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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 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Second-order nonlinear optical metamaterials: ABC-type nanolaminates

We demonstrate a concept for second-order nonlinear metamaterials that can be obtained from non-metallic centrosymmetric constituents with inherently low optical absorption. The concept is based on iterative atomic-layer deposition of three different materials, A = Al{sub 2}O{sub 3}, B = TiO{sub 2}, and C = HfO{sub 2}. The centrosymmetry of the resulting ABC stack is broken since the ABC and the inverted CBA sequences are not equivalent—a necessary condition for non-zero second-order nonlinearity. In our experiments, we find that the bulk second-order nonlinear susceptibility depends on the density of interfaces, leading to a nonlinear susceptibility of 0.26 pm/V at a wavelength of 800 nm. ABC-type nanolaminates can be deposited on virtually any substrate and offer a promising route towards engineering of second-order optical nonlinearities at both infrared and visible wavelengths.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Ab Initio Study of the Interface of the Solid-State Electrolyte Li 9 N 2 Cl 3 with a Li-Metal Electrode

Ab initio molecular dynamics study of an electrochemical interface between a solid-state-electrolyte Li 9 N 2 Cl 3 and a Li-metal is performed to analyze interphase formation due to electrochemical instability. An external electric field is applied to emulate the interface under charging conditions. At open circuit, we observed electrochemical stability at both sides of the interface, without electrolyte decomposition, or new phase formation, in good agreement with reported experiments. However, a structural re-accommodation takes place at both sides of the interface. Some oxidation of the surface Li-atoms takes place when Cl and N anions capture Li from the metal to complete octahedral structures, a Bader charge and electron density analyses explain the difference between Cl and N behavior. At the interface with the Li 9 N 2 Cl 3 (002) facet, the larger negative charge on low-coordinated N atoms is able to retain more Li atoms than the Cl atoms do. At charging conditions applying an external electric field (0.5 V/Å), no major changes are observed in atomic diffusion or structural changes at the interphase; however, the main difference is the change of the Li-metal crystal structure. Due to migration of Li from the metal anode to the electrolyte, vacancies are created on the anode, causing a structural re-arrangement at the anode surface.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Understanding the Role of Temperature and Cathode Composition on Interface and Bulk: Optimizing Aluminum Oxide Coatings for Li-Ion Cathodes

Here, surface coating of cathode materials with Al 2 O 3 has been shown to be a promising method for cathode stabilization and improved cycling performance at high operating voltages. However, a detailed understanding on how coating process and cathode composition changes the chemical composition, morphology and distribution of coating within cathode interface and bulk lattice, is still missing. In this study, we use a wet-chemical method to synthesize a series of Al 2 O 3 -coated LiNi 0.5 Co 0.2 Mn 0.3 O 2 and LiCoO 2 cathodes treated under various annealing temperatures and a combination of structural characterization techniques to understand the composition, homogeneity and morphology of coating layer and the bulk cathode. Nuclear magnetic resonance and electron microscopy results reveal that the nature of the interface is highly depended on the annealing temperature and cathode composition. For Al 2 O 3 -coated LiNi 0.5 Co 0.2 Mn 0.3 O 2 , higher annealing temperature leads to more homogeneous and more closely attached coating on cathode materials, corresponding to better electrochemical performance. Lower Al 2 O 3 coating content is found to be helpful to further improve the initial capacity and cyclability, which can greatly outperform the pristine cathode material. For Al 2 O 3 -coated LiCoO 2 , the incorporation of Al into the cathode lattice is observed after annealing at high temperatures, implying the transformation from “surface coatings” to “dopants”, which is not observed for LiNi 0.5 Co 0.2 Mn 0.3 O 2 . As a result, Al 2 O 3 -coated LiCoO 2 annealed at higher temperature shows similar initial capacity but lower retention compared to that annealed at a lower temperature, due to the intercalation of surface alumina into the bulk layered structure forming a solid solution.

