2,861 research outputs found

    Carbon Black Reinforcement of Tyre Tread Compounds

    Get PDF
    The tyre industry is the leading consumer of rubber materials, accounting for approximately 70% of annual natural rubber production. The inherent properties and strength of rubber makes it suitable for engineering applications. However, to have useful lifetimes, rubber needs to be reinforced with fillers such as carbon black or silica. Fillers account for approximately 30% of materials by weight used in tyre tread compounds with CB being the most widely used reinforcing filler in tyres and engineering rubber materials. Various studies have shown that CB generally enhances properties such as modulus, tensile and tear strength, crack growth and abrasion resistance. For tyre tread applications, CB also influences other properties such as rolling resistance and grip. Less understood though, is how the morphological properties of CB influence fatigue and fracture properties of the rubber composite. The aim of this thesis is to conduct a systematic study to understand how these CB morphological properties including the structure and surface area affect reinforcement in tyre tread compounds. Eight different CB fillers varying widely in their structure and surface area were examined. The wide variation of CBs allows quantitative correlations to be drawn to understand the extent the CB properties affect these parameters. The CBs had an equivalent loading of 50 parts per hundred (phr) in natural rubber. An unfilled equivalent was also included. A series of experiments including conventional static and dynamic mechanical tests, strain induced crystallisation estimations, heat build-up and energy dissipation characterisation, fatigue crack growth resistance measurements, intrinsic and critical tear strength tests were conducted. Abrasion resistance as well as cut and chip resistance experiments were also performed. The results show the controlling CB morphological property is influenced by parameters such as the applied strain level, strain rate, severity of loading and the predominant deformation type (strain-, energy- or stress- controlled) in the test or application. Increasing CB surface area generally increases heat build-up and energy dissipation while CB structure affects crystallinity due to strain amplification effects. There is a step change in crack growth resistance below certain tearing energies which is attributed to the kinetics of strain induced crystallisation. There is a flip in ranking of cut and chip damage, with high structure CB compounds preferred at low impact forces and low structure CB compounds preferred at high impact forces. Abrasion tests show the formation of smear wear causes better abrasion resistance. The formation of smear wear is a factor of both the CB structure and surface area. Overall, the results highlight the difficulty to simultaneously optimise different parameters in tyre tread design. However, this work provides the tyre design engineer greater clarity on which CB to use to obtain a desired performance

    Viscous Thin-film Models of Nanoscale Self-organization Under Ion Bombardment

    Get PDF
    For decades, it has been observed that broad-beam irradiation of semiconductor surfaces can lead to spontaneous self-organization into highly regular patterns, sometimes at length scales of only a few nanometers. Initial theory was largely based on erosion and redistribution of material occurring on fast time scales, which are able to produce good agreement with certain aspects of surface evolution. However, further experimental and theoretical work eventually led to the realization that numerous effects are active in the irradiated target, including stresses associated with ion-implantation and the accumulation of damage leading to the development of a disordered, amorphous layer atop the substrate. It was also shown that relaxation of this amorphous layer proceeds in a manner closer to viscous flow ratherthan surface diffusion on a crystal lattice. Observing the viscous character of the amorphous layer, it is natural to consider whether stress-based continuum models might help explain pattern formation under ion bombardment and the observations described above. Indeed, there are early indications from the experimental literature that this may be the case, and, at low energies (∼ 1keV), at least one experimental-theoretical study has shown that they may even dominate erosive and redistributive effects in their contribution to surface evolution. In this thesis, we develop a continuum model based on viscous thin-film flow and ion-induced stresses within the amorphous layer. This model is a composite of, and significant generalization of, a previously-studied “anisotropic plastic flow” (APF) mechanism and a previously-studied “ion-induced isotropic swelling” (IIS) mechanism. Previous work has shown that, with certain simplifying assumptions about the amorphous-crystalline interface and spatial homogeneity of anisotropic plastic flow, this mechanism produces an instability capable of predicting pattern formation beginning at 45◦ angle of incidence against the macroscopically-flat substrate, consistent with some experimental systems. Under similar simplifying assumptions, ion-induced swelling has been shown to be capable of suppressing pattern formation. Our generalizations allow the use of simulation data to inform both linear and nonlinear surface evolution due to the spatial localization of APF and IIS to certain regions of the bulk, improved treatment of the amorphous-crystalline geometry, andboundary conditions suitable to the physical systems of interest. We are then able to provide insight into several phenomena that have previously been difficult to explain, but seem to emerge naturally from a more detailed treatment of the physical system

