Sonogel Gelation

The gelation time is taken as the time interval between the end of the ultrasonic treatment and the transition from a viscous fluid to an elastic solid. The first outstanding effect of ultrasound on the gelation process is the drastic decrease of this time in comparison 

When the applied energy density is near f/s(niax), the gelation takes place in a few seconds, whereas in other cases this process takes several minutes or hours, depending on the ultrasonic energy dose. 

Kun-Hong et al. [54] studied the gelation time as a function of the pH value. However, they did not give any detail of the ultrasound transducer device. They prepared sonosols with = 10 with acidic water (HCl). As a first step, the pH was set to 2.7 and then increased to 4.7 with NH4OH. Instantaneous gelation occurred at a pH = 5.5 or higher. 

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Figure 20.4. Evolution of the gelation time for water/TEOS = 4 pure sonogels as a function of the ultrasrmic energy dose. Inset Evolution of the gelation time for water/TEOS = 4 pure sonogels as a function of l/T for different ultrasonic energy dose. Adapted with permission from [17].

It is possible to apply the sonogel approach in the field of OIHM based on ormosils (Chap. 13). This approach to these materials incorporates an organic phase, very commonly PDMS, in the inorganic precursor (TEOS or TMOS) sol in combination with sonication of the precursors, producing a sono-ormosil (also called hard ormosil) after gelation [41, 57-59]. The processing and properties of sono-ormosils and derived materials have been recently reviewed [60, 61]. 

The limitation to low C/W can be overcome, if gelation is assisted by ultrasonic irradiation, producing so-called sonogels (Tonanon et al., 2005). Then, also for C/... 

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