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At least 1,261 records · Page 70
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

CO DIFFUSION INTO AMORPHOUS H{sub 2}O ICES

The mobility of atoms, molecules, and radicals in icy grain mantles regulates ice restructuring, desorption, and chemistry in astrophysical environments. Interstellar ices are dominated by H{sub 2}O, and diffusion on external and internal (pore) surfaces of H{sub 2}O-rich ices is therefore a key process to constrain. This study aims to quantify the diffusion kinetics and barrier of the abundant ice constituent CO into H{sub 2}O-dominated ices at low temperatures (15–23 K), by measuring the mixing rate of initially layered H{sub 2}O(:CO{sub 2})/CO ices. The mixed fraction of CO as a function of time is determined by monitoring the shape of the infrared CO stretching band. Mixing is observed at all investigated temperatures on minute timescales and can be ascribed to CO diffusion in H{sub 2}O ice pores. The diffusion coefficient and final mixed fraction depend on ice temperature, porosity, thickness, and composition. The experiments are analyzed by applying Fick’s diffusion equation under the assumption that mixing is due to CO diffusion into an immobile H{sub 2}O ice. The extracted energy barrier for CO diffusion into amorphous H{sub 2}O ice is ∼160 K. This is effectively a surface diffusion barrier. The derived barrier is low compared to current surface diffusion barriers in use in astrochemical models. Its adoption may significantly change the expected timescales for different ice processes in interstellar environments.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

Early crystallization of amorphous selenium under high pressure studied by synchrotron XRD method

Abstract The amorphous selenium (a-Se) was studied via x-ray diffraction (XRD) under pressures ranging from ambient pressure up to 30 GPa at room temperature to study its high-pressure behavior. Two compressional experiments on a-Se samples, with and without heat treatment, respectively, were conducted. Contrary to the previous reports that a-Se crystallized abruptly at around 12 GPa, in this work we report an early partially crystallized state at 4.9 GPa before completing the crystallization at around 9.5 GPa based on in-situ high pressure XRD measurements on the a-Se with 70 °C heat treatment. In comparison, crystallization pressure on another a-Se sample without thermal treatment history was observed to be 12.7 GPa, consistent with the previously reported crystallization pressure. Thus, it is proposed in this work that prior heat treatment of a-Se can result in an earlier crystallization under high pressure, which helps to understand the possible mechanism caused by the previous controversial reports on pressure induced crystallization behavior in a-Se.

Physics↗
DOE OSTI2018

Transformation of 2D group-III selenides to ultra-thin nitrides: enabling epitaxy on amorphous substrates

Here, the experimental realization of two-dimensional (2D) gallium nitride (GaN) has enabled the exploration of 2D nitride materials beyond boron nitride. Here we demonstrate one possible pathway to realizing ultra-thin nitride layers through a two-step process involving the synthesis of naturally layered, group-III chalcogenides (GIIIC) and subsequent annealing in ammonia (ammonolysis) that leads to an atomic-exchange of the chalcogen and nitrogen species in the 2D-GIIICs. The effect of nitridation differs for gallium and indium selenide, where gallium selenide undergoes structural changes and eventual formation of ultra-thin GaN, while indium selenide layers are primarily etched rather than transformed by nitridation. Further investigation of the resulting GaN films indicates that ultra-thin GaN layers grown on silicon dioxide act as effective 'seed layers' for the growth of 3D GaN on amorphous substrates.

