Oksana Melikhova

Origin of green luminescence in hydrothermally grown ZnO single crystals

Combining photoluminescence and positron annihilation studies of hydrothermally grown ZnO crystals with stoichiometry varied by controlled annealing enabled us to clarify the origin of green luminescence. It was found that green luminescence in ZnO has multiple origins and consists of a band at 2.3(1) eV due to recombination of electrons of the conduction band by zinc vacancy acceptors coupled with hydrogen and a band at 2.47(2) eV related to oxygen vacancies. The as-grown ZnO crystals contain zinc vacancies associated with hydrogen and exhibit a green luminescence at 2.3(1) eV. Annealing in Zn vapor removed zinc vacancies and introduced oxygen vacancies. This led to disappearance of the green luminescence band at 2.3(1) eV and appearance of a green emission at higher energy of 2.47(2) eV. Moreover, the color of the crystal was changed from colorless to dark red. In contrast, annealing of the as-grown crystal in Cd vapor did not remove zinc vacancies and did not cause any significant change of green luminescence nor change in coloration.

Vacancy clusters in ultra fine grained metals prepared by severe plastic deformation

Severe plastic deformation of metals introduces not only dislocations, but also a high concentration of vacancies. In the present work we employed positron lifetime spectroscopy for investigation of deformation-induced vacancies in ultra fine grained metals prepared by high pressure torsion. It was found that in all metals studied deformation-induced vacancies agglomerate into small vacancy clusters. Experimental positron lifetime results were combined with ab-initio theoretical calculations of positron parameters for vacancy clusters of various sizes. This new approach described in this paper enables to determine the size distribution of vacancy clusters.

Investigation of spatial distribution of defects in ultra-fine grained copper

Ultra-fine grained copper prepared by high pressure torsion has been studied by means of slow positron implantation spectroscopy with Doppler broadening measurement. In addition, conventional positron lifetime and Doppler broadening spectroscopy have been utilised. Defects present in the specimens were identified, their spatial distribution and depth profile have been determined. The results are discussed in correlation with those obtained by XRD and TEM.

Modification of steel surfaces induced by turning: non-destructive characterization using Barkhausen noise and positron annihilation

This paper deals with the characterization of sub-surface damage caused by the machining of 100Cr6 roll bearing steel. The samples turned using tools with variable flank wears were characterized by two non-destructive techniques sensitive to defects introduced by plastic deformation: magnetic Barkhausen noise and positron annihilation. These techniques were combined with light and electron microscopy, x-ray diffraction and microhardness testing. The results of the experiment showed that damage in the sub-surface region increases with increasing flank wear, but from a certain critical value dynamic recovery takes place. The intensity of Barkhausen noise strongly decreases with increasing flank wear due to the increasing density of the dislocations pinning the Bloch walls and suppressing their motion. This was confirmed by positron annihilation spectroscopy, which enables the determination of the dislocation density directly. Hence, a good correlation between Barkhausen noise emission and positron annihilation spectroscopy was found.

Defect Behaviour in Yttria-Stabilised Zirconia Nanomaterials Studied by Positron Annihilation Techniques

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.

Positron annihilation study of yttria-stabilized zirconia nanopowders containing Cr2O3 additive

Yttria-stabilized zirconia compacted nanopowders, doped with trivalent chromium oxide, were studied by means of high-resolution positron lifetime and coincidence Doppler broadening techniques. The observed data suggest that positrons annihilate mainly in vacancylike defects at grain boundaries or in larger open volumes most likely located at triple points. The results also show that an addition of Cr2O3 leads to a decrease in grain size.

Characterization of point defects in yttria stabilized zirconia single crystals

Characterization of point defects in a fully stabilized ZrO2 + 9 mol.% Y2O3 single crystal with cubic structure was performed in this work. It was found that the crystal contains a high density of vacancy-like defects characterized by a lifetime of 175 ps. First principle theoretical calculations showed that this lifetime is comparable with lifetime of positrons trapped in zirconium vacancies associated with hydrogen. In particular, in the vicinity of the zirconium vacancy hydrogen forms an O-H bond with one of the nearest neighbour oxygen atoms. The calculated bond length is close to 1 A. Using nuclear reaction analysis it was found that the hydrogen concentration in the crystal is 0.3 at.%. This amount of hydrogen is sufficient to form zirconium vacancy – hydrogen complexes capable of saturated positron trapping.

