Surface Energy Barrier Research Articles

Modeling of Tritium Permeation Through Erbium Oxide Coatings

Tritium permeation through erbium oxide coatings has been modeled on the basis of experimental results. Permeation models were constructed step-by-step by the introduction of the following predominant parameters: surface coverage, grain size, and energy barrier. The surface-coverage model agreed with the imperfectly coated samples fabricated by filtered arc deposition as well as by metal-organic decomposition. The grain-boundary-diffusion model also agreed with the coatings fabricated by filtered arc deposition, though it was not applicable to the metal-organic decomposition coatings because of impurities and different layer structures. The energy-barrier model explains the contributions to the additional permeation reduction of the multilayer coatings. The discussion of permeation models provides new design concepts for the development of tritium permeation barriers.

Methodology for Calculating the Volume of Condensate Droplets on Topographically Modified, Microgrooved Surfaces

Liquid droplets on micropatterned surfaces consisting of parallel grooves tens of micrometers in width and depth are considered, and a method for calculating the droplet volume on these surfaces is presented. This model, which utilizes the elongated and parallel-sided nature of droplets condensed on these microgrooved surfaces, requires inputs from two droplet images at ϕ = 0° and ϕ = 90°--namely, the droplet major axis, minor axis, height, and two contact angles. In this method, a circular cross-sectional area is extruded the length of the droplet where the chord of the extruded circle is fixed by the width of the droplet. The maximum apparent contact angle is assumed to occur along the side of the droplet because of the surface energy barrier to wetting imposed by the grooves--a behavior that was observed experimentally. When applied to water droplets condensed onto a microgrooved aluminum surface, this method was shown to calculate the actual droplet volume to within 10% for 88% of the droplets analyzed. This method is useful for estimating the volume of retained droplets on topographically modified, anisotropic surfaces where both heat and mass transfer occur and the surface microchannels are aligned parallel to gravity to assist in condensate drainage.

Fluorinated mix in plain ZDDP oil and commercial oil using design of experiment analysis of all interactions and fundamental study of fluorinated mix in plain ZDDP oils under 2 different r/min test cycles and extreme boundary lubrication

Wear performance of engine oil was studied in the presence of submicron iron fluoride (FeF3) and polytetrafluoroethylene (PTFE) using 2 different r/min test cycles and a ball-on-ring wear tester apparatus under boundary lubrication. The submicron powder mix was called fluorinated mixture with FeF3 catalyst to PTFE ratio of [1:2]. X-ray photoelectron spectroscopy and differential scanning calorimetry analysis of PTFE and FeF3 presented clear evidence of the catalyst importance in preparing the surface and lowering energy barrier. An explicit design of experiment analysis was developed to investigate the interactions of the fluorinated mix in commercial fully formulated oil and in plain oils subject to 2 different r/min test cycles of 100 r/min for the first 5000 revolutions and a 700 r/min until failure or 100 000 revolutions whichever comes first. All tests were generated under extreme boundary lubrication (Hertzian contact pressure of 2.72 GPa) and compared with 1 cycle at 700 r/min. The tests cylinders were baked in oil for a specific period of time and this was included as a factor in the 2-level factorial of the design of experiment (DOE). A fundamental and analytical study of the friction and wear performance was carried out on the plain zinc dialkyldithiophosphates (ZDDP) oil+(FeF3, PTFE) optimized DOE combination to justify the model predictions of the DOE. The fundamental analysis did not include commercial oils in the study since there were many components that affect the wear performance and frictional behaviour. Friction coefficient under boundary lubrication is affected by many factors. In order to study the friction characteristic of optimized tests, DOE was used to simplify experimentations and to investigate the failure and wear responses with respect to fluorinated mix interactions in plain and fully formulated oils. The 2 different r/min cycles tests indicate better performance than the one continuous 700 r/min cycle tests. Findings indicate that the positive factors affecting friction and wear are fluorinated mix concentrations together with oil formulations. Scanning electron microscopy, energy dispersive spectroscopy, focus ion beam, X-ray of wear track, and Auger electron spectroscopy were used to characterize the tribofilm of the two optimized tests of fluorinated mix in plain ZDDP oil under 1 continuous 700 r/min and 2 different r/min test cycles. Results indicate that the thickness of the tribofilms is in the range of 220 nm and phosphorus exists in both wear tracks with more traces in the 2 r/min cycles sample.

