Surface Modification of Silica Fiber by Alumina and Its Thermal Stability

Abstract

Silica fibers coated with alumina layers have been prepared using a solution of aluminum nitrate dissolved in 1, 3-butanediol. Texture and crystallite structures of the alumina have been examined from a viewpoint of thermal stability of the modified silica fiber. Amorphous alumina layer could be uniformly prepared on the silica fiber by impregnating the fibers in the aluminum solution. The alumina layer transformed to crude assemblies of alumina particles after aging the alumina-modified silica fiber in deionized water at 100°C. The alumina particles were composed of fibrils with 6nm in diameter and 100-150nm in length. The phase of the alumina also changed from amorphous to boehmite by aging and to γ-alumina by reheating at 500°C. The texture of the alumina on the silica fiber was thermally stable up to 1200°C for the aged samples. The surface area, based on the weight of the alumina, of the samples calcined at 1200°C for 5h and 30h were 84m²·g^-1 and 80m²·g^-1, respectively. XRD analysis showed no α-alumina formation the after above treatment, indicating that the crude texture of the alumina might suppress sintering because of small contact area among the alumina fibrils. Furthermore, it was found that alumina surface was covered with silica, and the silica might also restrain the surface diffusion of alumina not to induce the sintering. Both the crude texture of alumina and silica on alumina structure might effectively improve the thermal stability of the silica fiber.