Pyrolytic Carbon Deposition Research Articles

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.

Synthetic Carbon Derived from Polyimide

Synthetic carbon was obtained from the porous copolymer of 4, 4'-bis(maleimidodiphenyl)methane and styrene by carbonization in inert and methane atmospheres. The samples were characterized by nitrogen adsorption and mercury intrusion using BET, αs, DR and Kelvin analysis. It was shown that carbonization in an inert atmosphere results in the formation of microporous and mesoporous carbon, while heat treatment in an atmosphere of methane decreases the porosity due to the deposition of pyrolytic carbon in small pores (R < 2–3 nm).

Influence of Carbonization Conditions on the Pyrolytic Carbon Deposition in Acacia and Eucalyptus Wood Chars

The amount of deposited pyrolytic carbon (resulting from the cracking of volatile matter) was found to depend on wood species and carbonization conditions, such as temperature and heating rate. Maximum pyrolytic carbon deposition in both the acacia and eucalyptus wood chars has been observed at a carbonization temperature of800°C. Rapid carbonization (higher heating rate) of wood significantly reduces the amount of deposited pyrolytic carbon in resulting chars. Results also indicate that the amount of deposited pyrolytic carbon in acacia wood char is less than that in eucalyptus wood char.

Formation of New Type of Porous Carbon by Carbonization in Zeolite Nanochannels

An attempt was made to prepare porous carbon by using the channels of Y zeolite as a template. Poly(acrylonitrile) and poly(furfuryl alcohol) were carbonized in the zeolite channels and the resultant carbon/zeolite complexes were subjected to acid treatment in order to extract carbon from the zeolite framework. In addition, pyrolytic carbon deposition in the channels was carried out by exposing the zeolite to propylene at high temperature, and then the carbon was liberated in the same manner as above. The morphology and structure of the carbon prepared in the channels were characterized, and the results were discussed in relation to the morphology and structure of the original zeolite template. It was found that the microscopic morphology of the resultant carbons reflects that of the corresponding zeolites. All of these carbons are highly porous, and some of the CVD carbons have BET surface areas as high as >2000 m2/g.

Preparation of Ultrafine Carbon Tubes in Nanochannels of an Anodic Aluminum Oxide Film

A template carbonization technique was applied to prepare carbon nanotubes and submicron-tubes in one dimensional channels. An anodic aluminum oxide film that has uniform and straight channels with a diameter at the nanometer level was used as a one-dimensional template. The pyrolytic carbon deposition on the channel wall was carried out by exposing the anodic oxide film to propylene at 800 °C. The carbon was then liberated from the anodic oxide film by HF washing. It was found that the resultant carbon is comprised only of uniform hollow tubes with open ends. The infiltration of poly(furfuryl alcohol) into the channels of the film followed by heat treatment led to the formation of bamboolike carbon tubes. For all the carbon tubes obtained here, the outer diameter (30 or 230 nm) and the length (60 or 75 μm) precisely reflect the channel diameter and the thickness of the template used, respectively. Furthermore, in the case of the carbon tubes from propylene, the wall thickness of the tubes was easily cont...

Microscopy studies of deposition of pyrolytic carbon on polished substracts

Microscopy studies of deposition of pyrolytic carbon on polished substracts

Microscopy Studies of Deposition of Pyrolytic Carbon on Polished Substrates

Microscopy Studies of Deposition of Pyrolytic Carbon on Polished Substrates

Mass Spectrometric Study of the Gas Phase During Chemical Vapor Deposition of Pyrolytic Carbon

The pyrolysis of CH 4 , of C 3 H 8 and of the mixture 20C 3 H 8 /80CH 4 was investigated in a hot wall CVD reactor by mass spectrometry. Experiments were conducted as a function of temperature and of surface-to-volume (S/V) ratio of the substrate. The main by-products of the pyrolysis were identified and reaction mechanisms were proposed taking into account homogenous decomposition/formation reactions. Cyclic species are possibly involved in the formation mechanism of pyrocarbon.

