Hlavním cílem této práce je studium aktuálního problému adheze mezi kompozitními materiály, a určení vlivu složení mezivrstvy v pevnosti adhezivního spoje mezi vláknové (FRC) a částicové (PFC) kompozity, používané ve stomatologii. FRC tyčinky byly vytvořené na bázi komerční dimetakrylatové pryskyřice a skleněných S vláken. PFC byl vytvořen ze směsi pryskyřic (bis-GMA a PEGDMA), plněných drceným barnatým sklem. Celkově bylo připraveno a vyzkoušeno 84 vzorků. Vzorky byly rozdělené do pěti hlavních skupin. První skupina byla vytvořena ze série vzorků bez mezivrstvy. Zbylé čtyři skupiny byly rozděleny dle složení mezivrstvy (tloušťky a druhu pryskyřic). Částicové kompozity vzorků se lišily obsahem plniva (0, 10, 40 hm %). FRC tyčinky byly stejné pro všechny vzorky. Univerzální testovací přístroj ZWICK Z010 byl použit k zjištění smykové pevnosti adhezivního spoje všech vzorků. Rastrovací elektronový mikroskop byl použít k pozorování místa porušení. Ze získaných výsledků vyplívá, že s přidáváním mezivrstvy mezi vláknovým a částicovým narůstá smyková pevnost spoje. Důležitost těchto výsledků je způsobená tím, že v posledních letech použití těchto materiálů ve stomatologii narůstá a adheze zůstává jeden z hlavních problémů při klinické praxi.

Die ITRS (International Technology Roadmap for Semiconductors) sagt voraus, dass die fortlaufende Miniaturisierung der Transistoren und Verdrahtungen auch neue Isolationsmaterialien mit äußerst niedrigen (ultralow) Dielektrizitätskonstanten k erfordern wird. Die Miniaturisierung der Bauteile der ULSI (Ultra Large Scale Integration) führt zu starken Anforderungen an die Fertigung der kritischen Bereiche (backend-of-line, BEoL). Die ITRS deutet darauf hin, dass die k-Werte bis zu 2.0 für die 45 nm Technologie reduziert werden müssen, und zu noch niedrigeren k-Werten (k  1.5) für die nachfolgenden Jahre. Ergänzend zur äußerst niedrigen dielektrischen Konstante müssen die Isolatoren auch über entsprechende mechanische Eigenschaften verfügen. Die vorliegende Arbeit stellt Forschungen vor, die das Ziel haben, mittels modernen ab-initio und halbempirischen theoretischen Methoden neuartige Isolationsmaterialien für zukünftige mikroelektronische Anwendungen zu entwerfen. Die umfangreichen eingesetzten Rechenmethoden wurden verwendet, um strukturelle und physikalische (mechanische, dielektrische und elektronische) Eigenschaften von entworfenen Zwischenschichtsdielektrika zu bestimmen. Eine neue Art von Materialien wird vorgestellt, die als ein möglicher Kandidat für isolierende ultralow-k dünne Schichte zwischen Metallleiterbahnen in zukünftigen CMOS (Complementary Metal-Oxide-Semiconductor) Technologien fungieren sollen. Die Struktur der neuartigen Materialien wird durch ein Modell beschrieben, das ein geordnetes dreidimensionales Netzwerk (Mosaikstruktur) darstellt. Dies besteht aus drei Hauptkomponenten: Knoten, Kanten und Topologie der Anordnung. Fullerenmoleküle (C60) werden als Knoten des Netzwerkes verwendet. Die Knoten werden durch Verknüpfermoleküle entlang der Kanten der Mosaikzelle angekoppelt. Dies wird durch kovalente Bindungen realisiert. Als Verknüpfermoleküle werden Kohlenwasserstoff- Kettenmoleküle verwendet. Einfache kubische, flächenzentrierte kubische und diamantähnliche Topologien werden für Anordnungen des Netzwerkes betrachtet. Das Innere einer Netzwerkzelle repräsentiert eine Nanopore der Größe in Bereich von 1 nm. Zunächst werden am Beispiel fluorierter Fullerene Probleme der molekularen Polarisierbarkeit untersucht. In Molekülen mit ionischem Beitrag zur Bindung kann der Beitrag der Kernverschiebungen (wegen des äußeren Feldes) zur statischen Polarisierbarkeit entscheidend sein. Mittels der Finite Field Methode wird die Struktur mit und ohne ein endliches äußeres elektrisches Feld optimiert. Dabei wird die Optimierung durch Minimierung der Gesamtenergie durchgeführt und die molekulare Polarisierbarkeit aus dem induzierten Dipolmoment bestimmt. In C60Fn erhöht meistens das Fluorieren die Polarisierbarkeit. Nur für n = 2 und 18, wobei das Molekül ohne ein äußeres Feld ein sehr großes Dipolmoment hat, wird die Polarisierbarkeit verringert. Für große Werte n (n = 20, 36 und 48) wird die Polarisierbarkeit pro zusätzliches Fluoratom wegen Kernverschiebungen deutlich erhöht. Die Modifizierung der Knoten des Netzwerkes wird betrachtet und die Anwendbarkeit des Additivitätsmodells diskutiert. Die Dielektrizitätskonstante des reinen flächenzentrierten kubischen Fullerengitters beträgt etwa 4.4. Die Einführung der Verknüpfermoleküle zwischen benachbarten Fullerenmolekülen und die gleichzeitige Verwendung von auf Kohlenstoffatomen basierten käfigförmigen Molekülen reduziert die Dichte des Materials. Dies ergibt eine beträchtliche Verringerung der makroskopischen Polarisierbarkeit des Materials. Die Struktureinheit, die aus zwei Fullerenmolekülen und einem Kohlenwasserstoff-Verknüpfermolekül besteht, wird mittels quantenchemischer Methoden (DFTB Molekulardynamik) optimiert. Es werden die Dichte der lokalen Dipole und elektronische Effekte betrachtet, um die effektive Dielektrizitätskonstante des Modells abzuschätzen. Die Berechnungen zeigen, dass k-Werte von etwa 1.4 erreicht werden können, wenn C6H12 Kettenmoleküle verwendet werden, um die C60-Moleküle im Netzwerk mit diamantähnlicher Symmetrie zu verknüpfen. Weiterhin werden molekulare Cluster mit angelegten periodischen Randbedingungen für einfache kubische und diamantähnliche Topologien konstruiert. Kombinationen der klassischen und quantentheoretischen Methoden werden eingesetzt, um die Struktur zu optimieren, Kompressionsmodule zu berechnen und die dielektrischen Eigenschaften der fullerenbasierten Materialien zu berechnen. Dies hat das Ziel, ultralow-k Isolatoren mit entsprechenden mechanischen Eigenschaften zu finden. Es wird die kovalente Verknüpfung der C60 Moleküle untersucht und sowohl die Länge und chemische Zusammensetzung des Verknüpfermoleküles als auch die Verknüpfungsgeometrie variiert. Gemäß dem entworfenen Modell werden Strukturen mit einfacher kubischer und diamantähnlicher Topologie des Netzwerkes als vielversprechende Kandidaten betrachtet. Die (statische) Dielektrizitätskonstanten k und Kompressionsmodule B sind für einige vorgeschlagene Materialien im Bereich von k = 1.7 bis 2.2 und beziehungsweise von B = 5 bis 23 GPa. Das Clausius-Mossotti Modell wird zur Bestimmung der Dielektrizitätskonstante der entworfenen Strukturen