Browsing by Author "Prochazka, Ivan"

Morphological, structural and chemical evolution in Ti/B/H-2 system is studied in detail as a function of mechanical treatment. Ti/B powder continuously changes both in composition and morphology during ball-milling in H-2 flow: The powder composition varies from Ti/B to TiH2-x/B causing a change in mechanical properties. The role of boron additive also changes from preventing the Ti nanoparticles from sticking together in the early stages to a matrix material participating in Ti - B interface reactions in the intermediate and final stages of the process. Boron atoms participating in the formation of nanoscopic holes give rise to new H states in the hydride by changing the local atomic state of Ti atoms. The dynamics of the formation of these sites and the redistribution of hydrogen between different types of occupation sites in dependence of phase composition and milling time of the powders are also studied.

The present work reports on microstructure investigations of hydrogen-loaded nanocrystalline Gd films by means of slow positron implantation spectroscopy combined with in situ synchrotron radiation X-ray diffraction. It is found that the virgin films contain a high density of vacancy-like open volume defects at grain boundaries which trap positrons. These defects represent trapping sites also for hydrogen. With increasing hydrogen concentration the transformation from the alpha-into the beta-phase (GdH(2)) takes place in the film. Accumulation of hydrogen at grain boundaries causes a decrease of positron localization at defects. The transformation into the beta-phase is completed at x(H) approximate to 1.6 H/Gd. Contrary to bulk Gd specimens, the gamma-phase (GdH(3)) is not formed in the nanocrystalline Gd films. (c) 2008 Elsevier B.V. All rights reserved.

Hydrogen interaction with defects in thin niobium (Nb) films was investigated using slow positron implantation spectroscopy (SPIS) combined with X-ray diffraction (XRD) and transmission electron microscopy (TEM). Thin Nb films on Si substrates were prepared using cathode beam sputtering at room temperature. Initially, the microstructure of the virgin (hydrogen-free) films was characterized. Subsequently, the films were step-by-step electrochemically charged with hydrogen and the evolution of the microstructure with increasing hydrogen concentration was monitored. Hydrogen loading leads to a significant lattice expansion which was measured by XRD. Contrary to free-standing bulk metals, thin films are highly anisotropic. The in-plane expansion is prevented because the films are clamped on the elastically hard substrate. On the other hand, the out-of-plane expansion is substantially higher than in the bulk samples. Moreover, an enhanced hydrogen solubility in the a-phase was found in nanocrystalline Nb films. It was found that most of positrons in the films are trapped at open-volume defects at grain boundaries (GBs). These defects represent trapping sites also for hydrogen atoms. Hydrogen trapping at vacancy-like defects like GBs leads to a local increase of the electron density and is reflected by a pronounced decrease of the S parameter in the hydrogen-loaded samples. In addition, it was found that new defects are introduced at higher concentrations of hydrogen due to the formation of NbH (beta-phase) particles. (c) 2005 Elsevier B.V. All rights reserved.

Various defect studies of hydrothermally grown (0001) oriented ZnO crystals electrochemically doped with hydrogen are presented. The hydrogen content in the crystals is determined by nuclear reaction analysis and it is found that already 0.3 at. % H exists in chemically bound form in the virgin ZnO crystals. A single positron lifetime of 182 ps is detected in the virgin crystals and attributed to saturated positron trapping at Zn vacancies surrounded by hydrogen atoms. It is demonstrated that a very high amount of hydrogen (up to similar to 30 at. %) can be introduced into the crystals by electrochemical doping. More than half of this amount is chemically bound, i.e., incorporated into the ZnO crystal lattice. This drastic increase of the hydrogen concentration is of marginal impact on the measured positron lifetime, whereas a contribution of positrons annihilated by electrons belonging to O-H bonds formed in the hydrogen doped crystal is found in coincidence Doppler broadening spectra. The formation of hexagonal shape pyramids on the surface of the hydrogen doped crystals by optical microscopy is observed and discussed. (c) 2008 American Institute of Physics.

