This master’s thesis deals with design and fabrication of MEMS microhotplate platform for chemical gas sensors. The theoretical part describes MEMS, sensors and processes and technologies needed for fabrication of micro hotplate. The practical part includes simulations, masks and step by step microhotplate fabrication. Fabricated heating membrane was characterized and compared to theoretical values from simulations and to similar devices presented in literature.

This thesis deals with the use of non-metallic highly thermally conductive materials, more concrete-ly special plastic materials, enriched with highly thermally conductive additives, for the purpose of passive cooling of a given heat source. The thesis compares the effectivity of these heat sinks with the classically used materials, specifically aluminium. The thesis is divided into two main sections, theoretical and practical. The theoretical part deals with a constantly growing need of LED (Light Emitting Diode) chips cooling in automotive head-lamps, where the new materials could be put into effect, analyses possible replacement of classic aluminium heat sinks with different materials with a significantly lower thermal conductivity and introduces problems of such materials. The practical part applies the problematic described in the theoretical one on the actually produced heat sinks, which are compared among themselves, with regard to their method of production, as well as with aluminium counterpart in different conditions. Furthermore, the problematic of de-signing a heat sink made from material which is characteristic for its highly anisotropic thermal con-ductivity is dealt with. The end of the thesis shows the importance of heat dissipation via radiation, which can have a great significance in case of plastic heat sinks and in a specific applications.

Riparian buffers have been created as a sustainable and effective way to combat the harmful effects of excess nitrogen and soil salinity in riparian settings. The goal of this research was to determine what species will I) germinate in saline environments and II) establish and produce sufficient biomass while being exposed to increased nitrogen. Incubation of eight native riparian graminoid species were evaluated for their ability to germinate in MgSO4-induced salinity. In a greenhouse study, seven riparian graminoid species were evaluated to quantify their ability to survive and take up nitrogen, mimicking buffer strips exposed to high inputs of runoff nitrogen. Slender wheatgrass and green needlegrass were able to germinate successfully when exposed to MgSO4 with EC levels up to 16 dS m-1 and 8 dS m-1, respectively. Of the graminoid species tested, smooth brome yielded sufficient biomass and nitrogen uptake percentages in a controlled setting.

Electrical insulating liquids, organic esters, conductivity, dielectric, permittivity

Polymer nanofibers have attracted a lot of industrial interest in the past decade. In general, these fibers need to be thermally stable for many applications, such as in the aerospace industry. However, most of these polymer nanofibers suffer from low temperature degradation, limiting their use in many potential applications. Graphene, which is one sheet of graphite, has unique properties such as high conductivity, and high thermal stability. This exceptional material can be incorporated into the polymer nanofibers as nanofillers in order to enhance their thermal properties. The aim of this dissertation is to investigate the effect of adding graphene nanofillers into the polymer fiber on the resulting fibers’ thermal properties. For that purpose, polyvinyl alcohol (PVA), a non-conductive polymer and a different source of graphene, namely graphene foam, expendable graphite and graphite powder were used. The growth technique was the electrospinning technique which offers a variety of parameters that need to be optimized. For this includes, the amount of PVA in the water solvent, the flow rate, the applied voltage, the growth time, and the tip/collector distance. In summary, it has been optimized that the best conditions for growth of fibers will be as follows: PVA concentration will be fixed at 10 wt%, flow rate will be 3 ml/h, applied voltage will be 30 kV, growth time of 60 s and tip/collector distance will be fixed at 12 cm. The resulted PVA fibers from these conditions were smooth continuous and hollow with diameter ranging between 190-340 nm, while PVA/graphene nano-fibers are much thinner with diameter ranging between 132 - 235 nm when the same parameters were used with only graphene concentration varied. The fiber obtained with PVA showed a hollow structure which is desirable for incorporation of graphene nanofillers. The dispersion of the different source of graphene sheets in the starting PVA solution showed enhanced thermal stability compared to the PVA fibers alone. Furthermore, an increase in the thermal stability is observed with increasing concentration of graphene nanofillers. This work shows the promising use of graphene as nanofillers for PVA fibers. This can be expended to other non-conductive and conductive polymers in order to broaden the application of these fibers in the industries, where thermal stability is a prerequisite. / Dissertation (MSc)--University of Pretoria, 2013. / gm2014 / Physics / unrestricted

