The high-pressure behaviour of the lanthanide series of metals has been the subject of study since the work of Percy Bridgman in the 1940s. Differences in said behaviour between the different lanthanide metals are attributed to the increasing occupation of the 4f electron shell as Z increases. Upon compression, or as Z decreases, the trivalent lanthanides (La to Lu, excluding Eu and Yb) undergo a common phase transformation sequence through various close packed structures: hcp → Sm-type (the structure adopted by samarium at ambient conditions) → dhcp → fcc → distorted fcc (d-fcc). Upon further compression, the lanthanide metals experience a first order transition to a "volume collapsed" phase. Many studies have focused on the low-Z members of the series, since the various phase transitions occur at much lower pressure where it is comparatively easy to collect high quality data. By contrast, the other members of the series have received comparability little attention, and there are even fewer reports of the structural behaviour of the lanthanide metals at high pressure and high temperature. This thesis contains the results of angle-dispersive x-ray powder diffraction experiments at high pressure and high temperature of the various members of the lanthanide metals. Ce has been the subject of many previous studies, but a systematic x-ray diffraction study of the fcc/d-fcc phase boundary has never been attempted. Furthermore, the location in P-T space of the high temperature fcc/bct/d-fcc triple point has only been inferred, due to the lack of data on the fcc/bct phase boundary at high temperature. The high-pressure high-temperature phase diagram of Ce is presented and discussed. La is unique amongst the lanthanide metals due to its empty 4f shell at ambient conditions. Despite this, La undergoes the common lanthanide transformation sequence up to the d-fcc phase, after which it undergoes a re-entrant transition back to the fcc phase at 60 GPa. The diffraction peaks of d-fcc La are shown in this thesis to undergo changes in intensity upon compression, indicating a transformation to the oI 16 structure found in Pr. La is one of the few elements whose behaviour has been unknown above 100 GPa, and results of La's structural behaviour upon compression to 280 GPa are presented and discussed. At 76 GPa, La begins a transition from the fcc phase to a new phase with the bct structure. Finally, the d-fcc→fcc re-entrant phase transition has been determined at various temperatures, and the d-fcc stability region has been mapped out. Finally, x-ray diffraction experiments were performed on Gd up to 100 GPa and ~700 K, to determine the structure of the d-fcc phase and the "volume collapsed" phase. While d-fcc Gd does not undergo pressure-induced changes similar to its low Z brethren, the d-fcc Gd remains stable up to 41 GPa at 700 K, putting a constraint on the d-fcc stability region. The data collected on Gd's "volume collapsed" phase cannot be fitted to the currently accepted mC4 structure. This has implications for our understanding of the lanthanide series as a whole, since most of of the heavier members, and some of the lighter lanthanides, are reported to adopt the mC4 structure.

This thesis project has focussed on the experimental study of simple molecular systems at extreme conditions. High-pressure and high-temperature techniques have been used in combination with Raman spectroscopy and X-ray diffraction diagnostics to characterise three simple molecular systems which are unified by the inclusion of nitrogen as a constituent element. The N2 molecule contains the only triple-bond amongst the elemental diatomics and is considered a model system for exploring the changes in structure and bonding induced by tuning pressure and temperature conditions. As such the nitrogen phase-diagram is a focus-point in current extreme conditions research and nitrogen has been found to exhibit a high-degree of polymorphism not observed in other simple molecular systems such as hydrogen or oxygen. Understanding molecular mixtures of nitrogen with other simple molecules at extreme conditions is significant to many scientific fields varying from chemistry to astronomy. The first system presented is the binary mixture of nitrogen and xenon which was studied as a function of pressure. The study constitutes the first comprehensive study of the xenon-nitrogen system at high-pressures. A new van der Waals compound was observed which underwent a phase transition at 14 GPa and was stable up to at least 180 GPa and 3000 K, conditions where pure nitrogen becomes amorphous. Optical measurements suggested possible metallization of the new compound around 120 GPa. The second system presented is the binary mixture of nitrogen and hydrogen which was studied both as a function of pressure and composition. Two known nitrogen-hydrogen structures were confirmed and a pressure-temperature path-dependent formation of hydrazine or ammonia was discovered. Additionally, one mixture was compressed to 242 GPa, the highest pressure investigated in the nitrogen-hydrogen system. The third system presented is the elemental nitrogen phase known as i-nitrogen, an elusive high-temperature polymorph which has hitherto eluded structure determination and proved challenging to access. i-nitrogen was successfully characterised as having an extraordinarily large unit cell containing 48 N2 molecules, making it the most complex molecular nitrogen structure to be determined unambiguously.

