Effects of mechanical pretreatment of precursor sols and gels on the formation of NiO/SiO₂ composites with a controlled microstructure

The effects of ball-milling a sol, prepared from Ni-Si bidentate alkoxides, on the segregation of NiO from SiO₂ during subsequent heating are studied for the purpose of fabricating composites in which the nanosized microstructure is controlled. After dissolving a nylon substrate on which gel films were deposited, the films were heated to 500°C in air. Transmission electron micrographs of substrateless gel films showed that the agglomeration of NiO particles, formed in the films, decreased by ball-milling a sol. A similar phenomenon was also observed on ball-milling gel powders. The infrared absorption bands due to Si-O asymmetric stretching vibration became broader by ball-milling. This broadening also occurred when NiO segregated from SiO₂ by heating a non-milled gel powder above 700°C. It is suggested that ball-milling produced a more homogeneously distributed nucleation of NiO particles through the breakage of bonds between O^2- and Ni²⁺ incorporated into SiO₂ networks.