Xerogels thermal treatment

In order to convert xerogels to dense, stable, solid materials, a further thermal treatment is required. In this stabilization step the gel is dehydrated, dehydroxylated and sintered. Thus all surface hydroxyls are removed and the material is fully densified. Since this work discusses the characterization and modification of porous silica xerogel we will not further discuss the processes in this step. The reader is... 

XRD analysis of the xerogels obtained by drying pure titanium dioxide sol at 70°C showed the presence of the nanocrystalline anatase phase [109]. Thermal treatment of this xerogel resulted in the growth of anatase crystallites up to 400°C. The anatase-to-rutile transformation began to occur at 450-500°C. This process was practically completed at 700°C, and only rutile phase existed at rcaic > 700°C. This feature of Ti02 xerogels is typical and well known (see, for example, [109]). Thus, it can be concluded that anatase-rutile transition temperature of nanosized particles is considerably lower than that of the... 

More recently, models were developed to achieve the elimination of the template under milder conditions. The first relies on the preparation of a dialkyl carbonate system by polycondensation/hydrolysis of the corresponding precursor family 26. Thermal treatment of a non-porous xerogel of 26 at 250-350 °C results in the simultaneous elimination of CO2 and hybrid materials with residual hydroxyalkyl and olefinic functions according to Scheme 18. It is interesting to note that this approach also allows the preparation of materials which cannot be prepared by other routes, such as direct polycondensation of the corresponding allyl and hydroxyalkyl precursors. 

In a few cases, the thermal treatment was performed as a pyrolysis at inert atmosphere, and a very small weight loss was observed for xerogels like 7795, 54 and 78150, due to the possible high cross-linking of these materials8. 

The products were synthesized by sintering xerogel with various metal atom stoichiometric amounts at different temperatures for different times and by using different thermal treatments. The results measured by XRD are shown in Table 1. 

FIGURE 12.2 (a) Comparison of the linear shrinkage of mullite (3Al203-2Si02) micronic powder, 600°C thermally treated gel (xerogel) and pristine gel. (After Reference 87.) (b) Plot of the R.T. microhardness for mullite and Nasicon (Na3Zr2Si3POi2) as a function of thermal treatment. (After Reference 27.)... 

Figure 6.12. Combined study of the evolution of the mean grain size and of the amount of microstrain rate, according to the temperature of the thermal treatment, in zirconia xerogels...

Thermal treatment.In many cases, the optimization of the pH value of the coprecipitation, and the aging time, does not lead to well crystallized LDH phases and thermal treatment often gives good results in improving the crystallinity of the amorphous xerogel or the badly crystallized materids. 

During the calcination with or without gases, there are decompositions of hydroxides, carbonates (gel, xerogel) transforming into oxides which are chemical reactions, transforming one compound to other compounds. During thermal treatments, there are following cases ... 

Thermal treatment of silica xerogel doped with Co2(CO)6L 2, where L is PPh2(CH2)2Si(OEt)3, gives a nanocomposite containing nanoclusters of C02P. A TEM... 

In another approaeh, A,A -bis-(3-triethoxysilylporpyl) diimides are first prepared by the transimidation method from 7V,7V -disubstituted diimides and aminopropyl-triethoxysilane (Scheme 50). The final hybrid material is attained by thermal treatment in the conditions of the sol-gel method. Transparent xerogels can be obtained by this procedure from a variety of bis-(triethoxysilyl) diimides based on classical dianhydrides [259]. 

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