Dissertationen zum Thema „MAX phase synthesis“

This Thesis explores synthesis and characterization of new MAX phase alloys (M = early transition metal, A = A-group element, and X = C or N), based on incorporation of M and X elements previously not considered. My primary focus is on M = Mn for attaining magnetic properties, and on X = O for potential tuning of the transport properties. A recent theoretical study predicted (Cr1-xMnx)2AlC MAX phase to be a stable magnetic nanolaminate. I aimed at realizing this material and through a combinatorial approach based on magnetron sputtering from elemental targets, the first experimental evidence of Mn incorporation (x = 0.16) in a MAX phase is presented. The corresponding MAX phase was also synthesized using cathodic arc film deposition (x = 0.20) and bulk synthesis methods (x = 0.06). The primary characterization techniques were X-ray diffraction and high-resolution (scanning) transmission electron microscopy in combination with energy dispersive X-ray spectroscopy and/or electron energy loss spectroscopy, to obtain a precise local quantification of the MAX phase composition and to perform lattice resolved imaging. For epitaxial film growth of (Cr1-xMnx)2AlC, evidence is presented for the formation of (Cr1-yMny)5Al8, exhibiting a bcc structure with an interplanar spacing matching exactly half a unit cell of the hexagonal MAX phase. Consequently, routinely performed X-ray diffraction symmetric θ-2θ measurements result in peak positions that are identical for the two phases. As (Cr1-yMny)5Al8 is shown to display a magnetic response, its presence needs to be taken into consideration when evaluating the magnetic properties of the MAX phase. Methods  to distinguish between (Cr1-yMny)5Al8 and (Cr1-xMnx)2AlC are also suggested. As different A-element in the MAX phase is theoretically predicted to influence phase stability, attainable level of Mn  incorporation, as well as magnetic properties, thin films of (Cr0.75Mn0.25)2GeC and bulk (Cr0.7Mn0.3)2GaC have also been synthesized. Vibrating sample magnetometry measurements display a magnetic response for all these materials, identifying (Cr,Mn)2AlC, (Cr,Mn)2GeC, and (Cr,Mn)2GaC as the first magnetic MAX phases. The results presented in this Thesis show that A = Al displays the highest magnetic transition temperature (well above room temperature) and A = Ga allows the highest Mn content. The attainable O incorporation in Ti2Al(C1-xOx)MAX phase was explored by arc deposition of Ti2AlC1-y thin films under high vacuum conditions, and solid-state reactions following deposition of understoichiometric TiCz on Al2O3. Ti2Al(C1-xOx)thin films with up to 13 at.% O (x = 0.52) were synthesized, and O was shown to occupy the C lattice site. The obtained O concentration is enough to allow future experimental investigations of the previously suggested (from theory) substantial change in anisotropic electronic properties with increasing O content. The experimental results obtained in this Thesis expand the MAX phase definition and the materials characteristics into new research areas, towards further fundamental understanding and functionalization.

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.

The objective of this Thesis is synthesis and characterization of new MAX phase alloys (M = early transition metal, A = A-group element, and X = C or N) based on incorporation of M and X elements previously not used in any known MAX phases. This is motivated by a search for optimized and unique materials properties, such as different magnetic states. Two synthesis routes have been used to attain Ti2AlC1-xOx: deposition of Ti2AlCy under high vacuum conditions with residual gas acting as O source, and solid-state reactions following deposition of understoichiometric TiCy on Al2O3. Detailed local quantification by analytical transmission electron microscopy (TEM) including electron energy loss spectroscopy (EELS) shows up to 13 at.% O within high quality MAX phase structure. According to previous theoretical work, the range of experimentally obtained O content is enough to observe drastic changes in the materials anisotropic electronic properties. Calculations on effect of substitutional O on shear deformation have also been performed. In a recent theoretical study by Dahlqvist et al., (Cr,Mn)2AlC has been predicted as a new stable magnetic nanoscale laminate. Inspired by this work, thin films of (Cr,Mn)2AlC, as well as of a neighboring system (Cr,Mn)2GeC, have been synthesized by magnetron sputtering. Incorporation of 8 and 12.5 at.% of Mn, respectively, has been detected by analytical TEM including EELS and energy dispersive X-ray spectroscopy (EDX). The total saturation moment of 0.36μB per Mn atom at 50 K has been measured by vibrating sample magnetometry (VSM) for a (Cr,Mn)2GeC sample, providing the first experimental evidence of a magnetic MAX phase. The experimental results obtained in this Thesis provide a base for expanding the MAX phase definition and materials characteristics into new areas, towards further fundamental understanding and functionalization.

