Biomedical Engineering Reference
In-Depth Information
Figure 1. Transport processes at solid-liquid-vapour interfaces.
the release of volatile compounds (oxides, sub-oxides, etc.); (b) adsorbed layers or
even new compounds can form at the solid surface, originated from metal vapours.
In the first case, the released components can enter the liquid phase, modifying the
liquid surface and interfacial tensions, while in the second case these new surface
layers can modify the contact angle, as a result of a new balance among the three
interfacial tensions, as shown by the Young equation (1).
When working with molten metallic phases, one of the most relevant effects
is caused by oxygen: this element, ubiquitous in high temperature processes, has
a tensioactive effect on the liquid-vapour as well as on the solid-liquid surface
tensions, through adsorption processes and reactions [2-6]. In particular, studies
taking into account both thermodynamics and transport phenomena have shown
that a molten metal surface can be kept oxygen-free despite the thermodynamic
driving forces foresee surface oxidation conditions [7-11]. This important effect,
which is substantiated by the definition of an 'effective oxygen pressure' [8], is
mainly due to oxygen and suboxides flux in the opposite direction to the oxygen
adsorption/reaction. On the other hand, once oxygen reaches the liquid surface,
it diffuses through the liquid phase, eventually reaching the solid-liquid interface
modifying its energetics by adsorption and/or formation of new phases [12].
Indeed, oxygen can also play a positive effect on the wetting of solid oxides, and
this effect is currently exploited in the 'air brazing technique' which makes use of
the strong effect of oxygen to increase the wettability of oxides (mainly those to be
used in force of their electrical properties which depend on oxygen stoichiometry)
by means of processes running in air or in atmospheres with high oxygen content
[13-15].
Under equilibrium conditions, the variation of surface and interfacial tensions as
a function of oxygen concentration in the liquid phase ( M ) can be interpreted in
terms of the Gibbs adsorption equation
d σ
=−
O d μ O
M d μ M
(3)
referred to the liquid/vapour or to the solid/liquid interfaces. Following the develop-
ment used by Chatain et al. [16] and provided that the dissolution of the metal cation
from the oxide substrate into the liquid matrix can be considered negligible, the ex-
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