Passivation Models

Passivation Models. The models based on surface passivation suggest that a passive layer, similar to the silicon oxide formed under an anodic potential, exists on 

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 passivation models are not in agreement with a number of experimental observations. First, it has been found that the (111) surfaee in KOH at the OCP is free of oxide but terminated by hydrogen and formation of an oxide film oeeurs only at more anodic potentials. Also, the passivation potential for the (100) surfaee in KOH solutions is more negative than that of the (111) surface indicating that it is easier to passivate the (100) surface than the (111) surface.Furthermore, the etching of silicon at the OCP in alkaline solution is mainly a chemical reaction etch rate changes 

The etch rate of the (111) surface, although much smaller than those of the (100) and (110) planes, shows definite values, in the range of 2-10 A/s in KOH solutions. It is still much larger than the dissolution current density on a passivated surface in KOH (a dissolution rate of 2-10 A/s is equivalent to a current density of several milliamperes per square centimeter). In alkaline solutions, the dissolution rate of silicon oxide is less than 0.01 A/s (see Chapter 4), which is several orders of magnitude smaller than the etch rates of a (111) surface. Thus, it is unlikely that the silicon surface of any orientation is covered by Si02 during etching. 

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On the basis of the results from XPS studies by Kanamura and co-workers that the SEI has a multilayered structure,Peled and co-workers modified their lithium electrode passivation model to include carbonaceous anodes and proposed a so-called mosaic model to describe the SEI structure on the anode, as Figure 15a shows.According to this model, multiple reductive decompositions occur between the negatively charged anode surface and the various electrolyte components simultaneously, depositing a mixture of insoluble products on the anode. This heteropolymicrophase SEI consists of many microregions that are of entirely different chemical... 

The passivation model, proposed by Palik et attributed the etch rate... 

On the other hand, the passivation model, proposed by Palik et al. [80, 149], attributed the etch rate reduction to the easier formation of an oxide film on highly doped silicon. It is supported by a number of experimental observations. First, the difference between passivation potential and OCP decreases with increasing doping concentration, implying easier passivation... 

In summary, the above described passivation model suggests that the composition and structure of the adsorbed expander layer, its affinity to Pb and PbS04 surfaces and its electric charge all exert strong influence on the delay of the passivation phenomena caused by the PbS04 layer. 

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