Atomic Continuum Research Articles

Molecular dynamics simulation of CoCrFeMnNi with twin boundaries under high-speed impact

Based on the hybrid modeling framework of atomic scale and continuum, molecular dynamics (MD) was the method to study the shock wave, elastic-plastic dual-wave, and microstructure of CoCrFeMnNi high entropy alloy (HEA) with twin boundary under high-speed impact. The interaction between dislocation and twin interfaces is significant to the plastic deformation of HEA, but research on them as initial defects is still limited. Therefore, this article simulates and analyzes the impact velocity and twin spacing and proves that HEA with twin boundaries can synergistically improve strength and flexibility by comparing the wave simulation process and variation of spalling strength between HEA with TB and crystal Ni. The simulation results show that TB affects the propagation of shock waves in HEA and provides deformation sites for the generation of dislocations. Unlike single-crystal HEA, there is a smoother impact response process, and the maximum tensile stress during the fracture stage can reach 41 GPa, which is about 34.12% higher than that of single-crystal HEA. The research results may contribute to a better understanding of HEA with TB and help apply complex load scenarios such as explosive loads.

The VANDELS ESO public spectroscopic survey: The spectroscopic measurements catalogue

VANDELS is a deep spectroscopic survey, performed with the VIMOS instrument at VLT, aimed at studying in detail the physical properties of high-redshift galaxies. VANDELS targeted ~2100 sources at 1 < z < 6.5 in the CANDELS Chandra Deep-Field South (CDFS) and Ultra-Deep Survey (UDS) fields. In this paper, we present the public release of the spectroscopic measurement catalogues from this survey, featuring emission and absorption line centroids, fluxes, and rest-frame equivalent widths obtained through a Gaussian fit, as well as a number of atomic and molecular indices (e.g. Lick) and continuum breaks (e.g. D4000), and including a correction to be applied to the error spectra. We describe the measurement methods and the validation of the codes that were used.

Study on Interfacial Interaction of Cement-Based Nanocomposite by Molecular Dynamic Analysis and an RVE Approach

There is an increased demand for cement nanocomposites in the twenty-first century due to their composition, higher strength, high efficiency, and multiscale nature. As carbon nanotubes (CNTs) possess extremely high strength, resilience, and stiffness, inclusion of carbon nanotubes in small quantities to the concrete mix makes them a multifunctional material. A molecular level understanding is significant to capacitate the macrolevel properties of these composites. In the proposed work, molecular dynamics (MD) simulations are used to understand the behaviour of the composites at the atomic level and continuum mechanics with representative volume element (RVE) homogenization modelling is carried out for interfacial interaction study of composites. The mechanical properties such as Young’s modulus, shear modulus, and poisons are evaluated using previous methods of simulations for different compositions of nanomaterials in cement matrix. The FORCITE module of MD simulation and square RVE model is used to determine the mechanical, electrical properties, and elastic constants of the cement nanocomposite. The MD simulation describes the linking effect of CNT into cement matric, and the RVE modelling study reveals the pull-out effect of CNT from matrix. From experimental and analytical studies, it is found that increase in CNT till 0.5% weight fraction increases the mechanical properties about 12% and further increasing of CNT weight fraction causes a reduction in mechanical properties about 5% due to the agglomeration of nanotubes. The density of states method in MD simulation indicates that mobility of the electrons increases with an increase in carbon nanotube proportion in the composites. The experimental test results substantiate the analytical studies, and the error obtained from both approaches is less than 20%. From the analytical study, the average maximum Young’s modulus, shear modulus, and bulk modulus are obtained as 46 GPa, 31 GPa, and 32 GPa for 0.5% weight fraction of CNT in cement matrix. Hence, it is concluded that 0.5% weight fraction of CNT is considered as optimum dosage to obtain better electrical and mechanical properties.

Calculation of Radiative Properties for [82%Ar-18%CO2]-Fe Plasmas in MAG Welding Arc.

