Reversible Trapping Research Articles

Hydrogen transport through nickel hydroxide film: current transient analysis

Hydrogen transport through electro-deposited nickel hydroxide films containing various fractions of cobalt hydroxide has been investigated by analysing build-up and decay current transients under the application of various voltage steps from 70 to 400 mV. The build-up and decay transients of hydrogen flux into and from the electrode film with reversible trap under the impermeable boundary condition were numerically simulated on the basis of the McNabb and Foster's physical model of hydrogen trapping. The simulated transients were compared to the measured data. The occurrence of current plateau upon hydrogen extraction from the completely hydrogen-injected films indicated that the hydrogen transport through the film proceeds by the movement of NiOOH/Ni(OH) 2 phase boundary. The velocity and mobility of the phase boundary movement upon hydrogen extraction were determined to be orders of 10 −6cm s −1 and 10 −5cm s −1 V −1 in magnitude, respectively. It is concluded that both the velocity and mobility of the NiOOH/Ni(OH) 2 phase boundary movement is raised by the Co(OH) 2 incorporation into the Ni(OH) 2 film. From the appearance of build-up and decay current transient curves, it is suggested that Co(OH) 2 incorporation into the Ni(OH) 2 film enhances the release of the trapped hydrogen.

Hydrogen transport through Pd electrode: current transient analysis

The hydrogen transport through a palladium electrode in the coexistence of two palladium hydride phases (α- and β-PdH) has been investigated by analysing current decay transients based on the modified McNabb and Foster's physical model of hydrogen trapping, considering interstitial sites in β-phase as reversible trap sites, supplemented by cyclic polarization curve and open-circuit potential transients. From the appearance of the three-staged current decay transients and hydrogen content in the electrode, it is inferred that the β-phase is formed just beneath the electrode surface during the hydrogen injection into the electrode at overpotentials between 0.08 and −0.02 V(rhe) as the β-PdH patches sporadically embedded in α-PdH matrix and below − 0.02 V(rhe) as the β-phase layer completely embedded in α-PdH matrix. During the hydrogen extraction from the electrode, it follows the hydrogen transport initially proceeds to decomposition of the complete β-phase layer into α-phase by “up-hill diffusion” of hydrogen from inner hydrogen-poor α-phase towards outer hydrogen-rich β-phase, accompanied by interface-controlled phase boundary movement, intermediately it proceeds to complete decomposition of the α-phase into Pd by simple diffusion through α-phase, finally followed by complete decomposition of the remaining sporadic β-PdH patches into α-PdH or Pd. The “up-hill diffusion” is accomplished by transferring interstitial hydrogen in the α-phase to hydrogen trapped in the β-phase. The stress gradient across the α β phase boundary developed during the hydrogen injection helps the hydrogen transport during the hydrogen extraction.

DNA gyrase, topoisomerase IV, and the 4-quinolones.

For many years, DNA gyrase was thought to be responsible both for unlinking replicated daughter chromosomes and for controlling negative superhelical tension in bacterial DNA. However, in 1990 a homolog of gyrase, topoisomerase IV, that had a potent decatenating activity was discovered. It is now clear that topoisomerase IV, rather than gyrase, is responsible for decatenation of interlinked chromosomes. Moreover, topoisomerase IV is a target of the 4-quinolones, antibacterial agents that had previously been thought to target only gyrase. The key event in quinolone action is reversible trapping of gyrase-DNA and topoisomerase IV-DNA complexes. Complex formation with gyrase is followed by a rapid, reversible inhibition of DNA synthesis, cessation of growth, and induction of the SOS response. At higher drug concentrations, cell death occurs as double-strand DNA breaks are released from trapped gyrase and/or topoisomerase IV complexes. Repair of quinolone-induced DNA damage occurs largely via recombination pathways. In many gram-negative bacteria, resistance to moderate levels of quinolone arises from mutation of the gyrase A protein and resistance to high levels of quinolone arises from mutation of a second gyrase and/or topoisomerase IV site. For some gram-positive bacteria, the situation is reversed: primary resistance occurs through changes in topoisomerase IV while gyrase changes give additional resistance. Gyrase is also trapped on DNA by lethal gene products of certain large, low-copy-number plasmids. Thus, quinolone-topoisomerase biology is providing a model for understanding aspects of host-parasite interactions and providing ways to investigate manipulation of the bacterial chromosome by topoisomerases.

