Biomedical Engineering Reference
In-Depth Information
tent of oxygen adsorption O with respect to M is given by: at the liquid/vapour
interface:
1
RT
d σ LV
dln X O
L O
=−
(4)
and, at the solid/liquid interface:
1
RT
d σ SL
dln X O .
S O
=−
(5)
Here X O represents the oxygen mole fraction in the liquid phase, and is calculated
after the Sievert's law (Eq. (6))
K( PO 2 ) 1 / 2 , (6)
where K is the equilibrium reaction constant of the dissolution reaction of oxygen
into the liquid metal.
In addition to oxygen, other elements can play a fundamental role in modify-
ing the wetting characteristics of an otherwise non-wetting metal. Thermodynamics
provides criteria for selection of the most effective solute for promoting wetting: ex-
periments and theoretical models are today available which demonstrate this effect,
also trying to understand, from more basic points of view, the underlying principles
[17-22].
Thus, a complex situation occurs at the solid-liquid interface.
In Fig. 1, a schematic picture is shown, where the liquid phase rests, at equilib-
rium, on a modified solid-liquid interface. In order to understand what may happen
at this specific interface, the following processes must be taken into consideration:
(a) dissolution of the solid into the liquid, (b) penetration/diffusion of the liquid
components into the solid, (c) adsorption of components of the liquid phase at the
solid-liquid interface, (d) reaction of some component of the liquid with the solid
and formation of new phases, (e) dynamic restructuring of the solid surface.
As all these processes depend strongly on atoms movements, each one of them
has its characteristic time (depending on diffusion coefficients and/or reaction rates
and, thus, on temperature) which in turn affects the equilibration kinetics, i.e., the
overall wetting process.
From the large amount of experimental results and from their thermodynamic in-
terpretation, it appears clearly that the adhesion between liquid metals and ceramic
materials is mainly due to two kinds of chemical bonds: those at the metal-ceramic
interface and the metal-metal ones. An efficient way to study these interactions
and to quantify their effects in terms of adhesion energy is offered by modelling
[23-25], through molecular dynamics approaches or by applying the Density Func-
tional Theory (DFT) [26-32]. Up to now, most efforts have been made to model
metal/oxides interfaces, but a few calculations exist on metal/carbides [33, 34],
metal/nitrides [35] and metal/borides [36] systems, arriving at a correct estimation
of the metal-ceramic bonding mechanisms. These efforts have indeed opened new
important insights into the basic solid-liquid interactions at high temperatures.
X O =
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