YTTRIUM-ALUMINUM-GARNET FIBERS FROM METALLOORGANIC PRECURSORS

Mixtures of yttrium acetate hydrate (Y(O2CCH3)(3) . 4H(2)O) and alumin um formate hydrate (Al(O2CH)(3) . 3H(2)O in H2O) or yttrium isobutyrat e (Y[O2CCH(CH3)(2)](3)) and aluminum isobutyrate (Al[O2CCH(CH3)(2)](3) ) in tetrahydrofuran were used as precursors to process yttrium alumin um garnet (YAG, AI(5)Y(3)O(12)) fibers. The pyrolytic decomposition pa tterns of Al(O2CH)(3) . 3H(2)O, Y(O2CCH3)(3) . 4H(2)O, and a [3Y(O2CCH 3)(3) . 4H(2)O/5Al(O2CH)(3) . 3H(2)O] YAG stoichiometry mixture were a ssessed by heating samples to selected temperatures and characterizing the products by thermogravimetric analysis, differential thermal anal ysis, X-ray diffractometry, and Fourier transform infrared spectroscop y. The YAG acetate/formate precursor decomposes to an amorphous interm ediate at temperatures >400 degrees C and crystallizes (at similar to 800 degrees C) to phase-pure YAG with a ceramic yield of 40% at 1000 d egrees C. YAG isobutyrate precursor fibers were extruded or hand drawn . YAG acetate/formate precursor fibers were formed using a commercial extruder, The pyrolysis behavior of both precursor fibers was studied to identify the best pyrolysis conditions for producing dense, defect- free ceramic fibers. Only thin (diameter of <30 mu m) precursor fibers could be processed to dense, defect-free, thin YAG fibers (diameter o f <20 mu m). For the YAG isobutyrate precursor, crack-free crystalline YAG fibers (diameter of similar to 7 mu m) were obtained at 1000 degr ees C. For YAG acetate/formate precursor fibers, dwell times of 2 h at temperatures of 400 degrees and 900 degrees C were necessary to proce ss fully dense, defect-free ceramic fibers. Heating the resulting 900 degrees C fibers (at a rate of 30 degrees C/min) to 1570 degrees-1650 degrees C gave dense fibers with grain sizes of 0.7-3.2 mu m and bend strengths of up to 1.7 +/- 0.2 GPa (for fibers that had a diameter of similar to 10 mu m and had undergone sintering at 1600 degrees C).