Rozprawy doktorskie na temat „MAX phase materials”

 

 

 

MAX-phase materials are a new type of material class. These materials are potentiallyt echnologically important as they show unique physical properties due to the combination of metals and ceramics. In this project, spectroscopic ellipsometry in the spectral range of 0.06 eV –6.0 eV was used to probe the linear optical response of MAX-phases in terms of the complexd dielectric function ε(ω) = ε1(ω) + iε2(ω). Measured data were fit to theoretical models using the Lorentz and generalized oscillator models. Data from seven different samples of MAX-phase materials were obtained using two ellipsometers. Each sample dielectric function was determined, including their infrared spectrum.

This thesis presents theoretical research on MAX phases (M=transition metal, A=A-group element, X=carbon and/or nitrogen), with focus on predictions of phase stability as well as of physical properties. The first part is an investigation of the phase stability of the MAX phases Ti2AlC, Ti3AlC2, and Ti4AlC3 at elevated temperatures, where the former two phases have been obtained experimentally. Phase stability calculations of MAX phases usually do not take temperature dependent effects such as electronic excitations and lattice vibrations into consideration due to significantly increased computational cost. The results have nevertheless so far been quite accurate, with good agreement between theory and experiments. Still, the question whether the inclusion of temperature into the calculations could significantly alter the results as compared to previous 0 K calculations needs to be investigated, since this has bearing on the reliability of future predictions of the stability of not yet known MAX phases. However, it is shown that for Tin+1AlCn, the different temperature dependent effects largely cancel each other. The results therefore suggest that to go beyond 0 K calculations for phase stability predictions of MAX phases is motivated only for borderline cases. The second part investigates the Mn2GaC MAX phase, which was recently predicted from theoretical phase stability calculations and subsequently synthesized. As a new member of the MAX phase family as well as being one of the first known MAX phases to exhibit magnetism, it is of interest to explore its physical properties. Here, we have used firstprinciples calculations to determine the electronic, vibrational and elastic properties. Analysis of the electronic band structure indicates anisotropy in transport properties, while the electronic and phonon density of states shows that the relative orientation of the Mn magnetic moments over the Ga layers affects the distribution of the electronic and vibrational states for both Mn and Ga. The Voigt bulk, Voigt shear, and Young's modulus is also investigated, together with the Poisson's ratio, the elastic anisotropy, and the  machinability via two machinability indices. In relation to experimental results of the moduli of other M2AC phases, the Voigt bulk and shear moduli are concluded to be fairly low, 157 and 93 GPa, respectively, while the magnitude of the Young's modulus at 233 GPa is intermediate. The Poisson's ratio, which is 0.25, on the other hand, is comparatively high. The phase is shown to be elastically quite isotropic, and, just as other M2GaC phases, also machinable. As all here investigated properties are affected by the choice of magnetic spin configuration, the results show the importance of identifying the correct magnetic ground state in future theoretical work on magnetic MAX phases.

The series name of this thesis Linköping Studies in Science and Technology Licentiate Thesis is incorrect. The correct name is Linköping Studies in Science and Technology Thesis.

The study of magnetic Mn+1AXn (MAX) phases (n = 1 − 3, M – a transition metal, A – an A group element, X – C or N) is a recently established research area, fuelled by theoretical predictions and first confirmed experimentally through alloying of Mn into the well-known Cr2AlC and Cr2GeC. Theoretical phase stability investigations suggested a new magnetic MAX phase, Mn2GaC, containing Ga which is liquid close to room temperature. Hence, alternative routes for MAX phase synthesis were needed, motivating a further development of magnetron sputtering from liquid targets. In this thesis, (Cr1-xMnx)2GaC 0 ≤ x ≤ 1  MAX phase thin films have been synthesized from elemental and/or compound targets, using ultra high vacuum magnetron sputtering. Initial thin film synthesis of Cr2GaC was performed using elemental targets, including liquid Ga. Process optimization ensured optimal target size and crucible geometry for containing the Ga. Films were deposited at 650 °C on MgO(111) substrates. X-ray diffraction and transmission electron microscopy confirms the growth of epitaxial Cr2GaC MAX phase with minor inclusions of Cr3Ga. To explore the magnetic characteristics upon Mn alloying, synthesis of (Cr0.5Mn0.5)2GaC thin films was performed from elemental Ga and C and a composite Cr/Mn target of 1:1 composition. Films were deposited on MgO(111), Al2O3(0001) (with or without NbN seed layer), and 4° off-cut 4H-SiC(0001) substrates. The films are smooth and of high structural quality as confirmed by X-ray diffraction and transmission electron microscopy. The film composition measured by high resolution energy dispersive X-ray spectroscopy confirms a composition corresponding to (Cr0.5Mn0.5)2GaC. The magnetic response, as measured with vibrating sample magnetometry, displays a ferromagnetic component, however, the temperature dependence of the magnetic moments and saturation fields suggests competing magnetic interaction and possible non-collinear magnetic ordering. Finally, inspired by theoretical predictions, a new member of the MAX phase family, Mn2GaC, was synthesized. This is the first MAX phase containing Mn as a sole M element. X-ray diffraction and transmission electron microscopy confirms the characteristic MAX phase structure with a 2:1:1 composition. Theoretical work suggests that the magnetic ground state is almost degenerate between ferromagnetic and anti-ferromagnetic. Vibrating sample magnetometry shows ferromagnetic response with a transition temperature Tc of 230 K. However, also for this phase, complex magnetism is suggested. Altogether, the results indicate a new family of magnetic nanolaminates with a rich variation of magnetic ground states.

