Temperature for graphitization

Fig. 13. Chemical erosion yield as a function of temperature for graphite.

The limiting temperature for graphite use in fusion systems is defined by tliermal sublimation (--1500-2000°C). However, a process which is very similar to thermal sublimation (in cause and in effect) appears to define the current temperature limit. This phenomenon, which is known as radiation enhanced sublimation (RES), is not clearly understood but dominates above a temperature of about 1000°C and increases exponentially with increasing temperatme. 

Graphitization processes are carried out at 2600 to 3000°C in inert atmospheres in direct or indirect processes. In direct processes (e.g. Acheson and Castner processes) the carbon articles between the electrodes are heated, either directly or indirectly with resistive materials in between, and the required temperature for graphite formation attained by resistive heating. In indirect processes there is no physical contact between the energy source and the carbon article. 

Figure 2. Rate of oxidation vs. surface temperature for graphite rods and graphite pellets containing 0.0IM cupric acetate...

Figure 3. Rate of gasification vs. temperature for graphite rods exposed to carbon dioxide discharge...

The pyrolysis of CellNFs is expected to result in the formation of CNFs. Due to their small diameter, cellulose-based CNFs may require lower temperature for graphitization [72]. However, little research has been reported on the production of nanoscale CFs by pyrolysis of CellNFs. It is expected that the molecular and morphological properties of precursors strongly affect those of the pyrolyzed carbon material. Ishida et al. investigated the carbonization of freeze-dried bacterial and tunicate CellNFs and found that the carbon residue retained its fibrous morphology by using HCl as... 

A typical feature of metals is that the lower the temperature the lower is the resistivity (Figure 16.1). For gold, silver, and copper metal, resistivity ranges between 10" -3 X 10" Q. cm (10 -3 X 10 Q m) at room temperature. For graphite the resistivity is 0.3 Q, cm. The electron-nuclear (electron-phonon) interactions are responsible for the resistivity. 

FIGURE 7.13 Line tension versus temperature for graphite-water and BN-water systems. (Adapted from Dutta, R. C., Khan, S., and Singh, J. K., Fluid Phase Equilibria, 302, 310, 2011.)... 

Carbon, Carbides, and Nitrides. Carbon (graphite) is a good thermal and electrical conductor. It is not easily wetted by chemical action, which is an important consideration for corrosion resistance. As an important stmctural material at high temperature, pyrolytic graphite has shown a strength of 280 MPa (40,600 psi). It tends to oxidize at high temperatures, but can be used up to 2760°C for short periods in neutral or reducing conditions. The use of new composite materials made of carbon fibers is expected, especially in the field of aerospace stmcture. When heated under... 

Mechanical Properties. The hexagonal symmetry of a graphite crystal causes the elastic properties to be transversely isotropic ia the layer plane only five independent constants are necessary to define the complete set. The self-consistent set of elastic constants given ia Table 2 has been measured ia air at room temperature for highly ordered pyrolytic graphite (20). With the exception of these values are expected to be representative of... 

Electrothermal vaporization can be used for 5-100 )iL sample solution volumes or for small amounts of some solids. A graphite furnace similar to those used for graphite-furnace atomic absorption spectrometry can be used to vaporize the sample. Other devices including boats, ribbons, rods, and filaments, also can be used. The chosen device is heated in a series of steps to temperatures as high as 3000 K to produce a dry vapor and an aerosol, which are transported into the center of the plasma. A transient signal is produced due to matrix and element-dependent volatilization, so the detection system must be capable of time resolution better than 0.25 s. Concentration detection limits are typically 1-2 orders of magnitude better than those obtained via nebulization. Mass detection limits are typically in the range of tens of pg to ng, with a precision of 10% to 15%. 

Wei and Robbins [10] have reviewed much of the work performed on the thermal physical properties of CBCF. Fhe emissivity parallel to the fibers was 0.8 over the temperature range from 1000 to 1800 °C. This value is higher than the emissivity of c-direction pyrolytic graphite (0.5-0.6), but is close to values for graphite and dense carbon-carbon composite (0.8-0.95). 

A key factor in the suitabihty of cokes for graphite production is their isotropy as determined by the coefficient of thermal expansion. After the calcined coke was manufactured into graphite, the axial CTE values of the graphite test bars were determined using a capacitance bridge method over a temperature range of 25 to 100°C. The results are summarized in Table 24. Also included in the table are bulk density measurement of calcined cokes and the resistivity values of their graphites. 

---