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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

A Study on the Surface Composition and Migration of Materials and Their Effect on Surface Microhardness during Micro-EDM of Ti-6Al-4V

This study aims to investigate the effect of machining parameters on the surface topography and composition of Ti-6Al-4V after micro-EDM as well as the effect of elemental composition on the surface microhardness. The surface topography has been studied in terms of quality of machined surface, crater sizes, and the presence of surface defects. Statistical analysis was carried out to study the influence of micro-EDM process parameters, i.e., voltage, capacitance, discharge energy, electrode rotational speed, and the electrode coating, on the elemental composition of the machined surface. It was found that the crater size, and hence the surface finish, was dependent on the electrical parameters only, with more pronounced effect of capacitance over voltage. The 60 V and 30 pF combination resulted in crater size as small as 1 µm, whereas crater size of 12 µm was obtained using a combination of 4700 pF and 112 V. Although smaller capacitance resulted in smaller crater sizes, it also increased the chances of defects or deposition of debris on the machined surface. There was migration of material from the tool electrode, dielectric oil, and atmosphere to the workpiece surface, which influenced the surface topography, crater sizes, and surface microhardness. The effects of non-electrical parameters on the relative percentage of various elements were found to be statistically insignificant. The surface microhardness was found to increase slightly due to the surface modification after EDM. The reason for increase in surface microhardness was identified as the migration of tungsten from the tool electrode and carbon from the dielectric. The maximum surface microhardness of 455 HV was obtained at a surface composition after micro-EDM containing 7.96% C and 9.69% W, where non-machined Ti-6Al-4V surface does not have any carbon or tungsten on the surface.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Anomalous Surface Doping Effect in Semiconductor Nanowires

Surface doping is being used as an effective approach to improve the mechanical, optical, electronic, and magnetic properties of various materials. For example, experimental studies have proven that rare-earth element doping can enhance the optical properties of silicon nanostructures. However, the majority of previous investigations focused on either bulk materials or nanosized spherical crystals. Here we present a comparative study on semiconducting germanium (Ge) nanowires with and without surface doping by using multiple integrated characterization probes, including high resolution scanning/transmission electron microscopy (SEM/TEM), in situ high pressure synchrotron X-ray diffraction (XRD), and Raman spectroscopy. Our results reveal that under pressure the stability of the Ge-I phase (diamond structure) in erbium (Er)-doped Ge nanowires is enhanced compared to undoped Ge nanowires. We also found an increased stability of the Ge-II phase (body centered tetragonal structure) in Er-doped Ge nanowires during decompression. Furthermore, the presence of Er doping elevates the transition kinetics by showing a smaller pressure span needed for a complete Ge-I to Ge-II phase transformation. In contrast, Er doping has a negligible impact on the mechanical properties of Ge nanowires under high pressure, exhibiting a very different mechanical behavior from other foreign element-doped nanostructures. This anomalous doping effect was explained based on surface modification and decoration. Furthermore, these findings are of both fundamental and applied significance, because they not only provide a thorough understanding of the distinct role of surface doping in nanoscale materials, but also yield insight with regard to a given material’s design for favorable properties in semiconductor nanostructures.

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

Simulation study of the effects of surface chemistry and temperature on the conformations of ssDNA oligomers near hydrophilic and hydrophobic surfaces

