V. Gottardi

Raman and infrared spectra on silica gel evolving toward glass

Abstract Infrared and Raman spectra of gels obtained from specially prepared solutions of Si(OC2H5)4 (TEOS) which have been thermally treated in the 40–800°C temperature range, are reported and discussed with reference to the spectra of fused quartz. The results show that the gel to glass transformation is an hydrolytic polycondensation process, which takes plase gradually and is practically completed in the samples treated at 800°C. Particularly revealing is the behaviour of the bands due to OH (associated and/or unassociated H-bonded) stretching modes, including those of water, which are weakened by increasing temperature; the behaviour of the band due to the SiOH stretching mode; the behaviour of the network bands, some of which become more intense with increasing temperature, due to asymmetric and symmetric stretching and the bending SiOSi modes. In the Raman spectra of gels heated at different temperatures two peaks also appear at ≈600 and ≈490 cm−1, which are also present in the Raman spectrum of fused quartz, they are related to network defects of the glass structure, for which no definite interpretation has yet been given in the literature.

Further investigations on Raman spectra of silica gel evolving toward glass

Abstract In a previous investigation a continuous evolution of silica gel structure toward that of v-SiO 2 was verified by Raman spectroscopy. Modifications on the intensity of some Raman bands usually associated in the literature to “defects” of the structure were observed. In the present work the study has been extended to the transformation with temperature of silica gels obtained from tetraethoxysilane (TEOS) with different H 2 O/TEOS molar ratios. Raman spectra as well as thermogravimetric data are discussed in terms of the effect of this ratio on the local structure and the sintering process of silica gel.

Phenylacetylene half-hydrogenation with Pd supported on vitreous materials having different chemical compositions

Pd metal has been supported on vitreous materials prepared by the gel route. Three supports with different chemical compositions of the SiO2, Al2O3 and Na2O systems were prepared; the corresponding catalysts, performed with very similar degrees of metal dispersion and Pd loading, have been used in the half-hydrogenation of phenylacetylene at different H2 pressures. The results display an analogous behaviour with modest changes of the catalytic properties on changing the chemical composition of the support. The relatively better catalytic activity of the catalyst with smaller alkaline content is discussed in terms of the possible electronic shift from the metal to the support.

Raman defect peaks in the spectra of Na2O silica gels evolving toward glass

Abstract In previous works the continuous thermal evolution toward v-SiO2 of gels obtained from tetraethoxysilane (TEOS) and with different H2O/TEOS molar ratios was studied by Raman spectroscopy; Raman defect peaks at ∼ 600 and ∼ 490 cm−1 were observed both in gels and in the v-SiO2. In this work the structural effects of small amounts of Na2O on silica gels evolving toward glass are studied in order to give a further experimental contribution to the problem of “Raman defect peaks”. Some considerations on the nature of these defect peaks are now presented considering all the results so far obtained.

Preparation of supports for catalysis by the “gel route”

Abstract The surface area thermal stability of SiO2/Al2O3/Na2O supports prepared from alkoxides was studied as a function of time and temperature. The results indicate substantial stability in time at temperatures ⩽ 250°C, whereas a progressive fall in porosity was observed at higher temperatures. New supports were prepared by coating glass microspheres with a thin layer of porous oxides. The formation of the porous layer was achieved by extending the gel method to the coating technique. Surface area, electron microscopy results and other features relevant to these systems are presented and discussed. The data suggest that these materials may be exploited as effective supports for metallic catalysts.

AZCS refractories prepared from gels

Abstract The material of chemical composition 1% Na2O, 28% Cr2O3, 28% Al2O3, 15% SiO2 and 28% ZrO2 was obtained by the gel route; this product is usually prepared by electrocasting and it is a well recognized refractory. The starting species dissolved in methanol/formic acid form the gel upon hydrolysis; this, on heating to 1500°C, forms a product of tetragonal and monocline ZrO2, ZrSiO4, Al2O3/Cr2O3 solid solution and vitreous phase. The evolution from gel to the final material was followed by X-ray analysis and DTA experiments: the separation and modification of the crystalline phases as a function of the temperature is demonstrated and discussed. Chemical analysis and scanning electron microscopy results allow a complete comparison to be made with the same composition obtained by electrocasting.

Interactions with Portland cement paste of glass fibres coated by the sol-gel method

Abstract E-glass fibres were coated with SiO2 and 80% SiO2-20% ZrO2 thin films by the sol-gel method to verify the possibility of protecting them from the alkaline attack of cement. Mechanical and chemical tests were performed on coated and uncoated fibres in cement, thin coatings on flat substrates, coated and uncoated glass rods and on gel-glass powder in order to investigate the interactions between gel-glass film and cement paste. The results indicate that the SiO2ZrO2 coating is effective in the protection of the substrate from the alkaline cement medium. Some evidence is presented that the coating reacts with calcium to give a thick and compact protective layer.

Mechanical properties of ion implanted glasses

Abstract It is well known that heavy ion implantation causes modifications of the surface mechanical characteristics of glasses. At first a compaction is induced with increasing ion fluence, then a relief is observed after a maximum tension stress. In this paper we report the results of a study on the mechanical resistance of Ar implanted soda-lime glasses. Vickers microindentation tests have been performed by measuring both the percentage of cracks, which develop after application of a loaded square pyramidal indenter on the glass, with respect to the number of tests and the length of cracks. In order to clarify the mechanisms responsible for the ion induced mechanical modifications, the influence of the implantation parameters and the thermal stability of the modified surface layers have been investigated.

Glasses of the same composition but different properties

Abstract Glasses with the same chemical composition may have quite different structures and physical properties. This concept is illustrated with examples. Glasses formed from gels are compared with glasses of the same composition formed from melts to demonstrate the effects of the preparation method. Glass structure is determined in part by the thermal history, particularly the temperature of the liquid from which the glass is formed. A third example is radiation damage and ion implantation can produce glasses of quite different structures and properties depending on dose.

Chemical evolution of the Na2OP2O5 = 1.4 gel: A 31P NMR investigation

Abstract The chemical evolution accompanying the gel heating was studied in the case of the Na 2 O P 2 O 5 = 1.4 system. In order to identify single chemical components of the prepared amorphous materials, the samples obtained by heating the crude product at different temperatures and relevant melted glasses were analyzed by 31P NMR spectroscopy using suitable solvents and in the presence of cation complexing agents, to simplify NMR analysis and to facilitate sample dissolution. Results indicate that the starting gel is composed of monomeric phosphoric units organized in an amorphous structure. These species, on heating, evolve to oligomeric compounds analogous to the products which may be obtained only by keeping the corresponding glasses prepared by melting at high temperature for long times. Gel-derived glass after melting is practically composed of the same species of the glass prepared by the melting method, although the NMR analysis indicates the presence of some residual -OH groups in the gel-derived material.