Laser Micro Sintering – A Quality Leap through Improvement of Powder Packing

Laser micro sintering, a modification of selective laser sintering for freeform fabrication of micro-parts, was continuously upgraded since its first application. Poor density of the powder layers has been a persisting problem that had to be dealt with from the beginning. One solution was the application of high intensity q-switched laser pulses. Compaction of the material and improvement of the sinter resolution was achieved. But with these pulse-regimes only limited density of the sintered body has been achievable. Recently special efforts have been made to get rid of or at least reduce these drawbacks by markedly higher compaction of the respective powder layers. There is clear evidence that with sufficiently compacted powder layers even laser micro sintering with continuous radiation should be feasible. Till recently laser sintering of metal had been applied mainly to produce monolithic components. With the upgraded technique direct generation of micro devices with freely movable subassemblies can be possible.Mechanical Engineerin

Skeletons in the Rhodes Cupboard: What Should Be Done about Them?

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Collaborating in No man's land : an enquiry towards creating an environment for 'equal' collaboration between international partners in an applied theatre project

Includes abstract.Includes bibliographical references (leaves 61-64).This dissertation is an enquiry towards creating an environment for 'equal' collaboration between international partners in an applied theatre project. As a direct case study, I used my master's fieldwork project, No-man's land, a theatre project involving performers from South Africa and The Netherlands. The problematics of international exchanges in which people, resources and art works are brought together over long distances, generates issues around power, culture and the performing arts which demand attention from project partners. The term 'No Man's Land' isthe metaphor developed throughout this dissertation in order to conceptualise the space of collaboration, as well as the mentality such a collaboration necessitates. The focus here is on international collaboration projects within the field of applied theatre that have the potential to unite artists from different backgrounds to explore issues of mutual interest through theatre processes and performances

Identifying benefits and challenges in the application of agile methodologies in software development

A dissertation submitted to the University of Bedfordshire in accordance with the requirements of the degree of Master of ScienceSoftware has generally been developed using the ‘Waterfall’ methodology which is now believed to be cumbersome and slow to react to change. This belief has spawned new ‘Agile’ methodologies that aim to deliver small pieces of working software on a frequent basis. Many claims have been made about Agile, but little academic research has been carried out to justify them. This study identifies the significance of benefits and challenges practitioners have found when adapting to Agile methodologies. Further, the benefits and challenges are linked to the Agile features in use and assesses what contributes to the level of user satisfaction. A survey instrument was employed to collect data from as many practitioners as possible followed by post survey interviews. Claims made for Agile by service providers are compared to the experiences of those canvassed in the survey. Results reveal that the most significant benefit is closer collaboration with other members of the development team. Significant challenges were the estimation of the time and effort Agile projects take to complete and that Agile only works with competent, motivated people. The insights gained in this study will be circulated for the benefit of future research on software development methodologies and Agile practice

DNA electrophoresis in designed channels

We present a simple description on the electrophoretic dynamics of polyelectrolytes going through designed channels with narrow constrictions of slit geometry. By analyzing rheological behaviours of the stuck chain, which is coupled to the effect of solvent flow, three critical electric fields (permeation field $E^{(per)} \sim N^{-1}$, deformation field $E^{(def)} \sim N^{-3/5}$ and injection field $E^{(inj)} \simeq N^0$, with $N$ polymerization index) are clarified. Between $E^{(per)}$ and $E^{(inj)}$, the chain migration is dictated by the driven activation process. In particular, at $E>E^{(def)}$, the stuck chain at the slit entrance is strongly deformed, which enhances the rate of the permeation. From these observations, electrophoretic mobility at a given electric field is deduced, which shows non-monotonic dependence on $N$. For long enough chains, mobility increases with $N$, in good agreement with experiments. An abrupt change in the electrophoretic flow at a threshold electric field is formally regarded as a nonequilibrium phase transition.Comment: 11 pages, 8 figure

