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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 901 records · Page 50
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Medial packing and elastic asymmetry stabilize the double-gyroid in block copolymers

Triply-periodic networks are among the most complex and functionally valuable self-assembled morphologies, yet they form in nearly every class of biological and synthetic soft matter building blocks. In contrast to simpler assembly motifs – spheres, cylinders, layers – networks require molecules to occupy variable local environments, confounding attempts to understand their formation. Here, we examine the double-gyroid network phase by using a geometric formulation of the strong stretching theory of block copolymer melts, a prototypical soft self-assembly system. The theory establishes the direct link between molecular packing, assembly thermodynamics and the medial map, a generic measure of the geometric center of complex shapes. We show that “medial packing” is essential for stability of double-gyroid in strongly-segregated melts, reconciling a long-standing contradiction between infinite- and finite-segregation theories. Additionally, we find a previously unrecognized non-monotonic dependence of network stability on the relative entropic elastic stiffness of matrix-forming to tubular-network forming blocks. The composition window of stable double-gyroid widens for both large and small elastic asymmetry, contradicting intuitive notions that packing frustration is localized to the tubular domains. This study demonstrates the utility of optimized medial tessellations for understanding soft-molecular assembly and packing frustration via an approach that is readily generalizable far beyond gyroids in neat block copolymers.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Lattice dynamics and elasticity for ε-plutonium [First-principles lattice dynamics for ε-plutonium]

Here, lattice dynamics and elasticity for the high-temperature ε phase (body-centered cubic; bcc) of plutonium is predicted utilizing first-principles electronic structure coupled with a self-consistent phonon method that takes phonon-phonon interaction and strong anharmonicity into account. These predictions establish the first sensible lattice-dynamics and elasticity data on ε-Pu. The atomic forces required for the phonon scheme are highly accurate and derived from the total energies obtained from relativistic and parameter-free density-functional theory. The results appear reasonable but no data exist to compare with except those from dynamical mean-field theory that suggest ε-plutonium is mechanically unstable. Fundamental knowledge and understanding of the high-temperature bcc phase, that is generally present in all actinide metals before melting, is critically important for a proper interpretation of the phase diagram as well as practical modeling of high-temperature properties.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Dissociated vacancies and screw dislocations in MgO and UO 2 : atomistic modeling and linear elasticity analysis

Understanding the effect of dislocations on the mass transport in ionic ceramics is important for understanding the behavior of these materials in a variety of contexts. In particular, the dissociated nature of vacancies at screw dislocations, or more generally, at a wide range of low-angle twist grain-boundaries, has ramifications for the mechanism of defect migration and thus mass transport at these microstructural features. In this paper, a systematic study of the dissociated vacancies at screw dislocations in MgO is carried out. The important role of stress migration in the atomistic modeling study is identified. Another aspect of the current work is a rigorous treatment of the linear elasticity model. As a result, good agreement between the atomistic modeling results and the linear elasticity model is obtained. Furthermore, we demonstrate that the proposed vacancy dissociation mechanism can also be extended to more complicated ionic ceramics such as UO 2 , highlighting the generality of the mechanism.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Synergy of elastic and inelastic energy loss on ion track formation in SrTiO 3

While the interaction of energetic ions with solids is well known to result in inelastic energy loss to electrons and elastic energy loss to atomic nuclei in the solid, the coupled effects of these energy losses on defect production, nanostructure evolution and phase transformations in ionic and covalently bonded materials are complex and not well understood due to dependencies on electron-electron scattering processes, electron-phonon coupling, localized electronic excitations, diffusivity of charged defects, and solid-state radiolysis. Here we show that a colossal synergy occurs between inelastic energy loss and pre-existing atomic defects created by elastic energy loss in single crystal strontium titanate (SrTiO 3 ), resulting in the formation of nanometer-sized amorphous tracks, but only in the narrow region with pre-existing defects. These defects locally decrease the electronic and atomic thermal conductivities and increase electron-phonon coupling, which locally increase the intensity of the thermal spike for each ion. This work identifies a major gap in understanding on the role of defects in electronic energy dissipation and electron-phonon coupling; it also provides insights for creating novel interfaces and nanostructures to functionalize thin film structures, including tunable electronic, ionic, magnetic and optical properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Atomic picture of elastic deformation in a metallic glass

The tensile behavior of a Ni₆₀Nb₄₀ metallic glass (MG) has been studied by using ab initio density functional theory (DFT) calculation with a large cell containing 1024 atoms (614 Ni and 410 Nb). We provide insight into how a super elastic limit can be achieved in a MG. Spatially inhomogeneous responses of single atoms and also major polyhedra are found to change greatly with increasing external stress when the strain is over 2%, causing the intrinsically viscoelastic behavior. We uncover the origin of the observed super elastic strain limit under tension (including linear and viscoelastic strains) in small-sized MG samples, mainly caused by inhomogeneous distribution of excess volumes in the form of newly formed subatomic cavities.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

