Near-surface Region Research Articles

A Layered Pork Model for Subdermal Antenna Tests at 433 MHz

Clinical or Biological Impact: Antennas for next generation implantable medical devices are likely to be distributed throughout the body, particularly in the near-surface subdermal regions, rather than tethered to the surface of the implantable device. A simple test bed for evaluating subdermal antennas during the design phase is needed. Objectives: The objectives of this paper are to create a simple biological test bed for subdermal implantable antennas that includes the normal expected variation of the tissues. Technology or Method: A layered model using pork products with skin, pork fat or lard, and pork loin or ground pork is used. Results: The ex vivo porcine tissues are similar to in vivo human tissues. Pork fat and loin have more variability than lard and ground pork but are more difficult in which to imbed subdermal antennas. This model provides an easy to use platform for testing subdermal antennas in the lab. This test bed was developed for 433 MHz: the industrial–scientific–medical band closest to the MedRadio band (402–405 MHz). Initial tests are demonstrated on a two-wire passive system for focusing power within the muscle region.

Near-Surface Mass Defect in Models of Locally Heterogeneous Solid Mechanics

Abstract This article deals with the model of the locally heterogeneous elastic body. The model accounts for long-range interaction and describes near-surface non-homogeneity and related size effects. The key systems of model equations are presented. From the viewpoint of the representative volume element, the boundary condition for density and the limits of applicability of the model are discussed. The difference of mass density in the near-surface body region from the reference value (near-surface mass defect) causes a non-zero stressed state. It is indicated on the strong dependence of the surface value of density from the curvature of the surface of thin fibres. The effect of the near-surface mass defect on the stressed state and the size effect of surface stresses have been investigated on an example of a hollow cylinder. Size effect of its strength has been studied as well.

Molecular-Dynamics Simulation of Relaxation Processes at Liquid–Gas Interfaces in Single- and Two-Component Lennard-Jones Systems

The formation of equilibrium liquid–gas interfaces in single- and two-component Lennard-Jones systems has been reproduced by molecular-dynamics simulation. The second component in the two-component system is a volatile impurity. The initial state is created by bringing in contact homogeneous liquid and gas phases having equal temperatures, pressures, and chemical potentials. The times required to establish equilibrium values of pressure, composition, shape and thickness of an interfacial layer, relative adsorption, and surface tension have been evaluated by the simulation. The calculations have been carried out at a temperature close to the triple point temperature of a solvent. It has been found that, in the course of relaxation, the maximum dynamic surface tension exceeds the equilibrium value by a factor of 1.2–1.6, while the relaxation time increases from 10 to 100 ns as the concentration of the volatile component in the solution grows to 0.25. In the two-component system with a limited volume of the gas phase, an equilibrium interfacial layer is formed in two stages. At the first stage, the volatile component is transferred into the interfacial layer from the near-surface regions of the liquid and gas phases. When an equilibrium partial density of the volatile component in the gas phase is achieved, the second stage begins, at which the surface layer is mainly supplied with liquid-phase particles. As a result, the relaxation times of relative adsorption and surface tension substantially increase. The role of the dynamic surface tension in the process of nucleation has been discussed.

Formation of Ag nanoparticles in Si (100) wafers by single and multiple low energy Ag ions implantation

