Biomedical Engineering Reference
In-Depth Information
12.5.1 Effects of mechanosynthesis in the Cu
2
(OH)
2
CO
3
-Al
reagent system
Figure 12.7 shows the XRD patterns (Philips X'Pert diffractometer Cu K
)
of the Cu
2
(OH)
2
CO
3
-Al system mechanically treated at different time
intervals (Pulverissette 6 Fritsch GmbH). The results indicate that the initial
components of the mixture studied still exist in the system after up to 10
hours of mechanical treatment. However, their intensities reduced
significantly in the fourth hour of milling. This indicates that
Cu
2
(OH)
2
CO
3
started to decompose and metallothermic reduction of
CuO with Al proceeded gradually with formation of metallic copper.
Furthermore, a broad diffraction peak appears in the range of 2
α
=42
8
to
y
45
after 10 hours of milling. Its intensity increases significantly with a
lengthening of the milling time. The shape of this X-ray indicates its
complexity and the deconvolution confirms the presence of copper in
addition to part of the Al
2
O
3
phase. After 15 hours and 20 hours of milling,
the position of the Cu(1 1 1) peak shifted from 2
8
,
respectively, while Al peaks completely disappeared. This may suggest that a
solid solution of Cu(Al) has been formed as a result of mechanical alloying
of two metals, Cu and Al. This is con-firmed by an increase of metallic
copper lattice parameter from 3.6151 A
˚
to 3.6203 A
˚
(Pearson, 1958).
According to Vegard's law, which states that the lattice parameter of the
solid solution is a function of the amount of solute and using parameters for
pure Cu and Al, the contents of the Al solute in the Cu matrix for the
mechanically treated Cu
2
(OH)
2
CO
3
-Al system are estimated on a level of
about 5%.
From a thermodynamic point of view, the reactions occurring in the
tested system are highly exothermic, for example the enthalpy of
aluminothermic reduction of CuO with Al equals
= 43.32
8
to 43.25
8
y
1179 kJ mol
1
.
However, in the hydroxosalt-active metal system, processes in the mill
occur in a controlled manner. Coming back to Fig. 12.3 illustrating the
temperature and pressure in a vial during milling process, it can be seen that
in the first hours of milling the temperature and pressure increase
simultaneously. The rise in pressure is due to mechanical decomposition
of Cu
2
(OH)
2
CO
3
to CuO with H
2
O and CO
2
evolving, while the
temperature increase is caused by the friction and impact of the balls.
When Cu-hydroxocarbonate is completely decomposed, any rise in pressure
is observed, whereas only a transitory rise in temperature occurs (Fig.
12.3b). On the T versus milling time curve, one can observe several small
rises in the gas temperature. However, all of them take place below 40
8
C.
This indicates that the aluminothermic reduction occurs locally, in a
repeatable fashion. Based on these data we can conclude that in this case
mechanochemical synthesis takes place in a controlled way, gradually, so
