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Biomedical subjects

A Migliori

Publications and source records attributed to A Migliori.

At least 19 recordsLinked to original sources
PubMed2002

Megagauss sensors.

Magnetic fields change the way that electrons move through solids. The nature of these changes reveals information about the electronic structure of a material and, in auspicious circumstances, can be harnessed for applications. The silver chalcogenides, Ag2Se and Ag2Te, are non-magnetic materials, but their electrical resistance can be made very sensitive to magnetic field by adding small amounts--just 1 part in 10,000--of excess silver. Here we show that the resistance of Ag2Se displays a large, nearly linear increase with applied magnetic field without saturation to the highest fields available, 600,000 gauss, more than a million times the Earth's magnetic field. These characteristics of large (thousands of per cent) and near-linear response over a large field range make the silver chalcogenides attractive as magnetic-field sensors, especially in physically tiny megagauss (10(6) G) pulsed magnets where large fields have been produced but accurate calibration has proved elusive. High-field studies at low temperatures reveal both oscillations in the magnetoresistance and a universal scaling form that point to a quantum origin for this material's unprecedented behaviour.

Journal Article↗
PubMed2001

Neutron diffraction study of phi-Bi(8)Pb(5)O(17): structure refinement and analysis of cationic ordering.

The triclinic crystal structure of the varphi phase in the structure of the Bi(2)O(3)-PbO phase diagram has been recently solved by the synergetic use of electron and X-ray diffraction on a polycrystalline Bi(8)Pb(5)O(17) sample. In the present work the problem is re-examined on the basis of powder neutron diffraction data: the structure has been confirmed and refined by the Rietveld method. The increased accuracy of the O-atom positions allowed the study of the cationic ordering in this structure by means of bond-valence calculations and charge distribution analysis. The results, confirmed by the refinement of the site occupancies, indicate that the structure is ordered to a large extent, with Bi and Pb atoms occupying preferentially different lattice sites. In this frame, the varphi phase being the most ordered one in the considered region of the Bi(2)O(3)-PbO phase diagram, it should represent the thermodynamically stable low-temperature polymorph.

Journal Article↗
PubMed2001

Spatial resolution and energy filtering of backscattered electron images in scanning electron microscopy.

Numerical simulations of energy filtering effects on backscattered electron images of semiconductor multilayers are reported. The theoretical investigation has been performed for a wide range of energies, 1-40 keV, and for beam incidence angles between 90 degrees (normal incidence) and 20 degrees. Quite a general purpose of this research concerns the investigation of the optimum energy conditions and of their implications. It will be shown that the optimum energy defines an operating context suitable to ensure a compositional contrast enhancement; i.e. a minimum threshold current and a maximum resolution, without energy filtering, independent of the beam incidence angle. This optimum energy, depending on the specimen and its details, is, however, of the order of a few keV or less for specimen details having a size of the order of few nm. When the performance of the electron gun does not allow to work at low energy it is necessary to operate at an energy higher than the optimum one, the energy filtering can produce positive effects. Yet in those circumstances there is an optimum energy loss window suitable to minimise the threshold current. It spreads from 10-30%, depending on the primary energy and size of the compositional detail, for normal incidence, to a few per cent for high incidence angles and high energy. The simulation results for these last conditions are in agreement with the well-known experimental results obtained with the low-loss methods.

Journal Article↗
PubMed2000

Structure determination of phi-Bi8Pb5O17 by electron and powder X-ray diffraction

The triclinic crystal structure of phi-Bi8Pb5O17, a ionic fast conductor material, has been determined by the synergy of both electron and powder X-ray crystallography. The heavy atom positions were found by direct methods on electron diffraction data and the structure was completed by iterative use of a priori information in direct methods and difference Fourier maps on both types of data. Structure refinement was performed by the Rietveld method on powder X-ray data. The results suggest that phi-Bi8Pb5O17 is an ordered phase, with Bi and Pb atoms occupying different sites of the lattice, at variance with the other structural phases known for similar composition in the Bi-Pb-O phase diagram, which are solid solutions characterised by a wide compositional range.

Journal Article↗
PubMed2000

QED v 1.0: a software package for quantitative electron diffraction data treatment.

A new software package for quantitative electron diffraction data treatment of unknown structures is described. No "a priori" information is required by the package which is able to perform in successive steps the 2-D indexing of digitised diffraction patterns, the extraction of the intensity of the collected reflections and the 3-D indexing of all recorded patterns, giving as results the lattice parameters of the investigated structure and a series of data files (one for each diffraction pattern) containing the measured intensities and the relative e.s.d.s of the 3-D indexed reflections. The software package is mainly conceived for the treatment of diffraction patterns taken with a Gatan CCD Slow-Scan Camera, but it can also deal with generic digitised plates. The program is designed to extract intensity data suitable for structure solution techniques in electron crystallography. The integration routine is optimised for a correct background evaluation, a necessary condition to deal with weak spots of irregular shape and an intensity just above the background.

Algorithms↗
PubMed1996

Comparison of spatial resolutions obtained with different signal components in scanning electron microscopy.

Comparative studies on the ultimate spatial resolution of the Scanning Electron Microscope, using different components of the electron signal have been performed on specimens providing compositional contrast. By operating the microscope in conventional way as well as with a specifically designed set-up we have ascertained that the delocalized components of the signal provide a spatial resolution of the order of the beam size, even if the practical use can be limited by the noise. To amplify the contribution of the delocalized components of the signal, as backscattered electrons by a bulk specimen or forward scattered electrons by a thin specimen, we used a device consisting of a plate of a material with high secondary yield placed above or below the sample. An important practical implication arises from this study. A detecting system consisting of a standard Everhart-Thornley detector coupled with a converter of backscattered or transmitted electrons represents a high performance detecting device for low voltage observations.

Microscopy, Electron, Scanning↗