Laser-Induced Thermal Decomposition of Uranium Coordination Compounds with Non-oxidic Ligands to Produce Nitride and Carbide Materials.

The production of ceramics from uranium coordination compounds can be achieved through thermal processing if an excess amount of the desired atoms (i.e., C or N), or reactive gaseous products (e.g., methane or nitrogen oxide) is made available to the reactive uranium metal core via decomposition/fragmentation of the surrounding ligand groups. Here, computational thermodynamic approaches were utilized to identify the temperatures necessary to produce uranium metal from some starting compounds─UI4(TMEDA)2, UCl4(TMEDA)2, UCl3(pyridine)x, and UI3(pyridine)4. Experimentally, precursors were irradiated by a laser under various gaseous environments (argon, nitrogen, and methane) creating extreme reaction conditions (i.e., fast heating, high temperature profile >2000 °C, and rapid cooling). Despite the fast dynamics associated with laser irradiation, the central uranium atom reacted with the thermal decomposition products of the ligands yielding uranium ceramics. Residual gas analysis identified vaporized products from the laser irradiation, and the final ceramic products were characterized by powder X-ray diffraction. The composition of the uranium precursor as well as the gaseous environment had a direct impact on the production of the final phases.

Crystallographic Study of Product Phases of Carbothermic Reduction and Nitridation of Hafnium Dioxide.

Details of the carbothermic reduction/nitridation to synthesize hafnium nitride (HfN) and hafnium carbide (HfC) are scarce in the literature. Therefore, this current study was carried out to evaluate two pathways for synthesizing these two refractory materials: direct nitridation and carbothermic reduction/nitridation. Two mixtures of hafnium dioxide and carbon with C/HfO2 molar ratios of 2.15 and 3.1 were nitridized directly using flowing nitrogen gas at elevated temperatures (1300-1700 °C). The 3.1 C/HfO2 molar ratio mixture was also carbothermically reduced under flowing argon gas to synthesize HfC, which was converted into HfN by introducing a nitridation step under both N2(g) and N2(g)-10% H2(g). X-ray diffraction results showed the formation of HfN at 1300 and 1400 °C and HfC1-yNy at ≥1400 °C under direct nitridation of samples using a C/HfO2 molar ratio of 2.15. These phase analysis data together with lower lattice strain and greater crystallite sizes of HfC1-yNy that formed at higher temperatures suggested that the HfC1-yNy phase is preferred over HfN at those temperatures. Carbothermic reduction of 3.1 C/HfO2 molar ratio samples under an inert atmosphere produced single-phased HfC with no significant levels of dissolved oxygen. Carbothermic reduction nitridation made two phases of different carbon levels (HfC1-yNy and HfC1-y'Ny', where y' < y), while direct nitridation produced a single HfC1-yNy phase under both N2 and N2-10% H2 cover gas environments.

Unconventional Pathways to Carbide Phase Synthesis via Thermal Decomposition of UI4(1,4-dioxane)2.

UI4(1,4-dioxane)2 was subjected to laser-based heating─a method that enables localized, fast heating (T > 2000 °C) and rapid cooling under controlled conditions (scan rate, power, atmosphere, etc.)─to understand its thermal decomposition. A predictive computational thermodynamic technique estimated the decomposition temperature of UI4(1,4-dioxane)2 to uranium (U) metal to be 2236 °C, a temperature achievable under laser irradiation. Dictated by the presence of reactive, gaseous byproducts, the thermal decomposition of UI4(1,4-dioxane)2 under furnace conditions up to 600 °C revealed the formation of UO2, UIx, and U(C1-xOx)y, while under laser irradiation, UI4(1,4-dioxane)2 decomposed to UO2, U(C1-xOx)y, UC2-zOz, and UC. Despite the fast dynamics associated with laser irradiation, the central uranium atom reacted with the thermal decomposition products of the ligand (1,4-dioxane = C4H8O2) instead of producing pure U metal. The results highlight the potential to co-develop uranium precursors with specific irradiation procedures to advance nuclear materials research by finding new pathways to produce uranium carbide.

Synthetic diversity in the preparation of metallic uranium.

