Occurrence of bimodal pore structure and leaching process in the preparation of porous glass from fluorine containing sodium borosilicate glass

Abstract

Leaching process in the preparation of porous glass from fluorine containing sodium borosilicate glass was intensively investigated under different conditions of phase-separation temperature, the thickness of specimens and the amount of acid leaching solution. Techniques employed are pore size distri-bution measurements and SEM observation with focusing on the leaching of SiO2 component from the B2O3-rich phase. The pores formed in the inner part of specimens, after being treated with 0.5 mol/l sulfuric acid solution of 200 ml/ (g-specimen) at 92 ± 1°C for 0.5h, had a smaller size than those formed in the surface layer of specimens. Those smaller pores contained a larger amount of silica-gel which precipitated from the B2O3-rich phase in the leaching process. This is the cause why a bimodal pore structure was inevitably obtained through the course of leaching: the silica-gel was removable by leaching with a larger amount of acid solution for a prolonged time. In the early stage of leaching process, the dissolution of the B2O3-rich phase proceeds, forming smaller pores with residual silica-gel, and the silica-gel is subsequently removed from the smaller pores resulting in enlargement. This diffusion-controlled process proceeds from the surface of the specimen to its inner part, indicating that a thin specimen is effective to accomplish the complete removal of the silica-gel with a minimum amount of the leaching solution.