Metal-oxide interfaces

In botli cases, tire anodic reaction occurs by oxidation at tire metal/oxide interface ... 

The logaritlrmic law is also observed when the oxide him is an electrical insulator such as AI2O3. The transport of elecuons tlrrough the oxide is mainly due to a space charge which develops between tire metal-oxide interface and the oxide-gas interface. The incorporation of oxygen in the surface of tire oxide requhes the addition of electrons, and if this occurs by a charging process... 

Case 3. Electros move very slowly. Oxide can grow (slowly) at metal-oxide interface or oxide-air interface depending on whether M diffuses faster than 0" or not. Example At... 

At elevated temperatures where titanium alloys could be the adherend of choice, a different failure mechanism becomes important. The solubility of oxygen is very high in titanium at high temperatures (up to 25 at.%), so the oxygen in a CAA or other surface oxide can and does dissolve into the metal (Fig. 12). This diffusion leaves voids or microcracks at the metal-oxide interface and embrittles the surface region of the metal (Fig. 13). Consequently, bondline stresses are concentrated at small areas at the interface and the joint fails at low stress levels [51,52]. Such phenomena have been observed for adherends exposed to 600°C for as little as 1 h or 300°C for 710 h prior to bonding [52] and for bonds using... 

Fig. 1.76 Potential energy of an interstitial ion near the metal/oxide interface...

Oxide movements are determined by the positioning of inert markers on the surface of the oxideAt various intervals of time their position can be observed relative to, say, the centreline of the metal as seen in metal-lographic cross-section. In the case of cation diffusion the metal-interface-marker distance remains constant and the marker moves towards the centreline when the anion diffuses, the marker moves away from both the metal-oxide interface and the centreline of the metal. In the more usual observation the position of the marker is determined relative to the oxide/ gas interface. It can be appreciated from Fig. 1.81 that when anions diffuse the marker remains on the surface, but when cations move the marker translates at a rate equivalent to the total amount of new oxide formed. Bruckman recently has re-emphasised the care that is necessary in the interpretation of marker movements in the oxidation of lower to higher oxides. 

Fig. 1.84 Surface of a Cu-IONi alloy after oxidation in oxygen at 500°C, showing blistering, probably associated with CuO formation over voids at the metal/oxide interface (courtesy Central Electricity Research Laboratories)...

Several authors " have suggested that in some systems voids, far from acting as diffusion barriers, may actually assist transport by permitting a dissociation-recombination mechanism. The presence of elements which could give rise to carrier molecules, e.g. carbon or hydrogen , and thus to the behaviour illustrated in Fig. 1.87, would particularly favour this mechanism. The oxidant side of the pore functions as a sink for vacancies diffusing from the oxide/gas interface by a reaction which yields gas of sufficiently high chemical potential to oxidise the metal side of the pore. The vacancies created by this reaction then travel to the metal/oxide interface where they are accommodated by plastic flow, or they may form additional voids by the mechanisms already discussed. The reaction sequence at the various interfaces (Fig. 1.87b) for the oxidation of iron (prior to the formation of Fe Oj) would be... 

Much of the difficulty in demonstrating the mechanism of breakaway in a particular case arises from the thinness of the reaction zone and its location at the metal-oxide interface. Workers must consider (a) whether the oxide is cracked or merely recrystallised (b) whether the oxide now results from direct molecular reaction, or whether a barrier layer remains (c) whether the inception of a side reaction (e.g. 2CO - COj + C)" caused failure or (d) whether a new transport process, chemical transport or volatilisation, has become possible. In developing these mechanisms both arguments and experimental technique require considerable sophistication. As a few examples one may cite the use of density and specific surface-area measurements as routine of porosimetry by a variety of methods of optical microscopy, electron microscopy and X-ray diffraction at reaction temperature of tracer, electric field and stress measurements. Excellent metallographic sectioning is taken for granted in this field of research. 

As the film dissolves more oxide film is formed, i.e. the metal/oxide interface progresses into the metal, and the overall rate may be low enough to be acceptable for a particular process. In other cases, the corrosion products precipitate on the surface of the oxide and either accelerate the overall rate by enhancing diffusion of ions through the porous outer layers or, when less porous layers are formed, access of hydrogen ions to the inner oxide surface is reduced thus decreasing the rate. 

A wide variety of in situ techniques are available for the study of anodic hhns. These include reflectance, eUipsometry, X-ray reflectivity, and SXRD. X-ray reflectivity can be used to study thick surface layers up to 1000 A. The reflectance technique has been used to study oxide growth on metals, and it yields information on oxide thickness, roughness, and stoichiometry. It the only technique that can give information on buried metal-oxide interfaces. It is also possible to get information on duplex or multiple-layer oxide hhns or oxide hhns consisting of layers with different porosity. Films with thicknesses of anywhere from 10 to 1000 A can be studied. XAS can be used to study the chemistry of dilute components such as Cr in passive oxide hhns. 

All Fe oxide films on Pt have strongly relaxed, unreconstructed bulk-terminated surfaces, but while the Fe304 and Fe203 oxide layers are similar to their respective bulk compounds, the ultrathin FeO layers are true 2D oxide phases that are different from the FeO bulk and stabilized by the metal-oxide interface. 

Metal/metal oxides are the materials of choice for construction of all-solid-state pH microelectrodes. A further understanding of pH sensing mechanisms for metal/metal oxide electrodes will have a significant impact on sensor development. This will help in understanding which factors control Nemstian responses and how to reduce interference of the potentiometric detection of pH by redox reactions at the metal-metal oxide interface. While glass pH electrodes will remain as a gold standard for many applications, all-solid-state pH sensors, especially those that are metal/metal oxide-based microelectrodes, will continue to make potentiometric in-vivo pH determination an attractive analytical method in the future. 

Templeton AS, Trainor TP, Spormann AM, Brown GE Jr (2003b) Selenium speciation and partitioning within Burkholderia cepacia biofilms formed on a-Al203 surfaces. Geochim Cosmochim Acta 67 3547-3557 Templeton AS, Trainor TP, Traina SJ, Spormann AM, Brown GE Jr (2001) Pb(II) distributions at bio film-metal oxide interfaces. Proc Natl Acad Sci USA 98 11897-11902... 

FIGURE 19.12 Considerations for the interpretation of SSITKA data. Case 1 Three formates can exist, including (a) rapid reaction zone (RRZ)—those reacting rapidly at the metal-oxide interface (b) intermediate surface diffusion zone (SDZ)—those at path lengths sufficient to eventually diffuse to the metal and contribute to overall activity, and (c) stranded intermediate zone (SIZ)—intermediates are essentially locked onto surface due to excessive diffusional path lengths to the metal-oxide interface. Case 2 Metal particle population sufficient to overcome excessive surface diffusional restrictions. Case 3 All rapid reaction zone. Case 4 For Pt/zirconia, unlike Pt/ceria, the activated oxide is confined to the vicinity of the metal particle, and the surface diffusional zones are sensitive to metal loading. 

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