Adiabatic Mechanism Research Articles

Особенности разрушения ударников из пористого сплава на основе вольфрама с упрочняющим наполнителем при взаимодействии с бронепреградами

In this work, computational and experimental studies of the process of destruction of composite firing pin of porous alloy tungsten+nickel+iron+cobalt with 10 % content of titanium tungsten carbide at high-speed collision with steel barriers. It is shown that at ballistic tests with the broad range of speeds, significant exceeding of penetration of these firing pins in steel barriers in comparison with a mass-dimensional analog of the W-Ni-Fe-90 alloy. Based on the analysis of the crater morphology and structure of the striker fragments after penetration into the barrier, the assumption of implementation of the self-sharpenings mode of the firing pin, by means of localization of plastic deformation is made that leads to decrease in the effective area of interaction and increase in depth of penetration. Modification of a mathematical model of a porous ideal elasto-plastic solid with complex structure for the description of destruction with a possibility of accounting of the adiabatic shift mechanism in the course of interaction of the firing pin and a barrier is carried out.

Development of a Misaligned Tropical Cyclone

AbstractA cloud-resolving model is used to examine the virtually shear-free evolution of incipient tropical cyclones initialized with different degrees of misalignment between the lower- and middle-tropospheric centers of rotation. Increasing the initial displacement of rotational centers (the tilt) from a negligible value to several hundred kilometers extends the time scale of hurricane formation from 1 to 10 days. Hindered amplification of the maximum tangential velocity υm at the surface of a strongly perturbed system is related to an extended duration of misalignment resulting from incomplete early decay and subsequent transient growth of the tilt magnitude. The prolonged misalignment coincides with a prolonged period of asymmetric convection peaked far from the surface center of the vortex. A Sawyer–Eliassen model is used to analyze the disparity between azimuthal velocity tendencies of selected pre–tropical storm vortices with low and high degrees of misalignment. Although no single factor completely explains the difference of intensification rates, greater misalignment is linked to weaker positive azimuthal velocity forcing near υm by the component of the mean secondary circulation attributed to heating by microphysical cloud processes. Of note regarding the dynamics, enhanced tilt only modestly affects the growth rate of kinetic energy outside the core of the surface vortex while severely hindering intensification of υm within the core for at least several days. The processes controlling the evolution of the misalignment associated with inefficient development are examined in detail for a selected simulation. It is found that adiabatic mechanisms are capable of driving the transient amplification of tilt, whereas diabatic processes are essential to ultimate alignment of the tropical cyclone.

Why Silicon Doping Accelerates Electron Polaron Diffusion in Hematite.

It is common that dopants enhance the conductivity of hematite Fe2O3, a popular photoanode, but the origin of the enhancement remains unclear. We establish the detailed mechanism by performing ab initio molecular dynamics simulations on electron polarons (EPs) in Fe2O3, obtained by an excess electron (e@EP) and a substitutional Si doping (Si@EP). For the first time, we observe EP hopping in both pristine and doped Fe2O3. We find that the neighboring Fe-Fe distance is the main driving force for the EP hopping, which occurs by the adiabatic charge transfer mechanism. The EP transport is determined by interplay of probabilities to reach a favorable configuration and to hop in that configuration. The hopping barrier decreases as the Fe-Fe distance decreases; however, hops can take place at larger Fe-Fe distances, because such configurations are easier to achieve. Importantly, we demonstrate that the Si dopant speeds up the EP transfer process by increasing the EP mobility. The origin can be ascribed to the following three factors: longer Fe-O bonds, smaller activation energies, and creation of low energy metastable EP states, in Si@EP Fe2O3 compared to e@EP Fe2O3. Interestingly, the EP hopping is random in e@EP Fe2O3, but quasi-random in Si@EP Fe2O3 with specific pathways promoting efficient EP transfer. These new findings greatly enrich the understanding of charge transport in Fe2O3 photoanodes.

