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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Nanocolumnar Pt:Ni Alloy Thin Films by High Pressure Sputtering for Oxygen Reduction Reaction

Self-supported nanocolumnar Pt:Ni thin films (TFs) with varying Pt:Ni atomic ratios and Pt mass loadings were produced on a microporous layer (MPL)-like surface composed of carbon particles by high pressure sputtering and examined as oxygen reduction reaction (ORR) electrocatalysts for polymer electrolyte membrane fuel cells. Cauliflower-like microstructures were observed from scanning electron microscopy imaging. Various Pt:Ni atomic ratios were obtained by simply changing the relative deposition power between Pt and Ni source and investigated by X-ray diffraction and quartz crystal microbalance analysis. Additionally, electrochemical characterization of the Pt:Ni-TF/MPL-like-layer/glassy-carbon samples was conducted through benchtop cyclic voltammetry and rotating disk electrode measurements. The electrochemically active surface area (ECSA) was found to be between 22–42 m 2 g -1 for different Pt:Ni atomic ratios. Lower Pt mass loadings exhibited a higher ECSA and the catalytic activity of all Pt:Ni ratios increased with the increase in Pt mass loading. The ORR activity of the Pt:Ni-TFs increased in the order of 3:1 < 1:1 < 1:3 with exhibiting a specific activity of 1781 μA cm -2 and mass activity of 0.66 A mg -1 for the Ni-rich film with 1:3 ratio. The catalytic performance of Pt:Ni-TFs were higher than traditional high surface area carbon supported Pt nanoparticles, elemental Pt nanorods, and Pt-Ni nanorods.

08 HYDROGEN↗
DOE OSTI2026

Translating Fundamental Insights into Ag-Based Bimetallic Electrocatalysts to Anion-Exchange Membrane Fuel Cells

To address challenges of high costs and scalability for fuel cells, it is essential to develop af-fordable and earth-abundant materials that are Pt-group-metal (PGM)-free. Recent advance-ments in PGM-free electrocatalysis for the oxygen reduction reaction (ORR) in alkaline media show that high catalyst loadings are needed to achieve high reaction rates; however, there are associated mass transport limitations. To address this issue, we develop low-loading ionomer-less Ag-bimetallic thin films by alloying with 3d block elements or Sn. We bridge knowledge from fundamental rotating disk electrode (RDE) studies to gas-diffusion cathodes in high-temperature anion-exchange membrane fuel cells (HT-AEMFCs), resulting in high-activity ORR catalysis and peak power densities up to 1.2 W cm-² geo for the Ag-Sn and Ag-Co alloys. Here, experimental post-characterization and theoretical calculations reveal small Co or Sn oxide nano-islands on Ag as likely active sites enhancing ORR activity, ultimately offering these low-loading PGM-free ORR materials as a viable path towards sustainable energy conversion.

Catalysts↗
DOE OSTI2020

Porous Membranes for Moisture Probe Protection (SR18024)

In the presence of contaminants, moisture probes can give false moisture readings and have shorter lifetimes. Absolute humidity probes are composed of porous, hygroscopic metallic oxide thin films (Al 2 O 3 ) coupled to electrodes (e.g. Au and Al). While these materials provide absolute humidity readings at low water levels at a relatively low cost, they are sensitive to the presence of contaminants, especially ammonia, due to its high hygroscopicity and ability to bind to the probe. Without frequent recalibration, especially in the presence of contaminants, the humidity levels are inaccurately reported due to measurement drift that occurs when the pores close, changing the impedance of the probe. The goal of this project was to improve the stability and reliability of these moisture probes by incorporating a molecular sieve into the probe to allow only the water to reach the sensor. Molecular sieves contain pores that allow molecule specific permeation. Two dimensional inorganic nanosheets (e.g. BN, MoO 2 , MoS 2 ) have recently been shown to behave as molecular sieves for water. Compared to zeolites, nanoporous materials have higher flux rates through the pores, leading to faster water permeation. Graphene oxide sheets have also been successfully employed for unimpeded water permeation; however, at lower humidity levels, the structure is unstable and the pores shrink, preventing the sieving of molecules. Compared to graphene oxide, sheets based off of inorganic materials, such as molybdenum, have been found to be more stable at lower humidity levels and have faster permeation rates. By creating a barrier in before the Al 2 O 3 sensor chip to prevent the permeation of contaminants into the probe, the lifetime of the probe will be extended. While the molybdenum sheets are a promising material for this application, their performance against ammonia and tritiated compounds have not yet been evaluated. This technology can be applied to other applications, including gas and liquid purification for environmental remediation, single molecule sensors, field effect transistors, and catalysts.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2025