Thin niobium (Nb) films (thickness 350-400 nm) were prepared on (100)Si substrate in a UHV chamber using the cathode beam sputtering. The sputtering temperature T, was varied from 40 up to 500 T and the influence of the sputtering temperature on the microstructure of thin No films was investigated. Defect studies of the thin Nb films sputtered at various temperatures were performed by slow position implantation spectroscopy (SPIS) with measurement of the Doppler broadening of the annihilation line. SPIS was combined with transmission electron microscopy (TEM) and X-ray diffraction (XRD). We have found that the films sputtered at T(s) = 40 degrees C exhibit elongated, column-like nanocrystalline grains. No significant increase of grain size with T, (up to 500 T) was observed by TEM. The thin Nb films sputtered at T, = 40 degrees C contain a high density of defects. It is demonstrated by shortened positron diffusion length and a high value of the S parameter for Nb layer compared to the well-annealed (defect-free) bulk Nb reference sample. A drastic decrease of defect density was found in the films sputtered at T(s) >= 300 degrees C. It is reflected by a significant increase of the positron diffusion length and a decrease of the S parameter for the Nb layer. The defect density in the Nb layer is, however, still substantially higher than in the well-annealed reference bulk Nb sample. Moreover, there is a layer at the interface between the Nb film and the substrate with very high density of defects comparable to that in the films sputtered at T(s) < 300 degrees C. All the Nb films studied exhibit a strong (1 1 0) texture. The films sputtered at T(s) < 300 degrees C are characterized by a compressive macroscopic in-plane stress due to lattice mismatch between the film and the substrate. Relaxation of the in-plane stress was observed in the films sputtered at Ts >= 300 degrees C. The width of the XRD profiles of the films sputtered at r(s) >= 300 degrees C is significantly smaller compared to the films sputtered at lower temperatures. This is most probably due to a lower defect density which results in reduced microstrains in the films sputtered at higher temperatures. (c) 2005 Elsevier B.V. All rights reserved.

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.

Defect studies of Nb irradiated with 10 MeV electrons were performed in the present work by means of positron annihilation spectroscopy. The lattice defects were characterized by positron lifetime spectroscopy. Moreover, defect depth profiles were studied by slow positron implantation spectroscopy. The experimental investigations were accompanied by first principles theoretical calculations of positron parameters. It was found that irradiation-induced vacancies in Nb specimens are surrounded by H, which causes a shortening of the lifetime of trapped positrons. The influence of a I'd and Cr over-layer on the H concentration in the Nb specimens was examined.

Hydrogen loading of thin films introduces very high compressive stresses which grow in magnitude with increasing hydrogen concentration. When the hydrogen-induced stresses exceed a certain critical in-plane stress value, the loaded film starts to detach from the substrate. This results in the formation of buckles of various morphologies in the film layer. Defect studies of a hydrogen loaded Pd film which undergoes a buckling process are presented, using slow positron implantation spectroscopy, in situ acoustic emission, and direct observations of the film structure by transmission electron and optical microscopies. It is found that buckling of the film occurs at hydrogen concentrations x(H) >= 0.1 and causes a significant increase of the dislocation density in the film. (c) 2008 Elsevier B.V. All rights reserved.

Our aim in the present work was to investigate changes of the defect structure of bulk niobium induced by hydrogen loading. The evolution of the microstructure with increasing hydrogen concentration was studied by x-ray diffraction and two complementary techniques of positron annihilation spectroscopy (PAS), namely positron lifetime spectroscopy and slow positron implantation spectroscopy with the measurement of Doppler broadening, in defect-free Nb (99.9%) and Nb containing a remarkable number of dislocations. These samples were electrochemically loaded with hydrogen up to x(H)=0.06 [H/Nb], i.e., in the alpha-phase region, and it was found that the defect density increases with hydrogen concentration in both Nb samples. This means that hydrogen-induced defects are created in the Nb samples. A comparison of PAS results with theoretical calculations revealed that vacancy-hydrogen complexes are introduced into the samples due to hydrogen loading. Most probably these are vacancies surrounded by 4 hydrogen atoms.

