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Research And Development On Advanced Graphite Materials. Volume XXXIX - Diamagnetic Susceptibility Of Graphite By The Faraday Method

Report Number: WADD TR 61-72 Volume 39
Author(s): D. E. Soule and C. W. Nezbeda
Corporate Author: Research Laboratory of National Carbon Company Division of Union Carbide Corporation
Laboratory: AF Materials Laboratory
Date of Publication: 1963-11
Pages: 60
Contract: AF 33(616)-6915
Project: 7350
Task: 735002
AD Number: AD0426781

Abstract:

The diamagnetic susceptibility of several types of graphite, both single and polycrystalline, was investigated at room temperature by the Faraday method. This method measures the entire sample; it can be used to obtain the three components for a trace; and it can account for ferromagnetic impurities. For single crystals, the conduction electron component (∥ c-axis) is χ= -21.8x10-6 emu/gm; whereas the isotropic ionic core component is χ= -0.32x10-6 emu/gm, resulting in a maximum anisotropy ratio of 68. Anisotropies found for polycrystals range from 25 for annealed pyrolytic down to 1.01 for lampblack-base grade CEP graphite. The susceptibility trace, representing the contribution of the truly graphitic portion of a sample, ranges from -22.8x10-6 emu/gm for single crystals down to -19.1x10-6 emu/gm for grade ZTA graphite. Direct orientation measurements showed that both single crystals and polycrystals display a true cosine-squared dependence, modified in amplitude by the degree of preferred crystallite orientation. These results are justified theoretically.

A study of the effect of annealing temperature up to 3360°C on the anisotropy and trace pyrolytic samples showed a competition of processes depending upon the properties of the "as received" material, where trace values vary from -18.2x10-6 emu/gm to -24.0x10-6 emu/gm according to the amount of non-graphic carbon and the degree of turbostraticity. Above ~ 3200°C, all sample traces tend toward the single crystal value.

A study of the effect of acceptor boron doping from 1 to 5000 ppm showed a decrease in the magnitude of χ with increasing boron concentration, following the lowering of the Fermi level, becoming asymptotic to the ionic core component at ~ 3x10-3 B/C. This effect correlates with the Hall coefficient, which shows a peak due to the transition at the bottom of the conduction band from mixed conduction to single hole cinduction. Both effects give an ionization efficiency for boron of 75 ±15%. Magnetoresistance and conductivity results show that there is predominantly impurity scattering in the measured range of 77°K to 299°K.


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