We investigated the structural and optical changes of Si (100) induced by single or multiple low energy silver ion irradiation with fluences up to 1016 ions/cm2. The irradiated samples were analyzed by the means of Rutherford backscattering spectrometry, X-ray photoelectron spectroscopy, transmission electron microscopy and spectroscopic ellipsometry measurements. The results revealed that after 60, 75 and multiple (75 + 60) keV ion implantation Ag atoms are preferentially situated in the near-surface region of the Si with the maximum distribution at 20, 26 and 42 nm, respectively. Spherical-like particles with diameters below 5 nm are homogenously distributed along the ion track of implanted ions. In the case of single ions implantations the Ag nanoparticles are distributed in the shallower regions of silicon, while subsequent implantation of silver with multiple ion energies results in a distribution in a wider region and again a shift of the Ag nanoparticles to larger depths of silicon substrate. As the ion fluence increases a monotonic decrease in the absorption in the ultraviolet region of the spectrum was observed, due to amorphization of the silicon. On the other hand, for the fluences in the range of 1014–1015 ions/cm2 in the long-wave regions wide absorption band appears at wavelengths between 700 nm and 1000 nm, which is due to plasmon resonance of Ag nanoparticles synthesized during implantation. The position of the resonance peak reflects not only the change in the ion fluence, but also the difference in the incident Ag ion energy. After implantation to the higher ion fluences of 1016 ions/cm2 this absorption band was shifted over 1000 nm, which is much above the known values for plasmon resonances of Ag nanoparticles.

Simultaneous scanning near-field optical and X-ray diffraction microscopy for correlative nanoscale structure-property characterization.

A multimodal imaging instrument has been developed that integrates scanning near-field optical microscopy with nanofocused synchrotron X-ray diffraction imaging. The instrument allows for the simultaneous nanoscale characterization of electronic/near-field optical properties of materials together with their crystallographic structure, facilitating the investigation of local structure-property relationships. The design, implementation and operating procedures of this instrument are reported. The scientific capabilities are demonstrated in a proof-of-principle study of the insulator-metal phase transition in samarium sulfide (SmS) single crystals induced by applying mechanical pressure via a scanning tip. The multimodal imaging of an in situ tip-written region shows that the near-field optical reflectivity can be correlated with the heterogeneously transformed structure of the near-surface region of the crystal.

Surface studies on the characterization of RAFM steel after deuterium plasma exposure

Previous works have identified the potential for preferential sputtering of low/middle-Z elements in the reduced-activation ferritic-martensitic (RAFM) steels, leaving behind a W-enriched surface, which could then potentially reduce further sputtering of Fe and other elements in the steel. Thus, it is of vital importance to observe the morphology and concentration of the W-enriched surface. In this work, we detail the near-surface analysis of China low-activation ferritic (CLF-1) steel which has been exposed to deuterium plasma (with an impinging ion energy of 150 eV per D, incident fluence of 7.2 × 1024 D/m2, and specimen temperature of 470 K). Transmission electron microscopy and Rutherford backscattering spectroscopy tests were applied with particular focus on assessing the structure of the near-surface region. Results revealed that a shallow W-enriched surface consisting of W, Fe, Cr and O with a thickness of approximately 15 nm formed in the near-surface region of CLF-1 steel after deuterium-plasma exposure. In this W-enriched surface, the areal density and concentration of W were approximately 2.0 × 1015 atoms/cm2 and 2.0 at.%, respectively, evaluated by simulation with SIMNRA. Moreover, most of the W crystalline grains were scattered on the outside edge of the accumulated layer.

Evolution of thermally stratified turbulent open channel flow after removal of the heat source

Evolution of thermally stratified open channel flow after removal of a volumetric heat source is investigated using direct numerical simulation. The heat source models radiative heating from above and varies with height due to progressive absorption. After removal of the heat source the initial stable stratification breaks down and the channel approaches a fully mixed isothermal state. The initial state consists of three distinct regions: a near-wall region where stratification plays only a minor role, a central region where stratification has a significant effect on flow dynamics and a near-surface region where buoyancy effects dominate. We find that a state of local energetic equilibrium observed in the central region of the channel in the initial state persists until the late stages of the destratification process. In this region local turbulence parameters such as eddy diffusivity $k_{h}$ and flux Richardson number $R_{f}$ are found to be functions only of the Prandtl number $Pr$ and a mixed parameter ${\mathcal{Q}}$, which is equal to the ratio of the local buoyancy Reynolds number $Re_{b}$ and the friction Reynolds number $Re_{\unicode[STIX]{x1D70F}}$. Close to the top and bottom boundaries turbulence is also affected by $Re_{\unicode[STIX]{x1D70F}}$ and vertical position $z$. In the initial heated equilibrium state the laminar surface layer is stabilised by the heat source, which acts as a potential energy sink. Removal of the heat source allows Kelvin–Helmholtz-like shear instabilities to form that lead to a rapid transition to turbulence and significantly enhance the mixing process. The destratifying flow is found to be governed by bulk parameters $Re_{\unicode[STIX]{x1D70F}}$, $Pr$ and the friction Richardson number $Ri_{\unicode[STIX]{x1D70F}}$. The overall destratification rate ${\mathcal{D}}$ is found to be a function of $Ri_{\unicode[STIX]{x1D70F}}$ and $Pr$.