This paper is dedicated to the calculation of the radiative properties of 82%argon-18%CO2 thermal plasmas with the addition of metallic vapors (iron, in the present case, due to workpiece and wire erosion), this mixture being representative of metal active gas (MAG) arc welding processes. These radiative properties are obtained in the frame of the net emission coefficient (NEC) theory, using the recent and accurate “line by line” method. All significant radiative contribution mechanisms are taken into account in the calculation: atomic lines, atomic continuum (radiative attachment, radiative recombination, and bremsstrahlung), molecular bands for diatomic and polyatomic molecules, and molecular continuum. Broadening phenomena (Doppler and pressure effects) are also carefully treated for bound-bound transitions (atomic lines and molecular bands). Regarding 82%Ar-18%CO2 plasma, the results obtained demonstrate the key role of molecular bands at low temperatures (T < 4 kK), whereas the atomic line and continuum prevailed at intermediate and high temperatures. With the addition of a few percentages of iron vapor, it was shown that the total NEC is significantly increased (especially at low temperatures) and that the atomic and ionic lines become dominant in all the studied temperature ranges (3–30 kK). This theoretical study will constitute a groundwork to build a diagnostic method (based on the calculation of partial NECs for accurately chosen spectral intervals) for the determination of plasma temperature and iron vapor concentration in welding arcs.

Origin of the Stokes–Einstein deviation in liquid Al–Si

ABSTRACT One goal in materials simulation is finding relationships between atomic scale and continuum properties. The Stokes–Einstein–Sutherland (SES) equation relates diffusion and viscosity in liquids through the effective diameter d, which is frequently observed to be constant with the ratio where T is the temperature, D is the diffusion coefficient, and η is the viscosity. The SES has a practical use in estimating diffusion from viscosity or vice versa as they can be difficult to measure experimentally. The constant effective diameter observed in the SES holds for many liquids within a temperature range, but at low temperatures, deviations where d is not constant have been observed. This work investigates SES in liquid Al–Si using molecular dynamics. We analyze the local order using Voronoi polyhedrons and agglomerative clustering. Clustering methods from machine learning allowed us to analyze the large amount of data generated from molecular dynamics trajectories in an efficient manner. We found that clusters have minimal effect on diffusion while increasing viscosity, which is a likely origin of the SES deviation for liquid Al–Si at low temperatures near the melting temperature.

Using data science to locate nanoparticles in a polymer matrix composite

Structural polymers such as epoxy are commonly reinforced with nano-sized reinforcement particles to increase the toughness and thermo-mechanical properties. Although qualitative measurements can be made to examine particle distribution on a scratched or abraded nanocomposite surface, it is difficult to quantify the three-dimensional locations of the particles relative to the visible surface. In this investigation, we developed a method that combines experimental data obtained from atomic force microscopy (AFM), data science, and continuum micromechanics to discover the 3D position of ∼142 nm diameter nanosilica (NS) particles relative to an abraded surface. Finite element analysis was used to develop a training set of modulus values as a function of NS particle position relative to the surface of the polymer. Bayesian optimization was then used to determine the particle position(s) by minimizing the error between simulated and experimental modulus (AFM) contours. The algorithm can consistently predict particle positions within 3 nm of the actual known positions in synthetically created AFM data. We implemented the algorithm on experimental AFM data to create simulated modulus contours that partially reproduce key features present in an experimental modulus contour. This method provides a powerful tool to map spherical particle distribution in a 3D space, allowing better processing-structure-property understanding that can arise from resin chemistry variations, surface functionality, processing conditions, and nanofiller particle properties.