Les phases MAX forment une famille de carbures et de nitrures nano-lamellaires de formule chimique Mn+1AXn, où M est un métal de transition des premières colonnes, A appartient aux colonnes 13-16 et X est soit C, soit N, ou une combinaison des deux éléments. Ces phases combinent les mérites des céramiques et des métaux, comme une bonne stabilité chimique, l’usinabilité, la résistance aux chocs mécaniques, de bonnes conductivités thermique et électrique, etc. Malgré tout, l’étude de leurs propriétés intrinsèques et de leurs anisotropies a été jusqu’à présent limitée par l’indisponibilité de monocristaux. Cette thèse traite de la réactivité de tels monocristaux de phases MAX. Grâce à la large taille des cristaux produits au LMGP, il a été possible d’évaluer directement l’anisotropie de la réactivité chimique et d’obtenir des données originales. Nous avons montré le rôle prépondérant joué par l’élément A pour initier des transformations chimiques menant parfois à la synthèse de matériaux originaux, et nous nous sommes concentrés sur quatre aspects différents : Tout d’abord, nous avons tenté de synthétiser des MXènes de grande taille, en profitant de la grande taille des cristaux disponibles. Un effort particulier a été porté sur la description de la réactivité chimique de phases MAX plongées dans diverses solutions d’attaque, avec un accent particulier mis sur l’utilisation de HF. En second lieu, nous avons étudié la chloruration de phases MAX : l’objectif initial était de former des MXènes, mais nous avons finalement développé une méthode pour synthétiser des carbures de chrome poreux avec des propriétés intéressantes. Troisièmement, nous avons utilisé des cristaux de grande taille pour évaluer l’anisotropie des propriétés électrochimiques. Une anisotropie significative a été trouvée, soit en mesurant le courant durant la polarisation électrochimique, soit par mesure de spectroscopie d’impédance. Divers mécanismes ont été proposés afin d’expliquer cette anisotropie des propriétés de corrosion. Enfin, nous avons montré que les résultats électrochimiques pouvaient être utilisés pour révéler indirectement la présence de défauts structurels récemment identifiés dans la littérature. De tels défauts, appelés « ripplocations », sont spécifiques aux matériaux nano-lamellaires
MAX phases are a family of layered ternary carbides and nitrides with chemical formula Mn+1AXn, where M is an early transition element, A is an element of groups 13 to16 and X is either C, N or both. These phases combine the merits of ceramics and metals, such as chemical stability, machinability, shock resistance, good electrical and thermal conductivity, etc. However, the investigation of their intrinsic properties and anisotropies has heretofore been limited by a lack of availability of single crystals. This thesis mainly deals with the chemical reactivity of MAX phase single crystals. Owing to the large size single crystals grown at LMGP, it was possible to directly assess the anisotropy of the chemical reactivity and to obtain original data. We showed that the prominent role played by the A element for initiating chemical transformations could lead to the synthesis of original materials, and we focused on four different aspects. First, we tried to synthesize MXenes from MAX phase single crystals: The purpose was to obtain large-scale MXenes by taking advantage of the large size of the single crystals. Effort was put on describing the chemical reactivity of MAX phases dipped in different etchants, focusing on HF. Secondly, we studied the MAX phase reactivity with chlorination: the initial purpose was to obtain MXenes, but we finally developed a method for synthesizing porous chromium carbides which exhibit several interesting properties. Thirdly, we used large size single crystals in order to assess the anisotropy of the electrochemical properties. A significant anisotropy was found, either by measuring the current during electrochemical polarization or by frequency-dependent impedance measurements. Several mechanisms were proposed in order to explain this anisotropy of the corrosion properties. Eventually, we showed that the electrochemical results could be used to indirectly evidence the presence of structural defects recently identified in the literature. Such defects, called ripplocations, are specific to nano-lamellar materials

