37 research outputs found
Approximated structured pseudospectra
Pseudospectra and structured pseudospectra are important tools for the analysis of matrices. Their computation, however, can be very demanding for all but small-matrices. A new approach to compute approximations of pseudospectra and structured pseudospectra, based on determining the spectra of many suitably chosen rank-one or projected rank-one perturbations of the given matrix is proposed. The choice of rank-one or projected rank-one perturbations is inspired by Wilkinson's analysis of eigenvalue sensitivity. Numerical examples illustrate that the proposed approach gives much better insight into the pseudospectra and structured pseudospectra than random or structured random rank-one perturbations with lower computational burden. The latter approach is presently commonly used for the determination of structured pseudospectra
Modeling multiphase flow and substrate deformation in nanoimprint manufacturing systems
Nanopatterns found in nature demonstrate that macroscopic properties of a surface are tied to its nano-scale structure. Tailoring the nanostructure allows those macroscopic surface properties to be engineered. However, a capability-gap in manufacturing technology inhibits mass-production of nanotechnologies based on simple, nanometer-scale surface patterns. This gap represents an opportunity for research and development of nanoimprint lithography (NIL) processes. NIL is a process for replicating patterns by imprinting a fluid layer with a solid, nano-patterned template, after which ultraviolet cure solidifies the fluid resulting in a nano-patterned surface. Although NIL has been demonstrated to replicate pattern features as small as 4 nm, there are significant challenges in using it to produce nanotechnology. Ink-jet deposition methods deliver the small fluid volumes necessary to produce the nanopattern, and drop volumes can be tuned to what the pattern requires. However the drops trap pockets of gas as they merge and fill the template, and due to relatively slow gas dissolution, reduce processing throughput. Capillary forces that arise from the gas-liquid interfaces drive non-uniform gap closure and the resulting variations in residual layer reduces process yield or degrades product performance.
This thesis develops reduced-order models for fluid flow and structural mechanics of the imprint process for NIL. Understanding key phenomena of gas trapping and residual layer non-uniformity drives model development to better understand how throughput and yield can be improved.
Reynolds lubrication theory, the \textit{disperse} type of multiphase flow, and a lumped-parameter model of dissolution unite to produce a two-phase flow model for NIL simulations of 10,000 drops per . Qualitative agreement between simulation and experiment provides a modicum of validation of this model for flow in NIL simulations. The two-phase model simulations predicts that both dissolution and viscous resistance affect throughput.
The coupling of a reduced-order model for 3D structural mechanics with the two-phase flow model enables simulations of drop merger on a free-span tensioned web. Challenges in improving the structural model lead to formulation of a 2D model for which sources of instability are more easily discovered and understood. Inextensible cylindrical shell theory and lubrication theory combine into a model for the elastohydrodynamics of a rolling-imprint modality of NIL. Foil-bearing theory describes the lubrication layer that forms between a thin, tensioned web moving past another surface. Reproduction of the results of foil-bearing theory validates this coupled model and reveals a highly predictable region of uniformity that provides low shear stress conditions ideal for UV-cure. These results show theoretical limitations that are used to construct a processing window for predicting process feasibility
Magnetic field directed self-assembly of gold Pickering emulsion for preparing patterned film.
