Kiyohito Okamura

Synthesis of continuous silicon carbide fibre: Part 4The structure of polycarbosilane as the precursor

The structure of polycarbosilane is represented by three structural elements, but their quantification is difficult. Polycarbosilanes were synthesized by three methods and the respective molecular structures were examined by measurements of the molecular weight and the intrinsic viscosity, infrared, ultraviolet,1H-,13C- and29Si-NMR spectral measurements, and chemical analysis. The three structural elements (SiC4, SiC3H, SiCxSi4−x) in the polycarbosilane molecule were determined quantitatively. By the comparison between1H-NMR spectral data and calculation assuming a linear chain structure, the number of linkages in the unit consisting of ten silicon atoms was estimated to be 3 to 4. This result is in agreement with the result from the intrinsic viscosity; it was found that the shape of the polycarbosilane molecule is planar.

Application of radiation curing in the preparation of polycarbosilane-derived SiC fibers

Polycarbosilane fibers were irradiated by gamma-rays under vacuum and by electron beam in He gas flow or under vacuum at room temperature. Free radicals on Si and C atoms were produced. Most radicals reacted with each other, causing cross-links between polycarbosilane molecules. Some radicals, which did not contribute to cross-linking, were fairly stable under vacuum or in inert gas at room temperature but oxidized on exposure to air. The number of stable radicals under vacuum could be decreased by annealing. The remaining radical concentration was about 1% after annealing at 513 K. By a combination of radiation curing and annealing, SiC fibers with smaller quantities of oxygen were prepared. The mechanical properties of the SiC fibers showed a high tensile strength of 2.5 GPa after heat treatment at 1773 K. On the other hand, polycarbosilane fibers could be cured by radiation oxidation at room temperature, that is, gamma-ray or electron irradiation in oxygen, and the oxygen content could be well controlled by irradiation dose and dose rate. The SiC fibers obtained by the radiation oxidation had an oxygen gradient from the surface to the center which was dependent on the radiation oxidation conditions.

Synthesis of continuous silicon carbide fibre: Part 5 Factors affecting stability of polycarbosilane to oxidation

Polycarbosilane (PC) was obtained by adding bolodiphenylsiloxane (BDPSO) as a reaction accelerator to poly(dimethylsilane) (PDMS), then the thermal decomposition and condensation at various conditions were determined. The molecular weight distribution and the reactivity with oxygen of PC differ with the quantity of BDPSO added, the reaction temperature and the reaction time. The larger the amount of BDPSO, the higher the reaction temperature and the longer the reaction time, the larger becomes the molecular weight of PC. In addition, the higher the reaction temperature, the more stable becomes PC for oxidation. The synthesized PC was spun and the fibre was heated in air at low temperature for curing. The cured fibre was then heat-treated to obtain the SiC fibre. Properties of the SiC fibre are closely related to the oxidation properties of the PC.

Preparation of SiC-C composite fibre by carbothermic reduction of silica

The SiC-coated carbon fibre (SiC-C composite fibre) was prepared by the carbothermic reduction of silica. The composite fibre was examined by X-ray diffraction, scanning electromicroscopy and chemical analysis. Thermogravimetry was used to determine the reduction rate of silica, and the formation mechanism of the SiC film was revealed by a kinetic study. The oxidation resistance of the composite fibre was evaluated by heating in an oxygen gas stream.

Production mechanism of polytitanocarbosilane and its conversion of the polymer into inorganic materials

The reaction of polycarbosilane with tetra-alkyltitanate proceeded at 300° C in nitrogen atmosphere by the condensation of Si-H bonds in polycarbosilane and the substituent groups of the tetra-alkyltitanate accompanied by evolution of alkan gas, and then the formation of Si-O-Ti bonds occurred. In this condensation reaction using tetra-isopropyl titanate, tetra-n-butyl titanate and tetra-2-ethylhexyl titanate, activation energies of the initial rate of the increase in molecular weight were 17.04, 20.07 and 31.07 kcal mol−1 respectively, and thus the more bulky the substituent group of tetra-alkyltitanate, the lower the reactivity became. Of these alkyltitanates, tetra-2-ethylhexyl titanate was found to be the most advantageous reactant for obtaining polytitanocarbosilane with a narrow molecular weight distribution, low gel fraction and high titanium concentration. Polytitanocarbosilane with high titanium concentration was converted into the densified amorphous inorganic material with high Si-C bonding energy in high yield. Titanium contained in the pyrolysed polytitanocarbosilane was furthermore found to have the effect of inhibiting crystalline grain growth of β-type SiC up to high temperature.