Anomalous magneto-elastic and charge doping effects in thallium-doped BaFe 2 As 2

Within the BaFe 2 As 2 crystal lattice, we partially substitute thallium for barium and report the effects of interlayer coupling in Ba 1-x Tl x Fe 2 As 2 crystals. We demonstrate the unusual effects of magneto-elastic coupling and charge doping in this iron-arsenide material, whereby Néel temperature rises with small x, and then falls with additional x. Specifically, we find that Néel and structural transitions in BaFe 2 As 2 (TN = T s = 133 K) increase for x = 0.05 (T N = 138 K, T s = 140 K) from magnetization, heat capacity, resistivity, and neutron diffraction measurements. Evidence from single crystal X-ray diffraction and first principles calculations attributes the stronger magnetism in x = 0.05 to magneto-elastic coupling related to the shorter intraplanar Fe-Fe bond distance. With further thallium substitution, the transition temperatures decrease for x = 0.09 (T N = T s = 131 K), and this is due to charge doping. Finally, we illustrate that small changes related to 3d transition-metal state can have profound effects on magnetism.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Ion irradiation induced structural modifications and increase in elastic modulus of silica based thin films

Ion irradiation is an alternative to heat treatment for transforming organic-inorganic thin films to a ceramic state. One major shortcoming in previous studies of ion-irradiated films is the assumption that constituent phases in ion-irradiated and heat-treated films are identical and that the ion irradiation effect is limited to changes in composition. Here, we investigate the effects of ion irradiation on both the composition and structure of constituent phases and use the results to explain the measured elastic modulus of the films. Our results indicated that the microstructure of the irradiated films consisted of carbon clusters within a silica matrix. It was found that carbon was present in a non-graphitic sp 2 -bonded configuration. It was also observed that ion irradiation caused a decrease in the Si-O-Si bond angle of silica, similar to the effects of applied pressure. A phase transformation from tetrahedrally bonded to octahedrally bonded silica was also observed. The results indicated the incorporation of carbon within the silica network. Finally, a combination of the decrease in Si-O-Si bond angle and an increase in the carbon incorporation within the silica network was found to be responsible for the increase in the elastic modulus of the films.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Programmable shape transformation of elastic spherical domes

We investigate mismatch strain driven programmable shape transformation of spherical domes and report the effects of different geometric and structural characteristics on dome behavior in response to applied mismatch strain. We envision a bilayer dome design where the differential swelling of the inner layer with respect to the passive outer layer in response to changes in dome surroundings (such as the introduction of an organic solvent) introduces mismatch strain within the bilayer system and causes dome shape transformation. Finite element analysis reveals that, in addition to snap-through, spherical domes undergo bifurcation buckling and eventually gradual bending to morph into cylinders with increasing mismatch strain. Besides demonstrating how the snap-through energy barrier depends on the spherical dome shape, our analysis identifies three distinct groups of dome geometries based on their mismatch strain-transformed configuration relationships. Our experiments with polymer-based elastic bilayer domes that exhibit differential swelling in organic solvents qualitatively confirm the finite element predictions. We establish that, in addition to externally applied stimuli (mismatch strain), bilayer spherical dome morphing can be tuned and hence programmed through its geometry and structural characteristics. Incorporation of an elastic instability mechanism such as snap-through within the framework of stimuli-responsive functional devices can improve their response time which is otherwise controlled by diffusion. Furthermore, our proposed design guidelines can be used to realize deployable, multi-functional, reconfigurable, and therefore, adaptive structures responsive to a diverse set of stimuli across multiple length scales.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Making metals linear super-elastic with ultralow modulus and nearly zero hysteresis

We demonstrate a novel materials design approach to achieve unprecedented properties by utilizing nanoscale chemo-mechanical coupling. In particular, by using computer simulations we demonstrate how to engineer ultralow modulus (12 GPa), nearly hysteresisfree, and linear super-elastic metals with a giant elastic strain limit (2.7%) by creating appropriate concentration modulations (CMs) at the nanoscale in the parent phase and by pre-straining to regulate the stress-induced martensitic transformation (MT). The nanoscale CMs created via spinodal decomposition produce corresponding phase stability modulations, suppress autocatalysis in nucleation, impose nano-confinements on growth, and hinder long-range ordering of transformation strain during the MT, which changes the otherwise sharp first-order transition into a smeared, macroscopically continuous transition over a large stress range. The pre-straining generates retained martensitic particles that are stable at the test temperature after unloading and act as operational nuclei in subsequent load cycles, eliminating the stress–strain hysteresis and offering an ultralow apparent Young’s modulus. Materials with a high strength and an ultralow apparent Young’s modulus have great potential for application in orthopaedic implants.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Elasticity and yielding of mesophases of block copolymers in water–oil mixtures