Nitrogen is required by plants in large quantities and its deficiency is mostly related to reduction in crop production. A study was conducted to assess the importance of nitrogen in tomato (Lycopersicon esculentum Mill.) transplant production. Transplants were propagated at 0, 30, 60, 90, and 120 mg∙L-1 N applied as NH4NO3 while 30 mg∙L-1 P applied as NaH2PO4 and 30 mg∙L-1 K as KCl were used. Fergitation was done by floating cavity trays in nutrient solution until the medium reached field capacity. The experiment was arranged in a randomized complete block design (RCBD) with four replications. Sampling was initiated at 21 days after sowing and was done weekly until the transplants were ready for transplanting (when transplants could be pulled out of the cavity easily without breaking) at 42 days after sowing. Nitrogen supply had a pronounce influence on the transplant root and shoot growth. Observations throughout the experiment indicated that increased nitrogen application favoured shoot growth which is an indication that most of the assimilates were partitioned to shoots rather than to roots. Nitrogen application of 120 mg∙L-1increased fresh shoot mass and subsequently enhanced dry shoot mass. As nitrogen was increased from 0 to 120 mg∙L-1, it further promoted relative growth rate, specific leaf area, leaf mass ratio, leaf area ratio, plant chlorophyll content, leaf tissue nitrogen and improved the pulling success. At 42 days after sowing, a quality transplant that was produced with 90 mg∙L-1 N, had a root to shoot ratio of 0.16, leaf mass ratio of 0.86, root mass ratio of 0.13, leaf area of 594 cm2, plant chlorophyll content of 33, leaf tissue nitrogen of 32 g∙kg-1, specific leaf area of 194 cm2∙mg-1, leaf area ratio of 167.7 cm2∙mg-1, relative growth ratio of 0.31 cm∙mg-1∙wk-1 and 100% pulling success. This transplant proved to be ideal for the production of tomato as compared to other treatment combinations that were employed. Another glasshouse experiment was conducted to determine the influence of electrical conductivity (EC) and or nutrient solution composition on growth, yield and quality parameters in tomato. The pots were arranged in a randomized complete block design (RCBD). One plant per pot represented an experimental unit. Four EC treatments were used that consisted of 1.12, 2.24, 4.48 and 6.72 mS∙cm-1. Each treatment was replicated six times. Distilled water was used for irrigation water to maintain the required pH, which was 5.5 to 6.2 throughout the duration of the study, and cocopeat was used as substrate. Salinity inhibited growth (shoot length) and yield (average fruit mass, fruit diameter and fruit circumference) at the highest concentration of 6.72 mS∙cm-1. However, it did not significantly affect number of trusses, number of fruits and stem diameter, rather tomato quality was improved in terms of total soluble solids. Although tomato fruits grown at 6.72 mS∙cm-1 were relatively smaller than fruits grown at 1.12, 2.24 and 4.48 mS∙cm-1 treatments respectively, they had higher acidity, increased soluble solids and higher sugar content which are all qualities required by the tomato processing industry. Increasing the concentration of the solution from 1.12 to 6.72 mS∙cm-1 increased the %Brix from 3.9 to 6.1% while titratable acidity was also increased from 3.3 to 5.7%, respectively. The incidents of blossom end rot were higher (6.3%) at concentration of 6.72 mS∙cm-1 as compared to 1.12 mS∙cm-1 concentration, which was 0.5%. / Dissertation (MScAgric)--University of Pretoria, 2014. / tm2015 / Plant Production and Soil Science / MScAgric / Unrestricted

