Zugl.: Aachen, Techn. Hochsch., Diss., 2002.

Das Ziel der vorliegenden Arbeit war, die Gefrierstrukturierung von Biopolymer-Keramik-Kompositen zur Nachahmung von osteochondralem Gewebe zu untersuchen. Dies diente der Forschung an alternativen Therapiemethoden zur Regeneration von osteochondralen Defekten, da durch derzeitige Therapien oftmals nur ein minderwertiger Reparaturknorpel gebildet wird und keine langfristigen Erfolge erzielt werden. Die Herstellung der Proben zur Nachahmung von osteochondralem Gewebe erfolgte mit der Technik der Gefrierstrukturierung, wodurch anisotrope und hoch geordnete Systeme erhalten wurden. Im Rahmen einer systematischen Untersuchung wurden mehrere Parameter, wie beispielsweise der externe Temperaturgradient, variiert und deren Auswirkungen auf die Proben untersucht. Im ersten Versuchsteil wurde die bidirektionale Gefrierstrukturierung untersucht, um die Morphologie der hergestellten Proben zu optimieren. Anschließend wurden zweischichtige Alginat- bzw. Kollagen-Bruschit-Systeme zur Nachahmung von osteochondralem Gewebe hergestellt. Die erste Schicht sollte Knochen imitieren, während die zweite Schicht Knorpel nachahmte. Die Morphologie der hergestellten Proben wurde unter dem Stereo- und Rasterelektronenmikroskop untersucht. Zur Untersuchung des mechanischen Verbundes zwischen den Schichten wurden Zugversuche durchgeführt. Alle hergestellten Systeme waren hoch geordnet und anisotrop. Die zweischichtigen Systeme wiesen einen Verbund beider Schichten auf und durch die Variation verschiedenster Parameter konnte ein näheres Verständnis des Einflusses dieser auf die Probenmorphologie erlangt werden. / The aim of the present work was to investigate the freeze-structuring of biopolymer-ceramic-composites to mimic osteochondral tissue. This served to research alternative therapy methods for the regeneration of osteochondral defects, as current therapies often do not achieve long-term success. The preparation of samples to mimic osteochondral tissue was performed using the technique of freeze-structuring, obtaining anisotropic and highly ordered systems. As part of a systematic investigation several parameters, such as the external temperature gradient, were varied and their effects on the samples were studied. In the first part of the experiment, bidirectional freeze-structuring was investigated to optimize the morphology of the prepared samples. Subsequently, two-layered alginate- or collagen-bruschite-systems were prepared to mimic osteochondral tissue. The first layer was intended to mimic bone, while the second layer mimicked cartilage. The morphology of the prepared samples was examined under the microscope. Tensile tests were performed to investigate the mechanical bond between the layers. All the fabricated systems were highly ordered and anisotropic. The two-layered systems had a composite of both layers and by varying a wide range of parameters, a more detailed understanding of the influence of these on the sample morphology could be obtained.

Nürnberg, Universiẗat, Diss., 2002--Erlangen.

The key hypothesis of this work represented the question, if mimicking the zonal composition and structural porosity of musculoskeletal tissues influences invading cells positively and leads to advantageous results for tissue engineering. Conventional approaches in tissue engineering are limited in producing monolithic “scaffolds” that provide locally variating biological key signals and pore architectures, imitating the alignment of collagenous fibres in bone and cartilage tissues, respectively. In order to fill this gap in available tissue engineering strategies, a new fabrication technique was evolved for the production of scaffolds to validate the hypothesis. Therefore, a new solidification based platform procedure was developed. This process comprises the directional solidification of multiple flowable precursors that are “cryostructured” to prepare a controlled anisotropic pore structure. Porous scaffolds are attained through ice crystal removal by lyophilisation. Optionally, electrostatic spinning of polymers may be applied to provide an external mesh on top or around the scaffolds. A consolidation step generates monolithic matrices from multi zonal structures. To serve as matrix for tissue engineering approaches or direct implantation as medical device, the scaffold is sterilized. An Adjustable Cryostructuring Device (ACD) was successively developed; individual