Bien que mon activité de recherche a été principalement axée sur la caractérisation structurale des phases d'Aurivillius en relation avec leurs propriétés ferroélectriques, je me suis aussi intéréssé à divers aspects liés à la synthèse de ces phases que ce soit sous forme de monocristaux, de poudres ou encore de films (voie chimique). Une première section consacrée à la synthèse ouvrira donc cette deuxième partie. Dès mon arrivée au SPCTS, une part importante de mon activité a été consacrée à l'étude des intercroissances à longues périodes qui ont pu être observées dans divers systèmes mais dont les caractéristiques structurales restaient inconnues. Sur la base de nos observations expérimentales, un modèle cristallographique généralisé utilisant le formalisme des groupes de super-espace a été développé pour les stuctures de type Aurivillius. Ce travail est issu d'une collaboration étroite avec l'équipe du Prof. J.M. Perez-Mato de l'Université de Bilbao. Cette nouvelle approche cristallographique et son utilisation constitueront la deuxième section. L'étude des relations structure/propriétés dans ces composés a constitué une autre part importante de mes recherches au SPCTS avec, notamment, le co-encadrement de la thèse de Jenny Tellier. Les divers aspects de la transition ferroélectrique-paraélectrique (FE-PE) rencontrés dans les composés de type Aurivillius ont été abordés que ce soit dans le cas des termes simples, des composés d'intercroissances que dans le cas, moins classique, des composés présentant un comportement ressemblant à celui de "relaxeurs" tel que Pb(Mg1/3Nb2/3)O3. Ce travail visant à décrire les mécanismes structuraux associés à la transition FE-PE fera l'objet de la troisième et dernière section.

Carbon nanostructures are a wide field with many applications. The use of carbon nanostructures as support in heterogeneous catalysis is a key development that led together with the use of nanoparticles to a significant cost reduction of catalysts. Catalysts designed in this way are widely applied in fuel cell technologies. For portable devices especially low temperature fuel cells are desirable with low hazards for the user. One technology which fulfills these requirements is the direct formic acid fuel cell (DFAFC). DFAFC have many promising characteristics, such as high electromotive force and easy fuel handling. However, they still suffer from too low power output and lifetime for commercialization. This thesis focusses on two main aspects: the synthesis of carbon nanostructures by chemical vapor deposition (CVD) and their application as catalyst support. The materials are investigated by many different techniques ranging from transmission electron microscopy (TEM) to fuel cell tests. Different carbon nanostructures could be synthesized by catalytic CVD on palladium (Pd) nanoparticles. Multi-walled carbon nanotubes (MWCNTs), carbon nanofibers (CNFs) and helical carbon nanofibers (HCNFs) were grown, selectively, dependent on temperature, using acetylene as carbon precursor. Especially HCNF raised further interest due to their unique structure. A growth model for HCNFs was developed based on an anisotropic extrusion model. The synthesis conditions for HCNFs were optimized until an almost 100 % purity with very high efficiency was obtained. The unique helical but fiber-like structure made the material very interesting as support for heterogeneous catalysis. Several catalysts based on Pd nanoparticle decorated HCNFs were developed. The synthesis methods ranged from standard methods like the polyol method to phase-transfer methods. The catalysts showed very promising results for the electro-oxidation of methanol, ethanol and formic acid. This makes them highly attractive for fuel cell applications. The catalysts were tested in DFAFC. The superiority of HCNF-based catalysts is attributed to the good attachment of nanoparticles to the defect-rich and easy to functionalize surface of HCNFs in combination with adequate film forming properties during electrode preparation. / Nanostrukturerat kol är ett mycket brett fält med ett stort antal tillämpningar. Användning av kolnanostrukturer som support för heterogena katalysmaterial har tillsammans med utvecklingen av nanopartiklar lett till en avsevärd minskning av kostnaden för katalysatorer. Katalysatorer designade på detta sätt används frekvent i bränsleceller. För portabla tillämpningar är utvecklingen av säkra och miljövänliga lågtemperaturceller mycket viktig. En teknologi som uppfyller dessa kriterier är bränsleceller som drivs med myrsyra (DFAFC). Sådana bränsleceller har många önskvärda egenskaper, såsom en hög elektromotorisk kraft och en enkel hantering av bränslet. Trots dessa goda egenskaper har de också en del nackdelar som hindrar en full kommersialisering. De två mest problematiska är en för låg genererad effekt samt en för kort livslängd på katalysatorerna. Denna avhandling fokuserar på två huvudpunkter som adresserar dessa problem; tillverkning och karaktärisering av kolnanostrukturer producerade med CVD, och deras tillämpningar som support för katalysatorer. Materialen karaktäriseras med en rad olika tekniker, allt från transmission-elektronmikroskopi till bränslecellstester. Olika kolnanostrukturer har syntetiserats med katalytisk CVD på palladium (Pd) nanopartiklar. Produktionen av flerväggiga kolnanorör, kolfibrer och heliska kolnanofibrer har tillverkats med acetylen som kolkälla och genom att variera temperaturen kunde innehållet av olika typer av nanostrukturerat kol kontrolleras. Särskilt stort intresse har de heliska kolnanofibrerna rönt på grund av deras unika struktur. Vi beskriver en tillväxtmekanism baserad på en anisotrop diffusionsmodell. Genom att justera produktionsparametrarna visar vi att heliska kolnanofibrer kunde tillverkas med nära 100 %-ig renhet och hög effektivitet. Den unika heliska och fiberlika strukturen är mycket intressant for tillämpningar som support för heterogena katalysatorer. Ett flertal kompositer för katalytiska tillämpningar har utvecklats baserade på heliska kolnanofibrer, dekorerade med heterogena katalysatorer genom en rad olika kemiska/fysikaliska tekniker. De syntetiserade materialen visar mycket goda katalytiska egenskaper för att oxidera metanol, etanol och myrsyra. Därigenom blir de mycket attraktiva för användning i bränsleceller. Vi korrelerar de goda katalytiska egenskaperna med en bra vidhäftning av nanopartiklarna på de heliska kolnanofibrerna defekter, deras goda ledningsförmåga, bra egenskaper för att förbereda elektroder, samt deras stora yta i förhållande till deras volym och vikt.

