Maintaining the safe operation of nuclear power plants (NPPs) is crucial. This requires fully understanding the mechanism of long term irradiation and thermal ageing, as well as their effects, on components including the reactor pressure vessel (RPV). The research community is collecting data that will be required to support the case for extending the operation of western-type NPPs beyond that of 60 years. One of the current dilemmas faced by the long-term operation of RPVs is the formation of nanometre scale precipitates. These precipitates are known to cause embrittlement where it increases the ductile-to-brittle transition temperature of the RPV steels. The chemistry of these precipitates is strongly dependent on the chemistry of the RPV steels. In general, these precipitates can be categorised into two types, copper-rich precipitates (CRPs) and manganese-nickel (-enriched) precipitates (MNPs) [1, 2]. The concentration of copper in the precipitates depends on the bulk content of the steel [3]. The formation mechanism of the precipitates under neutron irradiation and thermal ageing, and their influence on material degradation at high neutron fluence (Φt), is still unclear. To understand the long term precipitation under irradiation and thermal ageing, high nickel and copper containing RPV steels with a similar microstructure an chemical composition as those currently in service were subjected to either neutron irradiation (to high neutron fluences, Φt ≥ 5 x 10²³ neutrons.m^-2) or thermal ageing (for as long as ≈ 50,000 hours). CRPs and MNPs were both detected. The co-precipitation of the CRPs and MNPs were observed in thermally aged steels. The development of crystal structures in the CRPs is believed to be dependent on the size of the precipitates and the ambient temperature. When the CRPs reached a critical size, they underwent the martensitic transformation from BCC→9R→3R→FCC or FCT. The CRPs preferentially nucleate heterogeneously at the dislocation lines. Chemical analysis suggests that most of the CRPs are iron free. Under thermal ageing, the MNPs were found to precipitate at the interface of the CRPs and the matrix. These MNPs are found to be iron free too. Larger MNPs were often found to be at CPRs that were associated with dislocation lines. Also, based on the volume fraction observed, it is possible to suggest that the kinetics of nucleation and growth of the MNPs are relatively slow compared to the CRPs. This is in good agreement with the simulations reported in Refs. [4, 5]. It is the first time the MNPs are directly imaged from neutron irradiation low copper steels using electron microscopy. These irradiation-induced MNPs are densely populated in the neutron irradiated samples. It was found that the irradiation-induced MNPs are more sensitive to electron beams. It was thought that this was due to a relatively large amount of point defects present in the irradiation-induced MNPs.

The in-situ examination of historical structures for diagnostic and monitoring purposes is a troublesome work that necessitates the use of non-destructive investigation (NDT) techniques. The methods of quantitative infrared thermography (QIRT) and ultrasonic testing have distinct importance in this regard. The key concern of the study was developing the in-situ use of QIRT for assessment of stone masonry wall sections having different sublayer(s) and failures. For that purpose, the non-destructive in-situ survey composed of QIRT and ultrasonic testing was conducted on a 16th century monument, Cenabi Ahmet PaSa Camisi, suffering from structural cracks, dampness problems and materials deterioration. The combined use of these two methods allowed to define the thermal inertia characteristics of structural cracks in relation to their depth. The temperature evolution in time during the controlled heating and cooling process was deployed for the cracks/defects inspection. The superficial and deep cracks were found to have different thermal responses to exposed conditions which made them easily distinguishable by QIRT analyses. The depth of cracks was precisely estimated by the in-situ ultrasonic testing data taken in the indirect transmission mode. The inherently good thermal resistivity of the wall structure was found to have failed due to entrapped moisture resulting from incompatible recent plaster repairs. The IRT survey allowed to detect the wall surfaces with different sublayer configurations due to their different thermal inertia characteristics. The knowledge and experience gained on the experimental set-ups and analytic methods were useful for the improvement of in-situ applications of QIRT and ultrasonic testing.