parts were conceptualized by computer aided design (CAD) and assembled. During optimisation, a significant performance improvement of the ACDs accessible external temperature gradient was achieved, from (1.3 ± 0.1) K/mm to (9.0 ± 0.1) K/mm. Additionally, four different configurations of the device were made available that enabled the directional solidification of collagenous precursors in a highly controlled manner with various sample sizes and shapes. By using alginate as a model substance the process was systematically evaluated. Cryostructuring diagraphs were analysed yielding solidification parameters, which were associated to pore sizes and alignments that were determined by image processing. Thereby, a precise control over pore size and alignment through electrical regulation of the ACD could be demonstrated. To obtain tissue mimetic scaffolds for the musculoskeletal system, collagens and calcium phosphates had to be prepared to serve as raw materials. Extraction and purification protocols were established to generate collagen I and collagen II, while the calcium phosphates brushite and hydroxyapatite were produced by precipitation reactions. Besides the successive augmentation of the ACD also an optimization of the processing steps was crucial. Firstly, the concentrations and the individual behaviour of respective precursor components had to be screened. Together with the insights gained by videographic examination of solidifying collagen solutions, essential knowledge was gained that facilitated the production of more complex scaffolds. Phenomena of ice crystal growth during cryostructuring were discussed. By evolutionary steps, a cryostructuring of multi-layered precursors with consecutive anisotropic pores could be achieved and successfully transferred from alginate to collagenous precursors. Finally, very smooth interfaces that were hardly detectable by scanning electron microscopy (SEM) could be attained. For the used collagenous systems, a dependency relation between adjustable processing parameters and different resulting solidification morphologies was created. Dehydrothermal-, diisocyanate-, and carbodiimide- based cross linking methods were evaluated, whereby the “zero length” cross linking by carbodiimide was found to be most suitable. Afterwards, a formulation for the cross linking solution was elaborated, which generated favourable outcomes by application inside a reduced pressure apparatus. As a consequence, a pore collapse during wet chemical cross linking could be avoided. Complex monolithic scaffolds featuring continuous pores were fabricated that mimicked structure and respective composition of different areas of native tissues by the presence of biochemical key stimulants. At first, three types of bone scaffolds were produced from collagen I and hydroxyapatite with appropriate sizes to fit critical sized defects in rat femurs. They either featured an isotropic or anisotropic porosity and partly also contained glycosaminoglycans (GAGs). Furthermore, meniscus scaffolds were prepared by processing two precursors with biomimetic contents of collagen I, collagen II and GAGs. Here, the pore structures were created under boundary conditions, which allowed an ice crystal growth that was nearly orthogonal to the external temperature gradient. Thereby, the preferential alignment of collagen fibres in the natural meniscus tissue could be mimicked. Those scaffolds owned appropriate sizes for cell culture in well plates or even an authentic meniscus shape and size. Finally, osteochondral scaffolds, sized to either fit well plates or perfusion reactors for cell culture, were fabricated to mimic the composition of subchondral bone and different cartilage zones. Collagen I and the resorbable calcium phosphate brushite were used for the subchondral zone, whereas the cartilage zones were composed out of collagen I, collagen II and tissue mimetic contents of GAGs. The pore structure corresponded to the one that is dominating the volume of natural osteochondral tissue. Energy dispersive X-ray spectroscopy (EDX) and SEM were used to analyse the composition and pore structure of the individual scaffold zones, respectively. The cross section pore diameters were determined to (65 ± 25) µm, (88 ± 35) µm and(93 ± 42) µm for the anisotropic, the isotropic and GAG containing isotropic bone scaffolds. Furthermore, the meniscus scaffolds showed pore diameters of (93 ± 21) µm in the inner