No description available.

Heterogeneous ferromagnetic-superconducting systems such as a regular array of ferromagnetic nano dots deposited on the top of a superconducting thin film have attracted many research teams both experimental and theoretical. The interest in these systems does not only stem from being good candidates for technological applications, but also because they represent a new class of physical systems where two competing order parameters can coexist. This work focuses on the theoretica laspects of these systems by studying the static and dynamics of few model systems. In the first part, the static properties of a superconducting thin film interacting with a ferromagnetic texture are considered within the London approximation. In particular, the ferromagnetic textures considered here are a circular dot of submicrometer size with in-plane magnetization, an elliptical dot magnetized in the direction perpendicular to the superconductor, and a ferromagnetic dot magnetized in the direction normal to the superconducting film and containing non magnetic cavities. I also consider the interaction of vortices in the superconductor with a ferromagnetic columnar defect which penetrates the supercondcting film. In each case the vector potential and magnetic field of the ferromagnet in the presence of the superconductor are calculated. Afterward the presence of vortices in the superconductor is assumed and the energy of vortex-texture system is found. The pinning potential and force supplied by the texture are then derived from the energy of interaction between the ferromagnet and superconductor. I show that if the magnetization of the ferromagnet exceeds a critical value then vortices are spontaneously created in the ground state of the system. Such spontaneous creation of vortices is possible mostly in a close vicinity of the superconducting transition temperature Ts. For every case, the threshold value of the magnetization at which vortices start to be spontaneously created in the SC is calculated as a function of the parameters of the texture geometry. The phase diagrams for transitions from vortexless regime to regimes with one or more vortices are determined for all cases. In the second problem, the transport properties of a ferromagnetic superconducting bilayer with alternating magnetization and vortex density are studied within a phenomenological model. I show that pinning forces do not appear for continuous distribution of vortices, so a discrete model for the bilayer system is constructed. Afterward, I calculate the pinning forces acting on vortices and antivortices resulting from highly inhomogeneous distribution of flux lines and prove that this system has strong transport anisotropy. In the absence of random pinning, the system displays a finite resistance for the current in the direction perpendicular to the domains while its resistance vanishes for the parallel current. The transport anisotropy strongly depends on temperature. I study this dependence and show that the ratio of parallel to perpendicular critical current is largest close to the superconducting transition temperature Ts and the vortex disappearance temperature Tv while it has a minimum in between them.

Long-term applicability of current surgical interventions for the repair of articular cartilage is jeopardized by the formation of mechanically inferior repair tissue. Cartilage tissue engineering offers the possibility of developing functional repair tissue, similar to that of native cartilage, enabling long-lasting repair of cartilage defects. Current techniques, however, rely on the need for a large number of cells, requiring substantial harvesting of donor tissue or a separate cell expansion phase. As routine cell expansion methods tend to elicit negative effects on cell function, the following study describes an approach to generate large-sized engineered cartilage constructs (≥ 3 cm2) directly from a small number of immature rabbit chondrocytes (approximately 20,000), without the use of a scaffold. After characterizing the hyaline-like engineered constructs, the in vivo repair capacity was assessed in a chondral defect model in the patellar groove of rabbits. In vitro remodeling of the constructs developed in the bioreactor occurred as early as 3 weeks, with the histological staining exhibiting zonal differences throughout the depth of the tissue. With culturing parameters optimized (3 weeks growth under 15 mM NaHCO3), constructs were grown and implanted into critical-sized 4 mm chondral defects. Assessed after 1, 3 and 6 months (n=6), implants were scored macroscopically to evaluate integration and survival of the implants. Out of 18 rabbits, 16 received normal or nearly normal over-all repair assessment. Histological and immunohistochemical evaluation showed good integration with surrounding cartilage and underlying subchondral bone. Architectural remodeling of the constructs was present at each time point, with the presence of flattened chondrocytes at the implant surface and columnar arrangement of chondrocytes in deeper zones. The observation of in vivo remodeling was also supported by the changes in biochemical composition of the constructs. At each time point, constructs had a collagen to proteoglycan ratio similar to that of native cartilage (3:1 collagen to proteoglycan). In contrast, the repair tissue for each control group was inferior to that produced with treated defects. These initial results hold promise for the generation of engineered articular cartilage for the clinical repair of cartilage defects without the limitations of current surgical repair strategies. / Thesis (Master, Chemical Engineering) -- Queen's University, 2012-01-31 01:03:15.276