We study the effects of the presence of a hydrophilic and a hydrophobic surface on the conformations and interactions of a single-stranded DNA (ssDNA) oligomer using atomistic molecular dynamics, umbrella sampling, and temperature-replica exchange. Our simulations capture the expected interactions between the ssDNA and the two surfaces (e.g., hydrogen bonds, hydrophobic interactions), but we find that the surface chemistry does not strongly affect the exposure of the relatively hydrophobic nucleobases or the hydrophilic phosphate backbone in a 16-base ssDNA. Likewise, the surfaces do not strongly affect the preferred size of the ssDNA compared to bulk solution, although the hydrophilic surface does favor slightly more compact ssDNA conformations than the hydrophobic surface. In more compact conformations, the negative charge of the ssDNA is more concentrated, and the energetic interactions of the DNA and DNA-bound counterions with the hydrophilic surface are more favorable, which consequently favors smaller ssDNA sizes. Increasing temperature, regardless of the presence or chemistry of a surface, makes it less unfavorable for the ssDNA to assume both compact and extended conformations. With increasing temperature the free energy cost of assuming a compact conformation is reduced to a greater extent than the cost of assuming an extended conformation. The reason for this difference is the entropically favorable release of DNA-bound water molecules upon assuming a compact conformation. Increasing temperature decreases water-DNA interactions while surprisingly increasing counterion-DNA interactions, changes which are attributed to the relative balance of entropic and energetic contributions for water molecules and counterions bound to the ssDNA.

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

Effect of surface properties and polymer chain length on polymer adsorption in solution

In polymer nanoparticle composites (PNCs) with attractive interactions between nanoparticles (NPs) and polymers, a bound layer of the polymer forms on the NP surface, with significant effects on the macroscopic properties of the PNCs. The adsorption and wetting behaviors of polymer solutions in the presence of a solid surface are critical to the fabrication process of PNCs. In this study, we use both classical density functional theory (cDFT) and molecular dynamics (MD) simulations to study dilute and semi-dilute solutions of short polymer chains near a solid surface. Using cDFT, we calculate the equilibrium properties of polymer solutions near a flat surface while varying the solvent quality, surface–fluid interactions, and the polymer chain lengths to investigate their effects on the polymer adsorption and wetting transitions. Using MD simulations, we simulate polymer solutions near solid surfaces with three different curvatures (a flat surface and NPs with two radii) to study the static conformation of the polymer bound layer near the surface and the dynamic chain adsorption process. Additionally, we find that the bulk polymer concentration at which the wetting transition in the poor solvent system occurs is not affected by the difference in surface–fluid interactions; however, a threshold value of surface–fluid interaction is needed to observe the wetting transition. We also find that with good solvent, increasing the chain length or the difference in the surface–polymer interaction relative to the surface–solvent interaction increases the surface coverage of polymer segments and independent chains for all surface curvatures. Finally, we demonstrate that the polymer segmental adsorption times are heavily influenced only by the surface–fluid interactions, although polymers desorb more quickly from highly curved surfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Effects of surface chemistry and microstructure of electrolyte on oxygen reduction kinetics of solid oxide fuel cells

In this study, we report systematic investigation of the surface properties of yttria-stabilized zirconia (YSZ) electrolytes with the control of the grain boundary (GB) density at the surface, and its effects on electrochemical activities. The GB density of thin surface layers deposited on single crystal YSZ substrates is controlled by changing the annealing temperature (750-1450 °C). Higher oxygen reduction reactions (ORR) kinetics is observed in samples annealed at lower temperatures. The higher ORR activity is ascribed to the higher GB density at the YSZ surface where 'mobile' oxide ion vacancies are more populated. Meanwhile, oxide ion vacancies concurrently created with yttrium segregation at the surface at the higher annealing temperature are considered inactive to oxygen incorporation reactions. Our results provide additional insight into the interplay between the surface chemistry, microstructures, and electrochemical activity. They potentially provide important guidelines for engineering the electrolyte electrode interfaces of solid oxide fuel cells for higher electrochemical performance.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Mechanical Surface Treatments of AISI 304 Stainless Steel: Effects on Surface Microrelief, Residual Stress, and Microstructure

