Les cellules photovoltaïques en couches minces de silicium cristallin sont des candidates prometteuses pour réduire le prix du watt-crête de l'énergie photovoltaïque, grâce à une très faible utilisation de silicium de haute pureté. Dans notre cas, les couches actives de silicium sont supportées par des substrats, de bas coût et compatibles avec les conditions de haute température nécessaires à une croissance cristalline rapide et de bonne qualité des couches. La société S’TILE développe ces substrats, par frittage à partir de poudres de silicium, et en recristallisant les plaquettes ainsi obtenues. Le but de cette thèse est de valoriser ce substrat pour l’industrie photovoltaïque et de démontrer qu’il est adapté à la fabrication de cellules solaires à bas coût et rendement élevé. Ces travaux utilisent le procédé d’épitaxie de silicium, qui est central pour fabriquer des cellules minces. Ils s’articulent autour de deux axes principaux. Le premier est la fabrication de cellules solaires et leur optimisation sur des substrats de référence monocristallins. Dans ce cadre, de nombreuses voies ont été explorées : l’utilisation de réflecteurs de Bragg en silicium poreux, l’optimisation du dopage de l’émetteur, la formation de gradients de dopage dans la base et l’utilisation de structures à émetteur en face arrière. Ces études ont permis d’évaluer le potentiel de ces différentes voies ; des résultats prometteurs pour l’amélioration du rendement de conversion des cellules sur couches minces ont été obtenus. Le second axe de la thèse est la fabrication de cellules sur les substrats frittés préparés par S’TILE et l’application des moyens développés dans le cadre du premier axe pour améliorer ces cellules. Les rendements encoura-geants obtenus ont ainsi démontré la faisabilité de cellules solaires sur les substrats réalisés par le procédé de frittage à bas coût développé par la société S’TILE. / Crystalline silicon thin-film solar cells are promising candidates to reduce the watt-peak prices of photovoltaic energy, thanks to a much smaller use of high purity silicon. In our case, the active layers of silicon are supported by substrates. These substrates have low production costs and are compatible with the high temperature process steps, which are necessary to a rapid and high-quality crystalline growth. The company S’TILE develops these substrates, by sintering silicon powders and recrystallizing the obtained wafers. The objective of this PhD thesis is to pinpoint the relevance of this substrate for the photovoltaics industry and demonstrate that it is adapted to the fabrication of solar cells with low cost and high efficiency. This work uses the epitaxy process, which is central to fabricate these thin-film cells. It is organized in two main axes. The first one is the fabrication of solar cells and their optimization on monocrystalline reference substrates. Several optimization pathways have been tested: the use of porous silicon Bragg reflectors, the optimization of emitter doping, the base variable doping and the use of rear emitter structures. The studies permitted to unveil the potential of each pathway; promising results were obtained for the improvement of thin-film solar cell conversion efficiency. The second axis of the thesis is the cell fabrication on the substrates prepared by S’TILE and the application of the means developed in the first axis to improve these cells. Encouraging efficiencies have demonstrated the feasibility of solar cells on the substrates made by the low-cost process developed by S’TILE.

