<p> A necessity of tribological systems in aerospace applications is functioning with long lifetimes and high efficiencies. Such demanding applications can prohibit the use of traditional lubricants due to physical, chemical, thermal, or other environmental challenges. This dissertation proposed to fulfill the need for advanced performing bearing materials in challenging conditions by using the body of knowledge gathered on a particular alumina-PTFE composite to improve tribomaterials design. </p><p> The current model for alumina-PTFE necessitates a hard filler with multi-scale functionality and an operating environment that supports beneficial tribochemistry. This study proposed to satisfy the current model requirements by replacing alumina with a soft micro-sized filler that also supports tribochemistry in any environment. Several materials could meet these filler requirements, including micro-sized PEEK. A tribology study on PEEK-PTFE composites was implemented to test the proposed model for a relatively soft filler in lubricious matrix to display advanced bearing performance in conditions representing terrestrial and space operating environments. </p><p> To conduct the proposed investigation, prior work was necessary to increase sample testing capacity and improve assessment of results. First, there existed a logistical limitation to the number of polymer materials that could undergo tribology testing in a timely manner. This barrier was overcome by designing, fabricating, prototyping, and implementing a wear testing tribometer with high-throughput capabilities. Second, it was necessary to establish a standard for numerically assessing the success or failure of polymer materials in bearing applications. This goal was achieved by studying common polymers and polymer composites, with a wide range in bearing performance, to identify a quantitative metric to reliably predict polymer wear rate. Finally, the equipment and methodologies developed in this dissertation were applied to testing PEEK-PTFE composites. </p><p> This study identified PEEK-PTFE as the first solid lubricant to demonstrate ultralow wear rates and moderate friction in both dry and humid conditions, which support the high potential for this tribomaterial to fulfill the needs of the aerospace industry. The outcomes of this investigation have enhanced the understanding of tribological mechanisms driving the success of polymeric solid lubricants, and opened avenues for designing more composites to display advanced bearing performance in challenging environmental conditions.</p><p>

xvii, 183 p. : ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / In recent years, nanoscience has evolved from a multidisciplinary research concept to a primary scientific frontier. Rapid technological advancements have led to the development of nanoscale device components, advanced sensors, and novel biomimetic materials. However, potential negative impacts of nanomaterials are sometimes overlooked during the discovery phase of research. The implementation of green chemistry principles can enhance nanoscience by maximizing safety and efficiency while minimizing the environmental and societal impacts of nanomaterials. This dissertation introduces the concept of green nanosynthesis, demonstrating the application of green chemistry to the synthesis of nanornaterials. A comprehensive review of the synthesis of metal nanomaterials is presented, demonstrating how individual green chemistry principles can improve traditional synthetic routes as well as guide the design of new materials. Detailed examples of greener syntheses of functionalized gold nanoparticles with core diameters of 2-10 nm are described in subsequent chapters, beginning with a method for functionalizing citrate-stabilized gold nanoparticles that are desirable for advanced applications. Although citrate-stabilized gold nanoparticles can be easily produced from a classic procedure using mild reagents and benign methods, functionalization via ligand exchange is often unsuccessful. It was discovered that an ill-defined layer comprised of citrate and other ligands interferes with functionalization processes. By removing excess citrate in a manner where overall structure and stability is maintained, gold cores produced by this route are readily functionalized by incoming thiols, affording unprecedented control over surface composition and functionality. A direct route to functional nanomaterials using Bunte salt precursors is discussed next, describing the use of easily synthesized shelf-stable alternatives to thiols in the preparation of water-soluble gold nanoparticles. Control of core size and surface chemistry is demonstrated through simple manipulation of reagent ratios, yielding products similar to those produced by traditional direct syntheses which rely on the use of thiols. The use of functionalized nanoparticles as "building blocks" for more complex structures was demonstrated in self-assembly processes. Cationic gold particles were deposited upon DNA scaffolds to create linear arrays. A discussion of the future outlook of green nanosynthesis concludes this work, identifying immediate challenges and long-term goals. This dissertation contains previously published and co-authored materials. / Adviser: James E. Hutchison