La conception de matériaux sur mesure utilisant des approches innovantes permettant des processus de charge/décharge plus rapides pourrait être la clé pour l'avancement de la mobilité électrique. Cette thèse examine des matériaux novateurs pour les électrodes négatives de batteries Li-ion, en se concentrant sur les oxydes multicationiques à base de niobium et les carbures de métaux de transition à base de titane. Les travaux de recherche explorent la synthèse, la structure et les propriétés électrochimiques de ces matériaux, avec une attention particulière aux modifications structurales et aux mécanismes de stockage des charges. Les principaux résultats incluent l'étude de l'activation électrochimique in-situ des matériaux et du comportement unique de stockage du Li+ dans les matériaux pérovskite AgNbO 3 et Ag1-3xLa x□2xNbO 3 (avec 0 ≤ x ≤ 0,40 ; □ étant une lacune sur le site A).De plus, cette étude examine l'effet des lacunes du site A sur la structure et sur les propriétés d'insertion du Li+ dans les structures K1- 3xLa x□2xNbO 3 (avec 0 ≤ x ≤ 0,15 ; □ étant une lacune sur le site A). En outre, des informations sur la voie de synthèse polyacrylamide pour les phases MAX à base de Ti et Al sont fournies. Ce travail présente des approches pour ajuster les matériaux de manière à l’échelle atomique, sans sacrifier la phase initiale, suggérant l'utilisation potentielle de pérovskites de type ABO 3 peu étudiées comme électrodes négatives. De plus, il offre des perspectives mécanistiques sur la synthèse chimique en solution des phases MAX pour leur utilisation comme électrodes de batterie lithium-ion
Design of tailored materials using innovative approaches that allow faster charging/discharging processes could be the key for advancement of electric mobility. This thesis investigates novel materials for Li-ion battery negative electrodes, focusing on niobium-based multicationic oxides and titanium-based transition metal carbides. This research work explores the synthesis, structure, and electrochemical properties of these materials, with particular emphasis on atomic-scale structural modifications and Li+ storage mechanisms. Key findings include the investigation of in-situ electrochemical activation and unique Li+ storage behavior in AgNbO 3 model perovskite and Ag 1-3xLa x□2xNbO 3 (with 0 ≤ x ≤ 0.40; □ is an A -site vacancy) tailored materials. Additionally, the study examines the effect of A-site deficiency on the structure and Li+ insertion properties of K1-3xLa x□2xNbO 3 (with 0 ≤ x ≤ 0.15; □ is an A -site vacancy). Furthermore, insights into the polyacrylamide synthesis route for Ti and Al-based MAX phases are provided. These work present approaches to atomically tailor the materials without sacrificing the pristine phase, suggesting the potential use of less common ABO 3-type perovskites as negative electrodes. Additionally, it offers mechanistic insights into the wet chemical synthesis of MAX phases for their use as battery electrodes

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.

Les travaux exposés dans cet ouvrage portent sur l'élaboration, la caractérisation microstructurale et les propriétés mécaniques de solutions solides nanolamellaires de phases dites MAX. Les phases MAX représentent une classe exceptionnellement étendue de céramiques. Elles répondent à une formule générale du type Mn+1AXn (n=1, 2 ou 3) où M est un métal de transition, A est un métal des groupes IIIA ou IVA, et X est un métalloïde (C ou N). Nous avons dans un premier temps réalisé l'optimisation de la synthèse, par métallurgie des poudres, de Ti3AlC2 pur. Une nouvelle phase, Ti3SnC2, ayant été découverte au laboratoire en 2007, les travaux se sont alors focalisés sur la synthèse de solutions solides du type Ti3AlxSn(1-x)C2 par pressage isostatique à chaud. Nous nous sommes, par la suite, attachés à la caractérisation microstructurale de ces solutions solides en étudiant notamment les variations du paramètre de maille, du taux de distorsion des octaèdres [Ti6C] et des prismes trigonaux [Ti6AlxSn(1-x)]. Enfin, nous avons déterminé la dureté intrinsèque et le module d'élasticité des différentes solutions solides en fonction du taux de substitution en utilisant la nanoindentation. Par ailleurs, des essais de compression, uniaxiale et sous confinement de gaz, ont été réalisés à température ambiante, afin d'étudier et de comparer les mécanismes de déformation de Ti3AlC2 et de la solution solide Ti3Al0. 8Sn0. 2C2. Les relations entre modifications microstructurales et propriétés mécaniques sont discutées. Nous montrons notamment que Ti3AlC2 et Ti3Al0. 8Sn0. 2C2 peuvent être considérés comme des matériaux "Kinking Non-linear Elastic"
The work described in this thesis concerns the elaboration, the microstructural characterization and the mechanical properties of nanolaminated MAX phases solid solutions. The MAX phases represent a large class of ceramics. They are a family of ternary nitrides and carbides, with the general formula Mn+1AXn (n=1, 2 or 3), where M is an early transition metal, A is a metal of the groups IIIA or IVA, and X is either carbon or nitrogen. We performed at first the optimization of the synthesis, by powder metallurgy, of highly pure Ti3AlC2. Since a new MAX phase, Ti3SnC2, has been discovered in the laboratory in 2007, the study has been further focused on the synthesis of Ti3AlxSn(1-x)C2 solid solutions by hot isostatic pressing. In a second step, the microstructural characterization of these solid solutions has been carried out, by studying, in particular, the variation of the cell parameters, the distortion rates of [Ti6C] octahedrons and [Ti6AlxSn(1-x)] trigonal prisms. Finally, we have determined the intrinsic hardness and the elastic modulus of the various solid solutions as a function of the Al content by using the nanoindentation. Besides, uniaxial and gas confining compression tests were realized at room temperature, to study and compare the deformation mechanisms of Ti3AlC2 and Ti3Al0. 8Sn0. 2C2. The relationship between microstructural modifications and mechanical properties are discussed. We show in particular that Ti3AlC2 and Ti3Al0. 8Sn0. 2C2 can be considered as "Kinking Non-linear Elastic" materials

