Additive manufacturing of non plastic porcelain material by direct writing and freeze casting.

Two direct consolidation methods usually used for advanced ceramics have been combined in this project in order to develop a novel fabrication route for traditional ceramics. Specifically the method used is based on the Additive Manufacturing extrusion process using direct writing of high solid loading ceramic pastes and then freeze-casting to solidify the deposited material. This novel fabrication method, for which a patent has been granted, has been christened “Direct Writing Freeze-Casting” (DWFC). Although the DWFC process is the subject of investigation by other researchers for a range of different applications, including the production of medical implants with alumina, the research presented in this thesis focuses on its use in the manufacture of white wares, giftware, and applied arts and crafts in general. This new system will provide designers, potters, artists, craft makers and manufacturers with a flexible and automated way of manufacturing porcelain objects. One of the major challenges to be overcome to exploit the DWFC process is the development of suitable slurry material formulations. Initial trials demonstrated that it is not possible to use conventional clay based porcelain materials with a platelet shaped microstructure which inhibits freeze casting. In this thesis the development and characterisation of non plastic porcelain slurry, based on substitution of kaolin (clay) with a calcined clay material (molochite), which can be processed using this new method is presented. The new non plastic porcelain formulation, which has a high solid load of 75.47% wt., has been subjected to detailed analysis to assess its suitability at each stage of the process; extrusion, freeze-casting (solidification) and firing.National Consul of Sciences and Technology Mexico, CONACY

Processing of Preceramic Polymers for Direct-Ink Writing

Preceramic polymers are organosilicon polymers that, when pyrolyzed to above 1000°C, convert from a polymer to an amorphous ceramic. These polymers have been used for fiber spinning, polymer infiltration, and casting of materials but have recently gained interest for use as the feedstock material for additive manufacturing techniques. This work explores preceramic polymers being used for direct-ink writing (an additive manufacturing method) and many of the issues that occur with the polymers during curing and pyrolysis. The first chapter of this dissertation provides a review of preceramic polymers, while the second and third chapters focus on the development of inks made of preceramic polymers. The second chapter uses a polysilazane polymer mixed with up to 43.3 volume percent hexagonal boron nitride as the rheological modifier to enable printing. The pyrolyzed parts are tested with 3-point flexure and microhardness indentation to observe failure behavior. The third chapter uses a polycarbosilane polymer with zirconium diboride and silicon carbide fibers as constituents for printable inks. These polycarbosilane-based inks exhibit much more porosity and crack development during curing and pyrolysis than the inks in the second chapter. Defects are characterized with micro-computed tomography and scanning electron microscopy. From the measured defects, new suggestions for decreasing porosity and crack development are discussed. Building from the observations in the third chapter, the fourth chapter focuses on how the size of printed material influences the development of defects and overall strength. Two new inks, similar to those in chapter three, are used with the addition that one of the formulations utilizes fumed alumina as an added viscosity modifier. The final study investigates printed rods of varying diameters (0.45 to 1.7 millimeter) to observe the effects of off-gassing during curing on the development of porosity. Failure strength is measured with 3-point flexure and Weibull statistics are used to understand how specimen size and ink formulation affect final specimen strength. Overall, this dissertation shows that preceramic polymers are a viable option as a feedstock material for direct ink-writing and begins to quantify the degree to which part size and filler selection affect overall porosity development after curing and pyrolysis

