Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O efeito magnetocalórico, base da refrigeração magnética, é caracterizado por duas quantidades: a variação isotérmica da entropia (ΔST) e a variação adiabática da temperatura (ΔTS); que são obtidas sob variações na intensidade de um campo magnético aplicado. Em sistemas que apresentam anisotropia magnética, pode‐se definir o efeito magnetocalórico anisotrópico, o qual, por definição, é calculado sob variações na direção de aplicação de um campo magnético cuja intensidade mantém‐se fixa, e é caracterizado por duas quantidades: a variação anisotrópico‐isotérmica da entropia (ΔSan) e a variação anisotrópico‐adiabática da temperatura (ΔTan). O efeito magnetocalórico e o efeito magnetocalórico anisotrópico foram estudados nos compostos intermetálicos formados por terras e outros materiais não magnéticos: RNi2, RNi5, RZn e Gd1‐nPrnAl2. Os cálculos foram feitos partindo de hamiltonianos modelo que incluem as interações de troca, Zeeman, de campo cristalino e quadrupolar. / The magnetic refrigeration is based on the magnetocaloric effect. The magnetocaloric potential is characterized by the two thermodynamics quantities: the isothermal entropy change (ΔSiso) and the adiabatic temperature change (ΔTad), which are calculated upon a change in the intensity of the applied magnetic field. In anisotropic magnetic systems it is observed a change in the magnetocaloric effect, since this potential becomes strongly dependent on the direction in which the external magnetic field is applied. The anisotropy in such magnetic systems can lead to an inverse magnetocaloric effect, as well as to the definition of an anisotropic magnetocaloric effect, that by definition is calculated upon a magnetic field which intensity is kept fixed and which orientation is changed from a hard direction of magnetization to the easy direction of magnetization. This anisotropic magnetocaloric effect was performed for the RAl2 intermetallic compounds considering a microscopic model Hamiltonian that includes the Zeeman interaction, the exchange interaction (taken in the mean field approximation) and the crystalline electrical field, that is responsible for the anisotropy in the RAl2 compounds. The anisotropic magnetocaloric was fully investigated for the serie RAl2 and compared with the usual magnetocaloric effect and several curves of (ΔSiso) and (ΔTad) were obtained.

Out of the known aerospace metal and alloys, Aluminium (Al) and Titanium (Ti) are important due to their unique combination of properties, such as strength, ductility and corrosion resistance etc. For these reasons, welding of these two materials, especially in the butt and lap configuration, has a significant impact for structural applications. However, welding of Al to Ti is a challenge due to wide differences in their physical properties and properties of the brittle intermetallic that are formed. Such problems in Ti-Al weld can be minimized if the temperature of welding is reduced. Therefore, many solid-state welding processes have been introduced for this system in the past few decades. Amongst these processes, Friction Stir Welding (FSW) is among the most appropriate for dissimilar materials in the butt and lap configuration, as this process involves lower temperature of processing. The present thesis is an attempt to address the issues pertaining to the friction stir welding of commercially pure Al and Ti. Though these commercially pure materials are seldom used in actual applications, where alloys such as Ti-6Al-4V and Al 2219 (and their variants) are used, this work is done to get a fundamental understanding of the underlying mechanisms during Friction Stir Welding (FSW). The study has been extended to the effect of using a thin strip of other metallic materials between Al and Ti. These inserts are likely to play a role in the formation of intermetallic and control the after effects of the formation of these intermetallic. Two metals have been chosen for this purpose, namely Zinc (Zn) and Niobium (Nb). The thesis has 8 chapters that attempts to systematically understand the process of FSW of cp-Al to cp-Ti. In Chapter 1 of the thesis, the FSW process is introduced with an emphasis on important parameters that control the welding process. In addition, a brief introduction of Al-Ti binary system is also given. Literature related to conventional solid state welding processes and friction stir welding process is presented in Chapter 2. In this chapter, previous works on the FSW of various materials is reviewed, with more emphasis on welding of aluminium to titanium. At the end of the chapter the scope and motivation of the present investigation has been outlined Chapter 3 includes the experimental details involved in the present study. In addition to the details of the processes and various characterization techniques used in the present investigation, the basic principles involved in various techniques, names as X-ray tomography, Scanning Electron Microscopy (SEM) with Electron Back-Scattered Diffraction (EBSD), X-Ray Diffraction (XRD) and Electron Probe Micro-Analysis (EPMA) have also been given. Micro-hardness and tensile tests results are also reported in this chapter. A detailed study on FSW of Al and Ti is presented in chapter 4 of the thesis. The effect of process parameters on the evolution of microstructure and mechanical properties has been reported. A bottom-up approach on experimentally determining the “process window” is presented. The results emphasises on the distribution of titanium fragments and intermetallic particles in the nugget zone and their influence on mechanical properties of the weld. The microstructural evolution in the matrix is also detailed. The most noteworthy observation is substantial grain refinement in the nugget zone due to the presence of fine fragments of titanium and intermetallic. Cross-tensile tests of the samples welded under the optimised conditions fail in the retreating side of the aluminium material and has strength more than the parent material. The last section in this chapter deals with thermal stability of the microstructures. Chapter 5 deals with the use of Zn as interlayer between Al and Ti. The microstructural evolution and its effect on the mechanical properties have been examined. The investigations clearly show that FSW of Al and Ti with Zn interlayer has superior mechanical properties compared to Al-Ti welds without interlayer. The resulting microstructure has a better thermal stability. The use of Nb as interlayer has been studied in chapter 6. The microstructural investigation of the nugget zone reveals that Nb interlayer does not readily form solid solution with any of the base materials and Nb gets distributed more heterogeneously compared to Ti itself. This has led to a reduction in the strength of the weld, however, the ductility increases The thermal stability of the microstructure is poor compared to FSW of Al to Ti with Zn interlayer. In chapter 7, salient features of the different micro-mechanism operating during FSW of the investigated combinations has been discussed in detail. Finally, the outcome of the thesis has been summarized and scope for future investigation is outlined in chapter 8.

