The dispersion strengthening is a process that strengthens the material's matrix with use of the smallest precipitates. The most power comes from the character of the nanostructures, their chemical compositions, crystallographic structures, sizes, shapes and interfaces between matrix and precipitates. Through analytical methods it is possible to study and achieve the optimal mechanical properties of elements produced from high-manganese steels with dispersive nanoparticles such as carbides or carbo-nitrides. So far the most effective research methods are transmission electron microscopy and spectroscopy with use of multiple detectors such as Dual Electron Energy Loss Spectrometer (DualEELS), Energy Dispersion X-ray detector, (EDX) or Gatan Imaging Filter (GIF). The DualEELS, which is mostly developed during this thesis, is a technique which results in spectrum imaging of all elements presented in the analyzed area. The material studied during this Ph.D. is a high-manganese steel with vanadium and niobium carbides or carbo-nitrides with various sizes from few to dozen of nanometers, but the average is less than 20nm. The main goal of this Ph.D. is to advance the current available methods of nano-analysis using DualEELS of the smallest precipitates embedded in the matrix supported by improved and optimized sample preparation method with Focused ion Beam (FIB) lamella. The advancement is made in several fields including development of acquisition conditions, development of sample preparation technique using low voltages at FIB, and development and adaptation of the novel post-processing routines of the acquired data using Gatan DualEELS QuantumER post-column detector. The post-processing routine involved quantitative and qualitative analysis of precipitates, which required new values of partial cross-sections and mean free paths of inelastic scattering events estimated and calculated based on experimental (standards) approach. The results consist of fully quantified precipitates (V,Ti)(N,C) and ((Nb,Ti)C) supported by chemical profiles, comparison discussion between specimen, with a pseudo-3D reconstruction of spectrum imaging of these precipitates, which means that all elements have individually calculated thicknesses. The results gave a glimpse into the nucleation of the smallest precipitates, unexpectedly different than in most literature, showing that in this case there is no core-shell structure in mixed transition metal carbides. The Ti, which is not an intentional part of the alloy, acts as a nucleation sites, but is not built over core for the V or Nb mixed precipitates. These results are shown by four examples of V-steel and three of Nb-steel. They are presented as spectrum imaging signals, separated from matrix, with pseudo 3D representation and quantified in absolute manner with the accuracy down to a few atoms.

Solid state nuclear magnetic resonance (NMR) is a powerful probe of inorganic materials systems. Through carefully changing materials compositions and synthesis methods, the impact on the local structure can be investigated. These have been applied to three main materials sectors: Paramagnetic materials in NMR have traditionally suffered from poor resolution due to broadening experienced at the nuclei from localised unpaired electrons. In this work, a fast magic angle spinning (MAS) and low field approach has been applied to these paramagnetic cathode materials to improve this resolution, and elucidate structural information from the investigated materials. The resolution gained from these techniques has been used to highlight differences observed in the 7Li shifts of lithium iron phosphate (LFP) produced by different synthesis techniques. This was found to be related to the cell volume of the LiFePO4 phase. Furthermore, the investigation of V doped LFP by 7Li and 31P MAS NMR has resulted in the observation of many common impurities resulting from synthesis. Additionally, 31Presonances could be identified that were related to V near the phosphorus site, indicating successfully doping in some of the higher Li containing samples. Through 29Si and 17O MAS NMR, changes in the local structure between Ca and Zn doped Stöber nanoparticles are observed. Similarly to other Ca containing materials, incorporation of Ca into the Stöber network has been shown to disrupt Sibridging bonds promoting the formation of non-bridging bonds in the silica network. However, addition of Zn tells a different story. This is first observed in the static measurements, where incorporation of high amounts of Zn leads to no evidence of hydroxyls observed in the Stöber network. Whereas, high resolution transmission electron microscopy (HRTEM) and density functional theory (DFT) calculations confirm the presence of crystalline Zn2SiO4 -II in the nanoparticles. Finally, activation of two series of synthetic sodium- and aluminium substituted calcium silicate hydrate (C-(N)-(A)-S-H) geopolymers are investigated. Increasing the CaO has been shown to increase the disorder of the silica network, and also to promote the increase of crystallinity of the systems through observation of calcium aluminate phases. Additionally, increasing the amount of aluminium relative to the silicon in the system, promotes more of these crystalline phases to form.

