Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “phase engineering”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,729 records · Page 96
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Synthesis, Processing, and Use of Isotopically Enriched Epitaxial Oxide Thin Films

Isotopic engineering has emerged as a key approach to study the nucleation, diffusion, phase transitions, and reactions of materials at an atomic level. It aims to uncover mass transport pathways, kinetics, and operational and failure mechanisms of functional materials and devices. Understanding these phenomena leads to deeper insights into important physical processes, such as the transport of ions in energy conversion and storage devices and the role of active sites and supports during heterogeneous catalytic reactions. Likewise, isotopic engineering is being pursued as a means of modifying functionality to enable future technological applications. In this Account, we summarize our recent work employing isotope labeling (e.g., 18 O 2 and 57 Fe) during thin film synthesis and postgrowth processing to reveal growth mechanisms, defect chemistry, and elemental diffusion under working and extreme conditions. Isotope-resolved analysis techniques with nanometer-scale spatial resolution, such as time-of-flight secondary ion mass spectrometry and atom probe tomography, facilitate the accurate quantification of isotopic placement and concentration in our well-defined heterostructures with precisely positioned, isotope-enriched layers. By measuring the nanometer-scale redistribution between natural abundance and isotopically enriched oxygen layers during the deposition of Fe 2 O 3 and Cr 2 O 3 by molecular beam epitaxy, we identified intermixing processes driven by surface adatoms occurring both at the film growth surface and within the first few layers below the surface. Further insights into synthesis mechanisms were gained by studying the tungsten oxide thin films grown by evaporating WO 3 powder in the presence of background 18 O 2 , revealing minimal incorporation of background oxygen during the film formation process. Thermal and radiation-enhanced diffusion in epitaxial Fe and Cr oxides were precisely tracked using 18 O and 57 Fe tracer layers incorporated into model epitaxial oxide thin films. This approach has allowed us to access thermal diffusion behavior at lower temperatures than previously measured, revealing a potential changeover in diffusion mechanism. Understanding radiation-enhanced diffusion in model oxides that represent the surface layers on the structural components of nuclear reactors informs our understanding of their corrosion behavior under irradiation. Isotopic labeling can also provide unique insights into the surface exchange reactions and defect chemistry of electrocatalysts. For instance, tracking the change in 18 O concentration at the surface of an epitaxial LaNiO 3 thin film after the electrocatalytic oxygen evolution reaction revealed the participation of lattice oxygen, confirming a hypothesis that had been proposed previously. Lastly, we highlight a new direction wherein we perform in situ processing studies utilizing isotopic tracers in conjunction with model epitaxial thin films within the atom probe tomography instrument. Additionally, this Account illustrates the great potential of isotopic engineering to enable fundamental mechanistic insights into physical processes and engineer functional properties in epitaxial films, heterostructures, and superlattices.

36 MATERIALS SCIENCE↗
NASA NTRS2020

Europa Clipper Mission: Onward to Implementation!

Europa, the fourth largest moon of Jupiter, is believed to be one of the best places in the solar system to look for extant life beyond Earth, and the Europa Clipper Mission is preparing to investigate. Now entering the fifth year of life as a formal project, the Europa Clipper Mission has completed its major design phase and is preparing for implementation. The mission envisions sending a flight system, consisting of a spacecraft equipped with a payload of NASA-selected scientific instruments, to execute numerous flybys of Europa while in Jupiter orbit. The mission is being formulated and implemented by a joint Jet Propulsion Laboratory (JPL) and Applied Physics Laboratory (APL) Project team. At the Project Preliminary Design Review (PDR) in August 2018, it was determined that the unique integration of the REASON instrument’s antennas with the spacecraft’s solar arrays required additional time for maturity and review. The required maturity was demonstrated, and in June 2019 the Clipper Mission passed its Delta Preliminary Design Review, and formal confirmation for entry into Phase C was announced by NASA in August 2019. The earliest launch opportunity is now in July 2023, one year later than originally planned. The Project is now working toward its Critical Design Review in May 2020. This paper will describe the progress of the Europa Clipper Mission since January 2019, including maturation of the spacecraft, subsystem and instrument designs, the beginning of fabrication of engineering models and the first flight models, planning for the verification & validation phase, and planning for operations.