Des couches minces contenant des phases intermétalliques présent et des propriétés et de combinaisons de propriétés qui ne sont que partiellement explorées. Elles portent des solutions potentielles pour conférer des multifonctionnalités à des matériaux avancés requis par les secteurs industriels et sont source derupture et de l'innovation. Leur élaboration par dépôt chimique en phase vapeur à partir de précurseurs métallo-organiques (MOCVD) permet un dépôt conforme sur, et la fonctionnalisation de surfaces complexes, avec un temps de traitement court et à un coût modéré.Pour ceci, il est nécessaire de contrôler les réactions chimiques complexes et des mécanismes de transport impliqués. La modélisation informatique du procédé, alimentée avec des données obtenues par des expériences de dépôt ciblées, est un outil intégré pour l'étude et la compréhension des phénomènes qui se produisent à différentes échelles,de l’échelle macroscopique à celle nanométrique. La MOCVD de composés intermétalliques Al-Fe est étudiée en tant que paradigme de la mise en oeuvre d'une telle approche combinée, expérimentale et théorique. La phase approximanteAl13Fe4 est particulièrement ciblée,en raison de son intérêt comme alternative peu onéreuse aux catalyseurs à base de métaux noble dans l'industrie chimique. La mise au point du dépôt de la phase Al13Fe4est subordonnée à l'étude des proc /min à 185oC. La simulation du procédé prédit des vitesses de croissance en bon accord avec ces résultats, en particulier dans la gamme 139oC-227oC. La modélisation multi-échelle prédit la rugosité RMS avec précision, permettant ainsi le contrôle des propriétés telles que la résistivité électrique. La possibilité d'obtenir des films MOCVD de Fe à faible contamination en O et C est explorée dans la gamme 130oC-250oC à partir de fer pentacarbonyle, Fe(CO)5. La morphologie de la surface des films dépend fortement de la température de dépôt; elle devient plus lisse au-dessus de200oC, qui correspond aussi à la vitesse de croissance maximale, 60nm/min. La vitesse de dépôt diminue fortement lorsque la pression augmente. Les prédictions de la modélisation macroscopique reproduisent précisément ce comportement. Elles indiquent que la diminution de la vitesse de croissance à des températures et des pressions élevées est due à l’augmentation de la décomposition du précurseur en phase gazeuse et à l'inhibition de la réactivité de surface par le ligand CO. Le modèle multi échelle conduit à des valeurs RMS en bon accord avec les mesures expérimentales, en particulier à des températures plus élevées. Suite à l’étude des deux procédés, des co-dépôts d'Al-Fe effectués à 200oCrésultent en des films riches en Alavec une microstructure poreuse et rugueuse. Ceux-ci ne contiennent pas de phases intermétalliques et sont riches en oxygène dû à la réaction d'Al avec les ligandscarbonyles. Afin d’éliminer la contamination, des dépôts séquentiels d'Al et de Fe sont réalisés, ce dernier dans des conditions modifiées à 140oC, 40Torr et 10 min. Ces films sont exemptes d’hétéroélements et présentent un rapport atomique Al:Fe13:4. Diffraction des rayons X et microscopies électroniques révèlent qu’un recuit in situ à 575oC pendant 1 h conduit à des films à gradient de la composition sur l'épaisseur,composés de la phase approximantem-Al13Fe4 conjointement avec des phases intermétalliques Al-Fesecondaires. Il est ainsi démontré que des procédés MOCVD sont appropriés pour obtenir des films constitués d'alliages intermétalliques.Ces films multifonctionnels,appliqués de façon conforme sur des surfaces complexes sont utiles pour un grand nombre d'applications. / Films containing intermetallic compounds exhibit properties and combination of properties which are only partially explored. They carry potential solutions to confer multifunctionality to advanced materials required by industrial sectors and to become a source of breakthrough and innovation.Metalorganic chemical vapor deposition (MOCVD) potentially allows conformal deposition on, and functionalization of complex surfaces, with high throughput and moderate cost. For this reason, it is necessary to control the complex chemical reactions and the transport mechanisms involved in a MOCVD process. In this perspective, computational modeling of the process, fed with experimental information from targeted deposition experiments, provides an integrated tool for the investigation and the understanding of the phenomena occurring at different length scales, from the macro- to the nanoscale. The MOCVD of Al-Fe intermetallic compounds is investigated in the present thesis as a paradigm of implementation of such a combined, experimental and theoretical approach. Processing of the approximant phase Al13Fe4 is particularly targeted, due to its potential interest as low-cost and environmentally benign alternative to noble metal catalysts in the chemical industry. The attainment of the targeted Al13Fe4 intermetallic phase passes through the investigation of the MOCVD of unary Al and Fe films. The MOCVD of Al from dimethylethylamine alane (DMEAA) in the range 139oC-241oC results in pure films. Increase of the deposition temperature yields higher film density and decreased roughness. The Aldeposition rate increases to a maximum of 15.5 nm/min at 185oC and then decreases. Macroscopic simulations of the process predictdeposition rates in sufficient agreement with experimental measurements, especially in the range 139oC-227oC. At higher temperatures, competitive gas phase and surface phenomena cannot be captured by the applied model. Multiscale modeling of the process predicts the RMS roughness of the films accurately, thus allowing the control of properties such as electrical resistivity which depend on the microstructure. The MOCVD of Fe from iron pentacarbonyl, Fe(CO)5, is investigated in the range 130oC-250oC for the possibility toobtain fairly pure Fe films with low Oand C contamination. The surface morphology depends strongly on the temperature and changes are observed above 200oC. The Fe deposition rate increases up to 200oC, to a maximum of 60 nm/min, and then decreases. Moreover, the deposition rate decreases sharply with increasing pressure. Computational predictions capture accurately the experimental behavior and they reveal that the decrease athigher temperatures and pressures is attributed to the high gas phase decomposition rate of the precursor and to inhibition of the surface fromCO. The multiscale model calculates RMS roughness in good agreement with experimental data, especially at higher temperatures. Upon investigation of the two processes, aseries of Al-Fe co-depositions performed at 200oC results in Al-rich films with a loose microstructure. They contain no intermetallic phases and they are O-contaminated due to the reaction of the Al with the carbonyl ligands. Sequential deposition of Al and Fe followed by in situ annealing at 575oC for 1 h is applied to bypass the Ocontamination. The process conditions of Fe are modified to 140oC, 40 Torr and 10 min resulting in O-free films with Al:Fe atomic ratio close to the targeted 13:4 one. Characterization techniques including X-ray diffraction, TEM and