MAX phase ceramics are ternary ceramics with both metallic and ceramic properties. The existing backing materials in grinding wheels can be made of ceramics or metals. In these applications, ceramics have the disadvantage of low toughness, and most metals have the disadvantages of relatively high density and intolerance to some very high temperatures. The MAX phases have a combination of the main advantages of both metals and ceramics: they are soft and machinable yet also heat-tolerant, strong and lightweight. Cubic boron nitride (c-BN) is a widely used abrasive in grinding wheels, which is exceeded in hardness only by diamond. Composites of c-BN and selected MAX phases may result in materials of some interesting and useful properties for application in industry. Firstly MAX phases, Ti₃SiC₂; Ti₃AlC₂ and Ti₂AlC were synthesised, then reaction couples of MAX-cBN are made in order to investigate the best conditions for composite synthesis, and to analyse the interfacial phases which occur. Finally, the MAX-cBN composites were synthesised from the reaction couple studies. The following results were obtained: 1. Samples synthesised to obtain Ti₃AlC₂ were largely composed of the Ti₂AlC, and thus synthesis of the Ti₃AlC₂ MAX phase was deemed unsuccessful. 2. Nearly pure samples of Ti₂AlC and Ti₃SiC₂ were successfully synthesised with high densities, 99.16% and 98.21%, respectively, of the theoretical density. 3. Reaction couple studies revealed that the Ti₃SiC₂ /c-BN couple was successfully made at 1400°C, 10MPa pressure for 30 minutes, and Ti₂AlC/c-BN couple was successfully made at 1500°C, 10MPa pressure for 30 minutes. The interfacial phases characterised by XRD and SEM found here were TiN, TiC, TiB₂ and AlN for the latter and TiN, TiS₂ and TiB₂ for the former. 4. These conditions were used to successfully synthesise MAX/c-BN composites where both could react and still remain intact. The interfacial phases characterised by XRD and SEM found here were TiAl, TiC, TiB₂ and AlN for Ti₂AlC/c-BN and TiN, TiC, TiS₂ and TiB₂ for Ti₃SiC₂ /c-BN. From these results the following conclusion was drawn: Ti₂AlC and Ti₃SiC₂ are fully compatible with c-BN in order to synthesise a composite with notable properties such as the fracture toughness, suggested by the observed fracture mechanism seen from the fracture surface of these composites.

This Thesis presents two different deposition techniques for the synthesis of Ti₂AlC coatings. First, I have fabricated Ti₂AlC coatings by high velocity oxy-fuel (HVOF) spraying. Analysis with scanning electron microscopy (SEM) show dense coatings with thicknesses of ~150 µm when spraying with a MAXTHAL 211^TM Ti₂AlC powder of size ~38 µm in an H₂/O₂ gas flow. The films showed good adhesion to stainless steel substrates as determined by bending tests and the hardness was 3-5 GPa. X-ray diffraction (XRD) detected minority phases of Ti₃AlC₂, TiC, and Al_xTi_y alloys. The use of a larger powder size of 56 µm resulted in an increased amount of cracks and delaminations in the coatings. This was explained by less melted material, which is needed as a binding material. Second, magnetron sputtering of thin films was performed with a MAXTHAL 211^TM Ti₂AlC compound target. Depositions were made at substrate temperatures between ambient and 1000 °C. Elastic recoil detection analysis (ERDA) shows that the films exhibit a C composition between 42 and 52 at% which differs from the nominal composition of 25 at% for the Ti₂AlC-target. The Al content, in turn, depends on the substrate temperature as Al is likely to start to evaporate around 700 °C. Co-sputtering with Ti target at a temperature of 700 °C, however, yielded Ti₂AlC films with only minority contents of TiC. Thus, the addition of Ti is suggested to have two beneficial roles of balancing out excess of C and to retain Al by providing for more stoichiometric Ti₂AlC synthesis conditions. Transmission electron microscopy and X-ray pole figures show that the Ti₂AlC grains grow in two preferred orientations; epitaxial Ti2AlC (0001) // Al2O3 (0001) and with 37° tilted basal planes of Ti₂AlC (101̅7) // Al₂O₃ (0001).

Report code: LIU-TEK-LIC-2008:15.