Changes of the defect structure of niobium induced by hydrogen loading are presented in this work. It was found that annealing of virgin bulk Nb (99.9%) at 1000degreesC for 1h leads to a complete recovery of defects. Subsequently, the defect-free samples were step-by-step electrochemically loaded with hydrogen up to x(H) - 0.06 [H/Nb atom ratio], i.e. in the alpha-phase region, where the Nb-H system represents a single-phase solid solution. The evolution of the microstructure with increasing hydrogen concentration was studied by X-ray diffraction and two complementary techniques of positron annihilation spectroscopy (PAS), namely positron lifetime spectroscopy and slow positron implantation spectroscopy with measurement of Doppler broadening. It was found that new defects were created due to hydrogen loading. The concentration of these hydrogen-induced defects increases with increasing hydrogen concentration. A comparison of PAS results with theoretical calculations revealed that complexes consisting of a vacancy, surrounded likely by four hydrogen atoms, were introduced into the samples due to hydrogen loading.

Changes of the defect structure of Nb induced by hydrogen loading were studied by positron annihilation spectroscopy (PAS). Two sets of samples with different initial microstructure were studied: (i) well-annealed bulk samples, and (ii) thin nanocrystalline films. First, the microstructure of the virgin samples was characterized. Subsequently, the samples were step-by-step electrochemically loaded with hydrogen in the alpha-phase region, where the Nb-H system represents a single-phase solid solution. Two complementary PAS techniques, namely positron lifetime (PL) spectroscopy and slow positron implantation spectroscopy (SPIS), and in addition X-ray diffraction were applied to investigate the evolution of the microstructure with increasing hydrogen concentration. It was found that new defects were created in the bulk Nb sample due to hydrogen loading. Their concentration increases with increasing hydrogen concentration. A comparison of PAS results with theoretical calculations revealed that complexes consisting of a vacancy surrounded likely by four hydrogen atoms are formed. Hydrogen trapping in open-volume defects at grain boundaries was observed in the thin Nb films. (c) 2005 Elsevier B.V. All rights reserved.

The development of the microstructure in nanocrystalline, polycrystalline and epitaxial Pd films loaded with hydrogen is investigated. Structural changes in Pd films loaded with hydrogen were characterized by positron annihilation spectroscopy combined with electron microscopy and X-ray diffraction. It was found that hydrogen charging causes plastic deformation which leads to an increase of the defect density in all Pd films studied. Moreover, the formation of buckles was observed in nanocrystalline and polycrystalline Pd films loaded above a certain critical hydrogen concentration. Buckling leads to detachment of the film from the substrate and this is accompanied with in-plane stress relaxation and plastic deformation of the film. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

The aim of the present work was to investigate the microstructure of bulk niobium irradiated by 10 MeV electrons. Positron-annihilation spectroscopy was employed as a principal technique for the characterization of irradiation-induced defects. Experimental results were compared to first-principles theoretical calculations of positron characteristics. In addition to extended positron-annihilation studies, the specimens were characterized also by x-ray diffraction. It was found that irradiation-induced vacancies are surrounded by hydrogen. Complexes consisting of a Nb vacancy surrounded by one and two H atoms were identified in the irradiated specimens. The concentration of these vacancy-hydrogen complexes was estimated to be (18-24)x10(-5) at. %. Vacancy-2H complexes are found to represent the dominating type of defects. Hydrogen atoms surrounding a Nb vacancy cause a shortening of the lifetime of trapped positrons. Moreover, it was demonstrated that hydrogen attached to Nb vacancy can be identified by coincidence Doppler broadening technique. The effect of a thin Pd (or Cr) overlayer on the irradiation-induced defects was investigated also. It was found that the relative fraction of vacancy-2H complexes is higher in the specimens irradiated with such an overlayer.