Reversible and irreversible hydrogen trapping in metals: new computer-based code THYDA

Use of metallic materials for fusion reactor components and their performance in the presence of hydrogen isotopes require a development of a physical transport model that permits to explain the dynamics of hydrogen absorption and release. Trapping and release processes will strongly affect hydrogen isotopes inventories. The non-linear nature of hydrogen diffusion equations requires a numerical analysis. In this work a new computer code named THYDA was developed. In relation to thermal cycles, THYDA permits to calculate the concentrations of trapped and movable hydrogen atoms in metallic samples as well as hydrogen evolution rates during thermal desorption at elevated temperature with variable heating rates. In these calculations it is possible to take into account both the internal and external sources of hydrogen isotopes (i.e. generation in (n, p) nuclear reactions and by D-T ion implantation).

Evidence of Airway Obstruction in Children With Idiopathic Scoliosis

Restrictive pulmonary function abnormalities are reported in children and adolescents with idiopathic scoliosis. We hypothesized that spirometry alone, without more extensive testing, including the measurement of lung volumes, is inadequate in characterizing lung function in these children and may miss obstructive abnormalities including significant gas trapping. To examine this hypothesis, we reviewed the pulmonary function tests of 44 children (36 female, 8 male) between the ages of 10 and 18 years with idiopathic scoliosis prior to surgical correction. Spirometry, measurements of lung volumes with both plethysmographic and helium dilution techniques, and bronchodilator response were analyzed for evidence of reversible airway obstruction and gas trapping. Eighteen of 44 (41%) subjects had significant restriction. Only 3 (7%) subjects met standard criteria for airflow obstruction. However, 20 (46%) subjects had an elevated total gas volume by plethysmography-functional residual capacity by helium dilution ratio indicative of moderate or severe gas trapping, 10 (23%) subjects showed mild gas trapping, 8 (18%) subjects had a ratio suggestive of gas trapping, and only 6 (14%) subjects were normal. Additionally, significant improvement in airway mechanics was noted after bronchodilator administration. Specific conductance improved in all subjects, with a mean increase of 62% +/- 8.0 (p<0.001). The residual volume-total lung capacity ratio and total gas volume by plethysmography also decreased significantly (mean decrease, 22.5% +/- 3.0 and 15% +/- 1.0, respectively, p<0.001) in response to inhaled bronchodilators. In conclusion, although restrictive defects are commonly present in children with idiopathic scoliosis, significant gas trapping and responses to bronchodilators also commonly occur. These abnormalities may be missed without extensive pulmonary function testing.

Electrical properties of Ta2O5 films obtained by plasma enhanced chemical vapor deposition using a TaF5 source

High quality Ta2O5 thin films have been obtained from TaF5 and O2 using a microwave excited electron cyclotron resonance plasma at low pressure (∼2 mTorr). Physical and electrical measurements reveal that the as-deposited amorphous films have excellent properties: refractive indices ∼2.16, dielectric constants ∼25, and leakage currents &amp;lt;10−10 A cm−2 at 2.5 V (0.3 MV cm−1, 85 nm thick, 13 nm SiO2 equivalent). Trapping and conduction properties of these layers have also been investigated, showing a reversible electron trapping and a trap-limited Poole–Frenkel effect.