Carbon nanotubes (CNTs) have been proposed as a material for use in printed electronics for a number of years. The potential to exploit the unique electrical and mechanical properties of these structures on the macro-scale is appealing; however there are a number of hurdles to overcome. Printing allows the deposition of CNT networks, the properties of which are governed by the CNT type and network density. The formulation of a suitable ink and deposition of a film with specific properties is challenging, and the work described in this thesis is concentrated on two specific areas, CNT ink development and CNT based device production. The CNT ink was developed by identifying key ink and dried film parameters for characterisation and assessing the effect of several major variables, namely the resin material, resin concentration, processing temperature, CNT concentration, CNT functionality and processing energy. A suitable research ink was developed and optimised using N-methyl-2-pyrrolidone as the solvent and polyvinyl alcohol as the resin at a concentration ratio of 1:1 with the CNT content. The effects of CNT concentration, CNT functionality and processing energy are shown to be interdependent. This is among the first reported studies to investigate the dependence of these factors upon a CNT ink for roll-to-roll processing. This ink system was then used in the production of CNT based thin film transistor (TFT) devices using flexography. Initially the concept was proven using MWCNTs. The design was then refined and devices were produced using SWCNTs at varying network densities. It was seen that the printing of CNT based devices using flexography is feasible, though careful control of the CNT network density is required to achieve suitable device performance. This is the first reported production of TFTs using flexography, and the first reported use of flexograi)hy to deposit CNTs.

<p> The continuous improvement of modern electronics has been sustained by the scaling of silicon based MOSFETs over the last 4 decades. However, the frequency of the processors has been saturated since 2005 when the power dissipation in CPUs reached its cooling limit (100W/cm2). The thermionic emission in MOSFET limits the SS to 60 mV/dec, which prevents CMOS from further reducing the power consumption. Tunnel-FETs (TFETs) were proposed to solve this problem by removing thermal emission. Although, <i>SS</i> &lt; 60 has been demonstrated experimentally in conventional TFETs, they suffer from low ON current, orders of magnitude lower than MOSFETs. Hence achieving high ON-current and performance requires novel device structures. </p><p> Low dimensional materials have unique features which can be used to solve the challenges of TFETs. In this work, several novel TFETs based on low dimensional materials (new channel material candidates such as Bilayer graphene, Black Phosphorous and interlayer TFETs based by stacking TMD materials) have been proposed to solve the low ON current issue. Their device performance and the scalability have been studied by means of atomistic quantum transport simulations. </p><p>

The discovery of carbon nanotubes (CNTs) has sparked great interest in the scientific world because of their remarkable electrical and physical properties. Only a thorough understanding of these properties, however, will allow CNTs to be commercially viable. Essentially, CNTs are graphite-like surfaces of sp2 hybridized carbon atoms in the form of tubes. CNTs could range from single-walled carbon nanotubes (SWNTs), consisting of one cylindrical graphite sheet to multi-walled nanotubes (MWNTs) that have concentric sheets. Nanotubes can be synthesized using a number of techniques such as electric arc–discharge, laser ablation and catalytic chemical vapor deposition (CCVD). In this project the CCVD technique was used for the synthesis of CNTs because of it simplicity and availability. The source of carbon was not the conventional hydrocarbon gas, but was camphor, a botanical hydrocarbon that is a solid at room–temperature. Ferrocene was the catalyst, not only because it has been used before in the synthesis of nanotubes, but it appears to be one of the best catalysts during the CCVD synthesis of nanotubes. The presence of nitrogen gas is known to assist in the synthesis of CNTs that have a bamboo–like structure; hence the effect of carrier gases such as nitrogen, argon/hydrogen and argon on the quality of nanotubes synthesized was investigated. Initially, the optimal experimental method for the synthesis of CNTs was determined by varying the reaction path length, temperature, mixing the catalyst and carbon source together or keeping them separate and varying the %m/m of the catalyst to carbon source. It was found that either an increase in the reaction temperature or an increased path length resulted in an increase in the mass of product obtained, whereas mixing the catalyst and carbon source together as opposed to them being separated only caused a slight variation in the mass of product synthesized. The mass of product synthesized also increased as the catalyst concentration increased. The remainder of the project was aimed at investigating the role of different gases: nitrogen, argon and hydrogen (in argon) in the CCVD synthesis of CNTs. The resulting materials were characterized using transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and laser Raman analysis. The results indicated that this method could be tailored to synthesize either carbon spheres or carbon nanotubes of specific diameters and quality. Finally, in an attempt to synthesize aligned carbon nanotubes, catalyst supports {characterized using Brunauer-Emmett-Teller analysis (BET)} namely; silica, alumina and magnesium oxide were used. Although this was not successful for the synthesis of aligned CNTs under the conditions used, alumina showed the most promise. / Mr. S. Durbach Dr. R. W. Krause

