Fundamental study on low temperature synthesis of alumina fibers by sol-gel method

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A) Preparation of alumina fibers by sol-gel method The shape of particles present in the alumina sols prepared from inorganic salts was examined by TEM observation and related to the rheological property and fiber drawing behavior of the sols. It was found that fibers could be drawn in the viscosity range of 1 - 100 Pa・s from the sols in which long-shaped particles were found, while no fibers could be drawn from the sols containing round particles. It was also found that non- spinnable sols showed relatively large structural viscosity. Effect of Al_2O_3 and Cl^- ion concentration on the flow properties of the spinnable sols was also examined.B) Preparation of beta-alumina fibers by sol-gel method Viscous sols corresponding to Na_2O・11Al_2O_3, Na_2O・8.8Al_2O_3, Na_2O・7.3Al_2O_3 and Na_2O・5.5Al_2O_3 in oxide composition were prepared by heating slurry consisting of A;(NO_3)_3・9H_2O, Al powder, NaNO_3, H_2O and HNO_3 in reflux with stirrer. Fiber drawing from the viscous sols yielded gel fibers of about one meter long and 5 - 250 mum in diameter. Heating at 1000 ﾟC for 20 min and for 10 h of the gel fibers of oxide compositions Na_2O・8.8Al_2O_3 and Na_2O・5.5Al_2O_3 resulted in ceramic fibers consisting of beta-alumina crystals, respectively. The size of beta-alumina crystals in these fibers was about 0.2 mum and the electrical conductivity of fired fibers of latter oxide composition was 1 x 10^<-6> S・cm^<-1> (1 kHz) at 20 ﾟC. Conditions for the occurrence of spinnability in the starting sols and for the precipitation of beta-alumina crystals in gel fibers have been discussed.