The high-temperature stability of SiC-based ceramics has led to their use in high-temperature structural materials and composites 1-3 . In particular, silicon carbide fibres are used in tough fibre-reinforced composites. Here we describe a type of silicon carbide fibre obtained by sintering an amorphous Si-Al-C-O fibre precursor at 1,800 ЊC. The fibres, which have a very small aluminium content, have a high tensile strength and modulus, and show no degradation in strength or change in composition on heating to 1,900 ЊC in an inert atmosphere and 1,000 ЊC in air-a performance markedly superior to that of existing commercial SiC-based fibres such as Hi-Nicalon. Moreover, our fibres show better high-temperature creep resistance than commercial counterparts. We also find that the mechanical properties of the fibres are retained on heating in air after exposure to a salt solution, whereas both a representative commercial SiC fibre and a SiCbased fibre containing a small amount of boron were severely degraded under these conditions 4 . This suggests that our material is well suited to use in environments exposed to salts-for example, in structures in a marine setting or in the presence of combustion gases containing alkali elements.Si-Al-C-O fibre was synthesized using polyaluminocarbosilane prepared by the reaction of polycarbosilane (-SiH(CH 3 )-CH 2 -) with aluminium(III)acetylacetonate. The reaction of the last two compounds proceeded at 300 ЊC in a nitrogen atmosphere by the condensation reaction of Si-H bonds in polycarbosilane and the ligands of aluminium(III) acetylacetonate accompanied by the evolution of acetylacetone 5 ; the molecular weight then increased by the cross-linking reaction with the formation of a Si-Al-Si bond.Polyaluminocarbosilane was melt-spun at 220 ЊC, and then the spun fibre was cured in air at 160 ЊC. The cured fibre was continuously fired in inert gas up to 1,300 ЊC to obtain an amorphous Si-Al-C-O fibre. This fibre contained a non-stoichiometric excess of carbon and oxygen of ϳ12 wt%. The Si-Al-C-O fibre was converted into a sintered SiC fibre by decomposition accompanied by the release of CO gas at temperatures from 1,500 to 1,700 ЊC and sintering at temperatures over 1,800 ЊC. In this sintering process, aluminium plays a very important role as a sintering aid. However, to obtain a very strong sintered SiC fibre, the content of aluminium in the fibre has to be controlled under 1 wt %. As can be seen from Fig. 1, such a sintered SiC fibre (with Ͻ1 wt% Al) showed a smooth surface (Fig. 1a) and a densified structure. Moreover, in this case, the sintered SiC fibre showed transcrystalline fracture behaviour (Fig. 1b). On the other hand, a sintered fibre with a large amount of aluminium showed intercrystalline fracture behaviour (Fig. 1d). These phenomena are presumed to be related to the upper concentration limit of solid-soluble aluminium in the SiC crystal 6 . From the transmission electron microscopy (TEM) image (Fig. 1c) of the low-Al sintered SiC fibre, no obvious second phase is obser...

Ceramics are often prepared with surface layers of different composition from the bulk, in order to impart a specific functionality to the surface or to act as a protective layer for the bulk material. Here we describe a general process by which functional surface layers with a nanometre-scale compositional gradient can be readily formed during the production of bulk ceramic components. The basis of our approach is to incorporate selected low-molecular-mass additives into either the precursor polymer from which the ceramic forms, or the binder polymer used to prepare bulk components from ceramic powders. Thermal treatment of the resulting bodies leads to controlled phase separation ('bleed out') of the additives, analogous to the normally undesirable outward loss of low-molecular-mass components from some plastics; subsequent calcination stabilizes the compositionally changed surface region, generating a functional surface layer. This approach is applicable to a wide range of materials and morphologies, and should find use in catalysts, composites and environmental barrier coatings.

A sintered silicon carbide fiber-bonded ceramic, which consists of a highly ordered, close-packed structure of very fine hexagonal columnar fibers with a thin interfacial carbon layer between fibers, was synthesized by hot-pressing plied sheets of an amorphous silicon-aluminum-carbon-oxygen fiber prepared from an organosilicon polymer. The interior of the fiber element was composed of sintered beta-silicon carbide crystal without an obvious second phase at the grain boundary and triple points. This material showed high strength (over 600 megapascals in longitudinal direction), fibrous fracture behavior, excellent high-temperature properties (up to 1600 degreesC in air), and high thermal conductivity (even at temperatures over 1000 degreesC).

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