Spectra and Pitch-Angular Distributions of Relativistic Electrons Near the Outer Radiation Belt Maximum During the Magnetic Storm of December 19–22, 2015

Variations in the spectra and pitch-angle distribution of relativistic electrons are studied for the large magnetic storm on December 19–22, 2015, during which the position of the electron trapping boundary with an energy >100 keV was recorded inside the auroral oval according to the Meteor-M No. 2 satellite. The results of ECT-REPT observations in the RBSP/Van Allen Probes mission near the maximum of the newly formed belt are used. Changes in the spectra and pitch-angle distributions as a result of the development of individual substorms during the storm are considered. The results support effective particle acceleration at relatively short time scales (<6 h). The role of the adiabatic mechanism in the acceleration and loss of relativistic electrons is considered. Evidence has been obtained for the dominant role of processes at the latitudes of the auroral oval in the formation of the outer radiation belt.

Curious Mechanism of the Dissociative Chemisorption of Ammonia on Ru(0001)

Dissociative chemisorption of polyatomic molecules on metals, which is relevant to heterogeneous catalysis, usually proceeds through a rotationally adiabatic or rotational sudden mechanism. The reaction is usually either direct or proceeds through a trapped molecular chemisorbed state. Here, ab initio molecular dynamics is used to model the dissociative chemisorption of ammonia on Ru(0001). The reaction mechanism is neither rotationally adiabatic nor rotational sudden, with clearly distinct and nonstatistical initial and time-of-reaction orientation distributions. A reasonably good agreement is obtained between the computed and previously measured sticking probabilities. Under the conditions investigated, the reaction of NH3 goes through a molecular chemisorption-like state, but the reaction is direct.

The Influence of Microstructural Anisotropy on the Formation of Adiabatic Shearing Bands in Extruded 7003 Aluminum Alloy Profiles During High Strain Rate Impaction

The plastic deformation behavior of the extruded 7003‐T6 aluminum profiles during high strain rate impaction is investigated. The effect of the microstructural anisotropy on the formation of adiabatic shearing bands and related mechanisms are analyzed. The results show that the flow stress is much higher when the impaction is parallel to the extrusion direction. This is caused by strong texture existed in the raw profiles. The maximum Schmid factor for all the slip systems is much lower when the loading direction is parallel to this direction. Higher flow stress results in lower energy barrier for shear localization. Therefore, the formation of adiabatic shearing bands is susceptible when the impaction is parallel to the extrusion direction, but is not sensitive when the impaction is perpendicular to the extrusion direction. When the impaction is perpendicular to the extrusion direction, there are more flow path for the deformation which inhibits the initiation of the adiabatic shearing bands. Through finite element simulation, the deformation process is successfully simulated. This study provides important designing guidance for the application of such profiles.

Broadband on-chip polarization mode splitters in lithium niobate integrated adiabatic couplers.

We report, to the best of our knowledge, the first broadband polarization mode splitter (PMS) based on the adiabatic light passage mechanism in the lithium niobate (LiNbO3) waveguide platform. A broad bandwidth of ~140 nm spanning telecom S, C, and L bands at polarization-extinction ratios (PER) of >20 dB and >18 dB for the TE and TM polarization modes, respectively, is found in a five-waveguide adiabatic coupler scheme whose structure is optimized by an adiabaticity engineering process in titanium-diffused LiNbO3 waveguides. When the five-waveguide PMS is integrated with a three-waveguide "shortcut to adiabaticity" structure, we realize a broadband, high splitting-ratio (ηc) mode splitter for spatial separation of TE- (H-) polarized pump (700-850 nm for ηc>99%), TM- (V-) polarized signal (1510-1630 nm for ηc>97%), and TE- (H-) polarized idler (1480-1650 nm for ηc>97%) modes. Such a unique integrated-optical device is of potential for facilitating the on-chip implementation of a pump-filtered, broadband tunable entangled quantum-state generator.

