Raman studies of particle structure and graphitization in carbon blacks

Abstract

Raman spectroscopy is used to probe the in-plane layer dimensions (L$\sb{\rm a}$) of graphitic nanocrystalline regions within several carbon blacks. The empirical relationship of Tuinstra and Koenig is used to relate integrated peak intensities to L$\sb{\rm a}$. These data are compared to those derived from Raman spectra calculated using a theoretical phonon-confinement model developed by Richter and coworkers in their studies on silicon and not previously applied to measurements of L$\sb{\rm a}$ in graphitic materials. These Raman measurements yield L$\sb{\rm a}$ values which correlate very well with measurements performed using an accepted x-ray diffraction-based method. Graphitization was investigated in terms of the dependence of L$\sb{\rm a}$ on furnace, continuous wave laser, and pulsed laser heat-treatment. Nanocrystal environment and heat transfer in carbon black particles were identified as strong influences on the final state of graphitization. Graphitic crystal planes grow substantially on the microsecond time scale, and graphitization is completed within seconds. In addition, surface activity decreased dramatically in graphitized carbon blacks, indicating surface homogenization and removal of active surface sites through increases in L$\sb{\rm a}$ at the particle boundary. These results give new insight to the energetic processes which govern graphitic nanolayer realignment. Three-dimensional transmission electron microscope investigations of structure in graphitized carbon blacks demonstrate that aggregates are more or less flat entities, which may be contrasted with the expected isotropic structure. Anisotropy is quantified and discussed in terms of its potential effects on the reinforcement of elastomers by carbon black. These discoveries should contribute to the development of improved carbon blacks through new production technologies.