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At least 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Modification of Ni-20Cr corrosion dealloying behavior in molten fluorides via cold work induced plastic deformation

The corrosion dealloying behavior of cold-worked (CW) Ni20Cr alloy (wt%) was studied in molten LiF-NaF-KF (or FLiNaK) salts at 600 °C, equal to a homologous temperature (TH) of 0.52. Alloys were cold-rolled to achieve reductions of thickness of 10%, 30%, and 50% introducing plastic deformation and a high density of dislocations. Potentiostatic holds (Eapplied) were applied in two different electrode potential regimes. At 1.75VK+/K$${\text{V}}_{{\text{K}}^{+}/{\text{K}}}$$, Cr dealloying to Cr(II) and Cr(III) is predominant, while at 1.90 VK+/K$${\text{V}}_{{\text{K}}^{+}/{\text{K}}}$$, both Ni and Cr are oxidized in molten FLiNaK at 600 °C. In these potential regimes, dealloyed Ni20Cr displayed bicontinuous porosity within the grain interior and at grain boundaries, driven by the high driving force for Cr dissolution and sustained by defect mediated outward solid state diffusion of Cr in parallel with surface diffusion of Ni. The bicontinuous porous structure developed was observed to undergo further coarsening and densification of the Ni-rich ligaments at a higher electrode potential. The main effect of CW observed is the introduction of plastic deformation and dislocation substructures that serve as short-circuit paths for Cr solid state diffusion to surfaces exposed to FLiNaK. This modified the evolution of the bicontinuous porous structure which increased with CW substantially. Kinetic analysis reveals that the Cr dealloying at +1.75 VK+/K$${\text{V}}_{{\text{K}}^{+}/{\text{K}}}$$ and 1.90 VK+/K$${\text{V}}_{{\text{K}}^{+}/{\text{K}}}$$ is initially charge transfer controlled, except for the 50% CW condition at 1.90 VK+/K$${\text{V}}_{{\text{K}}^{+}/{\text{K}}}$$, where the process becomes limited by slow Cr defect mediated bulk diffusion. The rate determining factors are explored and compared to experimental results.

Chan, Ho Lun↗
DOE OSTI2018

Multiple Coulomb phase in the fluoride pyrochlore CsNiCrF 6

The Coulomb phase is an idealized state of matter whose properties are determined by factors beyond conventional considerations of symmetry, including global topology, conservation laws and emergent order. In theory, Coulomb phases occur in ice-type systems such as water ice and spin ice; in dimer models; and in certain spin liquids. Yet, apart from ice-type systems, more general experimental examples are very scarce. Here we study the partly disordered material CsNiCrF 6 and show that this material is a multiple Coulomb phase with signature correlations in three degrees of freedom: charge configurations, atom displacements and spin configurations. We use neutron and X-ray scattering to separate these correlations and to determine the magnetic excitation spectrum. Our results reveal how the structural and magnetic properties of apparently disordered materials may inherit, and be dictated by, a hidden symmetry—the local gauge symmetry of an underlying Coulomb phase.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Stratification of fluoride uptake among enamel crystals with age elucidated by atom probe tomography

Dental enamel is subjected to a lifetime of de- and re-mineralization cycles in the oral environment, the cumulative effects of which cause embrittlement with age. However, the understanding of atomic scale mechanisms of dental enamel aging is still at its infancy, particularly regarding where compositional differences occur in the hydroxyapatite nanocrystals and what underlying mechanisms might be responsible. Here, we use atom probe tomography to compare enamel from a young (22 years old) and a senior (56 years old) adult donor tooth. Findings reveal that the concentration of fluorine is elevated in the shells of senior nanocrystals relative to young, with less significant differences between the cores or intergranular phases. It is proposed that the embrittlement of enamel is driven, at least in part, by the infusion of fluorine into the nanocrystals and that the principal mechanism is de- and re-mineralization cycles that preferentially erode and rebuild the nanocrystals shells.