Previous studies have suggested that the material graphene might transition into an electron-electron interaction driven, unconventional, time-reversal-symmetry-breaking, (d+id)wave superconducting state upon either significant electron or hole doping, and, in particular, upon doping to the Van Hove singularity. As defects are likely to be introduced in the doping process, we are, in this text, concerned with the effects of defects on this superconducting state near the Van Hove singularity doping. To investigate the effects we use a mean-field treatment of a phenomenological resonant-valence-bond model. We find that the resonant-valence-bond amplitudes, which in the defect free graphene sheet are proportional to the superconducting pairing-potential, are suppressed near the defects, and that the recovery is well described by an exponential, yet anisotropic, recovery. In general, we find that the (d+id)-wave, superconducting state is quite resilient, and that even for strong defects, such as a vacancy, the recovery length is of the order of one lattice constant when extrapolated to weak pairing-potentials; this is compared to a conventional superconducting state of an attractive Hubbard model for which the same decay length is found to be of the order of a half lattice constant. For the defect free graphene sheet the (d+id)-wave state is a completely gapped state. The introduction of vacancies is, however, found to be accompanied by the appearance of midgap states. These states are shown to be localized around the vacancies. In accordance with the nature of this text, we will, for the benefit of students and non-experts, include an introductory section on the fundamental methods and concepts used. It gives a short and hopefully pedagogical introduction to the rudimentary concepts of solid state theory and the microscopic BCS theory of superconductivity.

No description available.

No description available.

In this thesis defects and impurities in CdTe have been studied with ab initio methods. CdTe is a II-VI semiconductor with many important applications such as γ- and X-ray detectors, solar cells and medical imaging. Even though CdTe has been studied for more than 70 years, some of its properties connected with defects and impurities, are still shrouded in mystery. Todays experimental techniques are highly developed and can provide rather detailed data, but require elaborate theoretical analysis. Here ab initio modelling comes into play and in particular density functional theory (DFT). When reviewing different theoretical studies of defects and impurities in CdTe, one finds a vast number of discrepancies between experiment and theory. Mismatches appear even between different theoretical studies. Although many problems, such as, for example, the semiconductor band gap underestimation or the spurious interaction between charged defects, are avoided by employing corrections or implementing new functionals, some of them still remain. Employing the hybrid functional HSE06, the following topics were studied in this thesis: - Te antisites: Experimental data predict the defect state to appear in the middle of the band gap, thus "pinning" the Fermi level. In contrast, our calculations show that Te antisite alone cannot be the reason for the Fermi level pinning, since it does not form a defect level in the middle of the band gap. Instead we propose that charge compensation between Te antisites in a (+2) state and Cd vacancies in a (-2) state explains the Fermi level pinning. - Cd vacancy: Electron paramagnetic resonance experiments clearly show the existence of a hole polaron for the (-1) charged vacancy. But DFT studies report a completely delocalised hole. In our studies, for the first time, this state was found in its proper geometrical configuration with a hole localisation stabilised by a Jahn-Teller distortion, thereby removing the discrepancy between experiment and theory. - Cd chalcogenides: Additionally, with particular focus on the hole localisation problem, the series of isovalent compounds (CdTe, CdSe and CdS) was studied to understand the mechanism of hole polaron formation. We explain the trend of the hole localisation in terms of Coulomb interaction, explicitly showing that the effect of electron correlation is negligible. - Cl-doped CdTe: The formation of a Cl - Cd vacancy complex explains the selfcompensation and selfpurification mechanism. We find Cl to annihilate the hole polaron. - Te antisite under deformation: In an attempt to tailor the energy position of the Te antisite defect level in the CdTe band gap, we studied CdTe under different deformations. It is shown that by a carefully chosen deformation the defect levels can be pushed closer to the valence and/or conduction band and hence the CdTe detector performance may be improved.