25 ENERGY STORAGE↗
DOE OSTI2015

Capacitance-voltage characteristics of (Al/Ti)/Al{sub 2}O{sub 3}/n-GaN MIS structures

The capacitance-voltage characteristics of (Al/Ti)/Al{sub 2}O{sub 3}/n-GaN metal—insulator-semiconductor (MIS) structures are measured and analyzed. n-Type GaN films are grown on sapphire (0001) substrates by the metal-organic chemical vapor deposition method. An aluminum-oxide layer with a thickness of 60 nm is deposited onto the surface of GaN by the method of atomic-layer deposition from the gas phase. Metallic contacts are deposited by the electron-beam evaporation of titanium and aluminum in vacuum. According to the measurement results, the breakdown-field strength of the oxide, its dielectric constant, and the integrated electron density of states at the oxide-semiconductor interface are 5 × 10{sup 6} V/cm, 7.5, and 3 × 10{sup 12} cm{sup −2}, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2020

(S)TEM/EDS study of native precipitates and irradiation induced Nb-rich platelets in high-burnup M5®

We have investigated microstructure and microchemistry of precipitates and dislocation loops in high-burnup M5® using (scanning) transmission electron microscopy ((S)TEM) equipped with energy dispersive X-ray spectroscopy (EDS). Two (S)TEM lamellae were made by cryo-FIB from the same cladding sample. The Nb-rich native precipitates were found in the metal, in the suboxide and in the oxide. Upon diffraction analysis, most of the Nb-rich native precipitates in the metal matrix remain as β-Nb phase, while no β-Nb native precipitates were found in the oxide. Nearby the oxide and metal (O/M) interface, the native precipitates in the oxide were already oxidized into t-NbO2 phase. At further distance away from the O/M interface, the Nb-rich native precipitates were gradually oxidized and became amorphous. Besides the native precipitates, Nb-rich irradiation induced precipitates (IIPs) were found in the metal matrix. Using g = <0002> vector for imaging, the length of the IIPs was aligned with dislocation loops. However, no Nb segregation to the dislocation loops themselves was observed. For the first time, we report that IIPs indeed exist in the oxide but only within about 1.5 µm away from O/M interface. However, the oxidation state of the IIPs in the oxide is still unclear. The presence of both native precipitates and IIPs in the oxide may indicate the Nb concentration in the oxide solid solution remain low nearby the O/M interface, which may explain the reduced corrosion kinetics of in-pile M5®. On the other hand, no IIPs were observed in the oxide at further distance and this may indicate that they have eventually dissolved back into the oxide. A few mechanisms related to IIPs stability in the oxide are presented.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2014

Effects of Oxygen-Containing Functional Groups on Supercapacitor Performance

Molecular dynamics (MD) simulations of the interface between graphene and the ionic liquid 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMIM OTf) were carried out to gain molecular-level insights into the performance of graphene-based supercapacitors and, in particular, determine the effects of the presence of oxygen-containing defects at the graphene surface on their integral capacitance. The MD simulations predict that increasing the surface coverage of hydroxyl groups negatively affects the integral capacitance, whereas the effect of the presence of epoxy groups is much less significant. The calculated variations in capacitance are found to be directly correlated to the interfacial structure. Indeed, hydrogen bonding between hydroxyl groups and SO3 anion moieties prevents BMIM+ and OTf- molecules from interacting favorably in the dense interfacial layer and restrains the orientation and mobility of OTf- ions, thereby reducing the permittivity of the ionic liquid at the interface. The results of the molecular simulations can facilitate the rational design of electrode materials for supercapacitors.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2014

Properties of the In{sub 2}O{sub 3}-Si interface: An ab initio study of a model geometry