Les problèmes inverses de localisation de sources en électroencéphalographie (EEG) consistent à retrouver le lieu d'origine dans le cerveau des signaux mesurés sur le scalp. La qualité du résultat de localisation dépend des modèles géométriques et de conductivité électrique utilisés pour la résolution du problème. Parmi les tissus composant la tête, le crâne est celui dont la conductivité est la plus influente, en particulier à cause de sa faible valeur. De plus, le crâne humain est un tissu osseux comportant des parties dures et spongieuses, d'épaisseurs variables. Sa composition est très variable selon les individus, en termes de géométrie et de valeurs des conductivités, d'où la nécessité de développer des technique d'estimation de conductivités inconnues dans le crâne. Le but de cette thèse est de réduire l'incertitude sur la conductivité du crâne, pour des géométries sphériques et réalistes, en particulier en vue d’améliorer les résultats d'estimation des sources dans le problème inverse EEG. Dans le cas d'un domaine sphérique à 3 couches, l'existence, l'unicité et la stabilité de la conductivité dans la couche intermédiaire (crâne) sont discutées, et une procédure de reconstruction est proposée. Puis deux modèles plus réalistes de tête sont étudiés, l'un pour lequel le crâne est modelisé par un seul compartiment, l'autre dans lequel les parties spongieuses et dure sont distinguées. Des simulations numériques mettent en évidence le rôle de la structure interne du crâne pour la détermination de sa conductivité. / One of the major issues related to electroencephalography (EEG) is to localize where in the brain signals are generated, this is so called inverse problem of source localization. The quality of the source localization depends on the accuracy of the geometry and the electrical conductivity model used to solve the problem. Among the head tissues, the skull conductivity is the one that influences most the accuracy of the source localization, due to its low value. Moreover, the human skull is a bony tissue consisting of compact and spongy bone layers, whose thickness vary across the skull. As the skull tissue composition has strong inter-individual variability both in terms of geometry and of individual conductivity, conductivity estimation techniques are required in order to determine the unknown skull conductivity. The aim of this thesis is to reduce the uncertainty on the skull conductivity both in spherical and realistic head geometries in order to increase the quality of the inverse source localization problem. Therefore, conductivity estimation is first performed on a 3-layered spherical head model. Existence, uniqueness and stability of the conductivity in the intermediate skull layer are discussed, together with a constructive recovery scheme. Then a simulation study is performed comparing two realistic head models, a bulk model where the skull is modelled as a single compartment and a detailed one accounting for the compact and spongy bone layers, in order to determine the importance of the internal skull structure for conductivity estimation in EEG.

A graphite foam was developed at Oak Ridge National Laboratory (ORNL) by Dr. James Klett and license was granted to POCO Graphite, Inc. to manufacture and market the product as PocoFoam™. Unlike many processes currently used to manufacture carbon foams, this process yields a highly graphitic structure and overcomes many limitations, such as oxidation stabilization, that are routinely encountered in the development of carbon foam materials. The structure, thermal properties, electrical resistivity, isotropy, and density uniformity of PocoFoam™ were evaluated. These properties and characteristics of PocoFoam™ are compared with natural and synthetic graphite in order to show that, albeit similar, it is unique. Thermal diffusivity and thermal conductivity were derived from Fourier's energy equation. It was determined that PocoFoam™ has the equivalent thermal conductivity of metals routinely used as heat sinks and that thermal diffusivity is as much as four times greater than pure copper and pure aluminum. SEM and XRD results indicate that PocoFoam™ has a high degree of crystalline alignment and near theoretical d spacing that is more typical of natural flake graphite than synthetic graphite. PocoFoam™ is anisotropic, indicating an isotropy factor of 0.5, and may yield higher thermal conductivity at cryogenic temperatures than is observed in polycrystalline graphite.