Decoupling the effects of material thickness and size scale on the transverse free vibration of BNNTs based on beam models

Here we establish a new model to decouple the material thickness and elastic property of single-walled boron nitride nanotubes, and thus we can evaluate natural frequencies of transverse free vibration by directly using the Timoshenko beam and Euler-Bernoulli beam models. Full atomic simulations regarding on the vibration behaviors are also presented for comparison. The determination procedure of thickness and the physical meaning of scale parameter for one-atomic thick material are clearly defined. In order to comprehensively unfold the influence of the selection of tubular thickness on natural frequencies, a series of assumed tubular thickness is also adopted to analyze the sources of error and its trend, and finally confirms the accuracy of our proposed result. The accuracy of the beam models is also found to be dependent on the type of boundary constraints. The precision of the beam theory improves for weaker boundary constraints due to the release of shear stresses. The paper heals the rift between atomic simulations and continuum theories.

An atomistic-continuum multiscale approach to determine the exact thickness and bending rigidity of monolayer graphene

In this paper, we employ the atomistic-continuum method to open a new way to precisely measure the thickness of one-atomic material. The wall thickness of graphene is exactly extracted to be 0.0739 nm by fitting the first eight natural frequencies of a series of simply-supported graphene using atomistic-continuum method with classical plate theory. The Young's modulus and bending rigidity are determined to be 3.1851 TPa and 0.8066 eV. By using this thickness, the free vibration studies of clamped case, which removes the effect of dangling bonds, by the atomistic-continuum method, molecular mechanics and classical plate theory are found to be in a good agreement, confirming its accuracy. Inherited it to the hollow tubular structure, it is found that both Timoshenko and Euler-Bernoulli beam models also give a quite close prediction of carbon nanotubes as compared with molecular mechanics. This means the discrepancy between atomic simulation and continuum mechanics is no other than the unreal wall thickness. Besides, the usage of scale parameter and its value is also discussed.

Suppressing deleterious effects of spontaneous emission in creating bound states in cold atom continuum

In a previous paper [Deb and Agarwal 2014 Phys. Rev. A 90 063417], it was theoretically shown that, magneto-optical manipulation of low energy scattering resonances and atom-molecule transitions could lead to the formation of a bound state in continuum (BIC), provided there is no spontaneous emission. We find that even an exceedingly small spontaneous decay from exited molecular states can spoil the BIC. In this paper, we show how to circumvent the detrimental effect of spontaneous emission by making use of vacuum-induced coherence (VIC) which results in the cancellation or suppression of spontaneous emission. VIC occurs due to the destructive interference between two spontaneous decay pathways. An essential condition for VIC is the non-orthogonality of two transition dipole moments associated with the radiative decays form two excited states. Furthermore, the interference between two decay pathways requires that the spacing between the two decaying states must be comparable to or smaller than the square root of the product of the two spontaneous linewidths. We demonstrate that these conditions can be fulfilled by optically manipulating appropriately chosen molecular excited states, opening a promising prospect for the experimental realization of BIC of cold atoms.

Quantum mechanics at high redshift – Modelling Damped Lyman–α absorption systems

Abstract For around 100 years, hydrogen spectral modelling has been based on Voigt profile fitting. The semi-classical Voigt profile is based on a 2-level atom approximation. Whilst the Voigt profile is excellent for many circumstances, the accuracy is insufficient for very high column density damped Lyman-α absorption systems. We have adapted the quantum-mechanical Kramers-Heisenberg model to include thermal broadening, producing a new profile, the KHT profile. Interactions involving multiple discrete atomic levels and continuum terms, not accounted for in the Voigt model, generate asymmetries in the Lyman line wings. If not modelled, this can lead to significant systematics in parameter estimation when modelling real data. There are important ramifications in particular for measurements of the primordial deuterium abundance. However, the KHT model is complicated. We therefore present a simplified formulation based on Taylor series expansions and look-up tables, quantifying the impact of the approximations made. The KHT profile has been implemented within the widely-used VPFIT code.