verwendet. In den nächsten Schritten der Arbeit werden die Wege der Verbesserungen für das vorgeschlagene Modell betrachtet. Es wird analysiert, auf welche Art Verknüpfermoleküle an die Knoten gebunden werden können, um die mechanischen und dielektrischen Eigenschaften der generierten ultralow-k Strukturen zu verbessern. Es gibt zwei mögliche verschiede Arten, die Verknüpfermoleküle > C = C < und > C – CH2 – CH2 – C < an das Käfigmolekül C60 anzukoppeln. Die Berechnungen zeigen, dass es im gegenwärtigen Verbesserungsschritt möglich ist, für die einfache kubische Topologie Eigenschaftskombinationen mit k = 2.2 und B = 33 GPa zu bekommen. In der vorliegenden Arbeit wurde eine theoretische Methode ¬¬– sogenannter molekularer Entwurf – entwickelt und erfolgreich angewandt. Die theoretische Behandlung ist kompliziert, weil Wechselwirkungen im atomaren Skalabereich und auf einem strukturellen Niveau von 1 nm zusammen betrachtet werden müssen. Dies Verfahren erfordert die Anwendung komplementärer theoretischen Methoden, um das gesamte Problem beschreiben zu können. Die Methoden schließen klassische, kontinuierliche theoretische und quantenchemische Näherungen ein. Der Vorteil dieser Methode ist, dass verschiedene mögliche Kandidaten für ultralow-k Dielektrika theoretisch getestet werden können, ohne teure und zeitaufwendige Experimente durchzuführen. / The International Technology Roadmap for Semiconductors (ITRS) predicts that continued scaling of devices will require insulating materials with ultralow dielectric constant k. The shrinking of device dimensions of ultra-large-scale integrated (ULSI) chips imposes strong demands on the backend of the line (BEoL) interconnect structures. The ITRS indicates that the k values need to be reduced to 2.0 for the 45 nm technology node or below (k  1.5) in the next few years. Additionally to extremely low dielectric constants, the insulating materials must have also suitable mechanical properties. The work represents research, which is aimed to support molecular design and investigations of modelled novel insulating materials for future application in microelectronics by means of theoretical ab-initio and semiempirical methods. A wide range of computational methods were used to estimate structural and physical (mechanical, dielectrical and electronic) properties of the designed interlayer dielectrics (ILDs). A new class of materials is presented that is supposed to be a potential candidate for isolating ultralow-k thin films between metal on-chip interconnects in future CMOS technology nodes. The structure of the novel materials is described by a model that assumes an ordered three-dimensional network (mosaic structure) consisting of three main components: nodes, edges and topology of arrangement. Fullerene (C60) molecules are used as the nodes of the network. The nodes are connected by linker molecules along the edges of the mosaic cells through a covalent bonding. Hydrocarbon chain molecules are used as the linkers. Simple cubic, face-centred cubic and diamond-like topologies of the network are considered. The interior of a network cell represents a nanopore of a 1-nm scale. At first problems of molecular polarizability are investigated considering the case of fluorinated fullerenes. In molecules with ionic contribution to the binding, the contribution of nuclear displacements (due to the external field) to the static polarizability can be decisive. Using the finite field method, the structure is optimized with and without a finite external electric field by a total energy minimization and the polarizability is calculated from the induced dipole moment. In C60Fn, fluorination mostly increases the molecular polarizability. Only for n = 2 and 18, where the molecule without an external field has a very large dipole moment, fluorination does decrease it. For large n (n = 20, 36, and 48), the polarizability per added F atom due to nuclear displacements is increased by a factor of about 2. The modification of the nodes of the network is considered and the validity of the additivity model is discussed. The dielectric constant of the pure fullerene face-centred cubic lattice is about 4.4. The introduction of bridge molecules between neighbouring fullerene molecules and the simultaneous usage of cage-like molecules based on carbon atoms reduces the density of the material. This results in a considerable decrease of the macroscopic polarizability of the material. The structural units of the models consisting of two fullerenes and a hydrocarbon bridge molecule are optimized by means of quantum chemical methods (DFTB molecular dynamics). The density of local dipoles and electronic effects are considered to estimate the effective dielectric constant of the models. It is shown that k values of about 1.4 can be obtained if C6H12 chain molecules are used to connect C60 molecules on a network with diamond-like symmetry. Further, molecular clusters with applied periodic boundary conditions are constructed for simple cubic and diamond-like topologies. Combinations of classical and quantum-theoretical approaches are used to optimize the structure, to calculate bulk moduli, and for the assessment of the dielectric properties of fullerene-based materials with the goal to find ultralow-k insulators with suitable mechanical properties. The covalent linking of C60 molecules is studied and the length and chemical composition of the linker molecule as well as the linkage geometry is varied. According to the molecular design-based model, structures with simple cubic and diamond-like topology of the network are proposed as promising candidates. The (static) dielectric constants k and elastic bulk moduli B of the proposed materials are in the range of k = 1.7 to 2.2 and B = 5 to 23 GPa, respectively. The Clausius-Mossotti-Model is used to estimate dielectric constants of the designed structures. In the next steps of the work the ways of improvements for the proposed model are considered. The way to connect linker molecules to the node molecules is analyzed, in order to improve the mechanical and dielectric properties of the generated ultralow-k structures. Two different types of bonding linker molecules to the cage C60 molecule with the > C = C < and > C – CH2 – CH2 – C < linker molecules are possible. It is shown that at the present improvement step it is possible to get property combinations with dielectric constant of k = 2.2 and bulk modulus of B = 33 GPa for the simple cubic topology. In this work a theoretical method called molecular design is developed and successfully applied. The theoretical treatment is difficult since interactions both on the atomic scale and on the structural level of 1 nm must be considered. This approach requires the application of complementary theoretical methods to describe the complex problems. The methods include classical, continuum theoretical and quantum-chemical approximations. The advantage of the present approach is that various possible candidates for ultralow-k dielectrics can be tested theoretically without performing expensive and time-consuming experiments.

