The fabrication and mechanical properties of Ti2AlC composites reinforced with the alumina oxide fibers, such as NextelTM 720 and ALBF1, were described in this thesis. Alumina fibers and Ti2AlC powders were dispersed in the water and slip cast in the molds to form green bodies. Sedimentation test were carried out to optimize pH of the slurry. It was found that suspensions prepared with PAA as a dispersant and has an excellent stability in the pH range of 4 ~ 5. Composite green bodies were densified by pressureless sintering or hot isotatic pressing (HIP) at different temperatures. The microstructure of fabricated samples was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), and porosimetry. It was found out that HIPing at 1300 oC for 4 hrs at 100 MPa results in almost fully dense composites with majority phases being alumina fibers and Ti2AlC. However, fully dense Ti2AlC composites could not be obtained by the pressurless sintering, even at temperature as high as 1400 oC at which reaction between Ti2AlC and NextelTM 720 was observed. The double torsion (DT) tests were carried out at room temperature to measure the fracture toughness of the HIPed pure and 5vol% alumina fiber reinforced Ti2AlC. DT results showed increase in the fracture toughness of Ti2AlC reinforcing with NextelTM 720 alumina fibers. However, fracture toughness of the samples reinforced with ALBF1 was lower than that of pure Ti2AlC because of the low relative densities of those composites. SEM study of the fracture surfaces after DT tests showed that toughening mechanisms by crack bridging and fiber pull outs at the crack tip are operative in all reinforced samples. In addition, elastic moduli of HIPed Ti2AlC measured by Resonant Ultrasound Spectroscopy (RUS) do not show significant change due to reinforcement with alumina fibers, while the Vickers hardness of composites was found to be larger for Ti2AlC reinforced with NextelTM 720 and lower for the samples reinforced with ALBF1.

Dans cette étude la céramique Ti2AlC a pu être brasée avec succès sur un substrat de Ni dans une gamme de température allant de 1000°C à 1100°C et pour des temps de maintien de 10 à 60 mn. Les mécanismes mis en œuvre pendant la brasure ont été mis en lumière en étudiant la microstructure du joint Ti2AlC/Ni. On montre ainsi que le nickel provenant du métal d’apport BNi-2 diffuse dans Ti2AlC, majoritairement le long des joints de grains et parfois aussi dans certains grains d’orientation favorable. Le phénomène d’interaction induit des variations de fluidité du métal d’apport liquide. En conséquence, l’épaisseur des joints réalisé sont fonction des différents paramètres de brasage. La résistance maximale au cisaillement est obtenue à 1100°C après un temps de maintien de 30 min. Dans un second temps, l’interaction entre les principaux éléments contenus dans le métal d’apport (c.-à-d. Ni, Cr et Si) et le substrat Ti2AlC a été expliqué à l’aide de modélisations DFT. Il est montré que ces trois éléments ont chacun leur site de substitution spécifique. Les atomes de Si se placent plutôt sur le site Al, le Cr sur le site Ti et les atomes de Si sur le site de l’aluminium. Deux voies de décomposition de Ti2AlC en présence de défauts ont également été identifiées soit en raison de modes de vibrations instables, soit en raison de la compétition avec d’autres phases. / In the present work, the brazing experiments of Ti2AlC ceramics to Ni substrate have been successfully performed in the temperature range between 1000°C and 1100°C, with the holding time varying from 15 min to 60 min. The corresponding mechanisms have been disclosed by studying the microstructure of the Ti2AlC/Ni joint. It is found that the Ni element originating from the BNi-2 filler can diffuse into the Ti2AlC substrate during the brazing process, mainly along the grain boundary, or in some grains with special orientations. The interaction process induces some differences in the fluidity of the liquid filler; consequently, the thickness of the Ti2AlC/Ni joints achieved finally is different at different brazing parameters. The maximum shear strength is measured to be 193 MPa, achieved at 1100°C holding for 30 min. Afterwards, the interaction behavior between the filler elements (including Ni, Cr and Si) and the Ti2AlC substrate, has been explained by DFT simulation. It is found that the three kinds of filler elements have their own preferred substitution sites: Ni atoms are most likely to be found at Al site, Cr atoms at Ti site, and Si atoms at Al site. Then, the decomposition mechanisms of the defective Ti2AlC model have been illustrated as well, which might occur in two possible means: unstable vibrational behaviors, and lower Gibbs energy of competing phases.

