MAGNETORESISTANCE AND MICROTEXTURE OF CARBON-FILMS PREPARED FROM POLYIMIDE FILM LARC-TPI

Structural changes in carbon films prepared from a commercially available polyimide, LARC-TPI, of 50 mum thickness, with heat-treatment temperatures between 2000-degrees and 2800-degrees-C were investigated by measuring magnetoresistance at liquid nitrogen temperature and by scannign electron microscopy. All the specimens exhibited granular microtextures, with mean grain diameters spanning 10-12 nm, similar to those of bulk glasslike carbons. The films heat treated between 2000-degrees and 2600-degrees-C showed a small negative magnetoresistance characteristic of bulk glasslike carbon. The carbon films heat treated at 2750-degrees and 2800-degrees-C gave a composite magnetoresistance attributed to granular microtexture (negative, isotropic) with graphite layer skins (positive, anisotropic). The fractured cross-sections of these films indicated layered skins about 100 nm thick. These graphitizable skins may be attributed to constraints that influenced the orientation of the molecules near the surfaces of the film when they were formed.