meniscus zone and (248 ± 63) µm inside the outer meniscus zone. Pore sizes of (82 ± 25) µm, (83 ± 29) µm and (85 ± 39) µm were present inside the subchondral, the lower chondral and the upper chondral zone of osteochondral scaffolds. Depending on the fabrication parameters, the respective scaffold zones were also found to feature a specific micro- and nanostructure at their inner surfaces. Degradation studies were carried out under physiological conditions and resulted in a mean mass loss of (0.52 ± 0.13) %, (1.56 ± 0.10) % and (0.80 ± 0.10) % per day for bone, meniscus and osteochondral scaffolds, respectively. Rheological measurements were used to determine the viscosity changes upon cooling of different precursors. Micro computer tomography (µ-CT) investigations were applied to characterize the 3D microstructure of osteochondral scaffolds. To obtain an osteochondral scaffold with four zones of tissue mimetic microstructure alignment, a poly (D, L-lactide-co-glycolide) mesh was deposited on the upper chondral zone by electrostatic spinning. In case of the bone scaffolds, the retention / release capacity of bone morphogenetic protein 2 (BMP-2) was evaluated by an enzyme linked immunosorbent assay (ELISA). Due to the high presence of attractive BMP binding sites, only less than 0.1 % of the initially loaded cytokine was released. The suitability of combining the cryostructuring process with 3D powder printed calcium phosphate substrates was evaluated with osteochondral scaffolds, but did not appear to yield more preferable results than the non-combined approach. A new custom build confined compression setup was elaborated together with a suitable evaluation procedure for the mechanical characterisation under physiological conditions. For bone and cartilage scaffolds, apparent elastic moduli of (37.6 ± 6.9) kPa and (3.14 ± 0.85) kPa were measured. A similar behaviour of the scaffolds to natural cartilage and bone tissue was demonstrated in terms of elastic energy storage. Under physiological frequencies, less than 1.0 % and 0.8 % of the exerted energy was lost for bone and cartilage scaffolds, respectively. With average relaxation times of (0.613 ± 0.040) sec and (0.815 ± 0.077) sec, measured for the cartilage and bone scaffolds, they respond four orders of magnitude faster than the native tissues. Additionally, all kinds of produced scaffolds were able to withstand cyclic compression at un-physiological frequencies as high as 20 Hz without a loss in structural integrity. With the presented new method, scaffolds could be fabricated whose extent in mimicking of native tissues exceeded the one of scaffolds producible by state of the art methods. This allowed a testing of the key hypothesis: The biological evaluation of an anisotropic pore structure in vivo revealed a higher functionality of immigrated cells and led finally to advantageous healing outcomes. Moreover, the mimicking of local compositions in combination with a consecutive anisotropic porosity that approaches native tissue structures could be demonstrated to induce zone specific matrix remodelling in stem cells in vitro. Additionally, clues for a zone specific chondrogenic stem cell differentiation were attained without the supplementation of growth factors. Thereby, the hypothesis that an increased approximation of the hierarchically compositional and structurally anisotropic properties of musculoskeletal tissues would lead to an improved cellular response and a better healing quality, could be confirmed. With a special focus on cell free in situ tissue engineering approaches, the insights gained within this thesis may be directly transferred to clinical regenerative therapies. / Die Schlüsselhypothese dieser Arbeit bestand darin zu überprüfen, ob eine Nachahmung der zonalen Zusammensetzungen und Porenstruktur muskulo-skelettaler Gewebe einwandernde Zellen beeinflusst und zu vorteilhafteren Ergebnissen im Tissue Engineering führt. Obwohl bereits zahlreiche konventionelle Ansätze existieren, so sind diese in ihrem Vermögen spezielle Zellträgermatrices („Scaffolds“) herzustellen limitiert. Insbesondere können dabei lokal variierende biologische Schlüsselreize nicht mit einer Porenstruktur, welche die Ausrichtung der Kollagenfasern in Knochen- und Knorpelgeweben imitiert, kombiniert werden. Um diese Lücke in den verfügbaren Tissue Engineering Strategien zu schließen, wurde ein neues Verfahren entwickelt. Dieses erlaubte die Herstellung