J. Zarzycki

Synthesis of glasses from gels: the problem of monolithic gels

The synthesis of glasses from gels has recently aroused considerable interest both from the theoretical and practical standpoint. The conditions of gel formation which determine the structure of the wet gel are first examined. During the drying process, which is a decisive step in the obtention of monolithic gels, an essential role is played by capillary forces. It is shown that, among the various methods of obtaining monolithic gels, that based on the hypercritical evacuation of the solvent is the most reliable.

Past and Present of Sol-Gel Science and Technology

In the last twenty years sol-gel science has undergone a spectacular development. The various stages of the sol-gel process have been scrutinised in considerable detail and a sound basis for future technological developments established. In the beginning the studies centred mainly on silica and silicate glasses and were progressively extended to many ceramics and composites. A turning point was reached with the emergence of ormosils and of organic-inorganic nanocomposites. This opened the gateway to whole classes of new materials. The present studies show a definite tendency towards very specialised high-tech applications.

Efficient tunable solid-state laser near 630 nm using sulforhodamine 640-doped silica gel

Lasing is observed near 630 nm in a sulforhodamine 640-doped silica gel pumped by frequency-doubled radiation from a Q-switched Nd:YAG laser. A conversion efficiency of 20% is obtained. The laser wavelength is tunable over 40 nm.

Crystallization of gels and glasses made from hot-pressed gels

Abstract Silica gels were prepared by two different methods: (1) destabilization of a silica hydrosol (gel 1); (2) hydrolysis and polycondensation of a tetra-methoxysilane (gel 2). The crystallization of the gels was then studied as a function of the temperature by means of X-ray diffraction. It was evident that the crystallization was strongly influenced by the amount of alkali oxides present in the gel. It is effectively the concentration of akali impurities which could explain the lower temperature of crystallization necessary for gel 1 compared with that of gel 2. During the crystallization of the gel containing Na2O the crystalline phase of silica which appears first is the cristobalite; with Li2O it is quartz. The effect of additives such as boric anhydride was studied. This oxide was found to reduce the tendency of the gels to crystallize. The glasses of the system SiO2B2O3 obtained by the hot-pressing of the gels confirmed this phenomenon. Above 10 mol% B2O3 it was impossible to crystallize the gels and the glasses of this system under two hours.

Synthesis of monolithic silica gels by hypercritical solvent evacuation

The method of obtaining monolithic dry gels by hypercritical solvent evacuation is presented. The influence of various parameters on the possibility of obtaining crack-free pieces is studied in some detail. The influence of these parameters on the final properties of the gel is also discussed.

Monolithic aerogels in the systems SiO2B2O3, SiO2P2O5, SiO2B2O3P2O5

Abstract The gels in the binary and ternary systems: SiO 2 B 2 O 3 , SiO 2 P 2 O 5 , SiO 2 B 2 O 3 P 2 O 5 ; were prepared by hydrolysis and polydensation of metalorganic compounds. The gelling times vary with the molar percentage of SiO 2 . The solvent was evacuated under hypercritical conditions in an autoclave in order to obtain aerogels free of cracks. The monolithicity of the aerogels is influenced by the method of preparation of the alcogels. The crystallization of BPO 4 was observed in the ternary system only. These materials can be converted into glasses by heat treatment. The structural evolution was followed by means of infrared spectroscopy and textural evolution by dilatometric measurements.

Preparation of xNa2O−(1−x)SiO2 gels for the gel-glass process: I. Atmospheric effect on the structural evolution of the gels

The gels of the binary system Na2OSiO2 are difficult to prepare under reproducible conditions because of the catalytic effect of the OH− ions. The gels were prepared in the whole range of composition which corresponds to the vitrification range. The structural analogy between the gels and the corresponding glasses was shown by X-ray diffraction and IR spectroscopy. The problems related to the preparation of these gels were studied, following their structural evolution during ageing. The action of atmospheric agents. (CO2 and H2O) induces a crystallization of carbonate compounds. The crystalline phase which appears is Na3H(CO3)2 2H2O in the case of gels containing less than 25 mol% Na2O. For a higher sodium content the compound which crystallizes is Na2CO3H2O. The interaction between the atmosphere and the gel starts at the “surface” of the samples as observed by the IR reflection spectroscopy.

Magnetic properties of a basalt glass and glass-ceramics

Abstract Mossbauer, ESR and magnetization measurements have been carried out on a basalt glass heat-treated at different temperatures (600, 650, 700, 800 and 900°C for 8 h). The as-annealed glass and the above five samples showed two-doublet Mossbauer spectra, while the last two samples also showed a six line magnetic hyperfine pattern at 300 K. At 4 K, the last four samples showed magnetic hyperfine patterns, while the as-annealed glass showed that there was already short range magnetic ordering present. High field Mossbauer data at 4 K showed that the surface spins are canted. The minimum quadrupole splitting and the maximum isomer shift around 700°C are related to the improved symmetry of the magnetite lattice. ESR spectra showed paramagnetic resonances at g = 4.3 and g = 2.0 for the first two samples, while the last four samples showed superparamagnetic resonance centred around g = 2.0 at 300 K. At lower temperatures, the 650 and 700°C samples showed ferrimagnetic resonance. Magnetization curves against H/T superpose well both at 300 and 77 K, showing the typical superparamagnetic behaviour of the small magnetite particles. The saturation magnetization (at 270 K) showed a sharp change around 700°C, showing the formation of magnetite. The magnetic structure of the small magnetite particles are discussed in terms of the above results.

