MEMS based mechanical switches are seen to be the likely replacements for CMOS based switches due to the several advantages that these mechanical switches have over CMOS switches. Mechanical switches can be used in systems under extreme conditions and also provide more reliability and cause less power loss. A major problem with mechanical switches is bouncing. Bouncing is an undesirable characteristic which increases the switching time and causes damage to the switch structure affecting the overall switch life. This thesis proposes a new switch design that may be used to mitigate bouncing by using two voltage sources using a double electrode configuration. The effect of many switch’s tunable parameters is also discussed and an effective tuning technique is also provided. The results are compared to the current control schemes in literature and show that the double electrode scheme is a viable control option.

Alternating current electrohydrodynamics (ac-EHD) has been reported as a promising technique for enhancing sensor performance by the intimate mixing of the analyte solution at the electrode surface. The lateral fluid motion created by the ac-EHD phenomenon can be tuned by changing the frequency, voltage, and electrode geometry. To date, various studies have been conducted on the use of 2D electrodes based ac- EHD devices for sensor applications. However, the use of 3D electrodes may provide better fluid mixing as compared to the 2D electrodes due to the high surface area of the electrodes. To test this hypothesis, 2D and 3D microelectrodes with different sizes were designed and fabricated for ac-EHD studies using standard lithography and etching processes. Previous methods to achieve 3D microstructures and common issues faced during fabrication are also discussed. The lateral fluid motion created by the 2D and 3D electrodes after the application of different voltages and frequencies was analyzed by tracking the motion of fluorescent beads present in the mixing fluid. Fluorescence microscopy technique was used to capture videos of the movement of fluorescent beads in the fluid. The videos were analyzed using ImageJ to calculate the speed of fluorescent beads in the case of 2D and 3D electrodes. Furthermore, a different pattern of the fluid motion was observed in the case of 3D electrodes, which highlights the complex fluid movement in the case of 3D electrodes as compared to the 2D electrodes.

No description available.

The purpose of this research project was to determine the processing conditions necessary for preparing flexible carbon nanofiber electrodes by electrospinning and to explore various applications for those electrodes. It was found that by varying only the relative humidity while electrospinning a poly(acrylonitrile) precursor, fragile or flexible freestanding carbon nanofiber electrodes were prepared. The relative humidity during electrospinning controlled the fiber diameter, the bulk porosity of the material, and flexibility of the final carbon electrode. Higher porosity mats electrospun in a high relative humidity environment prevented fiber sintering, which if not minimized, resulted in non-flexible carbon electrodes. Both flexible and fragile electrodes were freestanding, binderless, and collectorless. Additionally, they required no further processing before use and were 100 wt.% active material. When cycled galvanostatically as a lithium ion battery anode, the flexible electrode exhibited a specific capacity of 379 mAH g-1 at the 100th cycle and capacity retention was 97.4% relative to the fifth cycle. When applied as an active material support electrode for lithium ion battery cathodes, the carbon support was successfully utilized with both micron and nano structured active material and cycled for 100 cycles with limited capacity loss. The same electrodes were also found to be a viable replacement for Pt electrode based actuators/artificial muscles. However, this application requires much further research to understand better the required processing and effects of the physical properties of the electrode on actuator performance. In addition to this, the flexible electrodes have a wide variety of other potential applications including, electrochemical storage and conversion devices, chemical sensing, and filtration. The focus of this work was electrochemical storage and conversion devices in the form of lithium ion battery anodes and cathodes as well as ionic polymer composite actuators. / PHD / In this research, the processing conditions required to prepare flexible carbon nanofiber electrodes by electrospinning was determined. These carbon electrodes were then applied as the anode for lithium ion battery applications, as a support material for the cathode active material for lithium ion battery applications and as an electrode for electrically stimulated actuators, also known as artificial muscles. In addition to these applications, the carbon nanofibers developed here have potential uses for fuel cells, chemical sensors, and filtration. The method used to develop these electrodes was electrospinning, an industrially scalable manufacturing technique that produces nanofibers with diameters ranging from 100 nm to a few microns in diameter. To produce the flexible carbon nanofibers, it was found the precise control of all electrospinning variables had to be maintained. Specifically, the relative humidity of the electrospinning environment was found to be the most crucial. When the electrode was applied as a lithium ion battery anode, it was used without additional processing which made it 100 wt.% active material. When the performance of the battery was tested, a specific capacity, or the energy stored, was found to be 379 mAH/g on the 100th cycle. Relative to the 5th cycle, after the electrode had stabilized, this was a capacity retention of 97.4%. In addition to its successful use as an anode, the carbon nanofiber electrode was also applied as a support material for a flexible lithium ion battery cathode. For this application, two cathode types were examined, micron and nanostructured. Both were prepared by vacuum filtering a dispersion of the active material through the carbon nanofiber electrode support material. Both the micron and nano structured active material were successfully cycled for 100 cycles with limited capacity loss using this novel cathode support material. The same electrodes were also found to be a viable replacement for Pt electrode based actuators/artificial muscles. However, this application requires much further research to understand better the required processing and effects of the physical properties of the electrode on actuator performance.