The present thesis deals with Nb-Si alloy composites in both bulk and multilayer forms. The work has been divided into two parts. First part (chapter 4-6) deals with Nb based silicides binary and ternary alloys with alloying additions like Ga and Al. These alloys are synthesized by vacuum arc melting and suction casting (non-equilibrium processing techniques). The studies on intermetallic coatings of Nb-Si alloys and Nb/Si multilayer synthesized by pulsed laser deposition technique have been presented in the second part (chapter7-8). Nb-Si alloys are one of the candidate materials for the advanced structural and microelectronic applications. There are few issues with these materials like poor oxidation resistance, low fracture toughness and brittleness which need to be solved. Microstructure plays a crucial role to control these properties. The main focus of this work is to understand the process of phase transformation and thereby control the microstructure in both bulk alloys and thin films. We have also investigated in a limited manner mechanical and environmental properties of bulk alloys. This thesis is subdivided into nine chapters. After a brief introduction in the first chapter, a brief overview on Nb-Si phase diagram and literature reviews on Nb-Si based alloys are presented with emphasis on the current work in the second chapter. Literature reviews on the phase formations sequence and stability in Nb-Si alloys thin films and Nb/Si multilayers are also discussed in the same chapter. In the third chapter different experimental techniques, processing parameters and characterization tools like XRD, SEM, TEM etc. are briefly discussed. Special emphasis is given on two non-equilibrium techniques: laser deposition technique to deposit the thin film/multilayer and vacuum suction casting to produce the 3 mm diameter rods of different Nb-Si alloys. The fourth chapter discusses the microstructural aspects of Nb-Si alloys prepared by suction casting and its mechanical behavior. The samples have the compositions hypoeutectic (Nb-10at.%Si and Nb-14at.%Si), eutectic (Nb-18.7at.%Si) and hypereutectic (Nb-22at.% Si and Nb-25at.% Si). SEM microstructural analyses of all the samples clearly show the enhancement in the volume fraction of eutectic and decease in the eutectic spacings in microstructure due to large undercooling. Rod eutectic is observed in most of places with irregular eutectic a few places in all samples. First check of phases has been done by XRD in all samples. Phase confirmation using TEM showed the eutectic between Nbss and Nb3Si phases in all samples. The primary phase for hypoeutectic alloys is Nbss (dendritic structure), Nb3Si phase for eutectic composition and β-Nb5Si3 phase for hypereutectic alloys. Compositional analysis using EDS and EPMA also supported the above results. No signature of eutectoid reaction (Nb3Si→Nb+α-Nb5Si3) is observed. Mechanical properties like hardness, strength, ductility and indentation fracture toughness have been determined for above mention alloy compositions. SEM micrographs showed that silicides fractured by cleavage and Nb phase in a ductile manner during the compression tests carried out at room temperature. We attempt to explain how the above mention mechanical properties change with alloy compositions and processing. Chapter five deals with the effect of Ga addition on the microstructure and mechanical properties of the Nb-Si alloy. The composition selected for this study is Nb-20.2at.%Si-2.7at.%Ga. The results of ternary alloy have been compared with the binary alloy composition Nb-18.7at.%Si. Phase analysis has been carried out using TEM and XRD. Ga addition has suppressed the formation of Nb3Si phase and promoted the formation of β-Nb5Si3 phase. Ga addition also established the eutectic between Nbss and β-Nb5Si3, which is a metastable eutectic. Ga added ternary alloy, on suction casting, yields ultrafine eutectic with nanometer length scale (50-100nm). From the compression tests, it is concluded that the combination of ultrafine eutectic (Nbss-β-Nb5Si3) and primary β-Nb5Si3 in ternary alloy results in a high compressive strength ~2.8±0.1 GPa with 4.3% plasticity. In contrast binary alloy under identical conditions shows the compressive strength ~1.35±0.1 GPa and 0.2% plasticity. Ga addition also enhances the indentation fracture toughness from 9.2±0.05 MPa√m (binary) to 24.11±0.5 MPa√m (ternary). Composite hardness values of the ternary and binary alloys are 1064±20 Hv and 1031±20 Hv respectively. Chapter six deals with Al added Nb-Si ternary alloy. Here we have discussed microstructural and mechanical properties like in chapter 5 along with oxidation behavior for the alloy composition Nb-12.7at.%Si-9at.%Al. SEM micrograph shows the presence of primary dendrites structure with ultra fine lamellar eutectic (50-100nm). Detailed TEM studies confirm the Nbss as primary phase present in form of dendrites. These dendrites contain the plate shape precipitates of δ-Nb11Si4 (body centered orthorhombic structure) phase in Nb matrix (primary dendrites). Eutectic phases are Nbss and β-Nb5Si3. The analysis of the results indicates that Al addition promote the formation of β-Nb5Si3 phase in the eutectic. The results of this ternary composition were also compared with the binary alloy composition Nb-18.7at.%Si. Compression tests have been carried out at room and elevated temperatures to measure the strength of the material. Al added ternary alloy yields the compressive strength value 1.6±0.01 GPa whereas binary alloy yields the compressive strength value 1.1±0.01 GPa. Enhancement in indentation fractured toughness is observed in Al added ternary alloy (20.4±0.5MPa√m) compare to binary alloy (9.2±0.05 MPa√m). Thermal analysis by TGA and DTA were used to see the oxidation behavior of Al added ternary alloy. Chapter seven deals with the deposition characteristics and the TEM studies on the laser deposited Nb-Si thin films. Films were deposited on the NaCl crystals and Si single crystal substrates. The compositions chosen in this case are Nb-25at.%Si, Nb-37.5at.%Si and Nb-66.7at.%Si. These compositions correspond to the equilibrium intermetallic compounds Nb3Si, Nb5Si3 and NbSi2 respectively. In this chapter we have briefly discussed the microstructural and phase evolutions in the intermetallic coatings. The smooth films quenched from the vapor and/or plasma state show amorphous structure. The sequence of crystallization was studied by hot stage TEM experiments as well as by cross sectional TEM in the films deposited at the elevated temperatures (600oC and 700oC) on Si substrates. During the hot stage experiment, crystallization is observed in Nb-25at.