The failure mechanism at adiabatic shear bands of titanium alloy: High-precision survey using precession electron diffraction and geometrically necessary dislocation density calculation

The difficulty of obtaining key information, such as the crystal orientation and geometrically necessary dislocation (GND) density distribution, from a large deformation region in an adiabatic shear band (ASB) has hindered further study of the ASB failure mechanism in titanium alloys. In this work, the crystal orientation information of the failure position and surrounding region in ASB of a Ti-5Al-2.5Cr-0.5Fe-4.5Mo-1Sn-2Zr-3Zn alloy was obtained via transmission electron microscopy (TEM) and precession electron diffraction (PED) with a high spatial resolution. The GND density distribution was calculated and the adiabatic shear failure mechanism was revealed from the collected data. ASBs (original width: ∼3–4 µm) formed in the cylinder sample during dynamic compression (strain rate: ∼3000 s−1). In the transition region at the edge of ASB, the grains were severely elongated along the direction parallel to the ASB boundaries with an average grain size on the order of μm and an average GND density of 5.5374 × 1015/m2 in the α phase. Relatively strong < 0001 > textures and weak < -12–10 > textures were observed. However, significant dynamic-recrystallization-dominated grain refinement in ASB, especially at the crack tip, resulted in many ultrafine equiaxed recrystallized grains (10 nm level) and weakened textures. The highest average GND density (8.6242 × 1015/m2, in α phase) occurred in the crack tip region. Moreover, the vicinity of the "primary microcracks" in the main crack extension line was characterized by combinations of an extremely high GND density work hardening region and a group of low GND density recrystallized grains. This indicates that cracks in the ASB were initiated by the deformation incompatibility between the antecedent recrystallization region and the surrounding high work hardening region.

Highly efficient photogenerated electron transfer at a black phosphorus/indium selenide heterostructure interface from ultrafast dynamics

Highly efficient photogenerated electron transfer at a black phosphorus/indium selenide heterostructure interface by an adiabatic mechanism.

Barriers for interfacial back-electron transfer: A comparison between TiO2 and SnO2/TiO2 core/shell structures

Temperature dependent kinetics for back-electron transfer (BET) from electrons in TiO2 or SnO2/TiO2 core/shell nanoparticles to oxidized donor-bridge-acceptor (D-B-A) sensitizers is reported over a 110° range. Two D-B-A sensitizers (CF3-p and CF3-x) were utilized that differed only by the nature of the bridging ligand: a xylyl spacer that largely insulated the two redox active centers and a phenyl bridge that promoted strong electronic coupling and an adiabatic electron transfer mechanism. An Arrhenius analysis revealed that the activation energies were significantly larger for the core/shell oxides, Ea = 32 ± 4 kJ/mol, compared to TiO2 alone, Ea = 22 ± 6 kJ/mol. The barriers for BET on sensitized TiO2 were within the same range as previous literature reports, while this study represents the first quantification for SnO2/TiO2 core/shell materials. Two different models were proposed to rationalize the larger barrier for the core/shell materials: (1) a band edge offset model and (2) a low energy trap state model with recombination from the TiO2 rutile polymorph shell. The latter model was preferred and is in better agreement with the experimental data. The kinetic analysis also afforded the forward and reverse rate constants for the intramolecular equilibrium. In accordance with theoretical predictions and previous research, the absolute value of the free energy change was smaller for the adiabatic equilibrium provided by the phenyl bridge, i.e., |ΔGo ad| <|ΔGo|.

Adiabatic and nonadiabatic charge separation dynamics in graphene oxide quantum dots for overall water splitting

