Thesis (Ph. D.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaves 107-109). Also available on the Internet.

In this research, corrosion behaviour of A36 carbon steel under engineered disbonded coating was investigated in sulphuric acid solutions containing sodium chloride and iron (III) sulphate. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) analyses were carried out to study the morphology and phase composition of corrosion products formed on the carbon steel surface. The results of the SEM analysis showed that only general and pitting corrosion occurred on the carbon steel surface with the engineered crevice. The size of the pits increased as the sulphuric acid and sodium chloride concentrations increased. Moreover, the corrosion products had an open, irregular and loose structure at the pits mouth. The loose and open structure of the corrosion products facilitates diffusion of chloride ions, oxygen, water and contaminants into the carbon steel surface. In contrast, the corrosion products had a very compact and continuous structure outside the pits which provided a good protection against further corrosion. The x-ray diffraction analysis showed that the corrosion products layer mainly consisted of lepidocrocite (γ-FeOOH), goethite (α-FeOOH) and iron sulphide (FeS) on the crevice edges. The Pourbaix diagram of iron in sulphuric acid solution at room temperature indicates that iron sulphide is formed on the metal surface at different pH values. The akaganeite (β-FeOOH) diffraction peak was not identified in any spectrum which could be due to the low concentration of chloride ions in the solutions. Furthermore, the number of lepidocrocite peaks decreased as the sulphuric acid concentration increased from 10 g l-1 to 50 g l-1. The lepidocrocite is dissolved in the presence of sulphuric acid, and the dissolved ion acts as an oxidant to the metal and hence lower lepidocrocite peaks are identified. Electrochemical noise measurement (ECN) testing was also performed to investigate the corrosion process occurring on the carbon steel surface with the engineered crevice. The results of the ECN measurements showed that current increased during first few minutes and then decreased slightly. Also, the coupled potential did not change after an initial shift in negative direction. The low current flowing through the carbon steel electrodes and the constant potential showed that the crevice corrosion did not develop. These results imply that the crevice corrosion may not occur on the carbon steel surface in acidic solutions containing chloride ions.

This work introduces a combination of image processing and analysis methods that perform feature extraction, shape analysis and tomographic reconstruction of Electron Microscopy images. These have been implemented on images of the AIDS virus interacting with neutralizing molecules. The AIDS virus spike is the primary target of drug design as it is directly involved in infecting host cells. First, a fully automated technique is introduced that can extract sub-volumes of the AIDS virus spike and be used to build a statistical model without the need for any user supervision. Such an automatic feature extraction method can significantly enhance the overall process of shape analysis of the AIDS virus spike imaged through the electron microscope. Accurate models of the virus spike will help in the development of better drug design strategies. Secondly, a tomographic reconstruction method implemented using a shape based regularization technique is introduced. Spatial models of known features in the structure being reconstructed are integrated into the reconstruction process as regularizers. This regularization scheme is driven locally through shape information obtained from segmentation and compared with a known spatial model. This method shows reduced blurring, and an improvement in the resolution of the reconstructed volume was also measured. It performs better than popular current techniques and can be extended to other tomographic modalities. Improved Electron Tomography reconstructions will provide better structure elucidation and improved feature visualization, which can aid in solving key biological issues. / text

Carbon nanotubes (CNTs) have extraordinary electrical and mechanical properties, and many potential applications have been proposed, ranging from nanoscale devices to reinforcement of macroscopic structures. However, due to their small sizes, characterization of their mechanical properties and deformation behaviours are major challenges. Theoretical modelling of deformation behaviours has shown that multi-walled carbon nanotubes (MWCNTs) can develop ripples in the walls on the contracted side when bent above a critical curvature. The rippling is reversible and accompanied by a reduction in the bending stiffness of the tubes. This behaviour will have implications for future nanoelectromechanical systems (NEMS). Although rippling has been thoroughly modelled there has been a lack of experimental data thus far. In this study, force measurements have been performed on individual MWCNTs and vertically aligned carbon nanofibers (VACNFs). This was accomplished by using a custom-made atomic force microscope (AFM) inside a scanning electron microscope (SEM). The measurements were done by bending free-standing MWCNTs/VACNFs with the AFM sensor in a cantilever-to-cantilever fashion, providing force-displacement curves. From such curves and the MWCNT/VACNF dimensions, measured from SEM-images, the critical strain for the very onset of rippling and the Young’s modulus, E, could be obtained. To enable accurate estimations of the nanotube diameter, we have developed a model of the SEM-image formation, such that intrinsic diameters can be retrieved. We have found an increase in the critical strain for smaller diameter tubes, a behaviour that compares well with previous theoretical modelling. VACNFs behaved very differently, as they did not display any rippling and had low bending stiffnesses due to inter-wall shear. We believe that our findings will have implications for the design of future NEMS devices that employ MWCNTs and VACNFs. / <p>Artikel 2 Image formation mechanisms tidigare som manuskript, nu publicerad: urn:nbn:se:kau:diva-16425 (MÅ 150924)</p>

