Conductive behaviour of carbon nanotube based composites

This project was basically exploratory in the electrical properties of carbon nanotube (CNT) based materials. The direct current (DC) conductivity of CNT/polymer composites was computed by using equivalent circuit method and a three dimensional (3-D) numerical continuum model with the consideration of tunneling conduction. The effects of the potential barrier of polymer and the tortousity of CNTs on the conductivity were analyzed. It was found that both of percolation threshold and DC conductivity can be strongly affected by the potential barrier and the tortousity. The influence of contact resistance on DC conductivity was also computed, and the results revealed that contact resistance and tunneling resistance had significant influences on the conductivity, but did not affect the percolation threshold. The microstructure-dependent alternating current (AC) properties of CNT/polymer composites were investigated using the 3-D numerical continuum model. It was found that AC conductivity and critical frequency of CNT/polymer composites can be enhanced by increasing the curl ratio of CNTs. In the mid-range CNT mass fraction, with increasing curl ratio of CNTs, AC conductivity, interestingly, became frequency-dependent in low frequency range, which cannot be explained by reference to the percolation theory. A proper interpretation was given based on the linear circuit theory. It was also found that the critical frequency can also be affected by the size of CNT cluster. Series numerical formulas were derived by using a numerical capacitively and resistively junction model. In particular, this work introduced an equivalent resistor-capacitor (RC) circuit with simple definitions of the values of contact resistance and average mutual capacitance for CNT/polymer nanocomposites. Theoretical results were in good agreement with experimental data, and successfully predicted the effect of morphology on the AC properties of CNT/polymer composites. DC and AC conductivities of multi-walled carbon nanotube (MWCNT)/graphene oxide (GO) hybrid films were measured for selected MWCNT mass fractions of 10%, 33.3%, 50%, 66.7%, and 83.3% using four-probe method. The experimental results were fitted using scaling law, and relatively high percolation threshold was found. This high percolation threshold was understood in terms of the potential energy and intrinsic ripples and warping in the freestanding graphene sheets. The capacitance of these hybrid films were measured using the voltmeter-ammeter-wattmeter test circuit with different voltages and heat treatments. The MWCNT/GO film showed relatively high specific capacitance (0.192F/cm3 for the mass fraction of 83.3%) and power factor compared to conventional dielectric capacitors. Both of measured capacitance and power factor can be enhanced by increasing testing voltages. The capacitance of MWCNT/GO films rapidly decreased after heat treatments above 160 ℃. This decrease was caused by redox reaction in the GO sheets. The capacitive behaviour of MWCNT/GO hybrid films was also interpreted by using the equivalent circuit model. Single-walled carbon nanotube (SWCNT) and SWCNT/Poly(vinyl alcohol) (PVA) films were used to form a piezoresistive strain sensor. Both of static and dynamic strain sensing behaviours of SWCNT and SWCNT/PVA films were measured. It was found that the sensitivities of these films decreased with increasing their thicknesses. The SWCNT film with a thickness of 1900 nm and SWCNT/PVA film exhibited viscoelastic sensing behaviour, because van der Waals attraction force allowed axial slippages of the smooth surface of nanotubes. A numerical model was derived based on the dynamic strain sensing behaviour. This model could be useful for designing CNT strain sensors. Finally, thermoelectric power (TEP) of deformed SWCNT films with various thicknesses was measured. It was observed that positive TEP of SWCNT films increased with increasing stain above the critical point. The experimental results were fitted by using a numerical model in terms of a variation of Nordheim-Gorter relation and fluctuation induced tunneling (FIT) model. From the numerical model, it was found that the increase of TEP above the critical strain resulted from the positive term of the contribution from the barrier region, and the effect of barrier regions decreases with increasing the thickness of the film

Electroplating of Nanostructures

The electroplating was widely used to electrodeposit the nanostructures because of its relatively low deposition temperature, low cost and controlling the thickness of the coatings. With advances in electronics and microprocessor, the amount and form of the electrodeposition current applied can be controlled. The pulse electrodeposition has the interesting advantages such as higher current density application, higher efficiency and more variable parameters compared to direct current density. This book collects new developments about electroplating and its use in nanotechnology

Cationic surfactant modification and its impact on the engineering behaviors of montmorillonite.

