This paper describes the small‐angle scattering beamline built at the Brazilian Synchrotron Light Laboratory (LNLS). Vertical focusing of the synchrotron beam is achieved by an elastically bent gold‐plated cylindrical mirror. An asymmetric cut curved triangle‐shaped silicon single crystal (111 reflection) is used for monochromatization and horizontal focusing. The mirror, monochromator optics and 2θ arm were designed to cover the spectral range between 1.0 and 2.0 Å. Three slit sets, a secondary photon shutter, two beam monitors, filters and absorbers, a multi‐sample holder, a vacuum path, a beam‐stopper and a set of detectors are the basic components of the workstation. The stepping motors are equipped with specially designed encoders. All mechanical and pneumatic movements and detectors can be remotely controlled using a direct panel or a PC.
A composite material consisting of spherical Bi nanoclusters ͑nanocrystals and/or liquid nanodroplets͒ embedded in a 28Na 2 O-72B 2 O 3 glass was studied by the wide-angle x-ray scattering ͑WAXS͒ and small-angle x-ray scattering ͑SAXS͒ techniques over the temperature range in which the Bi crystal-liquid transition occurs. Because of the wide radius distribution of Bi clusters and due to the dependence of the melting temperature on crystal radius, the overall transition occurs over a wide range, from 365 up to 464 K. In this transition range, large Bi nanocrystals coexist with small liquid droplets. A weak contraction in a and c lattice parameters of rhombohedral Bi nanocrystals with respect to the bulk crystal was detected. As expected, the average radius of crystalline Bi clusters, deduced from WAXS data, increases for increasing temperatures over the whole solidto-liquid transition range. The SAXS spectrum recorded at different temperatures within the transition range is essentially invariant, indicating that the radius distribution of Bi nanoclusters ͑nanocrystals and nanodroplets͒ is temperature independent. The volume distribution of Bi nanoclusters is a single-mode function with the radius ranging from about 15 up to 41 Å with a maximum at 28 Å. The integral of Bragg peaks of Bi nanocrystals decreases for increasing temperatures as a consequence of the progressive melting of nanocrystals of increasing size. By combining the results of WAXS and SAXS experiments, we determined the melting temperature of the nanocrystals as a function their radius suppressing unwanted size dispersion effects. Our results clearly indicate a linear dependence of the melting temperature on nanocrystal reciprocal radius, thus confirming previous theoretical predictions.
Abstract:The Er,Cr:YSGG system is commonly employed in tissue removal, but recently it has also been clinically evaluated for caries prevention. The present work explains the clinical and pre-clinical observations on the basis of the crystallographic changes that this laser can produce in the dental enamel. The analyzed samples were obtained from sound human third molar teeth. The laser irradiation was conducted with a Er,Cr:YSGG laser with 12.5 mJ/pulse, 0.25 W, and 2.8 J/cm 2 . The laser device operates at a wavelength of 2.79 μm, and the pulse width duration is 140 μs, with a repetition rate of 20 Hz of spot size of 750 μm. The crystalline structure of the samples was evaluated by Xray diffraction at a synchrotron beamline The X-ray beam was configured at a grazing angle, to maximize the surface diffraction signal and to better detect the possible new crystallographic phase produced after the laser irradiation. It was observed that the crystallographic structure tetracalcium phosphate (TetCP, JCPDF 25-1137) exhibits several peaks that match more precisely with the new experimental peaks of the irradiated enamel. The present results suggesting the coexistence of tetracalcium phosphate with hydroxyapatite in enamel irradiated with Er,Cr:YSGG laser and can be the answer to the clinical and pre-clinical observations reported in the literature.
Two different vacuum tight sample cells for in situ temperature dependent small angle scattering from liquids are presented in this article. In the first one, the sample fills a 1 mm thickness gap sealed on both sides by two thin parallel mica windows (volume 300 μl). In the second one, the liquid is injected into a 1 mm cylindrical capillary tube (volume 130 μl). The cells are lodged into temperature controlled chambers directly connected to the beamline vacuum path. Several important improvements with respect to similar instrumentation previously reported are: (1) versatile application of the mica cell, that can be used for all types of samples (gels, liquid crystals, and dispersions in organic solvents) and (2) the design of the chamber for the capillary cell allows registration of wider angle data and a convenient replacement of the capillary tube after each experiment. Signal to background ratio and data reproducibility were tested using protein solutions. We give a brief report of scattering experiments performed with different protein samples and two-dimensional data collection.
A nanocomposite consisting of PbTe nanocrystals embedded in a silicate glass was studied by small-angle x-ray scattering during the early stage of isothermal annealing at 793 K. A theoretical function based on a model of spherical PbTe nanocrystals surrounded by a Pb and Te depleted shell fits well to all experimental curves. The time dependences of the nanocrystal radius and size of the depleted shell agree with the prediction of the theory of nucleation and growth by the classical mechanism of atomic diffusion.
In this paper, Au nanoparticles obtained by ion implantation and a subsequent thermal annealing have been elongated in a privileged direction by means of a postannealing 10 MeV Si ion irradiation. The modification and splitting of the surface plasmon resonance peak were determined by optical extinction spectroscopy, changing the polarization angle of the incident light. Moreover, the extinction spectra were accurately fitted with the T-matrix method, showing a good agreement with the results obtained by transmission electron microscopy, Rutherford backscattering spectrometry, and grazing-incidence small-angle X-ray scattering.
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