Na2CO3-SiO2 の反応

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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=Mt/M∞=1-Σ∞n=16L2e-βn2Dt/a2βn2(βn2+L(L-1))where Mt/M∞ is the fraction of reaction, D is the diffusion coefficient, βn are the positive root of the equation, βn·cotβn+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 SiO2Diffusion 2Na++O2- 2Na++O2-Phase boundary Na2CO3 Na2O·SiO2 SiO2reactions -CO2 +Na2O + Na2O=Na2O =2Na2O·SiO2 =Na2O·SiO2It is inferable, therefore, that the diffusion of oxygen ions through the product layer of silicate and the phase boundary reaction control the whole reaction.
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Na2CO3-SiO2 の反応

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