    Effective theories of phase transitions

    Get PDF
    In this thesis we study systems undergoing a superfluid phase transition at finite temperature and chemical potential. We construct an effective description valid at late times and long wavelengths, using both the holographic duality and the Schwinger-Keldysh formalism for non-equilibrium field theories. In particular, in chapter 2 we employ analytic techniques to find the leading dissipative corrections to the energy-momentum tensor and the electric current of a holographic superfluid, away from criticality. Our method is based on the symplectic current of Crnkovic and Witten [1] and extends on previous results [2, 3]. We assume a general black hole background in the bulk, with finite charge density and scalars fields turned on. We express one-point functions of the boundary field theory solely in terms of thermodynamic quantities and data related to the black hole horizon in the bulk spacetime. Matching our results with the expected constitutive relations of superfluid hydrodynamics, we obtain analytic expressions for the five transport coefficients characterising superfluids with small superfluid velocities. In chapter 3 we examine the hydrodynamics of holographic superfluids arbitrarily close to the critical point. The main difference in this case is that, close to the critical point, the amplitude of the order parameter is an additional hydrodynamic degree of freedom and we have to include it in our effective theory. For simplicity, we choose to work in the probe limit. Utilising the symplectic current once again, we find the equations that govern the critical dynamics of the order parameter and the charge density and show that our holographic results are in complete agreement with Model F of Hohenberg and Halperin [4]. Through this process, we find analytic expressions for all the parameters of Model F, including the dissipative kinetic coefficient, in terms of thermodynamics and horizon data. In addition, we perform various numerical checks of our analytic results. Finally, in chapter 4 we consider critical superfluid dynamics within the Schwinger-Keldysh formalism. As in chapter 3, we focus on the complex order parameter and the conserved current of the spontaneously broken global symmetry, ignoring temperature and normal fluid velocity fluctuations. We construct an effective action up to second order in the a-fields and compare the resulting stochastic system with Model F and our holographic results in chapter 3. A crucial role in this construction is played by a time independent gauge symmetry, called “chemical shift symmetry”. We also integrate out the amplitude mode and obtain the conventional equations of superfluid hydrodynamics, valid for energies well below the gap of the amplitude mode

    Advances and Applications of DSmT for Information Fusion. Collected Works, Volume 5

    Get PDF
    This fifth volume on Advances and Applications of DSmT for Information Fusion collects theoretical and applied contributions of researchers working in different fields of applications and in mathematics, and is available in open-access. The collected contributions of this volume have either been published or presented after disseminating the fourth volume in 2015 in international conferences, seminars, workshops and journals, or they are new. The contributions of each part of this volume are chronologically ordered. First Part of this book presents some theoretical advances on DSmT, dealing mainly with modified Proportional Conflict Redistribution Rules (PCR) of combination with degree of intersection, coarsening techniques, interval calculus for PCR thanks to set inversion via interval analysis (SIVIA), rough set classifiers, canonical decomposition of dichotomous belief functions, fast PCR fusion, fast inter-criteria analysis with PCR, and improved PCR5 and PCR6 rules preserving the (quasi-)neutrality of (quasi-)vacuous belief assignment in the fusion of sources of evidence with their Matlab codes. Because more applications of DSmT have emerged in the past years since the apparition of the fourth book of DSmT in 2015, the second part of this volume is about selected applications of DSmT mainly in building change detection, object recognition, quality of data association in tracking, perception in robotics, risk assessment for torrent protection and multi-criteria decision-making, multi-modal image fusion, coarsening techniques, recommender system, levee characterization and assessment, human heading perception, trust assessment, robotics, biometrics, failure detection, GPS systems, inter-criteria analysis, group decision, human activity recognition, storm prediction, data association for autonomous vehicles, identification of maritime vessels, fusion of support vector machines (SVM), Silx-Furtif RUST code library for information fusion including PCR rules, and network for ship classification. Finally, the third part presents interesting contributions related to belief functions in general published or presented along the years since 2015. These contributions are related with decision-making under uncertainty, belief approximations, probability transformations, new distances between belief functions, non-classical multi-criteria decision-making problems with belief functions, generalization of Bayes theorem, image processing, data association, entropy and cross-entropy measures, fuzzy evidence numbers, negator of belief mass, human activity recognition, information fusion for breast cancer therapy, imbalanced data classification, and hybrid techniques mixing deep learning with belief functions as well