Net emission coefficient of complex thermal plasmas used in SWNT synthesis

The calculation of net emission coefficient NEC is done for mixtures He–Ar/N2–[C100−x−y−z,Bz,Nix/Cox,Yy] used in the synthesis of single-wall carbon nanotubes SWNTs by arc discharge technique, at pressure of 60 kPa and at temperature interval going from 1 kK to 30 kK. These mixtures contain helium–argon as a gaseous atmosphere, nitrogen–boron as a doping atoms and a carbon anode filled with yttrium–nickel/cobalt as catalysis. Different proportions of metallic catalysts are considered going from 0.01 atomic% (at%) to 5 at%. Whereas for doping atoms the concentration amounts are much higher and do not exceed 8 at% for boron and 50% for nitrogen. The results of NECs are obtained by summing over all wavelengths, the contributions of atomic and molecular continuum, results of free–free and free–bound transitions, and line and molecular bands emissions corresponding to bound–bound transitions. Doppler broadening, resonance, van der Waals, electron and ion Stark broadening are taken into account for the calculation of the lines widths. As Stark broadening is the most important broadening mechanism at intermediate and high temperature, an empirical correction factors determined for some resonance lines, are added to the semi-empirical Stark formulas for a best reproducing of the experimental Stark widths. The comparisons of the NECs of the pure plasmas of argon, helium, nitrogen and carbon to those given in literature present an acceptable agreement. For the studied mixtures containing He–Ar/N2–[C100−x−y−z,zBz, Nix/Cox,Yy], the obtained results show that the addition of boron or the nitrogen induces a few little changes in the NEC, in contrary to the metal catalysts where we noted a considerable increase in the plasma emission especially at low temperatures. The effect of the addition of yttrium in carbon anode even in a small amount (<0.1 at%) affects strongly the NEC compared to that of cobalt or nickel which have given similar results.

Comparative analysis of breakdown spectra registered using optical spectrometry technique in biodegradable ester liquids and mineral oil

This study presents the results of optical spectrometry based measurements performed in three commercial dielectric liquids: synthetic ester, natural ester, and mineral oil. The optical spectra collected concern the breakdown initiated by standard lightning impulse voltage of positive polarity in a point-plane electrode system. The obtained results showed the differences between the spectra. The relationships between emission from atomic hydrogen (656.3 nm) and continuum concerning the wavelengths between 380 and 600 nm attributed to dissociation and fragmentation of molecules and recombination of molecular fragments were assessed as different for the distinctive liquids tested. The emission from atomic hydrogen was dominant in the case of mineral oil, however, for natural ester and first of all for synthetic ester, the light from the mentioned continuum had the highest intensity. Thus, this may be supposed that the differences in the spectra resulting from the different proportions of the physical processes responsible for the breakdown in a given liquid. It seems that in ester liquids the way for direct excitation and ionisation of liquid molecules is easier, so easier rotational and vibrational types of excitation of the high number density main molecules may take place giving a reflection in the optical spectra registered.

Towards First-Principles Design of Battery Components for Current and Next-Generation Li-Ion Batteries

Complete electrification of transportation and aviation requires more energy-dense batteries and requires a much more rapid design iteration loop.[1,2] Computation and data will play a crucial role towards achieving this goal. The design of Li-ion batteries components - anode, electrolyte and cathode require simulation capabilities at varying length and time scales. Here, we will discuss our group's progress in using first-principles modeling and data-driven methods for designing liquid electrolytes, solid electrolytes and cathodes. We highlight the importance of open source methods, especially in the context of phase-field modeling for describing Li metal based anodes. We will show the crucial role played by a tight integration between atomic scale and continuum scale simulations to identify material targets for various use-cases. We believe that a vertically integrated modeling approach is necessary to enable rapid development of next-generation Li-ion batteries to power electric semitrucks and aircrafts. [1] S. Sripad, V. Viswanathan, J. Electrochem. Soc. 164 (11), E3635-E3646.[2] S. Sripad, V. Viswanathan, ACS Energy Lett. 2 (7), 1669-1673.

