Multiscale Simulation Approach Research Articles

TiOx deposited by magnetron sputtering: a joint modelling and experimental study

This paper presents a 3D multiscale simulation approach to model magnetron reactive sputter deposition of TiOx⩽2 at various O2 inlets and its validation against experimental results. The simulation first involves the transport of sputtered material in a vacuum chamber by means of a three-dimensional direct simulation Monte Carlo (DSMC) technique. Second, the film growth at different positions on a 3D substrate is simulated using a kinetic Monte Carlo (kMC) method. When simulating the transport of species in the chamber, wall chemistry reactions are taken into account in order to get the proper content of the reactive species in the volume. Angular and energy distributions of particles are extracted from DSMC and used for film growth modelling by kMC.Along with the simulation, experimental deposition of TiOx coatings on silicon samples placed at different positions on a curved sample holder was performed. The experimental results are in agreement with the simulated ones. For a given coater, the plasma phase hysteresis behaviour, film composition and film morphology are predicted. The used methodology can be applied to any coater and any films. This paves the way to the elaboration of a virtual coater allowing a user to predict composition and morphology of films deposited in silico.

Prediction of degree of impregnation in thermoplastic unidirectional carbon fiber prepreg by multi-scale computational fluid dynamics

A multi-scale simulation approach was proposed to predict the degree of impregnation (DoI) in thermoplastic unidirectional carbon fiber prepreg (UD-CFP). The multi-scale approach included a two-dimensional (2D) micro-scale computational fluid dynamics (CFD) in a representative elementary volume (REV) of carbon fiber (CF) tow, a 3D macro-scale CFD of an entire impregnation die with 15 sliding CF tows, and a process-scale simulation assembling data from the micro- and macro-scale CFDs. In the macro-scale steady-state CFD, thermoplastic resin injection and CF tow insertion were considered for an impregnation die 10 cm in width. In the micro-scale transient CFD, impregnation mechanisms of resin into CF filaments 7 μm in diameter were identified in terms of surface coverage, capillary permeation, and penetration through CF filaments. The DoI as a function of pressure and time was obtained from the micro-scale CFD within a range of pressures found in the macro-scale CFD. In the process-scale simulation, the cumulative DoI of the 15 tows was predicted along the impregnation die length with the aid of the micro- and macro-scale CFD results. Combining the multi-scale models gives a potential to predict the uniformity of the transverse resin amount in the final UD-CFP product.

On Low-Resistance Contacts to 2-D MoTe2 by Crystalline Phase Junctions

Low contact resistance to 2-D semiconductor materials plays a critical role on their device applications. It has been experimentally demonstrated recently that the crystalline phase homojunctions between the 1T’ metallic and 2H semiconducting phases of 2-D transition metal dichacolgenide (TMDC) materials can be formed by a variety of fabrication techniques. A multiscale simulation approach that integrates atomistic ab initio simulations with quantum transport calculations based on the nonequilibrium Green’s function formalism is used to examine the contact properties of the crystalline phase junctions of monolayer MoTe2. It is shown that the following mechanisms can contribute to the low contact resistance of crystalline phase metal–semiconductor junctions of 2-D materials. First, the electric field is significantly enhanced at the 2-D phase junction interface due to the extremely thin body, which results in a thin Schottky barrier. Second, the coupling of electron wave functions cross the 1T’-2H junction interface is strong. Third, different from 3-D bulk metal–semiconductor junctions, metal-induced band gap states (MIGS) do not pin the Fermi level in the 2-D material junctions due to low dimensionality of the MIGS charge. The results provide insights into the possibility and limits of achieving low-contact-resistance contacts to 2-D TMDC semiconductors by using crystalline phase metal–semiconductor junctions.

A multiscale numerical simulation approach for chloride diffusion and rebar corrosion with compensation model

Refined analysis depicting mass transportation and physicochemical reaction and reasonable computing load with acceptable DOFs are the two major challenges of numerical simulation for concrete durability. Mesoscopic numerical simulation for chloride diffusion considering binder, aggregate and interfacial transition zone is unable to be expended to the full structure due to huge number of DOFs. In this paper, a multiscale approach of combining both mesoscopic model including full-graded aggregate and equivalent macroscopic model was introduced. An equivalent conversion of chloride content at the Interfacial Transition Layer (ITL) connecting both models was considered. Feasibility and relative error were discussed by analytical deduction and numerical simulation. Case study clearly showed that larger analysis model in multiscale model expanded the diffusion space of chloride ion and decreased chloride content in front of rebar. Difference for single-scale simulation and multiscale approach was observed. Finally, this paper addressed some worth-noting conclusions about the chloride distribution and rebar corrosion regarding the configuration of rebar placement, rebar diameter, concrete cover and exposure period.

