A numerical tool has been developed for particle-level simulations of fibre suspension flows, particularly forming of the fibre network structure of paper sheets in the paper machine. The model considers inert fibres of various equilibrium shapes, and finite stiffness, interacting with each other through normal, frictional, and lubrication forces, and with the surrounding fluid medium through hydrodynamic forces. Fibre–fluid interactions in the non-creeping flow regime are taken into account, and the two-way coupling between the solids and the fluid phases is included by enforcing momentum conservation between phases. The incompressible three-dimensional Navier–Stokes equations are employed tomodel themotion of the fluid medium. The validity of the model has been tested by comparing simulation results with experimental data from the literature. It was demonstrated that the model predicts well the motion of isolated fibres in shear flow over a wide range of fibre flexibilities. It was also shown that the model predicts details of the orientation distribution of   multiple, straight, rigid fibres in a sheared suspension. Furthermore, model predictions of the shear viscosity and first normal stress difference were in fair agreement with experimental data found in the literature. Since the model is based solely on first principles physics, quantitative predictions could be made without any parameter fitting.   Based on these validations, a series of simulations have been performed to investigate the basic mechanisms responsible for the development of the stress tensor components for monodispersed, non-Brownian fibres suspended in a Newtonian fluid in shear flow. The effects of fibre aspect ratio, concentration, and inter-particle friction, as well as the tendency of fibre agglomeration, were examined in the nonconcentrated regimes. For the case of well dispersed suspensions, semi-empirical relationships were found between the aforementioned fibre suspension properties, and the steady state apparent shear viscosity, and the first/second normal stress differences.   Finally, simulations have been conducted for the development of paper structures in the forming section of the paper machine. The conditions used for the simulations were retrieved from pilot-scale forming trial data in the literature, and from real pulp fibre analyses. Dewatering was simulated by moving two forming fabrics toward each other through a fibre suspension. Effects of the jet-to-wire speed difference on the fibre orientation anisotropy, the mass density distribution, and three-dimensionality of the fibre network, were investigated. Simulation results showed that the model captures well the essential features of the forming effects on these paper structure parameters, and also posed newquestions on the conventional wisdom of the forming mechanics.

Electrochromic iridium oxide (IrOx) and iridium-tantalum oxide (IrTaOx) thin films were prepared by reactive magnetron sputtering. Composition, density, and structure were determined using Rutherford backscattering spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and electron energy loss spectroscopy. The electronic density of states (DOS) and the solid phase chemical diffusion coefficient (D) were determined for hydrogen in IrOx and IrTaOx by potentiostatic intermittent titration technique (PITT), and electrochemical impedance spectroscopy (EIS). The complex refractive indices were determined for colored and bleached IrOx and IrTaOx by inverting transmission and reflectance, measured using spectrophotometry in the 300-2500 nm wavelength range. A very porous structure, with a stoichiometry of IrO2.2, was found for IrOx. It contained ~4 nm sized grains. The IrTaOx had a denser structure built up by ~4 nm sized grains. The composition of IrTaOx was found to vary on a nanometer scale, with an average composition of IrTa1.4O5.6. It was found that DOS can be measured using PITT and EIS in the presence of spontaneous side reactions, even for systems influenced by non-negligible charge transfer kinetics and Ohmic drops. It was found that the measured DOS is 30-50% of the theoretically calculated DOS and that D is in the 10-10 – 10-11 cm2/s range for both materials. The hydrogen diffusion mechanism was described by an anomalous diffusion model, possibly indicating percolation or diffusion paths described by a fractal network. The refractive indices were found to be ~1.3 and ~2 for IrOx and IrTaOx, respectively, and independent of coloration state, whereas the extinction coefficients were found to modulate by ~30% for IrOx and ~50% for IrTaOx, making IrTaOx more favorable for electrochromic applications. A modulation peak was found at ~660 nm for both IrOx and IrTaOx associated with the removal of intraband transitions within the Ir t2g band.