Mechanical properties of bimetallic crystalline and nanostructured nanowires

Nanowires are basic components of interconnects at the nanoscale level in electronic as well as in electromechanical devices. Presently, there is a fast growing interest in their synthesis as well as in their mechanical testing. Focused ion beams now allow machining pillars with diameters as small as a few tens of nanometres and nanoindenter systems allow measuring strains at the atomic scale and compressive stresses up to the 10 GPa range. Such pillars typically contain less than millions of atoms, which makes their modelling and the modelling of their mechanical properties at the atomic scale realistic. A few Molecular Dynamics studies are presently available, discussing deformation mechanisms in thin narrow crystalline nanowires, but the literature about nanoalloy wires and nanostructured wires, as they can be synthesized from clusters, is almost non-existent. In the latter, the dislocation activity may be inhibited, leading to specific mechanical properties. By means of large scale computations, we use Ni3A1 to discuss the mechanical properties of crystalline and nanostructured nanowires. We also compare wires to their bulk counterparts. Both isothermal and isoenergetic whereby mechanical work converts into heat in the system-deformation mechanisms are considered. The comparison between pair correlation functions, stress distributions, configuration analysis and strain stress relations capture most of the stress-induced evolution mechanisms of nanowires with different diameters and structures, including elastic properties, dislocation activity, grain rotation and boundary motion, local melting, superplasticity and fracture. A structural transition which may be martensitic is predicted for the first time at the nanoscale level, suggesting possible shape memory properties of nanoalloy nanowires.

Mechanical properties of AgCo nanostructured nanowires

The mechanical deformation properties of nanostructured AgCo nanowires were studied by Molecular Dynamics (MD) under uniaxial tensile and compressive stresses. The cohesion of the immiscible AgCo system was modeled by an Embedded Atom Model (EAM). Crystalline Co grains were embedded into a Ag crystalline matrix and the whole system was set to relax by MD at 300 K. The Ag crystal structure turned out to be unstable and to transform into the same layered structure as was already predicted in AgCo nanoclusters and AgCo Cluster Assembled Materials (CAM). Deformation was modeled by varying the length of the nanowire in the axial direction. Both isothermal and isoenergetic—whereby mechanical work converts into heat in the system—deformation mechanisms were considered. In contrast with the case of AgCo CAM where an elastic regime was identified, deformation of AgCo nanowires was never found fully elastic, whatever the magnitude of the deformation. In case of constant temperature deformation, Ag kept its layered structure and the Co grains kept crystalline. At constant energy, as a consequence of the rise of temperature, phase separation and melting took place whereby a coaxial structure was formed with the Co covered by Ag.

Hydrogen Interaction with Defects in Nanocrystalline, Polycrystalline and Epitaxial Pd Films

Hydrogen interaction with defects and structural development of Pd films with various microstructures were investigated. Nanocrystalline, polycrystalline and epitaxial Pd films were prepared and electrochemically loaded with hydrogen. Structural changes of Pd films caused by absorbed hydrogen were studied by in-situ X-ray diffraction combined with acoustic emission and measurement of electromotorical force. Development of defects during hydrogen loading was investigated by positron annihilation spectroscopy. It was found that hydrogen firstly fills open volume defects existing already in the films and subsequently it occupies also interstitial sites in Pd lattice. Absorbed hydrogen causes volume expansion, which is strongly anisotropic in thin films. This introduces high stress into the films loaded with hydrogen. Acoustic emission measurements revealed that when hydrogen-induced stress achieves a certain critical level rearrangement of misfit dislocations takes place. The stress which grows with increasing hydrogen concentration can be further released by plastic deformation and also by detachment of the film from the substrate.