Qing Su

Radiation damage in materials is an important reliability issue in applications ranging from microelectronic devices to nuclear reactors. However, the influence of atomic structure and specifically topological constraints on the ion damage resistance of amorphous dielectrics has until recently been largely neglected. We have investigated the 120 keV He+ ion damage resistance for a series of amorphous hydrogenated silicon carbide (a-SiC:H) thin films. Changes in elemental composition and atomic structure induced by He+ ion irradiation were monitored using nuclear reaction analysis, Rutherford backscattering spectroscopy, transmission Fourier-transform infrared spectroscopy, and transmission electron microscopy while changes in mechanical properties were investigated using nanoindentation measurements. We show that for 120 keV He+ ion doses producing up to one displacement per atom, significant hydrogen loss, bond rearrangement, film shrinkage, and mechanical stiffening were induced for films with mean atomic coordination (〈r〉) ≤ 2.7, while comparatively minor changes were observed for films with 〈r〉 > 2.7. The observed radiation hardness threshold at 〈r〉rad > 2.7 is above the theoretically predicted rigidity percolation threshold of 〈r〉c = 2.4. Based on the observed elimination of terminal CH bonds and SiCH2Si linkages, the higher radiation hardness threshold is interpreted as evidence that these bonds are too weak to function as constraints in high-energy ion collisions. Eliminating these constraints increased 〈r〉c to 2.7, in agreement with the observed 〈r〉rad = 2.7. These results demonstrate the key role of topological constraints in ion damage resistance and provide additional criteria for the design of ion-damage-resistant amorphous materials.

The management of irradiation defects is one of key challenges for structural materials in current and future reactor systems. To develop radiation tolerant alloys for service in extreme irradiation environments, the Fe self-ion radiation response of nanocomposites composed of amorphous silicon oxycarbide (SiOC) and crystalline Fe(Cr) were examined at 10, 20, and 50 displacements per atom damage levels. Grain growth in width direction was observed to increase with increasing irradiation dose in both Fe(Cr) films and Fe(Cr) layers in the nanocomposite after irradiation at room temperature. However, compared to the Fe(Cr) film, the Fe(Cr) layers in the nanocomposite exhibited ~50% less grain growth at the same damage levels, suggesting that interfaces in the nanocomposite were defect sinks. Moreover, the addition of Cr to α-Fe was shown to suppress its grain growth under irradiation for both the composite and non-composite case, consistent with earlier molecular dynamic (MD) modeling studies.

The management of radiation defects and insoluble He atoms represent key challenges for structural materials in existing fission reactors and advanced reactor systems. To examine how crystalline/amorphous interface, together with the amorphous constituents affects radiation tolerance and He management, we studied helium bubble formation in helium ion implanted amorphous silicon oxycarbide (SiOC) and crystalline Fe composites by transmission electron microscopy (TEM). The SiOC/Fe composites were grown via magnetron sputtering with controlled length scale on a surface oxidized Si (100) substrate. These composites were subjected to 50 keV He+ implantation with ion doses chosen to produce a 5 at% peak He concentration. TEM characterization shows no sign of helium bubbles in SiOC layers nor an indication of secondary phase formation after irradiation. Compared to pure Fe films, helium bubble density in Fe layers of SiOC/Fe composite is less and it decreases as the amorphous/crystalline SiOC/Fe interface density increases. Our findings suggest that the crystalline/amorphous interface can help to mitigate helium defect generated during implantation, and therefore enhance the resistance to helium bubble formation.