Here, amphiphilic block copolymers self-assemble at the water/oil interface to form different mesomorphic structures, such as lamellar, micellar cubic, normal hexagonal, and reverse hexagonal structures. Usually, these structures are polycrystalline and the value of their elastic modulus depends on the average orientation of their constituent's single crystals. We introduce a model to predict the elastic modulus and yielding of mesophases from their characteristic length and intermicellar interactions. Shear modulus of each structure is measured as a function of deformation (strain). Zero-shear modulus, G 0 , depends on the inverse of the intermicellar distance with a power law model. The power law index for each structure is approximately n + 2 where n is the degree of confinement in the mesophase: 1 for lamellar, 2 for both normal and reverse hexagonal, and 3 for micellar cubic structures. Rheological properties of different mesophases of Pluronic P84 in the presence of water and p -xylene are used as a case study. The model is found to be in good agreement with experimental data in the linear viscoelastic region. When compared to experimental data, the yield strain value obtained from the model is one order of magnitude higher than the limit of the linear viscoelastic regime and close to the strain at the cross-over point of storage and loss moduli. Frequency sweep measurements are done to characterize the relaxation and cooperative model behaviors of each mesophase structure.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Comparison of pseudo-static limit equilibrium and elastic wave equation analyses of dynamic earth pressures on retaining structures

The seismic earth pressure increment is typically computed using either pseudo-static limit equilibrium methods or elastic wave equation analyses of the interaction between a retaining structure and backfill material, yet current interpretations of the two methods provide conflicting recommendations. The focus of this study is to compare the seismic earth pressure increment computed using the two methods. This approach is demonstrated by subjecting an initially uniform prototype site selected from standard site classifications to harmonic excitation in one-dimensional equivalent linear analyses. Then, the seismic earth pressure resultant for a rigid wall is computed using the two methods. The limit equilibrium approach utilizes the acceleration records from the equivalent linear analysis to compute a seismic coefficient, whereas the elastic solution incorporates the reduced modulus and damping from the final iteration of the analysis, as well as the relative displacement records. The results presented herein corroborate the findings of recent centrifuge experiments and associated analyses.

58 GEOSCIENCES↗
DOE OSTI2013

Vibrational, elastic, and structural properties of cubic silicon carbide under pressure up to 75 GPa: Implication for a primary pressure scale

We present results of concomitant measurements of synchrotron x-ray diffraction (XRD), Brillouin, and Raman spectroscopy on the single crystal samples of cubic silicon carbide (3C-SiC) under quasi-hydrostatic pressures up to 65 GPa, as well as x-ray diffraction and Raman spectroscopy up to 75 GPa. We determined the equation of state of 3C-SiC and pressure dependencies of the zone-center phonon, elastic tensor, and mode Gruneisen parameters. Cubic SiC lattice was found to be stable up to 75 GPa, but there is a tendency for destabilization above 40 GPa, based on softening of a transverse sound velocity. By applying the concomitant density and elasticity measurements, we determined the pressure on the SiC sample without referring to any other pressure scale thus establishing a new primary pressure scale with a 2%–4% precision up to 65 GPa. We proposed corrections to the existing ruby and neon pressure scales, and also calibrated cubic SiC as a pressure marker for the x-ray diffraction and Raman experiments.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Properties of the low-spin high-spin interface during the relaxation of spin-crossover materials, investigated through an electro-elastic model

The present work is devoted to the spatio-temporal investigations of spin-crossover lattices during their thermal relaxation from high- to low-spin state. The analysis is performed using Monte Carlo simulations on a distortable 2D lattice the sites of which are occupied by high-spin (HS) or low-spin (LS) atoms. The lattice is circular in shape and the HS to LS transformation results in single domain nucleation followed by growth and propagation processes. The evolution of the LS:HS interface is monitored during the relaxation process, through the mapping of spin states, displacement fields, local stresses, and elastic energy. The results show a curved interface, the curvature of which is reversed at the mid-transformation. The local stresses and elastic energy peak at the vicinity of the HS:LS interface, with sizeable dependence upon the position along the front line which evidences the edge effects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Magnetic and elastic anisotropy in magnetorheological elastomers using nickel-based nanoparticles and nanochains

