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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Evaluating the demand response potential of ammonia plants

Demand-side management/demand response (DSM/DR) are key strategies for mitigating the inherent variability in electricity generation rates by renewable sources. This article represents—to our knowledge—the first foray into assessing the DR potential of ammonia plants. Ammonia plants are interesting candidates for DR initiatives because of their significant electricity use (for operating compressors driving the synthesis loop) and the ability to store the ammonia product relatively easily and safely. Our approach is based on formulating and solving an optimal DR scheduling problem for an ammonia plant while accounting for the process dynamics. To this end, we introduce a new Hammerstein–Wiener-inspired modeling framework based on injecting linear dynamics in a first-principles static nonlinear model of the process. The results are encouraging; for the cases considered, peak-time power consumption decreases between 3.57% and 7.40%, coupled with 1.39% to 3.70% reductions in operating cost.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Magnetic, dielectric properties, and scaling behaviors of Aurivillius compounds Bi{sub 6−x∕3}Fe{sub 2}Ti{sub 3−2x}(WCo){sub x}O{sub 18} (0 ≤ x ≤ 0.15)

We investigate the structural, magnetic, dielectric properties, and scaling behaviors of Aurivillius compounds Bi{sub 6−x∕3}Fe{sub 2}Ti{sub 3−2x}(WCo){sub x}O{sub 18} (0 ≤ x ≤ 0.15). The room-temperature weak ferromagnetism is observed for the W/Co co-doped samples. The results of the dielectric constant ε{sub r}, complex impedance Z″, the dc conductivity σ{sub dc}, and hopping frequency f{sub H} manifest that the dielectric relaxation of the x = 0 sample and the doped samples in the dielectric anomaly region (450–750 K) can be ascribed to the trap-controlled ac conduction around the doubly ionized oxygen vacancies and the localized hopping process of oxygen vacancies, respectively. The scaling behaviors reveal that the dynamic process of both electrons in the x = 0 sample and oxygen vacancies in the doped samples is temperature independent. The ferroelectric Curie-temperature T{sub c} decreases slightly from 973 K to 947 K with increasing the doping level of W/Co. In addition, the dielectric loss exhibits a dielectric relaxation above 800 K with the rather large activation energies (1.95 eV ≤ E{sub a} ≤ 2.72 eV)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Unified Entrainment and Detrainment Closures for Extended Eddy-Diffusivity Mass-Flux Schemes

The extended Eddy Diffusivity Mass Flux (EDMF) scheme is used as a unied parameterization that simulates subgrid-scale dynamics across a range of regimes.This work presents unied entrainment and detrainment closures that allow this scheme to simulate the dry convective boundary layer, shallow convection and deep convection , while adjacent works present other necessary closures for this unied scheme and test its performance for stable, stratocumulus topped and convective boundary layers. Entrainment and detrainment are modeled as a combination of turbulent and dynamical processes. The turbulent component is taken as downgradient diffusion between subdomains. The dynamical component is based on a ratio of buoyancy and vertical velocity that ensures consistent treatment of the ratio of mass-flux at vanishing area fraction. Dynamical detrainment is controlled by the difference in relative humidity between subdomains when liquid water is present. These closures are rst evaluated with entrainment and detrainment diagnosed from Large Eddy Simulations where decaying tracers are used to identify the different subdomains and mass exchanges between them. When implemented in a single column model (SCM), these closures are tested in simulations of several observationally motivated cases of dry convective boundary layer, shallow convection and deep convection. Results show good agreement between the SCM and Large Eddy Simulations in key properties for climate models and profiles of first, second and third moments. The development of diurnal cycles and the onset of and precipitation (with simplified microphysics) are well predicted by the SCM.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2020

Characterizing the Evolution of Trapped scCO 2 Curvature in Bentheimer and Nugget Sandstone Pore Geometry

