Formation Mechanism of Fibrous Silica from a Suspension of Monodispersed SiO2.

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Abstract
Fibrous silica was prepared by drying a silica suspension. Under the experimental conditions, it was possible to prepare a fibrous silica of 7cm in length and 40-200μm in width. The surface morphology of the silica fibers consisted of almost close-packed silica particles. When the suspension concentration was low or the contact angle of the meniscus was large, the film thickness was small and no fibers were generated. The direction of the silica fiber generation was always perpendicular to the water level. The formation mechanism of the fibers was elucidated as follows: (1) Formation of closely-packed homogeneous SiO2 film. (2) Generation of cracks by shrinkage of the film. (3) Formation of fibers by propagation of cracks. Some experimental conditions affected the fiber width. A concentrated silica solution (i.e., thick film), large silica particles, and low drying temperature gave a wide fiber. The particle diameter and drying temperature also influenced the fiber width. The larger silica particles and the lower drying temperature gave wider fibers. As prepared fibers were translucent and fragile, however, they became strong by calcination at 800°C for 1h. A measurement of the pore diameter distribution revealed that they have homogeneous pores with a diameter around 20nm. After calcination at 1000°C, they became transparent and the strength was much higher than that at RT. The pores observed until at 800°C disappeared at 1000°C.

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