Flexible electronics attract research and commercial interests in last 2 decades for its flexibility, low cost, light weight and etc. To develop and improve the electro-mechanical properties of flexible electrodes is the most critical and important step. In this work, we have performed nanomechanical and electro-mechanical characterization of materials for flexible electrode applications, including metallic nanowires (NWs), indium tin oxide (ITO)-based and carbon nanotube (CNT)-based electrodes. First, we designed and developed four different testing platforms for nanomechanical and electro-mechanical characterization purpose. For the nano/sub-micro size samples, the micro mechanical devices can be used for uni-axial and bi-axial loading tests. For the macro size samples, the micro tester will be used for in situ monotonic tensile test, while the fatigue tester can be used for in situ cyclic tensile or bending testing purpose. Secondly, we have investigated mechanical behaviors of single crystalline Ni nanowires and single crystalline Cu nanowires under uni-axial tensile loading inside a scanning electron microscope (SEM) chamber. We demonstrated both size and strain-rate dependence on yield stress of single-crystalline Ni NWs with varying diameters (from 100 nm to 300 nm), and themolecular dynamics (MD) simulation helped to confirm and understand the experimental phenomena. Also, two different fracture modes, namely ductile and brittle-like fractures, were found in the same batch of Cu nanowire samples. Finally, we studied the electro-mechanical behaviors of flexible electrodes in macro scale. We reported a coherent study integrating in situ electro-mechanical experiments and mechanics modeling to decipher the failure mechanics of ITO-based and CNT-based electrodes under tension. It is believed that our combined experimental and simulation results provide some further insights into the important yet complicated deformation mechanisms for nanoscale metals and fracture mechanism for flexible electrodes applications.