%Si film around 850oC with nucleation of metastable cubic Nb3Si phase. Occasionally metastable hexagonal Nb3Si3 phase has also been observed (close to Si substrate) along with cubic Nb3Si phase in the films at elevated temperatures. For Nb-37.5at.%Si film, crystallization is observed at 800oC with the nucleation of grains of metastable hexagonal Nb5Si3 phase. Cross-sectional TEM shows the presence of hexagonal Nb5Si3 phase along with few grains of NbSi3 (equilibrium) phase in the films deposited at elevated temperatures. Hot stage experiment of Nb-66.3at.%Si film showed the onset of crystallization much earlier at 400oC and complete crystallization at 600oC. This crystallization leads to the nucleation of grains of NbSi2 phase. Films of this composition deposited at elevated temperatures showed the presence of NbSi2 and metastable hexagonal Nb5Si3 phases (occasionally). The laser ablated films, besides the film matrix also contain the micron and submicron sized spherical droplets of different sizes. These droplets travel at very high velocities and impinge on the substrate resulting in a very high rate of heat transfer during solidification from liquid state. Therefore in this work we have also studied the microstructural evolution in the droplets for each composition. The phases observed in the droplets embedded in the matrix of Nb-25 at% Si alloy film are the bcc Nb and the cubic Nb3Si (metastable phase). The droplets in the matrix of Nb-37.5 at% Si alloy showed the bcc Nb and tetragonal β-Nb5Si3 phases. The phases observed in the droplets of in the Nb-66.3at.%Si alloy are the bcc Nb, tetragonal β-Nb5Si3 and the hexagonal NbSi2 (metastable phase). Chapter eight describes the synthesis and microstructural characterization using TEM of Nb/Si multilayers. The aim of this work is to check the stability and phase formation sequence in Nb/Si multilayer. Nb/Si multilayers were first annealed at different time intervals at 600oC and at different temperatures (for 2 hours) and then characterized by the cross-sectional transmission electron microscopy. As-deposited Nb layer is crystalline while Si layer is amorphous. Microstructural and compositional evidences suggest the intermixing between the Nb and Si layers at the interfaces. Nb/Si multilayer annealed at 600oC for 1 hour, NbSi2 was identified as the first crystalline nucleating phase. However amorphous silicide layers were also observed between Nb and NbSi2 layers. Metastable hexagonal Nb5Si3 was identified as the next crystalline phase that nucleated from the amorphous silicide layers at the interfaces of Nb and NbSi2 layers. Occasionally few grains of cubic Nb3Si phase were also observed after 8 hours of annealing at 600oC. In the chapter we have compared the results to the other reported works in Nb-Si bulk diffusion couples and also thin film couples. The final chapter summarizes the major conclusions of the present work and scope of future work.

Im Netz der Elektroenergieversorgung werden einzelne Netzkomponenten und Betriebsmittel durch Verbindungen elektrisch zusammengeschaltet. Dabei werden häufig Schraubenverbindungen mit Stromschienen eingesetzt. Diese müssen über mehrere Jahrzehnte zuverlässig hohe Ströme tragen können. Abhängig von der sich einstellenden Temperatur an den Verbindungen altern diese mit der Zeit. Die Alterung wird je nach Verbindungssystem von verschiedenen Mechanismen beeinflusst, die alle parallel ablaufen. Bei ruhenden, stationären elektrotechnischen Verbindungen, deren Kontaktpartner aus verschiedenen Materialien bestehen, können abhängig von der Paarung intermetallische Phasen (IMP) entstehen. Die sich bildenden IMP haben schlechtere elektrische und mechanische Eigenschaften als die reinen Metalle. Daraus resultiert ein höherer Verbindungswiderstand. Die erzeugte Verlustleistung sowie die Temperatur der Verbindung steigen an. Dies kann zum Ausfall der Verbindung führen. In der Elektroenergietechnik werden aufgrund ihrer guten elektrischen Leitfähigkeit häufig die Werkstoffe Aluminium und Kupfer sowie das Beschichtungsmetall Silber bei Temperaturen von üblicherweise 90 °C bis 200 °C eingesetzt. Speziell bei Aluminium-Kupfer-Verbindungen, die nicht langzeitstabil sind, wird als maßgebliche Ausfallursache das Bilden von IMP gesehen. Die IMP des Systems Al-Cu wurden in der Vergangenheit bereits vielfach untersucht. Das Übertragen der Ergebnisse auf die Problematik stromtragender Verbindungen der Elektroenergietechnik ist jedoch nicht ohne Weiteres möglich. Der relevante niedrige Temperaturbereich zwischen 90 °C und 200 °C spielt bei vielen Untersuchungen nur eine untergeordnete Rolle. Zusätzlich können die Eigenschaften der IMP bei unterschiedlichen Herstellungsverfahren voneinander abweichen. Zum System Al-Ag ist in der Literatur nur wenig bekannt. Deshalb wurden für diese Arbeit phasenreine IMP der Systeme Al-Cu und Al-Ag mit unterschiedlichen Herstellungsverfahren bei möglichst identischen Randbedingungen hergestellt. Diese wurden mit einer speziell für diese Proben entwickelten Messeinrichtung elektrisch charakterisiert und der ermittelte spezifische elektrische Widerstand der IMP und ihr Temperaturbeiwert mit Werten aus der Literatur verglichen. An verschiedenen Schraubenverbindungen mit Stromschienen aus Aluminium und Kupfer wurden Langzeitversuche von bis zu 3 Jahren durchgeführt. Der Verbindungswiderstand wurde abhängig von der Zeit ermittelt. An ausgewählten Verbindungen wurde zusätzlich in zwei identischen Versuchen der Einfluss der Belastung mit Dauer- und Wechsellast auf das Langzeitverhalten untersucht. Mithilfe der an den IMP ermittelten elektrischen Eigenschaf-ten wurde deren Einfluss auf den Verbindungswiderstand berechnet. Die Ergebnisse dieser Modellrechnung wurden mit den Ergebnissen aus den Langzeitversuchen verglichen. Ausgewählte Verbindungen wurden dazu mikroskopisch untersucht. Es wurde festgestellt, dass die IMP nicht ausschließlich das Langzeitverhalten stromtragender Verbindungen bestimmen. Es muss mindestens ein weiterer Alterungsmechanismus einen signifikanten Einfluss haben. Die Untersuchungen deuten darauf hin, dass dabei Sauerstoff eine zentrale Rolle spielen könnte. / In electrical power supply networks a huge number of electrical joints are used to connect transmission lines, conductors, switchgears and other components. During operation these joints are aging due to different aging mechanisms. Depending on the type of the joint several aging mechanisms can take place at the same time. For stationary joints with contact partners made of different materials, the formation of intermetallic compounds (IMC) may be an issue. These IMC have worse electrical and mechanical properties compared to