Sharp indentation behavior of carbon/carbon composites and varieties of carbon

Sharp indentation tests on carbon fiber and carbon matrix composites (C/C composite) were carried out over a wide load range from 0 to 2 N on three different cross sections: normal, parallel and inclined to the fiber axis. For comparison purposes, a variety of carbons including HOPG, glassy C, and pyrocarbon films was also examined. Both the fibers and the matrices displayed first a purely elastic response and second crack-induced damage. A purely elastic behavior was also observed with most of the varieties of carbon considered. Young’s modulus was extracted from the indentation curves either at maximum or at various forces, using the Sneddon equation of elastic response on loading (elastic indentation) or a classical equation based on elastic recovery on unloading (elastoplastic indentation). Results are discussed with respect to features of structure and heterogeneity of material in the stressed volume.     

Kinetics of pyrolytic carbon infiltration for the preparation of ceramic/carbon and carbon/carbon composites

Carbon/carbon and zeolite/carbon composites have been prepared by pyrolytic carbon infiltration of organic and inorganic substrates with different porous structures. The chemical vapour infiltration kinetics of these substrates has been studied in a thermogravimetric system at atmospheric pressure, using benzene as pyrolytic carbon precursor. The rate of pyrolytic carbon infiltration seems to depend on the porosity of the substrate available to the pyrolytic carbon precursor, irrespective of the nature of the substrate studied. Activation energy values of about 180 kJ/mol were found for the different substrates used in the temperature range of 700-800 °C, where the cracking reaction of benzene takes place, predominantly, in a heterogeneous form. At higher temperatures homogeneous reactions compete with heterogeneous ones and higher values of activation energies (280-380 kJ/mol) were obtained. The oxidation of the pyrolytic carbon deposited on the different substrates studied takes place in the same range of temperature, which suggests the presence of a similar pyrolytic carbon structure on substrates of different nature or a similar accessibility to the deposited layer.     

Synthesis and structure of carbon belts made of carbon nanofibers supported on carbon foams

Two-dimensional carbon belts (CBs) made of carbon nanofibers (CNFs) supported on a carbon foam (CFoam) substrate have been synthesized by a procedure involving carbonization of polyamic acid (PAA)/Ni(NO₃)₂ solution impregnated polyurethane foam in flowing H₂ at 700 °C and catalytic chemical vapor deposition (CCVD) using C₂H₄ as a carbon source and SO₂ as a promoter. The CBs, which are hundreds of micrometers in length, several micrometers in width and tens of nanometers in thickness, are made of CNFs with a low degree of graphitization that array with an orientation roughly parallel to the longitudinal axis of the CBs. The results show that the mass ratio of Ni to PAA, a H₂ atmosphere in carbonization and SO₂ in CCVD process are the three key factors governing the growth of the CBs.     

Particulate carbon and other components of soot and carbon black

Soot is an unwanted by-product of combustion or pyrolysis of organic compounds. At least four morphologically distinct forms of particulate carbon can be recognized in various types of soot:

1. (1) aciniform carbon (AC), composed of spheroidal carbon particles fused together in aggregates of colloidal dimensions

2. (2) carbonaceous microgel (CM), in which spheroidal carbon particles are embedded in carbon or carbonaceous material

3. (3) carbon cenospheres

4. (4) coke and char fragments.

We have examined 18 soots from various sources. Domestic chimney soots have very little AC but consist mainly of char fragments, CM, ash, and soluble organic fraction (SOF); whereas diesel soots consist mainly of AC and SOF. Carbon black is a manufactured product consisting almost entirely of AC. Carbon blacks can be distinguished from many soots by their morphology, and from all soots investigated in the present study by their purity. Thus, carbon blacks have less than 0.3% SOF removable by toluene, and less than 1% ash, whereas soots exceed at least one of these limits by several fold. Because of differences in origin, function, and composition, common nomenclature carbon black with soot is inappropriate.

Single-walled carbon nanotube buckypaper and mesophase pitch carbon/carbon composites

Carbon/carbon composites consisting of single-walled carbon nanotube (SWCNT) buckypaper (BP) and mesophase pitch resin have been produced through impregnation of BP with pitch using toluene as a solvent. Drying, stabilization and carbonization processes were performed sequentially, and repeated to increase the pitch content. Voids in the carbon/carbon composite samples decreased with increasing impregnation process cycles. Electrical conductivity and density of the composites increased with carbonization by two to three times that of pristine BP. These results indicate that discontinuity and intertube contact barriers of SWCNTs in the BP are partially overcome by the carbonization process of pitch. The temperature dependence of the Raman shift shows that mechanical strain is increased since carbonized pitch matrix surrounds the nanotubes.     

Oxidation behavior and protection of carbon/carbon composites

The oxidation behavior of C/C composite sheet materials in air has been studied over a wide range of temperature. Gasification was detectable at around 500°C and above about 900°C, under the flow conditions used in the experiments, the overall rates of gasification were controlled by gas phase diffusion. The presence of catalysts reduced the temperature for the onset of gasification but had no effect on the kinetics in the diffusion-controlled region. Borate-based coatings containing refractory particulates and silicon carbide coatings sealed with borates have been found capable of protecting C/C composites against air oxidation for extended periods to temperatures of at least 1200°C.     

