Key Engineering Materials Vols. 336-338

Abstract: Aluminum borate whiskers and magnetic ceramic particles (NiFe2O4 or Fe3O4) reinforced aluminum matrix composites were synthesized by a squeeze casting technique. Microstructures were observed using scanning electron microscopy (SEM). The tensile strength and tensile yield strength of the as-cast composites containing NiFe2O4 particles were higher than that of composites containing Fe3O4 particles. The tensile yield strength of the thermomagnetically treated composites was higher than that of the as-cast or the thermally treated composites. The effects of the magnetostrictive property of Fe3O4 or NiFe2O4, the interface and the thermal residual stresses on tensile behaviors of the composites with different treatments were discussed in this paper.

Abstract: The CNTs/Si3N4 ceramic matrix composites were prepared by the reaction bonded processing. The phase compositions, chemical compatibility, mechanical properties, and microwave attenuation properties of the composites were investigated. XRD analysis shows the composites consist mainly of the α- and β-Si3N4, with a trace of unreacted silicon. The SEM micrograph displays the fractured surface of the composites studs with intact CNTs, indicating that CNTs and Si3N4 are chemically compatible. The composites with 1.0wt.% CNTs have a strength of 280 MPa, hardness of 8.2 GPa and toughness of 2.3 MPa·m0.5. The average value of the transmission attenuation reaches 6 dB at X band, indicating the composites have a potential for application in electromagnetic adsorbing or shielding.

Abstract: Aluminum phosphate is one of the best inorganic binders, and the addition of chromium oxide can improve the properties of the binder. In this work, the phase evolutions in chromium phosphate system and aluminum-chromium-phosphate system during heat-treatment were investigated. The initial binder solutions were prepared by dissolving Al(OH)3 and Cr2O3 in aqueous H3PO4 solution. The binder solutions consolidated at 100°C. The as-consolidated products were heated at different temperature. The experimental results showed the phase evolution of the consolidated products as: amorphous phase in 300~800°C, Cr(PO3)3 and Cr4(P2O7)3 in 900~1200°C for chromium phosphate system binder; amorphous phase in 300~1000°C, Cr(PO3)3 and AlPO4 in 1100°C, Cr4(P2O7)3 and AlPO4 in 1200°C for aluminumchromium- phosphate system binder. The addition of chromium oxide demonstrates to strengthen amorphous network, which further improves the thermal stability.

Abstract: 3-D braided C fiber preform was used to reinforce SiC matrix by polymer infiltration and pyrolysis (PIP). The effect of PCS pyrolysis process on the uncoated carbon fiber was studied. During the pyrolysis, amorphous SiCxOy and some free silicon yielded. The Si element diffused into the C fiber from the matrix because of the concentration gradient at high temperature and destroyed the intrinsic structure of the uncoated C fiber. At the same time, the free Si reacted with the uncoated C fiber. Thus, strong bonding between the fiber and matrix was formed. As a result, bending strength of the composite was decreased.

Abstract: Due to the introduction of oxygen that works to keep the shape of the fibers and enhance the ceramic yield during high temperature pyrolysis, air-curing is a critical step during the preparation of the polymer-derived Si-Al-C-O fibers (KD-A). In this work, to investigate the evolvement of oxygen in the fibers and the influence of oxygen on the mechanical properties of the resulted KD-A fibers, FT-IR, TGA, XRD, SEM and element analysis were performed on the air-curing process of polyaluminocarbosilane (PACS) fibers and the pyrolysis process of the cured PACS fibers. The results showed that the oxygen in the cured PACS fibers was originated from aluminum aletylacetanate (Al(AcAc)3) and the air-curing process, which could be regarded as a constant mass during the pyrolysis. In addition, it was found that the mechanical properties of the achieved KD-A fibers were greatly effected by the amount of oxygen in the fibers due to its inhabiting the crystallization of silicon carbide. And the oxygen content which leaded to the KD-A fibers with the highest tensile strength was found to be 8~10%.

Abstract: 3D braided carbon fiber reinforced silicon carbide (3D-Cf/SiC) composites were fabricated by precursor infiltration and pyrolysis(PIP), with carbon coatings prepared by chemical vapor deposition (CVD) before PIP. The effect of 1873K heat treatment on the mechanical properties of Cf/SiC composites were investigated. The results showed that heat treatment before PIP can increase the density of composites and lead to excellent properties of Cf/SiC composites. The flexual strength of the Cf/SiC composites with one cycle of 1873 K heat treatment reached 571 MPa, shear strength 51 MPa, and fracture toughness 18 MPa⋅m1/2.

Abstract: A new technique of mass-producing silicon carbide whiskers at a low cost is introduced in this paper. Silicon carbide whiskers are synthesized by double-heating technique with the activated carbon and silica gel as raw material and CoCl2 as catalyst. The results indicate that the silicon carbide whiskers with the average diameter of 0.2μm, length of 10-50μm and high content of 81% can be obtained at a lower temperature of 1300°C and a shorter time of 1.5h. Compared with the conventional heating, the double-heating technique is suitable for realizing the scaled production because of the lower whiskerssynthesizing temperature, shorter reaction time and greater output.

Abstract: A new curing method, chemical vapour curing (CVC) for polycarbosilane (PCS) fibers using unsaturated hydrocarbon (cyclohexene) vapour was studied. The low oxygen content SiC fibers were prepared and when the oxygen content in fibers was lower than 4.0%, the maximum tensile strength reached to 3.8GPa. The relationship between the tensile strength and the oxygen content of the fibers was investigated. The structure and composition of SiC fibers were also characterized by element analysis, FT-IR, XRD and XPS. Compared with that by air curing method, SiC fibers with less oxygen and better mechanical properties were prepared by CVC method.

Abstract: In the preparation of polymer-derived SiC fibers, nanochannels are believed to be formed in the early pyrolysis stages due to loss of large volumes of pyrolysis gases. In this paper, small angle X-ray scattering (SAXS) was applied to the characterization and calculation of nanochannels in partiallypyrolyzed SiC fibers. The SAXS measurements showed that nanochannels with a radius of 1.0-20 nm were formed for fibers heat-treated at 973K and 1173K. But their distributions were not continuous and at lower part of the distribution, a peak value was observed at about 1.2nm. This means if the nanochannels are finely controlled, the partially-pyrolyzed SiC fibers have great potential application in the fields of hydrogen storage, gas separation, and so on.

Abstract: Silicon carbide whiskers (SiCw) as a kind of high strength fibrous material are widely used in the reinforcement of metal, ceramic and polymer. In this paper preparation of Silicon carbide whiskers by carbon thermal reduction reaction from the mixture of Silica and carbon was investigated with an emphasis on the study of the effect of catalyst. Fe, Co, Ni and their compounds were used as catalyst in this study. Since vapor–liquid–solid (VLS) mechanism was found to be responsible for the growth of whiskers, the formation of catalyst and its subsequent evolution on whisker nucleation, growth and appearance was a subject of extensive research. The catalyst that has the best effect among iron, nickel, cobalt and their compounds was made certain by comparing the products.