Large effect of polydispersity on defect concentrations in colloidal crystals

We compute the equilibrium concentration of stacking faults and point defects in polydisperse hard-sphere crystals. We find that, while the concentration of stacking faults remains similar to that of monodisperse hard sphere crystals, the concentration of vacancies decreases by about a factor two. Most strikingly, the concentration of interstitials in the maximally polydisperse crystal may be some six orders of magnitude larger than in a monodisperse crystal. We show that this dramatic increase in interstitial concentration is due to the increased probability of finding small particles and that the small-particle tail of the particle size distribution is crucial for the interstitial concentration in a colloidal crystal.Comment: 6 pages, 4 figure

Thermoforming of foam sheet

Thermoforming is a widely used process for the manufacture of foam sheet products. Polystyrene foam food trays for instance can be produced by first heating the thermoplastic foam sheet, causing the gas contained to build up pressure and expand, after which a vacuum pressure can be applied to draw the sheet in the required form on the mould. This production method appears to be a very sensitive process with respect to e.g. the sheet temperature, the pressures applied and the cooling time. More problems can be foreseen when for environmental reasons the blowing agent will be adapted (for instance replaced by a gas with a lower molecular weight). To gain more insight in the occuring phenomena the large deformations of a foam structure have been analysed using finite element modelling. To this end a constitutive model has to be defined. Starting from the basic theory given by Gibson & Ashby [1], the behaviour of a closed cubic cell has been elaborated for large strains. The total stiffness is then the sum of the contributions of the edges and faces of the cell and the gas contained in it. The large deformations cause anisotropy of the cells [2], which influences their tangential stiffness. The constitutive model developed here includes the effects of internal gas pressure and the evolving anisotropy

Research on the combination of water and membranes as a structural building material

p. 3024-3033The aim of this paper is to investigate the combination of water and membranes for temporary architectural applications. Water as a construction material, can be useful for three different purposes: first of all, thanks to its thermal mass, it can be used as a medium for cooling down or heating up buildings (Pronk et al [6]); secondly water is uncompressible and, in combination with air, can be used as part of a structural element; thirdly the mass of water could work as a sound barrier so it can be used as sound insulation material (Rodrigues and Coutinho [7]). This paper shows the result of the structural behaviour. There is another paper about sound insulation properties of water. The research in both structural and sound insulation fields was carried out in the laboratories of Technische Universiteit Eindhoven, The Netherlands. The prototype is a "waterbeam" of 2 m span. Starting from the Tensairity(R) technology principle (Luchsinger et al [8]) developed by Airlight, the purpose of this experiment is to replace the iron struts with a second membrane chamber filled with water. Water works well in compression and air prevents buckling. The result is a rigid structural element made by non-rigid material (water, air, membranes and cables) with a significant reduction of weight and cost of transportation compared to the traditional iron beam but also compared with the Tensairity(R) system. Different bending tests were carried out. In each test, the two chambers of the beam were filled with air or water to understand the material behaviour.The comparison between the results shows that water works slightly better than air (stiffness increase of a range of 8-13% in the elements filled with water). Water application in architecture showed promising results. Further investigation (pure compression tests on columns, multiple layers sound barrier) should be carried out. These results could give architects new design opportunities and solutions concerning temporary buildings and moveable architecture. Moreover the company and building construction industry could develop innovative structural elements and new insulation components.Pronk, A.; Maffei, R.; Martin, H. (2009). Research on the combination of water and membranes as a structural building material. Editorial Universitat Politècnica de València. http://hdl.handle.net/10251/670

A new approach to stochastic evolution equations with adapted drift

In this paper we develop a new approach to stochastic evolution equations with an unbounded drift $A$ which is dependent on time and the underlying probability space in an adapted way. It is well-known that the semigroup approach to equations with random drift leads to adaptedness problems for the stochastic convolution term. In this paper we give a new representation formula for the stochastic convolution which avoids integration of nonadapted processes. Here we mainly consider the parabolic setting. We establish connections with other solution concepts such as weak solutions. The usual parabolic regularity properties are derived and we show that the new approach can be applied in the study of semilinear problems with random drift. At the end of the paper the results are illustrated with two examples of stochastic heat equations with random drift.Comment: Minor revision. Accepted for publication in Journal of Differential Equation

Co-evaporation of Co-Cr at intermediate oblique incidence

The Co-evaPoration technique has been used for deposition of Co-Cr layers. Deposition has been done under intermediate angle of incidence of opposing vapour streams. The layers showed a single Phase hcP poly-crystalline structure. The (002) plane turned out to be tilted towards the direction of the CO source. The layers showed good perpendicular magnetic behaviour although the magnetic anisotropy axis was also inclined towards the Co-source. Because of the opposing angle of incidence for Co and Cr atoms, a process-induced segregation takes place which causes a relative high coercivity also at low Process temFeratures. A simple model for the segregation effect can explain the relation between the existance of a non-magnetic region and an increased coercivity of th Co-Cr film