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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 1,117 records · Page 62
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Magnetization dynamics and interface studies in ion-beam sputtered Si/CoFeB(8)/MgO(4)/CoFeB(8)/Ta(5) structures

The interface roughness, Boron distribution in bulk CoFeB and at interface, Gilbert damping constant (α), and inhomogeneous broadening in ion-beam sputtered Si/CoFeB(8)/MgO(4)/CoFeB(8)/Ta(5) structures are found to be sensitive to the MgO growth process. The ion-assist and reactive growth processes that result in sharper interfaces of width ∼0.5 nm lead to smaller α of 0.0050 ± 0.0003 and 0.0060 ± 0.0002 and inhomogeneous broadening ΔH{sub 0} of 3 ± 0.3 and 1 ± 0.3 Oe, respectively. On the other hand, the post-oxidation method results in rough interface and higher retention of Boron in CoFeB leading to higher values for α and ΔH{sub 0} as 0.0080 ± 0.0006 and 5 ± 0.3 Oe, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Degenerate Ising model for atomistic simulation of crystal-melt interfaces

One of the simplest microscopic models for a thermally driven first-order phase transition is an Ising-type lattice system with nearest-neighbour interactions, an external field, and a degeneracy parameter. The underlying lattice and the interaction coupling constant control the anisotropic energy of the phase boundary, the field strength represents the bulk latent heat, and the degeneracy quantifies the difference in communal entropy between the two phases. We simulate the (stochastic) evolution of this minimal model by applying rejection-free canonical and microcanonical Monte Carlo algorithms, and we obtain caloric curves and heat capacity plots for square (2D) and face-centred cubic (3D) lattices with periodic boundary conditions. Since the model admits precise adjustment of bulk latent heat and communal entropy, neither of which affect the interface properties, we are able to tune the crystal nucleation barriers at a fixed degree of undercooling and verify a dimension-dependent scaling expected from classical nucleation theory. We also analyse the equilibrium crystal-melt coexistence in the microcanonical ensemble, where we detect negative heat capacities and find that this phenomenon is more pronounced when the interface is the dominant contributor to the total entropy. The negative branch of the heat capacity appears smooth only when the equilibrium interface-area-to-volume ratio is not constant but varies smoothly with the excitation energy. Finally, we simulate microcanonical crystal nucleation and subsequent relaxation to an equilibrium Wulff shape, demonstrating the model's utility in tracking crystal-melt interfaces at the atomistic level.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Effect of boron concentration on recombination at the p-Si–Al{sub 2}O{sub 3} interface

We examine the surface passivation properties of Al{sub 2}O{sub 3} deposited on boron-doped planar 〈100〉 crystalline silicon surfaces as a function of the boron concentration. Both uniformly doped and diffused surfaces are studied, with surface boron concentrations ranging from 9.2 × 10{sup 15} to 5.2 × 10{sup 19} cm{sup −3}. Atmospheric pressure chemical vapor deposition and thermal atomic layer deposition are used to deposit the Al{sub 2}O{sub 3} films. The surface recombination rate of each sample is determined from photoconductance measurements together with the measured dopant profiles via numerical simulation, using the latest physical models. These values are compared with calculations based on the interface properties determined from capacitance–voltage and conductance measurements. It is found that the fundamental surface recombination velocity of electrons, S{sub n0}, which describes the chemical passivation of the interface, is independent of the surface boron concentration N{sub s} for N{sub s} ≤ 3 × 10{sup 19} cm{sup −3}, and in excellent agreement with values calculated from the interface state density D{sub it} and capture coefficients c{sub n} and c{sub p} measured on undiffused boron-doped surfaces. We conclude that the physical properties of the Si–Al{sub 2}O{sub 3} interface are independent of the boron dopant concentration over this range.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Qualitative link between work of adhesion and thermal conductance of metal/diamond interfaces

We report Time-Domain ThermoReflectance experiments measuring the Thermal Boundary Conductance (TBC) of interfaces between diamond and metal surfaces, based on samples consisting of [111]-oriented diamond substrates with hydrogen or with sp{sup 2} carbon surface terminations created using plasma treatments. In a concurrent theoretical study, we calculate the work of adhesion between Ni, Cu, and diamond interfaces with (111) surface orientation, with or without hydrogen termination of the diamond surface, using first-principles electronic structure calculations based on density functional theory (DFT). We find a positive correlation between the calculated work of adhesion and the measured conductance of these interfaces, suggesting that DFT could be used as a screening tool to identify metal/dielectric systems with high TBC. We also explain the negative effect of hydrogen on the thermal conductance of metal/diamond interfaces.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Interface conditions of two-shot molded parts

