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At least 1,207 records · Page 67
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Flexible dynamic boundary microgrid operation considering network and load unbalances

Flexible microgrids with dynamic boundaries have recently been introduced in the literature. With the ability to reconfigure the topology of the microgrids dynamically through remotely controlled switches, flexible microgrids with dynamic boundaries can further improve the resiliency and energy efficiency of microgrids with distributed energy resources (DERs). This paper focuses on the optimal operation considering one of the predominant characteristics of microgrids and distribution systems – unbalanced networks and loads. In existing literature, balanced modeling of microgrids is more common due to its attractive simplicity. The three-phase power unbalance has not been considered as a constraint on the generation units in a microgrid. Further, negative sequence constraints have also been neglected. In this article, we propose a set of constraints that is specifically related to the capabilities of inverter interfaced resources to supply unbalanced current/power when the microgrid is islanded from the main distribution grid. We incorporate the new set of constraints into two optimization formulations leveraging two convex relaxations of the three-phase power flow equations: mixed-integer linear programming (MILP) and mixed-integer semidefinite programming (MISDP) that optimize the dispatch of controllable switches and DERs in the microgrid. The algorithms are then extended to networked microgrids with grid-forming sources. We test the algorithms on a realistic community microgrid model in Puerto Rico as well as standardized IEEE distribution test feeders. The testing results demonstrate the performance of the proposed algorithms. The MILP is fast and scalable, and the MISDP enforces the negative sequence voltage constraints.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2026

Design trade-offs for residential retail tariffs and virtual power plants

Retail rate design and virtual power plants (VPPs) have the potential to shift customer electricity demand and provide economic benefits to utility customers. As the adoption of distributed energy resources (DERs) and flexible loads increases, retail tariff and program design can impact Bonbright's rate design principles including affordability, fairness, and economic efficiency. We model the effects of residential retail rates and VPP programs on power system costs in Massachusetts under a potential future system with high renewable energy and DER adoption. We model interactions among retail rate design, demand flexibility, and utility costs and identify trade-offs across different rate designs and VPP programs. We estimate that time-of-use (TOU) rates and VPP programs designed to avoid critical peak rates can lower overall system costs by 3.5 %-4.8 %. These lower costs translate to lower electricity bills for 62 %-91 % of customers, depending on the scenario. Although TOU rates with a critical peak VPP program can benefit all customer segments and are economically efficient, a VPP program with flat rates leads to the lowest overall bills for customers. We find that customers with loads that align with peak demand and who participate in critical peak VPP programs can underpay for their contribution to utility costs and shift costs to other customers. While our assumptions about mandatory TOU and/or critical peak pricing likely impact the magnitude of the results, the results highlight the trade-offs of these tariffs and programs and the importance of tariff and program design as demand becomes more flexible and responsive.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2022

Pressure dependent magnetic properties on bulk CrBr 3 single crystals

The van der Waals class of materials offer an approach to two-dimensional magnetism as their spin fluctuations can be tuned upon exfoliation of layers. Moreover, it has recently been shown that spin-lattice coupling and long-range magnetic ordering can be modified with pressure in van der Waals materials. In this work, the magnetic properties of quasi two-dimensional CrBr 3 are reported applying hydrostatic pressure. The application of pressure up to 0.844 GPa shows a 1.77% decrease in saturation magnetization with a decrease in the Curie temperature from 33.05 to 30.41 K. Density functional theory calculations with pressure up to 1 GPa show a reduction in volume and interplanar distance as pressure increases. To further understand the magnetic properties with applied pressure, the magnetocrystalline anisotropy energy (MAE) and exchange coupling parameter (J) are calculated. There is small decrease in MAE and the first nearest neighbor interaction (J 1 ) (U = 2.7 eV and J = 0.7 eV) is increasing with respect to increasing pressure. Overall, CrBr 3 displays ferromagnetic interlayer coupling and the calculated exchange coupling and MAE parameters match well with the observations from the experimental work.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Unexpected Conformational Behavior of Poly(poly(ethylene glycol) methacrylate)-Poly(propylene carbonate)-Poly(poly(ethylene glycol) methacrylate) (PPEGMA-PPC-PPEGMA) Amphiphilic Block Copolymers in Micellar Solution and at the Air-Water Interface

