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W. Wallidge Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
R.J. Newport School of Physical Sciences, University of Kent at Canterbury, Canterbury CT2 7NR, United Kingdom
M.E. Smith Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom (Received 24 January 2000; accepted 23 June 2000)
The small-angle x-ray scattering (SAXS) technique was used to investigate inhomogeneities on the scale of 10 to 600 Å in acid-catalyzed titania–silica and zirconia–silica xerogels. SAXS of (TiO2)x(SiO2)1−x and (ZrO2)x(SiO2)1−x xerogels with x < 0.1, in which there was no phase separation, showed the presence of two types of inhomogeneity. For Q < 0.05 Å−1 there was a clear departure from Porod scattering which showed that xerogel powder particle surfaces were rough. For 0.1 < Q < 0.4 Å−1 there was a plateau feature corresponding to micropores within the silica-based network, and this feature changes with heat treatment. SAXS of xerogels with x > 0.3 showed the presence of phase-separated regions of metal oxide, which were initially amorphous and crystallized at higher temperatures. A (TiO2)0.18(SiO2)0.82 xerogel that was not initially phase separated became phase separated after heat treatment at 750 °C due to reduced solubility of Ti in the silica network.
I. INTRODUCTION
Mixed titania–silica materials, (TiO2)x(SiO2)1−x, are useful in a number of technological applications,1 such as ultralow expansion glasses, antireflective thin film coatings, and catalytic materials.2 Mixed zirconia–silica materials, (ZrO2)x(SiO2)1−x, have low thermal expansion, tunable refractive index, and high physical and chemical durability3 and have potentially useful catalytic properties.4 However, the preparation of these mixed oxide materials by high-temperature methods is difficult because of the high melting temperatures involved. The sol-gel process5 has the advantages of using liquid precursors and involving relatively low processing temperatures. A sol of water and metal alkoxide(s) undergoes hydrolysis and condensation reactions to produce a gel. A gel is dried to produce a xerogel, generally an amorphous, porous, hydrated solid. This becomes similar to a melt-quenced glass during heat treatment (up to ∼1000 °C). The structures of xerogels are strongly dependent on the details of preparation and heat treatment. For example, base-catalyzed gels have a colloidal structure, whereas acid-catalyzed gels have a polymeric structure.5 Furthermore, conventionally aged/dried acid1998
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J. Mater. Res., Vol. 15, No. 9, Sep 2000 Downloaded: 12 Apr 2015
catalyzed gels, i.e., xerogels, are microporous, whereas mesoporous structures called aerogels can be produced by using special aging/drying conditions. The degree of homogeneity in xerogels is an important issue for investigation. Acid-catalyzed xerogels are commonly prepared with two-step hydrolysis and low water: alkoxide ratio, i.e., 1 艋 Rw 艋 5, to promote homogeneity. Titania– and zirconia–silica xerogels prepared in this
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