Volume 99, Issue 5

A laser assisted tomographic atom probe has recently been designed. The use of femtosecond laser evaporation pulses rather than of high-voltage pulses has opened the field of application of the technique to new materials such as semi-conductors and oxides. These classes of materials are of utmost importance in microelectronics for the design of nano-chips and nano-transistors in integrated devices. This type of instrument is the only 3D analytical microscope capable of mapping out the distribution of elements in a small volume (50 × 50 × 100 nm 3 ) on a nearly atomic scale. In addition, the tomographic atom probe enables us to get quantitative composition measurements. This review illustrates the potential of this new instrument (laser assisted wide angle tomographic atom probe LaWaTAP) when applied to study scientific topics in nano-sciences. As an example, both the distribution of addition elements (Pt) during inter-diffusive reaction in NiSi contacts of nano-transistors and the investigation of interfaces in MgO/Fe tunnel junctions are presented and discussed.

We have studied the structural properties and the magnetization density profiles in multilayers of the Heusler phase CoMnGe with three different spacer layers: V, Au, and AlO, using hard X-ray scattering, soft X-ray resonant magnetic scattering, and polarized neutron reflectivity. We have found that the spacer layer has a significant effect on the magnetization profile of the CoMnGe layer. In all cases the interlayers at the top and the bottom of the Heusler layer exhibit a reduced magnetic moment, the thickness of which depends on the spacer layer and increases from 0.5 nm for V, to 0.6 nm for Au and 1 nm for AlO.

The present study gives an overview of the fundamental material properties of perovskite-type hydrides. Formation ability of the perovskite-type hydrides has been reasonably explained on the basis of the geometric restrictions on the composed ions using the Goldschmidt tolerance factor. Characteristic crystal structures and charge density maps of the perovskite-type hydrides without/with transition metals, such as NaMgH 3 /CaNiH 3 , were also examined in detail to clarify the relationship between the structures (atomic and electronic) and the hydriding properties.

Based on the thermodynamics of inhomogeneous systems, it is expected that the chemical transition at an interface between two immiscible components cannot be atomically sharp. However, measurements by direct local chemical analysis are rare, as utmost spatial resolution on an atomic scale is required. In this article, the temperature dependence of the chemical width of layer interfaces in binary Cu/Ag and ternary Cu/Ni 81 Fe 19 specimens is studied experimentally. Atom probe techniques have been used to achieve sufficient spatial resolution. In both materials the interface width could be measured on a length scale of one to two nanometres. Its temperature dependence is found to be in agreement with the Cahn – Hilliard theory. From the measurements the gradient energy coefficient and the specific interfacial Gibbs energy are quantitatively determined.

The high temperature decomposition transformation of lithium amide into imide has been studied by anelastic spectroscopy and thermogravimetry. At the amide-to-imide phase transformation a huge elastic modulus increase takes place, whose change increases with the evolution of the decomposition. The modulus variation has been used to monitor the time and temperature progression of the reaction, and indications have been obtained that the reaction rates are faster than those usually reported. Moreover, formation of different phases is also detected at extremely low concentrations.

The migration of a faceted high angle tilt grain boundary 85° in Zn was investigated. The migration behavior complies with theoretical predictions of a recently proposed model on the joint motion of curved and faceted boundary segments. The results are compared to data published for a 30° boundary. It is shown that the observed dynamic facet length corresponds to a maximum dissipation rate of free energy.

The heterophase boundaries between precipitates such as spherical amorphous SiO particles inside a copper matrix as well as crystalline CuO precipitates inside a silver matrix and the surrounding metal matrix are examined with high spatial resolution using a scanning transmission electron microscope equipped with an electron energy loss spectrometer. Typical features of the electron energy loss near – edge fine structure of the oxygen O-K and of characteristic metal ionization edges allow determination of the bonding state of oxygen and thus detection of fine interlayers at the phase boundary. The results obtained are interpreted in a tentative way, supporting basic ideas of Kirchheim's structural vacancy model of oxygen segregation to metal – oxide interfaces. The model predicts that no interfacial accumulation of excess oxygen should occur to the phase boundary of amorphous precipitates. In Ag – Cu alloys there is the additional degree of freedom for the oxidation value of copper and we present indications that structural vacancies change the oxidation value at the phase boundary.