Mean absorption coefficients of He/Ar/N2/(C1−x−y, Nix, Coy) thermal plasmas for CNT synthesis

In this paper, we present the mean absorption coefficients (MACs) calculated for plasma mixtures of argon–helium–nitrogen–carbon–nickel–cobalt at 60 kPa and in a temperature range from 1 kK to 20 kK. These coefficients have been computed under the assumption of a local thermodynamic equilibrium (LTE), isothermal plasma, including atomic and molecular continuum, molecular bands and lines radiation splitted into nine spectral intervals. The results show that the continuum absorption coefficients strongly depend on photodissociation and photoionization processes of the molecular species N2, CN and C2, with a significant effect on photodetachment processes of C− in a frequency interval lower than 1 × 1015 Hz and for low temperature (<6 kK). While at high temperature, the main contribution in continuum absorption coefficient comes from radiative recombination processes except in the infrared region (<0.5 × 1015 Hz) where the inverse bremsstrahlung represents the most important component in continuum processes for all temperature values. On the other hand, the calculation of MAC shows that the role of molecular continuum, molecular bands and line absorption of the neutral catalysis species Ni/Co are only important in a small range of temperature and in a few spectral bands located in visible and infrared regions, while at high temperature and in UV and visible regions, the foremost contributions to MAC come from atomic continuum and line absorption.

SU-E-I-43: Photoelectric Cross Section Revisited

Purpose: The importance of the precision in photoelectric cross-section value increases for recent developed technology such as dual energy computed tomography, in which some reconstruction algorithms require the energy dependence of the photo-absorption in each material composition of human being. In this study, we revisited the photoelectric cross-section calculation by self-consistent relativistic Hartree-Fock (HF) atomic model and compared with that widely distributed as “XCOM database” in National Institute of Standards and Technology, which was evaluated with localdensity approximation for electron-exchange (Fock)z potential. Methods: The photoelectric cross section can be calculated with the electron wave functions in initial atomic state (bound electron) and final continuum state (photoelectron). These electron states were constructed based on the selfconsistent HF calculation, where the repulsive Coulomb potential from the electron charge distribution (Hartree term) and the electron exchange potential with full electromagnetic interaction (Fock term) were included for the electron-electron interaction. The photoelectric cross sections were evaluated for He (Z=2), Be (Z=4), C (Z=6), O (Z=8), and Ne (Z=10) in energy range of 10keV to 1MeV. The Result was compared with XCOM database. Results: The difference of the photoelectric cross section between the present calculation and XCOM database was 8% at a maximum (in 10keVmore » for Be). The agreement tends to be better as the atomic number increases. The contribution from each atomic shell has a considerable discrepancy with XCOM database except for K-shell. However, because the photoelectric cross section arising from K-shell is dominant, the net photoelectric cross section was almost insensitive to the different handling in Fock potential. Conclusion: The photoelectric cross-section program has been developed based on the fully self-consistent relativistic HF atomic model. Due to small effect on the Fock potential for K-shell electrons, the difference from XCOM database was limited: 1% to 8% for low-Z elements in 10keV-1MeV energy ranges. This work was partly supported by the JSPS Core-to-Core Program (No. 23003)« less

Analysis of macromolecular microtubules using the potential-based matrix displacement method

This work presents an efficient matrix displacement approach for simulation of microtubules, a typical polyatomic macromolecular bio-structure in eukaryotic cells. The microtubule is modeled as an interatomic potential-based mechanical system. Unlike most of the continuum-based methods, the description of material properties in this model is based on fictitious bond and the interatomic potential energy is considered as based on the atomic force integral between basic macromolecule components. Meanwhile, superior to the conventional atomic-based simulation, the equilibrium state is solved in an efficient matrix framework comparable to the continuum-based structural mechanics approach. Following this approach, free vibration behavior of microtubules is intensively investigated. Microtubules with different boundary restrictions are considered in case studies and vibration modes and frequencies are obtained and compared with available references. It is found that the proposed macromolecular model performs with both accuracy and efficiency, and is superior to the two simulation regimes in atomic scale and continuum level.

