TECNICA QUIRURGICA BIMANUAL

Damage in low density amorphous carbon (2.0g/cc) was simulated under the same conditions as the graphite cascades. Figure 5.24 is a snap-shot of the final lattice after a cascade through low density amorphous carbon with an initial PKA energy of 500 eV, initial PKA direction x = 1, y = 2 and z = 1 has stabilised.

Figure 5.24:A snap-shot of the final image during a low density amorphous (2.0g/cc) cascade. The PKA is given an initial energy of 500 eV and initial direction x = 1, y = 2 and z = 1. The initial temperature was set to 300◦C. Coloured atoms denote displaced atoms.

The low density carbon produced cascades mainly localised to the PKA col- lision origin. However, movement of other atoms around the cell can also be seen. This is partially due to the result of shock waves passing through the cell. The shock wave is similar to those seen in diamond. The shock waves do not have a major effect on the overall structure. Atoms are further displaced as a direct result of collision cascades during the simulation. The structure of low density amorphous carbon is such that atoms travelling on a certain path can move a distance in the cell before colliding with a subsequent atom. This phenomenon has also be seen in graphite due to the layers of graphene. Three initial PKA directions from graphite have been selected and simulated in low density carbon so an accurate comparison can be made. Due to the high volume of displaced atoms occurring in low density amorphous carbon cascades, energies in the range of 25 - 500 eV have been simulated. Figure 5.25 is an example of a cascade through low density amorphous carbon with an initial PKA energy 50 eV.

Figure 5.25:Low density amorphous carbon cascade with initial PKA energy 50 eV after: (a) 0.007 ps, (b) 0.026 ps, (c) 0.83 ps, (d) 0.155 ps, (e) 0.441 ps and (f) 2.621 ps. Initial PKA direction 1 from table 3.1. The colour of the scalar bar colour indicates the distance moved by atoms during the cascade. Initial cell temperature 300◦C.

The collision cascade remains local to the initial PKA site until 0.441 ps, figure 5.25 (e). The displacement of atoms outside the initial cluster is due to a shock wave passing through the lattice. The shock wave was created as a direct response to the initial collision. The initial collisions sent vibrations through the cell which have resulted in the displacement of atoms outside of the cluster. Throughout the simulation, a maximum of 15 atoms were displaced. Figure 5.25 (d), (e) and (f) show the average distance travelled by atoms in the cluster is 1Å. Due to the structure of low density amorphous carbon, a few atoms can travel a short distance through the lattice before colliding with another carbon atom. This results in final displacements of 5Å being observed (figure 5.25 (f)). Cascades with energies ranging from 25 - 100 eV saw a maximum of 25 atoms displaced. The average number of displaced atoms occurring in cascades at 100 eV was double the number observed in 25 eV cascades. Displaced atoms are mainly found in a cluster near to the original PKA position. Less than 20% of atoms displace outside of the main cluster in cascades with initial PKA energy in the range of 25 - 100 eV. The average final displacement of atoms in cascades with an initial energy below 100 eV is 2Å.

Figure 5.26 is an example of a cascade through low density amorphous carbon with an initial energy of 500 eV.

Figure 5.26:Low density amorphous carbon cascade with initial PKA energy 500 eV after: (a) 0.007 ps, (b) 0.040 ps, (c) 0.104 ps, (d) 0.359 ps, (e) 0.974 ps and (f) 2.491 ps. Initial PKA direction 1 from table 3.1. The colour of the scalar bar indicates the distance moved by atoms during the cascade. Initial cell temperature 300◦C.

Figure 5.26 (a) shows the initial path of the PKA. After 0.040 ps, there is ev- idence of secondary collision cascades (figure 5.26 (b)). Figures 5.26 (c) and (d) show the formation of main cluster site. A shock wave is created as a di- rect result of initial collisions. The shock wave has created further displaced atoms which can be seen in figures 5.26 (e) and (f). The maximum number of displaced atoms was 218.

The PKA has travelled 3Å from its initial lattice site after 0.007 ps, figure 5.26 (a). Figure 5.26 (b) shows atoms displaced as a direct result of an initial collision travel further through the cell. The final displacement of atoms in the main cluster is on average less than 1Å. Evidence can be seen in figures 5.26 (d), (e) and (f). Figure 5.26 (f) highlights displaced atoms away from the main defect cluster, all of which have a final displacement of less than 1Å.

Cascades with energies ranging from 250 - 500 eV saw a maximum of 218 atoms displaced during the cascade. The majority of displaced atoms occurred in a cluster. On average, a quarter of displaced atoms occurred away from the main clusters. Atoms displaced outside the clusters are formed from a combination of displaced atoms travelling through the cell without colliding with another carbon atom and the effects of a shock wave. Displaced atoms do not travel far from their original lattice site. In cascades with energies of the range 250 - 500 eV, the average distance travelled by a displaced atom is 1Å.

Quantitative cascade data for low density amorphous carbon has been taken over three initial PKA energies with initial PKA energies ranging from 25 - 500 eV.

Figure 5.27 presents the maximum number of displaced atoms occurring dur- ing cascades through low density amorphous carbon.

Figure 5.27:The maximum number of displaced atoms as a function of en- ergy. Error bars denote one standard deviation.

Figure 5.27 highlights how the number of displaced atoms increases as the initial PKA energy increases. The average number of displaced atoms in the lattice cell at 25 eV is nine compared to 193 in cascades with an initial energy of 500 eV. There is a 95% increase in the average number of displaced atoms in the final lattice cell at 25 eV compare to 500 eV. Low density amorphous carbon cascades have not produced one final lattice cell without any atoms being displaced when simulated with an initial energy of 25 eV or greater. The average number of displaced atoms during cascades through high density amorphous carbon shows a linear trend presented in equation 5.4.

Ndisplacements =0.3898E (5.4)