During a Geologic Carbon Storage process, supercritical CO 2 (scCO 2 ) is subjected to a series of dynamic and static conditions where the relationship between pore geometry and the trapped scCO 2 curvature remains to be established. To mimic the dynamic process, two sandstones, Bentheimer and Nugget, were subjected to two successive drainage and imbibition (D-I) cycles and X-ray computed tomography scanned at each residual state to capture the wettability evolution at static conditions in the same pore geometry. Both sandstones contain similar grain size distributions, pore size distributions, and pore interconnectivity but differ in that the Nugget formation contains approximately half the porosity of the Bentheimer sandstone, and the pore network contains dead-end pores. scCO 2 size distributions, strain calculations, and geometric contact angle measurements were used to characterize the curvature of scCO 2 in different pore types between cycles. An increase in geometric contact angle was the greatest when advancement along the pore network of the same ganglion occurred between cycles while strain increased the most with pore-filling trapping. Moreover, Nugget sandstone results in a greater aggregated residual saturation and shows a clear increase in scCO 2 sizes with an additional D-I cycle while scCO 2 in the Bentheimer core shows a more complex response with some ganglion increasing and some decreasing in size with an additional D-I cycle. From this work, we suspect the pore geometry is playing a role in scCO 2 size distributions and use this information to suggest using water pulses to enhance trapping capacity in lower porosity sandstones.

58 GEOSCIENCES↗
DOE OSTI2019

Thermal Energy Storage for Dispatchable Geothermal Power

Dispatchable generation refers to sources of electricity that can be turned on or off, or can adjust their power output accordingly to an order. Geothermal plants are usually used for base-load power rather than dispatchable power. Flexibility is a challenge for a geothermal plant because the heat source does not naturally respond quickly to changes in demand. The current state-of-the-art approach to address this challenge is to maintain the geothermal fluid flows at relatively steady rates while providing a bypass around the generation equipment as needed. Under part load, some of the geothermal fluid is pumped to the surface, bypasses the generation equipment, and is re-injected to the ground without extracting any useful enthalpy. This approach is robust but necessarily incurs high parasitic power draws at part load due to the constant, full-flow pumping power requirements. There are even more challenges and unknowns for steam and flash geothermal plants, which were the focus for this project. As a process improvement to this existing configuration of a dispatchable geothermal plant, we examined the use of a phase-change thermal-energy storage (TES) unit that will provide some “inertia” against temperature fluctuations. The TES phase change material (PCM) could be in the form of eutectic molten nitrate salts, organic sugar alcohols, or the like based on the required temperature. This configuration allowed the power plant to meet the ramp rate requirements while allowing the geothermal resource to drawn down at the duty-cycle-averaged extraction rate rather than the full-rated-output extraction rate. The project team performed an experimental and computational evaluation of a peak shaving technical approach that used thermal storage to switch between two nominal operating points: full rated power and 50% power. In the initial requirements definition phase, the team decided to examine the PCM/TES-enhanced geothermal plant in the context of a 40-MW two-stage flash plant operating in mid-Southern California. Steady-state process simulations were used to understand the equipment sizing and the energetics. Dynamic process simulations were used to understand the response rate of the system to changes in requested power output from the utility. The technical approach was also guided by a techno-economic analysis (TEA) and a high-level analysis of the reservoir. Recognizing that performance of the thermal storage was a key risk element, high-fidelity finite-element simulations of the PCM-TES heat exchanger were performed to better understand its performance and temporal response. These simulations were validated with a subscale PCM-TES experimental apparatus that mimicked the geometry of the full-scale system. The key outcomes of this Phase I project were that: 1) the PCM-TES heat exchanger met the target for gravimetric energy storage density, 2) the dispatchable geothermal plant met its target for response rate, 3) the peak shaving approach reduces the number of production wells and does not disturb the wellfield, and 4) the incentives for properly remunerating dispatchable operation were identified.

15 GEOTHERMAL ENERGY↗
DOE OSTI2018

A New Method for Optimizing Hot Processing Parameters of Mg-6.0Zn-0.5Mn-0.5Er Alloy Based on Kinetic Model of Dynamic Recrystallization and Processing Map