<p> Block copolymers are composed of chemically distinct polymer chains that can be covalently linked in a variety of sequences and architectures. They are ubiquitous as ingredients of consumer products and also have applications in advanced plastics, drug delivery, advanced membranes, and next generation nano-lithographic patterning. The wide spectrum of possible block copolymer applications is a consequence of block copolymer self-assembly into periodic, meso-scale morphologies as a function of varying block composition and architecture in both melt and solution states, and the broad spectrum of physical properties that such mesophases afford. </p><p> Materials exploration and discovery has traditionally been pursued through an iterative process between experimental and theoretical/computational collaborations. This process is often implemented in a trial-and-error fashion, and from the computational perspective of generating phase diagrams, usually requires some existing knowledge about the competitive phases for a given system. Self-Consistent Field Theory (SCFT) simulations have proven to be both qualitatively and quantitatively accurate in the determination, or forward mapping, of block copolymer phases of a given system. However, it is possible to miss candidates. This is because SCFT simulations are highly dependent on their initial configurations, and the ability to map phase diagrams requires a priori knowledge of what the competing candidate morphologies are. The unguided search for the stable phase of a block copolymer of a given composition and architecture is a problem of global optimization. SCFT by itself is a local optimization method, so we can combine it with population-based heuristic algorithms geared at global optimization to facilitate forward mapping. In this dissertation, we discuss the development of two such methods: Genetic Algorithm + SCFT (GA-SCFT) and Particle Swarm Optimization + SCFT (PSO-SCFT). Both methods allow a population of configurations to explore the space associated with the numerous states accessible to a block copolymer of a given composition and architecture. </p><p> GA-SCFT is a real-space method in which a population of SCFT field configurations &ldquo;evolves&rdquo; over time. This is achieved by initializing the population randomly, allowing the configurations to relax to local basins of attraction using SCFT simulations, then selecting fit members (lower free energy structures) to recombine their fields and undergo mutations to generate a new &ldquo;generation&rdquo; of structures that iterate through this process. We present results from benchmark testing of this GA-SCFT technique on the canonical AB diblock copolymer melt, for which the theoretical phase diagram has long been established. The GA-SCFT algorithm successfully predicts many of the conventional mesophases from random initial conditions in large, 3-dimensional simulation cells, including hexagonally-packed cylinders, BCC-packed spheres, and lamellae, over a broad composition range and weak to moderate segregation strength. However, the GA-SCFT method is currently not effective at discovery of network phases, such as the Double-Gyroid (GYR) structure. </p><p> PSO-SCFT is a reciprocal space approach in which Fourier components of SCFT fields near the principal shell are manipulated. Effectively, PSO-SCFT facilitates the search through a space of reciprocal-space SCFT seeds which yield a variety of morphologies. Using intensive free energy as a fitness metric by which to compare these morphologies, the PSO-SCFT methodology allows us to agnostically identify low-lying competitive and stable morphologies. We present results for applying PSO-SCFT to conformationally symmetric diblock copolymers and a miktoarm star polymer, AB₄, which offers a rich variety of competing sphere structures. Unlike the GA-SCFT method we previously presented, PSO-SCFT successfully predicts the double gyroid morphology in the AB-diblock. Furthermore, PSO-SCFT successfully recovers the A₁₅ morphology at a composition where it is expected to be stable in the miktoarm system, as well as several competitive metastable candidates, and a new sphere morphology belonging to the hexagonal space group 191, which has not been seen before in polymer systems. Thus, we believe the PSO-SCFT method provides a promising platform for screening for competitive structures in a given block copolymer system.</p><p>