Jackson, Maddalena↗
NASA NTRS1998

Space Shuttle Main Engine Quantitative Risk Assessment: Illustrating Modeling of a Complex System with a New QRA Software Package

During 1997, a team from Hernandez Engineering, MSFC, Rocketdyne, Thiokol, Pratt & Whitney, and USBI completed the first phase of a two year Quantitative Risk Assessment (QRA) of the Space Shuttle. The models for the Shuttle systems were entered and analyzed by a new QRA software package. This system, termed the Quantitative Risk Assessment System(QRAS), was designed by NASA and programmed by the University of Maryland. The software is a groundbreaking PC-based risk assessment package that allows the user to model complex systems in a hierarchical fashion. Features of the software include the ability to easily select quantifications of failure modes, draw Event Sequence Diagrams(ESDs) interactively, perform uncertainty and sensitivity analysis, and document the modeling. This paper illustrates both the approach used in modeling and the particular features of the software package. The software is general and can be used in a QRA of any complex engineered system. The author is the project lead for the modeling of the Space Shuttle Main Engines (SSMEs), and this paper focuses on the modeling completed for the SSMEs during 1997. In particular, the groundrules for the study, the databases used, the way in which ESDs were used to model catastrophic failure of the SSMES, the methods used to quantify the failure rates, and how QRAS was used in the modeling effort are discussed. Groundrules were necessary to limit the scope of such a complex study, especially with regard to a liquid rocket engine such as the SSME, which can be shut down after ignition either on the pad or in flight. The SSME was divided into its constituent components and subsystems. These were ranked on the basis of the possibility of being upgraded and risk of catastrophic failure. Once this was done the Shuttle program Hazard Analysis and Failure Modes and Effects Analysis (FMEA) were used to create a list of potential failure modes to be modeled. The groundrules and other criteria were used to screen out the many failure modes that did not contribute significantly to the catastrophic risk. The Hazard Analysis and FMEA for the SSME were also used to build ESDs that show the chain of events leading from the failure mode occurence to one of the following end states: catastrophic failure, engine shutdown, or siccessful operation( successful with respect to the failure mode under consideration).

Smart, Christian↗
DOE OSTI2016

Benchmark problems for numerical implementations of phase field models

Here, we present the first set of benchmark problems for phase field models that are being developed by the Center for Hierarchical Materials Design (CHiMaD) and the National Institute of Standards and Technology (NIST). While many scientific research areas use a limited set of well-established software, the growing phase field community continues to develop a wide variety of codes and lacks benchmark problems to consistently evaluate the numerical performance of new implementations. Phase field modeling has become significantly more popular as computational power has increased and is now becoming mainstream, driving the need for benchmark problems to validate and verify new implementations. We follow the example set by the micromagnetics community to develop an evolving set of benchmark problems that test the usability, computational resources, numerical capabilities and physical scope of phase field simulation codes. In this paper, we propose two benchmark problems that cover the physics of solute diffusion and growth and coarsening of a second phase via a simple spinodal decomposition model and a more complex Ostwald ripening model. We demonstrate the utility of benchmark problems by comparing the results of simulations performed with two different adaptive time stepping techniques, and we discuss the needs of future benchmark problems. The development of benchmark problems will enable the results of quantitative phase field models to be confidently incorporated into integrated computational materials science and engineering (ICME), an important goal of the Materials Genome Initiative.

36 MATERIALS SCIENCE↗
NASA NTRSDate not supplied

Development Of 12 Kw Hall Thrusters for Nasa Lunar Gateway Power And Propulsion Element

