Biomedical Engineering Reference
In-Depth Information
powders carried out in a non-explosive way based on copper-aluminium
and nickel-aluminium alloys reinforced by aluminium oxide using their salts
instead of oxides are described in Sections 12.5 and 12.6. For comparison,
the explosive reactions in metal oxide with active metal systems are also
presented.
12.3 Monitoring mechanochemical processes
A comprehensive study of the physical and chemical processes that occur
during mechanical treatment by high-energy ball milling appears only to be
possible if a reliable identification of solids and the quantitative phase
analysis of activated products is made. Because of the complexity of
mechanochemical reactions, the nature of the solids obtained closely relates
to the milling conditions and so they should be well defined. Moreover, it is
very important to determine the factors that influence the activation effects.
Another difficulty arises from the fact that the reactions are composed of
many successive stages which vary in different cases. The experimental
methods required to identify and characterize materials synthesized
mechanochemically involve not only techniques that are applicable to
solids, but also others, more particularly adapted to the nanostructured
character of the milling products. Thus, different types of analytical
methods must be applied (e.g. Wieczorek-Ciurowa et al., 2000, 2001; Bala´ zˇ ,
2008).
The best way to study the kinetics of mechanochemical transformations
would be to analyse continuously the milled products in situ. However, until
now this has been a difficult task. The only method that has been used in
some laboratories is the gas pressure-temperature measuring system (GTM)
to acquire in situ data during planetary ball milling; for example, Fritsch
GmbH mills, which enable indirect control and provision of synthesis of
chemical compounds by selection of appropriate parameters for milling
(http://www.fritsch.com). Recording from the beginning of the milling
process is especially useful when gaseous products evolve from the system
caused by mechanical decomposition of the compounds. It is also useful
when the self-propagating high-temperature syntheses occur in an explosive
manner owing to local overheating, for example by two impacting balls (e.g.
Murty and Ranganathan, 1998; Kwon et al., 2002). In such cases applying
less intensive milling could be considered, for example by using lower values
of rotation per minute (rpm) and/or ball-to-powder mass ratio and/or the
size and material of grinding balls, as well as controlling the milling
atmosphere and the type of control agent. Optimization of the milling
parameters chosen experimentally must be taken into account when
selecting mechanochemical syntheses conditions because not all
these
variables are completely independent.
