Preparation of Oxide Fibers by Unidirectional Freezing

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Titania and alumina fibers were prepared by unidirectional freezing of gels. Preparation conditions, formation mechanism, structure and properties of the fibers were investigated.A titania sol prepared by hydrolyzing Ti <Cl_4> and five kinds of alumina sols were used as starting materials. The sols were put into a cellulose tube 120mm in length and 20 or 8mm in diameter, and dialyzed in distilled water for 2-30 days to convert to gel. The gels, placed in a polyethylene cylinder, were unidirectionally frozen by inserting into -78゜C cold bath from the bottom at a rate between 1.0 and 13cm/h. Frozen bodies were thawed at room temperature. About 10cm long titania fibers were formed when the Ti <O_2> concentration of the sol was 1.0 and 2.0M and the rate of insertion was 2.0 and 4.5cm/h. One to 5cm long alumina fibers were formed only from the gels derived from the sols containing feather-like particles. Both titania and alumina fibers were formed only when the cellular growth of ice took place in gels. The occurrence of the cellular growth of ice fibrous crystals was found to depend on freezing rate R and temperature gradient G during unidirectional freezing. Fiber diameters were found to be in an inverse proportion to the product of G and R.The fibers thus prepared were both porous and charaterized by large specific surface area of 350 <m^2> /g in Ti <O_2> and 240 <m^2> /g in <Al_2> <O_3> and high water contents after dried at 110゜c. The tensile strength of alumina fibers increased to about 180MPa on heating to 1000゜C. These results indicate that titania and alumina fibers could be used as catalyst, catalyst carrier, and reinforcing fibers.