Self-assemblies (bulk micelles and Langmuir monolayers) of a new amphiphilic block copolymer, poly(poly(ethylene glycol) methacrylate)-poly(propylene carbonate)-poly(poly(ethylene glycol) methacrylate) (PPEGMA-PPC-PPEGMA), were investigated in dilute aqueous solution and at the air-water interface. An important feature that distinguishes PPEGMA-PPC-PPEGMA from conventional linear PEG-based polymer surfactants is that in the hydrophilic PPEGMA block the PEG moieties exist as side chains attached to the poly(methacrylate) (PMA) backbone. This study was performed using three PPEGMA-PPC-PPEGMA samples having an identical PPC molecular weight (5.6 kDa) and different PPEGMA molecular weights (7.3, 2.8 and 2.1 kDa on either side) (named as “G7C6G7”, “G3C5G3”, and “G2C6G2”, respectively). Cryo-TEM images revealed that G2C6G2 micelles form large clusters, whereas G7C6G7 micelles exist as unimers; the behavior of G3C6G3 is intermediate. The average core surface distance between two adjacent G2C6G2 micelles in a cluster was estimated to be only about 3.7 nm, which suggests that in the micelle corona layer only the PEG side chains (but not the PMA backbone) exist as hydrated brush chains. A calculation showed that at the small micelle separation distance observed, the van der Waals attractive forces become significant, and are thus likely responsible for the cluster formation. Steady shear rheometry measurements confirmed that G2C6G2 micelles are indeed only weakly bound in clusters, and these micelle clusters are thus prone to breakup even under mild steady shear. The micelle breakup force estimated from the shear viscosity data is in reasonable agreement with an estimate obtained from the theoretical van der Waals interaction potential. Langmuir monolayers formed at the air-water interface by PPEGMA-PPC-PPEGMA were also characterized; surface pressure-area measurements were performed. Detailed features of the surface pressure-area isotherms further supported that only the PEG side chains are hydrated in the subphase, while the PMA backbone remains unhydrated and situated on the water surface.

Lee, Jaewon↗
DOE OSTI2022

Layered topological semimetal GaGeTe: New polytype with non-centrosymmetric structure

GaGeTe is a layered van der Waals material composed of germanene and GaTe sublayers that has been recently predicted to be a basic Z 2 topological semimetal. To date, only one polytype of GaGeTe is known with trigonal centrosymmetric structure (α phase, space group R-3m, No. 166). Here we show that as-grown samples of GaGeTe show traces of at least another polytype with hexagonal non-centrosymmetric structure (β phase, space group P6 3 mc, No. 186). Moreover, we suggest that another bulk hexagonal polytype (γ phase, space group P-3m1, No. 164) could also be found near room conditions. Both α and β polytypes have been identified and characterized by means of X-ray diffraction and Raman scattering measurements with the support of ab initio calculations. We provide the vibrational properties of both polytypes and show that the Raman spectrum reported for GaGeTe almost forty years ago and attributed to the α phase, was, in fact, that of the secondary β phase. Additionally, we show that a Fermi resonance occurs in α-GaGeTe under non-resonant excitation conditions, but not under resonant excitation conditions. Theoretical calculations show that bulk β-GaGeTe is a non-centrosymmetric weak topological semimetal with even smaller lattice thermal conductivity than centrosymmetric bulk α-GaGeTe. In perspective, our work paves the way for the control and engineering of GaGeTe polytypes to design and implement complex van der Waals heterostructures formed by a combination of centrosymmetric and non-centrosymmetric layers of up to three different polytypes in a single material, suitable for a number of fundamental studies and technological applications.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Dispersion of modified fumed silica in elastomeric nanocomposites