Chemical functionalization and heteroatom doping are two effective strategies for improving the conductivity of a graphene lattice. Nitrogen-doped graphene oxide quantum dot (GOQD) has been reported to possess both p- and n-conductivity that is induced by an oxygen functional group and nitrogen doping, respectively, and is suitable for catalyzing hydrogen and oxygen evolution reactions for complete water splitting. The experimental study shows that the hydrogen evolution reaction occurs considerably faster than the oxygen evolution reaction. However, the mechanism of this phenomenon remains unknown, which poses a challenge to the chemical modification of such classes of materials. In the present work, we perform nonadiabatic ab initio molecular dynamics to explore the charge separation dynamics in N-doped GOQD with oxygen functional groups. Our results show that there exists multiple charge decay channels governed by different mechanisms, which complicates the overall charge separation dynamics in N-doped GOQD. The intramolecular electron transfer mainly occurs through the nonadiabatic channel, whereas the intramolecular hole transfer mainly occurs through the adiabatic channel. There is an additional adiabatic decay channel in the p-domain, which accelerates electron–hole recombination in the p-domain. We further calculated the decoherence times and found that the decoherence of intramolecular electron transfer occurs considerably faster than that of hole transfer, which slows the quantum transition of intramolecular electron transfer. Finally, we simulated the carrier relaxation times and found that the electron relaxation is approximately one order of magnitude longer than that of the hole relaxation. Our calculation rationalizes the experimentally observed higher catalytic activity towards the hydrogen evolution compared to the oxygen evolution. More importantly, the calculation explains the fundamental roles of nitrogen doping and oxygen functional groups in the adiabatic and nonadiabatic charge decay mechanisms.

An Adiabatic Mechanism for the Reduction of Jet Meander Amplitude by Potential Vorticity Filamentation

Abstract The amplitude of ridges in large-amplitude Rossby waves has been shown to decrease systematically with lead time during the first 1–5 days of operational global numerical weather forecasts. These models also exhibit a rapid reduction in the isentropic gradient of potential vorticity (PV) at the tropopause during the first 1–2 days of forecasts. This paper identifies a mechanism linking the reduction in large-scale meander amplitude on jet streams to declining PV gradients. The mechanism proposed is that a smoother isentropic transition of PV across the tropopause leads to excessive PV filamentation on the jet flanks and a more lossy waveguide. The approach taken is to analyze Rossby wave dynamics in a single-layer quasigeostrophic model. Numerical simulations show that the amplitude of a Rossby wave propagating along a narrow but smooth PV front does indeed decay transiently with time. This process is explained in terms of the filamentation of PV from the jet core and associated absorption of wave activity by the critical layers on the jet flanks, and a simple method for quantitatively predicting the magnitude of the amplitude reduction without simulation is presented. Explicitly diffusive simulations are then used to show that the combined impact of diffusion and the adiabatic rearrangement of PV can result in a decay rate of Rossby waves that is 2–4 times as fast as could be expected from diffusion acting alone. This predicted decay rate is sufficient to explain the decay observed in operational weather forecasting models.

Dynamic fracture response of pre-flawed elbow pipe subjected to internal hydrogen-oxygen detonation

A coupled fluid-structure-fracture approach was presented to study the dynamic fracture of pre-flawed elbow pipes subjected to internal hydrogen-oxygen detonation. The initial flaw is located at the extrados, crown and intrados of the elbow, respectively. The hydrogen-oxygen detonation was modeled by a user-programed burn method based on the CJ theory. The fracture of elbow pipe was simulated by a validated bivariate failure criterion which was deduced based on the adiabatic shear failure mechanism of materials at high strain rates. Results demonstrate that the presented approach can capture the detonation wave propagation and the complex crack extension and branching effectively. It is found the peak pressure at extrados is 2.9 times larger than that at intrados, but the hoop stresses and effective strains at the three initial flaw positions are comparable with each other, even the average amplitude of hoop stress at intrados is higher than those at extrados and crown. The increase of detonation pressure can lead to crack branching and makes more cracks propagate simultaneously. The crack initiating at crown tends to turn and run to the intrados when passing the transition sections of elbow, while the crack initiating at intrados is expected to branch here. The average crack speeds for forward cracks are 100–300 m/s, while the speeds are 70%–90% of the above for backward cracks. The branch crack speeds are generally less than 40% of those before branching. Furthermore, the elbow pipes with initial flaw at intrados generally have the largest total crack length, while elbow pipes with initial flaw at crown have the strongest resistance against crack propagation. It is also suggested that local bucking is an important feature that takes place in dynamic fracture processes of elbow pipes.

