Diffusion in metal oxidation

But it is necessary to further characterise D in different crystal structures (lattices or sub-lattices) and subsequently to derive expressions for the temperature and oxygen pressure dependence of diffusion in metal oxides. 

As described in the previous chapter on diffusion in metal oxides the driving force is given by the negative of the potential gradient. The force exerted on a charged particle of type i with charge Zie is given by... 

The diffusion coefficients of cations in metal oxides are usually measured through the use of radioactive isotopes. Because of the friable nature of oxides it is exU emely difficult to use the sectioning technique employed for metal samples. The need for this can be avoided by the application of radioisotopes which emit radiation having a well established absorption law in matter. Isotopes which emit y radiation are very useful when the cation has a relatively high diffusion coefficient because of the long-range peneU ation of y rays. The absorption law is... 

Anionic diffusion in the oxidation of a convex surface creates a situation which is the reverse of that just described. The oxide is in tension along planes parallel to the surface and fracture may be expected to occur readily in perpendicular directions and starting from the gas/metal interface. Although very thin films may have resistance to fracture, thick films frequently acquire the morphology shown in Fig. 1.83. 

Local accumulation of dirt on a steel structure in a damp environment is enough to set up an anodic area underneath it by excluding air. Similarly, chipped paintwork results in lateral spreading of anodic areas under the paintwork, radially outward from the chips. At the chipped site, air has relatively free access to the metal, but under the paint the oxygen is excluded and anodic activity becomes intense, spreading under the paint and leaving a trail of rust behind where air has slowly diffused in to oxidize the Fe2+(aq). 

In this chapter, diffusion in solid materials, that is, metals, oxides, and nanoporous crystalline, ordered, and amorphous materials is discussed. We first study diffusion in a phenomenological, general form afterward the diffusion of atoms in crystals by means of knowledge obtained from studies of diffusion in metals is discussed. Thereafter, those phenomena that are exclusive to oxides are separately discussed. Finally, diffusion in nanoporous materials is described. 

One of the most widely used materials for the fabrication of modern VLSI circuits is polycrystalline silicon, commonly referred to as polysilicon. It is used for the gate electrode in metal oxide semiconductor (MOS) devices, for the fabrication of high value resistors, for diffusion sources to form shallow junctions, for conduction lines, and for ensuring ohmic contact between crystalline silicon substrates and overlying metallization structures. 

Examples of Ihe deterniinalioii of self-diffusion coefficients in solids are Ihe diffusion of hydrogen ions and water molecules (labelled with T and O, respectively) in alums, of Cl (labelled with Cl) in AgCl, and of 1 (labelled with l) in Agl. Besides self-diffusion, many other diffusion coefficients of trace elements in metals, oxides, silicates and other substances have been determined by application of radio-tracers. Investigation of the migration of trace elements from solutions into glass revealed fast diffusion of relatively small monovalent ions such as Ag+. 

The unique structure and electronic properties of CNTs provide a tremendous potential for construction of CNTs and MOX hybrid materials in the field of gas-sensing applications. Advantages for mixing CNTs in metal oxides for gas sensors are the reduction of operating temperature and enhancement of sensitivity and selectivity due to the amplification effects of p-n heterojunctions with the gas reaction, formation of nanochannels for gas diffusion, high specific surface area, and increase of charge carrier on the surface. As a result of these advantages, the hybrid CNT/metal oxide gas sensor may be used instead of the popular commercial metal oxide gas sensors (such as TGS gas sensors) in the near future. 

In the processing of integrated circuits, silicon dioxide (SiOa) can be used as a mask during ion implantation or diffusion of impurity into silicon, for passivation, for protection of the device surface, as interlayers for multilevel metallization, or as the active insulating material — the gate oxide film in metal-oxide-semiconductor (MOS) devices [1, 2], At the present time, several methods have been developed for the formation of Si02 layers, including thermal and chemical oxidation, anodization in electrolyte solutions, and various chemical vapor deposition (CVD) techniques [2, 3],... 

To accurately model contaminant transport when Al, Fe, and Mn oxide minerals are present, intraparticle diffusivities are needed. Additionally, as we tried to point out in this ehapter, there are a number of implieations in using the diffusion model with amorphous oxides. Some of these implications of intraparticle diffusion have been observed by researchers in macroscopic studies of both model and real systems. However, as only a small number of studies have been conducted on metal eontaminant diffusion in aqueous oxide systems, many implications need yet to be addressed sueh as the long-term effect of contaminants sorbed in micopores of metastable minerals and desorption of contaminants from both coprecipitated oxides and oxides exposed to contaminants over long periods of time. Therefore, future studies are needed to study and improve our understanding of this slow sorption proeess, intraparticle diffusion. 

More interesting is the commonly encountered situation where an ion diffuses in a majority electronic conductor. Thus, diffusion in metallic and semiconducting alloys or of inserted species in transition metal oxides and chalcogenides fall into this category. Many electrode reactions are of this type. Lithium diffusion in j5-LiAl and other alloys is of interest in negative electrode reactions for advanced hthium batteries hydrogen and lithium diffusion in oxides (e.g. VeOn) and sulfides (e.g. TiSa) are of importance as positive electrode reactions for batteries and elec-trochromic devices. 

Epitaphial effects of a scale can influence diffusivity as does any defect such as porosity, grain boundaries, cracks, dislocation substructures, etc. Impurity cations can have a great effect on diffusivity in the oxide depending on the valence of the impurity ion and the semiconducting properties of the scale. Common scales formed from oxides, sulfides, and nitrides can be classified as p-type, n-type, or amphoteric semiconductors. The p-type, metal-deficit scales are nonstoichiometric with cation vacancies present. Impurity ions with valencies greater than the p-type semiconductor will tend to increase the concentration of cation vacancies and, hence, diffusivity. Lower vacancy ions will have the opposite effect. Impurity ions with the same valence should have little effect on diffusion. The n-type semiconductors... 

Defects are mandatory for additional transport processes in solids, among them diffusion and ionic conductance. The latter is an important property for sensors. It is encountered mainly in ionic crystals, e.g. in metal oxides. 

---