Novel effects of strains in graphene and other two dimensional materials

The analysis of the electronic properties of strained or lattice deformed graphene combines ideas from classical condensed matter physics, soft matter, and geometrical aspects of quantum field theory (QFT) in curved spaces. Recent theoretical and experimental work shows the influence of strains in many properties of graphene not considered before, such as electronic transport, spin-orbit coupling, the formation of Moir\'e patterns, optics, ... There is also significant evidence of anharmonic effects, which can modify the structural properties of graphene. These phenomena are not restricted to graphene, and they are being intensively studied in other two dimensional materials, such as the metallic dichalcogenides. We review here recent developments related to the role of strains in the structural and electronic properties of graphene and other two dimensional compounds.Comment: 75 pages, 15 figures, review articl

Investigation of key challenges facing aerogel composites development through multiscale approach

Error on title page. Date of award is 2022The aerogel particulate and fibre reinforced composites are becoming more and more popular due to their exceptional properties, nevertheless, they do face a range of challenges that need to be overcome for wider applications. The main ones include a lack of understanding of the interactions between aerogels and reinforcing fibre materials, lack of appropriate models to predict their performance, and finally, lack of property database, allowing for an informative selection of aerogel composites as a viable alternative to other materials. The primary goal of this work is to tackle those challenges and provide a better fundamental understanding of some cases of aerogel composites. In order to fulfil the thesis' goals, the aerogel influence on the various thermal and mechanical properties of epoxy and vinyl ester polymers were investigated. By incorporating various weight contents and sizes of silica and polyimide aerogel particles into these polymers, their thermal conductivity, compressive properties, and other thermomechanical properties in these particle-filled polymers have been evaluated. Overall, created composites presented a significant decrease in thermal conductivity, while the introduction of porous particles deteriorated composite mechanical response. Additionally, micromechanical testing of the interface between aerogel and fibre reinforcement has been performed for the first time to understand their bonding ability. By designing a method to deposit an aerogel droplet surrounding the fibre, the microbond tests were enabled, and the results revealed poor adhesion between aerogel and selected fibre type in general. In addition to the experimental part, this study also focused on modelling aerogels and aerogel composites, which provided insight into the interactions between aerogels and most common reinforcement materials using a multiscale approach. As a result, the nanoscale analysis using molecular dynamics allowed to estimate thermal and mechanical properties of low density silica and polyimide. What is more, the aerogel-fibre interfacial properties values have also been obtained though modelling. Finally, the microscale model was used to model the thermal and mechanical properties of epoxy composites. A close match between experimental and modelled thermal conductivity and compressive modulus of epoxy combined with low density silica or polyimide particles has been achieved by incorporating the nanoscale properties into the micromechanical model.The aerogel particulate and fibre reinforced composites are becoming more and more popular due to their exceptional properties, nevertheless, they do face a range of challenges that need to be overcome for wider applications. The main ones include a lack of understanding of the interactions between aerogels and reinforcing fibre materials, lack of appropriate models to predict their performance, and finally, lack of property database, allowing for an informative selection of aerogel composites as a viable alternative to other materials. The primary goal of this work is to tackle those challenges and provide a better fundamental understanding of some cases of aerogel composites. In order to fulfil the thesis' goals, the aerogel influence on the various thermal and mechanical properties of epoxy and vinyl ester polymers were investigated. By incorporating various weight contents and sizes of silica and polyimide aerogel particles into these polymers, their thermal conductivity, compressive properties, and other thermomechanical properties in these particle-filled polymers have been evaluated. Overall, created composites presented a significant decrease in thermal conductivity, while the introduction of porous particles deteriorated composite mechanical response. Additionally, micromechanical testing of the interface between aerogel and fibre reinforcement has been performed for the first time to understand their bonding ability. By designing a method to deposit an aerogel droplet surrounding the fibre, the microbond tests were enabled, and the results revealed poor adhesion between aerogel and selected fibre type in general. In addition to the experimental part, this study also focused on modelling aerogels and aerogel composites, which provided insight into the interactions between aerogels and most common reinforcement materials using a multiscale approach. As a result, the nanoscale analysis using molecular dynamics allowed to estimate thermal and mechanical properties of low density silica and polyimide. What is more, the aerogel-fibre interfacial properties values have also been obtained though modelling. Finally, the microscale model was used to model the thermal and mechanical properties of epoxy composites. A close match between experimental and modelled thermal conductivity and compressive modulus of epoxy combined with low density silica or polyimide particles has been achieved by incorporating the nanoscale properties into the micromechanical model

