Macroscopic Steps Research Articles

Thermally-activated locomotion of a bilayer polymer actuator

The development of smart actuators based on renewable or biocompatible materials, which are able for delivery of specific cargo is of great importance in robotics, medical and material science engineering, food industry, etc. Here, we report the original design of a bilayer polymer actuator consisting of two polymer materials with an interface adhesive layer between them, able for macroscopic locomotion step by step on a special ratchet substrate through repetitive bending and straightening. This was triggered by heat alternation due to incandescent lamp radiation on/off switching, with a rapid reaction time of the actuator to heat exposure of the order of few seconds. Specifically, a relatively fast locomotion of the actuator was achieved due to the large amplitude of its reversible bending deformation of up to ∼40% measured in terms of the actuator's elevation-to-length ratio. As a result, a typical actuator demonstrated the locomotion velocity of about 3 cm/min, where each cycle of contraction/expansion yielded a walking step of ∼1 cm for about 20 s. It was demonstrated that the actuator, while moving, is able to carry a cargo almost twice heavier than the mass of the carrier itself. Based on optical microscopy and atomic force - infrared spectroscopy data it was concluded that the adhesive interface layer plays an important role in the stable operation of the actuator as it retards linear expansion of the rear polymer layer and thus assists conversion of different linear expansion of the adjacent layers into their effective bending.

Step Structures and Adatom Diffusion on SiC Surfaces

Recent first-principles studies on step structures and adatom diffusion on SiC(0001) surfaces are reviewed. SiC is a promising material for next-generation power electronics, and SiC power devices are already on the market. The obtained formation energies of all the possible atomic steps provide a definite identification of the most relevant atomic step for two vicinal directions inclined toward either <1100> or <1120> direction on SiC(0001) surfaces. Relations between the energetics of surface steps and macroscopic step morphology are discussed. Diffusion mechanism for Si, C, and H adatoms which are three important chemical elements of gas sources of chemical vapor deposition (CVD) for SiC is revealed both on the surface terraces and near the surface steps.

A multiscale approach for the development of a nonlinear viscoelastic friction-and-cavitation-based model for solid propellants

Solid composite propellants present a highly nonlinear viscoelastic behavior when submitted to cyclic loadings. To model these nonlinearities, it is proposed to use two internal variables representing friction and cavitation. A homogenization approach is set to determine the evolution equations of these variables and their mathematical relationships with the material’s macroscopic properties. Then, a macroscopic step allows the construction of a thermomechanically consistent viscoelastic model containing the new internal variables. Numerical parameters are identified through propellant characterization tests. Validations on cyclic experiments show the model’s ability to reproduce and to predict the material’s behavior correctly outside this identification range.

A Beam-Splitting Bianisotropic Metasurface Designed by Optimization and Machine Learning

Electromagnetic metasurfaces have attracted significant interest recently due to their low profile and advantageous applications. Practically, many metasurface designs start with a set of constraints for the radiated far-field, such as main-beam direction(s) and side lobe levels, and end with a non-uniform physical structure for the surface. This problem is quite challenging, since the required tangential field transformations are not completely known when only constraints are placed on the scattered fields. Hence, the required surface properties cannot be solved for analytically. Moreover, the translation of the desired surface properties to the physical unit cells can be time-consuming and difficult, as it is often a one-to-many mapping in a large solution space. Here, we divide the inverse design process into two steps: a macroscopic and microscopic design step. In the former, we use an iterative optimization process to find the surface properties that radiate a far-field pattern that complies with specified constraints. This iterative process exploits non-radiating currents to ensure a passive and lossless design. In the microscopic step, these optimized surface properties are realized with physical unit cells using machine learning surrogate models. The effectiveness of this end-to-end synthesis process is demonstrated through measurement results of a beam-splitting prototype.

Application of Particular Solutions of the Burgers Equation to Describe the Evolution of Shock Waves of Density of Elementary Steps

Particular solutions of the Burgers equations (BE) with zero boundary conditions are investigated in an analytical form. For values of the shape parameter greater than 1, but approximately equal to 1, the amplitude of the initial periodic perturbations depends nonmonotonically on the spatial coordinate, i.e. the initial perturbation can be considered as a shock wave. Particular BE solutions with zero boundary conditions describe a time decrease of the amplitude of initial nonmonotonic perturbations, which indicates the decay of the initial shock wave. At large values of the shape parameter , the amplitude of the initial periodic perturbations depends harmoniously on the spatial coordinate. It is shown that over time, the amplitude and the spatial derivative of the profile of such a perturbation decrease and tend to zero. Emphasis was put on the fact that particular BE solutions can be used to control numerical calculations related to the BE-based description of shock waves in the region of large spatial gradients, that is, under conditions of a manifold increase in spatial derivatives. These solutions are employed to describe the profile of a one-dimensional train of elementary steps with an orientation near &lt;100&gt;, formed during the growth of a NaCl single crystal from the vapor phase at the base of a macroscopic cleavage step. It is shown that the distribution of the step concentration with distance from the initial position of the macrostep adequately reflects the shock wave profile at the decay stage. The dimensionless parameters of the wave are determined, on the basis of which the estimates of the characteristic time of the shock wave decay are made.