From cells to tissues

An essential prerequisite for the existence of multi-cellular life is the organization of cells into tissues. In this thesis, we theoretically study how large-scale tissue properties can emerge from the collective behavior of individual cells. To this end, we focus on the properties of epithelial tissue, which is one of the major tissue types in animals. We study how rheological properties of epithelia emerge from cellular processes, and we develop a physical description for the dynamics of an epithelial cell polarity. We apply our theoretical studies to observations in the developing wing of the fruit fly, Drosophila melanogaster. In order to study epithelial mechanics, we first develop a geometrical framework that rigorously describes the deformation of two-dimensional cellular networks. Our framework decomposes large-scale deformation into cellular contributions. For instance, we show how large-scale tissue shear decomposes into contributions by cell shape changes and into contributions by different kinds of topological transitions. We apply this framework in order to quantify the time-dependent deformation of the fruit fly wing, and to decompose it into cellular contributions. We also use this framework as a basis to study large-scale rheological properties of epithelia and their dependence on cellular fluctuations. To this end, we represent epithelial tissues by a vertex model, which describes cells as elastic polygons. We extend the vertex model by introducing fluctuations on the cellular scale, and we develop a method to perform perpetual simple shear simulations. Analyzing the steady state of such simple shear simulations, we find that the rheological behavior of vertex model tissue depends on the fluctuation amplitude. For small fluctuation amplitude, it behaves like a plastic material, and for high fluctuation amplitude, it behaves like a visco-elastic fluid. In addition to analyzing mechanical properties, we study the reorientation of an epithelial cell polarity. To this end, we develop a simple hydrodynamic description for polarity reorientation. In particular, we account for polarity reorientation by tissue shear, by another polarity field, and by local polarity alignment. Furthermore, we develop methods to quantify polarity patterns based on microscopical images of the fly wing. We find that our hydrodynamic description does not only account for polarity reorientation in wild type fly wings. Moreover, it is for the first time possible to also account for the observed polarity patterns in a number of genetically altered flies.:1 Introduction 1.1 The development of multi-cellular organisms 1.2 Biology of epithelial tissues 1.3 The model system Drosophila melanogaster 1.4 Planar cell polarity 1.5 Physical description of biological tissues 1.6 Overview over this thesis 2 Tissue shear in cellular networks 2.1 Geometry of tissue deformation on the cellular scale 2.2 Decomposition of the large-scale flow field into cellular contributions 2.3 Cellular contributions to the flow field in the fruit fly wing 2.4 Discussion 3 Rheological behavior of vertex model tissue under external shear 3.1 A vertex model to describe epithelial mechanics 3.2 Fluctuation-induced fluidization of tissue 3.3 Discussion 4 Quantitative study of polarity reorientation in the fruit fly wing 4.1 Experimentally quantified polarity patterns 4.2 Effective hydrodynamic theory for polarity reorientation 4.3 Comparison of theory and experiment 4.4 Discussion 5 Conclusions and outlook Appendices: A Algebra of real 2 × 2 matrices B Deformation of triangle networks C Simple shear simulations using the vertex model D Coarse-graining of a cellular Core PCP model E Quantification of polarity patterns in the fruit fly wing F Theory for polarity reorientation in the fruit fly wing G Boundary conditions for the polarity field in the fruit fly wing Table of symbols BibliographyEine wesentliche Voraussetzung für die Existenz mehrzelligen Lebens ist, dass sich einzelne Zellen sinnvoll zu Geweben ergänzen können. In dieser Dissertation untersuchen wir, wie großskalige Eigenschaften von Geweben aus dem kollektiven Verhalten einzelner Zellen hervorgehen. Dazu konzentrieren wir uns auf Epitheliengewebe, welches eine der Grundgewebearten in Tieren darstellt. Wir stellen theoretische Untersuchungen zu rheologischen Eigenschaften und zu zellulärer Polarität von Epithelien an. Diese theoretischen Untersuchungen vergleichen wir mit experimentellen Beobachtungen am sich entwickelnden Flügel der schwarzbäuchigen Taufliege (Drosophila melanogaster). Um die Mechanik von Epithelien zu untersuchen, entwickeln wir zunächst eine geometrische Beschreibung für die Verformung von zweidimensionalen zellulären Netzwerken. Unsere Beschreibung zerlegt die mittlere Verformung des gesamten Netzwerks in zelluläre Beitrage. Zum Beispiel wird eine Scherverformung des gesamten Netzwerks auf der zellulären Ebene exakt repräsentiert: einerseits durch die Verformung einzelner Zellen und andererseits durch topologische Veränderungen des zellulären Netzwerks. Mit Hilfe dieser Beschreibung quantifizieren wir die Verformung des Fliegenflügels während des Puppenstadiums. Des Weiteren führen wir die Verformung des Flügels auf ihre zellulären Beiträge zurück. Wir nutzen diese Beschreibung auch als Ausgangspunkt, um effektive rheologische Eigenschaften von Epithelien in Abhängigkeit von zellulären Fluktuationen zu untersuchen. Dazu simulieren wir Epithelgewebe mittels eines Vertex Modells, welches einzelne Zellen als elastische Polygone abstrahiert. Wir erweitern dieses Vertex Modell um zelluläre Fluktuationen und um die Möglichkeit, Schersimulationen beliebiger Dauer durchzuführen. Die Analyse des stationären Zustands dieser Simulationen ergibt plastisches Verhalten bei kleiner Fluktuationsamplitude und visko-elastisches Verhalten bei großer Fluktuationsamplitude. Neben mechanischen Eigenschaften untersuchen wir auch die Umorientierung einer Zellpolarität in Epithelien. Dazu entwickeln wir eine einfache hydrodynamische Beschreibung für die Umorientierung eines Polaritätsfeldes. Wir berücksichtigen dabei insbesondere Effekte durch Scherung, durch ein anderes Polaritätsfeld und durch einen lokalen Gleichrichtungseffekt. Um unsere theoretische Beschreibung mit experimentellen Daten zu vergleichen, entwickeln wir Methoden um Polaritätsmuster im Fliegenflügel zu quantifizieren. Schließlich stellen wir fest, dass unsere hydrodynamische Beschreibung in der Tat beobachtete Polaritätsmuster reproduziert. Das gilt nicht nur im Wildtypen, sondern auch in genetisch veränderten Tieren.:1 Introduction 1.1 The development of multi-cellular organisms 1.2 Biology of epithelial tissues 1.3 The model system Drosophila melanogaster 1.4 Planar cell polarity 1.5 Physical description of biological tissues 1.6 Overview over this thesis 2 Tissue shear in cellular networks 2.1 Geometry of tissue deformation on the cellular scale 2.2 Decomposition of the large-scale flow field into cellular contributions 2.3 Cellular contributions to the flow field in the fruit fly wing 2.4 Discussion 3 Rheological behavior of vertex model tissue under external shear 3.1 A vertex model to describe epithelial mechanics 3.2 Fluctuation-induced fluidization of tissue 3.3 Discussion 4 Quantitative study of polarity reorientation in the fruit fly wing 4.1 Experimentally quantified polarity patterns 4.2 Effective hydrodynamic theory for polarity reorientation 4.3 Comparison of theory and experiment 4.4 Discussion 5 Conclusions and outlook Appendices: A Algebra of real 2 × 2 matrices B Deformation of triangle networks C Simple shear simulations using the vertex model D Coarse-graining of a cellular Core PCP model E Quantification of polarity patterns in the fruit fly wing F Theory for polarity reorientation in the fruit fly wing G Boundary conditions for the polarity field in the fruit fly wing Table of symbols Bibliograph

