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The following research investigates the effect that axial cooling channels will have on the performance of the thermal management system of a hypothetical switched reluctance motor. A baseline motor with no axial cooling will be compared to an identical motor with the innovative cooling design implemented. This will allow for a direct comparison of the two designs, with a quantifiable performance increase determined through thermal simulations. The ability of a polymer composite to transfer heat to the axial cooling channel is also explored. A detailed material selection process is discussed with the result being an epoxy polymer composite. The material development of a thermally enhanced polymer composite is then investigated to achieve a maximum thermal conductivity material that can exist within the stator slot to achieve enhanced thermal energy transfer. / Thesis / Master of Applied Science (MASc) / The desire to increase the power density of electric machines is becoming an increasingly popular challenge, especially in the automotive industry. With the advent of electrified powertrains as an alternative solution to conventional internal combustion powered vehicles, the topic of increasing electric motor performance is becoming very attractive area of research. An important aspect of electric motor performance is the way in which the generated thermal energy is managed. Through material development and innovative motor design, there exists the opportunity to cool electric motors through cooling paths flowing axially through the stator. This ‘axial cooling’ design has the opportunity to greatly increase motor cooling by removing thermal energy directly from its main source, the motor windings. The following research is aimed at the thermal design of the axial cooling and the role in which thermally conductive polymer composites play in order to enhance motor cooling.

<p> An electrically conductive thermoplastic composite (ECTPC) consists of electrically conductive filler(s) at a concentration above percolation threshold distributed in an insulating polymer matrix. The high concentration of the filler required to achieve high electrical conductivity for ECTPC is usually accompanied with the deterioration of mechanical properties and a large increase in the viscosity which prevents feasible processing of these materials in common polymer processing equipments such as injection molding machinery. The initial focus of this work was to control these drawbacks by using combinations of conductive fillers namely carbon fiber (CF) and carbon black (CB) to create a hybrid-filler composite. Cyclic olefin copolymer (COC), an amorphous polyolefin, was used as the matrix material. It was found that carbon black and carbon fiber synergistically contribute to the transport of electrons through the matrix. The synergism exists at various filler concentrations including when one of the fillers was present below its percolation threshold, but not at high carbon fiber content. Results showed that where the concentration of CF was several fold higher than carbon black a good trade-off between viscosity and conductivity can be achieved so that the obtained composites can be reasonably processed tn common processing equipment e.g. in an injection molding machine </p> <p> Carbon fiber is preferred to carbon black as it leads to ECTPC with higher electrical conductivity and lower viscosity. However, the high aspect ratio fibers preferentially align in the flow direction leading to ECTPCs which have electrical conductivity several orders of magnitude greater in the in-plane rather than through-plane. We focused on foaming as a strategy to reorient the fibers toward the through-plane direction in foam injection molding. Through a fractional factorial experimental design, the effect of injection rate, melt temperature and mold temperature on electrical conductivity was screened at two levels for foam and nonfoam COC/CF(lO vol%)-CB(2 vol%) injection molded composites. It was found that foaming significantly enhanced the through-plane fiber orientation and through-plane conductivity of the hybrid composite at low injection rate and high melt temperature. The concurrence of the melt flow and bubble growth was considered to be the key mechanism for fiber reorientation while the cell size and shape should not disrupt the conductive path spanning the bulk of the material. </p> <p> The importance of the relative length scale of the fillers on cell size and subsequently, electrical conductivity was investigated by injection molding. Results showed that where the length scale of the filler was comparable to the cell size, as for foamed COC/CF composites, the conductivity considerably decreases with foaming. The drop was greater in the through plane direction and smaller in the in-plane direction for the composites with larger average fiber length. Also smaller cells led to a larger drop in the composite conductivity. It was observed that where the length scale of the filler was much smaller than the cell size as such for COC/CB composites, foaming enhanced the electrical conductivity particularly in the through-plane directions and its effects became more pronounced at lower carbon black concentrations. It was proposed that induced carbon black coagulation by foaming was the main reason for the observed improvement in conductivity. For COC/CF-CB hybrid composites, enhancement in through-plane conductivity, particularly at CB concentration below percolation, via foaming inferred that CB aggregates significantly contributed in improving fiber-fiber contacts. </p> <p> Reorientation of the fibers by foaming was found to be very dependent on processing conditions. High viscosity and fiber- fiber interactions can hinder fiber rotation. The general understanding of the investigation was that fiber reorientation may occur where the cells are much larger than the fibers. In comparison, a series of nonfoam injection molded composites containing CF, CB and CF-CB were foamed in a batch process to avoid flow effects. The insignificant change in fiber orientation with foaming proved that fibers can not rotate by the growth of an adjacent cell in the absence of shear. Also, a large drop in electrical conductivity with foaming as compared to the foam injection molded composites suggested that particle relocalization can not occur in batch foaming. </p> / Thesis / Doctor of Philosophy (PhD)