Dans l’optique de refroidir les parois des chambres de combustion aéronautiques le plus efficacement possible, un intérêt particulier est aujourd’hui porté à la technologie de refroidissement par transpiration. L’air de refroidissement s’écoule au travers d’une paroi poreuse dans laquelle une grande quantité de chaleur est échangée par convection. L’éjection de l’air profite ensuite de la distribution des pores pour former une couche limite protectrice relativement homogène.Les matériaux métalliques obtenus à partir de poudres partiellement frittées sont de bons candidats pour former ces parois poreuses. Ce travail se focalise sur les échanges internes et consiste à développer une méthodologie permettant de dégager les architectures partiellement frittées les plus adaptées à ce type d’application.L’écoulement et les échanges de chaleur lors du refroidissement par transpiration sont régis par quelques propriétés effectives des matériaux qui sont fonction de l’architecture : la conductivité thermique effective, le coefficient de transfert convectif volumique et les propriétés de perméabilité. A l’aide de travaux expérimentaux ou d’études numériques sur des échantillons numérisés par tomographie aux rayons X, des relations simples entre les propriétés effectives des matériaux partiellement frittés et leurs paramètres architecturaux sont tout d’abord développées. La porosité, la surface spécifique et le type de poudre utilisé sont retenus pour prédire les paramètres effectifs.Ces relations sont finalement intégrées dans un modèle de transfert de chaleur prédisant la performance d’une solution dans les conditions de fonctionnement du moteur. Une optimisation "multi-objectifs" et une analyse des designs optimaux permettent alors de mettre en valeur quelques architectures montrant un fort potentiel pour des applications de refroidissement par transpiration. Des matériaux peu poreux formés à partir de larges poudres irrégulières semblent assurer le meilleur compromis entre tous les critères pris en compte. / In order to cool aero-engine combustion chambers as efficiently as possible, there is today a special interest given to transpiration cooling technology. The cooling air flows through a porous liner in which a large amount of heat can be exchanged by convection. The air injection could then take benefit of the pore distribution to form a more homogeneous protective boundary layer.Partially sintered metallic materials are potential candidates to form these porous liners. The present work focuses on internal heat transfers. It aims to develop a methodology capable of highlighting the most adapted partially sintered architectures to this kind of application.During transpiration cooling, flows and heat transfers are governed by some effective material properties which depends on the porous architecture: the effective solid phase thermal conductivity, the volumetric heat transfer coefficient and the permeability properties. Thanks to experimental works and numerical studies on samples digitized by X-ray tomography, simple relationships are first developed between the effective material properties of partially sintered materials and their architectural parameters. The porosity, the specific surface area and the powder type are selected to predict the effective properties.These relationships are finally integrated into a heat transfer model predicting the thermal performance of a design at working engine conditions. A multi-objective optimization and an analysis of the optimal designs highlight some architectures as being potentially interesting for transpiration cooling. Materials with a low porosity and made of large irregular powders seem to ensure the best trade-off among the different criteria taken into consideration.

Made available in DSpace on 2014-10-09T12:51:30Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:02:10Z (GMT). No. of bitstreams: 1 11328.pdf: 13067508 bytes, checksum: 886d399df69a8146f0353092e99aa16a (MD5) / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:51:30Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:02:10Z (GMT). No. of bitstreams: 1 11328.pdf: 13067508 bytes, checksum: 886d399df69a8146f0353092e99aa16a (MD5) / O objetivo deste trabalho foi desenvolver potenciais materiais para aplicações automotivas, em particular como insertos para assento de válvula em motores de combustão interna à gasolina. O desenvolvimento compreendeu as etapas de processamento dos materiais via metalurgia do pó, tratamento térmico, caracterização