Development of an extended Macro Monte Carlo method for efficient and accurate dose calculation in magnetic fields.

Progress in the field of magnetic resonance (MR)-guided radiotherapy has triggered the need for fast and accurate dose calculation in presence of magnetic fields. The aim of this work is to satisfy this need by extending the macro Monte Carlo (MMC) method to enable dose calculation for photon, electron, and proton beams in a magnetic field. The MMC method is based on the transport of particles in macroscopic steps through an absorber by sampling the relevant physical quantities from a precalculated database containing probability distribution functions. To enable MMC particle transport in a magnetic field, a transformation accounting for the Lorentz force is applied for each macro step by rotating the sampled position and direction around the magnetic field vector. The transformed position and direction distributions on local geometries are validated against full MC for electron and proton pencil beams. To enable photon dose calculation, an in-house MC algorithm is used for photon transport and interaction. Emerging secondary charged particles are passed to MMC for transport and energy deposition. The extended MMC dose calculation accuracy and efficiency is assessed by comparison with EGSnrc (photon and electron beams) and Geant4 (proton beam) calculated dose distributions of different energies and homogeneous magnetic fields for broad beams impinging on water phantoms with bone and lung inhomogeneities. The geometric transformation on the local geometries is able to reproduce the results of full MC for all investigated settings (difference in mean value and standard deviation <1%). Macro Monte Carlo calculated dose distributions in a homogeneous magnetic field are in agreement with EGSnrc and Geant4, respectively, with gamma passing rates >99.6% (global 2%, 2mm and 10% threshold criteria) for all situations. MMC achieves a substantial efficiency gain of up to a factor of 21 (photon beam), 66 (electron beam), and 356 (proton beam) compared to EGSnrc or Geant4. Efficient and accurate dose calculation in magnetic fields was successfully enabled by utilizing the developed extended MMC transport method for photon, electron, and proton beams.

Electrochemical Emission during Discontinuous Creep of Aluminum‒Magnesium Alloy

It is experimentally established that the discontinuous creep of aluminum‒magnesium alloy AlMg6, deformed in an aqueous medium, is accompanied by discrete electrochemical emission: jumps of the sample electrode potential, which arise simultaneously with stress drops during the development of macroscopic step on the creep curve. It has been found (with the aid of high-speed video recording) that the electrode potential jump occurs simultaneously with nucleation and initial fast stage of deformation band widening. A possible mechanism of the occurrence of electrode potential jumps in an aluminum alloy under discontinuous creep conditions is discussed.

Evaluation of force term in lattice Boltzmann method with discrete external boundary force for flow over an immersed body

In this paper, lattice Boltzmann method is used for the simulation of two-dimensional flow over an immersed solid body using external boundary force. Fluid simulation is performed using the Eulerian grid for the entire computational domain, while the domain of solid body is discretized using a boundary-fitted Lagrangian grid, imposed on the uniform Eulerian grid. Fluid–solid interaction is modeled using external boundary force (EBF) technique. Two different methods of implementing boundary force are discussed including modification in (a) collision step of LBM and (b) both collision and computation of macroscopic velocity steps. Computations are performed for flow over a stationary circular object as well as particle sedimentation. Results of drag force and sedimentation velocity show good correspondence with previously published ones at Re =1.0 for both methods. No discrepancy in results was observed between two methods for higher Reynolds numbers. Results obtained for sedimentation of an elliptical object show that the type of implementation of boundary force has no significant effect on the predicted location and angular position of the elliptical object. Furthermore, it is observed that more accurate results are obtained by introducing a modification in EBF for which solutions were independent of the number of nodes on the surface boundary.