Characterizing the atmospheric conditions during the 2010 heatwave in Rio de Janeiro marked by excessive mortality rates

Global temperatures have increased considerably over the last decades, directly impacting the number, intensity and duration of extreme events such as heat waves. Climate model projections accounting for anthropogenic factors indicate that deadly mega-heat waves are likely to become more frequent in the future. Although the atmospheric features and social-economic related impacts of heat waves have already been documented in various regions around the world, for other highly populated regions, such as the Metropolitan Region of Rio de Janeiro (MRRJ), a similar objective assessment is still needed. Heat waves directly impact the public health sector and particularly the less wealthy and elderly population groups. During February 2010, an elevated mortality peak occurred during a 8-day period (from 2 to 9 Feb 2010) characterized as a heat wave episode in MRRJ. A total excess of 737 deaths was recorded with the elderly group registering the highest mortality incidence. During this heat wave period, a quasi-stationary anticyclonic anomaly forced in altitude by a Rossby wave train was established over the south Brazilian coast. At the surface, the meteorological scenario from January 2010 to the heat wave period was marked by clear sky conditions, large precipitation deficits, and enhanced diabatic heating. During the heat wave period, warm and dry air masses were advected from interior regions towards the MRRJ, exacerbating temperature conditions by pronounced subsidence and adiabatic heating mechanisms. All these conditions contributed to pronounced positive temperature anomalies, reinforced by land-atmosphere feedbacks.

Reassess study of high temperature electric transport properties of PbTiO3

In the present study we obtained the detailed dielectric properties of PbTiO3. The formation of the materials was confirmed through basic structural analysis using x-rays diffraction collected at room temperature. Differential scanning calorimetry confirms a first order phase transition at 763 K. Furthermore, DC conductivity as well as pyroelectricity measurements were conducted in the temperature range (400–823 K), from which the polarizability as a function of temperature was evaluated. On the other hand, AC conductivity behavior as a function of both temperature and frequency (0.12–102 kHz), shows a semiconducting like behavior indicating an adiabatic conduction mechanism. While the ferroelectric phase transition was established at 763 K from the permittivity data as a function of temperature. The behavior of the frequency and temperature exponents of the AC electrical conductivity confirm the correlated barrier hopping “CBH” mechanism of conduction. Thermal activation energy is obtained from DC and AC conductivity as well. Dielectric modulus was also studied as a function of both temperature and frequency.

Sensitivity of idealised baroclinic waves to mean atmospheric temperature and meridional temperature gradient changes

The sensitivity of idealised baroclinic waves to different atmospheric temperature changes is studied. The temperature changes are based on those which are expected to occur in the Northern Hemisphere with climate change: (1) uniform temperature increase, (2) decrease of the lower level meridional temperature gradient, and (3) increase of the upper level temperature gradient. Three sets of experiments are performed, first without atmospheric moisture, thus seeking to identify the underlying adiabatic mechanisms which drive the response of extra-tropical storms to changes in the environmental temperature. Then, similar experiments are performed in a more realistic, moist environment, using fixed initial relative humidity distribution. Warming the atmosphere uniformly tends to decrease the kinetic energy of the cyclone, which is linked both to a weaker capability of the storm to exploit the available potential energy of the zonal mean flow, and less efficient production of eddy kinetic energy in the wave. Unsurprisingly, the decrease of the lower level temperature gradient weakens the resulting cyclone regardless of the presence of moisture. The increase of the temperature gradient in the upper troposphere has a more complicated influence on the storm dynamics: in the dry atmosphere the maximum eddy kinetic energy decreases, whereas in the moist case it increases. Our analysis suggests that the slightly unexpected decrease of eddy kinetic energy in the dry case with an increased upper tropospheric temperature gradient originates from the weakening of the meridional heat flux by the eddy. However, in the more realistic moist case, the diabatic heating enhances the interaction between upper- and low-level potential vorticity anomalies and hence helps the surface cyclone to exploit the increased upper level baroclinicity.