This thesis presents actuators used in autonomous microsystems. Characteristic for all actuators presented is the low drive voltage and the low power consumption. Different motion mechanisms have been studied and applied in various locomotion modules for microrobots. High resolution movement of a monolithic piezoceramic PZT rotational arm module, using a quasi-static motion mechanism, was demonstrated in a 10x10x20 mm3 autonomous robot. The rotational arm comprises multilayer PZT bimorphs and is fabricated by a wet-building technology. The multilayer approach enables operation of the modules at the low drive voltages provided by the robot electronics. In addition a locomotion module has been designed and fabricated based on the above principles. A three-legged locomotion module with piezoceramic unimorphs, moving by tapping the legs against the floor, has been investigated. Characteristics such as low power consumption, high velocities, low drive voltages and a high weight carrying capability were demonstrated using a resonant motion mechanism. Highly miniaturized three-legged locomotion modules were developed for a 3x3x3 mm3 autonomous microrobot. The modules comprise a multilayer structure of the electroactive copolymer P(VDF-TrFE) on a flexible printed circuit board (FPC) substrate. A novel multilayer fabrication process suitable for mass production was used. It is based on sequential deposition of spun cast copolymer with evaporated aluminum electrodes. Reactive ion etching is used to microstructure the copolymer and the FPC. The mechanical deformability of the FPC is exploited when folding the 2D FPC-multilayer assembly into 3D locomotion modules. Locomotion was demonstrated by moving a glass slider corresponding to the robot weight. A modular building technology for microsystems is presented. It uses surface mounting technology and conductive adhesives to assemble modules on a double-sided FPC. Complex geometries were achieved by subsequent folding the FPC. The feasibility of the technology was demonstrated by assembly of the 3x3x3 mm3 autonomous microrobots.

This thesis deals with the physical modeling of the initiation and propagation of upward positive leader discharges from grounded structures during lightning strikes. It includes the analysis of upward leaders initiated under the influence of the electric field produced by a dominant negative cloud charge and due to the combined action of a negative thundercloud and a descending downward stepped negative leader. Thus, a self-consistent model based on the physics of leader discharges is developed for the evaluation of the attachment of lightning flashes to any kind of grounded structure. The predictions of the model have been found to be in good agreement with the results of laboratory long air gap experiments and with classical and altitude rocket triggered lightning experiments. Due to the high application level and predictive power of the developed model, several contributions to the physical understanding of factors influencing the initiation and propagation of upward positive leaders during thunderstorms have been made. For instance, it has been found that the initiation of upward connecting leaders is strongly affected by the average velocity of the downward stepped leader. Similarly, it is shown that the switching voltage impulses used in the laboratory do not “fairly approximate” the electric fields produced by a descending downward leader, as claimed by supporters of Early Streamer Emission (ESE) devices. Furthermore, it is found that the space charge layer created by corona at ground level significantly increases the thundercloud electric fields required to initiate upward lightning leaders from tall objects. On the other hand, it is also shown that the upward leader velocity depends on the downward leader average velocity, the prospective return stroke current, the lateral distance of the downward leader channel and the ambient electric field. By implementing the model to the analysis of complex structures, it has been observed that the corners of actual buildings struck by lightning coincide rather well with the places characterized by low leader inception electric fields. Besides, it has been found that the leader inception zones of the corners of complex structures do not define symmetrical and circular regions as it is generally assumed.