2D materials↗
DOE OSTI2021

Measurement of the ionization response of amorphous selenium with 122 keV γ rays

In this work, we performed a measurement of the ionization response of 200 μm-thick amorphous selenium (aSe) layers under drift electric fields of up to 50 V/μm. The aSe target was exposed to ionizing radiation from a 57 Co radioactive source and the ionization pulses were recorded with high resolution. Using the spectral line from the photoabsorption of 122 keV γ rays, we measured the charge yield in aSe and the line width as a function of drift electric field. From a detailed microphysics simulation of charge generation and recombination in aSe, we conclude that the strong dependence of recombination on the ionization track density provides the dominant contribution to the energy resolution in aSe. These results provide valuable input to estimate the sensitivity of a proposed next-generation search for the neutrinoless ββ decay of 82 Se that aims to employ imaging sensors with an active layer of aSe. We estimate the RMS line width of the integrated ionization signal from neutrinoless ββ decay events (of deposited energy 3 MeV) to be 2.0% for a drift field of 50 V/μm. The energy resolution can be improved to 1% by correcting for the charge yield as a function of ionization density along the imaged electron tracks.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

Impact of hydrogenation on the stability and mechanical properties of amorphous boron nitride

Abstract Interconnect materials with ultralow dielectric constant, and good thermal and mechanical properties are crucial for the further miniaturization of electronic devices. Recently, it has been demonstrated that ultrathin amorphous boron nitride (aBN) films have a very low dielectric constant, high density (above 2.1 g cm −3 ), high thermal stability, and mechanical properties. The excellent properties of aBN derive from the nature and degree of disorder, which can be controlled at fabrication, allowing tuning of the physical properties for desired applications. Here, we report an improvement in the stability and mechanical properties of aBN upon hydrogen doping. With the introduction of a Gaussian approximation potential for atomistic simulations, we investigate the changing morphology of aBN with varying H doping concentrations. We found that for 8 at% of H doping, the concentration ofsp 3 -hybridized atoms reaches to a maximum which leads to an improvement of thermal stability and mechanical properties by 20%. These results will be a guideline for experimentalists and process engineers to tune the growth conditions of aBN films for numerous applications.

Materials Science↗
DOE OSTI2021

Role of element-specific damping in ultrafast, helicity-independent, all-optical switching dynamics in amorphous (Gd,Tb)Co thin films

Ultrafast control of the magnetization in ps timescales by fs laser pulses offers an attractive avenue for applications such as fast magnetic devices for logic and memory. However, ultrafast helicity-independent all-optical switching (HI-AOS) of the magnetization has thus far only been observed in Gd-based, ferrimagnetic amorphous (a-) rare earth-transition metal (a-RE-TM) systems, and a comprehensive understanding of the reversal mechanism remains elusive. Here, we report HI-AOS in ferrimagnetic a-Gd 22-x Tb x Co 78 thin films, from x=0 to 18, and elucidate the role of Gd in HI-AOS in a-RE-TM alloys and multilayers. Increasing Tb content results in increasing perpendicular magnetic anisotropy and coercivity, without modifying magnetization density, and slower remagnetization rates and higher critical fluences for switching but still shows picosecond HI-AOS. Simulations of the atomistic spin dynamics based on the two-temperature model reproduce these results qualitatively and predict that the lower damping on the RE sublattice arising from the small spin-orbit coupling of Gd (with L=0) is instrumental for the faster dynamics and lower critical fluences of the Gd-rich alloys. Annealing a-Gd 10 Tb 12 Co 78 leads to slower dynamics which we argue is due to an increase in damping. These simulations strongly indicate that accounting for element-specific damping is crucial in understanding HI-AOS phenomena. The results suggest that engineering the element-specific damping of materials can open up new classes of materials that exhibit low-energy, ultrafast HI-AOS.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Diffusion coefficients predicting facet-dependent crystallization in amorphous silicon nitride

Amorphous silicon nitride is a common material in microelectronics devices, which acts as an insulating barrier. Extended annealing times at elevated temperature can initiate crystallization of α-Si 3 ⁢ N 4 , which does not possess the same barrier properties. Molecular dynamics can resolve the fundamental mechanism for α-Si 3 ⁢N 4 crystallization and the influence of local environments. Here, we compare two interatomic potentials and conclude that these models predict structural features (e.g., angular distributions and densities) which span the range of experimental measurements. We confirmed these models reproduce experimental estimates of activation energy and leveraged these models to identify crystallization drivers. We conclude that near-T g , facet-dependent silicon nitride crystal growth rates can be predicted directly by either bulk or interfacial diffusion properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

