Biomedical Engineering Reference
In-Depth Information
aluminium, the alloying process does not form NiAl but mainly proceeds to
Ni
3
Al formation. Presumably, the reason for this is the different nature and
structure of the nickel salt compared with copper salt. Ni-hydroxocarbonate
belongs to hydrated salts with low crystallinity (Wieczorek-Ciurowa et al.,
2002b; Wieczorek-Ciurowa and Gamrat, 2005). Moreover, crystalline water
stabilizes the structure of the nickel salt, making its mechanical decomposi-
tion more difficult, in effect slowing down the delivery of NiO in the
aluminothermic reaction [12.7]. The examples of two kinds of salts
presented here, Ni
2
(OH)
2
CO
3
·xH
2
O, almost amorphous, and
Cu
2
(OH)
2
CO
3
with high crystallinity showed that crystalline structures are
more vulnerable to mechanical treatment because their activity becomes
higher, for example creating structural defects and new surfaces, and
undergoing mass transfer (mixing).
Special attention should be paid to the important part being played by
water vapour in the milling system of Ni
2
(OH)
2
CO
3
.xH
2
O-Al during
mechanical decomposition of salt. The compound contains both crystalline
water (x=about three molecules) and OH groups. The evolved water
reacting rapidly (
949 kJ mol
1
) with mechanically activated
aluminium powder, see equation 12.6 decreases the amount of Al needed to
synthesise the intermetallics (see Section 12.6.1).
Δ
H
298
=
2Al
þ
6H
2
O
!
2Al
2
O
3
þ
3H
2
½
12
:
6
Too low a concentration of Al in the desired NiAl phase of the
Ni
2
(OH)
2
CO
3
.xH
2
O-Al system gave Ni
3
Al intermetallics in the composite
powder and a phase with a lower amount of aluminium.
Similar to the formation of copper-aluminium matrix/Al
2
O
3
composite
particles, which are the result of strong exothermic reactions, formation of
composite particles of nickel-aluminium/Al
2
O
3
is also typical of a self-
propagating high-temperature synthesis (with a combustible nature). The
enthalpy changes for reactions 12.7 and 12.8 are negative (Barin et al., 1977;
Kubaschewski et al., 1993; Takacs, 2002):
955 kJ mol
1
3NiO
þ
2Al
!
Ni
þ
Al
2
O
3
; D
H
298
¼
½
12
:
7
xNi
þ
Al
!
Ni
x
Al
177 KJ mol
1
where x
¼
1 or 3
; D
H
298
¼
ð
NiAl
Þ
and
153 KJ mol
1
ð
Ni
3
Al
Þ
½
12
:
8