Linking Friction and Surface Properties of Hydrogels Molded Against Materials of Different Surface Energies.

Biological tissues subjected to rubbing, such as the cornea and eyelid or articular cartilage, are covered in brushy, hydrated mucous structures in order to reduce the shear stress on the tissue. To mimic such biological tissues, we have prepared polyacrylamide (PAAm) hydrogels with various concentrations of un-cross-linked chains on their surfaces by synthesizing them in molds of different surface energies. The selected molding materials included hydrophilic glass, polyoxymethylene (POM), polystyrene (PS), polyethylene (PE), polypropylene (PP), and polytetrafluoroethylene (PTFE). After synthesis, demolding, and equilibration in water, the elastic modulus at the hydrogel surface decreased with increasing water contact angle of the mold. The softer, brushier surfaces did not completely collapse under compressive pressures up to 10 kPa, remaining better hydrated compared to their denser, cross-linked analogs. The hydrogels with brushier surfaces displayed an order of magnitude lower coefficient of friction than the cross-linked ones, which is attributed to the ability of their near-surface regions to retain larger amounts of liquid at the interface. The characteristic speed-dependent friction of the denser, cross-linked hydrogel surface is compared to the speed-independent friction of the brushy hydrogels and discussed from the perspectives of (elasto)hydrodynamic lubrication, permeability, and shear-induced hydrodynamic penetration depth.

Computer Simulation of Surface‐Initiated Controlled Radical Polymerization: Effect of Free‐Monomer Model on Brush Properties

AbstractMonte Carlo simulation is employed using the bond‐fluctuation model (BFM) to explore the role of free‐monomer model on surface‐initiated controlled polymerization from flat substrates. Three free‐monomer models differ in two aspects: 1) their extent of excluded volume interactions between free‐monomer/polymer segments and 2) monomer availability (finite or infinite) during the simulation. In the explicit monomer (EM) model, free‐monomers behave as a single BFM‑type units. In the phantom monomer (PM) model, free‐monomers act analogously to those in the EM model but lack excluded volume interactions with the growing polymers. In the implicit monomer (IM) model, no explicit monomers are included in the simulation box; the polymers can grow as long as space is available near active chain‐ends. It is found that the breadth of the molecular weight distribution of the grown polymers decreases from EM, to PM, to IM models. With the EM model, free‐monomers are excluded from the near‐surface region, while with the PM model they are not. Due to its excluded volume interactions, the EM model tends to compress the brush against the substrate. Finally, the relaxation of the shape of a polydisperse brush after the polymerization reaction ends has been reported.

Mean field solar surface dynamo in the presence of partially ionized plasmas and sub-surface shear layer

Abstract A non-linear α − Ω dynamo in the partially ionized turbulent plasma in the presence of sub-surface velocity shear is studied with mean-field electrodynamics. Such a dynamo is probably operational in the near-surface region of the Sun, where the presence of both neutrals and the velocity shear (due to sub-surface shear layer in the rotation profile) is observationally well established. In particular, we show that the inclusion of ambipolar diffusion leads to a saturation of magnetic field amplitudes in the α − Ω dynamo. We also demonstrate that the temporal evolution of large-scale global magnetic fields follows the well-known pattern similar to the ‘butterfly’ diagram displayed by sunspots. As usual the velocity shear converts part of the poloidal into the toroidal magnetic field which in turn is regenerated largely by the combined kinetic plus Hall helicity, thus closing the dynamo loop. In addition, by allowing temporal variation in the helicity and ambipolar diffusion coefficient we are able to reproduce the grand-minimum type behaviour of the solar dynamo. Details of theoretical model along with numerical computations of dynamo equations in the partially ionized plasma are outlined. The solar surface dynamo model envisaged in this work could operate in conjunction with the global dynamo present in the bulk of the convection zone.

