45 Search Results

Crystallization is a common problem for epoxy resins, which are ubiquitous in industrial and commercial use. Integration of crystallized epoxy monomers into cured thermosets has been shown to alter the thermosets’ final mechanical properties. However, no studies have investigated the impact of these crystals on the thermal stability of the thermoset. Here we investigate the degradation kinetics of a bisphenol F–based epoxy thermoset with and without crystallized monomers using Product-Specific Kinetic (PSK) analysis coupled with Evolved Gas Analysis-Mass Spectrometry (EGA-MS). PSK analysis revealed significant differences in evolved product ion kinetics, suggesting changes in the degradation kinetics between thermoset configurations. Further,more » it was concluded that early stages of degradation are influenced most by crystal presence due to the high concentration of unreacted epoxy monomers and lower cross-linking density of the cured network. After post-cure annealing, significant changes are observed in the degradation kinetics of thermosets without crystal inclusions. Conversely, post-cure annealing procedures of crystal integrated thermosets showed little change in the thermoset degradation kinetics across all conversion extents. These findings suggest that post-cure annealing of thermosets with crystals present at the onset does not alter the cross-linking density of the polymer network enough to significantly change the degradation kinetics. We hypothesize this is because the excess monomers from the melted crystals are unable to find suitable reaction sites for complete binding into the polymer network, which has direct implications for the material properties and final thermal stability of the thermoset.« less

Lanthanide AB₂ intermetallic compounds known as Laves phases have itinerant 3d and localized 4f electrons, which lead to interesting physical properties such as magnetic anisotropy and high Curie temperatures. Actinide Laves phases can display physical properties that are similarly intriguing. However, at reduced A–A spacing the C14 and C15 polytypes may exhibit larger wavefunction overlap for their 5f electron states and distinct characteristics for phases with more delocalized chemical bonding. The C36 polytype, on the other hand, is extraordinarily rare (<5% of known Laves phases). UAl₂ is the only known actinide Laves phase to show a pressure-controllable C15 → C36more » transition. Here, we apply first principles calculations to determine the origin of the C15 → C36 phase transition and reveal the differences between the corresponding properties of each phase. Pressure increases lead to bond compression–induced electron transfer from Al to U, which drives dynamic instability in the C15 phonon modes because of the uniform U–U bonding environment. Further, the opposite phenomena is observed in C36: varied U–U bonding environments are vibronically more stable after charge transfer. We find that the interplay between charge transfer, chemical bonding, and phononic stability are central to predicting phase transitions and corresponding changes in physical properties for both C15 and C36 UAl₂.« less

To develop strategies for incorporating transition metal taggants (Fe, Cr, and Ni) into oxide fuels and to understand how these taggant candidates persist through early fuel cycle processes, synthetic procedures are modified from established production routes to yield intentionally tagged early fuel cycle intermediates including uranyl nitrate hexahydrate (UNH, UO₂(NO₃)₂·6H₂O), uranyl peroxide tetrahydrate (studtite, UO₂O₂·4H₂O), and uranyl peroxide dihydrate (metastudtite, UO₂O₂·2H₂O). First, Fe, Cr, and Ni nitrate solutions are introduced to an aqueous solution of UNH followed by precipitation to produce tagged UNH. Then, studtite is precipitated from UNH followed by dehydration to metastudtite. Structural influences of taggant incorporation withinmore » all synthesized phases are investigated using powder X-ray diffraction (PXRD) and Raman spectroscopy to provide insight into crystallographic modifications resulting from the addition of tags to these early fuel cycle materials and elucidate the chemical form of taggants introduced at these stages. The possibility of segregation of taggant species into discrete phases within U matrices was examined using scanning electron microscopy with energy dispersive X-ray spectroscopy. Taggant concentrations in solid-phase materials were determined using inductively coupled plasma-optical emission spectroscopy. Observations from Raman spectroscopy and PXRD indicate that introducing transition metal tags during uranyl nitrate precipitation results in potential impurity phase segregation in UNH, but transition metal incorporation is suggested by results for tagged uranyl peroxide materials. Further, results from this study will inform strategies for optimizing taggant incorporation in UO₂.« less