NASA is embarking on a new and exciting era of human exploration to the Moon and its vicinity. As part of the Artemis program, NASA is developing the lunar Gateway, an orbiting platform in Near Rectilinear Halo Orbit (NRHO) about the Moon. The first two elements of the Gateway will be launched in 2024 and will be transferred from a high Earth orbit to the NRHO via the use of Solar Electric Propulsion (SEP). The SEP system is located on the Power and Propulsion Element (PPE) which will provide up to 60 kW of power. Aerojet Rocketdyne is developing the 12 kW flight Hall thrusters to be used for transferring the PPE and the Habitat and Logistics Outpost (HALO) module to NRHO. Work is continuing on the development testing, and these results will be described in the paper. The project has also recently completed its critical design review and is transitioning to the fabrication of the three flight thrusters. The 12 kW Hall thrusters are being developed under the NASA Advanced Electric Propulsion System (AEPS) program. As part of the AEPS program, AR built two Engineering Test Unit (ETU) Hall thrusters based on the NASA HERMeS engine and conducted a series of tests to validate the design. One series of tests focused on environments and functional testing, while the other series was mainly focused on long duration test blocks. Results from these tests were used to compare with NASA data from the HERMeS thruster. The comparison of the test results showed no change to the critical performance and lifetime parameters, which enabled the next phase of the program to proceed. Currently testing is continuing on the Engineering Development Units (EDU) as part of the flight qualification activities. The successful completion of the CDR serves as the starting point for the flight thruster fabrication, which is now underway to support the launch of the Gateway modules in 2024. The development and qualification of the AEPS 12 kW Hall thrusters is proceeding well. As the program completes CDR and transitions into flight builds of the three 12 kW Hall thrusters that will be used to transfer the combined PPE and HALO stack to the NRHO, the results of the development testing are reviewed in this presentation, and a status of the program and future plans is provided. The launch of the two Gateway modules and the transfer of this large payload to cislunar orbit will mark a significant milestone in electric propulsion, as it will demonstrate the ability of SEP to transfer very large payloads over great distances. This establishes the precedent that could lead to cargo transfers from the Earth to the Moon or even to Mars using SEP.

Hall thrusters↗
NASA NTRS2015

Next-Generation Ion Thruster Design Tool

Computational tools that accurately predict the performance of electric propulsion devices are highly desirable and beneficial to NASA and the broader electric propulsion community. The current state of the art in electric propulsion modeling relies heavily on empirical data and numerous computational "knobs." In Phase I of this project, Tech-X Corporation developed the most detailed ion engine discharge chamber model that currently exists. This kinetic model simulates all particles in the discharge chamber along with a physically correct simulation of the electric fields. In addition, kinetic erosion models are included for modeling the ion-impingement effects on thruster component erosion. In Phase II, Tech-X developed a user-friendly computer program for NASA and other governmental and industry customers. Tech-X has implemented a number of advanced numerical routines to bring the computational time down to a commercially acceptable level. NASA now has a highly sophisticated, user-friendly ion engine discharge chamber modeling tool.

Stolz, Peter↗
DOE OSTI2020

Phase-field simulations of intergranular fission gas bubble behavior in U 3 Si 2 nuclear fuel

U 3 Si 2 is a potential accident-tolerant fuel that shows promise due to its high thermal conductivity and higher uranium density relative to UO 2 . However, its swelling and fission gas release behavior in light water reactor (LWR) conditions is relatively unknown. To provide mechanistic insight and determine parameters for engineering-scale fuel performance modeling of pellet-form U 3 Si 2 , phase-field simulations of the growth, interconnection, and venting of intergranular fission gas bubbles were performed. The fractional coverage of the grain boundary and the fraction of bubble area that is vented were calculated as a function of time. From the simulation data, the fractional grain boundary coverage at saturation, an important parameter needed in engineering-scale modeling of swelling and fission gas release, was determined. Multiple simulations were run to determine the uncertainty in the calculated value. Finally, the effect of model assumptions and input parameters that are not well known was evaluated. Simulation results are compared to related theoretical and computational work. Based on the simulation results, a value of 0.60 for the fractional grain boundary coverage at saturation is recommended for U 3 Si 2 fuel.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2021

Thermodynamic properties of the Nd-Bi system via emf measurements, $\mathrm{DFT}$ calculations, machine learning, and $\mathrm{CALPHAD}$ modeling

Thermodynamic properties of the Nd-Bi system were investigated using a combination of experimental measurements, first-principles calculations based on density functional theory (DFT), data mining and machine learning (DM + ML) predictions, and calculation of phase diagrams (CALPHAD) modeling. The electromotive force (emf) of Nd-Bi alloys in molten LiCl-KCl-NdCl 3 at 773–973 K was measured via coulometric titration of Nd into Bi for the determination of thermochemical properties such as activity coefficients and solubilities of Nd in Bi. A new peritectic reaction of [liquid + NdBi 2 = Nd 3 Bi 7 ] at 774 K was confirmed using differential scanning calorimetry, structural (X-ray diffraction), and microstructural (scanning electron microscopy) analyses. The unknown crystal structure of NdBi2 was suggested to be a mixture of the anti-La 2 Sb configuration and the La 2 Te-type configuration based on ML predictions for over 26,000 data-mined AB 2 -type configurations together with DFT-based verifications. Using the newly acquired experimental data and DFT-based calculations, the thermodynamic description of the Nd-Bi system was remodeled, and a more complete Nd-Bi phase diagram was calculated, including the Nd 3 Bi 7 compound, invariant transition reactions, and liquidus temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Direct observation on supersonic microprojectile penetration of carbon fiber composites with ultrafast synchrotron X-ray phase contrast imaging

