Abstract
U–Si intermetallic compounds have drawn great attention due to their potential application as nuclear fuels. However, the thermodynamic properties and phase equilibria of this binary system from ambient to high temperature conditions are not fully understood. Via high temperature oxidative drop calorimetry and detailed characterization of the initial and final phases, we have experimentally determined the standard enthalpies of formation of USi and U3Si5.07 at 298 K to be −43.2 ± 6.2 and −43.8 ± 9.0 kJ/mol·atom, respectively. The energetics of the tetragonal USi (t-USi, space group I4/mmm) phase has also been calculated with Density Functional Theory (DFT) for the first time. Combining the obtained formation enthalpies with the heat capacities measured previously, we assessed the thermodynamic stability of t-USi relative to a phase assemblage of two other U–Si phases, U3Si5.07 and U3Si2, from ambient temperature to 1200 K. The tetragonal USi is thermodynamically more stable than U3Si5.07 + U3Si2, which supports previously published phase diagram (H. Okamoto and T. Massalski, 1990 [1]): specifically, at least one stable USi phase exists when the U content is 50 at.%. Further thermodynamic and phase equilibrium studies are needed for a more comprehensive understanding of the U–Si system across broader compositional and temperature ranges.
Original language | English |
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Pages (from-to) | 101-110 |
Number of pages | 10 |
Journal | Journal of Nuclear Materials |
Volume | 523 |
DOIs | |
State | Published - Sep 2019 |
Funding
Research presented in this article was supported by the Laboratory Directed Research and Development (LDRD) program of Los Alamos National Laboratory (LANL) under project number 20180007 DR. The DFT + U calculations were performed using EMSL (grid.436923.9), a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research . We thank T. M. Besmann for valuable discussions on the computational results from literature and N. Botto for his hwlp with the EPMA experiments. LANL, an affirmative action/equal opportunity employer, is managed by Triad National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under contract 89233218CNA000001 .
Keywords
- Calorimetry
- Density functional theory
- Intermetallics
- Nuclear reactor fuel
- Phase stability
- Thermodynamic properties