Atomic-level defects dictate the mechanical properties of carbon fibers and strong correlations have been established between the crystallite sizes and mechanical properties. We recently demonstrated similar correlations with hydrogen content, but reliably quantifying the hydrogen content is not possible using only inelastic neutron scattering experiments. Here, we present prompt-gamma activation analysis (PGAA) experiments collected on 20 commercially available carbon fibers to quantify the hydrogen content of carbon fibers and find correlations between fiber modulus and hydrogen content. We then evaluate the role of hydrogen defect type and connect the PGAA results to both newly acquired and recently reported inelastic neutronmore » scattering experiments. In conclusion, we find that intercalated hydrogen defects are preferentially removed at carbonization temperatures required for high-modulus fibers, potentially giving rise to voids within the carbon fibers that undermine their tensile strength.« less

Understanding the formation of uranium alloys with steel is important to advance nuclear technologies involving U metal fuels and machining U metal, and for nuclear forensics applications. No known phase diagram for the quaternary U-(M = Fe, Ni, Cr) system exists. For this work, we synthesize samples of U-304 L steel (nominal composition 70.1:18.3:10.4 at% Fe:Cr:Ni) across the U composition range 4.45—63.35 at%U by arc melting under inert conditions. Using the binary UFe phase diagram as a reference, we identify four U-steel alloy phases. We find the known U-Fe analogue phases UM₂ and U₆M, and two low-U composition phases withmore » nominal compositions UM₁₀ and U₂M₇. We apply a correlation length analysis to backscatter scanning electron microscopy images of sectioned and polished cross sections to quantify the domain formation length scale. We demonstrate that these depend heavily on the initial composition and range from 30 nm to 1.5 µm. This result, in particular, could be applicable to theoretical predictions of transport properties. Furthering our understanding of U alloy phase formation with important structural elements such as steel primaries is foundational in developing future nuclear technology.« less

Attenuation-based neutron computed tomography (CT) has been used to non-destructively characterize the uncoated tristructural-isotropic (TRISO) nuclear fuel kernels in this work. Particularly, the effect of two different types of carbon blacks (Raven 3500 and Mogul L) on the internal gelation process of UO₃-C kernels has been investigated. With 3D reconstructed kernel volumes and digital imaging processing techniques, heterogenous density distributions are mapped in both types of kernels. It is found that the kernels produced with Mogul L are ~ 20 % denser and ~ 10 % larger (in equivalent diameter) than the Raven 3500 kernels. Furthermore, less neutron attenuating regions,more » which are most likely to be carbon agglomerates as scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) results show, are observed in the Mogul L kernels. The size distribution of such carbon agglomerates (ranges from 50 μm to 850 μm with a peak at ~ 200 μm) has been determined by analyzing the CT data. Furthermore, multiple metrics, including equivalent diameter, surface area, volume, sphericity, have been extracted to evaluate the fuel kernels. This work demonstrates that neutron imaging is an excellent, nondestructive tool to efficiently characterize, understand, and explore fuel materials for nuclear material research and development.« less

Carbon fibers and carbon fiber composites are applied in high-performance applications, but a key consideration for application is their relative sensitivity to oxidative environments. To enable in-situ characterization of carbon fibers exposed to oxidative conditions, the Raman spectral response of T700 carbon fibers that have been exposed to a variety of dwell temperatures is reported herein with dwell times reaching up to 1 month. Here, we evaluate the spectra holistically by using integrated absolute difference analysis. By combining this analysis with straightforward kinetic models, we connect the total Raman spectral response to the temperature-time curve that could yield such amore » shift in spectral parameters. Our work connects the Raman spectral response of carbon fibers to their thermal history and can easily be extended to other graphitic materials, such as nuclear graphite.« less