Applications of chromium vary widely (refractories, chemicals and metallurgical); however, the greatest benefit of chromium is its ability to improve the corrosion resistance, strength and hardness of steel. South Africa possesses approximately 75% of the viable global chromite reserves and, as a result, dominates the ferrochrome market with production in excess of 5 million mega tonnes per year - making it an industry of extreme importance to the South African economy Submerged arc ferroalloy production furnaces mainly use Soderberg electrodes - self–baking continuous electrodes that are produced in situ during furnace operation. Electrode breakings may affect a furnace in a number of ways depending on the nature and location of the break. Low furnace power input, abnormal charging and tapping conditions, as well as loss of production are among the more common negative implications associated with electrode breaks. The successful operation of Soderberg electrodes is dependent on two main factors: high quality electrode paste and effective electrode management procedures. This study focused on electrode paste quality. The raw materials utilised in the production of Soderberg electrode paste consists of calcined anthracite mixed with a tar pitch binder. In this study the focus was on the development of an experimental procedure to measure the dimensional changes of electrode paste raw materials as a function of temperature by means of thermomechanical analysis (TMA). Three uncalcined anthracite (Zululand chips, Zululand duff, and Tendele duff) and two tar pitch samples (low and high softening point pitches, i.e. LSP and HSP) were obtained from a local paste producer. Electrode graphite samples were also obtained from a local pre–baked electrode supplier. The experimental procedure for both the anthracite and tar pitches consisted of two phases: sample preparation and TMA measurements. During the sample preparation procedure for the tar pitches, the two tar pitches were heat treated in order to prevent softening in the TMA (preventing possibly damage the instrument), where after pellets were pressed for TMA measurement. The anthracite samples were calcined at 1200, 1300 and 1400°C in the anthracite sample preparation phase. TMA sample pellets of calcined and uncalcined anthracite were pressed using only water as a binder. TMA was performed on pellets produced from the heat–treated tar pitch samples, uncalcined and calcined anthracite samples, as well as core drilled pellets of the pre–baked electrode graphite. The dimensional changes of these pellets were measured, as a function of temperature, through three consecutive heating (room temperature to 1300°C) and cooling (1300°C to approximately 100°C) cycles under a N2 atmosphere. A significant shrinkage (> 12%) for both the LSP and HSP tar pitches occurred during the first TMA heating cycle. During the second and third heating cycles of the LSP and HSP tar pitches, dimensional changes were approximately 2%. This indicates that substantial structural reordering of the carbonaceous binder takes place during the first heating cycle. TMA results obtained for all three the calcined anthracite samples investigated indicated thermal dimensional changes of less than 1%. The anthracite samples calcined at the highest experimental calcination temperature (1400°C) prior to TMA analysis had the smallest dimensional changes. This confirmed that higher calcination temperatures result in a higher level of structural ordering and dimensional stability. Considering the combined calcined anthracite and tar pitches TMA results, the importance of the initial baking of a Soderberg electrode at temperatures exceeding the baking isotherm temperature (475°C) becomes apparent - the dimensional behaviour of the tar pitch binder and the calcined anthracite differ dramatically, making the newly–formed electrode very susceptible to breakage. Once structural reordering of the pitch had taken place, thermal dimensional behaviours of the materials are much more similar, significantly reducing the risk of thermal shock–induced electrode breakages. In contrast to the relatively small dimensional changes measured for the calcined anthracite samples, the shrinkages measured for the uncalcined samples during the first TMA heating/cooling cycle were substantial (6–8%). This indicates the importance of the anthracite calcination process, before the electrode paste is formulated. Improperly calcined anthracite present in electrode paste would result in additional dimensional shrinkage that would have to be accommodated in the baking of a new electrode section. Considering the large shrinkage of the tar pitch that already takes place, it is unlikely that a strong enough electrode would be formed if this occurs. From the results, it also became apparent that the anthracite with the highest fixed carbon and lowest ash contents exhibited the smallest shrinkage during in situ TMA calcination. High fixed carbon, low ash type anthracites are therefore less prone to dimensional instabilities in Soderberg electrodes, as a result of poor calcination. The dimensional changes observed in the calcined anthracites were very similar to those observed for the electrode graphite samples. The expansions/shrinkages observed in the graphite samples were mostly less than 0.5%, whereas the expansions/shrinkages observed in the various calcined anthracites were approximately 0.6 to 0.9%. The difference in the magnitude of the dimensional behaviour between the calcined anthracites and the graphite can be attributed to the fact that the graphite had already undergone maximum structural ordering (having been pre–baked at 3000°C). / Thesis (M.Ing. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2012.

