Crystallization of amorphous silicates far from equilibrium part I: A versatile nitrate-based sol–gel synthesis of amorphous porous Ca,Mg-rich silicates

An easy-to-use, comprehensive sol–gel method is developed to produce amorphous calcium and magnesium silicates from nitrate precursors and TetraEthOxySilane (TEOS). Final products were designed to suit basic prerequisites of starting materials for experimental investigation of crystallization around the glass transition temperature range. After gelification, thermo-gravimetric methods and infrared-spectroscopy were used to follow dehydration, decarbonation and denitrification of the xerogel. A temperature of 500 °C is found to successfully remove volatiles without causing crystallization. The microstructure revealed by transmission electron microscopy (TEM) consists of 10–20 nm individual mesoparticles of 10–20 nm. Samples annealed at 500 °C were found entirely amorphous at the TEM scale. The porosity observed by TEM and characterized by nitrogen adsorption–desorption is homogeneous and varies from 4.6 to 8.5 nm as a function of the composition. Bulk analyses by ICPMS and local analyses by EDS-TEM demonstrate that the stoichiometry can be achieved and the homogeneity is confirmed at least down to 100 × 100 nm. At lower scale, irradiation by the electron beam produces a significant volatilization of Ca and Mg, which makes chemical analyses unreliable. Pros and cons of the method and special cares for specific applications are discussed. The method was also successfully used to produce a wider range of amorphous analogs having complex compositions or containing trace elements for applications in the field of mineral physics and chemistry.