Atomic Flux Research Articles

Analytical Estimation of the Atomic and Molecular Flux Ratio from the Tungsten Surface

Analytical Estimation of the Atomic and Molecular Flux Ratio from the Tungsten Surface

Explanation of Heliospheric Energetic Neutral Atom Fluxes Observed by the Interstellar Boundary Explorer

Interstellar neutral atoms propagating into the heliosphere experience charge exchange with the supersonic solar wind (SW) plasma, generating ions that are picked up by the SW. These pickup ions (PUIs) constitute ∼25% of the proton number density by the time they reach the heliospheric termination shock (HTS). Preferential acceleration of PUIs at the HTS leads to a suprathermal, kappa-like PUI distribution in the heliosheath, which may be further heated in the heliosheath by traveling shocks or pressure waves. In this study, we utilize a dynamic, 3D magnetohydrodynamic model of the heliosphere to show that dynamic heating of PUIs at the HTS and in the inner heliosheath (IHS), as well as a background source of energetic neutral atoms (ENAs) from outside the heliopause, can explain the heliospheric ENA signal observed by the Interstellar Boundary Explorer (IBEX) in the Voyager 2 direction. We show that the PUI heating process at the HTS is characterized by a polytropic index larger than 5/3, likely ranging between γ ∼ 2.3 and 2.7, depending on the time in solar cycle 24 and SW conditions. The ENA fluxes at energies >1.5 keV show large-scale behavior in time with the solar cycle and SW dynamic pressure, whereas ENAs < 1.5 keV primarily exhibit random-like fluctuations associated with SW transients affecting the IHS. We find that ≲20% of the ENAs observed at ∼0.5–6 keV come from other sources, likely from outside the heliopause as secondary ENAs. This study offers the first model replication of the intensity and evolution of IBEX-Hi ENA observations from the outer heliosphere.

Experimental and Numerical Investigation of Electromigration Behavior of Printed Silver Wire Under High Current Density

Abstract The electromigration (EM) damage is becoming a severe problem in the printed flexible electronics as the printed circuits are fabricated thinner and thinner due to the development of printing technology. In this work, the EM behavior of printed silver wires was investigated by EM experiments and numerical simulations. The EM tests showed that voids are generated in the cathode area and hillocks are formed in the anode area for a wire with a small length. However, with the increase of wire length, hillocks tend to occur on the two sides of the silver wire middle part. The results of numerical simulations based on the atomic flux divergence (AFD) method revealed that the formation of the hillocks on the printed wire is caused by not only the mechanism of electron wind but also the strong temperature gradient along the wire length and width direction. Also, it can be concluded that the temperature gradient induced by Joule heating plays a more important role than electron wind in the atomic migration of the printed silver wire subjected to a high current density. The influence of the printed silver wire size on the EM behavior was also analyzed by numerical simulation, and the results demonstrated that the printed silver wires with a larger length and a smaller width-to-thickness ratio are more likely to develop hillocks on the two sides of silver wire middle part while subjected to a high current density.

A phase-field model for void and gas bubble superlattice formation in irradiated solids

A phase-field model to simulate the formation of both void and gas bubble superlattices is derived from a grand potential functional, assuming 1D diffusion of self-interstitial atoms. The model is capable of accounting for superlattice formation by either a nucleation and growth or spinodal decomposition mechanism; in this work, we focus on the nucleation and growth mechanism, using a discrete nucleation approach in the phase-field model. In simulations of void formation, short aligned rows of voids were initially formed, followed by growth in the size of the aligned rows, and finally leading to superlattice formation, consistent with experimental observations. Void superlattice spacing as a function of model parameters was quantified. Increased nucleation rates led to smaller superlattice spacing, while increased vacancy diffusivity led to larger superlattice spacing. In simulations of gas bubble superlattice formation, superlattice spacing decreased with increasing gas atom flux, consistent with experimental observations. Simulations with increasing ratio of gas atom flux to vacancy–interstitial pair production led to increased superlattice spacing, which is inconsistent with experimental observations at high gas atom flux rates; this discrepancy is attributed to the fact that the model does not account for bound gas atom–vacancy pairs that may dominate at high gas flux rates.

