Radiation endurance in Al2O3nanoceramics

The lack of suitable materials solutions stands as a major challenge for the development of advanced nuclear systems. Most issues are related to the simultaneous action of high temperatures, corrosive environments and radiation damage. Oxide nanoceramics are a promising class of materials which may benefit from the radiation tolerance of nanomaterials and the chemical compatibility of ceramics with many highly corrosive environments. Here, using thin films as a model system, we provide new insights into the radiation tolerance of oxide nanoceramics exposed to increasing damage levels at 600 °C-namely 20, 40 and 150 displacements per atom. Specifically, we investigate the evolution of the structural features, the mechanical properties, and the response to impact loading of Al2O3 thin films. Initially, the thin films contain a homogeneous dispersion of nanocrystals in an amorphous matrix. Irradiation induces crystallization of the amorphous phase, followed by grain growth. Crystallization brings along an enhancement of hardness, while grain growth induces softening according to the Hall-Petch effect. During grain growth, the excess mechanical energy is dissipated by twinning. The main energy dissipation mechanisms available upon impact loading are lattice plasticity and localized amorphization. These mechanisms are available in the irradiated material, but not in the as-deposited films

Nucleation and growth mechanisms of Fe on Au(111) in the sub-monolayer regime

The growth of Fe on Au(111) at 300 K in the sub-monolayer regime has been investigated using scanning tunneling microscopy, focusing on the mechanisms of nucleation, coalescence and interlayer diffusion. Below a coverage of 0.1 ML, Fe growth proceeds in a well-ordered fashion producing regular arrays of islands, while approaching the island coalescence threshold (above 0.35 - 0.4 ML), we observed a consistent increasing of random island nucleation. These observations have been interpreted through rate equation models for the island densities in the presence of preferred nucleation sites. The evolution of the second layer fraction has also been interpreted in a rate equation scheme. Our results show that the ordered to random growth transition can be explained by including in the model bond breaking mechanisms due to finite Fe-Fe bond energy. A moderate interlayer diffusion has been inferred from data analysis between the second and the first layer, which has been used to estimate the energy barrier of the adatoms descending process

Corrosion and radiation resistant nanoceramic coatings for lead fast reactors

International audienceBare and Al2O3-coated austenitic steel samples are exposed to lead-fast-reactor relevant corrosive conditions.Selective leaching of Ni, Mn and Cr is observed in bare samples exposed to high temperature stagnant lead(550 DC, 10et8722;8 wt.percent oxygen, 1000 and 4000 h). By contrast, corrosion is not observed in either pristine (4000 h)or irradiated (1000 h) coated samples. Further characterization and testing methods include SEM, TEM, STEM,EDS, cyclic nanoimpact, microindentation, scratch, and thermal cycling. Overall, the results show that thecoatings retain structural integrity under the conditions investigated, which is a crucial prerogative for corrosionprotection with ceramic coatings

Strain effect on local electronic properties of Fe nanoislands grown on Au(111)

The electronic properties of Fe islands grown on reconstructed Au(111) were investigated by means of scanning tunneling spectroscopy and density functional theory calculations. A characteristic peak measured in the electron local density of states at Fermi level indicates the presence of a non-dispersive minority Fe dz2 state. The peak energy was measured to shift from occupied to unoccupied states as a function of position within Fe islands. We related this effect to the reconstruction-induced local Fe-Fe bond length variation

Extreme ion irradiation of oxide nanoceramics: Influence of the irradiation spectrum

International audienceOxide nanoceramics combine the enhanced radiation tolerance of nanocrystalline materials with the chemical inertness of oxides, and are promising materials for highly corrosive and intense radiation environments. In this work, nanocrystalline Al2O3 thin films are irradiated at 600 °C with either 12 MeV Au5þþ18 MeV W8þ or 4 MeV Ni2þ ions. The radiation damage exposure exceeds 450 displacements per atom. A comprehensive analysis of the irradiated samples is accomplished by X-Ray Diffractometry (XRD), Transmission Electron Microscopy (TEM) and Scanning-TEM (STEM). Results are compared in an effort to establish correlations between the irradiation spectrum and the response of this class of materials to radiation environments. The results show that grain growth is the main microstructural change induced by ion irradiation in the material, regardless of the ion utilized in this work. The phase evolution may be depth-dependent, and depends strongly on the ion utilized and on the irradiation spectrum.12 MeV Au5þþ18 MeV W8þ irradiations favor the formation of g-Al2O3 and a-Al2O3, while 4 MeV Ni2þ irradiations yield mainly d-Al2O3, accompanied by small a-Al2O3 centers. Molecular dynamics simulations of displacement cascades are used to support discussions on the mass effect brought about by the different ions

Extreme ion irradiation of oxide nanoceramics: Influence of the irradiation spectrum

Oxide nanoceramics combine the enhanced radiation tolerance of nanocrystalline materials with the chemical inertness of oxides, and are promising materials for highly corrosive and intense radiation environments. In this work, nanocrystalline Al 2 O 3 thin films are irradiated at 600 °C with either 12 MeV Au 5+ +18 MeV W 8+ or 4 MeV Ni 2+ ions. The radiation damage exposure exceeds 450 displacements per atom. A comprehensive analysis of the irradiated samples is accomplished by X-Ray Diffractometry (XRD), Transmission Electron Microscopy (TEM) and Scanning-TEM (STEM). Results are compared in an effort to establish correlations between the irradiation spectrum and the response of this class of materials to radiation environments. The results show that grain growth is the main microstructural change induced by ion irradiation in the material, regardless of the ion utilized in this work. The phase evolution may be depth-dependent, and depends strongly on the ion utilized and on the irradiation spectrum. 12 MeV Au 5+ +18 MeV W 8+ irradiations favor the formation of γ-Al 2 O 3 and α-Al 2 O 3 , while 4 MeV Ni 2+ irradiations yield mainly δ-Al 2 O 3 , accompanied by small α-Al 2 O 3 centers. Molecular dynamics simulations of displacement cascades are used to support discussions on the mass effect brought about by the different ions

Corrosion and radiation resistant nanoceramic coatings for lead fast reactors

Bare and Al2O3-coated austenitic steel samples are exposed to lead-fast-reactor relevant corrosive conditions. Selective leaching of Ni, Mn and Cr is observed in bare samples exposed to high temperature stagnant lead (550 °C, 10â\u88\u928 wt.% oxygen, 1000 and 4000 h). By contrast, corrosion is not observed in either pristine (4000 h) or irradiated (1000 h) coated samples. Further characterization and testing methods include SEM, TEM, STEM, EDS, cyclic nanoimpact, microindentation, scratch, and thermal cycling. Overall, the results show that the coatings retain structural integrity under the conditions investigated, which is a crucial prerogative for corrosion protection with ceramic coatings