Today, CMOS-compatible Flash memory technology dominates the non-volatile memory storage market due to high density and low fabrication costs. However, with CMOS approaching fundamental scaling limits, research into novel emerging non-volatile memory storage technologies that exploit materials properties including resistance, spin and polarisation, has significantly progressed. The ideal non-volatile memory technology would compete with Flash, offering high-density memory storage at low costs, however it would outperform Flash due to its faster operating speeds, lower energy requirements, greater endurance and greater potential for scaling. Of all the emerging technologies, resistive RAM (RRAM) elements, in which reproducible (switchable) and distinct high and low resistance states are the basis of memory storage, are considered most advantageous due to their superior potential for scaling, fastest exhibited operating speeds and extremely low energy requirements. Despite progress in the field of RRAM research, the underlying mechanisms that allow a device to switch between high and low resistance states remains unclear in many materials systems and is the key motivation behind this work. Here, Pulsed Laser Deposited (PLD) RRAM devices that incorporate resistive switching transition metal oxide thin films were studied using Electron Energy Loss Spectroscopy (EELS). Basic metal/oxide/metal RRAM heterostructures that incorporated strongly oxidising titanium electrodes and polycrystalline ZnO and manganese-doped ZnO were investigated in Chapter 3. These devices were designed for direct comparison to a device in presented the literature which displayed the simultaneous co-switching of resistance and magnetisation states. In the devices fabricated here, EELS analysis revealed Mn-phase segregation both at grain boundaries both above and below the top and bottom electrodes, which supported the proposed co-switching mechanism. In Chapter 4, epitaxial single crystal perovskite oxide Pr0.48Ca0.52MnO3 was incorporated into a novel metal/oxide/tunnel-oxide/metal RRAM structure, where the thickness of the interfacial Yttria-stabilised Zirconia tunnel oxide varied the output current density. In both the ZnO and Pr0.48Ca0.52MnO3 devices, EELS analysis revealed that the observed resistive switching was mediated by the field-induced exchange of oxygen vacancies between the bulk oxide and an interfacial oxide. Despite this similarity, the overall device resistance was governed by different effects: for the polycrystalline ZnO-based devices, this was the oxygen-vacancy induced formation and dissolution of a highly resistive TiO2 interfacial layer; in contrast, for the epitaxial Pr0.48Ca0.52MnO3 device, this was the oxygen-vacancy induced charge accumulation and dissipation in the tunnel oxide, which modulated the tunnel barrier height.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

<p> This dissertation contains investigations based on Molecular Dynamics (MD) of a variety of systems, from small atomic clusters to polymers in solution and in their condensed phases. The overall research is divided in three parts. First, I tested a new thermostat in the literature on the thermal equilibration of a small cluster of Lennard-Jones (LJ) atoms. The proposed thermostat is a Hamiltonian thermostat based on a logarithmic oscillator with the outstanding property that the mean value of its kinetic energy is constant independent of the mass and energy. I inspected several weak-coupling interaction models between the LJ cluster and the logarithmic oscillator in 3D. In all cases I show that this coupling gives rise to a kinetic motion of the cluster center of mass without transferring kinetic energy to the interatomic vibrations. This is a failure of the published thermostat because the temperature of the cluster is mainly due to vibrations in small atomic clusters This logarithmic oscillator cannot be used to thermostat any atomic or molecular system, small or large. </p><p> The second part of the dissertation is the investigation of the inherent structure of the polymer polyethylene glycol (PEG) solvated in three different solvents: water, water with 4% ethanol, and ethyl acetate. PEG with molecular weight of 2000 Da (PEG₂₀₀₀) is a polymer with many applications from industrial manufacturing to