Carbon vapor deposition

High process temperatures generally not achievable by other means are possible when induction heating of a graphite susceptor is combined with the use of low conductivity high temperature insulation such as flake carbon interposed between the coil and the susceptor. Temperatures of 3000°C are routine for both batch or continuous production. Processes include purification, graphitization, chemical vapor deposition, or carbon vapor deposition to produce components for the aircraft and defense industry. Figure 7 illustrates a furnace suitable for the production of aerospace brake components in a batch operation. 

Poly(m-phenylene isophthalamide) derived carbon fibers can be activated by carbon vapor deposition of benzene. The activated carbon fibers are suitable as molecular sieves for air separation. Carbon fibers can be obtained from the aramid by pyrolysis at 750-850°C. The pyrolysis may take place in Ar or CO2. 

Property Glassy carbon Vapor- deposited carbon LTI carbon LTI carbon with silicon (5-12%)... 

A novel approach in this field was the growing carbon nanofibers on the surface of conventional carbon fibers via carbon vapor deposition in the presence of a minuscule amount of metal catalyst [180], The presence of the carbon nanostructures on the carbon fiber surface was found to enhance the surface area of perform from 2 m /g up to over 400 m2/g and consequently increased the interfacial bonding between the fiber and the matrix. 

Manso et studied the formation of CMS by carbon vapor deposition (CVD) over activated carbons from four different rank coals. The deposition of carbon was carried out by pyrolyzing benzene vapors at 725°C. This produced gradual closing of the micropores, due to the formation of constrictions at their entrances. As a result the MSC with a narrow micropore-size distribution around 0.35 to 0.5 nm were obtained. Samples with diameters smaller than 0.33 mn obtained by a high degree of deposition were able to separate O2/N2 and CO2/CH4 mixtures. 

The uniqueness and versatility of carbonaceous porous materials is demonstrated by Mukai et al. (2004) in their attempt to reduce the phenomenon of irreversibility of the LIB. As indicated above, irreversibility is associated with the formation of solid electrolyte films on surfaces of carbons by an irreversible reaction of lithium ions with the electrolytes. For the isotropic porous carbons (not amorphous carbons as quoted by Mukai et al., 2004), the electrolyte film is formed preferentially in the entrances to the porosity (mainly microporosity). Should it be possible to prevent this deposition, then the irreversible component of battery performance could be reduced. It is established that increasing the heat treatment of carbons (normally beyond about 800 °C) decreases the pore dimensions, but at the same time there is reduction in volume of porosity which is available for lithium entry. Quite separately, Suzuki et al. (2003) report on the impossibility of bringing about a meaningful reduction in the irreversible component, maintaining the reversible component, by changing the porosity of the material. That is, an improvement automatically creates a deterioration. The use of an approach of carbon vapor deposition (as for pyrolytic carbons) has been tried whereby carbon is deposited in the entrances to the microporosity. There is no overall change to carbon structure. This method was successful but applications on an industrial scale are expensive. 

In addition to the conventional carbon vapor deposition method, our research group has used two additional procedures for the preparation of CMS i) controlled uncatalysed gasification of chars obtained from lignocellulosic precursors and ii) mild oxidation of a char and subsequent controlled removal of oxygen surface groups (this second procedure can also be applied to a previous CMS with wider micropore width, to reduce the pore width). 

Titanium carbide may also be made by the reaction at high temperature of titanium with carbon titanium tetrachloride with organic compounds such as methane, chloroform, or poly(vinyl chloride) titanium disulfide [12039-13-3] with carbon organotitanates with carbon precursor polymers (31) and titanium tetrachloride with hydrogen and carbon monoxide. Much of this work is directed toward the production of ultrafine (<1 jim) powders. The reaction of titanium tetrachloride with a hydrocarbon-hydrogen mixture at ca 1000°C is used for the chemical vapor deposition (CVD) of thin carbide films used in wear-resistant coatings. 

Carbon Composites. Cermet friction materials tend to be heavy, thus making the brake system less energy-efficient. Compared with cermets, carbon (or graphite) is a thermally stable material of low density and reasonably high specific heat. A combination of these properties makes carbon attractive as a brake material and several companies are manufacturing carbon fiber—reinforced carbon-matrix composites, which ate used primarily for aircraft brakes and race cats (16). Carbon composites usually consist of three types of carbon carbon in the fibrous form (see Carbon fibers), carbon resulting from the controlled pyrolysis of the resin (usually phenoHc-based), and carbon from chemical vapor deposition (CVD) filling the pores (16). 

Carbon Composites. In this class of materials, carbon or graphite fibers are embedded in a carbon or graphite matrix. The matrix can be formed by two methods chemical vapor deposition (CVD) and coking. In the case of chemical vapor deposition (see Film deposition techniques) a hydrocarbon gas is introduced into a reaction chamber in which carbon formed from the decomposition of the gas condenses on the surface of carbon fibers. An alternative method is to mold a carbon fiber—resin mixture into shape and coke the resin precursor at high temperatures and then foUow with CVD. In both methods the process has to be repeated until a desired density is obtained. 

Of the many forms of carbon and graphite produced commercially, only pyrolytic graphite (8,9) is produced from the gas phase via the pyrolysis of hydrocarbons. The process for making pyrolytic graphite is referred to as the chemical vapor deposition (CVD) process. Deposition occurs on some suitable substrate, usually graphite, that is heated at high temperatures, usually in excess of 1000°C, in the presence of a hydrocarbon, eg, methane, propane, acetjiene, or benzene. 

Easily decomposed, volatile metal carbonyls have been used in metal deposition reactions where heating forms the metal and carbon monoxide. Other products such as metal carbides and carbon may also form, depending on the conditions. The commercially important Mond process depends on the thermal decomposition of Ni(CO)4 to form high purity nickel. In a typical vapor deposition process, a purified inert carrier gas is passed over a metal carbonyl containing the metal to be deposited. The carbonyl is volatilized, with or without heat, and carried over a heated substrate. The carbonyl is decomposed and the metal deposited on the substrate. A number of papers have appeared concerning vapor deposition techniques and uses (170—179). 

Sihcon carbide fibers exhibit high temperature stabiUty and, therefore, find use as reinforcements in certain metal matrix composites (24). SiUcon fibers have also been considered for use with high temperature polymeric matrices, such as phenoHc resins, capable of operating at temperatures up to 300°C. Sihcon carbide fibers can be made in a number of ways, for example, by vapor deposition on carbon fibers. The fibers manufactured in this way have large diameters (up to 150 P-m), and relatively high Young s modulus and tensile strength, typically as much as 430 GPa (6.2 x 10 psi) and 3.5 GPa (507,500 psi), respectively (24,34) (see Refractory fibers). 

Graphite was tised as substrate for the deposition of carbon vapor. Prior to the tube and cone studies, this substrate was studied by us carefully by STM because it may exhibit anomalotis behavior w ith unusual periodic surface structures[9,10]. In particular, the cluster-substrate interaction w as investigated IJ. At low submonolayer coverages, small clusters and islands are observed. These tend to have linear struc-tures[12j. Much higher coverages are required for the synthesis of nanotubes and nanocones. In addition, the carbon vapor has to be very hot, typically >3000°C. We note that the production of nanotubes by arc discharge occurs also at an intense heat (of the plasma in the arc) of >3000°C. 

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