U. Geyer

Diffusion mechanisms in metallic supercooled liquids and glasses

The mechanisms of atomic transport in supercooled liquids and the nature of the glass transition are long-standing problems. Collective atomic motion is thought to play an important role in both phenomena. A metallic supercooled liquid represents an ideal system for studying intrinsic collective motions because of its structural similarity to the “dense random packing of spheres” model, which is conceptually simple. Unlike polymeric and network glasses, metallic supercooled liquids have only recently become experimentally accessible, following the discovery of bulk metallic glasses. Here we report a 9Be nuclear magnetic resonance study of Zr-based bulk metallic glasses in which we investigate microscopic transport in supercooled liquids around the glass transition regime. Combining our results with diffusion measurements, we demonstrate that two distinct processes contribute to long-range transport in the supercooled liquid state: single-atom hopping and collective motion, the latter being the dominant process. The effect of the glass transition is clearly visible in the observed diffusion behaviour of the Be atoms.

Decomposition and nanocrystallization in reactively sputtered amorphous Ta–Si–N thin films

The nanocrystallization process of reactively sputtered thin amorphous Ta–Si–N films is investigated by anomalous small angle x-ray scattering (ASAXS) and x-ray diffraction (XRD). Changes in the microstructure in Ta40Si14N46 films, density variations in the amorphous matrix, decomposition, formation, and growth of nanocrystals after vacuum anneals at different temperatures in the range between 800 and 1000 °C are observed and the results of the different techniques are compared. From a Fourier analysis of ASAXS intensities the nanostructure of the investigated ternary system is derived using a model of hard spheres according to Guinier and Fournet. ASAXS investigations indicate that the noncrystalline samples can be described by a monophase fit and the crystallized samples by a bimodal-phase fit, the latter results being consistent with XRD which identifies TaN and Ta5Si3 phases. Detailed analysis shows that TaN nanograins of approximately 2 nm size develop after a decomposition process. Larger grains of ...

Island growth and surface topography of epitaxial Y‐Ba‐Cu‐O thin films on MgO

High‐quality epitaxial Y1B2Cu3Ox thin films (Tc,o ≥ (R18) 90 K, jc (77 K)≥ (R18)3 ×106A/cm2) were in situ grown on MgO by KrF excimer laser ablation. The combination of in situ resistance measurements, x‐ray diffraction experiments, Tc measurements, scanning electron microscopy and scanning tunneling microscopy gives clear indications for an island growth on these substrates and shows growth steps and spirals at the film surface.

Hydrogen-induced stress in Nb single layers

Abstract Niobium films prepared by molecular beam epitaxy (MBE) and electron beam evaporation (EB) were loaded electrolytically with hydrogen. Out-of-plane strain and in-plane stresses during hydrogen loading were determined using X-ray diffraction and substrate bending measurements. Stress and strain development during loading can be explained using a one-dimensional elastic expansion model up to concentrations of X H =0.05 H/Nb. Deviations from elastic behavior were observed above X H =0.05 H/Nb and at X H =0.20 H/Nb for MBE and EB samples, respectively. These concentrations are where phase separation occurs. Additionally, the stress increase in the EB films deviates from a linear elastic dependence above X H =0.07 H/Nb within the α-phase. The maximum measured stress is about −2.6 GPa.

Determination of elastic constants in thin films using hydrogen loading

By measuring stress and strain that build up in thin films during hydrogen absorption, the elastic constants of the films can be determined, if a one-dimensional elastic behavior occurs only. This will be demonstrated for hydrogen absorption in Nb films. The in-plane stress is determined from the substrate curvature that is measured by using a two-beam laser setup. The out-of-plane strain is measured via x-ray diffraction. Furthermore, this method allows us to distinguish whether the film is plastically or elastically deformed by checking the reversibility of the stress–strain curve. In the case of a 250-nm-thick Nb film, the elastic constants obtained are similar to that of bulk Nb.