Die Einhaltung von Brandschutzanforderungen ist ein wichtiger Aspekt für sichere Baukonstruktionen. Beim innovativen Werkstoff textilbewehrter Beton, der einen Verbundwerkstoff aus einer Feinbetonmatrix und textiler Bewehrung darstellt, ist das Brandverhalten bisher nur unzureichend erforscht worden. Insbesondere das Tragverhalten der einzelnen Komponenten unter hohen Temperaturen stellt noch eine Wissenslücke in der heutigen Forschung dar. Dieser Artikel befasst sich mit den experimentellen Untersuchungen an einer Feinbetonmatrix, die ein Größtkorndurchmesser von 0,6 mm aufweist, sowie an AR-Glas- und Carbongarnen. Basierend auf instationären Versuchen werden das Spannungs- und Dehnungsverhalten unter hohen Temperaturen abgeleitet und Ansätze zur rechnerischen Beschreibung des Hochtemperaturverhaltens vorgeschlagen. / The design of structural members under fire attack is an important aspect for safe constructions. For the innovative material textile reinforced concrete (TRC), which is a composite material made of fine-grained concrete and textile reinforcement, the fire behavior has not been investigated insufficiently yet. Especially the load-bearing behavior under high-temperatures of the single components marks a gap in the state-of-the-art of science and technology today. This article deals with experimental investigations on a fine-grained concrete matrix, which has maximum grain size of only 0.6 mm, as well as yarns made of AR-glass and carbon. On the basis of transient tests the stress and strain behavior under high temperatures is derived. Finally, a calculative approach for the hightemperature behavior is presented.

Die Einhaltung von Brandschutzanforderungen ist ein wichtiger Aspekt für sichere Baukonstruktionen. Beim innovativen Werkstoff textilbewehrter Beton, der einen Verbundwerkstoff aus einer Feinbetonmatrix und textiler Bewehrung darstellt, ist das Brandverhalten bisher nur unzureichend erforscht worden. Insbesondere das Tragverhalten der einzelnen Komponenten unter hohen Temperaturen stellt noch eine Wissenslücke in der heutigen Forschung dar. Dieser Artikel befasst sich mit den experimentellen Untersuchungen an einer Feinbetonmatrix, die ein Größtkorndurchmesser von 0,6 mm aufweist, sowie an AR-Glas- und Carbongarnen. Basierend auf instationären Versuchen werden das Spannungs- und Dehnungsverhalten unter hohen Temperaturen abgeleitet und Ansätze zur rechnerischen Beschreibung des Hochtemperaturverhaltens vorgeschlagen. / The design of structural members under fire attack is an important aspect for safe constructions. For the innovative material textile reinforced concrete (TRC), which is a composite material made of fine-grained concrete and textile reinforcement, the fire behavior has not been investigated insufficiently yet. Especially the load-bearing behavior under high-temperatures of the single components marks a gap in the state-of-the-art of science and technology today. This article deals with experimental investigations on a fine-grained concrete matrix, which has maximum grain size of only 0.6 mm, as well as yarns made of AR-glass and carbon. On the basis of transient tests the stress and strain behavior under high temperatures is derived. Finally, a calculative approach for the hightemperature behavior is presented.

In der vorliegenden Arbeit wurden bruch- und schädigungsmechanische Eigenschaften keramischer Filterwerkstoffe, welche im Rahmen des SFB 920 entwickelt und erforscht werden, bestimmt. Es wurden zwei verschiedene Werkstoffsysteme - Aluminiumoxid und kohlenstoffgebundenes Alumniumoxid - mit entwickelten Miniaturprüfmethoden bei Prüftemperaturen bis 1500°C untersucht. Mit Hilfe des Ball-On-Three-Balls-Tests (B3B) wurde die biaxiale Festigkeit in Abhängigkeit der Prüftemperatur bestimmt. Für die kohlenstoffgebundenen Proben wurde weiterhin der Einfluss der Verkokungstemperatur als auch des Kohlenstoffgehalts auf die Festigkeit untersucht. Es konnte ein Festigkeitsmaximum ermittelt werden, wenn die Prüftemperatur der Verkokungstemperatur entspricht. Die schädigungsmechanischen Materialparameter des duktilen Verformungs- und Versagensverhalten beider Werkstoffe bei 1500°C wurden mit Hilfe von numerischen Simulationen des B3B identifiziert. Die Bestimmung der bruchmechanischen Kennwerte erfolgte mit einem Vier-Punkt-Biegeversuchsaufbaus mit Chevron-gekerbten Proben (CNB-Versuch). Numerische Untersuchungen dienten zur Bestätigung der Versuchsergebnisse und Bestimmung der Form der Rissfront sowie Risswachstum während des Versuches. / In the present work, fracture and damage mechanical properties of ceramic filter materials, which are developed and investigated within the framework of the CRC 920, are determined. Two different material systems - alumina and carbon-bonded alumina - are investigated using miniaturized test methods at temperatures up to 1500°C. The biaxial strength at different temperatures is determined using the Ball-On-Three-Balls-Test (B3B). The strength of carbon-bonded specimens is dependent on the coking temperature and the carbon content. A maximum strength can be obtained if the testing temperature equals the coking temperature. The damage-mechanical material parameters in order to describe the ductile deformation and failure behaviour of both materials at 1500°C are identified with the help of numerical simulations. The determination of fracture-mechanical properties are carried out with a four-point bending test setup with chevron-notched specimens (CNB). Numerical investigations are used to validate the test results and to simulate the shape of the crack front and crack growth during the experiment.

