Nanostructured ceramic powders by hydrothermal synthesis and their applications

Nanoparticles of 3:2 mullite (3Al(2)O(3).2SiO(2)) + 5 wt.% zirconia. boehmite (gamma-AlOOH) and pure ZrO2 suitable for applications in the field of monolithic/fibre reinforced ceramic composites, nanoceramics and ceramic fibre coating, respectively, have been produced by hydrothermal synthesis (HS) at moderately low temperatures. The effect of the process parameters or starting precursor materials properties on the particle size. shape and structure have been examined by particle size measurements, TEM and scanning electron microscopy. The mullite powders synthesised tend to be highly irregular in morphology with a wide particle size distribution from 55 nm to 7.1 mum, whereas the boehmite (gamma-AlOOH) particle morphology and size are determined by the pH value of the initial hydrothermal solution. As the pH changes from low (acid) to high (alkaline), the boehmite morphology changes from a "needle-like" one to a "platelet" one. At a pH value of 10. the boehmite particles exhibit platelet morphologies, with a maximum face dimension of about 40 nm and a thickness of approximately 5 nm. The synthesised boehmite particles were successfully used to produce monolithic alumina components at lower sintering temperatures with sub-micrometer grain sizes. Near-spherical shape monoclinic zirconia particles with dimensions in the range of 20-60 nm were also produced by HS. The successful application of the synthesised powders in the field of mesoporous fibre-reinforced ceramic composites, high-density monolithic ceramics and ceramic fibre coatings (formation of weak inter-phase between fibre and matrix) are also presented. (C) 2002 Elsevier Science Inc. All rights reserved.