Fully Reversible Transition from Wenzel to Cassie−Baxter States on Corrugated Superhydrophobic Surfaces

Liquid drops on textured surfaces show different dynamical behaviors depending on their wetting states. They are extremely mobile when they are supported by composite solid-liquid-air interfaces (Cassie-Baxter state) and immobile when they fully wet the textured surfaces (Wenzel state). By reversibly switching between these two states, it will be possible to achieve control over the fluid dynamics. Unfortunately, these wetting transitions are usually prevented by surface energy barriers. We demonstrate here a new, simple design paradigm consisting of parallel grooves with an appropriate aspect ratio that allows for the controlled, barrierless, reversible switching of the wetting states upon application of electrowetting. We report a direct observation of the barrierless dynamical pathway for the reversible transitions between the Wenzel (collapsed) and Cassie-Baxter (suspended) states and present a theory that accounts for these transitions, including detailed lattice Boltzmann simulations.

Surfactant effect of arsenic doping on modification of ZnO (0001) growth kinetics

The effect of arsenic doping on the growth kinetics of ZnO during metalorganic vapor phase epitaxy has been investigated. Arsenic was found to segregate to the growth surface and facilitate layer-by-layer growth. Such surfactant enhances the lateral expansion of the terraces preferential along [1¯1¯20] direction and also reduces the screw lattice distortion. Arsenic is expected to reduce the total surface energy and diffusion barrier of oxygen adatoms, hence producing Zn-rich surface condition on the growth front, in which two-dimensional growth is thermodynamically and kinetically favored. The origin of tiny hexagonal pits formed on the wide terrace is discussed in terms of the modified step-bunching mechanism.

Molecular Recognition between the Constituents of a Pseudorotaxane Studied by Scanning Tunneling Microscopy

The formation of intermolecular complexes of two large molecules—a macrocycle and a semiaxle, which have been used in templated syntheses of amide rotaxanes—was studied by scanning tunneling microscopy (STM) and density functional theory (DFT). These experiments mimic the so-called “threading process”, which is based on intermolecular recognition and which is essential for the rotaxane synthesis in solution. First, ordered monolayers of a tetralactam macrocycle (TLM), i.e. the rotaxane wheel, are prepared on a Au(111) surface. Then, semiaxles (SA) are deposited on top of these ordered TLM layers at ca. 140 K. In solution, the SA molecule threads into the TLM cavity by formation of three hydrogen bonds between the amide groups of both molecules. On the Au(111) surface, the scenario is similar, although different in detail due to geometric restrictions given by the underlying Au(111) surface and conformational energy barriers due to the confinement of the TLM geometry in the ordered monolayer structure. Thr...

Carbon Nanotube Based Chemical Sensors for Space and Terrestrial Applications

A nanosensor technology has been developed using nanostructures, such as single walled carbon nanotubes (SWNTs), on a pair of interdigitated electrodes (IDE) processed with a silicon-based microfabrication and micromachining technique. These sensors have been exposed to nitrogen dioxide, acetone, benzene, nitrotoluene, chlorine, and ammonia in the concentration range of ppm to ppb at room temperature. Due to the large surface area, low surface energy barrier and high thermal and mechanical stability, nanostructured chemical sensors potentially can offer higher sensitivity, lower power consumption and better robustness than the state-of-the-art systems, which make them more attractive for defense and space applications. Combined with MEMS technology, lightweight and compact size sensors can be made in wafer scale with low cost. Additionally, a wireless capability of such a sensor chip can be used for networked mobile and fixed-site detection and warning systems for military bases, facilities and battlefield areas.

