<p>Clay, quartz and feldspar are widely available in Uganda. The location and properties of various clay deposits are reported in the literature, but little is reported on feldspar and quartz deposits. In this work an extended literature on ceramics and porcelains in particular, is documented. Samples from two deposits of feldspar and two deposits of quartz are characterised and found to possess requisite properties for making porcelain insulators. Sample porcelain bodies are made from materials collected from selected deposits using different mixing proportions of clay, feldspar and quartz. Their properties in relation to workability, firing temperature, dielectric and bending strengths are studied. It is found that a mixture consisting of 30% Mutaka kaolin, 15% Mukono ball clay, 30% Mutaka feldspar and 25% Lido beach flint yields a body with highest mechanical strength (72MPa) and dielectric strength (19kV/mm) when fired at 1250°C. The strength (both mechanical and dielectric) is found to decrease with increasing firing temperature. At high firing temperatures, the undissolved quartz in the body decreased, the glass content increases and pores are formed. Mullite content on the other hand does not change at temperatures above 1200°C but there are significant differences in the morphologies of the mullite crystals in the samples. Optimum mechanical and electrical properties are found at maximum virtification and a microstructure showing small closely packed mullite needles.</p>

Clay, quartz and feldspar are widely available in Uganda. The location and properties of various clay deposits are reported in the literature, but little is reported on feldspar and quartz deposits. In this work an extended literature on ceramics and porcelains in particular, is documented. Samples from two deposits of feldspar and two deposits of quartz are characterised and found to possess requisite properties for making porcelain insulators. Sample porcelain bodies are made from materials collected from selected deposits using different mixing proportions of clay, feldspar and quartz. Their properties in relation to workability, firing temperature, dielectric and bending strengths are studied. It is found that a mixture consisting of 30% Mutaka kaolin, 15% Mukono ball clay, 30% Mutaka feldspar and 25% Lido beach flint yields a body with highest mechanical strength (72MPa) and dielectric strength (19kV/mm) when fired at 1250°C. The strength (both mechanical and dielectric) is found to decrease with increasing firing temperature. At high firing temperatures, the undissolved quartz in the body decreased, the glass content increases and pores are formed. Mullite content on the other hand does not change at temperatures above 1200°C but there are significant differences in the morphologies of the mullite crystals in the samples. Optimum mechanical and electrical properties are found at maximum virtification and a microstructure showing small closely packed mullite needles. / QC 20101122

Thermal spray coatings offer new alternatives in the production of electronic power modules that use alumina ceramic as an isolation layer. Current processes use direct bond copper (DBC) soldered to a nickel plated copper heat spreader. A coefficient of thermal expansion (CTE) mismatch exists between copper and alumina and leads to reliability issues that arise due to product failure during thermal cycling and lifetime operation. The substitution of an AlSiC metal matrix composite (MMC) heat spreader baseplate addresses the problem of CTE mismatch and will reduce the number of product failures related to cracking and delamination caused by this pronounced mismatch in the thermal expansion coefficient.. The substitution of an AlSiC (MMC) heat spreader baseplate also allows the production process to be achieved with a fewer number of metallization layers. Thermal spray can apply alumina ceramic coatings directly to the AlSiC (MMC) baseplates. A reduction in process steps will lead to a reduction in manufacturing costs, the main driving objective in Microelectronics Industries. Thermal spray coatings have a major problem since they have a porous microstructure which can trap undesired moisture. The moisture basically causes the coatings to have a lower dielectric breakdown voltage and a higher leakage current at normal operating voltages. This problem can be eliminated by manufacturing the electronic power modules in a controlled environment and packaging the devices in a hermetically sealed package. This thesis analyzes the data obtained from direct-voltage dielectric breakdown and direct-voltage leakage current tests conducted on coupons manufactured using the thermal plasma spray coating process and the thermal high-velocity oxyfuel (HVOF) coating process. ASTM specifications defining appropriate testing procedures are used in testing the dielectric strength of these coupons. Issues relating to the dielectric strength and dielectric leakage current are evaluated and validated at the Microelectronics Laboratory at Virginia Polytechnic Institute & State University. The objective to conduct this research study using plasma and HVOF alumina coatings as dielectric isolation layers is to support the Microelectronics Industries in developing a product with increased reliability at a lower manufacturing cost. / Master of Science

