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At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

MnBi-based magnets prepared from melt-spun alloys: Effect of $α → β$ phase transformation during field annealing

The temperature of the annealing treatment applied to the MnBi alloys with the intention to stabilize the α -MnBi “low-temperature” phase – a promising material for the rare-earth-free permanent magnets – is usually kept below the range of the β -Mn 1.08 Bi “high-temperature” phase. This work is focused on the investigation of the effects of partial and complete α → β phase transformation in bulk magnetic-field-annealed magnets on their structure and magnetic properties. MnBi alloys modified with In, Mg and Sb were melt-spun, compacted to their full density and then heated in a magnetic field of 30 kOe to temperatures sufficiently high for the beginning of the α → β transformation. A heating of 15 °C/min was used which was immediately followed by cooling at the rate of 7.5 °C/min; the degree of the phase transformation was measured by in situ monitoring of changes in the sample magnetization. In all the studied alloys, the α → β transformation was found to improve the [0 0 1] fiber texture induced by the applied magnetic field, although it simultaneously decreased the coercivity. A partial transformation has resulted in a 11% increase of the maximum energy product for the Mn 50 Bi 49 Sb 0.5 In 0.5 magnet, to 8.3 MGOe, and a 3% increase for the Mn 50 Bi 46 Mg 3 Sb 0.5 In 0.5 magnet, to 12.8 MGOe. In the latter alloy, the α → β and reverse transformations were found to initially occur in well-defined pockets leading to clusters of large, highly textured α crystallites. In the Mn 50 Bi 48.5 Sb 1.5 magnet, only part of the α phase could be recovered after the transformation, which instead promoted the paramagnetic MnBi 0.9 Sb 0.1 phase.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Magnetic properties of (Mo 2/3 Dy 1/3 ) 2 AlC arc melted polycrystalline samples

Here, this study investigates the structural and magnetic properties of arc-melted (Mo 2/3 Dy 1/3 ) 2 AlC polycrystalline samples, a member of the i-MAX phase family. Temperature-dependent magnetization and specific heat measurements confirm the low-temperature antiferromagnetic transitions around 14 K and 17 K. Neutron diffraction data collected at 4 K reveal the emergence of magnetic Bragg peaks that are not allowed in the paramagnetic space group C2/c, further confirming the presence of antiferromagnetic ordering. The detection of a secondary phase, DyAl 2 , is complicated by overlapping Bragg peaks with the monoclinic phase of (Mo 2/3 Dy 1/3 ) 2 AlC in powder XRD patterns. However, magnetization and neutron diffraction data suggest the presence of DyAl 2 , evidenced by a ferromagnetic phase transition around 62 K.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Electrochemical behavior and separation of iodide in molten LiF-NaF-KF melt

This work studied the electrochemical behavior of iodide in molten LiF-NaF-KF mixture and the separation of iodide from the molten salt at 973 K using a graphite working electrode. The cyclic voltammetry (CV) and chronoamperometry (CA) measurements indicated an anodic passivation layer was formed on the graphite electrode in this melt and the layer blocked the charge transfer across the electrolyte/electrode interface. However, the added I- ions could pass through the passivation layer and were oxidized at the graphite electrode. The characterization of the obtained products proved that iodide ions was efficiently oxidized to I2 gas during electrolysis. Furthermore, the graphite anode showed excellent stability without any obvious change in dimensions after electrolysis, which showed the high current efficiency of I2 evolution reaction. Thus, the graphite electrode was possible to be served as an excellent anode to separate other anions (such as Br) from molten fluoride salts.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Simulation of two nanoparticle melting to understand the conductivity drop of 3D-printed silver nanowires

The future flexible sensor technology will require low-temperature, fast processing 3D printing techniques that will rely heavily on the quality of nanoparticle (NP) Ink. Electrical conductivity has been found to decrease in majority of the 3D printed electronic wires. Herein we report a fundamental understanding in drop of electrical conductivity in processed silver (Ag) line wire, generally found in Ag-nanoparticle Ink, using large-scale atomistic molecular dynamics (MD) simulations of sintering of five different sizes of Ag NPs. To preserve the high conductivity of pure silver wires, the integrity of the pristine face-centered cubic (FCC) crystalline structure must be retained in the processed line wires. Simulations show that the pristine Ag FCC structures of the nanoparticles are not recovered after melting and resolidification, instead, the resolidified material is paracrystaline. The breakdown of pristine FCC structures might be the cause of the drop in conductivity of processed Ag wires. Simulation results suggest that the intermediate size nanoparticles retain highest percentage of the pristine silver face-centered cubic (FCC) structure after pulse treatments. Our results show that the most promising Ag-Ink should contain smaller to medium size silver NPs that can retain FCC structure after 3D printing of the Ag-Ink.