Hot deformation behavior of the cast-homogenized Mg-6.0Zn-0.5Mn-0.5Er alloy was studied using dynamic recrystallization (DRX) kinetic model and processing map. The compressing tests were conducted in the temperature range of 250-450 °C and strain rates of 0.001-1 s{sup −1}. According to the evolution of microstructures, under lower strain rates, the main DRX mechanism of Mg-6.0Zn-0.5Mn-0.5Er alloy is continuous DRX (CDRX); twinning induced DRX (TDRX) and CDRX both become the main DRX mechanism under higher strain rates. The DRX kinetics model of Mg-6.0Zn-0.5Mn-0.5Er alloy is calculated as X{sub DRX}=1−exp[−1.9463(((ε−ε{sub c})/(ε{sup ∗}))){sup 1.4608}], which is corresponding to the microstructure evolutions of DRX under different deformation conditions. The contour map of DRX was proposed based on the calculation results of DRX kinetics model. The processing maps are constructed to predict processing parameters of the alloy, and the predictability was evaluated combining with the contour map of DRX and dynamic materials model (DMM) processing map. It is deduced from the microstructures evolution and processing map that the optimum processing domain is mainly at 380-450 °C and 0.01-0.001 s{sup −1}, and 410-420 °C and 0.01-1 s{sup −1}.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Compression Algorithm Analysis of In-Situ (S)TEM Video: Towards Automatic Event Detection and Characterization

Precise analysis of both (S)TEM images and video are time and labor intensive processes. As an example, determining when crystal growth and shrinkage occurs during the dynamic process of Li dendrite deposition and stripping involves manually scanning through each frame in the video to extract a specific set of frames/images. For large numbers of images, this process can be very time consuming, so a fast and accurate automated method is desirable. Given this need, we developed software that uses analysis of video compression statistics for detecting and characterizing events in large data sets. This software works by converting the data into a series of images which it compresses into an MPEG-2 video using the open source “avconv” utility [1]. The software does not use the video itself, but rather analyzes the video statistics from the first pass of the video encoding that avconv records in the log file. This file contains statistics for each frame of the video including the frame quality, intra-texture and predicted texture bits, forward and backward motion vector resolution, among others. In all, avconv records 15 statistics for each frame. By combining different statistics, we have been able to detect events in various types of data. We have developed an interactive tool for exploring the data and the statistics that aids the analyst in selecting useful statistics for each analysis. Going forward, an algorithm for detecting and possibly describing events automatically can be written based on statistic(s) for each data type.

algorithm analysis↗
DOE OSTI2024

Stochastic representation and conditioning of process-based geological model by deep generative and recognition networks

Accurate and realistic geological modeling is the core of oil and gas development and production. In recent years, process-based methods are developed to produce highly realistic geological models by simulating the physical processes that reproduce the sedimentary events and develop the geometry. However, the complex dynamic processes are extremely expensive to simulate, making process-based models difficult to be conditioned to field data. In this work, we propose a comprehensive generative adversarial network framework as a machine-learning-assisted approach for mimicking the outputs of process-based geological models with fast generation. The main objective of our work is to obtain a continuous parametrization of the highly realistic process-based geological models which enables us to calibrate the models and condition the models to data. Numerical results are presented to illustrate the capability of our proposed methodology.

58 GEOSCIENCES↗
DOE OSTI2015

The microscopic Z-pinch process of current-carrying rarefied deuterium plasma shell

For insight into the microscopic mechanism of Z-pinch dynamic processes, a code of two-dimensional particle-in-cell (PIC) simulation has been developed in cylindrical coordinates. In principle, the Z-pinch of current-carrying rarefied deuterium plasma shell has been simulated by means of this code. Many results related to the microscopic processes of the Z-pinch are obtained. They include the spatio-temporal distributions of electromagnetic field, current density, forces experienced by the ions and electrons, positions and energy distributions of particles, and trailing mass and current. In radial direction, the electric and magnetic forces exerted on the electrons are comparable in magnitude, while the forces exerted on the ions are mainly the electric forces. So in the Z-pinch process, the electrons are first accelerated in Z direction and get higher velocities; then, they are driven inwards to the axis at the same time by the radial magnetic forces (i.e., Lorentz forces) of them. That causes the separations between the electrons and ions because the ion mass is much larger than the electron's, and in turn a strong electrostatic field is produced. The produced electrostatic field attracts the ions to move towards the electrons. When the electrons are driven along the radial direction to arrive at the axis, they shortly move inversely due to the static repellency among them and their tiny mass, while the ions continue to move inertially inwards, and later get into stagnation, and finally scatter outwards. Near the stagnation, the energies of the deuterium ions mostly range from 0.3 to 6 keV, while the electron energies are mostly from 5 to 35 keV. The radial components, which can contribute to the pinched plasma temperature, of the most probable energies of electron and ion at the stagnation are comparable to the Bennett equilibrium temperature (about 1 keV), and also to the highest temperatures of electron and ion obtained in one dimensional radiation magnetohydrodynamic simulation of the plasma shell Z-pinch. The trailing mass is about 20% of the total mass of the shell, and the maximum trailing current is about 7% of the driven current under our trailing definition. Our PIC simulation also demonstrates that the plasma shell first experiences a snow-plow like implosion process, which is relatively stable.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Toroidal modeling of runaway avalanche in DIII-D discharges

