Gas evolution from a sol-gel MgO-Al2O3-SiO2-Li2O powder during heat treatment analysed by mass spectrometry

Desorption and vaporisation from MAS-L (MgO-Al2O3-SiO2-Li2O) sol-gel powders are observed during heat treatment from room temperature to 1000 degreesC using thermogravimetry and Knudsen-cell mass spectrometry. The vaporisation gaseous products are H2O, CO2, NO, NO2 and LiNO2. Volatile hydroxides are not observed, but the evolution of the partial pressure of H2O(g) with temperature, when compared with equilibrium thermodynamic calculations, reveals that LiOH (solid or liquid) is present as an independent bulk phase and that Li2O is likely to be combined with another compound. The hydroxide Mg(OH)(2) is formed from the physically adsorbed and/or condensed water inside the porosity. From the evolution of the CO2(g) partial pressure, independent carbonated bulk phases could not be detected, although the measured partial pressures crossed the pressure range of thermodynamic stability of known carbonate phases. Besides, both decompositions of LiOH and of nitrates were seen to contribute to the loss of lithium during heat treatment.