Hydrogen plasma treatment of β -Ga2O3: Changes in electrical properties and deep trap spectra

The effects of hydrogen plasma treatment of β-Ga2O3 grown by halide vapor phase epitaxy and doped with Si are reported. Samples subjected to H plasma exposure at 330 °C developed a wide (∼2.5 μm-thick) region near the surface, depleted of electrons at room temperature. The thickness of the layer is in reasonable agreement with the estimated hydrogen penetration depth in β-Ga2O3 based on previous deuterium profiling experiments. Admittance spectroscopy and photoinduced current transient spectroscopy measurements place the Fermi level pinning position in the H treated film near Ec-1.05 eV. Annealing at 450 °C decreased the thickness of the depletion layer to 1.3 μm at room temperature and moved the Fermi level pinning position to Ec-0.8 eV. Further annealing at 550 °C almost restored the starting shallow donor concentration and the spectra of deep traps dominated by Ec-0.8 eV and Ec-1.05 eV observed before hydrogen treatment. It is suggested that hydrogen plasma exposure produces surface damage in the near-surface region and passivates or compensates shallow donors.

Milling-induced chemical decomposition of the surface of EuBaCo2O5.5 powders studied by means of soft X-ray absorption spectroscopy

We present results of soft X-ray absorption spectroscopy studies of EuBaCo2O5.5 powders subjected to mechanical impact (milling in a ball mill). We show that in the near-surface region (5–10 nm) of particles of the powder, EuBaCo2O5.5 decomposes into Co3O4, BaCO3, and EuCoO3. A knowledge of the low-spin state of Co3+ ions in EuCoO3 and Co3O4 and possibilities of surface-sensitive soft X-ray absorption spectroscopy have allowed to determine the composition of the EuBaCo2O5.5 sample subjected to mechanical impact near its surface, which is inaccessible to standard X-ray phase analysis.

On the origin of ‘fuzz’ formation in plasma-facing materials

Tungsten, the material used in the plasma-facing components (PFCs) of nuclear fusion reactors, develops a fuzz-like surface morphology under typical reactor operating conditions. This fragile ‘fuzz’ surface nanostructure adversely affects reactor performance and operation. Developing predictive models, capable of simulating the spatiotemporal scales relevant to the fuzz formation process is essential for understanding the growth of such extremely complex surface features and improving PFC and reactor performance. Here, we report the development of an atomistically-informed, continuous-domain model for the onset of fuzz formation in helium plasma-irradiated tungsten and validate the model by comparing its predictions with measurements from carefully designed experiments. Our study demonstrates that fuzz forms in response to stress induced in the near-surface region of PFCs as a result of plasma exposure and helium gas implantation. Our model sets the stage for detailed descriptions of this complex fuzz formation phenomenon and similar phenomena observed in other materials.

Identifying the Origin of the Limiting Process in a Double Perovskite PrBa0.5Sr0.5Co1.5Fe0.5O5+δ Thin-Film Electrode for Solid Oxide Fuel Cells.