Two-dimensional (2D) layered hexagonal boron nitride (h-BN) has become a very popular material in nanoelectronics in recent years because of its extraordinary chemical stability and thermal conductivity [1]. Recently, h-BN is also commonly used as a dielectric material [2], and research in this area is still in its early stages. The commonly used methods for fabricating h-BN include mechanical exfoliation and chemical vapor deposition (CVD). CVD is a recognized industry-compatible method for producing large-area h-BN. However, studies have shown that multilayer h-BN grown by CVD is polycrystalline and contains multiple local defects [3]. These defects and inhomogeneity cannot be avoided and lead to small amounts of atom-wide amorphous regions that have weak dielectric strength [3]. Although the general characteristics of h-BN prepared by these two fabrication methods can be learned from different works in the literature, it is difficult to study the quality of h-BN without systematically comparing the differences between the two growth methods under the same experimental conditions and with large number of samples. This also makes it difficult for researchers to choose the best-quality h-BN. In this work, the morphological characteristics and electrical properties of mechanically exfoliated h-BN and CVD-grown h-BN from different sources have been compared under different conditions. Commercially available h-BN flakes mechanically exfoliated from NIMS h-BN bulk crystal show no leakage current at electrical fields up to 25.9 MV/cm, and above this applied electrical force, the size of the conductive spots is extremely small (1.99 ± 1.81 nm2). On the contrary, “monolayer” CVD-grown h-BN samples from Graphene Supermarket were shown to be amorphous in ~20% of their area, which makes them appear discontinuous from an electrical point of view, plus they contain large thickness fluctuations up to 6 layers. Moreover, in nanoelectronic measurements collected with a conductive atomic force microscope (CAFM) working in vacuum, mechanically exfoliated h-BN showed better electrical homogeneity and presented later dielectric breakdown compared to the h-BN samples fabricated by the CVD method.

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This work describes the techniques of assembling colloidal gold on flexible polymeric substrates from solution. The process takes advantage of the strong affinity of gold to thiol and amino groups. Polymeric substrates were modified with silanes having these functional groups prior to Au attachment or in the case of poly(urethane urea) (PUU), no surface functionalization was required. This polymer has terminal amine and N-H groups on the polymer chain, which can act as coordination points for gold. Immersion in the colloidal gold solution led to the formation of a monolayer. Increased coverage was obtained by two methods. The first was a reduction or "seeding" process, where Au was reduced onto the attached particles on the surface. The second was using different linker molecules and creating a multilayered film by a layer-by-layer assembly. Three linker molecules of different lengths were used. Films fabricated using the smallest molecule had the least resistance whereas films fabricated with the longest molecule were not conductive. The resistance of these films may be varied easily by heating. Heating the films at temperatures as low as 120 °C caused a dramatic decrease in the resistance of over six orders in magnitude. Successful attachment of gold to PUU with very good adhesion properties was also demonstrated. The attachment of gold was stable in different solvents. Upon stretching the PUU-Au films, it was observed that there is a reversible resistance increase with strain and at a certain strain, the film becomes non-conductive. This sharp transition from conductive to insulating has potential applications in flexible switches and sensors. A hysteresis in the strain-resistance curves, analogous to the hysteresis in the stress-strain curves of the polymer was also observed. Using PUU as an adhesive agent, gold electrodes were successfully assembled on Nafion-based polymer transducers. These materials showed comparable actuation behavior to the electrodes made by the Pt-reduction method, with the added advantage of the ability to form patterned electrodes for distributed transducers. Patterning techniques were developed to form colloid-polymer multilayers for use in photonic crystal materials using selective deposition on patterned silane monolayers. Patterns of gold electrodes were also made on flexible polymers using a photoresist-based method. / Ph. D.