The HOPG targets are irradiated by the ion beams with energy density of 70 ~120 J/cm2. The target is set parallel to the cathode, and dAT was changed at 150 and 180 mm. Only one shot is carried out for each sample. The morphology of the ablated surfaces is observed by a scanning electron microscope (JEOL, JSM-6700F).
TABLE. Physical properties of the HOPG
EXPERIMENTAL RESULTS AND DISCUSSIONS
Figure 2 shows SEM images of the surface of (a) unirradiated, (b) irradiated HOPG targets and (c) the cross sectional view of the irradiated HOPG target. The irradiated sample was placed at dAT=150 mm, where the ion beams with ~ 120 J/cm2 of energy density were irradiated on the target. The observed area was the center of the irradiated region on the target surface. From Figures 2b and 2c, the irradiated sample has many sphere particles and whiskers. The diameter of these particles was approximately 0.5 ~1 mm.
The growth mechanism of the metallic whisker from solids was studied by many scientists. Furuta et al. [9] studied the growth of Sn whiskers from a solid Al-Sn alloy. The whiskers were formed by growing a topmost grain in the strained alloy. Fisher [10] and Hasiguti [11] observed the growth of whiskers from solid Sn plated Zn samples. The growth rate was increased with increasing applied pressure by exerted in tightening the clamp. In the present experiment of the intense pulsed ion beam irradiation on the graphite target, carbon whiskers were observed to be present (Figure 2b). Although, as far as the present authors know, no results have been reported on the growth of carbon whiskers from solid, it is likely that the growth of carbon whiskers will be enhanced by the hydraulic pressure. Hence, the presence of the carbon whiskers observed in Figure 2b on the pulsed ion beam irradiation on the graphite targets may give us an evidence of the generation of hydraulic pressure, which had been numerically predicted by the previous studies.
FIGURE 2. Surfaces of a)
unirradiated; b) irradiated HOPG targets; c) the
cross sectional view of the irradiated HOPG target observed by SEM
SUMMARY
The surface modification of the highly oriented pyrolytic graphite (HOPG) targets have been carried out by the pulsed ion-beam irradiation. In the SEM analysis, sphere particles and whiskers which approximately 0.5~1 mm in diameter were observed on the irradiated surface. These whiskers may be grown under the hydraulic pressure by the ablation-plasma radiation. We have found the ablation plasma produced by the intense pulsed ion-beam irradiation can be used for the surface modification of the target materials.
ACKNOWLEDGMENTS
This work was partly supported by the Grant-in-Aid for Scientific Research and the 21st. Century COE Program of the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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