Borate treatment of carbon fibers and carbon/carbon composites for improved oxidation resistance

Carbon fibers and carbon/carbon composites have been treated with borate additives and then cured at 500–600°C to produce a continuous film of boron oxide on all exposed surfaces.This treatment has been found to be highly effective in retarding oxidation of the carbonaceous substrate for extended periods in flowing air at temperatures up to 1000°C. At higher temperatures, and in the presence of water vapor, borate species were appreciably volatile and the oxidation protection provided by the coatings was less effective.     

Endor studies of carbon black and carbon black-polymer composites

Electron-nuclear double resonance (ENDOR) spectra have been obtained at the free proton frequency for a carbon black and a carbon black-polymer composite, at temperatures ranging from 130 K to 350 K. The ENDOR spectra show an unusual dependence on the saturation of the associated homogeneously broadened ESR resonance, as “non-saturating” microwave power gives rise to largest ENDOR enhancements. This behaviour is attributed to a “lineshift” ENDOR mechanism. In the composite, the polymer ¹H nuclei contribute very strongly to the ENDOR signal, and this signal shows a marked dependence on molecular motion within the polymer.     

CO与CO2甲烷化反应研究进展

介绍了甲烷化反应的催化剂,尤其是Ni基催化剂的研究进展,综述了助剂、载体以及制备方法对甲烷化催化剂催化性能的影响,不同催化剂上CO和CO2甲烷化反应过程的机理。     

Porous carbon/carbon composites produced from carbon black and petroleum pitch

Granular porous carbon/carbon composites were prepared by mixing carbon black, petroleum pitch and a solvent, followed by granulating the mixture and carbonization of the resulting pellets in an inert atmosphere. The pore structure of this material is studied by mercury porosimetry and scanning electron microscopy. Based on the obtained results, a model for it is proposed. The effects of carbon black type used, filler/binder ratio, heat treatment temperature and mixing time on surface area, total pore volume and strength of the finished pellets were investigated. Comparison with activated carbons indicates that the investigated material can find industrial applications as a catalyst support and as an adsorbent for adsorption of large molecules due to the meso- and macroporous structure and low ash content.     

Contact between catalyst and carbon in the tin- and lead- catalyzed gasification of carbon with carbon dioxide

The wettability and distribution of tin and lead metals on a phenol-folmaldehyde resin char were investigated by means of contact angle measurement and microscopic analysis. The wettability on the pelletized char was not so significant. However, stannic oxide was found to be distributed around a tin catalyst particle. This can be considered a result of the vapor phase transport of stannous oxide and its oxidation by CO₂. Stannous oxide was formed through a red-ox type catalysis cycle. On the other hand, in the case of lead-catalyzed gasification, the mobile species was the lead metal itself. Thus, the high catalytic activity of tin and lead metals was appropriately ascribed to an intimate contact between the catalyst and the carbon substrate caused by the vapor phase transportation of stannous oxide and lead metal.     

Transport properties of double-walled carbon nanotubes and carbon boronitride heteronanotubes

Using the density functional theory combined with the nonequilibrium Green's function, the transport properties of double-walled carbon nanotubes (DWCNTs) and carbon boronitride (CBN) heteronanotubes were investigated. As the hopping length increases, the conductance of DWCNTs shows a dramatic variation that is independent of the intertube space. The transport of the CBN heterojunctions also displays abnormal behavior when the hopping length is changed, which is very different from the behavior of DWCNTs. The currents of the forward in the CBN heterojunctions are about 3–15 times as large as those of the back under lower bias voltages. The negative differential resistance (NDR) effect occurs in the CBN heterojunctions, and the peak-to-valley ratio in the additional NDR regions is about 2–4 for the current–voltage relationship. The hopping length and BN parts have a great influence on the transport of the double-walled nanodevices.     

Microstructure and ablation performances of dual-matrix carbon/carbon composites

The ablation performances of dual-matrix carbon/carbon composites were tested on a DJ-21 arc heater, and the morphologies of specimens were investigated by scanning electron microscopy (SEM). It is found that the composites have typical rough laminar pyrocarbon structure and ablation always tends to start at interfaces, defects and pores. In addition, the work indicates that ablation performances of parallel ablation are better than vertical ablation and parallel ablation is mainly controlled by block denudation while vertical ablation is more likely to be controlled by particle denudation.     

Melt mixing of carbon fibers and carbon nanotubes incorporated polyurethanes

Polyurethane composites filled with carbon fibers (CF) and carbon nanotubes (CNT) were prepared by mixing and injection molding, and its mechanical as well as their thermal properties were investigated. Dynamic mechanical analysis (DMA), thermogravimetry analysis (TGA), and thermal conductivity tests were done, and the properties were evaluated as a function of the filler concentration. The storage modulus of the composites increased with fillers concentration, which also mean the increase of the stiffness, suggest a good adhesion between the polyurethane matrix and the fillers. Addition of more CF and CNT to the composites broadened and lowered the peak of tan δ specifies that the polyurethane composite became more elastic because there is a good adhesion between the fillers and the matrix. The addition of carbon fillers improves the thermal stability of the polyurethane. The inclusions of CNT show a better thermal stability when compared with CF. The addition of carbon fillers also increased the thermal conductivity of the polyurethane composites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008