Biomedical Engineering Reference
In-Depth Information
12.2
SEM microphotograph (BSE image) illustrating a high level of
structure homogeneity in a Cu-Al/Al
2
O
3
composite powder following
aluminothermic reaction with Cu
2
(OH)
2
CO
3
at the final stage of
reactive milling. The darker spots are finely dispersed Al
2
O
3
particles
whose average size is no larger then 1 μm. The brighter spots
correspond to the higher content of Cu-Al intermetallics (see Fig. 12.1)
(SEM Hitachi S-4700 with EDX Noran Vantage) (Wieczorek-Ciurowa
and Gamrat, 2007b).
.
a self-propagating reaction initiated when the reaction enthalpy is
sufficiently high.
These kinetics models depend on the nature, type of reactants and their
properties (e.g. hardness), thermodynamics of the reactions and technical
parameters of milling (Butyagin, 2000, 2003; Takacs et al., 2001; Takacs and
Mandal, 2001; Takacs, 2002; Lyakhov et al., 2008).
The first type of mechanical synthesis concerns reactions, which proceed
more slowly up to the point at which processes become a function of the
milling time. If ignition does not occur, the collisions between milled
material and the grinding medium contribute to comminution, mixing and
defect formation. The formation of the final product occurs gradually.
Figure 12.3 shows the first and final step of mechanochemical synthesis of
Cu-Al/Al
2
O
3
composite powder as a function of temperature and pressure
inside the mill (Wieczorek-Ciurowa et al., 2007b).
The second type requires a critical time for ignition of the reaction. When
the temperature of the vial was recorded during milling, it was observed that
initially the temperature increased slowly with time. After a certain period of
