Synthesis of boron-dispersed carbon by pressure pyrolysis of organoborane copolymer

Boron-dispersed carbon was synthesized by pressure pyrolysis of divinylbenzene-tris(allyl)-borane and styrene-tris(allyl)borane at 125 MPa below 650° C. Amorphous boron dispersed in a carbon matrix was oxidized easily to yield boric acid by heat treatment under air at 300° C. The BK image of the product showed that boron was dispersed uniformly in a carbon matrix. Boron-dispersed carbon had the morphology of coalescing spherulite and polyhedra depending upon the concentration of boron in the parent copolymer. The grain size of carbon polyhedra decreased from 2.0m to 0.2m with an increase in the boron concentration from 1.3 to 5.7 wt%. The presence of 0.5 wt% boron in a carbon matrix enhanced the graphitization at 4.0 GPa and 1200° C, decreasing the lattice spacing with an increase in the crystallite size. The crystallite sizes were comparable to each other after heat treatment at 1100° C and 4.0 GPa when the specimen contained boron from 0.5 to 2.5 wt%. The lattice constant (c ₀) and crystallite size (L _c) of boron-dispersed carbon containing 2.5 wt% boron were 677.0 pm and 30 nm, respectively, after heat treatment at 1200° C and 4.0 GPa.     

Fabrication of multi-walled carbon nanotube-reinforced carbon fiber/silicon carbide composites by polymer infiltration and pyrolysis process

Multi-walled carbon nanotube (MWNT)-reinforced carbon fiber/silicon carbide (C_f/SiC) composites were prepared using a polymer infiltration and pyrolysis (PIP) process. The MWNTs used in this study were modified using a chemical treatment. The MWNTs were found to be well dispersed in the matrix after ultrasonic dispersion, and the mechanical properties of the C_f/SiC composite were significantly improved by the addition of MWNTs. The addition of 1.5 wt.% of MWNTs to the C_f/SiC composite led to a 29.7% increase in the flexural strength, and a 27.9% increase in the fracture toughness.     

Synthesis and properties of platinum-dispersed carbon by pressure pyrolysis of organoplatinum copolymer

Platinum-dispersed carbon was synthesized by pressure pyrolysis of divinylbenzenebis (2-allylphenyl)platinum (APPt) and phenylacetylene-APPt at 550 °C and 125 MPa. The crystallinity of platinum dispersed in the carbon matrix synthesized from phenylacetylene(PA)-APPt was higher than that from divinylbenzene(DVB)-APPt. Platinum particles less than 60 nm were dispersed in the carbon matrix synthesized from DVB-APPt at 550 °C and 125 MPa. The carbon matrix formed from PA-APPt contained platinum particles of about 120 nm. The specific area of platinum-dispersed carbon synthesized at 550 °C and 125 MPa increased on subsequent heat treatments in argon, and reached 90 m² g^–1 after heat treatment at 800 °C for 1 h. The activity of platinum-dispersed carbon for the hydrogenation of cyclohexene increased with increasing specific area. Platinum-dispersed carbon formed from DVBAPPt was more active for hydrogenation reaction than that from PA-APPt. The highly active platinum-dispersed carbon could be synthesized from DVB-APPt at 520 °C. The surface area reached 154 m² g^–1 after heat treatment at 800 °C.     

Synthesis and properties of magnetite-dispersed carbon by pressure pyrolysis of divinylbenzene-vinylferrocene with water

Magnetite-dispersed carbon was synthesized by pressure pyrolysis of the divinylbenzene-vinylferrocene system in the presence of water at 125 MPa below 700°C. Supercritical water influenced the phase separation of oligomers formed during the pyrolysis to give carbons with various morphologies, such as spherulitic, coalescing spherulitic and polyhedral carbon, depending upon the concentration of water. Carbon spherulites from 5 to 10 μm diameter dispersed with magnetite particles (<100 nm) were synthesized by pyrolysis of divinylbenzene-5.1 mol% vinylferrocene and 20.0 wt% water at 550°C and 125 MPa. The specific area of magnetite-dispersed carbon synthesized at 600°C and 125 MPa was 92 m²g⁻¹ after heat treatment at 800°C for 1 h. The specific area of the carbon specimen increased with decreasing pyrolysis temperature of the parent copolymers from 700 to 550°C. The Curie temperature of magnetite-dispersed carbon was 585°C. Magnetite dispersed in the carbon matrix was reduced to wüstite during the further heat treatment in vacuum. The saturation magnetization of magnetite-dispersed carbon was 79% of the theoretical value, and changed in proportion to the concentration of iron in the carbon matrix.     

Thermophysical properties of ceramic matrix composites produced by polymer pyrolysis

A unidirectional composite and a series of bidirectionally reinforced composites were fabricated using carbon fibre reinforcement in a silicon carbide matrix, which was produced by the pyrolysis of a polymer precursor. The thermal expansion over the temperature range 20–1000 °C has been measured and the thermal diffusivity measured over the temperature range 200–1200 °C. Thermal diffusivity data was converted to conductivity data using measured density and literature specific heat data. Metallographic examination has been carried out on the composites and the results are discussed in terms of the observed microstructural features.     