42 ENGINEERING↗
DOE OSTI2023

Development of multifunctional nylon 6,6-based nanocomposites with high electrical and thermal conductivities by scalable melt and dry blending methods for automotive applications

With the advent of intelligent automobile technology, the demand for multifunctional composites with higher electrical and thermal conductivities is increasing. We have herein reported for the first time the effect of polyolester on the dispersion of carbon nanotube (CNT) in the short-carbon fiber (CF)/nylon 6,6 (PA66) composite using a scalable melt-mixing and dry-blending method and its subsequent improvement in electrical and thermal conductivities. Masterbatches were prepared with a higher concentration of CNT (both untreated and polyolester-treated) followed by dry blended with reference composite (30 wt% CF and 10 wt% mineral fiber filled PA66) before making coupons via injection molding. A 500% and 63% improvements in electrical (3.14 S cm -1 ) and thermal conductivities (in-plane) (1.99 W m -1 K -1 ) conductivities were observed by adding 1.5 wt% treated CNT compared to the reference composite. The improvements were even higher than that of untreated CNT due to better dispersion. Furthermore, the electromagnetic shielding effectiveness reached 61 dB for 1.5 wt% treated CNT-filled composite (39% enhancement). In conclusion, these results are significant in imparting multiple functionalities to the automotive composite parts.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Microstructural evolution and phase stability in Nb-containing interstitial Fe-Mn-Co-Cr-C high-entropy alloys: An in-situ synchrotron X-ray diffraction study during laser melting

The influence of Nb on phase stability and microstructural evolution in an interstitial Fe-Mn-Co-Cr-C high-entropy alloy was investigated using in-situ synchrotron X-ray diffraction (SXRD) during laser melting. Scheil-Gulliver simulations predict the formation of σ and γ-f.c.c. phases in all three alloys, along with NbC in Nb-containing compositions. SXRD confirmed the presence of most predicted phases, but the σ phase was absent. Nb promotes crystallite refinement and increases dislocation density, though excessive additions reduce refinement efficiency due to solubility limits and secondary phase formation. Furthermore, Nb addition also enhances ε-h.c.p. phase formation by reducing stacking fault energy through NbC-induced carbon depletion. Analysis of intensity peak evolution reveals that Nb alters preferred grain orientations, reducing {111} γ intensity while enhancing {220} γ , leading to a more isotropic grain distribution. Overall, Nb plays a key role in phase selection, microstructure refinement, and preferred orientation evolution, allowing the tailored microstructure of high-entropy alloys via rapid solidification.

Interstitial high entropy alloys↗
DOE OSTI2025

Melting down a tetraquark: D*D(*) interactions and T c c ( 3875 ) + in a hot environment

We discuss the modification of the properties of the tetraquark-like and its heavy quark spin partner, immersed in a hot bath of pions. We consider these exotic states as purely isoscalar DD* and D*D* S-wave bound states, respectively. Finite temperature effects are incorporated through the D and D* state-of-the-art thermal spectral functions calculated in [G. Montana et al., Phys. Rev. D, 102 (2020) 096020], using the imaginary-time formalism. We find important modifications of the DD* and D*D* scattering amplitudes already for MeV, and show that the hot-bath lineshapes of these tetraquark-like states strongly depend on their Weinberg molecular content. We find that the thermal and spectral functions change more rapidly with temperature for high molecular probabilities P0. For large values of P0, the widths significantly increase with temperature, leading to the melting of these exotic states for temperatures larger than 80 MeV. For small molecular components, the changes in the spectral functions of these states due to temperature become significantly less important. All these results show that any future experimental determination of the D(*)D* scattering amplitudes at finite temperature will provide valuable insights into the molecular content of the and exotics.