Frequency multiplication is an effect that arises in electronic components that exhibit a non-linear response to electromagnetic stimuli. Barriers to achieving very high frequency response from electronic devices are the device capacitance and other parasitic effects such as resistances that arise from the device geometry and are in general a function of the size of the device. In general, smaller device geometries and features lead to a faster response to electromagnetic stimuli. It was posited that the small size of the silicon nanowires (SiNWs) would lead to small device capacitance and spreading resistance, thus making the silicon nanowires useful in generating microwave and terahertz radiation by frequency multiplication. To verify this hypothesis, silicon nanowires based devices were fabricated and investigated using two experimental setups. The setups were designed to allow the investigation of the nanowire based devices at low frequencies and at high frequencies. Both setups consisted of an RF/microwave source, filters, waveguide, and a spectrum analyzer. They also allowed the characterization of the samples with a semiconductor parameter analyzer. The first step in the investigation of the SiNW devices was to install them in the waveguides and perform Current-Voltage (I-V) sweeps using the semiconductor parameter analyzer. The devices that exhibited the non-linear I-V characteristics typical of diodes were further investigated by first exposing them to 70MHz and 500MHz frequencies in the low frequency setup and then to 50GHz microwaves in the high frequency setup. The response of the devices was captured with a spectrum analyzer. The results demonstrate that the non-linear effect of frequency multiplication is present in nanowire devices from DC to 100GHz. The HF setup provides a platform that with an appropriate detector can be used to detect harmonics of the SiNWs in sub-millimeter/THz region of the electromagnetic spectrum.

No description available.

Magister Scientiae - MSc / Characterization of nanophase electrocatalysts, which are an essential part in the direct methanol fuel cell (DMFC) and solid-polymer electrolyte (SPE) electrolyser, have been studied in this work. Their nanoparticulate size raises significant challenges in the analytical techniques used in their structural and chemical characterization. Hence, the applicability of analytical protocols for the qualitative and quantitative characterization of structural and chemical properties of nanophase platinum and platinum-ruthenium electrocatalysts was investigated. Also, fabricated carbon-supported platinum, platinum-ruthenium, iridium oxide, and mesoporous silica-templated platinum electrocatalysts were screened on the basis of their electrocatalytic activity. A set of structural and chemical parameters influencing the performance of nanophase electrocatalysts was identified. Parameters included crystallinity, particle size, particle size distribution, agglomeration, aggregation, surface area, thermal stability, chemical speciation, electrocatalytic activity, and electrochemically-active surface area. A large range of analytical tools were employed in characterizing the electrocatalysts of interest. High accuracy and precision in the quantitative and qualitative structural characterization of nanophase electrocatalysts, collected by x-ray diffractometry and transmission electron microscopy, was demonstrated. Selected-area electron diffraction was limited to a rapid qualitative evaluation of electrocatalyst polycrystallinity and crystal symmetry. Scanning electron microscopy was limited to the qualitative evaluation of the agglomeration state of supported electrocatalysts. High-performance particle sizing was unable to resolve the particle size of the electrocatalyst from that of the support and was therefore employed in the quantitative investigation of aggregate size and size distribution in supported electrocatalysts. The technique produced high precision data illustrating the reproducibility of the aggregate size data. N2-physisorption produced surface area and pore size distribution data of high quality, but was unable to determine surface areas specific to the metal phase in supported electrocatalysts. The technique was deemed inconsistent in the accurate determination of average pore size. The resolution of scanning electrochemical microscopy and proton-induced x-ray emission spectroscopy (SECM) did not allow for an investigation of characteristics at the nanoscale. Quantitative chemical information was difficult to extract from SECM maps and the technique was limited to the qualitative characterization of surface topography. Thermogravimetry was suitable for the qualitative investigation of the thermal stability of the nanophase electrocatalysts of interest. In this study, temperature-programmed reduction was able to qualitatively speciate the surface chemical state and investigate the strength of the metal-support interaction in supported nanophase electrocatalysts. Cyclic voltammetry and linear-sweep voltammetry were employed in the electrochemical characterization of nanophase electrocatalysts and both qualitative and quantitative information were obtained. The techniques were able to discriminate between various commercial and fabricated electrocatalysts and identify new highly-active materials. Preparation variables could be critically evaluated for the fabrication of cost-effective highly-active nanophase electrocatalysts. Certain techniques were deemed to be highly applicable in discriminating between high and low activity nanophase electrocatalysts based on their structural and chemical properties. The electrocatalyst characterization strategy and methodology was developed and will be implemented for future characterization of nanophase electrocatalysts. / South Africa

