Defect and Diffusion Forum Vol. 331

Abstract: Hydrogen absorption in Pd causes a significant volume expansion. In free-standing bulk Pd, the hydrogen-induced volume expansion is isotropic. However, the situation becomes more complicated in thin Pd films. Contrary to bulk samples, thin films are clamped to an elastically stiff substrate, which prevents in-plane expansion. Hence, the volume expansion of a thin film is strongly anisotropic because it expands in the out-of-plane direction only. Internal stresses introduced by absorbed hydrogen may become so high that detachment of a film from the substrate is energetically favorable and buckles of various morphologies are formed. In the present work, we studied hydrogen-induced buckling in a nanocrystalline thin Pd film deposited on a sapphire substrate. Slow positron implantation spectroscopy (SPIS) was employed as a principal tool for the characterization of defects and investigation of defect interactions with hydrogen. SPIS studies were combined with X-ray diffraction and direct observations of buckling by light microscopy. It was found that buckling of thin Pd film occurs at hydrogen concentrations x_H > 0.1 and is accompanied by a strong increase of dislocation density.

Abstract: Oxide-dispersion-strengthened (ODS) FeCrAl steel is a class with promising materials to be applied for future nuclear applications. However, radiation damage, especially the formation of vacancy clusters or gas-filled bubbles, may result in hardness increase and the loss of ductility. Positron annihilation spectroscopy (PAS) is demonstrated to be a very useful and non-destructive analysis method to detect and to determine open volume defects of sub-nm size in ODS alloy. Synchronized dual beam implantation of Fe and He ions is performed to simulate the radiation damage caused by (n, α) reactions and to avoid induced activation. For room temperature implantation, i.e. without significant point defect recombination, the differences in the defect formation are shown by comparison between irradiation of ODS alloy and pure Fe bulk. The open volume defects created in ODS alloy are vacancy clusters closely connected with dispersed Y oxide nanoparticles. Their profiles are in reasonable qualitative agreement with the hardness profiles, indicating a relationship between sub-nm vacancy clusters or He bubbles and the hardness of the material. In heat-treated ODS alloy, containing larger vacancy clusters, the radiation induced hardness increase is more distinctive than for as-received ODS alloy. For irradiation at a moderately enhanced temperature of 300°C open volume defects are drastically reduced. The few remaining defects are vacancy clusters of the same type as in as-received ODS alloy. Close to the surface the open volume defects completely disappear. These results are in agreement with the hardness measurements showing little hardness increase in this case. The suitability of ODS-based materials for nuclear applications was verified.

Abstract: The aim of this work was to investigate the effect of Cr on the microstructure of neutron-irradiated Fe-Cr alloys. Neutron irradiation-induced damage at its early stage was simulated by ion implantation. Positron Annihilation Spectroscopy was applied to identify irradiation-induced defects depending on the Cr-content in Fe-Cr alloys. Different irradiation scenarios were used to investigate the influences of irradiation step by step.