MOlecular MAterials Property Prediction Package (MOMAP) 1.0: a software package for predicting the luminescent properties and mobility of organic functional materials

ABSTRACTMOlecular MAterials Property Prediction Package (MOMAP) is a software toolkit for molecular materials property prediction. It focuses on luminescent properties and charge mobility properties. This article contains a brief descriptive introduction of key features, theoretical models and algorithms of the software, together with examples that illustrate the performance. First, we present the theoretical models and algorithms for molecular luminescent properties calculation, which includes the excited-state radiative/non-radiative decay rate constant and the optical spectra. Then, a multi-scale simulation approach and its algorithm for the molecular charge mobility are described. This approach is based on hopping model and combines with Kinetic Monte Carlo and molecular dynamics simulations, and it is especially applicable for describing a large category of organic semiconductors, whose inter-molecular electronic coupling is much smaller than intra-molecular charge reorganisation energy.

Multi-scale performance simulation and effect analysis for hydraulic concrete submitted to leaching and frost

The long-term behavior and safety performance of hydraulic concrete structures are influenced by chemo-mechanical deterioration due to leaching and frost. The single-scale experimental investigations on the certain deterioration phenomenon of concrete structure submitted to leaching or frost have been performed. In this work, the multi-scale numerical simulation approach is introduced to analyze the deterioration phenomena due to leaching or/and frost in hydraulic concrete and the structural responses to the above deterioration phenomena. The three-dimensional (3D) multi-scale modeling and homogenization methods of hydraulic concrete submitted to leaching or/and frost are developed. The effect analysis of leaching or/and frost on the mechanical properties of hydraulic concrete is implemented. First, according to the multi-scale and multi-phase characteristics of hydraulic concrete as composite material, the scale separation standard of hydraulic concrete is given. The hydration model and random aggregate model are introduced to reconstruct the representative volume element model of hydraulic concrete with different scales, namely micro-scale (cement paste), meso-scale (mortar), macro-scale (concrete). The numerical homogenization method is presented to determine the mechanical properties of the hydraulic concrete. Second, the methods are developed to implement the 3D multi-scale simulation for random leaching process and thermo-mechanical coupling frost damage process of hydraulic concrete. Regarding seepage leaching as the leading role, a numerical method investigating the combined effect of leaching and frost on the mechanical properties of hydraulic concrete is proposed. Finally, the water-saturated hydraulic concrete is taken as an example. The deterioration phenomena due to leaching or/and frost in selected concrete are analyzed. The structural responses to the above deterioration phenomena are identified. It is indicated that the proposed multi-scale progressive analysis approach can capture the potential deterioration phenomena of hydraulic concrete. The variation mechanism of concrete structural behavior can be given a deeper insight. It can be seen that the nonlinear superimposed effect of leaching and frost can easily cause serious damage of water-saturated hydraulic concrete.

SELF-HEATING EFFECTS IN SILICON NANOSCALE MOSFET(A MULTISCALE MODELING APPROACH)

In this work we present a novel multiscale simulation approach that combines circuit level with device level simulations together with experimental measurements to uncover the temperature of the hot-spot at nanoscale MOSFET devices. The circuit level simulation is performed with COMSOL simulation software. This allows one to perform electro-thermal modeling given the input Joule heating terms, the magnitude and the location of which is determined with more physically-based electro-thermal Monte Carlo device simulator. Simulation results obtained with this simulator are in agreement with experimental measurements from IMEC using specialized heater-sensor test structures in common-source and common-drain configurations.