This study focuses on the microstructure of organoclays after organic surfactant modifications and the potential use in geoenvironmental engineering applications including waste containment in earthen barrier, rheological control agents for drilling fluid and soil stabilization. Organoclays, or clays modified by organic matters, are often synthesized by exchanging the naturally occurring interlayer inorganic cations (e.g., Na+, Ca2+) of the clay with organic cationic surfactants. In this research, montmorillonites intercalated with two quaternary ammonium surfactants, hexadecyltrimethylammonium (HDTMA+) and bis (hydrogenated tallow alkyl) dimethyl ammonium were used as the representative organo-rich, partitioning clays. The laboratory characterization techniques including XRD, TEM and FT-IR were employed to examine the interactions between montmorillonite minerals, surfactants, and organic sorbates. The results of XRD and TEM showed the successive interlayer expansion of montmorillonite because of intercalation of surfactants and hydrocarbons sorbates. The FT-IR results further confirmed the arrangement of the intercalated surfactant and the organic sorbates due to primary and secondary sorption. To understand the engineering behaviors of organoclays in earthen barriers, the free swelling and hydraulic conductivity tests were conducted to evaluate the permeability of compacted clay and geosynthetic clay liner (GCL) amended with HDTMA-organoclay. The results suggested that the addition of organoclay (less than 10%) in compacted clay slightly increased the permeability of the mixture to water. However, due to the interaction between the organophilic phase in organoclays and non-polar liquids, low amounts of organoclay within the compacted clay admixture significantly decreased its permeability for non-polar liquids such as gasoline. Moreover, it was observed that low weight percentages of HDTMA-bentonite (up to 20% by weight) had little or no impact on the hydraulic conductivity of the Na-bentonite or Ca-bentonite GCL. However, higher dosages of organoclay in GCLs could reduce the permeability to organic fluids such as gasoline. The impact of the amount of organoclay additives, pressure and temperature on the rheological behavior of organoclay/oil-based drilling fluids was investigated. The obtained results from XRD test suggested that the oil molecules entered the PM199 interlayer, resulting in swelling and exfoliation of PM199. It was observed that the viscosity of 5% PM199 suspension slightly decreased by increasing the temperature from 25 to 60 °C, and then the viscosity increased when the temperature raised from 60 to 150 °C. Moreover, it was found that the viscosity of 5% PM199 suspension increased when the pressure increased from 0 to 200 bar due to physical changes on both oil and organoclay particles. The effectiveness of organoclay and Portland cement for the solidification and stabilization (S/S) of contaminated soils was investigated in the laboratory. The results indicated that the addition of cement (5% or 10% by weight) reduced the hydraulic conductivity and increased the compressive strength of the solidification and stabilization soil specimens. Additionally, the leaching test results indicated that the addition of organoclay during solidification and stabilization significantly reduced the leaching of naphthalene and phenanthrene from the stabilized soil specimen. The results suggested that organoclay particles sorbed the organic contaminates and consequently reduced the naphthalene and phenanthrene leachate concentration. Also, it was observed that the naphthalene and phenanthrene leachate concentration decreased by increasing the curing time of S/S products. Overall, this study performed laboratory tests to obtain information regarding the microsturcture of cationic surfactant modifed bentonties and their engineering behaviors (e.g. hydrophobicity, swelling, permeability, stability in oil suspension). The obtained results are expected to yield significant insights into their potential applications as sorbents in hydraulic and sorptive barriers for organic compounds; rheological control additives in oil-based drilling fluid; agents for stabilization and solidification of contaminated soils

Development of a Low-Current Plasma-Based Cathode using the Emitter Material C12A7 Electride for Space Applications

