Skeletal densification

These data (2, 80, 83, 84) indicate that skeletal densification at intermediate temperatures is observed in general for metal alkoxide derived silicate xerogels, whereas aqueous silicates have skeletal densities comparable to silica glass, and thus, like the fumed silica gel discussed earlier, exhibit little skeletal densification at intermediate temperatures. 

Figure 12. Linear shrinkage and skeletal density of a multicomponent borosilicate xerogel during heating in air at 2 °C/min. Skeletal densification accounts for all the shrinkage observed between 250 and 550 °C. (Reproduced with permission from reference 84. Copyright 1986.)...

The shrinkage in region II can be attributed solely to densification of the solid skeletal phase of the gel. Alternatively, the removal of very fine pores by... 

We would expect skeletal densification to be insignificant for particulate gels in which the solid network has the same structure as the corresponding melt-prepared glass and to make the greatest contribution to shrinkage for weakly cross-linked gels. This is fully borne out by the results of Fig. 5.29. 

Table 5.7 summarizes the dominant shrinkage mechanisms and their relative contribution to the overall shrinkage for the borosilicate gel described in Fig. 5.30. The significant contribution due to skeletal densification by condensation polymerization and structural relaxation should not go unnoticed. 

Schematic representation of structural changes resulting from skeletal densification. Shrinkage occurs isotropically causing no change in the surroundings of the structural elements in either the cylindrical array (top) or the random-close-packed array (bottom) [1451.

Uthgenannt, B.A. et oL (2007) Skeletal self-repair stress fracture healing by rapid formation and densification of woven bone. J. Bone Miner. Res., 22 (10), 1548-1556. 

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