Die hochempfindliche Methode der „Microwave Detected Photoconductivity“ (MDP) wird eingesetzt, um technologisch relevante Halbleiterparameter wie die Ladungsträgerlebensdauer, Photoleitfähigkeit und Defektkonzentrationen über viele Größenordnungen der optischen Anregung hinweg zu untersuchen. Durch die Entwicklung und die Anwendung eines neuartigen Modellierungssystems für die Ladungsträgerdynamik in Halbleitern können wichtige Defektparameter quantitativ aus MDP Messungen in Abhängigkeit der Anregungsintensität bestimmt werden. Ein Verfahren zur Charakterisierung von Haftstellen (Konzentration, Energielage, Einfangsquerschnitt) bei konstanter Temperatur wird vorgestellt. Das technologisch relevante Verfahren des quantitativen Eisennachweises in p-dotiertem Silizium wird für die MDP Methode angepasst und entsprechende Messergebnisse mit DLTS Resultaten verglichen. Ein detaillierter Vergleich der gängigsten kontaktlosen Messverfahren QSSPC und MW-PCD mit der MDP zeigt, dass entgegen gängiger Annahmen die unterschiedlichen Anregungsbedingungen zu drastischen Unterschieden in den gemessenen Werten der Ladungsträgerlebensdauer führen. Dies wird sowohl durch theoretische Berechnungen als auch durch praktische Messergebnisse belegt.

While group VII 4d Tc and 5d Re have hexagonally close-packed (hcp) ground states, 3d Mn adopts the complex chi-phase which exhibits non-collinear magnetism. Density functional theory (DFT) calculations have shown that without magnetism the chi-phase remains the ground state of Mn implying that magnetism is not the critical factor, as is commonly believed, in driving the anomalous stability of the chi-phase over hcp. Using a tight-binding (TB) model it is found that while harder potentials stabilise close-packed hcp, a softer potential stabilises the more open chi-phase. By analogy with the structural trend from open to close-packed phases down the group IV elements, the anomalous stability of the chi-phase in Mn is shown to be due to 3d valent Mn lacking d states in the core which leads to an effectively softer atomic repulsion between the atoms than in 4d Tc and 5d Re. Subsequently an analytic Bond-Order Potential (BOP) is developed to investigate the structural and magnetic properties of elemental Mn at 0 K. It is derived within BOP theory directly from a new short-ranged orthogonal d-valent TB model of Mn, the parameters of which are fitted to reproduce the DFT binding energy curves of the five experimentally observed phases of Mn, alpha, beta, gamma, delta, and epsilon-Mn. Not only does the BOP reproduce qualitatively DFT binding energy curves of the five different structure types, it also predicts the complex collinear antiferromagnetic (AFM) ordering in alpha-Mn, the ferrimagnetic (FiM) ordering in beta-Mn and the AFM ordering in the other phases that are found by DFT. A BOP expansion including 14 moments is sufficiently converged to reproduce most of the properties of the TB model with the exception of the elastic shear constants which require further moments. Magnetic analytic BOPs are also developed for Fe and Fe-Mn. The Fe model correctly reproduces trends in the structural stabilities of the common metallic structures except that AFM hcp is overstabilised. Reproduction of the elastic constants with a 9-moment BOP is reasonable although as is found for the Mn BOP the elastic shear constants require more moments to converge. Vacancy formation energies are close to those determined by experiment and DFT and the relative stabilities of self-interstitial atom (SIA) defects in ferromagnetic bcc Fe are correctly reproduced. The SIA formation energies are found to be better than those calculated with existing BOP models. The Fe-Mn TB and BOP models were challenging to fit and nonmagnetic face-centred cubic (fcc) structures are overstabilised. Furthermore within BOP an incorrect magnetic solution is predicted for one fcc structure resulting in poor reproduction of the DFT stacking fault energies. Refitting the bond integrals might help to better reproduce the nonmagnetic hcp-fcc energy differences while an environment-dependent Stoner parameter could help provide the flexibility needed to correctly capture the magnetic energy differences.

Die hochempfindliche Methode der „Microwave Detected Photoconductivity“ (MDP) wird eingesetzt, um technologisch relevante Halbleiterparameter wie die Ladungsträgerlebensdauer, Photoleitfähigkeit und Defektkonzentrationen über viele Größenordnungen der optischen Anregung hinweg zu untersuchen. Durch die Entwicklung und die Anwendung eines neuartigen Modellierungssystems für die Ladungsträgerdynamik in Halbleitern können wichtige Defektparameter quantitativ aus MDP Messungen in Abhängigkeit der Anregungsintensität bestimmt werden. Ein Verfahren zur Charakterisierung von Haftstellen (Konzentration, Energielage, Einfangsquerschnitt) bei konstanter Temperatur wird vorgestellt. Das technologisch relevante Verfahren des quantitativen Eisennachweises in p-dotiertem Silizium wird für die MDP Methode angepasst und entsprechende Messergebnisse mit DLTS Resultaten verglichen. Ein detaillierter Vergleich der gängigsten kontaktlosen Messverfahren QSSPC und MW-PCD mit der MDP zeigt, dass entgegen gängiger Annahmen die unterschiedlichen Anregungsbedingungen zu drastischen Unterschieden in den gemessenen Werten der Ladungsträgerlebensdauer führen. Dies wird sowohl durch theoretische Berechnungen als auch durch praktische Messergebnisse belegt.

