[en] We investigated the formation of UC_x from UO_2_+_x and graphite in situ using neutron diffraction at high temperatures with particular focus on resolving the conflicting reports on the crystal structure of non-quenchable cubic UC_2. The agents were UO_2 nanopowder, which closely imitates nano grains observed in spent reactor fuels, and graphite powder. In situ neutron diffraction revealed the onset of the UO_2 + 2C → UC + CO_2 reaction at 1440 °C, with its completion at 1500 °C. Upon further heating, carbon diffuses into the uranium carbide forming C_2 groups at the octahedral sites. This resulting high temperature cubic UC_2 phase is similar to the NaCl-type structure as proposed by Bowman et al. Our novel experimental data provide insights into the mechanism and kinetics of formation of UC as well as characteristics of the high temperature cubic UC_2 phase which agree with proposed rotational rehybridization found from simulations by Wen et al.

[en] A team of Los Alamos researchers supported a final campaign to use the Trident laser to produce neutrons, contributed their multidisciplinary expertise to experimentally assess if laser-driven neutron sources can be useful for MaRIE. MaRIE is the Laboratory's proposed experimental facility for the study of matter-radiation interactions in extremes. Neutrons provide a radiographic probe that is complementary to x-rays and protons, and can address imaging challenges not amenable to those beams. The team's efforts characterize the Laboratory's responsiveness, flexibility, and ability to apply diverse expertise where needed to perform successful complex experiments.

No abstract available

[en] In view of the looming energy crisis facing our planet, attention increasingly focuses on materials potentially useful as a basis for energy saving technologies. The discovery of giant magnetocaloric (GMC) compounds--materials that exhibit especially large changes in temperature as the externally applied magnetic field is varied--is one such compound. These materials have potential for use in solid state cooling technology as a viable alternative to existing gas based refrigeration technologies that use choro-fluoro - and hydro-fluoro-carbon chemicals known to have a severe detrimental effect on human health and environment. Examples of GMC compounds include Gd5(SiGe)4, MnFeP1-xAsx and Ni-Mn-Ga shape memory alloy based compounds. Here we explain how the properties of one of these compounds (Ni2MnGa) can be tuned as a function of temperature by adding dopants. By altering the free energy such that the structural and magnetic transitions coincide, a GMC compound that operates at just the right temperature for human requirements can be obtained. We show how Cu, substituted for Mn, pulls the magnetic transition downwards in temperature and also, counterintuitively, increases the delocalization of the Mn magnetism. At the same time, this reinforces the Ni-Ga chemical bond, raising the temperature of the martensite-austenite transition. At 25 percent doping, the two transitions coincide at 317 K

[en] New in situ data for the U-C system are presented, with the goal of improving knowledge of the phase diagram to enable production of new ceramic fuels. The none quenchable, cubic, @@-phase, which in turn is fundamental to computational methods, was identified. Rich datasets of the formation synthesis of uranium carbide yield kinetics data which allow the benchmarking of modeling, thermodynamic parameters etc. The order-disorder transition (carbon sublattice melting) was observed due to equal sensitivity of neutrons to both elements. This dynamic has not been accurately described in some recent simulation-based publications.

[en] An in-situ high-pressure neutron diffraction experiment was conducted on a δ-phase stabilized plutonium alloy with isotope ²⁴²Pu. Upon room-temperature compression, neither of the previously reported pressure-induced transformation paths, δ → α′ or δ → γ’ → α′, was observed up to 1.2 GPa. Instead, a drastic reduction in the diffraction intensity of the δ phase was observed when the pressure was above 0.8 GPa. At the highest pressure of the experiment (1.2 GPa), the diffraction data appear to be characteristic of an amorphous state, manifested by the diminishing intensities of all diffraction lines. In addition, no evidence was found to support the transformation to a body-centered tetragonal structure (δ’), which was previously inferred from the diffraction line broadening during initial compression to 0.1 GPa. The discrepancies between the present and previous experiments suggest a substantial strain-induced stabilization of the δ-phase and is presumably attributed to the different stress states in the high-pressure environments. From the pressure - volume measurements, the determined isothermal bulk modulus for the δ-phase is in the range of 31.9 ± 1.3–34.8 ± 1.8 GPa using different pressure scales, comparable to those obtained from the resonant ultrasound spectroscopy measurements of the alloys of similar composition. The pressure-induced elastic softening is neither convinced in the present work, nor can it be resolved from the diffraction experiments if it is intrinsically weak.

[en] Time-of-flight neutron diffraction measurements were conducted at ambient conditions to study microstructures of the δ-phase ²³⁹Pu-2at% Ga alloy. Based on the line profile analysis of diffraction data we derived dislocation densities using a correction routine to account for the anisotropic strain broadening. Our results show that the average dislocation density in the as-received sample, previously treated at cryogenic temperatures, is five times higher than the dislocation density measured after annealing the sample, indicating that the neutron instrumentation used in this work has sufficient resolution to detect the dislocation density changes in different sample processing. These results are also in reasonably good agreement with the dislocation densities previously reported based on TEM observations and x-ray diffraction data, suggesting that the simple correction routines applied in this work have a fairly good fidelity for deriving important microstructural information such as dislocation density for elastically highly anisotropic δ-phase ²³⁹Pu-Ga alloys.