High-speed penetration into carbon fiber composites is of fundamental importance to materials science and impact engineering, but research along this line suffers considerably from the lack of direct experimental observations. Here we investigate such penetration dynamics of a unidirectional carbon fiber reinforced epoxy (UCFRE) composite, with a combination of in situ, ultrafast, synchrotron phase contrast imaging and finite element (FE) analysis. The experiments yield the first direct observation on projectile trajectories and fiber-scale deformation and damage in the UCFRE composites during supersonic microprojectile penetration, for different fiber orientations (0 degrees - 90 degrees from the impact direction) and projectile velocities (600 - 850 m.s -1 ), at unprecedented temporal (~ 100 ps) and spatial (5 μm) resolutions. The maximum penetration depth decreases with increasing fiber orientation angles, as a result of anisotropic damage evolution in the composite sample. Strain localizations are prone to develop along a direction perpendicular to the fiber orientation, while the damage or cavity region, along the fiber direction. FE modeling with a three-dimensional Hashin criterion yields consistent projectile trajectory and cavity morphology with the experimental results. Finally, with increasing fiber orientation angles, damage analyses show a transition in the damage mode from fiber compression to matrix compression damage, in line with the increasing maximum penetration depth.

36 MATERIALS SCIENCE↗
NASA NTRSDate not supplied

NASA Hybrid Thermally Efficient Core (HyTEC) Project Overview

The HyTEC Project goal is to mature compact core aircraft engine technologies as part of the NASA-led Sustainable Flight National Partnership (SFNP) that contributes to the United States Aviation Climate Action Plan goal for aviation net zero carbon emissions by 2050. Individual technologies were proposed and defined by industry partners, and then selected technology maturation activities are cost shared. Phase 1 contracts were awarded in 2021, and since then, the project has matured numerous technologies to TRL 4-5. Some of the Phase 1 efforts have been completed with successful results. An initial systems analysis study has been completed to estimate the impact of Phase 1 technologies against project performance metrics, with results indicating significant progress is being made toward meeting those metrics. HyTEC will culminate with an engine core demonstration that will integrate many of the Phase 1 technologies into a large-scale integrated ground demonstration that will achieve TRL 6 to enable industry to transition the technologies into the next single-aisle engine architecture. The goal of the demonstration is to fully meet the project performance metrics that define a compact engine core with substantial improvements in efficiency, durability, performance, hybridization, and sustainability over the baseline established in the year 2020. HyTEC will also incorporate hybrid electric technology into the engine core with the intention to bring the first electrified turbofan engines into production. The core demonstration has been awarded with a cost-share partnership and is planned to occur by the end of 2028.

Anthony L Nerone↗
DOE OSTI2026

Bypassing the yellow phase for extremely stable formamidinium lead iodide perovskite solar cells

