Aspect Ratio Dependence Research Articles

Shape Anisotropy of Grains Formed by Laser Melting of (CoCuFeZr)17Sm2

For permanent magnetic materials, anisotropic microstructures are crucial for maximizing remanence Jr and maximum energy product (BH)max. This also applies to additive manufacturing processes such as laser powder bed fusion (PBF-LB). In PBF-LB processing, the solidification behavior is determined by the crystal structure of the material, the substrate, and the melt-pool morphology, resulting from the laser power PL and scanning speed vs. To study the impact of these parameters on the textured growth of grains in the melt-pool, experiments were conducted using single laser tracks on (CoCuFeZr)17Sm2 sintered magnets. A method was developed to quantify this grain shape anisotropy from electron backscatter diffraction (EBSD) analysis. For all grains in the melt-pool, the grain shape aspect ratio (GSAR) is calculated to distinguish columnar (GSAR < 0.5) and equiaxed (GSAR > 0.5) grains. For columnar grains, the grain shape orientation (GSO) is determined. The GSO represents the preferred growth direction of each grain. This method can also be used to reconstruct the temperature gradients present during solidification in the melt-pool. A dependence of the melt-pool aspect ratio (depth/width) on energy input was observed, where increasing energy input (increasing PL, decreasing vs) led to higher aspect ratios. For aspect ratios around 0.3, an optimum for directional columnar growth (93% area fraction) with predominantly vertical growth direction (mean angular deviation of 23.1° from vertical) was observed. The resulting crystallographic orientation is beyond the scope of this publication and will be investigated in future work.

Physical insights from the aspect ratio dependence of turbulence in negative triangularity plasmas

In this work, we study the impact of aspect ratio A=R0/r (the ratio of major radius R 0 to minor radius r) on the confinement benefits of negative triangularity (NT) plasma shaping. We use high-fidelity flux tube gyrokinetic GENE simulations and consider several different scenarios: four of them inspired by TCV experimental data, a scenario inspired by DIII-D experimental data and a scenario expected in the new SMART spherical tokamak. The present study reveals a distinct and non-trivial dependence. NT improves confinement at any value of A for ITG turbulence, while for TEM turbulence confinement is improved only in the case of large and conventional aspect ratios. Additionally, through a detailed study of a large aspect ratio case with pure ITG drive, we develop an intuitive physical picture that explains the beneficial effect of NT at large and conventional aspect ratios. This picture does not hold in TEM-dominated regimes, where a complex synergistic effect of many factors is found. Finally, we performed the first linear gyrokinetic simulations of SMART, finding that both NT and PT scenarios are dominated by micro-tearing-mode (MTM) turbulence and that NT is more susceptible to MTMs at tight aspect ratio. However, a regime where ITG dominates can be found in SMART, and in this regime NT is more linearly stable.

Vertical convection regimes in a two-dimensional rectangular cavity: Prandtl and aspect ratio dependence

Vertical convection is the fluid motion that is induced by the heating and cooling of two opposed vertical boundaries of a rectangular cavity (see e.g. Wang et al., J. Fluid Mech., vol. 917, 2021, A6). We consider the linear stability of the steady two-dimensional flow reached at Rayleigh numbers of O( $10^8$ ). As a function of the Prandtl number, $Pr$ , and the height-to-width aspect ratio of the domain, $A$ , the base flow of each case is computed numerically and linear simulations are used to obtain the properties of the leading linear instability mode. Flow regimes depend on the presence of a circulation in the entire cavity, detachment of the thermal layer from the boundary or the corner regions and on the oscillation frequency relative to the natural frequency of oscillation in the stably temperature-stratified interior, allowing for the presence of internal waves or not. Accordingly, the regime is called slow or fast, respectively. Either the global circulation or internal waves in the interior may couple the top and bottom buoyancy currents, while their absence implies asymmetry in their perturbation amplitude. Six flow regimes are found in the range of $0.1 \leq Pr \leq 4$ and $0.5 \leq A \leq 2$ . For $Pr \lessapprox 0.4$ and $A>1$ , the base flow is driven by a large circulation in the entire cavity. For $Pr \gtrapprox 0.7$ , the thermal boundary layers are thin and the instability is driven by the motion along the wall and the detached boundary layer. A transition between these regimes is marked by a dramatic change in oscillation frequency at $Pr = 0.55 \pm 0.15$ and $A <2$ .