This Thesis presents two different deposition techniques for the synthesis of Ti2AlC coatings. First, I have fabricated Ti2AlC coatings by high velocity oxy-fuel (HVOF) spraying. Analysis with scanning electron microscopy (SEM) show dense coatings with thicknesses of ~150 µm when spraying with a MAXTHAL 211TM Ti2AlC powder of size ~38 µm in an H2/O2 gas flow. The films showed good adhesion to stainless steel substrates as determined by bending tests and the hardness was 3-5 GPa. X-ray diffraction (XRD) detected minority phases of Ti3AlC2, TiC, and AlxTiy alloys. The use of a larger powder size of 56 µm resulted in an increased amount of cracks and delaminations in the coatings. This was explained by less melted material, which is needed as a binding material. Second, magnetron sputtering of thin films was performed with a MAXTHAL 211TM Ti2AlC compound target. Depositions were made at substrate temperatures between ambient and 1000 °C. Elastic recoil detection analysis (ERDA) shows that the films exhibit a C composition between 42 and 52 at% which differs from the nominal composition of 25 at% for the Ti2AlC-target. The Al content, in turn, depends on the substrate temperature as Al is likely to start to evaporate around 700 °C. Co-sputtering with Ti target at a temperature of 700 °C, however, yielded Ti2AlC films with only minority contents of TiC. Thus, the addition of Ti is suggested to have two beneficial roles of balancing out excess of C and to retain Al by providing for more stoichiometric Ti2AlC synthesis conditions. Transmission electron microscopy and X-ray pole figures show that the Ti2AlC grains grow in two preferred orientations; epitaxial Ti2AlC (0001) // Al2O3 (0001) and with 37° tilted basal planes of Ti2AlC (101̅7) // Al2O3 (0001).
Report code: LIU-TEK-LIC-2008:15.

Les objectifs de ce travail sont d'une part d'étudier la structure électronique de carbures de titane bidimensionnels appartenant à la famille des MXènes, et d'autre part de synthétiser des films minces pour caractériser certaines de leurs propriétés. L'étude de la structure électronique a été réalisée sur le système Ti3C2T2 avec une attention particulière portée aux groupements de surface T (T=OH, F ou O) en comparant les résultats obtenus par spectroscopie de perte d'énergie des électrons à ceux des calculs ab initio. Cette étude, portée à la fois sur les excitations du gaz d'électrons de valence et des électrons de coeur, a permis de mettre en évidence la localisation des groupements de surface, ainsi que leur influence sur la structure électronique du MXene. La comparaison des simulations et des spectres expérimentaux a également permis de caractériser la nature chimique des groupements de surface. Enfin, la limite d'une telle étude est discutée en considérant les phénomènes d'irradiation responsables de la perte d'atomes d'hydrogène. La synthèse d'échantillons modèles nécessite la synthèse préalable d'un film mince de phase MAX précurseur pour le MXene : nous avons choisi la phase Ti2AlC, précurseur de Ti2C. La synthèse de Ti2AlC a été réalisée par recuit ex-situ de systèmes multicouches déposés à température ambiante. Les films ont été caractérisés par diffraction des rayons X et microscopie électronique en transmission. Au-delà de l'obtention d'un film mince de Ti2AlC texturé, cette étude a permis de montrer que la phase recherchée était obtenue via des mécanismes d'interdiffusions induisant la formation d'une solution solide métastable vers 400°C qui se transforme en phase MAX vers 600°C. Enfin, l'application de ce procédé à la phase V2AlC a permis de montrer l'importance de l'orientation de la phase initiale pour l'obtention d'un film mince texturé
The aim of this work is at first to study the electronic structure of bidimensional titanium carbide systems, belonging to the MXene family and also to synthesize thin films of such new materials to characterize their properties. The study of the electronic structure has been performed for the Ti3C2T2 MXene with a special attention to the T surface groups by using a combination of electron energy loss spectroscopy and ab initio calculations. This study, focused on both valence and core electrons excitations, enabled the identification of the surface group localization, their influence on the MXene electronic structure as well as their chemical nature. The limits of our TEM-based study is also discussed in view of irradiation phenomena which induce the loss of hydrogen atoms. The synthesis of a MXene thin film requires, beforehand, that of a MAX phase thin film: we opted for Ti2AlC, the precursor for the Ti2C MXene. The MAX phase thin film synthesis was carried out by ex-situ annealing of a multilayer layers. X-ray diffraction experiments and cross-sectional transmission electron microscopy observations show that a highly textured Ti2AlC thin film is obtained above 600°C after the formation, at 400°C, of a metastable solid solution. Finally, by using the same process for V2AlC, we demonstrate that the initial phase orientation plays a key role for the texture of the thin film so obtained

The search for alternate and renewable energy resources as well as the efficient use of energy and development of such systems that can help to save the energy consumption is needed because of exponential growth in world population, limited conventional fossil fuel resources, and to meet the increasing demand of clean and environment friendly substitutes. Hydrogen being the simplest, most abundant and clean energy carrier has the potential to fulfill some of these requirements provided the development of efficient, safe and durable systems for its production, storage and usage. Chemical hydrides, complex hydrides and nanomaterials, where the hydrogen is either chemically bonded to the metal ions or physiosorbed, are the possible means to overcome the difficulties associated with the storage and usage of hydrogen at favorable conditions. We have studied the structural and electronic properties of some of the chemical hydrides, complex hydrides and functionalized nanostructures to understand the kinetics and thermodynamics of these materials. Another active field relating to energy storage is rechargeable batteries. We have studied the detailed crystal and electronic structures of Li and Mg based cathode materials and calculated the average intercalation voltage of the corresponding batteries. We found that transition metal doped MgH2 nanocluster is a material to use efficiently not only in batteries but also in fuel-cell technologies. MAX phases can be used to develop the systems to save the energy consumption. We have chosen one compound from each of all known types of MAX phases and analyzed the structural, electronic, and mechanical properties using the hybrid functional. We suggest that the proper treatment of correlation effects is important for the correct description of Cr2AlC and Cr2GeC by the good choice of Hubbard 'U' in DFT+U method. Hydrogen is fascinating to physicists due to predicted possibility of metallization and high temperature superconductivity. On the basis of our ab initio molecular dynamics studies, we propose that the recent claim of conductive hydrogen by experiments might be explained by the diffusion of hydrogen at relevant pressure and temperature. In this thesis we also present the studies of phase change memory materials, oxides and amorphization of oxide materials, spintronics and sulfide materials.

