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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 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

In Situ Study of Silicon Electrode Lithiation with X-ray Reflectivity

Surface sensitive X-ray reflectivity (XRR) measurements were performed to investigate the electrochemical lithiation of a native oxide terminated single crystalline silicon (100) electrode in real time during the first galvanostatic discharge cycle. This allows us to gain nanoscale, mechanistic insight into the lithiation of Si and the formation of the solid electrolyte interphase (SEI). We describe an electrochemistry cell specifically designed for in situ XRR studies and have determined the evolution of the electron density profile of the lithiated Si layer (Li x Si) and the SEI layer with subnanometer resolution. We propose a three-stage lithiation mechanism with a reaction limited, layer-by-layer lithiation of the Si at the Li x Si/Si interface.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Fluoroethylene Carbonate Induces Ordered Electrolyte Interface on Silicon and Sapphire Surfaces as Revealed by Sum Frequency Generation Vibrational Spectroscopy and X-ray Reflectivity

Here, the cyclability of silicon anodes in lithium ion batteries (LIBs) is affected by the reduction of the electrolyte on the anode surface to produce a coating layer termed the solid electrolyte interphase (SEI). One of the key steps for a major improvement of LIBs is unraveling the SEI’s structure-related diffusion properties as charge and discharge rates of LIBs are diffusion-limited. To this end, we have combined two surface sensitive techniques, sum frequency generation (SFG) vibrational spectroscopy, and X-ray reflectivity (XRR), to explore the first monolayer and to probe the first several layers of electrolyte, respectively, for solutions consisting of 1 M lithium perchlorate (LiClO 4 ) salt dissolved in ethylene carbonate (EC) or fluoroethylene carbonate (FEC) and their mixtures (EC/FEC 7:3 and 1:1 wt %) on silicon and sapphire surfaces. Our results suggest that the addition of FEC to EC solution causes the first monolayer to rearrange itself more perpendicular to the anode surface, while subsequent layers are less affected and tend to maintain their, on average, surface-parallel arrangements. This fundamental understanding of the near-surface orientation of the electrolyte molecules can aid operational strategies for designing high-performance LIBs.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Probing the In Situ Pseudocapacitive Charge Storage in Ti 3 C 2 MXene Thin Films with X-ray Reflectivity

MXenes are a large family of two-dimensional materials that are attractive for energy storage due to their high-rate charging capabilities as well as for electrochemical actuators, water purification, and many other technologies. Ion intercalation during electrochemically driven charge and discharge processes is the fundamental process associated with MXene functionality, which we have characterized here using in situ and operando X-ray reflectivity (XRR). Experiments performed at the Advanced Photon Source at Argonne National Laboratory monitored the changes in the structure of a Ti 3 C 2 MXene film on a platinum current collector as a function of static applied potential between 0.3 and –0.7 V vs Ag/AgCl in an aqueous 0.1 M Li 2 SO 4 electrolyte. Negative potential sweeps lead to a contraction of 1.2 Å in the interlayer spacing and a loss of electron density between the layers, likely due to Li + ion insertion and water removal. The change in lattice spacing includes a continuous variation vs potential as well as an additional discrete contraction that occurs near –0.35 V that has the characteristics of a first-order transition. The continuous change in the MXene interlayer spacing is associated with the capacitive charge, while the discrete change in structure correlated to the weak feature in the cyclic voltammogram at –0.35 V can be interpreted as either a pseudocapacitive charging process or a potential-dependent change in capacity.

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

Predicting Pulsed-Laser Deposition SrTiO 3 Homoepitaxy Growth Dynamics Using High-Speed Reflection High-Energy Electron Diffraction