Aziridines. 69. Reactions of N-Acylaziridines with Sodium Metal and Sodium Naphthalenide. Elimination of olefines

Reactions of N-acylaziridines 1a–g (N-benzoyl except 1d) with sodium or naphthalenide N.− in THF provide a variety of products that usually arise via the aziridino ketyls 2. Homolytic ring opening of 2 generates the amidatoalkyl radicals 3. Only with a very short reaction time were small amounts of benzil or benzoylnaphthalenes obtained indicating a reversible trapping of 2 by dimerization or coupling with N.−. Homolysis of 2 produced always the more stable 3 apart from reactions of monomethylaziridines 1c,d where the primary radical i-3c,d is kinetically favoured. The amides R1CONHCHR4CHR2R3 (9, isopropylamides i-9c,d from 1c,d) were usually the main products. 9 arise from 3 either by H atom abstraction from THF (probably in sodium metal runs) or by reduction of 3 to carbanions 5 that abstract a proton from THF (N.− runs). Addition of 5a (R2−4 = H) to 1a gives finally the ketone 8a. Self reaction of primary radical 3a is dimerization. Self reaction of tertiary or secondary radicals is disproportionation when an allylamide arises. This isomerizes to an enamide unless it is conjugated. R2R3CCHR4 and R1CONH2 arise (probably) always. The mechanism, possibly a cyclic process of anion 6, is not clear.

Forward and reverse electronic energy transport and trapping in solution. I. Theory

A theoretical study of forward and reverse incoherent transport and trapping of electronic excitations in a two-component disordered system is carried out. The method used is the diagrammatic expansion of the configuration averaged Green function of the master equation for this problem. A new summation method based on the fractal structure of multigraphs with nodes allowing the calculation of the investigated infinite series is introduced. The results of an exact diagrammatic analysis are presented. The donor fluorescence decay function, relative quantum yield and emission anisotropy are obtained using the self-consistent approximation.

Forward and reverse electronic energy transport and trapping in solution. II. Numerical results and Monte Carlo simulations

Abstract This paper deals with the forward and reverse electronic excitation energy transport and trapping in a three-dimensional two-component disordered system. The numerical results for the donor fluorescence decay function and the relative quantum yield are obtained using the self-consistent diagrammatic method (SCDM) discussed in detail in the preceding paper (Part I). The results for the Forster dipole-dipole transfer are evaluated within the framework of the SCDM model in the two- and three-body approximation. The fluorescent observables depend on the ratios ξD = RODA/RODD and ξΛ = ROAD/ROAA of the critical radii as well as on the reduced concentrations γ XY = 4 3 π (R O XY ) 3 ϱ γ , X, Y ϵ { D, A } . A comparison to oth theoretical treatments as well as to the Monte Carlo simulations and experimental results is presented.

Hydrogen Embrittlement of Cast Al-AL2O3 Particulate Composites

The effect of hydrogen on the mechanical properties of cast Al-Al 2 O 3 composites, containing 2, 4 and 8 wt.% Al 2 O 3 , prepared by vortex casting process was assessed by tensile testing hydrogen pre-charged specimens. Hydrogen charging reduced both ductility and ultimate tensile strength of the composites. The degree of hydrogen embrittlement was lower in composites compared with the base metal. The hydrogen embrittlement behaviour is explained by considering the reversible and irreversible trap sites in the composites

Analysis of Olefinic Gasolines with Multidimensional Gas Chromatography

This paper evaluates the strengths and weaknesses of a multidimensional Paraffin-lso/normal-Olefin-Naphthene-Aromatic (PIONA) analyzer for olefinic naphthas. These complex materials, which can overwhelm conventional capillary gas chromatographic (GC) techniques, are analyzed with reasonably good accuracy (generally within 0.2 wt % for nonolefinic compounds, and approximately 0.5 wt % for olefinic species) using the PIONA instrument. Although not a speciation technique, this multidimensional GC method provides accurate class and carbon number information from C4 through C11. Using traditional single capillary GC, it is difficult to resolve all the species present in olefinic sample types beyond approximately C6. Limitations of the PIONA instrument, including non-idealities of the reversible olefin trap, are discussed. Suggestions for improved aromatic characterization and analysis of oxygenated gasolines are also presented.

