Biomedical Engineering Reference
In-Depth Information
To produce these materials, combustive reactions need to be suppressed
during mechanochemical synthesis. Ying and Zhang (2000a,b; 2003) show
that this can be achieved utilizing a technique in which the Al powder is first
diluted by Cu and then the resultant Cu-Al alloy powder is reacted with
CuO. The dilution was done through mechanical alloying of Al and Cu into
a Cu(Al) solid solution or Cu-Al intermetallics compounds (depending on
the amount of Al needed in the mixture). In the next step, powders of Cu-Al
alloy and CuO were milled to form a composite structured powder. Finally,
this composite powder was heated (
<
8
C) causing the reaction between
CuO and Cu(Al) or Cu
9
Al
4
. Subsequent heating at a temperature below
800
450
C led to formation of Cu and Al
2
O
3
. Mechanical treatment enhances
the kinetics of both reactions by refining the composite particles structure.
Another technique, also a combination of oxidation and mechanical
alloying, has been used to produce Cu-Al
2
O
3
, metal matrix-ceramic
composite powders. With this method, Cu powder is first partially oxidized
and then the powder obtained is mechanically alloyed with Al powder to
facilitate the aluminothermic reaction of Al
2
O
3
particles
8
formation
(Bobrova and Besterci, 1994).
Particulate reinforced Cu/Al
2
O
3
metal matrix-ceramic composites can be
mechanochemically synthesized by using several methods which include
mixing of Cu melt and Al
2
O
3
powder followed by calcination and internal
oxidation of Cu-Al alloy powders (Shi and Wang, 1998). The calcination
process is limited (Liang et al., 2004) because the Al
2
O
3
particle size has to
be large enough to allow effective milling, while internal oxidation can only
produce composites with a low volume fraction of Al
2
O
3
particles, such as
oxide dispersion strengthened alloys. Oxidation was used in conjunction
with mechanical alloying (Ogbuji, 2004). In this process, Cu-Al alloy
powder was milled under an oxidizing atmosphere to produce the composite
powder. The advantage of this process is that very small oxide particles can
be achieved.
Another approach that avoids too high activities of the reactants has been
made by Venugopal et al. (2005) who introduced toluene as the milling
medium (PCA) in copper-alumina composite synthesis by reactive milling
of CuO/Cu
2
O and Al. In fact, the transformation of a combustible reaction
to a progressive one yields nanocomposite particles of Cu and Al
2
O
3
with
both components with a crystallite size in the range of about 20 nm.
Hwang and Lee (2005) demonstrated that Cu/Al
2
O
3
nanocomposite
powders with various vol% of alumina as a reinforcement phase have been
successfully produced by mechanochemical synthesis in a high-energy
attritor mill using mixtures of Cu, Cu
2
O and Al powder components. It is
important to note that in this case the heat generated during milling was
removed by a cooling tube coil attached to the outside of the milling
chamber. Moreover, in every stoichiometric reaction, excess Cu was added
