Takemi Yamamura

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.

Development of a new continuous Si-Ti-C-O fibre using an organometallic polymer precursor

A polytitanocarbosilane, which is useful as the precursor polymer for ceramic fibre, was synthesized using polydimethylsilane, polyborodiphenylsiloxane and titanium tetraisopropoxide. The polytitanocarbosilane was melt-spun and using the continuous heat-treatment process from the polymer fibre to ceramic fibre, flexible Si-Ti-C-O fibre was produced. The density, tensile strength and Youngs modulus of this amorphous ceramic fibre were found to be 2.35 g cm−3, 3.0±0.2 and 220±10 GPa, respectively. The Si-Ti-C-O fibre retained its high tensile strength to higher temperatures (about 1200° C). The specific resistance of this ceramic fibre covered a wide range of 107 to 10−1ωcm. This ceramic fibre is considered to be useful as reinforcement fibre for composites.

Synthesis of a polytitanocarbosilane and its conversion into inorganic compounds

The novel polytitanocarbosilane, formed by the cross-linking of polycarbosilane with titanium tetra-alkoxide, was synthesized to examine the process of converting a multielement organometallic polymer into an inorganic compound. The chemical structure of this polymer was investigated by the techniques of infra-red spectroscopy (IR), gel permeation chromatography (GPC), number average molecular weight measurements and29Si nuclear magnetic resonance (NMR) measurements. The pyrolysis products in N2 gas at 1400° C and 1700° C were the microcrystalline and crystalline states of silicon carbide and titanium carbide, respectively.

High-performance SiC/SiC composites by improved PIP processing with new precursor polymers

Abstract As they are potential candidates for fusion reactor structural materials, R&D are being conducted on SiC-based composite materials (CREST-ACE program). To improve the efficiency of the polymer impregnation and pyrolysis (PIP) process for SiC/SiC composite fabrication, a new precursor polymer, poly-vinylsilane (PVS) with SiC filler addition, was adopted as a matrix precursor and process optimization was performed. Consequently, high-density SiC/SiC composite with high mechanical properties was efficiently fabricated. Importantly, near-stoichiometric SiC matrix was developed by blending of poly-carbosilane (PCS) and poly-methylsilane (PMS). Remarkable improvements in tensile properties and fatigue characteristics (at 1573 K) were attained when inorganic powder fillers, BMAS or ZrSiO 4 , were added to the polymer mixture as the matrix precursor. These results are encouraging to make economically and environmentally attractive fusion reactors utilizing SiC/SiC composites as major structural materials.

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.

The conversion process from polydimethylsilane to polycarbosilane in the presence of polyborodiphenylsiloxane

The pyrolytic reaction of polydimethylsilane to polycarbosilane in the presence of polyborodiphenylsiloxane have been studied by the use of organosilicon intermediates obtained during the initial pyrolytic stage of polydimethylsilane. In this reaction, the cleavage of the framework of Si-Si bonds occurred. Si-CH2-Si and Si-H bonds were subsequently formed by the rearrangement reaction between radicals formed and Si-CH3 bonds. The formation of Si-Si bonds by dehydrogenation between Si-H bonds then proceeded simultaneously or stepwise. It is assumed that polyborodiphenylsilane plays a role in accelerating the polycondensation reaction, that is, the propagation reaction occurred by dehydrogenation between the Si-H bonds, but takes no part in the radical rearrangement reaction from Si-Si bonds to Si-CH2-Si bonds.

A SAXS study of the nanometer scale hybrid structure of SiTiCO amorphous fibers

Abstract A small-angle X-ray scattering (SAXS) study was performed to reveal the nanometer scale hybrid structure of SiTiCO fibers prepared by the pyrolysis of polytitanocarbosilane. The SAXS profile for SiTiCO fibers is attributed to two different types of scattering entities: an anisotropic contribution from long filaments with diameters hundreds to thousands of Angstroms and an isotropic contribution from β-SiC fine clusters of about a nanometer in diameter. A drastic degradation in the tensile strength of the fibers is correlated to the characteristic variations in their long-and medium-range structure.

Temperature-dependence mechanism of tensile strength of Si-Ti-C-0 Fiber-Aluminum matrix composites

The mechanism for the temperature dependence of the tensile strength of unidirectional hybrid type Si-Ti-C-O (Tyranno) fiber-reinforced aluminum matrix composite, in which SiC-particles are dispersed in the matrix, is discussed, focusing on the temperature dependencies of the stress concentration arising from broken fibers and critical length and their influences on the composite strength, by means of a shear-lag analysis and a Monte Carlo simulation. The main results are summarized as follows. The softening of the matrix at high temperatures raises the composite strength from the point of decrease in stress concentration, but on the other hand, it also reduces strength from the point of increase in critical length, which reduces the stress-carrying capacity of broken fibers over a long distance. The reason why the measured strength of composite decreased with increasing temperature could be attributed to the predominacy of the latter effect over the former one. The results of the simulation indicated that the hybridization of the composites improved room-temperature and high-temperature strengths through the strengthening of the matrix.

Properties of Si-Zr-C-O fibre/S1ZrC composites dispersed ZrSiO4 particles in the matrix

Si-Zr-C-O fibre (Tyranno ZMI®fiber)/SIZrC mincomposites dispersed ZrSiO4 particles in the matrix were fabricated by the PIP (Polymer Impregnation and Pyrolysis) method. The minicomposites with 30wt% ZrSiO4 particles in the matrix showed extremely high tensile strength up to 1573K in air than when otherwise. From the observation of microstructures of the minicomposites with dispersed ZrSiO4 particles, it is found that any fibre did not contact each other within a distance of a few micrometers in the matrix. In addition, the matrix included fewer cracks and was of apparently higher density than those of the matrix without ZrSiO4 particles. ZrSiO4 particles dispersed matrix was applied to the 2D composites.

Fracture behavior of notched unidirectional Si-Ti-C-O/BMAS composite

Experimental and modeling studies on tensile fracture behavior of notched unidirectional Si-Ti-C-O (Tyranno fiber) reinforced BMAS (barium magnesium aluminosilicate) glass matrix composite were carried out. The longitudinal crack arose at the tip of the transverse notch before overall fracture. The critical energy release rate around at initiation of the longitudinal cracking was estimated to be nearly 100 J/m2 by application of the present model to the experimentally observed relation between the stress of the composite in the very early stage of longitudinal cracking and the notch size. The notched strength was higher than that predicted by the fracture mechanical criterion due to the blunting arising from the premature longitudinal cracking, but it was lower than that predicted by the net stress criterion due to the constraint effect arising from the bridging of the fibers and the spalling of the segmented matrix into the longitudinal crack.