Enhanced biochemical sensing with high- Q transmission resonances in free-standing membrane metasurfaces

Optical metasurfaces provide solutions to label-free biochemical sensing by localizing light resonantly beyond the diffraction limit, thereby selectively enhancing light–matter interactions for improved analytical performance. However, high-Q resonances in metasurfaces are usually achieved in the reflection mode, which impedes metasurface integration into compact imaging systems. Here, we demonstrate a metasurface platform for advanced biochemical sensing based on the physics of the bound states in the continuum (BIC) and electromagnetically induced transparency (EIT) modes, which arise when two interfering resonances from a periodic pattern of tilted elliptic holes overlap both spectrally and spatially, creating a narrow transparency window in the mid-infrared spectrum. We experimentally measure these resonant peaks observed in transmission mode (Q ~ 734 at λ ~ 8.8 µm) in free-standing silicon membranes and confirm their tunability through geometric scaling. We also demonstrate the strong coupling of the BIC-EIT modes with a thinly coated PMMA film on the metasurface, characterized by a large Rabi splitting (32 cm -1 ) and biosensing of protein monolayers in transmission mode. Our new photonic platform can facilitate the integration of metasurface biochemical sensors into compact and monolithic optical systems while being compatible with scalable manufacturing, thereby clearing the way for on-site biochemical sensing in everyday applications.

Rosas, Samir [Univ. of Wisconsin, Madison, WI (Uni↗
DOE OSTI2025

Ultrahigh-power-density flexible piezoelectric energy harvester based on freestanding ferroelectric oxide thin films

Flexible piezoelectric nanogenerators are emerging as a promising solution for powering next-generation flexible electronics by converting mechanical energy into electrical energy. However, traditional ferroelectric ceramics, despite their excellent piezoelectric properties, lack flexibility; while piezoelectric polymers, although highly flexible, have low piezoelectricity. The quest to develop materials that combine high piezoelectricity with exceptional flexibility has thus become a research focus. Herein, we present a breakthrough in this field with the fabrication of freestanding (111)-oriented PbZr 0.52 Ti 0.48 O 3 single crystalline thin films, which exhibit remarkable flexibility and a high converse piezoelectric coefficient (~585 pm/V). This is achieved through water-soluble sacrificial layer to relieve substrate clamping and controlling the crystal orientation to further enhance the piezoelectric response. Our nanogenerators, constructed using these freestanding nanoscale membranes, demonstrate a record-high output power density (~63.5 mW/cm 3 ), excellent flexibility (with a strain tolerance >3.4%), and superior mechanical stability in cycling tests (>60,000 cycles). These advancements pave the way for high-performance, flexible electronic devices utilizing ferroelectric oxide thin films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Fusion Blanket and Fuel Cycle Research at PNNL: FY22 Year-end Report

During the reporting period, research at PNNL focused on two tasks within the DOE Fusion Blanket and Fuel Cycle Program. Research on Task 1, Tritium Extraction from Pb-Li and He Using a Vacuum Permeator, focused on atomistic modeling to better understand tritium transport in Pd-coated V vacuum permeators. As a lower cost alternative to Pd permeators, thin coatings of Pd (or other noble metals) can be deposited over a substrate like V. However, the permeation performance of composite metal membranes degrades over time, due to the formation of intermetallics at the coating-substrate interface. Computational studies were performed to better understand tritium transport through these Pd-V intermetallics. The results of the FY22 Pd-V modeling study were recently submitted for publication in Computational Materials Science and presented at the Technology of Fusion Energy conference. Future work in this area will focus on interdiffusion barriers to prevent intermetallic formation that is deleterious to tritium transport. There are opportunities for collaboration with researchers at the Colorado School of Mines, who are manufacturing and testing candidate interdiffusion barriers. Research on Task 3, Solid Breeder Materials, included ion irradiation and post-irradiation characterization of lithium orthosilicate (Li 4 SiO 4 ) and lithium metasilicate (Li 2 SiO 3 ) to improve fundamental understanding of irradiation effects, in combination with atomistic modeling focused on the energetics of He clustering in these two ceramic phases. The results of the study suggested that the Li 4 SiO 4 phase, which is more desirable as a solid breeder due to its higher Li density, was amorphized during ion irradiation while the Li 2 SiO 3 phase appeared to be more resistant to irradiation damage. It is possible that Li loss contributed to the poor irradiation performance of the Li 4 SiO 4 , and some thoughts are provided regarding coatings that could be applied to solid breeders like this to prevent Li loss at elevated temperature while not hindering tritium diffusion. The results of the FY22 ion irradiation study were recently submitted for publication in Journal of Nuclear Materials and presented at the 22nd International Conference on Ion Beam Modification of Materials. Future work in this area will focus on Li-rich ceramics such as Li 5 AlO 4 and Li 8 ZrO 6 that have high Li density and should provide rapid tritium release based on previous work with less Li-rich ceramics.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Atomically dispersed iron sites with a nitrogen–carbon coating as highly active and durable oxygen reduction catalysts for fuel cells

