Field emission characteristics and structure of carbon-containing cathode materials

Field emission and structural characteristics of carbon nanotube fibers, polyacrylonitrile fibers, pyrolytic graphite and micrograined dense graphite were experimentally studied before and after their operation as a field cathode using registration of the current-voltage characteristics, optical microscopy, scanning electron microscopy and Raman spectroscopy in the spectral range from 1000 to 2000 cm$^{-1}$. The experiments carried out showed large and small structural rearrangements of carbon-containing cathodes and their surfaces in the process of field emission. In addition to the G, D and D’ lines characteristic of carbon materials, a line was detected in the range of 1450–1480 cm$^{-1}$, which is observed in the starting materials of pyrolytic graphite, carbon nanotube fibers and persists after operation, and also appears in the micrograined dense graphite sample after operation in as a cathode. The relative integral intensity of line D in pyrolytic graphite, micrograined dense graphite, and carbon nanotube fibers undergoes the greatest change. In pyrolytic graphite and carbon nanotube fiber, its increase is observed, and in micrograined dense graphite its decrease is observed after operation as a cathode. This made it possible to use the relative integral intensity of the D-line to quantify the change in the surface properties of carbon materials as a result of field emission when using these materials as cathodes, in particular. to assess changes in crystallite sizes. Thus, the possibility of using Raman spectra to control the surface structure of carbon-containing materials has been demonstrated, which significantly facilitates the possibility of further analysis of the relationship between the surface structure and its emission characteristics. The prospects for improving the field emission characteristics of carbon-containing cathodes were discussed.