Extensive CGMD Simulations of Atactic PS Providing Pseudo Experimental Data to Calibrate Nonlinear Inelastic Continuum Mechanical Constitutive Laws

In this contribution, we present a characterization methodology to obtain pseudo experimental deformation data from CG MD simulations of polymers as an inevitable prerequisite to choose and calibrate continuum mechanical constitutive laws. Without restriction of generality, we employ a well established CG model of atactic polystyrene as exemplary model system and simulate its mechanical behavior under various uniaxial tension and compression load cases. To demonstrate the applicability of the obtained data, we exemplarily calibrate a viscoelastic continuum mechanical constitutive law. We conclude our contribution by a thorough discussion of the findings obtained in the numerical pseudo experiments and give an outline of subsequent research activities. Thus, this work contributes to the field of multiscale simulation methods and adds a specific application to the body of knowledge of CG MD simulations

The Hybrid Capriccio Method: A 1D Study for Further Advancement

Polymers and, in particular, polymer composites are known for the enormous adjustability of their mechanical, chemical, and thermal behavior. Multiscale methods are increasingly employed to unravel the polymer microstructure's impact on the material properties. These methods combine the accuracy of particle-based techniques with the efficiency of continuum mechanical approaches. Amorphous polymers pose a special challenge since their microstructure does not continue periodically, and therefore special attention needs to be paid to the particle domain boundary. In this study, we introduce a coupling via an interface between the continuum and the particle domain. Padding atoms as particle representations of the continuum, which serve as interaction partners for the atoms in the particle region, allow for the transfer of displacements and forces between the domains. We present a straightforward 1D example with simple interactions, evaluate the scheme's performance, discuss the resulting energy contributions, and identify an optimal set of coupling parameters. Eventually, this forms the basis for future 3D implementations

Multiscale FE-MD Coupling: Influence of the Chain Length on the Mechanical Behavior of Coarse-Grained Polystyrene

Polymers have become increasingly essential to cope with modern engineering challenges. To better understand their microstructure's influence, multi-scale approaches that couple molecular dynamics and continuum mechanics are emerging progressively. However, these simulation techniques require detailed knowledge of the material behavior, which is commonly derived from molecular dynamics simulations. Reducing the degrees of freedom by coarse graining enables the investigation of sufficiently large samples and thus we focus on coarse-grained (CG) polystyrene as a model material. The goal of this contribution is two-fold: Firstly, we identify the minimum sample size necessary to analyze the mechanical properties. Secondly, we quantify the influence of chain length on the material behavior of polystyrene. To this end, we investigate density, end-to-end distance, stress-strain behavior, Young's modulus, and Poisson's ratio. In conclusion, we were able to verify that we can use significantly smaller samples for our investigations without affecting their structure or mechanical behavior. The chain length has a drastic influence on the mechanical properties, with a loss of stiffness in the range of 15 % for very short-chain specimens

Duplicate Table Detection with Xash

Data lakes are typically lightly curated and as such prone to data quality problems and inconsistencies. In particular, duplicate tables are common in most repositories. The goal of duplicate table detection is to identify those tables that display the same data. Comparing tables is generally quite expensive as the order of rows and columns might differ for otherwise identical tables. In this paper, we explore the application of Xash, a hash function previously proposed for the discovery of multi-column join candidates, for the use case of duplicate table detection. With Xash, it is possible to generate a so-called super key, which serves like a bloom filter and instantly identifies the existence of particular cell values. We show that using Xash it is possible to speed up the duplicate table detection process significantly. In comparison to SimHash and other competing hash functions, Xash results in fewer false positive candidates

Correlative analysis on InGaN/GaN nanowires: structural and optical properties of self-assembled short-period superlattices

