Hydrated surface, etching

Uranium tarnishes readily in the atmosphere at room temperature. Electropolishing inhibits the process whilst etching in nitric acid activates the surface. Uranium dioxide and hydrated UO3 are the principal solid products. 

The transport velocity of Li+ is faster than that of Na+ and K+ due to the size of the cation. The data are consistent with a hopping transport mechanism of the cations accompanied by a non-specific co-transport of the anions. The transport rates for N03 > Cl- > C104 are related to the adjacent hydrate shell and not yet fully understood. Anyway, a path in the center of the supramolecular tubes, where the crown ethers assemble, must exist and allow for the co-transport of the anions. By forming the membranes in the pores of track-etched membranes, the transport rates could be improved by an order of magnitude due to the orientation of the channels perpendicular to the membrane surface. 

Approaches for the development of water-resistant surface treatments include application of inhibitors to retard the hydration of oxides or the development of highly crystalline oxides as opposed to more amorphous oxides. Standard chemical etching procedures, which remove surface flaws, also result in improved resistance to high humidity. 

A New Model. The results of the studies on anodic oxide films (see section 5.9 and chapter 3 on passive film and anodic oxides) show that anodic oxide properties (oxidation state, degree of hydration, 0/Si ratio, degree of crystallinity, electronic and ionic conductivities, and etch rate) are a function of the formation field (the applied potential). Also, they vary from the surface to the oxide/silicon interface, which means that they change with time as the layer of oxide near the oxide/silicon interface moves to the surface during the formation and dissolution process. The oxide near the silicon/oxide interface is more disordered in composition and structure than that in the bulk of the oxide film. Also, the degree of disorder depends on the formation field which is a function of thickness and potential. The range of disorder in the oxide stmcture is thus responsible for the variation in the etch rate of the oxide formed at different times during a period of the oscillation. The etch rate of silicon oxides is very sensitive to the stmcture and composition (see Chapter 4). 

In a slightly different angle, Kendall reasoned that beeause the 111 surfaee is oxidized thermally more rapidly than other low-index surfaees, the silicon surface ean be covered with a silicon oxide (or a hydrated silieon oxide) during etehing in aqueous solution, mueh faster than other planes. The formation of the oxide film passivates the (111) plane and blocks the dissolution reaetions. This model implies that the etch rate of a (111) surface should be similar to that of silieon oxide in KOH solu-... 

The models based on surface passivation suggest that a passive layer, similar to the silicon oxide formed under an anodic potential, exists on (111) silicon at OCP but not on other planes [82, 126, 156]. Instead of oxide, formation of inactive hydration complexes of K+ and OH-has been proposed to block the (111) surface [49, 134]. The fact that the relative etch rates of the different planes vary with the type of solution was attributed to the orientation-dependent adsorption of solvation complexes on the surface. 

The diffusion of chemical species is the predominant mechanism driving corrosion in neutral solutions. After water diffusion into the glass network, hydration of alkaline oxides present in all glass formulations (even the most resistant) leads to the diffusion of Na" and OH ions towards the surface and the aqueous medium (Equation [12.66]). Hydroxide ions may then lead to the hydrolysis of siloxane bonds (etching) without being consumed, as shown in Equations [12.67] and [12.68] (Ishai, 1975). This is an autocatalytic process, as the rate of dissolution of the glass increases with time. 

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