The focus of this work is on interfaces of two-shot molded parts. It is well known that e.g. material combination, process parameters and contact area structures show significant effects on the bond strength of multi-component injection molded parts. To get information about the bond strength at various process parameter settings and material combinations a test mold with core back technology was used to produce two-component injection molded tensile test specimens. At the core back process the different materials are injected consecutively, so each component runs through the whole injection molding cycle (two-shot process). Due to this consecutive injection molding processes, a cold interface is generated. This is defined as overmolding of a second melt to a solidified polymer preform. Strong interest lies in the way the interface conditions change during the adhesion formation between the individual components. Hence the interface conditions were investigated by computed tomography and Raman spectroscopy. By analyzing these conditions the understanding of the adhesion development during the multi-component injection molding was improved.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Charge transfer effects of ions at the liquid water/vapor interface

Charge transfer (CT), the movement of small amounts of electron density between non-bonded pairs, has been suggested as a driving force for a variety of physical processes. Herein, we examine the effect of CT on ion adsorption to the water liquid-vapor interface. Using a CT force field for molecular dynamics, we construct a potential of mean force (PMF) for Na{sup +}, K{sup +}, Cl{sup −}, and I{sup −}. The PMFs were produced with respect to an average interface and an instantaneous interface. An analysis of the PMF relative to the instantaneous surface reveals that the area in which the anions experience a free energy minimum is quite narrow, and the cations feel a steeply repulsive free energy near the interface. CT is seen to have only minor effects on the overall free energy profiles. However, the long-ranged effects of ions are highlighted by the CT model. Due to CT, the water molecules at the surface become charged, even when the ion is over 15 Å away from the surface.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Molecular dynamics study of Si(100)-oxidation: SiO and Si emissions from Si/SiO{sub 2} interfaces and their incorporation into SiO{sub 2}

Dynamics of Si(100)-oxidation processes at the Si/SiO{sub 2} interface and in the SiO{sub 2} region are investigated focusing on SiO and Si emissions from the interface and the following incorporation into the SiO{sub 2} and/or substrate. Classical molecular dynamics (MD) simulations with variable charge interatomic potentials are performed to clarify these atomic processes. By incorporating oxygen atoms, two-folded Si atoms are formed after structural relaxation at the interface and are emitted as SiO molecules into SiO{sub 2}. The energy barrier of the SiO emission is estimated to be 1.20 eV on the basis of the enthalpy change in an MD simulation. The emitted SiO molecule is incorporated into the SiO{sub 2} network through a Si-O rebonding process with generating an oxygen vacancy. The energy barrier of the SiO incorporation is estimated to be 0.79–0.81 eV. The elementary process of oxygen vacancy diffusion leading to the complete SiO incorporation is also simulated, and the energy barriers are found to be relatively small, 0.71–0.79 eV. The energy changes of Si emissions into the substrate and SiO{sub 2} are estimated to be 2.97–7.81 eV, which are larger than the energy barrier of the SiO emission. This result suggests that, at the ideally flat Si/SiO{sub 2} interface, the SiO emission into the SiO{sub 2} region occurs prior to the Si emission, which is consistent with previous theoretical and experimental studies. The above mentioned typical atomic processes are successfully extracted from some (or one) of MD simulations among many trials in which a statistical procedure is partly employed. Our results give a unified understanding of Si oxidation processes from an atomistic point of view.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
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 OSTI2014

Superconductivity observed in platinum-silicon interface

We report the discovery of superconductivity with an onset temperature of ∼0.6 K in a platinum-silicon interface. The interface was formed by using a unique focused ion beam sputtering micro-deposition method in which the energies of most sputtered Pt atoms are ∼2.5 eV. Structural and elemental analysis by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy reveal a ∼ 7 nm interface layer with abundant Pt, which is the layer likely responsible for the superconducting transport behavior. Similar transport behavior was also observed in a gold-silicon interface prepared by the same technique, indicating the possible generality of this phenomenon.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Bandgap properties of diamond structure photonic crystal heterostructures with inclined and curved interfaces