Eukaryotic cells face a considerable challenge organizing a complicated interior with spatial and temporal precision. They do so, in part, through the deployment of the microtubule- based molecular motors kinesin and dynein, which translate chemo-mechanical force production into the movement of diverse cargo. Many aspects of kinesin’s motility mechanism are now known in detail, whereas fundamental aspects of dynein’s motility mechanism remain unclear. An important unresolved question is how dynein couples rounds of ATP binding and hydrolysis to changes in affinity for its track, a requisite for a protein that takes steps. Here we report a sub- nanometer cryo-EM reconstruction of the high affinity state of dynein’s microtubule binding domain in complex with the microtubule. Using molecular dynamics flexible fitting, we determined a pseudoatomic model of the high affinity state. When compared to previously reported crystal structure of the free microtubule binding domain, our model revealed the conformational changes underlying changes in affinity. Surprisingly, our simulations suggested that specific residues within the microtubule binding domain may tune dynein’s affinity for the microtubule. We confirmed this observation by directly measuring dynein’s motile properties using in vitro single molecule motility assays, which demonstrated that single point mutations of these residues dramatically enhance dynein’s processivity. We then sought to understand why dynein has been selected to be a restrained motor, and found that dynein-driven nuclear oscillations in budding yeast are defective in the context of highly processive mutants. Together, these results provide a mechanism for the coupling of ATPase activity to microtubule binding and release by dynein, and the degree to which evolution has fine-tuned this mechanism. I conclude with a roadmap of future approaches to gain further insight into dynein’s motility mechanism, and describe our work developing materials and methods towards this goal.

Oxidation developed from the surfaces into the cores of the microwaved particles. Metallic particles were also formed during microwave exposure. Lihir gold ore, in which gold was finely disseminated in pyrite and marcasite, was an extremely refractory gold ore. Without pretreatment, only 37-39% of the gold could be extracted with sodium cyanide. However, this was improved after the head ores or floatation concentrates were pretreated by microwave radiation. 74.581.2% of the gold was extracted from the microwave treated head ore. The hydrometallurgical pretreatment of pyrite and marcasite in a microwave field and a conventional heating environment was also investigated.I,n a nitric acid solution, pyrite and marcasite can be rapidly leached. Reaction temperature and the concentration of HNO3 had a significant influence on decomposition rate. Marcasite had a substantially higher i decomposition rate than pyrite. Microwave heating could promote the dissociation of marcasite and pyrite, compared with conventional heating. This was caused by special volumetric heating during microwave exposure that may induce local overheating or improve the interactions between the high dielectric loss minerals and the leaching solution. Kinetic investigations show that the decomposition of both the minerals in a nitric acid medium is controlled by chemical reactions on the surfaces of particles. The decomposition is a second order reaction with respect to nitric acid concentration. Less than 5- 7% of the decomposed sulphur was transformed into elemental sulphur during the leaching of both the minerals

As silicon-based transistor technology continues to scale ever downward, anticipation of the fundamental limitations of ultimately-scaled devices has driven research into alternative device technologies as well as new materials for interconnects and packaging. Additionally, as power dissipation becomes an increasingly important challenge in highly miniaturized devices, both the implementation and verification of high mobility, high thermal conductivity materials, such as low dimensional carbon nanomaterials, and the experimental investigation of heat transfer in the nanoscale regime are requisite to continued progress. This work furthers the current understanding of structure-property relationships in low dimensional carbon nanomaterials, specifically carbon nanotubes (CNTs) and graphene, through use of combined thermal conductance and transmission electron microscopy (TEM) measurements on the same individual nanomaterials suspended between two micro-resistance thermometers. Through the development of a method to measure thermal contact resistance, the intrinsic thermal conductivity, [kappa], of multi-walled (MW) CNTs is found to correlate with TEM observed defect density, linking phonon-defect scattering to the low [kappa] in these chemical vapor deposition (CVD) synthesized nanomaterials. For single- (S) and double- (D) walled (W) CNTs, the [kappa] is found to be limited by thermal contact resistance for the as-grown samples but still four times higher than that for bulk Si. Additionally, through the use of a combined thermal transport-TEM study, the [kappa] of bi-layer graphene is correlated with both crystal structure and surface conditions. Theoretical modeling of the [kappa] temperature dependence allows for the determination that phonon scattering mechanisms in suspended bi-layer graphene with a thin polymeric coating are similar to those for the case of graphene supported on SiO₂. Furthermore, a method is developed to investigate heat transfer through a nanoscale point contact formed between a sharp silicon tip and a silicon substrate in an ultra high vacuum (UHV) atomic force microscope (AFM). A contact mechanics model of the interface, combined with a heat transport model considering solid-solid conduction and near-field thermal radiation leads to the conclusion that the thermal resistance of the nanoscale point contact is dominated by solid-solid conduction. / text