    Amplification and Oscillations in the Curvature Power Spectrum from Features in the Inflaton Potential

    Get PDF
    Στη διδακτορική διατριβή γίνεται μελέτη των χαρακτηριστικών του πληθωριστικού δυναμικού που προκαλούν σημαντική ενίσχυση, καθώς και ταλαντώσεις, στο φάσμα διαταραχών καμπυλότητας. Τα χαρακτηριστικά που παρουσιάζουν ιδιαίτερο ενδιαφέρον είναι οι απότομες μεταπτώσεις (σκαλοπάτια) στο δυναμικό και οι στροφές στον εσωτερικό χώρο πολλών πεδίων. Και στις δύο περιπτώσεις δίνεται ιδιαίτερη έμφαση στην αθροιστική επίδραση περισσότερων του ενός τέτοιων χαρακτηριστικών, ενώ η ανάλυση που διεξάγεται είναι τόσο αριθμητική όσο και αναλυτική. Στην περίπτωση του πληθωρισμού από ένα πεδίο γίνεται μελέτη συγκεκριμένων μοντέλων εμπνευσμένων από το πλαίσιο των α-attractors. Σε πολλές από τις μελετώμενες περιπτώσεις η αύξηση του φάσματος διαταραχών καμπυλότητας είναι αρκετή ώστε να οδηγεί σε σημαντική παραγωγή αρχέγονων μελανών οπών καθώς και στοχαστικών βαρυτικών κυμάτων, πιθανά ανιχνεύσιμων στο μέλλον. Οι ταλαντώσεις στο φάσμα διαταραχών καμπυλότητας αποτυπώνονται στο φάσμα των στοχαστικών βαρυτικών κυμάτων.In this thesis we study features of the inflaton potential that can lead to a strong enhancement of the power spectrum of curvature perturbations. In single-field inflation, which constitutes the biggest part of this work, we focus on models with steep step-like features in the potential that result in the temporary violation of the slow-roll conditions during the inflaton evolution. These features enhance the power spectrum of the curvature perturbations by several orders of magnitude at certain scales and also produce prominent oscillatory patterns. Our analysis regarding the inflationary dynamics is both analytical and numerical. We describe quantitatively the size of the enhancement, as well as the profile of the oscillations, which are shaped by the number and position of the features in the potential. The models that we use include some simplified potentials, as well as potentials that are constructed within the framework of α\alpha-attractors in supergravity. We also demonstrate that the induced tensor power spectrum inherits the distinctive oscillatory profile of the curvature spectrum and is potentially detectable by near-future space interferometers. In addition, the enhancement of the power spectrum by several step-like features may trigger the production of a sizeable number of primordial black holes under suitable conditions. We also extend our work on the enhancement of the curvature spectrum during inflation to the two-field case. Our emphasis here is given on sharp turns in field space, which is a clearly multi-field phenomenon that occurs when the component of the slow-roll parameter perpendicular to the background trajectory grows large. Our focus is mainly on the additive eeffect of several turns, leading to the resonant growth of the curvature spectrum. Three or four features in the evolution of η are sufficient in order to induce an enhancement of the power spectrum by six or seven orders of magnitude, which can lead to the significant production of primordial black holes and stochastic gravitational waves, in analogy with the case of steps in the potential