乙醇热解制备C／C复合材料探索研究

以可再生的资源无水乙醇为前驱体,在负压条件下,沉积温度为900℃～1200℃,采用压力梯度CVI工艺制备C/C复合材料.考察了沉积时间与密度的变化规律,采用偏光显微镜和扫描电镜观察了材料的组织结构和断口形貌,利用三点弯曲测定了材料的弯曲强度.结果表明:采用乙醇为前驱体,可大幅度提高致密化效率,96h制备出密度为1.47g/cm3的C/C复合材料;易于获得高织构的组织,制备试样的热解炭组织以粗糙层为主,断裂方式为假塑性断裂.乙醇是一种很有应用前景的制备C/C复合材料的前驱体.     

Carbon fibre reinforced carbon produced by polymer impregnation and pyrolysis

Carbon fibre reinforced carbon (C/C) is an attractive material for intermediate and high temperature applications due to its specific properties like low density, high strength and chemical stability. Unfortunately the material oxidizes, so that in an oxidative environment a protective coating has to be applied. Polymer impregnation and pyrolysis is a cost effective production technique to produce C/C materials. In the present work, an abstract of a research program funded by the German Science Foundation (DFG), the mechanical properties of C/C as a function of processing temperature and test temperature have been described. In the program the behaviour of two-dimensionally reinforced (2D) material and unidirectional reinforced (1D) materials has been investigated. All materials experience a strength reduction as a result of carbonization of the polymer matrix at temperatures up to 1000°C. An additional heat treatment above 1000°C causes a partial recovery of the strength. The 1D C/C material shows up to testing temperatures of 1800°C a 10 % loss of strength, whereas for the 2D C/C the strength increases by 10 % at 1500°C in comparison with the room temperature results.     

Multiwalled carbon nanotube reinforced polymer composites

Due to their high stiffness and strength, as well as their electrical conductivity, carbon nanotubes are under intense investigation as fillers in polymer matrix composites. The nature of the carbon nanotube/polymer bonding and the curvature of the carbon nanotubes within the polymer have arisen as particular factors in the efficacy of the carbon nanotubes to actually provide any enhanced stiffness or strength to the composite. Here the effects of carbon nanotube curvature and interface interaction with the matrix on the composite stiffness are investigated using micromechanical analysis. In particular, the effects of poor bonding and thus poor shear lag load transfer to the carbon nanotubes are studied. In the case of poor bonding, carbon nanotubes waviness is shown to enhance the composite stiffness.     

Synthesis of iron-dispersed carbons by pressure pyrolysis of divinylbenzene-vinylferrocene copolymer

Versatile carbons with finely dispersed iron were synthesized by pressure pyrolysis of a copolymer prepared from divinylbenzene and vinylferrocene at temperatures below 680? C and pressures of 125 MPa. The pyrolysis conditions of the copolymer were found to influence the final morphology of carbons to give fibrils, spheres and polyhedra. The resulting carbons contained uniformly fine particles of cementite (Fe₃C) which were less than 30 nm in size, whereas the magnetite was dispersed in the carbon matrix by pressure pyrolysis in the presence of water. Highly dispersed cementite in carbon was found to decompose into metallic iron by further heat treatment above 850? C. Porous spherulitic carbons were also synthesized by heat treatment of magnetite containing carbon spherulites.     

Synthesis of cementite-dispersed carbons by pressure pyrolysis of organoiron copolymers

Cementite-dispersed carbons were synthesized by pressure pyrolysis of divinylbenzene-vinylferrocene and styrene-vinylferrocene copolymer at temperatures below 600° C and the pressure of 125 MPa. The pyrolysis process of both copolymers was analysed by infrared spectra and magnetization of the pyrolysed substances. The absorption band of iron-carbon bond of divinylbenzene-vinylferrocene copolymer decreased on increasing its pyrolysis temperature from 300 to 450° C and finally disappeared at 500° C. The carbonization of divinylbenzene-vinylferrocene proceeded more rapidly than styrene-vinylferrocene at temperatures between 450 and 500° C. Styrene-vinylferrocene was heat-treated at 250° C for 2 h under 100 MPa affording a paramagnetic product, whereas the paramagnetic character of divinylbenzene-vinylferrocene was revealed after heat-treatment at 380° C. The saturation magnetization of cementite-dispersed carbon synthesized from both kinds of copolymers was comparable when the pressure pyrolysis was carried out at temperatures between 520 to 600° C at 125 MPa. The saturation magnetization of cementite-dispersed carbon formed at 550° C under 125 MPa was correlated linearly with the iron content in carbon. Threedimensional cross-linked divinylbenzene-vinylferrocene copolymer gave the highly dispersed cementite particles less than 50 nm with the coercive force of 950 Oe. On the other hand, the larger particle size of cementite up to 120 nm and the lower coercive force about 400 Oe were obtained in carbon matrix prepared by the pressure pyrolysis of styrene-vinylferrocene copolymer.     