Antenna near-field wave studies in magnetized plasmas as part of the optimization of auxiliary heating in fusion devicesAntenna near-field wave studies in magnetized plasmas as part of the optimization of auxiliary heating in fusion devices

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Multiscale modelling on material properties and mechanical behaviours of graphene reinforced polymer nanocomposites

Graphene possesses many superior properties, such as ultrahigh mechanical stiffness and strength, exceptional thermal and electrical conductivities as well as excellent optical properties. In many of the envisioned applications, graphene or its derivatives are incorporated into the polymer matrix to form graphene based nanocomposite systems in which the polymer matrices can work synergically with graphene fillers as functional components providing supports and protections to the embedded graphene. Two types of additive manufacturing (AM) techniques have been developed for the graphene reinforced polymer nanocomposites. One is the layer-by-layer (LbL) assembly technique which is a versatile process and capable of manipulating material composition and architectures at the nanoscale. The other AM technique is conventionally known as the extrusion-based 3D printing. This research focuses on the computational method and numerical modelling of material properties and mechanical behaviours of graphene-based polymeric nanocomposites. A hierarchical multiscale analysis approach is adopted and tailored specifically for the graphene-based polymeric nanocomposites fabricated using the AM techniques. Some of the important material characteristics at nano- and meso-scales such as molecular interactions and microstructure morphologies are simulated and discussed in details. The nonlinear mechanical behaviours e.g., bending, post-buckling and vibration of functionally graded graphene reinforced nanocomposite (FG-GRC) beams fabricated by LbL technique are subsequently carried out. Numerical analysis with various macroscaled parameters such as functionally graded patterns, temperature rises as well as foundation stiffnesses are presented and discussed. This study is crucial for engineering applications to evaluate mechanical behaviours of such nanocomposite materials with optimal arrangements and manufactured by using these two above-mentioned methods

Tuning dynamical properties of nanoscale systems via atomic-level modifications: insights from all-atom Molecular Dynamics simulations

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Teórica de la Materia condensada. Fecha de lectura: 14-01-202