In this Thesis Ti-Al-C thin films deposited by cathodic arc at 700, 800 and 900 °C were investigated with respect to composition, structure and mechanical properties. The highest growth temperature resulted in close to single crystalline Ti2AlC MAX phase.   A high oxygen incorporation of 7-12 at.% was detected in all the films, likely originating from residual gas and the Al2O3 substrate. It was evident that the characteristic nanolaminated MAX phase structure was retained upon deflection from the ideal MAX phase stoichiometry.   Hardness and elastic modulus of the sample grown at 900 °C were 16 and 259 GPa, respectively, as determined by nanoindentation using a Berkovich tip. Nanoindentation measurements with a cube corner tip were also performed on all three samples in order to extract elastic moduli.   Analysis of loading-unloading curves and SPM images revealed no relation between pop-in events and pile-ups around the residual imprints, indicating that other mechanisms than formation of kink bands may be responsible for formation of pile-ups. This was also confirmed by cross-sectional TEM investigation of an indent: Ti2AlC MAX phase deformed without kinking and delamination, as opposed to the observations in single crystalline Ti3SiC2 films. Several possible reasons for the different deformation mechanism observed are discussed.    These results are of importance for the fundamental understanding of the origin of material characteristics, and serve as an initial study initiating further investigations of the influence of defects on MAX phase properties.

<p>In this Thesis Ti-Al-C thin films deposited by cathodic arc at 700, 800 and 900 °C were investigated with respect to composition, structure and mechanical properties. The highest growth temperature resulted in close to single crystalline Ti₂AlC MAX phase.</p><p> </p><p>A high oxygen incorporation of 7-12 at.% was detected in all the films, likely originating from residual gas and the Al₂O₃ substrate. It was evident that the characteristic nanolaminated MAX phase structure was retained upon deflection from the ideal MAX phase stoichiometry.</p><p> </p><p>Hardness and elastic modulus of the sample grown at 900 °C were 16 and 259 GPa, respectively, as determined by nanoindentation using a Berkovich tip. Nanoindentation measurements with a cube corner tip were also performed on all three samples in order to extract elastic moduli.</p><p> </p><p>Analysis of loading-unloading curves and SPM images revealed no relation between pop-in events and pile-ups around the residual imprints, indicating that other mechanisms than formation of kink bands may be responsible for formation of pile-ups. This was also confirmed by cross-sectional TEM investigation of an indent: Ti₂AlC MAX phase deformed without kinking and delamination, as opposed to the observations in single crystalline Ti₃SiC₂ films. Several possible reasons for the different deformation mechanism observed are discussed. </p><p> </p><p>These results are of importance for the fundamental understanding of the origin of material characteristics, and serve as an initial study initiating further investigations of the influence of defects on MAX phase properties.</p>

