Li2CO3-SiO2 Reaction

Abstract

The development of lithium silicate by solid state reaction in mixed powders of Li2CO3 and SiO2 in various mole ratios is followed by thermogravimetric analysis (TGA) and quantitative X-ray diffraction measurement. The powders are used consisting of coarse particle of SiO2 in a fine grained matrix of Li2CO3. Isothermal TGA shows that reaction does not exactly follow either diffusion mechanism or boundary reaction mechanism at the temperature range 680-700°C. The results of X-ray diffraction measurement, however, suggest that reaction probably proceeds by diffusion controlled mechanism at the lower temperature range, 637-662°C. The activation energies obtained by TGA and X-ray measurement are 38 and 130kcal/mole respectively. Such difference of reaction mechanism and activation energy might be attributed, at least partially, to the formation of liquidus phase at the temperature range 680-700°C. Activation energy obtained by dynamic TGA at the temperature range 640-670°C is 87.5kcal/mole.In atmosphere, Li2O·SiO2 is primary reaction product and 2Li2O·SiO2 appears somewhat later. In vacuum, on the contrary, 2Li2O·SiO2 is predominant product and Li2O·SiO2 is scarcely detectable. Moreover, the rate of reaction is much smaller in vacuum than in air.The studies with the use of the reaction couple with platinum markers indicate that Li is the main diffusing species.When the reaction proceeds by the diffusion of Li ion, migration of oxygen ion should accompany it. Therefore, the reaction rate must be controlled not only by the diffusion of Li but by the diffusion of oxygen. The activation energies obtained experimentally are considered to be too high to be attributed to the diffusion of Li even if the lowest value, 38kcal/mole, is taken. Thus, it is probable that the activation energy is connected with the diffusion of oxygen. The driving force of the diffusion-controlled reaction is the concentration difference of the diffusing species between the surface of the reaction product layer and the reaction boundary. In vacuum, oxygen concentration on the surface of the product layer is so much lowered that the rate of reaction decreases considerably. Preferential formation of 2Li2O·SiO2 in vacuum may be the result of accumulation of Li2O on the reaction boundary which is caused by the slow migration of Li into SiO2.The whole aspect of reaction in air will be represented as follows:Li2CO3 2Li2O·SiO2 Li2O·SiO2 SiO2Diffusion 2Li++O2- 2Li++O2-Phase boundary Li2CO3 Li2O·SiO2 SiO2reactions -CO2 +Li2O +Li2O=Li2O =2Li2O·SiO2 =Li2O·SiO2