Mechanisms of strain localisation in the lithosphere

This thesis examines the development of shear-zone localisation in the continental lithosphere. I use non-Newtonian, viscous models to examine the controls on strain localisation with depth and on the development of horizontal shear-zones in regions away from strength contrasts. I show how the vertical extent of strain localisation is principally controlled by lithology and geothermal gradient, and how the horizontal extent of localisation is a consequence of strain-weakening and the geometry of strength contrasts. I explore how strain localisation develops from an initial isolated weak inclusion. The progress of strain localisation follows a power-law growth that is strongly non-linear. When applied to the rheological laws for common lithospheric minerals, the temperature and stress-dependence provide a direct means of predicting the depth below which localisation does not occur. I apply the calculations to four major continental strike-slip zones and find observations from seismic data agree with the calculations. Localisation to the base of the lithosphere is not supported by the calculations or the geophysical data. I use a model configured to resemble the India-Asia convergence that includes an isolated weak region within the Tibetan Plateau area and, in selected experiments,strong regions representing the Tarim and Sichuan Basins. I rotate a strong India region into a weaker Asia and observe the evolving strain. Shear zones develop adjacent and propagate outwards from the weak region. Where the Basins are present then high strain- rate zones develop adjacent to them and the overall distribution of strain within the model is altered. A high strain-weakening component enables shear-zones to localise. Micro-plate models assume the pre-existence of such high strain regions but I show how a continuum model can initiate and grow localised deformation within a region of generally diffuse deformation

Development of Superplastic Structural Ceramics

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65140/1/j.1151-2916.1990.tb06734.x.pd

The influence of fire retardant additives on the properties of HIPS and PBT

Halogen compounds and antimony-based synergists are used at low loading levels in many polymers systems to impart high levels of flame retardancy. This study used a range of brominated flame retardants (BFR) and the most commonly used synergist for halogen-based flame retardance, antimony trioxide (Sb2O3), to investigate the effects on mechanical and physical properties of flame retardants in HIPS and PBT. The polymers used were High Impact Polystyrene (HIPS) and Polybutylene Terephthalate (PBT). Initially each of the additives was used individually, before being combined to study the effect of the complete package. This was achieved by producing a series of compounds using a twin-screw extruder, and then an injection moulder to produce impact, tensile and fracture toughness specimens. The compounds were also analysed using rheological testing and thermal analysis. Also the effects of Stereon impact modifier and Fyrebloc masterbatches were determined in HIPS. [Continues.

The optimisation of flexible impact-protection systems for varying strain rates and energies.

The need for smarter and active, energy absorbing systems designed especially for human protection applications has sparked interest in highly strain rate sensitive compounds. This thesis describes the iterative design, development and optimisation of a novel form of energy absorbing, body worn protection. The original contribution to knowledge is the development of a novel strain rate sensitive protection system incorporating synergetic internal architecture. Co-continuous blends of silicone based dilatant and thermoplastic elastomer have been developed through a recursive design process to develop a new material specifically optimised for body worn protection. Failure mechanisms were analysed, and from these results techniques have been developed to mitigate internal fracture mechanisms. This enabled the development of a strain rate sensitive material utilised with an internal architecture. The novel material properties were examined and developed using monolithic samples, tested at a variety of energies, speed and environmental conditions. Methods for designing and developing auxetic structures that work synergistically with the new material have been developed. The novel system has also been combined with textiles, and the merit of this combination explored. An improvement in performance has been validated, as well as a design improvement through being able to attach parts directly to garments. The resulting impact protectors are applicable over a range of strain rates. Systems have been designed to incorporate this novel technology in pre-production prototypes in three selected market areas, which typify low, medium and high impact speeds. The work also explores the systems ability to manage multiple impacts at the same location with a surprisingly low loss in performance, effectively making a protector that can withstand repeat impacts. This work has contributed to the methods previously used in testing personal protective equipment. The techniques developed in this work have enabled new revision of these PPE standards, as well as directly contributing to two new standards.Open Acces