monolithischer Scaffolds, welche eine Validierung der Hypothese ermöglichten. Das neue Plattform-Verfahren basiert auf der gerichteten Erstarrung mehrerer fließfähiger Vorstufen, um somit eine kontrollierte anisotrope Porenstruktur vorzubereiten. Ein Entfernen der erstarrten Lösungsmittel durch Lyophilisation führt zu porösen Scaffolds. Optional besteht die Möglichkeit, Polymere mittels elektrostatischem Verspinnen als umhüllendes Vlies zu inkorporieren. Nach einem Vernetzungsschritt resultieren monolithische Matrices, bestehend aus mehreren Zonen mit unterschiedlichen Zusammensetzungen. Vor einer Verwendung als Tissue Engineering Matrix oder implantierbares Medizinprodukt erfolgt eine Sterilisation. Hierfür wurde ein “Adjustable Cryostructuring Device“ (ACD) entwickelt, einzelne Bauteile mit Computer Aided Design entworfen und zu einer Apparatur montiert. Die Optimierung der Anlage ermöglichte eine signifikante Erhöhung des verfügbaren externen Temperaturgradienten von (1.3 ± 0.1) K/mm auf (9.0 ± 0.1) K/mm. Außerdem erlauben vier unterschiedliche Konfigurationen des ACD die gerichtete Erstarrung von kollagenen Vorstufen in einer besonders kontrollierten Art und Weise bei einer Vielzahl an Probengrößen und Formen. Die systematische Evaluation des Prozesses erfolgte mit Alginat als Modell-Substanz. Aus den zeitlichen Verläufen der Gefrierstrukturierung resultierten Erstarrungsparameter, die mittels Bildverarbeitung den entstandenen Porengrößen und -ausrichtungen zugeordnet wurden. Dies demonstrierte eine präzise Kontrolle der Ergebnisse durch elektrische Ansteuerung der ACD. Zur Erzeugung von Rohmaterialien war eine Etablierung von Extraktions- und Aufreinigungsprotokollen für Kollagen I und Kollagen II notwendig, während eine Herstellung der Calciumphosphate Bruschit und Hydroxylapatit mittels Präzipitations-Reaktionen verlief. Neben der sukzessiven Verbesserung des ACD, stellte auch die Optimierung einzelner Prozessschritte wichtige Aspekte dar. Die Untersuchung und Diskussion des Verhaltens einzelner Vorstufenkomponenten sowie der Erstarrungs-phänomene von Kollagenlösungen führte zu einem Verständnis welches die Produktion von komplexeren Scaffolds zuließ. Somit war es auch möglich eine Abhängigkeitsrelation der einstellbaren Prozessparameter zu den resultierenden Erstarrungsmorphologien der verwendeten Kollagensysteme abzuleiten. Die Gefrierstrukturierung von mehreren Lagen unterschiedlicher Vorstufen konnte erfolgreich von Alginat- auf Kollagenvorstufen transferiert werden. Nach einer Optimierung der jeweiligen Grenzflächenübergänge, waren diese selbst mittels Rasterelektronenmikroskopie kaum noch zu erkennen. Eine Evaluierung von dehydrothermal-, diisocyanat- und carbodiimid- basierten Quervernetzungs-methoden zeigte die vorteilhaftesten Ergebnisse für die Vernetzung durch Carbodiimide. Zusätzlich wurde eine Zusammensetzung der Vernetzungslösung ermittelt, welche beim Einsatz in einer Unterdruckapparatur einen Porenstrukturkollaps durch nasschemische Vernetzung vermeidet. Eine erweiterte Kontrolle der Gefrierprozesse erlaubte es Struktur und Zusammensetzung verschiedener Zonen nativer Gewebe durch eine monolithische Zellträgermatrix mit durchgängiger Porenstruktur und biochemischen Schlüsselreizen nachzuahmen. Zuerst wurden drei Arten von Knochenscaffolds aus Kollagen I und Hydroxylapatit hergestellt, die Defekten kritischer Größe in Rattenoberschenkel-knochen entsprachen. Diese zeichneten sich durch eine isotrope oder eine anisotrope Porenstruktur aus und enthielten teilweise Glycosaminoglycane (GAGs). Weiterhin erfolgte die Produktion von Meniskusscaffolds aus zwei Vorstufen mit biomimetischen Anteilen an Kollagen I, Kollagen II und GAGs. Dabei verlief die Gefrierstrukturierung unter Grenzbedingungen, welche ein nahezu senkrechtes Eiskristallwachstum zu dem äußeren Temperaturgradienten erlaubten. Somit konnte der bevorzugte Verlauf von Kollagenfasern in nativem Meniskusgewebe nachgeahmt werden. Die Scaffolds waren entweder passend für „Well Plates“ der Zellkultur bemaßt oder besaßen sogar Form und Größe von authentischen Menisken. Zuletzt wurden osteochondrale Scaffolds hergestellt, deren Zusammensetzung den jeweiligen Bereichen von Subchondralzone und verschiedenen Gelenkknorpelzonen entsprach. Kollagen I und die bioresorbierbare Calciumphosphatphase Bruschit fanden Verwendung in der Subchondralzone, während die Knorpelzonen aus Kollagen I, Kollagen II und entsprechenden biomimetischen