<div>The response time of magnetization switching in current spintronic devices is limited to nanosecond timescales due to the precessional motion of the magnetization during reversal. To overcome this limit two routes of investigation leading to novel recording and logic devices are considered in this thesis: 1) Magnetic tunnel junction structures where the recording and reference layers are replaced by synthetic ferrimagnets and switching is induced by spin transfer torque and 2) Hybrid magneto-photonic devices where switching is induced by plasmon-enhanced all-optical switching. To circumvent limitations of the materials and magnetic properties of CoFeB, the most utilized alloy in spintronics, hcp-CoCrPt, a material that exhibits superior perpendicular anisotropy and thermal stability, is chosen as the ferromagnetic electrode in this work. Whereas actual devices based on the two schemes aforementioned are still in the process of being fabricated, through collaborative work with our international collaborators, this thesis describes fundamental magnetic and structural characterization needed for the realization of said ultrafast switching devices. The magnetic switching behavior of CoCrPt-Ru-CoCrPt synthetic ferrimagnets with perpendicular magnetic anisotropy have been studied in the temperature range from 2K to 300K. It was found that two sets of magnetic transitions occur in the CoCrPt-Ru-CoCrPt ferrimagnet systems studied. The first set exhibits three magnetization states in the 50K – 370K range, whereas the second involves only two states in the 2K and 50K range. The magnetic hysteresis curves of the synthetic ferrimagnet are assessed using an energy diagram technique which accurately describes the competition between interlayer exchange coupling energy, Zeeman energy, and anisotropy energy in the system. This energy diagram analysis is then used to predict the changes in the magnetic hysteresis curves of the synthetic ferrimagnet from 200K to 370K. This represents the potential operation temperature extrema that a synthetic ferrimagnet could be expected to operate at, were it to be utilized as a free layer in a memory or sensor spintronic device in the device configuration described in this dissertation.</div><div>Circularly polarized fs laser pulses generate large opto-magnetic fields in magnetic materials, through the inverse Faraday effect. These fields are attributed to be largely responsible for achieving ultrafast all-optical magnetization switching (AOS). All experimental demonstrations of AOS thus far have been realized on thin films over micron-sized irradiated regions. To achieve magnetization switching speeds in the ps and potentially fs time regimes, this work proposes the use of surface plasmon resonances at the interface of hybrid magneto-photonic heterostructures. In addition to the ability of plasmon resonances to confine light in the nm scale, the resonant excitation can largely enhance induced opto-magnetic fields in perpendicular magnetic anisotropy materials. This requires strong spin-photon coupling between the plasmonic and the magnetic materials, which thus requires the minimization of seed layers used for growth of the magnetic layer. This work reports on the development of ultrathin (1 nm thick) interlayers to control the growth orientation of hcp-Co alloys grown on the refractory plasmonic material, TiN, to align the magnetic axis out-of-plane. CoCrPtTa seed layers down to 1 nm were developed to seed the growth of CoCrPt, and the dependence of the quality of the CoCrPt is investigated as Ta composition is varied in the seed layer. Whereas bismuth iron garnet (BIG) meets the magneto-optical requirements for a hybrid magneto-photonic material, its magnetic and structural properties are highly sensitive to the Bi:Fe ratio and must be grown epitaxially on single crystalline substrates. Therefore, in this work we have investigated alternative materials that offer superior magnetic properties and are amenable to growth on inexpensive substrates. Opto-magnetic field enhancements up to 2.6x in Co-ferrite magneto-photonic heterostructures have been obtained via finite element analysis modelling. Alternative materials for plasmon-enhanced all-optical switching such as Co/Pd multilayers have also been investigated. Successful growth of Co/Pd multilayers on TiN using ultrathin Ti interlayers has been achieved. </div><div><br></div>

Anti-ferromagnetic interlayer exchange coupling between iron layers in Fe/MgO superlatices have recently been of interest among researchers for its potential application in 3D memory storage in computers (hard drives) and in the research field of spintronics. Thinfilms and bi-layers between the 10-100 ångström range in thicknesses were analyzed for its resistive properties during different conditions. Both mono-layers and superlatices were experimented with.The resistivity of the films was measured with both the four-probe method and the Van der Pauw method. Because of mechanical limitations with respect to the direction of the external magnetic field, the Van der Pauw method had to be used for the superlattice measurements. The collected data from all the measurments of the superlattice suggest that the electric current is not only passing the capping layer of the lattice but did not however show any sign of resisitive properties changing with changing magnetic profile. The resistivity of thesuperlattice was similar to that of the resistivity calculated from data when the external magnetic field was off. The results from this study show for the first time that it is possible to measure the electronic transport within an Fe/MgO superlattice which provides a basis for futher investigations.

