Pyrolytic carbon structure

Metals such as titanium, stainless steel, nitinol, cobalt-chrome alloys, etc., are used in many devices. Generally, these are metals with passive surfaces or surfaces that can be passivated. Silver has been used as a coating designed to resist infection. Glassy carbons have also been used as coatings to render surfaces thromboresistant. Pyrolytic carbon structures or coatings on graphite have been utilized in the fabrication of bileaflet heart valves. These are the most popular mechanical valves in use today. 

Bokros, J.C., Deposition, structure, and properties of pyrolytic carbon, Chem. Phys. Carbon, 1969,5, 1 IIS. 

Figure 7.4. Structure of high-density ( 2.0 g/cm ) isotropic pyrolytic carbon, observed by transmission electron microscopy. Viewing plane is parallel to deposition plane (x = 23 600). (Photograph Courtesy J. L. Kaae, General Atomics, San Diego, CA)...

The pores of the silica template can be filled by carbon from a gas or a liquid phase. One may consider an insertion of pyrolytic carbon from the thermal decomposition of propylene or by an aqueous solution of sucrose, which after elimination of water requires a carbonization step at 900°C. The carbon infiltration is followed by the dissolution of silica by HF. The main attribute of template carbons is their well sized pores defined by the wall thickness of the silica matrix. Application of such highly ordered materials allows an exact screening of pores adapted for efficient charging of the electrical double layer. The electrochemical performance of capacitor electrodes prepared from the various template carbons have been determined and are tentatively correlated with their structural and microtextural characteristics. 

High Resolution Transmission Electron Microscopy (HRTEM, Philips CM20, 200 kV) was applied to get structural and nanotextural information on the fibers, by imaging the profile of the aromatic carbon layers in the 002-lattice fringe mode. A carbon fiber coated with pyrolytic carbon was incorporated in epoxy resin and a transverse section obtained by ultramicrotomy was deposited on a holey carbon film. An in-house made image analysis procedure was used to get quantitative data on the composite. 

The HRTEM observation of the cross section of a coated fiber showed that the core is constituted of aromatic layers highly misoriented, whereas they are preferentially oriented in parallel for the thin coating pairs of stacked layers form mainly Basic Structural Units (BSUs) in which the average interlayer distance is smaller than between the aromatic layers in the bulk of the fiber. Since the nanotexture is more dense for the pyrolytic carbon than for the fiber itself, it acts as a barrier which prevents the diffusion of the large solvated lithium ions to the core of the fiber, allowing the passivation layer to be less developed after this treatment. Hence, the major amount of lithium inserted is involved in the reversible contribution therefore this composite material is extremely interesting for the in-situ 7Li NMR study of the reversible insertion. 

A critical factor here is the reactivity of the hydrogen by-product that is not only able to gasify the initial surface termination of the carbon fiber but also to etch away the newly formed pyrolytic carbon. This effect is desirable for optimization of the growing structure but additionally slows down the reaction. 

Nanocarbon emitters behave like variants of carbon nanotube emitters. The nanocarbons can be made by a range of techniques. Often this is a form of plasma deposition which is forming nanocrystalline diamond with very small grain sizes. Or it can be deposition on pyrolytic carbon or DLC run on the borderline of forming diamond grains. A third way is to run a vacuum arc system with ballast gas so that it deposits a porous sp2 rich material. In each case, the material has a moderate to high fraction of sp2 carbon, but is structurally very inhomogeneous [29]. The material is moderately conductive. The result is that the field emission is determined by the field enhancement distribution, and not by the sp2/sp3 ratio. The enhancement distribution is broad due to the disorder, so that it follows the Nilsson model [26] of emission site distributions. The disorder on nanocarbons makes the distribution broader. Effectively, this means that emission site density tends to be lower than for a CNT array, and is less controllable. Thus, while it is lower cost to produce nanocarbon films, they tend to have lower performance. 

A—Pyrolytic carbon showing ribbon-like structure in vitrinoid bands. B—Faint gray lines define compression cracks in a bright micrinoid particle. C—Pyrrhotite (white) formed by the thermal decomposition of pyrite impregnating semifusinoids (gray). D—Bright coke particles in a baked-bone coal layer... 

Electron Diffraction and Electron Microscopy. A limited amount of information regarding graphite structure has been obtained by the use of electron beams. Grisdale (27) has measured the degree of orientation using electron diffraction methods on films of pyrolytic carbon deposited on a silica surface under a variety of conditions. Oxidation of the graphite causes an increase in the degree of orientation. 

Carbon deposition occurs on the surface of a substrate inserted into the carbonization system using hydrocarbon gases, such as methane and propane [45], This process is a kind of chemical vapor deposition (CVD) and the products are called pyrolytic carbons. In order to control the structure, the deposition conditions have to be controlled. The deposition can occur on either static or dynamic substrates. In the former, the substrate is placed in a furnace, which is heated either by direct passing of electric currents or from the surroundings. In the latter, small substrate particles are fluidized... 

Endo, M., K. Takeuchi, S. Igarashi, K. Kobori, M. Shiraishi, and H. W. Kroto. 1994. The production and structure of pyrolytic carbon nanotubes (PCNTs). J. Phys. Chem. Solids 54 1841-1848. 

X-ray diffraction analysis of the samples is performed on a DRON-4 apparatus with Cr Ka radiation. As a monohromator, we applied a crystal of pyrolytic graphite. The carbon structure morphology is investigated with a REM-200 electron microscope. The infrared spectra of the optical transmission of the pressed sample tablet in KBr are measured on a Specord M80 spectrophotometer. 

On the microstructural level, several types of pyrolytic carbons may be deposited each with one of four distinctly different structures, ranging from layered, highly anisotropic forms to structures with very small, randomly-oriented crystallites with no preferred orientation. All of these structural variations are a result of modifications in processing conditions. In this particular study, only the isotropic forms of both pure LTI carbon and co-deposited LTI carbon-silicon alloyed carbon (Pyrolite registered trademark of Carbo-Medics, Inc., San Diego, California) were investigated. 

Although pyrolytic carbons have been produced for about fifty years and studied by X-ray, electron and neutron diffraction, which are the most direct probes of their structure, interpretation of experimental data and explanationof different behaviour of these materials upon annealing is rather complicated, due to interplay of order and disorder in interatomic correlations. 

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