The pursuit of high quality, large area graphene has been a major research focus of contemporary materials science research, in the wake of the discovery of the multitude of exceptional properties exhibited by the material. The DPhil project was undertaken with the objective of developing an understanding of the growth of large graphene sheets by chemical vapour deposition (CVD), and also in the subsequent characterisation of their material properties. By conducting atmospheric pressure CVD growth at high methane flow rates, it was found that few-layered graphene (FLG) could be deposited on a copper catalyst. It is demonstrated that the self-limiting property of a copper catalyst is not universal to all deposition conditions, and shown that FLG grows in a terrace-like configuration. In depth transmission electron microscopy (TEM) studies were carried out on FLG. By selective image reconstruction from the inverse power spectrum of the TEM images, it was possible to elucidate the inter-grain connectivity of few-layer graphenes. It was determined that there were two possible inter-grain configurations possible; specifically an overlap of graphene layers or a discrete atomic bonding edge. The perturbation of the few-layer structure when subject to an out of plane distortion was found to incur a shift in the conventional AB-Bernal stacking of FLG. By utilising the aberration corrected TEM (AC-TEM) at Oxford it was possible to resolve atomic detail in CVD synthesised monolayer films, including atomic bond rotations and vacancies. The use of a high current density at low accelerating voltage (80 kV) was demonstrated to allow for the controlled defect creation in graphene sheets. This permitted the creation of monovacancies and iron doped vacancy complexes suitable for further study. The behaviour of these two defect types under electron beam irradiation was subsequently studied.

Microscopic beams of penetrating synchrotron radiation provide a unique tool for the analysis of material structure and deformation. This thesis describes my contributions to the development of new synchrotron X-ray micro-beam diffraction experimental techniques and data interpretation, and the use of experimental results for the validation of material deformation models. To study deeply buried material volumes in thick samples, the micro-beam Laue technique was extended to higher photon energies. Through-thickness resolution was achieved either by a wire scanning approach similar to Differential Aperture X-ray Microscopy (DAXM), or by applying tomographic reconstruction principles to grain-specific Laue pattern intensity. Both techniques gave promising first results. For reliable micro-beam Laue diffraction measurements of elastic strains in individual grains of a polycrystal, understanding of the error sources is vital. A novel simulation-based error analysis framework allowed the assessment of individual contributions to the total measurement error. This provides a rational basis for the further improvement of experimental setups. For direct comparison of experimental measurements and dislocation dynamics simulations, diffraction post-processing of dislocation models in two and three dimensions was developed. Simulated diffraction patterns of two-dimensional dislocation cell/wall type structures captured correctly some of the features observed experimentally in reciprocal space maps of a large-grained, lightly deformed aluminium alloy sample. Crystal lattice rotations computed from three-dimensional dislocation dynamics simulations of a Frank-Read source showed anisotropic orientation spread similar to that observed in micro-beam Laue experiments. For the experimental study of crystal lattice distortion, a novel technique was proposed that combines micro-beam Laue diffraction with scanning white-beam topography. Diffraction topography allows the study of lattice rotation at scales smaller than the scanning beam size. The new technique makes it possible to apply classical topography methods to deformed samples.

This thesis describes investigations into the role of non-stoichiometry in the surface and bulk properties of SrTiO₃ single crystals. A family of (n×n) reconstructions, where n = 2, 3, 4, 5, 6 are produced by argon ion sputtering of the SrTiO₃ (111) single crystals and subsequent annealing in UHV or in an oxygen rich environment. The sputtering process introduces defects or oxygen vacancies in the surface region of the sample, whilst the annealing gives rise to surface reconstructions. The surface preparation conditions such as sputtering time, annealing temperature and environment are optimized to obtain various reconstructions in a controlled and reproducible manner. High resolution STM images of these reconstructions are also obtained and utilized in the investigation of the surface reactivity. Fullerene molecules are deposited on the reconstructed surfaces to elucidate the surface reactivity through template assisted growth. Fullerene molecules are first deposited with substrate surfaces held at room temperature. Being the most highly reduced among the (n×n) family, the 5×5 reconstruction significantly influenced the growth of fullerenes. Both C₆₀ and C₇₀ adsorb as individual molecules and produce clusters with magic numbers. The 4×4 and 6×6 reconstructed surfaces encourage the formation of close-packed structures upon the deposition at room temperature. When the surface covered with fullerenes is heated to a temperature of around 200 °C, epitaxial islands are observed. The 6×6 reconstructed surface appeared to be less reactive than the 4×4. Electrical transport, cathodoluminescence (CL) and electron spin resonance (ESR) experiments are also carried out to investigate the effect of oxygen vacancies on the bulk properties of UHV annealed SrTiO₃ single crystals. Thermal reduction leads to carrier doping of the material, which not only gives rise to electrical conduction but also induces room temperature luminescence. Both the electrical conductivity and CL intensity increases with annealing time. The work presented in this thesis provides insight into the defect driven properties in both the surface and bulk of SrTiO₃ single crystals, which could play an important role in the development of oxide-based electronic devices.