Abstract A toroidal modeling tool is developed to study the runaway electron (RE) avalanche production process in tokamak plasmas, by coupling the Rosenbluth–Putvinski avalanche model (Rosenbluth and Putvinski 1997 Nucl. Fusion 37 1355) with an n = 0 magneto-hydrodynamic (MHD) solver. Initial value numerical simulations are carried out for two DIII-D discharges with different plasma shapes (one near circular, and the other with high elongation). It is found that, assuming the same level of about 1% seed current level, the Rosenbluth–Putvinski model somewhat underestimates the RE plateau current for the circular-shaped plasma, as compared with that measured in DIII-D experiments. For an elongated, higher current plasma, simulations find strong runaway current avalanche production despite the lack of measured plateau RE current in experiments. A possible reason for this discrepancy is a lack of additional RE dissipation physics in the present two-dimensional model. Systematic scans of the plasma boundary shape, at fixed pre-disruption plasma current, find that the plasma elongation helps to reduce the RE avalanche production, confirming recent results obtained with an analytic model (Fülöp et al 2020 J. Plasma Phys. 86 474860101). The effect of the plasma triangularity (either positive or negative), on the other hand, has a minor effect. On the physics side, the avalanche process involves two competing mechanisms associated with the electric field. On the one hand, a stronger electric field produces a higher instantaneous avalanche growth rate. On the other hand, a fast growing RE current quickly reduces the fraction of the conduction current together with the electric field, and hence a faster dissipation of the poloidal flux. As a final result of these two dynamic processes, the runaway plateau current is not always the largest with the strongest initial electric field. These results lay the foundation for future self-consistent inclusion of the MHD dynamics and the RE amplification process.

Physics↗
DOE OSTI2022

Environmental cell for in situ X-ray synchrotron micro-CT imaging with simultaneous acoustic measurements

Synchrotron radiation provides the necessary spatial and temporal resolution for non-invasive operando studies of dynamic processes under complex environmental conditions. Here a new environmental cell for simultaneous in situ dynamic X-ray imaging and measuring acoustic properties of geological samples is presented. The primary purpose of this cell is to study gas-hydrate formation in porous geo-materials and its influence on their acoustic properties. The cell is designed for cylindrical samples of 9 mm in diameter, confining and pore pressures up to 12 MPa, and temperatures from –20°C to room temperature. The cell is portable and can be easily assembled and operated at different X-ray sources. This cell enables a wide range of experiments studying physical/chemical processes in the Earth subsurface that change the mechanical properties of rocks (geochemical reactions, phase transitions, etc.).

47 OTHER INSTRUMENTATION↗
DOE OSTI2019

Dynamic imaging of crystalline defects in lithium-manganese oxide electrodes during electrochemical activation to high voltage

Crystalline defects are commonly generated in lithium-metal-oxide electrodes during cycling of lithium-ion batteries. Their role in electrochemical reactions is not yet fully understood because, until recently, there has not been an effective operando technique to image dynamic processes at the atomic level. In this study, two types of defects were monitored dynamically during delithiation and concomitant oxidation of oxygen ions by using in situ high-resolution transmission electron microscopy supported by density functional theory calculations. One stacking fault with a fault vector b/6[110] and low mobility contributes minimally to oxygen release from the structure. In contrast, dissociated dislocations with Burgers vector of c/2[001] have high gliding and transverse mobility; they lead to the formation, transport and release subsequently of oxygen related species at the surface of the electrode particles. This work advances the scientific understanding of how oxygen participates and the structural response during the activation process at high potentials.

