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The surface of nano-silicon dioxide (nano-SiO2) particles was modified by small molecular coupling agent KH-560 and macromolecular coupling agent SEA-171, respectively, to change the surface activity and structure. The modified nano-SiO2 was then used for reinforcing cyanate ester resin (CE). Influences of the content of nano-SiO2 and the interfacial structure over the thermal and frictional properties of nano-SiO2/CE composites were investigated. The mechanism of the surface modification of silicon dioxide by KH-560 and SEA-171 was discussed. The experimental results show that the addition of coupling agents increased the interfacial bonding between nano-SiO2 particles and the CE resin so that the heat resistance and friction properties of the composites were improved. After surface treatment of nano-SiO2 by SEA-171, the thermal decomposition temperature of the 3.0 wt% nano-SiO2/CE composites increased nearly by 75 °C and the frictional coefficient was reduced by 25% compared with that of the pure CE, and the wear resistance increased by 77%. Keywords: Cyanate ester resin, Silicon dioxide, Silane coupling agent, Frictional properties

The microstructure and mechanical properties of two kinds of dual-matrix C/C composites were studied by polarized light microscopy (PLM), scanning electron microscopy (SEM) and mechanical properties tests techniques respectively. PLM results indicate that the matrix carbon exhibits the smooth laminar structure of the pyrocarbon, the isotropic, mosaics and flow domains of the pitch carbon. The TEM results show the normal pitch carbon is the grape structure; the mesophase pitch carbon is the lamellar banded structure. Materials with multi-interface structure can improve the flexural strength and fracture toughness, the load-displacement curve shows the load drop is decreased for step type, the composites show a pseudo-plastic fracture characteristics. The flexural strengths of the material A and material B are 206.68MPa and 243.66MPa, the fracture toughness are 8.06MPa·m1/2 and 9.66MPa·m1/2, respectively. The flexural strength and fracture toughness of material B are both superior than that of material A.

A density (ρ) gradient thermally-conductive (λ) phenolic foam material was initially prepared and it was used as a precursor to prepare a carbon foam (CF) material (ρ-λ-CF). The precursor was carbonized at the temperature between 900 and 1100 °C under the protection of N2 gas and graphitized between 2100 and 2500 °C under the protection of Ar gas, respectively. The thermogravimetric analysis was carried out to investigate the influence of temperature on the thermal stability of the achieved ρ-λ-CF during carbonization and graphitization. The deflection of ρ-λ-CF reinforced with different types of carbon fibers by mass fractions of 0 wt%, 3 wt%, 6 wt%, and 9 wt% was investigated. The microstructure of the composites was observed and analyzed by scanning electron microscopy. The results show that the prepared ρ-λ-CF consisted of three parts: reticulated carbon, carbon microspheres, and voids. It had the graphite structure and exhibited a well structured obturator microsphere shape. The toughening mechanism of ρ-λ-CF reinforced with carbon fibers was preliminarily discussed. Keywords: Phenolic resin, Carbon foam, Carbonization, Graphitization, Thermal stability, Deflection

The microstructure of carbon/carbon(C/C) composites with different matrix carbon was studied by polarized light microscopy (PLM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and XRD techniques respectively. PLM results indicate that the different matrix carbon exhibits different optical reactivity, and the average optical reactivity is gradually enhanced from normal pitch carbon, smooth laminar of pyrocarbon, rough laminar of pyrocarbon to mesophase pitch carbon; SEM results show that the normal pitch carbon is mainly of grapy structure, the pyrocarbon exhibits like-crinkle lamellar structure, while the mesophase pitch carbon exhibits lamellar banded structure with different shapes. Under HRTEM, the lattice fringes of the mesophase pitch carbon are arranged regularly, is a long range ordered crystal structure, and the preferred orientation is high. The degree of the graphite and the interlayer spacing of the material B (mesophase pitch-based C/C composites) are better than that of the material D (pyrocarbon-based C/C composites).

Chopped carbon fiber-reinforced cement composite (CFRC) has become one of the potential smart materials in recent years. The key process of preparing this kind of composite at the early stage is how to disperse carbon fibers evenly into the cement matrix to achieve CFRC composite with good properties. In this study, a three-step mixing process and a six-step mixing process for the preparation of CFRC were suggested, respectively. The fracture morphology was observed by scanning electron microscopy. The influence of mass fractions of three common dispersants methyl cellulose (MC), carboxymethyl cellulose sodium (CMC) and hydroxyethyl cellulose (HEC) on the dispersion of short carbon fibers in water under certain temperature was investigated prior to the manufacture of CFRC. The correlation between mass fraction and viscosity was discussed, and the dispersion effect was assessed from the structure of dispersants and molding process. A hypothetical capsule theory was proposed to reasonably explain the dispersion effect of MC, CMC, and HEC. The experiments showed that pre-dispersion by ultrasonic vibration improved the dispersion of carbon fibers greatly. The dispersivity of carbon fibers was closely related to the category and mass fractions of dispersants. With the same mass fraction of dispersants at a certain temperature, the dispersion effect was in order of HEC > CMC > MC. Meanwhile, when the mass fraction of HEC was between 0.6 and 0.8 wt% by weight of cement and the mass fraction in the aqueous solution was between 1.65 and 1.80 wt%, carbon fibers dispersed most ideally and distributed further uniformly in the cement matrix.