A numerical model of the steady state and alternating current behaviour of a solid-oxide fuel cell is presented to explore the possibilities to diagnose and identify degradation mechanisms in a minimally invasive way using impedance spectroscopy. This is the first report of an SOFC impedance model to incorporate degradation, as well as the first one to include the ribbed interconnect geometry, using a 2-D approximation. Simulated degradation modes include: electrode/electrolyte delamination, interconnect oxidation, interconnect/electrode interface detachment, and anode sulfur poisoning. Detailed electrode-level simulation replaces the traditional equivalent circuit approach, allowing the simulation of degradation mechanisms that alter the shape of the current path. The SOFC impedance results from calculating the cell response to a small oscillatory perturbation in potential. Starting from the general equations for mass and charge transport, and assuming isothermal and isobaric conditions, the system variables are decomposed into a steady-state component and a small perturbation around the operating point. On account of the small size of the imposed perturbation, the time dependence is eliminated, and the original equations are converted to a new linear, time independent, complex-valued system, which is very convenient from a numerical viewpoint. Geometrical and physical modifications of the model simulate the aforementioned degradation modes, causing variations in the impedance. The possibility to detect unique impedance signatures is discussed, along with a study of the impact of input parameter inaccuracies and parameter interaction on the presented results. Finally, a study of pairs of concurrent degradation modes reveals the method’s strengths and limitations in terms of its diagnosis capabilities. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

Part I of this dissertation is concerned with the problem of the magnetic field dependence of the surface impedance of superconductors, with particular reference to tin. In chapter 1 the predictions of the simple theory for the behaviour of the surface impedance for different frequencies and fieid configurations are described and compared with the results of previous experiments. Chapter 2 deals with the experimental side of the present work, where a frequency of 170 Mc/s was used, and in chapter 3 the results obtained are discussed. In chapter 4 the attempts which have been made to account theoretically for the experimental observations are reviewed. Part II of the dissertation is concerned with another problem in superconductivity, namely the behaviour of superconducting systems partitioned by thin barriers of substances which in bulk are not superconducting. The theory of such systems is developed, the consequences investigated in some detail and the present experimental situation reviewed.

As modern power systems operate closer to the limits due to load growth and financial imperatives, voltage stability becomes a more important issue and there have been more incidents caused by voltage collapse. For example, there have been 11 outages affecting more than 4000MW between 1984 and 2000 in North America [1]. In power systems, load voltages decrease as the supplied loads increase until the maximum power transfer point is reached. The voltage will collapse if the load is increased above this limit. Therefore, it is important to monitor the loadability of a system to avoid voltage collapse. The loadability of a system can be calculated when the Thevenin impedance is available as the maximum power transfer occurs when the Thevenin impedance and the load impedance are the same in magnitude. This thesis suggests a method to estimate the Thevenin impedance of a system. ABB corporation suggests the Voltage Stability Predictor (VIP) method to estimate the Thevenin impedance, but there are problems with this method and it is not gaining popularity in industry. In this thesis, a method is suggested to estimate the Thevenin impedance by taking advantage of the existance of negative sequence components in the system. The concept of this method has been proved mathematically. Simulations were performed on simple systems and on the modified IEEE 13 bus power flow test case to verify the feasibility of the method and the results are promising. Then, the method was verified with field measurements for a 25kV substation. The voltages and currents were analyzed to estimate the Thevenin equivalent impedance of the power system and the results were compared with the design Thevenin equivalent impedance. The result confirms the viability of the method as the estimated Thevenin impedance matched the design value. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

No description available.