Nickel (Ni) based nanoparticles and nanochains were incorporated as fillers in polydimethylsiloxane (PDMS) elastomers and then these mixtures were thermally cured in the presence of a uniform magnetic field. In this way, macroscopically structured-anisotropic PDMS-Ni based magnetorheological composites were obtained with the formation of pseudo-chains-like structures (referred as needles) oriented in the direction of the applied magnetic field when curing. Nanoparticles were synthesized at room temperature, under air ambient atmosphere (open air, atmospheric pressure) and then calcined at 400 °C (in air atmosphere also). The size distribution was obtained by fitting Small Angle X-ray Scattering (SAXS) experiments with a polydisperse hard spheres model and a Schulz-Zimm distribution, obtaining a size distribution centered at (10.0 ± 0.6) nm with polydispersivity given by σ = (8.0 ± 0.2) nm. The SAXS, X-ray powder diffraction, and Transmission Electron Microscope (TEM) experiments are consistent with single crystal nanoparticles of spherical shape (average particle diameter obtained by TEM: (12 ± 1) nm). Nickel-based nanochains (average diameter: 360 nm; average length: 3 μm, obtained by Scanning Electron Microscopy; aspect ratio = length/diameter ∼ 10) were obtained at 85 °C and ambient atmosphere (open air, atmospheric pressure). The magnetic properties of Ni-based nanoparticles and nanochains at room temperature are compared and discussed in terms of surface and size effects. Both Ni-based nanoparticles and nanochains were used as fillers for obtaining the PDMS structured magnetorheological composites, observing the presence of oriented needles. Magnetization curves, ferromagnetic resonance (FMR) spectra, and strain-stress curves of low filler's loading composites (2% w/w of fillers) were determined as functions of the relative orientation with respect to the needles. The results indicate that even at low loadings it is possible to obtain magnetorheological composites with anisotropic properties, with larger anisotropy when using nanochains. For instance, the magnetic remanence, the FMR field, and the elastic response to compression are higher when measured parallel to the needles (about 30% with nanochains as fillers). Analogously, the elastic response is also anisotropic, with larger anisotropy when using nanochains as fillers. Therefore, all experiments performed confirm the high potential of nickel nanochains to induce anisotropic effects in magnetorheological materials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

On the origin of elastic strain limit of bulk metallic glasses

All bulk metallic glasses exhibit a large and almost universal elastic strain limit. Here, we show that the magnitude of the yield strain of the glass state can be quantitatively derived from a characteristic property of the flow state typical in running shear bands (the root cause of yielding). The strain in the shear flow is mostly plastic, but associated with it there is an effective elastic atomic strain. The latter is almost identical for very different model systems in our molecular dynamics simulations, such that the corresponding yield strain is universal at any given homologous temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Effect of dynamics on the elastic softening of vacancies in Si

Recently, elastic softening at temperatures below 20 K has been observed in nondoped floating zone silicon. From the experimental analysis, it has been suggested that this softening is caused by an intrinsic vacancy defect through the Jahn-Teller (JT) effect. We have theoretically studied the relations between softening and the vacancies. The ground state of the JT distortion is stiff. However, by considering atomistic dynamical and anharmonic effects, it is found that low-energy excitations exist in the E-mode distortion and that different polarizations of the E-distortion can be easily interchanged. The calculated energy barriers for the reorientation of JT distortions are consistent with other experiments and calculations. This low-lying mode can be the cause of softening in the elastic responses.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Elastically Cooperative Activated Hopping Theory of Relaxation in Viscous Liquids. I. General Formulation and Application to Hard Sphere Fluids.

We generalize the force-level nonlinear Langevin equation theory of single particle hopping to includecollective effects associated with long range elastic distortion of the liquid. The activated alpharelaxation event is of a mixed spatial character, involving two distinct, but inter-related, local and collectivebarriers. There are no divergences at volume fractions below jamming or temperatures abovezero Kelvin. The ideas are first developed and implemented analytically and numerically in the contextof hard sphere fluids. In an intermediate volume fraction crossover regime, the local cage processis dominant in a manner consistent with an apparent Arrhenius behavior. The super-Arrhenius collectivebarrier is more strongly dependent on volume fraction, dominates the highly viscous regime,and is well described by a nonsingular law below jamming. The increase of the collective barrieris determined by the amplitude of thermal density fluctuations, dynamic shear modulus or transientlocalization length, and a growing microscopic jump length. Alpha relaxation time calculations arein good agreement with recent experiments and simulations on dense fluids and suspensions of hardspheres. Comparisons of the theory with elastic models and entropy crisis ideas are explored. Thepresent work provides a foundation for constructing a quasi-universal, fit-parameter-free theory forrelaxation in thermal molecular liquids over 14 orders of magnitude in time.

Mirigian, Stephen↗