Influence of the Carbon Allotropes on Dilatometric Properties …
667
curves 2 (Figs. 12 and 13) is in accordance with the diagram of Fe–C system states,
as it is consistent with the shift to lower values of temperature of the α-Fe → γ-Fe
transformation with carbon content increasing [27].
The further increase of the MWCNT content (Fe–Cu-MWCNT(1.0) composite)
suppresses the described processes, first of all, as we believe due to the formation
of carbides in the surface layers of a large number of iron grains, and leads to a
certain separation of the carbides, iron and coppers grains and carbon nanotubes.
As a result, the temperature behavior of the L/L 0 (T) and α(T) characteristics for
samples Fe–Cu and Fe–Cu-MWCNT(1.0) becomes quite similar.
The incorporation into the composite of twice concentration of carbon nanotubes
(sample Fe–Cu-MWCNT(2.0)) enhances, in our opinion, the processes described
above lead to the formation of the iron carbide and the products of their decomposition
layer between metallic grains. It is known that CNTs in the axial and radial directions
have the CTE of opposite sign: −1.2 · 10
−5 K
−1 and (1.6–2.6)·10
−5 K
−1 [56, 57]).
This means that in their disordered state, and that is, obviously, characteristic of
the nanotubes state in the investigated composites, carbon nanotubes should have
a low, or close to zero, CTE. Thus, the above-mentioned MWCNT’s layers in the
significant amount of the MWCNT (2 vol.%) actually damp an expansion of metal
fractions of composites, which is illustrated by the behavior of the curve 4 in Figs. 12
and 13. The results discussed are in agreement with data on some other similar CNT
nanomaterials, where their CTEs become practically equal to the CTE of carbon
nanotubes [38, 39, 51, 58, 59]. Such a strong decrease in CTE is associated with
the formation of iron oxides and the formation of M–O-CNT bonds [60, 61] or with
the formation of iron carbides due to carbon tube atoms [55, 62]. Both types of
formations have relatively low CTEs at the grains boundaries and thus, they block
the possible expansion of the metal matrix of the composite.
Our study of macro-Raman light scattering in near-surface layers of the samples
actually revealed a number of lines in the frequency range 150–800 cm
−1 , which
correspond to internal vibrations of various types of iron and copper oxides (see
Fig. 17). (In fact, the lines at this spectral range are located on a wide background
of the excitation line wing.) The goethite (α-FeO 2 H), hematite (α-Fe 2 O 3 ), hydrohematite (H 2 Fe 4 O 7 ), protogemate (Fe 2 O 3 ), and other its derivatives [63] can be such
iron oxides, for example Their phases are unstable to mechanical influences, and
therefore their content can considerably vary for composite samples with the same set
but with different content of components. The data give in Fig. 16 from the frequency
range lower than 500 cm
−1 are consistent with this assertion. The described results,
obviously, confirm the possibility of implementation of the above noted “oxide”
mechanism of influence on the temperature behavior of the nanocomposites under
study.
As for the “carbide” mechanism of influence, the MWCNT state is important
for its implementation, as well as for the activity of all other processes that determine the dilatometric behavior of Fe–Cu-MWCNT nanocomposites. Obviously, the
formation of carbides, as well as the procedure of mechanical–chemical activation,
is detrimental to CNT. Thus, the loss of carbon atoms that diffuse to grains and then
combine with iron forming carbides, at the same time, is the source of defects, and
667
curves 2 (Figs. 12 and 13) is in accordance with the diagram of Fe–C system states,
as it is consistent with the shift to lower values of temperature of the α-Fe → γ-Fe
transformation with carbon content increasing [27].
The further increase of the MWCNT content (Fe–Cu-MWCNT(1.0) composite)
suppresses the described processes, first of all, as we believe due to the formation
of carbides in the surface layers of a large number of iron grains, and leads to a
certain separation of the carbides, iron and coppers grains and carbon nanotubes.
As a result, the temperature behavior of the L/L 0 (T) and α(T) characteristics for
samples Fe–Cu and Fe–Cu-MWCNT(1.0) becomes quite similar.
The incorporation into the composite of twice concentration of carbon nanotubes
(sample Fe–Cu-MWCNT(2.0)) enhances, in our opinion, the processes described
above lead to the formation of the iron carbide and the products of their decomposition
layer between metallic grains. It is known that CNTs in the axial and radial directions
have the CTE of opposite sign: −1.2 · 10
−5 K
−1 and (1.6–2.6)·10
−5 K
−1 [56, 57]).
This means that in their disordered state, and that is, obviously, characteristic of
the nanotubes state in the investigated composites, carbon nanotubes should have
a low, or close to zero, CTE. Thus, the above-mentioned MWCNT’s layers in the
significant amount of the MWCNT (2 vol.%) actually damp an expansion of metal
fractions of composites, which is illustrated by the behavior of the curve 4 in Figs. 12
and 13. The results discussed are in agreement with data on some other similar CNT
nanomaterials, where their CTEs become practically equal to the CTE of carbon
nanotubes [38, 39, 51, 58, 59]. Such a strong decrease in CTE is associated with
the formation of iron oxides and the formation of M–O-CNT bonds [60, 61] or with
the formation of iron carbides due to carbon tube atoms [55, 62]. Both types of
formations have relatively low CTEs at the grains boundaries and thus, they block
the possible expansion of the metal matrix of the composite.
Our study of macro-Raman light scattering in near-surface layers of the samples
actually revealed a number of lines in the frequency range 150–800 cm
−1 , which
correspond to internal vibrations of various types of iron and copper oxides (see
Fig. 17). (In fact, the lines at this spectral range are located on a wide background
of the excitation line wing.) The goethite (α-FeO 2 H), hematite (α-Fe 2 O 3 ), hydrohematite (H 2 Fe 4 O 7 ), protogemate (Fe 2 O 3 ), and other its derivatives [63] can be such
iron oxides, for example Their phases are unstable to mechanical influences, and
therefore their content can considerably vary for composite samples with the same set
but with different content of components. The data give in Fig. 16 from the frequency
range lower than 500 cm
−1 are consistent with this assertion. The described results,
obviously, confirm the possibility of implementation of the above noted “oxide”
mechanism of influence on the temperature behavior of the nanocomposites under
study.
As for the “carbide” mechanism of influence, the MWCNT state is important
for its implementation, as well as for the activity of all other processes that determine the dilatometric behavior of Fe–Cu-MWCNT nanocomposites. Obviously, the
formation of carbides, as well as the procedure of mechanical–chemical activation,
is detrimental to CNT. Thus, the loss of carbon atoms that diffuse to grains and then
combine with iron forming carbides, at the same time, is the source of defects, and