Brownsche-Dynamik-Simulation der Migration von DNA in strukturierten Mikrokanälen

Streek MA. Brownian dynamics simulation of migration of DNA in structured microchannels. Bielefeld (Germany): Bielefeld University; 2005.Elektrophorese ist eine der Haupttechniken, um DNA-Stränge nach Länge zu trennen. Die Mobilität von DNA in freier Lösung ist unabhängig von der Größe des Moleküls. Daher wird diese normalerweise in Polymergelen durchgeführt. Leider sind diese nicht in der Lage, lange DNA-Stränge effektiv zu trennen. Ein neuer Ansatz besteht darin, die Fortschritte in der Mikroelektronik zur Herstellung neuer Trennstrukturen zu nutzen. In dieser Arbeit wurden diese Strukturen mittels Computersimulation eines Kugel-Feder-Ketten-Modells untersucht. Eine Struktur, die bereits weitgehend untersucht wurde, sind sogenannte "entropische Fallen". Diese bestehen aus einer periodischen Folge von breiten und engen Regionen. Die Engstellen haben eine Breite, die mit der Persistenzlänge vergleichbar ist. Daher ist für das Passieren der Engstellen eine Entknäuelung der DNA notwendig. Dieses reduziert die entropische freie Energie, daher der Name. Die Entknäuelung wird eingeleitet durch eine initiale Schlaufe, die in den engen Bereich reicht und dann das Polymer nachzieht. Da größere Ketten eine größere Wahrscheinlichkeit haben, diese Schlaufe zu bilden, migrieren diese schneller. Bislang wurde ein Einfang ausschließlich an der plötzlichen Verengung angenommen. Anhand der Simulationsdaten konnte ein neuer Einfangmechanismus identifiziert werden: Ketten können in die weiten Bereiche der Struktur diffundieren und dort aufgrund des fehlenden elektrischen Feldes verharren. Auch dieser Effekt bewirkt eine schnelle Migration der langen Ketten, da die Diffusion längenabhängig ist. Ein Vergleich der Simulationsdaten mit einem einfachen Modell zeigt, dass dieser Effekt mindestens ebenso wichtig ist wie das Hängenbleiben an der Verengung. Eine andere Struktur, die bislang wenig untersucht wurde, sind geometrisch strukturierte Mikrokanäle. Diese sind sehr ähnlich den entropischen Fallen, allerdings ist die Breite der Engstelle vergleichbar mit dem Gyrationsradius und damit wesentlich breiter. Ein Entknäueln ist nicht mehr erforderlich, um diese Region zu passieren. Trotzdem kann der neue Einfangmechanismus aus entropischen Fallen hier ebenfalls auftreten. Die Simulationsdaten zeigen jedoch eine Umkehrung der Mobilität als Funktion der Kettenlänge bei schwachen Feldern. Bei starken Feldern treten zwei Migrationszustände auf, ein langsamer und ein schneller, die unter geeigneten Bedingungen auch experimentell beobachtet werden konnten. Die Gründe für das Auftreten der zwei Zustände sowie der Übergangsmechanismus wurden untersucht. Zur Untersuchung von Ratscheneffekten wurde ein gepulstes elektrisches Feld an die geometrisch strukturierten Mikrokanäle angelegt. Zunächst wurden die Driftweiten in einem symmetrischen gepulsten Feld analysiert. Dabei zeigte sich, dass der langsame Zustand eine Orientierung besitzt: Ein Teil der Kette ist verknäuelt in der breiten Region, der andere ist in der Verengung und zieht das Knäuel in die Verengung. Bei der Umkehr des Feldes benötigt der langsame Zustand also eine gewisse Zeit, um sich an die neue Migrationsrichtung anzupassen. In einem zweiten Schritt wurde ein asymmetrisches Feld angelegt, um Ratscheneffekte bei schnellem Antrieb zu untersuchen. Eine Ausnutzung der zwei Migrationszustände als solche ist nicht möglich, da die Lebensdauer der zwei Zustände zu lang ist. Da der schnelle Zustand weder Orientierung noch Wechselwirkungen mit der Wand hat, zeigt dieser keine Ratscheneffekte. Dennoch existieren Ratscheneffekte im langsamen Zustand, welche diskutiert werden.DNA electrophoresis is one of the main techniques to separate DNA molecules by size. Since the mobility of DNA molecules does not depend on its size in free solution, it is usually performed in gels. Unfortunately, electrophoresis in gels becomes ineffective for long DNA strands. Therefore, effort is being spent on the separation in microfluidic devices, which can be produced very easily due to the advances in microelectronics. In