Light emitting diodes (LEDs) and laser diodes (LDs) have many advantages over conventional light sources. Current commercial LEDs span the spectrum from IR to near- UV. There are a variety of applications for devices that extend into the deep-UV, including biological agent detection and optical storage. The nitride material system is a set of semiconducting compounds that have wavelengths that span a broad range, from yellow to deep-UV. AlGaN has a direct bandgap that extends into the deep-UV range; we will try to grow device-quality material, deposited epitaxially using metalorganic chemical vapor deposition on sapphire substrates.

Light emitting diodes (LEDs) and laser diodes (LDs) have many advantages over conventional light sources. Current commercial LEDs span the spectrum from IR to near- UV. There are a variety of applications for devices that extend into the deep-UV, including biological agent detection and optical storage. The nitride material system is a set of semiconducting compounds that have wavelengths that span a broad range, from yellow to deep-UV. AlGaN has a direct bandgap that extends into the deep-UV range; we will try to grow device-quality material, deposited epitaxially using metalorganic chemical vapor deposition on sapphire substrates.

No description available.

No description available.

The main purpose of this thesis is to improve the fundamental knowledge of non-isothermal tempering of martensite phase and its effects on the reduction in hardness (softening) with respect the base metal occurring at the heat affected zone (HAZ) of resistance spot welded dual-phase (DP) steels. This thesis also aims at understanding the influence of HAZ-softening on the joint performance of various DP steel grades. The tempering of martensite occurring at the sub-critical HAZ (SC-HAZ) of resistance spot welded DP600, DP780 and DP980 steels has been systematically evaluated by microhardness testing through Vickers indentation and the degree of tempering has been correlated to the HAZ-softening. From the joint performance analysis of similar and dissimilar steel grade combinations assessed through standardized testing methods, three important issues have been targeted: a) the joint strength (maximum load to failure), b) the location of failure (failure mode), and c) the physical characteristic of the weld that determines certain type of failure (weld nugget size). In addition, a partial tensile test has been conducted in order to evaluate the initiation of failure in dissimilar steel grade combinations. It has been shown that HAZ-softening lowered the weld size at which transition from interfacial to pullout failure mode takes place along with increased load-bearing capacity and higher energy absorption. Thus, it is concluded from mechanical testing that HAZ-softening benefits the lap-shear tensile joint performance of resistance spot welded DP steels by facilitating pullout failures through failure initiation at the SC-HAZ (tempered region). Instrumented nanoindentation testing was employed to further investigate HAZ-softening along the SC-HAZ by evaluating individual phases of ferrite matrix and tempered martensite islands. Although the ferrite matrix presented a slight reduction in hardness at nanoscale, higher reduction in hardness (softening) resulted for tempered martensite; thus confirming that tempered martensite is the major contributor to softening at micro-scale. A comparison between nanohardness and microhardness testing made at different distances from the line of lower critical temperature of transformation (Ac1) allowed revealing the actual extension of the SC-HAZ. In this regard, good correlation was obtained between nanohardness results along the SC-HAZ and the microstructural changes analyzed by electron microscopy (i.e., the tempering of martensite occurring at various distances far from Ac1 was correlated to low temperature tempering of dual phase steels). An in-depth analysis of the tempering of martensite phase at high temperature in DP steel subjected non-isothermal conditions i.e., rapid heating, extremely short time at peak temperature and rapid cooling (resistance spot welding), has been carried out mainly through analytical transmission electron microscopy (TEM). In addition, an isothermal tempering condition (i.e., slow heating and long time at peak temperature) in DP steel has been evaluated for complementing the analysis. Both non-isothermal and isothermal conditions have been correlated to the softening behaviour. TEM analysis of the base metal in the DP steel indicated that the morphology of the martensite phase is dependent on its carbon content, and its tempering characteristics are similar to that of equal carbon containing martensitic steel. The isothermally tempered structure is characterized by coarsening and spheroidization of cementite (θ) and complete recovery of the martensite laths; whereas precipitation of fine quasi-spherical intralath θ-carbides, coarser plate-like interlath θ-carbides, decomposition of retained austenite into elongated θ-carbides, and partial recovery of the lath structure were observed after non-isothermal tempering of DP steel. This difference in tempering behaviour is attributed to synergistic effect of delay in cementite precipitation due to higher heating rate, and insufficient time for diffusion of carbon that delays the third stage of tempering process (cementite coarsening and recrystalization) during non-isothermal. The finer size and the plate-like morphology of the precipitated carbides along with the partial recovery of the lath structure observed after non-isothermal tempering strongly influenced the softening behaviour of DP steel. The chemical analysis of θ-carbides through extraction replicas for three different DP steels revealed that the chemistry of the carbides is inherited from the parent DP steel during non-isothermal tempering at high temperature confirming that non-isothermal tempering DP steel is predominantly controlled by carbon diffusion.