microestrutural e mecânica, usinagem e desgaste dos mesmos. O desenvolvimento objetivou principalmente a redução de custo e a aplicação de elementos menos poluentes, excluindo-se, por exemplo, a aplicação do Co devido ao seu alto custo e do Pb devido aos seus efeitos toxicológicos e prejudiciais ao meio ambiente. A realização de uma pesquisa minuciosa na busca de patentes relacionadas indicou que os materiais estudados apresentam composições particulares, e, portanto inéditas. Os resultados encontrados nos ensaios de dureza e de resistência à compressão radial nas amostras tratadas termicamente apresentaram resultados superiores aos da liga comercial atualmente em uso. Testes de usinabilidade em amostras sem tratamento térmico apresentaram comportamento similar ao da liga comercial, indicando que a usinabilidade do material praticamente não foi afetada com a mudança na composição química. Após a etapa de tratamento térmico, as ligas obtidas apresentaram valores de esforços de corte superiores aos do material comercial. Os resultados dos testes de desgaste abrasivo em amostras tratadas termicamente apresentaram menores valores de coeficientes de atrito e perda de massa em todos os casos em relação ao material comercial. Este comportamento é devido aos benefícios introduzidos pelo tratamento térmico executado e pela adição de alguns elementos resistentes ao desgaste na forma de carbonetos de Nb e Ti/W. Os resultados dos esforços de corte apresentaram boa concordância com os resultados dos ensaios de desgaste. Os materiais produzidos apresentam-se como potenciais candidatos para substituir, com vantagens, inserto para assento de válvula a base de Fe-Co em motores de combustão interna a gasolina. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

The pores in pressed and sintered components constitute weak points in the material since the stress concentration is larger than the nominal stress there. Therefore, fatigue cracks initiate at the pores. Specifically, it can be assumed that the fatigue cracks initiate at the largest pore in the stressed volume. Studies have previously looked at finding ways to model the fatigue strength of the material based on the largest pore. This thesis looks at a model previously derived for hardened pressed and sintered materials that is based on linear elastic fracture mechanics and investigates if said model can be modified to include non-hardened pressed and sintered materials that do not necessarily behave linear elastically. A model describing the influence of the size of the largest pore on the fatigue limit using empirical coefficients is suggested. Furthermore, the area of the largest pore is modelled using extreme value statistics. The model proved successful in modelling the density effect of the porosity on the fatigue strength for two materials with different microstructures: one with a homogeneous microstructure and one with a heterogeneous microstructure. For the material with the homogenoeous microstructure the model also accounted well for the notch effect when tested on samples with a different geometry. However, for the heterogeneous material the model did not account for the notch effect. Deformation hardening due to local plastic deformation in the softer phases was suggested as a possible explanation and was supported by tensile tests.

The diploma work deals with comparison of sintered carbides in product assortment of Pramet Tools a.s., Sandvik-Coromant and Ceratizit companies in term of their cutting speed, feed and depth of cut for the turning operation. The comparison of cutting parameters is made for P, M, K, N, S and H groups. For better comparison there are always two sintered carbides from each of the manufacturers mentioned for each of the groups P, M, K. They are mentioned for groups N, S and H only if they are suggested by the manufacturer.

