Alkali diffusion

Near-Surface Alkali Diffusion into Glassy and Crystalline Silicates at 25 0 C to 100 0 C... 

Of potentially greater significance is surface hydration which occurs concurrently with alkali diffusion at relatively low temperature. The average activation energy of water diffusion in obsidian can be estimated at 75kJ between 95° and 245°C (25). A nuclear resonance hydration profile of obsidian at 25°C has yielded a diffusion coefficient of 5xlO-20 cm2-s 1... 

Comparison with alkali diffusion coefficients and activation energies reported in the present study indicates that a hydration front most likely precedes the diffusion of Rb, Cs and Sr into the glass surface. 

Foland K. A. (1974). Alkali diffusion in orthoclase. In Geochemical Transport and Kinetics, Hoffman, Giletti, Yoder, and Yund, eds. Carnegie Institution of Washington. 

Silicic acid,1 in a form which is soluble and chemically pure, can be obtained by employing a divided cell with alkali silicate in the anode compartment. Perforated electrodes are fitted against the diaphragm wall, and during electrolysis alkali diffuses into the cathode compartment whilst silicic acid remains in the anode compartment. Hydrated silica is thus separated in a pure form specially suitable for stabilising colloids. 

Lipatov reported studies of alkali diffusion in starch 472-474 He discovered that the adsorption velocity from dilute solutions is higher than that from concentrated solutions and obeyed the empirical relation... 

Alkali diffusion coefficient Major, lower by a factor 10 ... 

Since alkali diffusion from a substrate into a crystallizing film requires time, the influence on the resulting structure type decreases from substrate/film boundary to film/air boundary depending on film thickness d. Thus, thin 2 films deposited on alkali glasses exhibit lower mean refractive indices n than thicker films. This effect is shown in Fig. 5 for 2 films made from butyl titanate solution and baked 1 h at 450°C and 500°C respectively [28]. The refractive index of 2 layers fabricated in this way is dependent on various production parameters it is clear... 

To animate the alkali diffusion process in the glasses, the coordinates of each atom in the simulation box must be recorded each timestep, or each several timesteps, for thousand or tens of thousands of timesteps. Those time-dependent coordinates are then extracted and decomposed to multiple files, each file recording the coordinates of the atoms at a specific time for input into the graphical software, which can then be used to produce a 3-D solid model picture these can be run together to produce a movie. 

Most of the current models of the mixed-alkali effect are based on the experimental observation of certain macroscopic properties, such as electrical conductivity, alkali ion diffusion and viscosity. However, the secret of the mixed-alkali effect must lie in the structure of the glass and in how the structure controls the mechanisms of alkali diffusion in the glass. 

Front surface attack (frontal attack). This is direct attack at the glass/refractory interface. Its mechanisms include alkali diffusion into the refractory with consequent fluxing and dissolution of the refractory crystals. Porous refractories are generally more susceptible. 

The variation of D is shown for alkali metal ions (Na" ) in Figure M.4 and shows a good fit with a constant pre-exponential Dq. Interestingly when the concentration of ions increases the diffusion rate increases as well. This is attributed to more lattice network discontinuities that fecilitate ion transport. Alkali diffusion is important to consider since electrical conduction of most commercial glasses results from sodium ions. While in many applications glass serves as an electrical insulator, ionic transport and hence conduction plays an important role in the manufecture of glass. Ionic conduction can be approximately estimated from Einstein s law ... 

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