Oxygen reduction reaction in a double perovskite material, PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF), was studied for application as a cathode in a solid oxide fuel cell (SOFC). Electrochemical measurements were performed on a geometrically well-defined dense thin-film (0.8-2 μm thickness) electrode, fabricated as a symmetric cell. In combination with density functional theory (DFT) and molecular dynamics (MD) simulations, experiments provided an insight into the operating mechanism of the SOFC material tested at an open-circuit voltage. The dense thin-film electrode of PBSCF showed a thickness-dependent electrochemical performance, suggesting bulk diffusion limitation. To understand the origin of this diffusion-limiting electrochemical performance, DFT calculations were utilized to calculate the surface (γ) and oxygen vacancy formation (EOV) energies. For example, EOV in the Pr plane (190 kJ/mol) of PBSCF was measured to be lower than that of the BaSr plane (EOV = 297 kJ/mol). In addition, oxygen vacancies were difficult to be created in the BaSr/CoFe terminal surface (EOV = 341.6 kJ/mol) as compared to other terminal surfaces. MD simulations further elaborated on the nature of cation disordering in the surface and subsurface regions, consequently leading to the preferential segregation of the Ba cations to the surface, which is a known phenomenon in such double perovskite materials. Because of cation disordering and segregation of Ba species, the oxygen anion diffusivity (∼10-12 cm2 s-1), calculated from MD, in the near-surface region was observed to be 2 orders of magnitude lesser than that of the bulk (D = 2.98 × 10-10 cm2 s-1) of the material at 973 K. Surface characterization of the thin-film electrode using X-ray photoelectron spectroscopy was indicative of a nonperovskite Ba2+ phase on the electrode surface. The segregation of Ba cations was linked with the transport of oxygen anions, which was limiting the electrochemical performance of the electrode.

Observations of Diurnal Coastal-Trapped Waves with a Thermocline-Intensified Velocity Field

AbstractUsing 18 days of field observations, we investigate the diurnal (D1) frequency wave dynamics on the Tasmanian eastern continental shelf. At this latitude, the D1 frequency is subinertial and separable from the highly energetic near-inertial motion. We use a linear coastal-trapped wave (CTW) solution with the observed background current, stratification, and shelf bathymetry to determine the modal structure of the first three resonant CTWs. We associate the observed D1 velocity with a superimposed mode-zero and mode-one CTW, with mode one dominating mode zero. Both the observed and mode-one D1 velocity was intensified near the thermocline, with stronger velocities occurring when the thermocline stratification was stronger and/or the thermocline was deeper (up to the shelfbreak depth). The CTW modal structure and amplitude varied with the background stratification and alongshore current, with no spring–neap relationship evident for the observed 18 days. Within the surface and bottom Ekman layers on the shelf, the observed velocity phase changed in the cross-shelf and/or vertical directions, inconsistent with an alongshore propagating CTW. In the near-surface and near-bottom regions, the linear CTW solution also did not match the observed velocity, particularly within the bottom Ekman layer. Boundary layer processes were likely causing this observed inconsistency with linear CTW theory. As linear CTW solutions have an idealized representation of boundary dynamics, they should be cautiously applied on the shelf.

Modification of metal surface under the radiation exposure in the atomic-scale

Modification of the surface structure by radiation exposure in metallic materials was investigated by field ion microscopy. Irradiation regimes for the formation of amorphized and nanostructured states in the near-surface platinum regions were determined. Established modes of radiation processing of engineering alloys Pd50Cu30Ag20 to obtain in the near-surface volume structures like microduplex-type structure (the particle size of the phases to 100 nm) with the required combination of properties.

Forming Process during Electron Beam Surfi-Sculpt<sup>TM</sup> on Ti-6Al-4V Alloy

Electron beam Surfi-SculptTM is a novel surface treatment technique applied to produce high level performance Composite-Metal-Weld (ComeldTM) joints. Investigation on forming process during electron beam Surfi-SculptTM on Ti-6Al-4V alloy showed protrusions were formed via a layer-by-layer mode like additive manufacturing process. The near-surface region of electron beam Surfi-Sculpted Ti-6Al-4V alloy was occupied by fusion zone, heat-affected zone and base metal from the outermost surface to the underlying bulk alloy. The microstructure of fusion zone was characterized by a high density of fine acicular martensite phase, leading to a higher micro-hardness. A heat-affected zone was sandwiched between fusion zone and the underlying base metal, with different microstructural features compared to both fusion zone and the base metal.