: The influence of self-assembled short-period superlattices (SPSLs) on the structural and optical properties of InGaN/GaN nanowires (NWs) grown by PAMBE on Si (111) was investigated by STEM, EDXS, µ-PL analysis and k·p simulations. STEM analysis on single NWs indicates that in most of the studied nanostructures, SPSLs self-assemble during growth. The SPSLs display short-range ordering of In-rich and In-poor InxGa1-xN regions with a period of 2-3 nm that are covered by a GaN shell and that transition to a more homogenous InxGa1-xN core. Polarization- and temperature-resolved PL analysis performed on the same NWs shows that they exhibit a strong parallel polarized red-yellow emission and a predominantly perpendicular polarized blue emission, which are ascribed to different In-rich regions in the nanostructures. The correlation between STEM, µ-PL and k·p simulations provides better understanding of the rich optical emission of complex III-N nanostructures and how they are impacted by structural properties, yielding the significant impact of strain on self-assembly and spectral emission

Optische Analyse von InN und InGaN-Nanostrukturen

Nanostructures of group-III-V materials, such as quantum wells (QWs) or nanowires (NWs), are widely deployed in modern solid-state technology. The applicability of the III-nitride system originates in the direct fundamental bandgap covering the whole visible range and expanding even into the near-infrared and the ultraviolet (UV). Lighting applications based on these materials exhibit extraordinary efficiency over most emission wavelengths. However, the green spectral region is governed by an efficiency drop. Miscibility problems leading to strong compositional fluctuations and disorder, combined with a large quantum-confined Stark effect, are the main problems in devices relying on InGaN QWs. NWs can potentially solve some of these problems, but the technology requires further advancement. The nanoscopic dimensions necessitate experimental techniques able to resolve subwavelength features. This work investigates InN surfaces and InGaN single quantum well (SQW) samples to explore the reason for the occurrence of the additional modes in tip-enhanced Raman spectroscopy (TERS). A connection with charge carrier accumulations close to the surface is found. Multi-wavelength Raman spectroscopic (RS) studies are employed to resolve the interrelationship between charge carriers and vibrational states. The observation of a similar near-field effect in InGaN SQW samples suggests the existence of locally degenerate regions. Supported by · -calculations, photoluminescence (PL), and UV-RS, as well as transmission electron microscopy (TEM) with nano-cathodoluminescence, the sensitivity of the SQW towards compositional and thickness fluctuations, defects, strain, and surface distance is analyzed and highlighted. The interplay between structural and optical properties in InGaN nanostructures is continued with the characterization of self-assembled InGaN-GaN NWs. The origin of the emission redshift observed in the room temperature PL is examined in PL series dependent on temperature, intensity, and polarization. Calculations of the surface band bending emphasize a bulk carrier concentration beyond 10¹⁸ cm⁻³ to enable a Stark effect comparable to the observed redshift. A negative depolarization ratio, UV-RS, and TEM results demonstrate the presence of structural disorder associated with InGaN and the spontaneous formation of inclusions during the growth.III-Nitrid-Halbleiter-Nanostrukturen wie Quantenfilme (QW) oder Nanodrähte (NW) sind Standardmaterialien in der modernen Halbleitertechnologie. Der Erfolg beruht auf ihrer mit der Komposition durchstimmbaren direkten Bandlücke, die vom nah-infraroten bis in den ultravioletten (UV) Bereich reicht. Allerdings sinkt bei Anwendungen mit InGaN QWs die Quanteneffizienz sowohl für längere Wellenlängen als auch für höhere Betriebsströme. Zunehmende lokale Inhomogenitäten und der “quantum-confined Stark-effect” (QCSE) führen zu Effizienzeinbußen. InGaN NWs können einige der Probleme zumindest teilweise beheben, allerdings ist diese Technologie noch nicht so weit etabliert wie die Quantenfilme. Die nanoskopischen Größenordnungen verlangen nach speziellen Techniken, um Eigenschaften im Sub-Wellenlängenbereich aufzulösen. In dieser Arbeit werden InN Oberflächen und InGaN Einfachquantenfilmproben (SQW) untersucht, um die Ursachen für neue, zusätzliche Vibrationsmoden in spitzenverstärkter Raman-Spektroskopie zu ergründen. Eine Verbindung zu Ladungsträgerakkumulationen nahe der Oberfläche wird hergestellt und mithilfe einer Multi-Wellenlängen-Raman-Untersuchung die Interaktion mit den Schwingungseigenschaften analysiert. Die Beobachtung einer ähnlichen Wechselwirkung bei InGaN-SQWs lässt auf lokal entartete Regionen an der Oberfläche schließen. Diese Vermutung wird durch · - Rechnungen, Photolumineszenz- (PL) und UV-Raman-Spektroskopie, sowie Nano-Kathodolumineszenz (STEM-CL) gestützt und verdeutlicht die Sensitivität von SQWs in Bezug auf Komposition, Schichtdickenfluktuation, Defekte, Verspannungen und dem Abstand zur Oberfläche. Der Zusammenhang zwischen Struktur und optischen Eigenschaften wird mit der Analyse von InGaN-GaN NWs weiter untersucht. Die Ursache für die Rotverschiebung der Raumtemperatur-Lumineszenz wird in PL-Studien in Abhängigkeit der Temperatur, Anregungsintensität und Polarisation geprüft. Berechnungen der Bandverbiegung zeigen, dass eine Volumenladungsträgerdichte von mehr als 10¹⁸ cm⁻³ vorhanden sein muss, um einen Stark-Effekt von der Größenordnung der Rotverschiebung zu erzeugen. Ein negatives Depolarisationsverhältnis zusammen mit UV-Raman-Spektroskopie sowie TEM Bildern weisen auf die Existenz von Unordnung hin, die mit der Inhomogenität von InGaN und der spontanen Bildung von Einschlüssen zusammenhängt.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Extending a generic and fast coarse-grained molecular dynamics model to examine the mechanical behavior of grafted polymer nanocomposites