Magister Scientiae - MSc / The main objective of this work it to produce membrane electrode assemblies (MEAs) that have improved performance over MEAs produced by the conventional manner, by producing highly efficient, electroactive, uniform catalyst layers with lower quantities of platinum electrocatalyst. The catalyst coated membrane (CCM) method was used to prepare the MEAs for the PEM fuel cell as it has been reported that this method of MEA fabrication can improve the performance of PEM fuel cells. The MEAs performances were evaluated using polarisation studies on a single cell. A comparison of polarisation curves between CCM MEAs and MEAs produced in the conventional manner illustrated that CCM MEAs have improved performance at high current densities (>800 mA/cm2). / South Africa

The enhanced functionality and tunability of electronic nanomaterials enables the development of next-generation photovoltaic, optoelectronic, and electronic devices, as well as biomolecular tags. Design and efficient synthesis of such semiconductor nanomaterials require a fundamental understanding of the underlying process-structure/composition-property-function relationships. To this end, this thesis focuses on a systematic, comprehensive analysis of the physical and chemical phenomena that determine the composition and properties of semiconductor nanomaterials. Through synergistic combination of computational modeling and experimental studies, the thesis addresses the thermodynamics and kinetics that are relevant during synthesis and processing and their resulting impact on the properties of silicon thin films and ternary quantum dots (TQDs) of compound semiconductors. The thesis presents a computational study of the growth mechanisms of plasma deposited a-Si:H thin films based on kinetic Monte Carlo (KMC) simulations according to a transition probability database constructed by first-principles density functional theory (DFT) calculations. Based on the results, a comprehensive model is proposed for a-Si:H thin-film growth by plasma deposition under conditions that make the silyl (SiH3) radical the dominant deposition precursor. It is found that the relative roles of surface coordination defects are crucial in determining the surface composition of plasma deposited a-Si:H films and should be properly accounted for. The KMC predictions for the temperature dependence (over the range from 300 K to 700 K) of the surface concentration of SiHx(s) (x = 1,2,3) surface hydride species, the surface hydrogen content, and the surface dangling-bond coverage are in agreement with experimental measurements. In addition, the thesis details a systematic analysis of equilibrium compositional distribution in TQDs and their effects on the electronic and optoelectronic properties. Formation of hetero-nanostructures, such as core/shell-like structures, through atomic-scale assembly driven by equilibrium surface segregation is studied as a function of nanocrystal size, composition, and temperatures for TQD morphologies that include faceted equilibrium nanocrystal shapes for ZnSe1-xTex and InxGa1-xAs TQDs; the results are based on coupled compositional, structural, and volume relaxation of the nanocrystals according to Monte Carlo and conjugate-gradient methods employing a DFT-parameterized description of interatomic interactions. A phenomenological species transport theory also is developed that explains the concentration profiles due to surface-segregation-induced ordering of constituent and dopant atoms in the dilute limit. The nm-scale diffusion lengths in nanocrystals introduce an interesting interplay between the kinetic and thermodynamic stability of interfaces. The thermodynamic stability of such interfaces in ZnSe 1-xSx TQDs are investigated based on DFT calculations combined with X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectra of TQDs that are synthesized and annealed using colloidal methods. The results demonstrate the possibility of compositional redistribution that causes degradation over time of core/shell TQD electronic properties, with far reaching implications for the use of such nanostructures in devices. Electronic structure calculations of ZnSe1-xSx (type-I) and ZnSe1-xTe x (type-II) TQDs elucidate the impact of composition and compositional distribution on the electron density distribution, density of states, and band gap of the TQDs. The resulting relationships with respect to the distributions in the TQDs of constituent/dopant/impurity atoms (core/shell vs. alloyed TQDs) provide an interpretation for the key features observed in the PL spectra, as well as useful guidelines for improving the design and device performance of TQDs.