Abstract: Recent experimental and theoretical investigations on a variety of yttria-stabilised zirconia (YSZ) nanomaterials are reviewed. The investigations were conducted within the frame of a collaboration of three institutions: (i) Charles University in Prague, (ii) Helmholtz Centre Dresden-Rossendorf and (iii) Donetsk Institute for Physics and Engineering of the NAS of Ukraine, Materials studied involved pressure-compacted nanopowders of binary and ternary (with Cr₂O₃ additive) YSZ and YSZ ceramics obtained by sintering the nanopowders. The nanopowders were prepared by the co-precipitation technique. Positron annihilation spectroscopy including the conventional positron lifetime (LT) and coincidence Doppler broadening (CDB) techniques was employed as the main experimental tool. Slow positron implantation spectroscopy (SPIS) was used in investigation of commercial YSZ single crystals for reference purposes. Extended state-of-art theoretical ab-initio calculations of positron response in the ZrO₂ lattice were carried out for various vacancy-like defect configurations. It was suggested by these calculations that none of the oxygen-vacancy related defects are capable to trap positrons. On the other hand, zirconium vacancy was demonstrated by the calculations to be a deep positron trap, even in the case that a hydrogen atom is attached to the vacancy. The measured positron LT data clearly indicated that positrons annihilate in nanopowders predominantly from trapped states at defects of two kinds: (a) the vacancy-like misfit defects concentrated in layers along the grain boundaries and characterised with lifetimes of 0.180 ns, and (b) the larger defects of open volume comparable to clusters of a few vacancies which are situated at intersections of three (or more) grain boundaries (characteristic lifetimes of 0.380 ns). The intensity ratio of LT components corresponding to these two kinds of defects was found to be correlated with the mean particle size. This correlation reconfirms the above interpretation of LT components and, moreover, the measured ratios could be used to estimate changes of the mean particle size with chromia content or sintering temperature. It was shown in this way that chromia addition to the YSZ nanopowder leads to a smaller particle size compared to the binary YSZ. Similarly, grain growth during sintering could be monitored via this intensity ratio. A portion of 10 % of positrons was found to form positronium (Ps) in compacted binary YSZ nanopowders. The observed ortho-Ps lifetimes correspond to Ps pick-off annihilation in cavities of 3 nm size which may be expected to occur between the primary nanoparticles. On the other hand, an addition of chromia at a concentration as low as 0.3 mol.% appeared to be sufficient to suppress Ps formation below the detection limit. Similarly, Ps formation could not be detected in binary YSZ sintered for 1 hour at a temperature of 1000 °C or higher. The former effect indicates an enhanced concentration of Cr cations at the particle surfaces, while the latter one appears to be due to a decrease of cavity concentration induced by sintering. The measured CDB data supported the idea that vacancy-like trapping centres are similar to zirconium vacancies and gave further evidence of a strong segregation of Cr segregation at particle interfaces. SPIS was further involved in a trial experiment on binary YSZ nanopowders and sintered ceramics. This experiment clearly demonstrated that SPIS may reveal valuable information about changes of depth profiles of microstructure during sintering, e.g. a sintering induced diffusion of defects from sample interior to its surface.

Abstract: The application of variable energy positron annihilation spectroscopy (VE-PAS) methods to the study of perovskite oxide, ABO₃, material thin films and near-surface regions is reviewed. The primary focus is on ferroic perovskite titanate oxide materials SrTiO₃ and Pb (Zr_xTi_1-x)O₃, but studies of BaTiO₃, La_xSr_1-xCoO₃, La_0.67Sr_0.33MnO₃ and YBa₂CuO_7-δ are also included. Characterization of single layer and multilayer structures is discussed. The methods, in particular positron annihilation lifetime spectroscopy, allow the identification of cation vacancy defects at both the A-and B-sites with parts per million sensitivity. Varying oxygen deficiency is often observed to result in marked changes in PAS spectra; these effects are reviewed and discussed.

Abstract: After a general introduction to the field of resistive switching and spin electronics and the role of defects therein, recent investigations on the above mentioned topics including positron beams are reviewed. An ongoing project at the Helmholtz Centre Dresden-Rossendorf to further extend such investigations is briefly outlined and expected benefits are mentioned.

Abstract: Positron chemistry refers to chemical processes of high-energy positrons injected into molecular substances, the most interesting of which is the formation of positronium (Ps), the hydrogen-like bound state between a positron and an electron. Ps is formed predominantly by fast intra-track radiation chemical processes. In polymers it tends to be localized in intra/inter-molecular open space in the sparsely packed amorphous structure. Whilst short-lived singlet para-positronium (p-Ps) undergoes self-annihilation, the positron in long-lived triplet ortho-positronium (o-Ps) annihilates with one of the spin opposite electrons bound in the surrounding polymer molecules. This process is called pick-off annihilation. The pick-off annihilation lifetime reflects the polymer chain packing through the size of the volume, where Ps is localized. Positrons are used to probe the amorphous structure of various polymeric systems. In this article, basic concepts and experimental techniques of positron chemistry in polymers as well as applications to the characterization of functional polymeric materials are overviewed.

Abstract: Positron annihilation spectroscopy (PAS) is a novel method that can provide molecular-level information about complex biological and macromolecular structure in a manner which is different, but complementary, to conventional medical and biochemical research methodology. Positron annihilation lifetime spectroscopy (PALS) and Doppler broadening energy spectroscopy (DBES), coupled with a slow positron beam have been extensively applied to the life science research recently. These techniques provide new information about the atomic and molecular level free-volume and void sizes, and their distributions 0.1 nm to a few nm, molecular bonding, structures at depth-layers, and phase transitions. This paper is to review recent research on positron annihilation spectroscopy applied to the area of life science and also focus on current bioscience-related work in the positron group at the University of MissouriKansas City (UMKC).