the pure metals. Therefore, the presence of IMC in the contact area results in a higher joint re-sistance and the temperature and the thermal power losses increase. Typical temperatures for high current joints are between 90 °C and 200 °C. Due to their good electrical conductivity aluminum and copper are often used as conductor materials and silver as a coating material. Especially bimetal joints made of aluminum and copper are not long term stable. The formation of Al-Cu IMC is held responsible as a cause of failure. The IMC of the System Al-Cu have already been studied by several authors. However, it is difficult to apply the results directly to electrical joints in power supply networks. In many studies the low temperature range between 90 °C and 200 °C is not regarded. In addition, the properties of the IMC may vary due to different preparation processes. There is only little information about the system Al-Ag in the literature. For this work, phase pure IMC of the systems Al-Cu and Al-Ag were prepared by different preparation processes using similar process parameters. These IMC samples were electrically characterized with a specially developed measuring device. The specific electric resistivity and the temperature coefficient of resistance were determined and compared to values taken from the literature. Various combinations of bus bar joints made of aluminum and copper were investigated in long term tests for up to three years. The joint resistance was determined as a function of time. In addition, for selected joints two identic setups were operated with continuous load and alternating load. The long term behavior was investigated with regard to the load ap-plied. Using the results of the electrical characterization of the IMC their influence on the joint resistance was calculated theoretically. The results of the calculation were compared to the results determined in the long term tests. Selected joints were examined microscopi-cally after termination of the long term tests. It was found, that the long term behavior of bimetal electrical joints with the combination Al-Cu and Al-Ag cannot be exclusively described by the growth of IMC. At least there is one further aging mechanism involved. The studies suggest, that oxygen may have a significant influence.

<p>  </p> <p>Magnetism plays a vital role in the technologies of today, and materials used for magnetic applications largely consist of solid state phases. Intermetallic chalcogenides are one such material which have exhibited a full range of properties useful for a variety of applications requiring soft magnets, superconductors, magnetocalorics, and even rarer magnetic phenomenon such as 1D Heisenburg magnetic chains. Solid state chemists continue to develop new synthesis methods for chalcogenides as they produce both new phases and modifications of existing phases, usually with the express intent of improving their physical and chemical properties. Low dimensional chalcogenides often have predictable structure-property relationships which when understood aids in these efforts of optimizing existing materials.</p> <p>In this work, we have synthesized novel, low-dimensional Tl1-xAxFe3Te3 (A = K, Na)-based magnetocalorics for magnetic refrigeration technologies utilizing a variety of synthetic methods. Doping of alkali metals into the thallium site simultaneously reduces the toxicity and cost of the material, and also modifies their crystal structures leading to changes in their magnetic properties including ordering temperature, magnetic anisotropy, magnetic hysteresis, coercivity, and magnetic entropies. Most notably, the magnetic ordering temperature has been boosted from 220 K of the prior known TlFe3Te3 phase up to 233 K in the new Tl0.68Na0.32Fe2.76Te3.32 phase, further towards room temperature which is required for the commercialization of magnetic refrigerants for home appliances. There exist strong magnetostructural correlations for most of the alterations in the magnetic properties, and relationships have been modelled where trends exist to match the magnetism to the changes in the unit cell of the structure.</p> <p>New synthetic methods were also developed for the ternary TBi4S7 (T = transition metal) phase which exhibits a pseudo-1D structure of Heisenberg antiferromagnetic chains. These synthetic techniques resulted in more consistent high purity of phases than methods reported previously in literature. Attempts at synthesizing new phases were made, and crystallographic and composition analysis methods suggested the synthesis of a new Mn1-xCoxBi4S7 phase, though magnetic impurities prevented characterization of this new material’s magnetic properties. </p>

Les structures magnétiques de deux familles de composés sont déterminées et discutées : (i) celles des hydrures RTXH (R = terre rare, T = métal de transition et X = Si, Ge) cristallisant dans la structure de type ZrCuSiAs et obtenus par insertion d’hydrogène dans les intermétalliques quadratiques de type CeFeSi. L’hydrogénation induit des transitions magnétiques variées engendrées par la compétition entre la dilatation anisotrope de la maille cristalline causée par l’absorption d’hydrogène et l’apparition de la liaison chimique R-H; (ii) celles des siliciures ternaires R6T1.67Si3 (R = Ce, Nd, Gd, Tb et T = Co, Ni) présentant des propriétés magnétocaloriques significatives autour de leur température de Curie, en particulier ceux à base de gadolinium Gd6T1.67Si3. Les composés à base de Ce, Nd et Tb présentent des comportements magnétiques originaux qui sont expliqués par la détermination de leurs structures magnétiques. La nature complexe de ces propriétés résulte en partie de la présence de deux sites magnétiques pour R et d’un désordre atomique de l’élément T dans la structure cristallographique. / Magnetic structures of two families of compounds are determined and discussed : (i) those of the hydrides RTXH (R = Rare earth, T = transition metal and X = Si, Ge) crystallizing in the ZrCuSiAs-type structure and obtained by hydrogen insertion in the intermetallics adopting the tetragonal CeFeSi-type structure. Hydrogenation induces various magnetic transitions governed by the competition between the anisotropic unit cell expansion linked to hydrogen absorption and the occurrence of the R-H chemical bonding ; (ii) those of the ternary silicides R6T1.67Si3 (R = Ce, Nd, Gd, Tb and T = Co, Ni) which show significant magnetocaloric properties around their Curie temperature, especially those based on gadolinium Gd6T1.67Si3. The compounds based on Ce, Nd and Tb present original magnetic behaviors which are explained by their magnetic structures determination. The origin of these complex properties results especially from the presence of two magnetic sites for R and from an atomic disorder of the T element in the crystallographic structure.

