Biomedical Engineering Reference
In-Depth Information
Table 12.2 Examples of induction time intervals for selected SHS
mechanochemical reactions occurring under different milling conditions
Type of
mill
Induction
time (min) Reference
Reaction
BPR
ZnO
þ
Mg
!
Zn
þ
MgO
Vibratory
7:1
45
Yang and
McCormick,
1993
2FeO
þ
Ti
!
2Fe
þ
TiO
2
Vibratory
8:1
20
Takacs, 2002
8CuO
þ
3Fe
!
4Cu
2
O
þ
Fe
3
O
4
Planetary
30:1
153
Shen
et al.,
1992
2CuO
þ
Zn
!
Cu
2
O
þ
ZnO
Vibratory
-
44
Takacs, 2002
Cu
2
S
þ
Fe
!
2Cu
þ
FeS
Vibratory
8:1
-
McCormick
et al.,
1989
Source: literature data.
reactant crystallinity could be observed as well as a small amount of
product, whereas after 37 min milling time the aluminothermic reaction was
completed. Traditionally, this reaction would be thermally initiated during
high-temperature treatment. However, under mechanical activation condi-
tions a 'flash point' is the direct consequence of previous energy
accumulation in the crystalline structure of powder particles.
In the case of milling time less than 30 min a certain amount of active
product precursor forms from the energy accumulated in the solid phase.
The precursor releases this energy by reacting at lower temperatures during
thermal treatment. This effect is close to the one observed in the TiO
2
-Al
system, but in this case, the aluminothermic reaction does not occur during
milling. This phenomenon of solid activation, because of milling, can be
seen in syntheses other than in metallic-ceramic composites ones.
The kinetics of mechanochemical reactions depends not only on the
reactant properties but also on the synthesis conditions. The synthesis can
be explosive or can occur in a controlled steady way. Changing the
conditions of the activation process can prevent the reaction taking place at
all. As well as the ball-to-powder ratio (BPR), the type of ball material is
also important. Moreover, too low heat conductivity of the milling medium
can hinder formation of hot spots in which the reaction is initiated. Table
12.2 gives induction times (in minutes) for SHS reactions carried out under
the given conditions of mechanochemical syntheses.
On the other hand, in order to control explosive effects during self-
propagating reactions, that is to suppress their kinetics, it is possible for
example to use a process control agent (PCA) such as toluene, which does
not stop the reduction reaction but allows it to run in a controlled manner
(Murty and Ranganathan, 1998; Takacs et al., 2001; Takacs and Mandal,
2001; Takacs, 2002).
The mechanochemical syntheses of metallic-ceramic nanocomposite