The In{sub 2}O{sub 3}(001)-Si(001) heterojunction is studied by means of the ab initio density functional theory, quasiparticle corrections, and the supercell method. We construct a model interface based on the idea of a coincidence lattice, only Si-O interface bonds and biaxially strained In{sub 2}O{sub 3}. The properties of the interface and their consequences for the junction are mainly described in terms of electronic band levels and charge redistribution. The results indicate a type II heterostructure caused by interface dipole alteration due to electron rearrangements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

First-principles study of graphene under c-HfO{sub 2}(111) layers: Electronic structures and transport properties

We investigated the electronic properties, stability, and transport of graphene under c-HfO{sub 2}(111) layers by performing first-principles calculations with special attention to the chemical bonding between graphene and HfO{sub 2} surfaces. When the interface of HfO{sub 2}/graphene is terminated by an O layer, the linear dispersion of graphene is preserved and the degradation of transport is suppressed. For other interface structures, HfO{sub 2} is tightly adsorbed on graphene and the transport is strictly limited. In terms of the stability of the interface structures, an O-terminated interface is preferable, which is achieved under an O-deficient condition.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Effects of nitrogen on the interface density of states distribution in 4H-SiC metal oxide semiconductor field effect transistors: Super-hyperfine interactions and near interface silicon vacancy energy levels

The performance of silicon carbide (SiC)-based metal-oxide-semiconductor field-effect transistors (MOSFETs) is greatly enhanced by a post-oxidation anneal in NO. These anneals greatly improve effective channel mobilities and substantially decrease interface trap densities. In this work, we investigate the effect of NO anneals on the interface density of states through density functional theory (DFT) calculations and electrically detected magnetic resonance (EDMR) measurements. EDMR measurements on 4H-silicon carbide (4H-SiC) MOSFETs indicate that NO annealing substantially reduces the density of near interface SiC silicon vacancy centers: it results in a 30-fold reduction in the EDMR amplitude. The anneal also alters post-NO anneal resonance line shapes significantly. EDMR measurements exclusively sensitive to interface traps with near midgap energy levels have line shapes relatively unaffected by NO anneals, whereas the measurements sensitive to defects with energy levels more broadly distributed in the 4H-SiC bandgap are significantly altered by the anneals. Using DFT, we show that the observed change in EDMR linewidth and the correlation with energy levels can be explained by nitrogen atoms introduced by the NO annealing substituting into nearby carbon sites of silicon vacancy defects.

42 ENGINEERING↗
DOE OSTI2023

Charge trapping at Al/Al 2 ⁢O 3 interface facilitates hydrogen-induced superabundant metal vacancy formation

Hydrogen is a ubiquitous impurity that deteriorates the metal/oxide interface, causing challenges such as current leakage and blistering. Here, in this work, we uncover the mechanisms behind hydrogen-induced damage at the prototypical Al/A⁢l 2 ⁢O 3 interface. The precursor that we identify is charge trapping at the interface, which facilitates hydrogen segregation to the interface plane as H – . Presence of hydrogen increases aluminum vacancy concentration at the interfacial metal and oxide planes. Each Al vacancy can trap multiple hydrogen atoms. As a result, the interfacial aluminum vacancy and hydrogen concentrations increase by orders of magnitude. Additionally, hydrogen behaves as an n-type dopant and increases electronic conductivity at the oxide layer adjacent to the interface by forming H + , and the [V Al –H] complex is the predominant defect at this layer. These findings provide the precursor mechanisms of hydrogen-induced damage at the metal-oxide interfaces and have implications for advancing Al/A⁢l 2 ⁢O 3 –based coatings and electronic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Parabolic oxidation kinetics of a plutonium alloy at room temperature