Kinetics of the Thermal Decomposition of 2,4,6-Triazido-3,5-Difluoropyridine

The kinetics and products of the thermal decomposition of 2,4,6-triazido-3,5-difluoropyridine in melt at temperatures of 120–160°C have been studied using pressure measurements, differential thermal analysis, and IR spectroscopy. The reaction occurs in two macroscopic steps, each described by a first-order kinetic equation. In the first, the activation energy and the common logarithm of the pre-exponential factor are 35.6 ± 1.2 kcal/mol and 15.1 ± 0.6 s–1, respectively. For the studied compound (and certain other heterocyclic azides, such as 2,4,6-triazido-1,3,5-triazine and 2,4,6-triazidopyrimidine), the pre-exponential factor is anomalously high. This is due to the fact that the 2,4,6-triazido-3,5-difluoropyridine molecule contains no hydrogen atoms. For such azides, the usual decomposition mechanism (cleavage of the azide group with subsequent attack of nitrene on a hydrogen atom of a neighboring molecule) is impossible, which favors the stability of the formed nitrene. In this case, the reaction occurs through a complex chain polymerization mechanism, which leads to the formation of specific condensed products: packs of planar polyconjugate carbon–nitrogen networks with a porphyrin-like structure.

SEM, Chemical Etching and THz Wave Generation of 4-Dimethylamino-N-methyl-4-stilbazolium Tosylate (DAST) Single Crystals

The 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal which was candidate for generating and detecting terahertz (THz) wave was grown by slope nucleation method (SNM). The functional groups of DAST were identified by Fourier Transform Infrared (FTIR) studies. Macroscopic steps and streaks on the (001) face were found by scanning electron microscope (SEM). Different patterns of etch pits under different chemical etchants were studied by optical microscope. Widely tunable THz waves ranging from 0.38 to 18.10 THz were generated from 0.5 mm-thick DAST crystal. The maximum output energy was 345.4 nJ/pulse at 3.39 THz and the conversion efficiency was 8.44 × 10–5.

Mechanism of low-frequency discrete acoustic emission during intermittent creep of aluminum alloy

A correlation between the dynamics of deformation bands and the discrete acoustic emission during the intermittent creep of the AlMg6 alloy using a high-speed video recording with a time resolution to 50 μs has been studied. A trigger of a macroscopic deformation step in the creep curve is the nucleation and the broadening of the primary deformation band that generates a characteristic acoustic emission signal with duration of several milliseconds. The results confirm the mechanism of generating an acoustic emission signal related to the cooperative dislocations outcrop on the specimen external surface.

Effect of an aggressive medium on discontinuous deformation of aluminum–magnesium alloy AlMg6

It is experimentally shown that the molecular (chemical) process of surface etching of deformed aluminum–magnesium alloy AlMg6 causes the development of a macroscopic plastic strain step with an amplitude of a few percent. Using numerical simulation of the polycrystalline solid etching process, it is shown that the corrosion front morphology varies during etching from Euclid (flat) to fractal (rough). The results obtained show the key role of the surface state on the development of macroscopic mechanical instability of a material exhibiting the Portevin–Le Chatelier effect.

TU‐H‐CAMPUS‐TeP3‐03: Dose Enhancement by Gold Nanoparticles Around the Bragg Peak of Proton Beams

Purpose:To make clear the spatial distribution of dose enhancement around gold nanoparticles (GNPs) located near the proton Bragg peak, and to evaluate the potential of GNPs as a radio sensitizer.Methods:The dose enhancement by electrons emitted from GNPs under proton irradiation was estimated by Geant4 Monte Carlo simulation toolkit in two steps. In an initial macroscopic step, 100 and 195 MeV proton beams were incident on a water cube, 30 cm on a side. Energy distributions of protons were calculated at four depths, 50% and 75% proximal to the Bragg peak, 100% peak, and 75% distal to the peak (P50, P75, Peak, and D75, respectively). In a subsequent microscopic step, protons with the energy distribution calculated above were incident on a 20 nm diameter GNP in a nanometer‐size water box and the spatial distribution of dose around the GNP was determined for each energy distribution. The dose enhancement factor (DEF) was also deduced.Results:The dose enhancement effect was spread to several tens of nanometers in the both depth and radial directions. The enhancement area increased in the order of P50, P75, Peak, and D75 for both cases with 100 and 195 MeV protons. In every position around the Bragg peak, the 100 MeV beam resulted in a higher dose enhancement than the 195 MeV beam. At P75, the average and maximum DEF were 3.9 and 17.0 for 100 MeV, and 3.5 and 16.2 for 195 MeV, respectively. These results indicate that lower energy protons caused higher dose enhancement in this incident proton energy range.Conclusion:The dose enhancement around GNPs spread as the position in the Bragg peak region becomes deeper and depends on proton energy. It is expected that GNPs can be used as a radio sensitizer with consideration of the location and proton beam energy.