Glass fibre composites are common materials used in high voltage applications as insulating materials that provide good structural integrity. The aim of this thesis is to develop a method of studying the failure in such materials by measuring the dielectric strength on micro- and meso- scale samples, consisting of single fibre filaments and fibre bundles respectively embedded in epoxy resin. To do this, a body of relevant knowledge has been amassed, which is complemented with finite element analysis giving detailed insight into the electric field distribution in the microstructure of fibre composites. A method of producing virtually defect free single fibre samples has been developed where a filament is hung down tubes and cast in epoxy resin. A similar method was developed for producing bundle samples, however this needs some slight correction in order to prevent exothermic reactions. The dielectric strength of these samples are measured by applying a continuously increasing voltage until discharge is recorded. To evaluate the method micro- and meso- scale samples were prepared of three different fibres and their dielectric strengths measured. This evaluation showed that the method can be used to measure a definitive lower bound in the dielectric strength of fibre composites. However, the method can not definitively determine the location of the discharge, which is necessary to verify conclusions about the materials properties. To progress the method, the dielectric strength of neat epoxy samples of the same dimensions as the fibre composite samples should be investigated. Increasing the tolerance of the measurement setup should also be investigated as this could help by increasing the power of the discharge leading to more severe damage in the material.

Semiconductor die attach materials for high voltage, high reliability analog devices require high thermal conductivity and retention of dielectric strength. A comparative study of effective thermal conductivity and dielectric strength of selected thermoset/ceramic composites was conducted to determine the effect of ceramic particle size and ceramic particle loading on thermoset polymers. The polymer chosen for this study is bismaleimide, a common aerospace material chosen for its strength and thermal stability. The reinforcing material chosen for this study is a ceramic, hexagonal boron nitride. Thermal conductivity and dielectric breakdown strength are measured in low and high concentrations of hexagonal boron nitride. Adhesive fracture toughness of the composite is evaluated on copper to determine the composite’s adhesive qualities. SEM imaging of composite cross-sections is used to visualize particle orientation within the matrix. Micro-indentation is used to measure mechanical properties of the composites which display increased mechanical performance in loading beyond the percolation threshold of the material. Thermal conductivity of the base polymer increases by a factor of 50 in 80%wt loading of 50µm hBN accompanied by a 10% increase in composite dielectric strength. A relationship between particle size and effective thermal conductivity is established through comparison of experimental data with an empirical model of effective thermal conductivity of composite materials.

Wide bandgap (WBG) devices and power electronic converters (PEC) that enable the dynamic control of energy and high-power density designs inevitably contain defects including sharp edges, triple points, and cavities, which result in local electric field enhancements. The intensified local electric stresses cause either immediate dielectric breakdown or partial discharge (PD) that erodes electrical insulators and accelerates device aging. With the goal of addressing these dielectric challenges emerging in power-dense applications, this dissertation focuses on 1) modeling the dielectric characteristics of supercritical fluids (SCFs), which is a new dielectric medium with high dielectric strength and high cooling capability; and 2) establishing the optimal fabrication conditions of electrets, which is a new dielectric solution that neutralizes locally enhanced electric fields. In this dissertation, the dielectric breakdown characteristics of SCFs are modeled as a function of pressure based on the electron scattering cross section data of clusters that vary in size as a function of temperature and pressure around the critical point. The modeled breakdown electric field is compared with the experimental breakdown measurements of supercritical fluids, which show close agreement. In addition, electrets are fabricated based on the triode-corona charging method and their PD mitigation performance is evaluated through a series of PD experiments. Electrets are fabricated under various charging conditions, including charging voltage, duration, polarity, and temperature with the goal of identifying the optimal condition that leads to effective PD mitigation. The PD mitigation performance of electrets fabricated based on these charging conditions is further assessed by investigating the impact of various power electronics voltage characteristics, including dv/dt, polarity, switching frequency, and duty cycle. Electret based electric field neutralization approach is further utilized in increasing the critical flashover voltage associated with the surface flashover voltage. Moreover, due to the high mechanical strength of epoxy composites at cryogenic temperatures, in this dissertation, epoxy-based electrets are fabricated as a solution to PD in high temperature superconducting cables. The experimental demonstrations conducted with electret in this dissertation is dedicated for the establishing the electret based electric field neutralization approach as a dielectric solution for the dielectric challenges in power electronics driven systems.

