Supercritical Drying Temperature

Concerning the effect of the SCD temperature, which must be 20°C higher than the critical temperature of the solvent [28], many research groups claimed that higher SCD 

It is also important to note that results obtained by different researchers indicated that surface areas of calcined zirconia aerogels were not much higher than those of precipitated samples when supercritical CO2 was used for drying. In fact, after calcination at 500°C surface areas were mostly below 100 m g [17, 25, 27, 29]. 

Compared io xerogeis, zirconia aerogel samples showed higher surface areas and pore volumes and exhibited smaller crystallites. In fact, the collapse of the gel framework due to the surface tension in the liquid occurs during rapid drying of the gels. However, SCD avoids this collapse and preserves the porous network. 

Many researchers noted that independent of the zirconium precursor and the preparation method (precipitation, sol-gel, etc.), obtained zirconia xerogeis, dried at moderate temperature, were X-ray amorphous and crystallized only after calcination, at least, at 300°C, while the high-temperature SCD led to samples exhibiting tetragonal nanocrystallites. 

Baiker et al. [25] also reported that zirconia xerogeis and aerogels, obtained by sol-gel process using ethanol as solvent, exhibited Bronsted and Lewis acid sites. However, with xerogeis, the density of acid sites on the surface was significantly lower. This behavior was 

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In each case, a pure and its corresponding doped oxide were prepared with a weight content of yttria of 8 %, the volumetric ratio of precursor to solvent was 1/1, and the supercritical drying temperature was 265 °C while a stoichiometric quantity of water was reacted for the hydrolysis reactions. All reactants were vigorously stirred at ambient temperature to carry out the sol-gel achievement. Table 1 gives the precursors and corresponding solvent of the synthesis. No catalyst was added to the reactant mixtures. 

It is less well known, but certainly no less important, that even with carbon dioxide as a drying agent, the supercritical drying conditions can also affect the properties of a product. Eor example, in the preparation of titania aerogels, temperature, pressure, the use of either Hquid or supercritical CO2, and the drying duration have all been shown to affect the surface area, pore volume, and pore size distributions of both the as-dried and calcined materials (34,35). The specific effect of using either Hquid or supercritical CO2 is shown in Eigure 3 as an iHustration (36). 

Fig. 1. Crack-free monolithic titania-silica aerogel photos, (a) aerogel prepared by hi -temperature ethanol supercritical drying, (b) aerogel prepared by low-temperature CO2 supercritical drying.

Distortions of patterns due to capillary forces can be avoided altogether by applying supercritical drying (SCD) [83,84]. In a supercritical liquid, the surface tension becomes neghgible (y 0) and the capillary force vanishes. However, depending on the solvent, high pressure (1-10 MPa) and elevated temperatures (40-400 °C), may be required [85]. This factor renders this method not imiversally applicable for wet processing. For example, if SCD was to be used for isopropanol removal from SU-8, temperatures over 235.2 °C and pressures over 4.8 MPa would be needed [85]. 

Solgel processing specifies a type of solid material synthesis method performed in a liquid and at low temperatures [152], The produced inorganic solids, mostly oxides or hydroxides, are formed by chemical transformation of chemical solutes termed precursors. The solid is formed as the result of a polymerization process that involves the establishment of M-OH-M or M-O-M bridges between the metallic atoms M of the precursor molecules [152], The drying process, after the gel formation, is carried out at a relatively low temperature to produce a xerogel, or by a supercritical drying process. 

Three different MgO samples were used in the experiments. AP-MgO with a surface area of 385 m2/g was prepared by a sol-gel technique involving high-temperature supercritical drying described in detail earlier.4,13 CP-MgO (281 m2/g) was obtained by decomposition of Mg(OH)2 prepared by hydration of commercial MgO. In both cases the final preparation step was overnight evacuation at 500°C followed by storage under ambient conditions. Their performance was compared to that of a commercial low surface area sample CM-MgO (10 m2/g). 

Drying to remove solvent from a gel drying method (e.g. evaporative vs. supercritical vs. freeze drying) temperature and heating rate pressure and pressurization rate time... 

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