Figure 27 gives Raman spectra for unirradiated and irradiated graphite as well as for baked carbon.

In the spectral range shown, there is a prominent G-peak at 1580 cm-1 associated with the basal plane bond stretching of V axis sp2 atoms. The D-peak at 1350 cm-1 is associated with the breathing mode of sp2 atoms and disordered carbon structure. The sec­ond D-peak at 2700 cm-1 is indicative of the crystal­line structure of the graphite.

In Figure 28 the normalized positions of the G — and D-peaks, and the ratio of the peak intensities are compared for various graphites (unirradiated and irradiated). HOPG is obviously the most ordered structure followed by the PGA needle coke graphite and then the medium grained graphite grades. The most disordered materials are the baked carbon (NBG-18 baked) followed by irradiated BEPO (a UK test reactor) graphite.

Fluence (1020 ncm-2 EDND)
c-axis

Figure 29 qualitatively demonstrates that there is a relationship between Raman spectra and crystal structure disorder. The higher the disorder, the higher are the D — and G-peak wave number and 1(D/G) ratio.

In Figure 29(a), the crystal length La has been calculated from the full width at half maximum (FWHM) using the method proposed by Tuinstra and Koenig.56 Both figures demonstrate that Raman can be used to quantify the disorder in the graphite structure, either as manufactured or due to irradiation.