Auger electron spectroscopy (AES) was used to characterize the oxide scale formed on an ~0.5 wt. % Ga plutonium metal alloy. Analysis of changes to the characteristic Auger transitions for Pu and O during sputter depth profiles were used in combination with previously measured sputter yields to determine the thickness of the oxide scale and provide insight into its chemistry. From this data, the parabolic kinetics for the Pu-Ga alloy were determined as a function of relative humidity, RH = 14–92%, and oxygen partial pressure, p(O 2 ) = 31–157 Torr, at 25°C. For each relative humidity investigated, the parabolic rate constant was found to be independent of oxygen partial pressure. Furthermore, power-law fits to the relative humidity effect on the rate constant was found to have a power of n = 1.265, indicating the diffusion species which rate-limits the reaction isn’t molecular water, which subsequent dissociates at the oxide-metal interface, or hydroxyl from dissociation at the gas-surface interface.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Epitaxial ferromagnetic oxide thin films on silicon with atomically sharp interfaces

A bottleneck in the integration of functional oxides with silicon, either directly grown or using a buffer, is the usual formation of an amorphous interfacial layer. Here, we demonstrate that ferromagnetic CoFe{sub 2}O{sub 4} films can be grown epitaxially on Si(111) using a Y{sub 2}O{sub 3} buffer layer, and remarkably the Y{sub 2}O{sub 3}/Si(111) interface is stable and remains atomically sharp. CoFe{sub 2}O{sub 4} films present high crystal quality and high saturation magnetization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Oxide-nitride heteroepitaxy for low-loss dielectrics in superconducting quantum circuits

Superconducting qubits show great promise for the realization of fault-tolerant quantum computing, but lossy, amorphous dielectrics limit current technology. Identifying highly crystalline and stoichiometric dielectrics with intrinsically low microwave loss is therefore a central materials challenge, yet experimentally validated platforms remain scarce. In this work, we integrate a crystalline dielectric into a heteroepitaxial TiN/$γ$-Al$_2$O$_3$/TiN trilayer grown via pulsed laser deposition. Correlative high-resolution imaging, diffraction, and spectroscopy measurements confirm the single-crystal quality and chemical integrity of all layers, with minimal defects and limited anion interdiffusion across the oxide-nitride interfaces. Using microwave lumped-element resonators with parallel-plate capacitors, we report the first direct measurement of the dielectric loss of epitaxial $γ$-Al$_2$O$_3$, for which we find a low intrinsic two-level system loss, $δ_{\text{TLS}}^0 = (2.8 \pm 0.1) \times 10^{-5}$. These results establish heteroepitaxial oxides on transition metal nitrides as an attractive materials platform for superconducting quantum circuits, particularly for integration into compact device architectures such as merged-element transmons and microwave kinetic inductance detectors.

Garcia-Wetten, David A. [Northwestern U.]↗
DOE OSTI2014

Study on the Ge{sub 1−x}Sn{sub x}/HfO{sub 2} interface and its impacts on Ge{sub 1−x}Sn{sub x} tunneling transistor

In this paper, we employ first-principle calculation to investigate the Ge{sub 1−x}Sn{sub x}/HfO{sub 2} interface, and then evaluate its impacts on Ge{sub 1−x}Sn{sub x} tunneling field-effect transistor (TFET). First-principle calculations of Ge{sub 1−x}Sn{sub x}/HfO{sub 2} interfaces in the oxygen-rich process atmosphere indicate that the interface states originate from the Ge and Sn dangling bond, rather than Hf-bond. The total density of state shows that there are more interface states in the semiconductor bandgap with increasing Sn fraction. By further incorporating the material and interface parameters from density functional theory calculation into advanced device simulation, the electrical characteristics of Ge{sub 1−x}Sn{sub x} TFET are investigated. Removing the Sn atom from the first atom layer of Ge{sub 1−x}Sn{sub x} in device processes is found to be beneficial to reduce the degradations. For the degradation mechanisms, the trap-assisted-tunneling is the dominant mechanism at the low Sn fraction, and enhanced Shockley-Read-Hall recombination induced by traps becomes the dominant mechanism with increasing Sn fraction. The results are helpful for the interface optimization of Ge{sub 1−x}Sn{sub x} TFET.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Multilayer interface tracking model of pure tungsten oxidation