Anteilen an GAGs bestanden. Außerdem bildete die Scaffoldporenstruktur die Volumendominierende in natürlichem Osteochondralgewebe nach, wobei die Dimensionierungen der Scaffolds Well Plates oder Perfusionsreaktoren der Zellkultur angepasst waren. Mittels energiedispersiver Röntgenspektroskopie und Rasterelektronenmikroskopie erfolgte die Analyse von Zusammensetzung und Porenstruktur der jeweiligen Scaffoldzonen. Die Größe der Porenquerschnitte betrug (65 ± 25) µm, (88 ± 35) µm und (93 ± 42) µm für die anisotropen, die isotropen und die GAG-haltigen isotropen Knochenscaffolds. Die Meniskusscaffolds besaßen Porendurchmesser von (93 ± 21) µm in der inneren Meniskuszone und (248 ± 63) µm innerhalb der äußeren Meniskuszone. Im Falle der osteochondralen Scaffolds wurden Porengrößen von (82 ± 25) µm, (83 ± 29) µm und (85 ± 39) µm in der subchondralen, der unteren chondralen und der oberen chondralen Zone gemessen. In Abhängigkeit von den Prozessparametern zeigten die inneren Oberflächen der jeweiligen Scaffoldzonen eine spezifische Mikro- und Nanostruktur. Eine Prüfung des Degradationsverhaltens unter physiologischen Bedingungen ergab einen mittleren Massenverlust von (0.52 ± 0.13) %, (1.56 ± 0.10) % und (0.80 ± 0.10) % pro Tag für die Knochen-, Meniskus- und osteochondralen Scaffolds. Die Untersuchung der Viskositätsveränderungen während der Abkühlung unterschiedlicher Vorstufen geschah mit rheologischen Messungen. Weiterhin wurde die 3D Mikrostruktur von osteochondralen Matrices mit Mikro Computer Tomographie charakterisiert. Um einen osteochondralen Scaffold mit vier Zonen gewebeähnlich ausgerichteter Mikrostruktur zu erhalten, konnte die Scaffoldoberfläche durch ein elektroversponnenes Poly (D, L-Lactid-co-Glycolid) Vlies modifiziert werden. Ein „enzyme linked immunosorbent assay“ (ELISA) diente zur Evaluation des Rückhalte- bzw. Freisetzungsverhaltens von „bone morphogenetic protein 2“ (BMP-2) in Knochenscaffolds. Bedingt durch die hohe Präsenz von attraktiven BMP Bindungsstellen betrug die freigesetzte Menge des initial beladenen Zytokins nur weniger als 0.1 %. Die Eignung einer Kombination des Gefrierstrukturierungsprozesses mit 3D gedruckten Calciumphosphatsubstraten wurde anhand von osteochondralen Scaffolds überprüft, aber zeigte keine vorteilhafteren Resultate als die nicht kombinierte Vorgehensweise. Für die mechanische Charakterisierung unter physiologischen Bedingungen konnte ein neues Test-Setup mitsamt Auswertungsverfahren entwickelt werden. Die gemessenen Elastizitätsmoduln betrugen (37.6 ± 6.9) kPa für Knochen- und (3.14 ± 0.85) kPa für Knorpelscaffolds. Da unter physiologischen Frequenzen nur weniger als 1.0 % der eingebrachten Energie verloren ging, entsprach die Fähigkeit der Zellträgermatrices zur elastischen Energiespeicherung dem von natürlichem Knochen- und Knorpelgewebe. Bei mittleren Relaxationszeiten von (0.613 ± 0.040) sec und (0.815 ± 0.077) sec für Knorpel- und Knochenscaffolds reagieren diese vier Größenordnungen schneller als die nativen Gewebe. Außerdem waren alle produzierten Matrices dazu in der Lage zyklischen Kompressionen bei unphysiologisch hohen Frequenzen von 20 Hz zu wiederstehen, ohne an struktureller Integrität zu verlieren. Mit dem vorgestellten neuen Verfahren konnten Scaffolds hergestellt werden, deren Ausmaß in der Nachahmung nativer Gewebe mit etablierten Methoden nicht erreichbar war und welche eine Überprüfung der Schlüsselhypothese erlaubten: Die biologische Evaluation einer anisotropen Porenstruktur in vivo zeigte eine höhere Funktionalität eingewanderter Zellen, was zu vorteilhafteren Heilungsergebnissen führte. Darüber hinaus demonstrierte eine Imitation der lokalen Zusammensetzungen in Kombination mit einer durchgängigen anisotropen Porenstruktur, welche an diejenige in nativen Geweben angenähert ist, eine Induktion von zonenspezifischer Matrixremodellierung von Stammzellen in vitro. Außerdem waren Hinweise auf eine zonale chondrogene Stammzelldifferenzierung ohne eine gesonderte Zugabe von Wachstumsfaktoren zu beobachten. Somit konnte die Hypothese, dass eine verbesserte Nachahmung der hierarchischen Zusammensetzung und anisotroper Struktur von muskuloskelettalen Geweben zu einer optimierten zellulären Reaktion und somit einer besseren Heilungsqualität führt, bestätigt werden. Mit einem speziellen Fokus auf zellfreies in situ Tissue Engineering, könnten die Erkenntnisse dieser Arbeit direkt für klinische Therapien eingesetzt werden.

