-ray diffraction spectrum to the experimental spectrum, and its formation during the fabrication of UN from UF4 is supported by the simulated phase diagram. It is calculated that UN2 can be produced by the ammonolysis of UF4, but requires elevated temperatures, high NH3(g) partial pressure, and large amounts of flowing NH3(g) to avoid solid fluoride impurities in the uranium nitride. Likewise, U2N3 can be produced instead at temperatures greater than 980 K. The use of silane (SiH4) gas was investigated as a potential additive in the ammonolysis fabrication route to speed removal of fluorine. The addition of SiH4(g) offers little advantage to the removal of fluorine, and adds the complication of Si3N4 formation. The use of DFT to fill in missing data to perform CALPHAD calculations demonstrated here allows for the determination of more comprehensive and trustworthy phase diagrams than the use of existing experimental data alone.