Stress development in thin yttrium films on hard substrates during hydrogen loading

The present drive to make munitions as safe as is feasible and to develop predictive models describing their constitutive response, has led to the development and production of plastic bonded explosives and propellants. There is a range of elastomers used as binder materials with the energetic components. One of these is known as Kel-F-800™ (poly-chloro-trifluroethylene) whose structure is in some ways analogous to that of poly-tetrafluoroethylene (PTFE or Teflon). Thus, it is of interest to assess the mechanical behavior of Teflon and to compare the response of five different production Teflon materials, two of which were produced in pedigree form, one as-received product, and two from previous in-depth literature studies. The equations of state of these variants were quantified by conducting a series of shock impact experiments in which both pressure-particle velocity and shock velocity-particle velocity dependencies were measured. The compressive behavior of Teflon, based upon the results of this study, appears to be independent of the production route and additives introduced.

Long-time effects in a simulation model of sputter erosion

A simple (2 + I )-dimensional discrete model is introduced to study the evolution of solid surface molphologies during ion beam sputtering. The model is based on the same assumptions about the erosion process as the existing analytic theories. Due to its simple structure. simulations of the model can he performed on time scales where effects beyond the linearized theory become important. Whereas for short times we observe the formation of ripple structures in accordance with the linearized theory, we find a roughening surface for intermediate times. The long-time behavior of the model strongly depends on the surface relaxation mechanism.

Influence of deposition conditions on Ir/IrO2 oxygen barrier effectiveness

The influence of the deposition temperature during the reactive sputtering process on the microstructure of thin Ir and IrO2 films deposited on oxidized Si substrates was investigated and related to the oxygen barrier effectiveness. For this purpose differential thermal analysis combined with residual gas analysis by mass spectrometry was used for the investigation of the microstructural and chemical behavior of the as-sputtered IrO2 films upon heating. Moreover, in situ stress relaxation analyses up to 900 °C, in and ex situ x-ray diffraction measurements were done for various annealing conditions. The investigated polycrystalline IrO2 films exhibited a large compressive stress and a distorted lattice due to the sputter deposition process. It is demonstrated that a high deposition temperature involves a delayed relaxation of the IrO2 grains which is causing an extrinsic, enhanced defect controlled oxygen mobility for the annealing temperatures below the recrystallization. The well-known low intrinsic oxy...

Internal interfaces and intrinsic stress in thin amorphous Cu-Ti and Co-Tb films

A combined investigation of intrinsic stress formation by in situ substrate curvature measurements and of surface morphology evolution by scanning tunneling microscopy during the growth of amorphous Cu-Ti and Co-Tb films is reported. Intrinsic tensile stress and surface morphology are clearly correlated: all films that show intrinsic tensile stress formation in the late growth stages exhibit a cluster-like surface morphology and vice versa. Both the magnitude of the intrinsic tensile stress and the cluster size at the surface depend systematically on the reduced substrate temperature during film preparation. This dependence fits Hoffman’s model for tensile stress formation in thin films. Thus, the observed surface clusters are probably the top domes of growth columns, and the atomic mismatch at the column boundaries gives rise to tensile stress formation.

Oxygen tracer diffusion in IrO2 barrier films

The 18O tracer diffusion method was used to investigate oxygen diffusion in reactively dc-sputtered IrO2 films. The profile measurements were done by secondary ion mass spectrometry. For the investigation of the oxygen diffusivity in the samples a temperature range from 600 to 765 °C was chosen. The oxygen tracer diffusion in IrO2 films was found to be described by an Arrhenius law with D0=(2.8±2.5)10−6 m2 s−1 and an activation energy of Ea=(2.73±0.07) eV. It was also shown that the extrinsic oxygen diffusion is strongly influenced by the film preparation conditions, which is especially important for the application of IrO2 films as an oxygen barrier in future memory device applications.