Self-Consistent Calculation of the Rates of Dissociative Adsorption and Desorption with the Adsorption Isotherm on the Rough Surface of an Adsorbent

The fulfillment of the self-consistency condition in the description of adsorption and desorption the rates of dissociating vapor molecules on the rough surface of an adsorbent (equilibrium and tempered from an equilibrium state) is considered. The adsorption process causes the reconstruction of the equilibrium near-surface region of the adsorbent, but does not change the state of the tempered surface. The adsorption–desorption rates are calculated taking into account the interaction of the nearest neighbors in the quasi-chemical approximation. Four models are considered for the description of a nonuniform surface: the initial averaged multilayer model with different types of adsorption sites (and Henry constants), a model containing all the sites available for adsorption, and three types of its averaging. The first type of averaging is associated with the transition to a single-layer nonuniform surface containing different types of adsorption sites with different Henry constants, the second is associated with the transition to a set of effective uniform monolayers in the multilayer transition region, and the third is associated with the transition to a single-layer effective uniform surface. The condition of self-consistency is fulfilled in the last two types of averaging and is violated in the first type of averaging, as well as in the averaged model taking into account the differences between the types of sites. It is found that the state of a nonuniform surface (equilibrium or tempered) does not affect the self-consistency of the description of the rates of adsorption and desorption in the latter two types of models. The influence of taking into account the effects of correlation of the interaction of species on the condition of self-consistency is shown. The neglect of the effects of correlation of interacting species leads to the violation of the self-consistency condition in all models.

Anomalous Behavior of Lateral C–V Characteristic of an MNOS Transistor with an Embedded Local Charge in the Nitride Layer

The C–V characteristic of the lateral source–substrate junction in a metal–nitride–oxide–semiconductor transistor has been simulated. With a certain voltage across the junction that depends on the dopant concentration in the substrate, a local trapped charge embedded in the nitride layer causes an anomalous rise or fall of the junction capacitance. Such a capacitance variation is associated with charge carrier redistribution in the near-surface region of the substrate when the trapped charge is embedded. This feature of the C–V characteristic can be used to detect a local charge embedded in the insulating layer of a field-effect transistor.

Effect of thermal post-treatment on surface plasmon resonance characteristics of gold nanoparticles formed in glass by UV laser irradiation

Effects of thermal post treatment and of thickness of the initial gold film coated the silicate glass surface, on localized surface plasmon resonance (SPR) of gold nanoparticles formed in glass by UV laser irradiation were studied to develop effective technique for production of gold nanoparticles and their arrays with required and stable characteristics of SPR. Optical spectra of the obtained Au/glass samples showed that SPR of gold nanoparticles formed by irradiation of glasses sputter coated with gold films of thickness 6–10 nm depends of T-treatment, while irradiation of glass coated with gold film ∼70 nm leads to SPR stable of temperature. The origin of such dependences was studied by the structural analysis of gold nanoparticles in Au/glass samples before and after thermal treatment using XRD, EXAFS methods, TEM-EDX images and direct calculations of optical spectra. It was revealed that the changes or stability of SPR parameters under thermal post-treatment of Au/glass samples depend respectively of the presence or absence of tin atoms in the near-surface region (shell) of the gold particles. Thus, if tin atoms are presented in the shell of gold particles (as in “as prepared” Au/glass samples initially coated with gold films of 6–10 nm) then the leave of tin atoms from the particle's volume during the heating leads to partial dissolution of the particles shell and consequently, to decreasing of the particles size, which in turn leads to the observed changes in SPR. The most probable explanation of the presence of tin atoms in the shell of gold particles formed by irradiation of samples initially coated with gold films of thickness 6–10 nm and their absence in the case of sample with film ∼70 nm was proposed, basing on the difference in the effects of the first laser pulse actions on these films on the tin-bath side of the glass samples. The sizes of coherent scattering region in gold nanoparticles before and after thermal treatment of Au/glass samples were determined and the dependence of these sizes upon the thickness of the gold film coating the sample's surface before laser irradiation was revealed.