Efficient electron sources are crucial for any space-based mission, especially when using electric thrusters. In many respects, hollow cathodes are a baseline technology due to their power-efficient electron emission in the desired current range and the potentially long lifetime of these emitters. However, the delicate design of the heater, with the associated constraints on its operation, and the high degradation of state-of-the-art materials to new propellant options under evaluation for electric space propulsion systems, are severe limitations of current systems. To address some of the most pressing challenges with cathodes, a heaterless plasma-based cathode using the emitter material C12A7 electride has been developed and is described in this thesis. The cathode has been developed with the requirements of an electrodynamic tether demonstration mission in mind. C12A7 electride is an electrically conductive ceramic that has recently attracted much attention as a potential electron emitter in hollow cathodes. However, there appear to be significant challenges with the material itself, requiring careful design evaluation and thorough testing to gain a sufficient understanding of the material's behavior. Most importantly, material degradation in the harsh environment of a plasma. Throughout the thesis, an optimized electride material was developed and tested, yielding a ceramic-metal composite with greatly improved plasma performance compared to pure C12A7 electride material. In addition, a special design of a plasma-based cathode was developed and described, which respects the unique properties of the material and allows convenient operation, and thus characterization and optimization of the cathode. Several milestones have been achieved, including endurance operation for nearly \num{1000} hours, successful operation with a Hall-effect thruster, characterization of the cathode in the discharge current range of \qtyrange{0.2}{2}{\A}, reduction of the flow rate required for ignition and operation down to \qty{2}{\sccm}, and heaterless ignition cycling for up to \num{3300} cycles with a single insert. The observed performance of the cathode was eventually compared with performance data reported in the literature using state-of-the-art materials and showed reasonable comparability. In particular, advantages over state-of-the-art cathodes were identified in terms of ignition behavior: Requiring only \qty{2}{\sccm} of krypton and a potential of less than \qty{400}{\V}, and reaching steady-state operation in less than a few tens of milliseconds, the performance was better than reported in the literature. Combined with the acceptable discharge performance, these results motivate the further development of such an electride cathode for space applications. Due to the simplicity of such a cathode, applications for a wide range of industrial processes may also be considered.:1 - Introduction 2 - Cathode Theory 3 - C12A7 Electride 4 - Scope of Development 5 - Design Development 6 - Thruster Operation 7 - Endurance Operation 8 - Electride Cathode for Low Current EDT Operation 9 - Additional Tests with the Electride Cathode 10 - Discussion of Results and Further Steps 11 - Conclusion Bibliography Appendi

Microscopy Conference 2017 (MC 2017) - Proceedings

Das Dokument enthält die Kurzfassungen der Beiträge aller Teilnehmer an der Mikroskopiekonferenz "MC 2017", die vom 21. bis 25.08.2017, in Lausanne stattfand

Microscopy Conference 2017 (MC 2017) - Proceedings

Das Dokument enthält die Kurzfassungen der Beiträge aller Teilnehmer an der Mikroskopiekonferenz "MC 2017", die vom 21. bis 25.08.2017, in Lausanne stattfand

Hydroxyapatite Nanoparticles and Nanobiocomposite Scaffold for Protein Adsorption and Release

Spherical, rod and fibrous hydroxyapatite (HA) nanoparticles synthesize through a common co-precipitation technique and fabricate macroporous HA-gelatin nanobiocomposite scaffolds for protein adsorption/release study. Three fundamental processing parameters such as solution pH, temperature and Ca/P ratio synchronize the morphology and crystallinity of nano HA from identical precursors Ca(CH3COO)2 and KH2PO4. Dispersion study illustrates the HA nanoparticle suspension stability phenomenon in aqueous media. Rod shaped HA exhibits relatively better bovine serum albumin (BSA) protein adsorption efficacy with compare to other two morphologies. In aqueous media, one gram nanorod HA particle adsorb 28 mg BSA within a time frame of 48 h and subsequently 75 wt.% release after 96 h in phosphate buffer solution. Low temperature freeze casting of homogenous aqueous slurry of HA nanoparticles, gelatin and biocompatible polyvinyl alcohol binder develops nano HA – gelatin nanobiocomposite macroporous scaffolds. Freeze casted nanorod HA-gelatin macroporous (70 vol.%) scaffold demonstrate highest yield compressive strength of ~2 MPa compare to other scaffolds prepared from spherical and fibrous HA because of high surface area and the effective anchoring. An optimum cryogenic treatment time at 77K promotes the mechanical response of this low strength scaffold and designates as cryo-treated hydroxyapatite–gelatin macroporous scaffold (CHAMPS). CHAMPS has a high degree of interconnected pores of 50-200 μm in size, compressive strength up to 5.6 MPa and larger strain failure up to 25%. L929 mouse fibroblast cell interaction supports the cytotoxicity and cell adherence behaviour with CHAMPS. Porous scaffold exhibits bioactivity in simulated body fluid (SBF) solution through preferable deposition of carbonated apatite layer around the pores. Biodegradation of scaffold in tris-HCl solution reveals a slow but systematic decrease in weight over incubation up to 7 days. Importantly, the excellent adsorption (upto 50 wt.%) and release (upto 60 wt.% of adsorbed protein) of BSA within 48 h has been uniquely attributed to the inherent porous microstructure of the CHAMPS. Protein adsorption behaviour for both of the particles and scaffolds follow the classical Langmuir isotherm. The extensive micro-computed tomography (micro-CT) analysis establishes cancellous bone-like highly interconnected and complex porous architecture of the protein loaded and original CHAMPS. Overall, the present study provides an assessment of the interaction of protein with HA nanoparticles and their cryotreated HA-gelatin scaffold in vitro to support as drug delivery media and tissue engineering, respectively