This PhD thesis addresses the threat posed to society by sources that can produce high power electromagnetic pulses (HEPM) and be used maliciously to disturb or damage electronic equipment. The vulnerability from intentional electromagnetic interference (IEMI) has increased in the recent decades due to the widespread dependence of the civil society on sensitive electronic systems and proliferation of radiation sources. As the characteristics of the disturbances associated with IEMI often have very high frequency content, the existing mitigation measures and protection components may not be adequate. It was seen that for ultra wideband (UWB) transients low voltage protection components may not work as intended, due to parasitic components that arises from the packaging of the device. The large spatial distribution of many civilian facilities and critical infra-structures (e.g., power generation, communications, train system, etc.) presents many unexpected ports for an attacker as the majority of the parts of these systems are not protected or secure. As the new European Rail Traffic Management System (ERTMS) will utilize wireless communication for communication and control of the trains the vulnerability from different radiating HPEM sources was investigated. Angles of incidence and frequencies that are a threat in a given situation are identified. Due to the possibility of unexpected ports, the propagation of differential mode ultra wideband transients in low voltage power networks, when injected into a power socket of a facility, was studied. The effects on the transient propagation from cable bends, switches and junctions were studied, both in a laboratory setup and in the network of a facility. Also, as modern electronic equipment and systems may not be tested for waveforms and disturbances other than standardized EMC tests, experiments on some common commercial-off-the-shelf (COTS) equipment were performed with non-standard test situation. It was seen that these could easily be disturbed or even permanently damaged. In addition, due to the inherent difficulties with IEMI, a new method for classifying facilities from IEMI is suggested. It is based on available terminology of accessibility (A), susceptibility (S) and consequence (C), but expands these and forms the so called IEMI/ASC-cube.

This Thesis describes the development of an electron-impact microfocus x-ray source and its application for phase-contrast imaging. The source is based on a novel, liquid-jet target concept. Stable and continuous operation can be achieved at substantially higher electron-beam power densities than conventional solid target based systems. The maximum x-ray brightness can potentially be increased by a factor of 10-1000, which would provide significantly improved performance in applications such as imaging. In order to reach the high x-ray brightness, comparable performance from the electron gun is needed. A LaB6-cathode-based electron gun is analyzed in terms of achievable e-beam brightness and beam quality and is found capable to deliver power densities in the 10-100 MW/mm2 range using optimized electro-optics. A proof-of-principle microfocus source has been developed. Experiments show that the liquid-metal-jet target can be operated at more than an order of magnitude higher e-beam power densities than modern solid-metal targets. This brightness enhancement has been utilized to acquire in-line phase-contrast images of weakly absorbing objects. The source potentially enables the application of high-resolution phase-contrast x-ray imaging with short exposure times in clinics and laboratories. Different liquid-jet-target materials have been tested. The Sn-jet (Ka=25.3 keV) could be suitable for mammography, whereas the Ga-jet ((Ka=9.2 keV) may be utilized for x-ray diffraction studies. In addition, a non-metallic methanol jet has been the demonstrated in stable x-ray operation. All materials and compounds found in liquid form can, thus, potentially be used for electron-impact liquid-jet-target x-ray generation. Scaling to higher e-beam power density and x-ray brightness levels is discussed and is determined to be feasible. Potential difficulties, such as debris emission and instabilities of the x-ray emission spot, are investigated in some detail. Larger and/or faster jets could overcome the present limitations because of their inherently higher heat load capacities. Dynamic-similarity experiments show that liquid jets can in principle be operated in a stable manner at much higher speeds than previously shown. / QC 20100915

The superior characteristics of Silicon Carbide as a wide band gap semiconductor have motivated many industrial and non-industrial research groups to consider SiC for the next generations of high power semiconductor devices. The SiC Bipolar Junction Transistor (BJT) is one candidate for high power applications due to its low on-state power loss and fast switching capability. However, to compete with other switching devices such as Field Effect Transistors (FETs) or IGBTs, it is necessary for a power SiC BJT to provide a high current gain to reduce the power required from the drive circuit. In this thesis implantation free 4H-SiC BJTs with linearly graded base layer have been demonstrated with common-emitter current gain of 50 and open-base breakdown voltage of 2700 V. Also an efficient junction termination extension (JTE) with 80% of theoretical parallel-plane breakdown voltage was analyzed by fabrication of high voltage PiN diodes to achieve an optimum dose of remaining JTE charge. Surface passivation of 4H-SiC BJT is an essential factor for efficient power BJTs. Therefore different passivation techniques were compared and showed that around 60% higher maximum current gain can be achieved by a newsurface passivation layer with low interface trap density that consists of PECVD oxide followed by post-deposition oxide anneal in N2O ambient. This surface passivation along with doublezone JTE were used for fabrication of high power BJTs that result in successful demonstration of 2800 V breakdown voltage for small area (0.3 × 0.3 mm) and large area (1.8 × 1.8 mm) BJTs with a maximum dc current gain of 55 and 52, respectively. The small area BJT showed RON = 4mΩcm2, while for the large are BJT RON = 6.8 mΩcm2. Finally, a Darlington transistor with a maximum current gain of 2900 at room temperature and 640 at 200 °C is reported. The high current gain of the Darlington transistor is achieved by optimum design for the ratio of the active area of the driver BJT to the output BJT. / QC 20110216

