Nos processos industriais envolvendo eletrólise, como na produção de alumínio, cobre, zinco, níquel, manganês, carbeto de silício, cloro e seus derivados, as correntes envolvidas em geral atingem patamares que excedem o valor nominal de um único semicondutor, exigindo a associação de componentes em paralelo com uma distribuição de corrente equilibrada. Este trabalho apresenta uma análise comparativa da distribuição de corrente em associações de semicondutores em paralelo em vinte retificadores de alta potência com diferentes tipos de arquitetura física de barramentos. A particularidade deste estudo é a medição simultânea das correntes em todos os semicondutores de todos os ramos do retificador obtendo-se uma imagem da distribuição real de correntes do conversor. / The industrial electrolytic processes to produce, aluminum, cooper, zinc, nickel, manganese, silicon carbide, chlorine and its derivative, require current values which greatly exceed the capability of single rectifying devices and paralleled combinations of semiconductors or equipments are necessary. This study evaluated through a comparative analyzes twenty rectifiers, with different busbar geometries, in order to verify the current distribution in paralleled devices. The differential of this study is the simultaneous measurement of currents in all semiconductors of each branch in order to obtain a real representation of the rectifiers current distribution.

Nos processos industriais envolvendo eletrólise, como na produção de alumínio, cobre, zinco, níquel, manganês, carbeto de silício, cloro e seus derivados, as correntes envolvidas em geral atingem patamares que excedem o valor nominal de um único semicondutor, exigindo a associação de componentes em paralelo com uma distribuição de corrente equilibrada. Este trabalho apresenta uma análise comparativa da distribuição de corrente em associações de semicondutores em paralelo em vinte retificadores de alta potência com diferentes tipos de arquitetura física de barramentos. A particularidade deste estudo é a medição simultânea das correntes em todos os semicondutores de todos os ramos do retificador obtendo-se uma imagem da distribuição real de correntes do conversor. / The industrial electrolytic processes to produce, aluminum, cooper, zinc, nickel, manganese, silicon carbide, chlorine and its derivative, require current values which greatly exceed the capability of single rectifying devices and paralleled combinations of semiconductors or equipments are necessary. This study evaluated through a comparative analyzes twenty rectifiers, with different busbar geometries, in order to verify the current distribution in paralleled devices. The differential of this study is the simultaneous measurement of currents in all semiconductors of each branch in order to obtain a real representation of the rectifiers current distribution.

In polymer electrolyte membrane (PEM) fuel cells one of the most important components is the flow field. The flow field distributes reactant gasses to the active area and also delivers electrons from the outer circuit so that the electrochemical reaction may be completed. Optimizing flow field design is extremely important in order to increase the overall power density of the fuel cell. It is particularly important to understand the ways in which the different portions of the flow field, namely the land and channel sections, interact with the gas diffusion layer (GDL), catalyst layer and membrane; this study focuses on those interactions. The most common type of flow field design currently used in PEM fuel cells is the serpentine flow field. It is used for its simplicity of design, its effectiveness in distributing reactants and its water removal capabilities. The knowledge about where current density is higher, under the land or the channel, is critical for flow field design and optimization. Yet, no direct measurement data are available for serpentine flow fields. In this study a fuel cell with a single channel serpentine flow field is used to separately measure the current density under the land and channel, which is either catalyzed or insulated on the cathode. In this manner, a systematic study is conducted under a wide variety of conditions and a series of comparisons are made between land and channel current density. Results show that under most operating conditions, current density is higher under the land than that under the channel. However, at low voltage, a rapid drop off in current density occurs under the land due to concentration losses. The mechanisms for the direct measurement results and general guidelines for serpentine flow field design and optimizations are provided. In addition the same technique is utilized to separately measure current density under the land and channel on a variety of serpentine flow field geometries. Each flow field is tested under a wide variety of operating conditions thereby providing guidance for the optimum design geometry. Experimental results show that generally flow fields with both thinner lands and thinner channels provide better overall performance. However, the optimal flow field designs are highly dependent on fuel cell operating parameters. Finally, it is critical not only to know where the current density is greater, under the land or under the channel, but to understand the fundamental mechanisms driving these differences. Resistance was measured, ex-situ, between the GDE and flow plate under the land of the flow field and under the channel separately. The contact resistance between the gas diffusion electrode (GDE) and the graphite flow plate were measured using an ex-situ technique. The resistance was measured under different land and channel widths. Cyclic Voltammetry tests were also conducted in order to determine if there is any different in electrochemically active area(ECA) under the land and under the channel and what the cause of this difference might be. Results show that the compression of the gas diffusion electrode not only affects the electronic resistance but the ECA as well and that these are key factors in current density variations under the land and channel.

