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• W2000724365 abstract "Abstract The molecular organization of the anthracene coke, in the first stages of charring and coking was studied. Anthracene coke was prepared for the French Group of Carbon Studies (GFEC) by charring very pure anthracene crystals at 450°C under argon pressure. The powder was then coked, after crushing, at various temperatures under flow of inert gas (nitrogen or argon) during two hours. The crystallographic results were obtained by comparing the experimental interference functions (Fig. 1) with the calculated ones (Fig. 2). We calculated an interference function, first subtracting the contribution of the (002) reflection. This (002) reflection was calculated assuming a parallel stacking of N macromolecules with a gd 2 mean square-distance and a thermal agitation factor M . We chose the values for the parameters N , gd 2 and M , for the best fitting of the experimental curves with the theoretical ones (Table 1). By subtracting this (002) reflection, we were able to apply the same treatment to all of the ( hk ) diffraction peaks. The assumption made in these calculations was that the macromolecules are planar, rectangular or square with dimensions L × l or L × L , and we used the Warren's method [6]. Figure 2 explains why it is necessary to drop the assumption of isometric molecules and to take a rectangular form in the case of 600°C HTT. The values of L and l giving the best peak by peak fitting are given in Table 2. A comparison between the entire calculated diffractogram and the experimental one is given in Fig. 3 for the 600°C HTT sample. Using the chosen values of the sizes, we can well represent the macromolecules drawn on Fig. 4. The error of stacking and the dispersion of distances between planes increase with the increase of the sizes and originate from the absence of rigid bonds between the monomers giving rise to the mosaic effect. By EPR we observe the usual maximum in the paramagnetic susceptibility for 750°C heat treatment (Fig. 5). This maximum corresponds to 1.5 × 10 20 localized spins/g. The temperature dependence of the paramagnetic susceptibility decreases with HTT between 400°C and 1000°C. On the drawing (Fig. 5), χ p seems to be almost zero at 1200°C. In fact, it is not: χ p is about 0.8 × 10 −8 uemcgs, and although the signal is weak, it is quite observable the linewidth remaining small (0.8 G). At 1500°C HTT, χ p increases to about 5 × 10 −8 uemcgs with a linewidth of 4G. Throughout the full temperature range studied, the lines are narrow and frequently show an anisotropy of the factor of Lande g observable by the dissymmetry of the lines. On Fig. 7 the half of the peak to peak linewidth of the isotropic part of the spectrum is plotted for samples outgassed (200°C, 10 −6 torr, 12 hr) and in air. The very strong oxygen effect observed for HTT 650°C, corresponds to the maximum of the gd 2 mean square distances between the aromatic planes detected by X-ray diffraction. The T 1 , electronic relaxation times were measured by saturation and goniometric technique at various frequencies. Figure 8 shows that for HTT > 700°C the anthracene cokes behave like metals with T 1 ~ T 2 and that the T 1 are frequency independent as long as the frequency used is smaller than the exchange frequency. On Fig. 10 the static magnetic susceptibility χ d or the mean susceptibility χ m , together with the g  and g ⊥ values of the Lande factor are plotted (in detail on Fig. 11). The g values were obtained by comparing the calculated curves with experimental ones (an example is given in Fig. 12 where the sign of the g -anisotropy clearly changes between 900 and 1200°C (HTT)). g  and g ⊥ vary independently one from the other, g  being directly related to the increase of the diameters of the aromatic planes while g ⊥ is related to the coupling between the planes. Recapitulating our results, we can say that by parallel X-ray diffraction and E.P.R. studies, we have followed the first stages of charring and coking of anthracene. At 400°C the material is essentially composed of radical bisanthene molecules which stack in a quite regular manner. At 600°C these radical molecules group together horizontally 3 × 3 to give rectangles. The stacking of these rectangles presents parallelism defects which lead to many open micropores. Finally, at 800°C, the rectangles regroup themselves roughly into 25 A × 25 A squares which have planarity defects and consequently the spaces between the stackings are even more irregular. The electronic properties corroborate this evolution. χ p shows the disappearance of free radicals during polymerisation; at the same time modifications in the relaxation times and in the principal values of the g -factor are occurring. All the properties studied confirm the progressing electronic delocalization in aromatic planes and the buildup of planar systems that will produce a very high quality graphite with further heat-treatment." @default.
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• W2000724365 date "1977-01-01" @default.
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• W2000724365 title "Etude des semi-cokes d'anthracene: Relation entre leurs proprietes structurales et electroniques" @default.
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• W2000724365 doi "https://doi.org/10.1016/0008-6223(77)90002-1" @default.
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