Synthesis of solid and spirally cracked TiO 2 fibers by a liquid mix process
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Titania fibers were synthesized by a liquid mix process, starting by complexing titanium isopropoxide with a chelating agent solution to form a precursor resin for further fiber drawing. The as-drawn continuous precursor fibers underwent a weight change of 80% and a volume change of 75% after heat treatment at 800 °C in nitrogen followed by an additional treatment at 600 °C in air. The fibers consisted mainly of rutile with 5–10% anatase. Further treatment at 700 °C transformed the anatase completely into rutile. Fibers with finished diameter less than about 15 m were solid with smooth surfaces. Fibers with finished diameter greater than about 15 m were hollow and spirally cracked in a uniform manner. Fibers treated at 600 °C showed no visible grains. Additional annealing at 800 °C grew grains to an average size of 0.3–0.4 m. The fibers appeared solid and dense. I. INTRODUCTION
Fibrous materials have been used for many purposes for centuries. More recently, polymer fibers have been used for clothing and for industrial purposes. However, polymer fibers have limitations in higher temperature uses. For high temperature uses, glass and ceramic fibers are important.1–5 Since glass is amorphous and has a relatively low softening point, drawing fibers from glass through proper control of temperature is done regularly.5 Ceramic fibers are much more difficult to pull from a melt due to their polycrystalline nature and low viscosity at their melting point. It is difficult, though not impossible, to draw ceramic fibers from a melt with existing technology. An alternate approach is to form precursor fibers first and then transform them into ceramic fibers by proper chemical and physical treatments.3,4,6–8 Mechanical properties and thermal resistance are usually the most important physical parameters for fibers. Because of this, it is important to generate fibers with a microstructure designed to minimize flaws and grain size. Generally, fine-grained fibers with no pores or other flaws will have high strength while fibers of small diameter and low elastic modulus will be more flexible. There are some commercial alumina and mullite fibers. Fiber FP and PRD-166 from DuPont (Wilmington, DE), Alf fiber from Sumitomo (Tokyo, Japan), Safimax from ICI (London, United Kingdom), and Nextel 312, 440, and 480 from 3M (St. Paul, MN) are examples. Fiber FP was the first small-diameter (20 m) ceramic fiber and became available in 1979. This fiber is made by blending -alumina powder with water and other compounds to form a mixture that can be spun. The filaments are then extruded through spinnerets, and fired J. Mater. Res., Vol. 18, No. 3, Mar 2003
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in two steps to achieve densification and surface flaw healing. The fabrication process for the other fibers is similar to the Fiber FP process. Another approach to synthesize ceramic fibers is by the precursor polymer method.1–3,9 A good example is the work of Yajima,9 which was published in the mid-1970s, and gave rise to the first fin
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