Forward and reverse excitation energy transport and concentration depolarization in two-component systems

A theoretical study of forward and reverse electronic excitation energy transport and trapping in a two-component disordered system is presented. The method used is an extension of the diagrammatic technique applied by Loring, Andersen, and Fayer to the system with no reverse energy transfer. The results of an exact diagrammatic analysis of the configuration-averaged Green function of the master equation are presented. The following steady-state observables: relative donor quantum yield and emission anisotropy, are obtained using the self-consistent approximation. The numerical results for Förster dipole—dipole transfer are evaluated within the framework of the improved two-body approximation introduced herein (the two-body approximation and the three-body treatment of the energy migration process in the donor ensemble). A comparison with the experimental data and Monte Carlo results is presented.

Theoretical fluorescence induction curves derived from coupled differential equations describing the primary photochemistry of photosystem II by an exciton-radical pair equilibrium

Fluorescence induction curves were calculated from a molecular model for the primary photophysical and photochemical processes of photosystem II that includes reversible exciton trapping by open (PHQ(A)) and closed (PHQ(-) (A)) reaction centers (RCs), charge stabilization as well as quenching by oxidized (P(+)HQ((-)) (A)) RCs. For the limiting case of perfectly connected photosynthetic units ("lake model") and thermal equilibrium between the primary radical pair (P(+)H(-)) and the excited singlet state, the primary reactions can be mathematically formulated by a set of coupled ordinary differential equations (ODE). These were numerically solved for weak flashes in a recursive way to simulate experiments with continuous illumination. Using recently published values for the molecular rate constants, this procedure yielded the time dependence of closed RCs as well as of the fluorescence yield (= fluorescence induction curves). The theoretical curves displayed the same sigmoidal shapes as experimental fluorescence induction curves. From the time development of closed RCs and the fluorescence yield, it was possible to check currently assumed proportionalities between the fraction of closed RCs and either (a) the variable fluorescence, (b) the complementary area above the fluorescence induction curve, or (c) the complementary area normalized to the variable fluorescence. By changing selected molecular rate constants, it is shown that, in contrast to current beliefs, none of these correlations obeys simple laws. The time dependence of these quantities is strongly nonexponential. In the presence of substances that quench the excited state, the model predicts straight lines in Stern-Volmer plots. We further conclude that it is impossible to estimate the degree of physical interunit energy transfer from the sigmoidicity of the fluorescence induction curve or from the curvature of the variable fluorescence plotted versus the fraction of closed RCs.

Hydrogen Trapping in Fe&amp;ndash;Ti Alloys

The hydrogen trapping effects in Fe-Ti alloys were studied by repetitive electrochemical permeation measurements and gas pressure oscillating permeation measurements. A theoretical analysis has been made of the effects of both reversible and irreversible trapping sites on hydrogen diffusion using the Crank-Nicolson finite difference method. The break-through times are different for the first and second electrochemical polarizations in Fe-Ti alloys. The difference of the break-through times for the first and second polarizations is due to the irreversible trapping sites in the alloys, and the difference of break-through times in the second polarization for the alloys and pure iron is due to the reversible trapping sites

Calculation of the trapping force in a strongly focused laser beam

We present what is, to the best of our knowledge, the first detailed calculation of the reverse trapping force acting on a dielectric sphere when it is illuminated by a strongly focused laser beam. The calculation is carried out within the geometrical optics approximation. The phenomenon of laser trapping was discovered experimentally by Ashkin et al. [ Opt. Lett.11, 288 ( 1986)] and is of great practical interest in view of the possibility it offers for freely manipulating biological particles, such as viruses and bacteria, in a nondestructive manner. We support our calculations by a qualitative experiment that clearly shows the accessibility of the trapping effect in practice. We use, as an experimental improvement, an objective with a central field stop producing a conical dark field. This enhances the relative contribution from high-N.A. illumination and makes it easier to achieve optical trapping.

The effect of prior mechanical deformation on hydrogen transport through 13% chromium martensitic stainless steel

Hydrogen permeation measurements have been made on a 13% chromium martensitic stainless steel which had been previously mechanically deformed. Tensile deformation was applied up to strains of 8%, corresponding to the UTS, and cold rolling was used to generate higher levels of deformation up to 50%. Analysis of the permeation transients for strains below UTS has shown a small but significant increase in the number of reversible trap sites, which is consistent with their association with dislocations. There was little change in the number of irreversible trap sites by virtue of maintaining the strain below the level at which significant interfacial separation could occur. The results of this study confirm that the major reversible trap sites in this steel are associated with dislocations.

