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Abstracts 299 62. (Laboratoire de Chimie Physique (ERA 312), Universite’ Bordeaux I, 33405 Talence, France). L’application de la methode de London a des rubans graphitiques conduit a des variations thermiques des susceptibilites magnetiques analogues a des variations thermiques des susceptibilites magnetiques analogues a celles des carbones prtgraphitiques. Ces dernihes peuvent done &tre interpretees a partir d’un modele forme de niveaux d’tnergie discrets et l’introduc- tion d’un diamagnetisme de Landau est inutile. Un tel modele simplifie permet d’obtenir une expression de la susceptibilite analogue a celle obtenue Pacault et Mar- chand a partir d’un modtle de bandes. 63. Magnetisation measurements on pyrolytic graphite in fields up to 11 tesla G. Chouteau and A. Briggs (Service National des Champs Intenses and Centre de Recherches sur les Tres Basses Temperatures, Centre National de Recherche Scientifique, B. P. 166, 38042 Grenoble, France). 64. Relation of structure with some E.P.R. properties in anthracene semi-cokes C. Simon, H. Estrade, D. Tchoubar et J. Conard (CRSOCI, CNRS, Orleans, France). Experimental X-ray bands are compared with calculated one obtained from various shapes of elementary patterns. In the first step, anthracene molecules are associated by pairs with a 3.40 A distance between planes, and high value of g 1 c. Then, they associate with non rigid bonding, with a gradual loss of regularity in c spacing and a lowering of g 1 c value. At 800°C the growing of aromatic plane reaches about 25 A and II electrons get delocalized. 65. Effect of interstitial carbons in graphite, on the band structure J. Conard et H. Estrade (CRSOCI, CNRS, Orleans: France). Using the covalent model of bonding yet proposed for interstitial carbon in graphite, we discuss (1) the origin of the high stability of this point defect, (2) the creation of a gap bounded with a small packet of partially localized states, (3) the explanation of destabilizing these defects by substitutional boron, (4) the relation between Hall effect anomaly and the presence of interstitial carbons. 66. ESR-Untersuchungen an unter verschiedenen Bedingungen Statisch abgeschiedenem Pyrokohfenstoff Fr. Rozploch (Nikolaus-Corpemicus- University, Torun’, Poien ), H. Luhleich, H. Nickel und B. Scholand (Kemforschungsanlage Jiilich, Institut fur Reaktorwerk- stoffe, D-5170 Jiilich, Postfach 1913). Pyrokohlenstoff wurde im Druckbereich von 100bis 400mbar bei Tem- peraturen zwischen 1600-2100°Cstatisch auf Grafitstaben abgeschieden. Bruckstticke, Pulver und kleine Bereiche des beschteten Grafitstabes wurden bei Raumtemperatur und 100K mittels Elektronenspin-Resonanz-Messungen (ESR) untersucht. Ebenso wurde ESR-Messungen an Pyrokohlenstoff vorgenommen, der aufgrund einer hohen elektrischen Spannung entstanden war. Die Ergebnisse dieser Untersuchungen werden mitgeteilt und eriirtert. 67. Paramagnetische Elektronenresonanz (EPR) in polykristailinen Graphiten bei hohen Temperaturen M. J. Paersch and W. Schroller (Institut fur Eisenhiittenkunde, RWTH Aachen, Intzestr. 1, D-5100 Aachen) and F. H. FRanke (Rheinische Braunkohlen- werke AC, D-5000 Kiiln 1). EPR-measurements were carried out on polycrystalline graphites in a temperature range of 300-1300°K. It is possible to obtain the crystallite tensor components of the g-factor and the crystallite line-width parameters from the asymmetrical powder spectrum, if there is an axial symmetry of the crystallites within the grains. A simplified statement of the density of states was used for computing the paramagnetic suscepti- bility of the free charge carries as a function of temperature. 68. SpeciRc heat studies of carbons at very low and at intermediate temperatures S. Mrozowski, A. S. Vagh, E. Michael and B. Myers (Ball State University, Department of Physics and Astronomy, Muncie, Indiana, U.S.A.). Detailed studies of specific heat of variously heat-treated soft carbons and of neutron irradiated polycrystalline graphite were carried out in the temperature range 0.4-4S”K and reported in a series of papers. In comparison to crystalline graphite, two types of anomalies appear when defects and disorder are present in the material-a linear temperature contribu- tion of noncarrier origin and a specific heat peak at around 0.65”K. The two anomalies were found to vary in parallel, the linear part increasing with increase in height of the peak. The exact position of the peak varies slightly with its height. The suspicion that these two anomalies are in some way connected with the presence of free electronic spins localized at defects, found support in studies of the influence of magnetic field on these anomalies. On the other hand, various observations led us to anticipate the presence of additional anomalies at still lower tempera- ture. Thanks to the kind permission of the NASA Lewis Research Center in Cleveland to use their dilution refrigerator, we have carried out measurements to temperature as low as 70 milidegrees, and found for a soft carbon heat-treated to 1250°Ca second specific heat peak at around 0.3”K. This finding was reported at the 12th American Carbon Conference in Pittsburgh. 69. Thermal expansion and coke microstructure J. E. Zimmer and J. L. White (The Aerospace Corporation, P.O. Box 92957, Los Angeles, Calif. 90009, U.S.A.). Thermal expansion measurements have been made in the directions perpendicular and parallel to the length of individual particles of needle coke having a distinct acicular structure, and on other selected cokes, as a function of heat-treatment temperature. These data are used to assess the relationship between thermal expan- sion, accommodation, and the coke microstructure.