The 'defects problem' has demanded considerable attention in recent years, with much emphasis given to the technical causes of failure. This research project examines the problem from a different point of view - that of human error. Taking as a starting point, technical publications in the construction industry, the research reviews human error literature from a variety of industries and perspectives and synthesises a model of error causation covering organisations in a construction project context. This model is then progressively tested in four studies, a general preliminary survey and three more detailed studies of house-building. Conclusions support the view that errors leading to failure in complex socio-technical systems often exhibit systems characteristics and involve the whole managerial structure. An improved model is proposed, which emphasises the importance of both project and general management errors.

Information Technology (IT) can exploit strategic opportunities for new ways of facilitating information and data exchange and the exchange of expert and specialist opinions in any field of engineering. Knowledge-Based Systems are sophisticated computer programs which store expert knowledge on specific subject and are applied to a broad range of engineering problems. Integrated Database applications have facilitated the essential capability of storing data to overcome an increasing information malaise. Integrating these areas of Information Technology (IT) can be used to bring a group of experts in any field of engineering closer together by allowing them to communicate and exchange information and opinions. The central feature of this research study is the integration of these hitherto separate areas of Information Technology (IT). In this thesis an adaptable Graphic User Interface Centred application comprising a Knowledge-Based Expert System (DEMARECEXPERT), a Database Management System (REPCON) and Evaluation program (ECON) alongside visualisation technologies is developed to produce an innovative platform which will facilitate and encourage the development of knowledge in concrete repair. Diagnosis, Evaluation, MAintenance and REpair of Concrete structures (DEMAREC) is a flexible application which can be used in four modes of Education, Diagnostic, Evaluation and Evolution. In the educational mode an inexperienced user can develop a better understanding of the repair of concrete technology by navigating through a database of textual and pictorial data. In the diagnostic mode, pictures and descriptive information taken from the database and performance of the expert system (DEMAREC-EXPERT) are used in a way that makes problem solving and decision making easier. The DEMAREC-EXPERT system is coupled to the REPCON (as an independent database) in order to provide the user with recommendations related to the best course required for maintenance and in the selection of materials and methods for the repair of concrete. In the evaluation mode the conditions observed are described in unambiguous terms that can be used by the user to be able to take engineering and management actions for the repair and maintenance of the structure. In the evolution mode of the application, the nature of distress, repair and maintenance of concrete structures within the extent of the database management system has been assessedT. he new methodology of data/usere valuation could have wider implications in many knowledge rich areas of expertise. The benefit of using REPCON lies in the enhanced levels of confidence which can be attributed to the data and to contribution of that data. Effectively, REPCON is designed to model a true evolution of a field of expertise but allows that expertise to move on in faster and more structured manner. This research has wider implications than within the realm of concrete repair. The methodology described in this thesis is developed to provide tecýnology transfer of information from experts, specialists to other practitioners and vice versa and it provides a common forum for communication and exchange information between them. Indeed, one of the strengths of the system is the way in which it allows the promotion and relegation of knowledge according to the opinion of users of different levels of ability from expert to novice. It creates a flexible environment in which an inexperienced user can develop his knowledge in maintenance and concrete repair structures. It is explained how an expert and a specialist can contribute his experience and knowledge towards improving and evolving the problem solving capability of the application.

Bibliography: leaves 361-370. / xvii, 451 leaves : ill. (some col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Determines the effects of welding and metallurgical variables on the formation of hollow bead and provides a detailed study of the characteristics of hollow bead in order to understand their formation. / Thesis (Ph.D.)--University of Adelaide, Dept. of Mechanical Engineering, 1998