In polymer nanocomposites, surface modification of silica aggregates can shield Coulombic interactions that inhibit agglomeration and formation of a network of agglomerates. Surface modification is usually achieved with silane coupling agents although carbon-coating during pyrolytic silica production is also possible. Pyrogenic silica with varying surface carbon contents were dispersed in styrene-butadiene (SBR) rubber to explore the impact on hierarchical dispersion, the emergence of meso-scale structures, and the rheological response. Pristine pyrogenic silica aggregates at concentrations above a critical value (related to the Debye screening length) display correlated meso-scale structures and poor filler network formation in rubber nanocomposites due to the presence of silanol groups on the surface. Here, in the present study, flame synthesized silica with sufficient surface carbon monolayers can mitigate the charge repulsion thereby impacting network structural emergence. The impact of the surface carbon on the van der Waals enthalpic attraction, a * , is determined. The van der Waals model for polymer nanocomposites is drawn through an analogy between thermal energy, k B T, and the accumulated strain, γ. The rheological response of the emergent meso-scale structures depends on the surface density of both carbon and silanol groups.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Closing the Gap Between Modeling and Experiments in the Self-assembly of Biomolecules at Interfaces and in Solution

Molecular self-assembly is a powerful tool in materials design, wherein non-covalent interactions like electrostatic, hydrophobic, hydrogen bonding, and van der Waals can be exploited to produce supramolecular nanostructures that are functional and highly tunable. Biomolecules are attractive building blocks, as they are biocompatible, biodegradable and adopt a wide array of higher order structures. Moreover, naturally occurring protein systems display a manifold of structures and interactions that can be replicated in synthetic biomolecules. In this perspective, we highlight advances in multiscale simulation techniques across broad spatiotemporal scales that can aid in characterizing self-assembly of hybrid and hierarchical bionanomaterial systems, with an emphasis on physics-based simulation approaches currently employed to study biomolecules at mineral interfaces. The power of these approaches is highlighted across a few recent areas where molecular simulations have advanced our understanding of self-assembly spanning peptides to protein self-assembly. Looking forward, we discuss how in the near future emerging methods in statistical and machine learning will advance this research field in all areas from expanding the capabilities of physics-based simulation methods to enabling new analyses of high throughput experiments. These advances will pave the way for understanding the molecular recognition patterns in systems that are dictated by self-assembly - biomineralizing peptides, hierarchical peptoids, and large protein assemblies, and will aid in the development of a new synthesis science for achieving precise molecular control in materials design

Sampath, Janani↗
DOE OSTI2023

Effect of Flexibility in Molecular Simulations of Carbon Dioxide Adsorption and Diffusion in a Cuprous Triazolate Framework

Using fixed atom force fields to model gas adsorption in flexible metal organic frameworks (MOFs) is known to pose difficulties in accurately reproducing and predicting experimental results. This paper studies the difference in accuracy between flexible and fixed atom force fields in reproducing CO 2 adsorption measurements in MAF-2 ([Cu(etz)]∞ (MAF-2, Hetz) 3,5-diethyl-1,2,4-triazole), an NbO-type triazolate scaffold with a bcu cavity system and attached ethyl groups. The flexible force field used to run the hybrid molecular dynamics and grand canonical Monte Carlo calculations were generated using the QuickFF software incorporating van der Waals parameters from the Universal Force Field (UFF) and density derived electrostatic and chemical (DDEC) charges. The fixed atom force field used was composed of UFF van der Waals parameters together with DDEC charges. The calculations were run at 298 K and at pressures of 0.1, 0.3, 0.5, 0.8, and 1 bar. It was observed that for this MOF the rigid force field overpredicted gas adsorption, whereas the flexible force field values closely matched experimental data. In the flexible structure, the freely moving ethyl groups of MAF-2 hindered adsorption, reducing the interaction energy between CO 2 and the N atoms of the triazolate framework as well as reducing the size of the largest cavity diameter. Here, the combination of these factors led to improved prediction of adsorption values with the flexible forcefield as compared to the rigid forcefield, demonstrating the need for modeling MOFs in a way more indicative of their behavior.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Proximity-Induced Exchange Interaction: A New Pathway for Quantum Sensing Using Spin Centers in Hexagonal Boron Nitride