In situ spectroscopic study of the plastic deformation of amorphous silicon under nonhydrostatic conditions induced by indentation

Indentation-induced plastic deformation of amorphous silicon (a-Si) thin films was studied by in situ Raman imaging of the deformed contact region of an indented sample, employing a Raman spectroscopy-enhanced instrumented indentation technique (IIT). The occurrence and evolving spatial distribution of changes in the a-Si structure caused by processes, such as polyamorphization and crystallization, induced by indentation loading were observed. Furthermore, the obtained experimental results are linked with previously published work on the plastic deformation of a-Si under hydrostatic compression and shear deformation to establish a model for the deformation behavior of a-Si under indentation loading.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Shear-transformation-zone theory of yielding in athermal amorphous materials

Yielding transitions in athermal amorphous materials undergoing steady-state shear flow resemble critical phenomena. Historically, they have been described by the Herschel-Bulkley rheological formula, which implies singular behaviors at yield points. In this paper, I examine this class of phenomena using an elementary version of the thermodynamic shear-transformation-zone (STZ) theory, focusing on the role of the effective disorder temperature, and paying special attention to scaling and dimensional arguments. I find a wide variety of Herschel-Bulkley-like rheologies but, for fundamental reasons not specific to the STZ theory, conclude that the yielding transition is not truly critical. Specifically, for realistic many-body models with short-range interactions, there is a correlation length that grows rapidly but ultimately saturates near the yield point.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Coarse graining atomistic simulations of plastically deforming amorphous solids

The primary mode of failure in disordered solids results from the formation and persistence of highly localized regions of large plastic strains known as shear bands. Continuum-level field theories capable of predicting this mechanical response rely upon an accurate representation of the initial and evolving states of the amorphous structure. We perform molecular dynamics simulations of a metallic glass and propose a methodology for coarse graining discrete, atomistic quantities, such as the potential energies of the elemental constituents. A strain criterion is established and used to distinguish the coarse-grained degrees-of-freedom inside the emerging shear band from those of the surrounding material. A signal-to-noise ratio provides a means of evaluating the strength of the signal of the shear band as a function of the coarse graining. Finally, we investigate the effect of different coarse graining length scales by comparing a two-dimensional, numerical implementation of the effective-temperature description in the shear transformation zone (STZ) theory with direct molecular dynamics simulations. These comparisons indicate the coarse graining length scale has a lower bound, above which there is a high level of agreement between the atomistics and the STZ theory, and below which the concept of effective temperature breaks down.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Multiple rare-earth ion environments in amorphous ( G d 2 O 3 ) 0.230 ( P 2 O 5 ) 0.770 revealed by gadolinium K -edge anomalous x-ray scattering

A Gd K-edge anomalous X-ray scattering (AXS) study is performed on the rare-earth (R) phosphate glass, (Gd 2 O 3 ) 0.230 (P 2 O 5 ) 0.770 , in order to determine Gd…Gd separations in its local structure. The minimum rare-earth separation is of particular interest given that the optical properties of these glasses can quench when rare-earth ions become too close to each other. To this end, a weak Gd…Gd pairwise correlation is located at 4.2(1) Å which is representative of a meta-phosphate R…R separation. More intense first neighbor Gd…Gd pairwise correlations are found at the larger radial distributions, 4.8(1) Å, 5.1(1) Å and 5.4(1) Å. These reflect a mixed ultra-phosphate and meta-phosphate structural character, respectively. A second neighbor Gd…Gd pairwise correlation lies at 6.6(1) Å which is indicative of meta-phosphate structures. Meta- and ultra-phosphate classifications are made by comparing the R…R separations against those of rare-earth phosphate crystal structures, R(PO 3 ) 3 and RP 5 O 14 respectively, or difference pair distribution function (ΔPDF) features determined on similar glasses using difference neutron scattering methods. The local structure of this glass is therefore found to display multiple rare-earth ion environments, presumably because its composition lies between these two stoichiometric formulae. These Gd…Gd separations are well resolved in the ΔPDFs that represent the AXS signal. Indeed, the spatial resolution is so good that it also enables the identification of R…X (X = R, P, O) pairwise correlations up to r ~ 9 Å; their average separations lie at r ~ 7.1(1) Å, 7.6(1) Å 7.9(1) Å, 8.4(1) Å and 8.7(1) Å. This is the first report of a Gd K-edge AXS study on an amorphous material. Its demonstrated ability to characterize the local structure of a glass up to such a long-range of r, heralds exciting prospects for AXS studies on other ternary non-crystalline materials. However, the technical challenge of such an experiment should not be underestimated, as is highlighted in this work where probing AXS signal near the Gd K-edge is found to produce inelastic X-ray scattering that precludes the normal AXS methods of data processing. Furthermore, it is shown that AXS results are not only tractable but they also reveal local structure of rare-earth phosphate glasses that is important from a materials-centered perspective and which could not be obtained by other materials characterization methods.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Accurate band alignment at the amorphous Al 2 O 3 /p-Ge(100) interface determined by hard x-ray photoelectron spectroscopy and density functional theory

