Electrical Conduction in Graphite along c Axis

Abstract

A study of the electrical conduction in the c-direction of graphite has been made of some pyrolytic carbons and two kinds of highly-orientated pyrolytic graphite through the measurements of galvanomagnetic effects at 77°K. Since anything that disturbs the perfect alignment of graphitic planes, like mosaic structure, will lead to a zigzag flow and decrease in the resistivity along c axis, experiments were based on the following expectations for the presence of electron transport along c axis;1. Magretoresistance under a constant magnetic field should not indicate a large maximum when the field is perpendicular to the layer plane.2, Hall voltage becomes observable only if the cyclotron motion of carriers takes place so as to pass through several layers.The field dependence of magnetoresistance was investigated with changing a parameter θ, the angle between the magnetic field and the axis normal to the deposition plane . All the pyrolytic carbon specimens exhibited negative magnetoresistance, while it was positive forthe pyrolytic graphites. In both cases, the maximum absolute value of magnetoresistance was found when the field direction coincided with the normal axis (;θ=0, longitudinal situation).Change of this longitudinal magnetoresistance with graphitization is nearly the same as those found for the inplane transverse magnetoresistance of usual soft carbons. Hall voltages were never detectable for each specimen except for the specimen of the most highly-orientated pyrolytic graphite, for one could not cut out a specimen of sufficient size for the measurements of Hallcoefficient from it. However, one can reasonably conclude that the Hall voltages in the plane perpendicular to the layer plane is never detectable for the specimen which has anything that disturbs the perfect alignment of the layer planes. Based on these observations, the electron transport found in the present experiments is concluded to take place only in the layer planes. The resistivity in the c-direction has been estimated to be of the order of 103 Ω cm from the acuracy of the magnetoresistance measurements. On the other hand, unusual field dependence of the magnetoresistance has been observed for the most highly-orientated pyrolytic graphite specimen in the high field region.