Defects near pore surfaces

Positronium formation and annihilation behavior in Si and Si02 thin films are reviewed. Positronium is highly sensitive to pore (or void) sizes, surface properties of pores, defects near pore surfaces, etc., in various Si and Si02 samples. Therefore, not only positron annihilation spectroscopy but also positronium annihilation spectroscopy is useful for characterization of Si and Si02 materials. 

When a gel is formed in the presence of a dye, it seems likely that most of the combined dye will be situated on or near the surface, since such interactions should generate defects in the gel structure... It seems reasonable to suppose that in the process of methanol extraction, which follows gel formation and drying, dye molecules on or near the surface are removed leaving micropores ( footprints ) of geometry and properties characteristic of the dye. The number of such pores should increase with the basicity of the dyes." [33]... 

Van Cappellen and Qiu (1997a) have shown that the dissolution of biogenic silica in the highly undersaturated water column is inherently different than that in the seabed, where pore waters reach near-saturation levels. In areas of high undersaturation the dissolution rate of biogenic silica increases exponentially with the increasing departure from undersaturation. In contrast, most sedimentary environments exhibit a linear relationship between departure from undersaturation and dissolution rate. The difference in dissolution mechanism is believed to be related to the onset of localized dissolution centered on surface defects (Van Cappellen and Qiu, 1997a). 

Several investigators have used combined approaches, particularly in the in situ precipitation of active material in the pores of sintered substrates, using cathodic polarization and caustic precipitation in simultaneous or nearly simultaneous steps. A considerable amount of the reported information on the chemistry, electrochemistry, and crystal structure of the nickel electrode has been obtained on thin films (qv) made by the anodic corrosion of nickel surfaces. However, such films do not necessarily duplicate the chemical and/or crystallographic condition of active material in practical electrodes. In particular, the high surface area, space charge region, and lattice defect structure are different. Some of the higher (3.5+) valence state electrochemical behavior seen in thin films has rarely been reproduced in practical electrodes. 

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