Chemical gas sensors can be used in a variety of applications such as process control, security systems and medical diagnosis. In the research for new functions and new sensing materials a “breadboard” would be useful. A technique that has been investigated for such a purpose is the grid-gate device which is a metal-oxide-semiconductor (MOS) based gas sensor. It is a MOS capacitor consisting of a passive grid-gate with depositions of sensing materials overlapping the grid. The measuring is carried out with a light addressable method called scanning light pulse technique (SLPT) which enables the detection of spatially distributed gas response. A development of the grid-gate sensor would be to separate the sensing materials from the chip. In this thesis the aim was to see if this was possible by depositing the sensing material on a slide of micro pillars which was put on top of a biased grid-gate chip. The test was made with palladium depositions in an ambient of synthetic air and 2500 ppm hydrogen, and the measuring technique was SLPT as for the preceding device. The result of the test was that the new device showed a combined gas response of both charge content shift at flat-band voltage and at inversion voltages. The conclusion is therefore that the sensing material can be separated from the grid-gate chip and that the response will be caused by several mechanisms. The two-dimensional image response utilized for the preceding grid-gate device will instead be a multi-dimensional response consisting of the curve for the charge content shift at every measuring position.

Electric energy conversion is an important issue in today's society as our daily lives largely depend on the supplies of energy. Two energy sources are studied for conversion in the present thesis, ocean waves and hydropower. The work focuses on the generator and the transmission of its output to the electric grid. Different approaches have been used, over the years, to convert the energy in ocean waves, and the method presently used is based on a point absorber (buoy) directly coupled to a linear generator on the seabed. A varying alternating voltage is induced with such configuration, where both the amplitude and the frequency changes continuously. The target is to connect several units in a farm, and thereby decrease the fluctuations in power production. This is shown to be possible to accomplish with a rectifier connected to each generator. Transmission systems can be designed with converters and transformers to connect the farm to the electric grid onshore. Several aspects of the concept are considered as well as interconnection issues. Analytical calculations verified by finite element simulations and measured data are used to model the behaviour of a linear generator. A series expanded expression for the ideal no-load flux and EMF (electromotive force) is derived, which can be developed into an analytical transmission design tool. Hydropower has been used for more than a century. Today many of the stations from the mid 1900's are up for refurbishment. Studies with finite element calculations show that a higher electric efficiency can be obtained with a high voltage cable wound generator.

Spacecraft on interplanetary missions or advanced satellites orbiting the Earth all require propulsion systems to complete their missions. Introducing microelectromechanical systems technology to the space industry will not only reduce size and weight of the propulsion system, but can also increase the performance of the mission. Fluid handling systems are used in chemical and electric propulsion. Some components incorporated in a fluidic handling system are presented and evaluated in this work. Microsystems are very sensitive to contamination. Reliable, robust, and easily integrated filters were modeled, manufactured, and experimentally verified. A fluid connector, designed to withstand large temperature variations and aggressive propellants was manufactured and characterized. Similar designs was also be used as a thermally activated minute valve. The feasibility of a cold gas system for precise attitude control has been demonstrated. Steps towards improving the performance (from specific im-pulse 45 s) have been taken, by the integration of suspended heater elements. For electric propulsion, two thermally regulated flow restrictors have been characterized. These devices can fine-tune the propellant flow to e.g. an ion engine. A single-use valve using a soldered seal has also been successfully dem-onstrated within a pressure range of 5 to 100 bar. The microsystem-based propulsion systems of tomorrow’s spacecraft need to be demonstrated in space, in order to gain necessary credibility.