Pulsed-laser deposition (PLD) is a powerful technique for growing complex oxides with controlled stoichiometry. To understand growth dynamics therein, it is common to leverage in situ spectroscopies, such as reflection high-energy electron diffraction (RHEED), to monitor surface crystallinity. Most commercial systems rely on video-rate cameras operating at 60-120 Hz that lack sufficient temporal resolution to capture growth dynamics at practical deposition frequencies. Here, a high-speed platform to record in situ dynamics via RHEED at >500 Hz is implemented. An open-source analysis package is designed to fit diffraction spots to 2D Gaussians, allowing single-pulse surface reconstruction kinetics extraction. Using homoepitaxially deposited (001)-oriented SrTiO 3 as a model system, we demonstrate how high-speed RHEED can provide real-time insight into growth processes obscured by slower acquisition systems. By fitting the single-pulse intensity to a set of exponential functions, we observe changes in the characteristic decay time and mechanism correlated to the substrate step width and surface termination. We observe distinct surface effects, with diffraction intensity decaying on lower-energy TiO 2 -terminated surfaces and stabilizing on SrO- or mixed-terminated surfaces. Similarly, using an exponential model, the extracted characteristic time of adatom deposition decreases with increased density of bonding sites associated with mixed termination and narrower step widths. Ultimately, this work shows how increasing RHEED temporal resolution can uncover new insights into growth processes, with practical implications for the design and control of PLD processes. This experimental platform provides new capabilities to enable data-driven machine learning analysis and autonomous control systems to enhance the complexity and fecundity of PLD.

(SrO)↗
DOE OSTI2013

Is the Calcite-Water Interface Understood? Direct Comparisons of Molecular Dynamics Simulations with Specular X-ray Reflectivity Data

New insights into the structure of the calcite-water interface are obtained through direct model-independent comparison of multiple classical molecular dynamics (MD) simulations with high-resolution specular X-ray reflectivity (XR) data. This set of comparisons, with four different state-of-the-art force fields (including two non-polarizable, one polarizable, and one reactive force field), reveal new insights into the absolute accuracy of the simulated structures and the uniqueness of the XR-derived structural results. These four simulations, while qualitatively similar, have visibly distinct interfacial structure, and are distinguished through a quantitative comparison of the XR signals calculated from these simulations with experimental XR data. The results demonstrate that the simulated calcite-water interface structures, as a whole, are not consistent with the XR data (i.e., within their precision and accuracy). This disagreement is largely due to the simulation of the calcite lattice. The simulated interfacial water profiles show substantially different levels of agreement with the XR data. Of these, the rigid-ion model (RIM) simulations show the best consistency with the experimental XR data. Further model-dependent comparisons of the structural parameters that describe the interfacial structure (derived from both the MD simulations and the XR data) provide further insight into the sources of differences between these two approaches. Using the new insights from the RIM simulations, new structures of the calcite-water interface consistent with both the experimental data and the simulation are identified and compared to recent results.

Calcite-water interface↗
DOE OSTI2012

Interfacial Properties and Iron Binding to Bacterial Proteins That Promote the Growth of Magnetite Nanocrystals: X-ray Reflectivity and Surface Spectroscopy Studies

Surface sensitive X-ray scattering and spectroscopic studies have been conducted to determine structural properties of Mms6, the protein in Magnetospirillum magneticum AMB-1 that is implicated as promoter of magnetite nanocrystals growth. Surface pressure versus molecular area isotherms indicate Mms6 forms stable monolayers at the aqueous/vapor interface that are strongly affected by ionic conditions of the subphase. Analysis of X-ray reflectivity from the monolayers shows that the protein conformation at the interface depends on surface pressure and on the presence of ions in the solutions, in particular of iron ions and its complexes. X-ray fluorescence at grazing angles of incidence from the same monolayers allows quantitative determination of surface bound ions to the protein showing that ferric iron binds to Mms6 at higher densities compared to other ions such as Fe{sup 2+} or La{sup 3+} under similar buffer conditions.

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

Three-Dimensional Orientation Determination of Stationary Anisotropic Nanoparticles with Sub-Degree Precision under Total Internal Reflection Scattering Microscopy

Single-particle and single-molecule orientation determination plays a vital role in deciphering nanoscale motion in complex environments. Previous attempts to determine the absolute three-dimensional orientation of anisotropic particles rely on subjective pattern matching and are inherently plagued by high degrees of uncertainty. Herein, we describe a method utilizing total internal reflection scattering microscopy to determine the 3D orientation of gold nanorods with subdegree uncertainty. The method is then applied to the biologically relevant system of microtubule cargo loading. Finally, we demonstrate the method holds potential for identifying single particles versus proximate neighbors within the diffraction limited area.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2011

X-ray reflectance studies of interface in ion beam sputtered CoFeB/MgO bilayers

This letter reports specular and diffuse x-ray reflectance studies of the interface in the ion beam sputtered CoFeB({approx}5-8 nm)/MgO({approx}2-5 nm) bilayers. The study reveals the specific roles of oxygen ion energy, substrate temperature, and reactive gas ambient on controlling the interface width in the bilayers. Depending on oxidation-method employed for MgO growth the interface width varies between 0.51 and 0.20 nm. Energy of assist ions ({<=}50 eV) plays a key role compared to the growth temperature in determining the sharpness of the interface. The bottom grown CoFeB exhibited significant increase in coercivity (H{sub C}), which also depends on MgO-growth process.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Experimental Investigation of the Reflection Mode Micro Laser Propulsion under Highly Frequent and Multi Pulse Laser

