Thesis (Ph.D.)--Virginia Commonwealth University, 2008. / Prepared for: Dept. of Pharmaceutics. Title from thesis description page. Includes bibliographical references.

Thesis (Ph.D.(Chemical Engineering))--University of Pretoria, 2003. / Includes summary in English. Includes bibliographical references.

Single Point Diamond Turning (SPDT) is a technology widely applied for the fabrication of contact lenses. One of the limiting factors in polymer machining is wear of the diamond tool due to electrostatic discharge resulting in poor surface quality of the machined products. The research work presented in this dissertation highlights the electrostatic properties of contact lenses during machining operations and the effects these properties have on the surface quality of the work piece materials. Two contact lens samples were experimented on, Definitive 74 (Silicone Hydrogel) and Tyro 97 (Rigid Gas Permeable). The electrostatic surface potentials (ESPs) were measured during turning operations using an electrostatic voltmeter and the surface roughness measurements were taken using a surface profilometer. Response Surface Methodology (RSM) techniques were employed to create predictive models for both surface roughness and ESPs with respect to the cutting speed, feed rate and depth of cut. Predictive surface roughness models were successfully generated for both materials and the cutting speed and feed rate were identified as the parameters with most effect on surface roughness. In addition, an electrostatic model was successfully generated for the Definitive 74 contact lens material which cited the cutting speed and feed rate as the most effective parameters on the material’s electrostatic behaviour. However, no relationship was evident between the machining parameters and electrostatic behaviour of Tyro 97.

Dans un système nano-électronique quantique, l'énergie électrostatique représente souvent la plus grand échelle d'énergie. Pourtant, dans les travaux théoriques ou les simulations quantiques, l'environnement électrostatique est tout aussi souvent considérée comme un potentiel externe, ce qui peut conduire à une mauvaise représentation de la physique. Le développement d'outils numériques capables de traiter correctement l'électrostatique et son interaction avec la mécanique quantique est d'une importance capitale pour la compréhension des dispositifs quantiques, pax exemple dans les matériaux semi-conducteurs ou le graphène.Cette thèse est consacrée au problème de la physique quantique et électrostatique autocohérente. Ce problème (également connu sous le nom de Poisson-Schr"odinger") est notoirement difficile dans des situations où la densité des états varie rapidement avec l'énergie. A basse température, ces fluctuations rendent le problème hautement non linéaire, ce qui rend les schémas itératifs profondément instables. Dans cette thèse, nous présentons un algorithme stable qui apporte une solution à ce problème avec une précision contrôlée. La technique est intrinsèquement convergente, y compris dans les régimes très non linéaires. Il fournit ainsi une voie viable pour la modélisation prédictive des propriétés de transport des dispositifs de nanoélectronique quantique.Nous illustrons notre approche par un calcul de la conductance différentielle d'un point de contact quantique.Nous réexaminons également le problème des bandes compressibles et incompressibles dans le régime de l'effet Hall quantique entier. Nos calculs révèlent l'existence d'une nouvelle phase "hybride" pour les champ magnétiques intermédiaires, qui sépare la phase à faible champ des bandes (in)compressibles à champ élevé.Dans une deuxième partie, nous construisons une théorie qui décrit la propagation des excitations collectives (plasmons) qui peuvent être excitées dans des gaz électroniques bidimensionnels. Notre théorie, qui se réduit au liquide de Luttinger en une dimension, peut être directement reliée au problème électrostatique quantique microscopique, ce qui nous permet de faire des prédictions sans aucun paramètre libre. Nous discutons des expériences récemment faites à Grenoble, qui visent à démontrer la faisabilité de bits quantiques volants. Nous constatons que notre théorie concorde quantitativement avec les données expérimentales. / Electrostatic energy is very often the largest energy scale in quantum nanoelectronic systems. Yet, in theoretical work or numerical simulations, the electrostatic landscape is equally often taken for granted as an external potential, which may result in a wrong physical picture. Developing numerical tools that can properly handle the electrostatics and its interplay with quantum mechanics is of utter importance for the understanding of quantum devices in e.g. semi-conducting or graphene like materials.This thesis is devoted to the self-consistent quantum-electrostatic problem. This problem (also known as Poisson-Schr"odinger) is notoriously difficult in situations where the density of states varies rapidly with energy. At low temperatures, these fluctuations make the problem highly non-linear which renders iterative schemes deeply unstable. In this thesis, we present a stable algorithm that provides a solution to this problem with controlled accuracy. The technique is intrinsically convergent including in highly non-linear regimes. Thus, it provides a viable route for the predictive modeling of the transport properties of quantum nanoelectronics devices.We illustrate our approach with a calculation of the differential conductance of a quantum point contact geometry.We also revisit the problem of the compressible and incompressible stripes in the integer quantum Hall regime. Our calculations reveal the existence of a new ”hybrid” phase at intermediate magnetic field that separate the low field phase from the high field stripes.In a second part we construct a theory that describes the propagation of the collective excitations (plasmons) that can be excited in two-dimensional electron gases. Our theory, which reduces to Luttinger liquid in one dimension can be directly connected to the microscopic quantum-electrostatic problem enabling us to make predictions free of any free parameters. We discuss recent experiments made in Grenoble that aim at demonstrating electronic flying quantum bits. We find that our theory agrees quantitatively with the experimental data.