Typical X-Ray Outburst Light Curves of Aql X-1

We show that the typical X-ray outburst light curve of Aql X-1 can be reproduced by accretion onto a neutron star in the frame of the disk instability model without invoking partial accretion or the propeller effect. The knee and the subsequent sharp decay in the X-ray light curve can be generated naturally by taking into account the weak dependence of the disk aspect ratio, h/r, on the disk mass-flow rate, Ṁin , in the X-ray irradiation flux calculation. This Ṁin dependence of h/r only slightly modifies the irradiation temperature profile along the hot disk in comparison to that obtained with a constant h/r. Nevertheless, this small difference has a significant cumulative effect on the hot disk radius, leading to a much faster decrease in the size of the hot disk and thereby to a sharper decay in the X-ray outburst light curve. The same model also produces the long-term evolution of Aql X-1, consistent with its observed outburst recurrence time and typical light curve. Our results imply that the source accretes matter from the disk in the quiescent state as well. We also estimate that the dipole moment of the source μ ≲ 4 × 1026 G cm3 (B ≲ 4 × 108 G at the surface).

Elliptical vibration chiseling: a novel process for texturing ultra-high-aspect-ratio microstructures on the metallic surface

Highlights Elliptical vibration chiseling is proposed based on a game-changing process principle for the high-efficient texturing of ultrahigh-aspect-ratio surface microstructures. Uniformed microstructures with an aspect ratio of 2–12 in the spacing scale of 1–10 μm have been successfully fabricated using elliptical vibration chiseling. The developed process model of elliptical vibration chiseling has been verified by the measured results of the microstructures’ geometric parameters. An inclined elliptical trajectory of tool vibration is more suitable for elliptical vibration chiseling than the standard elliptical trajectory. The deterministic process effects on the surface generation of microstructure in elliptical vibration chiseling have been demonstrated.

Incremental Analysis of Magnetic Domains in Multiple Types of Ferromagnetic CoFe Nanolayer Patterns

In this article, the results of incremental analysis on the thickness, aspect ratio, and crystallographic substrate orientation dependence of magnetic domain formation in CoFe nanolayer patterns (NPs) on GaAs (001) substrates by magnetic force microscopy are reported. All the 35 nm thick CoFe NPs show multiple magnetic domains under as‐deposition condition whereas the 13 nm thick CoFe NPs have a single magnetic domain in the case of the patterns with a relatively small size and high aspect ratio. Subsequently, the magnetic fields are applied in the direction parallel to the <110> and <1–10> directions of the substrates to analyze a magnetization switching behavior in the patterns. The results reveal that the magnetic fields required for magnetization switching markedly tend to increase, with decreasing the thickness of the patterns and increasing their aspect ratio. A general tendency is obtained, which a higher magnetic field is needed for a higher aspect ratio and a thinner thickness of the NPs. Even in the case of the patterns with an aspect ratio of unity and a thickness of 35 nm (but, a smaller size), a single magnetic domain is formed at a relatively high applied magnetic field of 1500 Gauss.

Bias-supply timing tailored to the aspect ratio dependence of silicon trench etching in Ar plasma with alternately injected C4F8 and SF6

In semiconductor device fabrication, the feature profiles of a high-aspect-ratio (HAR) Si trench is needed to be controlled considering aspect-ratio-dependent etching (ARDE). This is achieved by a cyclic process in which Ar plasma is sustained while C4F8 and SF6 are alternately injected and a short-period supply of a substrate bias is provided. Presently, the transient behaviors of gaseous and surface reactions are dynamically revealed by measuring the plasma parameters using a surface wave probe and optical emission spectroscopy. When the etched surface is fluorinated during the cycle, an ARDE-free Si-trench feature profile can be fabricated by controlling the bias-supply timing.

Impact of fin aspect ratio on enhancement of external quantum efficiency in single AlGaN fin light-emitting diodes pixels

Previously, we showed within a sub-micron fin shape heterojunction, as current density increases, the non-radiative Auger recombination saturates mediated by the extension of the depletion region into the fin, resulting in a droop-free behavior. Here, we investigate the dependence of the fin aspect ratio (height to width ratio) on external quantum efficiency (EQE) of single n-AlGaN fin/p-GaN heterojunctions. Fins are arranged in an array format varying in width from 3000 to 200 nm. In this architecture, an n-metal contact is interfaced with the non-polar side facet of the fin. At a fixed current density, as the aspect ratio increases from 0.2 to 3 (the fin width reduces), we systematically observe an increase in the ultraviolet (UV) excitonic emission of the AlGaN fin and a 7× enhancement in the EQE. We explain this phenomenon by conserving the volume of the carrier depletion region within a fin. As the fin gets thinner, the base area of the depletion volume shrinks, whereas its height increases within the fin. This geometrical advantage allows a 200 nm wide fin to operate at 1/3rd the current density compared to a 3000 nm wide fin while generating a UV emission with a comparable power of 1 μW. These findings show additional parameters that can be used for developing brighter light sources, including the shape and aspect ratio of a heterojunction at the micro- or nano-scale.

