ABSTRACTS 381

94. Contribution B la cin&ique de la grapbitation

A. Pacault, A. Marchand, M. Mazza, H. Gasparoux, A. Forchioni et J. Amiell (Lu~o~ut~~re de Chimies Physigue, Facultk des Sciences de Bordeaux). Un carbone est le plus souvent caracterist par la plus haute temperature a laquelle il a CtC trait6 (H.T.T.), sans que soit pris en consideration le temps pendant lequel il a CtC trait6 a cette H.T.T. Or, on montre que suivant la valeur de H.T.T., il faut parfoisplusieurs semaines pour que soit atteint un &at d’equilibre. La cinetique de transformation d’un carbone a CtC suivie, par mesure de sa susceptibilite X, de son coefficient Hall A, et de son degre de graphitation g. On donne en fonction du temps, les courbes representant R, A, X (mesures g 298°K) de carbones preparb a H.T.T.= 1520, 1620,1720,1820,1920,2025,2230,2850”C. Signalons par exemple, que la susceptibilite d’un carbone (H.T.T.= 1520”) passe de -1,66.10-8 (lH30 de traitement) a sa valeur limite - 3,40.10-6 (120H de traitement), et que le degre de graphitation g d’un carbone (H.T.T. = 2025”) passe de 0,40 (lH30 de traitement) B 0,67 (6H de traitement). En Ctudiant la variation thermique de la susceptibilite magdtique, on donne l’evolution en fonction du temps des parametres X0,&, 0. (Voir E. Poquet et A. Pacault-Mise au point sur le magn~tisme des carbones). Les courbes X, A, g= f(t) ant et& analystes et la cinetique de transformation est sensiblement du premies ordre. On determine les constantes de vitesse et les energies d’activation, et on constate qu’il existe au moins deux domaines distincts de graphitation, correspondant l’un a une croissance biperiodique (H.T.T. > 2000°C> et l’autre a une organisation simultanee dans fes troid dimensions H.T.T.>2000°C).

95. Investigations about the influence of graphitization time and temperature on the ash Content of electro graphite

E. Wege (~~~gsdo~~ u/e& GmH, Bud Godesb~g-~ehZem, Gurneys. Carbon products which are supposed to insure a stronger polishing action are frequently made of raw materials with a high ash content. Since, during the graphitization process, a large portion of the ash vaporizes, the knowledge of the influence of the grapbitizing conditions on the ash content is of importance. We have analysed cube-like samples of grade “VR680” of Ringsdorff Werke GmbH, which contains a larger quantity of natural graphite. The ash content of this grade amounts to 4.98”/“; the most important minerals are mica, orthosilicates, pyrrhotite and quartz. The samples were placed into a carbon-tube resistance furnace operating at a reaction temperature, and were taken out after pre-defined time periods for an ash analysis. The flow speeds of the protective gas as well as the size of the cubes had no influence on the ash content within the investigated range. The measurements disclose an exponential reduction of the de-ashing speed with time. For the ash content (c) as compared with the time (t), the following equation applies: c=co exp (+‘t). The speed constant (K) amounts to 0.0052 min-1 at 15OO”C, and 0.132 min-l at 2700°C. If we apply (0) for the ash content of the grain surface, then the diffusion coefficient of the transport of substance within the grain can be estimated if the grain size is known. Thus, a microscopically ascertained median grain diameter (a)=30.4 would result in D150,,oc 3.3 x 10-10cm2/seC, D2,000c 8.5 x lV9cmZ/sec. The diffusion coefficient, as ascertained by J. Bromley and N. R. Large during tests with fission products, lies within the same order of magnitude. The rela- tiveIy small activation energy of about 28 kcaljmol leads to the assumption that, in addition to grain diffusions, border-surface diffusions also become active.

96. Studies of radiation in single crystal graphite

C. Baker and A. Kelly (Metallurgy Department, Cambridge University, Cambridge, England). Neutron irradiation to a dose of N 10ls nvt (Ni) results in a fourfold increase in the elastic modulus of single crystal graphite. The increase is attributed to a reduction in the anelastic strain by the radiation induced defects. Electron microscope studies of the graphite irradiated in this range show characteristic dislo- cation configurations which afford indirect evidence for dislocation pinning. Quantitatively the damage concentration is sufficient to produce the necessary effects. Annealing after irradiation leads to full recovery of the modulus change and the temperature of recovery depends upon the neutron dose. The