Na₂CO₃-SiO₂ の反応

抄録

The reaction in a solid state of mixture of powdered sodium carbonate and silica systems in various mole ratios was followed by thermogravimetric analysis (TGA), X-ray diffraction and RI-tracer technique. The powders consisting of coarse particles of silica in a fine grained matrix of sodium carbonate were used.
The isothaml TGA shows that the reaction does not exactly follow simple diffusion mechanism at the temperature range 750∼835°C. The rate data are best described by the Crank equation which is given to the reaction which is controlled both by the diffusion of one of the reactant through a product layer on the surface of the second spherical reactant and by the rate of phase boundary reaction at their reactant interface:
α=M_t/M_∞=1-Σ^∞_n=16L²e-β_n²Dt/a²β_n²(β_n²+L(L-1))
where M_t/M_∞ is the fraction of reaction, D is the diffusion coefficient, β_n are the positive root of the equation, β_n·cotβ_n+L-1=0, L=a·k/D, a is the radius of the particle, k is the rate constant of the phase boundary reaction.
The diffusion coefficient calculated from penetration of ²²Na into the fused silica is 5, 0.10^-11cm²/sec and well agrees with the diffusion coefficient calculated by the Crank equation from the present data. Since the self-diffusion coefficients of sodium ions in the glass Na₂O-SiO₂ system are in the order of 10^-7-10^-8cm²/sec at the temperature range 300∼450°C and their activation energies are about 15kcal/mole, it is found that the diffusion of sodium ions does not control this reaction.
In atmosphere, meta-silicate is the primary reaction product but this disappears somewhat later. In vacuum, meta-silicate is scarcely detectable, and the rate of its reaction is much smaller than in air, and their behavior are similar to the reaction of Li₂CO₃-SiO₂ system.
The reaction scheme in air will be represented as follow:
Na₂CO₃ 2Na₂O·SiO₂ Na₂O·SiO₂ SiO₂
Diffusion 2Na⁺+O^2- 2Na⁺+O^2-
Phase boundary Na₂CO₃ Na₂O·SiO₂ SiO₂
reactions -CO₂ +Na₂O + Na₂O
=Na₂O =2Na₂O·SiO₂ =Na₂O·SiO₂
It is inferable, therefore, that the diffusion of oxygen ions through the product layer of silicate and the phase boundary reaction control the whole reaction.