    Multifrequency atomic force microscopy for the in-plane and out-of-plane nanomechanical characterization of graphitic surfaces

    Get PDF
    Graphene is considered as one of the most promising materials for numerous applications such as electronics, photonics, membranes, sensors, heat dissipators, lubricants and many more [1, 2]. In addition to its outstanding electronic properties, in particular, the extraordinary mechanical properties of graphene have become the focus of scientific attention. For example, it has been shown that defect-free graphene has an enormously high Young’s modulus of about 1 TPa [3]. However, little research has been done on the local influence of defects on the nanomechanical properties of graphene. Several challenges come to mind as possible reasons, such as (i) imaging graphene with atomic resolution, (ii) simultaneous analysis of sample properties parallel and perpendicular to the sample plane, (iii) preparation of adsorbate-free graphene samples, and (iv) targeted introduction of defects. To overcome the aforementioned challenges, first, an atomic force microscopy-based method was developed within this work that enables imaging of graphene in air under ambient conditions with atomic resolution. In addition, the method was designed to allow quantification of interaction forces, both, perpendicular and parallel to the graphene surface. This is particularly important to access the complete set of elastic constants of graphene. An important finding of this work is that different adsorbate types could be observed on the graphene surface shortly after the preparation of graphene samples. Therefore, a detailed analysis of the adsorbates was performed using the developed multifrequency atomic force microscopy method. Furthermore, the extent to which oxygen-plasma treatment can be used to remove adsorbates from a graphene sample stored under laboratory air conditions was examined. Adsorbate removal is a basic requirement for the targeted introduction of defects, as well as for the investigation of their influence on the local nanomechanical properties. The effect of oxygen-plasma treatment on different graphene-/graphite-samples was additionally investigated by Raman spectroscopy

    Singularity Formation in the High-Dimensional Euler Equations and Sampling of High-Dimensional Distributions by Deep Generative Networks

    Get PDF
    High dimensionality brings both opportunities and challenges to the study of applied mathematics. This thesis consists of two parts. The first part explores the singularity formation of the axisymmetric incompressible Euler equations with no swirl in ℝⁿ, which is closely related to the Millennium Prize Problem on the global singularity of the Navier-Stokes equations. In this part, the high dimensionality contributes to the singularity formation in finite time by enhancing the strength of the vortex stretching term. The second part focuses on sampling from a high-dimensional distribution using deep generative networks, which has wide applications in the Bayesian inverse problem and the image synthesis task. The high dimensionality in this part becomes a significant challenge to the numerical algorithms, known as the curse of dimensionality. In the first part of this thesis, we consider the singularity formation in two scenarios. In the first scenario, for the axisymmetric Euler equations with no swirl, we consider the case when the initial condition for the angular vorticity is Cα Hölder continuous. We provide convincing numerical examples where the solutions develop potential self-similar blow-up in finite time when the Hölder exponent α &lt; α*, and this upper bound α* can asymptotically approach 1 - 2/n. This result supports a conjecture from Drivas and Elgindi [37], and generalizes it to the high-dimensional case. This potential blow-up is insensitive to the perturbation of initial data. Based on assumptions summarized from numerical experiments, we study a limiting case of the Euler equations, and obtain α* = 1 - 2/n which agrees with the numerical result. For the general case, we propose a relatively simple one-dimensional model and numerically verify its approximation to the Euler equations. This one-dimensional model might suggest a possible way to show this finite-time blow-up scenario analytically. Compared to the first proved blow-up result of the 3D axisymmetric Euler equations with no swirl and Hölder continuous initial data by Elgindi in [40], our potential blow-up scenario has completely different scaling behavior and regularity of the initial condition. In the second scenario, we consider using smooth initial data, but modify the Euler equations by adding a factor ε as the coefficient of the convection terms to weaken the convection effect. The new model is called the weak convection model. We provide convincing numerical examples of the weak convection model where the solutions develop potential self-similar blow-up in finite time when the convection strength ε &lt; ε*, and this upper bound ε* should be close to 1 - 2/n. This result is closely related to the infinite-dimensional case of an open question [37] stated by Drivas and Elgindi. Our numerical observations also inspire us to approximate the weak convection model with a one-dimensional model. We give a rigorous proof that the one-dimensional model will develop finite-time blow-up if ε &lt; 1 - 2/n, and study the approximation quality of the one-dimensional model to the weak convection model numerically, which could be beneficial to a rigorous proof of the potential finite-time blow-up. In the second part of the thesis, we propose the Multiscale Invertible Generative Network (MsIGN) to sample from high-dimensional distributions by exploring the low-dimensional structure in the target distribution. The MsIGN models a transport map from a known reference distribution to the target distribution, and thus is very efficient in generating uncorrelated samples compared to MCMC-type methods. The MsIGN captures multiple modes in the target distribution by generating new samples hierarchically from a coarse scale to a fine scale with the help of a novel prior conditioning layer. The hierarchical structure of the MsIGN also allows training in a coarse-to-fine scale manner. The Jeffreys divergence is used as the objective function in training to avoid mode collapse. Importance sampling based on the prior conditioning layer is leveraged to estimate the Jeffreys divergence, which is intractable in previous deep generative networks. Numerically, when applied to two Bayesian inverse problems, the MsIGN clearly captures multiple modes in the high-dimensional posterior and approximates the posterior accurately, demonstrating its superior performance compared with previous methods. We also provide an ablation study to show the necessity of our proposed network architecture and training algorithm for the good numerical performance. Moreover, we also apply the MsIGN to the image synthesis task, where it achieves superior performance in terms of bits-per-dimension value over other flow-based generative models and yields very good interpretability of its neurons in intermediate layers.</p