Nitrogen-coordinated single atom iron sites (FeN 4 ) embedded in carbon (Fe–N–C) are the most active platinum group metal-free oxygen reduction catalysts for proton-exchange membrane fuel cells. Still, current Fe–N–C catalysts lack sufficient long-term durability and are not yet viable for practical applications. Here we report a highly durable and active Fe–N–C catalyst synthesized using heat treatment with ammonia chloride followed by high-temperature deposition of a thin layer of nitrogen-doped carbon on the catalyst surface. We propose that catalyst stability is improved by converting defect-rich pyrrolic N-coordinated FeN 4 sites into highly stable pyridinic N-coordinated FeN 4 sites. The stability enhancement is demonstrated in membrane electrode assemblies using accelerated stress testing and a long-term steady-state test (>300 h at 0.67 V), approaching a typical Pt/C cathode (0.1 mg Pt cm -2 ). The encouraging stability improvement represents a critical step in developing viable Fe–N–C catalysts to overcome the cost barriers of hydrogen fuel cells for numerous applications.

08 HYDROGEN↗
DOE OSTI2022

Three-Electrode Study of Electrochemical Ionomer Degradation Relevant to Anion-Exchange-Membrane Water Electrolyzers

Among existing water electrolysis (WE) technologies, anion-exchange-membrane water electrolyzers (AEMWEs) show promise for low-cost operation enabled by the basic solid-polymer electrolyte used to conduct hydroxide ions. The basic environment within the electrolyzer, in principle, allows the use of non-platinum-group metal catalysts and less-expensive cell components compared to acidic-membrane systems. Nevertheless, AEMWEs are still underdeveloped, and the degradation and failure modes are not well understood. To improve performance and durability, supporting electrolytes such as KOH and K 2 CO 3 are often added to the water feed. The effect of the anion interactions with the ionomer membrane (particularly other than OH – ), however, remains poorly understood. We studied three commercial anion-exchange ionomers (Aemion, Sustainion, and PiperION) during oxygen evolution (OER) at oxidizing potentials in several supporting electrolytes and characterized their chemical stability with surface-sensitive techniques. We analyzed factors including the ionomer conductivity, redox potential, and pH tolerance to determine what governs ionomer stability during OER. Specifically, we discovered that the oxidation of Aemion at the electrode surface is favored in the presence of CO 3 2– /HCO 3 – anions perhaps due to the poor conductivity of that ionomer in the carbonate/bicarbonate form. Sustainion tends to lose its charge-carrying groups as a result of electrochemical degradation favored in basic electrolytes. PiperION seems to be similarly negatively affected by a pH drop and low carbonate/bicarbonate conductivity under the applied oxidizing potential. Furthermore, the insight into the interactions of the supporting electrolyte anions with the ionomer/membrane helps shed light on some of the degradation pathways possible inside of the AEMWE and enables the informed design of materials for water electrolysis.

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

Analytical Model for Atomic Relaxation in Twisted Moiré Materials

By virtue of being atomically thin, the electronic properties of heterostructures built from two-dimensional materials are strongly influenced by atomic relaxation. The atomic layers behave as flexible membranes rather than rigid crystals. Here we develop an analytical theory of lattice relaxation in twisted moiré materials. We obtain analytical results for the lattice displacements and corresponding pseudo gauge fields, as a function of twist angle. We benchmark our results for twisted bilayer graphene and twisted WSe 2 bilayers using large-scale molecular dynamics simulations. Our single-parameter theory is valid in graphene bilayers for twist angles 𝜃 ≳ 0.7°, and in twisted WSe 2 for 𝜃 ≳ 1.6°. Furthermore, we also investigate how relaxation alters the electronic structure in twisted bilayer graphene, providing a simple extension to the continuum model to account for lattice relaxation.