The 3D (dimensional) diamond structure photonic crystal heterostructures with different lattice constants were prepared using rapid prototyping and gel casting with alumina. In this paper, heterostructures with inclined and curved interfaces were designed and its bandgap properties were studied. The normalized resonant intensity of electromagnetic wave in heterostructure with inclined and curved interface is stronger than that in the ordinary heterostructure without modified interface. The influence of curved interface on transmission properties of electromagnetic wave was investigated with the radius of curvature ranging from 17 mm to 37 mm at 5 mm interval. The results show that two resonant modes appear in the photonic band gap, being similar to the band gap characteristics of the photonic crystals with two defects inside. With the increasing of the radius of curvature, the resonant mode shift to higher frequency. In the structure with a radius of curvature of 32 mm, a guiding band appears in the photonic band gap. Further increase in the radius of curvature, the guiding band will split into two resonant modes again and the two resonant modes shift to lower frequencies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Efficient simulation of the impact of interface grading on the transport and optical properties of semiconductor heterostructures

An efficient model is proposed to evaluate the impact of interface grading on the properties of semiconductor heterostructures. In the plane-wave approximation, the interface grading is taken into account by simply multiplying the Fourier components of the potential by a Gaussian function, which results only in a very small increase of the computation time. We show that the interface grading may affect the transition energies, the field strength for resonant coupling of subbands, and even the miniband formation in complex systems such as quantum-cascade lasers. This model provides a convenient tool for the incorporation of interface grading into the design of heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Atomic and electronic structure of the ferroelectric BaTiO{sub 3}/Ge(001) interface

In this study, we demonstrate the epitaxial growth of BaTiO{sub 3} on Ge(001) by molecular beam epitaxy using a thin Zintl template buffer layer. A combination of density functional theory, atomic-resolution electron microscopy and in situ photoemission spectroscopy is used to investigate the electronic properties and atomic structure of the BaTiO{sub 3}/Ge interface. Aberration-corrected scanning transmission electron micrographs reveal that the Ge(001) 2 × 1 surface reconstruction remains intact during the subsequent BaTiO{sub 3} growth, thereby enabling a choice to be made between several theoretically predicted interface structures. The measured valence band offset of 2.7 eV matches well with the theoretical value of 2.5 eV based on the model structure for an in-plane-polarized interface. The agreement between the calculated and measured band offsets, which are highly sensitive to the detailed atomic arrangement, indicates that the most likely BaTiO{sub 3}/Ge(001) interface structure has been identified.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Simultaneous probing of bulk liquid phase and catalytic gas-liquid-solid interface under working conditions using attenuated total reflection infrared spectroscopy

Design and performance of a reactor set-up for attenuated total reflection infrared (ATR-IR) spectroscopy suitable for simultaneous reaction monitoring of bulk liquid and catalytic solid-liquid-gas interfaces under working conditions are presented. As advancement of in situ spectroscopy an operando methodology for gas-liquid-solid reaction monitoring was developed that simultaneously combines catalytic activity and molecular level detection at the catalytically active site of the same sample. Semi-batch reactor conditions are achieved with the analytical set-up by implementing the ATR-IR flow-through cell in a recycle reactor system and integrating a specifically designed gas feeding system coupled with a bubble trap. By the use of only one spectrometer the design of the new ATR-IR reactor cell allows for simultaneous detection of the bulk liquid and the catalytic interface during the working reaction. Holding two internal reflection elements (IRE) the sample compartments of the horizontally movable cell are consecutively flushed with reaction solution and pneumatically actuated, rapid switching of the cell (<1 s) enables to quasi simultaneously follow the heterogeneously catalysed reaction at the catalytic interface on a catalyst-coated IRE and in the bulk liquid on a blank IRE. For a complex heterogeneous reaction, the asymmetric hydrogenation of 2,2,2-trifluoroacetophenone on chirally modified Pt catalyst the elucidation of catalytic activity/enantioselectivity coupled with simultaneous monitoring of the catalytic solid-liquid-gas interface is shown. Both catalytic activity and enantioselectivity are strongly dependent on the experimental conditions. The opportunity to gain improved understanding by coupling measurements of catalytic performance and spectroscopic detection is presented. In addition, the applicability of modulation excitation spectroscopy and phase-sensitive detection are demonstrated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Characterization of a Fe/Y{sub 2}O{sub 3} metal/oxide interface using neutron and x-ray scattering

The structure of metal/oxide interfaces is important to the radiation resistance of oxide dispersion-strengthened steels. We find evidence of gradual variations in stoichiometry and magnetization across a Fe/Y{sub 2}O{sub 3} metal/oxide heterophase interface using neutron and x-ray reflectometry. These findings suggest that the Fe/Y{sub 2}O{sub 3} interface is a transitional zone approximately ∼64 Å-thick containing mixtures or compounds of Fe, Y, and O. Our results illustrate the complex chemical and magnetic nature of Fe/oxide interfaces and demonstrate the utility of combined neutron and x-ray techniques as tools for characterizing them.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Role of the dielectric for the charging dynamics of the dielectric/barrier interface in AlGaN/GaN based metal-insulator-semiconductor structures under forward gate bias stress