The surface roughness, residual stress, and microstructure of AISI 304 stainless steel specimens after laser shock peening (LSP), water jet cavitation peening (WjCP), water jet shot peening (WjSP), and multi-pin ultrasonic impact treatment (UIT) were studied in this work. Compared to the initial state, the surface roughness (Ra) was, respectively, decreased by approx. 5.5, 7.8, 38.2, and 91.1% after the LSP, WjCP, WjSP, and UIT processes. The volume fraction of ε-martensite of ~ 3-5% was observed in all treated specimens except for the LSP-treated ones. The volume fraction of α′-martensite was increased in the following sequence: WjCP (~ 5%), LSP (~ 5%), WjSP (~ 25%), UIT (~ 50%). The studied mechanical surface treatments promote a significant reduction in grains size of both austenite (~ 15-20 nm) and martensite (~ 20-37 nm) leading to essential hardening. All studied processes result in the formation of compressive residual stresses (− 377…693 MPa) and the improvement in the bearing curve parameters. The microhardness estimated accounting for the contributions of different hardening mechanisms to the yield strength magnitude correlates well with the experimental data. The grain boundary hardening and dislocation hardening are concluded to be the most influential mechanisms.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of surface hydroxyl groups on heat capacity of mesoporous silica

This paper quantifies the effect of surface hydroxyl groups on the effective specific and volumetric heat capacities of mesoporous silica. To achieve a wide range of structural diversity, mesoporous silica samples were synthesized by various methods, including (i) polymer-templated nanoparticle-based powders, (ii) polymer-templated sol-gel powders, and (iii) ambigel silica samples dried by solvent exchange at room temperature. Their effective specific heat capacity, specific surface area, and porosity were measured using differential scanning calorimetry and low-temperature nitrogen adsorption-desorption measurements. The experimentally measured specific heat capacity was larger than the conventional weight-fraction-weighted specific heat capacity of the air and silica constituents. The difference was attributed to the presence of OH groups in the large internal surface area. A thermodynamic model was developed based on surface energy considerations to account for the effect of surface OH groups on the specific and volumetric heat capacity. The model predictions fell within the experimental uncertainty.

Marszewski, Michal↗
DOE OSTI2013

Effects of surface properties on barrier height and barrier inhomogeneities of platinum contacts to n-type 4H-SiC

We investigated the Schottky barrier of Pt/4H-SiC contact as a function of 4H-SiC surface properties which effectively controlled by electronic cyclotron resonance hydrogen plasma pretreatment for different periods and annealing. It is found that the effective barrier height monotonically increases with decreasing the degree of Fermi level pinning. Electrically homogeneous contacts are observed when the Fermi level (FL) is 'pinned (Bardeen limit)' and 'free-pinned (Schottky limit).' However, a partial pinning of FL leads to Gaussian distribution of inhomogeneous barrier height. These results could be correlated with changes in the magnitude and spatial distribution of surface state density after different pretreatments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Surface smoothing effect of an amorphous thin film deposited by atomic layer deposition on a surface with nano-sized roughness

Previously, Lau (one of the authors) pointed out that the deposition of an amorphous thin film by atomic layer deposition (ALD) on a substrate with nano-sized roughness probably has a surface smoothing effect. In this letter, polycrystalline zinc oxide deposited by ALD onto a smooth substrate was used as a substrate with nano-sized roughness. Atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (XTEM) were used to demonstrate that an amorphous aluminum oxide thin film deposited by ALD can reduce the surface roughness of a polycrystalline zinc oxide coated substrate.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Structure Effects on the Energetics of the Electrochemical Reduction of CO2 by Copper Surfaces

Polycrystalline copper electrocatalysts have been experimentally shown to be capable of reducing CO{sub 2} into CH{sub 4} and C{sub 2}H{sub 4} with relatively high selectivity, and a mechanism has recently been proposed for this reduction on the fcc(211) surface of copper, which was assumed to be the most active facet. In the current work, we use computational methods to explore the effects of the nanostructure of the copper surface and compare the effects of the fcc(111), fcc(100) and fcc(211) facets of copper on the energetics of the electroreduction of CO{sub 2}. The calculations performed in this study generally show that the intermediates in CO{sub 2} reduction are most stabilized by the (211) facet, followed by the (100) facet, with the (111) surface binding the adsorbates most weakly. This leads to the prediction that the (211) facet is the most active surface among the three in producing CH{sub 4} from CO{sub 2}, as well as the by-products H{sub 2} and CO. HCOOH production may be mildly enhanced on the more close-packed surfaces ((111) and (100)) as compared to the (211) facet, due to a change in mechanism from a carboxyl intermediate to a formate intermediate. The results are compared to experimental data on these same surfaces; the predicted trends in voltage requirements are consistent between the experimental and computational data.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Effect of surface disorder on the chiral surface states of a three-dimensional quantum Hall system