In it´s first part, this presented master´s thesis focuses on the problematics of actions taking place in dielectric materials and phenomenons on the surface of real insulators placed in electric field. This research is followed by selection and rating of appropriate ceramic materials for the actual design of the metal – ceramic assembly. The greatest emphasis is given to design of the optimal configuration of the insulator and electrode, which is supported by simulations from the software that implements finite element method. Eventually, the high voltage tests of the assembly used for measuring leakage currents are performed.

This semestral paper deals with the issues of testing and requirements examinations for cables and cable harnesses. These terms are defined in the work and the difference in their meaning is described. The work also describes the characteristic features of cables and cable harnesses, explains their importance and focuses in more detail on the variable properties that may change over time or that are dependent on manufacturing errors. The principles and procedures of tests of these properties are covered in the work and are the essence of the subsequent practical part. In this part, the work deals with the design of the measuring circuit prototype for testing of the insulation resistance of cables and their desired continuity in bundles. This design is then physically implemented, the processes of designing and implementing are described in detail and finally its functionality is tested and presented to the reader.

Conventional microelectronic and power electronic packages based on Si devices usually work below 150°C. The emergence of wide-bandgap devices, which potentially operate above a junction temperature of 250°C, results in growing research interest in high-density and high-temperature packaging. There are high-temperature materials such as encapsulants on the market that are claimed for capability of continuous operation at or above 250°C. With an objective of identifying encapsulants suitable for packaging wide-bandgap devices, some of commercial high-temperature encapsulants were obtained and evaluated at the beginning of this study. The evaluation revealed that silicone elastomers are processable for various types of package structure and exhibit excellent dielectric performance in a wide temperature range (25 - 250°C) but are insufficiently stable against long-term aging (used by some manufacturers, e.g., P²SI, to evaluate polymer stability) at 250°C. These materials cracked during aging, causing their dielectric strength to decrease quickly (as soon as 3 days) and significantly (60 - 70%) to approximately 5 kV/mm, which is below the value required by semiconductor packaging. The results of this evaluation clearly suggested that silicone needs higher thermal stability to reliably encapsulate wide-bandgap devices. Literature survey then investigated possible methods to improve silicone stability. Adding fillers is reported to be effective possibly due to the interaction between filler surface and polymer chains. However, the interaction mechanism is not clearly documented. In this study, the effect of Al₂O₃ filler on thermal stability was first investigated by comparing the performance of unfilled and Al₂O₃-filled silicones in weight-loss measurements and dielectric characterization. All test results on composites filed with Al₂O₃ micro-rods indicated that thermal stability increased with increasing filler loading. Thermogravimetric analysis (TGA) test demonstrated that the temperature of degradation onset increased from 330 to 379°C with a 30 wt% loading of Al₂O₃ rods. In isothermal soak test, unfilled and 30-wt%-filled silicones lost 10% of polymer weight in 700 and 1800 hours, respectively. The dielectric characterization found that both Weibull parameters, characteristic dielectric strength (E₀, representing the electric field at which 62.3% of samples are electrically broken down) and shape parameter (β, representing the spread of data. The larger the β, the narrower the distribution) can reflect the thermal stability of polymers. Both of them were influenced by microstructure evolution, to which β was found to be more sensitive than E₀. The characteristic dielectric strength of unfilled silicone decreased significantly after 240 hours of aging at 250°C, whereas that of Al₂O₃/silicone composites exhibited no significant change within 560 hours. The shape parameter of Al₂O₃-filled silicone decreased slower than that of unfilled silicone, also indicating the positive effect of Al₂O₃ micro-rods on thermal stability. Improved thermal stability can be explained by restrained chain mobility caused by interfacial hydrogen bonds, which are formed between hydroxyl groups on Al₂O₃ surface and silicone backbone. In this study, the effect of hydrogen bonds was investigated by dehydrating Al₂O₃ micro-rods at high temperature in N₂ to partially destroy the bonds. Removal of hydrogen bonds impaired thermal stability by increasing the initial weight-loss rate from 0.025 to 0.036 wt%/hour. The results explained the importance of interfacial hydrogen bond, which effectively reduced the average chain mobility, hindered the formation of degradation products, and led to higher thermal stability. The main discoveries of this study are listed below: 1. Al₂O₃ micro-rods were found to efficiently improve the thermal stability of silicone elastomer used for high-temperature encapsulation. 2. Characteristic dielectric strength and shape parameter obtained from Weibull distribution reflected the change of material microstructure caused by thermal aging. The shape parameter was found to be more sensitive to microscale defects, which were responsible for dielectric breakdown at low electric field. 3. Hydrogen bonds existing at filler/matrix interface were proven to be responsible for the improvement of thermal stability because they effectively restrained the average chain mobility of the silicone matrix. / Ph. D.

