Charting nanocluster structures via convolutional neural networks

A general method to obtain a representation of the structural landscape of nanoparticles in terms of a limited number of variables is proposed. The method is applied to a large dataset of parallel tempering molecular dynamics simulations of gold clusters of 90 and 147 atoms, silver clusters of 147 atoms, and copper clusters of 147 atoms, covering a plethora of structures and temperatures. The method leverages convolutional neural networks to learn the radial distribution functions of the nanoclusters and to distill a low-dimensional chart of the structural landscape. This strategy is found to give rise to a physically meaningful and differentiable mapping of the atom positions to a low-dimensional manifold, in which the main structural motifs are clearly discriminated and meaningfully ordered. Furthermore, unsupervised clustering on the low-dimensional data proved effective at further splitting the motifs into structural subfamilies characterized by very fine and physically relevant differences, such as the presence of specific punctual or planar defects or of atoms with particular coordination features. Owing to these peculiarities, the chart also enabled tracking of the complex structural evolution in a reactive trajectory. In addition to visualization and analysis of complex structural landscapes, the presented approach offers a general, low-dimensional set of differentiable variables which has the potential to be used for exploration and enhanced sampling purposes.Comment: 28 pages, 13 figure

Atomistic modelling of metamagnetic transition in FeRh with four-spin exchange

The metamagnetic transformation of FeRh from antiferromagnetic (AFM) to ferromagnetic (FM) ordering makes it suitable for a wide scope of applications, ranging from magnetic recording media to antiferromagnetic spintronics and magnetic refrigeration. Exchange spring systems of FeRh coupled with a hard magnetic layer (FePt) are a promising approach for heat-assisted magnetic recording technologies that assure high density of stored information. It has been shown that different temperature scalings of the exchange interactions can lead to a first-order phase transition in FeRh systems (Barker, J., & Chantrell, R. W. (2015). Higher-order exchange interactions leading to metamagnetism in FeRh. Physical Review B, 92(9), 094402). This model assumes the presence of a higher-order exchange term in the form of four-spin exchange, that arises from four consecutive hops of electrons from one spin configuration to the spin-flipped one, the higher order four-spin interaction being mediated in FeRh by the Rh atoms. At small temperatures the four-spin exchange is responsible for the AFM ordering, while, at higher temperatures the FM ordering is given by the bilinear exchange since the four-spin term decreases more rapidly with temperature than the bilinear term. In this work, the first-order phase transition that appears in FeRh is systematically studied via the four-spin parametric model that was previously given in literature. A degeneracy in the ground-state of the four-spin exchange system is found. The effect of the parameters entering into the spin Hamiltonian was systematically analysed. The model has been implemented for FeRh and then developed in order to consider other materials with different crystal structures. As nanoscale applications of the FeRh systems are more practical due to the high cost of Rh, the finite size effects of FeRh grains and thin films are systematically investigated. As a further test of the model, ultrafast simulations have been performed. In accordance to the literature, it is found that, by laser-heating the FeRh system, the ferromagnetic ordering is generated in picosecond time-scales. Additionally the dynamical and equilibrium properties of FePt/FeRh bilayers have been systematically investigated, as this system is of particular interest for recording media applications

Investigating the micromechanics of soil reinforced with recycled tyres using the Discrete Element Method

This PhD program aims to provide insights into the behaviour of tyre reinforced sand, and to further the understanding of the reinforcement mechanisms. The discrete element method (DEM) using the software PFC3D version 5.0, is used to investigate the micro- mechanics of tyre-reinforced sand based on pull-out tests. A series of numerical direct shear tests have been performed. A full investigation of the individual particle shape descriptors (e.g., elongation index, flatness index, convexity and roundness) is presented, highlighting their influence on the macroscopic behaviour (e.g., failure mode and volumetric response). MorphologI G3 at UCL helps to obtain the quantitative particle shape information on Fujian Standard Sand. The sand information is incorporated in the results of particle shape studies to propose a representative particle shape for Fujian Standard Sand to model direct shear tests, leading to the calibration of pure sand. Then, a new three-dimensional DEM model for tyre rubber is developed based on uniaxial tensile tests. The model can capture the key volume change characteristics of tyres with a Poisson’s ratio of 0.5 independent of the tyre dimensions. Tyre rubber is modelled using body-centred-cubic (BCC) packing with linear inter-particle bonds. Next, a systematic parametric study is presented, which includes the effects of different packings, particle overlapping, particle radii and sample aspect ratios on the mechanical response of the tyre model using Young’s modulus and Poisson’s ratio. The DEM parameters are set to match corresponding experimental Young’s modulus data, completing the calibration of tyre. Lastly, the tyre – sand interface coefficient friction is calibrated from numerical and laboratory interface direct shear tests. All DEM parameters are used in tyre reinforced sand pull-out tests simulations. Micro analyses (such as particle displacement, velocity, contact force) presented in simulations show that the DEM tyre-sand pullout test simulation can capture the progress failure of tyre rubber during the pull-out process, and the intrusive capabilities of the discrete element method are used to gain insight into the reinforcement mechanisms between tyre and sand