The high density of defect states at the dielectric/III-N interface in GaN based metal-insulator-semiconductor structures causes tremendous threshold voltage drifts, ΔV{sub th}, under forward gate bias conditions. A comprehensive study on different dielectric materials, as well as varying dielectric thickness t{sub D} and barrier thickness t{sub B}, is performed using capacitance-voltage analysis. It is revealed that the density of trapped electrons, ΔN{sub it}, scales with the dielectric capacitance under spill-over conditions, i.e., the accumulation of a second electron channel at the dielectric/AlGaN barrier interface. Hence, the density of trapped electrons is defined by the charging of the dielectric capacitance. The scaling behavior of ΔN{sub it} is explained universally by the density of accumulated electrons at the dielectric/III-N interface under spill-over conditions. We conclude that the overall density of interface defects is higher than what can be electrically measured, due to limits set by dielectric breakdown. These findings have a significant impact on the correct interpretation of threshold voltage drift data and are of relevance for the development of normally off and normally on III-N/GaN high electron mobility transistors with gate insulation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Seeking to quantify the ferromagnetic-to-antiferromagnetic interface coupling resulting in exchange bias with various thin-film conformations

Ni{sub 3}Fe/(Ni, Fe)O thin films with bilayer and nanocrystallite dispersion morphologies are prepared with a dual ion beam deposition technique permitting precise control of nanocrystallite growth, composition, and admixtures. A bilayer morphology provides a Ni{sub 3}Fe-to-NiO interface, while the dispersion films have different mixtures of Ni{sub 3}Fe, NiO, and FeO nanocrystallites. Using detailed analyses of high resolution transmission electron microscopy images with Multislice simulations, the nanocrystallites' structures and phases are determined, and the intermixing between the Ni{sub 3}Fe, NiO, and FeO interfaces is quantified. From field-cooled hysteresis loops, the exchange bias loop shift from spin interactions at the interfaces are determined. With similar interfacial molar ratios of FM-to-AF, we find the exchange bias field essentially unchanged. However, when the interfacial ratio of FM to AF was FM rich, the exchange bias field increases. Since the FM/AF interface ‘contact’ areas in the nanocrystallite dispersion films are larger than that of the bilayer film, and the nanocrystallite dispersions exhibit larger FM-to-AF interfacial contributions to the magnetism, we attribute the changes in the exchange bias to be from increases in the interfacial segments that suffer defects (such as vacancies and bond distortions), that also affects the coercive fields.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

The influence of molecular interface modification on the charge dynamics of polymeric semiconductor:ZnO heterostructure

We demonstrate an enhancement of photocurrent in hybrid photovoltaic cells based on nanoparticles of zinc oxide (ZnO) and poly(3-hexylthiophene) (P3HT), through molecular interface modification with and without cis-bis(4,4′-dicarboxy-2,2′bipyridine) ruthenium (II) (N3-dye) and α-Sexithiophen-2-yl-phosphonic Acid (6TP) as interface modifiers. We identify the formation of long-lived polarons at P3HT:ZnO interface by means of quasi-steady-state photoinduced absorption (PIA) spectroscopy. Furthermore, by probing the pump-modulation-frequency-dependent PIA signal, we find that P3HT:ZnO-N3 and P3HT:ZnO-6TP exhibit more sharply decaying density with increasing modulation frequency, which is indicative of a longer average lifetime, approximating 1 ms as opposed to ∼0.2 ms without ZnO surface modification. This highlights the importance of the molecular interface modification in the steady-state polaron dynamics in hybrid solar cells.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Ferromagnetic-nonmagnetic and metal-insulator phase transitions at the interfaces of KTaO{sub 3} and PbTiO{sub 3}

We studied the electronic and magnetic properties of hole doped KTaO{sub 3}/PbTiO{sub 3} interface using density functional theory methods. Ferromagnetic-nonmagnetic phase transition and metal-insulator phase transition occur simultaneously at the interface with ferroelectric polarization reversal. Furthermore, these two transitions are coupled with each other because hole doping with large concentration of holes gives rise to ferromagnetism. The interfacial magnetization, which is proportional to hole concentration at the interface, can be tuned by ferroelectric polarization, leading to strong intrinsic magnetoelectric effect at the interface of originally nonmagnetic KTaO{sub 3} and PbTiO{sub 3}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