Compacted film

If a compact film growing at a parabolic rate breaks down in some way, which results in a non-protective oxide layer, then the rate of reaction dramatically increases to one which is linear. This combination of parabolic and linear oxidation can be tenned paralinear oxidation. If a non-protective, e.g. porous oxide, is fonned from the start of oxidation, then the rate of oxidation will again be linear, as rapid transport of oxygen tlirough the porous oxide layer to the metal surface occurs. Figure C2.8.7 shows the various growth laws. Parabolic behaviour is desirable whereas linear or breakaway oxidation is often catastrophic for high-temperature materials. 

Plain carbon steels rust in wet environments and oxidise if heated in air. But if chromium is added to steel, a hard, compact film of CrjOj will form on the surface and this will help to protect the underlying metal. The minimum amount of chromium needed to protect steel is about 13%, but up to 26% may be needed if the environment is particularly hostile. The iron-chromium system is the basis for a wide range of stainless steels. 

In view of its position in the e.m.f. series ( °aj3+/ai = 166V (SHE)), aluminium would be expected to be rapidly attacked even by dilute solutions of relatively weak acids. In fact, the rate of chemical attack is slow, owing to the presence on the aluminium of a thin compact film of air-formed oxide. When a voltage is applied to an aluminium anode there is a sudden initial surge of current, as this film is ruptured, followed by a rapid fall to a lower, fairly steady value. It appears that this is due to the formation of a barrier-layer. Before the limiting thickness is reached, however, the solvent action of the electrolyte initiates a system of pores at weak points or discontinuities in the oxide barrier-layer. 

The protective action of sulphuric films is mainly controlled by the anodising conditions, compact films formed at temperatures below 20° C in 7<7o v/v sulphuric acid being more resistant than the films formed at higher temperatures in more concentrated acid. The wider pores of the latter result in less... 

After the potential step, polymeric oxidation is followed by an oxidation charge to open, swell, and oxidize the compact film. At the start, the charge consumed to relax the compacted polymeric structure is the only component of the oxidation charge. Thus any quantitative information about the... 

The surface-phase layers will difier in character depending on the stractures of metal and oxide. On certain metals (zinc, cadmium, magnesium, etc.), loose, highly porous layers are formed which can attain appreciable thicknesses. On other metals (aluminum, bismuth, titanium, etc.), compact layers with low or zero porosity are formed which are no thicker than 1 pm. In a number of cases (e.g., on iron), compact films are formed wfiicfi fiave a distorted lattice, owing to the influence of substrate metal stracture and of the effect of chemical surface forces. The physicochemical and thermodynamic parameters of such films differ from tfiose of ordinary bulk oxides. Because of the internal stresses in the distorted lattice, such films are stable only when their thickness is insignificant (e.g., up to 3 to 5 nm). 

At a certain anodic potential, the compact film breaks down and lets electrons pass through without much resistance, causing oxygen evolution at a high rate. This dielectric breakdown is discussed in more detail in Section V. 

FIG. 1.6 A schematic diagram of colloidal processing of ceramic specimens. The figure illustrates some of the ways in which a dispersion is densified and transformed into porous or compact films or bulk objects. (Adapted and modified from Brinker and Scherer 1990.)... 

Eq. 1. h Planck s constant e dielectric constant m, electron mass m hole effective mass this is indeed the case for isolated silicon particles [11] as well as for compact films of nc-Si (25]... 

The radiative decay time of the red PL is of the order of several tens of microseconds in all cases. The lack of the blue shift in the compact films has been explained in terms of the surface state model [19]. The theory predicts an increase of the transition probability for absorption and PL with decreasing crystallite size [9, 10, 13]. Fig. 5 shows the measured dependence of the PL intensity on the average crystallite size in the compact ncSi/SiO films [19,25]. 

This phenemcnon is thought to occur by reason that, once the surface of each copper particle, the aggregation of which constitutes the powder, is covered with the compact film made up of copper oxides, the film thus formed hinders gaseous oxygen from attaining to the surface of the unoxidized copper particle. 

Similarly to saturated lipids, increasing concentration of DPHC in benzene resulted in the formation of a compact film and a stronger blocking of the interfacial ET. The limiting ET rate across the complete monolayer of DPHC was 2.4 times higher than that obtained with C-16 (Fig. 14). 

The third method involved equilibration of dioxin present in a compact film on a glass slide, (formed by evaporation of dioxin dissolved in hexane), with water in a 2 liter volumetric flask. A glass stirrer was used for mixing the contents. To avoid problems with the formation of an interfacial film, the flask was rigorously cleaned with boiling concentrated nitric acid, and the cleaned flask was filled to the neck with water for equilibration. A capillary GC/MS method was used for dioxin analysis. Solubility results using this method are given in Table III. 

Table III. Solubility of Unlabeled Dioxin in Water from a Compact Film ...

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