[pt] Neste trabalho, o comportamento tridimensional de tubos multicamadas com escorregamento entre camadas, sob grandes deslocamentos, para aplicação em análise global de risers e dutos é avaliado. Foi desenvolvido um novo elemento finito, considerando o modelo de viga de Timoshenko em cada camada. O elemento contempla os graus de liberdade axial, flexional e torcional, todos variando ao longo do elemento de acordo com as funções de interpolação de Hermite: carregamentos axial e torcional constantes e momentos fletores lineares. As deformações de cisalhamento também foram consideradas na formulação do elemento através de graus de liberdades generalizados, constantes ao longo do elemento. A formulação também considera modelos de contato não-lineares para representar várias possibilidades de atrito entre camadas, através da representação apropriada da relação constitutiva para as tensões de cisalhamento no material adesivo. O trabalho também apresenta os carregamentos hidrostáticos e hidrodinâmicos devidos aos fluidos interno e externo, atuando nos graus de liberdade das respectivas camadas. As forças de arrasto e de inércia devidas ao fluido externo foram calculadas através da fórmula de Morison. As matrizes de massa e amortecimento, associadas a cada camada do elemento, são obtidas através da consideração das respectivas contribuições na expressão do trabalho virtual desenvolvido pelo carregamento externo. O elemento finito desenvolvido permite a representação numérica de risers com camadas aderentes ou não aderentes, incluindo os efeitos de pequenos deslocamentos entre camadas. O problema de interação solo-estrutura também é tratado neste trabalho, sendo que dois modelos de contato entre o solo e o duto são propostos. A formulação do elemento e o seu desempenho numérico são avaliados através de alguns exemplos de aplicação e os resultados são comparados com outros resultados numéricos ou analíticos disponíveis na literatura. Os resultados mostram que o novo elemento é uma solução simples, robusta e confiável para análise de tubos em multicamadas. / [en] This work addresses the behavior of three-dimensional multilayered pipe beams with interlayer slip condition, under general three-dimensional large displacements, in global riser and pipeline analysis. A new finite element model, considering the Timoshenko beam for each element layer, has been formulated and implemented. It comprises axial, bending and torsional degrees-of-freedom, all varying along the element length according to discretization using Hermitian functions: constant axial and torsional loadings, and linear bending moments. Transverse shear strains due to bending are also considered in the formulation by including two generalized constant degrees-of-freedom. To represent various friction conditions between the element layers, nonlinear contact models are considered. These conditions are accounted in the model through a proper representation of the constitutive relation for the shear stresses behavior in the binding material. Derivations of hydrostatic and hydrodynamic loadings due to internal and external fluid acting on respective element layers are presented. The drag and inertia forces due to external fluid are calculated by using the Morison equation. Mass and damping matrices, associated to each element layer, are properly derived by adding their respective contributions to the expression of the virtual work due to external loading. The FE implementation allows for the numerical representation of either bonded or unbonded multilayered risers, including small slip effects between layers. Effects of the pipe-soil interaction are also addressed in this work with two contact models considering either no or full interaction between friction forces in longitudinal and lateral directions, respectively. The element formulation and its numerical capabilities are evaluated by some numerical testing, which are compared to other numerical or analytical solutions available in the literature. These tests results show that the proposed element provides a simple yet robust and reliable tool for general multilayered piping analyses.

Im Rahmen der vorliegenden Arbeit wird eine Tight-Binding-Formulierung der Korringa-Kohn-Rostoker-Greenschen-Funktionsmethode vorgestellt. Dabei werden mittels eines geeignet gewählten Referenzsystems abgeschirmte Strukturkonstanten konstruiert. Es werden die Vorteile und Grenzen dieser Transformation des Formalismus diskutiert. Es wird gezeigt, daß der numerische Aufwand zur erechnung der Elektronenstruktur von Systemen mit langgestreckter Elementarzelle linear mit der Systemgröße wächst. Damit ist eine Behandlung von Systemen mit 500 und mehr Atomen pro Elementarzelle möglich. Anhand von umfangreichen Testrechnungen kann demonstriert werden, daß das neue Verfahren bezüglich seiner Genauigkeit mit dem traditionellen KKR-Verfahren vergleichbar ist. Es werden Anwendungen zur Berechnung der Elektronenstruktur sowie zur Zwischenlagenaustauschkopplung von Co/Cu(100)-Vielfachschichten vorgestellt. / A newly developed ab initio tight-binding-formulation of the Korringa-Kohn-Rostoker-Green's function method for layered systems is presented. Screened structure constants are calculated by means of a repulsive reference system. Advantages and limits of this transformation of the formalism are discussed in detail. The numerical effort for self consistent electronic structure calculations of systems with a large prolonged supercell scales linearly with the system size. Systems with up to 500 atoms per unit cell can be treated easily. The accuracy of the new method is of the same order as the traditional KKR method. Applications to electronic structure calculations and magnetic interlayer exchange coupling in Co/Cu(100) multilayers are presented.

