Shrinkage, framework

Applied forces can also induce mass flow between interfaces. When tensile forces are applied, atoms from an unloaded free surface will tend to diffuse toward internal interfaces that are normal to the loading direction this redistribution of mass causes the system to expand in the tensile direction. Applied compressive forces can superpose with capillary forces to cause shrinkage. In this chapter, we introduce a framework to treat the combined effects of capillary and applied mechanical forces on mass redistribution between surfaces and internal interfaces. 

Since the a //-distance varies so little with framework vibration, particularly in high barrier cases 09). The anti-peak is accordingly not suited for direct determination of 00. On the other hand, the torsional motion leads to an asymmetry in the anti-peak due to the functional relation between r and . For a low barrier case this asymmetry may be appreciable, while in a high barrier case it may be observed only as a shrinkage effect for the anti-distance. The asymmetry or the shrinkage may be used to derive a value for 00. 

The hydrogel has an open structure (i.e. a low particle coordination number) and is mechanically weak. The removal of the aqueous liquid phase normally leads to a drastic shrinkage of the silica framework and die formation of additional siloxane bonds (Fenelonov et al., 1983). The resulting xerogel is therefore much stronger and more compact, but inevitably has a lower pore volume. 

Table III shows XRD and porosimetry data for calcined USY and AFS zeolites. All samples show shrinkage of the unit cell to comparable values following calcination. As a result, calcined samples are compared at similar silica-alumina framework ratios. All calcined samples have well developed microporous structures and comparable total pore volumes. These porosimetry data confirm that the hydrothermally dealuminated materials contain a significant fraction of mesopores relative to chemically dealuminated materials. The extensive washing given to AFS-1 results in higher micropore surface area and volume compared to AFS-2 and suggest that AFS-2 contains occluded fluoroaluminate and fluorosilicate compounds within the microporous structure.

XRD studies showed that both starting NH4 -forms of Y zeolite and of modified Y zeolites contains no amorphous phase and has the well-crystallized framework, the shrinkage of the unit cell of modified Y zeolites. The tto values determined by IR spectroscopy and XRD are very close however, the second value slightly exceeds, as a rule, the first one. 

In another approach Subramanian et al. [32] infiltrated the polymeric opal template directly with ultrafine particles instead of employing metal alkoxides or salts. In this approach infiltration with a nanocrystalhne material of known crystal phase is possible, and therefore materials with a predetermined crystal structure of the walls can be obtained even at mild processing conditions. For instance, Ti02 frameworks with a rutile phase of the wall material could be obtained without the necessity to resort to sintering at high temperatures. Additionally, shrinkage, which commonly occurs in the case of condensation of metal alkoxides or conversion of metal salts, is largely reduced. Finally, this route opens a pathway to obtain porous metal oxide materials, which are barely accessible by wet chemistry approaches. 

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