Многомасштабный подход к моделированию сложных переходных процессов движения жидкостей в технических системах

The paper presents a multiscale approach for simulation of the two-phase flows processes in complex technical systems. The multiscale approach is based both on the division of the computational domain into subdomains with their own system of equations, as well as on the splitting of the initial system of equations into several subsystems each is valid to the corresponding scale under consideration. The problem of the far field acoustic noise calculation during the launch of a rocket vehicle is considered as an example of the possible use of a multiscale model. The case is studied with account to noise suppression due to water supply into the gas jets of the propulsion system. Other areas of application of the multiscale model include the cases of the oil and gas industry: killing gas-producing wells located at great depth, killing oil wells with a high gas factor at the fields. The proposed multi-scale mathematical model includes 5 sub-models: 1) gas dynamics of high-speed multicomponent gas mixture flows; 2) the hydrodynamics of a two-phase mixture flow in a homogeneous approximation with the account for the compressibility of the gas phase and the mass exchange between the phases; 3) the liquid-gas interface transport; 4) the transport of a cloud of droplets and its interaction with a gas-liquid medium; 5) noise calculation in the far field using the Ffowks Williams-Hawking acoustic analogy. The model can be extended to include additional sub-models, such as the Eulerian-Lagrange Jet Atomization. The implementation of the submodels can be done on the basis of open source packages: OpenFOAM, Nektar ++, ITHACA-FV. The acoustics library and the hybrid algorithm for compressible homogeneous two-phase flow are implemented as libAcoustics and hybridCentralSolvers modules based on the OpenFOAM open package. The source code of the developed model is freely available through the GiHub project https://github.com/unicfdlab.

Feasible Device Architectures for Ultrascaled CNTFETs

Feasible device architectures for ultrascaled carbon nanotubes field-effect transistors (CNTFETs) are studied down to 5.9 nm using a multiscale simulation approach covering electronic quantum transport simulations and numerical device simulations. Schottky-like and ohmiclike contacts are considered. The simplified approach employed in the numerical device simulator is critically evaluated and verified by means of comparing the results with electronic quantum simulation results of an identical device. Different performance indicators, such as the switching speed, switching energy, the subthreshold slope, I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</sub> /I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</sub> -ratio, among others, are extracted for different device architectures. These values guide the evaluation of the technology for different application scenarios. For high-performance logic applications, the buried gate CNTFET is claimed to be the most suitable structure.

Integrated computational material engineering model development for tube drawing process

In order to be able to understand the tube drawing process in a better way and also to analyze more complex situations, multiscale simulation approach based on the Integrated Computational Materials Engineering was used and an FEM model was developed. The responsibility of this model was to get the eccentricity and residual stresses of the tubes before the drawing process and simulate the same parameters and their variations during the drawing. For simulation, the multiscale methodology based on the idea of Integrated Computational Materials Engineering was used to study the process. The main reason for using the multiscale simulation methodology was to study plastic behavior of the material in the FEM model considering texture of the material.

Multiscaled Simulation Methodology for Neuro-Inspired Circuits Demonstrated with an Organic Memristor

Organic memristors are promising molecular electronic devices for neuro-inspired on-chip learning applications. In this paper, we present a numerically efficient compact model suitable for $Fe(bpy)_3^{2+}$ organic memristors operating according to an intramolecular charge transfer switching mechanism. This compact model, being physics-based and relying on electrical characterizations and parametric extractions performed on test structures, is especially efficient in pulsed mode and describes the conductance variations for both SET and RESET regimes. Using this model, a dynamic multiscale simulation approach has been set-up to extend the model from individual devices to larger model systems that learn progressively through time. To verify the soundness and highlight emergent properties of the organic memristors, instances of the compact model have been simulated within a simple neuromorphic design that co-integrates with CMOS neurons. In addition, a larger supervised learning system using the new compact model is demonstrated. These successful tests suggest our model might be of interest to neuromorphic designers.

Effective thermal properties of an isotactic polypropylene (α‐iPP) injection moulded part by a multiscale approach

AbstractIn injection moulding processes, the melt undergoes a complex deformation and cooling history, which results in an inhomogeneous distribution of crystalline superstructures in semi‐crystalline thermoplastics, which significantly influences their final mechanical and thermal properties. In this paper we describe the determination of local effective thermal properties of a moulded part via a multiscale simulation approach. First a macroscopic filling and heat transfer simulation is performed followed by a microstructure evolution calculation on the micro‐scale. Then, the effective thermal properties are derived via a two‐level homogenization scheme. The results show that the effective thermal conductivity is anisotropic and that it varies asymmetrically over the analysed plate sections.

Rational In Silico Design of an Organic Semiconductor with Improved Electron Mobility.

