Biomedical Engineering Reference
In-Depth Information
Table 12.3 Formation enthalpy values of selected
metal oxides
-ΔH (kJ mol
1
)
Oxide
Al
2
O
3
1675.7
NiO
239.7
Cu
2
O
173.2
CuO
161.9
Source: Kubashewski
et al.
, 1993
Table 12.3. From a thermodynamic point of view, the aluminothermic
reactions in the systems studied are significantly favoured.
In Sections 12.5.1 and 12.5.2, the in situ formation of a nanostructural
metallic-ceramics nanocomposite Cu-Al/Al
2
O
3
powder is described starting
from two different mixtures, Cu
2
(OH)
2
CO
3
and CuO with Al, respectively
(Wieczorek-Ciurowa et al., 2002a; Wieczorek-Ciurowa and Gamrat, 2007a,
b; Wieczorek-Ciurowa et al., 2007a,b; 2008). Composite powders were
synthesized by high-energy ball milling in a laboratory planetary mill with
vial and balls made of hardened steel. The component systems were
prepared as physical mixtures in which the amount of Al was calculated
assuming the formation Cu
9
Al
4
with Al
2
O
3
and CuAl
2
with Al
2
O
3
as
components of the synthesis products. Among the intermetallics in the
binary Al-Cu phase diagram (Hansen, 1958) are CuAl
2
,Cu
9
Al
4
, Cu(Al) and
Al(Cu) solid solutions. It was found that if Cu
9
Al
4
was the expected phase,
mechanosynthesis in both reagents used brings about the formation of a
composite material consisting of Cu(Al) solid solution and Al
2
O
3
. The
amount of Al dissolved into the Cu matrix is considerably higher for the
CuO-Al system than for Cu
2
(OH)
2
CO
3
-Al. This is probably caused by the
different kinetics of mechanochemical reactions in these two systems.
According to Avvakumov et al. (2001), compounds containing groups of
atoms with oxygen and/or hydrogen (like acidic or basic salts, hydrates)
take part in reactions evolving water and/or volatile compounds. These
compounds are characterized by 3-4 times lower hardness in comparison to
their anhydrous oxides, probably providing the mechanochemical process
with lower mechanical loading. Processes taking place with these types of
substances are known as soft mechanochemical reactions.
Mechanochemical syntheses where the mixtures are provided with
stoichiometric proportions of reagents to form CuAl
2
result in the
formation of two intermetallic phases, CuAl
2
and Cu
9
Al
4
,aswe las
Al
2
O
3
. It is evident that in the systems with CuO as a reagent for
aluminothermic reaction CuAl
2
appears after only 1 hour of milling. This
indicates the rapid alloying process of both metals: copper and aluminium.
The experimental results are described in detail in Sections 12.5.1 and 12.5.2.
