Preparation and Characterization of Bioactive Silica-Based Ceramics Derived from Rice Husk Ash

Abstract

This thesis deals with the preparation and characterization of amorphous silica based bioactive ceramics using rice husk ash (RHA) as silica source. Three types of silica precursors were prepared depending on impurity and forms. Ceramics were fabricated through conventional powder compaction, polymeric sponge replication and gelcasting
methods. Bioglass-ceramics and mesoporous silica aerogel were also prepared using silica precursor. Mechanical, in vitro bioactivity and biodegradability properties of above
ceramics were investigated.Brown ash (BA), obtained by burning husk at 700 oC, contains about 96 wt.% amorphous SiO2 and the rest 4% impurities like CaO, Fe2O3, K2O, ZnO, and Mn2O3.White ash (WA), prepared by burning acid-leached husk, and contains almost pure silica(99.86%). Silica gel (SG) powder was prepared from BA through the alkaline extraction of silica from ash followed by acid neutralization. SG contains about 99.79% silica.During sintering, amorphous silica transformed into cristobalite phase at 1000, 1200, and 1300 oC, respectively for BA, SG, and WA ceramics. The earlier phase transformation in BA was due to its highest impurity content. There was a fall in compressive strength of all three (BA, SG and WA) sintered silica ceramics as and when amorphous silica transformed to cristobalite. This was due to the crack formation by the high-low displacive phase transformation of cristobalite.
Amorphous silica based scaffolds were fabricated by slurry impregnation processusing polymeric sponge as the replica. The aqueous slurry with 40 wt.% solid loading showed good thixotropic behaviour in presence of polyvinyl alcohol(PVA) binder. The strength of scaffold decreased above a sintering temperature where the amorphous silica transformed into cristobalite. In vitro bioactivity test showed the formation of apatite layer on silica scaffold surface.