Micro laser propulsion used for some space tasks of micro-satellites are preferred to providing small thrust and high specific impulse while keeping power consumption low. Most previous work on micro laser propulsion are about transmission mode (T-mode) using a CW laser. In this article, a pulsed fiber laser is used to study the micro laser propulsion performance under reflection mode. Multi pulse (ranged from 100 to 2000) tests are conducted on a double base propellant with the vacuum less than 10 Pa. The laser frequency is 20 kHz and two kinds of instantaneous power density 4.77x10{sup 6} W/cm{sup 2} and 2.39x10{sup 7} W/cm{sup 2} are used. It is found that the momentum coupling coefficient C{sub m} and the mean thrust F increases with the increasing pulse numbers, which is different to the previous work. By adjusting the irradiation time T, it is easy to get a large mean thrust, up to mN. When the energy density is the same, C{sub m}, I{sub sp}, F and {eta} increase with the increasing power density. Also I{sub sp} and {eta} are very low, laser ablation is insufficiently under the current condition. 3D Morphology of the ablation hole is obtained by confocal microscope for the first time.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Measurement of absolute diffraction efficiency of a variable line spaced grating using reflectivity beamline

First and second order diffraction efficiency of a variable line spacing grating of average 1200 lines/mm density is measured at the reflectivity beam line of Indus-1. This grating is blazed at 3.2 deg. and has a blaze wavelength of 10 nm. The measurement has been carried out in the spectral range of 4.5 nm to 80 nm at 3o grazing angle of incidence. The maximum diffraction efficiency of the first order at {approx}10 nm has been found to be {approx} 23% and for the second order it is {approx}13%.

43 PARTICLE ACCELERATORS↗
DOE OSTI2012

Characterization of the local crystallinity via reflectance of very slow electrons

The reflectance of very slow electrons from solids and its electron energy dependence are shown as characteristic for the crystal system and its spatial orientation so they can serve, e.g., to fingerprinting the orientation of grains in polycrystals. Measurements on single crystals and polycrystals are validated via electron backscatter diffraction analyses. Sensitivity of the method to fine details of crystallinity is demonstrated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Infrared reflectivity spectra of gas-source molecular beam epitaxy grown dilute InN{sub x}As{sub 1-x}/InP (001)

Vibrational spectra of gas-source molecular beam epitaxy grown dilute InN{sub x}As{sub 1-x}/InP (001) alloys are obtained using a Fourier-transform infrared (IR) spectroscopy. A triply degenerate N{sub As} local vibrational mode of T{sub d}-symmetry is observed near 438 cm{sup -1} corresponding to the In-N bond energy. The analysis of composition dependent infrared reflectivity spectra in InNAs has predicted a two-phonon-mode behavior. In In(Ga)-rich GaInNAs alloys the observed splitting of the N{sub As} local mode into a doublet for the N{sub As}-Ga{sub 1}(In{sub 1})In{sub 3}(Ga{sub 3}) pair-defect of C{sub 3v}-symmetry is consistent with our simulated results based on a sophisticated Green's function theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

A reverse Monte Carlo method for deriving optical constants of solids from reflection electron energy-loss spectroscopy spectra

A reverse Monte Carlo (RMC) method is developed to obtain the energy loss function (ELF) and optical constants from a measured reflection electron energy-loss spectroscopy (REELS) spectrum by an iterative Monte Carlo (MC) simulation procedure. The method combines the simulated annealing method, i.e., a Markov chain Monte Carlo (MCMC) sampling of oscillator parameters, surface and bulk excitation weighting factors, and band gap energy, with a conventional MC simulation of electron interaction with solids, which acts as a single step of MCMC sampling in this RMC method. To examine the reliability of this method, we have verified that the output data of the dielectric function are essentially independent of the initial values of the trial parameters, which is a basic property of a MCMC method. The optical constants derived for SiO{sub 2} in the energy loss range of 8-90 eV are in good agreement with other available data, and relevant bulk ELFs are checked by oscillator strength-sum and perfect-screening-sum rules. Our results show that the dielectric function can be obtained by the RMC method even with a wide range of initial trial parameters. The RMC method is thus a general and effective method for determining the optical properties of solids from REELS measurements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Adsorption calorimetry during metal vapor deposition on single crystal surfaces: Increased flux, reduced optical radiation, and real-time flux and reflectivity measurements