Les travaux exposés dans cet ouvrage décrivent la synthèse, la caractérisation microstructurale et les propriétés physiques de solutions solides nanolamellaires des phases MAX. Les phases Mn+1AXn (M : métal de transition, A : un métal des groupes IlIA ou IV A, et X: carbone ou azote) constituent une famille de nitrures et de carbures ternaires (n = 1 à 3), qui possèdent les meilleures propriétés des métaux et les meilleures propriétés des céramiques.Lors d'une première étape, nous nous concentrons sur la synthèse de solutions solides pures et denses de Ti2AICxNy par compression isostatique à chaud. Les variations des paramètres de maille sont étudiée et discutée an fonction du taux de substitution (carbone-azote) et du taux de lacune (sur le site X). Lors d'une seconde étape, nous étudions les propriétés mécaniques et les propriétés de transport électronique des solutions solides Ti2AICxNy et des phases Ti2AICx et Ti2AINy. La technique de nanoindentation pour déterminer la dureté et le module élastique en fonction du taux de substitution et de lacune. Nous démontrons que la substitution conduit à une amélioration des propriétés mécaniques tandis que l'introduction de lacune conduit à une détérioration de ces propriétés. La résistivité électrique augmente lorsque des lacunes et/ou un effet de substitution sont introduits. Dans le cas de la substitution, nous démontrons que le désordre introduit est faible et que seule la diminution du temps de relaxation explique l'augmentation de la résistivité (interaction électron-phonons). Dans le cas de l'introduction de lacunes, nous montrons que ces dernières conduisent à une modification du temps de relaxation et probablement à une modification de la densité de porteurs.Enfin, l'anisotropie des propriétés de transport électronique a été mise en évidence par des mesures de résistivité réalisée avec le courant électrique circulant dans le plan de base et avec le courant électrique circulant selon l'axe c. Nous démontrons les propriétés de transport dans le plan de base peuvent être comprises en utilisant un modèle à une bande et un mécanisme de conduction assuré par des électrons ayant le comportement de trous
The work discussed in this thesis concerns the synthesis, the microstructural characterization and the physical properties of nanolaminated MAX phase's solid solution. The Mn+1AXn phases (M: transition metal, A: IlIA or IV A group element, and X: either carbon or nitrogen) are a class of ternary nitrides and carbides (n=l to 3), which possess sorne of the best properties ofmetal and sorne of the best properties of ceramics.In a first step, we focus on the synthesis of highly pure and dense Ti2AICxNy solid solutions by hot isostatic pressing. The influence of the substitution of C atoms by N atoms and the influence of vacancy content on the solid solution lattice parameters is discussed. In a second step, we investigate the mechanical and transport properties of Ti2AICxNy solid solutions and oftheir related Ti2AICx and Ti2AINy end-members. Hardness and elastic modulus has been studied using nanoindentation tests. It is demonstrated that sol id solution effect leads to a hardening effect whereas the presence vacancy leads to a softening effect. The electrical resistivity is shown to increase with vacancy content and substitution rate. Such an effect is discussed in terms of disorder and relaxation time variation. Finally, the anisotropic transport properties of MAX phases is studied and discussed. The anisotropy of transport properties has been evidenced by direct measurement of the resistivity along the basal plane and along the c-axis. It is demonstrated that transport property in the basal plane can be understood in the framework of a single band model with hole-like states as charge carrier

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.