Reversible trapping on a cubic lattice: Comparison of theory and simulations

Simulations of the kinetics of the reversible diffusion-influenced reaction A+B⇄AB on a cubic lattice, with initial conditions [A] = [B], [AB]=0, are compared with the predictions of two approximate theoretical formalisms. The first involves a simple rate equation with rate coefficients that are proportional to the time-dependent rate coefficient for an irreversible reaction. The second, which is based on a superposition approximation, contains a rate coefficient that explicitly depends on the bulk concentrations. Both reduce to the Smoluchowski approach in the irreversible limit. The results obtained using the modified rate equation formalism are exact at short times, but tend to approach equilibrium too rapidly. The predictions of the computationally more demanding superposition formalism agree remarkably well with the simulations for all times for the range of parameters examined.

Spatial profiles of trapped hydrogen or centers excited by hydrogen diffusion

Nearly step-function-like profiles of trapped hydrogen or of properties, such as donor concentration, related to hydrogen diffusion are often observed in semiconductors. Expressions are derived for the depth of such profiles and their finite width as a function of time for arbitrary ratios of hydrogen concentration to trap concentration and for nonzero detrapping rates. The depth scales as t1/2 for normal trapping, but as (t ln t)1/2 if the observed quantity is excited by hydrogen passing in the vicinity without being trapped. The width of the profile is time-independent if hydrogen traps irreversibly. For reversible trapping or excitation without trapping the width is proportional to t1/2 or (t ln t)1/2, respectively.

The influence of palladium on the hydrogen-assisted cracking resistance of PH 13-8 Mo stainless steel

We compare the hydrogen-assisted cracking resistance of wrought PH 13-8 Mo stainless steel alloyed with 0.4 to 1.0 wt pct palladium to the conventional alloy when aged to yield strengths of 1170 to 1250 MPa. Intergranular hydrogen cracking is suppressed with Pd in both static load and constant extension rate tests conducted with electrochemical hydrogen charging. These results are analyzed to elucidate the role of Pd in suppressing intergranular cracking. Palladium is found both in substitutional solid solution in the martensitic phase and also in the form of randomly distributed PdAl precipitates in all Pd-modified alloys. Interfacial segregation of Pd to grain boundaries and lath boundaries is not observed at any levels above a detection limit of approximately 0.5 monolayers. Hydrogen permeation analyses indicate that hydrogen ingress is not inhibited by Pd but that apparent diffusion coefficients are lowered relative to the conventional alloy. Lower diffusion coefficients are consistent with the creation of a strong but reversible hydrogen trap, identified as the uniformly distributed PdAl phase. We hypothesize that PdAl trap sites force a redistribution of trapped hydrogen, which lowers the amount of interfacially segregated hydrogen at prior austenite grain boundaries for the electrochemical conditions applied. These assertions are supported by a simplistic trapping model for PH 13-8 Mo which shows that both the hydrogen trap binding energy and the trap density for the PdAl trapping site are greater than the hydrogen trap binding energy and density for prior austenite grain boundaries.

Hot-carrier-induced degradation of the back interface in short-channel silicon-on-insulator MOSFETS

The tolerance of silicon-on-insulator MOSFETs to hot-carrier injection into the buried oxide is investigated. It is shown that stressing of the back channel results in reversible electron trapping and formation of localized defects at the buried interface. This damage is responsible for the transconductance overshoot, large threshold voltage shift, and attenuated kink effect. It is also noticed that even in moderately thin films the back oxide damage does not affect the front-channel operation and, conversely, stressing the front channel does not generate defects at the buried interface. These findings indicate that the hot-carrier degradation of the buried oxide might be chosen as a sensitive criterion for optimizing SIMOX (separation by implantation of oxygen) structures.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>