EIT is a method of imaging that exists for a century, initially in geophysics and in recent years in medical imaging. Even though the practical applications of EIT go back to the early 20th century the systematic study of the inverse conductivity problem started in the late 1970s, hence many aspects of the problem remain unexplored. In the study of the inverse conductivity problem usually Finite Element Models are used since they can be easily adapted for bodies of irregular shapes. In this work though we use an equivalent approximation, the electrical resistor network, for which many uniqueness results as well as reconstruction algorithms exist. Furthermore resistor networks are important for EIT since they are used to provide convenient stable test loads or phantoms for EIT systems. In this thesis we study the transfer resistance matrix of a resistor network that is derived from n-port theory and review necessary and sufficient conditions for a matrix to be the transfer resistance of a planar network. The so called “paramountcy” condition may be useful for validation purposes since it provides the means to locate problematic electrodes. In the study of resistor networks in relation to inverse problems it is of a great importance to know which resistor networks correspond to some Finite Element Model. To give a partial answer to this we use the dual graph of a resistor network and we represent the voltage by the logarithm of the circle radius. This representation in combination with Duffin’s non-linear resistor network theory provides the means to show that a non-linear resistor network can be embedded uniquely in a Euclidean space under certain conditions. This is where the novelty of this work lies.

>Magister Scientiae - MSc / Velocity modelling forms an integral part of the seismic interpretation process initially completed in two-way time. In order for a representative depth conversion, it is obligatory to construct a velocity model that serves the bridge between velocity and respective two-way time. This study deals with the investigation of subsurface heterogeneities and its impact on the variation of velocities. Interpretation of time domain reflection data results in one or more seismic horizons, however these horizons should represent the variation in subsurface geology as a result of acoustically different layers displaying varying reflection amplitudes. The purpose of this study was fulfilled by examining the variation of these velocities in relation to the geology and its significance towards building a velocity model. It is evident that complexities, such as an existing heterogeneous subsurface is present in the study area. Using velocities only considered at formation well tops, as a result, does not completely honour the variation in these velocities. The velocity profile as calculated from the sonic log was characterized into zones representing unique velocity trends. The analyses to understand the impact of subsurface heterogeneities on the velocities was completed by the application of seismic facies analysis which entailed the study of the seismic reflector patterns and amplitudes; a study of the lithologies present and the generation of mineral plots using available wireline logs, all of which in close relation to the variation in velocities. The characterized zones, as a result have shown that shaly sediments are typically associated with higher velocities (~2800 – 4600m/s) compared to sandstones of lower densities. Mineral plots however, have also indicated that where quartz minerals were present (specifically zone L), sandstones as a result have shown higher velocities (~4800m/s) as compared to the shales (~3600m/s). These higher velocities are also associated with more organised seismic reflectors with brighter amplitudes and strong contrasts in acoustic impedance as shown by the seismic. Uniform velocities were observed in zones such as zone Ia, typically associated with a low acoustic impedance contrast and minimal variation in its lithological make-up. The integrated investigation of subsurface heterogeneities has shown that velocities vary to a substantial degree as a result of existing subsurface heterogeneities. The variation of these velocities are hence significant enough that it should be considered when constructing a velocity model which aims to respect the geology of the study area. The result of understanding the relation between the geology and resultant velocities may prove to advance the results of the velocity model in a manner that it is more complete and representative of the subsurface.

Per-Fluoro-Sulphonic-Acid (PFSA) ionomers have been singled out as the preferable ionomers for making the Polymer Electrolyte Membrane Fuel Cells (PEMFC) membranes owing to their extensive intrinsic chemical stability and super sulfonic acid strength which is core to the PEMFC proton conductivity. This thesis presents a deeper analysis into these PFSA ionomer membrane electrode assemblies (MEA), presenting an electrochemical-analytical comparative analysis of the two basic types, which are the Long-Side-Chain (LSC) Nafion® and the ShortSide-Chain (SSC) Aquivion® ionomer MEA with emphasis on performance and durability which are currently not well understood. In particular, electrochemical circuit models and semiempirical models were employed to enable distinguishable comparative analysis. Also, in this thesis, we present a further probe into the effect of ionomer ink making processes, critically investigating the effect of the High Share Dispersion (HSD) process on both the Nafion® and Aquivion® ionomer membrane electrode assembly (MEA). The findings in this research provides a valuable insight into the performance and durability of PFSA ionomer membrane under various application criteria. The effect of operating parameters and accelerated stress testing (AST) on the PFSA ionomers was determined using electrochemical impedance spectroscopy (EIS) and electronic circuit model (ECM) analysis. The result of this study, shows that the ionomer ink making process for Nafion® and Aquivion® MEAs are not transferrable. Analysis of the PEMFC performance upon application of the high shear dispersion (HSD) process showed that Nafion® MEA had a 10.47% increase in voltage while the Aquivion® MEA had a 2.53% decrease in voltage at current density of 1.14A/cm2 . Also, upon accelerated stress testing, the Nafion® showed a 10.49% increase in its voltage while the Aquivion® on the other hand had a 7.16% decrease in voltage at 0.66A/cm2 . Thus indicating the HSD process enhances the performance of the Nafion® MEA and inhibits the performance of the Aquivion® MEA.