The insatiable global demand for energy cannot be sustained by fossil fuels without irreparable damage to the environment. Various alternative energy sources are being investigated to provide renewable clean energy. One promising technology is the hydrogen fuel cell, which uses hydrogen and oxygen to produce electricity. However, the currently used catalyst support material, carbon black, corrodes in the low pH and oxidative environment. Therefore, new catalyst support materials are being sought. A new class of material, called MAX phases, shows potential because some possess a combination of properties of metals and ceramics. Three of them, Ti2AlC, Ti3AlC2, and Ti3SiC2, show good electrical conductivity and oxidation resistance. These MAX phases have been investigated using density functional theory (DFT) in this thesis to determine their properties. The density of states show that they are electrically conductive, with a continuous band over the Fermi level primarily from the Ti d orbital. Calculating the Boltzmann transport properties, yielded electrical resistivity values of 0.460 µΩ m for Ti2AlC, 0.370 µΩ m for Ti3AlC2, and 0.268 µΩ m for Ti3SiC2 at 300 K. Therefore, Ti3SiC2 should be the most electrically conductive of the three. The vacancy formation energy of an A group atom was investigated using a 2 x 2 x 2 supercell. The vacancy formation energies were calculated to be 2.882 eV for Ti2AlC, 2.812 eV for Ti3AlC2, and 2.167 eV for Ti3SiC2. The formation of a vacancy increases the electrical resistivity of the bulk MAX phases. As a catalyst support material, a MAX phase particle will have surfaces present. Due to the layered structure of the MAX phases, multiple terminations of (0 0 0 1) surfaces could be possible, which were investigated. It was shown that terminations where the Ti-C cage structure remained intact produced the lowest cleavage energies. For Ti2AlC, the two low cleavage energy surfaces are Al(Ti) and Ti(C), for Ti3AlC2, Al(Ti2) and Ti2(C), and for Ti3SiC2, Si(Ti2) and Ti2(C). The surfaces with the lowest cleavage energy should be more stable than other surfaces and would therefore be expected to be present on a MAX phase particle. Vacancies were also formed in the surface systems. The surfaces with the vacancy in the surface layer had the lowest vacancy formation energy, with that of Si(Ti2) being positive. The surface slabs generally showed a higher electrical resistivity than the bulk systems, while the formation of a vacancy generally increased the resistivity, in agreement with the bulk vacancy trend. These MAX phases are electrically conductive, however a quantifiable oxidation resistance was not able to be calculated. They do however show signs of being good electrocatalyst support materials.

The main goal of this work is to synthetize and characterize new and innovative ceramic materials that can be used for energetic catalysis. The work is split in two main branches, the first one focused on TiOxCy ceramics, the second one on Max phases. Both of them appear to be excellent anodic materials for fuel cells, with the first one specifically developed within the European DECORE Project. The titanium oxycarbide was developed to work at the anode of direct ethanol fuel cells because of its predicted stability in acidic and moderate high temperature ambient. The initial requirements that have to be satisfied for the European project were to have a pure powder with high surface area, that can be scaled easily to industrial scale. Several and different paths were used to satisfy and outdo the starting requirements, obtaining a process and the resulting powder that show excellent results in terms of purity, surface area, reproducibility and scalability. All of the requirements were fully satisfied. New catalysts were also synthetized to optimize the efficiency of the anode, using platinum and platinum-tin nanoparticles. Especially the latter showed very promising results, that have to be further analysed with more complete and in-depth experiments. The Max phases are a class of innovative ceramics with nanolamitated structures. They mix the best properties of the ceramics, like acid and high temperature resistance, with the best ones of the metals, as electrical conductivity and malleability. They were studied in the last two decades, but few works aimed to discover their utility in fuel cells. Due to their very promising qualities we tried to produce them for future works aimed to use them as catalyst support. We concentrated our work on Ti3SiC2, Ti2AlC and Ti3AlC2 due to their ease of synthesis, but we obtained pure powders barely, so additional and further studies are needed. On that powders, we made a preliminary study on the feasibility of decoration with platinum nanoparticles and on the electrochemical behaviour in mild conditions. The results were promising, but require more experiments.
L'obiettivo principale di questo lavoro è la sintesi e la caratterizzazione di nuovi ed innovativi materiali ceramici che possono essere utilizzati per catalisi energetica. Il lavoro è diviso in due rami principale, il primo focalizzato sulle ceramiche TiOxCy, il secondo sulle Max Phases. Entrambe sembrano essere ottimi supporti anodici per celle a combustibile, con il primo sviluppato espressamente all'interno del progetto europeo DECORE. L'ossicarburo di titanio è stato sviluppato per lavorare all'anodo di celle a combustibile a etanolo per la sua stabilità in ambienti acidi e a medie-alte temperature. Le richieste iniziali da soddisfare per il progetto europeo erano di avere una polvere pura con grande area superficile, che può essere prodotta in grande scala facilmente. Differenti e varie strade sono state percorse per soddisfare e superare gli obiettivi iniziali, ottenendo un processo e una polvere finale che mostrano eccellenti risultati in purità, area superficiale, riproducibilità e scalabilità. Tutte le richieste sono state soddisfatte. Nuovi catalizzatori sono stati sintetizzati per ottimizzare l'efficienza dell'anodo, usando nanoparticelle di platino e di platino-stagno. Le ultime in particolare hanno mostrato risultati promettenti che devono essere analizzati ulteriormente con studi più completi e dettagliati. Le MAX phases sono una classe di ceramici innovativi con strutture nanolaminate. Uniscono le migliori proprietà dei metalli, come la conducibilità elettrica e malleabilità, con quelle dei ceramici, come la resistenza agli acidi e alle alte temperature. Sono state studiate negli ultimi venti anni, ma pochi lavori si sono focalizzati sul loro uso nelle celle a combustibile. Ci siamo concentrati su Ti3SiC2, Ti2AlC e Ti3AlC2 per la loro facilità di sintesi, ma abbiamo ottenuto scarsi risultati. Per questo sono richiesti ulteriori studi. Sulle polveri ottenute abbiamo svolto uno studio preliminare per la decorazione con nanoparticelle di platino e per il comportamento elettrochimico in condizioni blande. I risultati sono incoraggianti, ma richiedono un studio più approfondito.

