Papers by Keyword: Positron Lifetime

Abstract: The development of algorithms and simulation codes as well as the ever increasing comput ing power available have have enabled many-body simulations to emerge as a viable alternative to the two-component density-functional theory to study positron states and annihilation in solids, defects and on solid surfaces. One practical many-body method is quantum Monte Carlo. This article will aim to present the practitioners of positron annihilation methods the basic ideas of the variational and diffusion quantum Monte Carlo, provide examples of their past and recent applications in positron physics of atomic/molecular and solid-state systems as well as an outlook into the future.

Abstract: 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.

Abstract: Zirconium alloys, such as Zr-1Nb are widely used as cladding materials for nuclear fuel elements of light water reactors. Hydrogen embrittlement problem causes degradation of these parts of nuclear reactors. It is known, that hydrogen uptake causes changes in microstructure and defect structure of metals. The aim of this work is study of Zr-1Nb alloys’ defect structure after hydrogen saturation up to the concentration of 600 ppm. Saturation of hydrogen was carried out from the gas phase under high temperature and pressure. This study reveals the increase of average positron lifetime with the increase of hydrogen concentration. Value of the average positron lifetime achieves plateau when the concentration of hydrogen is about 600 ppm. Also the following effects were detected in the material after hydrogen uptake up to different concentrations: crystal lattice expansion, dislocations and vacancy-like defects formation, as well as the defect-hydrogen complexes formation.

Abstract: The influence of hydrogen sorption-desorption cycles on defect structure of Zr-1Nb alloy was investigated. Specimens were hydroganated from gas atmosphere at temperature 500 °C and pressure 2 atm up to the hydrogen concentration equal to 0.05 wt.% for each cycle. The hydrogen concentration during saturation was determinate by the volumetric method. Then samples were annealed at temperature 900 °C with the heating rate of 4 °C/s. The lattice defects were studied by means of positron lifetime spectroscopy (PLS) and doppler broadening spectroscopy (DBS). New experimental data about the evolution of the average positron lifetieme τ_avg, as well as the relative changes in the parameters S, W depending treatment stage in Zr-1Nb alloy during thermo hydrogen proccessing was obtained.

Abstract: This paper presents the results of a defect structure investigation in commercially pure titanium alloy after hydrogen charging in a gaseous atmosphere at the temperature of 873 K up to the concentration of 5.1 at. %. Structure of samples was studied by positron lifetime, Doppler broadening and X-ray diffraction spectrometry. Several processes, corresponding to the different ranges of hydrogen concentrations were revealed. It was shown that hydrogen, penetrating in the material, expands its crystal lattice, initiates formation of vacancy-like defects of different dimensions and reacts with the last ones, forming the defect-hydrogen complexes.

Abstract: Coincidence Doppler broadening spectra measurements on ZnO-based varistor doped with semiconductor additives have been investigated, and the proposed samples exhibit different characteristics. The peak of the ratio curve of pure zinc is the highest, the Positron Lifetime in sample a03 doped with ZT is the longest. The maximum probability of the positron-3d electron occurs in sample doped with semi-ZnO (sintered at 1300 °C for 2h with conventional ceramic processing method). The electrical characteristics of the samples and the behavior of positrons in ZnO-based varistor have been discussed.

Abstract: This paper is devoted to the defect structure study of commercially pure titanium after hydrogen sorption-desorption cycles by means of positron lifetime (PL) and Doppler broadening spectrometry. Material was loaded with hydrogen from the gas phase till the concentration of hydrogen reached the value of 0.05 wt.% for each cycle. The essential changes in the positron annihilation characteristics of the sample are occurred after the each stage of treatment.

Abstract: Positron Annihilation Lifetime Spectroscopy (PALS) is used to study the nanoporosity and fractional free volume in Ultra High Molecular Weight Polyethylene (UHMWPE) and composites with the addition of Martian Regolith (UHMWPE-MR) as-made and irradiated with ⁵⁶Fe heavy ions at an energy of 600 MeV/u to three different doses (10, 32, 64 Gy). The positron lifetime spectra were obtained using ²²Na positron source and the spectra were analyzed to two lifetime components using POSFIT program. First short lifetime component around 0.28 ns is related to positron annihilation in material including vacancy defects and the second long lived component around 1.7 ns is due to Positronium formation in free volume pores. UHMWPE-MR composites were shown to be less porous with much lower nanopores concentration compared to the UHMWPE polymer. The average size of the nanopores is around 0.5 nm (obtained from a simple model). Larger variations in positron lifetime parameters are observed with increasing irradiation dose for UHMWPE polymer compared to UHMWPE+MR composites. The 3-point bend test results also showed larger variations with increasing irradiation dose for the UHMWPE polymer. The variations in PALS parameters may indicate an increasing competition between two processes at higher irradiation doses: 1) vacancy defects aggregation and 2) escape of vacancy defects as the local temperature increases at higher doses resulting in increased vacancy defects mobility. Present results clearly indicate a qualitative inverse relationship between nanoscale porosity measured by positron life time and mechanical properties of UHMWPE and its composite with MR.

Abstract: Ultra-high-temperature ceramics (UHTC) such as ZrB₂ and HfB₂ with SiC nanofiller are useful for propulsion and thermal protection systems. ZrB₂ and HfB₂ with 10-20 wt% SiC were prepared using ultra-sonication, rotary evaporation, and spark plasma heat treatment to high temperatures (~2,000°C) and pressures (50-60 MPa). We used positron annihilation lifetime spectroscopy (PALS) to study the nanoporosity, SEM for particle size distribution, and microhardness tester for Vickers hardness. The PALS studies were performed using a ²²Na positron source and the positron lifetime spectra were analyzed to three components using POSFIT program. The first and second components are related to positrons annihilating in bulk and in vacancy clusters, respectively; and the third component to positronium annihilation in nanopores within the granules. The PALS results indicate that HfB₂ has larger vacancy clusters and nanopores with lesser concentrations compared to ZrB₂ and SiC. The SEM observations showed that HfB₂ has larger particles compared to ZrB₂ and SiC showed wide range of size distribution. The Vickers-Hardness Number (VHN) is measured for spark plasma heat treated composites using a microhardness tester and the results indicate that 10wt%SiC composite has higher hardness compared to 20wt%SiC in both ZrB₂-SiC and HfB₂-SiC composites. HfB₂-SiC composites seem to be more brittle compared to ZrB₂-SiC composites. This may be due to larger size and smoother surface of HfB₂ particles (600 nm) compared to ZrB₂ particles (240 nm).

Abstract: In this work the goal was to study the behavior of titanium-hydrogen system with hydrogen concentration of 1 wt. % at high temperature vacuum annealing by means of positron lifetime (PL) spectroscopy and electrical resistivity measurements. The range of hydrogen concentrations under investigation requires the γ phase formation. The study is related with the X-ray diffraction (XRD) and thermal desorption spectra (TDS) analysis. The registered positron lifetime spectra were analyzed by multiexponential decomposition into two components. Correlation of changes of the parameters of the PL spectrum and the electrical resistivity is revealed in this paper.