Evidence of shear-dependent boundary slip in newtonian liquids

The European Physical Journal E - The flow of Newtonian fluids was studied by directly measuring the hydrodynamic drainage force acting on a sphere approaching a flat surface. Our force...     

Optical properties of a high-efficiency glass ceramic X-ray storage phosphor

The turbidity, photoluminescence, and photo-stimulated luminescence (PSL) of fluorozirconate glass containing barium chloride nano- and micro-crystals have been measured for samples prepared by isochronal (70 min) annealing over a temperature range of 220–283°C, and correlated with the microstructure as determined by X-ray diffraction measurements. Crystallization of hexagonal phase barium chloride commences at around 220°C, but until 275°C the material retains excellent transparency although it displays negligible PSL. Between 275°C and 277°C, the hexagonal phase converts to the orthorhombic phase, the transparency abruptly decreases, and the PSL rises to a value of around 13% of that found for the commercial storage phosphor BaFBr:Eu. For a slightly higher temperature of 280°C, new phases appear which correspond to the onset of bulk crystallization, and at 283°C the relative PSL rises to 33%, while the transparency falls further. The trade-off between optical transparency and PSL over this narrow temperature window for X-ray imaging plate applications is briefly discussed.     

New developments in X-ray storage phosphors

We found a significant PSL effect in Eu²⁺-doped fluorozirconate glasses (ZBLAN) which were additionally doped with Br⁻ or Cl⁻ ions. The PSL is attributed to the characteristic emission of Eu²⁺ present in nano-crystallites of BaBr₂ or BaCl₂, which form in the glass upon annealing. The metastable hexagonal form of BaX₂ (X=Br,Cl) is always formed first before it is converted into the stable orthorhombic form. The particle size increases upon annealing and so does the PSL efficiency of the glass ceramic. However, there is a saturation of the PSL efficiency, which is for Br⁻ doping about 9% and for Cl⁻-doping about 80% of the Eu-doped BaFBr standard. The particle size was determined by transmission electron microscopy (TEM). The TEM results show a clear tendency for bigger particles for longer annealing at the expense of its number. The particle size for the most efficient phosphor is about 100 nm.     

Evolution of the high power THz source program at Jefferson Lab

We report the evolution of the high power THz source program at Jefferson Lab. The source is based on coherent synchrotron radiation in which short bunches of relativistic electrons radiate when traversing a dipolar magnetic field. In our first accelerator we produced 20 W of broadband THz light. Our upgraded accelerator with higher current and improved THz extraction optics will considerably enhance the output power to >100 W. In this paper we describe the source in some detail and present theoretical calculations for the upgrade.     

An Experimental Investigation into the Acoustic Characteristics of Fluid-filled Porous Structures—A Simplified Model of the Human Skull Cancellous Structure

In nature, shape and structure evolve from the struggle for better performance. Often, biological structures combine multiple beneficial properties, making research into mimicking them very complex. Presented here is a summary of observations from a series of experiments performed on a material that closely resembles the human skull bone’s cancellous structure under acoustic loads. Transmission loss through flat and curved open-cell polyurethane foam samples is observed using air and water as the two interstitial fluids. Reduction in strength and stiffness caused by porosity can be recovered partially by filling the interstitial pores with a fluid. The test findings demonstrate the influence of the interstitial fluid on the mechanical characteristics of a porous structure in a quantitative manner. It is also demonstrated that the transmission loss does not depend only on the mass per unit area of the structure as predicted by acoustic mass law. Current tests also demonstrate that the transmission loss is more sensitive to the interstitial fluid than the shape and support conditions of the structures. Test observations thus support the concepts of “moisture-sensitivity of biological design” and the “law of hierarchy in natural design”.     

The Elimination of Grains and Grain Boundaries in Grain Growth

The topological changes that occur during coarsening of 2D and 3Dcellular structures, such as polycrystals, areinvestigated. Particular attention is given to the elimination ofgrain boundaries and grains with more than the minimum number oftopological elements. A thermodynamic criterion is introduced tofind out which topological transformations are favoured, based on theevaluation of the Gibbs free energy of the initial and finalconfigurations. In general, elimination of grains is possible only ifthe number of their neighbours is below a critical value, which maybe affected by geometry.