Chemistry Reference
In-Depth Information
3-7 nm a-Fe particles on carbon support and 3-4 nm a-Fe particles in alumina [
96
].
It is also important that some in situ experiments carried out on
57
Fe coated nano-
particles to enhance the surface signal: indeed, as natural Fe contains 2.2 at. %
57
Fe
isotope, the prevailing contribution to the hyperfine structure is due to the surface
while that of the bulk becomes negligible [
97
]. Additional Mössbauer studies versus
temperature on 100 nm a-Fe
2
O
3
nanoparticles led to distinguish clearly surface to
volume hyperfine structure contributions [
97
].
4.5 Nanocrystalline Alloys
In 1990s, numerous studies were devoted to nanocrystalline alloys after their
discovery by Yoshizawa with that of the so-called FINEMET [
98
]. These nano-
crystalline alloys typically originate from as-quenched amorphous precursor met-
glasses submitted to a subsequent annealing, when they are characterized by the
presence of two crystallization peaks, contrarily to most of metallic glasses for which
a single crystallization peak allows to transform directly into a microcrystalline
structure. In the first case, the annealing treatment yields to the emergence of
nanocrystalline grains embedded within a residual amorphous phase: the proportions
of these two components (crystalline giving rise to the volumetric fraction) and the
morphology and the size of the crystalline grains can be a priori well controlled from
the annealing conditions (time and temperature). But it is also obvious that the final
nanocrystalline state is rather dependent on the atomic composition of the amorphous
precursor, its amorphicity, i.e. the quenching conditions, and the relative diffusivities
of the different atomic constituents. The transformation from the amorphous into the
nanocrystalline state, the intergranular amorphous remainder is thus subjected to
substantial microstructural and chemical evolutions due to the migration of some
atomic species and its enrichment in some elements which are expelled from the first
emerged precipitates. But it is important to emphasize that the high energy milling
route was also considered to prepare amorphous and/or nanostructured powders with
similar chemical composition to compare the physical properties.
The interest of the nanocrystalline alloys is mainly due to their unusual soft
magnetic properties combining high magnetic permeability and large saturation
magnetization, making them very attractive candidates for applications as induc-
tive devices, loss free transformers [
99
]. In addition, their two phase structural and
magnetic behavior provided them very relevant and fascinating examples from a
fundamental point of view [
100
,
101
]. The most prominent nanocrystalline alloys
as Finemet (FeCuNbSiB) [
98
], Nanoperm (FeZr(Cu)B) [
102
,
103
] and Hitperm
(FeCoZr(Cu)B) [
104
] were widely investigated during the 1990s. In those nano-
crystalline systems, crystalline grains consist of FeSi with DO
3
type structure, bcc-
Fe and bcc-FeCo phase, respectively.
In addition, the magnetic properties of nanocrystalline alloys are found to be
strongly dependent on the microstructure of nanocrystalline alloys, i.e. the volu-
metric crystalline fraction and the size and morphology of crystalline grains created
