Klaus J. Hüttinger

Surface-oxidized carbon fibers: I. Surface structure and chemistry

Abstract The surface structure and chemistry of various surface oxidized HT carbon fibers, an IM and a HM carbon fiber were studied by SEM, STM, CAM (contact angle measurement), XPS and TPD with special reference to adsorbed oxidation products and adsorbed water. It is shown that a real image of surface structure and surface chemistry is only obtained after removal of adsorbed oxidation products by extraction with water and careful drying of the fibers without changing the surface chemistry. A comparison on the usefulness of the various surface analytical methods will be given in Parts II–IV of this paper.

Chemistry and kinetics of chemical vapor deposition of pyrocarbon—II pyrocarbon deposition from ethylene, acetylene and 1,3-butadiene in the low temperature regime

Abstract Pyrocarbon deposition from ethylene, acetylene and 1,3-butadiene was studied with a vertical hot-wall reactor at ambient pressure and 1000 °C; initial partial pressures of the hydrocarbons and residence time were varied. Steady-state pyrocarbon deposition rates and corresponding compositions of the gas-phase were determined. Reaction models describing homogeneous gas-phase and heterogeneous pyrocarbon deposition reactions were derived and applied for simulation of pyrocarbon deposition rates and the inhibiting effect of hydrogen. This latter effect is ascribed to a blocking of active sites at the growing pyrocarbon surface.

Consideration of reaction mechanisms leading to pyrolytic carbon of different textures

Abstract 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).

Chemistry and kinetics of chemical vapour deposition of pyrocarbon: I. Fundamentals of kinetics and chemical reaction engineering

Open scientific problems of chemical vapour deposition of pyrocarbon are discussed on the basis of brief historical and literature reviews. Some thermodynamic considerations are followed by detailed analyses of reaction pathways in hydrocarbon pyrolysis. Reaction schemes are derived, and corresponding differential equations have been solved to show the influence of (i) initial partial pressure of the hydrocarbon and (ii) residence time on the rate of pyrocarbon deposition in general as well as carbon deposition profiles along the reactor axis. Finally, some suggestions are presented for model-based analyses of pyrocarbon deposition. In the following parts of this paper, these fundamentals will be applied for an as good as possible analysis and simulation of pyrocarbon deposition kinetics. It will be the goal to correlate pyrocarbon structures not with the deposition conditions, but with the dominating growth species.

Micro- and nanostructure of the carbon matrix of infiltrated carbon fiber felts

The structural properties of carbon/carbon-composites fabricated by chemical vapor infiltration (CVI) were studied by polarized light microscopy (PLM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) on a micrometer and nanometer scale. The types of carbon bonds were estimated by electron-energy-loss spectroscopy (EELS). Using a methane/hydrogen gas mixture at a temperature of 1100°C two different methane partial pressures were applied. The carbon fibers are surrounded by ring-shaped layers with different optical reflectance. The SEM analyses of fracture surfaces revealed differences in the micromechanical behavior depending on the matrix morphology. Particular emphasis was put on the distinction of individual forms of pyrolytic carbons with similar optical behavior, which reveals significant structural differences in detailed SEM and TEM analyses.

Chemistry and kinetics of chemical vapor deposition of pyrocarbon : IV. Pyrocarbon deposition from methane in the low temperature regime

Abstract Pyrocarbon deposition from methane was studied at ambient pressure and 1100 °C using a vertical hot-wall deposition reactor; the methane initial partial pressure was varied up to 75 kPa and the residence time up to 1 second. The influence of hydrogen partial pressure was studied at constant methane partial pressure. Steady-state pyrocarbon deposition rates and corresponding compositions of the gas-phase were determined. Reaction models of homogeneous gas-phase and heterogeneous pyrocarbon deposition reactions were derived and used for simulation of pyrocarbon deposition kinetics influenced by residence time, initial partial pressures of methane and hydrogen, and concentrations of active sites. The importance of active sites and their blocking by hydrogen is discussed with special emphasis.