Ultra-fine grained (UFG) Cu (grain size 80 nm) containing 0.5 wt.% Al(2)O(3) nanoparticles (size 20nm) was prepared by high pressure torsion (HPT). Positron lifetime spectroscopy was employed to characterize the microstructure of this material, especially with respect to types and concentration of lattice defects. The evolution of microstructure with increasing temperature was studied by positron lifetime spectroscopy and X-ray diffraction measurements. The thermal stability of the Cu + 0.5 wt.% Al(2)O(3) nanocomposite was compared with that of pure UFG Cu prepared by the same technique. The processes taking place during thermal recovery of the initial nanoscale structure in both studied materials are described.

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. (C) 2002 Elsevier Science B.V. All rights reserved.

H interaction with defects in thin Nb films was investigated in this work. Thin Nb films were prepared by the cold cathode beam sputtering. First, microstructure of the as deposited films was characterized. The films sputtered at room temperature exhibit nanocrystalline grains, while those sputtered at high temperature (T=850 degrees C) are epitaxial. Subsequently, the films were step-by-step electrochemically charged with H. Development of microstructure and evolution of defect structure with increasing H concentration was investigated by slow positron implantation spectroscopy combined with X-ray diffraction. It was found that H is trapped at open-volume defects in the thin films of both kinds. The nanocrystalline films exhibit significantly extended H solubility in the alpha-phase. Formation of the hydride-phase (Nb-H) at higher H concentrations leads to introduction of new defects. These are most probably dislocation loops that are emitted by growing hydride-phase particles. (C) 2007 Elsevier B.V. All rights reserved.

Thermal stability of ultra-fine grained (UFG) nickel (mean grain size 114 nm) prepared by high pressure torsion was studied by means of positron-lifetime spectroscopy (PLS) combined with TEM. The experimental results obtained by PLS are interpreted using the diffusion trapping model, which allows for determination of important physical parameters characterizing the specimens. The microstructure of the material studied is strongly inhomogeneous. The grain interiors with low dislocation density are separated by distorted regions with high numbers of dislocations. We have found that positrons are trapped at dislocations inside the distorted regions and in the microvoids situated inside the grains. Structure evolution with increasing temperature was studied in details using isochronal annealing of the specimen. We have found that recovery of the UFG structure involves the abnormal grain growth followed by further recrystallization in the whole volume of samples. It was shown that PLS is sensitive to structure changes, caused by the magnetostriction phenomenon.

Positron annihilation spectroscopy was employed for defect studies of "Ti"-based nanocomposites prepared by high-energy ball milling and consisting of Ti nanoparticles separated by hexagonal boron nitride (h-BN) or boron (B) additive. The size distribution of nanoscopic holes in nanocomposites was determined directly from measurement of ortho-positronium (Ps) lifetimes. Chemical environment of defects was characterized using coincidence Doppler broadening. It was found that size of nanoscopic holes is reduced with increasing milling time in H(2)/He atmosphere and also probability of Ps formation in holes decreases. At the same time the Ti content in the vicinity of holes increases. This can be explained by (i) increased intermixing of Ti particles with h-BN or B additive and by (ii) filling the nanoscopic holes with absorbed hydrogen. Analysis of obtained results showed that both these processes take place during milling of nanocomposites. In addition, it was found that the effect of filling the nanoscopic pores with hydrogen is enhanced in TiH(2) milled with h-BN or B in He atmosphere. Comparison of nanocomposites with h-BN and B additive showed that sufficiently long milling time leads to a similar size distribution of nanoscopic holes in Ti/h-BN and Ti/B, despite the fact that it differs substantially in the initial powders. However, density of nanoscopic holes in Ti/B is significantly lower than in Ti/h-BN nanocomposites.