Integrated Relocation Model for Free-Floating Carsharing Systems

This paper presents an integrated relocation model for free-floating car-sharing (FFCS) systems. A historical data analysis conducted monthly generates demand indicators. A zone categorization identifies zones with a historical vehicle shortage or surplus and enables the estimation of the optimal vehicle distribution for the target time period. Given the current vehicle positions, the model identifies vehicle imbalances. Five macroscopic and microscopic steps using optimization and rule-based methods recommend relocations and service trips on an individual vehicle level. This process results in detailed staff operation plans. To assess the impact of relocations on the system's operation, the model was applied to an FFCS system in Munich, Germany. Three real-world field tests conducted at stages of the model's development exhibited different degrees of automation. The evaluation showed promising results. All tests had positive impacts on the main measures of success. The earnings per vehicle were increased by up to 18%. The mean idle time per trip end of all vehicles in the feet was decreased by up to 18%. The difference in idle times between the relocated vehicles and neighboring vehicles that were not relocated was decreased by up to 31%. Most important, the profit of the operator was between 4.7% and 5.8% higher than without relocations. The automation of the whole model in the last field test led to slightly lower but still very positive impacts while facilitating the whole relocation process for the operator.

Discrete scale invariance in supercritical percolation

Recently it has been demonstrated that the connectivity transition from microscopic connectivity to macroscopic connectedness, known as percolation, is generically announced by a cascade of microtransitions of the percolation order parameter (Chen et al 2014 Phys. Rev. Lett. 112 155701). Here we report the discovery of macrotransition cascades which follow percolation. The order parameter grows in discrete macroscopic steps with positions that can be randomly distributed even in the thermodynamic limit. These transition positions are, however, correlated and follow scaling laws which arise from discrete scale invariance (DSI) and non self-averaging, both traditionally unrelated to percolation. We reveal the DSI in ensemble measurements of these non self-averaging systems by rescaling of the individual realizations before averaging.

A practice-ready relocation model for free-floating carsharing systems with electric vehicles – Mesoscopic approach and field trial results

This paper introduces a relocation model for free-floating Carsharing (FFCS) systems with conventional and electric vehicles (EVs). In case of imbalances caused by one-way trips, the approach recommends profit maximizing vehicle relocations. Unlike existing approaches, two types of relocations are distinguished: inter zone relocations moving vehicles between defined macroscopic zones of the operating area and intra zone relocations moving vehicles within such zones. Relocations are combined with the unplugging and recharging of EVs and the refueling of conventional vehicles. In addition, remaining pure service trips are suggested. A historical data analysis and zone categorization module enables the calculation of target vehicle distributions. Unlike existing approaches, macroscopic optimization steps are supplemented by microscopic rule-based steps. This enables relocation recommendations on the individual vehicle level with the exact GPS coordinates of the relocation end positions. The approach is practice-ready with low computational times even for large-scale scenarios.To assess the impact of relocations on the system’s operation, the model is applied to a FFCS system in Munich, Germany within three real world field tests. Test three shows the highest degree of automation and represents the final version of the model. Its evaluation shows very promising results. Most importantly, the profit is increased by 5.8% and the sales per vehicle by up to 10%. The mean idle time per trip end is decreased by 4%.

Identifying the active sites for CO dissociation on Fe–BCC nanoclusters

CO dissociation rate is fundamental for determining selectivity and yield for growing carbon chains on iron catalysts in, e.g., Fischer–Tropsch and carbon nanotube syntheses. CO dissociation is a highly structure sensitive reaction; on crystal surfaces and large particles CO dissociation is facile only at step sites. However, small (<3nm) nanoparticles cannot contain steps and even in macroscopic clusters step concentration is small. Here, density functional theory is used systematically to map several CO dissociation pathways to find and explain the nature of active sites in small BCC iron nanoclusters. It is shown that BCC(100-110) edges of the particle are responsible for the reactivity of small Fe–BCC clusters and that low CO dissociation barrier is achievable even without stepped surfaces. Careful analysis proves the essential role of local geometry at the active site leading to strong interaction between the metal and 2π* orbitals and accessible, empty states. Furthermore, CO dissociation on Fe–BCC particles is more facile than on smooth crystal surfaces and icosahedral and amorphous iron particles exemplifying the importance of nanoparticle geometry on reactivity. Providing suitable particle geometry is used, CO dissociation on Fe–BCC nanoparticles occurs even under mild conditions, enabling controlled synthesis of carbon structures from CO.

