C. Dauwe

Positronium trapping in free volume of polymers.

Abstract Positronium trapping process caused by slow thermalization of Ps inside intermolecular spaces of polymer is considered. The pickoff annihilation rate of Ps decreases from an initial value, characteristic for the delocalized Ps, to a final value, relating to its localized state. The elastic thermalization lifetime analysis of polymers spectra with a new ETLT program is proposed to determine the total Ps intensity ( I Ps ), Ps trapping rate ( γ ), its initial ( τ 0 ) and final ( τ ∞ ) pickoff lifetimes as well as the relative e + –e − contact density ( η ). The model is tested through the analysis of polyurethane spectra measured in a wide range of temperature, containing the glass transition point ( T g ). In contrast to the conventional analysis, I Ps does not display any change at T g . The results indicated slow trapping rate (about 1 ns −1 ) of Ps in the cavities.

Room-temperature grain growth in sputter-deposited Cu films

The microstructure of sputter-deposited Cu films is shown to be unstable at room temperature (RT). The average grain size increases significantly during RT storage for a couple of hours after deposition. This RT grain growth is shown to be very dependent on the deposition parameters (substrate temperature and sputter gas pressure) and hence on the microstructure of the as-deposited film. The microstructure of sputter-deposited films is usually summarized in a structure-growth-zone model. It is found that significant RT grain growth only occurs for Cu layers with a zone-T-type, as-deposited microstructure.

On the Mossbauer parameters in BiFeO3

The Mossbauerspectrum of BiFeO3 is shown to consist of two super-imposed six-line spectra. The main distinction between the two iron sites is made up by the different ΔEQ values, even having opposite sign. A qualitative explanation is proposed.

Positron lifetimes and lineshape factors in deformed copper

Measurements of positron annihilation lifetime and lineshape factors in deformed OFHC copper, for 14 different pairs of specimen, ranging up to 66% deformation, are discussed. The influence of annihilation in the source is carefully examined and the data are consequently corrected. The lifetime varies from (124±2) ps for annealed copper to (179±2) ps for dislocation saturated samples.Trapping cross sections and trapping efficiencies are calculated. The possibility of interference of vacancies formed during the deformation is discussed.

Mössbauer spectroscopy study of MgAl2O4-matrix nanocomposite powders containing carbon nanotubes and iron-based nanoparticles

Materials involved in the catalytic formation of carbon nanotubes are for the first time systematically studied by Mossbauer spectroscopy between 11 K and room temperature. Mg1−xFexAl2O4 (x=0.1, 0.2, 0.3, 0.4) solid solutions are transformed into carbon nanotubes–Fe/Fe3C–MgAl2O4 composite powders by reduction in a H2–CH4 gas mixture. The oxides are defective spinels of general formulae (Mg1−x2+Fex−3α2+Fe2α3+□αAl23+)O42−. Ferromagnetic α-Fe, ferromagnetic Fe3C and a γ-Fe form, the latter possibly corresponding to a γ-Fe–C alloy, are detected in the composite powders. An attempt is made to correlate these results with the microstructure of the powder. It seems that the nanoparticles, which catalyze the formation of the carbon nanotubes, are detected as Fe3C in the post-reaction Mossbauer spectroscopy analysis.

Evolution of the positron annihilation lifetime for ageing in β phase Cu-Al-Ni-(Ti)-(Mn) shape memory alloys

During the last years, the positron annihilation technique has proven to be a sensitive tool for the characterization of phase transitions, and in particular of the martensitic transformation in Cu-based alloys. The differences in structure between the high temperature phase (cubic) and the martensite (monoclinic) produce distinctive positron annihilation characteristics which allow the determination of the transformation temperature range. In the present article the influence of these parameters on the positron annihilation characteristics of three Cu-Al-Ni shape memory alloys is discussed. Two of the chosen compositions contain Ti and Mn, which have been added to the ternary Cu-Al-Ni alloy in order to improve its mechanical properties. The effect of those elements has also been evaluated.

On the effective magnetic hyperfine field in naturally occuring goethite

Abstract The asymmetric broadening of the Mossbauer absorption lines frequently observed in goethite is shown to be due to relaxation effects in the surface layers rather than being an intrinsic property of this material. The particle size determines entirely the shape of the observed M-spectrum at room temperature which can vary from a magnetically ordered spectrum to a pure paramagnetic one.

Positron annihilation study of nanocrystalline iron.

Abstract Positron lifetime measurements on different nanocrystalline iron samples show that the microstructure, with respect to the inherent free volumes in the samples, can be determined. Three distinct defect types can be detected: i) a vacancy-like defect in the interface, ii) a microvoid at the intersection of interfaces and iii) larger free volumes (missing grains) where the annihilation of ortho-positronium is observed. The effect of the crystallite size and preparation conditions of the samples are discussed.

Incomplete magnetic ordering in Fe2(1−y)Mg1+y Ti y O4 spinels with intermediate compositiony

Static magnetization measurements on the ferrimagnetic spinels Fe2(1−y)Mg1+yTiyO4 withy=0.3, 0.4, 0.5, and 0.6 show that these compounds have no well-defined orderdisorder transition temperature and that their ferrimagnetism may not be described in terms of the Néel theory. From the Mössbauer spectra we conclude that a temperature dependent number of the ferric ions does not participate in the ferrimagnetism of those compounds with compositiony≧0.4. The explanation of the observed magnetic and Mössbauer properties is based on the assumption that each ferric ion must have at least two magnetic linkages of the type FeA3+−O2−−FeB3+ in order to couple its magnetic moment to the neighbouring ones over the entire temperature interval between 0 K and the respective Néel temperature.

A new view on positronium in polymers

Abstract A new model is presented describing the annihilation of positronium in polymers. Ps is formed in the bulk and is trapped into the free volume holes with epithermal initial energy where it thermalizes towards its ground state. This model was verified by interpreting lifetime spectra in an amorphous cross-linked polyurethane.