Solar cells convert sunlight into electrical energy using the photo effect. With a mar-ket share of 60%, multicrystalline silicon (mc-Si) is the most frequently used absorber material. Standard mc-Si ingots are directionally solidified in a fused silica (SiO2) crucible, which exhibits a silicon nitride (Si3N4) inner coating. After the entire raw material has been melted, the nucleation takes place on the Si3N4 inner coating at the bottom of the crucible. This results in an inhomogeneous initial grain structure and an increased fraction of dislocation clusters in the upper part of the ingot, which decrease the quality of standard mc-Si. Therefore, the global goal is the development of a cost-effective technology that reduces the formation of clusters and enhances the quality of mc-Si ingots. One way of achieving that goal is to produce the so-called \"high performance multi crystalline silicon\" (HPM-Si). During the directional solidification silicon raw material remains unmelted at the bottom of the SiO2 crucible, whereby crystallization does start on the silicon feedstock a few millimeters above the crucible bottom. Compared to standard mc-Si, a finer grained structure with many small grains is formed, which are separated by so-called random grain boundaries. Since the movement of dislocations across this grain boundary type has rarely been observed, the risk of formation of dislocation clusters, which have a negative impact on the efficiency of solar cells, is greatly reduced for HPM-Si. However, the disadvantage of the HPM-Si compared to the mc-Si is the yield loss resulting from the unmelted raw material at the crucible bottom. Hence, the aim of the present work is to produce mc-Si with a fine-grained structure in combination with a high fraction of random grain boundaries without the disad-vantage of yield loss. In order to investigate the grain structure in dependence of the nucleation conditions G1 ingots having a mass of 14.5 kg and dimensions of 220x220x130 mm³ were directional solidified in a furnace. The analysis of the grain structure with respect to the grain size, grain orientation and the random grain boundary length fraction and the comparison with the HPM-Si reference crystal took place on horizontal wafers with a thickness of 3mm. One possibility to influence the grain structure of mc-Si could be the variation of the cooling conditions before the start of crystallization at the crucible bottom. In a first series of experiments, a gas-flowed cooling plate, positioned below the crucible, was used. An increased gas flow increases the axial heat flow downwards and the cooling rate below the crucible bottom in the same direction. The detected cooling rate, measured by a thermocouple in the silicon melt 5 mm above the crucible bottom, varied in a range between 0.06-1.5 K/min. An increased cooling rate increases the supercooling, with a maximum of 2K. The analysis of the grain structure shows that a reduction in the cooling rate in combination with the lowest supercooling minimizes the average grain size and increases the fraction of random grain boundaries. However, an HPM-Si like grain structure (grain size and fraction of random grain boundaries comparable to HPM-Si) could not completely produced. Furthermore, due to the extended process time, the wafer yield is reduced, whereby the reduction of the cooling rate is not a preferable method for the industrial process. In a second experimental series, which took place under constant cooling rates, the influence of an additional nucleation layer on the initial grain structure was investigated. For this purpose, the additional nucleation layer was applied on the already existing Si3N4 inner coating on the crucible bottom. In order to adjust a HPM-Si like grain structure, the contact angle of the silicon melt on the additional nucleation layer should be lower than on the Si3N4 inner coating. The theoretical basis for this hypothesis is the relationship between the contact angle and the nucleation energy, which states that a reduced contact angle lowers the nucleation energy and can ultimately lead to more nuclei. Furthermore, in order to avoid melting, the additional nucleation layer must have a higher melting point than silicon. Suitable materials for the application as a foreign seed sample are SiC, SiO2 and Al2O3, which are used in the form of particles with different sizes. The production of the additional nucleation layer was carried out by a spraying as well as by an embedding procedure. These layers exhibit different thermal conductivity as well as surface roughness. Embedded nucleation layers generate higher roughness values than sprayed nucleation layers. The analysis of the grain structure identified the surface roughness as the main influencing factor on the initial grain size. While an increased surface roughness (Rq>100μm) results in a fine-grained structure (average grain size: <2mm²) comparable to HPM-Si, the average grain size increases (>2 mm²) with a reduced surface roughness (Rq<100μm). However, the analysis of the grain boundary relationship shows that the fraction of random grain boundaries does not correlate with the average grain size. Only a ma-terial dependency was detected. All SiO2 nucleation layers generate an increased fraction of random grain boundaries, comparable to the HPM-Si material. In contrast, the fraction of random grain boundaries was reduced for all SiC nucleation layers. This result is probably established with the different thermal conductivities of the used materials. The increased thermal conductivity of the sample with the SiC nucleation layers increases the cooling rate, promoting dendritic growth. In contrast the lower thermal conductivity of the SiO2 nucleation layers reduces the cooling rate and dendritic growth is suppressed. Since dendrites exhibit a Σ3 grain boundary relationship in the center, the fraction of this grain boundary type increases for SiC nucleation layers and the fraction of random grain boundaries decreases. In this thesis, various possibilities for influencing the grain structure have been pre-sented. A SiO2 nucleation layer with a roughness value Rq> 200μm represents an industrially relevant solution for the production of mc-Si with comparable properties to the HPM-Si without the disadvantages of yield loss. Hence, it was possible to in-crease the yield with comparable material quality, whereby the production costs could be reduced. Some first crucible manufacturers have already transferred the use of the SiO2 nucleation layers on top of the already existing Si3N4 inner coating at the crucible bottom to production.