INTRODUCTION Formamidinium lead iodide (FAPI) emerged as an ideal material for single-junction perovskite solar cells owing to its near optimal bandgap of 1.45 to 1.5 eV and outstanding thermal stability. However, the photoactive cubic α-phase (3C-FAPI) of FAPI is structurally unstable and undergoes a reconstructive phase transition to the nonperovskite yellow hexagonal δ-phase (2H-FAPI) at ambient temperature. The phase reconstruction from 3C-FAPI to 2H-FAPI could be prevented by alloying methylammonium (MA) or Cs or both at the A-site and Br at the halide site, but this limits long-term durability owing to phase segregation or materials instability. Addressing these challenges requires a rational design strategy to stabilize 3C-FAPI by restricting lattice reconstruction without compromising thermal stability. RATIONALE Two main strategies have emerged to improve the phase stability and film quality of FAPI. Here, the first is a lattice-templating approach, which enables the slow formation of perovskite but it eventually degrades through the formation of yellow phases. The second approach, which has been widely explored, involves additive engineering, using alkyl ammonium halide or mostly chloride-based additives, which provide better control over the crystallization route. However, the FAPI films fabricated using these additives are often alloyed and compromise long-term stability. Moreover, the exact role of Cl has been unclear and speculative, specifically when Cl-based additives are used. Even after using a high additive concentration, the incorporation of Cl in perovskite lattice is rare. RESULTS Guided by synergistic modeling and experimental studies, we developed a coadditive strategy using 15 mol % FACl and 0.5 mol % BA 2 PbI 4 perovskites in combination (where BA is butylammonium) to enable a highly oriented (100) Cl-doped FAPI film with exceptional durability. Synchrotron-based in situ wide-angle x-ray scattering revealed a favorable transition for the coadditive-treated FAPI (FAPI-CA) to the corner-sharing 3C black phase through a progressive transformation through the 2H, 4H, 6H, and 8H phases. Moreover, solid-state 35 Cl nuclear magnetic resonance (NMR) revealed Cl incorporation in the perovskite lattice and, as predicted by modeling, indicated that Cl plays a key role in altering the energetics of both the formation and degradation pathways. The Cl-doped perovskite can completely bypass the expected and energetically favorable degradation pathway via the yellow phase or the 2H-PbI 2 phase. Instead, it undergoes degradation only upon exposure to harsh conditions such as 15-sun illumination and 90°C through the energetically uphill 3R-PbI 2 phase path. A p-i-n device fabricated with FAPI-CA film demonstrated a power conversion efficiency (PCE) of 25.1% with an average of 24.1% (40 devices). The notable film stability translated to other devices and retained 98% of its initial PCE under open-circuit conditions at 85° ± 5°C for 1200 hours. CONCLUSION Our study highlights the decisive role of chloride in regulating both the formation and degradation pathways. This regulation is critical for creating a perovskite film with commercially relevant durability.

Garai, Rabindranath [Rice Univ., Houston, TX (Unit↗
DOE OSTI2021

Understanding Melting and Freezing/Solidification and the Associated Effects on Heat Transfer

Phase change (solid/liquid or liquid/vapor) is a process that enables a large heat transfer capacity for a broad range of engineering applications, such as steam generators, condensers, thermal energy storage, thermosyphons, and heat pipes. Water has been the most usable phase-change cooling medium. Advanced applications are proposing alkali metals (e.g., sodium, potassium, lithium) to increase the total heat transfer rate for moderate and high-temperature applications. Sodium and potassium have the advantage of relatively low melting temperatures (98 and 64°C, respectively) make them suitable for high-temperature applications (700–1100 K). At too high of temperatures (above 1200 K), lithium is an excellent choice as a working fluid because of its high latent heat vaporization and high-surface tension. The low vapor pressure of alkali metals makes them candidates for closed spaces applications, such as thermosyphons and heat pipes. Lithium has the lowest vapor pressure than potassium and sodium, which can work at a much higher temperature without over-pressurizing the container.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
NASA NTRS1982

A technology data base for the design of 500 to 5000-lb thrust class liquid rocket engines utilizing hydrogen and oxygen as propellants

This paper presents an overview of the results of experimental evaluations of candidate designs for igniters, injectors, and propellant-cooled thrust chambers applicable to restartable high-performance, high-reliability upper-stage engines and to pulsing-type reaction control engines (RCE). Injection element types best suited for liquid, gas, and liquid/gas phase propellant supply are identified. The resulting interactions between element type, combustion efficiency, and chamber wall heating are compared. The distinction between thrust chamber design requirements for upper stage vs RCE applications as measured by cycle life requirements is translated into design configurations consisting of all-film-cooled, all-regeneratively-cooled, and composites of the two cooling approaches. The validity of the design approaches is confirmed by data from engine durability testing involving over 90,000 starts and 9,000 thermal cycles on RCE-type designs and multiple long-duration burns (up to 2,000 sec) on regeneratively cooled upper-stage designs.

