Chemistry Reference
In-Depth Information
fraction of the atoms residing in grain boundaries. The growing interest during the
last two decades is essentially due to their unusual and unexpected physical
properties, which are strongly influenced by the confinement of microcrystalline
into nanocrystalline structural domains, giving rise to an increasing contribution of
grain boundaries. During the last decades, numerous studies have been essentially
devoted to binary and ternary metallic and oxide nanostructured powders prepared
by different methods (mechanical alloying, mechanical milling, mechanochemis-
try, grinding, in situ consolidation of nanoscale atomic clusters,…).
Due to their high mixing efficiency at the atomic scale, the mechanical alloying
which was firstly developed in the 1960s by Benjamin to produce oxide dispersion
strengthened materials, consists in a top-down approach based on high energy ball
milling giving rise to ultra-fine disordered or nanostructured powders [
146
-
150
].
Indeed, the elaboration of a large variety of materials such as intermetallics, extended
solid solutions, quasicrystals and amorphous phases can be successfully achieved by
high energy ball milling, since they cannot be produced by means of conventional
procedure [
151
]. The milling conditions strongly influence the structural and con-
sequently the physical properties of nanostructured powders which need to be
characterized in terms of size, shape of crystalline grains, surface area, phase con-
stitution and microstructuration including grain boundaries [
151
-
153
].
Consequently, the main questions are essentially concerned by the packing of
nanocrystalline grains through the grain boundaries: do the grain boundaries exist?
Do their structural and chemical natures differ from that of polycrystalline regime?
Do they behave as frozen-gas with lacking both short and long range order? How
do they influence the physical properties of nanostructured powders?
Thus, the main goal is to investigate first the structural and microstructural prop-
erties involving diffraction techniques and electron microscopies, then atomic scale
approach using local probe techniques and finally thermodynamic, electric, magnetic,
vibrational properties in order to model the nanostructured state and to understand the
confinement effect on physical properties and its relation with the elaboration proce-
dure (milling conditions as example). We do emphasize that the elaboration of
nanocrystalline materials and nanostructured powders, and their handling, storage and
sampling for experimental studies require special attention because of the surface
contamination and/or oxidation processes: these aspects cannot be neglected as they do
strongly influence the physical properties and their evolution with ageing.
Below, we report two selected studies which illustrate how the use of
57
Fe
Mössbauer spectrometry brings relevant information on the structural and mag-
netic modelling of nanostructured powders. It is clear that the chemical nature of
systems under investigation influences remains a fundamental issue: in this con-
text, the case of nanostructured Fe powders which have been widely investigated
and debated [
154
-
157
], seem quite interesting based on its chemical simplicity
while the example of nanostructured ferric fluoride powders illustrates the role of
magnetic interactions dependent on the cationic topology, taking into account the
combined knowledge of polymorphic crystalline phases and amorphous varieties.
