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Large size silica articles, in particular optical glass components, are made by forming a silica colloidal solution; after gelation, the gel is cured, dried and sintered to form a dense glass having near net shape dimensions thereby reducing expensive polishing steps needed to obtain the desired final dimensions and surface quality. The process is characterized by the use of a two step acid-base procedure for the hydrolysis and condensation of tetralkoxides, addition to the sol of fumed silica in order to increase its concentration, hypercritical drying of the alcogel and sintering of the aerogel so obtained. Densification is uniform with three-dimensional shrinkage of the aerogel to a final shaped component of clear silica of the same properties as silica glass on the market, such as bulk density, Vickers hardness and thermal expansion coefficient.

Impedance spectrum measurements of an ionic conductor material constitute a well known method for the optimization of these materials. Today the most adopted method consists, essentially, in the vectorial comparison between two sinusoidal signals, one taken as reference, over a wide range of frequencies. In this work, after some considerations about practical electrical connections, an evaluation criterion to eliminate the so-called ‘parasitic impedance’, noticeable chiefly at high frequencies (10 kHz), is proposed. A method is also reported which permits the voltage at the extremes at the measuring cell to remain constant.

Vitreous silica containing titania has received considerable attention due to its anomalous thermal expansion behaviour, together with high optical quality. The effect and the role of TiO2 in the properties of glass are directly correlated with the structural position of titanium; thus several attempts have been made to elucidate the structural aspects of gels and glasses containing titania. Several SiO2-TiO2 gels containing up to 30 mol% TiO2 prepared using (i) normal drying; (ii) supercritical drying in alcohol; (iii) supercritical drying in CO2 were studied in this work. The aim of this work is to establish the titanium coordination during thermal treatments during the gel/glass system evolution. X-ray diffraction (XRD) and dilatometric analysis have been used to detect the presence of a crystal phase of TiO2 (anatase) in gel/glass samples. The crystal size of anatase was evaluated using the Scherrer relation from XRD patterns. Infrared and diffuse reflectance spectroscopy in the UV-visible region have been used to elucidate the structural aspects and the electronic transitions due to different titanium coordination. These results indicate that drying conditions strongly influence the silicon-oxygen-titanium structure.