Pores current density

From polarization curves the protectiveness of a passive film in a certain environment can be estimated from the passive current density in figure C2.8.4 which reflects the layer s resistance to ion transport tlirough the film, and chemical dissolution of the film. It is clear that a variety of factors can influence ion transport tlirough the film, such as the film s chemical composition, stmcture, number of grain boundaries and the extent of flaws and pores. The protectiveness and stability of passive films has, for instance, been based on percolation arguments [67, 681, stmctural arguments [69], ion/defect mobility [56, 57] and charge distribution [70, 71]. 

Hard plating is noted for its excellent hardness, wear resistance, and low coefficient of friction. Decorative plating retains its brilliance because air exposure immediately forms a thin, invisible protective oxide film. The chromium is not appHed directiy to the surface of the base metal but rather over a nickel (see Nickel and nickel alloys) plate, which in turn is laid over a copper (qv) plate. Because the chromium plate is not free of cracks, pores, and similar imperfections, the intermediate nickel layer must provide the basic protection. Indeed, optimum performance is obtained when a controlled but high density (40—80 microcrack intersections per linear millimeter) of microcracks is achieved in the chromium lea ding to reduced local galvanic current density at the imperfections and increased cathode polarization. A duplex nickel layer containing small amounts of sulfur is generally used. In addition to... 

As expected, the polarization parameter, = x x r is added to the pore length, / (see Section 2.2.5). The polarization resistanee is dependent on the current density [Eq. (2-35)]. For pure activation polarization, it follows from Eq. (2-45) ... 

The behavior of metal electrodes with an oxidized surface depends on the properties of the oxide layers. Even a relatively small amount of chemisorbed oxygen will drastically alter the EDL structure and influence the adsorption of other snb-stances. During current flow, porous layers will screen a significant fraction of the surface and interfere with reactant transport to and product transport away from the surface. Moreover, the ohmic voltage drop increases, owing to the higher current density in pores. All these factors interfere with the electrochemical reactions, particularly with further increase in layer thickness. 

Porous electrodes are systems with distributed parameters, and any loss of efficiency is dne to the fact that different points within the electrode are not equally accessible to the electrode reaction. Concentration gradients and ohmic potential drops are possible in the electrolyte present in the pores. Hence, the local current density, i (referred to the unit of true surface area), is different at different depths x of the porous electrode. It is largest close to the outer surface (x = 0) and falls with increasing depth inside the electrode. 

Fig. 2.9.13 Qu asi two-dimensional random ofthe percolation model object, (bl) Simulated site percolation cluster with a nominal porosity map of the current density magnitude relative p = 0.65. The left-hand column refers to simu- to the maximum value, j/jmaK. (b2) Expedited data and the right-hand column shows mental current density map. (cl) Simulated NMR experiments in this sample-spanning map of the velocity magnitude relative to the cluster (6x6 cm2), (al) Computer model maximum value, v/vmax. (c2) Experimental (template) for the fabrication ofthe percolation velocity map. The potential and pressure object. (a2) Proton spin density map of an gradients are aligned along the y axis, electrolyte (water + salt) filling the pore space...

The sixth factor is conditioning. Similar to the changes in current density, the pore fluid at the anode and cathode compartments can be conditioned to a specific pH or chemistry to increase the efficiency of the process. 

It should be noted here that the barrier-film-promoting electrolytes are also characterized by VA(t) curves similar to those of the pore-forming ones, if comparatively small current densities are used (less than 0.5mA/cm2).20... 

Such a behavior of the UA(t) or ja(t) functions is consistent with a fairly well established pore growth mechanism.4 According to this mechanism, the linear potential growth (and current density... 

The steady-state potential (or current density) is related to a steady growth of the porous oxide into the solution, maintaining a constant number of pores and a constant pore radius. This scheme is supported by electron microscopic observations reported by Xu et a/.102... 

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. 

Characteristic of both dependences is a decrease in the number of pores with an increase in either the current density or the steady-state voltage. To date, no clear explanation for this phenomenon is available. 

Figure 7 shows that PS thickness increases linearly with time up to certain thickness.16,17 Such constant growth rate at a constant current density means that the PS formed is uniform in thickness (Effective surface area remains constant assuming reaction kinetics is the same). At a large thickness the growth may deviate from linearity due to the effect of diffusion in the electrolyte within the pores.19,25 It has been found that for a very thick PS layer (150 pm) there is about 20% difference in HF concentration between that at the tips of pores and that in the bulk solution.19... 

Pore diameter generally increases with increasing potential and current density.9 24,60 80 Figure 10 shows that the diameter of pores formed on both p-Si and n-Si increases with current... 

Figure 10. Pore diameter of the PS formed in 5% HF + ethanol (1 1) as a function of doping concentration and current density. After Lehmann, et.al.w...

For very deep pores the diameter may increase with a decreased growth rate due to the effect of diffusion process inside pores. The depth at which this occurs depends on current density and HF concentration.87 Low FiF concentration, low temperature and low growth rate favour formation of deep uniform pores. 

There is a correlation between the occurrence of two-layer PS and the saturation photo current value.78 Only a single micro PS layer forms at a photo current density below the photo saturation value while two-layer PS forms at current densities above the saturation current as shown in Figure 17. Also, macro PS layer forms only after a certain amount of charge determined by the amount of etch required for the initiation of macro pores.77... 

For a stably growing PS the reactions and the rates are different on the pore walls and on the pore bottoms. Furthermore, they are different at different positions of a pore bottom due to the difference in the radius of curvature. The current is the largest at the pore tip because there the radius of curvature is the smallest. It decreases from the pore tip to the pore wall as radius of curvature increases. On the other hand, since the reactions depend on the current density, for a given condition, direct dissolution of silicon dominates at a relatively low current range while oxide formation and dissolution dominate at a higher current range. Thus, oxide formation and dissolution tend to occur at the pore tips at a lower potential than at the side of the pore bottom. There is a distribution of the kind of reactions along the pore bottom. 

For a pore to propagate under a steady state the current density on the side of the pore bottom, is, and that at the pore tip, it, as illustrated in Figure 25 have the relation. 

As illustrated in Figure 26, which is a varied presentation for a single pore from the scheme shown in Figure 19, there are five possible phases in the current path in which significant potential drops may occur. The distribution of the applied potential in the different phases of the current path depends on doping type and concentration, HF concentration, current density, potential, illumination intensity and direction. The phases in the current path... 

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