A Spatially Resolved Study of the Role of Surface Motifs in the Photoelectrochemical Conversion on Prospective 2-D Layered Chalcogenide Photoelectrodes for Solar-Fuel Generation

2-D Layered chalcogenide materials, with the general formula, MX2(where, generally, M= Mo, W and X=S, Se), have been of sustained research interest, over the past few decades, in the area of solar-fuel generation. They have been looked at as light absorbers as they exhibit favorable bandgaps, high absorption coefficients for capturing solar energy and have good photoelectrochemical stability. More recently, they have been also found to be highly active electrocatalysts for hydrogen evolution reaction (HER). The surfaces of these materials are highly heterogeneous displaying macroscopic terraces and step edges; along with other micro/nano-scale defects. The optimal role of these materials in solar-energy conversion is decisively determined by their surface morphology. For HER catalysis, material forms such as nanoparticles and amorphous powders that have high concentration of step edges are preferable as the dangling bonds/exposed atoms at the step edges are instrumental in catalyzing the reaction. On the other hand, in their role as light absorbers, the surfaces of these materials should, ideally, be composed of large flat terraces since the absorption of light and the separation of carriers at the semiconductor/electrolyte interface occur most efficiently at the terraces. The step edges and other defects are thought to be detrimental as they act as sites for carrier recombination. It is clear that, in employing these materials for solar-fuel generation, it is of fundamental importance to delineate the specific roles of the surface motifs of 2-D layered chalcogenides in the photoelectrochemical conversion processes. Addressing the above-mentioned knowledge gap, we were able to spatially probe the local photoelectrochemical response of the surface motifs on the layered chalcogenide photoelectrodes by employing the technique of laser beam induced current microscopy (LBIC). LBIC involves the photogeneration of carriers locally using a laser beam and the resulting photocurrent could then be spatially correlated to the excitation spot on the surface of the electrode. Hence, a detailed spatial map of the local photoconversion efficiencies on the photoelectrodes could be obtained. An important observation of the spatially resolved photocurrent measurements was that there were significant variations in the photoconversion efficiencies of different terraces and that, more than the macroscopic step edges, it was the presence of low performing terraces that crucially determined the overall performance of these materials as light absorbers. The source of the low efficiencies on some terraces were further explored by studying and comparing the topography of the various terraces at the micro/nano-scale using AFM and STM. These studies revealed that the terraces were invariably textured at the micro/nano-scale and that the texturing differed from terrace to terrace. Further, local spectral response measurements at terraces of differing photo-conversion efficiencies provided evidence for the existence of sub-bandgap surface states on the low-performing terraces; possibly, explaining the variations in the efficiencies between terraces. Differences in work-function values across terraces measured by Kelvin probe force microscopy also supported the prevalence of surface states on the low performing terraces. In conclusion, employing a suite of spatially resolved techniques to study in-depth the surface motifs of layered chalcogenide photoelectrodes, we have been able to construct a detailed picture of the photoconversion processes occurring on these materials at the microscopic level. The results from our study crucially highlight the sources of performance losses in these materials and will serve to guide efforts in optimization of this material class for application in solar-fuel generation.

Mechanism of the catalytic synthesis of water on silver

A mechanism of the macroscopic step of water synthesis in the oxidation of hydrogen on silver has been revealed.

Coupling the finite element method and molecular dynamics in the framework of the heterogeneous multiscale method for quasi-static isothermal problems

Multiscale models are designed to handle problems with different length scales and time scales in a suitable and efficient manner. Such problems include inelastic deformation or failure of materials. In particular, hierarchical multiscale methods are computationally powerful as no direct coupling between the scales is given. This paper proposes a hierarchical two-scale setting appropriate for isothermal quasi-static problems: a macroscale treated by continuum mechanics and the finite element method and a microscale modelled by a canonical ensemble of statistical mechanics solved with molecular dynamics. This model will be implemented into the framework of the heterogeneous multiscale method. The focus is laid on an efficient coupling of the macro- and micro-solvers. An iterative solution algorithm presents the macroscopic solver, which invokes for each iteration an atomistic computation. As the microscopic computation is considered to be very time consuming, two optimisation strategies are proposed. Firstly, the macroscopic solver is chosen to reduce the number of required iterations to a minimum. Secondly, the number of time steps used for the time average on the microscale will be increased with each iteration. As a result, the molecular dynamics cell will be allowed to reach its state of thermodynamic equilibrium only in the last macroscopic iteration step. In the preceding iteration steps, the molecular dynamics cell will reach a state close to equilibrium by using considerably fewer microscopic time steps. This adapted number of microsteps will result in an accelerated algorithm (aFE-MD-HMM) obtaining the same accuracy of results at significantly reduced computational cost. Numerical examples demonstrate the performance of the proposed scheme.