[en] In situ time-resolved synchrotron X-ray diffraction experiments were conducted to study the fine-scale phase evolution of U-6Nb. Upon rapid heating from 125 °C to 400 °C, a reverse martensitic transformation sequence, α″ → γ^o → γ_s, was observed in less than 4 seconds, which represents the first direct observation of the γ^o → γ_s transformation in diffraction-based measurements. Consistent with previous ex situ metallography experiments, our isothermal hold experiments at 526 °C, 530 °C and 565 °C reveal two distinct reactions for the phase separation, γ_s → α-U + γ₁ (general precipitation) followed by (α-U + γ₁) → α-U + γ_1-2 (discontinuous precipitation). For the first-stage precipitation, the incubation time is determined to be ~ 50 and 100 seconds, respectively, for the isothermal aging at 526-530 °C and 565 °C. At this stage, the phase transformation is characterized by the simultaneous growth of α-U and γ₁ at the expense of γ_s. As expected from the Arrhenius equation for the reaction rate, the determined times (~ 23 minutes) for the completion of the first-stage reaction at 526 ± 3 °C and 530 ± 3 °C are nearly twice longer than that at 565 ± 4 °C (~ 13 minutes). Over these periods of time, the Nb contents derived from a Vegard’s-type relationship for γ₁ are in the 30.2 to 32.1 and 29.2 to 30.6 at. pct ranges, and the kinetics of the precipitation at 565 ± 4 °C can be described by the classic Avrami rate equation and one-dimensional growth of a surface or grain-boundary nucleation. During the second-stage precipitation, the γ₁ phase continues to enrich in Nb as it gradually evolves toward the α + γ_1-2 metastable state (up to 47 at. pct over a period of 172 minutes at 530 °C). These new and time-resolved measurements can be used to better constrain the time–temperature–transformation diagram, solute (Nb) redistribution, and transformation kinetics during the early stages of the diffusional phase transformation.

[en] The crystal structure of the U₃Si₂ line compound in the U-Si system was investigated as a function of temperature from room temperature t o1373 K using high temperature neutron time-of-flight diffraction on the HIPPO diffractometer at LANSCE. The U-Si system is actively researched due to its promise as an accident tolerant nuclear fuel. The simultaneous Rietveld refinement of five histograms from the five HIPPO detector rings provided fundamental datasets for the lattice parameters, anisotropic atomic displacement parameters, and atomic positions as a function of temperature. To explore the possibility of minority phases as a result of the synthesis route and especially due to hyper-stoichiometry, a stoichiometric U₃Si_2.00 sample and a hyper-stoichiometric U₃Si_2.01 sample were studied. While minor differences in the anisotropic atomic displacement parameters between the two samples were observed, over the entire investigated temperature range no additional phases were observed. However, significant differences in the thermal expansion behavior were identified between the two compositions that warrant future investigations.

[en] The development of neutron diffraction under extreme pressure (P) and temperature (T) conditions is highly valuable to condensed matter physics, crystal chemistry, materials science, and earth and planetary sciences. We have incorporated a 500-ton press TAP-98 into the HiPPO diffractometer at the Los Alamos Neutron Science Center (LANSCE) to conduct in situ high-P-T neutron diffraction experiments. We have developed a large gem-crystal anvil cell, ZAP, to conduct neutron diffraction experiments at high P. The ZAP cell can be used to integrate multiple experimental techniques such as neutron diffraction, laser spectroscopy, and ultrasonic interferometry. More recently, we have developed high-P low-T gas/liquid cells in conjunction with neutron diffraction. These techniques enable in situ and real-time examination of gas uptake/release processes and allow accurate, time-dependent determination of changes in crystal structure and related reaction kinetics. We have successfully used these techniques to study the equations of state, structural phase transitions, and thermo-mechanical properties of metals, ceramics, and minerals. We have conducted researches on the formation/decomposition kinetics of methane, CO₂ and hydrogen hydrate clathrates, and hydrogen/CO₂ adsorption of inclusion compounds such as metal-organic frameworks (MOFs). The aim of our research is to accurately map out phase relations and determine structural parameters (lattice constants, atomic positions, atomic thermal parameters, bond lengths, bond angles, etc.) in the P-T-X space. We are developing further high-P-T technology with a new 2000-ton press, TAPLUS-2000, and a ZIA (Deformation-DIA type) cubic anvil package to routinely achieve pressures up to 20 GPa and temperatures up to 2000 K. The design of a dedicated high-P neutron beamline, LAPTRON, is also underway for simultaneous high-P-T neutron diffraction, ultrasonic, calorimetry, radiography, and tomography studies. Studies based on high-pressure neutron diffraction are important for multidisciplinary sciences, particularly for theoretical/computational modeling/simulations. (orig.)