Biomedical Engineering Reference
In-Depth Information
structures that are more homogeneous than those synthesized using
conventional ex situ techniques (Section 12.7).
In metallic-ceramic composite powder formation, the preparation of
metals and alloys by reducing their salts or oxides with more reactive metals
is commonly known as metallothermic reduction. This reaction is expressed
in general by the equation:
M
A
X
þ
M
B
>
M
A
þ
M
B
X
½
12
:
2
where a metal M
A
is reduced by a more reactive metal M
B
(reductant) to the
pure metal MA. M
A
XandM
B
X are oxides, chlorides, sulphides and other
salts.
Metallothermic reactions are characterized by a large negative free-energy
change and therefore they are thermodynamically feasible at room
temperature (see for example reactions 12.4 and 12.5. Moreover, the
mechanical activation of reagents significantly increases the synthesis
kinetics. A salt such as M
A
hydroxocarbonate can be used as a 'source' of
M
A
O, as described in Sections 12.5 and 12.6.
12.5 Copper-based composite powders with Al
2
O
3
The formation of a copper-based composite powder with aluminium is a
consequence of many complex, simultaneous and subsequent chemical
reactions that occur in the milling of a Cu
2
(OH)
2
CO
3
and Al mixture.
They are schematically reported below (equations 12.3-12.5) together
with the enthalpy values (
H
298
/kJ mol
1
; Barin et al., 1977):
Δ
.
mechanochemical decomposition of salt: +82
Cu
2
ð
OH
Þ
2
CO
3
ð
s
Þ
!
2CuO
ð
s
Þ
þ
CO
2
ð
G
Þ
þ
H
2
O
ð
G
Þ
½
12
:
3
.
aluminothermic reduction of CuO to Cu with Al
2
O
3
formation:
1179
2Al
0
ð
s
Þ
!
3Cu
0
3CuO
ð
s
Þ
þ
ð
s
Þ
þ
Al
2
O
3
ð
s
Þ
½
12
:
4
.
mechanical alloying of Cu and Al (in the presence of Al
2
O
3
):
8
(Cu
9
Al
4
)and
4 (CuAl
2
)
Cu
0
ð
s
Þ
þ
Al
2
O
3
ð
s
Þ
þ
Al
0
ð
s
Þ
!
Cu
0
Al
0
þ
Al
2
O
3
½
12
:
5
subscript (S) indicates solid and (G) indicates gas. The product is a metallic-
ceramic composite powder. Cu
0
-Al
0
alloy can exist as intermetallics and/or
a solid solution.
The formation enthalpy of oxide M
B
X should be higher than that for
M
A
X (see equation 12.2). The values for Cu, Ni and Al oxides are given in