In this thesis multidomain states in magnetically ordered systems with competing long-range and short range interactions are under consideration. In particular, in antiferromagnetically coupled multilayers with perpendicular anisotropy unusual multidomain textures can be stabilized due to a close competition between long-range demagnetization fields and short-range interlayer exchange coupling. These spatially inhomogeneous magnetic textures of regular multidomain configurations and irregular networks of topological defects as well as complex magnetization reversal processes are described in the frame of the phenomenological theory of magnetic domains. Using a modified model of stripe domains it is theoretically shown that the competition between dipolar coupling and antiferromagnetic interlayer exchange coupling causes an instability of ferromagnetically ordered multidomain states and results in three possible ground states: ferromagnetic multidomain state, antiferromagnetic homogeneous and antiferromagnetic multidomain states. The presented theory allows qualitatively to define the area of existence for each of these states depending on geometrical and material parameters of multilayers. In antiferromagnetically coupled superlattices with perpendicular anisotropy an applied magnetic bias field stabilizes specific multidomain states, so-called metamagnetic domains. A phenomenological theory developed in this thesis allows to derive the equilibrium sizes of metamagnetic stripe and bubble domains as functions of the antiferromagnetic exchange, the magnetic bias field, and the geometrical parameters of the multilayer. The magnetic phase diagram includes three different types of metamagnetic domain states, namely multidomains in the surface layer and in internal layers, and also mixed multidomain states may arise. Qualitative and quantitative analysis of step-like magnetization reversal shows a good agreement between the theory and experiment. Analytical equations have been derived for the stray field components of these multidomain states in perpendicular multilayer systems. In particular, closed expressions for stray fields in the case of ferromagnetic and antiferromagnetic stripes are presented. The theoretical approach provides a basis for the analysis of magnetic force microscopy (MFM) images from this class of nanomagnetic systems. Peculiarities of the MFM contrast have been calculated for realistic tip models. These characteristic features in the MFM signals can be employed for the investigations of the different multidomain modes. The methods developed for stripe-like magnetic domains are employed to calculate magnetization processes in twinned microstructures of ferromagnetic shape-memory materials. The remarkable phenomenon of giant magnetic field induced strain transformations in such ferromagnetic shape memory alloys as Ni-Mn-Ga, Ni-Mn-Al, or Fe-Pd arises as an interplay of two physical effects: (i) A martensitic transition creating competing phases, i.e. crystallographic domains or variants, which are crystallographically equivalent but have different orientation. (ii) High uniaxial magnetocrystalline anisotropy that pins the magnetization vectors along certain directions of these martensite variants. Then, an applied magnetic field can drive a microstructural transformation by which the martensitic twins, i.e. the different crystallographic domains, are redistributed in the martensitic state. Within the phenomenological (micromagnetic) theory the equilibrium parameters of multivariant stripe patterns have been derived as functions of the applied field for an extended single-crystalline plate. The calculated magnetic phase diagram allows to give a detailed description of the magnetic field-driven martensitic twin rearrangement in single crystals of magnetic shape-memory alloys. The analysis reveals the crucial role of preformed twins and of the dipolar stray-field energy for the magnetic-field driven transformation process in magnetic shape-memory materials. This work has been done in close collaboration with a group of experimentalists from Institute of Metallic Materials of IFW Dresden, Germany and San Jose Research Center of Hitachi Global Storage Technologies, United States. Comparisons between theoretical and experimental data from this cooperation are presented throughout this thesis as vital part of my work on these different subjects.

