88. A comparison of gas flow and self-diffusion in porous graphite

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  • ABSTRACTS 379

    par lair ou Ioxygene, en milieu set ou humide, suivent les memes Iois: proportion~aIit~ avec la surface developpee de lenchantillon et recouvrements plus faibles dans le cas des noirs graphites. Dautre part nous avons observe lexistence de micro pores dans nos Cchantillons et nous nous sommes demand& sil existait une relation entre ce fait et les precedents. Nous avons CtudiC par la methode de condensation capillaire (B.J.H.) la repartition de ces micropores suivant leurs diametres avant et aprPs oxydation a divers taux dusure, puis linfluence, sur cette repartition, de traitements sous vide a lOOO-1300~ et 37OOC, enfin nous avons mesure les vitesses doxydation aprb ces divers traitements. Nous avons ainsi pu CtabIir sur tout produit oxyde une evolution de micropores pendant le traitement thermique, evolution qui influence la vitesse de loxydation. La cinetique de Ioxydation de ces carbones est done like de facon certaine B lexistence de ces micropores.

    88. A comparison of gas flow and self-diffusion in porous graphite

    K. R. Weller (~~~v~~~~ ctf Adelaide, Ade~~de~ South A~trQ~ia) and H. Watts (South A~t~a~~a~ Iputitute of ~&n~ology). The permeability coefficient K, for gas flowunder a pressuregradient, and the self-diffusion coefficient D, under zero pressure gradient, have been measured for the transport of xenon and krypton through a porous graphite. Measurements were made on the same graphite, in the same cell, using xenon-133 and krypton-85 isotopes as tracers, in the pressure range 25-400 mm of mercury. At the high end of the pressure range, the value of Ii: is much larger than the value of D, while at low pressures, K and D tend to the same value. Flow me~uremen~ are sometimes used as a basis for the comparison of the behaviour of different porous solids toward diffusing gases. Our results indicate that this procedure is sometimes justified, but at other times may lead to serious errors.

    $9. Stress induced graphitization of pyrolytic graphite

    R. H. Bragg, D. D. Crooks, R. Fenn, Jr., and M. Hammond (Lockheed Research Lizbo~ato~y, Palo Alto, Cu~z~o~n~a). Detailed X-ray diffraction studies have been made of the effects of heat treatment with concurrent strain on the crystal structure and texture of pyrolytic graphite. There is virtually no ductility below 26OOC, but above this temperature the tensile strength and elongation parallel to the basal planes increase with temperature and strains up to 150 per cent are observed at 3000C. Concurrent straining up to about 8 per cent enhances the stacking order of adjacent layers in crystallites (graphi- tization), produces a sharper (002) texture, and promotes crystallite growth, but larger strains (up to 119 per cent) have little additional effect except on crystallite growth. The grap~tiza~on temperature is lowered by about 150C. An orientation dependence of layer stacking order reported previously was observed in these experiments .* The crystallites tilted the largest amount relative to the deposition surface are the least graphitized, but these are also most affected by strain. These observations show that mechanical stress produces some effects similar to those produced by high temperatures, and are evidence in support of a thermal stress-relief mechanism of graphitization.

    *R. H. BRAGG and C. R/I. PACKER, Nature 195, 1080 (1962).

    90. The graphitization of pyrolytic carbons

    D. B. Fischbach (Jet Pro@ulsion Laboratory ,* California Institute of Technology, Pasadena, California). When pyrolytic carbons (PC) are heat treated at temperatures exceeding their deposition temperatures, they undergo a complex graphitization transformation which involves layer ordering, crystallite growth, increase in preferred orientation, and relief of internal stresses. This transformation has been studied in a number of pyrolytic carbons as a function of both isothermal and isochronal heat treatment at temperatures up to 3200C. The carbons used were deposited essentially isothermally at temperatures ranging from 1600 to 2300C. Magnetic susceptibility, X-ray crystal structure and, in some cases, electrical resistivity, magnetoresistance and Hall effect measurements were made. The diamagnetic susceptibility is especially sensitive to the state of graphitization. The total susceptibility increases with increased layer disorder and crystallite diameter (up to about 150 A), and the susceptibility anisotropy

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