medicine that in bulk is a paste. However, its structure in very dilute solutions deserved a thorough study, important for the onset of aggregation with other polymer chains. I introduced a modification to the GROMOS 54A7 force field parameters for modeling PEG₂₀₀₀ and ethyl acetate. Both force fields are new and have now been incorporated into the database of known residues in the molecular dynamics package Gromacs. This research required numerous high performance computing MD simulations in the ARGO cluster of GMU for systems with about 100,000 solvent molecules. My findings show that PEG₂₀₀₀ in water acquires a ball-like structure without encapsulating solvent molecules. In addition, no hydrogen bonds were formed. In water with 4% ethanol, PEG₂₀₀₀ acquires also a ball-like structure but the polymer ends fluctuate folding outward and onward, although the general shape is still a compact ball-like structure. </p><p> In contrast, PEG₂₀₀₀ in ethyl acetate is quite elongated, as a very flexible spaghetti that forms kinks that unfold to give rise to folds and kinks in other positions along the polymer length. The behavior resembles an ideal polymer in a &thetas; solvent. A Principal Component Analysis (PCA) of the minima composing the inherent structure evidences the presence of two distinct groups of ball-like structures of PEG₂₀₀₀ in water and water with 4% ethanol. These groups give a definite signature to the solvated structure of PEG₂₀₀₀ in these two solvents. In contrast, PCA reveals several groups of avoided states for PEG₂₀₀₀ in ethyl acetate that disqualify the possibility of being an ideal polymer in a &thetas; solvent. </p><p> The third part of the dissertation is a work in progress, where I investigate the condensed phase of PEG₂₀₀₀ and study the interface between the condensed phase and the three different solvents under study. With a strategy of combining NPT MD simulations at different temperatures and pressures, PEG₂₀₀₀ condensed phase displays the experimental density within a 1% discrepancy at 300 K and 1 atm. This is a very encouraging result on this ongoing project. </p><p>

The radiofrequency (rf) pulses used in NMR are subject to a number of imperfections, such as those resulting from the inhomogeneity of the rf field or an offset of the transmitter frequency from exact resonance. In spin-echo experiments, these imperfections yield spectra with reduced signal intensity and distorted phase. Composite pulses, which have tailored bandwidth properties with respect to experimental frequency parameters such as the rf field strength or resonance offset, offer a route to improving the amplitude of the spin-echo signal. However, the symmetry of the pulse sequence must be carefully considered to prevent the introduction of phase errors into the spin-echo signal. Here, composite pulses will be studied as a means to improving one of the most common techniques for 1H background suppression in MAS NMR, the ”Depth” sequence. Novel composite 180° pulses will be presented for this application and verified experimentally. The composite pulse Depth experiment yields spectra with improved amplitude of the 1H signals of interest, while successfully maintaining good suppression of background signals. Novel families of dual-compensated 180° composite pulses for I = 1/2 will also be designed for use in NMR spin-echo experiments. These pulses are capable of simultaneously compensating for resonance offset and rf inhomogeneity problems. Crucially, unlike many composite pulses that have been presented before, these new pulses have the correct symmetry properties to form a spin echo without phase distortion. Composite pulses have found wide usage in solution-state NMR, and although in principle the same pulses can be applied in solid-state NMR experiments, complications can arise under magic angle spinning (MAS). The effects of MAS on composite pulse performance will be explored both through computer simulations and 31P experiments. Finally, on a different theme, we will investigate spin-locking of half-integer quadrupolar nuclei in solids. Spin-locking is an important feature of many NMR experiments, yet the complex behaviour observed for quadrupolar nuclei is not fully understood. Using the theoretical model introduced by Ashbrook and Wimperis, we will investigate the far off-resonance case of spinlocking for I = 3/2 and I = 5/2 nuclei.