Les phases MAX sont des carbures et nitrures ternaires dont les propriétés remarquables sont intermédiaires entre celles des métaux et des céramiques. Elles présentent une grande variété de composition sous la même structure cristalline. Ce travail de thèse a pour objectif de mesurer leur constante diélectrique complexe ["(!)] en fonction de la composition. La variation de celle-ci en fonction de l'orientation cristallographique est 'également étudiée. Les échantillons utilisées sont des monocristaux élaborés en couches minces et des polycristaux massifs. Les mesures ont été effectuées par spectroscopie de pertes d'énergie des électrons (EELS) et par l'ellipsom'etrie V-UV. L'ellipsom'etrie possède une très bonne résolution en 'énergie et angulaire toutefois fenêtre en énergie accessible expérimentalement est réduite (1.5 'a 5.5 eV). L'EELS, permet de couvrir une gamme d'énergie allant du proche IR aux X mous ( 1 'a 100 eV). Les résultats issus de ces deux méthodes de mesure sont tr'es comparables. L'utilisation d'un monochromateur avant 'echantillon s'av'ere toutefois indispensable en EELS pour mesurer correctement le signal dans la région des très basses énergies. Les propriétés diélectriques des composés Ti2AlC et Ti2AlN sont remarquablement différentes suivant l'orientation cristallographique de la sollicitation. du faisceau électronique incident dans le plan de base ou le long de l'axe de c. Les propriétés diélectriques très différentes en fonction de l'orientation du cristal ont été mises en 'évidence et ont donc confirmé l'anisotropie montre par la théorie à l'origine de cette 'étude. L'étude d'autres composés en couches minces tel que Ti2GeC, Ti2SnC dont la synthèse a été réalisée nous a permis d'étudier l'influence des éléments A sur les propriétés diélectriques et optiques de ces matériaux. Un modèle phénoménologique semi-classique de Drude-Lorentz a été utilisé pour reproduire la fonction ". Dans ce modèle la réponse d'un métal à une excitation electromagnétique est décrite comme celle d'un gaz d'électrons libres et d'une somme d'oscillateurs représentant les transitions interbandes possibles. Après ajustement de ce modèle aux mesures expérimentales, il a été possible d'extraire la densité électronique et d'estimer un temps de relaxation des électrons libres. Ces mesures ont ensuite été utilisées pour déduire la conductivité statique qui a été comparée aux meures macroscopiques. Les résultats sont en bon accord avec les valeurs publiées et nous permettent de confirmer que la conductivité statique présente une forte anisotropie dans Ti2AlC et Ti2AlN. Enfin, une approche ab initio a été mise en œuvre afin de modéliser les constantes diélectriques complexes. Ces calculs basées sur la théorie de la fonctionnelle de la densité dépendant du temps (TDDFT) reproduisent l'anisotropie de la réponse diélectrique et montrent qu'elle est intimement liée aux champs locaux.