Patterning plays a vital in role in sensor-based devices like surface-enhanced Raman spectroscopy (SERS), surface-enhanced infrared absorption (SEIRA), radio frequency (RF) antennas and many others. The linear array spacing and width of gold strips has been shown to increase the local intensity through near-field coupling with diffracted electromagnetic waves. This rise in local charge boosts vibrational energies of molecules in close-surface contact or proximity, resulting in increased IR absorption. The strip-like or any other types of patterns are efficiently achieved through top-down nanofabrication processes like atomic-force-deposition, nanoimprinting, UV-Lithography etc., which involve high capital cost, complex processing and occasionally low throughput. This research was therefore undertaken with the aim of reducing the process complexities and improving scalability, by applying a magnetic and spin coating directed self-assembly (MSCDS) to prepare optically sensitive dipole-dipole chain-like ordered arrays of the gold nanoparticle Pickering ferrofluid in polyvinyl alcohol (PVA) emulsion, in the form of a thin film on glass and silicon substrate. Previously-conducted MSCDS processes lacked the control over the dimensions of the prepared patterns. Here, the static magnetic field approach was taken to modify the MSCDS process to overcome the limitation of pattern dimension control, providing tuneability for optical applications. Quantitative image analysis of the patterned thin film allowed for the measurement of pattern geometrical dimension (chain length-CL, chain gap-CG and chain thickness-CT), which was then correlated with processing parameters such as magnetic field configurations (single, compound and concentric), spinning speeds and viscosities of Pickering emulsion. Upon optimization, spectroscopical characterisation was performed on prepared patterned thin film to demonstrate the capability of the modified MSDS process in enhancing the molecular detection at low concentrations. The UV-vis spectra of the patterns demonstrated the impact of CT and CG on the degree of gold-iron oxide nanoscale interactions leading to tuneability of absorption bands between 390-700nm. The coupling of the increased optical sensitivity through enhanced charge transfer dynamics with the mid-infra-red range grating order (CT+CG) resulted in an amplification in vibrational band excitation of molecular bonds. For example, SEIRA measurements of thin film patterns showed a vibrational signal enhancement in asymmetric vibration of -CH2 (2920cm-1) bonds of PVA by 40%, as CT increased by 178% from 1.2μm at probing 45 degree grazing angle. Furthermore, the magneto-optical SERS phenomenon - involving local polarization of gold nanoparticles through the neighbouring magnetised iron oxide nanoparticle in the presence of external magnetic field - was exploited to reveal the varying degree of enhancement in peaks related to Rhodamine 6G (R6G) coated on thin film nanostructure, which was dependent on magnetized CT/CG morphology; especially the C-C-C ring (671 cm-1), for which the Raman peak increased by 12,000% when magnetized by a 43mT field. In summary, the modified MSCDC process is cheap with an expandable throughput rate ( > 0.1 m2/h) and flexible designs, offering both nanoscale and microscale tuneability of pattern dimensions. Even with higher defectivity (~14%) in comparison to the nanoimprinting method, this method can potentially be used to create repetitive array-like structure. Furthermore, the use of iron oxide reduces the cost without sacrificing the optical performance and thus contributes to the optical tuneability of the thin film nanostructure, thereby making the entire product a potential absorbing antenna and microfluidics thin film for biomolecule detection
5 Post-processing methods for passivity enforcement
Many physical systems are passive (or dissipative): they are unable to generate energy on their own, but they can store energy in some form while exchanging power with the surrounding environment. This chapter describes the most prominent approaches for ensuring that Reduced Order Models are passive, so that their math- ematical representation satisfies an appropriate dissipativity condition. The main focus is on Linear and Time-Invariant (LTI) systems in state-space form. Different conditions for testing passivity of a given LTI model are discussed, including Linear Matrix Inequalities (LMIs), Frequency-Domain Inequalities, and spectral conditions on associated Hamiltonian matrices. Then we describe common approaches for perturbing a given non-passive system to enforce its passivity. Various examples from electronic applications are used to demonstrate both theory and algorithm performance
Novel in vitro systems elucidating metabolic health and disease
