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 b y 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 degrees 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 infor mation about the structural changes during thermal treatment was provided b y Si-29 and Ba-137 MAS NMR spectroscopy, showing that the feldspar framewor k of celsian begins to be established by about 500 degrees C but the Ba is still moving into its polyhedral lattice sites about 400 degrees C after th e sluggish onset of crystallization. Lanthanum hexaluminate crystallises sh arply at 1230 degrees C via gamma-Al2O3. (C) 2000 Elsevier Science Ltd. All rights reserved.