Aluminium surface layers

Finally, in 1985, the results of an extensive investigation in which adsorjDtion took place onto an aluminium oxide layer fonned on a film of aluminium deposited in vacuo onto a silicon wafer was published by Allara and Nuzzo 1127, 1281. Various carboxylic acids were dissolved in high-purity hexadecane and allowed to adsorb from this solution onto the prepared aluminium oxide surface. It was found that for chains with more than 12 carbon atoms, chains are nearly in a vertical orientation and are tightly packed. For shorter chains, however, no stable monolayers were found. The kinetic processes involved in layer fonnation can take up to several days. 

In tenns of an electrochemical treatment, passivation of a surface represents a significant deviation from ideal electrode behaviour. As mentioned above, for a metal immersed in an electrolyte, the conditions can be such as predicted by the Pourbaix diagram that fonnation of a second-phase film—usually an insoluble surface oxide film—is favoured compared with dissolution (solvation) of the oxidized anion. Depending on the quality of the oxide film, the fonnation of a surface layer can retard further dissolution and virtually stop it after some time. Such surface layers are called passive films. This type of film provides the comparably high chemical stability of many important constmction materials such as aluminium or stainless steels. 

Evidence for the solvated electron e (aq) can be obtained reaction of sodium vapour with ice in the complete absence of air at 273 K gives a blue colour (cf. the reaction of sodium with liquid ammonia, p. 126). Magnesium, zinc and iron react with steam at elevated temperatures to yield hydrogen, and a few metals, in the presence of air, form a surface layer of oxide or hydroxide, for example iron, lead and aluminium. These reactions are more fully considered under the respective metals. Water is not easily oxidised but fluorine and chlorine are both capable of liberating oxygen ... 

Silica gel and aluminium oxide layers are highly active stationary phases with large surface areas which can, for example, — on heating — directly dehydrate, degrade and, in the presence of oxygen, oxidize substances in the layer This effect is brought about by acidic silanol groups [93] or is based on the adsorption forces (proton acceptor or donor effects, dipole interactions etc) The traces of iron in the adsorbent can also catalyze some reactions In the case of testosterone and other d -3-ketosteroids stable and quantifiable fluorescent products are formed on layers of basic aluminium oxide [176,195]... 

In the case of alloys having one constituent considerably more reactive to oxygen than the others, conditions of temperature, pressure and atmosphere may be selected in which the reactive element is preferentially oxidised. Price and Thomas used this technique to develop films of the oxides of beryllium, aluminium, etc. on silver-base alloys, and thereby to confer improved tarnish resistance on these alloys. If conditions are so selected that the inward diffusion of oxygen is faster than outward diffusion of the reactive element, the oxide will be formed as small dispersed particles beneath the surface of the alloy. The phenomenon is known as internal oxidation and is of quite common occurrence, usually in association with a continuous surface layer of oxides of the major constituents of the alloy. 

Dilute binary alloys of nickel with elements such as aluminium, beryllium and manganese which form more stable sulphides than does nickel, are more resistant to attack by sulphur than nickel itself. Pfeiffer measured the rate of attack in sulphur vapour (13 Pa) at 620°C. Values around 0- 15gm s were reported for Ni and Ni-0-5Fe, compared with about 0-07-0-1 gm s for dilute alloys with 0-05% Be, 0-5% Al or 1-5% Mn. In such alloys a parabolic rate law is obeyed the rate-determining factor is most probably the diffusion of nickel ions, which is impeded by the formation of very thin surface layers of the more stable sulphides of the solute elements. Iron additions have little effect on the resistance to attack of nickel as both metals have similar affinities for sulphur. Alloying with other elements, of which silver is an example, produced decreased resistance to sulphur attack. In the case of dilute chromium additions Mrowec reported that at low levels (<2%) rates of attack were increased, whereas at a level of 4% a reduction in the parabolic rate constant was observed. The increased rates were attributed to Wagner doping effects, while the reduction was believed to result from the... 

Calorised and heat-treated mechanically-clad products have coating structures similar to hot-dip aluminised coatings, but the degree of alloying with iron is variable (Fig. 13.5). With Calorised products the surface layers usually contain 25-50% aluminium. 

At elevated temperatures in the presence of oxygen the aluminium oxide layer catalyzes the formation of blue fluorescent aluminium oxide surface compounds with 4-hydroxy-3-oxo-A -steroid structures [4]. Aluminium oxide acts as an oxidation catalyst for an activated methylene group. 

The latter interpretation of data is more in accord with the recent Al and Si NMR findings of Ellison Warrens (1987), who found that the structure of an appreciable fraction of the glass changed under acid attack with some loss of aluminium including all in fivefold coordination (see Section 5.9.2). Thus, acid attack was not entirely confined to the surface layer of a glass particle. If this is so then silicic acid as well as ions must migrate from the body of the particle and it is reasonable to suppose that silicic acid deposits as siliceous gel at the particle-matrix interface. 

The Natural Passivation and Corrosion of Metals and Alloys XPS studies of the air-formed natural passive layer on aluminium surfaces have identified a number of hydroxides as well as alumina (Barr, 1977). The oxidation of pure iron and of stainless steels and other iron alloys have also been extensively... 

Mixtures containing sodium explode only on impact, while potassium explodes on contact with the chloride. Aluminium foil ignites after prolonged contact (probably after the surface layer of oxide has been dissolved). 

The shine from the aluminium is lost as atoms on the surface of the foil are oxidized to form Al3+ ions (Equation (7.1)), which diffuse into solution. Because the aluminium touches the silver, the electrons generated by Equation (7.1) enter the silver and cause electro-reduction of the surface layer of Ag20 (Equation (7.2)). 

When controlled nitridation of surface layers is required, as for example in the modification of the chemical properties of the surface of a support, the atomic layer deposition (ALD) technique can be applied." This technique is based upon repeated separate saturating reactions of at least two different reactants with the surface, which leads to the controlled build-up of thin films via reaction of the second component with the chemisorbed residues of the first reactant. Aluminium nitride surfaces have been prepared on both alumina and silica supports by this method wherein reaction cycles of trimethylaluminium and ammonia have been performed with the respective supports, retaining their high surface areas." This method has been applied to the modification of the support composition for chromium catalysts supported on alumina." ... 

The above results show that post synthesis alumination of PSM with AlfNOjfi improves the hydrothermal stability of the resulting AMM material. Similar effect has been observed by Mokaya et al. [12], who reported that the hydrothermal stability of MCM-41 could be enhanced by reaction with chlorohydrate of aluminium. Moreover, from the study of high Si/Al ratio of Y zeolite, Lutz et al. [13] reported that the hydrothermal stability of Y zeolite was enhanced by an external introduction of non-structural aluminum species onto the surface of Y zeolite. The surface layer of Al-rich aluminosilicate or aluminum oxide was suggested to block the terminal OH groups and energy-rich =Si-0-Si= bonds on the surface of Y zeolite, hence minimizing the attack of water molecules on the framework. Due to these properties, the non-structural... 

Cohen et al. [361] studied, amongst other things, the influence of heat on monolayers of OTS and of arachidic acid adsorbed on surfaces of aluminium oxide formed on aluminium surfaces. They used wetting studies and infrared spectroscopy and concluded that, whereas arachidic acid layers deteriorated in an irreversible manner at about 100°C, OTS layers survived intact to about 140°C. This result is attributed by these authors to polymerisation of the latter material but could also be due to a rather different bonding mechanism between the organic material and the aluminium oxide. 

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