Seishi Yajima

Synthesis of continuous silicon carbide fibre with high tensile strength and high Young's modulus: Part 1 Synthesis of polycarbosilane as precursor

Polycarbosilane as the precursor of continuous SiC fibre was synthesized by thermal decomposition of polydimethylsilane. The structure of the polycarbosilane is concluded to be similar to that of polysilapropylene by the measurements of i.r. spectra, NMR spectra and chemical analyses. Its formation mechanisms are initially the formation of carbosilane by thermal decomposition of polydimethylsilane and then the increase in molecular weight by dehydrogenation-condensation of the carbosilane. Molecular structure and molecular weight distribution of the polycarbosilane depend on the reaction temperature.

Development of a silicon carbide fibre with high tensile strength

MUCH work has been done on preparing heat-resistant silicon carbide materials in fibrous form, since plastics or metals can be reinforced with them to obtain very heat-resistant material of great mechanical strength. SiC whiskers1 are, however, impractical because of their shortness (several mm), their non-uniform diameter and high cost of production. SiC-on-W (ref. 2) and SiC-on-C (ref. 3) filaments have been produced by chemical vapour methods. These coated filaments are more expensive, and the treatment for making such composite materials requires careful control. We report here on the synthesis of continuous β-SiC fibres by a new process: the conversion of organometallic polymers to inorganic substances. We have studied the transformation process and the structure and mechanical properties of these fibres.

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.

Synthesis of continuous silicon carbide fibre with high tensile strength and high Young's modulus

Polycarbosilane as the precursor of continuous SiC fibre was synthesized by thermal decomposition of polydimethylsilane. The structure of the polycarbosilane is concluded to be similar to that of polysilapropylene by the measurements of i.r. spectra, NMR spectra and chemical analyses. Its formation mechanisms are initially the formation of carbosilane by thermal decomposition of polydimethylsilane and then the increase in molecular weight by dehydrogenation-condensation of the carbosilane. Molecular structure and molecular weight distribution of the polycarbosilane depend on the reaction temperature.

High-temperature strengths of aluminium composite reinforced with continuous SiC fibre

Aluminium alloys were reinforced unidirectionally with 30, 35, 40 and 50 vol %SiC fibres by a liquid-pressing method. The SiC fibres for reinforcement were produced from a polycarbosilane and were “yarns” consisting of 500 continuous filaments of length 300 m and diameter 13μm, having a tensile strength of 2000 MPa. High-temperature tensile and bending strengths of the composites were examined in air in the temperature range from room temperature to 500° C. The strengths were not influenced by temperature up to 400° C, but were decreased at 500° C. The decrease is considered to be caused by the reduction in transfer efficiency of the stress accompanying the decrease in adhesion between fibres and matrix.

Mössbauer study of amorphous BaO−Fe2O3−B2O3 system

Abstract Homogeneous BaO − Fe 2 O 3 − B 2 O 3 glasses containing maximum Fe 2 O 3 content of 63 mol.% are prepared by splat cooling technique. Mossbauer study reveals that the glass mainly consists of tetrahedral network of Fe 3+ O 4 . Mossbauer spectrum of the glass shows a well defined hyperfine structure at low temperatures. Magnetic ordering temperature estimated is about 130 K for the most iron-rich specimen, being much lower than that of the corresponding crystalline phases, BaO;·;Fe 2 O 3 and BaO·;2Fe 2 O 3 . The magnetic structure is suggested to be of a short-range antiferromagnetic ordering.

STRUCTURAL FEATURES OF PYROLYTIC GRAPHITE.

The structure of pyrolytic graphite has been examined on several samples deposited under various conditions at 1340 to 2310° C and 5 to 200 torr by means of an X-ray technique. The structural features investigated include interlayer spacing, ordering, preferred orientation, intercrystallite porosity, and crystallite size. These features are closely related to the microstructure and density. In particular, the temperature dependence of crystallite size explains three types of formation mechanism, the cause of the occurrence of a minimum density at a certain temperature, and the cause of a preferred orientation in pyrolytic graphite.

Mechanical properties changes of Fe-Cr alloys by fast neutron irradiation

Abstract The changes in the mechanical properties of Fe-Cr ferritic, single α-phase alloys, containing 0–15 wt% Cr, by fast neutron irradiation were studied by means of tensile tests in the temperature range between 77 and 283 K, mainly with regard to the effects of the Cr content. The temperature dependence of the yield stress decreased with increasing Cr content. The irradiation raised the thermal activated component of the yield stress in low Cr alloys severely, but not in high Cr alloys. The fracture stress in the brittle fracture mode increased with Cr content up to 10 wt%, beyond which it decreased. The frequency of twinning deformation showed an opposite tendency to the fracture stress with or without irradiation. The transition temperature of the unstable plastic flow, which occurred intensively after irradiation, decreased with increasing Cr content. The last condition mainly governed the irradiation embrittlement of Fe-Cr alloys determined from absorbed energy-temperature curves in tensile testing. Thus, high Cr alloying can make the steel more resistant to irradiation embrittlement.

Ln-M-O glasses obtained by rapid quenching using a laser beam

A rapid quenching apparatus employing a laser beam was developed to obtain the glassy state of oxides which have high melting temperatures. Using this apparatus, the glassy state in Ln-Ti-O, Ln-Nb-O and Ln-Ta-O (Ln=lanthanides) systems was investigated. The range of compositions which produced glasses and the crystallization process were studied in detail, and the metastable phases, which appear during the crystallization process, were examined by differential thermal and X-ray diffraction analysis.

Siliconated pyrolytic graphite

By X-ray diffraction, surface oxidation, X-ray microanalysis, electron diffraction, and electron microscopy, the state of silicon present in Siliconated pyrolytic graphite has been examined on several samples prepared under a variety of conditions.In the siliconated pyrolytic graphite prepared at the deposition temperatures below 1730° C, the greater part of the silicon occurs as β-SiC, It does not segregate in the cone boundaries but disperses uniformly. It exists as flake-like single crystals, whose size increases with decreasing temperature. The (111) plane of β-SiC is parallel to the (00l) planes of graphite.