Solar cells convert sunlight into electrical energy using the photo effect. With a mar-ket share of 60%, multicrystalline silicon (mc-Si) is the most frequently used absorber material. Standard mc-Si ingots are directionally solidified in a fused silica (SiO2) crucible, which exhibits a silicon nitride (Si3N4) inner coating. After the entire raw material has been melted, the nucleation takes place on the Si3N4 inner coating at the bottom of the crucible. This results in an inhomogeneous initial grain structure and an increased fraction of dislocation clusters in the upper part of the ingot, which decrease the quality of standard mc-Si. Therefore, the global goal is the development of a cost-effective technology that reduces the formation of clusters and enhances the quality of mc-Si ingots. One way of achieving that goal is to produce the so-called \"high performance multi crystalline silicon\" (HPM-Si). During the directional solidification silicon raw material remains unmelted at the bottom of the SiO2 crucible, whereby crystallization does start on the silicon feedstock a few millimeters above the crucible bottom. Compared to standard mc-Si, a finer grained structure with many small grains is formed, which are separated by so-called random grain boundaries. Since the movement of dislocations across this grain boundary type has rarely been observed, the risk of formation of dislocation clusters, which have a negative impact on the efficiency of solar cells, is greatly reduced for HPM-Si. However, the disadvantage of the HPM-Si compared to the mc-Si is the yield loss resulting from the unmelted raw material at the crucible bottom. Hence, the aim of the present work is to produce mc-Si with a fine-grained structure in combination with a high fraction of random grain boundaries without the disad-vantage of yield loss. In order to investigate the grain structure in dependence of the nucleation conditions G1 ingots having a mass of 14.5 kg and dimensions of 220x220x130 mm³ were directional solidified in a furnace. The analysis of the grain structure with respect to the grain size, grain orientation and the random grain boundary length fraction and the comparison with the HPM-Si reference crystal took place on horizontal wafers with a thickness of 3mm. One possibility to influence the grain structure of mc-Si could be the variation of the cooling conditions before the start of crystallization at the crucible bottom. In a first series of experiments, a gas-flowed cooling plate, positioned below the crucible, was used. An increased gas flow increases the axial heat flow downwards and the cooling rate below the crucible bottom in the same direction. The detected cooling rate, measured by a thermocouple in the silicon melt 5 mm above the crucible bottom, varied in a range between 0.06-1.5 K/min. An increased cooling rate increases the supercooling, with a maximum of 2K. The analysis of the grain structure shows that a reduction in the cooling rate in combination with the lowest supercooling minimizes the average grain size and increases the fraction of random grain boundaries. However, an HPM-Si like grain structure (grain size and fraction of random grain boundaries comparable to HPM-Si) could not completely produced. Furthermore, due to the extended process time, the wafer yield is reduced, whereby the reduction of the cooling rate is not a preferable method for the industrial process. In a second experimental series, which took place under constant cooling rates, the influence of an additional nucleation layer on the initial grain structure was investigated. For this purpose, the additional nucleation layer was applied on the already existing Si3N4 inner coating on the crucible bottom. In order to adjust a HPM-Si like grain structure, the contact angle of the silicon melt on the additional nucleation layer should be lower than on the Si3N4 inner coating. The theoretical basis for this hypothesis is the relationship between the contact angle and the nucleation energy, which states that a reduced contact angle lowers the nucleation energy and can ultimately lead to more nuclei. Furthermore, in order to avoid melting, the additional nucleation layer must have a higher melting point than silicon. Suitable materials for the application as a foreign seed sample are SiC, SiO2 and Al2O3, which are used in the form of particles with different sizes. The production of the additional nucleation layer was carried out by a spraying as well as by an embedding procedure. These layers exhibit different thermal conductivity as well as surface roughness. Embedded nucleation layers generate higher roughness values than sprayed nucleation layers. The analysis of the grain structure identified the surface roughness