Montesinos, V. (ORCID:0000000261862777)↗
DOE OSTI2023

Comparison of nanocomposite dispersion and distribution for several melt mixers

Breakup (dispersion) and distribution of nanoparticles are the chief hurdles towards taking advantage of nanoparticles in polymer nanocomposites for reinforcement, flame retardancy, conductivity, chromaticity, and other properties. Microscopy is often used to quantify mixing, but it has a limited field of view, does not average over bulk samples, and fails to address nano-particle hierarchical structures. Ultra-small-angle X-ray scattering (USAXS) can provide a macroscopic statistical average of nanoscale dispersion (breakup) and emergent hierarchical structure, as well as the distribution on the nanoscale. Here, this work compares several common mixer geometries for carbon black-polystyrene nanocomposites. Two twin-screw extruder geometries, typical for industrial processing of melt blends, are compared with a laboratory-scale single screw extruder and a Banbury mixer. It is found that for a given mixer, nanoscale distribution increases following a van der Waals function using accumulated strain as an analogue for temperature while macroscopic distribution/dispersion, using microscopy, does not follow this dependency. Breakup and aggregation in dispersive mixing follow expected behavior on the nanoscale. Across these drastically different mixing geometries an unexpected dependency is observed for nanoscale distributive mixing (both nano and macroscopic) as a function of accumulated strain that may reflect a transition from distributive turbulent to dispersive laminar mixing as the mixing gap is reduced.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Focused Ion Beam Preparation of Low Melting Point Metals: Lessons Learned From Indium

Indium (In) and other low melting point metals are used as interconnects in a variety of hybridized circuits and a full understanding of the metallurgy of these interconnects is important to the reliability and performance of the devices. Here, this paper shows that room temperature focused ion beam (FIB) preparation of cross-sections, using Ga + or Xe + can result in artifacts that obscure the true In microbump structure. The use of modified milling strategies to minimize the increased local sample temperature are shown to produce cross-sections that are representative of the In bump microstructure in some sample configurations. Furthermore, cooling of the sample to cryogenic temperatures is shown to reliably eliminate artifacts in FIB prepared cross-sections of In bumps allowing the true bump microstructure to be observed.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Thermodynamics and Structure of Poly[n]catenane Melts.

Motivated by recent achievements in the synthesis of interlocking polymers, the structural features of poly[n]catenanes, polymers composed entirely of interlocking rings (or macrocycles), are studied by extensive molecular dynamics simulations in the melt state. The degree of polymerization (number of links) is varied from n = 1 25 and the number of beads per macrocycle is varied from m = 15-50; the results are compared to linear chains of degrees of polymerization N = 15-175. The mechanical bonds in the system cause significant topological contributions to the pressure and potential energy density not seen in other polymer systems. The polymers themselves possess many unusual structural features at short and intermediate length scales, which can be attributed to density inhomogeneities along the polymer contour. Furthermore, the conformations of the individual macrocycles within poly[n]catenanes are quite different from those of ordinary ring polymers and depend on the topology of the macrocycle, that is, whether it is threaded by one ring (chain end) or two (chain center). At larger length scales, the poly[n]catenanes are conformationally similar to ideal linear chains, but unlike traditional (covalent) polymers, they are highly globular at low degrees of polymerization and are extremely flexible relative to their size, which inhibits interchain entanglement. Implications for poly[n]catenane material properties and synthesis are discussed.

Rauscher, Phillip M.↗
DOE OSTI2024

Low Melting Temperature Gallium–Indium Liquid Metal Anode for Solid-State Li-Ion Batteries

Solid-state Li-ion batteries are attracting attention for their enhanced safety features, higher energy density, and broader operational temperature range compared to systems based on liquid electrolytes. However, current solid-state Li-ion batteries face performance challenges, such as suboptimal cycling and poor rate capabilities, often due to inadequate interfacial contact between the solid electrolyte and electrodes. To address this issue, we incorporated a gallium–indium (Ga–In) liquid metal as the anode in a solid-state Li-ion battery setup, employing Li 6 PS 5 Cl as the solid electrolyte. Operating at room temperature, this configuration achieved an initial capacity of 389 mAh g –1 and maintained 88% of this capacity after 30 cycles at a 0.05 C rate. It also demonstrated a capacity retention of 66% after 500 cycles at a 0.5 C rate. In comparison to solid anode materials, such as tin, the Ga–In liquid metal exhibited superior cycling stability and rate capacity, which is due to the self-healing and fluid properties of the alloy that ensure stable interfacial contact with solid electrolytes. In situ X-ray diffraction (XRD) and ex situ scanning electron microscope (SEM) analyses revealed that indium does not directly participate in the lithiation/delithiation process. Instead, it helps maintain the alloy’s low melting point, facilitating its return to a liquid state after delithiation. In a comparative analysis of stack pressure during cycling in cells utilizing Ga–In liquid metal and tin, the Ga–In liquid metal cell demonstrated an ability to buffer pressure increases associated with deformation. In conclusion, these findings suggest a promising approach for enhancing solid-state batteries by integrating liquid metal anodes, which improve interfacial contact and stability.