The triboelectrification of particles by contact or frictional charging is known to be an operational challenge in the polyolefin industry. Particularly in polyethylene production, gas-solid fluidized bed reactors are known to be susceptible to electrostatic charging due to the rigorous mixing of polyethylene and catalyst particles in a dry environment. The presence of charged particles coupled with a highly exothermic polymerization reaction results in sheet formation on the reactor walls. This behaviour can decrease reactor performance and obstruct the system, consequently forcing a shutdown for reactor maintenance. The generation of electrostatic charge in fluidized beds has been widely studied throughout the years; however, limited attention has been paid to the simulation and modeling of this phenomenon. Since it is difficult to accurately quantify the charge generation in industrial fluidized beds, developing an electrostatic model based on material properties would considerably aid in providing insight on this occurrence and its effects. A computational fluid dynamics (CFD) model that incorporates this electrostatic model can then be used as a predictive tool in research and development. Simulating electrostatic charging in gas-solid fluidized beds would be a cost-effective alternative to running experiments on them, especially for industrial-scale test runs. In this thesis, an electrostatic charging model was developed to be used in conjunction with an Euler-Euler Two-Fluid CFD model to simulate triboelectrification and its effects in gas-solid flows. The electrostatic model was first established for mono-dispersed gas-particle flows and was validated using past experimental findings of particle charging for gas-solid fluidization runs. With the goal of providing a realistic representation of gas-solid fluidization of polyethylene resins with a wide particle-size distribution, the electrostatic model was extended to consider bi-dispersed particulate flow systems. Simulation results using this model show the prediction of bipolar charging when the particles have different sizes, even though they are made of the same material. This phenomenon is analyzed and is shown to be driven by the electric field produced by the charge accumulated on the particles. Experimental studies of particle-wall and particle-particle contact charging were performed to investigate the electrostatic and mechanical parameters that are crucial for modeling the magnitude and direction of charge transfer in gas-solid flow systems. Particle-wall contact charging due to single and repeated collisions were tested with various particles, including commercial linear low-density polyethylene, to determine their rates of charging as well as their charge saturation limits when colliding with a metal surface. Plotting the charge saturation value of the particles against their respective surface areas revealed a linear trend which could be used to calculate the charge saturation of the particle for a given particle size. Additional particle-wall charging studies include the effect of initial charge, collision frequency, particle type, impact angle, impact velocity and the presence of impurities on particle charging. To study particle-particle contact charging, a novel apparatus was designed, built, and tested to determine the magnitude and direction of charge transfer due to the individual particle-particle collisions of insulator particles. This apparatus was the first of its kind, and it ensured that the measured charge transfer for each experimental trial was solely due to the binary collision between the particles. It was observed that the direction of charge transfer in identical particle collisions is not dictated by the net initial charges of the particles, but the localized charge difference at the particles’ contacting surface. Moreover, particle-particle collisions of nylon particles of varying sizes confirmed the bipolar charging phenomena, where the direction of charging was dictated by the relative size of the colliding particles. These findings, among others, contradict the charge transfer behavior predicted by electrostatic charging models currently proposed for particle-particle collisions. As such, it was concluded that an empirically accurate charge transfer model needs to be established to simulate the electrostatic charging of particles in poly-dispersed gas-solid flow systems.