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.

Im Rahmen dieser Arbeit wurden die seltenerdfreien Laves-Phasen Ti2M3Si mit M = Mn, Fe, Co, Ni möglichst energieschonend durch feldaktivierte Synthesemethoden dargestellt und strukturell und magnetisch charakterisiert.Darüber hinaus wurde die Mischkristallreihe Ti2(Co1-xFex)3Si mittels einer kombinierten Syntheseroute aus Lichtbogenofen und Spark-Plasma-Sintern synthetisiert, sowie ihr magnetisches Verhalten diskutiert. Neben den Laves-Phasen stand die Synthese und Charakterisierung aluminiumbasierter MAX-Phasen im Fokus dieser Arbeit. Hierbei wurden diese ternären Carbide erstmals durch feldaktivierte Methoden hergestellt. Besonders die phasenreine Darstellung von MAX-Phasen zeigte sich für die synthetische Festkörperchemie als Herausforderung. Das suszeptorgestützte Mikrowellen-Heizen erlaubt die Herstellung von qualitativ hochwertigen Proben, was in dieser Arbeit an M2AlC (M = Ti, V, Cr) und V4AlC3 gezeigt wurde. Darüber hinaus gelang - größtenteils zum ersten Mal - die Dotierung dieser Materialien mit Mn und Fe. Neben der strukturellen Charakterisierung der neuen Phasen werden in dieser Arbeit besonders die Mikrostruktur und magnetischen Eigenschaften diskutiert.Anhand dieser dotierten Verbindungen als auch anhand der Verbindung V4AlC3konnte gezeigt werden, dass feldaktivierte Synthesemethoden, insbesondere das suszeptorgestützte Mikrowellen-Heizen, sehr gute Syntheserouten darstellen, um Verbindungen zu erhalten, die durch konventionelle Methoden nur schlecht oder bisweilen gar nicht zugänglich sind.

The synthesis of several perflourinated ethers of pentaerythritol, dipentaerythritol, and tripentaerythritol by direct fluorination in solution is described. These ethers were perfluorinated using elemental fluorine in a two step process. In the first step, up to 95 percent of the hydrogens were replaced by fluorine while the ether was dissolved in a chlorofluorocarbon slolvent. The remaining hydrogens were replaced by exposing the partially fluorinated product to pure fluorine at elevated temperature. The hydrocarbon ethers used as starting material were prepared by applying the use of phase transfer catalysis to the Williamson ether synthesis. Six of the perfluorinated ethers prepared have been previously synthesized by other methods: perfluoro-5, 5-bis (ethoxymethyl)-3, 7-dioxanonane, perfluoro-6, 6-bis(propyloxymethyl)-4, 8-dioxaundecane, perfluoro-7, 7-bis(butyloxymethyl)-5, 9-dioxatridecane, perfluoro-8, 9-bis(pentyloxymethyl)-6, 10-dioxapentadecane, perfluoro-7, 7-bis(2-methoxyethoxymethyl)-2, 5, 9, 12-tetraoxatridecane, and perfluoro-4, 4, 8, 8-tetrakis (methoxymethyl)-2, 6, 10-trioxaundecane. In addition, the following compounds were isolated and characterized: Perfluoro-2, 12-dimethyl-7, 7-bis (2-methylbutyloxymethyl)-5, 9-dioxatridecane, perfluoro-9, 9-bis (hexyloxymethyl)-7, 11-dioxaheptadecane, perfluoro-10, 10-bis (heptyloxymethyl)-8, 12-dioxanonadecane, perfluoro-11, 11-bis(octyloxymethyl)-9, 13-dioxaheneicosane, perfluoro-5, 5, 9, 9-tetrakis (ethoxymethyl)-3, 7, 11-trioxatridecane, perfluoro-6, 6, 10, 10,-tetrakis (propyloxymethyl)-4, 8, 12-trioxapentadecane, perfluoro-7, 7, 11, 11-tetrakis (butyloxymethyl) -5, 9, 13-trioxaheptadecane, perfluoro-7, 7, 11, 11-tetrakis (2-methoxyethoxymethyl)-2, 5, 9, 13, 16-pentaoxaheptadecane, perfluoro-4, 4, 8, 8, 12, 12-hexakis (methoxymethyl )-2, 6, 10, 14-tetraoxapentadecane and perfluoro-5, 5, 9, 9, 13, 13-hexakis (ethoxymethyl)-3, 7, 11, 15-tetraoxaheptadecane