Polymer nanocomposites are an important class of materials for engineering applications due to their high versatility and good mechanical properties combined with low density. By directly attaching the polymer chains to the nanofillers, the so-called grafting, a better load transfer between matrix and filler is achieved, and, in addition, a better dispersion of the fillers is obtained. Both result in enhanced mechanical properties. Since experimental investigations on the nanoscale are extremely challenging, complementary numerical studies are needed to unravel the mechanical behavior of polymer nanocomposites. To this end, molecular dynamics is ideally suited since it captures the microstructure, but is also numerically expensive. Therefore, this contribution presents a fast coarse-grained molecular dynamics model for the investigation of the mechanical behavior of grafted polymer nanocomposites. For this purpose, we extend an existing model by grafting bonds, which allows us to compare the effect of untreated and grafted fillers directly. In particular, we investigate the influence of filler content, grafting degree, and filler size on the stiffness and strength of the polymer (grafted) nanocomposites. We conclude that the grafting bonds have little effect on the stiffness, while the strength is significantly improved compared to the untreated fillers, which is in agreement with the literature. The presented molecular dynamics model for polymer grafted nanocomposites provides the basis for further investigations, particularly of the crucial matrix-filler interphase. In addition, this contribution translates molecular dynamics insights into mechanical properties, which bridges the gap to the engineering scale and thus represents a step towards exploiting the full potential of polymer (grafted) nanocomposites

On equilibrating non-periodic molecular dynamics samples for coupled particle-continuum simulations of amorphous polymers

In the context of fracture simulations of polymers, the molecular mechanisms in the vicinity of the crack tip are of particular interest. Nevertheless, to keep the computational cost to a minimum, a coarser resolution must be used in the remaining regions of the numerical sample. For the specific case of amorphous polymers, the Capriccio method bridges the gap between the length and time scales involved at the different levels of resolution by concurrently coupling molecular dynamics (MD) with the finite element method (FEM). Within the scope of the Capriccio approach, the coupling to the molecular MD region introduces non-periodic, so-called stochastic boundary conditions (SBC). In similarity to typical simulations under periodic boundary conditions (PBC), the SBC MD simulations must reach an equilibrium state before mechanical loads are exerted on the coupled systems. In this contribution, we hence extensively study the equilibration properties of non-periodic MD samples using the Capriccio method. We demonstrate that the relaxation behavior of an MD-FE coupled MD domain utilizing non-periodic boundary conditions is rather insensitive to the specific coupling parameters of the method chosen to implement the boundary conditions. The behavior of an exemplary system equilibrated with the parameter set considered as optimal is further studied under uniaxial tension and we observe some peculiarities in view of creep and relaxation phenomena. This raises important questions to be addressed in the further development of the Capriccio method