While the laws of ionization and conductivity in polar systems are well understood and appreciated in the art, theories related to nonpolar systems and their conductivities remain elusive. Currently, multiple conflicting models, with limited experimental verification, exist that aim to explain and predict the mechanisms by which ionization occurs in nonpolar media. A historical overview of the field of nonpolar electrochemistry, dating back to the 1800’s, is presented herein with a focus on the work of prominent scientists such as Fuoss, Bjerrum, and Onsager, who’s pioneering discoveries serve as the scientific foundation for our understanding of ionization in nonpolar systems, from which our knowledge of ion-pairs (i.e. re-associated solvated ions which do not contribute to the overall conductivity of the system) stems. Recent work in the field of nonpolar electrochemistry has focused on ionization models such as the Disproportionation Model and the Fluctuation Model, which ignore ion-pair formation and the existence of these neutral entities altogether. While, the “Ion-Pair Conductivity Model”, the novel conductivity model introduced and explored herein, primarily focuses on the critical role these neutral entities play with regards to the electrochemistry in these systems. The “Ion-Pair Conductivity Model” for predicting and modeling the conductivity of mixtures of nonpolar liquids, arises from the Dissociation Model, a third ionization model for predicting conductivity in nonpolar systems which accounts for ion-pair formation, and is rooted in the foundational work of Fuoss, Bjerrum, Onsager, and others. The Ion-Pair Conductivity Model’s relationship to and derivation from the Dissociation Model is provided, including the justified assumptions and simplifications of the model to create a more concise approach for predicting conductivity in nonpolar systems that can be readily applied to real-world scenarios. In order to substantiate this model as a reliable method for predicting conductivity in nonpolar solutions, experimental results from studies examining various two-component solutions comprised of an amphiphile and nonpolar liquid, analyzed under a variety of conditions, across the entire concentration spectra, from pure nonpolar liquid to pure amphiphile, are compared to the Ion-Pair Conductivity Model. The role additional properties such as water contamination, ion size, concentration of free ions, range of ion-pair existence, shear and longitudinal rheology in these nonpolar solutions are also explored. The fitting of the experimental data and theoretical model demonstrates the ability of the Ion-Pair Conductivity Model to accurately predict conductivity in two-component solutions of an amphiphile and nonpolar liquid. Furthermore, it emphasizes the importance that ion-pairs play in the conductivity of such a system. Lastly, it reinforces the usefulness of the Ion-Pair Conductivity Model as a tool for investigating the mechanisms by which ionization in nonpolar media occur. Ultimately, the goal of this study is to validate the Ion-Pair Conductivity Model as a robust and precise model for characterizing conductivity in nonpolar solutions across a diverse set of nonpolar systems. Allowing us, and others, to better understand and exploit the intrinsic properties of conductivity in these systems to advance a number of technologies in the field electrochemistry, such as a-polar paints, electrophoretic inks, and electrorheological fluids.

In this thesis, two important thermo-physical properties of nanofluids: thermal conductivity and viscosity together with shelf stability of them are investigated. Nanofluids are defined as colloidal suspension of solid particles with the size of lower than 100 nanometer. Thermal conductivity, viscosity and stability of nanofluids were measured by means of TPS method, rotational method and sedimentation balance method, respectively. TPS analyzer and viscometer were calibrated in the early stage and all measured data were in the reasonable range. Effect of some parameters including temperature, concentration, size, shape, alcohol addition and sonication time has been studied on thermal conductivity and viscosity of nanofluids. It has been concluded that increasing temperature, concentration and sonication time can lead to thermal conductivity enhancement while increasing amount of alcohol can decrease thermal conductivity of nanofluids. Generally, tests relating viscosity of nanofluids revealed that increasing concentration increases viscosity; however, increasing other investigated parameters such as temperature, sonication time and amount of alcohol decrease viscosity. In both cases, increasing size of nanofluid results in thermal conductivity and viscosity reduction up to specific size (250 nm) while big particle size (800 nm) increases thermal conductivity and viscosity, drastically. In addition, silver nanofluid with fiber shaped nanoparticles showed higher thermal conductivity and viscosity compared to one with spherical shape nanoparticles. Furthermore, effect of concentration and sonication time have been inspected on stability of nanofluids. Test results indicated that increasing concentration speeds up sedimentation of nanoparticles while bath sonication of nanofluid brings about lower weight for settled particles. Considering relative thermal conductivity to relative viscosity of some nanofluids exposes that ascending or descending behavior of graph can result in some preliminary evaluation regarding applicability of nanofluids as coolant. It can be stated that ascending trend shows better applicability of the sample in higher temperatures while it is opposite for descending trend. Meanwhile, it can be declared that higher value for this factor shows more applicable nanofluid with higher thermal conductivity and less viscosity. Finally, it has been shown that sedimentation causes reduction of thermal conductivity as well as viscosity. For further research activities, it would be suggested to focus more on microscopic investigation regarding behavior of nanofluids besides macroscopic study.