We investigate the effect of surface disorder on the chiral surface states of a three-dimensional quantum Hall system. Utilizing a transfer-matrix method, we find that the localization length of the surface state along the magnetic field decreases with the surface disorder strength in the weak disorder regime but increases anomalously in the strong disorder regime. In the strong disorder regime, the surface states mainly locate at the first inward layer to avoid the strong disorder in the outmost layer. The anomalous increase of the localization length can be explained by an effective model, which maps the strong disorder on the surface layer to the weak disorder on the first inward layer. Our work demonstrates that surface disorder can be an effective way to control the transport behavior of the surface states along the magnetic field. We also investigate the effect of surface disorder on the full distribution of conductances P(g) of the surface states in the quasi-one-dimensional (1D) regime for various surface disorder strengths. In particular, we find that P(g) is Gaussian in the quasi-1D metal regime and log-normal in the quasi-1D insulator regime. In the crossover regime, P(g) exhibits highly nontrivial forms, whose shapes coincide with the results obtained from the Dorokhov-Mello-Pereyra-Kumar equation of a weakly disordered quasi-1D wire in the absence of time-reversal symmetry. Our results suggest that P(g) is fully determined by the average conductance, independent of the details of the system, in agreement with the single-parameter scaling hypothesis.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Optimum quantum yield of the light emission from 2 to10 nm hydrosilylated silicon quantum dots

Here, optimizing the light-emitting efficiency of silicon quantum dots (Si QDs) has been recently intensified by the demand of the practical use of Si QDs in a variety of fields such as optoelectronics, photovoltaics, and bioimaging. It is imperative that an understanding of the optimum light-emitting efficiency of Si QDs should be obtained to guide the design of the synthesis and processing of Si QDs. Here an investigation is presented on the characteristics of the photoluminescence (PL) from hydrosilylated Si QDs in a rather broad size region (≈ 2-10 nm), which enables an effective mass approximation model to be developed, which can very well describe the dependence of the PL energy on the QD size for Si QDs in the whole quantum-confinement regime, and demonstrates that an optimum PL quantum yield (QY) appears at a specific QD size for Si QDs. The optimum PL QY results from the interplay between quantum-confinement effect and surface effect. The current work has important implications for the surface engineering of Si QDs. To optimize the light-emission efficiency of Si QDs, the surface of Si QDs must be engineered to minimize the formation of defects such as dangling bonds at the QD surface and build an energy barrier that can effectively prevent carriers in Si QDs from tunneling out.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Chemical reactivity of twin-modified copper nanowire surfaces

Effect of twin boundary (TB) spacing on atomic surface structure and chemical reactivity of nanotwinned Cu nanowires (NWs) is investigated. Post-etching surface structure and wire diameter of Cu NWs were examined by transmission electron microscopy. When TB spacing is less than 10 nm, the Cu NWs remain almost intact after chemical attack and show faceted surface structures with low atomic step density. A mechanism based on surface tension torque acting on TB/surface triple junctions is proposed to explain the faceted structure formation and enhanced corrosion resistance of nanotwinned Cu NWs.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Stabilization of metastable phases in hafnia owing to surface energy effects