Thin films of metals and other materials are often grown by physical vapor deposition. To understand such processes, it is desirable to measure the adsorption energy of the deposited species as the film grows, especially when grown on single crystal substrates where the structure of the adsorbed species, evolving interface, and thin film are more homogeneous and well-defined in structure. Our group previously described in this journal an adsorption calorimeter capable of such measurements on single-crystal surfaces under the clean conditions of ultrahigh vacuum [J. T. Stuckless, N. A. Frei, and C. T. Campbell, Rev. Sci. Instrum. 69, 2427 (1998)]. Here we describe several improvements to that original design that allow for heat measurements with ∼18-fold smaller standard deviation, greater absolute accuracy in energy calibration, and, most importantly, measurements of the adsorption of lower vapor-pressure materials which would have previously been impossible. These improvements are accomplished by: (1) using an electron beam evaporator instead of a Knudsen cell to generate the metal vapor at the source of the pulsed atomic beam, (2) changing the atomic beam design to decrease the relative amount of optical radiation that accompanies evaporation, (3) adding an off-axis quartz crystal microbalance for real-time measurement of the flux of the atomic beam during calorimetry experiments, and (4) adding capabilities for in situ relative diffuse optical reflectivity determinations (necessary for heat signal calibration). These improvements are not limited to adsorption calorimetry during metal deposition, but also could be applied to better study film growth of other elements and even molecular adsorbates.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

Barrier performance optimization of atomic layer deposited diffusion barriers for organic light emitting diodes using x-ray reflectivity investigations

The importance of O{sub 3} pulse duration for encapsulation of organic light emitting diodes (OLEDs) with ultra thin inorganic atomic layer deposited Al{sub 2}O{sub 3} layers is demonstrated for deposition temperatures of 50 °C. X-ray reflectivity (XRR) measurements show that O{sub 3} pulse durations longer than 15 s produce dense and thin Al{sub 2}O{sub 3} layers. Correspondingly, black spot growth is not observed in OLEDs encapsulated with such layers during 91 days of aging under ambient conditions. This implies that XRR can be used as a tool for process optimization of OLED encapsulation layers leading to devices with long lifetimes.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Development of ultralow energy (1–10 eV) ion scattering spectrometry coupled with reflection absorption infrared spectroscopy and temperature programmed desorption for the investigation of molecular solids

Extremely surface specific information, limited to the first atomic layer of molecular surfaces, is essential to understand the chemistry and physics in upper atmospheric and interstellar environments. Ultra low energy ion scattering in the 1–10 eV window with mass selected ions can reveal extremely surface specific information which when coupled with reflection absorption infrared (RAIR) and temperature programmed desorption (TPD) spectroscopies, diverse chemical and physical properties of molecular species at surfaces could be derived. These experiments have to be performed at cryogenic temperatures and at ultra high vacuum conditions without the possibility of collisions of neutrals and background deposition in view of the poor ion intensities and consequent need for longer exposure times. Here we combine a highly optimized low energy ion optical system designed for such studies coupled with RAIR and TPD and its initial characterization. Despite the ultralow collision energies and long ion path lengths employed, the ion intensities at 1 eV have been significant to collect a scattered ion spectrum of 1000 counts/s for mass selected CH{sub 2}{sup +}.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Characterization of 2.3 μm GaInAsSb-based vertical-cavity surface-emitting laser structures using photo-modulated reflectance

We report angle dependent and temperature dependent (9 K–300 K) photo-modulated reflectance (PR) studies on vertical-cavity surface-emitting laser (VCSEL) structures, designed for 2.3 μm mid-infrared gas sensing applications. Changing the temperature allows us to tune the energies of the quantum well (QW) transitions relative to the VCSEL cavity mode (CM) energy. These studies show that this VCSEL structure has a QW-CM offset of 21 meV at room temperature. Consequently the QW ground-state transition comes into resonance with the CM at 220 ± 2 K. The results from these PR studies are closely compared with those obtained in a separate study of actual operating devices and show how the PR technique may be useful for device optimisation without the necessity of having first to process the wafers into working devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