Carbon materials heat-treatment temperatures

Polyacene is classified as a material which does not belong to either soft or hard carbons [84], It is also made by heat-treatment of phenol resin. As the heat-treatment temperature is lower than about 1000 °C, polyacene contains hydrogen and oxygen atoms. It has a conjugated plane into which lithium ions are doped. It was reported that the discharge capacity of polyacene is more than 1000 mAhg. However, there are no practical lithium-ion batteries using polyacene. 

Depending on the precursor and the heat-treatment temperature, the carbonaceous materials discussed so far contain heteroatoms in addition to the prevailing carbon atoms. Even highly crystalline graphite is saturated with heteroatoms at dislocations in the crystallites and at the edges... 

The parent materials differ from each other in many aspects the differences are being related both to their origin (coal or pitch) and heat treatment temperature. Clearly, coal should be classified as a polymeric type precursor while the others, such as carbonaceous precursors of relatively low, except for AC, carbonization degree. Specific of pitch-derived materials is distinctly lower mineral matter and heteroatoms content. Anisotropic appearance with predominating flow type texture proves the superior extent of structural ordering in pitch-derived materials. 

FIG U RE 12.2 Typical voltage profiles from carbon materials with different heat-treatment temperatures and different structural variations. The profiles were obtained at the second cycle, and (a) and (b) show discharge and charge cycles by using graphitizable carbon heat-treated at 3000°C, graphitizable carbon heat-treated at 2000°C, and nongraphitic carbon obtained at 700°C. (Reprinted from Endo, M., et al., Carbon, 38, 183, 2000. With permission.)... 

Figure 10. Examples of partially graphitizable carbonaceous materials (asphaltenes), ranging from Pmax 0 (non-graphitizing carbon ) with 3QQ2min. 3.44 A, to P x (highly graphi-tizing carbon) with HQQ2min. 3.36 A. qqo is plotted as a function of heat treatment temperature (25,3/).

The carbonization of natural or industrial hydrocarbon mixtures annually produces about 2.2 billion tons of solid carbon which, depending on the final heat treatment temperature, is called coke or graphite. A wide range of premium carbons that are not based directly on coal, produced by plastic phase pyrolysis of fusible or liquid isotropic hydrocarbons, constitute about one percent (22 million t/a) of the total solid carbon material produced included are ... 

The mechanical properties as a function of heat treatment temperature are shown in Figure 9 for fibers prepared by the Kyukoshi method. Fiber heat-treated to 2000°C or higher have strengths above 3GPa (435 kpsi) and tensile moduli of the order of 500 GPa (72 Mpsi). The present authors now believe that, in the near future, it will be possible to produce carbon fiber of equivalent properties by selection of suitable raw pitch materials and by development of specialized pretreatment procedures for the pitch to replace the extensive hydrogenation technique described here. 

The char precursors were carbonized at 823 K for 90 minutes under a nitrogen pressure of 0.68 MPa in tubing bomb reactors. The carbonaceous residues were subsequently heat treated at 1273 K for 1 hour under argon in a tube furnace to drive off residual volatile matter. The carbonization was conducted to increase char yield direct heat treatment of the precursors would result in very low char yeilds. The high heat treatment temperature was chosen to ensure that at the lower temperature used in the oxidation experiments, there would be no volatile material, thus ensuring that the reaction was heterogeneous. 

Organic materials undergo pyrolytic decomposition when heated in an inert atmosphere. Polyaromatic ring structures are developed in the early stages of carbonization. As the heat-treatment temperature (HT1) is increased the solid char or coke begins to acquire short-range order with the formation of distorted graphitic lamellae. In addition, localized and anisotropic densification leads to the development of free space between the lamellae. 

The H grade of austenitic steel, which may be specified for elevated temperature use, has a controlled carbon content and heat-treatment temperature designed to insure optimum elevated-temperature properties. The significance of this grade of austenitic steel is not always understood by European fabricators and steelmakers and for such material it may be necessary, in European purchase, to spell out the specification requirements for carbon content and heat treatment. 

The ability to produce high suriace area carbons with phosphoric acid in an inert atmosphere depends primarily on the structure of the starting material, the acid-to-precursor ratio and the heat treatment conditions (time-temperature profile and maximum heat treatment temperature). As with ZnCl2 activation, wood and some agricultural by-p roducts (such as almond shells and olive stones) as well as carbonaceous materials with a high volatile content are the precursors most commonly used in this method of activation. 

Fig. 13a) illustrates the difference in the surface coverage of a porous material with molecules of different sizes. In Fig. 13b), changes in fractal dimension with heat treatment temperature are shown for two activated carbon fibers derived fixjm different precursors. With inerting heat treatment temperature, Da approaches a value of 2.0, which corresponds to a smooth surface of pores. 

Today, we know that it is possible to produce these Fe- and/or Co-based electrocatalysts by adsorbing related metal-N4 macrocycles on a carbon support and heat-treating this material at about 600°C, the optimum temperature in terms of activity. More stable, but less active catalysts are, however, obtained for heat-treatment temperatures > 800 C. Similar catalysts may also be obtained with cheaper metal and nitrogen precursors (like metal salts and ammonia, for instance). For aU these catalysts, it is generally now believed that two types of catalytic sites are obtained simultaneously, but not in the same proportions. 

Figure 7.8 Effect of final heat treatment temperature on tensile strength of PAN, mesophase and isotropic pitch fibers. PAN A MP1 [41] MP2 IP [42], Source Reprinted from MatsumotoT, Mesophase pitch and its carbon fibers, Pure AppI Chem, 57(11), 1533,1985, Shen Z, Guo H et al, Carbon and Carbonaceous Composite Materials Structure-Property Relationship, Abstr and Proc, Malenovice, Czech Republic, 31, Oct 10-13, 1995.

---