Biomedical subjects

J Härtwig

Publications and source records attributed to J Härtwig.

16 recordsLinked to original sources

PubMed2006

In situ and real-time probing of quasicrystal solidification dynamics by synchrotron imaging.

Quasicrystal growth remains an unsolved problem in condensed matter. The dynamics of the process is studied by means of synchrotron live imaging all along the solidification of icosahedral AlPdMn quasicrystals. The lateral motion of ledges driving faceted growth at the solid-melt interface is conclusively shown. When the solidification rate is increased, nucleation and free growth of new faceted grains occur in the melt due to the significant interface recoil induced by slow attachment kinetics. The detailed analysis of the evolution of these grains reveals the crucial role of aluminum rejection, both in the poisoning of their growth and driving fluid flow.

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PubMed2005

Origin of the opalescence at the alpha-beta transition of quartz: role of the incommensurate phase studied by synchrotron radiation.

The origin of the light scattering observed at the alpha-beta transition of quartz is still a subject of controversy. We present structural studies performed during the coexistence of the alpha and the intermediate incommensurate (inc) phases using simultaneously synchrotron x-ray diffraction and optical techniques. The small and large angle light scatterings are due, respectively, to the orientation domains of the 3q inc phase and to the alpha phase twins revealed by diffuse x-ray scattering. In the vicinity of the interphase boundary, the two light scattering regions, both with perturbed properties, form a complex multiscale structure.

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PubMed2002

X-ray diffraction topography using a diffractometer with a bendable monochromator at a synchrotron radiation source.

The different properties of laboratory- and synchrotron-based double-crystal setups for X-ray topographic applications are discussed as a basis for the realization of a versatile instrument allowing the investigation of all kinds of crystals with high strain sensitivity and without any reduction in image size. It appears that the use of a bendable highly perfect monochromator (silicon) achieves this goal, through the local adaptation of Bragg angles, to compensate either dispersion or a bending of the sample.

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PubMed2002

Present state and perspectives of synchrotron radiation diffraction imaging.

The modern third-generation synchrotron radiation sources offer enhanced possibilities for all variants of imaging techniques. The quantitative and qualitative improvements with respect to previous synchrotron diffraction imaging work, which include the investigation in transmission of bulky samples, the use, as an additional parameter, of the sample-to-detector distance, and the use of the coherence of the beam, are illustrated by several examples. Emphasis is given to the possibilities associated with modern electronic detectors for this type of imaging. The new techniques implemented at the ESRF that take full advantage of new capabilities, and more particularly that of 'topo-tomography', are presented.

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PubMed2001

Applications of dynamical diffraction under locally plane wave conditions: defects in nearly perfect crystals and X-ray refractometry.

In a previous paper, the concept of a locally plane wave was explained theoretically. In such a configuration, the fringe pattern recorded on the film can be considered as a phase analyser. Here the experimental analysis is presented, showing examples of interesting applications to X-ray refractometry and to the visualization of the strain field around isolated defects.

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PubMed2000

Phase imaging using highly coherent X-rays: radiography, tomography, diffraction topography.

Several hard X-rays imaging techniques greatly benefit from the coherence of the beams delivered by the modern synchrotron radiation sources. This is illustrated with examples recorded on the 'long' (145 m) ID19 'imaging' beamline of the ESRF. Phase imaging is directly related to the small angular size of the source as seen from one point of the sample ('effective divergence' approximately microradians). When using the ;propagation' technique, phase radiography and tomography are instrumentally very simple. They are often used in the 'edge detection' regime, where the jumps of density are clearly observed. The in situ damage assessment of micro-heterogeneous materials is one example of the many applications. Recently a more quantitative approach has been developed, which provides a three-dimensional density mapping of the sample ('holotomography'). The combination of diffraction topography and phase-contrast imaging constitutes a powerful tool. The observation of holes of discrete sizes in quasicrystals, and the investigation of poled ferroelectric materials, result from this combination.

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PubMed1999

Theory of moiré fringes on X-ray diffraction topographs of bicrystals.

The theory of moiré fringes on X-ray diffraction topographs of bicrystals is derived from the dynamical theory of X-ray diffraction for the reflection (Bragg) and the transmission (Laue) case. The influence on the moiré fringes of the diffraction geometry, of the geometry of the sample, of its optical properties and of the topographic method is investigated. The perfect-crystal theory is also expanded to weakly deformed bicrystals.

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PubMed1999

X-ray diffraction moiré topography as a means to reconstruct relative displacement fields in weakly deformed bicrystals.

X-ray diffraction topographs of wafers produced by separation by implanted oxygen (SIMOX) show moiré fringes in both reflection and transmission geometry. These fringes reveal deformations of the order of 10(-6) to 10(-8) between the layer and the substrate of the SIMOX material. A new method for a quantitative analysis of moiré fringes is developed and allows reconstruction with a high sensitivity of the three components of the relative displacement field between layer and substrate directly from a set of topographs. This method is used for the interpretation of moiré topographs of entire 4 in SIMOX wafers and of regions around crystal defects. Finally, the capabilities of an analysis of moiré fringes are compared with those of the usual diffraction topo-graphy.

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PubMed1998

Conserving the coherence and uniformity of third-generation synchrotron radiation beams: the case of ID19, a 'long' beamline at the ESRF.

The lateral coherence length is of the order of 100 micron at the 'long' (145 m) ID19 beamline of the ESRF, which is mainly devoted to imaging. Most of the optical elements located along the X-ray path can thus act as ;phase objects', and lead to spurious contrast and/or to coherence degradation, which shows up as an enhanced effective angular size of the source. Both the spurious contrast and the coherence degradation are detrimental for the images (diffraction topographs, tomographs, phase-contrast images) produced at this beamline. The problems identified and the way they were solved during the commissioning of ID19 are reported. More particularly, the role of the protection foils located in the front end, the beryllium windows, the filters and the monochromator defects (scratches, dust, small vibrations) is discussed.

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PubMed1996

New features of dislocation images in third-generation synchrotron radiation topographs.

Some aspects of the dislocation contrast observed at third-generation synchrotron radiation set-ups are presented. They can be explained by taking into account angular deviation effects on the beam propagation, which are visible because of the ;almost plane-wave' character of these sources. In particular, we show how the evolution of the direct image width of a dislocation as a function of the sample-to-film distance can allow a complete determination of the Burgers vector, i.e. in sign and modulus. In addition, experimental results obtained in monochromatic beam topography are compared with simulated images calculated assuming plane-wave illumination and are demonstrated to show a satisfactory agreement. The utility of the weak-beam technique in enhancing the spatial resolution is demonstrated and a criterion for the selection of experimental conditions depending upon the required spatial resolution, signal-to-noise ratio and exposure time is presented.

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PubMed1995