Gesintertes Silber (SAG) stellt eines der vielversprechendsten Materialien für Hochtemperaturanwendungen in der Leistungselektronik dar. Im Vergleich zu konventionellen Loten sind die mechanischen und thermischen Vorteile enorm, allerdings hochgradig prozessabhängig. Zusammen mit den relativ zeitintensiven Ermüdungstestmethoden ist das die Ursache, dass es aktuell nur wenige Lebensdauermodelle dazu gibt. In dieser Arbeit wird am Beispiel solcher SAG-Proben ein mechanisch beschleunigter, isothermer Biegeversuch vorgestellt, welcher das Potenzial hat, die zeitkritischen Temperatur- oder Lastwechselversuche zu ersetzen. Zum Vergleich wurde ein Temperaturwechseltest als Referenzversuch durchgeführt. Hierzu wird zunächst der Stand der Technik des Silber-Sinterns aufgezeigt, wobei der Schwerpunkt auf der mechanischen Materialcharakterisierung liegt.Wo das elastische Verhalten als näherungsweise allein porositätsabhängig gelten kann, ist die inelastische Dehnung noch unzureichend untersucht. Besonders die zeitabhängige inelastische Dehnung (Kriechen) zeigt noch kein vollständig konsistentes Bild, wodurch auch die Fehlermechanismen und deren Gewichtung noch nicht grundsätzlich als geklärt gelten können. Die gängigsten Belastungstests, welche in der Literatur zu finden sind, haben schwerwiegende Nachteile. Der hohe Zeitbedarf, die teils schwer quantifizierbaren Fehlerparameter und die fehlende Einstellmöglichkeit des Verhältnisses Kriechdehnung zu plastischer Dehnung sind hier im Besonderen zu nennen. Der rein dehnungsgesteuerte Biegeversuch hat diese Nachteile nicht. Über die Biegegeschwindigkeit ließe sich der Kriechanteil nahezu beliebig erhöhen (ggf. unter Nutzung von Haltezeiten). Die Biegeversuche wurden isotherm bei fünf Temperaturen von 22◦C bis 125◦C mit je drei Amplituden und drei Biegegeschwindigkeiten durchgeführt. Schlecht gesinterte Proben machten sich reproduzierbar als Frühausfall bemerkbar, so dass sich die Methode bereits gleich zu Beginn als hervorragender Qualitätstest bewährte. In puncto Ermüdung konnte ein stabiles und reproduzierbares Ausfallverhalten in Form von vergleichbaren Weibull-Formfaktoren und Ausfallbildern in den metallografischen Schliffen gefunden werden. Mit den Daten der Biegeversuche wurde ein fehlerphysikalisches Lebensdauermodell (Coffin-Manson) aufgestellt, welches erfolgreich den Ausfall des Temperaturwechseltests als Referenzversuch vorhersagen konnte. / Sintered silver (SAG) as die attach material is one of the promising solutions to exploit the advantages of high-gap semiconductors in power electronics. The mechanical and thermal properties are far superior to solders, but severely process-dependent. Combined with the time requirements of the state of the art (SoA) fatigue test methods this is most likely the reason for the lack of profound reliability studies yet. This thesis presents an isothermal bending test, which has the ability to replace the time-consuming thermal shock test as primary fatigue experiment for physics of failure based (PoF) lifetime models. A benchmark against a conventional thermal cycling test was done. The state of the art of the silver-sintering technique will be given with focus on the mechanical material characterization. While the elastic properties are mostly porosity-dependent, the inelastic properties are insufficiently examined yet. Especially the creep does not show a consistent image, what leads to many questions regarding the failure mechanism. The most common fatigue tests in the literature do have serious disadvantages. The time-consumption is high, the failure parameter can hardly be quantified and the ratio of plasticity to creep cannot be adjusted easily. The pure mechanical bending test does not have those disadvantages. By changing the bending-speed and the addition of holding times, the creep can be adjusted almost at will. The bending-experiments were conducted at five different temperatures between 22°C and 125°C and with three bending amplitudes as well as three speeds. Insufficiently sintered samples could be identified very early. This already proofed the value of the test as a quality test. Furthermore, a stable and repeatable fatigue behaviour could be observed, what was given by stable Weibull-exponents and repeatable cross sections. A lifetime-model was established by usage of the bending-test-data, what eventually predicted successfully the lifetime of a thermal cycling reference test.

In der Dissertation werden Konzepte vorgestellt, mit denen die Abschätzung der Dauerfestigkeit von Bauteilen auf Grundlage von Werkstoffkennwerten möglich ist. Aus den sich dabei ergebenden Problemen sowie den speziellen Anforderungen der Werkstoffgruppe Sinterstahl wird ein neues Konzept aus bruchmechanischen Überlegungen abgeleitet. Damit ist eine Anwendung ohne prinzipielle Einschränkung bezüglich der Bauteilgeometrie und unter Berücksichtigung der individuellen Kerbempfi ndlichkeit des Werkstoffs möglich. Die Validierung und statistische Auswertung anhand einer breiten Datenbasis belegt eine gute Treffsicherheit im Vergleich zu anderen Verfahren. Empfehlungen zur praktischen Anwendung und den Grenzen der verschiedenen Konzepte werden gegeben.

