Oxide films pore formation

Figure 15. Electrocapillary energy for the formation of a breakthrough pore in a thin surface oxide film on metals as a function of pore radius.7 AE E - Epzc, where Epzc is the potential-of-zero charge of the film-free metal. Al is the activation banier for the formation of a breakthrough pore and r is its critical radius. M, metal OX, oxide film EL, electrolyte solution, h a 2 x I O 9 m, am = 0.41 J m-2, a = 0.01 J m-2, ACj= 1 F m"2. a, AE=0.89 V b, AE=0.9 V c,A = 1.0 V. (From N. Sato, J. Electmckem. Soc. 129,255,1982, Fig. 2. Reproduced by permission of The Electrochemical Society, Inc.)...

The capacitance determined from the initial slopes of the charging curve is about 10/a F/cm2. Taking the dielectric permittivity as 9.0, one could calculate that initially (at the OCP) an oxide layer of the barrier type existed, which was about 0.6 nm thick. A Tafelian dependence of the extrapolated initial potential on current density, with slopes of the order of 700-1000 mV/decade, indicates transport control in the oxide film. The subsequent rise of potential resembles that of barrier-layer formation. Indeed, the inverse field, calculated as the ratio between the change of oxide film thickness (calculated from Faraday s law) and the change of potential, was found to be about 1.3 nm/V, which is in the usual range. The maximum and the subsequent decay to a steady state resemble the behavior associated with pore nucleation and growth. Hence, one could conclude that the same inhomogeneity which leads to pore formation results in the localized attack in halide solutions. 

When the results for oxide growth and anion incorporation172,160 are compared with the kinetics of space charge accumulation in barrier and porous alumina films [see Section IV(1)], it can be concluded that anion incorporation modifies the electrostatics of the external oxide interface, thus influencing oxide dissolution and pore formation.172... 

When the surface is completely covered by an oxide film, dissolution becomes independent of the geometric factors such as surface curvature and orientation, which are responsible for the formation and directional growth of pores. Fundamentally, unlike silicon, which does not have an atomic structure identical in different directions, anodic silicon oxides are amorphous in nature and thus have intrinsically identical structure in all orientations. Also, on the oxide covered surface the rate determining step is no longer electrochemical but the chemical dissolution of the oxide.1... 

A schematic view of the cold cathode fabrication process is shown in Fig. 10.18. The cold cathode is fabricated by low pressure chemical vapor deposition (LPCVD) of 1.5 pm of non-doped polysilicon on a silicon wafer or a metallized glass substrate. The topmost micrometer of polysilicon is then anodized (10 mA cnT2, 30 s) in ethanoic HF under illumination. This results in a porous layer with inclusions of larger silicon crystallites, due to faster pore formation along grain boundaries. After anodization the porous layer is oxidized (700 °C, 60 min) and a semi-transparent (10 nm) gold film is deposited as a top electrode. 

Figures 10.1.3a and 10.1.3b show the SEM and TEM (transmission electron microscope) photographs of the resultant carbon sample, respectively. The SEM photograph indicates the formation of tubular carbon, whose diameter is almost equal to the channel diameter (230 nm) of the anodic oxide film template. Each tube ramifies into several thin tubes near the end. This branchlike structure originates from a similar branching in pore structure of the commercial anodic oxide film near its surface. A more clear view of the structure of the carbon samples is given by...

Martin and coworkers tried to prepare carbon tubes from the carbonization of polyacrylonitrile (PAN) in the channels of anodic oxide film (10). A commercially available film with a pore diameter of 260 nm was immersed in an aqueous acrylonitrile solution. After adding initiators, the polymerization was carried out at acidic conditions under N2 flow at 40°C. The PAN formed during the reaction was deposited both on the pore walls and on both sides of the film. Then the Film was taken from the polymerization bath, followed by polishing both faces of the film to remove the PAN deposited on the faces. The resultant PAN/alumina composite film was heat-treated at 250°C in air, and then it was heat-treated at 600°C under Ar flow for 30 min to carbonize the PAN. Finally, this sample was repeatedly rinsed in I M NaOH solution for the dissolution of the alumina film. The SEM observation of this sample indicated the formation of carbon tubes with about 50 xm long, which corresponds to the thickness of the template film. The inner structure of these tubes was not clear because TEM observation was not done. The authors claim that it is possible to control the wall thickness of the tubes with varying the polymerization period. 