Beurteilung von nativen und aufgetauten Spermatozoen fertiler und subfertiler Hengste mit Hilfe der Phasenkontrast- und Transmissionselektronenmikroskopie. Institut für Veterinär-Pathologie der Veterinärmedizinischen Fakultät, Universität Leipzig Reproduktionsmedizinische Einheit der Kliniken der Tierärztlichen Hochschule Hannover In dieser Arbeit wurde die Ultrastruktur von nativen und tiefgefrorenen Spermien mittels Phasenkontrast- und Transmissionselektronenmikroskopie (TEM) untersucht. Für die Beurteilung der Spermienmotilität und der Morphologie von in Formolzitrat fixierten Spermien standen jeweils drei Ejakulate von 50 Hannoveraner Hengsten des Niedersächsischen Landgestüts Celle zur Verfügung. Aus dieser Gruppe wurden drei fertile, drei subfertile Hengste und 6 Hengste mittlerer Fertilität ausgewählt, von denen sowohl die Nativ-Proben als auch eine Tiefgefrierprobe (TG-Probe) für die TEM im Institut für Veterinär-Pathologie der Universität Leipzig gemäß des Standardprotokolls des Institutes aufbereitet wurden. Die Spermien wurden gewaschen und das Seminalplasma der nativen Proben oder der Verdünner der TG-Proben abpipettiert und durch eine 5%-ige Glutaraldehydlösung in einem 0,1 M Kakodylatpuffer (pH 7,2) ersetzt. Die Fixierungslösung wurde anschließend entfernt und das Pellet gewaschen und danach mit Gelatine gemischt. Die spermienreichen Stellen wurden aus der Gelatine herausgeschnitten und in Glutaraldehyd aufbewahrt. Nach einer Nachfixierung in OsO4 und einer Entwässerung in Ethanollösungen erfolgte eine Einbettung in einer Eponmischung. Nach einer Polymerisation von 5 Tagen wurden die eingebetteten Eponblöckchen angetrimmt und die Semi- und Ultradünnschnitte angefertigt. Die Ultradünnschnitte wurden auf ein Kupfergrid gelegt, mit Uranylazetat und Bleizitrat kontrastiert und mit dem Transmissionselektronenmikroskop (Zeiss EM 900, Oberkochem) bei 80 kV analysiert. In den nativen Proben wurden insgesamt 360 Spermien pro Hengst beurteilt, in den TG-Proben 120 Spermien pro Hengst. Die Qualität der elektronenmikroskopischen Aufnahmen war sehr gut, doch die Plasmamembran zeigte fixierungsbedingte Artefakte. Nach dem Auftauen waren die Bilder heller und der Kontrast etwas geringer. Es gab eine Zunahme an Akrosomdefekten, akrosomreagierten Spermien und Beschädigungen der Plasmamembran, der Mitochondrien, sowie der Mantel- und Ringfasern. Durch die Membranbeschädigungen trat auch eine Verringerung der Anzahl proximaler und distaler Zytoplasmatropfen auf. Sowohl geschwollene Akrosome mit einer niedrigeren Dichte der akrosomalen Matrix als auch Mitochondrien mit einer zu hellen mitochondrialen Matrix waren typische Befunde in den TG-Proben. Die Studie der Ultrastruktur und die wahrgenommenen Defekte führten zur Erstellung eines Standardprotokolls für die transmissionselektronenmikroskopische Beurteilung von Hengstspermien. Die Beurteilung mittels TEM sollte aber nicht zu einer quantitativen, sondern zu einer qualitativen Aussage führen. Sie ermöglicht die Diagnose von Kern- (Kerndeformationen und Taschenbildung im Kern) und Akrosomabweichungen (deformierte Akrosome mit oder ohne Vakuolenbildung, abgehobene Akrosome und akrosomreagierte Spermien), Anomalien der Mitochondrien (Unterbrechung der Mitochondrienscheide, zu viele Mitochondrien, anormale Dichte der mitochondrialen Matrix), Defekten des Axonemas (Ordnung oder Anzahl der Mikrotubuli, Mantel- und Ringfasern) und der Anwesenheit immaturer Spermienvorstufen. Diese Methode eignet sich für die Diagnostik subfertiler Hengste mit normalen Spermienparametern bei der routinemäßige Spermienbeurteilung und kann sowohl in nativen als auch in TG-Proben angewendet werden. Im Vergleich zur Phasenkontrastmikroskopie waren die elektronenmikroskopischen Bilder wegen ihrer stärkeren Vergrößerung und der Darstellung innerer Spermienstrukturen viel aussagekräftiger. Für die Beurteilung von Halsansatzdefekten, abweichende Geißelformen und Mehrfachmißbildungen ist die Phasenkontrastmikroskopie