Generalized Sturmian Functions applied to double continuum problems

The Generalized Sturmian Functions method aims to deal with atomic physics problems. It has seen application to two and three-body problems, and its flexibility enables one to work with bound systems as well as with particles in the continuum. In the present contribution we analyze how the method expands the atomic double continuum in collision problems, using the double ionization of Helium by fast electrons as a showcase. We first test the robustness of the method in a particularly challenging situation, the zero energy case. We then present fully differential cross sections for a scattering problem which after 15 years of continued efforts has not been satisfactorily solved: the double ionization of Helium by electron impact in the fast projectile regime, as measured by the Orsay group.

Radio-frequency spectrum of the Feshbach molecular state to deeply bound molecular states in ultracold 40K Fermi gases

Spectroscopic measurements are made and obtained for three molecular levels within 50 MHz of the atomic continuum, along with their variation of magnetic field in ultracold 40K Fermi gases. We use spectroscopic measurements to modify the scattering properties near magnetic Fano–Feshbach resonances with a radio-frequency (RF) field by measuring the loss profile versus magnetic field. This work provides the high accuracy locations of ground molecular states near the s-wave Fano–Feshbach resonance, which can be used to study the crossover regime from a Bose–Einstein condensate to a Bardeen–Cooper–Schrieffer superfluid in the presence of an RF field.

Radiative properties of argon–helium–nitrogen–carbon–cobalt–nickel plasmas used in CNT synthesis

This work presents the radiative properties of argon–helium–nitrogen–carbon–nickel–cobalt thermal plasmas by the computation of net emission coefficients (NECs) under the assumption of a local thermodynamic equilibrium and at temperature range 1000–20 000 K. These mixtures were often used in the study of carbon nanotubes (CNTs) synthesis with arc plasma which becomes one of the most useful techniques in terms of flexibility of carbon nanostructures produced with fewer defects. The values of NEC allow estimation of total radiation losses in plasmas, by taking into account the emission radiation resulting from the atomic continuum, the molecular continuum, the atomic lines and some molecular bands. Free–free transitions (Bremsstrahlung) and free–bound (electron–ion recombination), have been considered for the calculation of atomic continuum. For bound–bound transitions, natural, resonance, Van der Waals, Stark and Doppler effects have been taken into account in the calculation of the lines broadenings while the self-absorption of the resonance lines has been treated using their escape factors. Molecular continuum has been only considered for N2, C2 and CN molecules whereas we have only taken into account diatomic systems N2, , CN and C2 for the emission of the molecular bands. The results obtained show that even for low concentrations of Ni and Co in the plasma, the NECs are modified and considerably increase only at a low temperature (T < 8000 K) and the major contribution in the total radiation arises from the lines emission. However, the effect of the thickness of the plasma on plasma radiation has been analysed based on the self absorption phenomenon of resonance lines.

The calculation of mechanical behavior for metallic devices at nano-scale based on Atomic–Continuum Coupled model

Atomic–Continuum Coupled (ACC) model is a nonlocal model which couples the continuum mechanics with atomic model. It is proposed by Xiang et al. (2011). The model is modified and improved to study the mechanical behavior for metallic devices at nano-scale in this paper. Some essential concepts coupling molecular dynamics with continuum mechanics are defined, such as representative volume element (RVE), atomic occupation coefficient, basic deformation element, deformation environment and so on. The computable formulae of some mechanical quantities, including deformation gradient, strain and stress tensor, etc., are derived in detail. And all those for the single-crystal Cu nanowire under tension and bending are numerically calculated. The results show the inhomogeneity of strain and stress tensor in Cu nanowire under tension and bending.