Xerogels cracking

Ru(dpp)3]2+) sequestered within the xerogels. The results of SEM and luminescence measurements shown that certain ([Ru(dpp)3]2) doped Octyl-triEOS/TEOS composites form uniform, crack-free xerogel films that can be used to construct high-sensitivity O2 sensors that have linear calibration curves and excellent long-term stability (over a period of 11 months). 

Scanning electron microscopy (SEM) used to investigate the structure of similar organically modified silicate (ORMOSIL) films shows that certain [Ru(dpp)3]2+-doped octyl-triethylorthosilicate (triEOS)-tetra-ethylorthosilicate (TEOS) composites form uniform, crack-free xerogel films (Figure 6.6) that can be used to construct high-sensitivity oxygen... 

There exist a maximum allowable thickness of the supported gel layers above which it is not possible to obtain crack-free membranes after calcination. For Y-alumina membranes this thickness depends on a number of (partly unknown) parameters and has a value between 5 and 10 /im. One of the important parameters is certainly the roughness and porosity of the support system, because unsupported membranes (cast on teflon) are obtained crack-free up to 100 )xm. The xerogel obtained after drying was calcined over a wide range of temperatures. At 390°C the transition of boehmite to y-AljOj takes place in accordance with the overall reaction... 

Porous membrane Colloidal CdS particles converted to Xerogels and dried at about 10 3 Torr at 30-40 °C to produce crack-free, optically transparent membranes... 

Gels dried by conventional methods, known as xerogels (xero- = dry), generally have lost the very open structure of the wet gel. The reason for this is that the liquid phase, as it evaporates and shrinks, exerts powerful capillary forces on the pore walls through the surface tension at the meniscus, resulting in collapse or cracking of the large-scale structure of the gel. 

Local variations in pore size and/or porosity as well as rigid inhomogeneities will give rise to local variations in the stress distributions and to residual stresses in the final xerogel. This happens because larger pores are emptied first by evaporation after the critical point with the consequence that the wall between adjoining pores is subjected to uneven stress which can cause cracking. Very few studies has been devoted to this aspect due to the absence of the necessary data. 

It appears that cracks may be avoided if the liquid evaporation rate is very slow. This was the first way to achieve monolithicity in gels, but it is evident that this drying process is very long (months). Moreover, the dry gel (xeroget) exhibits a very low permeability, which may present many disadvantages when the xerogel must be heated further. 

An explanation may be offered to take account of this behavior. The escape of gas by-products is hindered by the low xerogel permeability. The produced gas cannot escape easily. Its pressure increases, and thus the xerogel expands. A fast heating rate can lead to a cracked xerogel. To avoid this phenomenon, the thermal schedule must be optimized. However, more experiments are necessary to confirm this proposed explanation. 

Hydrolysis of a mixture of Si(OR>4 and Ti(OR)4 results in the immediate precipitation of titania, because Ti(OR)4 is much more reactive than Si(OR)4. When (R0)3Si(CH2)3C[C(Me)0]2Ti(0R)3 or (R0)3Si(CH2)3C[C(Me)0]2 2Ti(0R)2 is hydrolyzed under basic conditions, transparent and crack-free monolithic xerogels are formed instead. Mixed-oxide powdCTS with the nominal composition Ti02 SiO2 or Ti02-2Si02 and a low crystallization tendency are obtained when the organic groups were removed by calcination at 550 °C. 

Mosquera MJ, de los Santos DM, Valdez-Castro L, Esquivias L (2008) New route for producing crack-free xerogels obtaining uniform pore size. J Non-Cryst Solids 354 645-650... 

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