    Decision-making with gaussian processes: sampling strategies and monte carlo methods

    Get PDF
    We study Gaussian processes and their application to decision-making in the real world. We begin by reviewing the foundations of Bayesian decision theory and show how these ideas give rise to methods such as Bayesian optimization. We investigate practical techniques for carrying out these strategies, with an emphasis on estimating and maximizing acquisition functions. Finally, we introduce pathwise approaches to conditioning Gaussian processes and demonstrate key benefits for representing random variables in this manner.Open Acces

    The OpenMolcas Web: A Community-Driven Approach to Advancing Computational Chemistry

    Get PDF
    The developments of the open-source OpenMolcas chemistry software environment since spring 2020 are described, with a focus on novel functionalities accessible in the stable branch of the package or via interfaces with other packages. These developments span a wide range of topics in computational chemistry and are presented in thematic sections: electronic structure theory, electronic spectroscopy simulations, analytic gradients and molecular structure optimizations, ab initio molecular dynamics, and other new features. This report offers an overview of the chemical phenomena and processes OpenMolcas can address, while showing that OpenMolcas is an attractive platform for state-of-the-art atomistic computer simulations

    Anomalous shift and optical vorticity in the steady photovoltaic current

    Full text link
    Steady illumination of a non-centrosymmetric semiconductor results in a bulk photovoltaic current, which is contributed by real-space displacements (`shifts') of charged quasiparticles as they transit between Bloch states. The shift induced by interband excitation via absorption of photons has received the prevailing attention. However, this excitation-induced shift can be far outweighed (\ll) by the shift induced by intraband relaxation, or by the shift induced by radiative recombination of electron-hole pairs. This outweighing (\ll) is attributed to (i) time-reversal-symmetric, intraband Berry curvature, which results in an anomalous shift of quasiparticles as they scatter with phonons, as well as to (ii) topological singularities in the interband Berry phase (`optical vortices'), which makes the photovoltaic current extraordinarily sensitive to the linear polarization vector of the light source. Both (i-ii) potentially lead to nonlinear conductivities of order mAV2mAV^{-2}, without finetuning of the incident radiation frequency, band gap, or joint density of states.Comment: 14+45 pages, 7+7 Figure
    corecore