The recent empirical observation of ferroelectricity in hafnia is rather surprising since all of its known ground-state phases are nonpolar. In this letter, we show that finite size effects, relevant under experimental conditions, may themselves lead to this unexpected phenomenon due to stabilization of metastable polar phase(s). Using surface energies computed from first principles, we determine the thermodynamic stability of a parallelepiped shaped particle constructed from various low energy nonpolar and polar phases of hafnia. We find that at small dimensions, surface effects may stabilize either one of the polar phases or the nonpolar tetragonal phase (the parent phase of the polar phases), suggesting a possible explanation of the ferroelectric behavior observed in pure hafnia films. These results also explain the stabilization of the metastable tetragonal phase in nanoparticles of zirconia, the twin oxide of hafnia. While a comprehensive understanding of the origins of ferroelectricity in hafnia will require inclusion of other relevant factors (such as strain and dopants), this work highlights the importance of finite size effects as a possible key factor.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Representing Fine‐Scale Topographic Effects on Surface Radiation Balance in Hyper‐Resolution Land Surface Models

Land surface models are increasingly used to simulate land surface processes at hyper-spatial resolutions (e.g., ∼1 km). As model resolution increases, grid-scale topographic effects on surface radiation fluxes and their interactions between adjacent grids become more pronounced. However, current land surface models routinely neglect the fine-scale topographic effects on surface radiation balance. This study developed physically-based and computationally-efficient parameterizations (fineTOP) that explicitly resolve fine-scale topographic effects on downward shortwave and longwave radiation as well as land surface radiative properties. The newly developed parameterizations were implemented and tested in the Energy Exascale Earth System Model (E3SM) Land Model (ELM). Multi-decadal km-resolution ELM simulations over the California Sierra Nevada show that fine-scale topography significantly impacts the surface energy balance and snow processes across seasons. Slope determines the magnitude of topographic effects, while aspect controls their sign. For slopes larger than 30°, topography-induced change in annual surface temperature can be as large as 3.3 K. Regionally, the mean value and standard deviation of topography-induced changes in annual surface temperature are −0.22 ± 0.38 K and +0.25 ± 0.37 K over north-facing and south-facing slopes, respectively. Topography-induced changes in surface radiative properties account for 3.5% ± 13.8% of total topographic effects on annual net radiation. With fineTOP, ELM captures the aspect-dependence of snow cover fraction, snow water equivalent, and land surface temperature found in MODIS satellite observations and a snow reanalysis data set, while the default ELM fails to capture this phenomenon. The enhanced capability to represent fine-scale topographic effects on surface radiation balance can be used to advance understanding of the role of fine-scale topography in land surface processes and land-atmosphere interactions over mountainous regions.

Hao, Dalei [Pacific Northwest National Laboratory ↗
DOE OSTI2018

Surface Charge Effects on Fe(II) Sorption and Oxidation at (110) Goethite Surfaces

Iron(III) oxides and oxyhydroxides are among the most reactive minerals in the environment, with surfaces that become charged when immersed in water. The governing role of surface charge over interfacial processes such as metal sorption is well understood. However, its role in interfacial redox reactions, such as when metal sorption is coupled to interfacial electron transfer (ET), is not. This is mainly because surface charge affects not only the types and densities of surface complexes formed but also their respective driving forces for ET. An important case is Fe(II)-catalyzed recrystallization of Fe(III)-oxyhydroxides, in which Fe(II) sorption and interfacial ET are closely linked. We used replica-exchange constant-pH molecular dynamics simulations (Zarzycki, P.; Smith, D. M.; Rosso, K. M. J. Chem. Theory Comput. 2015, 11, 1715–1724) to calculate the distance-dependent electrostatic potential at charged (110) surfaces of goethite particles, assessing its effect on previously computed Fe(II) sorption and interfacial ET free energies (Zarzycki, P.; Kerisit, S.; Rosso, K. M. J. Phys. Chem. C 2015, 119, 3111–3123). We show that Fe(II) adsorbs preferentially as an inner-sphere complex on the negatively charged surface, and as an outer-sphere complex on the positively charged surface because of both electrostatic repulsion and high energy barriers that arise from ordered water layers at the interface. The separation distance between adsorbed Fe(II) and the surface largely dictates adiabatic versus nonadiabatic ET regimes for this interface. The findings help unravel the pH dependence of Fe(II)-catalyzed recrystallization of goethite.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