Magnesium and its alloys are extensively used for various industries such as aerospace, automobile and electronics due to their excellent properties such as low density, high strength and stiffness and electromagnetic shielding. However, the wide spread applications of these alloys are limited due to the undesirable properties such as poor corrosion, wear and creep resistance and high chemical reactivity. These alloys are highly susceptible to galvanic corrosion in sea water environment due to their high negative potential (-2.37 V vs SHE). The effective way of preventing corrosion is through the formation of a protective coating, which acts as a barrier between the corrosive medium and the substrate. Many surface modification methods such as electro/ electroless plating, conversion coating, physical and chemical vapour depositions, thermal spray coating etc., are available currently to improve the corrosion resistance of Mg alloys. Of these methods, the electroless nickel plating has gained considerable importance because of its excellent properties such as high hardness, good wear and corrosion resistance. The properties of binary electroless nickel coating have been further improved by the addition of a third element such as cobalt, tungsten, tin and copper etc. It has been reported that the addition of tungsten as the third element in the Ni-P improves the properties such as hardness, wear and corrosion resistance, thermal stability and electrical resistance. Magnesium alloys are categorized as a “difficult to plate metal”, because of their high reactivity in the aqueous solution. They react vigorously with atmospheric oxygen and water, resulting in the formation of the porous oxide/ hydroxide film which does not provide any protection in the corrosive environment. Further, the presence of this oxide film prevents the formation of a good adhesive bond between the coating and the substrate. The surface treatment process for removal of the oxide layer is very much essential before plating the Mg alloy. Currently two processes such as zinc immersion and direct electroless nickel plating are adopted to plate Mg alloys. Etching in a solution of chromate and nitric acid followed by immersion in HF solution to form a conversion film is necessary for direct electroless nickel (EN) plating of Mg alloy. However, strict environmental regulations restrict their usage because of hazardous nature. Expensive palladous activation treatment is a well-known process as a replacement for chromate-HF pretreatments for Mg alloys. It has been reported that EN plating has been carried out over Mg alloys by using conversion coating followed by HF treatment. Formation of an intermediate oxide layer by electrolytic methods is also one of the ways these toxic pretreatments can be avoided. Microarc oxidation (MAO) is an environment friendly surface treatment technique which provides high hardness, better corrosion and wear resistance properties for the Mg alloys. EN coating has been prepared on MAO layer for improving the corrosion resistance. These MAO/EN composite coatings have been prepared using chromic acid and HF pretreatment process. As the replacement for the chromate-HF pretreatment, SnCl2 and PdCl2 sensitization and activation procedures respectively were adopted over MAO layer for the deposition of Ni-P coating. From the above reported literature, it can be inferred that for the activation of inert MAO layer to deposit electroless nickel coating, the hazardous chromate/HF and highly expensive PdCl2 activation processes were followed. Therefore, there is a need for identifying an alternative simple and cost effective pretreatment process for the deposition of electroless nickel. It is well known that borohydride is a strong reducing agent that has been used for the deposition of Ni-B coatings. In the present study, an attempt has been made to utilize borohydride in the pretreatment process for the reduction of Ni2+ ions over the MAO interlayer, which provides the nucleation sites for the deposition of Ni-P coating. Ni-P and Ni-P/Ni-W-P duplex coatings were deposited from stabilizer free carbonate bath on AZ31B Mg alloy to improve the corrosion resistance of the base substrate. The conventional chromate and HF pretreatment processes were followed for the deposition of electroless nickel coating. In order to improve the corrosion resistance of the duplex coating, post treatments such as heat treatment (4 h at 150°C) and chromate passivation were adopted. EDX analysis of AZ31B Mg alloy showed the presence of 2.8 wt.% of Al and 1.2 wt. % Zn with the balance of Mg for AZ31B Mg alloy. After the chromic acid and HF treatment, the magnesium content was reduced from 90.0 wt % to 54.9 wt%, which could be due to the incorporation of chromium on the surface layer. The surface showed about 17.8 wt. % of F. The alloy exhibited the roughness of about 0.29± 0.01µm after mechanical polishing. The roughness value was significantly changed after the chromic acid treatment processes. The maximum roughness of about 1.28±0.06 µm was obtained after the HF activation. XPS analysis confirmed the existence of chromium in +3 oxidation state after the chromic acid treatment. The Ni-P coating thickness of about 25 microns was obtained in 1 h and 15 min. In the case of duplex coatings, Ni-P plating was done for 45 min. to obtain approx. 17 microns thickness and Ni-W-P plating was done for 1.15 h to obtain a thickness of approx. 10 microns, resulting in a total thickness of 25 ± 5 microns. Ni–P coating exhibited nodular morphology with porosity. The size of these cluster nodules were of about 10 µm in diameter. On the other hand, the duplex coating exhibited a less nodular, dense and smooth appearance. From the compositional analysis it was found that Ni–P coating contained about 6 wt. % P. In the case of duplex coating, the P content was reduced to 3 wt % due to the incorporation of about 2 wt% of tungsten. In corrosion studies, the potentiodynamic polarization data obtained for bare Ni-P coating in 0.15 M NaCl solution exhibited a higher current of about 218 μA/cm2 as compared to the substrate due to the porosity of the coating. However, the Ni-P/Ni-W-P duplex showed 55 times improvement in corrosion resistance, vis-a-vis Ni-P due to the dense nature of the coating. The corrosion resistance of the coatings increased in the following order: Ni-P < bare alloy < duplex < duplex-passivated < duplex-heat treated passivated. In EIS study, the Nyquist plot obtained