36 MATERIALS SCIENCE↗
DOE OSTI2017

3D-printed poly(vinylidene fluoride)/carbon nanotube composites as a tunable, low-cost chemical vapour sensing platform

We report the production of flexible, highly-conductive poly(vinylidene flouride) (PVDF) and multi-walled carbon nanotube (MWCNT) composites as filament feedstock for 3D-printing. This account further describes, for the first-time, fused deposition modelling (FDM) derived 3D-printed objects with chemiresistive properties in response to volatile organic compounds. The typically prohibitive thermal expansion and die swell characteristics of PVDF were minimized by the presence of MWCNTs in the composites enabling straightforward processing and printing. The nanotubes form a dispersed network as characterized by helium ion microscopy, contributing to excellent conductivity (1 x 10-2 S / cm). The printed composites contain little residual metal particulate relative to parts from commercial PLA-nanocomposite material visualized by micro X-ray computed tomography (μ-CT) and corroborated with thermogravimetric analysis. Printed sensing strips, with MWCNT loadings up to 15 % mass, function as reversible vapour sensors with the strongest responses arising with organic compounds capable of readily intercalating, and subsequently swelling the PVDF matrix (acetone and ethyl acetate). A direct correlation between MWCNT concentration and resistance change was also observed, with larger responses (up to 161 % after 3 minutes) generated with decreased MWCNT loadings. These findings highlight the utility of FDM printing in generating low-cost sensors that respond strongly and reproducibly to target vapours. Furthermore, the sensors can be easily printed in different geometries, expanding their utility to wearable form factors. The proposed formulation strategy may be tailored to sense diverse sets of vapour classes through structural modification of the polymer backbone and/or functionalization of the nanotubes within the composite.

3D-Printing↗
DOE OSTI2020

Gas-phase molybdenum-99 separation from uranium dioxide by fluoride volatility using nitrogen trifluoride

Production of the important 99m Tc medical isotope parent, molybdenum-99 ( 99 Mo), via the fissioning of high- and low-enriched uranium (HEU/LEU) targets followed by target dissolution in acid and solution-phase purification of 99 Mo is time-consuming, generates quantities of corrosive radioactive waste, and can result in the release of an array of radionuclides to the atmosphere. An alternative 99 Mo purification method has been devised that has the potential to alleviate many of these issues. Herein, we demonstrate the feasibility of a rapid Mo/Tc gas-phase separation from UO 2 . The results indicate that volatile [ 99 Mo]Mo can be captured downstream of the reacted solid mixture on a column bed (trap) of alumina; the majority of the captured [ 99 Mo]Mo can be subsequently eluted from the alumina trap with a few milliliters of water. >1.0 × 10 5 single pass decontamination of U and the collected [ 99 Mo]Mo product is demonstrated. This simple thermo-fluorination technique has the potential to provide a rapid methodology for routine 99 Mo production.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Bond dissociation energies of low-valent lanthanide hydroxides: lower limits from ion–molecule reactions and comparisons with fluorides

Despite that bond dissociation energies (BDEs) are among the most fundamental and relevant chemical properties they remain poorly characterized for most elementary lanthanide hydroxides and halides. Lanthanide ions Ln + = Eu + , Tm + and Yb + are here shown to react with H 2 O to yield hydroxides LnOH + . Additionally, under low-energy conditions such reactions must be exothermic, which implies a lower limit of 499 kJ mol -1 for the Ln + –OH BDEs. This limit is significantly higher than previously reported for YbOH + and is unexpectedly similar to the BDE for Yb + –F. To explain this apparent anomaly, it is considered feasible that the inefficient hydrolysis reactions observed here in a quadrupole ion trap mass spectrometer may actually be endothermic. More definitive and broad-based evaluations and comparisons require additional and more reliable BDEs and ionization energies for key lanthanide molecules, and/or energies for ligand-exchange reactions like LnF + OH ↔ LnOH + F. The hydroxide results motivated an assessment of currently available lanthanide monohalide BDEs. Among several intriguing relationships is the distinctively higher BDE for neutral LuF versus cationic LuF + , though quantifying this comparison awaits a more accurate value for the anomalously high ionization energy of LuF.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Poly (vinylidene fluoride-trifluoroethylene)/barium titanate nanocomposite for ferroelectric nonvolatile memory devices

