Why do
γ
-Fe nanoparticles
inside carbon nanotubes
abnormally expanse
at high temperature?
The thermal expansion of
γ
-Fe (austenite)
nano-crystals confined inside multiwalled
carbon nanotubes (MWCNTs) is studied
in situ, using synchrotron x-ray diffraction,
as a function of temperature. A peculiar
behaviour is evidenced for these nano-
crystals: the thermal expansion becomes
abnormally high above ~450°C. A scenario
involving progressive carbon uptake into
γ
-Fe gives a satisfactory understanding
of the phenomenon, and allows one to
propose a value of the carbon solubilization
rate in
γ
-Fe particles confined in MWCNTs.
Carbon nanotubes can be viewed
as nano-containers, where confined
matter can exhibit unusual properties.
Iron nano-crystals inside CNTs are formed
during the synthesis of MWCNT samples
by aerosol-assisted CCVD [1]. In this study,
we focused on the thermal expansion
behaviour of
γ
-Fe particles contained
in the inner core of MWCNTs (Figure
➊
)
by means of an in situ x-ray scattering
experiment as a function of temperature
conducted on the Diffabs beamline.
The existence of
γ
-Fe crystals under
ambient conditions may appear surprising
as this crystalline phase is not supposed
to be stable. It has already been discussed
that after formation at high temperature,
where
γ
-Fe is stable, the narrow space
available in the CNT does not allow
the structural phase transition from
the
γ
phase to the low temperature stable
α
-Fe phase (ferrite) to take place.
The
γ
-Fe structure would consequently
be conserved at low temperature [2].
Figure
➋
shows the relative thermal
expansion of
γ
-Fe deduced from the 200
peak position. The thermal expansion
coefficient, A, is defined by:
where
d
is the reticular distance
at temperature
T
and
d
0
is the reference
reticular distance at temperature
T
0
(
T
0
is equal to 50°C in our experiment).
The thermal evolution of
γ
-Fe nano-
crystals exhibits a peculiar behaviour,
featuring two thermal regions in which
the thermal expansion is different.
For temperatures lower than 450°C,
the thermal expansion coefficient
is equal to 2.0.10
-5
K
-1
. It is similar to
that measured for
γ
-Fe nano-inclusions in
FeCu crystals [3]. However, the large value
of 3.7.10
-5
K
-1
measured at temperatures
above 450°C is very unusual. We propose
that the increased thermal expansion
could originate from the gradual
intercalation of carbon atoms in the fcc
lattice of
γ
-Fe as interstitial atoms. Such
a phenomenon is likely, considering
the large quantity of carbon available
in the vicinity of iron-based particles.
Based on the work by Onink et al. [4]
dedicated to the intercalation of carbon
atoms in austenite, it is possible to derive
the solubilization rate of carbon into
γ
-Fe nanoparticles from the measured
excess of lattice expansion. A maximum
solubilization of 2.1 Carbon atoms / 100 Fe
atoms is deduced from our data at 750°C.
We therefore determine a carbon thermal
solubilization rate of 8.4.10
-3
Carbon
at.K
-1
in the 500-750°C temperature
range. Moreover, it is worth noting that
the same slope change in the variation
of lattice parameter with temperature
appears both during heating and cooling.
The phenomenon of carbon uptake is thus
reversible. The possibility of tuning the
carbon contents in the crystalline lattice
of
γ
-Fe nanoparticles could open
interesting perspectives for applications
in the field of magnetism.
Introduction
Results and Discussion
PHYSICS AND CHEMISTRY OF CONDENSED MATTER, EARTH SCIENCES
d - d
0
= A
•
(T- T
0
)
d
0
98
SOLEIL
HIGHLIGHTS
2013
