Chemistry Reference
In-Depth Information
V (mm/s)
V (mm/s)
-
8
-6
-4
-2
0
2
4
6
8
-
8
-6
-4
-2
0
2
4
6
8
1.00
1.00
0.98
0.98
0.96
0.94
0.96
0.92
77 K
77 K
0.94
0.90
1.00
1.00
0.99
0.98
0.98
0.97
0.96
0.96
0.94
0.95
150 K
240 K
0.92
0.94
1.00
1.00
0.98
0.98
0.96
0.96
0.94
0.94
260 K
375 K
0.92
1.00
0.99
0.98
0.96
0.96
0.93
0.94
373 K
475 K
0.90
0.92
1.00
1.00
0.96
0.98
0.92
0.96
0.88
473 K
575 K
0.94
0.84
1.00
1.00
0.96
0.98
0.92
0.96
0.88
0.94
625 K
573 K
0.84
0.92
-8
-6
-4
-2
0
2
4
6
8
-8
-6
-4
-2
0
2
4
6
8
V (mm/s)
V (mm/s)
Fig. 4.14 Mössbauer spectra recorded at given temperatures on nanocrystalline NANOPERM
Fe
80.5
Nb
7
B
12.5
alloys
annealed
at
783
and
883 K
for
1 h,
left-
and
right-hand
column,
respectively [
110
]
modeling, particularly in the case of FINEMET alloys [
111
-
117
]. Indeed, the
structure of FeSi grains is dependent on the Si content: Si atoms are randomly
distributed in bcc Fe lattice up to 10 at. % Si while in the range 12-31 at. % Si, one
gets an ordered or disordered DO
3
structure, both giving rise to a complex
hyperfine structure with several Fe sites [
118
]. Consequently, the best strategy
consists in recording of a series of Mösbauer spectra versus temperature on each
nanocrystalline sample and then describing the hyperfine structure of all spectra on
the basis of a unique model: an illustration is given in Fig.
4.14
for 2 nanocrys-
talline NANOPERM alloys annealed at 2 different temperatures for 1 h. Such a
route is a difficult task but it provides a great number of data such as the tem-
perature dependencies of isomer shift, quadrupolar shift, quadrupolar splitting and
hyperfine field corresponding to Fe probes located in the nanocrystalline grain, the
amorphous remainder and the resulting interfacial zone, and finally the tempera-
ture independent atomic crystalline fraction (which can be compared to the vol-
umetric crystalline fraction). The hyperfine structure is generally described by
means of three components ascribed to the nanocrystalline grains (narrow lines
magnetic sextet), the residual amorphous phase (broad line sextet which pro-
gressively collapses into a quadrupolar doublet) and the interfacial zone (broad
line sextet which roughly follows the same temperature dependence as that of the
first component) [
119
-
121
]. In addition, to check the free-texture behaviour for