Here, we have investigated the band alignment at the interface of amorphous aluminum oxide (am-Al 2 O 3 ) grown by atomic layer deposition on p-Ge(100) and the effects of postgrowth annealing using hard x-ray photoelectron spectroscopy and density function theory (DFT). Accurate determination of the valence-band offsets was obtained by comparing the experimentally measured valence bands with DFT-calculated densities of states. The am-Al 2 O 3 density of states calculated from a weighted ensemble of crystalline Al 2 O 3 structures gives excellent agreement with experiment, sufficiently capturing the nonlinear shape of the valence-band edge. We report a valence-band offset of 2.60 ± 0.1 eV for am-Al 2 O 3 /p-Ge, which is reduced by 0.20 eV upon annealing as interfacial GeOx is formed.

47 OTHER INSTRUMENTATION↗
DOE OSTI2019

Role of atomic coordination on superconducting properties of boron-doped amorphous carbon

We study the effect of atomic coordination (orbital hybridization) on superconducting properties of boron-doped amorphous carbon. The ratio of threefold coordinated (sp 2 -hybridized) and fourfold coordinated (sp 3 -hybridized) atoms in the system is found to have an impact on their electronic, vibrational, and superconducting properties. Our findings show that a high proportion of fourfold coordination in both carbon and boron atoms is important for realizing a high superconducting transition temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Unveiling the mechanism of phase and morphology selections during the devitrification of Al-Sm amorphous ribbon

The complex interplay between energetic and kinetic factors that governs the phase and morphology selections can originate at the earliest stage of crystallization in the amorphous parent phases. Because of the extreme difficulties in capturing the microscopic nucleation process, a detailed picture of how initial disordered structures affect the transformation pathway remains unclear. Here, we report the experimental observation of widely varying phase selection and grain size evolution during the devitrification of a homogeneous melt-spun glassy ribbon. Two different crystalline phases, θ-Al 5 Sm and ε-Al 60 Sm 11 , are found to form in the different regions of the same metallic glass ribbon during the devitrification. The grain size of ε-Al 60 Sm 11 phase shows a strong spatial heterogeneity. Coarse-grained ε-Al 60 Sm 11 phase coupled with the small volume fraction of θ-Al 5 Sm phase is preferably formed close to wheel side of the melt-spun ribbon. Combining experimental characterization and computational simulations, we show that phase selection and microstructure evolution can be traced back to different types and populations of atomic clusters that serve as precursors for the nucleation of different crystalline phases. Inhomogeneous cooling rates cause different structure orders across the glass sample during the quenching process. Our findings provide direct insight into the effect of structural order on the crystallization pathways during the devitrification of metallic glass. It also opens an avenue to study the detailed nucleation process at the atomic level using the metallic glass as a platform and suggests the opportunity of microstructure and property design via controlling the cooling process.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Persistent Octahedral Coordination in Amorphous GeO 2 Up to 100 GPa by K β ' ' X-Ray Emission Spectroscopy