The concept of Chemical Confinement is introduced to describe a relaxation mode occurring in copolymers with a non-random distribution of monomer units. Poly(ethene-co-methacrylic acid) copolymer turns out to be an appropriate test system. By suitable selection of the polymerization conditions, the randomness of the distribution of methacrylic acid units along the polymer backbone may be tuned without significantly affecting the degree of polymerization and the methacrylic content of the copolymer. By interaction via hydrogen bonds, associating methacrylic units can confine the relaxation mode of chain segments positioned in between two such associating sites. Variation of the methacrylic content results in a shift of the relaxation mode. Mechanical spectroscopy and differential scanning calorimetry are used to validate the model of Chemical Confinement.

In this study, two different nano-structured samples of the FeTi compound were prepared by mechanical alloying and mechanical grinding. For these samples, kinetics of the initial rate of hydrogen absorption, and the equilibrium hydrogen pressure as a function of hydrogen concentration were measured. Mechanical alloying of Fe and Ti atoms produced the FeTi compound powder samples with microstructures of a mixture of nano-structured FeTi grains and amorphous phases. This sample exhibited a high initial rate of hydrogen absorption even at 298 K, however, a strongly reduced hydrogen storage capacity. Mechanical grinding of the FeTi produced samples of particles with a particular microstructure: surface layers with a mixture of nano-structured FeTi grains and amorphous phases, and a single crystalline phase of FeTi below the surface layers for each particle. This sample exhibited a high initial rate of hydrogen absorption without a significant reduction of the hydrogen storage capacity compared with that of the standard FeTi sample. This mechanical grinding treatment was found to be an effective method of surface modification to improve the initial activation of the FeTi hydrogen storage alloy.

The nucleation, growth, and coarsening kinetics of Cu-rich precipitates in a multicomponent Fe – Cu based steel, containing 1.17 at.% Cu, aged at 500 °C for up to 1024 h are investigated. The temporal evolution of the precipitate, heterophase interface, matrix compositions and precipitate morphology are presented. Coarsening temporal exponents are determined for mean radius, number density, and supersaturations, and are compared to the Lifshitz – Slyzov – Wagner model for coarsening, modified for multicomponent alloys by Umantsev and Olson. The experimental results indicate that the alloy does not strictly follow Umantsev – Olson model behavior. Additionally, we compare the results to an investigation of a similar multicomponent steel containing 1.82 at.% Cu and to results in the literature. Furthermore, we present a Thermo-Calc determined phase diagram.

In this work the hydrogen absorption behaviour of two types of Pd-clusters, different in structure but similar in size (3.1 nm), will be presented. First, icosahedral Pd clusters stabilised in surfactants; second, cubic Pd clusters stabilised in a Teflon-AF matrix. The phase transition in these samples was monitored by in-situ X-ray diffraction. It will be shown that the hydrogen uptake ability depends strongly on the lattice structure, which is affected by the type of stabiliser. Teflon AF stabilised clusters (cubic clusters) show the phase transition which is common for bulk Pd, whereas the surfactant stabilised clusters (icosahedral clusters) show only weak lattice dilatation upon hydrogen absorption. Pressure – lattice parameter isotherms show that the cubic clusters absorb large amounts of hydrogen in comparison to the icosahedral clusters. The measured lattice expansion is 0.130 Å at 10 5 Pa and 300 K, which is about 320 % the amount measured for icosahedral clusters (0.04 Å). This suggests that surface sites are available for hydrogen in the Pd – Teflon-AF samples which are not accessible for Pd – surfactant-clusters, and that the icosahedral lattice absorbs less hydrogen for similar external pressures.

We used resonant ultrasound spectroscopy to measure the three independent elastic constants of PdH 0.71 and PdH 0.65 in the temperature range 1.4 K < T < 296 K. Measurements were performed as the crystals were rapidly cooled (∼10 K min − 1 ) and then slowly heated (∼0.1 K min − 1 ). During slow heating, the temperature dependence of the shear modulus C′ = (C 11 – C 12 )/2 shows a small peak at ∼55 K, but this peak is absent during fast cooling. We associate the peak with the so-called “50 K anomaly” in palladium hydride. In contrast, the other shear modulus, C 44 , shows no peak during slow heating. We propose that the 50 K anomaly observed in the elastic constants, and in other thermodynamic and transport properties of Pd-hydride, is a kinetic effect arising from changes in the hydrogen atom short-range order as the alloy is heated.