Effect of the surface energy state on the dynamics of dislocations in ionic crystals

The effect of the surface state and boundary physicochemical conditions on the starting stresses and dynamics of near-surface dislocations in lithium fluoride and sodium chloride crystals has been investigated by the method of prick dislocation rosettes arising in alkali halide single crystals upon microindentation. It has been shown that crystals with ionic bonds are characterized by a surface energy (potential) barrier that hinders the movement of near-surface dislocations and influences their starting stresses in crystal lattices.

Thermodynamic and kinetic considerations of nucleation and stabilization of acoustic cavitation bubbles in water

Qualitative explanation for a homogeneous nucleation of acoustic cavitation bubbles in the incompressible liquid water with simple phenomenological approach has been provided via the concept of the desorbtion of the dissolved gas and the vaporization of local liquid molecules. The liquid medium has been viewed as an ensemble of lattice structures. Validity of the lattice structure approach against the Brownian motion of molecules in the liquid state has been discussed. Criterion based on probability for nucleus formation has been defined for the vaporization of local liquid molecules. Energy need for the enthalpy of vaporization has been considered as an energy criterion for the formation of a vaporous nucleus. Sound energy, thermal energy of the liquid bulk (Joule–Thomson effect) and free energy of activation, which is associated with water molecules in the liquid state (Brownian motion) as per the modified Eyring’s kinetic theory of liquid are considered as possible sources for the enthalpy of vaporization of water molecules forming a single unit lattice. The classical nucleation theory has then been considered for expressing further growth of the vaporous nucleus against the surface energy barrier. Effect of liquid property (temperature), and effect of an acoustic parameter (frequency) on an acoustic cavitation threshold pressure have been discussed. Kinetics of nucleation has been considered.

Multiple separation of self-running drops by pinching a three-phase contact line

We produced multiple minute self-running drops from an original drop without external input power by pinching a three-phase contact line. By branching into surface energy barriers, the drop is forced to separate while translating on a flat surface. A symmetry in the drop front wetted perimeter conduces to equal division of the drop. Our experiments suggest that at least seven divisions (128 drops) can be conducted sequentially to give equal final volumes of approximately 4nl. A division of carrier liquid volume by 1∕27 is expected to enable multiple analysis on many separate stations. This is the only drop separation method at present that does not require external power input.

Investigation of Nitrogen Induced closely coupled Sb based Quantum Dots for Infrared Sensors Application

ABSTRACTWe report the growth and structural properties of InSb and InSb:N quantum dots on InAs and GaAs substrates. Strain induced, self-assembled quantum dots are grown using solid-source molecular beam epitaxy. For improved growth control, we developed a growth technique similar to atomic layer epitaxial methods. InSb and InSb:N multiple quantum dots formed on both InAs and GaAs. We explain the formation of multiple quantum dots by the anisotropic distribution of strain energy within the quantum dot, the long adatom lifetime during atomic layer epitaxy, and the low bond energy of InSb. Nitrogen incorporation during formation of quantum dots changes surface energy barrier and causes anisotropic distribution of strain energy, results in formation of closely coupled multiple quantum dots in <110> orientation. We obtained mid infrared luminescence around 3.6 μm from InNSb QDs grown on InAs substrate, where it exhibits relatively low efficiencies of nitrogen incorporation compared to the quantum well structure.

Magnetic Flux Penetration and Motion in Antiferromagnetic Superconductors

AbstractFor Abstract see ChemInform Abstract in Full Text.

Magnetic Flux Penetration and Motion in Antiferromagnetic Superconductors

The paper reviews a concept of induced spin-flop domain inside vortices in an antiferromagnetic superconductor. Such phenomenon may occur when an external magnetic field is strong enough to flip over magnetic moments in the core of the vortex from their ground state configuration. The formation of the domain structure inside vortices modifies the surface energy barrier of the superconductor. During this process the entrance of the flux is stopped and a newly created state exhibits perfect shielding. Such behavior should be visible as a plateau on the dependence of flux density as a function of the external magnetic field. The end of the plateau determines the critical field, which has been called the second critical field for flux penetration. Moreover, it is predicted and described how this phenomenon modifies flux creep in layered superconductors. The various scenarios of changing the creep regime from thermal to quantum and vice versa at constant temperature are discussed.

