Consideration of reaction mechanisms leading to pyrolytic carbon of different textures

A distinction between a growth and a nucleation mechanism is not sufficient to draw direct conclusions in relation to the texture of pyrolytic carbon. This is determined by the carbon formation mechanisms, which are analogous or at least similar to the mechanisms of aromatic growth. The latter mechanisms are reviewed in the first part of the paper with special consideration of structural chemical aspects. The relevance of the individual mechanisms is analyzed in the second part based on experimentally determined reaction products. Most important mechanisms are aryl-aryl combination, intramolecular dehydrocyclization and ethine addition reactions. The influence of mechanisms concerning an inhibition of the formation of five-membered rings and a transformation of five- into six-membered rings is difficult to estimate. The results indicate that a high textured carbon is formed from a gas phase exhibiting an optimum ratio of aromatic to small linear hydrocarbons (ethine). This model is called the particle-filler model (aromatic hydrocarbons: molecular particles; ethine: molecular filler).     

Regenerative laminar pyrocarbon

This work reports on a dense laminar pyrocarbon infiltrated at low temperature, not previously described. By using pulse-CVI and/or toluene for example, a pyrocarbon can be developed with a density as high as that of rough laminar pyrocarbon (RL) but with a regenerative texture. It will be referred to as regenerative laminar (ReL). This pyrocarbon is characterized by a high density (d=2.11), a high bireflectance and the maximum value for the extinction angle: A_e=22°, the latter being related to its high anisotropy. Contrary to rough laminar, regenerative laminar exhibits a smooth extinction of the Maltese-cross in polarized light. This is related, by means of TEM, to the regeneration of thin cones all along the growth. Rough laminar is not regenerative: it contains only primary cones. The regeneration of cones is found to be due to the lateral extent of the layers; on the contrary, rough laminar is grown from small layers which cannot propagate lattice defects (no regeneration). The experimental evidence is based on the infiltration of preforms by using toluene as precursor and pressure-pulsed chemical vapor infiltation (P-CVI). A reference carbon/carbon, RL, is also obtained with the same preform (3D PAN-based needled preform) by using the classical isobaric–isothermal chemical vapor infiltration with a methane–propane mix (I-CVI).     

Kinetics of chemical vapor infiltration of carbon nanofiber-reinforced carbon/carbon composites

Preforms containing 0, 5, 10, 15 and 20 wt.% carbon nanofibers (CNFs) were fabricated by spreading layers of carbon cloth, and infiltrated by using the technique of isothermal chemical vapor infiltration (ICVI) at the temperature of 1100 °C under the total pressure of 1 kPa and with the flow of the mixture of propane/nitrogen in a ratio of 13:1. The infiltration rates increased with the rising of CNF content, and after 580 h of infiltration, the achievable degree of pore filling was the highest when the CNF content was 5 wt.%, but the composite could not be densified efficiently as the CNF content ranged from 10 to 20 wt.%. An analysis of the results, based on the effective diffusion coefficient and on the in-pore deposition rates, shows that the CNFs, due to their higher aspect ratio, accelerate overgrowth at pore entrances and thus lead to incomplete pore filling.     

C/C复合材料显微结构的微米、纳米尺度研究

利用扫描电子显微镜、高分辨透射电子显微镜以及拉曼光谱对化学气相渗透C/C复合材料的形貌特征、显微结构以及石墨化度进行了系统表征.结果表明,热解炭是由厚度在几十到几百纳米的片层结构组成,这些片层结构围绕炭纤维呈层状排列;热解炭为粗糙层结构,在离子束的轰击下很容易撕裂和扭折.炭纤维/热解炭界面上存在一个宽度大约为15nm的无序区,该区域的形成可能与热解炭的沉积工艺有关.炭纤维和热解炭的石墨化度均很低,热解炭的R(R=ID/IG D')值由内层到外层逐渐降低.     

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.     

Nanostructures of pyrolytic carbon from a polyacetylene thin film

Pyrolysis of a polyacetylene thin film has been performed in order to carbonize at temperatures of 500 to 1000 °C in vacuum. A trans-polyacetylene thin film was synthesized using a Ziegler-Natta catalyst. A black char below 20% in weight of the original PA film remained after pyrolysis. Structural properties and morphology of the black chars were investigated using Raman scattering spectrum, X-ray diffraction measurements, and scanning and transmission electron microscopy. Dehydrogenation and carbonization of the PA film were almost finished at a pyrolysis temperature of 800 °C. However, hollow spherical or elliptical nano-particles of tens of nanometers in size, which are composed of graphite structure, were included in the black chars obtained at all pyrolysis temperatures in this work. The formation mechanism of a graphite crystal in nanometer size from a PA crystal was discussed.     

Magnetic properties of CNX whiskers

Carbon–nitrogen whiskers have been prepared by pyrolysis of 1,2-diaminopropane at 950 °C or of allylamine at 900 °C followed by quenching. They are scrolls of carbon film typically 250 nm thick and up to 1 mm long with about five layers in a structure like a “cigare russe” or “brandy snap”, about 50 μm in diameter. Approximately 8 wt% of nitrogen is incorporated into the carbon films, which are practically amorphous, exhibiting a broad diffraction peak at d = 0.34 nm. The whiskers are on the border of metallic conductivity with a resistivity of about 10⁻⁶ Ωm, and they may show either a positive or a negative temperature coefficient of resistance. The pyrolysis produces either whiskers, soot or both. Magnetization measurements of the whiskers made from 1,2-diaminopropane reveal a large diamagnetic susceptibility of χ = −170 × 10⁻⁹ m³  kg⁻¹ and a small ferromagnetic component of unknown origin with σ_S of up to 0.2 A m² kg⁻¹, whereas the soot shows a purely diamagnetic signal, with χ ≈ −40 × 10⁻⁹ m³ kg⁻¹.     

Evidence for the benefit of adding a carbon interphase in an all-carbon composite

T-800 polyacrylonitrile-based carbon fibres were coated with pyrolytic carbon and carbon/carbon/carbon (C/C/C) composites were prepared from them using coal tar pitch as the matrix precursor. Composites were characterised regarding pyrolysis behaviour, mechanical properties, microtexture, nanotexture, and fracture behaviour. All of the composite components, including interfacial areas, were characterised by high resolution transmission electron microscopy. It was checked that the presence of the carbon interphase did not affect the development of the matrix texture, nor deeply modified the fibre surface energetics. However, adding a pyrolytic carbon interphase resulted in improved mechanical properties for the C/C/C composites with respect to the similarly prepared but interphase-free composites (C/C), such as the increase in the flexural strength by a factor of five, and that of the flexural modulus a factor of two. It is shown that such a benefit brought by adding a carbon interphase is possible merely through the appropriate texture of the latter. Typically, the interphase has to exhibit features from both the matrix (anisotropic) and the fibres (isotropic) so that to reduce the discontinuity effect at the fibre/matrix contact, and should also exhibit features (elongated porosity) that promote multiple micro-cracking from the primary cracks, so that to somewhat absorb part of the fracture energy.