68. Specific heat studies of carbons at very low and at intermediate temperatures

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Abstracts 299

62.

(Laboratoire de Chimie Physique (ERA 312), Universite’ Bordeaux I, 33405 Talence, France). L’application de la methode de London a des rubans graphitiques conduit a des variations thermiques des susceptibilites magnetiques analogues a des variations thermiques des susceptibilites magnetiques analogues a celles des carbones prtgraphitiques. Ces dernihes peuvent done &tre interpretees a partir d’un modele forme de niveaux d’tnergie discrets et l’introduc- tion d’un diamagnetisme de Landau est inutile. Un tel modele simplifie permet d’obtenir une expression de la susceptibilite analogue a celle obtenue Pacault et Mar- chand a partir d’un modtle de bandes.

63. Magnetisation measurements on pyrolytic graphite in fields up to 11 tesla

G. Chouteau and A. Briggs (Service National des Champs Intenses and Centre de Recherches sur les Tres Basses Temperatures, Centre National de Recherche Scientifique, B. P. 166, 38042 Grenoble, France).

64. Relation of structure with some E.P.R. properties in anthracene semi-cokes

C. Simon, H. Estrade, D. Tchoubar et J. Conard (CRSOCI, CNRS, Orleans, France). Experimental X-ray bands are compared with calculated one obtained from various shapes of elementary patterns. In the first step, anthracene molecules are associated by pairs with a 3.40 A distance between planes, and high value of g 1 c. Then, they associate with non rigid bonding, with a gradual loss of regularity in c spacing and a lowering of g 1 c value. At 800°C the growing of aromatic plane reaches about 25 A and II electrons get delocalized.

65. Effect of interstitial carbons in graphite, on the band structure

J. Conard et H. Estrade (CRSOCI, CNRS, Orleans: France). Using the covalent model of bonding yet proposed for interstitial carbon in graphite, we discuss (1) the origin of the high stability of this point defect, (2) the creation of a gap bounded with a small packet of partially localized states, (3) the explanation of destabilizing these defects by substitutional boron, (4) the relation between Hall effect anomaly and the presence of interstitial carbons.

66. ESR-Untersuchungen an unter verschiedenen Bedingungen Statisch abgeschiedenem Pyrokohfenstoff

Fr. Rozploch (Nikolaus-Corpemicus- University, Torun’, Poien ), H. Luhleich, H. Nickel und B. Scholand (Kemforschungsanlage Jiilich, Institut fur Reaktorwerk- stoffe, D-5170 Jiilich, Postfach 1913). Pyrokohlenstoff wurde im Druckbereich von 100 bis 400mbar bei Tem- peraturen zwischen 1600-2100°C statisch auf Grafitstaben abgeschieden. Bruckstticke, Pulver und kleine Bereiche des beschteten Grafitstabes wurden bei Raumtemperatur und 100 K mittels Elektronenspin-Resonanz-Messungen

(ESR) untersucht. Ebenso wurde ESR-Messungen an Pyrokohlenstoff vorgenommen, der aufgrund einer hohen elektrischen Spannung entstanden war. Die Ergebnisse dieser Untersuchungen werden mitgeteilt und eriirtert.

67. Paramagnetische Elektronenresonanz (EPR) in polykristailinen Graphiten bei hohen Temperaturen

M. J. Paersch and W. Schroller (Institut fur Eisenhiittenkunde, RWTH Aachen, Intzestr. 1, D-5100 Aachen) and F. H. FRanke (Rheinische Braunkohlen- werke AC, D-5000 Kiiln 1). EPR-measurements were carried out on polycrystalline graphites in a temperature range of 300-1300°K. It is possible to obtain the crystallite tensor components of the g-factor and the crystallite line-width parameters from the asymmetrical powder spectrum, if there is an axial symmetry of the crystallites within the grains. A simplified statement of the density of states was used for computing the paramagnetic suscepti- bility of the free charge carries as a function of temperature.

68. SpeciRc heat studies of carbons at very low and at intermediate temperatures

S. Mrozowski, A. S. Vagh, E. Michael and B. Myers (Ball State University, Department of Physics and Astronomy, Muncie, Indiana, U.S.A.). Detailed studies of specific heat of variously heat-treated soft carbons and of neutron irradiated polycrystalline graphite were carried out in the temperature range 0.4-4S”K and reported in a series of papers. In comparison to crystalline graphite, two types of anomalies appear when defects and disorder are present in the material-a linear temperature contribu- tion of noncarrier origin and a specific heat peak at around 0.65”K. The two anomalies were found to vary in parallel, the linear part increasing with increase in height of the peak. The exact position of the peak varies slightly with its height. The suspicion that these two anomalies are in some way connected with the presence of free electronic spins localized at defects, found support in studies of the influence of magnetic field on these anomalies. On the other hand, various observations led us to anticipate the presence of additional anomalies at still lower tempera- ture. Thanks to the kind permission of the NASA Lewis Research Center in Cleveland to use their dilution refrigerator, we have carried out measurements to temperature as low as 70 milidegrees, and found for a soft carbon heat-treated to 1250°C a second specific heat peak at around 0.3”K. This finding was reported at the 12th American Carbon Conference in Pittsburgh.

69. Thermal expansion and coke microstructure J. E. Zimmer and J. L. White (The Aerospace

Corporation, P.O. Box 92957, Los Angeles, Calif. 90009, U.S.A.). Thermal expansion measurements have been made in the directions perpendicular and parallel to the length of individual particles of needle coke having a distinct acicular structure, and on other selected cokes, as a function of heat-treatment temperature. These data are used to assess the relationship between thermal expan- sion, accommodation, and the coke microstructure.