for the bare substrate and Ni–P coating showed the presence of inductance behavior at the lower frequency region due to the adsorption of electroactive species over the substrate through the porous oxide layer. However, the passivated and duplex passivated coatings exhibited only capacitive behavior due to their compact nature. From the above, it can be concluded that, direct deposition of Ni-P coating over the chosen Mg alloy using chromic acid and HF pretreatment process resulted in porous morphology, which affected the corrosion resistance of the coating. As an alternative strategy, the microarc oxidation conversion coating was developed on Mg alloy and characterized. The MAO coating was developed using silicate electrolyte at three different current densities (0.026, 0.046 and 0.067 A/cm2) for about 15 min. With respect to the MAO coating, an increase in the current density increased the pore diameter and decreased the pore density. The surface of the coating became coarser and rough. The cross-sectional morphology of the coating showed two district layers namely the dense and thin inner layer and a porous thick outer layer. The thickness of the coating increased with increase in current density. MAO coating prepared at an intermediate current density of 0.046 A/cm2 exhibited a higher thickness of about 12 µm and a further increase in current density showed a decrease in thickness, due to the greater rate of dissolution of Mg, relative to the rate of deposition. The surface roughness of the MAO coatings also increased with increase in current density. The Ra value increased from 1.39±0.06 to 3.52±0.17 µm with increase in current density. XRD peaks obtained for the Mg substrates corresponded predominately to magnesium. However, the coated specimens showed the presence of peaks corresponding to Mg2SiO4 along with Mg and MgO. The corrosion measurements for the bare substrate and MAO coatings were carried out in 3.5% NaCl medium (0.6 M). Based on potentiodynamic polarization studies, the MAO coating prepared at 0.046 A/cm2 exhibited a lower corrosion current density with a higher Rp value, which was about five orders of magnitude higher than the bare substrate, due to the dense nature of the coating. In EIS study, MAO coatings were fitted with the two time constants equivalent circuit containing outer porous layer and inner barrier layer. The barrier layer resistance values were higher than that of porous layer resistance, which indicated that the resistance offered by barrier layer was higher than the porous layer. The total resistance value obtained for the coating prepared at 0.046 A/cm2 were higher compared to the other coatings, which attested to its better corrosion resistance. The electrochemical noise measurement was carried out for longer immersion durations upto 336 h in 3.5% NaCl solution. The noise resistance value obtained for the base Mg alloy was about 100 Ω at 1h immersion, whereas for the MAO coating prepared at 0.04 A/cm2 a maximum value of about 34.8 MΩ was achieved and it was retained even after 96 h of immersion. Mott–Schottky analysis showed that the oxide layer on magnesium substrate acted as a n-type semiconductor, whereas the MAO coatings exhibited p-type semiconductor behavior. The MAO coating obtained at an intermediate current density showed a higher acceptor density and the flat band potential, which resulted in the better performance of the coating in corrosive environment. In another set of investigations, the Ni-P and Ni-P/Ni-W-P coatings were deposited on AZ31B Mg alloy with MAO coating as an interlayer. The MAO layer was activated by a simple borohydride pretreatment process. During the pretreatment process, the MAO coating was subjected to mild alkali treatment, immersion in the Ni-P plating solution and finally immersion in borohydride solution. During each pretreatment step, the sample was characterized for their surface morphology and composition. The surface morphology showed the distribution of spherical particles over the surface of MAO coating after immersion in the Ni-P plating solution. EDX analysis showed the presence of 2.4 wt. % of Ni, which confirmed that Ni ions were adsorbed over the surface of the MAO coating during the pretreatment process. XPS analysis carried out after immersion in the Ni-P plating solution indicated that Ni existed in +2 oxidation state. The surface became smooth and uniform with flake- like morphology after the borohydride treatment, which indicated that the surface was etched by the borohydride solution. EDX analysis showed the presence of 1.8 wt.% of Ni after borohydride reduction. XPS analysis confirmed the reduction of nickel to the zero oxidation state. Additionally, MAO/Ni-P and MAO/Ni-P/Ni-W-P duplex coatings were developed on MAO coating after a simple borohydride pretreatment. Ni-P and duplex coatings showed uniform and dense nodular morphology without any defects, which clearly indicated that the borohydride treatment provided a uniform and homogeneous active surface for the deposition of electroless nickel based coatings. Borohydride pretreatment process resulted in excellent bonding between MAO/Ni-P layers in the cross section. Based on potentiodynamic polarization studies, the corrosion current values obtained for MAO/ Ni-P and MAO/Ni-P/Ni-W-P duplex coatings were about 1.44 and 1.42 µA/cm2, respectively. The coating showed about 97 times improvement in corrosion resistance compared to the bare substrate, attesting to the dense nature of the coating. In EIS study, the single time constant equivalent circuit was used for fitting the spectra, which pertained to the coating /electrolyte interface. The single time constant could be attributed to the pore-free dense, uniform coatings developed over the MAO interlayer. For the MAO/Ni-P and MAO/Ni-P-Ni-W-P duplex coatings, the charge transfer resistance of about 15 and 11 kΩcm2 were obtained for duplex and Ni-P coatings, which reinforce the better corrosion protective ability of the coating. The above investigation confirms that MAO coatings have good corrosion resistance in the aggressive chloride medium. Consequently, they can serve as an ideal interlayer for the deposition of the electroless nickel coating. Even if the electroless nickel coating is found to fail in harsh environments, the MAO interlayer can protect the base substrate due to its higher corrosion resistance. It is also noteworthy that the borohydride treatment provides better adhesion between the MAO/Ni-P interlayer.