Applications of chromium vary widely (refractories, chemicals and metallurgical); however, the greatest benefit of chromium is its ability to improve the corrosion resistance, strength and hardness of steel. South Africa possesses approximately 75% of the viable global chromite reserves and, as a result, dominates the ferrochrome market with production in excess of 5 million mega tonnes per year - making it an industry of extreme importance to the South African economy Submerged arc ferroalloy production furnaces mainly use Soderberg electrodes - self–baking continuous electrodes that are produced in situ during furnace operation. Electrode breakings may affect a furnace in a number of ways depending on the nature and location of the break. Low furnace power input, abnormal charging and tapping conditions, as well as loss of production are among the more common negative implications associated with electrode breaks. The successful operation of Soderberg electrodes is dependent on two main factors: high quality electrode paste and effective electrode management procedures. This study focused on electrode paste quality. The raw materials utilised in the production of Soderberg electrode paste consists of calcined anthracite mixed with a tar pitch binder. In this study the focus was on the development of an experimental procedure to measure the dimensional changes of electrode paste raw materials as a function of temperature by means of thermomechanical analysis (TMA). Three uncalcined anthracite (Zululand chips, Zululand duff, and Tendele duff) and two tar pitch samples (low and high softening point pitches, i.e. LSP and HSP) were obtained from a local paste producer. Electrode graphite samples were also obtained from a local pre–baked electrode supplier. The experimental procedure for both the anthracite and tar pitches consisted of two phases: sample preparation and TMA measurements. During the sample preparation procedure for the tar pitches, the two tar pitches were heat treated in order to prevent softening in the TMA (preventing possibly damage the instrument), where after pellets were pressed for TMA measurement. The anthracite samples were calcined at 1200, 1300 and 1400°C in the anthracite sample preparation phase. TMA sample pellets of calcined and uncalcined anthracite were pressed using only water as a binder. TMA was performed on pellets produced from the heat–treated tar pitch samples, uncalcined and calcined anthracite samples, as well as core drilled pellets of the pre–baked electrode graphite. The dimensional changes of these pellets were measured, as a function of temperature, through three consecutive heating (room temperature to 1300°C) and cooling (1300°C to approximately 100°C) cycles under a N2 atmosphere. A significant shrinkage (> 12%) for both the LSP and HSP tar pitches occurred during the first TMA heating cycle. During the second and third heating cycles of the LSP and HSP tar pitches, dimensional changes were approximately 2%. This indicates that substantial structural reordering of the carbonaceous binder takes place during the first heating cycle. TMA results obtained for all three the calcined anthracite samples investigated indicated thermal dimensional changes of less than 1%. The anthracite samples calcined at the highest experimental calcination temperature (1400°C) prior to TMA analysis had the smallest dimensional changes. This confirmed that higher calcination temperatures result in a higher level of structural ordering and dimensional stability. Considering the combined calcined anthracite and tar pitches TMA results, the importance of the initial baking of a Soderberg electrode at temperatures exceeding the baking isotherm temperature (475°C) becomes apparent - the dimensional behaviour of the tar pitch binder and the calcined anthracite differ dramatically, making the newly–formed electrode very susceptible to breakage. Once structural reordering of the pitch had taken place, thermal dimensional behaviours of the materials are much more similar, significantly reducing the risk of thermal shock–induced electrode breakages. In contrast to the relatively small dimensional changes measured for the calcined anthracite samples, the shrinkages measured for the uncalcined samples during the first TMA heating/cooling cycle were substantial (6–8%). This indicates the importance of the anthracite calcination process, before the electrode paste is formulated. Improperly calcined anthracite present in electrode paste would result in additional dimensional shrinkage that would have to be accommodated in the baking of a new electrode section. Considering the large shrinkage of the tar pitch that already takes place, it is unlikely that a strong enough electrode would be formed if this occurs. From the results, it also became apparent that the anthracite with the highest fixed carbon and lowest ash contents exhibited the smallest shrinkage during in situ TMA calcination. High fixed carbon, low ash type anthracites are therefore less prone to dimensional instabilities in Soderberg electrodes, as a result of poor calcination. The dimensional changes observed in the calcined anthracites were very similar to those observed for the electrode graphite samples. The expansions/shrinkages observed in the graphite samples were mostly less than 0.5%, whereas the expansions/shrinkages observed in the various calcined anthracites were approximately 0.6 to 0.9%. The difference in the magnitude of the dimensional behaviour between the calcined anthracites and the graphite can be attributed to the fact that the graphite had already undergone maximum structural ordering (having been pre–baked at 3000°C). / Thesis (M.Ing. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2012.

