Yttrium Aluminum Garnet Fibers from Metalloorganic Precursors

Mixtures of yttrium acetate hydrate (Y(O2CCH3)3?4H2O) and aluminum formate hydrate (Al(O2CH)3?3H2O in H2O) or yttrium isobutyrate (Y[O2CCH(CH3)2]3) and aluminum isobutyrate (Al[O2CCH(CH3)2]3) in tetrahydrofuran were used as precursors to process yttrium aluminum garnet (YAG, Al5Y3O12) fibers. The pyrolytic decomposition patterns of Al(O2CH)3?3H2O, Y(O2CCH3)3?4H2O, and a [3Y(O2CCH3)3?4H2O/5Al(O2CH)3?3H2O] YAG stoichiometry mixture were assessed by heating samples to selected temperatures and characterizing the products by thermogravimetric analysis, differential thermal analysis, X-ray diffractometry, and Fourier transform infrared spectroscopy. The YAG acetate/formate precursor decomposes to an amorphous intermediate at temperatures >400°C and crystallizes (at ∼800°C) to phase-pure YAG with a ceramic yield of 40% at 1000°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 μm) precursor fibers could be processed to dense, defect-free, thin YAG fibers (diameter of <20 μm). For the YAG isobutyrate precursor, crack-free crystalline YAG fibers (diameter of ∼7 μm) were obtained at 1000°C. For YAG acetate/formate precursor fibers, dwell times of 2 h at temperatures of 400° and 900°C were necessary to process fully dense, defect-free ceramic fibers. Heating the resulting 900°C fibers (at a rate of 30°C/min) to 1570°–1650°C gave dense fibers with grain sizes of 0.7–3.2 μm and bend strengths of up to 1.7 ± 0.2 GPa (for fibers that had a diameter of ∼10 μm and had undergone sintering at 1600°C).