SYNTHESIS OF ULTRAFINE SIC POWDERS FROM CARBON-SILICA HYBRIDIZED PRECURSORS WITH CARBOTHERMIC REDUCTION

Inorganic-organic hybrid gels were prepared by simultaneous condensation of the liquid mixture of ethyl silicate, ethyl borate and water soluble phenol resin with low molecular weight. The liquid mixtures were changed into lightly colored transparent gels after a catalyst addition and stirring. The obtained gels were crushed, dried and fired in controlled conditions to yield the inorganic precursors. Since the tailored inorganic precursors mainly consisted of SiO2 and carbon, silicon carbide was formed with the heat treatments in an Ar atmosphere beyond 1773 K with carbothermic reduction. The properties and morphology of the formed silicon carbide powders in terms of the starting precursor compositions and the conditions of the carbothermic reduction were investigated with SEM, XRD and TG-DTA analysis.

High-temperature stability of Nicalon under Ar or O2 atmosphere

The thermal stability of Nicalon NL400 at 1573, 1673 and 1773 K has been studied by TGA and XRD. Under an Ar atmosphere, Nicalon pyrolysed severely to crystallize into β-SiC involving the generation of both SiO and CO. Under an O2 atmosphere, the oxide film which formed around Nicalon retarded the pyrolytic reaction of the core. Nicalon coated with a silica film by the previous oxidation treatment pyrolysed hardly at all under an Ar atmosphere, because the film restricted the escape of SiO and CO. Nicalon which was oxidation-treated at 1773 K retained 63% of its original strength after heating at 1773 K under an Ar atmosphere. The amorphous silica film was found to resist rapid thermal cycling between room temperature and elevated temperatures.

Development of silicon nitride fiber from Si-containing polymer by radiation curing and its application

Abstract A silicon nitride fiber (Si3N4) was synthesized from polycarbosilane (PCS) fiber by radiation application. PCS fibers were cured by electron beam (EB) irradiation in a helium gas atmosphere prior to the pyrolysis. The cured PCS fiber was converted to Si3N4 ceramic fiber with flexibility by nitridation in ammonia gas at a high temperature of 500–1000°C. The obtained Si3N4 fibre showed a high heat resistance up to 1300°C, a high tensile strength of 2 GPa and excellent electrical resistivity of 1013 Ω cm. The ceramic fiber was fabricated to cloth and applied for electric wire insulator. The wire cable is flexible and can be applied at a high temperature atmosphere of around 1000°C.

Electrical resistivity of Si-Ti-C-O fibres after rapid heat treatment

Two types of Si-Ti-C-O fibres were heat treated in a preheated graphite furnace at temperatures between 1273 and 1973 K, and the change in the electrical resistivity was measured after removing the fibres from the furnace. The resistivity of the fibres decreased monotonically with increasing heat-treatment temperature, but showed a significant increase of the order of 101–102 in the temperature range of gas evolution from the fibres. The resistivity of the fibre which has an amorphous character began to increase at a lower temperature than that of the fibre with a crystalline character. This increase in resistivity did not occur during heat treatment in a pure oxygen atmosphere, because the oxide layer formed on the fibre surface suppressed gas evolution from the fibres. The X-ray diffraction patterns of heat-treated fibres in nitrogen or oxygen atmospheres revealed that β-SiC crystals began to precipitate from the amorphous state as the heat-treatment temperature increased. The β-SiC crystal growth, however, did not always correspond with the decrease in the fibre resistivity.

The polycarbosilane-to-SixC1−x conversion studied by inelastic neutron scattering and infrared absorption

Abstract Polycarbosilane (PC) is a precursor which is converted to SixC1−x fibers by pyrolysis in an inert gas atmosphere. The changes in the atomic vibrational spectrum during the conversion process from PC to SixC1−x have been examined by means of inelastic neutron scattering (INS) and infrared absorption (IR). The existence of transverse-optical(TO)- and longitudinal-optical(LO)-like phonon modes due to amorphous SiC clusters was established in the original and pyrolyzed PC by INS measurements. After pyrolyzing at 700–800°C, these modes appear distinctly around 730 and 930 cm−1, respectively, in the INS spectra. Pyrolysis at 1000°C makes these TO- and LO-like phonon modes sharper.