Petr Hruška

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.

Quantum chaos in the nuclear collective model: Classical-quantum correspondence.

Spectra of the geometric collective model of atomic nuclei are analyzed to identify chaotic correlations among nonrotational states. The model has been previously shown to exhibit a high degree of variability of regular and chaotic classical features with energy and control parameters. Corresponding signatures are now verified also on the quantum level for different schemes of quantization and with a variable classicality constant.

Quantum chaos in the nuclear collective model. II. Peres lattices

This is a continuation of our paper [Phys. Rev. E 79, 046202 (2009)] devoted to signatures of quantum chaos in the geometric collective model of atomic nuclei. We apply the method by Peres to study ordered and disordered patterns in quantum spectra drawn as lattices in the plane of energy vs average of a chosen observable. Good qualitative agreement with standard measures of chaos is manifested. The method provides an efficient tool for studying structural changes in eigenstates across quantum spectra of general systems.

Influence of Deformation on Precipitation Kinetics in Mg-Tb Alloy

Precipitation effects in age-hardenable Mg-13wt.%Tb alloy were investigated in this work. The solution treated alloy was subjected to isochronal annealing and decomposition of the supersaturated solid solution was investigated by positron annihilation spectroscopy combined with transmission electron microscopy, electrical resistometry, differential scanning calorimetry and microhardness measurements. Peak hardening was observed at 200°C due to precipitation of finely dispersed particles of β phase with the D019 structure. Vacancy-like defects associated with β phase particles were detected by positron annihilation. At higher temperatures precipitation of β and subsequently β phase takes place. Formation of these phases lead to some additional hardening and introduces open volume defects at precipitate/matrix interfaces. To elucidate the effect of plastic deformation on the precipitation sequence we studied also a Mg-13wt.%Tb alloy with ultra fine grained structure prepared by high pressure torsion. In the ultra fine grained alloy precipitation of the β phase occurs at lower temperature compared to the coarse grained material and the peak hardening is shifted to a lower temperature as well. This effect can be explained by enhanced diffusivity of Mg and Tb atoms due to a dense network of grain boundaries and high density of dislocations introduced by severe plastic deformation. Moreover, dislocations and grain boundaries serve also as nucleation sites for precipitates. Hence, precipitation effects are accelerated in the alloy subjected to severe plastic deformation.

Positron annihilation study of cavities in black Au films

Defects in a black Au film were studied using variable energy positron annihilation spectroscopy. Black Au films exhibit porous morphology similar to cauliflower. This type of structure enhances the optical absorption due to a multiple reflections in the micro-cavities. A nanostructured black Au film was compared with conventional smooth Au films with high reflectivity. The black Au film exhibited a remarkably enhanced S-parameter in sub-surface region. This is caused by a narrow para-Positronium contribution to the annihilation peak.

Hydrogen-Induced Defects in Titanium

The aim of this work was investigation of hydrogen interaction with defects in Ti. Well-annealed Ti samples were loaded with hydrogen either electrochemically or using H2 gas phase. The hydrogen content and the phase composition of hydrogen loaded samples was determined by thermogravimetric analysis and by X-ray diffraction (XRD) respectively. Positron lifetime (LT) spectroscopy was employed for investigation of defects created by hydrogen loading. High-temperature H2 gas loading led to complete transformation of the whole sample into δ-TiH2 while low-temperature H2 gas loading and the electrochemical loading resulted in a non-uniform structure with most of the hydrogen absorbed in a sub-surface layer. All hydrogen-loaded samples exhibit positron lifetime component of ≈ 170 ps corresponding to positrons trapped at dislocations. Vacancy clusters were observed in the electrochemically loaded sample and the H2 gas phase loaded sample at low temperature.

Characterisation of Porosity in Zirconia-Based Nanopowders

Porosity in several zirconia-based pressure compacted nanopowders was studied using the positron lifetime technique combined with the mass-density measurements. Two kinds of pores were identified: (i) the larger pores of ≈ 10 to 19 nm diameter arising likely from a formation of secondary particle aggregates, and (ii) the smaller ones (≈ 1 nm) which are obviously of a more complex origin.

Influence of dislocations on precipitation processes in hot-extruded Al–Mn–Sc–Zr alloy

Abstract Solute clustering and precipitation processes in hot-extruded Al–Mn–Sc–Zr alloy were studied. Positron annihilation studies were combined with electrical resistometry, microhardness testing and microstructural characterization. Extrusion introduced dislocations which arranged into a cell structure. Sc solutes segregated at dislocations. Moreover, fine coherent Al3(Sc,Zr) particles precipitated during extrusion and subsequent cooling. During the isochronal annealing Sc agglomerated forming Sc-rich dispersoids. Agglomeration of Sc solutes is facilitated by pipe diffusion along dislocations. Zr atoms become mobile above 240 °C and form shells surrounding Al3Sc particles. This process is not affected by dislocations. The maximum fraction of positrons annihilated by Sc and Zr electrons occurred at 300 °C, coinciding with hardening of the alloy. The resistivity changes above 300 °C are caused by precipitation of Mn-containing particles.

Study of Defects in High Energy Ion Implanted ZnO Crystals

Positron annihilation spectroscopy (PAS) was employed for characterization of defects in the hydrothermally (HT) grown zinc oxide single crystals irradiated by high energy ions. Defects created in ZnO crystals by 2.5 MeV protons, 7.5 MeV N3+ and 167 MeV Xe26+ ions were compared. The virgin ZnO crystals contain Zn-vacancies associated with hydrogen. Ion implantation introduced additional defects, namely Zn+O di-vacancies in crystals irradiated by protons and small vacancy clusters in samples implanted by N and Xe ions.

Effects in Mg-Zn-based alloys strengthened by quasicrystalline phase

Magnesium Mg-based alloys are promising lightweight structural materials for automotive, aerospace and biomedical applications. Recently Mg-Zn-Y system attracted a great attention due to a stable icosahedral phase (I-phase) with quasicrystalline structure which is formed in these alloys. Positron lifetime spectroscopy and in situ synchrotron X-ray diffraction were used to study thermal stability of I-phase and precipitation effects in Mg-Zn-Y and Mg- Zn-Al alloys. All alloys containing quasicrystalline I-phase exhibit misfit defects characterized by positron lifetime of ~ 300 ps. These defects are associated with the interfaces between I- phase particles and Mg matrix. The quasicrystalline I-phase particles were found to be stable up to temperatures as high as ~ 370°C. The W-phase is more stable and melts at ~ 420°C. Concentration of defects associated with I-phase decreases after annealing at temperatures above ~ 300°C.