Uranium metal is associated with several aspects of nuclear technology; it is used as fuel for research and power reactors, targets for medical isotope productions, explosive for nuclear weapons and precursors in synthetic chemistry. The study of uranium metal at the laboratory scale presents the opportunity to evaluate metallic nuclear fuels, develop new methods for metallic spent fuel reprocessing and advance the science relevant to nuclear forensics and medical isotope production. Since its first isolation in 1841, from the reaction of uranium chloride and potassium metal, uranium metal has been prepared by solid-state reactions and in solution by electrochemical, chemical and radiochemical methods. The present review summarizes the methods outlined above and describes the chemistry associated with each preparation.

Semi-Automated Creation of Density Functional Tight Binding Models through Leveraging Chebyshev Polynomial-Based Force Fields.

Density functional tight binding (DFTB) is an attractive method for accelerated quantum simulations of condensed matter due to its enhanced computational efficiency over standard density functional theory (DFT) approaches. However, DFTB models can be challenging to determine for individual systems of interest, especially for metallic and interfacial systems where different bonding arrangements can lead to significant changes in electronic states. In this regard, we have created a rapid-screening approach for determining systematically improvable DFTB interaction potentials that can yield transferable models for a variety of conditions. Our method leverages a recent reactive molecular dynamics force field where many-body interactions are represented by linear combinations of Chebyshev polynomials. This allows for the efficient creation of multi-center representations with relative ease, requiring only a small investment in initial DFT calculations. We have focused our workflow on TiH2 as a model system and show that a relatively small training set based on unit-cell-sized calculations yields a model accurate for both bulk and surface properties. Our approach is easy to implement and can yield reliable DFTB models over a broad range of thermodynamic conditions, where physical and chemical properties can be difficult to interrogate directly and there is historically a significant reliance on theoretical approaches for interpretation and validation of experimental results.

Comparing results of X-ray diffraction, µ-Raman spectroscopy and neutron diffraction when identifying chemical phases in seized nuclear material, during a comparative nuclear forensics exercise.

This work presents the results for identification of chemical phases obtained by several laboratories as a part of an international nuclear forensic round-robin exercise. In this work powder X-ray diffraction (p-XRD) is regarded as the reference technique. Neutron diffraction produced a superior high-angle diffraction pattern relative to p-XRD. Requiring only small amounts of sample, µ-Raman spectroscopy was used for the first time in this context as a potentially complementary technique to p-XRD. The chemical phases were identified as pure UO2 in two materials, and as a mixture of UO2, U3O8 and an intermediate species U3O7 in the third material.

Discriminating factors affecting incorporation: comparison of the fate of Eu3+-Cm3+ in the Sr carbonate-sulfate system.

The aim of this work is to assess the effect of ligand strength, symmetry, and coordination number on solid solution formation of trivalent actinides and lanthanides in carbonate and sulfate minerals. This is of particular importance in radionuclide migration where trivalent actinides such as Pu, Am, and Cm are responsible for the majority of radiotoxicity after 1000 years. Time-resolved laser fluorescence spectroscopy was used to study trace concentrations of the dopant ion after interaction with the mineral phase. This study expands on previous work with aragonite and gypsum where it was found that aragonite incorporates Eu(3+) and Cm(3+) while only surface sorption is observed in gypsum. This study uses isostructural minerals strontianite (SrCO(3)) and celestite (SrSO(4)) to decouple the effect of structure from that due to the anion. It is demonstrated that while distribution coefficients can predict the amount of dopant ion associated with the mineral phase, they do not have any correlation with solid solution formation. This substitution mechanism is most likely dictated by the symmetry of the site being substituted and the electronic structure of the dopant atom.

A new incorporation mechanism for trivalent actinides into bioapatite: a TRLFS and EXAFS study.

One of the most toxic byproducts of nuclear power and weapons production is the transuranics, which have a high radiotoxicity and long biological half-life due to their tendency to accumulate in the skeletal system. This accumulation is inhomogeneous and has been associated with the chemical properties and structure of the bone material rather than its location or function. This suggests a chemical driving force to incorporation and requires an atomic scale mechanistic understanding of the incorporation process. Here we propose a new incorporation mechanism for trivalent actinides and lanthanides into synthetic and biologically produced hydroxyapatite. Time-resolved laser fluorescence spectroscopy and extended X-ray absorption fine structure have been used to demonstrate that trivalent actinides and lanthanides incorporate into the amorphous grain boundaries of apatite. This incorporation site can be used to explain patterns in uptake and distribution of radionuclides in the mammalian skeletal system.