Self-assembly of granular particles

Granular particles are ubiquitous in nature and daily life, and have wide applications in various disciplines such as infrastructure engineering, architecture, agriculture, etc. Yet, their fundamentals have not been fully understood by scientists. This is mainly because the structure of granular particles, which determines their properties, is complicated and can experience critical changes from disorder to ordered state. In recent years, understanding the fundamentals of such critical structural transitions of granular materials has become a hot multidisciplinary research topic attracting both scientists and engineers. Generally the transition from disordered to ordered structure can be regarded as a self-assembly process, which happens at different scales. In the nucleation of crystals, atoms or molecules can self-assemble due to thermal energy. For such thermodynamics systems, the theory of self-assembly is well established and is dependent on the Gibbs free energy. However, granular particles are much bigger and can dissipate energy quickly with the collision between particles, so they are normally at athermal or low-thermal states. The granular packings are prone to be disordered in structure, whereas they can also self-assemble with the input of external energy via vibration or shear, which can densify the granular packings and hence improve their properties for different applications. This thesis is devoted to advancing the knowledge of the self-assembly of granular spheres, particularly in better understanding the effects of the energy input and the boundary shape. The thesis has revealed a rich and deep picture for the effect of various factors on the self-assembly of granular particles, including the vibration mode, the container shape, material properties, different wall motions and gravity. The obtained results can improve the current understanding of the structural evolution and phase transition of the granular packings with or without vibration. The findings of this study enhance the knowledge on the self-assembly of granular systems and help take a step forward toward stablishing the mechanism behind the phenomenon. Thorough comprehension of the structure of the granular particles are essential for controlling the behaviour and properties of the granular materials, which can be of paramount importance for both the science and technology and have sensible influence on the mankind’s life

The Construction of Conforming-to-shape Truss Lattice Structures via 3D Sphere Packing

Truss lattices are common in a wide variety of engineering applications, due to their high ratio of strength versus relative density. They are used both as the interior support for other structures, and as structures on their own. Using 3D sphere packing, we propose a set of methods for generating truss lattices that fill the interior of B-rep models, polygonal or (trimmed) NURBS based, of arbitrary shape. Once the packing of the spheres has been established, beams between the centers of adjacent spheres are constructed, as spline based B-rep geometry. We also demonstrate additional capabilities of our methods, including connecting the truss lattice to (a shell of) the B-rep model, as well as constructing a tensor-product trivariate volumetric representation of the truss lattice - an important step towards direct compatibility for analysis.RYC-2017-2264

The Controlled Synthesis of Hydrogen Electrocatalysts for Alkaline Exchange Membrane Fuel Cell and Electrolysis Applications via Chemical Vapor Deposition

The development of catalysts for the electrochemical processes of hydrogen systems (e.g., fuel cells and electrolyzer systems) continues to be an attractive area of research for renewable energy technologies. One significant challenge has been developing hydrogen catalysts suitable for alkaline environments, mainly due to the sluggish kinetics of hydrogen reactions. In alkaline environments, the kinetics are decreased by two orders of magnitude when compared to acidic environments. Chemical vapor deposition (CVD) is a conventional method used to synthesize these types of catalysts. This effort discusses extending work being done using a modified CVD process known as “Poor Man’s” CVD (PMCVD) to tailor catalyst properties and inherently further its impact in electrochemical catalyst application. PMCVD utilizes an inexpensive vacuum oven to sublime commercially available and easily synthesized metal-organic salt precursors, lowering synthesis cost while providing better control of the catalyst properties such as particle size and distribution. Herein, we describe the various efforts conducted to characterize electrocatalysts produced via the PMCVD process to further elucidate this modified CVD method\u27s versatility and control. Additionally, we utilize a myriad of electrochemical characterization techniques to measure the electrocatalytic activates of catalyst prepared using this modified CVD method for the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) in alkaline media through half-cell reactions. The study begins with the development of rate laws and deposition mechanism for the PMCVD process facilitated by measuring particle growth as a function of metal loading. We investigate the impact varying reaction conditions have on the crystal structure of the deposited nanoparticles. We determine the PMCVD’s efficiency in alloying identical and mixed crystal structured metals through the development of bifunctional catalysts for the HOR/HER. We studied the impact varying properties of carbon supports has on the deposition method as well as how these properties impact the hydrogen kinetics. We close this work by investigating the deposition of transition metals and begin the necessary steps to elucidating the impact the addition of water has on the deposition mechanism. These results provide a clear description regarding the tuning of both physical and catalytic properties of nanoparticles produced through this deposition method