The effect of barium titanate (BaTiO{sub 3}) nanoparticles (particle size <100 nm) on the ferroelectric properties of poly (vinylidenefluoride-trifluoroethylene) P(VDF-TrFE) copolymer has been studied. Different concentrations of nanoparticles were added to P(VDF-TrFE) using probe sonication, and uniform thin films were made. Polarisation - Electric field (P-E) hysteresis analysis shows an increase in remnant polarization (P{sub r}) and decrease in coercive voltage (V{sub c}). Piezo-response force microscopy analysis shows the switching capability of the polymer composite. The topography and surface roughness was studied using atomic force microscopy. It has been observed that this nanocomposite can be used for the fabrication of non-volatile ferroelectric memory devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Strong induced polarity between Poly(vinylidene fluoride-co-chlorotrifluoroethylene) and α-SiC and its influence on dielectric permittivity and loss of their composites

Interface polarization and interface zone have been widely utilized to account for the abnormally improved dielectric properties of composites although their formation is rather vague and their influence has never been directly measured. In this work, micro α-SiC was designed as the filler particles incorporated into poly(vinylidenefluoride-co-chlorotrifluoroethylene) with internal double bonds (P(VDF-CTFE-DB)) to construct polymer micro composites through solution casting method. The dielectric constant of the composites is found to be increasing linearly as SiC content increases at lower content and the highest value is obtained as 83 at 100 Hz, which is unusually higher than both pristine polymer (13@100 Hz) and SiC filler (17@100 Hz). By studying the dielectric properties of a bilayer model composite, the real dielectric permittivity of SiC sheet and P(VDF-CTFE-DB) layer has been directly measured to be significantly enhanced than their original value. The induced polarity between high polar PVDF units in polymer matrix and the electron-hole dipoles in α-SiC is responsible for the elevated dielectric properties of both components, which could address the failure of binary series and parallel models in predicting the dielectric permittivity of 0-3 composites as well. The strong dependence of induced polarity on the volume content, thickness, and polar nature of both components strongly suggests establishing promising high induced polarity between polymer matrix and fillers may provide an alternative strategy for fabricating high-k composites.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Shock-induced melting of [100] lithium fluoride: Sound speed and Hugoniot measurements to 230 GPa

Although [100] lithium flouride (LiF) is the most widely used optical window material in dynamic compression experiments, its high stress (>100 GPa) shock compression response, including melting, is not well understood. To address this need, we measured wave profiles in plate impact experiments to determine the Hugoniot states and longitudinal sound speeds in [100] LiF crystals shock compressed to 231 GPa. The measured peak states are fitted well by a linear shock velocity-particle velocity relation, providing an accurate determination of the LiF Hugoniot curve to significantly higher stresses than previous experiments. The longitudinal sound speeds show a near linear increase with density compression to 182 GPa. Between 182 GPa and 195 GPa, the sound speed and the longitudinal modulus decrease abruptly, due to shock-induced melting. The increasing sound speeds and moduli at higher stresses suggest that shock compressed LiF is fully liquid at 195 GPa and above, allowing determination of the Gruneisen parameter for liquid LiF. Here, the melt stress determined here differs from that predicted by current multiphase equations of state for LiF. Our results provide important insight into the high stress solid and liquid states of shock compressed LiF and point to the need for an improved multiphase equation of state at high pressures and high temperatures.

36 MATERIALS SCIENCE↗