36 MATERIALS SCIENCE↗
DOE OSTI2022

In Situ Tracking of Nonthermal Plasma Etching of ZIF-8 Films

Surface characterization is critical for understanding the processes used for preparing catalysts, sorbents, and membranes. Nonthermal plasma (NTP) is a process that achieves high reactivity at low temperatures and is used to tailor the surface properties of materials. In this work, we combine the capabilities of infrared reflection absorption spectroscopy (IRRAS) with NTP for the in situ interrogation of zeolitic imidazolate framework-8 (ZIF-8) thin films to probe modifications in the material induced by oxygen and nitrogen plasmas. The IRRAS measurements in oxygen plasma reveal etching of organic ligands with sequential removal of the methyl group and imidazole ring and with the formation of carbonyl moieties (C═O). In contrast, nitrogen plasma induces mild etching and grafting of nitrile groups (−C≡N). Scanning electron microscopy imaging shows that oxygen plasma, at prolonged times, significantly degrades the ZIF-8 film at the grain boundaries. Treatment of ZIF-8 membranes using mild plasma conditions yields a fivefold enhancement for H 2 /N 2 and CO 2 /CH 4 ideal selectivities and an eightfold enhancement for CO 2 /N 2 ideal selectivity. Additionally, the new tools described here can be used for spectroscopic in situ tracking of plasma-induced chemistry on thin films in general.

IRRAS↗
DOE OSTI2022

Graphene Nanopattern for Single-Crystal Film Growth, Defect Reduction and Layer Transfer

Thin film heterostructures are key building blocks for advanced electronic and optoelectronic devices. For this, direct heteroepitaxy has been pursued for decades, although it has been challenging to reduce crystal defects stemming from lattice mismatches and thermal mismatches between materials. The layer transfer method has been proposed as an alternative approach, wherein dissimilar materials are separately grown and then hetero-integrated. However, the applicability of layer transfer techniques is limited by several technical challenges, such as controllability, throughput, and damage to the substrate. Remote epitaxy, which is a recently developed method to produce single-crystalline membranes, is a promising approach but cannot be applied to elemental materials such as Si and Ge. In this work, we report graphene nanopattern as a universal template for the growth of single-crystal thin films that can be exfoliated as a freestanding form. This is realized by the chemical inertness of graphene, which allows selective nucleation at the exposed region, followed by lateral overgrowth onto graphene to form a planarized thin film. By employing graphene nanopattern, both group IV and III-V materials are utilized as the substrate as well as the epilayer. The epilayer can be exfoliated at the graphene interface because partially covered graphene effectively weakens the interface, which is corroborated by theoretical analyses of spalling theory. We reveal that graphene nanopattern not only works as a weakened interface for exfoliation, but also allows for dislocation reduction in heteroepitaxial films. This is because of the flexibility and the dangling-bond-free nature of graphene, which provides an additional path for strain relaxation. Therefore, these results represent a meaningful step toward production of high-quality single-crystal membranes that can be hetero-integrated.

Kim, Hyunseok↗
DOE OSTI2022

Graphene Nanopattern for Single-Crystal Film Growth, Defect Reduction and Layer Transfer

Thin film heterostructures are key building blocks for advanced electronic and optoelectronic devices. For this, direct heteroepitaxy has been pursued for decades, although it has been challenging to reduce crystal defects stemming from lattice mismatches and thermal mismatches between materials. The layer transfer method has been proposed as an alternative approach, wherein dissimilar materials are separately grown and then hetero-integrated. However, the applicability of layer transfer techniques is limited by several technical challenges, such as controllability, throughput, and damage to the substrate. Remote epitaxy, which is a recently developed method to produce single-crystalline membranes, is a promising approach but cannot be applied to elemental materials such as Si and Ge. In this work, we report graphene nanopattern as a universal template for the growth of single-crystal thin films that can be exfoliated as a freestanding form. This is realized by the chemical inertness of graphene, which allows selective nucleation at the exposed region, followed by lateral overgrowth onto graphene to form a planarized thin film. By employing graphene nanopattern, both group IV and III-V materials are utilized as the substrate as well as the epilayer. The epilayer can be exfoliated at the graphene interface because partially covered graphene effectively weakens the interface, which is corroborated by theoretical analyses of spalling theory. We reveal that graphene nanopattern not only works as a weakened interface for exfoliation, but also allows for dislocation reduction in heteroepitaxial films. This is because of the flexibility and the dangling-bond-free nature of graphene, which provides an additional path for strain relaxation. Therefore, these results represent a meaningful step toward production of high-quality single-crystal membranes that can be hetero-integrated.