Defects in hexagonal boron nitride (hBN), a two-dimensional van der Waals material, have attracted a great deal of interest because of its potential in various quantum applications. Due to hBN’s two-dimensional nature, the spin center in hBN can be engineered in the proximity of the target material, providing advantages over its three-dimensional counterparts, such as the nitrogen-vacancy center in diamond. Here, in this work, we propose a novel quantum sensing protocol driven by exchange interaction between the spin center in hBN and the underlying magnetic substrate induced by the magnetic proximity effect. By first-principles calculation, we demonstrate that the induced exchange interaction dominates over the dipole–dipole interaction by orders of magnitude when in the proximity. The interaction remains antiferromagnetic across all stacking configurations between the spin center in hBN and the target van der Waals magnets. Additionally, we explored the scaling behavior of the exchange field as a function of the spatial separation between the spin center and the targets.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Unexpected Thermomechanical Behavior of Off-Stoichiometry Epoxy/Amine Materials

Recent studies on off-stoichiometric thermosets reveal unique viscoelastic behavior derived from increased free volume and physical interactions between chain ends. To understand structural characteristics arising from cure and its effect on properties, we developed a Monte Carlo model based on step-growth polymerization. Our model accurately predicted structure–property trends for a two-component system of EPON 828 (EPON) and ethylenediamine. A second epoxy monomer, D.E.R. 732 (DER), was investigated to modulate T g . Binary mixtures of EPON and DER in off-stoichiometric, amine-rich formulations resulted in nonlinear evolution of thermomechanical properties with respect to initial formulation stoichiometry. Modifying our model with kinetic parameters allowing for differential epoxide/amine reaction kinetics only partially accounted for trends in T g , suggesting that spatiotemporal contributions-not captured by our model-were significant determinants of material properties compared to polymer architecture for three-component systems. Here these findings underpin the importance of spatial awareness in modeling to inform the development of dynamic thermosets.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Anomalous Second Harmonic Generation from Atomically Thin MnBi 2 Te 4

MnBi 2 Te 4 is a van der Waals topological insulator with intrinsic intralayer ferromagnetic exchange and A-type antiferromagnetic (AFM) interlayer coupling. Theoretically, it belongs to a class of structurally centrosymmetric crystals whose layered antiferromagnetic order breaks inversion symmetry for even layer numbers, making optical second harmonic generation (SHG) an ideal probe of the coupling between the crystal and magnetic structures. Here, we perform magnetic field and temperature dependent SHG measurements on MnBi 2 Te 4 flakes ranging from bulk to monolayer thickness. We find that the dominant SHG signal from MnBi 2 Te 4 is unexpectedly unrelated to both magnetic state and layer number. As a result, we suggest that surface SHG is the likely source of the observed strong SHG, whose symmetry matches that of the MnBi 2 Te 4 -vacuum interface. Our results highlight the importance of considering the surface contribution to inversion symmetry breaking in van der Waals centrosymmetric magnets.

2D magnets↗
DOE OSTI2023

Enhanced Exciton-to-Trion Conversion by Proton Irradiation of Atomically Thin WS 2

Defect engineering of van der Waals semiconductors has been demonstrated as an effective approach to manipulate the structural and functional characteristics toward dynamic device controls, yet correlations between physical properties with defect evolution remain underexplored. Here, using proton irradiation, we observe an enhanced exciton-to-trion conversion of the atomically thin WS 2 . The altered excitonic states are closely correlated with nanopore induced atomic displacement, W nanoclusters, and zigzag edge terminations, verified by scanning transmission electron microscopy, photoluminescence, and Raman spectroscopy. Density functional theory calculation suggests that nanopores facilitate formation of in-gap states that act as sinks for free electrons to couple with excitons. The ion energy loss simulation predicts a dominating electron ionization effect upon proton irradiation, providing further evidence on band perturbations and nanopore formation without destroying the overall crystallinity. This study provides a route in tuning the excitonic properties of van der Waals semiconductors using an irradiation-based defect engineering approach.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2024

Spin–Phonon Coupling and Magnetic Transition in an Organic Molecule Intercalated Cr 2 Ge 2 Te 6