Schoenman, L.↗
DOE OSTI2017

Characterization of Rare Earth Element Minerals in Coal Utilization Byproducts

The United States currently produces over 100 million tons of coal utilization byproducts (CUB) per year in the form of fly ash, bottom ash, slag, and flue gas (American Coal Ash Association (ACCA), 2015). But this “waste material” also contains potentially useful levels of rare earth elements (REE). Rare earth elements are crucial for many existing and emerging technologies, but the U.S. lacks a domestic, sustainable REE source. Our project explored the possibility of developing a supply of REEs for U.S. technologies by extracting REEs from CUBs. This work offers the potential to reduce our dependence on other countries for supply of these critical elements (NETL, REE 2016 Project Portfolio). Geologic and diagenetic history, industrial preparation methods, and the specific combustion process all play major roles in the composition of CUB. During combustion, inorganic mineral phases of coal particles are fluidized at temperatures higher than 1400oC, so inorganic mineral materials are oxidized, fused, disintegrated, or agglomerated into larger spherical and amorphous (non-crystalline) particles. The original mineralogy of the coal-containing rock and heating/cooling of the material significantly affects the composition and morphology of the particles in the combustion byproduct (Kutchko and Kim, 2006). Thus, different types of coal/refuse/ash must be characterized to better understand mineral evolution during the combustion process. Our research focused on developing a working model to address how REE minerals behave during the combustion process: this research should help determine the most effective engineering methods for extracting REEs from CUBs. We used multimodal imaging and image processing techniques to characterize six rock and ash samples from different coal power plants with respect to morphology, grain size, presence of mineral phases, and elemental composition. The results of these characterization activities provided thresholds for realizing the occurrence of REE mineral phases in CUB and allowed us to calculate structural and volumetric estimates of REE. Collectively, the rock and coal ash samples contained minerals such as quartz, kaolinite, muscovite/illite, iron oxide (as hematite or magnetite), mullite, and clinochlore. Trace minerals included pyrite, zircon, siderite, rutile, diopside, foresterite, gypsum, and barite. We identified REE phosphate minerals monazite (Ce,La,Nd,Th)(PO 4 ,SiO 4 ), xenotime (YPO 4 ,SiO 4 ), and apatite (Ca 5 (PO 4 ) 3 (F,Cl,OH) via SEM and electron microprobe analysis: these materials generally occurred as 1-10 μm-long crystals in the rock and ash samples. As has been shown in other studies, amorphous material-aluminosilicate glass or iron oxyhydroxide-are the major components of coal fly and bottom ash. Trace amounts of amorphous calcium oxide and mixed element (e.g., Al-Si-Ca-Fe) slag are also present. Quartz, mullite, hematite, and magnetite are the crystalline phases present. We found that REEs are present as monomineralic grains dispersed within the ash, as well as fused to or encapsulated by amorphous aluminosilicate glass particles. Monazite and xenotime have relatively high melting points (>1800 °C) compared to typical combustion temperatures; our observations indicate that the REE-phosphates, which presumably contribute a large percentage of REE to the bulk ash REE pool, as measured by mass spectroscopy, are largely unaltered by the combustion. Our study shows that conventional coal combustion processes sequester REE minerals into aluminosilicate glass phases, which presents a new engineering challenge for extracting REE from coal ash. The characterization work summarized in this report provides a semi-quantitative assessments of REE in coal-containing rock and CUB. The data we obtained from 2- and 3-D imaging, elemental mapping, volumetric estimates, and advanced high-resolution pixel classification successfully identified the different mineral phases present in CUB. Further, our characterization results can guide techniques for extracting REEs from CUB, or other geologic and engineered materials. Whilst, interpretations will inform future REE separation and extraction techniques and technologies practical for commercial utilization of combustion byproducts generated by power plants.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2025

Optomechanical self-organization in a mesoscopic atom array

Increasing the number of particles in a system often leads to qualitative changes in its properties, such as breaking of symmetries and the appearance of phase transitions. This renders a macroscopic system fundamentally different from its individual microscopic constituents. Lying between these extremes, mesoscopic systems exhibit microscopic fluctuations that influence behaviour on longer length scales, leading to critical phenomena and dynamics. Therefore, tracing the properties of well-controlled mesoscopic systems can help bridge the gap between an exact description of few-body microscopic systems and the emergent description of many-body systems. Here we explore the mesoscopic signatures of an optomechanical self-organization phase transition using arrays of cold atoms inside an optical cavity. By precisely engineering atom–cavity interactions, we reveal how critical behaviour depends on the atom number, identify characteristic dynamical behaviours in the self-organized regime and observe a finite optomechanical susceptibility at the critical point. These findings advance our understanding of particle-number- and time-resolved properties of phase transitions in mesoscopic systems.