Fundamental studies for the improvement of photo-microbial fuel cells (pMFCs) within this work comprised investigations into ceramic electrodes, toxicity of metal-organic frameworks (MOFs) and hot-pressing of air-cathode materials. A novel type of macroporous electrode was fabricated from the conductive ceramic Ti2AlC. Reticulated electrode shapes were achieved by employing the replica ceramic processing method on polyurethane foam templates. Cyclic voltammetry of these ceramics indicated that the application of potentials larger than 0.5 V with regard to a Ag/AgCl reference electrode results in the surface passivation of the electrode. Ti2AlC remained conductive and sensitive to redox processes even after electrochemical maximisation of the surface passivation, which was shown electrochemically and with four terminal sensing. Application of macroporous Ti2AlC ceramic electrodes in pMFCs with green algae and cyanobacteria resulted in higher power densities than achieved with conventional pMFC electrode materials, despite the larger surface area of the Ti2AlC ceramic. The effect of electrode surface roughness and hydrophobicity on pMFC power generation and on cell adhesion was examined using atomic force and confocal microscopy, contact angle measurements and long-term pMFC experiments. The high surface roughness and fractured structure of Ti2AlC ceramic was beneficial for cell adhesion and resulted in higher pMFC power densities than achieved with materials such as reticulated vitrified carbon foam, fluorine doped tin oxide coated glass or indium tin oxide coated plastic. Toxicity of the MOF MIL101 and its amine-modified version MIL-101(Cr)-NH2 on green algae and cyanobacteria was assessed on the basis of both growth in liquid culture and by exclusion zones of agar colonies around MOF pellets. MOF MIL101 was found harmless in concentrations up to 480 mg L-1 and MIL-101(Cr)-NH2 did not exhibit toxic effects at a concentration of 167 mg L-1. Air-cathodes were produced from a range of carbon materials and ion-exchange membranes. Hot-pressing of Zorflex Activated Carbon Cloth FM10 with the proton-selective Nafion® 115 membrane provided the best bonding quality and pMFC performance.

Mn+1AXn-Phasen sind thermodynamisch stabile nanolaminierte Ternärcarbide und -nitride, die sowohl metallische als auch keramische Eigenschaften aufweisen. Der Buchstabe M steht für ein frühes Übergangsmetall, der Buchstabe A für ein A-Element aus den Gruppen IIIA – VIA und X für Kohlenstoff und/oder Stickstoff. Die M-Atome bilden Oktaederschichten mit X-Atomen in ihren Zentren. Der Index n beschreibt die Dicke der Mn+1Xn-Lage, die zwischen zwei hexagonalen A-Schichten eingebettet ist. Die außergewöhnlichen Eigenschaften dieser Materialien haben ihren Ursprung in ihrer Mikrostruktur. Um einen Einblick auf atomarer Ebene zu gewinnen wird die Messmethode der gestörten γ-γ-Winkelkorrelation (PAC) angewendet. Die radioaktiven Sonden 111In/111Cd und 181Hf/181Ta werden durch Ionenimplantation und/oder durch Neutronenaktivierung in das Wirtsmaterial eingebracht, um den elektrischen Feldgradienten (EFG) zu messen, der am Gitterpatz des Sondenatoms herrscht. Das erste Ziel der Arbeit ist die Suche nach optimalen Ausheilparametern, mit denen ein möglichst hoher Anteil der Sonden die gleiche lokale Umgebung spürt. Der nächste Schritt ist die Bestimmung des Gitterplatzes der Sonden in der MAX-Struktur. Als Ergebnis kann festgestellt werden, dass 111In in den In- und Al-basierten MAX-Phasen fast ausschließlich den A-Platz besetzt, während 181Hf in Hf2InC auf dem M-Platz eingebaut wird. Als überraschendes Ergebnis zeigt diese Arbeit, dass die PAC-Methode bei Phasen mit gleichen Konstituenten, aber unterschiedlicher Mn+1Xn-Schichtdicke sensitiv auf die Änderung der Stapelfolge ist. Die Experimente werden mit umfangreichen Rechnungen auf Basis der Dichtefunktionaltheorie (DFT) verglichen, die hier erstmalig für nahezu alle Mitglieder der Familie der MAX-Verbindungen durchgeführt wurden. Die DFT-Rechnungen reproduzieren die gemessenen EFGs mit hoher quantitativer Genauigkeit und stützen die Hypothese, dass sich die Sonden auf den prognostizierten Gitterplätzen befinden.