36 MATERIALS SCIENCE↗
DOE OSTI2020

High Speed In-situ X-ray Imaging of 3D Freeze Printing of Aerogels

3D freeze printing (3DFP) combines drop-on-demand (DOD) inkjet printing with freeze casting to fabricate lightweight and multifunctional aerogels with customized geometries. Freeze casting is an efficient and easily implemented method capable of fabricating porous, sponge-like structures for many different applications. This process enables tailoring the microstructure of the final product (i.e., pore morphology, alignment, average size distribution, etc.) by controlling the fabrication conditions and freezing kinetics. Furthermore, its combination with DOD printing provides the capability of engineering the macrostructure without relying on a mold as reported for 3D freeze-printed aerogels made from graphene, silver nanowires, and other nanocomposites. In this paper, we performed in-situ X-ray imaging to understand the inside process dynamics in 3DFP using a commercially available colloidal silica ink. We investigated the 3DFP process with the following hierarchy: first, single droplets; then, uniform lines obtained from coalescence of droplets; and finally, three consecutive lines deposited layer by layer. With the help of X-ray imaging, the importance of the balance between material deposition and freezing rates was shown in-situ by the observation inside of the freeze front following the tip of the printed line. The effects of the substrate temperature on the elimination of undesired interfacial boundaries were also shown by the observed ice crystals penetrating from lower to upper layer.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Investigating the mechanisms responsible for the lack of surface energy balance closure in a central Amazonian tropical rainforest

This work investigates the diurnal and seasonal behavior of the energy balance residual (E) that results from the observed difference between available energy and the turbulent fluxes of sensible heat (H) and latent heat (LE) at the FLUXNET BR-Ma2 site located in the Brazilian central Amazon rainforest. The behavior of E is analyzed by extending the eddy covariance averaging length from 30 min to 4 h and by applying an Information Flow Dynamical Process Network to diagnose processes and conditions affecting E across different seasons. Results show that the seasonal turbulent flux dynamics and the Bowen ratio are primarily driven by net radiation (R n ), with substantial sub-seasonal variability. The Bowen ratio increased from 0.25 in April to 0.4 at the end of September. Extension of the averaging length from 0.5 (94.6% closure) to 4 h and thus inclusion of longer timescale eddies and mesoscale processes closes the energy balance and lead to an increase in the Bowen ratio, thus highlighting the importance of additional H to E. Information flow analysis reveals that the components of the energy balance explain between 25 and 40% of the total Shannon entropy with higher values during the wet season than the dry season. Dry season information flow from the buoyancy flux to E are 30–50% larger than that from H, indicating the potential importance of buoyancy fluxes to closing E. While the low closure highlights additional sources not captured in the flux data and random measurement errors contributing to E, the findings of the information flow and averaging length analysis are consistent with the impact of mesoscale circulations, which tend to transport more H than LE, on the lack of closure.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

Validation and Verification for INL Modelica-based TEDS models Via Experimental Results

This report provides an overview on the verification and validation (V&V) of the Thermal Energy Distribution System (TEDS) model developed in the Modelica process modeling ecosystem using experimental data. Model development has led to the creation of a dynamic process model of the experimental TEDS facility housed within the Energy Systems Laboratory (ESL) at Idaho National Laboratory (INL). The model was then used during the preconstruction phase of the experimental effort to inform experimental design (e.g., insulation requirements, bypass line placement, expected performance of components) and to test innovative control schemes prior to the initial operation. The TEDS model developed in Modelica includes the primary components of the TEDS experimental unit: a 200kW Chromalox heater; a single-tank packed-bed thermal energy storage system filled with 0.125-inch alumina (Al2O3) beads; an ethylene-glycol-to-Therminol-66 heat exchanger; system piping; five control valves; and all associated temperature, pressure, and volumetric flow sensors. Using the Institute of Electrical and Electronics Engineers (IEEE) V&V methodologies, considered the gold standard in the engineering field, the model was verified using a combination of static analysis, spatial convergence, and regression tests. Then using dynamic time warping (DTW) initial runs to validate and tune the TEDS model versus the experiment were conducted. This tuning method was accomplished using the INL Risk Analysis Virtual ENvironment (RAVEN) software package. Tuning is required to account for physical phenomena that are less understood within the empirical heat transfer correlations. Through the commencement of this work, a systems-level model of TEDS with associated control systems, sensors, piping diameters, and component capabilities has been created. This model was utilized in the pre-experimental phase to inform system design, insulation thicknesses, and potential control schemes to operate the system effectively and safely. Then, initial experimental startup and operational data were used to demonstrate the validation and tuning methodology. This process demonstrates the classical two-step approach of a model informing experimental design followed by the experiment validation and tuning the model.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2021

