Kiyoshi Kumagawa

High-strength alkali-resistant sintered SiC fibre stable to 2,200 °C

The high-temperature stability of SiC-based ceramics has led to their use in high-temperature structural materials and composites. 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 °C. 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 °C in an inert atmosphere and 1,000 °C 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 SiC-based fibre containing a small amount of boron were severely degraded under these conditions. 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.

Production mechanism of polyzirconocarbo- silane using zirconium(IV)acetylacetonate and its conversion of the polymer into inorganic materials

The reaction of polycarbosilane with zirconium(IV)acetylacetonate proceeded at 573 K in nitrogen atmosphere by the condensation reaction of the Si–H bonds in polycarbosilane and the ligands of zirconium(IV)acetylacetonate accompanied by the evolution of acetylacetone, and then the molecular weight increased by the cross-linking reaction with a formation of Si–Zr bond. The obtained polyzirconocarbosilane showed higher ceramic yield than the polycarbosilane. Zirconium contained in the pyrolysed polyzirconocarbosilane was furthermore found to have the effect of inhibiting crystalline grain growth of β-type SiC up to high temperature, so Si–Zr–C–O fibre, which was obtained by the use of polyzirconocarbosilane as precursor, showed high tensile strength up to high temperature.

Excellent heat resistance of Si-Zr-C-O fibre

In order to obtain the high heat-resistant fibre, Si-Zr-C-O fibre has been developed. Si-Zr-C-O fibre was produced by the use of polyzirconocarbosilane as the precursor polymer. In this paper, the difference in heat-resistance between Si-Ti-C-O and Si-Zr-C-O fibres was clarified. Si-Zr-C-O fibre showed excellent heat resistance (up to 1773 K) compared with Si-Ti-C-O fibre (up to 1573 K). Generally speaking, decomposition reaction of this type of fibre proceeds accompained by the release of CO gas which was formed by the reaction between excess carbon and oxygen included in the fibre. In the case of Si-Zr-C-O fibre, Zr can strongly capture the oxygen atoms, so that the aforementioned decomposition hardly proceeds up to 1873 K (ZrO2 + 3C = ZrC + 2CO; ΔG < 0 at over 1906 K).

Characterization of the medium-range structure of Si-Al-C-O, Si-Zr-C-O and Si-Al-C Tyranno fibers by small angle X-ray scattering

The medium-range structures of Si-Al-C-O, Si-Zr-C-O and Si-Al-C Tyranno fibers, which were prepared by pyrolysing polymetalocarbosilane organic fibers, were observed by means of small angle X-ray scattering using a point-collimated Cu Kα incident beam and a two-dimensional imaging plate detector. Single Si-Al-C-O and Si-Zr-C-O Tyranno fibers of a few μm in diameter have an anisotropic structure comprising thousands of fine filaments of about 10 nm in diameter bundled together along with the long axis of the fibers. The anisotropic structure is not sensitive to the pyrolysing temperature during the organic-to-inorganic conversion process. However, the anisotropic structure of the Si-Al-C-O fibers is totally modified to an isotropic one in Si-Al-C fibers prepared by heating the Si-Al-C-O fibers above 1700°C in an argon gas stream, because of the formation of an aluminum oxide-rich phase at the grain boundaries of β-SiC nanoclusters.

Medium-range structural change in the ceramization from polymer precursors

Abstract Small-angle neutron scattering (SANS) experiments have been performed for the Si–Ti–C–O and polyborosilazane precursors during ceramization and showed the existence of two types of scattering entities different in size over the medium-range structure. Structural features during the ceramization process were obtained from the fits of the experimental small-angle scattering data to a model function based on the neutron scattering results. The two types of scattering entities do not change in size appreciably, whereas the volume fractions of the scattering entities decreases systematically with increasing heat-treatment temperature up to 1073 K. On heat-treatment at 1273 K, one typical particle scattering which follows the Porod law and probably due to the precursors of β-SiC crystallites begins to be detected in the place of two kinds of scattering patterns.