One of the world’s largest energy user is the building sector, where much of the energy goes to cooling of buildings. There is need for novel technology to reduce this energy usage, and one way is to install electrochromic windows. They have the ability to vary the transmittance of visible and infrared light by the application of a small electrical voltage, and hence to save large amounts of energy and money and to increase indoor comfort by avoiding strong glares from sunlight. This study concentrates on thin-film electrochromic devices that are based on flexible polyester foils. The conventional design is to make rectangular devices with contacts placed on transparent conductor layers opposite of each other. An electric potential is applied between the contacts, generating an electric field which causes ions to move between films of electrochromic active materials. Since there is an interest on the market for electrochromic windows of other geometries, such as triangular, there is a need to know how to place the contacts in order to obtain a rapid and uniform colouring and bleaching of the device. In order to investigate this, a mathematical model describing the current distribution over the device is a great tool. The model used in this study takes secondary current distribution into account, which includes ohmic effects and electrode kinetics, but neglects diffusive effects due to the assumption that the electrolyte is homogeneous. It describes the two dimensional ohmic flow through the transparent conductor films, the local current due to electrochemical effects in the electrochromic active materials, and a correlation between the optical properties and the injected charge over time. The model is simulated using FlexPDE, which solves the system of differential equations using a Finite Elements Method (FEM). To adjust model parameters, model simulation results are compared to experimental data from rectangular electrochromic devices. Initially, experiments are done on small are devices on which current distribution effects are small. The model is then further developed and validated using large area rectangular devices with 67 cm between the contacts. The model is shown to meet the aims of this study, which is to obtain a simulation tool which can predict the trend in the transmittance distribution. The strength of having this model at hands is that it becomes possible to simulate the transmittance behaviour over time for full size electrochromic windows of different geometries, without having to manufacture expensive devices for experiments. It provides a great design tool for optimizing a rapid and uniform colouring and bleaching, and to investigate how to reduce material costs without affecting performance too much. In this study an example which shows the strength of the model is given. The placement of contacts and its effect on the transmittance distribution in triangular electrochromic windows is examined. It shows that the current distribution model enables time-efficient and cheap design of electrochromic windows.

<p>To obtain the power levels required from high RF power transistors, the size of the chip has often to be made so large that inductance of electrical connections inside the package cannot be neglected. This may have the effect that various parts of the transistor chip are not connected exactly parallel, i.e. drain and gate voltages and currents densities will not be the same on different parts of the chip. This may result in degraded output power and efficiency. The same effect may occur when more than one chip are connected in parallel in a transistor package to obtain even higher output power.Often the connections to the transistor package are approximated as a number of electrical point connections (normally three: gate, drain, source); meaning that each of them can be described by a single electrical potential and current. In reality, they may be large enough that voltage and current distributions have to be considered. These distributions will be affected by different mountings of the transistor and other connected components.In this work, the LDMOS power transistor MRF6S21140HR3 was modeled using the segmentation method in high frequency signal simulation HFSS which is a 3D Full-Wave Electromagnetic Field Simulation, and utilized the advanced design system ADS to find a parameterized lumped model. Both the electromagnetic and lumped models showed consistent results. Non-ideal parallel connection of sub-transistors on chip is very important, but further studies are needed for definite conclusion. It was verified through modeling that non ideal parallel connection of different chips in the package does have an effect; the effect however is quiet small which proves that the signal is slightly non-uniformly distributed between the three chips in the package. External connection to PCB (drain connection is considered in this work) can effectively be taken as a point connection to some approximation. The electrical behavior of the modeled transistor was studied through the design of a class B power amplifier in order to estimate the importance of performance degradation due to non-ideal parallel connections and how these non ideal connections degrade efficiency and output power. The modeled transistor can deliver a maximum output power of 147 watts and efficiency of 65%. We have also studied the current distribution between the three chips in a three stage class B power amplifier. Again, the difference in the current distribution between the three chips turned out to be quiet small. All these results are presented through this work. The final conclusion regarding the current distribution between multichips cannot be made just based on these simulation results. The next step should be aimed at considering other effects, the thermal effect for example, in order to know exactly whether it is uniformly or not uniformly distributed.</p>