L'isolation thermique est essentielle dans de nombreux types de MEMS (micro-systèmes électro-mécaniques). Selon le type de dispositif, l'isolation permet de réduire la consommation d'énergie, diminuer le temps de réponse, ou augmenter sa sensibilité. Les matériaux d'isolation thermique actuellement disponibles sont difficiles à intégrer en couche épaisse dans des dispositifs en silicium. À cause de cela, l'approche la plus utilisée pour l'isolation est d'intégrer les zones à isoler sur des membranes minces (~ 1 µm). Cela assure une bonne isolation, mais est restrictif pour la conception du dispositif et la fragilité des membranes complique la fabrication et l'utilisation de celui-ci. Le silicium poreux est facile à intégrer puisqu'il est fabriqué par gravure électrochimique de substrats de Si cristallin. On peut aisément fabriquer des couches épaisses (100 µm) et sa conductivité thermique est 2-3 ordres de grandeur plus faible que celle du Si massif. Par contre sa porosité cause des problèmes : mauvaise résistance chimique, structure instable au-delà de 400°C, et tenue mécanique réduite. La facilité d'intégration des semiconducteurs poreux est un atout majeur, et nous visons donc de réduire les désavantages de ces matériaux afin de favoriser leur intégration dans des dispositifs en silicium. La première approche qui a été développée consiste à amorphiser le Si poreux en l'irradiant avec des ions à haute énergie (uranium, 110 MeV). Nous avons montré que l'amorphisation, même partielle, du Si poreux entraîne une diminution de sa conductivité thermique, sans endommager sa structure poreuse. On peut atteindre ainsi une réduction de conductivité thermique jusqu’à un facteur de trois. La seconde approche est de développer un nouveau matériau. Le SiC poreux a été choisi, puisque le SiC massif a des propriétés physiques exceptionnelles et supérieures à celles du silicium. Nous avons mené une étude systématique de la porosification du SiC en fonction de la concentration en HF et le courant, ce qui nous a permis de fabriquer des couches poreuses uniformes d’une épaisseur d’environ 100 µm. Nous avons implémenté un banc de mesure de la conductivité thermique par la méthode « 3 oméga » et l'avons utilisé pour mesurer la conductivité thermique du SiC poreux. Nos résultats montrent que la conductivité thermique du SiC poreux est environ deux ordres de grandeur plus faible que celle du SiC massif. Nous avons aussi montré que le SiC poreux est résistant à tous les produits chimiques typiquement utilisés en microfabrication et est stable jusqu'à au moins 1000°C. / Thermal insulation is essential in several types of MEMS (Micro electro mechanical systems). Depending on the device, insulation can reduce the device’s power consumption, decrease its response time, or increase its sensitivity. Existing thermal insulation materials are difficult to integrate as thick layers in silicon-based devices. Because of this, the most commonly used approach is to integrate the areas requiring insulation on thin membranes. This provides effective insulation, but restricts the design of the device and the membrane’s fragility makes the device’s fabrication and use more complicated. Poreux silicon is easy to integrate as it is made by electrochemical etching of crystalline silicon substrates. 100 µm thick layers can easily be fabricated and its thermal conductivity is 2-3 orders of magnitude lower than that of bulk silicon. However, its porosity causes other problems : low chemical resistance, its structure is unstable above 400°C, and reduced mechanical stability. The ease of integration of porous semiconductors remains a major advantage, so we aim to reduce the disadvantages of these materials in order to help their integration in microfabricated devices. The first approach we developed was to amorphise porous Si by irradiating it with heavy ions. We have shown that amorphisation of porous Si, even partial, causes a reduction of its thermal conductivity without damaging its porous structure. In this way a reduction in thermal conductivity by up to a factor of three can be achieved. The second approach was to develop a new material. Porous SiC was chosen, as bulk SiC has exceptional physical properties which are superior to those of silicon. We carried out a systematic study of the porosification process of SiC versus HF concentration and current, which enabled us to make thick (100 µm) and uniform layers. We have implemented a system for measuring thermal conductivity using the “3 omega” technique and used it to measure the thermal conductivity of porous SiC. Our results show that the thermal conductivity of porous SiC is about two orders of magnitude lower than that of bulk SiC. We have also shown that porous SiC is resistant to all chemical commonly used in microfabrication, and is stable up to at least 1000°C.