Structural Framework for Flight II: NASAs Role in Development of Advanced Composite Materials for Aircraft and Space Structures

This monograph is organized to highlight the successful application of light alloys on aircraft and space launch vehicles, the role of NASA in enabling these applications for each different class of flight vehicles, and a discussion of the major advancements made in discipline areas of research. In each section, key personnel and selected references are included. These references are intended to provide additional information for technical specialists and others who desire a more in-depth discussion of the contributions. Also in each section, lessons learned and future challenges are highlighted to help guide technical personnel either in the conduct or management of current and future research projects related to light-weighting advanced air and space vehicles

Development of microtubular solid oxide fuel cells design, fabrication and performance.

Solid oxide fuel cells (SOFCs) are the most efficient energy conversion devices known. Many designs exist, with most current ones based on planar, tubular or so-called hybrid geometries. Tubular designs have many advantages over planar ones, including robustness and simpler sealing. They suffer from somewhat lower area-specific power density and considerably lower volume-specific power density. The miniaturization of tubular cells offers great improvement to both, and more besides. Pushing the boundaries of state-of-the-art manufacture to ever thinner films increases performance further, greatly advancing the long road to large scale commercialisation of SOFCs. This is only possible via the rigorous selection of materials and careful design – both for optimal performance and for mass manufacture. Previous work by the author established the potential of a novel anode fabrication route as well as showing that even un-optimized electron beam physical vapour deposition (EB-PVD) was capable of creating demonstrator cells. In this work these manufacturing processes receive at least two passes of optimization towards both reproducible fabrication and maximising microtubular SOFC performance. The former was achieved by creating statistically significant quantities to assess reproducibility and studying the underlying science, and the latter was investigated in three aspects: gas transport, electrical and electrochemical. The unique oxidation-reduction route creates robust, highly reproducible anodes with excellent through porosity offering as much as 5 orders of magnitude superior gas permeance to published sources. Nickel tubes (Ni200 5.9 mm OD, 125 μm wall thickness, 100 mm long) were oxidised in air at 1,100 for 42 h and reduced in pure hydrogen at four different temperatures. The extremes (400 °C and 1,000 °C) proved sufficiently promising that both were considered in subsequent stages of experiments and analysis for the final anode design. The morphology of the electrolyte (in particular with respect to gas-tightness) is a critical aspect of SOFC miniaturisation, and a challenge to achieve via mass-manufacture-friendly EB-PVD. The yttria-stabilized zirconia (YSZ) electrolyte deposition was optimized as far as proved possible with the available equipment. While results are more than encouraging there are a number of important concerns to be addressed in future to assure successful commercialization of the design. Accurately measuring gas permeance through the anode-electrolyte tube (sometimes called a half-cell) provides quantified justification. Finally a porous platinum cathode film 300 nm thick was successfully magnetron-sputtered onto the YSZ electrolyte at p Aᵣ100 mTorr, demonstrating the fabrication process and creating complete cells for electrical and electrochemical characterisation.PhD in Manufacturin