Abstract The qualitative and quantitative characteristics of each component layer constituting the structure of organic bulk heterojunction solar cells (OSC-BHJ) contribute significantly towards its overall performance. One of the prevalent issues resulting in reduced device efficiency is due to the conformational inhomogeneities in the active and buffer layers. The mechanical stress, extended thermal exposure and presence of mutually reactive component layers etc., affects negatively on the device stability. Effective methods to address these issues will be extensively benefited by the industry since the current commercialisation of the technology is hindered owing to the lower efficiency and stability of these devices. This dissertation focuses on methods to coherently enhance the performance and longevity of the OSC-BHJ devices. The efficiency enhancements of the devices in this work were achieved through two main routes. The first route was through morphological improvement of the active layer. The second route was through boosting the electrical characteristics of hole transporting conducting polymer layer (HTL) by controlled annealing conditions. The introduction of a suitable additive in the active layer was found to reduce unfavourable phase segregation thus resulting in enhanced morphology. Further, the annealing conditions in different atmospheres (air, nitrogen and vacuum) were found to have a clear influence on the optimum functioning of the HTL in the device. Regarding the stability improvement study done in this work, a method of employing suitable interlayer was developed to effectively abate the internal degradation occurring in the device due to etching reaction on the indium tin oxide (ITO) anode by the HTL. Moreover, experimental investigations were carried out for drawing fundamental understanding of stability degenerating issues such as the influence of mechanical defects on transparent conducting metal oxide (ITO) anode on the performance of the device and heat induced degradations in the low band gap polymer-fullerene active layer. The highlight of this research is that the discovered methods are inexpensive, efficient, and easy to adopt. The results of the study could help the technology to overcome some of its limitations and accelerate its progress towards commercialisation. / Tiivistelmä Orgaanisten heteroliitosaurinkokennojen kerrosrakenteen ominaisuudet ja laatu vaikuttavat merkittävästi aurinkokennojen toiminnallisuuteen. Erityisesti rakenteelliset epähomogeenisuudet aktiivi- ja puskurikerroksissa heikentävät kennon hyötysuhdetta. Kennojen stabiilisuutta tarkasteltaessa myös mekaanisella rasituksella, pitkittyneellä lämpöaltistuksella ja materiaalien reagoinneilla keskenään kerrosten välillä, on selkeä negatiivinen vaikutus kennojen stabiilisuuteen. Orgaanisen aurinkokennoteknologian kaupallistamisen rajoitteina ovat kennojen heikko hyötysuhde ja stabiilisuus, joten menetelmät jotka tarjoavat ratkaisuja edellä mainittuihin ongelmiin, ovat erittäin tärkeitä teknologiaa kaupallistavalle teollisuudelle. Tämä väitöskirja keskittyy johdonmukaisesti selvittämään tapoja, joilla voidaan parantaa heteroliitosaurinkokennojen hyötysuhdetta ja elinikää. Hyötysuhteen tehostamiseksi valittiin kaksi eri lähestymistapaa, joista ensimmäisessä keskityttiin aktiivikerroksen morfologian parantamiseen ja toisessa aukkoja kuljettavan kerroksen sähköisten ominaisuuksien parantamiseen lämpökäsittelyprosessin avulla. Sopivan lisäaineen avulla aktiivikerroksen ei-toivottua kiteytymistä voidaan pienentää ja parantaa näin kerroksen morfologiaa. Lisäksi työssä todettiin, että lämpökäsittelyn aikaisella ympäristöolosuhteella (ilma, typpi, tyhjiö) on merkittävä vaikutus puskurikerroksen optimaaliseen toimintaan aurinkokennossa. Stabiilisuuden parantamiseksi kehitettiin välikerroksen hyödyntämiseen perustuva menetelmä, jolla voidaan tehokkaasti vähentää kennojen sisäisessä rakenteessa tapahtuvaa toiminnallisuuden heikkenemistä, joka aiheutuu aukkoja kuljettavan kerroksen syövyttävästä vaikutuksesta indiumtinaoksidi (ITO) pohjaiseen anodiin. Tämän lisäksi työssä tutkittiin kokeellisesti stabiilisuuteen heikentävästi vaikuttavia tekijöitä, kuten mekaanisen rasituksen aiheuttamia vaurioita metallioksidi (ITO) anodissa ja lämpöaltistuksesta aiheutuvia vikoja polymeeri-fullereeni rakenteeseen perustuvassa aktiivikerroksessa. Tutkimuksen keskeisin tulos on, että esitellyt keinot aurinkokennojen hyötysuhteen ja stabiilisuuden parantamiseen ovat edullisia, tehokkaita ja helppoja hyödyntää. Tulokset voivat merkittävästi edistää orgaanisten aurinkokennojen teknistä kehitystä ja kiihdyttää niiden tuloa kaupallisiksi tuotteiksi.

Die Bildung von CoSi$_2$-Schichten mittels TIME-Verfahren (TIME: Ti-Interlayer Mediated Epitaxy) wurde untersucht. Dabei wurde die Ti-Zwischenschicht durch eine Hf-Zwischenschicht ersetzt. Der Einfluss der Prozessparameter (Tempertemperatur, Temperzeit, Aufheizrate und Ausgangsschichtdicken) und des Metalls (Hf, Ti, Zr) der Zwischenschicht auf die Reaktion der metallischen Doppelschichten mit Si(100) wurde ermittelt. Zur Charakterisierung der Proben wurden RBS-, TEM-, XRD- und AES-Untersuchungen durchgefuehrt. Die Ausbildung eines Mehrschichtsystems nach der Temperung der Doppelschichten in Abhaengigkeit der Prozessparameter wird dargestellt. Es wird gezeigt, dass die Prozessparameter die Temperatur bestimmen, bei der die CoSi$_2$-Keimbildung stattfindet. Anhand dieser Untersuchungen wird nachgewiesen, dass sich mit Erhoehung der CoSi$_2$-Keimbildungstemperatur die epitaktische Qualitaet der gebildeten CoSi$_2$-Schicht verbessert. Die Erklaerung des Reaktionsablaufs der metallischen Doppelschichten mit Si(100) erfolgt anhand eines aufgestellten Reaktionsschemas. Zur Entfernung der Deckschicht wurden verschiedene Aetzverfahren angewandt und deren Wirkung verglichen. / The formation of a CoSi$_2$ layer by solid phase reaction of metallic bilayer with Si (TIME: Ti-Interlayer Mediated Epitaxy) was investigated. In this work the Ti was replaced by Hf. The influence of the annealing temperature, the annealing time, the heating rate and the thicknesses of the metallic layers on the reaction of the bilayer with Si was determined. The samples were characterised by Rutherford-backscattering (RBS), Transmission-Electron-Microscopy (TEM), X-ray-Diffraction and Auger-Electron-Spectroscopy (AES) studies. During the annealing of the samples a system of layers is formed. It was shown, that the annealing parameters and the thicknesses of the layer determine the temperature, on which the nucleation of CoSi$_2$ occurs. A decrease of this nucleation temperature leads to an improvement of the quality of the epitaxial CoSi$_2$ layer. A model of reaction is presented, which explains the reaction of the metallic bilayer with Si. The removal of the top layer by several etching procedures was investigated and the results were compared.