Here, we present a numerical model to predict oxide scale growth on tungsten surfaces under exposure to oxygen at high temperatures. The model captures the formation of four thermodynamically-compatible oxide sublayers, WO 2 , WO 2.72 , WO 2.9 , and WO 3 , on top of the metal substrate. Oxide layer growth is simulated by tracking the oxide/oxide and oxide/metal interfaces using a sharp-interface Stefan model coupled to diffusion kinetics. The model is parameterized using selected experimental measurements and electronic structure calculations of the diffusivities of all the oxide subphases involved. We simulate oxide growth at temperatures of 600°C and above, extracting the power law growth exponents in each case, which we find to deviate from classical parabolic growth in several cases. We conduct a comparison of the model predictions with an extensive experimental data set, with reasonable agreement at most temperatures. While many gaps in our understanding still exist, this work is a first attempt at embedding the thermodynamic and kinetic complexity of tungsten oxide growth into a comprehensive mesoscale kinetic model that attempts to capture the essential features of tungsten oxidation to fill existing knowledge gaps and guide and enhance future tungsten oxidation models.

36 MATERIALS SCIENCE↗
DOE OSTI2017

In situ atomic-scale imaging of the metal/oxide interfacial transformation

Directly probing structure dynamics at metal/oxide interfaces has been a major challenge due to their buried nature.Using environmental transmission electron microscopy,here we report observations of the in-place formation of Cu 2 O/Cu interfaces via the oxidation of Cu, and subsequently probe the atomic mechanisms by which interfacial transformation and grain rotation occur at the interfaces during reduction in an H 2 gas environment. The Cu 2 O→Cu transformation is observed to occur initially along the Cu 2 O/Cu interface in a layer-by-layer manner.The accumulation of oxygen vacancies at the Cu 2 O/Cu interface drives the collapse of the Cu 2 O lattice near the interface region, which results in a tilted Cu 2 O/Cu interface with concomitant Cu 2 O island rotation. Lastly, these results provide unprecedented microscopic detail regarding the redox reactions of supported oxides,which differs fundamentally from the reduction of bulk or isolated oxides that requires the formation of new interfaces between the parent oxide and the reduced phase.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Effect of Si substrate on interfacial SiO{sub 2} scavenging in HfO{sub 2}/SiO{sub 2}/Si stacks

The scavenging kinetics of an ultra-thin SiO{sub 2} interface layer (SiO{sub 2}-IL) in an HfO{sub 2}/SiO{sub 2}/Si stack is discussed by focusing on the substrate effect in addition to oxygen diffusion. {sup 18}O tracing experiments demonstrate that the O-atom moves from the SiO{sub 2}-IL to the HfO{sub 2} layer during scavenging. SiO{sub 2}-IL scavenging with various substrates (Si, SiC, and sapphire) has been found to be significantly different, which suggests that the Si in the substrate is also necessary to continuously cause the scavenging. Based on these findings and thermodynamic considerations, a kinetic model where oxygen vacancy (V{sub O}) transferred from the HfO{sub 2} reacts with the SiO{sub 2}, which is in contact with the Si-substrate, is proposed for the SiO{sub 2}-IL scavenging.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Abnormal bipolar resistive switching behavior in a Pt/GaO{sub 1.3}/Pt structure

A stable and repeatable abnormal bipolar resistive switching behavior was observed in a Pt/GaO{sub 1.3}/Pt sandwich structure without an electroforming process. The low resistance state (LRS) and the high resistance state (HRS) of the device can be distinguished clearly and be switched reversibly under a train of the voltage pulses. The LRS exhibits a conduction of electron tunneling, while the HRS shows a conduction of Schottky-type. The observed phenomena are considered to be related to the migration of oxygen vacancies which changes the space charge region width of the metal/semiconductor interface and results in a different electron transport mechanism.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