die am besten geeignete Methode. / Evaluation of fresh and frozen-thawed semen samples of fertile and subfertile stallions by light microscopy and transmission electron microscopy. Institut of Pathology of the Faculty of Veterinary Medicine, University of Leipzig Reproduktionsmedizinische Einheit der Kliniken der Tierärztlichen Hochschule Hannover In this study the ultrastructure of fresh and frozen-thawed semen samples of 50 stallions from the National Stud of Lower Saxony (Celle, Germany) were evaluated by light microscopy and transmission electron microscopy (TEM). Three ejaculates of each stallion were available for the motility analysis and the morphological analysis by lightmicroscopy after fixation in formol citrate. Based on the fertility data, the ejaculates of 12 stallions (3 fertile stallions, 3 subfertile stallions and 6 stallions of average fertility) were selected for the morphological analysis by TEM. The native samples and one frozen-thawed sample from these stallions were prepared for the TEM at the Institute of Pathology of the Faculty of Veterinary Medicine, Uni-versity of Leipzig. The sperm cells were washed and the seminal plasma from the native samples and the diluents of the frozen-thawed samples were replaced by a 5%-glutaraldehyde solution in a 0,1 M cacodylate buffer pH 7,2. The fixative was removed, the pellet was washed again and mixed with gelatin. The sperm rich fraction in the gelatin mass was excised and stored in glutaraldehyde. A second fixation in OsO4 was followed by a dehydratation in ethanol and a polymerization phase in epon. After 5 days of polymerization the starred samples were used for semi- and ultratight cuts. The latter were placed on a copper grid, contrasted with uranyl acetate and lead citrate and analyzed with the transmission electron micro-scope (EM 900) by 80 kV. In the fresh samples, 360 sperm cells were examined per stallion, whereas in the frozen-thawed samples only 120 sperm cells per stallion were evaluated. The microscopic pictures were of a high quality. However, the sperm plasma membrane showed some fixation artifacts. In the thawed samples a lower contrast was noticed than in the fresh samples. The sperm cells in the frozen-thawed samples showed an increase in acrosome defects, acrosome reactions, damage of the cell plasma membrane, mitochondria, fibrous sheet and outer dense fibers. The latter defect was associated with a decrease in proximal and distal cytoplasmatic droplets. Swollen acrosomes with a lower matrix density and a bright mitochondrial matrix were typically present in the cryopreserved samples. The ultrastructural defects in these samples, examined by TEM, have led to the development of a standard evaluation protocol with the most common sperm defects in stallion semen. TEM is an expensive and time consuming technique, which cannot be used to obtain quantitative results, but is considered as an accurate method for the qualitative examination of semen samples in cases of unexplained subfertility. TEM can especially be recommended for the diagnosis of nuclear (nuclear malformations and pouches) and acrosomal defects (acrosome deformations, acrosome vacuoles, detached acrosomes and acrosome reactions), mitochondrial (mitochondrial sheet defects, mitochondrial proliferation, decrease in mitochondrial matrix density) and axonema malformations (anormal position or quantity of microtubules and fibrous sheet or outer dense fibers defects) and the detection of immature sperm cells in ejaculates. The results of this study state that TEM can be useful for the evaluation of both fresh and frozen-thawed semen samples. Compared to the light microscopic evaluation of stallion sperm, the TEM images give more precise information because of their higher magnification rate and the ability to reveal internal sperm structures. However, light microscopy remains the best method to detect sperm neck defects, deformed tailes and sperm cells with multiple heads or tails.