Non-destructive testing (NDT) of glass-fibre reinforced polyester (GRP) composite materials has been becoming increasingly important due to their wide applications in engineering components and structures. Electronic Speckle Pattern Interferometry (ESPI) has promising potential in this context because it is a non-contact, whole-field and real-time measurement system. This potential has never been fully exploited and there is only limited knowledge and understanding available in this area. This reality constrains the wide popularity and acceptance of ESPI as a novel NDT technique. Therefore it is of considerable importance to develop an understanding of the capability of ESPI with respect to damage evaluation in GRP composite materials. The research described in this thesis is concerned with an investigation into the applicability of ESPI in the NDT of GRP composite materials. Firstly, a study was carried out to determine excitation techniques in terms of practicality and effectiveness in the ESPI system. Three categories of defects were artificially introduced in GRP composite materials, namely holes, cracks and delaminations each with different geometrical features. ESPI was then employed to evaluate the three kinds of defects individually. It has been found that cracks and holes on back surfaces can be defined when the technique is used in conjunction with thermal excitation. Internal Temperature Differential (ITD) induced fringe patterns were more efficient than External Thermal Source (ETS) induced fringe patterns with regard to detecting the presence of holes and cracks. In the case of delamination, ESPI was found to be capable of detecting the damage when used in combination with mechanical excitation originating from a force transducer hammer. The geometrical features and magnitudes of delaminations were also established as being quantifiable. The validation of ESPI as an NDT technique was carried out in an attempt to establish a better understanding of its suitability and have more confidence in its applications. Four damaged specimens were Subjected to ESPI examination in conjunction with visual inspection, ultrasonic C-scan and sectioning techniques. The geometrical features and magnitudes of damage evaluated using ESPI showed a good correlation with those evaluated by conventional techniques. Poor visibility and readability is an inherent problem associated with ESP! due to an overlapping between the noise and signal frequencies. An improvement of image quality is expected in an attempt to achieve a wide acceptance of ESPI as a novel NDT technique. It has also been demonstrated that this problem can be tackled using optical phase stepping techniques in which optical phase data can be extracted from the intensity fringes. A three-frame optical phase stepping technique was employed to produce the "wrapped" and "unwrapped" phase maps which are capable of indicating internal damage with high visibility and clarity. Finally ESPI was practically employed to evaluate damage in GRP composites introduced by quasi-static and dynamic mechanical loading. It was found that ESP! was capable of monitoring the progressive damage development of specimens subjected to incremental flexural loading. The initial elastic response, damage initiation, propagation and ultimate failure of specimens were clearly characterised by the abnormal fringe pattern variations. In a similar manner, ESPI was employed to evaluate the low velocity falling weight impact induced damage. A correlation was established between the magnitude of damage and the impact event parameters as well as the residual flexural properties.

Les applications de puissance dans lesquelles la température ambiante est élevée, provoquent l’augmentation de la température dans les dispositifs électroniques. De ce fait, il est important de développer les dispositifs électroniques pour pouvoir supporter des densités de courant et de puissance plus élevées. Dans cette thèse, nous avons pour objectif de jeter les bases d’une technologie en totale rupture avec celles existantes pour la fabrication d’une nouvelle génération de contacts électriques à base de Ti3SiC2, stables, fiables et reproductibles sur le Carbure de Silicium pour les applications à très hautes températures (300 – 600ºC). Deux méthodes d’élaborations seront étudiées, dans cette thèse, pour synthétiser le Ti3SiC2. La première est par voie réactionnelle, et la deuxième approche consistera à utiliser la technique Pulsed Laser Deposition (PLD), en utilisant une cible de Ti3SiC2. Le but est de développer des contacts ohmiques de bonne qualité. Des caractérisations physico-chimiques, électriques (TLM) et mécaniques (W-H et RSM) ont été effectuées sur les contacts de Ti3SiC2. Ces échantillons ont subi un vieillissement, à 600ºc pendant 1500h sous Argon, dans le but d’étudier la stabilité et la fiabilité des contacts électriques aux hautes températures. Les résultats des caractérisations ont montré que la fiabilité et la stabilité chimique entre Ti3SiC2 et SiC ont permis aux contacts de garder le comportement ohmique avec une faible résistivité électrique et un bon comportement mécanique, même après 1500h de vieillissement. De plus, les simulations réalisées ont servi à déterminer l’effet des ITR sur la dissipation de la chaleur et sur les contraintes mécaniques exercées sur une diode PN haute puissance. Dans cette thèse, nous avons montré qu’un contact ohmique, à base de Ti3SiC2, peut rester stable et fiable sur un substrat 4H-SiC, dans des températures allant jusqu’à 600ºC
Power applications in which the ambient temperature is high, cause the increase of temperature in electronic components. Therefore, it is important to develop electronic devices that are able to withstand high current and high-power densities. In this thesis, our objective is to lay the foundations of a new technology for the manufacture of a new generation of Ti3SiC2 MAX phase-based electrical contacts, stable, reliable and reproducible on Silicon Carbide for very high temperature applications (300 - 600ºC). To synthesize Ti3SiC2 on SiC, two elaboration methods were studied in this thesis. The first approach is a reaction method, and the second approach consists on using a Ti3SiC2 target via the Pulsed Laser Deposition (PLD) technique. Our goal is to develop a good quality ohmic contacts. Physico-chemical, electrical (TLM) and mechanical (W-H and RSM) characterizations were performed on the Ti3SiC2 contacts. These samples underwent a thermal aging test at 600°C for 1500 hours under Argon, in order to study the stability and reliability of the electrical contacts at high temperatures. The obtained results showed that the reliability and the chemical stability between Ti3SiC2 and SiC allowed the contacts to keep an ohmic behavior with low electrical resistivity, in addition to a good mechanical behavior, even after 1500 hours of aging at 600ºC. Furthermore, the thermomechanical simulations performed were used to determine the effects of Interfacial Thermal Resistances on the heat dissipation and the mechanical stresses exerted on a high power PN diode. In this thesis, we have shown that an ohmic contact, based on Ti3SiC2, can remain stable and reliable on a 4H-SiC substrate, in temperatures up to 600ºC

