Seminars Archive
Lucia Mancini
Abstract
Friday, April 6, 2000, 14:00
Seminar Room, ground floor, Building "T"
Sincrotrone Trieste, Basovizza
Phase imaging using highly coherent X-rays: new insights in the micro-structural
characterization of quasicrystals
Lucia Mancini
(IIstituto LAMEL-CNR, Bologna)
ABSTRACT
A large class of phenomena in condensed matter physics can be investigated
by using X-ray imaging techniques, which greatly benefit from the coherence
of the beam delivered by the modern synchrotron radiation sources. In particular,
the synchrotron beam quality of the ESRF source in Grenoble (France) allows
the use of novel techniques, as phase-sensitive radiography and tomography,
being instrumentally very simple when using the beam propagation from the
sample to the detector. Moreover, the combination of X-ray diffraction
topography and phase-contrast imaging constitutes a powerful tool to establish
a direct correlation between structural properties and phase inhomogeneities
observed in the investigated systems. This approach complemented by computer
simulations of topographic images permits, in some cases, a complete characterization
of the strain field around crystalline defects.
The application of the abovementioned imaging techniques to the characterization
of defects in quasicrystals allowed us to give new insights on the growth
mechanism and structure stability of these materials. Indeed, defects in
quasicrystals remain a controversial topic. They exhibit a complex associated
strain field, which has to be described in a higher dimensional space.
Besides defects analogous to crystalline ones, we observed peculiar structural
defects and inhomogeneities (dodecahedron-shaped holes displaying discrete
sizes and lamellar precipitates) in the volume of the investigated quasicrystalline
alloys. We could also establish a link between these structural defects
and the observed inhomogeneities. The effect of annealing (up to 750C)
on the structural perfection of stable quasicrystals has been studied,
for the first time, in large grains of the icosahedral AlPdMn phase. It
has revealed that the quasicrystalline quality improves as long as no precipitates
are formed. The improvement was mainly ascribed to the relaxation of the
strain-field around the dodecahedral holes present in the as-grown samples.
By means of X-ray diffractometry and topography, dynamical effects like
anomalous transmission (Borrmann effect) were confirmed to exist in selected
quasicrystalline grains, but even though the structural quality is still
far from being highly perfect.
