F. Devreux

Insight into silicate-glass corrosion mechanisms.

The remarkable chemical durability of silicate glass makes it suitable for a wide range of applications. The slowdown of the aqueous glass corrosion kinetics that is frequently observed at long time is generally attributed to chemical affinity effects (saturation of the solution with respect to silica). Here, we demonstrate a new mechanism and highlight the impact of morphological transformations in the alteration layer on the leaching kinetics. A direct correlation between structure and reactivity is revealed by coupling the results of several structure-sensitive experiments with numerical simulations at mesoscopic scale. The sharp drop in the corrosion rate is shown to arise from densification of the outer layers of the alteration film, leading to pore closure. The presence of insoluble elements in the glass can inhibit the film restructuring responsible for this effect. This mechanism may be more broadly applicable to silicate minerals.

A simplified model for glass dissolution in water

Numerical simulations of the water dissolution of a random ternary solid are presented. The three elements represent silica, soluble oxides (alkalis and boron) and quasi-insoluble oxides (Al2O3, ZrO2, Fe2O3,...). The soluble species are dissolved immediately when they are in contact with the solution. Their proportion is kept below the percolation threshold. For the other species, one introduces a model of dissolution-recondensation. It is shown that the dissolution rate constants should be dependent on the bonding environment in order to include surface tension. The condensation fluxes are proportional to the concentration of each species in solution. In the dynamic regime (no recondensation), one observes the congruent dissolution of silica and soluble species, after a short initial phase of selective extraction of the soluble species. The common rate of dissolution decreases with the proportion of insoluble species and increases sharply with that of soluble species. This is mainly due to the formation of a porous hydrated layer whose active surface area increases markedly with the proportion of soluble species. In the static regime (finite solution volume), the equilibrium solubility of silica decreases with the proportion of insoluble species and is practically independent of the proportion of soluble species. The porous hydrated layer is rearranged and almost free of soluble species. The ripening of the surface layer makes it protective and inhibits further extraction of the soluble species. These results are in general agreement with the experimental observations on the dissolution of durable glasses.

Sol-gel polymerization in alkoxysilanes: 29si nmr study and simulation of chemical kinetics

Abstract We report on the results of 29 Si NMR monitoring of alkoxysilane/ethanol/acid water sol-gels with tetraethoxysilane (TEOS), methyltriethoxysilane (MTEOS) and vinyltriethoxysilane (VTEOS). The gel time of the TEOS solution was 49 days at the degree of condensation C = 0.81, whereas MTEOS and VTEOS do not gel in spite of the high C = 0.95 achieved. The polymerization kinetics progressively slows with the time owing to the growth of sterical restrictions. To account for the effect we developed a kinetic model based on the reaction rates which explicitly decay as a function of the degree of condensation. The model describes the complete sol-gel process in all the studied systems including a gel transition in TEOS and aging of the gel.

Contribution of Monte Carlo Modeling to Understanding the Alteration of Nuclear Glasses by Water

Abstract This paper proposes a contribution to understanding the alteration of high-level waste glasses. Numerical simulations, based on a Monte Carlo model, have been performed in parallel to static dissolution tests on simplified glasses. The leaching of borosilicate glasses has been investigated for various compositions containing three or four oxides, which have been derived from the French nuclear glass composition. The comparison between experimental data and simulations allows a precise understanding of the role of each element. The degree of alteration is shown to result from a competition between the irreversible extraction of the soluble species (boron and alkalis) and the reversible dissolution-condensation dynamics of silica, which make possible the restructuring of the surface layer into a passivating layer. The model explains how the surface layer is responsible for the blocking or, at least, for a considerable slowing down of the alteration. It is also able to explain a quite unexpected result, namely, the fact that the replacement of silica by more insoluble oxides (zirconium or aluminum oxides) actually induces an increase of the degree of alteration. This is due to the slowing down of the surface layer reconstruction that delays the alteration blocking.