Research Doctorate - Doctor of Philosophy (PhD)
This thesis is primarily concerned with a series of experimental investigations into the synthesis of materials for energy conversion and related applications in hostile environments. The M_n+1AX_n (MAX) phases contain an early transition metal (M-element) a group 3 or 4 element (A-element) and either C or N (X-element) and are a group of ceramics with interesting properties that make them perfectly suited to many difficult and demanding applications. The high potential of the MAX phases has been largely frustrated by difficulties in large scale, economic synthesis. The formation of M_n+1AX_n phases was extensively studied throughout this thesis. Use of M-element oxides as reactants has been intensively investigated with great success. The processing involved in obtaining the metallic form of the M-elements contribute considerably to the high cost of the MAX phases, along with complex and small scale synthesis methodologies currently used. Methods have been developed throughout this work as a means of reducing the M-element oxides, considerably cheaper starting materials, and producing MAX phases via a single step pressureless reactive sintering process. Aluminium has been extensively explored as a reducing agent and aluminothermic reduction was proven capable of forming the majority of tested systems. Separation of the MAX phase alumina composite formed by the exchange reaction has been demonstrated in simple sedimentation experiments, allowing for purification of the MAX phase product. Alternatively carbothermal reduction has been shown in selected systems to produce self-separating products. This process has been shown to produce pure MAX phase products in a single step reaction, a highly desirable trait, and the first time a pure MAX phase has been produced by carbothermal reduction. Additionally investigations into the synthesis and stability of MAX phases in general lead to the discovery of two new compounds belonging to this family, Ti₃GaC₂ and Ti₃InC₂. Issues of energy conversion have been addressed in two ways, through the creation of a novel thermal energy storage material using immiscible materials known as Miscibility Gap Alloys and through the development of a thermionic converter for conversion of heat directly into electricity. Thermal energy storage is critical as it allows for the intermittency of a concentrator solar power plant to be overcome. Misibility Gap Alloys provide high energy density, constant temperature storage in a highly thermally conductive material. A thermionic converter, although in its most preliminary stages with very low power output and efficiency, was designed for high temperature energy conversion. This device can be used as a test bed for the design of a system which could be used as a topping cycle on a concentrated solar thermal power plant. To enhance the power output of the thermionic device, low work function hexaboride materials were investigated and synthesised at considerably lower temperatures than conventionally used. Overall several contributions have been made in novel and potentially economic methods for the production of MAX phases. The development of these synthesis methodologies may alloy for these materials to fulfil their long desired place in demanding environments such as efficient energy conversion. A new class of thermal storage materials was developed which can be used for overcoming the intermittency of concentrated solar thermal power production, which could be coupled with low work function materials in a thermionic energy conversion device in order to improve the efficiency of electricity generation.

Research Doctorate - Doctor of Philosophy (PhD)
The research presented in this thesis describes a series of studies on new synthesis techniques which were applied to two related families of nano-laminated ternary transition metal ceramics, MAX and MAB phases. For the first time, the laser cladding process was investigated as a potential time and cost-effective method for producing MAX phase coatings in situ. The studies found that laser cladding is incredibly effective at depositing coatings in the Ti-Al-C system onto titanium substrates, producing mm-scale thick coatings in a matter of seconds to minutes, with the production of sample sizes up to ~ 90 x 90 mm2. While the direct deposition of these coatings showed some success in the in situ formation of the MAX phases, post-deposition furnace annealing treatments were also developed to improve the phase-purity. Due to the recent discovery of MAB phases as potential materials for use in demanding applications, some basic characterisation of these materials and their properties was undertaken using standard powder metallurgy techniques. In particular, experimental investigations of solid-solution (Mo,W)AlB MAB phases were completed as a way to approach the bulk synthesis of MAB phases close to the W end of the composition range. This was motivated by the fact that the WAlB MAB phase has not previously been synthesised in the bulk form and may prove to be a beneficial material for use in nuclear fusion applications. Research was also undertaken for the first time, to investigate the high temperature radiation tolerance of MoAlB and Fe2AlB2, which were found to be comparable to existing radiation-tolerant materials including MAX phases and SiC. Finally, inspired by some unusual observations during and after preparation of MAB phase samples, assessment of the electrocatalytic performance of Fe2AlB2 and MoAlB powders for the reduction of nitrogen was conducted. MoAlB was found to have a high Faradaic efficiency and cycle stability for NH3 production, offering the potential for an environmentally friendly pathway towards ambient-condition NH3 synthesis, compared with the Haber-Bosch process.

The "MAX" phases are a class of ductile nanolaminated ternary nitrides and car- bides, including about 60 known phases. The chemical formula for the MAX phase is Mn+1AXn where M is an early transition metal. They consists of the 211, 321 and 412 groups. There are over 40 M2AX phases in existence.The M2AC MAX phase category is divided into two subgroups, the V(B) and VI(B) transition metals. There are three M3AX2 (Ti3SiC2, Ti3GeC2 and Ti3AlC2) and one M4AX3 (Ti4AlN3): As for the 321 MAX- phases the chemical formula can be written as Ti3AC2, where A = Al, Si, Ge. This group comprises of (Ti3SiC2, Ti3GeC2 and Ti3AlC2). MAX phase properties are of more than two elements. They can be "sliced or diced" using a man- ual hacksaw. These materials are used in applications such as industrial robots, disc drives, electronics and MEMS devices.