The convoluted nature of biology warrants improved models that further insights into health and disease. Bioengineered features and platforms allow the modulation and study of a range of biological phenomena. However, there remains a lack of versatile and well-characterised organotypic models and microphysiological systems that recapitulate phenotypes of interest. We have developed and comprehensively characterised a chemically defined, high throughput and stable 3D human adipose model comprising adipocyte spheroids. Adipocyte spheroids exhibit physiologically relevant gene expression signatures and improved phenotypes compared to conventional monolayer counterparts, with 4704 genes being differentially expressed compared to 2D cultures. Moreover, the model closely resembles freshly isolated human in vivo mature adipocytes. Such organotypic models and cellular phenomena can be manipulated using nanotopographies and structured polymer devices. We have demonstrated that nanostructures fabricated via nanoimprint lithography enabled precise modulation of cellular attachment and behaviour. Specific and high resolution structuring of microfluidic platforms is achieved using a novel fabrication approach, NanoRIM, allowing high fidelity generation of systems for organotypic hepatic cultures. The physiological crosstalk between liver and human pancreatic islets was then efficiently captured in an original microfluidic system featuring reciprocal perfusion. The versatile nature of these ensuing models and platforms enables the provision of a toolbox with which biology can be manipulated and studied across the vast temporal and spatial scales on which it exists
System- and Data-Driven Methods and Algorithms
An increasing complexity of models used to predict real-world systems leads to the need for algorithms to replace complex models with far simpler ones, while preserving the accuracy of the predictions. This two-volume handbook covers methods as well as applications. This first volume focuses on real-time control theory, data assimilation, real-time visualization, high-dimensional state spaces and interaction of different reduction techniques
Structural Defects in Nanotechnology: Production, Characterization, Applications: Transport Properties in Mechanically Ground Nanocrystalline Ceramics & Hydrogen Storage in Metallic Hydrides
Structural defects play a major role in nanotechnology as they influence most properties, thus largely motivating the special interest in studying materials at the nano scale. The present Thesis work contributes to this broad and diversified research field with emphasis on the characterization of nanocrystalline ceramic materials and their lattice defects. In particular, main efforts were addressed to develop new and more comprehensive approaches to the study of nanocrystalline powders, combining different techniques for a better and deeper understanding of materials.
The specific applications selected in this work are basically two: nanocrystalline fluorite as a promising ionic conductor and nanocrystalline magnesium hydride for hydrogen storage applications. Chapters II and III were dedicated to investigate nanocrystalline fluorite produced by two different methods: a bottom-up approach based on co-precipitation of Ca and F precursors yielding loosely bound nanocrystals, and a popular top-down approach, high energy ball milling, giving nanocrystals of comparable sizes but strongly agglomerated and densely populated with dislocations. As a major achievement reported in this part of the Thesis work, a new approach was proposed and tested for the simultaneous modelling of X-ray Diffraction (XRD) peak profiles and solid state NMR spin-lattice relaxation data. With the valuable support of Transmission Electron Microscopy (TEM), this work offers a new understanding of the complex defect structure of nanocrystalline fluorite, and is also a demonstration of the power of combining different techniques in a consistent way.
One of the most debated aspects of nanotechnology concerns the stability of the nanostructure, and the mechanisms of defect annealing and grain growth with temperature. This topic was the object of chapter V, dedicated to study the influence of lattice defects on the grain growth kinetics of nanocrystalline fluorite. This chapter was preceded by chapter IV, on the furnace recently installed at the MCX beamline for in-situ high temperature fast data collection; besides providing useful details for the in-situ study on nanocrystalline fluorite shown in the following chapter, the activity reported in chapter IV is a tangible sign of the special involvement during the Thesis work in supporting standard operation as well as development of the ELETTRA beamline MCX. The growth kinetics was studied on two samples, among those discussed in Chapter III, with comparable crystalline domain size but drastically different lattice defect content, so to highlight the role lattice defects – dislocations in this case – in the growth process.
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Last two chapters (chapter VI and VII) were dedicated to nanocrystalline magnesium hydride, and how the performance, in particular the hydrogen desorption kinetics, can be improved by adding a nanocrystalline tin oxide. Besides general aspects on phase composition of the system and hydrogen storage capability, the work also addressed the problem of obtaining activation energy values in the thermal decomposition of magnesium hydride powders, presenting an interesting review of results given by the most known and well-assessed TG-MS coupled measurements, with details on the use of different equations of the literature on thermal analysis.
Although research work can rarely be considered as finished, a sound conclusion of this Thesis work is toward the use of different characterization techniques, also within the same data analysis procedure, to support a better, and more reliable investigation of nanomaterial properties