Crystallization of fibre-coating compounds of potential use in fibre-reinforced oxide ceramics

Mixed oxide compounds of potential usefulness for fibre coatings (hexagonal celsian, BaAl2Si2O8 and lanthanum hexaluminate, LaAl11O18) 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. Both the gels convert to the crystalline phase below about 1200°C, via amorphous intermediates in which the Al shows an NMR resonance at 36–38 ppm sometimes ascribed to Al in five-fold coordination. Additional information about the structural changes during thermal treatment was provided by 29Si and 137Ba MAS NMR spectroscopy, showing that the feldspar framework of celsian begins to be established by about 500°C but the Ba is still moving into its polyhedral lattice sites about 400°C after the sluggish onset of crystallization. Lanthanum hexaluminate crystallises sharply at 1230°C via γ-Al2O3.