In this contribution Atom Probe Tomography has been applied systematically to characterise the homogeneity of mechanically alloyed Fe – Cu powders for two different compositions (Fe-10 at.% Cu, and Cu-2.5 at.% Fe). Fe – Cu is among the most attractive systems for technical application since it exhibits the Invar effect. Additionally it serves as a binary model system and hence is representative for those immiscible systems characterised by a large positive heat of mixing. Results on the extension of the solubility limits up to 10 at.% and on the homogeneity of the alloy – depending on composition and milling time – are presented, discussed and compared to earlier works. Chemical homogeneity of the sample as measured by the atom probe was never fully reached on the nano-scale, even for the longest milling time and for those samples which appear to be homogeneous via X-ray analysis. Furthermore, impurities such as oxygen, stemming from the preparation procedure, were frequently detected and analysed for the different samples. In addition, structural characterisation was performed using transmission electron microscopy and diffraction measurements to confirm the transition of Fe from bcc to fcc. Thus, nano-scale information on the local concentration, segregation effects and the distribution of impurities for the different nominal compositions and processing parameters could be gained.

The microstructure of solid-oxide fuel cell anodes, consisting of nickel particles embedded in either doped zirconia or ceria, was characterized in three different stages: (i) as sintered, (ii) after reduction, and (iii) after fuel cell operation for 500 h in an atmosphere of moist hydrogen. After 500 h of operation, we found that both types of anodes, those based on zirconia and those based on ceria, contained nanoparticles of elemental nickel. We propose that these nanoparticles form from nickel hydroxide, synthesized during fuel cell operation and precipitating elemental nickel where the partial pressure of water is low at the periphery of triple-phase boundaries or on cooling down from the operating temperature to room temperature.

Ball milling is an effective method for the synthesis and modification of light complex hydrides by changing the milling conditions. Using specially designed vials, in-situ variations of the hydrogen pressure and vial temperature taking place during the milling of LiAlH 4 , with and without dopants, and of LiBH 4 + MgCl 2 were monitored. This provided information about the efficiency of the reactions before removing the powder from the vial. The thermodynamic properties of the milled hydrides were analysed by differential scanning calorimetry and intelligent gravimetric analysis. Additional structural characterisation was carried out by X-ray diffraction and Raman spectroscopy. The results showed that the decomposition of LiAlH 4 during milling is strongly affected by the use of TiCl 3 as dopant. The partial formation of Mg(BH 4 ) 2 from LiBH 4 + MgCl 2 was demonstrated by Raman spectroscopy.

Gaseous hydrogen transport at pressures of 15 MPa is envisaged as a means of hydrogen delivery from central production facilities to refueling stations for the planned hydrogen economy. The study of the hydrogen embrittlement of medium or mild strength steels, which are under consideration for pipeline materials, has not as of yet led to methods to design safe and reliable pipelines. The most important failure modes in hydrogen containment components are due to subcritical cracking. However, current design guidelines for pipelines only tacitly address subcritical cracking by applying arbitrary, conservative safety factors on the applied stress. In the present work, we investigate the interaction of hydrogen transport with material elastoplasticity in the neighborhood of an axial crack in a steel pipeline. For all practical purposes, we find that the stress, deformation, and hydrogen fields exhibit a small scale character which allows for the use of the standard modified boundary layer approach to the study of the fracture behavior of steel pipelines. The approach is based on constraint fracture mechanics methodology whereby a two-parameter characterization – the stress intensity factor and the T-stress – is used to describe the interaction of the stress and deformation fields with the diffusing hydrogen under conditions of hydrogen uptake from the crack faces and outgassing through the outer boundaries, as in the pipeline. Employing the Rice and Tracey model of void growth, we find that hydrogen-induced softening can accelerate void growth in a small region confined at the crack tip by as much as 70 % relative to the case of a hydrogen-free material. We close by suggesting that one can ascertain the hydrogen effects on fracture at an axial pipeline crack with the use of a laboratory fracture mechanics specimen tested in hydrogen gas and subjected to the same intensity factor, and hydrostatic constraint, T-stress, as the real-life pipeline.