Crystallization behavior of silica fibers

The authors concentrate on crystallization behavior of high purity silica fibers. There are three types of such fibers used in experiment: type A prepared with acid leaching method, type B prepared sol-gel method, and type C, which is made by high purity silica fusion method. The authors concentrate further on the individual effects of impurities Al₂O₃, B₂O₃, and Na₂O and coupled effect of Al₂O₃ and B₂O₃ on the crystallization of silica fibers. The structure of the silica fibers is investigated by using X-ray diffraction, Raman spectrum, nuclear magnetic resonance, and X-ray radial distribution function. Results indicate that fibers made by different methods possess different crystallization tendencies. Contrary to previous results from bulk silica glasses, type C fiber possesses much higher crystallization tendency than types A and B. Type B silica fiber is the most crystallization resistant one. Small amount of Al₂O₃ in the fibers is mainly in the form of [AlO₄] with a H⁺ neutralizing the extra negative charge. This [AlO₄] promotes the crystallization of the fibers, and its effect grows stronger at higher temperatures. A model that such [ALO₄] will break down at high temperatures by combination of H⁺ and OH in the fibers is proposed, which can be used to explain satisfactorily the effect of [AlO₄] on the crystallization of the fibers.