Two-step flux penetration in classic antiferromagnetic superconductor

The influence of antiferromagnetic order on the mixed state of a superconductor may result in creation of spin-flop domains along vortices. This may happen when an external magnetic field is strong enough to flip over magnetic moments in the vortex core from their ground state configuration. The formation of domain structure causes modification of the surface energy barrier, and creation of the new state in which magnetic flux density is independent of the applied field. The modified surface energy barrier has been calculated for parameters of the antiferromagnetic superconductor DyMo$_{6}$S$_{8}$. The prediction of two-step flux penetration process has been verified by precise magnetization measurements performed on the single crystal of DyMo$_{6}$S$_{8}$ at milikelvin temperatures. A characteristic plateau on the virgin curve $B(H_0)$ has been found and attributed to the modified surface energy barrier. The end of the plateau determines the critical field, which we call the second critical field for flux penetration.

Two-step flux penetration in layered antiferromagnetic superconductor

A layered antiferromagnetic superconductor in the mixed state may possess magnetic domains created along the Josephson vortices. This may happen when an external magnetic field is strong enough to flip over magnetic moments, lying in the phase core of the Josephson vortex, from their ground state configuration. The formation of the domain structure of the vortices modifies the surface energy barrier of the superconductor. During this process the entrance of the flux is stopped and a newly created state exhibits perfect shielding. Such behavior should be visible as a plateau on the dependence of flux density as a function of the external magnetic field. The end of the plateau determines the critical field, which we call the second critical field for flux penetration.

Effect of doping of TiO 2 support with altervalent ions on physicochemical and catalytic properties in oxidative dehydrogenation of propane of vanadia–titania catalysts

Vanadia phase (one monolayer) was deposited on TiO 2 anatase doped with Ca 2+, Al 3+, Fe 3+ and W 6+ ions and the catalysts thus obtained (VMeTi) were characterized by XPS, work function technique, decomposition of isopropanol (a probe reaction for acido–basic properties) and tested in oxidative dehydrogenation of propane. The doping of the TiO 2 support modifies physicochemical and catalytic properties of the active vanadia phase with respect to the undoped TiO 2. The specific activity in the propane oxydehydrogenation decreases in the order: VFeTi>VWTi>VTi>VAlTi>VCaTi (3), whereas the selectivity to propene follows the sequence: VWTi<VTi<VFeTi<VAlTi<VCaTi (4). No clear correlation has been observed between the activity of the catalysts and the acido–basic properties. On the other hand, the rate of the isopropanol dehydration (the acidity) decreases in the same order as the selectivity increases (4). The increase in the selectivity with the decrease in the acidity is ascribed to the easier desorption of propene from the less acidic surface, preventing the consecutive total combustion of propene. The selectivity to propene can be also correlated with the work function values which decrease in the series: VWTi>VTi>VFeTi>VAlTi>VCaTi. This implies that the lower is the surface energy barrier for transfer of electrons from the catalyst to the reacting molecules the higher is the selectivity to the partial oxidation product. It is argued that owing to the decrease in this energy barrier the reoxidation step in the catalytic reaction, involving such a transfer: O 2+4e→2O 2− is fast, thus, preventing the presence of intermediate non-selective electrophilic oxygen species on the surface.

Effect of self-assembled Ge nanostructures on Si surface electronic properties

Incorporating self-assembled Ge islands on Si surfaces into electronic devices has been suggested as a means of forming small features without fine-scale litho- graphy. For use in electronic devices, the electrical properties of the deposited Ge and their relation to the underlying Si substrate must be known. This report presents the results of a surface photovoltage investigation of the surface energy barrier as increasing amounts of Ge are added to a Si surface by chemical vapor deposition. The results are interpreted in terms of band discontinuities and surface states. The surface barrier increases as a wetting layer is deposited and continues to increase as defect-free islands form. It saturates as the islands grow. As the amount of Ge continues increasing, defects form, and the surface barrier decreases because of the resulting allowed states at the Ge/Si interface. Qualitatively similar behavior is found for Si(001) and Si(111). Covering the Ge with Si reduces the surface-state density and possibly modifies the wetting layer, decreasing the barrier to one more characteristic of Si. Initial hydrogen termination of the surface decreases the active surface-state density. As the H desorbs, the surface barrier increases until it stabilizes as the surface oxidizes. The behavior is briefly correlated with scanning-tunneling spectroscopy data.

