The strength of metal oxide adhesion effectively determines the wetting of the metal-oxide interface. The strength of this adhesion is important, for instance, in production of light bulbs and fiber-matrix composites that depend on the optimization of wetting to create metal-ceramic interfaces. The strength of adhesion also determines the extent of dispersion on catalytically active metal. Metal oxide adhesion is important for applications such as complementary metal oxide semiconductor devices. These devices make possible the high packing densities of modern integrated circuits.
Oxide thermodynamics Metal oxides are formed consistent with minimizing surface energy and minimizing system entropy. The formation reactions are chemical in nature, forming bonds between oxygen dimers and pure metals or metal alloys. The reactions are endothermic for transition metals and semi-metals. At isothermic and isobaric conditions at atmosphere, the probability for a free metal surface to bind an oxygen dimer via oxidation is a function of the partial pressure of oxygen, the surface energy between the crystal and the liquid or vapor phase (see heat of formation), and time. In standard conditions, the determining factors for phase change are temperature and pressure. The idea here is that oxygen is making a phase change from gas to solid, and at the same time a bond is forming between oxygen and a metal. The instantaneous breaking of one bond and forming a different one required an energy contribution higher than the enthalpy of bond dissociation for molecular gaseous oxygen at 298K is +498.34 kJ/mol and is typically expressed as ∆Hf since it is also the heat of formation. The majority of contributed entropy in the formation of metal-oxides is from O2(g). Gaseous oxygen molecules have high translation entropy, due to the excited vapor phase. This allows the transport of oxygen from the system to the interface or reaction surface. The change in entropy (ΔS) for oxidation is negative (exothermic) for semi-metals, transition metals, alkali earth metals and lanthanides/actinides. This fact is due to the elevated surface energy of an exposed pure metal and the ability of the tiny oxygen dimer to attract to high energy sites. The trend for oxide formation is that the reaction rate increases as atomic number increases. Areas with elevated surface electron density will always oxidize preferentially, as is demonstrated beautifully in the formation of electro-anodized titanate. The formation of oxides is dominated by interactions between the Gibbs free energy surfaces of constituents. The intersections of Gibbs surfaces at a given temperature and pressure would be represented in 2D space as phase diagrams. In real world applications, Gibbs surfaces are subject to the additional dimension entropy. This third dimension constitutes a Cartesian coordinate space and the surface mapped out by the Gibbs energy for a given reaction gives a threshold energy needed for a phase transition. These values can be found in ASM library volumes, or online as the "standard heats of formation." ∆G=∆H-T∆S standard state change of enthalpy is independent and thus the gradient of the change in Gibbs free energy as a function of temperature is linear. This dictates that an oxide becomes less thermodynamically stable with increasing temperature. An important distinction between equilibrium wetting and non-equilibrium wetting is that the non-equilibrium condition occurs when a chemical reaction is taking place. This non-equilibrium wetting is an irreversible thermodynamic process that accounts for the changes of the chemical potential when forming a new boundary phase, such as an oxide.
Work of adhesion The ideal work of separation Wsep is the reversible work needed to separate the interface into two free surfaces. Important as a state function depending on the mechanical properties. It is referred to as ideal because when the two free surfaces create an interface, the concentration of the interface will only be identical to the bulk at the instant the surface is created. In order to reach chemical equilibrium, the process of diffusion will take place which will increase any measurement of the work of separation. The work of adhesion is the reversible free energy change for making free surfaces from interfaces. It is represented by the equation:
W a d = γ m + γ o − γ m o {\displaystyle W_{ad}=\gamma _{m}+\gamma _{o}-\gamma _{mo}}
where: Wad is the work of adhesion γm and γo are the respective surface energies of the metal and oxide γmo is the surface energy between the two materials in contact The following table gives some common metals and their corresponding surface energies. All the metals are face-centered cubic crystal structure and these surface energies correspond to the (100) surface plane.
Oxide stability Ellingham diagrams are generated according to the second law of thermodynamics and are a graphical representation of the change in the Gibbs free energy with respect to changing temperature for the formation of oxides.
Solid-gas interface
Structure Real surfaces may be macroscopically homogeneous, but their microscopic heterogeneity plays a crucial role in the relationship between the metal and its oxide.
Transition metal oxides Certain transition metals form multiple oxide layers that have different stoichiometric compositions. This is because the metal has multiple valency states with fewer or more electrons in the valence shell. These different valency states allow for multiple oxides to be formed from the same two elements. As the local composition of the material changes through diffusion of atoms, different oxides form as layers, one on top of another. The total adhesion in this situation involves the metal-oxide interface and oxide-oxide interfaces, which adds increasing complexity to the mechanics.
Roughness Increasing surface roughness increases the number of dangling bonds at the metal-oxide interface. The surface free energy of a crystal face is:
γ = E − T S {\displaystyle \gamma =E-TS}
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