Atomic and molecular interactions with photons↗
DOE OSTI2023

Development and evaluation of a skeletal mechanism for EHN additized gasoline mixtures in large Eddy simulations of HCCI combustion

Advanced Low Temperature Combustion modes, such as the Sandia proposed Additive-Mixing Fuel Injection (AMFI), can unlock significant potential to boost fuel conversion efficiency and ultimately improve the energy conversion of internal combustion engines. This is a novel improved combustion process that is enabled by supplying small (<5%) variable amounts of autoignition improver to the fuel to enhance the engine operation and control. Common, diesel-fuel ignition-quality enhancing additive, 2-ethylexyl nitrate (EHN), is doped into gasoline to enable Sandia LTGC + AMFI combustion. This manuscript focuses on the development of a reduced sub-mechanism for EHN chemical kinetics at engine relevant conditions that is implemented into a skeletal mechanism for chemical kinetic studies of gasoline surrogate fuels. The mechanism validation utilized zero-dimensional numerical simulations and comparison to shock tube ignition-delay data of pure and EHN-doped n-heptane. Additional validation is presented with Homogeneous Charge Compression-Ignition (HCCI) engine data of pure and EHN-doped research-grade E10 gasoline. Then, the mechanism was deployed in a 3-D computational fluid dynamics (CFD) using Large Eddy Simulations (LES) to model the HCCI engine experiments of 0.4% vol EHN additized E10 gasoline at several equivalence ratios. Simulations showed a very good performance of the mechanism, and the model accurately reproduced (a) the ignition point, (b) combustion phasing, (c) combustion duration, and (d) the peak of the heat release rates of the engine experiments. The results show that EHN promotes Low-Temperature Heat Release, ultimately driving the gasoline to autoignite at thermodynamic conditions where the fuel would not otherwise ignite. Overall, this work demonstrates a viable reduced chemical-kinetic mechanism for EHN and shows that it can be combined with a skeletal gasoline mechanism for CFD-LES analysis of well-mixed LTGC that matches well with experimental results. The CFD-LES analysis also shows the spatial distribution of EHN-fuel interactions that control the autoignition throughout the combustion chamber.

Guleria, Gaurav↗
DOE OSTI2023

Phase-field simulation of domain size effect on dielectric and piezoelectric responses in K 0.5 Na 0.5 NbO 3 epitaxial thin films with superdomain structures

Size effects of mesoscale ferroelectric domains on the macroscopic dielectric and piezoelectric responses in domain-engineered bulk piezocrystals have been extensively studied for more than two decades. However, less is known about the domain size effects in ferroelectric epitaxial thin films, especially for films consisting of low-symmetry ferroelectric phases and exhibits hierarchical superdomain structures. Herein, using phase-field simulations, we systemically evaluate the effective out-of-plane dielectric and piezoelectric coefficients, κ$^{*}_{33}$ and d$^{*}_{33}$, as a function of the domain periods for two types of superdomain structures in ferroelectric K 0.5 Na 0.5 NbO 3 epitaxial thin films. In one type of the superdomain structures, we find that more than 70% increase of κ$^{*}_{33}$ and nearly 20% increase of d$^{*}_{33}$ can be achieved by tuning the domain period by a few tens of nanometers. Dissimilar behaviors are found when the domain period varies along different lateral directions, suggesting anisotropic domain size effects in thin films. By analyzing the local dielectric and piezoelectric responses from each domain variants and domain walls, we reveal that the domain size effect is governed by the variation of out-of-plane polarization inside the domains. Moreover, we also demonstrate enhanced domain size effects by modulating the misfit strains and temperature to approach the polymorphic phase boundaries, suggesting tunability of the size effect by doping and strain engineering. Furthermore, our results reveal multimodal domain size dependence of dielectric and piezoelectric responses in low-symmetry ferroelectric epitaxial thin films, implying that domain size engineering can be used to tune macroscopic properties of thin-film ferroelectrics, similar to their bulk counterparts.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Perspective—Emergent Phases in Rare Earth Nickelate Heterostructure

The prediction of high T c superconductivity in layers of LaNiO 3 through orbital engineering has led to extensive research efforts over the last fifteen years. During this period, a plethora of thin films and heterostructures based rare-earth nickelate family with perovskite structure has been synthesized and explored. Here, in this short perspective, we briefly review the complexity of bulk RENiO 3 , spotlighting several recent findings of emergent phenomena in heterostructures containing the interface between RENiO 3 and another transition metal oxide. Finally, we outline potentially interesting future directions linked to time-domain dynamics to harness new Mott and topological phases in artificial structures of RENiO 3 .

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