The idea is now well established that light possesses angular momentum and that this comes in two distinct forms, namely spin and orbital angular momentum which are associated with circular polarisation and helical phase fronts respectively. In this thesis, we explain that this is, in fact, a mere glimpse of a much larger picture: light possesses an infinite number of distinct angular momenta, the conservation of which in the strict absence of charge reflects the myriad rotational symmetries then inherent to Maxwell's equations. We recognise, moreover, that many of these angular momenta can be identified explicitly in light-matter interactions, which leads us in particular to identify new possibilities for the use of light to probe and manipulate chiral molecules.

a-tricalcium phosphate (a-TCP, Ca3(PO4)2) demonstrates both bioactive and resorbable characteristics. Substitution of SiO4 4- for PO4 3- in a-TCP (Si-a-TCP) is found to stabilize the structure at lower temperatures and improve mechanical (and possibly bioactive) properties. The mechanism of electroneutrality in the Si-a-TCP structure is not fully understood, though is thought to take place through the creation of O2- vacancies or through excess Ca2+. This study addresses some structural properties of a-TCP using 31P MAS NMR at intermediate B0 fields (11.7 T) and 43Ca DOR NMR at multiple fields (20.0 T, 14.1 T, 11.7 T), and via correlation of the measured 31P and 43Ca isotropic chemical shifts (Oiso) against calculated values obtained with GIPAW DFT methods using the CASTEP code. These results show that the structure has high short range order and clearly support the monoclinic P21/a (12 P site/18 Ca site) model. In contrast, solid state 31P MAS and 43Ca DOR NMR studies of Si-a-TCP demonstrate that significant disorder broadening is characteristic of these data, however the corresponding 29Si MAS NMR data affords reasonably resolved resonances, a low intensity Q2 resonance at 0iso {84.5 ppm, and multiple resonances in the range 0iso - {70 -{75 ppm, despite this shift range normally associated with Q0 speciation, 29Si refocused-INADEQUATE data shows that Q1 resonances can also exist in this region. 31P-29Si HETCOR data from these systems suggests that, despite the intrinsic disorder, explicit PO43- framework species can be associated with the different Q0/Q1 Si species, and while there is some dispersion of the silicon throughout the structure it is predominantly associated with a small number of P sites. DFT calculations for the Si-a-TCP system suggest that the more favourable mechanism for charge balance is Ca2+ excess, despite this, the 29Si NMR data is in greater agreement with the DFT calculations for the O2- mechanism, whereby two adjoining cation-anion-columns in the structure are bridged together by a Q1 unit , 29Si NMR data also suggests the existence of Ca2SiO4 and Ca3Si3O9 existing as solid solutions within the a-TCP structure.

We have developed simplified microfluidic droplet generators and employed them to fabricate anisotropic polymer particles and capsules in the size range of 100–500 μm. We used cheap and generally available materials and equipment to design and assemble microfluidic devices. All our devices were made of standard wall borosilicate capillaries (OD 1.0mm, ID 0.58mm), steel dispensing needles without bevel (30 G, 32 G), microscopy glass slides, fast-curing epoxy glue (Araldite-80805) and diamond scribe to process the glass. We designed four different geometries for each device, which can be separated for two groups: single and double droplet generators. The performance of the devices was validated using computational fluid dynamics and laboratory experiments. First of all, we tried to fabricate intricate single emulsion droplets and then moved on to double emulsion droplets. The range of the fabricated particles and capsules includes anisotropically-shaped amphiphilic polymer “microbuckets”, biphasic particles, capsules with various fillers and stimuli responsive polymer vesicles. To produce such objects we employed different functional monomers, for instance “clickable” glycidyl methacrylate or hydrophilic 2-hydroxyethyl methacrylate. We also utilized several chemical and physical phenomena such as internal phase separation, wettability or polymer chain cross-linking to tune the properties of the synthesized particles. We investigated properties of the above mentioned particles and capsules. For example, “microbuckets” which are hydrophilic at the exterior surface, but hydrophobic inside the cavity, were able to withdraw oil droplets from an aqueous phase and “arrest” them inside the cavity.