The phase transformation of anodized Ti film has been studied. Although X-ray diffraction detected only the amorphous TiO2 phase, transmission electron microscopy analysis showed that TiO nanocrystallites less than 10 nm in size were also present, which was further supported by x-ray photoelectron spectroscopy analysis. Anatase was found to appear gradually by annealing the as-anodized specimen in air at 500¡V550 oC, which was accompanied by a simultaneous disappearance of TiO nanocrystallites. In contrast, only rutile is formed by annealing the Ti film at the same temperatures. The results indicate that TiO can induce the formation of anatase, which is explained by the close similarity between their structures. (Chapter 1) Anatase phase of TiO2 has been shown to have very good biocompability. It was frequently observed on Ti surfaces after anodizing and thermal annealing treatments. In this report the mechanisms of the Ti to TiO and the TiO to anatase phase transitions in anodizing and annealing treatments of Ti have been studied by transmission electron microscopy. Ti thin films of two strong textures were first grown on the (001)NaCl substrates. In addition to amorphous TiO2, the anodization treatment caused the formation of TiO with an orientation relationship of (11-20)Ti // (220)TiO with Ti. The subsequent thermal annealing treatment caused the TiO to anatase transition with an orientation relationship of {200}TiO //{200}A. Pure anatase film was prepared by this method. (Chapter 2)

The orientation relationship and interfaces of PtSn4 and PtSn with the Pt (001) and (111) surfaces have been studied with transmission electron microscopy. Pt was evaporated onto the NaCl (001) and (111) surfaces to form epitaxial Pt thin films and Sn was evaporated onto the Pt films at different temperature to form PtSn4 and PtSn. Pt was evaporated onto the NaCl (001) and (111) surfaces at 350 ¢J to form epitaxial Pt thin films of [001] and [111] zone axes, respectively. Some grains are in random orientation and other as ring pattern. The grain size was at about 10-20 nm. Sn was evaporated onto the Pt surface at 150 ¢J to form PtSn4 and at 200 ¢J to form PtSn. No good orientation relationships were formed on both the PtSn4/ Sn and the PtSn/ Sn interfaces. Heterogeneous nucleation theory, predicts that PtSn should form before PtSn4, but PtSn4 was observed to the first to form. The possible reasons were discussed. Keywords: PtSn4/ Sn interface, PtSn/ Sn interface, orientation relationships, thin films, evaporator, transmission electron microscopy