To obtain the power levels required from high RF power transistors, the size of the chip has often to be made so large that inductance of electrical connections inside the package cannot be neglected. This may have the effect that various parts of the transistor chip are not connected exactly parallel, i.e. drain and gate voltages and currents densities will not be the same on different parts of the chip. This may result in degraded output power and efficiency. The same effect may occur when more than one chip are connected in parallel in a transistor package to obtain even higher output power.Often the connections to the transistor package are approximated as a number of electrical point connections (normally three: gate, drain, source); meaning that each of them can be described by a single electrical potential and current. In reality, they may be large enough that voltage and current distributions have to be considered. These distributions will be affected by different mountings of the transistor and other connected components.In this work, the LDMOS power transistor MRF6S21140HR3 was modeled using the segmentation method in high frequency signal simulation HFSS which is a 3D Full-Wave Electromagnetic Field Simulation, and utilized the advanced design system ADS to find a parameterized lumped model. Both the electromagnetic and lumped models showed consistent results. Non-ideal parallel connection of sub-transistors on chip is very important, but further studies are needed for definite conclusion. It was verified through modeling that non ideal parallel connection of different chips in the package does have an effect; the effect however is quiet small which proves that the signal is slightly non-uniformly distributed between the three chips in the package. External connection to PCB (drain connection is considered in this work) can effectively be taken as a point connection to some approximation. The electrical behavior of the modeled transistor was studied through the design of a class B power amplifier in order to estimate the importance of performance degradation due to non-ideal parallel connections and how these non ideal connections degrade efficiency and output power. The modeled transistor can deliver a maximum output power of 147 watts and efficiency of 65%. We have also studied the current distribution between the three chips in a three stage class B power amplifier. Again, the difference in the current distribution between the three chips turned out to be quiet small. All these results are presented through this work. The final conclusion regarding the current distribution between multichips cannot be made just based on these simulation results. The next step should be aimed at considering other effects, the thermal effect for example, in order to know exactly whether it is uniformly or not uniformly distributed.

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The thesis presents a modeling framework for simulating three dimensional effects in lithium-ion batteries. This is particularly important for understanding the performance of large scale batteries used under high power conditions such as in hybrid electric vehicle applications. While 1D approximations may be sufficient for the smaller scale batteries used in cell phones and laptops, they are severely limited when scaled up to larger batteries, where significant 3D gradients can develop in concentration, current, temperature, and voltage. Understanding these 3D effects is critical for designing lithium-ion batteries for improved safety and long term durability, as well as for conducting effective design optimization studies. The model couples an electrochemical battery model with a thermal model to understand how thermal effects will influence electrochemical behavior and to determine temperature distributions throughout the battery. Several modeling example results are presented including thermal influences on current distribution, design optimization of current collector thickness and current collector tab placement, and investigation of lithium plating risk in three dimensions. / text

A two-dimensional isotropic metamaterials formed by crossed split-ring resonators (CSRRs) are studied in this paper. The effective characteristic parameters of this media are determined by quasi-static Lorentz theory. The induced current distributions of a single CSRR at the resonant frequency are presented. Moreover, the dependence of the resonant frequency on the dimensions of single CSRR and the spaces of the array are also discussed. / Singapore-MIT Alliance (SMA)

The production of chlorate, chlorine and sodium hydroxiderelies heavily on electrical energy, so that savings in thisarea are always a pertinent issue. This can be brought aboutthrough increased mass transfer of reacting species to therespective electrodes, and through increased catalytic activityand uniformity of current density distribution at theseelectrodes. This thesis will present studies involvingmathematical modelling and experimental investigations of theseprocesses. They will show the effect that hydrodynamicbehaviour has on the total current density and cell voltages,along with the effects on current density distributions andindividual overpotentials atthe respective electrodes. Primary, secondary and psuedo-tertiary current densitydistribution models of a chlor-alkali anode are presented anddiscussed. It is shown that the secondary model presentsresults rather similar to the pseudo-tertiary model, when thecurrent density distribution is investigated, although thepotential distribution differs rather markedly. Furthermore, itis seen that an adequate description of the hydrodynamicsaround the anode is required if the potential distribution, andthereby the prevalence of side-reactions, is to be reasonablepredicted. A rigorous tertiary current density distribution model ofthe chlorate cell is also presented, which takes into accountthe developing hydrodynamic behaviour along the height of thecell. This shows that an increased flowrate gives more uniformcurrent density distributions. This is due to the fact that theincreased vertical flowrate of electrolyte replenishes ioncontent at the electrode surfaces, thus reducing concentrationoverpotentials. Furthermore, results from the model lead to theconclusion that it is the hypochlorite ion that partakes in themajor oxygen producing side-reaction. A real-scale cross-section of a segmented anode-cathode pairfrom a chlorate cell was designed and built in order to studythe current density distribution in industrial conditions.These experiments showed that increased flowrate brought aboutmore even current density distributions, reduced cell voltageand increased the total current density. An investigation ofthe hydrodynamic effects on the respective electrodeoverpotentials shows the anode reactions being more favoured byincreased flowrate. This leads to the conclusion that theuniform current density distribution, caused by increasedflowrate, occurs primarily through decreasing the concentrationoverpotential at the anode rather than by decreasing thebubble-induced ohmic drop at the cathode. Finally, results from experiments investigating thebubble-induced free convection from a small electrochemicalcell are presented. These experiments show that Laser DopplerVelocimetry is the most effective instrument for investigatingthe velocity profiles in bubble-containing electrochemicalsystems. The results also show that the flow can transform fromlaminar to turbulent behaviour on both the vertical andhorizontal planes, in electrochemical systems where bubbles areevolved.