Alloys↗
DOE OSTI2020

Quantifying Single-Ion Transport in Percolated Ionic Aggregates of Polymer Melts

Single-ion conducting polymers such as ionomers are promising battery electrolyte materials, but it is critical to understand how rates and mechanisms of free cation transport depend on the nanoscale aggregation of cations and polymer-bound anions. We perform coarse-grained molecular dynamics simulations of ionomer melts to understand cation mobility as a function of polymer architecture, background relative permittivity, and corresponding ionic aggregate morphology. In systems exhibiting percolated ionic aggregates, cations diffuse via stepping motions along the ionic aggregates. These diffusivities can be quantitatively predicted by calculating the lifetimes of continuous association between oppositely charged ions, which equal the time scales of the stepping (diffusive) motions. In contrast, predicting cation diffusivity for systems with isolated ionic aggregates requires another time scale. Finally, our results suggest that to improve conductivity the Coulombic interaction strength should be strong enough to favor percolated aggregates but weak enough to facilitate ion dissociation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Shear Response of Ionizable Polymer Melts at the Crossover from Ionomers to Polyelectrolytes

Nonlinear shear response of polymers is affected by inherent barriers for diffusive motion, including entanglements and topology. In ionizable polymers, ionic clusters further constrain the intrinsic dynamics of the polymers, significantly enhancing their viscosity. Here, using fully atomistic molecular dynamics simulations, the nonlinear shear response of ionizable polymers is presented, across the transition from the ionomer regime where distinctive clusters dominate the structure to the polyelectrolyte regime where clusters percolate, in polystyrene randomly sulfonated with fractions of SO 3 − groups of f = 0.20 and 0.35, in pristine and tetrahydrofuran (THF) swollen polyelectrolyte melts. For f = 0.20, the ionic clusters first fracture into smaller clusters followed by splitting into individual ionic groups and eventually reform. At higher f, the clusters morph in shape but do not break under high shear. At very high shear rates, all of the chains stretch and recoil rapidly. As the shear rate is reduced, some chains stretch and recoil, while others remain largely unaffected by the shear. Macroscopically, for all systems, the shear viscosity displays initially an elastic response, followed by nonlinear shear stress overshoot and, eventually, a steady state. The evolution of viscosity with time and shear reflects that of the ionizable domains.

cluster chemistry↗
DOE OSTI2024

Chain-End Controlled Depolymerization Selectivity in α,α-Disubstituted Propionate PHAs with Dual Closed-Loop Recycling and Record-High Melting Temperature

In this article, within the large poly(3-hydroxyalkanoate) (PHA) family, C3 propionates are much less studied than C4 butyrates, with the exception of α,α-disubstituted propionate PHAs, particularly poly(3-hydroxy-2,2-dimethylpropionate), P3H(Me) 2 P, due to its high melting temperature (T m ~ 230 °C) and crystallinity (~76%). However, inefficient synthetic routes to its monomer 2,2-dimethylpropiolactone [(Me) 2 PL] and extreme brittleness of P3H(Me) 2 P largely hinder its broad applications. Here, we introduce simple, efficient step-growth polycondensation (SGP) of a hydroxyacid or methyl ester to afford P3H(Me) 2 P with low to medium molar mass, which is then utilized to produce lactones through base-catalyzed depolymerization. The ring-opening polymerization (ROP) of the 4-membered lactone leads to high-molar-mass P3H(Me) 2 P, which can be depolymerized by hydrolysis to the hydroxyacid in 99% yield or methanolysis to the hydroxyester in 91% yield, achieving closed-loop recycling via both SGP and ROP routes. Intriguingly, the chain end of the SGP-P3H(Me) 2 P determines the depolymerization selectivity toward 4- or 12-membered lactone formation, while both can be repolymerized back to P3H(Me) 2 P. Through the formation of copolymers P3H(Me/R) 2 P (R = Et, n Pr), PHAs with high tensile strength and ductility, coupled with high barriers to water vapor and oxygen, have been created. Notably, the PHA structure–property study led to P3H( n Pr) 2 P with a record-high T m of 266 °C within the PHA family.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Direct Observation of Shock‐Induced Disordering of Enstatite Below the Melting Temperature