The ever-increasing demand for superior devices with a smaller footprint in electronics calls for research on novel materials as a potential replacement of or integration to the existing silicon-based technology. The emergence of two-dimensional semiconductors paved a promising path in this direction. Easy isolation of atomically thin and flat layers with dangling bond free surfaces enables these materials to not only form 2D vertical heterostructures with novel properties but also facilitates advanced transistor, diode, and tunnel-device design with characteristics such as unprecedented gate-control of the channel, extremely high mobility of charge carriers, high current density, and high on-off ratios. However, like any other technology at the early development phase, 2D semiconductor research also faces numerous challenges which are needed to be addressed. In this work, we address two such challenges in the field–modeling of vertical electrostatics in these complex novel devices which enables better understanding and prediction of their characteristics and overcoming the contact resistance issue in a promising 2D semiconductor, WSe2, which enables the advancement of these devices towards near-deal characteristics. To predict and analyze the electrical characteristics of 2D vertical heterostructures, we need to develop solid understanding of the potential landscape, charge distribution, and energy band diagrams in these devices. Conventional modeling approaches and simulation tools that have been used so far to simulate the transport characteristics obscure our intuition as the devices get more arbitrary and complex. Here, we developed a circuit equivalent model to simulate the vertical electrostatics in these novel and arbitrary heterostructures in a simple and intuitive manner. In our model, all the parameters of the energy band diagram are represented by equivalent circuit elements involving capacitors and voltage sources. We also provide an elegant approach to solve these circuits by using Gauss law in electrostatics and charge-neutrality conditions in quasi-equilibrium. With a computationally efficient algorithm developed to solve these structures, we further built an opensource tool 2dmatstack on nanohub.org that enables researchers to predict and analyze the characteristics of novel heterostructures to maximize research output. In the next section, we focus on a major bottleneck in realizing these vertical devices experimentally. Fermi-level pinning and process-induced surface damage cause large Schottky barriers between metal contacts and these ultrathin 2D semiconducting layers resulting in large contact resistance and poor, non-ideal device performance. The solution to this problem is much more developed in the most widely studied n-type candidate, MoS2, compared to the common the p-type candidate, WSe2. In this work, we develop a UV-ozone-based oxidation technique that transforms the top layer of WSe2 into a nonstoichiometric oxide, TOS, that degenerately dopes the layers underneath p-type. This high hole-doping decreases the Schottky barrier width at the contacts and has resulted in the lowest p-type contact resistance to ultrathin WSe2 reported thus far. We show that this doping is stable in the ambient, remains active at low temperatures, repeatable, robust, and area selective for contact-doping without altering the channel properties. The high-performance ohmic contacts we demonstrate not only sets us in the path to realize near-ideal channel-dominated devices but also is pivotal to understand these devices better by eliminating the effect of contacts from the gate-controlled channel characteristics.

This work posits Layered Assembly as a novel, additive manufacturing method which usesvoxels as feedstock to fabricate multi-material objects at order-of-magnitude faster build rates than established additive manufacturing methods. Instead of using resins, filaments, and powders as raw materials, Layered Assembly uses premanufactured bits of matter called voxels, to fabricate truly multi-material, multi-functional parts. The implementation of Layered Assembly in this work is carried out by parallel electrostatic grippers. Electrostatic grippers are chosen as the gripping mechanism as they are solid-state, material-agnostic, adept at grasping millimeter-scale parts, and parallelize well to enable scalable high deposition rates. Most importantly, electrostatic grippers can apply localized electrostatic fields which results in highly selective grasping capability at the millimeter and sub-millimeter scale. The parallel gripping capabilities of electrostatic grippers were characterized for gripping repeatability, and then demonstrated by the fabrication of increasingly complex multi-material parts. Fabricated parts include a letter “C” comprised of 8 voxels, an 18 voxel pyramids and two parts comprised of tens of thousands of voxels. Experiments determined a > 95% gripping reliability independent of array size. The experiments in work have shown parallel electrostatic grippers to be a promising method for both material deposition and parallel pick-and-place manipulation.