Les travaux présentés dans ce manuscrit portent sur la synthèse et la caractérisation de nouveaux matériaux magnétocaloriques à basse de terres rares pour la réfrigération magnétique. Le premier chapitre constitue une introduction aux notions d’effet magnétocalorique et de réfrigération magnétique et dresse un état de l’art des matériaux magnétocaloriques existants. Dans le but d’obtenir des matériaux à forte capacité de réfrigération (RC) et d’identifier des stratégies d’amélioration de ce critère de performance, deux voies de recherche ont été explorées : l’élargissement de la transition magnétique et l’effet de l’élément de transition M et de l’élément p (X) dans les verres métalliques Gd60M30X10 (M = Mn, Fe, Co, Ni, Cu et X = Al, Ga, In) d’une part, et la synthèse de nouveaux siliciures ternaires dans les systèmes R-M-Si (R = Nd, Gd, Tb et M = Co, Ni) à fort potentiel magnétocalorique, d’autre part.Le second chapitre de cette thèse présente les propriétés magnétiques des rubans amorphes à base de gadolinium synthétisés par la technique de melt-spinning, dans lesquels le désordre structural induit un très fort élargissement de la transition magnétique (vis-à-vis de celle du gadolinium par exemple). Il montre dans un premier temps, la faible influence de l’élément p (X) sur les propriétés magnétiques des rubans Gd60Mn30X10 (X = Al, Ga, In). Une seconde partie présente la très forte influence de l’élément de transition M, tant sur la nature de la transition magnétique que sur les propriétés magnétocaloriques des verres métalliques Gd60M30In10 (M = Mn, Fe, Co, Ni, Cu), avec en particulier une température de Curie variant entre 86 (M = Ni) et 220 K (M = Fe) et l’existence d’un phénomène de type cluster-glass en dessous de 35 K lorsque M = Mn. Le chapitre trois de cette thèse se décline en trois parties. La première décrit les conditions de synthèse parfois délicates, notamment dans le choix des températures de recuit, des siliciures R5MSi2, Gd5Si3 et du composé à domaine d’existence Gd3Co2,5 ± xSi1,5 ± y. L’utilisation de la méthode Rietveld pour l’affinement des diffractogrammes de rayons X sur poudre et monocristaux et neutrons a permis de montrer que les composés R5MSi2 adoptent une structure de type Cr5B3 avec la particularité de l’occupation mixte du site 8h par Co et Si à 50 %/50 % et que Gd3Co2,5 ± xSi1,5 ± y adopte une structure de type Er3Ge4 avec des sites mixtes Co/Si en positions 4a et 4c. La seconde partie présente les propriétés magnétiques et magnétocaloriques du siliciure Gd5CoSi2. Ce composé subit une transition ferromagnétique à la température de Curie de 169 K qui s’accompagne d’une variation d’entropie magnétique calculée par l’application de la relation de Maxwell, de -4,7 et 8,7 J kg-1 K-1 pour des variations de champ magnétique respectives de 2 et 5 T. Le troisième volet de ce chapitre décrit les propriétés magnétiques de Nd5CoSi2 et Nd5NiSi2 qui présentent une transition ferromagnétique respectivement à 55 et 44 K. Il décrit également l’affinement de la structure ferromagnétique cantée de Nd5CoSi2 obtenue par des mesures de diffraction neutronique.Il ressort de ces travaux que l’évaluation des matériaux magnétocaloriques par le seul critère de capacité de réfrigération ne mène pas vers les matériaux les plus adaptés à l’application. Il faudrait cibler plus spécifiquement, pour chaque type de cycle de réfrigération envisagé, des critères pragmatiques tels qu’une fenêtre de température d’utilisation autour de la température de Curie ou une valeur de chaleur spécifique optimale afin de mieux guider la recherche de nouveaux matériaux magnétocaloriques. / The studies presented in this manuscript deal with the synthesis and characterization of new rare-earth based magnetocaloric materials for magnetic refrigeration applications. The first chapter is an introduction to the concepts of magnetocaloric effect and magnetic refrigeration and establishes a review of the magnetocaloric materials existing today. Two research axes were explored in order to obtain materials with a high refrigeration capacity (RC) and to identify strategies for improving this performance criterion: the enlargement of magnetic transition and the effect of transition element M and p-element X in the metallic glasses Gd60M30X10 (M = Mn, Fe, Co, Ni, Cu et X = Al, Ga, In) on one hand, and the synthesis of new ternary silicides in the RE-M-Si systems (RE = Nd, Gd, Tb et M = Co, Ni) with high magnetocaloric potential on the other hand. The second chapter of this thesis presents the magnetic properties of Gd-based amorphous ribbons synthesized by the melt-spinning technique, in which the structural disorder induces a very strong enlargement of the magnetic transition (compared to that of pure Gd for instance). In a first part, it shows the weak influence of the p element (X) on the magnetic properties of Gd60Mn30X10 (X = Al, Ga, In) ribbons. A second part presents the very strong influence of the transition element M, either on the nature of the magnetic transition and on the magnetocaloric properties of Gd60M30In10 (M = Mn, Fe, Co, Ni, Cu) metallic glasses with, in particular, a Curie temperature varying between 86 (M = Ni) and 220 K (M = Fe) and the occurrence of a cluster-glass behavior below 35 K when M = Mn. The third chapter of this thesis is composed of three parts. The first one describes the synthesis conditions of RE5MSi2 (RE = Nd, Gd, Tb), Gd5Si3 and of the compound with existence domain Gd3Co2.5 ± xSi1.5 ± y. These syntheses are sometimes delicate, particularly in the choice of annealing temperatures. The use of the Rietveld method to refine the X-ray and neutron powder diffraction patterns allowed showing that RE5MSi2 compounds adopt a Cr5B3 type structure, with a mixed occupation of 8h site by Co and Si at 50 %/50 % and that Gd3Co2.5 ± xSi1.5 ± y adopts an Er4Ge4 type structure with mixed Co/Si occupation in 4a et 4c positions. The second part presents the magnetic and magnetocaloric properties of the Gd5CoSi2 silicide. This compound exhibits a ferromagnetic transition at the Curie temperature TC = 169 K that is accompanied by a magnetic entropy change of -4.7 and 8.7 kg-1 K-1 at 2 and 5 T, respectively, as calculated by the application of Maxwell’s relationship. The third part is this chapter describes the magnetic properties of Nd5CoSi2 and Nd5NiSi2 which order ferromagnetically at 55 and 44 K, respectively. It also presents the refinement of the canted ferromagnetic structure on Nd5CoSi2, obtained by neutron diffraction measurements.These study show that evaluating the magnetocaloric materials by only considering the criterion of refrigeration capacity does not lead to the elaboration of the best materials for the applications. It could be more efficient to target more pragmatic criteria, for each considered refrigeration cycle, such as a temperature window of use around the Curie temperature or an optimal specific heat value in order to lead the research of new magnetocaloric materials at best.