Composite thin Pd/Ag alloy membranes with long-term thermal and chemical stabilities have potential applications for H2 separation via catalytic membrane reactors and may be one of the key determinants to achieve the 21st century's global hydrogen economy. This work provides a detailed microstructure characterization study and a better understanding of the fundamental principles involved in the synthesis of a novel Pd/Ag intermetallic diffusion barrier formed by the bi-metal multi-layer (BMML) deposition technique. The BMML deposition technique formed an extremely effective Pd/Ag intermetallic diffusion barrier and significantly improved the thermal and long-term stability of the composite Pd and Pd/alloy membranes over a temperature range of 500-600oC. In addition, high temperature annealing studies over a temperature range of 500-800oC in H2 atmosphere led a thorough understanding of the surface interactions and the phase changes between the Pd and Ag metals and the porous metal support elements (Fe, Cr and Ni) and it was shown by the SEI, EDX and X-ray phase analyses that the Ag/Fe and Ag/Ni binary systems exerted complete immiscibility compared to the completely miscible solid solutions of Pd/Fe and Pd/Ni phases. A novel characterization method of in-situ time-resolved high temperature X-ray diffraction (HTXRD) analysis was used to elucidate the mechanistic details of the isothermal nucleation and growth kinetics of the Pd/Ag alloy phase over a temperature range of 500-600oC in H2. The nucleation of the Pd/Ag alloy phase was instantaneous where the growth mechanism was through diffusion-controlled one-dimensional thickening of the Pd/Ag alloy layer. The Pd/Ag alloy phase growth was strongly dependent upon the deposition morphology of the as-synthesized Pd and Ag layers due to the presence of the heterogeneous nucleation sites. Based on the empirical rate constants derived from the solid-state reaction models, the estimated activation energies for the Pd/Ag alloy phase transformation were 236.5 and 185.6 kJ/mol and in good agreement with the literature values of 183-239.5 kJ/mol. The successful utilization of surface modification techniques and modified plating conditions led to the synthesis of several dense Pd/Ag layers, which were as thin as 5-15 µm with a bulk Ag content in the 10-40 wt% range. The long-term testing of the composite Pd/Ag membranes (5-15 µm) at 500oC showed stable hydrogen permeances as high as 30 to 54 m3/m2-h-atm0.5 with H2/He selectivities ranging from 200 to 14000. Furthermore, the atomic absorption flame analysis was used for the first time to elucidate the effects of temperature, initial metal ion concentration, initial hydrazine concentration and bath agitation on the electroless plating rates of Pd and Ag. The electroless plating of both Pd and Ag were strongly affected by the external mass transfer in the absence of bath agitation. The external mass transfer limitations for both Pd and Ag deposition have been overcome at or above an agitation rate of 400 rpm, resulting in a maximum conversion of the plating reaction and dramatically shortened plating times with the added advantage of uniform deposition morphology as evidenced by the SEI micrographs. Finally, the agitation rate of 400 rpm was successfully employed for the synthesis of composite Pd and Pd/Ag membranes. The H2 permeance for a 4.7 µm thick pure-Pd membrane at 400oC was as high as 61 m3/m2-h-atm0.5 with H2/He selectivity over 310 after a total testing period of 690 hours.

Die Filtration unter aggressiven Einsatzbedingungen, z.B. Einsatz in korrosiven Medien, Abgasfiltration, stellt besondere Anforderungen an das Filtermaterial. Sogenanntes "Liquid Phase Sintered Silicon Carbide" (LPS-SiC) erfüllt die Anforderungen sehr gut. Deshalb bestand das Ziel der Arbeit besteht darin, erstmals aus LPS-SiC asymmetrische keramische Membranen (grobporöses Substrat mit dünner, feinporiger Membran) herzustellen. Als Additivsystem fanden Yttriumoxid und Aluminiumoxid Verwendung. Es wurde Siliciumcarbid der Körnung F1200 auf Substrat der Körnung F500 abgeschieden. Dem Herstellungsverfahren kommt für die