The Properties of Carbon Contacts

Microphone carbon has been produced by deposition of pyrolytic carbon films over the surfaces of small spherules of silica. The properties of contacts between these spherules are shown to be dependent on the structure and geometry of the carbon surface as determined by electron diffraction and microscopic studies. The graphite-like crystallites in pyrolytic carbon surfaces are more or less preferentially oriented with their basal planes parallel to the surface, and the contact properties depend systematically on the degree of orientation. This is explained in terms of the anisotropy in properties of these crystallites which are closely approximated by those of single crystal graphite which were determined. The contact resistance and its temperature coefficient and the ``burning voltage'' for carbon contacts are explicable on this basis. However, the microphonic sensitivity of carbon contacts is independent of the surface structure and depends only on the surface geometry.