Reactivity of Free Radicals on Hydroxylated Quartz Surface and Its Implications for Pathogenicity of Silicas: Experimental and Quantum Mechanical Study

We studied the adsorption of hydroxyl radicals and superoxide anion radicals on a hydroxylated alpha-quartz surface using cluster and periodic slab models by means of density functional calculations. Models of two hydroxylated alpha-quartz surfaces--(0001) and (0111)--have been used in the simulations. The hydroxyl radical adsorbs readily on both surfaces. The subsurface Si-O bonds are weakened during the adsorption resulting in surface layer destabilization. This destabilization leads directly to surface disintegration in the case of *OH/(0111) adsorption. The product of the surface disintegration and reconstruction is a surface terminated by silanol groups (Si-OH) and siloxyl radicals (Si-O*). The model calculations suggest that adsorption of *OH on a hydroxylated quartz surface transforms a chemically inert, aged, silanol terminated surface to a very reactive, silicon-based radical terminated surface. The activated surface may then cause oxidative damage to the adsorbed biomaterial. The superoxide anion radical adsorbs on both surfaces, but the adsorption products are only weakly bonded to the surface. The calculated energy barrier for the O2*- activated subsurface Si-O bond dissociation is 10 kcal/mol, which is higher than for the *OH activated process (4 kcal/mol). The calculated weaker bonding to the surface and higher activation energy barrier suggest that the superoxide anion radical will be less efficient in reactivation of an aged, hydroxylated quartz surface than the hydroxyl radical. The importance of the specific geometry of the surface silicon atoms on the surface reactivity and adsorption properties is also discussed. The theoretical predictions are supported experimentally using chemiluminescence to monitor reactivation of the aged silica surface by superoxide anion radicals.

Energy barrier for the growth transition step-flow/step-bunching during epitaxy of InP/InP

We report the growth mode evolution of homoepitaxially grown InP by metal organic vapour phase epitaxy examined by ex-situ atomic force microscope (AFM). We varied the growth temperature between 500°C and 630°C and used substrates with different miscut angle from 0.15° to 2° off towards (111)A. After annealing under phosphine above 500°C, the wafer surface recovers its terrace structure with nominal width terraces. At growth temperatures of 580–600°C, the step flow is dominantly observed. At higher temperatures of 630°C, the step flow growth evolves towards step bunching when increasing the miscut angle from 0.2° to 2° off and the terrace width saturates at ≈40 nm. The effect of the growth temperature is analysed taking account for the different surface energy barrier for the transition step flow/step bunching. A clear evidence of the dependence of the activation energy with miscut angle will be shown. The Schwoebel's barrier evolution is shown from the growth onto differently misoriented substrates.

Change in the Oxidation State of the Adsorbed Oxygen Equilibrated at 25°C on ZnO Surface during Room Temperature Annealing after Rapid Quenching

The porous ZnO quenched from 500°C in air showed very low electrical resistance with non-ohmic current–voltage behavior. This characteristic was resulted from the surface energy barrier formed by negatively charged chemisorbed oxygen. The resistance and the non-ohmic exponent of the quenched specimen gradually decreased by holding the specimen at room temperature. Electron spin resonance measurements showed a change in the oxidation state of chemisorbed oxygen from its high temperature type, O2-, into low temperature types, O2- and O- while being kept at room temperature. From this viewpoint, it was judged that the decrease in the resistance and the non-ohmic exponent resulted from the release of the trapped electron by this change in the oxidation state. The resistance and the non-ohmic exponent of the sample doped with CoO were higher than those of the undoped sample, and their decrease for the CoO-doped sample while being kept at room temperature was slower than those for the undoped sample. The rate of change in the oxidation state was also slower in the CoO doped sample.