Electrically detected magnetic resonance (EDMR) is a sensitive spectroscopic technique, which can be used to readout few to single electron spins in semiconductor and carbon nanodevices for applications in solid state quantum information processing (QIP). Since only electrically active defects contribute to the EDMR signal, this technique can be used further to investigate defects and impurities in photovoltaic devices, in which they limit the sunlight-to-energy conversion efficiency significantly. Here, I employ X-band EDMR for semiconductor defect analysis and identify the most important recombination centres in Czochralski silicon with oxide precipitates, which can be intentionally grown to confine detrimental metallic impurities to inactive regions of the wafer in order to serve as a defect-free substrate for modern silicon photovoltaic devices. Those experiments show that oxide precipitation is accompanied by the formation of silicon dangling bonds. Furthermore, I describe a very promising route towards the fabrication and readout of few to single electron spins in carbon nanotube devices, which can be characterised structurally via transmission electron microscopy in order to relate their electrical and spin properties with their structure. Finally, I employ EDMR to read out electron spin states in donor-doped silicon field-effect transistors as a prerequisite for their application in QIP. I report on a novel cryogenic probe head for EDMR experiments in resonant microwave cavities operating at 0.35 T (9.7 GHz, X-band) and 3.34 T (94 GHz, W-band). This approach overcomes the inherent limitations of conventional X-band EDMR and permits the investigation of paramagnetic states with a higher spectroscopic resolution and signal intensity. Both advantages are demonstrated and discussed. I further report on a novel mechanism giving rise to the EDMR effect in donor-doped silicon field-effect transistors, which is capable of explaining why the EDMR signal intensities of the conduction electrons are enhanced by a factor of ∼100, while the donor resonance signals increase by a factor of ∼20 from X- to W-band only. The spin-relaxation and dephasing times are extracted from a series of pulsed-EDMR measurements and confirm this model. The author gratefully acknowledges funding from Trinity College Oxford, Department of Materials, EPSRC DTA, and Konrad-Adenauer-Stiftung e.V. (Begabtenförderung).

Silicon nitride is a bulk and a coating material exhibiting excellent mechanical properties. The understanding of the complex processes at the nanometre scale gained through experimental research will be enhanced by the existence of a computationally efficient and accurate model that is able to describe the mechanical properties of silicon nitride. Such a model has yet to be proposed. In this thesis we present a transferable reduced tight-binding (TB) model for the silicon nitride system. More precisely, this model consists of a reduced TB model for elemental silicon, a reduced TB model for elemental nitrogen, and a reduced TB model for silicon nitride. These models are developed within the framework of coarse-graining the electronic structure from density functional theory (DFT) to tight binding (TB) to bond-order potentials (BOPs), and can therefore be used in the future as the stepping stone to develop BOPs for the application in large scale simulations. The bond integrals employed in the reduced TB models are obtained directly from mixed-basis DFT projections of wave functions onto a minimal basis of atom-centred orbitals. This approach reduces the number of overall parameters to be fitted and provides models which are transferable through the different coarse-graining levels. We provide an example by using the same bond integrals in the reduced TB model for silicon and the preliminary bond-based BOP for silicon. DFT binding energies of ground state and metastable crystal structures are used as the benchmark to which the TB and BOP repulsive parameters are fitted. In addition to model development, we present an improved methodology when going from TB to reduced TB. By weighting all four σ TB bond integrals equally, we provide a new parameterisation (Eqs. (2.73) and (2.74)) and show that the quality of the silicon reduced TB model can be increased by choosing one of the reduced TB parameters to be distance invariant. The ingredients, the development methodology, and the quality of each of the four models are discussed in a separate chapter. The quality of the reduced TB models and BOP is demonstrated by comparing their predictions for the binding energies, heats of formation, elastic constants, and defect energies with DFT and experimental values.