Direct Writing of 3D Micro/Nanostructures Based on Nanoscale Strong Electric Field of Electron Beam

3D micro/nanostructures have a wide range of applications in metamaterials, integrated circuits and optoelectronic coupling devices, etc. However, 3D nanofabrication has always been a difficult problem in materials science and nanoengineering fields due to the influence of surface tension or capillary action at nanoscale. Here, using nanoscale strong electric field (NSEF) of the electron beam as a driving force, the diffusion flux, migration direction, and aggregation process of material atoms or nanoclusters can be accurately controlled to realize the direct writing of 3D micro/nanostructures. By this method, several ZnO nanocharacters and a vertical ZnO nanowire with a diameter of 18 nm are direct written successfully, and a spatial contact with a characteristic size of 8 nm is created. The direct writing process of a bud‐like nanostructure is also demonstrated on a NiCo2O4 nanorod. Finally, the electron beam direct writing mechanism is analyzed based on the electric field forces and electric potential. This 3D nanofabrication method shows the advantages of flexibility, controllability, and high real‐time ability, which are of great significance to solve the problems of direct writing of 3D nanostructures, material molding control at nanoscale, and electrical interconnection in integrated circuits.

Phonon trapping states as a witness for generation of phonon blockade in a hybrid micromaser system

In a hybrid micromaser system consisting of an optical cavity with a moving mirror connected to a low temperature thermal bath, we demonstrate, both analytically and numerically, that for certain interaction times between a random atomic flux and the optomechanical cavity, vacuum phonon trapping states are generated. Furthermore, under the approach of the master equation with independent phonon and photon thermal baths, we show that the trapping of the phonons and photons is achieved for the same interaction times. The results also indicate that by increasing the cavity-oscillator coupling one may generate a coherent phonon state aside from the trapping states. Within the same hybrid system, but now connected to the squeezed phonon reservoir, a phonon blockade effect can be engineered. Moreover, we identify an interconnection between the trapping and blockade effects, particularly if one approaches the vacuum trapping state, strong phonon blockade can be achieved when the system is connected with a weakly squeezed phonon reservoir.

Role of Metallic Adlayer in Limiting Ge Incorporation into GaN.

Atomically thin metal adlayers are used as surfactants in semiconductor crystal growth. The role of the adlayer in the incorporation of dopants in GaN is completely unexplored, probably because n-type doping of GaN with Si is relatively straightforward and can be scaled up with available Si atomic flux in a wide range of dopant concentrations. However, a surprisingly different behavior of the Ge dopant is observed, and the presence of atomically thin gallium or an indium layer dramatically affects Ge incorporation, hindering the fabrication of GaN:Ge structures with abrupt doping profiles. Here, we show an experimental study presenting a striking improvement in sharpness of the Ge doping profile obtained for indium as compared to the gallium surfactant layer during GaN-plasma-assisted molecular beam epitaxy. We show that the atomically thin indium surfactant layer promotes the incorporation of Ge in contrast to the gallium surfactant layer, which promotes segregation of Ge to the surface and Ge crystallite formation. Understanding the role of the surfactant is essential to control GaN doping and to obtain extremely high n-type doped III-nitride layers using Ge, because doping levels >1020 cm−3 are not easily available with Si.

Application of a dual-thermopile radical probe to expanding hydrogen plasmas

We compare the performance of a hydrogen radical probe to historic data determined via two-photon absorption laser induced fluorescence using a comparable cascaded arc source under similar operating conditions. This probe has dual heat flux sensors (DHFS) each coated with materials with different catalytic properties for hydrogen atoms. In the ideal situation, the hydrogen radical flux can be deduced based on the difference between the heat loads measured by these two sensors. The influence of DHFS temperature on the performance was also assessed. The experimental results showed measurement errors of <10% could be obtained regardless of the probe temperature during plasma exposures. To convert heat fluxes into atomic fluxes, we calibrated the difference of the recombination coefficients using a vacuum ultraviolet absorption technique, which is more reliable than modeled values based on assumptions or scattered values reported in literature. As a result, we measured the hydrogen plasma and radical parameters at various settings using both a double Langmuir probe and the DHFS. The typical atom flux in the 1022 m−2s−1 range was in good agreement with those obtained using optical techniques. We also observed that the ion and atom fluxes are both sensitive to the background gas pressure. These findings validate application of the DHFS to the cascaded arc source, and could pave the way for optimization of the source performance in the plasma material processing experiments.