this work, I investigated these structures by a computer simulation of a simple bead-spring model. One device, that has already been investigated thoroughly before, is called "entropic traps". These consist of a periodic sequence of narrow and wide regions. The narrow regions exhibit a width which is comparable to the persistence length and therefore force the polymer into an uncoiled state when passing through. The process of uncoiling reduces the entropic free energy, hence the name. The escape process is initiated by a loop in the narrow region, which drags the remaining polymer through the constriction. As larger polymers exhibit a larger probability of forming the initial loop, they migrate faster. Up to now, this was the only trapping mechanism reported. With the simulation data, a new trapping mechanism was identified: polymers may also diffuse into the wide regions of the device, where the electric field is low and remain there for a long time. This effect also favors the fast migration of long chains, as these exhibit a much less pronounced diffusion. A comparison of the simulation data to a simple model shows that this effect is at least as important as trapping at the sudden narrowing of the device. Another device, which is very similar to entropic traps, is a geometrically structured microchannel. These have not been investigated in detail before. The main difference, compared to entropic traps, is that the width of the narrow region is comparable to the radius of gyration. Therefore a passing of that region does not require an uncoiling of the polymer. Nevertheless, the new trapping mechanism presented above is still applicable. However, the simulation data show a reversal of the mobility as a function of the chain length at weak fields. At strong electric fields, two states of migration occur, one of which is fast and the other one slow, which can be observed experimentally under appropriate conditions. The reasons for the occurence of the two states as well as the transition mechanism are discussed. To investigate ratchet effects, a pulsed electric field was applied to the geometrically structured microchannels. At first, drift lengths during a single pulse in a symmetric field were investigated. It showed that the slow state exhibits an orientation: one part of the chain is coiled in the wide region, and the other is located in the narrow region, pulling the coil into the narrowing. At a sudden reversal of the electric field the slow state needs some time to adapt to the migration direction. In a second step, an asymmetric field was applied to investigate ratchet effects with a fast driving force. A direct usage of the two states turned out to be not useful as a ratchet mechanism, because the two states exhibit a long life time. Additionally, the fast state does not exhibit an orientation or interact with the walls, and therefore does not exhibit any ratchet effects. However, ratchet effects do occur in the slow state, which are discussed

Kuyper\u27s Legacy and Multiculturalism: Gender in his Conception of Democracy and Sphere Sovereignty

This paper was prepared for the Conference on Christianity and Culture: The Heritage of Abraham Kuyper on Different Continents, June 9-11, 1998, Amsterdam, The Netherlands

Processing of Silicon Carbide by Laser Micro Sintering

Silicon carbide – a solid with covalent bonds - is conventionally synthesized via the Acheson process. Usually solid bodies of silicon carbide with definite shapes are generated from the grained material via hot isostatic pressing or liquid phase sintering. Both processes are conducted under well-controlled temperature regimes. Applying the freeform fabrication technique “Laser Micro Sintering” poses a big challenge to experimental skill due to the nonequilibrium conditions that are characteristic features of laser material processing. Successive layers SiC layers with a thickness of 1μm were processed with coherent radiation of 1064 nm. The specific behavior of two different silicon carbide powders - one of them blended with additives - are reported along with interpretational approaches.Mechanical Engineerin