ABSTRACTS 399

(comprises entre 1500 et 2800°C) a CtC Ctudie par resonance paramagnetique Clectronique entre 80 et 700°K. La susceptibilite 1s du coke trait4 a 1500” suit presque la loi de Curie, mais s’en ecarte legerement auz hautes temperatures. Lorsque HTT croit, la variation thermique du paramagn&isme cesse de se conformer a la loi de Curie pour tendre vers une loi du type xp= x0 (1 - exp. (- To/T) ) caracteristique des porteurs de charges libres. Cependant, la susceptibilite devient presque independante de la temperature pour HTT=2800”, et se rapproche ainsi du paramagnetisme du graphite, qui augmente legerement avec la temperature (L. S. Singer and G. Wagoner -5bme Conference du Carbone, Penn State, 1961). L’ensemble de ces variations thermiques peut Ctre interprete quantitativement g&e a la formule (WILSON, The T&ory of Metals, Cambridge Univ. Press, p. 151, 1958).

s

co

xP= -pB' N(E) (af~~~)~E oii pE est le magn~ton de Bohr, iV(E) est la densite des niveaux --co

Clectroniques et f(E) la fonction de distrobution de Fermi, en representant les carbones pr6graphitiques par un modele phenomenologique oh les bandes de valence et de conduction sont separees par une bande d’energie interdite AE. La comparaison des courbes theoriques et expkrimentales permet de determiner BE et l’energie de Fermi so.

154. Electron spin resonance in doped and in irradiated polycrystalline graphites*

$3. Mrozowski (State University of New York at’Buffalo, BuJalo, New York). Studies of temperature dependence of electron spin resonance in carbons and graphites were continued, using a greatly improved experimental arrangement. By using an incompletely graphitized P33 carbon black which shows both conduction and localized spin centers present (temperature dependence intermediate between metallic and Curie type, the influence of doping and of neutron irradiation could be studied in detail. It was found that doping increases the number of conduction carriers (increases by as much as a factor of 7 for K+ and by 2.2 for Na+ were observed) and makes the temperature dependence more metallic, and that irradiation increases the number of localized spin centers, the increase being proportional to the irradiation dose (for not too heavy irradiations). Doping increases slightly the line width and irradiation leads to a decrease in line width, The two kinds of spin centers interact in such a way that they possess a common g-value at all times, but the character of the temperature dependence of the g-value depends on the ratio of their paramagnetic contributions. This seems to be an effect of mixing the g-values by exchange between the conduction carriers (g>g free) and spin centers localized at imperfections and crystal boundaries (with g=g free).

“Supported by the U.S. Office of Naval Research.

155. Electron spin resonance in chars, carbons and graphites+

S. Mrozowski (State ~~~v~~t~ of iV’ York at Banal, B~~ulo, Newt York). It has been found that spin resonance of normal and anomalous (sulfur containing) chars loses completely its oxygen sensitivity after the char is heated in potassium or sodium vapor. Such treatment does not affect the spin concentra- tion or g-value; although the dependence of line width on temperature is changed, the temperature dependence of intensity remains essentially Curie type. The desensitization by potassium seems to be permanent but the effect of sodium wears off in air in less than 2 weeks. Heating a desensitized char in vacua to 400-500°C results in complete disappearance of the resonance line. It is believed that the alkali atoms, which protected the carbon spin centers from oxygen molecules, have reacted with the carbon centers. Some interesting changes in intensity and shape of the line were observed to occur with time for a potassium treated with sulfur char. As chars (untreated) are heat treated to high temperatures and change first into carbons and then graphite, deviations from Curie law temperature dependence set in with a gradual change into a metallic behavior occu~ng between 2000 and 3000°C. A study of these deviations was carried out for various carbons in order to clarify their extent, nature and relationship to variations of other parameters.

*Supported by the U.S. Of&e of Naval Research.