Reflections on the effect of an external flux in surface physics

Sublimating surfaces are out of equilibrium. It has been proposed that sublimation can be compensated by an impinging atomic flux to obtain equilibrium. This work concerns the effect of such an impinging flux on the stability of surfaces in various situations. For this purpose we combine Kinetic Monte Carlo Simulations with analytical developments based on the Burton-Cabrera-Frank (BCF) classical theory. We show that a perfect compensation of the sublimation is possible for vicinal surfaces but not when 2D islands are present on a surface. We thus study the effect of an impinging flux on the dynamic of a 2D island on a surface. We show that the 2D island area generally varies with time t as −tα. In absence of any impinging flux the value of the exponent α enables to identify the main mechanism at work (diffusion limited or attachment-detachment limited). On the contrary, in presence of an impinging flux the value of the exponent α is not enough to identify the main mechanism limiting the area change.

A cold cesium beam source based on a two-dimensional magneto-optical trap

A beam source is proposed for the production of an intense cold cesium atomic beam that can be used in cesium beam atomic clocks. The source is based on a two-dimensional magneto-optical trap (2D-MOT), but introduces hollow cooling and pushing lights in the axial direction to create a 2D+-MOT, which separates the cooling and pushing functions while the low-power pushing light pushes atoms out to form a cold atomic beam. This cold cesium atomic beam source reduces the light shift due to leakage light and retains longitudinal cooling to increase the flux of the cold atomic beam compared with that of the conventional 2D+-MOT scheme. The specifics of the design are investigated, the atomic velocity and beam flux are calculated, and the results are experimentally verified. The results demonstrate that when the power of the pushing light is 180 µW and when its frequency resonates with the 4 → 5′ transition of the Cs D2 line, the most probable longitudinal velocity of the outgoing cold atomic beam, the width of velocity distribution, and the atomic beam flux are 19.38 m/s, 8.1 m/s, and 1.7 × 1010 atoms/s, respectively.

Improving the stress corrosion cracking resistance of aged SA508-309L dissimilar metal weld joints by pulsed electric current

The effect of pulsed electric current on the stress corrosion cracking resistance of aged SA508-309L welds was investigated. After the weld joints were aged at 475 °C for 2000 h, spinodal decomposition and G-phase resulted in the formation of Cr-depleted zones and hardening phenomena in δ-ferrite, which reduced the stress corrosion cracking resistance of the weld joints. After pulsed treatment, the spinodal decomposition and G-phase were eliminated, the chemical composition was homogenized, and the residual stress was removed. Therefore, the stress corrosion cracking resistance of the pulsed weld joints was even higher than that of the original unaged weld joints. Thermodynamic and kinetic analysis clarified that the pulsed electric current processing can lower the thermodynamic dissolution barrier of the precipitates and increase the atomic diffusion flux. Thus, the stress corrosion cracking resistance of the aged weld joint can be improved by pulsed electric current.

Modelling and experimental evidence of the cathode erosion in a plasma spray torch

The lifetime of tungsten cathodes used in plasma spray torches is limited by processes leading to a loss of cathode material. It was reported in the literature that the mechanism of their erosion is the evaporation. A model of the ionization layer of a cathode is developed to study the diffusive transport of evaporated tungsten atoms and tungsten ions produced due to ionization by electron impact in a background argon plasma. It is shown that the Stefan–Maxwell equations do not reduce to Fick law as one could expect for the transport of diluted species, which is due to significant diffusion velocities of argon ions. The ionization of tungsten atoms occurs in a distance of a few micrometers from the cathode surface and leads to a strong sink, which increases the net flux of tungsten atoms far beyond that obtained in absence of tungsten ions. This shows that the tungsten ions are driven by the electric field towards the cathode resulting in no net diffusive flux and no removal of tungsten species from the ionization layer even if convection is accounted for. A possible mechanism of removal is found by extending the model to comprise an anode. The extended model resolves the inter-electrode region and provides the plasma parameters for a current density corresponding to the value at the center of the cathode under typical arc currents of 600 A and 800 A. The presence of the anode causes a reversal of the electric field on the anode side, which pulls the ions away from the ionization layer of the cathode. The net flux of tungsten ions can be further fortified by convection. This model allows one to evaluate the loss of cathode material under realistic operating conditions in a quantitative agreement with measured values.