De nos jours, la durée de vie des modules d’électronique de puissance est désormais limitée par les technologies standards de conditionnement, telles que le câblage par fils et le brasage. Ainsi une optimisation des technologies actuellement employées n’est pas suffisante pour satisfaire les futures exigences de fiabilité. Pour dépasser ces limites, un nouveau module de puissance remplaçant les fils de connexion par des clips en cuivre a été développé. Ce design innovant vise à améliorer la fiabilité du module puisqu’il empêche la dégradation des fils de connexion, constituant bien souvent la principale source de défaillance. La contrepartie de ce gain de fiabilité réside dans la complexification de la structure interne du module. En effet, l’emploi d’un clip en cuivre nécessite une brasure supplémentaire fixant le clip à la puce. Ainsi, le comportement thermomécanique et les différents modes de rupture auxquels le composant est soumis lors de son utilisation doivent être caractérisés. Cette étude utilise la simulation numérique pour analyser avec précision le comportement de chaque couche de matériaux lors des cycles actifs de puissance. De plus, une étude de sensibilité à la fois expérimentale et numérique concernant les paramètres de tests est réalisée. Les zones critiques du module ainsi que les combinaisons critiques des paramètres de tests pour les différents modes de rupture sont mis en évidence. Par ailleurs, une analyse en mécanique de la rupture est conduite et la propagation des fissures à différentes zones clés est analysée en fonction des différents paramètres de tests. Les résultats obtenus permettent la définition de modèles de prédiction de durée de vie. / Today a point has been reached where safe operation areas and lifetimes of power modules are limited by the standard packaging technologies, such as wire bonding and soft soldering. As a result, further optimization of used technologies will no longer be sufficient to meet future reliability requirements. To surpass these limits, a new power module was designed using Cu clips as interconnects instead of Al wire bonds. This new design should improve the reliability of the module as it avoids wire bond fatigue failures, often the root cause of device failures. The counterpart for an improved reliability is a quite complicated internal structure. Indeed, the use of a Cu clip implies an additional solder layer in order to fix the clip to the die. The thermo-mechanical behavior and failure mechanisms of such a package under application have to be characterized. The present study takes advantage of numerical simulations to precisely analyze the behavior of each material layer under power cycling. Furthermore an experimental and numerical sensitivity study on tests parameters is conducted. Critical regions of the module are pointed out and critical combinations of tests parameters for different failure mechanisms are highlighted. Then a fracture mechanics analysis is performed and the crack growth at different locations is analyzed in function of different tests parameters. Results obtained enable the definition of lifetime prediction models.

Die vorliegende Arbeit beschäftigt sich mit der Herstellung von CrNi-Stahl/Aluminium-Mischverbunden mit dem Ziel der Verbesserung der mechanischen Eigenschaften und der Erhöhung der Lebensdauer der Lötverbindungen. Da sich die Eigenschaften der Fügepartner stark unterscheiden, ist ein geeignetes Fügeverfahren erforderlich. Die wesentliche Herausforderung besteht in der Vermeidung der Bildung von dicken intermetallischen Schichten in der Reaktionszone, welche die mechanischen Eigenschaften der resultierenden Lötverbindungen verschlechtern. Dementsprechend wird ausgehend vom Stand der Technik ein Konzept zur vollständigen Untersuchung der Mikrostruktur-Eigenschafts-Beziehungen der Mischverbunde erarbeitet und umgesetzt. Die Mischverbunde werden durch Induktionslöten hergestellt, was einen lokalen Wärmeeintrag in die Fügestelle ermöglicht. Dadurch können Lötverbindungen mit dünnen Reaktionszonen erzeugt werden. Das Potenzial der Lötverbindungen wird anhand von Zugscher- und Ermüdungsversuchen aufgezeigt, die mit den Ergebnissen der Mikrostrukturanalyse und der fraktografischen Bewertung korreliert werden.:Abkürzungen und Formelzeichen iii Abkürzungen iii Formelzeichen iv Abbildungsverzeichnis v Tabellenverzeichnis xi 1 Einleitung und Motivation 1 2 Stand der Technik 2 2.1 Anwendungen von Stahl/Aluminium-Mischverbunden 2 2.2 Stoffschlüssiges Fügen von Stahl/Aluminium-Mischverbunden 3 2.2.1 Schweißen von Stahl/Aluminium-Mischverbunden 3 2.2.2 Löten von Stahl/Aluminium-Mischverbunden 8 2.2.2.1 Grundlagen 8 2.2.2.2 Verfahren und Lotwerkstoffe 10 2.3 