There are a number of applications, from hearing aids to microfluidic pumps, which utilize micro-scale actuating diaphragms. These MEMS (micro-electromechanical system) devices can be actuated by electrostatic forces, which utilize an induced electric field to pull two charged plates towards one another. Such devices were fabricated and electrostatic actuation of the diaphragms was performed to analyze its viability as a micro-speaker. The long-term performance of such products requires adequate diaphragm deflection to create audible pressure waves with relatively low maximum stresses to ensure a high cycle fatigue life. With these requirements, initial calculations and FEA (finite element analysis) were performed to establish the optimal square diaphragm side length combined with an attainable gap between electrodes to achieve an audible response. Optical and acoustic testing was then performed on 4, 5, and 7 mm side length square diaphragms with 10 μm thickness and a 70 μm electrode gap. For the 5 mm device and a 300 V applied potential, deflection was calculated to be 4.12 μm theoretically and 3.82 μm using FEA, although deflections based on optical test data averaged 30.53μm under DC conditions. The DAQ used for optical testing was extremely limiting due to its fastest sampling interval of 89 milliseconds, so this testing was performed at 2 and 5 Hz. Although the 7 mm device generated audible noise at 300 V and 2 kHz when the observer was within approximately 6 inches of the device, acoustic testing with a microphone placed 1 inch from the device did not yield any definitive results.

While the search for a practical fusion energy source has been pursued for decades, ultimate success remains elusive, and the need is apparent for continued research into alternative experimental fusion techniques. A method that has received fair attention over the past few years is Inertial Electrostatic Confinement (IEC) fusion, a system in which a convergent ion focus is obtained solely through electrostatic fields. During device operation where the electrostatic field is supported by very high voltages (over 20,000V), the resulting ion focus sustains a dense plasma capable of generating a high rate of fusion reactions and neutron production. However, present limited theoretical knowledge and engineering issues prevent the capacity ofIEC systems to produce net power. Rather, intermediate applications center on using the device as a portable neutron source. The work performed here involves an IEC device using spherical geometry, and includes the introduction of a new variable, the addition of high frequency electromagnetic (EM) fields into its operation. The goal is to predict theoretically, observe and explain any beneficial compressive effects of added microwave EM fields on the potential fusion reaction rate in an experimental reactor. An optical determination of the enhancement is performed while using lower voltages under 1 000V, and is based on the relative intensity of the confined plasma during glow discharge operation. It is hypothesized that imposing EM fields will enhance the potential fusion reaction rate conservatively by a factor of two. Presently, IEC systems operate just under neutron production levels needed for practical applications such as landmine detection and medical isotope production, and any enhancement from EM fields should prove highly beneficial.

Metal - metal and to an extent metal - insulator contact or triboelectric charging are well known phenomena with good theoretical understanding of the charge exchange mechanism. However, insulator – insulator charging is not as well understood. Theoretical and experimental research has been performed that shows that the surface charge on an insulator after triboelectric charging with another insulator is rapidly dissipated with lowered atmospheric pressure. This pressure discharge is consistent with surface ions being evaporated off the surface once their vapor pressure falls below the saturation vapor pressure. A two-phase equilibrium model based on an ideal gas of singly charged ions in equilibrium with a submonolayer adsorbed film was developed to describe the pressure dependence of the surface charge on an insulator. The resulting charge density equation is an electrostatic version of the Langmuir isotherm for adsorbed surface particles, which describes well the experimental observations.