Influence of the Carbon Allotropes on Dilatometric Properties …
677
5 Conclusions
Dilatometric characteristics (relative expansion, L/L 0 (T), and linear thermal expansion coefficient, CTE or α(T)), and calorimetric data of the concentration set of
composite materials Fe–Cu-MWCNT (concentration of the MWCNT = 0, 0.5, 1.0,
and 2.0 vol.%; Fe: Cu ratio was 4:1) were discussed together with data on the composition, structure and morphology of the samples, for that the studies of their X-ray
diffraction, Raman spectroscopy, IR absorption, optical and electron microscopy
were carried out.
Temperature dependences of dilatometric characteristics revealed a number of
specific components that can not be accurately interpreted up to now due to their
significant superposition. Several temperature ranges were distinguished for the Fe–
Cu composite by the character of the L/L 0 (T) and α(T) dependences. The average
values of the coefficient of thermal expansion <α i (T)> calculated for each of the
temperature ranges were of ~4–6 times smaller than the known literature data for
Fe–Cu and Fe–Cu-CNT composites.
The addition of the MWCNTs to the Fe–Cu composite results in changes in the
L/L 0 (T) and α(T) dependencies), and the effect of MWCNTs being different for
different temperature ranges; moreover, this effect depends on the content of the
nanotubes. This is evidence of the manifestation of various mechanisms of thermal
expansion of the investigated composites and, at the same time, of the various effects
of carbon nanotubes on these mechanisms.
Temperature changes of crystalline lattice constants; multivariate interaction of
components; transformation of the lattice structure; phase transformation of components; reduction/shrinkage of the sample due to the temperature-activated release of
adsorbed gases and gases of adsorbed compounds formed as a result of the thermal
decomposition; sintering; and, conversely, the disintegration of grains and particles,
as well as the destruction of MWCNTs, were discussed as possible mechanisms of
thermal expansion of the studied composites.
In particular, it was shown that mechanism of “pure” thermal expansion of Fe–Cu
composite dominates in the temperatures range 375–550 °C, if to approximate the
experimental dependence of L/L 0 (T) on Fe–Cu composites with the assumption
of its linear expansion and to use the calculated <α i (T)> values.
The processes related to the change in composition and structure of composites can
be conventionally divided into two groups, such as “oxidation” and “carbidization,”
and carbon atoms of nanotubes play an important role both for each of them and in
ensuring their interconnection. Thus, the analysis of the IR and Raman spectroscopy
data allowed us to reveal carboxyl, carbonyl, and hydroxyl groups and the appearance
of which in the samples is due to the impossibility of avoiding contact with the
atmosphere in the processes of mechano-chemical activation and subsequent heat
treatment and rolling of composites. In the presence of nanotubes in the samples, the
number of molecular groups increases due to their adsorption by tubes and products of
their destruction. The mentioned molecular groups desorption occurs during heating,
which is accompanied by displacement and shrinkage of the composite grains. The
677
5 Conclusions
Dilatometric characteristics (relative expansion, L/L 0 (T), and linear thermal expansion coefficient, CTE or α(T)), and calorimetric data of the concentration set of
composite materials Fe–Cu-MWCNT (concentration of the MWCNT = 0, 0.5, 1.0,
and 2.0 vol.%; Fe: Cu ratio was 4:1) were discussed together with data on the composition, structure and morphology of the samples, for that the studies of their X-ray
diffraction, Raman spectroscopy, IR absorption, optical and electron microscopy
were carried out.
Temperature dependences of dilatometric characteristics revealed a number of
specific components that can not be accurately interpreted up to now due to their
significant superposition. Several temperature ranges were distinguished for the Fe–
Cu composite by the character of the L/L 0 (T) and α(T) dependences. The average
values of the coefficient of thermal expansion <α i (T)> calculated for each of the
temperature ranges were of ~4–6 times smaller than the known literature data for
Fe–Cu and Fe–Cu-CNT composites.
The addition of the MWCNTs to the Fe–Cu composite results in changes in the
L/L 0 (T) and α(T) dependencies), and the effect of MWCNTs being different for
different temperature ranges; moreover, this effect depends on the content of the
nanotubes. This is evidence of the manifestation of various mechanisms of thermal
expansion of the investigated composites and, at the same time, of the various effects
of carbon nanotubes on these mechanisms.
Temperature changes of crystalline lattice constants; multivariate interaction of
components; transformation of the lattice structure; phase transformation of components; reduction/shrinkage of the sample due to the temperature-activated release of
adsorbed gases and gases of adsorbed compounds formed as a result of the thermal
decomposition; sintering; and, conversely, the disintegration of grains and particles,
as well as the destruction of MWCNTs, were discussed as possible mechanisms of
thermal expansion of the studied composites.
In particular, it was shown that mechanism of “pure” thermal expansion of Fe–Cu
composite dominates in the temperatures range 375–550 °C, if to approximate the
experimental dependence of L/L 0 (T) on Fe–Cu composites with the assumption
of its linear expansion and to use the calculated <α i (T)> values.
The processes related to the change in composition and structure of composites can
be conventionally divided into two groups, such as “oxidation” and “carbidization,”
and carbon atoms of nanotubes play an important role both for each of them and in
ensuring their interconnection. Thus, the analysis of the IR and Raman spectroscopy
data allowed us to reveal carboxyl, carbonyl, and hydroxyl groups and the appearance
of which in the samples is due to the impossibility of avoiding contact with the
atmosphere in the processes of mechano-chemical activation and subsequent heat
treatment and rolling of composites. In the presence of nanotubes in the samples, the
number of molecular groups increases due to their adsorption by tubes and products of
their destruction. The mentioned molecular groups desorption occurs during heating,
which is accompanied by displacement and shrinkage of the composite grains. The