Self similarity of alumino-silicate aerogels

Abstract SANS data are presented concerning the structure of aluminosilicate aerogels prepared from the (BuO) 2 AlOSi (OEt) 3 complex precursor and heated at different temperatures. As in the silica system, aerogels clearly exhibit a mutually self similar fractal structure over a wide range of densities (40–160 kg/m 3 ). The structure consists of primary homogeneous rough units (size of 13 A) attached into volume fractal clusters ( D = 2.1). When heated at 800°C, the size of the fractal domain and the roughness of the unit particles decrease.

Monte Carlo Modelling of Glass Dissolution: Comparison with Experiments

One presents the results of numerical simulations of glass leaching. The glass is modelled as a random mixture of partly and totally soluble species, which represent silica, and boron or alkali oxides, respectively. It is shown that the dissolution rate and the thickness of the altered surface layer are strongly dependent on the glass composition, whereas the equilibrium solubility is not. The dependence of the layer thickness on the glass surface area to solution volume ratio is also emphasized. The protective role of the surface layer is shown to arise from its restructuring after the extraction of the soluble species. The simulation results are compared to an experimental study performed on series of SiO 2 -B 2 O 3 -Na 2 O glasses.

Effect of ZrO 2 on the glass durability

Borosilicate glasses were prepared with the molar composition 70 SiO 2 -15 Na 2 O-15B 2 O 3 -n ZrO 2 with n ranging from 0 to 10. The glasses were studied by conventional static dissolution tests of powders at 90°C in pure water and in buffered solutions for long times (months) and short times (minutes). During the first minutes of alteration in a buffered solution, sodium is rapidly leached until its loss becomes controlled by the silicon hydrolysis. The experimental data show that the introduction of zirconium drastically reduces the initial dissolution rate (Vo) of the glass. Zirconium strengthens the silica network but also strongly modifies the porous layer morphology. In the case of glasses with small Zr contents (less than 2%), the silica dissolution rate decreases but the formation of a passivating alteration layer is also delayed. As a result, small amounts of zirconium paradoxically decrease the loss of silica but increase the final loss of sodium and boron in the static leaching tests. Larger zirconium contents (above 5%) increase the durability of the glass regarding the initial dissolution rate and the final concentration of all elements.

Pseudo-equilibrium between a random system and a solution: a Monte-Carlo study of glass dissolution in water

The dissolution of a random binary glass with strongly different solubilities is studied by Monte-Carlo simulations in two dimensions. The soluble species represents Na2O or B2O3. The other species, SiO2, has a local environment-dependent solubility. For a finite volume of liquid a rapid dissolution is found until an apparent equilibrium is reached. Nevertheless, a steady state porosification continues at a lower but constant speed with restructuration of the porous surface. The simulation reproduces the restarting of dissolution at apparent equilibrium, recently observed experimentally. Dynamical percolation permits us to explain the strong dependence of the dissolution rate on the concentration.

29Si NMR and viscosity study of the sol-gel transition and evolution after the gel time

Abstract In order to study the sol to gel transition and the early stage of aging, 29 Si NMR and viscosity measurements were performed on acid-catalyzed TEOS in excess of water, where evaporation is hindered. NMR spectra evolution was followed over an interval running from 0.5 t g to 4 t g ( t g is gelation time). The difference between static spectra and magic angle spinning (MAS) spectra, analyzed in terms of anisotropy of the chemical shift, allowed evaluation of the gel weight-fraction. Its evolution appeared to be quite slow. A kinetic model of post-gelation aggregation accounts for this time dependance. NMR measurements were not very accurate close to the transition. Rheology experiments over a frequency range from 10 −2 Hz to 100 Hz were performed in this region. Scalar percolation transition was observed within a very narrow interval.

Nuclear spin relaxation in alumino-silicate aerogels☆

Spin relaxation allows one to probe lattice dynamics at a local level. Measurements in aerogels and xerogels reveal an identical behaviour as compared with glasses obtained by thermal quenching. While all thermal variations can be accounted for by a large number of activated local vibrations, these cannot explain the observed density dependence. When the density is lowered without modification of the local structure, relaxation is accelerated.