664
M. C. Bouleklab et al.
> CO, hydroxyl, −OH, etc.). Thus, the line at 3240 cm
−1 is associated with the O–H
bond of the CO 2 H group [50]. The doublet at 2850 and 2920 cm
−1 corresponds to the
symmetric and antisymmetric valence vibrations of the CH 2 group [51]. The lines at
2345 and 2355 cm
−1 are a manifestation of the > C=O bonds. Two peaks at 1642 and
1523 cm
−1 are due to C=C bonds that confirm their origin from carbon nanotubes.
In our opinion, the appearance of the mentioned molecular groups in the content of
the Fe–Cu composite should be considered as a consequence of certain participation
of the environment atmosphere in the processes of mechano-chemical activation
and subsequent heat treatment and rolling of the investigated materials. Increase
in the IR absorption intensity caused by such groups in the composites containing
carbon nanotubes (Fig. 14, curves 2–4) clearly shows that tubes and products of
their destruction can adsorb this type of molecular groups and their radicals from
the environment in conditions of high local temperatures, which are realized on the
surface of iron and copper grains under milling. It is obvious that under the heating
of the Fe–Cu composite in the temperature range of 35–200 °C, desorption of the
mentioned molecular groups occurs, that is accompanied by the displacement and
shrinkage of the composite. The further increase in temperature, as already shown
above, leads to the thermal expansion of the composite, which begins to dominate
the effect of shrinkage, and as a result, in the range of temperatures from 350 to 500
°C, the dimension of the sample varies, mainly due to the “pure” thermal expansion.
The shrinkage effect in the region of 35–200 °C is weakened for the Fe–CuMWCNT(0.5) composite containing the smallest amount of the MWCNT (Fig. 12,
curve 2), and for samples with a higher content of nanotubes, the reduction of samples
at these temperatures is not observed at all. This result, in our opinion, is due to the
fact that in the presence of a large number of the tubes in the samples, adsorption of the
mentioned molecular groups occurs predominantly on carbon nanotubes. The binding
energy of such molecules with MWCNT is quite significant and the desorption of
molecules is reduced at temperatures below 200 °C, and therefore the associated
shrinkage is minimized. (In particular, the binding energy is 0.92–1.72–3.0 eV for
the −CO 2 H, = CO and −OH groups, respectively, according to various data [53–
55]. A reduction of the Fe–Cu and Fe–Cu-MWCNT(0.5) samples can result from the
desorption of these molecules from the MWCNT surface, but this desorption already
can be occurred in the range of higher temperatures: 500–650 °C (Fig. 12, curves 1,
3). If for the last curve the reduction is obvious, then the effect of shrinkage of the
Fe–Cu sample is shown by the difference between the experimental curve 1 (Fig. 12)
and the model straight line (2) for this temperature range, which is a dashed–dotted
line in the Fig. 12.
At the same time, the phase transformation processes are already competitive in
this temperature range, in our opinion. Additional arguments regarding the role of
structural and phase transformations in the dilatometric behavior of Fe–Cu-MWCNT
composites, as expected have been provided by the XRD data (Fig. 15) (see also [31,
41]). Indeed, according to these data, the samples of the iron and copper powders
compacts, Fe–Cu, after their mechano-chemical activation are multicomponent and
contain the α-Fe phases with a solid Cu (α-Fe, Cu) solution and the composition of
which varies from 95 wt% of iron and 5% of copper to 83 wt% of iron and 17% of
M. C. Bouleklab et al.
> CO, hydroxyl, −OH, etc.). Thus, the line at 3240 cm
−1 is associated with the O–H
bond of the CO 2 H group [50]. The doublet at 2850 and 2920 cm
−1 corresponds to the
symmetric and antisymmetric valence vibrations of the CH 2 group [51]. The lines at
2345 and 2355 cm
−1 are a manifestation of the > C=O bonds. Two peaks at 1642 and
1523 cm
−1 are due to C=C bonds that confirm their origin from carbon nanotubes.
In our opinion, the appearance of the mentioned molecular groups in the content of
the Fe–Cu composite should be considered as a consequence of certain participation
of the environment atmosphere in the processes of mechano-chemical activation
and subsequent heat treatment and rolling of the investigated materials. Increase
in the IR absorption intensity caused by such groups in the composites containing
carbon nanotubes (Fig. 14, curves 2–4) clearly shows that tubes and products of
their destruction can adsorb this type of molecular groups and their radicals from
the environment in conditions of high local temperatures, which are realized on the
surface of iron and copper grains under milling. It is obvious that under the heating
of the Fe–Cu composite in the temperature range of 35–200 °C, desorption of the
mentioned molecular groups occurs, that is accompanied by the displacement and
shrinkage of the composite. The further increase in temperature, as already shown
above, leads to the thermal expansion of the composite, which begins to dominate
the effect of shrinkage, and as a result, in the range of temperatures from 350 to 500
°C, the dimension of the sample varies, mainly due to the “pure” thermal expansion.
The shrinkage effect in the region of 35–200 °C is weakened for the Fe–CuMWCNT(0.5) composite containing the smallest amount of the MWCNT (Fig. 12,
curve 2), and for samples with a higher content of nanotubes, the reduction of samples
at these temperatures is not observed at all. This result, in our opinion, is due to the
fact that in the presence of a large number of the tubes in the samples, adsorption of the
mentioned molecular groups occurs predominantly on carbon nanotubes. The binding
energy of such molecules with MWCNT is quite significant and the desorption of
molecules is reduced at temperatures below 200 °C, and therefore the associated
shrinkage is minimized. (In particular, the binding energy is 0.92–1.72–3.0 eV for
the −CO 2 H, = CO and −OH groups, respectively, according to various data [53–
55]. A reduction of the Fe–Cu and Fe–Cu-MWCNT(0.5) samples can result from the
desorption of these molecules from the MWCNT surface, but this desorption already
can be occurred in the range of higher temperatures: 500–650 °C (Fig. 12, curves 1,
3). If for the last curve the reduction is obvious, then the effect of shrinkage of the
Fe–Cu sample is shown by the difference between the experimental curve 1 (Fig. 12)
and the model straight line (2) for this temperature range, which is a dashed–dotted
line in the Fig. 12.
At the same time, the phase transformation processes are already competitive in
this temperature range, in our opinion. Additional arguments regarding the role of
structural and phase transformations in the dilatometric behavior of Fe–Cu-MWCNT
composites, as expected have been provided by the XRD data (Fig. 15) (see also [31,
41]). Indeed, according to these data, the samples of the iron and copper powders
compacts, Fe–Cu, after their mechano-chemical activation are multicomponent and
contain the α-Fe phases with a solid Cu (α-Fe, Cu) solution and the composition of
which varies from 95 wt% of iron and 5% of copper to 83 wt% of iron and 17% of