Frontier nonequilibrium materials science enabled by ultrafast electron methods

Most innovation in materials science and engineering resides in our ability to understand and control the intimate relationship between the structure of materials and their properties. Conventionally, the only route to discovering innovative new material properties has been to explore the structural and compositional phase space that is accessible at (or near) thermodynamic equilibrium. The characterization, manipulation and, ultimately, control of material properties far from equilibrium offers almost completely untapped possibilities for uncovering novel states and phases of materials. Investigations of materials far from equilibrium require the development of new techniques, which are capable of following dynamic processes in materials with extreme spatiotemporal resolution. Thus, ultrafast electron-based methods have become a major new frontier in materials science due to the capability of following dynamics on time scales as short as femtoseconds with the high spatial resolution and sensitivity afforded by electrons. The articles in this issue provide an exciting cross section of the novel research directions that have been enabled at this frontier. Imaging on ultrafast time scales carries critical information on phase transitions, fundamental processes involved in light-harvesting, magnetic, and plasmonic dynamics, the coupling of electronic and nuclear degrees of freedom in materials and has revealed previously hidden, nonequilibrium metastable states of matter that have no equilibrium analog.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Toward Quantum Chemical Free Energy Simulations of Platinum Nanoparticles on Titania Support

Platinum nanoparticles (Pt-NPs) supported on titania surfaces are costly but indispensable heterogeneous catalysts because of their highly effective and selective catalytic properties. Therefore, it is vital to understand their physicochemical processes during catalysis to optimize their use and to further develop better catalysts. However, simulating these dynamic processes is challenging due to the need for a reliable quantum chemical method to describe chemical bond breaking and bond formation during the processes but, at the same time, fast enough to sample a large number of configurations required to compute the corresponding free energy surfaces. Density functional theory (DFT) is often used to explore Pt-NPs; nonetheless, it is usually limited to some minimum-energy reaction pathways on static potential energy surfaces because of its high computational cost. In this work, we report a combination of the density functional tight binding (DFTB) method as a fast but reliable approximation to DFT, the steered molecular dynamics (SMD) technique, and the Jarzynski equality to construct free energy surfaces of the temperature-dependent diffusion and growth of platinum particles on a titania surface. In particular, we present the parametrization for Pt-X (X = Pt, Ti, or O) interactions in the framework of the second-order DFTB method, using a previous parametrization for titania as a basis. The optimized parameter set was used to simulate the surface diffusion of a single platinum atom (Pt 1 ) and the growth of Pt 6 from Pt 5 and Pt 1 on the rutile (110) surface at three different temperatures (T = 400, 600, 800 K). The free energy profile was constructed by using over a hundred SMD trajectories for each process. We found that increasing the temperature has a minimal effect on the formation free energy; nevertheless, it significantly reduces the free energy barrier of Pt atom migration on the TiO 2 surface and the transition state (TS) of its deposition. In a concluding remark, the methodology opens the pathway to quantum chemical free energy simulations of Pt-NPs’ temperature-dependent growth and other transformation processes on the titania support.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Computational approaches to model X-ray photon correlation spectroscopy from molecular dynamics

X-ray photon correlation spectroscopy (XPCS) allows for the resolution of dynamic processes within a material across a wide range of length and time scales. X-ray speckle visibility spectroscopy (XSVS) is a related method that uses a single diffraction pattern to probe ultrafast dynamics. Interpretation of the XPCS and XSVS data in terms of underlying physical processes is necessary to establish the connection between the macroscopic responses and the microstructural dynamics. To aid the interpretation of the XPCS and XSVS data, we present a computational framework to model these experiments by computing the X-ray scattering intensity directly from the atomic positions obtained from molecular dynamics simulations. We compare the efficiency and accuracy of two alternative computational methods: the direct method computing the intensity at each diffraction vector separately, and a method based on fast Fourier transform that computes the intensities at all diffraction vectors at once. Furthermore, the computed X-ray speckle patterns capture the density fluctuations over a range of length and time scales and are shown to reproduce the known properties and relations of experimental XPCS and XSVS for liquids.

36 MATERIALS SCIENCE↗