Biomedical Engineering Reference
In-Depth Information
12.4
Variations in temperature and pressure (GTM results) during self-
propagating high-temperature synthesis of Cu-Al/Al
2
O
3
composite
powder from a CuO-Al mixture. The milling conditions are the same
as used for a Cu
2
(OH)
2
CO
3
-Al mixture treatment (see Fig. 12.3)
(planetary ball mill Fritsch GmbH
Pulverisette 6
) (Wieczorek-Ciurowa
et al.
, 2007a).
2007a). The ignition temperature is a function of the enthalpy change and
the microstructure parameters of the particles, for example the inter-facial
area between the reactants. Continuing milling is necessary to form the
correct desired structure of synthesis products. Generally, a self-propagating
high-temperature synthesis (SHS) reaction consists of three stages:
component activation, chemical reaction initiation and a synthesis process
in bulk with product formation. Schaffer and McCormick (1992) stated that
SHS reactions occur when the temperature created by the colliding balls in a
ball mill is higher than initiation temperature, which depends on enthalpy
changes in the process. The decreasing of this temperature can be the effect
of mechanical activation by diminution of particle size and increasing the
surface between interfacial contacts.
The explanation of this phenomenon is as follows. It was assumed that
after a period of comminution, mixing and activation, agglomerates begin to
form and increase in size. The reaction starts in a single agglomerate or in
the powder layer coating a milling ball or the wall of the vial. One reaction
front propagates into other parts of the powder layer. The powder layer can
be attached to the surface of a milling ball or the inner wall of the container.
When a ball hits this layer, part of the kinetic energy is transferred to the
powder as heat and a local increase of temperature can occur. The stresses