Le incredibili proprietà termomeccaniche dei materiali biologici derivano dalla scala microscopica a causa di un complesso meccanismo gerarchico, che è regolato da microinstabilità a livello molecolare. La descrizione di strutture così complesse è consentita sia dal know-how introdotto dall'avvento degli esperimenti di spettroscopia di forza a singola molecola, che dà la possibilità di studiare tali sistemi in diverse condizioni termiche e meccaniche, sia dalla possibilità di imitare correttamente il loro comportamento al scala più bassa introducendo modelli matematici basati su energie non convesse. In questa tesi, vengono introdotte diverse classi di modelli per descrivere le caratteristiche importanti della transizione di fase, della decoesione e del danno in diverse condizioni di forze applicate e spostamento, campi termici e velocità di carico. Aumentando il livello di complessità di tali modelli, sono stati analizzati diversi fenomeni. Ad esempio, introducendo una catena di unità bistabili per imitare il comportamento di una molecola di titina in fase di dispiegamento, è stato descritto l'effetto del dispositivo di manipolazione in esperimenti su singole molecole, che influenza fortemente la risposta meccanica del sistema, portando a grandi errori nel misura della forza o spostamento risultante. Gli effetti della temperatura sono considerati all'interno di un quadro di Meccanica Statistica, anche nel caso in cui siano introdotte interazioni non locali. Infatti, fenomeni come la presenza di un picco di stress nel diagramma forza-estensione e corrispondente alla generazione e nucleazione di una fase sono osservati sperimentalmente in prove di trazione su nanofili a memoria di forma o materiali polimerici e possono essere descritti come una competizione tra energia interfacciale termini ed effetti entropici. La cooperatività delle interazioni deboli, come i legami idrogeno, è stata anche studiata per evidenziare fenomeni come la decoesione e la frattura nei sistemi biologici. Infatti, gli amminoacidi semplici sono disposti in modo multiscala dando luogo a materiali e strutture gerarchiche ad alte prestazioni, con elevate proprietà meccaniche. Nello specifico, considerando molle elastiche accoppiate con unità fragili, in questa tesi è stato dedotto un modello micromeccanico di sistemi come l'elica del DNA a doppio filamento oi fasci di microtubuli (MT) e le proteine ​​tau disposte all'interno degli assoni con effetti termici e di velocità. Il processo di decoesione risulta essere fortemente regolato dalla rigidità relativa delle due unità pseudoelastiche e il tipo di frattura può variare da un brusco collasso (comportamento fragile) ad un distacco sequenziale dei legami (comportamento duttile). Questo effetto è potenziato anche quando si considera la velocità di carico, dove diventa cruciale la capacità di superare le barriere energetiche che separano gli stati metastabili. I risultati ottenuti nella tesi vengono confrontati con evidenze provenienti da un'ampia rassegna della letteratura e dai comportamenti sperimentali dei sistemi descritti, e vengono dedotte leggi analitiche costitutive microscopiche che illustrano il comportamento complessivo di tali sistemi complessi regolati da microinstabilità multiscala.
The incredible thermo-mechanical properties of biological materials arise from the microscopic scale due to a complex hierarchical mechanism, which is regulated by microinstabilities at the molecular level. The description of such complex structures is allowed by both the know-how introduced by the advent of single molecule force spectroscopy experiments, which gives the possibility of studying such systems in different thermal and mechanical conditions, and the possibility of correctly mimicking their behaviour at the lowest scale by introducing mathematical models based on non-convex energies. In this thesis, different classes of models are introduced to describe the important features of phase transition, decohesion and damage under different conditions of applied forces and displacement, thermal fields and rates of loading. By increasing the level of complexity of such models, different phenomena have been analyzed. For instance, by introducing a chain of bistable units to mimic the behaviour of a titin molecule undergoing unfolding, it has been described the effect of the handling device in single molecule experiments, which strongly affects the system's mechanical response, leading to large errors in the measure of the resulting force or displacement. Temperature effects are considered within a Statistical Mechanics framework, also in the case when non local interactions are introduced. Indeed, phenomena such as the presence of a stress peak in the force-extension diagram and corresponding to the generation and nucleation of a phase is experimentally observed in tensile tests on memory shape nanowires or polymer materials and can be described as a competition between interfacial energy terms and entropic effects. The cooperativity of weak interactions, such as hydrogen bonds, has been also studied to highlight phenomena such as decohesion and fracture in biological systems. Indeed, simple amino acids are arranged in a multiscale fashion resulting in high performing hierarchical materials and structures, with elevated mechanical properties. Specifically, considering elastic springs coupled with breakable units, in this thesis a micromechanical model of systems such as the double-stranded DNA helix or the bundles of microtubules (MT) and tau proteins arranged within the axons with thermal and rate effects has been deduced. The decohesion process is found to be highly regulated by the relative stiffness of the two pseudo-elastic units, and the type of fracture may range from an abrupt collapse (fragile behaviour) to a sequential detachment of the bonds (ductile behaviour). This effect is also enhanced when the loading rate is considered, where the ability to overcome energy barriers separating the metastable states becomes crucial. The results obtained in the thesis are compared to pieces of evidence from an extensive literature review and to the experimental behaviours of the systems described, and microscopic constitutive analytic laws are deduced illustrating the overall behaviour of such complex systems regulated by multiscale microinstabilities.