A transparent conductive electrode (TCE) is an important component in many of our modern optoelectronic devices like photovoltaics, light emitting diodes and touch screens. These devices require good current injection and spreading as well as a high transparency. In this thesis we explore the use of graphene as an alternative to the current widely used indium tin oxide (ITO) as TCE in gallium nitride (GaN) based light emitting diodes (LEDs). Monolayer crystalline graphene can be produced on copper foils using chemical vapor deposition (CVD), where metals (especially copper) has a catalysing effect on the formation of graphene. However, transfer of graphene from copper foils is not suitable for an industrial scale and it results in a poor contact with the target substrate. We investigate the possibility of directly integrating graphene on GaN-based LEDs by using plasma-enhanced chemical vapor deposition (PECVD). We try to obtain the optimal conditions under these catalyst-free circumstances and propose a recipe adapted for the setup that we used. We will also study ideas of using a metal (we tried copper and nickel) to assist the direct growth that could help to increase the fraction of sp2 carbon bonds and reduce the sheet resistance. The metals are evaporated onto our samples either before or after we grow a carbon film to either assist the growth or rearrange the carbon respectively. The focus was not on trying to optimize the conditions for one metal treatment but rather to briefly explore multiple methods to find a suitable path for further studies. The direct grown pristine carbon films shows indications from Raman measurements of being nanocrystalline graphene with a sheet resistance ranging from about 20-50 kΩ/sq having a transmittance of approximately 96 % at 550 nm. A transmittance at this level is closely related to the value of an ideal monolayer graphene, which indicates that our carbon films could be close to one atom in thickness while being visually homogeneous and complete in coverage. Due to the use of a temperature close to the melting point of copper we struggled to keep the assisting copper from evaporating too fast or staying homogeneous after the treatment. Nickel has a higher melting temperature, but it appears as if this metal might be diffusing into the GaN substrate which changes the properties of both the GaN and carbon film. Even though the metal treatments that we tested did not provide any noticeable improvements, there is need for further investigations to obtain suitable treatment conditions. We suggest that the treatments involving copper are a more promising path to pursue as nickel seem to cause unavoidable intermixing problems.

Zinc coated steels are widely used in the automotive industry, because of the improved protection against corrosion. Their use has consequences for the resistance welding process, which is the most widely used method of joining body panels. The zinc coating alloys with the copper electrode, resulting in increased electrode wear, and a reduction in electrode life. The welding current must be increased, because of the reduced contact resistance and thus heavier cables and power sources are required. A novel form of power source, the Medium Frequency Direct Current inverter, offers advantages over the traditional AC transformer. The higher operating frequency results in a lighter transformer, and a smaller welding current may be used, because the DC welding current generates heat at a constant rate, and is thus more effective than an AC power source. A potential advantage of this technology is that the increased frequency allows improved resolution in monitoring and control. Novel signal conditioning circuitry was developed, allowing significant improvement in the time resolution of the voltage and resistance signals. A series of welding trials was conducted, while monitoring the welding process. The correlation between weld quality and various process variables was assessed, and a control algorithm to compensate for electrode wear was proposed. This algorithm, based on a constant voltage principle, was implemented on a bespoke welding timer. A significant improvement in electrode life was obtained using this technique. The control algorithm was shown effective experimentally, but practical limitations do not permit testing under all possible conditions. A numerical model of the spot welding process, using Finite Difference technique, was developed. Following successful validation, the model was used to predict the performance of the control algorithm under various conditions of electrode wear. The results indicate that a constant voltage algorithm can compensate for an increase in electrode tip diameter, but that a change in contact resistance may result in unsatisfactory welds.

No description available.