Kim, Hyunseok↗
DOE OSTI2024

Theoretical Prediction and Experimental Verification of IrO x Supported on Titanium Nitride for Acidic Oxygen Evolution Reaction

Reducing iridium (Ir) catalyst loading for acidic oxygen evolution reaction (OER) is a critical strategy for large-scale hydrogen production via proton exchange membrane (PEM) water electrolysis. However, simultaneously achieving high activity, long-term stability, and reduced material cost remains challenging. To address this challenge, we develop a frame-work by combining density functional theory (DFT) prediction using model surfaces and proof-of-concept experimental ver-ification using thin films and nanoparticles. DFT results predict that oxidized Ir monolayers over titanium nitride (IrO x /TiN) should display higher OER activity than IrO x while reducing Ir loading. Further, this prediction is verified by depositing Ir monolayers over TiN thin films via physical vapor deposition. The promising thin film results are then extended to commercially viable powder IrO x /TiN catalysts, which demonstrate a lower overpotential and higher mass activity than commercial IrO 2 , and a long-term stability of 250 hours to maintain a current density of 10 mA cm -2 . The superior OER performance of IrO x /TiN is further confirmed using proton exchange membrane water electrolyzer (PEMWE), which shows a lower cell voltage than commercial IrO 2 to achieve a current density of 1 A cm -2 . Both DFT and in situ X-ray absorption spectroscopy reveal that the high OER performance of IrO x /TiN strongly depends on the IrO x - TiN interaction via direct Ir-Ti bonding. This study highlights the importance of close interaction between theoretical prediction based on mechanistic understanding and experimental verification based on thin film model catalysts to facilitate the development of more practical powder IrO x /TiN catalysts with high activity and stability for acidic OER.

08 HYDROGEN↗
DOE OSTI2022

Absolute vs Convective Instabilities and Front Propagation in Lipid Membrane Tubes

Here we analyze the stability of biological membrane tubes, with and without a base flow of lipids. Membrane dynamics are completely specified by two dimensionless numbers: the well-known Föppl-von Kármán number Γ and the recently introduced Scriven-Love number SL, respectively quantifying the base tension and base flow speed. For unstable tubes, the growth rate of a local perturbation depends only on Γ, whereas SL governs the absolute versus convective nature of the instability. Furthermore, nonlinear simulations of unstable tubes reveal an initially localized disturbance result in propagating fronts, which leave a thin atrophied tube in their wake. Depending on the value of Γ, the thin tube is connected to the unperturbed regions via oscillatory or monotonic shape transitions-reminiscent of recent experimental observations on the retraction and atrophy of axons. We elucidate our findings through a weakly nonlinear analysis, which shows membrane dynamics may be approximated by a model of the class of extended Fisher-Kolmogorov equations. Our study sheds light on the pattern selection mechanism in axonal shapes by recognizing the existence of two Lifshitz points, at which the front dynamics undergo steady-to-oscillatory bifurcations.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Solvent Processing and Ionic Liquid-Enabled Long-Range Vertical Ordering in Block Copolymer Films with Enhanced Film Stability

Rapid and reliable processing methods for forming ordered block copolymer (BCP) materials with low defect density in a thin film geometry are required for many nanotechnology applications. Vertically aligned BCP structures, in particular, have applications ranging from nanolithography for electronics and photonics to nanoporous membranes for water remediation and novel batteries for flexible electronics. However, the attainment of nearly complete vertical orientational order of the BCP ordered phase remains challenging. Solvent-based techniques, such as direct immersion annealing (DIA) and solvent vapor annealing (SVA), have immense potential for these applications of BCP films, as it allows for tuning of their thermodynamic, structural, and chain mobility driven kinetic properties. We first demonstrate that DIA, using a judicious choice of binary solvent mixtures along with a relatively hydrophobic ionic liquid (IL), induces the rapid vertical ordering in polystyrene-b-polymethylmethacrylate (PS-PMMA) block copolymer in lamellar films. The IL establishes synergy to the binary solvent mixture of toluene and heptane to 1 create a near-neutral solvent environment for attaining the vertical microstructure of the BCP. Next, we show that IL can suppress the de-wetting of the PS-PMMA films to achieve long-range order using SVA in cylindrical films for long annealing times. Both vertical and horizontal morphology is attained in these films by selecting different solvent environments. Furthermore, attaining enhanced vertical and horizontal BCP structure with long-range defect-free order by tuning solvent quality and using additives like IL can render them useful for many nanotech applications.

