Effects of boron doping for the structural evolution of vapor-grown carbonfibers studied by Raman spectroscopy

The structural deviation of boron-doped vapor-grown carbon fibers (VGCFs) w ith diameters around 10 mu m relative to their undoped counterparts was inv estigated by polarized microprobe Raman spectroscopy and field-omission sca nning electron microscopy as a function of heat-treatment temperature (HTT) . Boron doping induces the formation of dislocation loops in the surface, w hich combine into larger loops with increasing HTT. The depolarization rati o, D-p, of the E-2g2 mode for VGCFs increases gradually with increasing HTT , and finally approaches the value of highly oriented pyrolytic graphite, w hich is consistent with the asymmetric shape of the peak at similar to 2725 cm(-1) in the second-order Raman spectra. On the other hand, the D-p ratio s of the E-2g2 mode for boron-doped VGCFs show no deviations up to an HTT o f 2100 degrees C, as compared to that of VGCFs, and decrease with increasin g HTT, whereas the D-p ratios of the D peak show a maximum value at 2100 de grees C, and decrease gradually with increasing HTT. Consistent with these Raman results, boron atoms in the graphite lattice introduce a decreased d( 002) spacing (accelerating graphitization), but also hinder two-dimensional structural development and increase the amount of disorder. This is done b y introducing tilt boundaries and vacancies, which make the D-p ratio of th e E-2g2 mode lower than the value for polycrystalline graphite, even though the fibers are heat treated at 2800 degrees C.