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, $BaAl_2Si_2O_8$ and lanthanum hexaluminate, $LaAl_{11}O_{18}$) or for matrix materials (yttrium aluminium garnet, $Y_3Al_5O_{12}$) 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 $12200^{\circ}C$, via amorphous intermediates in which the Al shows and 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 $500^{\circ}C$ but the Ba is still moving into its polyhedral lattice sites about $400^{\circ}C$ after the sluggish onset of crystallization. Lanthanum hexaluminate and YAG crystallise sharply at 1230 and $930^{\circ}C$ respectively, the former via $\gamma-Al_2O_3$, the latter via $YAlO_3$. Yttrium moves into the garnet lattice sites less than $100^{\circ}C$ after crystallisation.