The oxygen also has high enough speed of diffusion l,6TO 10 m/s, but the size of its atomic radius is only 10 % more than the size of an octahedral pore a-Ti. Therefore, already at a room temperature the atom of the oxygen, which has got into an octahedral pore, stretches it, and its exit from the pore is difficult. Thus, formation of a monolayer of oxide film on a surface of a particle of a powder is possible, and its thickness changes with time. 

Corrosion of reinforcement is probably the most widespread cause of deterioration in concrete. The expansion produced by rust formation causes the surrounding concrete to crack and spall. In a sound concrete, rusting is prevented by the high pH of the pore solution, which stabilizes an oxide film on the steel that inhibits further attack. This film is unstable at lower pH values, which can result from carbonation or leaching, or in the presence of Cl . Sources of the latter include sea water or salt spray, de-icing salts used on roads, certain aggregates, especially those available in desert climates, and CaClj used as an accelerator. Though now widely prohibited, this can... 

The distribution of chemical reactions which do not involve charge carriers in the semiconductor is not affected by surface curvature. Thus, formation of pores does not occur in KOH solutions where the dissolution of silicon is of almost 100% chemical nature. Also, the effect of surface curvature is little when the surface is covered with an oxide film which masks the semiconductor properties of silicon, e.g., during electropolishing in HF. 

Figure 12. Schematic diagram of the evolution of an anodic Ti02 nanotube array (a) Formation of a compact oxide layer, (b) Formation of pits due to the dissolution and breakdown of the barrier oxide film, (c) The barrier layer at the bottom of the pits is relatively thin and this leads to the enhanced electric field assisted dissolution of Ti02, which results in further pore growth, (d) Voids formed in the inter-pores region, (e) Fully developed nanotube array with a corresponding top view [51].

Generally, if the crack width is modest (e. g. it is below 0.3-0.5 mm), after the initiation of corrosion on the steel surface, the corrosion rate is low. Chemical processes in the cement paste and formation of corrosion products may seal the crack near the reinforcement and allow the protective oxide film to form again. For car-bonation-induced corrosion, repassivation can take place when the migration of alkalinity from the surrounding concrete brings the pH of the pore solution in contact with the corrosion products to values above 11.5. Repassivation may have trouble taking place or may not take place at all in the following situations ... 

For applications in adhesive bonding research or technology, ellipsometry is useful for the quantitative determination of film thicknesses. Especially aluminum is a metal that has been studied extensively. It lends itself well to oxide thickness measurements because AI2O3 is transparent, whieh is a requirement. The thickness of the oxide formed in certain media can be determined [92]. Other studies reported on the use of ellipsometry to investigate the corrosion or rate of oxide film dissolution in certain environments in situ. As the film dissolves, the formation of pores and differences between the densities of different layers in the oxide film ean be distinguished and related to the conditions of the anodizing process [93]. 

Carbonic acid formation decreases local pore solution pH from 14 to 8. This fall initiates passive hydrated ferric oxide film destruction and hydrated calcium carbonate formation ... 

Similar to aluminum, submicrometer size pores can also be formed on titanium surface. Titanium surface can be anodized to generate pores in the presence of fluoride which required very long anodization time. Anodization of titanium in sulfuric and phosphoric acid also leads to the formation of oxide layer, however it is very dense, but at very high voltage breakdown of oxide layer leads to the formation of pores. The size and number of pores can be controlled by controlling the process parameters. Fig. 12.12 shows the surface of titanium with porous oxide film containing submicron size... 

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