Chemistry and kinetics of chemical vapor deposition of pyrocarbon — III pyrocarbon deposition from propylene and benzene in the low temperature regime

Abstract Pyrocarbon deposition from propylene and benzene was studied at ambient pressure and 1000 °C using a vertical hot-wall reactor. Initial partial pressures of the hydrocarbons and residence time were varied; the influence of hydrogen in the feed gas was additionally studied. Gaseous and liquid reaction products were analyzed by on-line gas chromatography. The results are discussed in terms of homogeneous gas-phase and heterogeneous pyrocarbon deposition reactions. Reactions models have been derived and used for simulation of pyrocarbon deposition rates and their inhibition by hydrogen. Pyrocarbon deposition from propylene is shown to be controlled by complex pyrolysis reactions in the gas-phase, whereas benzene forms pyrocarbon directly i.e. without significant formation of intermediate products in the gas-phase.

CVD in Hot Wall Reactors—The Interaction Between Homogeneous Gas‐Phase and Heterogeneous Surface Reactions

The paper is concerned with chemical vapor deposition in hot wall reactors and in particular with the interaction of homogeneous gas-phase and heterogeneous surface reactions. Based on model considerations it is shown that this interaction has a tremendous influence on the deposition chemistry and kinetics in all cases in which the precursor gas undergoes complex gas-phase reactions. The decisive parameter determining the interaction is given by the ratio of free volume of the deposition space and size of the surface area of the substrate; it is termed the “third parameter” of chemical vapor deposition. Predictions from the model are experimentally confirmed using results on chemical vapor deposition of pyrolytic carbon from methane. The importance of this “third parameter” of chemical vapor deposition in investigations of kinetics and for technical processes of chemical vapor deposition and infiltration is discussed in the conclusion.

Influence of pressure, temperature and surface area/volume ratio on the texture of pyrolytic carbon deposited from methane

Abstract The kinetics of carbon deposition from methane were studied over broad ranges of pressures, temperatures and reciprocal surface area/volume ratios. Based on these results, it was possible to distinguish between a growth and a nucleation mechanism of carbon deposition and to select conditions for the preparation of well-defined samples for texture analysis by transmission electron microscopy and selected area electron diffraction. Maximal texture degrees were obtained at medium or high values of the above parameters, but never at low values, at which carbon formation is based on the growth mechanism and dominated by small linear hydrocarbons. High-textured carbon resulting from the growth mechanism is concluded to be formed from a gas phase with an optimum ratio of aromatic to small linear hydrocarbons, which supports the earlier proposed particle-filler model of carbon formation. High-textured carbon may also be formed from a gas phase dominated by polycyclic aromatic hydrocarbons (nucleation mechanism) provided that the residence time is sufficiently long that fully condensed, planar polycyclic aromatic hydrocarbons can be formed in the gas phase.

The carbon-steam reaction at elevated pressure: Formations of product gases and hydrogen inhibitions

The reaction between carbon and steam was studied at a temperature of 1000°C, a total pressure of 10 bar and various partial pressures of steam and hydrogen. The concentrations and stabilities of the intermediate surface complexes were measured with the aid of temperature-programmed desorption and transient kinetic experiments. Possible homogeneous formations of carbon dioxide and methane were studied separately. Hydrogen inhibition results from an irreversible reaction: C( ) + 12H2 → C(H), and two reversible reactions: C( ) + H2O ⇌ C(O) + H2 and C( ) + H2 ⇌ C(H)2. Carbon monoxide is formed from a C(O) surface complex: C(O) → CO + C( ) + ∼ ∼C( ), where ∼ ∼( ) represents a dangling carbon atom. Carbon dioxide clearly is a consecutive product of carbon monoxide and thus is not formed at the carbon surface. Methane is formed at the carbon surface, but only from dangling carbon atoms ∼ ∼C( ): ∼ ∼C( ) →H2 ∼ ∼CH2→H2 CH4 + C( ).