18 Search Results

Thin film deposition from the vapor phase is a complex process involving adatom adsorption, movement, and incorporation into the growing film. While simulations have been able to capture key aspects of these processes to reproduce film morphology, these models typically ignore any subsurface diffusion that occurs. The results of these diffusion and intermixing processes are often observed experimentally, but cannot be explained by invoking bulk lattice diffusion at the deposition temperature. In this work, we present quantitative experimental data that reveals anion and cation intermixing over long length scales during the deposition of epitaxial Fe₂O₃ and Cr₂O₃ films and heterostructuresmore » by oxygen-plasma-assisted molecular beam epitaxy. We track this diffusion by incorporating well-defined tracer layers containing ¹⁸O and/or ⁵⁷Fe, and measure their redistribution on the nanometer scale with atom probe tomography. Molecular dynamics simulations suggest potential intermixing events, which are then examined via nudged elastic band calculations. These reveal that adatoms on the film surface act to “pull up” subsurface O and Fe. Subsequent ring-like rotation mechanisms involving both adatom and subsurface anions then facilitate their mixing. A simple model incorporating these events qualitatively agrees with the experimental intermixing results. In addition to film deposition, these intermixing mechanisms may to be operant during other surface-mediated processes such as heterogeneous catalysis and corrosion.« less

Cr₂O₃ is a ubiquitous passivating film on alloys that is vital to their stability and performance in hostile environments. In this study, we directly observe and measure how anion transport through the oxide is affected by 400 keV Ar²⁺ ion irradiation as a function of temperature (30–500 °C) and dose (0.33–2.00 dpa) in single-crystal Cr₂O₃ films using embedded ¹⁸O isotopic tracers and atom probe tomography. Diffusion coefficients are experimentally determined and compared to a chemical rate theory model. Experimental and theoretical results are broadly in agreement, showing anion diffusion increases by at least 4–5 orders of magnitude upon irradiation withmore » a weak dependence on temperature and a stronger dose-driven sink-strength dependence. These results reveal that radiation could significantly reduce the protectiveness of Cr₂O₃ films when passivity relies upon limited anion diffusivity, even at relatively modest levels of irradiation and low temperatures.« less

The influence of radiation-induced (1 MeV energy H⁺ to ~0.1 displacements per atom (dpa) at 450 °C), nonequilibrium point defect populations on mass transport is studied with an integrated campaign of experimental and theoretical methods. Using epitaxial thin films of hematite (α-Fe₂O₃) with embedded ¹⁸O tracer layers and nanoscale atom probe tomography measurements, it is shown that anion self-diffusion is enhanced by at least 2 orders of magnitude under irradiation compared to thermal diffusion alone. Complementary scanning transmission electron microscopy of vacuum-annealed and irradiated specimens reveals associated microstructural changes near the surface of the oxide films, including local phase transformationmore » to Fe₃O₄ and the development of nanoscale voids from vacancy coalescence. Point defect formation and migration energies were computed from density functional theory and applied within the context of the chemical rate theory to analyze contributions from both interstitial and vacancy mechanisms to self-diffusion in thermal and irradiation conditions. Herein, comparisons are made between calculated, literature, and newly measured self-diffusion values, revealing good agreement on the magnitude of radiation-enhanced anion diffusion. Further, the model suggests a transition from vacancy to interstitialcy mechanisms at low temperatures and high oxygen activity, providing an explanation for the varied activation energies reported from prior studies.« less

Dynamic phase segregation, driven by the instability of Ni ²⁺ , occurs during SrNiO ₃ heteroepitaxy growth.

In this study, radiation induced segregation and precipitation phenomena in an in-service boiling water reactor 304 stainless steel component were investigated using directly correlated 3D-atom probe tomography and scanning transmission electron microscopy. Significant quantitative differences in measured segregation at grain boundaries were found between the atom probe and energy dispersive spectroscopy measurements of the exact same locations. In particular, a much stronger Si segregation (~10 atomic% via atom probe versus ~4 atomic% via electron microscopy) and different Cr profile shapes were detected that are critical to models of radiation induced segregation and stress corrosion cracking behavior. These quantitative differences highlightmore » the need for comparative microscopy and critical evaluation of limitations in each analytical method. Elemental segregation to dislocations and conjoined-clusters were also highlighted by atom probe; confirming and expanding upon what has been observed in test reactor neutron and accelerator-based ion irradiations.« less

The geochemical cycling of iron and carbon can couple in unique ways in the euphotic zone of aquatic systems.

We report Northwest Africa (NWA) 7042 is an intermediate, permafic shergottite consisting of two generations of zoned olivine (early olivine Fo_41-76, and late-stage olivine Fo_46-56), complexly zoned pyroxene (En_35-64Fs_22-46Wo_5-34), maskelynitized feldspar and shock melt (An_5-30Ab_16-61Or_1-47), and accessory merrillite, apatite, ilmenite, titanomagnetite, Fe-Cr-Ti spinels, pyrrhotite, and baddeleyite. The olivine grains have been pervasively altered resulting in orange-brown stained cores with dark-brown margins, surrounded by significantly less altered overgrowth rims. This textural relationship suggests that the alteration of the cores took place at magmatic temperatures prior to crystallization of the rims on Mars. While this type of olivine alteration is rare inmore » shergottites, it is similar to deuterically altered olivine in basalts and gabbros on Earth, caused by residual water-rich magmatic fluids. Launch-generated shock veins in NWA 7042 crosscut and displace several of the altered olivine grains indicating that the alteration occurred before ejection of the meteorite from Mars. However, NWA 7042 has also been subjected to extensive terrestrial weathering, with terrestrial calcite/barite veins occurring throughout the sample. Transmission electron microscopy analysis of the olivine alteration also reveals an absence of high-temperature phases typically associated with hydrous deuteric alteration on Earth, complicating our interpretation. The potential presence of deuterically altered olivine in NWA 7042 has significant implications, as it is the third martian meteorite where deuteric alteration of olivine has been observed (the others being NWA 10416, ALH 77005). The different mantle sources for the parental melts of these three meteorites would suggest many, if not all martian mantle reservoirs have the potential to produce water-rich magmas.« less

Novel isotopic labelling and imaging techniques are used to directly observe the autocatalytic reaction and facet-selective adsorption of Fe( ii ) onto individual hematite (Fe ₂ O ₃ ) crystallites.