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

ACS Spotlight: Bipolar Membranes for Electrochemical Energy Conversion, Chemical Manufacturing, and Separations

Sustainable energy conversion, chemical manufacturing, and separations are central to addressing the world’s energy and environmental challenges. Electrochemical platforms stand as a cornerstone in addressing these challenges because they are low exergy and can be powered on renewable electrons. In electrochemical systems, bipolar membranes (BPMs) are emerging as a unique class of ion exchange membranes poised to revolutionize various electrochemical processes via pH control of anode and cathode chambers and in situ pH adjustment. In this Spotlight Review, we provide a comprehensive review of electrochemical platforms utilizing BPMs for energy conversion (water electrolyzers for hydrogen production, fuel cells, and flow batteries), chemical manufacturing (electrolyzers that convert carbon dioxide into value-added chemicals and nitrate into ammonia), and separations. The motivation for using BPMs, as well as their performance and durability, in electrochemical platforms are disseminated. We also discuss current challenges that impede BPM electrochemical systems from competing with state-of-the-art electrochemical systems using monopolar ion-exchange membranes (e.g., anion/hydroxide exchange membranes and cation/proton exchange membranes). Here, the review also covers molecular modeling and continuum modeling efforts to understand the basic mechanisms that govern BPM performance.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2021

Role of transferred graphene on atomic interaction of GaAs for remote epitaxy

Remote epitaxy is a recently discovered type of epitaxy, wherein single-crystalline thin films can be grown on graphene-coated substrates following the crystallinity of the substrate via remote interaction through graphene. Although remote epitaxy provides a pathway to form freestanding membranes by controlled exfoliation of grown film at the graphene interface, implementing remote epitaxy is not straightforward because atomically precise control of interface is required. Here, we unveil the role of the graphene–substrate interface on the remote epitaxy of GaAs by investigating the interface at the atomic scale. By comparing remote epitaxy on wet-transferred and dry-transferred graphene, we show that interfacial oxide layer formed at the graphene–substrate interface hinders remote interaction through graphene when wet-transferred graphene is employed, which is confirmed by an increase of interatomic distance through graphene and also by the formation of polycrystalline films on graphene. On the other hand, when dry-transferred graphene is employed, the interface is free of native oxide, and single-crystalline remote epitaxial films are formed on graphene, with the interatomic distance between the epilayer and the substrate matching with the theoretically predicted value. The first atomic layer of the grown film on graphene is vertically aligned with the top layer of the substrate with these atoms having different polarities, substantiating the remote interaction of adatoms with the substrate through graphene. These results directly show the impact of interface properties formed by different graphene transfer methods on remote epitaxy.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Impact of large-scale effects on mass transfer and concentration polarization in Reverse Osmosis membrane systems

Here, we present well-resolved computational fluid dynamics simulations of a large-scale reverse osmosis membrane-spacer configuration (1 m). Our computational model solves the flow and transport equations with variable solute-dependent properties. We utilize a high resolution computational mesh to resolve all relevant length scales associated with spacer-induced mixing and thin concentration boundary layers. An important contribution of this work is the development of a modified mass-transfer correlation that accounts for the development of the concentration boundary layer along the channel. A set of 2D axisymmetric simulations were performed for a spiral wound module layer with varying cross-flow conditions and spacer diameters which indicate a significant entrance length effect for concentration profile development at lower flow rates while mixing effects dominate at higher flow rates. The mass-transfer correlations at higher flow rates compare well with published correlations while a surrogate model for Sherwood number was obtained that depends on an additional similarity variable that accounted for entrance length effects at lower flow rates. Finally, a large-scale membrane-spacer design relevant to high-pressure reverse osmosis is studied with a non-uniform arrangement of spacers, which indicate a substantial saving in pressure drop (~40%) compared to traditional uniformly spaced pattern with minor variations (~2%) in concentration polarization, product water quality (~1%) and water recovery (~7%) compared to a uniform spacer pattern.

97 MATHEMATICS AND COMPUTING↗