Organic semiconductors find a wide range of applications, such as in organic light emitting diodes, organic solar cells, and organic field effect transistors. One of their most striking disadvantages in comparison to crystalline inorganic semiconductors is their low charge-carrier mobility, which manifests itself in major device constraints such as limited photoactive layer thicknesses. Trial-and-error attempts to increase charge-carrier mobility are impeded by the complex interplay of the molecular and electronic structure of the material with its morphology. Here, the viability of a multiscale simulation approach to rationally design materials with improved electron mobility is demonstrated. Starting from one of the most widely used electron conducting materials (Alq3 ), novel organic semiconductors with tailored electronic properties are designed for which an improvement of the electron mobility by three orders of magnitude is predicted and experimentally confirmed.

In Silico Prediction of Growth and Dissolution Rates for Organic Molecular Crystals: A Multiscale Approach

Solution crystallization and dissolution are of fundamental importance to science and industry alike and are key processes in the production of many pharmaceutical products, special chemicals, and so forth. The ability to predict crystal growth and dissolution rates from theory and simulation alone would be of a great benefit to science and industry but is greatly hindered by the molecular nature of the phenomenon. To study crystal growth or dissolution one needs a multiscale simulation approach, in which molecular-level behavior is used to parametrize methods capable of simulating up to the microscale and beyond, where the theoretical results would be industrially relevant and easily comparable to experimental results. Here, we review the recent progress made by our group in the elaboration of such multiscale approach for the prediction of growth and dissolution rates for organic crystals on the basis of molecular structure only and highlight the challenges and future directions of methodic development.

Multiscale simulation approach to heat and mass transfer properties of nanostructured materials for sorption heat storage

Thermal storage devices are becoming crucial for the exploitation of solar energy. From the point of view of seasonal energy storage, the most promising technology is represented by adsorption thermal batteries, which allow storing energy without heat loss with time. The improvement of thermal batteries design is related to a better understating of transport phenomena occurring in the adsorption/desorption phases. In this work, we discuss an efficient computational protocol to characterize adsorbent materials, in terms of both heat and mass transfer proprieties. To this purpose, a hybrid Molecular Dynamics and Monte Carlo method is developed. The proposed model is then tested on two types of 13X zeolite, with 76 and 88 Na cations. The results obtained, such as adsorbate diffusivity, adsorption curves, and heat of adsorption are validated with the literature. Finally, in the view of a multiscale analysis of sorption thermal storage devices, the possible use of the simulation outputs as inputs of thermal fluid dynamics models of adsorbent beds is discussed.

A quantum wave based compact modeling approach for the current in ultra-short DG MOSFETs suitable for rapid multi-scale simulations

Source-to-drain (SD) tunneling decreases the device performance in MOSFETs falling below the 10nm channel length. Modeling quantum mechanical effects including SD tunneling has gained more importance specially for compact model developers. The non-equilibrium Green’s function (NEGF) has become a state-of-the-art method for nano-scaled device simulation in the past years. In the sense of a multi-scale simulation approach it is necessary to bridge the gap between compact models with their fast and efficient calculation of the device current, and numerical device models which consider quantum effects of nano-scaled devices. In this work, an NEGF based analytical model for nano-scaled double-gate (DG) MOSFETs is introduced. The model consists of a closed-form potential solution of a classical compact model and a 1D NEGF formalism for calculating the device current, taking into account quantum mechanical effects. The potential calculation omits the iterative coupling and allows the straightforward current calculation. The model is based on a ballistic NEGF approach whereby backscattering effects are considered as second order effect in a closed-form. The accuracy and scalability of the non-iterative DG MOSFET model is inspected in comparison with numerical NanoMOS TCAD data for various channel lengths. With the help of this model investigations on short-channel and temperature effects are performed.

Modeling of ultrasonic wave propagation in composite laminates with realistic discontinuity representation

This paper presents a method for embedding realistic defect geometries of a fiber reinforced material in a finite element modeling environment in order to simulate active ultrasonic inspection. When ultrasonic inspection is used experimentally to investigate the presence of defects in composite materials, the microscopic defect geometry may cause signal characteristics that are difficult to interpret. Hence, modeling of this interaction is key to improve our understanding and way of interpreting the acquired ultrasonic signals. To model the true interaction of the ultrasonic wave field with such defect structures as pores, cracks or delamination, a realistic three dimensional geometry reconstruction is required. We present a 3D-image based reconstruction process which converts computed tomography data in adequate surface representations ready to be embedded for processing with finite element methods. Subsequent modeling using these geometries uses a multi-scale and multi-physics simulation approach which results in quantitative A-Scan ultrasonic signals which can be directly compared with experimental signals. Therefore, besides the properties of the composite material, a full transducer implementation, piezoelectric conversion and simultaneous modeling of the attached circuit is applied. Comparison between simulated and experimental signals provides very good agreement in electrical voltage amplitude and the signal arrival time and thus validates the proposed modeling approach. Simulating ultrasound wave propagation in a medium with a realistic shape of the geometry clearly shows a difference in how the disturbance of the waves takes place and finally allows more realistic modeling of A-scans.

