Jasper Plaisier

ABSTRACT Laser Shock Peening (LSP) is a material enhancement process used to introduce compressive residual stresses in metallic components. This investigation explored the effects of different combinations of LSP parameters, such as irradiance (GW/cm(2)) and laser pulse density (spots/mm(2)), on 3.2 mm thick AA6056-T4 samples, for integral airframe applications. The most significant effects that are introduced by LSP without a protective coating include residual stress and surface roughness, since each laser pulse vaporizes the surface layer of the target. Each of these effects was quantified, whereby residual stress analysis was performed using X-ray diffraction with synchrotron radiation. A series of fully reversed bending fatigue tests was conducted, in order to evaluate fatigue performance enhancements with the aim of identifying LSP parameter influence. Improvement in fatigue life was demonstrated, and failure of samples at the boundary of the LSP treatment was attributed to a balancing tensile residual stress.

ABSTRACT In the zirconium-rich part of the Zr–Te system the stoichiometric compounds Zr5Te4and Zr3Te exist. The crystal structures of both compounds have been investigated. The crystal structure of Zr5Te4has been resolved by Rietveld refinement of neutron powder diffraction data. It has a tetragonal body-centered unit cell with lattice parametersa=1.07649(6) nm,c=0.38403(3) nm, spacegroupI4/m, and is isostructural with Ti5Te4. The profile agreement factors areRp=4.38%,Rwp=5.91% andS=2.22. The crystal structure of Zr3Te has been resolved by Rietveld refinement of the X-ray powder diffraction data. It has a tetragonal body-centered unit cell with lattice parametersa=1.13382(6) nm andc=0.56265(5) nm, spacegroup I4, isostructural with Ni3P, with profile agreement factorsRp=12.5,Rwp=13.6% andS=1.06. The structural similarities between both compounds are discussed.

ABSTRACT Ti–Zn mixed oxide thin films, with thickness less than 50 nm, were grown with atomic layer deposition (ALD) technique at low temperature (90 °C) varying the composition. ALD is a powerful chemical technique to deposit thin films with thickness of few atomic layers. ALD oxide material growth is achieved by dosing sequentially the metal precursor and the oxidizing agent. Thanks to ALD nature of layer by layer growth it was possible to realize mixed metal, Ti and Zn, oxide thin films with controlled composition, simply by changing the number of cycles of each metal oxide layer. Structural and electrical properties of the prepared thin films were studied as a function of their composition. Synchrotron radiation X-ray diffraction technique was used to follow thin film crystallization during sample annealing, performed in situ. It was observed that the onset temperature of crystallization raises with Ti content, and sample structure was Zn2TiO4 phase. Electrical resistivity measurements were performed on crystalline samples, annealed at 600 °C, revealing an increase in resistivity with Ti content.

The growth mechanism of semiconductor nanowires is still an argument of high interest, and it is becoming clearer, investigation after investigation, that simple pictures fail to describe the complex behaviors observed under different growth conditions. We report here on the growth of semiconductor nanowires, maintaining control over the chemical composition and the physical state of the metallic seeds, and tuning the growth mechanism by varying the growth conditions. We focused on Au-assisted ZnSe nanowires grown by molecular beam epitaxy on GaAs(111)B substrates. We show that at sufficiently high temperatures, the Au seed is strongly affected by the interaction with the substrate and that nanowire growth can occur through two different mechanisms, which have a strong impact on the nanowire's morphology and crystal quality. In particular, ZnSe NWs may exhibit either a uniformly oriented, straight morphology when the nanoparticle seed is liquid, or a kinked, worm-like shape when the nanoparticle seed is switched to a solid phase. This switch, which tunes the nanowire growth mechanism, is achieved by controlling the Zn-to-Se beam pressure ratio at the Au-seed surface. Our results allow a deeper understanding of particle-assisted nanowire growth, and an accurate control of nanowire morphology via the control of the growth mechanism.

ABSTRACT We report optimized photoluminescence of ZnSe nanowires grown by molecular beam epitaxy, obtained by lowering the growth temperature down to 300 °C. The low-temperature growth method has been developed using Si(111) and GaAs(111)B substrates. On the latter, vertical oriented blue-emitting nanowires have been obtained. The growth mecha- nism is discussed with the help of in-situ and ex-situ electronic and structural measurements. We also report strong blue luminescence from ZnSe nanowires grown on ITO-coated glasses, demonstrating that ZnSe nanowires are optimal candidates for transparent optoelectronics. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A structural study of Sm- and Gd-doped ceria was performed with the aim to clarify some unexplained structural features. (Ce1-xREx)O2-x/2 samples (RE ≡ Sm, Gd; x = 0, 0.1, ..., 1) were prepared by coprecipitation of mixed oxalates and subsequent thermal treatment at 1473, 1173, or 1073 K in air; they were then analyzed at room temperature both by synchrotron X-ray diffraction and μ-Raman spectroscopy. Two structural models were adopted to fit the experimental data, namely, a fluoritic one, resembling the CeO2 structure at low RE content, and a hybrid one at higher RE content, intermediate between the CeO2 and the RE2O3 structures. Two main transitions were detected along the compositional range: (a) an RE-dependent transition at the boundary between the fluoritic and the hybrid regions, of a chemical nature; (b) an RE-independent transition within the hybrid region at ∼0.5, having a purely geometrical nature. The presence of two finely interlaced F- and C-based structures within the...

ABSTRACT The structural determination of the CeO2/Gd2O3 mixed system is a non-trivial problem because of the close resemblance between the ionic sizes of Ce4+ and Gd3+ and between the crystal structures of CeO2 and Gd2O3. (Ce1−xGdx)O2−x/2 powder samples with x ranging between 0 and 1 have been synthesized by coprecipitation of mixed oxalates and subsequent thermal decomposition in air at 1200 °C followed by slow cooling. Synchrotron powder X-ray diffraction data were collected and refined by the Rietveld method. Lattice parameters do not follow Vegard's law and no peak splitting has been observed for any composition, meaning that no biphasic regions exist over the whole compositional range. The same hybrid structural model – a proper mixture of the structures of the two pure oxides – was used for the refinements, allowing to account for the data observed.

ABSTRACT The fabrication of P3HT nanopatterns (lamellae and fibres) within a few minutes, in standard laboratory conditions (i.e., at room temperature and in air) and over areas as large as cm2, is reported. The nanostructures are prepared using a wet-processing method. A satisfactory control over the pattern topology (lamellae, hierarchically connected and parallel fibres, entangled but disconnected and quasi-parallel fibres, randomly oriented fibres) is obtained by simply changing one process parameter. UV-vis spectroscopy and X-ray diffraction analyses carried out over the so-fabricated structures evidence a very high degree of supramolecular organization of the polymeric chains. Such a degree of order is similar or even better than that of P3HT samples treated with thermal or solvent annealing procedures. © 2013 The Royal Society of Chemistry.

Cyclops is a new computer program designed as a graphical front-end that allows easy control and interaction with tasks and programs for 3D reconstruction of biological complexes using cryo-electron microscopy. Cyclops' current plug-ins are designed for automated particle picking and include two new algorithms, automated carbon masking and quaternion based rotation space sampling, which are also presented here. Additional plug-ins are in the pipeline. Cyclops allows straightforward organization and visualization of all data and tasks and allows both interactive and batch-wise processing. Furthermore, it was designed for straightforward implementation in grid architectures. As a front-end to a collection of programs it provides a common interface to these programs, thus enhancing the usability of the suite and the productivity of the user.