Xerogels characteristics

Table 4 is devoted to xerogel characteristics. The semi-continuous mixing of silica reactants favors the formation of large pores for addition durations below 600 s. Beyond this value the effects are weak. Materials prepared at pH value 5.5 have smaller pore volume and surface than those prepared at pH values 9.5. For the two pH values the mean pore radii are nearly similar. 

For other adsorptives the experimental evidence, though less plentiful than with nitrogen, supports the view that at a given temperature the lower closure point is never situated below a critical relative pressure which is characteristic of the adsorptive. Thus, for benzene at 298 K Dubinin noted a value of 017 on active carbons, and on active charcoals Everett and Whitton found 0-19 other values, at 298 K, are 0-20 on alumina xerogel, 0-20-0-22 on titania xerogel and 017-0-20 on ammonium silicomolybdate. Carbon tetrachloride at 298 K gives indication of a minimum closure point at 0-20-0-25 on a number of solids including... 

Both xerogels and aerogels are characteristically high surface area materials (surface areas normally exceed 500 m2/g). Unlike wet gels, many uses exist for dried gels due to their high surface areas and small pore sizes (typically, < 20 nm diameters). Examples include catalyst supports (12.). ultrafiltration media (18), antireflective coatings (19-20), and ultra-low dielectric constant films. (Lenahan, P. M. and Brinker, C. J., unpublished results.)... 

TABLE 4. Textural characteristics of xerogels prepared at different temperature... 

The nature and structure of the surface of the xerogel are important characteristics with consequences which determine its chemical reactivity and stability. 

In a previous study [5], we showed that some materials, in particular the low density silica xerogels, exhibit a remarkable behavior when submitted to mercury porosimetry. At low pressure, the volume variation observed is entirely due to a crushing mechanism, generally irreversible with sometimes a weak elastic component. At high pressure, these xerogels are invaded by mercury which intrudes the pore network. The transition fi om the crushing mechanism to intrusion is sudden at a pressure Pi, characteristic of the material. This particular point can be easily located on the curve of cumulative volume versus logarithm of pressure by... 

Figure 1 shows mercury porosimetry curves on high dispjersive precipitated silica and on a low density xerogel previously examined [5]. The volume variation as a function of logarithm of pressure shows the same behavior. On both curves, one can see a sharp increase of the curve slope for a characteristic transition pressure P,. The value of this transition pressure is 45 MPa for precipitated silica and 27 MPa for the low density xerogel sample. The value of transition pressure Pt is dependent of the compressive strength of the sample. 

Figure2. Voltage-current characteristics of EL structure fabricated on PAA of 15 urn thick xerogel Eu203-In203 (curve 1), Eu203-Sn02 (curve 2). Sample dimensions are 5><5 mm. ...

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