A multi-scale three-dimensional Cellular Automata fracture model of radiolytically oxidised nuclear graphite

A multi-scale approach for fracture simulation, based on the Cellular Automata technique, has been developed and then applied to a nuclear graphite that is used in structural components of the UK Advanced Gas-cooled Reactors (AGR). High resolution X-ray computed tomographs of Gilsocarbon grade graphite, with up to 68% weight loss by radiolytic oxidation, provide quantitative descriptions of the porosity within its constitutive filler particles and their surrounding matrix. The statistical distributions for tensile strength and elastic modulus obtained from small models of the filler and matrix are introduced to a large scale model of the heterogeneous microstructure. These microstructure-derived simulations achieve a good agreement with experimental data. The computationally efficient analysis is then used to investigate the stochastic effects on mechanical properties of possible variations in the microstructure between individual small test specimens. As these may represent material of nominally the same microstructure, there is an apparent increase in the variability of strength and modulus at high weight loss.

Multiscale simulations and experiments on water jet atomization

Numerical simulation of primary atomization at high Reynolds number is still a challenging problem. In this work a multiscale approach for the numerical simulation of liquid jet primary atomization is applied, using an Eulerian-Lagrangian coupling. In this approach, an Eulerian volume of fluid (VOF) method, where the Reynolds stresses are closed by a Reynolds stress model is applied to model the global spreading of the liquid jet. The formation of the micro-scale droplets, which are usually smaller than the grid spacing in the computational domain, is modelled by an energy-based sub-grid model. Where the disruptive forces (turbulence and surface pressure) of turbulent eddies near the surface of the jet overcome the capillary forces, droplets are released with the local properties of the corresponding eddies. The dynamics of the generated droplets are modelled using Lagrangian particle tracking (LPT). A numerical coupling between the Eulerian and Lagrangian frames is then established via source terms in conservation equations. As a follow-up study to our investigation in Saeedipour et al. (2016a), the present paper aims at modelling drop formation from liquid jets at high Reynolds numbers in the atomization regime and validating the simulation results against in-house experiments. For this purpose, phase-Doppler anemometry (PDA) was used to measure the droplet size and velocity distributions in sprays produced by water jet breakup at different Reynolds numbers in the atomization regime. The spray properties, such as droplet size spectra, local and global Sauter-mean drop sizes and velocity distributions obtained from the simulations are compared with experiment at various locations with very good agreement.

Multiscale approach for simulation of silicon etching using SF6/C4F8 Bosch process

This study is dedicated to the development of a multiscale approach for the simulation of silicon etching using the Bosch process. The etching simulator is composed of three modules: plasma kinetic model, sheath model, and surface model. The top down connection of these three modules allows us to track the etch profile evolution through the mask versus the reactor parameters. Complex reaction schemes are used for both SF6 and C4F8 plasmas to quantify the neutral and ion species densities and fluxes. For the SF6 plasma used in the etching cycle and in our pressure and power ranges, the fluorine flux is still preponderant compared to the positive ion flux, indicating that the silicon etching is mainly ensured by the chemical processes. For the C4F8 plasma used in the deposition cycle, the simulation results reveal that CFx species are preponderant in comparison with atomic fluorine, confirming that, in our operating conditions, CFx species promote the deposition of a polymer layer instead of the chemical etching by fluorine F. Plasma parameters in terms of the reactive neutral and ion fluxes and sheath parameters in terms of ion energy distribution functions are used in the surface model to track the etch profile evolution. The duty cycle corresponding to the SF6 to C4F8 plasma pulse ratio is a crucial parameter in controlling the Bosch process performances in terms of the etch rate, anisotropy, and scalloping defect development. A parametric study is performed to show the role of the duty cycle in the etch anisotropy and the scalloping propagation along the sidewall. While the scalloping defect is more important when the duty cycle is higher, the diminution of this parameter may reduce the global etch rate of silicon.