as the main influencing factor on the initial grain size. While an increased surface roughness (Rq>100μm) results in a fine-grained structure (average grain size: <2mm²) comparable to HPM-Si, the average grain size increases (>2 mm²) with a reduced surface roughness (Rq<100μm). However, the analysis of the grain boundary relationship shows that the fraction of random grain boundaries does not correlate with the average grain size. Only a ma-terial dependency was detected. All SiO2 nucleation layers generate an increased fraction of random grain boundaries, comparable to the HPM-Si material. In contrast, the fraction of random grain boundaries was reduced for all SiC nucleation layers. This result is probably established with the different thermal conductivities of the used materials. The increased thermal conductivity of the sample with the SiC nucleation layers increases the cooling rate, promoting dendritic growth. In contrast the lower thermal conductivity of the SiO2 nucleation layers reduces the cooling rate and dendritic growth is suppressed. Since dendrites exhibit a Σ3 grain boundary relationship in the center, the fraction of this grain boundary type increases for SiC nucleation layers and the fraction of random grain boundaries decreases. In this thesis, various possibilities for influencing the grain structure have been pre-sented. A SiO2 nucleation layer with a roughness value Rq> 200μm represents an industrially relevant solution for the production of mc-Si with comparable properties to the HPM-Si without the disadvantages of yield loss. Hence, it was possible to in-crease the yield with comparable material quality, whereby the production costs could be reduced. Some first crucible manufacturers have already transferred the use of the SiO2 nucleation layers on top of the already existing Si3N4 inner coating at the crucible bottom to production.

Magnetische Formgedächtnsilegierungen haben die besondere Fähigkeit, sich im äußeren Magnetfeld zu verformen. Dies geschieht aufgrund von Zwillingsgrenzenbewegung in der martensitischen Tieftemperaturphase. Da der Effekt bislang an Einkristallen untersucht wurde, ist es das Ziel dieser Arbeit, den Effekt an polykristallinen Proben nachzuweisen. Dafür wurden Proben nach dem Prinzip der gerichteten Erstarrung präpariert. Deren Kristallstruktur wurde durch geeignete Zusammensetzung und Wärmebehandlung einphasig eingestellt. Mechanisches Training und weitere Wärmebehandlungen ermöglichten schließlich die Demonstration der magnetischen Dehnung von ca. 1 % an polykristallinen Proben. Durch zusätzliche Einkopplung akustischer Wellen konnte die Dehnung auf 2,2 % gesteigert werden.

Nürnberg, University, Diss., 2004--Erlangen.

Magnetische Formgedächtnsilegierungen haben die besondere Fähigkeit, sich im äußeren Magnetfeld zu verformen. Dies geschieht aufgrund von Zwillingsgrenzenbewegung in der martensitischen Tieftemperaturphase. Da der Effekt bislang an Einkristallen untersucht wurde, ist es das Ziel dieser Arbeit, den Effekt an polykristallinen Proben nachzuweisen. Dafür wurden Proben nach dem Prinzip der gerichteten Erstarrung präpariert. Deren Kristallstruktur wurde durch geeignete Zusammensetzung und Wärmebehandlung einphasig eingestellt. Mechanisches Training und weitere Wärmebehandlungen ermöglichten schließlich die Demonstration der magnetischen Dehnung von ca. 1 % an polykristallinen Proben. Durch zusätzliche Einkopplung akustischer Wellen konnte die Dehnung auf 2,2 % gesteigert werden.

Melt flow plays an important role in directional solidification of multicrystalline silicon influencing the temperature field and the crystallization interface as well as the transport of impurities. This work investigates the potential of a traveling magnetic field (TMF) for an active control of the melt flow. A system of 3D numerical models was developed and adapted based on open-source software for calculations of Lorentz force, melt flow, and related phenomena. Isothermal and non-isothermal model experiments with a square GaInSn melt were used to validate the numerical models by direct velocity measurements. Several new 3D flow structures of turbulent TMF flows were observed for different melt heights. Further numerical parameter studies carried out for silicon melts showed that already a weak TMF-induced Lorentz force can stir impurities near to the complete mixing limit. Simultaneously, the deformed temperature field leads to an increase of the deflection of crystallization interface, which may exhibit a distinct asymmetry. The numerical results of this work were implemented in a research-scale silicon crystallization furnace. Scaling laws for various phenomena were derived allowing a limited transfer of the results to the industrial scale.