Preparation of CaO-SiO2 gel by sol-gel method and its structural evolution by heat treatment

CaO-SiO2 gel was prepared from Ca(NO3)(2) and TEOS. The mixture of TEOS, Ca(NO3)(2), H2O and EtOH was immiscible at first, but became miscible by stirring for several hours, Wet gels containing 10-60 mol%CaO were formed and dry gels were amorphous at the composition of 10-40 mol%CaO. It was found that a small amount of Ca(NO3)(2) crystal precipitated in dry gels can react with SiO2 gel by heat treatment and the heat-treated gels were amorphous, The reaction of Ca(NO3)(2) with SiO2 gels was confirmed by disappearance of IR absorption peak attributed to NO3- ion after heat treatment. The gels in the immiscibility region in this system (10, 20 mol%CaO) caused phase separation by heat treatment at 600-800 degrees C. The tendency of phase separation was high for high CaO content gels, The crystallization behavior of gels in the immiscibility region was found similar to that of CaO-SiO2 amorphous films prepared by sputtering method.