Scanning electron microscopic study of acacia and eucalyptus wood chars

SEM studies of acacia and eucalyptus wood chars, prepared under different carbonization conditions, were undertaken to provide information on what happens in the transformation of wood to chars. The material normally lost as volatiles contributes totally to the formation of pores, cracks and pyrolytic carbon. Both woods exhibited similar devolatilization behaviour in pore structure development, crack formation and pyrolytic carbon deposition, showing a decrease in pore size with an increase in carbonization temperature and cracks/voids formation during rapid carbonization at higher temperatures (i.e. 800–1050 °C). Slow carbonization led to pyrolytic carbon deposition in resulting wood char structures and did not disturb the fibrous nature and cell structures of the wood, even at a high carbonization temperature of 1200 °C. Prolonged heating at carbonization (slow) temperatures of 800 and 1000 °C caused sintering of the adjacent fibres resulting in the formation of compacted mass.

Microstructural investigation of low-density carbon-carbon composites

The microstructure of low-density (0.13–0.64 Mg m−3) carbon-carbon composites was investigated using optical microscopy, scanning electron microscopy and image analysis. All samples initially contained varying proportions of rayon precursor carbon fibres, recycled fibrous material and phenolic resin precursor matrix, and were manufactured utilizing a vacuum moulding technique. Some of the composites were densified using the chemical vapour deposition (CVD) of pyrolytic carbon. All the composites were shown to have a two-dimensional planar random microstructure, with a distinct layering effect being seen on the microscopic (and sometimes macroscopic) level. The degree of layering in the composites was quantified utilizing image analysis and was found to be most pronounced in samples containing no recycled material, and least pronounced in samples containing all of its fibrous constituent as recycled material. The composites were found to be very porous, the pores consisting of mainly interconnecting open pores (typically 65–85% of the sample volume). In non-CVD samples the fibrous material was held together by thin (<5 μm) discrete “matrix bonds”, with a few large (typically 100 μm×200 μm×800μm) fibre bundles also existing within the structure. In the CVD-processed material the deposit coat on the fibres was of even thickness throughout the composite and joined together fibrous material that was not previously in contact.

Chemical vapor deposition of pyrolytic carbon on polished substrates

Pyrolytic carbon thin (4-100 nm) films were obtained from methane in a hot wall reactor on optically polished inert substrates by varying the deposition time and temperature. They were characterized by all modes of TEM. They are composed in majority of lamellar pyrocarbon whose thickness and disorder increases with increasing temperature. Isotropic carbon islands are also observed at the upper surface of the film

Pyrolytic Carbon Coating Enhances Teflon and Dacron Fabric Compatibility with Endothelial cell Growth

Compatibility with endothelial cell attachment and growth appears to be an important requisite of vascular prosthetic materials, possibly influencing thrombosis, pseudointimal hyperplasia, and accelerated atherosclerosis at the site of blood-material interaction. Since deposition of pyrolytic carbon (PC) on prosthetic surfaces has been associated with enhanced hemocompatibility, in the present study we assessed whether a thin layer (0.5 microns) of PC deposited onto materials such as knitted Teflon and Dacron enhanced endothelial cell attachment and growth. Cultured human umbilical vein endothelial cells (HUVEC) were seeded at a density of 4.5 x 10(4) cells/cm2 on PC-coated and uncoated grafts. In order to quantify endothelial cell attachment on the fabrics, the area of Teflon and Dacron fabrics covered by endothelial cells was estimated on day 2 after seeding using the point counting method in scanning electron micrographs. Subsequently, on days 2 and 4 after seeding, endothelial cell proliferation was measured both as number of endothelial cells and as total proteins of the endothelial cells covering the fabrics. On day 2 endothelial cell growth on PC-coated fabrics was greater (mean +/- SE; area 42.3 +/- 9.9 mm2, n = 6; cell number 3.9 x 10(4) +/- 3.03 x 10(3) cells, n = 4; total proteins 14.9 +/- 1.2 micrograms, n = 4) than on uncoated fabrics (area 10.6 +/- 4.6 mm2, n = 6; cell number 2.9 x 10(4) +/- 4.3 x 10(3) cells, n = 4; total proteins 11.3 +/- 1.7 micrograms, n = 4; P less than 0.001, less than 0.05 and less than 0.05, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical vapor deposition of pyrolytic carbon on carbon substrates: I: Effect of substrate surface characteristics on the kinetics of deposition