Using a Synthetic Borehole Model to Determine the Pore Aspect Ratio Dependence of Velocities from Acoustic Well-Logging Data

Revealing the elastic wave properties of carbonate rocks with complex pore structures and improving the reliability of carbonate reservoir descriptions have always been a global challenge in the field of carbonate geophysical exploration. In this study, we established a synthetic borehole model by selecting different particle sizes of cement, carbonate cuttings, and micro-silicon as the matrix, and silicon disks as the pores in carbonate rocks. We conducted four sets of low-porosity (0-3%) borehole models with different pore aspect ratios (ARs) and measured the P- and S-wave velocities ( V P and V S ) at the well-logging scale obtained from an acoustic logging system with one source and two receivers. The results indicate that the relationship between velocities and porosities in these borehole models follows a linear relation, with the pore AR significantly influencing the velocities at any given porosity. The velocity variation caused by pore AR reaches 560 m/s and 410 m/s at 3% porosity for the P-wave and S-wave within the AR range of 0.017-0.13. The theoretical DEM models provide a high and broad estimation of V P and V S at the well-logging scale in our measurement. They could perform better in fractured formation than in dissolved porous formation in carbonate reservoirs. The linear relation of V P and V S is independent of the pore AR and is effective for both fractured and dissolved porous formations. The change of V P / V S in different pore AR is more responsive to porosity and nonlinear dependent on the pore AR. The relationship between the defined normalized V P and V S indicates the pore AR has a more significant effect on V S than V P in our model. The constructed borehole models provide a unique opportunity for evaluating the availability of rock physics models at an acoustic logging scale. The study’s findings have significant implications for improving the reliability of carbonate reservoir descriptions and enhancing the accuracy of geophysical exploration in carbonate rocks with complex pore structures.

Magnetic domain control and its dependence on aspect ratio and thickness in Ni nanolayer patterns for nanowire spintronic devices

We investigate the aspect ratio and thickness dependence of magnetic domain formation in multiple types of ferromagnetic Ni nanolayer electrode patterns. Controlling magnetic domains is critical for spintronic devices using a group IV semiconductor, e.g. Si and Ge, nanowire as the electrodes with magnetic tunnel junction require parallel and anti-parallel magnetization configurations. Single magnetic domains are obtained in the Ni nanolayer electrode patterns on SiO2/Si substrate with an aspect ratio of 20 and a thickness of 40 nm even under the as-deposition condition, while other electrode patterns are mostly showing multiple magnetic domains. The results obtained by magnetic force microscopy also show that the magnetization switching is observed in the electrode pattern with a single magnetic domain. The results in this study show that the suitable design of the dimensions of nanolayer patterns is key to constructing a single magnetic domain in a ferromagnetic electrode for nanowire spintronic devices.

Effect of aspect ratio on three-dimensional Rayleigh-Bénard flow of a gas mixture with density maximum in rectangular containers

In order to understand the temperature distribution of Rayleigh-Bénard (R–B) flow of cyclohexane/oxygen mixture to avoid the potential hazard of deflagration, we presented a series of three-dimensional numerical simulations on the aspect ratio dependence of three-dimensional R–B flow of cyclohexane/oxygen mixture with the density maximum in rectangular containers. Results show that with increasing the aspect ratio (A), the sidewall's restriction weakens gradually, the critical Rayleigh (Ra) numbers for the steady flow onset and the oscillatory convection appearance decrease gradually. When A = 0.25 and 0.5, all the critical flow patterns are the single straight roll state. The types of the stable flow patterns are few and the evolutions of flow patterns are relatively simple. When A ≥ 1, the critical flow patterns change with the increasing density inversion parameter (Θm) and the evolution becomes more complicated. With the increasing Ra, the oscillation gets more apparent and the oscillatory frequency increases. When Θm = 0.4, the flow rolls swing back and forth along the sidewalls and the fundamental frequency is the highest at A = 0.25. With the increasing Ra and A, and the decreasing Θm, the heat transfer strengthens. Finally, a scaling relationship for the average Nusselt number in terms of Ra, Θm and A is proposed.

