Titania drying temperature

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. 3. Effect of using either liquid or supercritical carbon dioxide on the textural properties of titania aerogels calcined at the temperatures shown. (—), dried with Hquid carbon dioxide at 6 MPa and 283 K (-------), dried with supercritical carbon dioxide at 30 MPa and 323 K. Reproduced from Ref. 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.

The presence of sulfate ions markedly affects the nanopore structure of titania-sulfate aerogels. In Ti02-S042 materials, unlike in zirconia-sulfate aerogels, the larger sulfate load stimulates formation of a more consolidated structure. The XRD analysis shows that even a crystalline phase (anatase) may be present in fresh, dry aerogels, which, perhaps, is the first observation of this phase in sol-gel titania obtained from the low temperature drying process. 

We report herein LPD titania films deposited on three kinds of polyimides two high-temperature polyimide resins (PMR-15 and Kapton ) and a bismaleimide glass fiber composite (BMI). We have examined different solution conditions and different surface priming strategies. We have also learned to minimize film cracking by carefully controlling the drying process. 

Using the sol-gel technique five titania gels were prepared at final pH for the formation of the gels ranging from pH 1-6.7. The nomenclature adopted in this study starts with the letters SG followed by the pH of formation and finally a letter to indicate the final heat-treatment temperature a, b, c, d, or e being equal to 60°C, 200°C, 300°C, 400°C or 500°C, respectively. The original five materials were prepared at pH s of 1.0, 3.0,4.5, 5.7 and 6.7. The results obtained on these five materials dried at 60°C will be first discussed. The sample prepared at a pH of 5.7 was then calcined at various temperatures. The effects on the Sbet, porosity and phase composition will be discussed separately. 

Grain growth of zinc oxide occurs at much lower temperatures than in titania, so that production of fairly crystalline ZnO nanopowders needs special precautions. The thermal decomposition of freeze-dried Zn(N03)2 at 260 to 270°C results in the formation of poorly crystalline, but coarse-grained ZnO powders. " At the same time, insulation of ZnO particles in the inert NaCl matrix by the method described before - led to no change in the size of the ZnO particles. The ZnO particles were obtained from freeze-dried precipitate of zinc hydroxide of 15 to 20 nm, and this size was maintained at least up to 600°C. "... 

Amorphous silica has also been mentioned as a starting metal oxide material for the preparation of particulate mesoporous membranes. These membranes were prepared from commercial sols, Ludox (DuPont) or Cecasol (Sobret), and coated on a macroporous a-alumina support [35]. In contrast to crystalline membrane materials such as alumina, titania or zirconia, the evolution of pore size with temperature of amorphous silica membranes was revealed to be more sensitive to drying conditions than to firing temperature (Table 7.1). When heat-treated for several hours at 800°C the silica top layer transformed from an amorphous state to cristobalite. 

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