Preparation of microencapsulated powders by an aerosol spray method and their optical properties

Microencapsulated powders were prepared by an aerosol spray method using a precursor comprised of a mixture of two types of sols as well as a sol-aqueous mixture precursor solution. Two different types of sols were used as precursors to prepare microencapsulated small-SiO2/large-SiO2 and Al2O3/SiO2 powders. A microencapsulated ZrO2/SiO2 powder was also prepared from a mixture of SiO2 sol and an aqueous precursor solution of zirconyl nitrate dihydrate. For preparing small-SiO2/large-SiO2 powders, two different sizes, i.e. 6 and 109 nm, of SiO2 sols were used as precursors. The results show that the larger SiO2 particles were encapsulated with the smaller ones. A similar result was also observed in the preparation of an Al2O3/SiO2 powder prepared from a mixture of Al2O3 (18 nm) and a SiO2 (109 nm) sol precursor. The Al2O3 particles appeared on the surface of the prepared powder. In the case of the ZrO2/SiO2 powder, the morphology indicates that the SiO2 particles (109 nm) were encapsulated by a ZrO2 shell. The effect of precursor ratios on the morphology of powders was also investigated. The light scattering characteristics of the prepared particles were also investigated using a laser particle counter coupled with a pulse height analyzer. The effective refractive indices of the microencapsulated powders varied considerably with the content of coating material.