The manipulation of spin–phonon coupling in both formations and explorations of magnetism in two-dimensional van der Waals ferromagnetic semiconductors facilitates unprecedented prospects for spintronic devices. The interlayer engineering with spin–phonon coupling promises controllable magnetism via organic cation intercalation. Here, in this study, spectroscopic evidence reveals the intercalation effect on the intrinsic magnetic and electronic transitions in quasi-two-dimensional Cr 2 Ge 2 Te 6 using tetrabutyl ammonium (TBA + ) as the intercalant. The temperature evolution of Raman modes, E g 3 and A g 1 , along with the magnetization measurements, unambiguously captures the enhancement of the ferromagnetic Curie temperature in the intercalated heterostructure. Moreover, the E g 4 mode highlights the increased effect of spin–phonon interaction in magnetic-order-induced lattice distortion. Combined with the first-principle calculations, we observed a substantial number of electrons transferred from TBA + to Cr through the interface. The interplay between spin–phonon coupling and magnetic ordering in van der Waals magnets appeals for further understanding of the manipulation of magnetism in layered heterostructures.

2D magnet↗
DOE OSTI2026

Enhanced Interlayer Coupling and Excitons in Twin-Stacked Two-Dimensional Magnetic CrSBr Bilayers

The degree of electronic coupling between individual layers in van der Waals heterostructures offers a route to engineer their magnetic, electronic, and optical functionalities. Using state-of-the-art first-principles calculations, in this work, we demonstrate that the electronic coupling between two monolayers of CrSBr─an anisotropic two-dimensional magnetic semiconductor─is highly nonlinear and nonmonotonic with respect to their relative twist angle, exhibiting a pronounced maximum at the twin-stacking configuration. The coupling strength scales with both the degree of overlap of Br orbitals adjacent to the van der Waals gap and the cosine of half of the interlayer spin angle. This enhanced interlayer electronic coupling leads to excitons delocalized across the two layers, with a polarization dependence that reflects the interlayer spin alignment. Our results reveal a sensitive interplay among twist angle, magnetism, and excitonic properties in twin-stacked CrSBr bilayers, suggesting twin stacking as an effective means for engineering interlayer coupling.

2D materials↗
DOE OSTI2022

Atomic-Scale Insights into the Lateral and Vertical Epitaxial Growth in Two-Dimensional Pd 2 Se 3 –MoS 2 Heterostructures

Two-dimensional (2D) materials form heterostructures in both the lateral and vertical directions when two different materials are interfaced, but with totally different bonding mechanisms of covalent in-plane to van der Waal’s layered interactions. Understanding how the competition between lateral and vertical forces influences the epitaxial growth is important for future materials development of complex mixed layered heterostructures. In this study, we use atomic-resolution annular dark-field scanning transmission electron microscopy to study the detailed atomic arrangements at mixed 2D heterostructure interfaces composed of two semiconductors with distinctly different crystal symmetry and elemental composition, Pd 2 Se 3 :MoS 2 , in order to understand the role of different chemical bonds on the resultant epitaxy. Pd 2 Se 3 is grown off the step edge in bilayer MoS 2 , and the vertical and lateral epitaxial relationships of the Pd 2 Se 3 –MoS 2 heterostructures are investigated. We find that the similarity of geometry at the interface with one metal (Pd or Mo) atoms bonded with two chalcogens (S or Se) are the crucial factors to make the atomically stitched lateral junction of 2D heterostructures. In addition, the vertical van der Waal interactions that are normally dominant in layered materials can be overcome by in-plane forces if the interfacial atomic stitching is high in quality and low in defect density. This knowledge should help guide the approaches for improving the epitaxy in mixed 2D heterostructures and seamless stitching of in-plane 2D heterostructures with various complex monolayer structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Deterministic Fabrication of Large-Area, High-Crystallinity Oxide Moiré Superlattices