This master´s thesis deals about a measuring structures of elements working at a current mode. There were outlined structures for measuring input and output impedance. These structures were simulated for DACA element. It deals with a voltage to current and current to voltage converters. All structures and converters were simulated at computer program PSpice. Selected variant were practically verified and control measuring was realized.

ackground: The changes in body composition markers (weight, fat mass, lean mass, and BMI) over time can be associated with TB treatment outcome among HIV-infected patients. The aim of this study was to investigate whether changes in fat mass and lean mass were associated with the treatment response among patients with HIV infection and pulmonary tuberculosis. Materials and Methods: This was a prospective cohort study. Data from HIV-infected patients commencing TB therapy were analyzed. This included body weight measurement using bioimpedance equipment at baseline, one month, and two months after starting TB treatment. Results: The study was conducted in 125 patients, 17 patients (13.6%) died during treatment, of which 5 died during the first month of treatment, 4 during the second month and 8 after the second month. The group of patients with good response, increased their weight by 1.3 kg (p <0.001) at the end of the first month of TB treatment and 2.6 kg in the second month (p <0.001), and body fat increase was 1.2 Kg (p <0.001) and 2.3 kg (p <0.001), the first and second month respectively. The group of patients who died had lost 2.1 kg fat mass after the first month (p <0.001) and 3.7 kg in the second month (p <0.001). Conclusions: Our results show that the weight change during TB treatment (increased fat mass) helps us predict therapeutic response. Weight loss during the first month of starting therapy should be evaluated thoroughly to identify the probable cause of treatment failure. / Revisión por pares

Fiber-reinforced composite (FRC) materials are ideal for the aerospace and automotive industries which require high-strength structures with exceptional specific properties. The unfortunate reality is composite materials are susceptible to complex failure modes and difficult-to-predict damage growth as a result of their heterogeneity and anisotropy. Thus, robust structural health monitoring (SHM) for in-operation tracking of damage formation and accumulation is important for these materials. Self-sensing materials are a strong candidate to replace traditional composite SHM because they do not suffer from the disadvantages of point-based sensing. The piezoresistive effect in nanofiller-modified materials is a common approach to material self-sensing. Research to date in piezoresistivity has predominantly focused on the direct current (DC) response of such materials. This is an important limitation because alternating current (AC) has important advantages – it inherently possesses more information (AC data can relate both impedance magnitude and phase to damage), AC effects can be leveraged for improved damage sensitivity, and AC interrogation can reduce power requirements. Therefore, to develop knowledge that will facilitate the transition to AC, this work explores the effect of high-cycle<br>fatigue loading on the AC response of carbon nanofiber (CNF)-modified glass fiber/epoxy laminates. In this study, impedance magnitude and phase angle are measured along the length and through the thickness of composite specimens with an open-hole stress concentration<br>subjected to tension fatigue-loading up to 10 MHz. The collected impedance data is fit to an equivalent circuit model as a function of cycle. These results show that high-cycle fatigue loading does indeed have an appreciable effect on the equivalent circuit behavior of the material. However, clear and definitive trends were not observed thereby suggesting that further research is needed into the basic mechanisms of AC transport in nanocomposites if frequency-dependent transport is to be used to track fatigue loading. <br>