We measure valence-to-core x-ray emission spectra of compressed crystalline GeO 2 up to 56 GPa and of amorphous GeO 2 up to 100 GPa. In a novel approach, we extract the Ge coordination number and mean Ge-O distances from the emission energy and the intensity of the Kβ" emission line. Here, the spectra of high-pressure polymorphs are calculated using the Bethe-Salpeter equation. Trends observed in the experimental and calculated spectra are found to match only when utilizing an octahedral model. The results reveal persistent octahedral Ge coordination with increasing distortion, similar to the compaction mechanism in the sequence of octahedrally coordinated crystalline GeO 2 high-pressure polymorphs.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Flux Switching Permanent Magnet Motor with Metal Amorphous Nanocomposite Soft Magnetic Material and Rare Earth Free Permanent Magnets

The power losses in high power-density motors due to high rotational speed and/or high pole counts may be reduced adopting metal amorphous nanocomposites (MANCs) featuring low power loss and a relatively high flux density (~1.3 T). We recently proposed a flux switching permanent magnet (FSPM) motor with a rating of 2.5 kW at 1400 Hz electrical speed by incorporating low loss (< 3 W/kg at 1 kHz) FeNi-based MANC material with projected iron loss of about 5 W. The motor design allows use of rare earth-free permanent magnets. For incorporation of MANC materials, an axial flux motor where the rotor and stator core are made from a wound ribbon core has been developed. We have performed a 3-d FEA to optimize the design and built and tested a prototype demonstrating that the low loss MANC material can be used for motor applications.

33 ADVANCED PROPULSION SYSTEMS↗
DOE OSTI2016

Metastable tantalum oxide formation during the devitrification of amorphous tantalum thin films

Microstructural evolution during the devitrification of amorphous tantalum thin films synthesized via pulsed laser deposition was investigated using in situ transmission electron microscopy (TEM) combined with ex situ isothermal annealing, bright-field imaging, and electron-diffraction analysis. The phases formed during crystallization and their stability were characterized as a function of the chamber pressure during deposition, devitrification temperature, and annealing time. A range of metastable nanocrystalline tantalum oxides were identified following devitrification including multiple orthorhombic oxide phases, which often were present with, or evolved to, the tetragonal TaO 2 phase. While the appearance of these phases indicated the films were evolving to the stable form of tantalum oxide—monoclinic tantalum pentoxide—it was likely not achieved for the conditions considered due to an insufficient amount of oxygen present in the films following deposition. Nevertheless, the collective in situ and ex situ TEM analysis applied to thin film samples enabled the isolation of a number of metastable tantalum oxides. As a result, new insights were gained into the transformation sequence and stability of these nanocrystalline phases, which presents opportunities for the development of advanced tantalum oxide-based dielectric materials for novel memristor designs.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Atomistic Simulation of Glasses and Amorphous Materials: Challenges and Opportunities for the Next Decade

Atomistic simulations have become indispensable tools for understanding glass structure, dynamics, and properties, yet persistent challenges limit their predictive power. This perspective examines three interconnected issues, namely glass formation procedures, interatomic potential development, and machine learning applications, which emerged from the 5th International Workshop on Challenges of Atomistic Simulations of Glasses and Amorphous Materials. We identify convergent community priorities for (i) standardized validation protocols, (ii) curated benchmark datasets with complete metadata, and (iii) open repositories for glasses. A systematic was forward is provided by a hierarchical validation framework for assessing the structural fidelity, property prediction, and behavioral realism of simulation techniques. Looking ahead, transformative advances are promised by the fusion of classical techniques with machine learning based approaches, for instance, by integrating swap Monte Carlo with machine-learning (ML) potentials, leveraging foundation models through transfer learning, and finetuning ML potentials with experimental data. Progress depends on the community committing to validated models, reproducible protocols, and sustained data sharing.

Krishnan, N. M. Anoop↗