OTHER FORMS OF CARBON AND GRAPHITE

Carbon—graphite foam is a unique material that has yet to find a place among the various types of commercial specialty graphites. Its low thermal conductivity, mechanical stabiHty over a wide range of temperatures from room temperature to 3000°C, and light weight make it a prime candidate for thermal protection of new, emerging carbon—carbon aerospace reentry vehicles. 

The open ceU stmcture of carbon foam with its greater than 90% porosity and chemical inertness at temperatures below 500°C suggests its use as a filtration media for corrosive Hquids and a dispersant for gases. 

Pyrolytic graphite was first produced in the late 1800s for lamp filaments. Today, it is produced in massive shapes, used for missile components, rocket nozzles, and aircraft brakes for advanced high performance aircraft. Pyrolytic graphite coated on surfaces or infiltrated into porous materials is also used in other appHcations, such as nuclear fuel particles, prosthetic devices, and high temperature thermal insulators. 

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. 

The largest quantity of commercial pyrolytic graphite is produced in large, inductively heated furnaces in which natural gas at low pressure is used as the source of carbon. Deposition temperatures usually range from 1800 to 2000°C on a deposition substrate of fine-grain graphite. 

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Processing of baked and graphitized carbon. Properties of manufactured graphite, Appbcations of baked and graphitized carbon. Other forms of carbon and graphite. 

Baked and Graphitized Products, Uses" under "Carbon" in ECT2nd ed., VoL 4, pp. 202—243, by W. M. Gaylord, Union Carbide Corp. "Other Forms of Carbon and Graphite Carbon" under "Carbon (Carbon and Artificial Graphite)" in ECT 3rd ed., Vol. 4, pp. 628—631, by R. M. Bushong, Union Carbide Corp. 

CARBON - CARBON AND ARTIFICIALGRAPHITE - OTHER FORMS OF CARBON AND GRAPHITE] (Vol 4)... 

Market for Carbon and Graphite Products. Table 1.3 lists the estimated markets for the various forms of carbon and graphite reviewed in Chs. 5 to 10. The oid and well-estabiished industry of molded carbon and graphite stiii has a major share of the market but the market for others such as carbon fibers is expanding rapidiy. 

Fullerenes are described in detail in Chapter 2 and therefore only a brief outline of their structure is presented here to provide a comparison with the other forms of carbon. The C o molecule, Buckminsterfullerene, was discovered in the mass spectrum of laser-ablated graphite in 1985 [37] and crystals of C o were fust isolated from soot formed from graphite arc electrodes in 1990 [38]. Although these events are relatively recent, the C o molecule has become one of the most widely-recognised molecular structures in science and in 1996 the codiscoverers Curl, Kroto and Smalley were awarded the Nobel prize for chemistry. Part of the appeal of this molecule lies in its beautiful icosahedral symmetry - a truncated icosahedron, or a molecular soccer ball, Fig. 4A. 

At high pressures, diamond is the stable form of carbon, since it has a higher density than graphite (3.51 vs 2.26 g/cm3). The industrial synthesis of diamond from graphite or other forms of carbon is carried out at about 100,000 atm and 2000°C. 

Dissolution procedures are described for gram samples of graphite or pyrolytic carbon, milligram samples of irradiated fuel particles, and for more readily oxidised forms of carbon, such as charcoal. The first two methods involve heating the samples with mixtures of 70% perchloric and 90% nitric acids (10 1), and must only be used for graphite or pyrolytic carbon. Other forms of carbon must not be oxidised in this way (to avoid explosions), but by a preliminary treatment with nitric acid alone and in portions. 

Graphite produces the large beam currents desirable in electrostatic accelerator dating of radiocarbon. However, samples to be dated can be converted to other forms of carbon with less effort, and still provide satisfactory results. 

Fullerenes are the third natural form of carbon. These have been found to exist in interstellar dust and in geological formations on Earth, but only in 1985 did Smalley, Kroto and co-workers discovered this class of carbon solids and their unusual properties [447, 448]. It has been shown that Ceo, the most common fullerene, could be transformed under high pressure into the other forms of carbon, diamond, and graphite [449] or, at moderately high pressures and temperatures, into new various metastable forms [450 53]. Ceo crystals, fullerites, have/cc structure with weak van der Waals interactions. This structure is stable at ambient temperature up to 20 GPa and at ambient pressure up to 1800 K [454, 455]. 

If you were asked to produce at ordinary temperatures a sample consisting of the free atoms of an element, your only choice would be a Group 18 element, one of the noble gases. All the other elements occur with their atoms linked together in some way. The nonmetallic elements exist as molecules, such as the diatomic species H2, N2, 02, F2, Cl2, Br2, and I2, and the polyatomic species P4 and S8. Elements near the border between metals and nonmetals can form solids with an extended network of atoms, such as the graphite or diamond forms of carbon and crystalline silicon. There are also countless examples of diatomic, polyatomic, and extended network compounds between different nonmetallic elements, including the millions of organic compounds. 

The standard enthalpy of formation, AHf°, of a substance is the standard reaction enthalpy for the formation of a substance from its elements in their most stable form. (Phosphorus is an exception white phosphorus is used because it is much easier to obtain pure than the other, more stable allotropes.) Standard enthalpies of formation are expressed in kilojoules per mole of the substance (kj-mol-1). We obtain AHf for ethanol, for instance, from the thermochemical equation for its formation from graphite (the most stable form of carbon) and gaseous hydrogen and oxygen ... 

Well-characterized vanadium carbide thin films can be prepared by exposing a clean V(110) surface to ethylene or 1,3-butadiene at 600 K. The formation of vanadium carbide, rather than other forms of carbon-containing species such as graphite or carbonaceous overlayers, was confirmed by the characteristic AES and NEXAFS spectra.4 The stoichiometry and average thickness of the thin carbide films can be estimated by... 

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