Oxide twistronics extends moiré engineering beyond van der Waals materials, offering a promising platform for accessing emergent interfacial phenomena arising from the strong coupling of lattice, charge, and orbital degrees of freedom in complex oxides. However, deterministic fabrication of high-crystallinity oxide moiré superlattices over large lateral dimensions remains challenging due to the three-dimensional bonding network of oxides. Here, we demonstrate a scalable, generalized fabrication strategy that enables the formation of high-crystallinity oxide moiré superlattices with clean, chemically bonded interfaces and precisely controlled twist angles down to nominal values of 0.1°, achieving subdegree twist-angle accuracy across large contiguous lateral dimensions approaching the millimeter scale. Using NaNbO3 as a model system, we show that the resulting interlayer coupling drives pronounced structural reconstruction that modifies both the phase structure and ferroelectric domain configuration. Synchrotron-based X-ray 3D reciprocal space mapping reveals the emergence of a single-phase state in twisted bilayers, in contrast to the mixed-phase structure observed in single-layer membranes prior to twist assembly. The structural signatures are further consistent with gradual lattice rotation distributed along the thickness direction that may accommodate interfacial shear strain, distinct from reconstruction observed in van der Waals moiré systems which primarily occurs through in-plane stacking rearrangement. This collective lattice response is correlated with twist-dependent nanoscale electromechanical modulations observed by piezoresponse force microscopy. These results establish a scalable materials platform for oxide twistronics and support the implementation of twist-engineered functionalities in practical, macroscale device architectures.

Ghanbari, Reza [North Carolina State University (N↗
DOE OSTI2020

Electronically Coupled 2D Polymer/MoS 2 Heterostructures

Emergent quantum phenomena in electronically coupled two-dimensional heterostructures are central to next-generation optical, electronic, and quantum information applications. Tailoring electronic band gaps in coupled heterostructures would permit control of such phenomena and is the subject of significant research interest. Two-dimensional polymers (2DPs) offer a compelling route to tailored band structures through the selection of molecular constituents. However, despite the promise of synthetic flexibility and electronic design, fabrication of 2DPs that form electronically coupled 2D heterostructures remains an outstanding challenge. Here, we report the rational design and optimized synthesis of electronically coupled semiconducting 2DP/2D transition metal dichalcogenide van der Waals heterostructures, demonstrate direct exfoliation of the highly crystalline and oriented 2DP films down to a few nanometers, and present the first thickness-dependent study of 2DP/MoS 2 heterostructures. In this work, control over the 2DP layers reveals enhancement of the 2DP photoluminescence by two orders of magnitude in ultrathin sheets and an unexpected thickness-dependent modulation of the ultrafast excited state dynamics in the 2DP/MoS 2 heterostructure. These results provide fundamental insight into the electronic structure of 2DPs and present a route to tune emergent quantum phenomena in 2DP hybrid van der Waals heterostructures.

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

Hard Ferromagnetism Down to the Thinnest Limit of Iron-Intercalated Tantalum Disulfide

Two-dimensional (2D) magnetic crystals hold promise for miniaturized and ultralow power electronic devices that exploit spin manipulation. In these materials, large, controllable magnetocrystalline anisotropy (MCA) is a prerequisite for the stabilization and manipulation of long-range magnetic order. In known 2D magnetic crystals, relatively weak MCA typically results in soft ferromagnetism. Furthermore, we demonstrate that ferromagnetic order persists down to the thinnest limit of Fe x TaS 2 (Fe-intercalated bilayer 2H-TaS 2 ) with giant coercivities up to 3 T. We prepare Fe-intercalated TaS 2 by chemical intercalation of van der Waals-layered 2H-TaS 2 crystals and perform variable-temperature transport, transmission electron microscopy, and confocal Raman spectroscopy measurements to shed new light on the coupled effects of dimensionality, degree of intercalation, and intercalant order/disorder on the hard ferromagnetic behavior of Fe x TaS 2 . More generally, we show that chemical intercalation gives access to a rich synthetic parameter space for low-dimensional magnets, in which magnetic properties can be tailored by the choice of the host material and intercalant identity/amount, in addition to the manifold distinctive degrees of freedom available in atomically thin, van der Waals crystals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