Cubic InxGa1−xN/GaN quantum wells grown by Migration Enhanced Epitaxy (MEE) and conventional Molecular Beam Epitaxy (MBE)

We report the growth of InxGa1−xN/GaN quantum wells (QWs) in metastable-cubic phase through two well-controlled methods, Migration Enhanced Epitaxy (MEE) and conventional Molecular Beam Epitaxy (conventional MBE) on GaAs substrate. An increment of In mole fraction in the c-InxGa1−xN QWs was found with the decrease of growth temperature for both methods. The MEE implemented in this work, which consists of alternating the atomic flux periods at temperatures, approximately 100 ∘C lower than in conventional MBE, successfully addresses the cubic InxGa1−xN/GaN QW growth challenge. The QWs grown by this method present lower In segregation than those grown using conventional MBE. Excitonic transitions in the visible spectrum range from violet (414 nm) to green (544 nm) wavelengths were obtained in the QWs, by varying In content for both methods. These excitonic emissions are in good agreement with the theoretical calculations performed. Likewise, we identified the chemical bonds present in each c-InxGa1−xN QW, which corresponded to In–N and Ga–N, and their binding energies through In 3d, Ga 3d, and N 1s core levels by x-ray photoelectron spectroscopy (XPS). Therefore, this work provides an important understanding of In segregation, In incorporation mechanism, and radiative excitonic transitions of cubic QWs, that can be considered in complex heterostructures for novel optoelectronic applications.

Evidence for interface-rate limited densification kinetics at Al2O3-GdAlO3 interfaces characterized by in situ ultrahigh temperature transmission electron microscopy

The sintering behavior of single Al2O3 particles resting on GdAlO3 is characterized using in situ transmission electron microscopy imaging. In bicrystal experiments, particles with lower Al2O3-GdAlO3 phase boundary energies shrink via Ostwald ripening with no evidence of densification that would result from an atomic flux along the interface. Particles with higher Al2O3-GdAlO3 phase boundary energies undergo intermittent rigid body motion associated with an interfacial flux mediated by interfacial line defects, e.g. disconnections. Long incubation periods elapse between these comparatively rapid rigid body motion events. The interfacial diffusivity during each event agrees with the magnitude of interfacial diffusion obtained from independent measurements of interfacial diffusivity. The results from the model experiments suggest that capillarity driven interfacial strain mediation, e.g. densification during sintering, is interface rate limited at the driving forces investigated.

Bluntness-Dependent Hypersonic Boundary-Layer Modes’ Excitation

Boundary-layer disturbances on a cone in a hypersonic flow are investigated by heat flux sensors. The cone has a half-angle of 10.9 deg and is situated with a 0 deg angle of attack in a Mach 10 region of a conical nozzle in a shock-tunnel facility. There are three atomic layer thermopile heat flux sensors mounted at different chordwise positions in order to investigate the receptivity to freestream disturbances and the emergence of unstable modes of the boundary layer, that is, Mack first and second modes. The cone’s nose is exchangeable, and there are three differently sized radii to investigate the bluntness effect on transition with respect to entropy/boundary-layer displacement. It is found that each nose type tested shows a different set of amplified spectral bands in the receptivity region of the first two sensors, as well as at the third sensor’s location downstream of the critical point. There is a unique wavelength, scaling with boundary-layer height, that is most receptive to disturbances. For the two sharper cones, this excitation lies in the range of Mack second modes, which are observed in the unstable region. For the blunter cone, this excitation lies near the first mode frequency; as a result, only first modes are amplified and detected at sensor 3, although linear stability theory predicts higher growth rates for second modes.

Phase-defined growth of In2Se3 thin films using PLD technique for high performance self-powered UV photodetector

Self-powered photodetectors have attracted enormous attention for their high sensitivity, fast response with low power consumption. The unique set properties defined for various polymorphs of In2Se3 offers wide range of potential applications for optoelectronic and memory devices. Here, threephases of In2Se3 including, layered α-In2Se3and β-In2Se3, and non-layered γ-In2Se3, are grown using pulsed laser deposition (PLD) technique. Deposition gas pressure is found to govern the formation ofIn2Se3phasesattributed to flux of Se atoms reaching the heated substrate. Band structure, density of states and work function for each phase iscalculated using first principles calculations to understand their electronic properties. Based on the obtained work functions, it is found that Schottky junction based ultraviolet photodetectors usingas grownIn2Se3 films exhibit high performance with photo-responsivity of1310 mA/W for α-In2Se3, 260 mA/W for β-In2Se3and 240 mA/W for γ-In2Se3 in self-poweredmode under an illumination of ultraviolet radiation. The obtained high self-powered photodetectionis attributed to the non-centrosymmetric structure of α-In2Se3and γ-In2Se3, which produce a built-in field due to in-plane spontaneous polarization. Thus, In2Se3based self-powered photodetector prepared using PLD technique pave the way forlow power optoelectronic device applications, environmental monitoring and military applications.