Developing high performance RF heating scenarios on the WEST tokamak

High power experiments, up to 9.2 MW with LHCD and ICRH, have been carried out in the full tungsten tokamak WEST. Quasi non inductive discharges have been achieved allowing to extend the plasma duration to 53 s with stationary conditions in particular with respect to tungsten contamination. Transitions to H mode are observed, and H-modes lasting up to 4 s have been obtained. The increase in stored energy is weak since the power crossing the separatrix is close to the L–H threshold. Hot L mode plasmas (central temperature exceeding 3 keV) with a confinement time following the ITER L96 scaling law are routinely obtained. The weak aspect ratio dependence of this scaling law is confirmed. Tungsten accumulation is generally not an operational issue on WEST. Difficulty of burning through tungsten can prevent the discharge from accessing to a hot core plasma in the ramp-up phase, or can lead to rapid collapse of the central temperature when radiation is enhanced by a slight decrease of the temperature. Except a few pulses post-boronization, the plasma radiation is rather high (P rad/ P tot ∼ 50%) and is dominated by tungsten. This fraction does not vary as the RF power is ramped up and is quite similar in ICRH and/or LHCD heated plasmas. An estimate of the contribution of the RF antennas to the plasma contamination in tungsten is given.

Aspect-ratio dependence of heat and angular momentum transport in turbulent Taylor-Couette flows with axial thermal forcing

• Scaling law of transports determined in finite systems can be extrapolated to large-scale flows. • Coherent turbulent structures in form of Taylor vortices promote heat and angular transports. • Magnitudes of both heat and angular momentum transport depend on domain aspect-ratio. We report numerical studies of the effects of geometry factors, i.e. the aspect ratio Γ and radius ratio η , on the axial heat transport N u and the radial angular momentum transport N ω by turbulent Taylor-Couette flows that are subjected to a vertically destabilizing temperature gradient. For a given Reynolds number R e the magnitudes of both N u and N ω exhibit a pronounced aspect-ratio dependence. The angular momentum transport is reduced in long cylinders, since endwall effects are weakened as Γ increases. For the heat transfer in large- Γ cylinders, a significant N u -reduction appears for low- R e regime, whereas N u can be greatly enhanced in the regime of turbulent Taylor vortex (TV). The former N u -reduction is associated with the process that the thermal plumes are swept away by the Ekman vortices near the boundaries, and the fluid exchange is severely restricted between adjacent vortices in the bulk region. The latter N u -enhancement is attributed to the strengthened shears arising in large- Γ cylinders, resulting in a more turbulent boundary layer. Moreover, the pronounced inter-mixing of turbulent TVs continuously pumps the hot (cold) fluids into the bulk, intensifying the transport processes. It is found that although the constraints from the endwalls impact the global heat and angular momentum transport in the low- R e regime, they hardly affect their scaling properties in the high- R e regime with large Γ . We thus expect that the scaling relationship of N u ( R e ) and N ω ( R e ) determined with finite Γ can be extrapolated to large-scale, and even vertically unbounded industrial and geophysical flows.

Coercivity limits in nanoscale ferromagnets

It has been a puzzle for a century about how ``hard'' (coercive) a ferromagnet can be. Seven decades ago, W. Brown gave his famous theorem to correlate coercivity of a ferromagnet to its magnetocrystalline anisotropy field. However, the experimental coercivity values are far below the calculated level given by the theorem, which is called Brown's Coercivity Paradox. The paradox has been considered to be related to the complex microstructures of the magnets in experiments because coercivity is an extrinsic property that is sensitive to any imperfections in the specimens. To date, coercivity cannot be predicted and calculated by quantitative modeling. In this investigation, we carried out a case study on the high magnetic coercivity of Co nanowires exceeding the magnetocrystalline anisotropy field as predicted by Brown's theorem. It is found that the aspect ratio and diameter of the nanocrystals have a strong effect on the coercivity. When the nanocrystals have an increased aspect ratio, the coercivity is significantly higher than the magnetocrystalline anisotropy field of a hcp Co crystal. Micromagnetic simulations give a coercivity aspect-ratio dependence that is well consistent with the experimental results. It is also revealed that a coercivity limit exists based on the geometrical structures of the nanocrystals that govern the demagnetizing process. The quantitative correlation obtained between the structure and coercivity enables material design of advanced permanent magnets in the future.

