Cracks in gels

In another explanation, it has been proposed that cracking in gels occurs as a result of local stresses produced by a distribution of pores sizes (39). As sketched in Fig. 5.25, after the critical point, liquid is removed first from the largest pores. It has been suggested that the tension in the neighboring small... 

Fig. 5. 23 Occurrence of cracks in gels dried with supercritical CO2 depending on washing parameters as defined in Fig. 5.22b (taken from van Bommel and de Haan (1994)). (The shaded region marks the supercritical state of the single component CO2).

RC90 Rhone-Poulenc 50%TEOS oligomers + methylphenylsiloxane White spirit + 8% toluene Tin catalyst Fewer cracks in gel, used for coating stained glass... 

Kozuka H., Takenaka S., Tokita H., Hirano T., Higashi Y., Hamatani T. Stress and cracks in gel-derived ceramic coatings and thick film formation. J. Sol-Gel Sci. Technol. 2003 26 681-686... 

Several groups used sol-gel transition to immobilize the beads packed in a capillary. For example, Dulay et al. [102] packed a slurry of ODS beads in tetraethylorthosilicate solution and heated it to 100 °C to achieve the sol-gel transition and create the monolithic structure shown in Fig. 17. This technology is extremely sensitive and even a small deviation from the optimal conditions leads to cracks in the monoliths and a rapid deterioration in the column performance. However, even the best efficiency of 80,000 plates/m achieved with these column was relatively low. Henry et al. modified the original procedure and increased the efficiencies to well over 100,000 plates/m [103,104]. 

The checkers eluted the columns with a slight positive air pressure on the solvent reservoir to prevent formation of gas bubbles and cracks in the chromatographic medium. Fractions were collected in 25-mL test tubes (Note 1), analyzed by TLC on silica gel, eluting with the column solvent, and visualized with a phosphomolybdic acid solution. The checkers observed a nonvolatile hydrocarbon material (not substrate related) which was eluted in the fractions just prior to the products, which are quite nonpolar themselves and are eluted in the early fractions, ahead of any unreacted ester. Colored, metal-containing components usually remain near the top of the column, although some colored material may accompany the... 

Procedure Carefully add the Sample Preparation to the prepared column. Open the stopcock, and adjust the flow rate to about 2 mL/min, discarding the eluate. Rinse the sample beaker with 5 mL of chloroform, and add the rinsing to the column when the level drops to 2 cm above the silica gel. Never allow the column to become dry on top, and maintain a flow rate of 2 mL/min throughout the elution. Avoid interruptions during elution as they may cause pressure buildup and result in leakage through the stopcock or cracks in the silica gel packing. 

POSTER TITLE Characterization of Periodic and Curvilinear Cracking in Sol-Gel Si02 Thin Films... 

As a practical matter the following sequence of solvents is recommended in an investigation of unknown mixtures elute first with petroleum ether then ligroin, followed by ligroin containing 1%, 2%, 5%, 10%, 25%, and 50% ether pure ether ether and dichloromethane mixtures, followed by dichloromethane and methanol mixtures. A sudden change in solvent polarity will cause heat evolution as the alumina or silica gel adsorbs the new solvent. This will cause undesirable vapor pockets and cracks in the column. 

Place the gel carefully on the membrane (no air bubbles) so that the gel and mask overlap by at least 2 mm and fit the top frame. Small cracks in the gel should be repaired with molten agarose. Switch on the vacuum pump (trap between the blotting unit and the pump to avoid solutions reaching the pump) and adjust to SO mbar. Note that all solutions should be ready since the gel is covered with solutions whenever the vacuum is on. 

Gel batteries require an additional separator to fix the plate distance and to prevent electronic shorts. The most effective protection against shorts is achieved by means of separators with low pore size ideally, microporous materials should be used (pore size less than 1 pm). Additionally, the separator should have a low acid-displacement since the fumed silica and the cracks in the gel already reduce the volume available for electrolyte. To minimize the internal resistance of the battery, the electrical resistance of the separator should be as low as possible. These two requirements, viz., low acid-displacement and low electrical resistance, translate into a need for separators with good wettability, high porosity, and low geometrical volume, i.e., rib configuration and backweb thickness should both be optimized. 

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