Growth behavior and morphology evolution of interfacial (Cu,Ni)6Sn5 in (001)Cu/Sn/Ni micro solder joints

A novel reflow method with an additional temperature gradient (TG) was performed to bonding single crystal (001)Cu and polycrystal Ni substrates with pure Sn as interlayer. The growth behavior and morphology evolution of interfacial intermetallic compound (IMC) in (001)Cu/Sn/Ni micro solder joints during reflow with and without TG were investigated. (Cu,Ni)6Sn5 phase was found to dominate the IMC growth at all the interfaces, showing significant Cu-Ni cross-interaction. For as-soldered state, prism-type (Cu,Ni)6Sn5 grains formed on the (001)Cu substrate. After isothermal reflow or reflow under TG with Ni as hot end, a single crystal (Cu,Ni)6Sn5 layer formed at the (001)Cu side remaining the prism-type morphology. After reflow under TG with (001)Cu as hot end, the dominant polycrystal (Cu,Ni)6Sn5 IMC formed at the Ni cold end and directionally grew towards the hot end until forming full IMC joint with a much faster growth rate than the other two reflow cases. Cu-Ni cross-interaction and TG played a significant role in IMC growth and morphology/orientation evolution, which was discussed in detail from the respects of lattice misfit and atomic diffusion flux.

Metal evaporation study using the GANIL high temperature oven for intense metal ion beams production

A new resistive oven dedicated to the production of metal ion beams using ECR ion sources has been developed at GANIL. It aims to produce beams such as 58Niq+, 50Tiq+, 50Vq+ or 238Uq+ with ECR4 ion source on cyclotrons injectors and with Phoenix V3 ion source on Spiral 2facility. Typical intensity about 1.2 1013 pps is requested for the Super Separator Spectrometer (S3). Evaporated atom fluxes and their angular distributions have been measured on a dedicated test bench thanks to a quartz microbalance. These investigations were carried out using three crucibles with various exit caps geometries for which the fluxes and angular distributions display distinct behaviors. Based on those results, crucible is designed to obtain a high flux of atoms overlapping the ECR plasma while minimizing the atoms losses on the ion source chamber.

Design and thermal simulations towards a high intensity radioactive ion source for ISOL@MYRRHA

MYRRHA will be the world’s first large-scale Accelerator Driven System project at power levels scalable to industrial systems. ISOL@MYRRHA will produce Radioactive Ion Beams (RIBs) using the Isotope Separation On-Line (ISOL) technique, with increased isotope production by high intensity primary beams over a long period while maintaining a high-quality RIB. Higher atom flux produced prevalently affects the ISOL ion source. A surface ion source is chosen as a first source because of its reliability and simple design. To understand the hot cavity’s behaviour, finite element thermal-electric simulations were performed. To start, a heating system study with experimental results from the SPES project was reproduced. This concept was then modified by: electrically insulating the source from its support, adding a feedthrough, transforming a passive thermal screen into an active part. With this heating system upgrade, the ion source temperature profile can be adjusted, especially at its exit part where high temperature is expected to play a crucial role in ion production and extraction.

Large-Area Nanopillar Arrays by Glancing Angle Deposition with Tailored Magnetic Properties.

Ferromagnetic films down to thicknesses of tens of nanometers and composed by polycrystalline Fe and Fe2O3 nanopillars are grown in large areas by glancing angle deposition with magnetron sputtering (MS-GLAD). The morphological features of these films strongly depend on the growth conditions. Vertical or tilted nanopillars have been fabricated depending on whether the substrate is kept rotating azimuthally during deposition or not, respectively. The magnetic properties of these nanopillars films, such as hysteresis loops squareness, adjustable switching fields, magnetic anisotropy and coercivity, can be tuned with the specific morphology. In particular, the growth performed through a collimator mask mounted onto a not rotating azimuthally substrate produces almost isolated well-defined tilted nanopillars that exhibit a magnetic hardening. The first-order reversal curves diagrams and micromagnetic simulations revealed that a growth-induced uniaxial anisotropy, associated with an anisotropic surface morphology produced by the glancing angle deposition in the direction perpendicular to the atomic flux, plays an important role in the observed magnetic signatures. These results demonstrate the potential of the MS-GLAD method to fabricate nanostructured films in large area with tailored structural and magnetic properties for technological applications.