Electron crystallography is powerful for determination of complex structures. The newly-developed 3D electron diffraction (ED) methods make structure determination from nano- and micron-sized crystals much easier than using other methods, for example X-ray diffraction. Almost complete 3D ED data can be collected easily and fast from crystals at any arbitrary orientations. Dynamical effects are largely reduced compared to zonal ED patterns. 3D ED is powerful for phase identification and structure solution from individual nano- and micron-sized crystals, while powder X-ray diffraction (PXRD) provides information from all phases present in the samples. 3D ED methods and PXRD are complementary and their combinations are promising for studying multiphasic samples and complicated crystal structures. In this thesis, the feasibility and capability of 3D ED methods, specifically rotation electron diffraction (RED), in phase identification and structure determination of different kinds of crystalline materials with nano- or submicrometer-sized crystals are investigated. Experimental conditions for RED data collection and data processing in relation to data quality, as well as the challenges in the applications of RED are discussed. RED was combined with PXRD to identify phases from as-synthesized samples and to characterize atomic structures of eleven crystalline compounds. It was shown to be possible to identify as many as four distinct compounds within one sample containing submicron-sized crystals in a Ni-Se-O-Cl system. RED was also used to determine unit cell and symmetry of isoreticular metal-organic frameworks (SUMOF-7) and solve five zeolite structures with new frameworks, ITQ-51, ITQ-53, ITQ-54, EMM-23 and EMM-25 and that of a metal-organic framework (MOF), SUMOF-7I. The structure of an open-framework germanate SU-77 was solved by combining RED with PXRD. The structures of the zeolites and SU-77 were confirmed by Rietveld refinement against PXRD. High-resolution transmission electron microscopy was used to confirm the structure models of ITQ-51, EMM-25 and SUMOF-7I. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 4: Accepted. Paper 6: Manuscript. Paper 7: Epub ahead of print. Paper 9: Manuscript. Paper 11: Manuscript.</p>

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Titanium carbide (TiC) and titanium carbonitride (Ti(C,N)) are both common components in hard, wear resistant ceramic-metal composites, or cermets. In this study the intermetallic nickel aluminide (Ni3Al) has been used as a binder for the production of TiC and Ti(C,N) based cermets. These cermets offer several improved characteristics relative to conventional WC-based ‘hardmetals’, such as lower mass and improved oxidation resistance, which are also combined with high fracture resistance, hardness and wear resistance. The cermets were produced using an in-situ, reaction sintering procedure to form the stoichiometric Ni3Al binder, with the binder contents varied from 20 to 40 vol%. However, for high N content Ti(C,N) cermets, the wettability of molten Ni3Al is relatively poor, which leads to materials with residual porosity. Therefore various amounts of Mo2C (1.25, 2.5, 5 and 10 vol%) were incorporated into the Ti(C0.3,N0.7)-Ni3Al cermets, with the aim of improving the densification behaviour. Mo2C was found to improve upon the wettability during sintering, thus enhancing the densification, especially at the lower binder contents. The tribological behaviour of TiC and Ti(C,N) cermets have been evaluated under reciprocating sliding conditions. The wear tests were conducted using a ball-on-flat sliding geometry, with a WC-Co sphere as the counter-face material, for loads from 20 to 60 N. The wear response was characterised using a combination of scanning electron microscopy, energy dispersive X-ray spectroscopy, and focused ion beam microscopy. Initially, two-body abrasive wear was observed to occur, which transitions to three-body abrasion through the generation of debris from the cermet and counter-face materials. Ultimately, this wear debris is incorporated into a thin tribolayer within the wear track, which indicates a further transition to an adhesive wear mechanism. It was found that Mo2C additions had a positive effect on both the hardness and indentation fracture resistance of the samples, but had a detrimental effect on the sliding wear response of the cermets. This behaviour was attributed to increased microstructural inhomogeneity with Mo2C additions.