Qualität der Membran eine große Bedeutung zu. Daher wurden in dieser Arbeit folgende Beschichtungsmethoden untersucht, um die optimale Methode zu identifizieren: Tauchbeschichtung, elektrophoretische Abscheidung, Druckfiltration und Einsatz von Transfertapes (Transfertapes: Mischung aus Polyacrylatkleber und Pulver). Im Mittelpunkt stand dabei die Druckfiltration. Hierfür wurde eine neue Apparatur konzipiert und aufgebaut. Für die schlickerbasierten Methoden wurde ein wässriges System entwickelt, bei dem auf den Einsatz von organischen Hilfsstoffen verzichtet werden konnte. Die elektrostatische Stabilisierung konnte durch gezieltes Anlösen von Yttriumoxid, Ausfällen von feinskaligem Yttriumhydroxid und Belegung des Siliciumcarbids mit dem Yttriumhydroxid erreicht werden. Die Elektrophorese führte zu keinen befriedigenden Ergebnissen aufgrund des undefinierten spezifischen Widerstandes des Substrats (siehe Dissertation Jan Ihle, Bergakademie Freiberg 2004). Die Druckfiltration erwies sich als das geeignetste Verfahren. Mit ihr konnten ohne Einschränkungen hochwertige Membranen erzeugt werden. Druck und Zeit sind bei gegebenen Feststoffgehalt frei wählbar. Der Druck wurde zwischen 2*10E4 und 1*10E5 Pa variiert. Höherer Druck führte zu feineren Porengrößen (mittlere und maximale Porengröße). Mit der Druckfiltration konnten Membranen ohne makroskopische Defekte erzeugt werden. Sie führte im Vergleich aller Verfahren zu der geringsten Rauhtiefe der Membranen. Die Tauchbeschichtung ließ sich in diesem System nur über den Feststoffgehalt steuern. Membranen aus der Tauchbeschichtung wiesen makroskopische Fehler (große oberflächliche Poren) auf. Die Methode führte hinsichtlich Porengrößen und Rauhtiefe zu den schlechtesten Werten. Die Transfertape-Methode als neuartiger Ansatz erwies sich für das LPS-SiC System als noch nicht ausgereift. Das direkte Bekleben der Substrate war möglich. Hinsichtlich der Membrandicke sind aber Grenzen bei ca. 50 µm gesetzt. Darüber hinaus reißen die Membranen. Es wurden Schwankungen in der Entbinder- und Sinterschwindung verzeichnet. Weiterhin werden große Hohlräume im Substrat nicht von den Transfertapes abgeformt. Beide Effekte erhöhen die Spannungen beim Sintern, so dass bei geringeren Schichtdicken Risse entstehen. / Silicon Carbide (SiC) fulfills many requirements, e.g. a high robustness in terms of corrosion, which makes it a suitable Material for ceramic membranes. The aim of this work was to produce ceramic membranes out of porous liquid phase sintered Silicon Carbide (LPS-SiC). As additives Alumina and Yttria were used. The SiC based on commercial abrasive powders F1200 (Membrane) and F500 (Substrate). Different techniques of membrane formation were applied in order to find the optimum processing procedure: Dip Coating, Electrophoretic Deposition (EPD), Pressure Filtration and the usage of so called Transfer Tapes, a blend of Polyacrylate and ceramic powders). For the slip based methods a water based system was developed without the need of organic additives. A pure electrostatic stabilization was facilitated by solving Yttria with Hydrochloride Acid and precipitation, resulting in the coverage of the SiC particles with finely dispersed Yttria. The EPD was not successful due to a undefined specific resistance of the substrate. The pressure filtration turned out to be the best, most versatile method, leading to defect free membranes with the lowest measured surface roughness. The pressure ranged between 2*10E4 and 1*10E5 Pa. Higher pressure lead to finer pores. The Dip Coating was controlled only by the solids content. Membranes by Dip Coating showed macroscopic defects. As a new concept for ceramic membrane fabrication the Transfer Tapes needed further investigation. The direct gluing on the substrate was possible. The thickness of the membrane was limited to 50 microns in order to keep free of cracks. The Transfer Tapes exhibited pronounced fluctuations in the debinding and sintering shrinkage, leading to increased tension during sintering. Furthermore cavities, (e.g. big pores) were bridged. Both effects lead to increased tension during sintering.