Carbon specimens

The absorption property exhibited by active carbon certainly depends on the large specific surface area of the material, though an interpretation that it is based solely on this is incomplete. This is borne out by the fact that equal amounts of two activated carbon specimens, prepared from different raw materials or by different processes and having the same total surface area, may behave differently with regard to adsorption. Such differences can be partly explained in terms of the respective surface properties of the carbon samples and partly in terms of their relative pore structure and pore distribution. Every activated carbon particle is associated with at least two types of pores of distinctly different sizes. They are the macropores and the micropores. The macropores completely permeate each particle and act as wide pathways for the diffusion of material in and out of carbon, but they contribute very little to the total surface area. The micropores are more important since they... 

Van der Waals forces between solid/gas interactions and the liquid/gas surface tension forces represent the limiting cases, but in general both the forces competitively affect the adsorption process. Therefore, in determining the surface fractal dimension of the carbon specimen, it is very important to use appropriate relation between C and dFSF. According to Ismail and Pfeifer,111... 

Predicted Relative Rates of Carbon Gasification in Reaction Zone III for Similar Shapes of Carbon Specimens and Constant Linear Gas Flow Rate... 

It is of interest to note that several workers (99, 116) have assumed AI2/to represent the rate of reaction of a carbon specimen only when the reaction is proceeding entirely on the external surface. The above reasoning shows that AR/At can be a constant and represent the over-all rate of reaction when the reaction is occurring internally and the utilized surface area is far greater than the external surface area. Graham and co-workers (116), studying the carbon-steam reaction under high-velocity conditions. 

Figure 1. Plots of differential pore volume against pore diameter calculated from the N2 gas adsorption isotherms obtained from meso/macroporous carbon specimens I (-0-), II (- -), and III (-A-) using Barrett-Joyner-Halenda (BJH) method. Reprinted with permission from G. -J. Lee and S. -I. Pyun, Carbon, 43 (2005) 1804. Copyright 2005, with permission from Elsevier.

Figure 7 demonstrates on a logarithmic scale the dependence of perimeter P on area A of the pores obtained from the binary TEM image of CAS30 in Figure 6b. The (log P - log A) plots obtained from the carbon specimen displayed two straight lines with different slopes that can be divided into region I and II, indicating multifractal geometiy of the carbon specimen. The individual surface fractal dimensions in regions I and II were determined from Eqs. (26) and (27) to be 2.08 + 0.018 and 2.72 + 0.046, respectively. The transition area Ab from region I to II were determined to be 108 nm2, which corresponds to the pore diameter of 12 nm based upon spherical pore shape.

Recently, Lee and Pyun17 have intensively investigated the effect of microcrystallite structures on electrochemical characteristics of the mesoporous carbon electrodes for EDLC. They effectively controlled the microcrystallite sizes of the carbon specimens in a- and c-axis direction, i.e., Za (parallel to the basal... 

Figure 13. (a) Nyquist plots of the impedance spectra and (b) plots of reduced capacitance Cred vs. frequency CO experimentally measured on carbon specimens A (-o-), B (- -), and C (-A-) at an applied potential of 0.2 V (vs. SCE) in a 30 wt.% H2S04 solution. Here, the solution resistance was subtracted from the measured impedance spectra. The reduced capacitance in (b) was determined from the normalization of the capacitance with respect to the value of the capacitance calculated from the impedance spectra at 10 Hz. Reprinted from Ref. 17, Copyright (2006), with permission from Elsevier. 

Figure 15 presents the plots of the rate capability ycap against the scan rate v calculated from the CVs experimentally measured on carbon specimens A, B, and C.17 The higher the value of was, the lower exhibited the rate capability yoap, regardless of the scan rate v. Besides, as LJL, increased, the rate capability ycap... 

The Mass of the Electrode m, the BET Surface AreaABET, the Total Pore Volume Vtot, the Average Pore Diameter Z)ave, the Size of the Microcrystallite La (Parallel to the Basal Plane), Lc (Parallel to the Edge Plane), the Ratio of LJL the CPE Exponent a and the Specific Double-Layer Capacitance Cd for Carbon Specimens A, B and C. Reprinted from Ref.17, Copyright (2006), with... 

Figure 14. Dependence of the current density difference Ai between anodic and cathodic peak currents of the cyclic voltammograms (CVs) on the scan rate v for carbon specimens A (o), B ( ), and C (A). Reprinted from Ref. 17, (2006), with permission from Elsevier.

Figure 8.6. SEM micrograph taken from above of an Al/vitreous carbon specimen cooled from 1100K. showing an AI4C3 layer close to the triple line (Landry and Eustathopoulos 1996b).

Fig. 4.4.3. The effect of two types of carbon specimen holders for carbon grids on continuous white X-ray emission. (Saka and Thomas 1982).

Langmuir (JJ[) determined the melting point as 3540 K from intrinsic brilliance measurements this was later corrected to 3655 30 K by Jones et al. (14), and Pirani and Alterthum ( 5) from pyrometer measurements on a black body hole found 3660 60 K. Using the same technique Zalabak (1 ) reports 3680 K on a low carbon specimen. He reports a decrease of the melting point with increasing carbon content. Rudy and Windisch (j ) report a melting point of 3696 20 K. The value adopted is 3680 K 20 K. 

Gam and Flaschen have used the method to detect a number of polymorphic phase transformations. Fujita and Damask studied the precipitation of carbon in iron which caused an increase in conductivity. The samples were carburised at high temperature, quenched in brine at 0° C and stored under liquid nitrogen. All resistance measurements were made at liquid nitrogen temperature, and compared with a well aged iron/carbon specimen and a standard resistance. Jaky... 

Polymer mitrix Rav 7 S - polyallyl di glycol carbonate Specimen jilmvi 1cm. ihicbea I. it mm Application method. thermal transfer... 

Figure 12.18 Comparison of weight loss of infiltrated and non-infil-trated carbon/carbon specimens.

Table 12.3 BET Surface Areas (Abet) and Micropore (Vmi) and Mesopore (V meso) Volumes of Virgin and Metal-Supported Activated Carbon Specimens...

After five months, in the carbonated specimens protected at 10 mA/m, the potential difference between the two activated titanium electrodes increased significantly and reached a value up to 200 mV, suggesting that the atkahnity produced by the cathodic reaction at the steel surface induced locaHzed reaUsaUzation of the concrete in the vicinity of the rebar. For the other specimens (i. e. in carbonated concrete at 2 and 5 mA/m ) realkalisation of concrete and repassivation of steel did not occur even after five years of testing [45]. In the same study, it was found that the application of a start-up current density of 70 mA/m for 1 month proved to be an effective way for achieving repassivation of steel in carbonated concrete [46]. The tests showed that, once repassivation is induced, even a current density of 5 mA/m is sufficient to guarantee protection. 

Those results were obtained for BaCOj but Mandayo et aL (2011) also relate the influence of humidity in a BaliOj-CuO layer to the same reaction. In their case, sensitivity of BaliOj-CuO to CO2 is enhanced in the presence of water. According to the high-temperature reactions, CO2 detection needs carbonate specimens, whose presence was demonstrated by the authors in their BaTi03-Cu0 thin films. The sensitivity enhancement measured in humid air can be explained because the presence of water makes the second of the two high-temperature reactions to take place, so an extra amount of carbonate specimens increases the sensor response. When tests are carried out in humid air, both reactions take place, so sensitivity is higher than in dry air, where only one reaction happens. 

---