Abstract We report in situ structural measurements of shock‐compressed single crystal orthoenstatite up to 337 ± 55 GPa on the Hugoniot, obtained by coupling ultrafast X‐ray diffraction to laser‐driven shock compression. Shock compression induces a disordering of the crystalline structure evidenced by the appearance of a diffuse X‐ray diffraction signal at nanosecond timescales at 80 ± 13 GPa on the Hugoniot, well below the equilibrium melting pressure (>170 GPa). The formation of bridgmanite and post‐perovskite have been indirectly reported in microsecond‐scale plate‐impact experiments. Therefore, we interpret the high‐pressure disordered state we observed at nanosecond scale as an intermediate structure from which bridgmanite and post‐perovskite crystallize at longer timescales. This evidence of a disordered structure of MgSiO 3 on the Hugoniot indicates that the degree of polymerization of silicates is a key parameter to constrain the actual thermodynamics of shocks in natural environments.

Hernandez, J. ‐A.↗
DOE OSTI2024

Plutonium oxide melt structure and covalency

Advances in nuclear power reactors include the use of mixed oxide fuel, containing uranium and plutonium oxides. The high-temperature behaviour and structure of PuO 2-x above 1,800 K remain largely unexplored, and these conditions must be considered for reactor design and planning for the mitigation of severe accidents. Here, in this study, we measure the atomic structure of PuO 2-x through the melting transition up to 3,000 +/- 50 K using X-ray scattering of aerodynamically levitated and laser-beam-heated samples, with O/Pu ranging from 1.57 to 1.76. Liquid structural models consistent with the X-ray data are developed using machine-learned interatomic potentials and density functional theory. Molten PuO 1.76 contains some degree of covalent Pu-O bonding, signalled by the degeneracy of Pu 5f and O 2p orbitals. The liquid is isomorphous with molten CeO 1.75 , demonstrating the latter as a non-radioactive, non-toxic, structural surrogate when differences in the oxidation potentials of Pu and Ce are accounted for. These characterizations provide essential constraints for modelling pertinent to reactor safety design. The molten structure of plutonium oxide-a component of mixed oxide nuclear fuels-is measured, showing some degree of covalent bonding. Its atomic structure is similar to that of cerium oxide, which could be a non-radioactive structural surrogate.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2025

Multiple freezing–melting pathways of high-density ice through ice XXI phase at room temperature

Various metastable ice phases and their complicated transition pathways have been found by pressurization at low temperatures at which slow kinetics and high metastability are easily achieved. By contrast, such diversity is less expected at room or elevated temperatures. Here, using a combination of a dynamic diamond anvil cell and X-ray free electron laser techniques, we demonstrate that supercompressed water transforms into ice VI through multiple freezing–melting pathways at room temperature, hidden within the pressure region of ice VI. These multiple transition pathways occur via a metastable ice (more specifically, ice XXI with body-centred tetragonal structure ($I\bar{4}2d$)) discovered in this study and a metastable ice VII that exists within the pressure range of ice VI. We find that supercompressed water structurally evolves from high-density water to very-high-density water, causing multiple transition pathways. These findings provide an insight to find more metastable ice phases and their transition pathways at elevated temperatures.

Lee, Yun-Hee [Korea Research Institute of Standard↗
DOE OSTI2020

Spectroscopic fingerprint of charge order melting driven by quantum fluctuations in a cuprate

Copper oxide high-T C superconductors possess a number of exotic orders that coexist with or are proximal to superconductivity. Quantum fluctuations associated with these orders may account for the unusual characteristics of the normal state, and possibly affect the superconductivity. Yet, spectroscopic evidence for such quantum fluctuations remains elusive. Here in this paper, we use resonant inelastic X-ray scattering to reveal spectroscopic evidence of fluctuations associated with a charge order in nearly optimally doped Bi 2 Sr 2 CaCu 2 O 8+δ . In the superconducting state, while the quasielastic charge order signal decreases with temperature, the interplay between charge order fluctuations and bond-stretching phonons in the form of a Fano-like interference increases, an observation that is incompatible with expectations for competing orders. Invoking general principles, we argue that this behaviour reflects the properties of a dissipative system near an order–disorder quantum critical point, where the dissipation varies with the opening of the pseudogap and superconducting gap at low temperatures, leading to the proliferation of quantum critical fluctuations, which melt charge order.

36 MATERIALS SCIENCE↗