Bisphenol F (BPF) epoxy resin is a prepolymer component of thermosets used in construction of reinforced carbon fiber composites (RCFC). The fluorescence of this resin can provide information about chemical and structural alterations of the thermoset from thermal or mechanical damage. While some work has investigated the fluorescence spectra of bisphenol A epoxy resin, a structural analog of BPF, none have studied BPF. The purpose of this study is to investigate the baseline fluorescence spectra of the BPF epoxy resin for features that could be utilized for monitoring damage and structural alterations. Bisphenol F epoxy resin was found to havemore » two emission peaks with a broad peak centered at 410 nm and a sharp peak centered at 550 nm. The broad emission peak was identified as the fluorescence from BPF epoxy monomer while the sharp emission peak was found to be from an aggregate structure present in the epoxy resin. It is hypothesized that this structure is an H-aggregate based on the emission features and structural stability of the aggregate compared to the alternative J-aggregate which was determined via density functional theory calculations. The H-aggregate was found to persist into one of the cured thermoset formulations evaluated but was lost when exposed to elevated temperatures below thermal degradation of the thermoset. This loss of aggregate structure was due to polymer network rearrangement. This new fluorescence signature can be used as an indicator of polymer network rearrangement from thermal exposure which may alter the expected material performance and precede thermal degradation.« less

AbstractPlutonium(IV) oxalate hexahydrate (Pu(C2O4)2 ⋅ 6 H2O; PuOx) is an important intermediate in the recovery of plutonium from used nuclear fuel. Its formation by precipitation is well studied, yet its crystal structure remains unknown. Instead, the crystal structure of PuOx is assumed to be isostructural with neptunium(IV) oxalate hexahydrate (Np(C2O4)2 ⋅ 6 H2O; NpOx) and uranium(IV) oxalate hexahydrate (U(C2O4)2 ⋅ 6 H2O; UOx) despite the high degree of unresolved disorder that exists when determining water positions in the crystal structures of the latter two compounds. Such assumptions regarding the isostructural behavior of the actinide elements have been used to predict the structure of PuOx for use in a widemore » range of studies. Herein, we report the first crystal structures for PuOx and Th(C2O4)2 ⋅ 6 H2O (ThOx). These data, along with new characterization of UOx and NpOx, have resulted in the full determination of the structures and resolution of the disorder around the water molecules. Specifically, we have identified the coordination of two water molecules with each metal center, which necessitates a change in oxalate coordination mode from axial to equatorial that has not been reported in the literature. The results of this work exemplify the need to revisit previous assumptions regarding fundamental actinide chemistry, which are heavily relied upon within the current nuclear field.« less

Laves phase alloys possess unique thermal and electrical conduction properties, yet the factors governing phase stability in these systems remain an open question. The influence of phonons in particular has been broadly overlooked. Here, we investigate the UCo_2xNi_2(1-x) chemical space using density functional theory, which offers a unique opportunity to explore the factors influencing Laves phase stability as all three primary Laves phases (C14, C15, C36) can be stabilized by changing the ratio of Co to Ni. Calculations of the thermodynamic and dynamical stability of pure UCo₂ and UNi₂ in each of three primary Laves phases confirm the stability ofmore » experimentally known Laves phases for UNi₂ and UCo₂. A decrease in bonding strength is identified in UNi₂ compared to UCo₂, aligned with redshifts observed in the UNi₂ phonon density of states and a decoupling of the U and Ni vibrational modes. Phonon calculations of C14 UCo₂ reveal dynamical instabilities. Efforts to remove the unstable mode at the Γ point in UCo₂ via atomic displacements break the symmetry of the C14 phase, revealing a lower energy P2/c structure. Vibrational contributions to the free energy were calculated and did not change the thermodynamically stable Laves phase below 1000 K. The temperature-dependent free energies of single phase UCo₂ and UNi₂ were used to interpolate the relative stability of ternary UCo_2xNi_2(1-x) in each of the three Laves phases at varying temperatures and stoichiometries. The ternary C36 phase is only predicted to be thermodynamically stable over a narrow stoichiometric range below 600 K.« less