Chemical vapor deposition (CVD) of pyrolytic carbon on 25 different carbon substrates has been investigated at 1273–1338 K, using a mixture of 10% methane in argon flowing at 150 cc/min (STP). The substrates covered a wide spectrum of carbons (porous/nonporous, graphitic/nongraphitic, and small/large surface areas). The results indicate that there are at least four different categories of carbon substrates. The first category includes nonporous, graphitized, or carbonized substrates having a small surface area. The deposition rate increases with surface area. The second category includes microporous carbons with a small external surface area. The early deposition of pyrolytic carbon blocks the micropores, and the reaction continues on the external surface in a manner similar to the first category. The third category includes nonporous, graphitized, and carbonized substrates having a large surface area. The deposition rate on the graphitized substrates is smaller than the carbonized substrates provided that their initial surface area is the same. With carbonized substrates, the total and active surface areas are contributing differently to the CVD rate. However, the contribution of active surface area (ASA) is less pronounced than that of the total surface area (TSA). The fourth category includes activated graphitized carbons. The activation process enhances the ASA and develops microporosity. The newly developed ASA is apparently located inside the micropores since the deposition rate drops substantially after the commencement of reaction. When the micropores are blocked, the entrances to internal ASA are blocked and the reaction continues on the external surface in a manner similar to that of the third category.

Graphite coating by fluidized-bed CVD

Carbon films were formed on silica sand particles by the fluidized-bed CVD method using propylene pyrolysis at temperatures ranging from 1388 to 1573 K. The films were found to be graphite with high density from ultimate analysis and other measurement of their physical properties. The film growth rate was proportional to the concentration of propylene above the fluidizing gas velocity of 0.9 m/s, and was obtained as a function of temperature and propylene concentration. Under these experimental conditions, the deposition of pyrolytic carbon was found to be controlled by gas-phase decomposition of propylene.

Pyrolytic Carbon Deposition on Graphitic Surfaces

AbstractThe present study outlines the importance of carbon active sites in controlling the kinetics of pyrolytic carbon deposition on a graphitized pitch fiber at 1025°C. Blockage of active sites with chemisorbed hydrogen retards deposition rates substantially. Removal of the chemisorbed hydrogen from the occupied sites raises deposition rates to the normal values noted on a fresh “clean” surface. The effect of surface activation prior to deposition is discussed. Activating the surface generates additional active sites and enhances the rates. However, not all the newly developed sites contribute to the kinetics. After the deposition of the first carbon layer, a fraction of those newly developed sites is instantaneously blocked and does not further contribute to subsequent deposition. The remaining fraction, along with the original sites available before activation, keeps replicating during the remaining course of deposition. That is, the disappearance of one active site after carbon deposition is associated with the generation of a new site. This trend continues up to the deposition of 60 carbon layers.

Pyrolytic carbon deposition from methane: An analytical approach to the chemical process

The present work was conducted in order to investigate the effects of three important constraints, namely, temperature, flow-rate, and pressure, in the deposition process of pyrolytic carbon from methane, in the vicinity of 1000–1100°C, and at pressures between 150 and 400 mm Hg. The properties of the deposited carbon were studied, and the experimental setup enabled us to obtain a fairly complete qualitative and quantitative description of all the important intermediate reaction products. It is found that more intense pyrolysis conditions (higher temperature and pressure or lower flowrate) accelerate the later steps of the process more than the early ones, and that some particular intermediate polyaromatic hydrocarbons (e.g., anthracene) lead more efficiently to carbon than other ones (e.g., acenaphtylene). The pyrolysis mechanism remains essentially the same when methane is diluted with hydrogen.