Dielectric insulation are constantly exposed to various stresses, while also being expected to last for long periods of time. Naturally, they degrade with use and in demanding operational environments, this degradation phenomena can be accelerated, leading to costly consequences. With the constant evolution within the vehicle industry, high standards are established. For ignition coils, one of the challenges lies in achieving high performance, with the trade-off being a reduced insulation space. Furthermore, the operation of ignition coils at elevated temperatures could affect the insulation and its capabilities. Thus, it is important to investigate how the dielectric strength of an insulation behaves under these increased temperature conditions and constrained operational space, both in the short and long term. In this master thesis, the primary objective was to ascertain whether the oil would maintain its dielectric strength under prolonged elevated temperatures. The transformer liquid MIDEL 7131 was examined and assessed, through breakdown voltage tests from room temperature and up to temperatures of 140◦C. Subsequently analysing the results using descriptive statistics, employing both Gaussian and Weibull distributions. The observations from these measurements provide more data to the field of liquid dielectrics and allow a better understanding of how the condition and operating environment could affect the dielectric capabilities of transformer oils, with a specific focus on MIDEL 7131. / Dielektrisk insulation utsätts ständigt för diverse påkänningar, samtidigt som dem förväntas vara i drift under en lång tid. Naturligtvis degraderas isolationen efter användning och under krävande arbetsmiljöer, kan denna degradering fenomen acceleras, vilket kan leda till dyra konsekvenser. Med den ständiga utvecklingen inom fordonindustrin, etableras höga standarder. För tändstift, är en av svårigheterna att uppnå hög prestanda, i utbyte mot reducerad insulation utrymme. Dessutom är det möjligt att tändstift i drift under höga temperaturer kan påverka insulationen samt sina förmågor. Det är därför viktigt att undersöka hur dielektrisk styrka hos insulation beter sig vid höga temperaturer och minskat utrymme, både i korta perioder samt i långa. I detta master arbete, var det primära målet att fastställa om oljan skulle upprätthålla sin dielektriska hållfastighet under långa perioder vid höga temperaturer. Transformator oljan MIDEL 7131 undersöktes samt analyserades, genom bland annat tester av genombrottspänningar vid rumstemperatur upp till 140 ◦C. Därefter analyserades resultaten genom Normal- och Weibull fördelning. Observationerna från mätningarna tillhandahåller med mer data till flytande dielektrika området och ger en bättre förståelse om hur tillstånd samt driftmiljö kan påverka den dielektriska hållfastigheten av transformatorolja, med speciell fokus på MIDEL 7131.