Thermal Development from Hybrid Gels of Compounds for Use in Fibre-Reinforced Oxide Ceramics

Mixed oxide compounds of potential usefulness for fibre coatings (hexagonal celsian, BaAl₂Si₂O 8 and lanthanum hexaluminate, LaAl 11 O 18 ) or for matrix materials (yttrium aluminium garnet, Y₃Al 5 O 12 ) were prepared by hybrid sol-gel synthesis and their thermal crystallisation was monitored by thermal analysis, X-ray diffraction and multinuclear solid state MAS NMR. All the gels convert to the crystalline phase below about 1200℃, via amorphous intermediates in which the Al shows an NMR resonance at 36-38 ppm sometimes ascribed to Al in 5-fold coordination. Additional information about the structural changes during thermal treatment was provided by 29 Si, 137 Ba and 89 Y MAS NMR spectroscopy, showing that the feldspar framework of celsian begins to be established by about 500℃ but the Ba is still moving into its polyhedral lattice sites about 400℃ after the sluggish onset of crystallization. Lanthanum hexaluminate and YAG crystallise sharply at 1230 and 930℃ respectively, the former via γ-Al₂O₃, the latter via YAlO₃. Yttrium moves into the garnet lattice sites less than 100℃ after crystallisation.