The infiltration of carbon fiber felts and composites by pyrolytic carbon deposition from propylene

A flow system has been developed to simultaneously obtain data to determine kinetic parameters for the conversion of propylene and the resulting deposition of carbon on carbon fiber substrates. A kinetic scheme is proposed which involves homogeneous initiation of a carbon precursor by a first order reaction of propylene, growth of the precursor by a radical propagation reaction, and ultimately deposition of the precursor on the substrate. The model developed permits the prediction of initial carbon deposition rates based solely on operating conditions. The kinetic parameters determined include an activation energy of approximately 87 kcal/mole for carbon deposition in open structured, carbonized rayon and polyacrylonitrile felts, and a lower activation energy of 68 kcal/mole for a less porous composite substrate which may indicate significant diffusional limitations. The rate of carbon deposition in all cases investigated was found to increase with temperature, residence time, and propylene feed concentration. Also, propylene was found to be converted with an overall order of 1.45, an activation energy of 52.6 kcal/mole, and a preexponential factor of 10 12.8 ml .45/mole .45/s for propylene feeds of 5 to 15%.

Surface Modification of FRS Carbon Dental Implant by Low Temperature Plasma

Surface modification of an FRS carbon dental implant (FRS implant) with a fine Rahmen structure surface, was investigated in order to give wettability to the FRS implant surface. The wettability was given by oxidation of FRS implant surface only. The FRS implant was prepared by the deposition and infiltration of low temperature pyrolytic carbon (LTPC) from cis-1, 2-dichloroetylene. Surface oxidation onto the LTPC plate was carried out by several methods. The LTPC plate surface was not oxidized by reagent oxidation with hot concentrated nitric acid at 95°C. Then the LTPC plate surface was treated by electrolytic oxidation with phosphoric acid, which gave hydrophilic property. But micro-Vickers hardness of the LTPC plate surface decreased from 500 to 290kgf/mm2, while the LTPC plate surface was found to be strongly etched by scanning electron microscopy. So surface oxidation by low temperature plasma of oxygen (13.56MHz, 25W, 10min) was carried out to the LTPC plate. This treatment modified the surface of LTPC plate to highly hydrophilic, and the hardness showed the same value before plasma treatment. Surface condition of the LTPC plate was not changed by this treatment. Accordingly the low temperature plasma treatment is regarded as the best modifying method for the FRS carbon dental implant.

Mechanism of deposition of laminar pyrolytic carbon deposited in a tumbling bed

Pyrolytic carbon was deposited in a tumbling bed under various pyrolysis conditions by varying the process variables such as the speed of rotation of the reactor, total flow-rate, number of bed particles and propane concentration. The structures of pyrolytic carbons were characterized under an optical microscope with polarizing light and a scanning electron microscope. A laminar structure is obtained when the propane concentration is low, a large number of bed particles is provided and the speed of rotation of the reactor is high. Isotropic and columnar structures are also obtained under different pyrolysis conditions. It is suggested that a laminar structure is formed as a result of the continual deposition of gas-borne droplets by eddy current of pyrolysing gas and the surface growth of droplets at deposition sites. The mechanism of deposition of columnar and isotropic pyrolytic carbon is also explained in terms of the suggested mechanism for laminar pyrolytic carbon.

A study on the formation of soot in pyrolytic carbon deposited in a tumbling bed and its effects on mechanical properties

The formation of soot particles during the deposition of pyrolytic carbon in a tumbling bed and its effects on the mechanical properties of the pyrolytic carbon deposited have been investigated under various deposition conditions. The soot particles formed during the deposition were mainly distributed in the form of clusters in the deposit. The fraction of soot particles decreased with the decrease of the bed temperature and the propane concentration, and the increase of the total flow rate and the amount of bed particles. The variation of soot fraction in the deposits with the rotation speed of the reactor tube depended on the effective tumbling action of the substrate. Mechanical properties of Young's modulus, fracture strength and strain energy to fracture, which were measured by three-point bending test, all increased as the fraction of soot particles decreased.