Nonlinear scale-dependent deformation behaviour of beam and plate structures

Improving the knowledge of the mechanics of small-scale structures is important in many microelectromechanical and nanoelectromechanical systems. Classical continuum mechanics cannot be utilised to determine the mechanical response of small-scale structures, since size effects become significant at small-scale levels. Modified elasticity models have been introduced for the mechanics of ultra-small structures. It has recently been shown that higher-order models, such as nonlocal strain gradient and integral models, are more capable of incorporating scale influences on the mechanical characteristics of small-scale structures than the classical continuum models. In addition, some scaledependent models are restricted to a specific range of sizes. For instance, nonlocal effects on the mechanical behaviour vanish after a particular length. Scrutinising the available literature indicates that the large amplitude vibrations of small-scale beams and plates using two-parameter scaledependent models and nonlocal integral models have not been investigated yet. In addition, no twoparameter continuum model with geometrical nonlinearity has been introduced to analyse the influence of a geometrical imperfection on the vibration of small-scale beams. Analysing these systems would provide useful results for small-scale mass sensors, resonators, energy harvesters and actuators using small-scale beams and plates. In this thesis, scale-dependent nonlinear continuum models are developed for the time-dependent deformation behaviour of beam-shaped structures. The models contain two completely different size parameters, which make it able to describe both the reduction and increase in the total stiffness. The first size parameter accounts for the nonlocality of the stress, while the second one describes the strain gradient effect. Geometrical nonlinearity on the vibrations of small-scale beams is captured through the strain-displacement equations. The small-scale beam is assumed to possess geometrical imperfections. Hamilton’s approach is utilised for deriving the corresponding differential equations. The coupled nonlinear motion equations are solved numerically employing Galerkin’s method of discretisation and the continuation scheme of solution. It is concluded that geometrical imperfections would substantially alter the nonlinear vibrational response of small-scale beams. When there is a relatively small geometrical imperfection in the structure, the small-scale beam exhibits a hardeningtype nonlinearity while a combined hardening- and softening-type nonlinearity is found for beams with large geometrical imperfections. The strain gradient influence is associated with an enhancement in the beam stiffness, leading to higher nonlinear resonance frequencies. By contrast, the stress nonlocality is related to a remarkable reduction in the total stiffness, and consequently lower nonlinear resonance frequencies. In addition, a scale-dependent model of beams is proposed in this thesis to analyse the influence of viscoelasticity and geometrical nonlinearity on the vibration of small-scale beams. A nonlocal theory incorporating strain gradients is used for describing the problem in a mathematical form. Implementing the classical continuum model of beams causes a substantial overestimation in the beam vibrational amplitude. In addition, the nonlinear resonance frequency computed by the nonlocal model is less than that obtained via the classical model. When the forcing amplitude is comparatively low, the linear and nonlinear damping mechanisms predict almost the same results. However, when forcing amplitudes become larger, the role of nonlinear viscoelasticity in the vibrational response increases. The resonance frequency of the scale-dependent model with a nonlinear damping mechanism is lower than that of the linear one. To simulate scale effects on the mechanical behaviour of ultra-small plates, a novel scale-dependent model of plates is developed. The static deflection and oscillation of rectangular plates at small-scale levels are analysed via a two-dimensional stress-driven nonlocal integral model. A reasonable kernel function, which fulfil all necessary criteria, is introduced for rectangular small-scale plates for the first time. Hamilton and Leibniz integral rules are used for deriving the non-classical motion equations of the structure. Moreover, two types of edge conditions are obtained for the linear vibration. The first type is the well-known classical boundary condition while the second type is the nonclassical edge condition associated with the curvature nonlocality. The differential quadrature technique as a powerful numerical approach for implementing complex boundary conditions is used. It is found that while the Laplacian-based nonlocal model cannot predict size influences on the bending of small-scale plates subject to uniform lateral loading, the bending response is remarkably size-dependent based on the stress-driven plate model. When the size influence increases, the difference between the resonance frequency obtained via the stress-driven model and that of other theories substantially increases. Moreover, the resonance frequency is higher when the curvature nonlocality increases due to an enhancement in the plate stiffness. It is also concluded that more constraint on the small-scale plate causes the system to vibrate at a relatively high frequency. In addition to the linear vibration, the time-dependent large deformation of small-scale plates incorporating size influences is studied. The stress-driven theory is employed to formulate the problem at small-scale levels. Geometrical nonlinearity effects are taken into account via von Kármán’s theory. Three types of edge conditions including one conventional and two nonconventional conditions are presented for nonlinear vibrations. The first non-classical edge condition is associated with the curvature nonlocality while the second one is related to nonlocal in-plane strain components. A differential quadrature technique and an appropriate iteration method are used to compute the nonlinear natural frequencies and maximum in-plane displacements. Molecular dynamics simulations are also performed for verification purposes. Nonlinear frequency ratios are increased when vibration amplitudes increase. Furthermore, the curvature nonlocality would cause the small-scale pate to vibrate at a lower nonlinear frequency ratio. By contrast, the nonlocal in-plane strain has the opposite effect on the small-scale system. The outcomes from this thesis will be useful for engineers to design vibrating small-scale resonators and sensors using ultra-small plates.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 202

Liquid Phase Electron Microscopy of Soft Specimens

In the last decade, liquid-phase electron microscopy (LPEM) has provided a new strategy for investigating samples immersed in their media at the nanoscale.1–8 The main focus of previous research have mainly revolved around inorganic matter (e.g. metallic nanoparticles);9 nonetheless, the field of soft materials, classified as organic synthetic (i.e. polymers and gels), and biological (i.e. membranes and protein) structures have rapidly grown interest in LPEM to study fundamental questions.2 Soft materials deform easily or undergo dynamic changes by thermal fluctuations and external forces. Despite the great advantages LPEM provides, electron beam damage and image contrast present still an issue, particularly in sensitive samples. New technological and methodological advances may attenuate these issues. There is a need to employ these advancements to develop strategies to image soft materials. This thesis focuses on the development of methodologies for the investigation of soft materials using LPEM. Amongst the different conducted studies, there are three main sections of focus: (i) the reconstruction of three-dimensional (3D) structures via Brownian tomography (BT) and Brownian particle analysis (BPA), enabling the investigation of the 3D conformational space of single unit of the specimen, via BT, and an average reconstruction of several specimens, via BPA; (ii) the dynamic studies of biological and synthetic soft materials, specifically oxidant-sensitive polymeric micelles and viruses, focusing on their disassembly via external factors, reactive-oxygen species (ROS) and virucidal nanoparticles respectively; and (iii) the imaging of intracellular ultrastructure via organometallic, cyclometalated complexes for intracellular targeting, particularly actin and nuclear DNA, via correlative light and liquid phase electron microscopy (CLLEM)