Search results for "Diffraction"

Synthesis and processing of nanocrystalline tungsten carbide: Towards cemented carbides with optimal mechanical properties

2011

Abstract Nanocrystalline tungsten carbide has been obtained by reduction/carburization at low temperature from precursors obtained by freeze-drying of aqueous solutions. Nanocrystalline WC powders with a adequate content of carbon were mixed with submicrometric Cobalt powder (12 wt.%), obtained by same synthesis method, and sintered in vacuum furnace. The cemented carbides fabricated from experimental powders were compared with both commercial ultrafine and nanocrystalline WC-12Co mixtures consolidated by the same route. The synthesised powders were characterized by X-ray powder diffraction, elemental analysis and scanning and high resolution transmission electron microscopy. On the other h…

Diffractive corneal inlay for presbyopia

2017

[EN] A conceptually new type of corneal inlays for a customized treatment of presbyopia is presented. The diffractive inlay consists on a small aperture disc having an array of micro-holes distributed inside the open zones of a Fresnel zone plate. In this way, the central hole of the disc lets pass the zero order diffraction and produces an extension of the depth of far focus of the eye, while the diffracted light through the holes in the periphery produce the near focus. Additionally, the micro-holes in the inlay surface fulfill the essential requirement of allowing the flow of nutrients through it to the cells of the corneal stroma. Theoretical and optical-bench experimental results for t…

Ultrafast diffraction of tightly focused waves with spatiotemporal stabilization

2008

Experimental studies of ultrafast beam shaping have come about from the need to compensate diffraction-induced dispersive effects in femtosecond laser beams. From a theoretical point of view, chromatic matching of diffracted spherical waves in the vicinity of the geometrical focus is attained by applying conveniently dispersive boundary conditions in the far-field zone, a subject thoroughly analyzed in the paraxial regime. For applications demanding high spatial resolution, however, high-numerical-aperture microscope objectives may be employed instead and would lead to nonparaxiality of the focal wavefields. These circumstances have motivated our investigation. Concretely we report on prere…

Superresolved imaging based on wavelength multiplexing of projected unknown speckle patterns

2015

We propose a method for resolution enhancement of a diffraction limited optical system based on the capture of a set of low resolution images. These images are obtained after projection of an ensemble of unknown speckle patterns on top of the high resolution object that is to be imaged. Each speckle pattern is generated by the same thin (and unknown) diffuser, but illuminated with a slightly different wavelength. From the ensemble of low resolution images, we obtain a system of equations that can be solved in an iterative manner that enables reconstruction of the high resolution object. As a result, we also achieve the projected high resolution speckle patterns used for the encoding.

Single-Crystal-to-Single-Crystal Anion Exchange in a Gadolinium MOF: Incorporation of POMs and [AuCl4]−

2016

The encapsulation of functional molecules inside porous coordination polymers (also known as metal-organic frameworks, MOFs) has become of great interest in recent years at the field of multifunctional materials. In this article, we present a study of the effects of size and charge in the anion exchange process of a Gd based MOF, involving molecular species like polyoxometalates (POMs), and [AuCl4]−. This post-synthetic modification has been characterized by IR, EDAX, and single crystal diffraction, which have provided unequivocal evidence of the location of the anion molecules in the framework.

Oxidative Addition of Halogens on Open Metal Sites in a Microporous Spin-Crossover Coordination Polymer

2009

Illustrating the processability of magnetic layered double hydroxides: layer-by-layer assembly of magnetic ultrathin films.

2013

We report the preparation of single-layer layered double hydroxide (LDH) two-dimensional (2D) nanosheets by exfoliation of highly crystalline NiAl-NO3 LDH. Next, these unilamellar moieties have been incorporated layer-by-layer (LbL) into a poly(sodium 4-styrenesulfonate)/LDH nanosheet multilayer ultrathin film (UTF). Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible light (UV-vis), and X-ray diffraction (XRD) profiles have been used to follow the uniform growth of the UTF. The use of a magnetic LDH as the cationic component of the multilayered architecture enables study of the resulting magnetic properties of the UTFs. Our magnetic data show the appearance of spontaneous …

Polystyrene Sulfonate–Porphyrin Assemblies: Influence of Polyelectrolyte and Porphyrin Structure

2011

In this study, electrostatic self-assembly of different polystyrene sulfonates and a set of tetravalent cationic porphyrins is investigated. It is shown that association of linear polystyrene sulfonates of different molar masses yields finite size nanoscale assemblies that are stable in aqueous solution. Aggregates are compared to the ones of cylindrical brushes, revealing that both form assemblies in the 100 nm range with the charge ratio (molar ratio of porphyrin charges to polyelectrolyte charges) being determining, while the morphology of the resulting network-like assemblies is different for both polyelectrolyte architectures. For the smallest 8k polystyrene sulfonate, in addition, sto…

Phase behaviour of Ag2CrO4 under compression: Structural, vibrational, and optical properties

2013

We have performed an experimental study of the crystal structure, lattice dynamics, and optical properties of silver chromate (Ag2CrO4) at ambient temperature and high pressures. In particular, the crystal structure, Raman-active phonons, and electronic band gap have been accurately determined. When the initial orthorhombic Pnma Ag2CrO4 structure (phase I) is compressed up to 4.5 GPa, a previously undetected phase (phase II) has been observed with a 0.95% volume collapse. The structure of phase II can be indexed to a similar orthorhombic cell as phase I, and the transition can be considered to be an isostructural transition. This collapse is mainly due to the drastic contraction of the a ax…

Forming pressure dependence of the ferroelectric domain structure in green barium titanate pellets

1995

Abstract Modifications of the X-ray diffraction line profiles by die pressing of fine grained powders of barium titanate have been studied. Two main parameters have been considered: the forming pressure and the mechanical characteristics of the binder. The higher the forming pressure, the greater the modifications of the line profiles. Furthermore, the phenomena seems to be related to pressing only; no significant effect related to the nature of the binder has been shown. The modifications of the diffraction line profiles are interpreted by considering a reduction of the size of the ferroelectric domains during pressing. In order to confirm this interpretation, the behaviour of the domain s…