Li He

Although there exists a correlation between autoclave and in-reactor zirconium alloy performances, consistent oxidation kinetics discrepancies in these two environments have been observed and a fundamental understanding of the oxidation kinetics enhancement under irradiation is still lacking. Recent results obtained at the Advanced Test Reactor by the Naval Nuclear Laboratory show that photon irradiation significantly affects zirconium corrosion kinetics. In reactors, various photon sources are present in the core from ultraviolet (UV) to gamma (γ) rays. This study aims at characterizing the effect of UV and γ rays on the corrosion mechanism of Zircaloy-4. To this end, a state-of-the-art autoclave equipped with sapphire windows and connected to a recirculation loop has been installed. Zircaloy-4 coupons were exposed for 7 days at 260°C with and without recirculation or UV irradiation (or both). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) oxide characterizations show the presence of iron (Fe)-rich oxide deposits on top of the zirconium oxide where the sample has been irradiated by UV. The deposit concentration is larger in the static corrosion case and does not significantly influence the zirconium oxidation kinetics. A mechanism is proposed to explain the nucleation of these deposits and the relationship to Chalk River Unidentified Deposit nucleation is discussed. In another experiment, Zircaloy-4 coupons have been irradiated at the MIT reactor in neutron+gamma, gamma, and unirradiated loop conditions. The in-core specimens were exposed to ~1021 n/m2 fast neutron fluence in 290°C water at 7 MPa. Oxide layers have been characterized by SEM and TEM. The oxide grain size, t-ZrO2 fraction, fiber texture, and m-ZrO2 twin boundaries’ density were characterized. The results indicate that, at low dpa, the neutron + γ irradiated sample has a more protective oxide than the γ-irradiated sample, which has a more protective oxide than the nonirradiated sample.

Electron correlation microscopy (ECM) is a new technique that utilizes time-resolved coherent electron nanodiffraction to study dynamic atomic rearrangements in materials. It is the electron scattering equivalent of photon correlation spectroscopy with the added advantage of nanometer-scale spatial resolution. We have applied ECM to a Pd₄₀Ni₄₀P₂₀ metallic glass, heated inside a scanning transmission electron microscope into a supercooled liquid to measure the structural relaxation time τ between the glass transition temperature T_g and the crystallization temperature, T_x. τ determined from the mean diffraction intensity autocorrelation function g₂(t) decreases with temperature following an Arrhenius relationship between T_g and T_g+25 K, and then increases as temperature approaches T_x. The distribution of τ determined from the g₂(t) of single speckles is broad and changes significantly with temperature.

We have used fluctuation electron microscopy (FEM) to measure the medium range order in the molecular packing of 40 nm thick indomethacin glass films. Vapor deposition of indomethacin can create glasses with extraordinary kinetic stability and high density. We find peaks in the FEM variance at diffraction vector magnitudes between 0.03 and 0.09 Å^-1, corresponding to intermolecular packing distances of 1-3 nm. FEM experiments were performed with a 13 nm diameter electron probe, so these data are sensitive to medium-range order in intermolecular packing. The FEM variance from an indomethacin glass with normal stability cooled from the liquid is significantly smaller than the variance from the ultrastable glass, suggesting that ultrastable glass is more structurally heterogeneous at a 13 nm length scale. A dose of ∼7×10⁵ e^-/nm² with a very low beam current of ∼ 2.5 pA at 200 kV was used to minimize electron beam damage to the sample, and the average electron diffraction from the sample is unchanged at total electron doses fourteen times larger than required for a FEM experiment. These preliminary results on medium-range order in molecular glasses suggest that we may be able to provide insight into the structural differences between the remarkable ultrastable thin films and ordinary glasses.

Spintronic devices generally require the spin of carriers to be utilized in the storage or manipulation of data. One theoretical model for ferromagnetism in dilute magnetic semiconductors (DMS) results from the percolation of ferromagnetic regions around dilute dopants such as Mn atoms in III-V or group IV materials through the interaction of Mn atoms with carriers. Our work employed Mn implantation in Ge with subsequent rapid thermal annealing or TEM in-situ annealing to study the correlation between structure and magnetic properties. The magnetic properties of 300-350 ºC implanted Ge:Mn (which produced crystalline Ge films) varied significantly with implantation dose and annealing condition due to precipitation and transformation of different Mn_xGe_1-x secondary phases. It was found that Mn substitution of Ge and Mn_xGe_1-x secondary phases can both result in ferromagnetic properties. By combining TEM in-situ annealing and ex-situ magnetic characterization, we have demonstrated detailed correlation of magnetic properties with nanoscale structures in Mn implanted Ge DMS materials.

We introduced 1.1 at. % of Mn ions into Ge thin films in order to explore the ferromagnetism in Mn implanted Ge. Rapid thermal anneal (RTA) was applied after the implantation to recrystallize the Ge and enhance the incorporation of Mn ions into the Ge lattice. A maximum saturation moment of 0.7 μB/Mn at 5 K was reached when the sample was annealed at 300 °C for 1 min, and the moment decreased with higher annealing temperatures. Two transitions temperatures Tc and Tcl were observed corresponding to the global ferromagnetism in Mn:Ge bulk and short range magnetic ordering in Mn-rich clusters. Both critical temperatures increased with RTA temperatures and Tcl even persisted close to room temperature for the 400 °C, 1 min anneal. No secondary phases were observed.

A combination of molecular dynamics simulations and experiments has been used to investigate the use of various low energy ion assisted vapor deposition approaches for controlling the interfacial structures of a model copper∕tantalum multilayer system. Films were grown using argon ion beam assistance with either a fixed or modulated ion energy during metal deposition. The effect of sequential ion assistance (after layer’s deposition) was also investigated. The argon ion energy was varied between 0 and 50eV and the effect on the atomic scale structure of Ta∕Cu film interfaces and the film electrical resistivity were studied. The use of simultaneous argon ion assistance with an ion energy of ∼10eV and an ion∕metal atom flux ratio of ∼6 resulted in atomically sharp interfaces with little intermixing, consistent with simulation predictions. Ion impacts in this range activated surface atom jumping and promoted a step flow film growth mode. Higher energies were also successful at interface flattening, but they caused significant intermixing between the layers and increased film’s resistivity. This could be reduced using modulated ion energy and sequential ion beam assistance. This was again consistent with atomic scale simulations, which indicated that metal layers deposited over an interface before ion assistance was initiated impeded atom exchange across interfaces and therefore intermixing.