Formation of non-graphitizable isotropic spherulitic carbon from poly-divinylbenzene by pressure pyrolysis

Isotropic spherulites of carbon stable at 2000° C were synthesized by the pressure pyrolysis of divinylbenzene polymer sealed in a capsule. The morphology of the synthesized carbon was pressure and temperature dependent being influenced by the state of polymerization of the starting polymer. Using a polymer prepared at atmospheric pressure and 150° C without catalyst, isolated spherulitic carbon was formed at 700° C and pressures of 1000 to 1250 kg cm^–2. These spherulitic carbons were optically isotropic, hard and non-graphitizable after heat treatment at 2000° C. Such carbons originate in the co-existence of higher and lower molecular weight products of pressure pyrolysis and the survival cross-linkages in the original polymer.     

The strength of carbon fibre-reinforced nickel composites produced by electro-deposition and hot-pressing

Carbon fibre-nickel composites were made by electro-deposition and hot-pressing. The composites with low volume fraction of carbon fibres had a higher strength than the value obtained by the law of mixtures using the strength of annealed nickel. The current density became lower when the deposited nickel became thicker. The effects of annealing on deposited nickel were also observed and were related to the current density. Nickel deposited at low current density was hard to anneal.     

Synthesis and properties of nickel-dispersed carbons by pressure pyrolysis of nickelocene-divinylbenzene

Nickel-dispersed carbon was synthesized by pressure pyrolysis of nickelocene-divinylbenzene at temperatures below 700° C at 125 MPa. The carbon so produced contained uniformly dispersed metallic nickel particles less than 40 nm in size with low crystallinity. The magnetization of nickelocene-divinylbenzene polymer increased abruptly at 280° C. The morphology of carbon changed from coalescing polyhedra to filaments via coalescing spherulites as the temperature increased from 550 to 700° C. Carbon tubes of 30 nm diameter were formed by pyrolysis of nickelocene-divinylbenzene at 650° C and 125 MPa. The Curie point of nickel-dispersed carbon was 360° C. The uniform dispersion of nickel with comparable crystallinity in the carbon matrix gave a linear relation between the saturation magnetization and the nickel concentration. The saturation magnetization of nickel-dispersed carbons synthesized at temperatures below 650° C and at 700° C were 60 and 85% of the theoretical value, respectively. The saturation magnetization of the nickel-dispersed carbon could be increased to reach 90% of the theoretical value with an increase in the crystallinity of dispersed nickel particles by subsequent heat treatment at 700° C for 7 h.     

Reinforcing polymer composites with epoxide-grafted carbon nanotubes

An in situ functionalization method was used to graft epoxide onto single-walled carbon nanotubes (SWNTs) and improve the integration of SWNTs into epoxy polymer. The characterization results of Raman, FT-IR and transmission electron microscopy (TEM) validated the successful functionalization with epoxide. These functionalized SWNTs were used to fabricate nanocomposites, resulting in uniform dispersion and strong interfacial bonding. The mechanical test demonstrated that, with only 1?wt% loading of functionalized SWNTs, the tensile strength of nanocomposites was improved by 40%, and Young's modulus by 60%.These results suggested that efficient load transfer has been achieved through epoxide-grafting. This investigation provided an efficient way to improve the interfacial bonding of multifunctional high-performance nanocomposites for lightweight structure material applications.     

Time-dependent damage in carbon fibre-reinforced polymer composites

Time-dependent damage (matrix cracks) evolution in AS4/3501-6 cross-ply laminates was studied using constant strain rate and constant stress tests. First ply failure stress and strain as well as the matrix crack density at a given stress level were found to be strongly dependent on strain rate. Matrix crack density increased with creep time at a constant stress level. The compliance and creep rate of the laminate increased in the presence of these cracks. These results emphasize the importance of the knowledge of time-dependent damage evolution in a lamina/laminate of a polymer composite for reliable prediction of creep and creep rupture.     

Synthesis of submicron carbon spheres by the ultrasonic spray pyrolysis method

Submicron carbon spherical particles were obtained by polycondensation of resorcinol and formaldehyde in a solution and subsequent ultrasonic spray pyrolysis of the prepared sol. Microscopic characterization indicates the regular spherical shape of the obtained particles and sphere diameters in 200-700 nm range. The carbon spheres are amorphous as confirmed by electron diffraction, EELS, XRD and HREM characterization. Activation procedure was performed with H₂O in a nitrogen flow for 15 and 30 min at 800 °C. The activation procedure preserved the initial spherical shapes of the particles while the particle porosity and specific surface area were increased. The amount of surface oxygen functionalities was also increased by activation procedure as indicated by FTIR analysis.     

Multiscale modeling of carbon nanotube reinforced polymer composites

This article examines the effect of interfacial load transfer on the stress distribution in carbon nanotube/polymer composites through a stress analysis of the nanotube/matrix system. Both isostrain and isostress loading conditions are investigated. The nanotube is modeled by the molecular structural mechanics method at the atomistic level. The matrix is modeled by the finite element method, and the nanotube/matrix interface is assumed to be bonded either perfectly or by van der Waals interactions. The fundamental issues examined include the interfacial shear stress distribution, stress concentration in the matrix in the vicinity of nanotube ends, axial stress profile in the nanotube, and the effect of nanotube aspect ratio on load transfer.