Gel → glass transformation

Abstract The possibility of converting a gel into glass by sintering depends on a competition between the phenomena which lead to densification and those which promote crystallization. Sintering kinetics are presented for a close-pore texture and an open-pore texture and the influence of various parameters is discussed. The devitrification versus compaction problem is studied using TTT diagrams. It is shown that in the case of gels, a heterogeneous nucleation mechanism is active which explanins the variations of behaviour observed for gels prepared by different techniques.

Protection of 316L stainless steel against corrosion by SiO2 coatings

Oxide films prepared by sol-gel methods and presenting high resistance to heat, corrosion, friction and wear, as well as excellent mechanical properties, have recently been developed and put into practical use as structuraI materials [1-5]. The process of preparation offers potential advantages for modifying the properties of surfaces by low-temperature treatment without altering the original properties of strength and toughness of the substrates. A number of reports on sol-gel coatings concerning the prevention of chemical corrosion and oxidation have been published [6-10]. All of these films increase the protection of metal substrates from air corrosion (tested up to 800 °C) and acid attack (tested up to 80 °C). The most promising corrosion prevention for stainless steel has been studied by our group using sol-gel films of ZrO2, SiO2, SiO2-TiO2 and SiO 2A1203 prepared by dip-coating using sonocatalysed sols [11-14]. The. properties of these coatings have been studied by electrochemical techniques in NaC1 and H2SO 4 solutions. Although preliminary measurements have shown that SiO2 films are not the best protective coatings [13], they provide a very adequate model system to correlate corrosion protection with the physical structure of the sol-gel films. In this work, amorphous coatings of SiO 2 were deposited on 316L stainless steel by the dip-coating technique using a sol preparation involving sonocatalysis. The films were prepared through hydrolysis polymerization of metal alkoxide solutions and conversion to an oxide layer by heating at relatively low temperatures. The effect of the time of heat treatment of the SiO2 films on the corrosion resistance of stainless steel was studied in 15% H2SO 4 through potentiodynamic polarization curves at 25 °C. The substrate used in the experiments was 316L stainless steel (SS 316L, Caseurop, France) with chemical composition (wt %): 67.25 Fe, 18.55 Cr, 11.16 Ni, 2.01 Mo, 0.026 Cu, 1.71 Mn and 0.028 C. The specimens were machined into dimensions of 30 mm × 15 mm x I mm, degreased ultrasonically in acetone, cleansed by distilled water then dried in air. For silica films, tetraethylorthosilicate Si(OC2H5) 4 (TEOS) was used as the source of silica, absolute ethanol (C2HsOH) as solvent and glacial acetic acid CH3COOH as catalyst. The silica sonosol was prepared by dissolving Si(OCzHs) 4 in absolute ethanol to which a small amount of acetic acid CH3COOH was added. The volume ratios of Si(OC2Hs)4/C2HsOH and Si(OC2Hs)4/CH3COOH were, respectively, 1 and 5. The mixture was submitted to intense ultrasonic irradiation (20 kHz) produced by a transducer (Heat Systems Ultrasonics W385). After 25 min the resulting sol was homogenized and remained stable for about five weeks at room temperature when kept in a closed vessel. Coating films were formed on the substrates by dipping into the clear sonosol and withdrawing at a speed of 10 cmmin -1. The resulting gel films were dried at 60 °C for 15 min and densified in a furnace with air atmosphere by increasing the temperature at a rate of 5 °C min -1 up to 450 °C when an isothermal holding of 1 h was applied in order to remove the organic residues. The temperature was then increased again at the same rate up to either 600 or 800 °C and maintained at that value for variable lengths of time to complete the densification and obtain adherent coatings. The average thickness of the heat-treated films at 800 °C was around 0.4/zm. X-ray diffraction (XRD) analysis of the substrate and coatings was done with a Philips diffractometer using CuKo, The diffractogram of SS 316L shows the existence of a crystalline phase which corresponds to the cubic phase of the alloy containing Cr, Fe and Ni [4]. When the steel was heated at 800 °C for 2 h in air, additional XRD peaks appear corresponding to a mixture of cubic and hexagonal CrzO 3 [4]. In contrast, samples coated with SiO2 analysed after oxidation tests in air at 800 °C for 2 h showed only the peaks of the original substrate, indicating that the silica coating remains amorphous and inhibits any oxidation of the base material. A Bomem Fourier transformation infrared (F-FIR) spectrometer was used to obtain high resolution spectra of the coatings in the 400-4000 cm -1 range; the measurements were carried out at room temperature by reflection at an incident angle of 30 ° . The spectrum of a coating deposited on SS 316L and