Besonderheiten bei Stahl/Aluminium-Mischverbunden 15 2.3.1 Intermetallische Fe-Al-Verbindungen in der Reaktionszone 15 2.3.2 Kontrolle der Bildung und des Wachstums der Reaktionszone 20 2.3.3 Problematik bei mechanischer Charakterisierung der Mischverbunde 23 3 Folgerungen und Zielstellung 27 4 Experimentelle Durchführung 29 4.1 Grund- und Lotwerkstoffe 29 4.2 Benetzungs- und Lötversuche 31 4.3 Mikrostrukturelle Charakterisierung 34 4.3.1 Mikrostrukturanalyse 34 4.3.2 Mikrohärtemessung und Nanoindentation 35 4.3.3 Thermische Auslagerung 37 4.4 Mechanische Charakterisierung 39 4.4.1 Zugscher- und Warmzugscherversuche 39 4.4.2 Ermüdungsversuche 40 5 Ergebnisse 43 5.1 Benetzungsverhalten 43 5.2 Mikrostrukturelle Untersuchungen 45 5.2.1 Mikrostruktur der Lötverbindungen und Bildung der Reaktionszone 45 5.2.2 Mechanische Charakteristika der Gefügebestandteile 55 5.3 Festigkeitsuntersuchungen 59 5.3.1 Quasistatische Untersuchungen gelöteter Mischverbunde 59 5.3.2 Ermüdungsverhalten gelöteter Mischverbunde 69 5.4 Korrelation zwischen Reaktionszonendicke und Festigkeit 81 6 Diskussion der Ergebnisse 88 7 Zusammenfassung und Ausblick 92 8 Anlagen 93 8.1 Übersicht der Benetzungsproben auf Stahl X5CrNi18-10 93 8.2 Übersicht der Benetzungsproben auf AlMn1Cu 94 8.3 Beispiel der Ergebnisse der EDX-Analyse 95 8.4 Härteverlauf über die Reaktionszone 96 8.5 EBSD-Analyse der Lötverbindung 97 8.6 Mikrozugversuche 98 8.7 TEM-Untersuchungen der hergestellten Lötverbindungen 99 9 Literaturverzeichnis 102 10 Normenverzeichnis 112 11 Publikationen 113 / This thesis deals with the production of aluminum/stainless steel mixed joints in order to improve the mechanical properties and to extend the lifetime of the joints. Because of the different physical properties of the joining partners, a suitable joining technique is necessary. In comparison to welding, brazing offers significant advantages due to the lower liquidus temperature of the used brazing fillers. The main challenge is to prevent the formation of thick intermetallic layers in the reaction zone. These layers deteriorate the mechanical properties of the resulting joints predominantly. Correspondingly, a concept for the complete investigation of the microstructure-property relationships of the brazed joints is investigated. The joints are produced by induction brazing, which takes place in a short process time and allows a local heat input into the joint. Therefore, joints with a thin intermetallic layer in the reaction zone can be manufactured. The potential of the joints is demonstrated using monotonic tensile shear tests as well as fatigue tests. The achieved results are correlated with the results of the microstructural and fractographic analysis.:Abkürzungen und Formelzeichen iii Abkürzungen iii Formelzeichen iv Abbildungsverzeichnis v Tabellenverzeichnis xi 1 Einleitung und Motivation 1 2 Stand der Technik 2 2.1 Anwendungen von Stahl/Aluminium-Mischverbunden 2 2.2 Stoffschlüssiges Fügen von Stahl/Aluminium-Mischverbunden 3 2.2.1 Schweißen von Stahl/Aluminium-Mischverbunden 3 2.2.2 Löten von Stahl/Aluminium-Mischverbunden 8 2.2.2.1 Grundlagen 8 2.2.2.2 Verfahren und Lotwerkstoffe 10 2.3 Besonderheiten bei Stahl/Aluminium-Mischverbunden 15 2.3.1 Intermetallische Fe-Al-Verbindungen in der Reaktionszone 15 2.3.2 Kontrolle der Bildung und des Wachstums der Reaktionszone 20 2.3.3 Problematik bei mechanischer Charakterisierung der Mischverbunde 23 3 Folgerungen und Zielstellung 27 4 Experimentelle Durchführung 29 4.1 Grund- und Lotwerkstoffe 29 4.2 Benetzungs- und Lötversuche 31 4.3 Mikrostrukturelle Charakterisierung 34 4.3.1 Mikrostrukturanalyse 34 4.3.2 Mikrohärtemessung und Nanoindentation 35 4.3.3 Thermische Auslagerung 37 4.4 Mechanische Charakterisierung 39 4.4.1 Zugscher- und Warmzugscherversuche 39 4.4.2 Ermüdungsversuche 40 5 Ergebnisse 43 5.1 Benetzungsverhalten 43 5.2 Mikrostrukturelle Untersuchungen 45 5.2.1 Mikrostruktur der Lötverbindungen und Bildung der Reaktionszone 45 5.2.2 Mechanische Charakteristika der Gefügebestandteile 55 5.3 Festigkeitsuntersuchungen 59 5.3.1 Quasistatische Untersuchungen gelöteter Mischverbunde 59 5.3.2 Ermüdungsverhalten gelöteter Mischverbunde 69 5.4 Korrelation zwischen Reaktionszonendicke und Festigkeit 81 6 Diskussion der Ergebnisse 88 7 Zusammenfassung und Ausblick 92 8 Anlagen 93 8.1 Übersicht der Benetzungsproben auf Stahl X5CrNi18-10 93 8.2 Übersicht der Benetzungsproben auf AlMn1Cu 94 8.3 Beispiel der Ergebnisse der EDX-Analyse 95 8.4 Härteverlauf über die Reaktionszone 96 8.5 EBSD-Analyse der Lötverbindung 97 8.6 Mikrozugversuche 98 8.7 TEM-Untersuchungen der hergestellten Lötverbindungen 99 9 Literaturverzeichnis 102 10 Normenverzeichnis 112 11 Publikationen 113