Lead-free KNNS–BNZH-based piezo-active composites with an improved acoustic impedance and related figure of merit

New modeling results on the volume-fraction and aspect-ratio dependences of the acoustic impedance Z*, related figure of merit FE* and other parameters of piezo-active lead-free 1–3-type composites are analysed to show ways for improving Z* and FE*. In these composites, the main component (aligned poled dense ferroelectric ceramic rods) is characterised by the 0.965(K0.45Na0.55)(Nb0.96Sb0.04)O3–0.035Bi0.5Na0.5Zr0.85Hf0.15O3 composition, and the matrix is piezo-passive (either polyethylene or 0–3 corundum ceramic/polyethylene). Changes of Z*∼ (1–10) MRayl, non-monotonic behaviour of FE* and a large anisotropy of piezoelectric coefficients |d33*/d31*|≥ 5 and electromechanical coupling factors |k33*/k31*|≥ 5 are predicted and discussed for a 1–0–3 composite wherein its 0–3 matrix contains components with a large difference of their elastic properties. Results on the acoustic impedance and related parameters show that the studied composites are of value in acoustic transducer, sensor and structural health monitoring applications.

Tailoring Magnetic Domains and Magnetization Switching in CoFe Nanolayer Patterns with Their Thickness and Aspect Ratio on GaAs (001) Substrate

The thickness and aspect ratio dependence of magnetic domain formation in CoFe nanolayer patterns on GaAs (001) substrates are investigated by means of a direct approach using magnetic force microscopy at room temperature. Magnetic force microscope observations under as‐deposition condition show that magnetic domain formation in the patterns depends strongly on the aspect ratio and thickness of the patterns and the crystallographic orientations of the substrates. A single magnetic domain is more easily formed in the patterns with a higher aspect ratio, with a thinner nanolayer thickness, and along the ⟨110⟩ direction of the substrates. The magnetic fields are next applied in the direction parallel to the ⟨1–10⟩ and ⟨110⟩ orientations of the substrates to characterize a magnetization switching behavior in the patterns. The aspect ratio and thickness of the patterns and the crystallographic orientations of the substrates strongly affect the magnetic fields needed for magnetization switching in the patterns. A higher magnetic field is required for a higher aspect ratio and a thinner nanolayer thickness of the patterns. All the direct observations confirm that the magnetic domain structures and magnetization switching are tuned by controlling aspect ratio and thickness of the patterns and the crystallographic orientations of the substrates.

A Source Physics Interpretation of Nonself-Similar Double-Corner-Frequency Source Spectral Model JA19_2S

AbstractWe investigate the relation between the kinematic double-corner-frequency source spectral model JA19_2S (Ji and Archuleta, 2020) and static fault geometry scaling relations proposed by Leonard (2010). We find that the nonself-similar low-corner-frequency scaling relation of JA19_2S model can be explained using the fault length scaling relation of Leonard’s model combined with an average rupture velocity ∼70% of shear-wave speed for earthquakes 5.3 < M< 6.9. Earthquakes consistent with both models have magnitude-independent average static stress drop and average dynamic stress drop around 3 MPa. Their scaled energy e˜ is not a constant. The decrease of e˜ with magnitude can be fully explained by the magnitude dependence of the fault aspect ratio. The high-frequency source radiation is generally controlled by seismic moment, static stress drop, and dynamic stress drop but is further modulated by the fault aspect ratio and the relative location of the hypocenter. Based on these two models, the commonly quoted average rupture velocity of 70%–80% of shear-wave speed implies predominantly unilateral rupture.

Aspect-Ratio Dependence of Heat and Angular Momentum Transport in Turbulent Taylor-Couette Flows with Axial Thermal Forcing

Aspect-Ratio Dependence of Heat and Angular Momentum Transport in Turbulent Taylor-Couette Flows with Axial Thermal Forcing

Absorption Features of CdTe Nanoclusters: Aspect Ratio Dependency of the Singlet/Doublet from First-Principles Calculations

Two categories of absorption features, singlet and doublet, have been observed in semiconductive II–VI magic-size clusters (MSCs) experimentally; however, the underlying mechanism resulting in the ...

Aspect Ratio Dependence of Thermocapillary Flow Instability of Moderate-Prandtl Number Fluid in Annular Pools Heated from Inner Cylinder

Aspect Ratio Dependence of Thermocapillary Flow Instability of Moderate-Prandtl Number Fluid in Annular Pools Heated from Inner Cylinder