Electron radiation belt dynamics during magnetic storms and in quiet time

The paper discusses the dynamics of the outer electron belt, adiabatic and nonadiabatic mechanisms of replenishment and losses of energetic electrons. Under undisturbed conditions, the outer electron belt gradually empties: in the inner magnetosphere due to electron precipitation in the atmosphere and in the quasi-trapping region due to losses at the magnetopause because drift shells of electrons are not closed there. The latter process does not occur in normal years due to the masking replenishment by freshly accelerated particles, but in years of extremely low activity it leads to a significant decrease in the electron population of the belt. During the magnetic storm main phase, the first reason for the decrease in the electron flux intensity is the adiabatic cooling associated with conservation of adiabatic invariants and complemented by precipitation of electrons into the atmosphere and their dropout at the magnetopause. Electron flux increases involve EB electron injection by the induction electric field of substorm activation and by the large-scale solar wind electric field, with pitch energy diffusion along with adiabatic heating in the recovery phase. The rate of electron flux recovery after a storm is determined by the ratio of nonadiabatic increases and losses; hence the electron flux represents a continuous series from low to very high values. The combination of these processes determines the individual character of radiation belt development during each magnetic storm and the behavior of the belt in the quiet time.

Динамика электронного радиационного пояса во время магнитных возмущений и в спокойное время

The paper discusses the dynamics of the outer electron belt, adiabatic and nonadiabatic mechanisms of replenishment and losses of energetic electrons. Under undisturbed conditions, the outer electron belt gradually empties: in the inner magnetosphere due to electron precipitation in the atmosphere and in the quasi-trapping region due to losses at the magnetopause because drift shells of electrons are not closed there. The latter process does not occur in normal years due to the masking replenishment by freshly accelerated particles, but in years of extremely low activity it leads to a significant decrease in the electron population of the belt. During the magnetic storm main phase, the first reason for the decrease in the electron flux intensity is the adiabatic cooling associated with conservation of adiabatic invariants and complemented by precipitation of electrons into the atmosphere and their dropout at the magnetopause. Electron flux increases involve EB electron injection by the induction electric field of substorm activation and by the large-scale solar wind electric field, with pitch energy diffusion along with adiabatic heating in the recovery phase. The rate of electron flux recovery after a storm is determined by the ratio of nonadiabatic increases and losses; hence the electron flux represents a continuous series from low to very high values. The combination of these processes determines the individual character of radiation belt development during each magnetic storm and the behavior of the belt in the quiet time.

Adiabatic suppression of the axion abundance and isocurvature due to coupling to hidden monopoles

The string theory predicts many light fields called moduli and axions, which cause a cosmological problem due to the overproduction of their coherent oscillation after inflation. One of the prominent solutions is an adiabatic suppression mechanism, which, however, is non-trivial to achieve in the case of axions because it necessitates a large effective mass term which decreases as a function of time. The purpose of this paper is twofold. First, we provide an analytic method to calculate the cosmological abundance of coherent oscillation in a general situation under the adiabatic suppression mechanism. Secondly, we apply our method to some concrete examples, including the one where a string axion acquires a large effective mass due to the Witten effect in the presence of hidden monopoles.

Ultrafast coherent vibrational dynamics in dimethyl methylphosphonate radical cation.

Femtosecond pump-probe measurements of the nerve agent simulant dimethyl methylphosphonate (DMMP) demonstrate the preparation of a robust coherent vibrational state in the corresponding radical cation. The oscillations in the transient ion yields have a period of 45 fs (750 cm-1), which is at least 3 times faster than any previously observed oscillations in polyatomic radical cations. Use of 1200-1600 nm, as opposed to 800 nm, wavelengths for ionization increases the oscillation amplitude by a factor of 5 and doubles the number of visible oscillation periods from 6 to 12, indicating that an adiabatic ionization mechanism significantly enhances preparation of the coherent state. The coherent motion is assigned to a bending mode in DMMP+ with frequency in the range of 742.2-754.7 cm-1 based on the results of DFT calculations. The observation of coherent nuclear dynamics in the dissociation of DMMP+ suggests the potential utility of coherent control schemes for controlling the dissociation of DMMP and related molecules, which has important implications for developing detection and decontamination technologies for organophosphorus chemical warfare agents.