Preparation of Monodisperse Silica by a Multicycle Stepwise Seed Growth Method and Its Wave-Transparent Properties

Large-size monodisperse silica microspheres are important lightweight and highly efficient wave-transparent filling materials. However, the monodispersity will more or less deteriorate with increasing size of microspheres in the growth process. Herein, we propose, using the product from the previous step as seeds, a multicycle stepwise growth strategy to prevent secondary particle formation. The research revealed that the monodispersity and spherical morphology of silica microspheres in every growth cycle mainly depend on the number of binding sites available for growth on the seed surface and the concentration of TEOS hydrolysis products near the growth sites, which can be precisely controlled via regulating the amount of seed addition, concentration, and dropping speed of TEOS alcohol solution. As a result, we demonstrate that the particle size of monodisperse silica microspheres can be significantly enlarged from only 0.6 to 5.3 μm after only a few growth cycles, representing an 8-fold increase in size compared to its initial size. Additionally, the wave-transparent performance of the prepared silica microspheres was characterized and shows that a 5 μm grade microsphere with heat treatment at 550 °C for 2 h, realizing a lower dielectric constant and dielectric loss, achieves a wave-transmission rate of 90% in the range of 2-18 GHz. This work suggests that preparing micrometer-sized monodisperse silica microspheres and applying them to low-filling-density, lightweight wave-transparent materials are feasible.