Abstract
Alumina fibers were prepared by two new precursor methods based on the systems of Al(OPri)3-Citric acid-H2O and Al(OPri)3-AlCl3-Citric acid-H2O. In the former system, precursor fibers were obtained from a viscous solution in a compositional region of Al(OPri)3 mol/citric acid mol_??_1.25. In the latter system, they were obtained from a viscous solution in two compositional regions, one was a region of 6_??_total Al mol/citric acid mol_??_30 and 1_??_Al(OPri)3 mol/AlCl3 mol_??_3, and the other was a region of total Al mol/citric acid mol_??_1.5 and 1_??_Al(OPri)3 mol/AlCl3mol. The citric acid plays a very important role in developing a good spinnable solution by promoting the decomposition of Al137+ polycation, by inhibiting three-dimensional polymerization of Al-O bonds and by combining with [Al(H2O)6]3+ and Al-containing polymer. The structure of the polymers and cations was investigated by 27Al-NMR spectra. The intensity of the-4--6 ppm peak assigned to a AlO6 poly-structure decreased with citric acid content, and that of 0 ppm peak broaden by the coordination of citric acid to [Al(H2O)6]3+ The compositional and structural conversion from a precursor fiber to alumina fiber was achieved from amorphous to α-phase through γ-phase. The alumina fibers obtained from the former process always separated into double layers of margin and core, and those from the latter process were very homogeneous. This difference is related to the citric acid content in viscous solutions.
