| A melt extraction process with a new approach to making ceramic fibers was used to produce amorphous fibers in the Y2O3-Al 2O3 system within the 20–30 micron size range. Three different compositions were studied, E1, Y3A5 and E2 with 79, 62.5 and 57.5 mol% Al2O3, respectively. Such fibers were X-ray amorphous and transparent, regardless of composition in the as-produced state, and showed all the principal characteristics of the fibers made by this process: a cross section which is almost circular, a line of contact with the wheel and a clean and defect-free surface. However, the quality of the fibers was controlled by the wheel edge and rotational speed, with both having a significant effect on fiber diameter and avoidance of irregularities and instabilities along the fiber length. Moreover, compositional distribution analysis showed that the melt-extracted fibers were chemically homogeneous between the core and the edge of the fiber and, in addition, were very close to theoretical stoichiometry.; Thermal analysis was performed in order to determine the glass transition, crystallization and phase transformation temperatures. E1 fibers showed two exothermic peaks at 949 and 1040°C that correspond to the crystallization of YAG and δ-Al2O3 phases, respectively, and one small exothermic peak at 1300°C due to the phase transformation of δ-Al 2O3 to α-Al2O3. In the case of Y3A5 and E2 fibers, both showed one single exothermic peak at 942 and 939°C due to the crystallization of YAG and YAP cubic phases, respectively. YAP orthorhombic and YAG phases appeared in E2 fibers after heat-treating at 1400°C for 1h with no identifiable exothermic peak in the DTA scan.; Differential thermal analysis was used to study the crystallization phenomena in the as-extracted fibers. The activation energy values were calculated using both Kissinger and Augis-Bennett equations by measuring the variation of the peak temperature in the differential thermal scans with heating rate, and the crystallization mode was identified using the Ozawa analysis. After crystallization, there was a conversion of the transparent glass fiber to an opaque polycrystalline material and both E1 and Y3A5 fibers had a nanocrystalline microstructure with a grain size typically of 100–500 nm. In the case of E2, the grain size was 1–2 μm.; Tensile strengths and elastic moduli of the fibers in the glassy state varied from 606 to 776 MPa and from 95 to 121 GPa, respectively. Furthermore, fibers exhibited brittle fracture initiated from flaws on the fibers surface. |
