Key Engineering Materials Vols. 368-372

Abstract: The Mg,Al-hydrotalcite/PE-EVA flame-retardant nanocomposites were prepared by the co-mixing method at atmospheric pressure successfully. Moreover, the preparation of Mg,Alhydrotalcite/ PE-EVA nanocomposites was investigated according to the mechanical and flame-retardant tests of the samples. And the results show that the Mg,Al-hydrotalcite nanocrystalline is a highperformance inorganic flame-retardant agent.

Abstract: After removal of the resin from PAN-carbon fibers, the fibers were coated with C or SiC by chemical vapor deposition (CVD) and the effects of these CVD coatings on the mechanical and chemical properties of carbon fibers were investigated. DTG analysis was conducted at different temperatures (993K, 1053K and 1113K) to establish the oxidation reaction models of the C/C or C/SiC coated fibers. The results showed that the CVD coatings enhance the oxidation resistance of the carbon fibers. The surface morphology of the CVD C or SiC fibers was investigated by SEM. The tensile strength of the fibers was found to decrease rapidly after CVD, while the Young’s modulus remained almost constant. These changes can be explained by the interface performance of the carbon fibers and the mechanism of CVD C or SiC deposition. The difference in dilatability between the CVD C or SiC layers and the carbon fibers was the main reason for the tensile failure.

Abstract: C/SiC, C/Si-O-C and C/C composites reinforced with T300 carbon fiber were fabricated via polycarbosilane (PCS), polysiloxane (PSO), and phenolic resin precursor polymers infiltration/pyrolysis, respectively. Flexural strength and fracture toughness of the composites were evaluated. The results showed that all the composites had poor mechanical properties, less than 160 MPa in flexural strength and 5 MPa•m1/2 in fracture toughness. Deep investigation illuminated that the fiber was damaged severely during the preparation of the composites, especially in the first cycle of precursor pyrolysis. Great degradation of the fiber has relationship with coarsening of the microstructure. Bad in-situ strength of the fiber resulted in poor performance of the composites.

Abstract: Two-dimensional carbon fiber cloth reinforced silicon oxycarbide (2D-Cf/Si-O-C) composites were fabricated with silicone resin (SR) as precursors, ethanol as solvent and SiC as inert fillers by precursor infiltration pyrolysis (PIP). Effects of the pyrolysis temperatures in the first cycle and the last but third cycle on the microstructure and mechanical properties of 2D-Cf/Si-O-C composites were investigated. The results showed that, when the pyrolysis temperature of the first cycle was 1200°C, 2D-Cf/Si-O-C composites exhibited good mechanical properties, which can be attributed to the better fiber/matrix interfacial bonding. When the pyrolysis temperature of the last but third cycle was 1400°C, the mechanical properties of 2D Cf/Si-O-C composites were further enhanced. The flexural strength and fracture toughness of the composites reached 263.9MPa and 12.8 MPa·m1/2, respectively.

Abstract: Carbon fiber cloth reinforced silicon carbide (2D-Cf/SiC) composites were prepared through polycarbosilane(PCS) /divinylbenzene(DVB) pyrolysis with SiC as inactive filler. Effects of the molding pressure on the microstructure and mechanical properties of 2D-Cf/SiC composites were investigated. With increasing molding pressure from 0MPa to 3MPa, the fiber volume fraction of the composites was increased. As a result, the strengths of the composites were enhanced. But when the molding pressure exceeded 3MPa, SiC particles would damage the carbon fibers seriously. Therefore, although the fiber fraction of the composites was increased further, the flexural strengths of the composites were decreased. It was found that the composites fabricated with the molding pressure of 3 MPa exhibited highest flexural strength, reached 319.4 MPa.

Abstract: Three-dimensional textile SiC fiber reinforced SiC composites with pyrolytic carbon interfacial layer (3D-SiC/C/SiC) were fabricated by chemical vapor infiltration. The microstructure and complex permittivity of the 3D textile SiC/C/SiC composites were investigated. The flexural strength of the 3D textile SiC/C/SiC composites was 860 MPa at room temperature. The real part (ε′) and imaginary part (ε″) of the complex permittivity of the 3D-SiC/C/SiC composites are 9.11~10.03 and 4.11~4.49, respectively at the X-band frequency. The 3D-SiC/C/SiC composites would be a good candidate for structural microwave absorbing material.

Abstract: A novel composite, 3D C/SiC-Cu, which contained copper as transpiration agent, was designed and prepared. The influence of copper contents (2.18, 4.86, 6.53vol %) upon the mechanical and anti-ablative properties was investigated. The flexural strengths of three composites were over 450MPa, and fracture toughness over 15.0MPa•m1/2. After being ablated for 35 seconds in flowing oxyacetylene torch environment, the composites remained integral, and the flexural strength and strength retention ratio of the composite increased with the copper content increase. The maximum recession rate of the samples in oxyacetylene torch test was as low as 0.0490mm/s.

Abstract: The microstructure and its evolution of 3D-Cf/SiC composites derived from organic precursor are studied by using scanning electronic microscopy, mercury intrusion porosimetry, isothermal N2 sorption and bubble point method, etc. As the results shown, MIP is preferable to N2 sorption for the characterization of pore size distribution (PSD) because of its wider effective probing ranges. The typical porosity of fabricated 3D-Cf/SiC composites is 10-15vol.%, and all the pores distribute in a quite wide size ranging from some dozens of nanometers to hundreds of microns and can be divided into three groups, according to their sizes, contents and locations: the inter-bundle macro-pores/paths, the intra-bundle pores and the micro-pores/cracks around the interfaces or in the matrixes. The macro-pores/paths constitute a porous network, which is partially open throughout the composites.

Abstract: An improved chemical liquid vaporized infiltration process was developed to fast densify carbon/carbon (C/C) composites. A disc-shaped carbon felt was chosen as preform, whose upper and lower sides were fixed and heated simultaneously by two flat surfaces of two heat sources, and the precursor was heated by the third heat source separated. By this method, carbon felts (bulk density ~0.2 g/cm3) were densified to C/C composites with density of 1.29, 1.61 and 1.72 g/cm3 when prepared for 3h at 900°C, 1000°C and 1100°C, respectively. Scanning electron microscopy (SEM) reveals that the carbon fibers of the composite are surrounded by ring-shaped pyrocarbon. The deposition process is analyzed by dividing the reactor into four regions associated with specific functions and the reasons for the rapid fabrication are proposed as the short convection and diffusion path for the precursor and the existing thermal gradients across the preform.

Abstract: In this paper, 2D C/SiC composites with different carbon cloth filaments (1K, 3K) were prepared via precursor infiltration and pyrolysis (PIP) process. The flexural strength of 2D-1K C/SiC composites was 380MPa, and fracture toughness was 16.8MPa-m1/2, while those of 2D-3K C/SiC were 305MPa and 14.4MPa-m1/2, respectively. The differences of these two composites were analyzed from fiber volume ratio in the composites, density, and fracture surface (SEM) of the samples.