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[en] Highlights: • Defect annealing in FeCrCoNi high entropy alloy is studied using positron lifetime and Doppler broadening. • Variation of positron lifetime with temperature show the annealing of defects beyond 770 K. • XRD shows that the alloy forms FCC solid solution and the phase remains stable even after annealing at 1373 K. • Recrystallization assisted grain growth occurs beyond 973 K. - Abstract: Defect annealing in FeCrCoNi high entropy alloy is studied using positron lifetime and Doppler broadening spectroscopic techniques. Variation of positron lifetime with temperature show the annealing of defects beyond 770 K. Theoretical positron lifetime and electron momentum distributions were computed for the alloy to understand the nature of defects present in the arc melted alloy. X-ray diffraction measurements show that the arc melted alloy forms FCC solid solution and the phase remains stable even after annealing at 1373 K. Recrystallization assisted grain growth occurs beyond 973 K
[en] Defect characterization of room temperature 1.5 MeV Ni ion implanted high entropy FeCrCoNi alloy for two fluences (1 × 10"1"5 ions/cm"2 and 5 × 10"1"6 ions/cm"2) was carried out using the variable low energy positron beam. The FCC solid solution remains robust and stable under 100 dpa irradiation and high temperature annealing. The change in the defect sensitive S-parameter upon implantation reveals the presence of monovacancies for both the doses. The changes in the defect microstructure upon thermal annealing are found to be dose dependent. The high dose shows the formation of stable stacking fault tetrahedrons (SFT's) from the aggregates of monovacancies at higher annealing temperatures while the low dose shows the annealing of monovacancies with temperature. - Highlights: • Defect characterization of room temperature Ni implanted high entropy FeCrCoNi alloy carried out using the slow positron beam. • The FCC FeCrCoNi remains structurally stable under both irradiation and high temperature annealing. • Defect evolution upon thermal annealing is found to be dose dependent.
[en] Depth-resolved positron beam measurements were carried on Pd/Si, Co implanted Si and bulk nickel silicides. The uniqueness of the technique in picking up features attributable to the annealing of defects, inter-mixing, silicide phase formation and the evolution of vacancy defects is highlighted in the thesis work. In addition to this, other complementary characterization techniques have enabled a comprehensive understanding of the silicide formation. (author)
[en] Positron beam based Doppler broadening measurements have been carried out on Pd/Si thin film system to investigate various aspects of silicidation. Ab-inito calculations of the positron lifetime and Doppler lineshape parameters of Pd, Si and Pd2Si were also carried out. A comparison of experimental and calculated positron annihilation parameters has been presented. (author)
[en] Bulk cobalt silicide samples (Co2Si, CoSi and CoSi2) are studied using experimental positron lifetime and Doppler Broadening techniques. Ab-initio calculations of positron lifetime have been carried out and these are compared with the experimental values, which reveal the presence of vacancy defects. (author)
[en] Silicidation in Ni/Si thin-film junction has been investigated using depth-resolved positron annihilation spectroscopy (PAS) and Rutherford backscattering spectrometry (RBS). Identification of various silicide phases from an analysis of the positron annihilation parameters is consistent with the RBS results. Absence of vacancy defects in the silicide region is clearly brought out by PAS
[en] Micro channel plate (MCP) is used for detecting charged particles such as electrons, positrons, ions, photons and amplifies the detected signal. The working of MCP is similar to secondary electron multiplier. The MCP comprises of an array of single channel electron multipliers wherein a single electron entering a channel produces multiple secondary electrons upon striking the inner wall of the channel. In this work, a chevron type, 2 stage MCP with phosphor screen as a readout device (Photonis make) is mounted on 6 inches CF flange located at the end of a UHV chamber. The MCP is effectively configured by means of a circuit to derive timing signal from the MCP end. Here, a 60Co source is placed between the MCP and the BaF2 detector and the corresponding timing signals are fed to the digital storage oscilloscope having 20 GSa/s. The histogram of the time differences between the MCP and the BaF2 detector signals is deduced to obtain the timing resolution of the MCP- BaF2 system. Preliminary investigation has shown a time resolution of ~460ps with 60Co source. This study has been done to explore the applicability of MCP with phosphor screen configuration for timing measurements
[en] Positron beam studies on the silicidation in Pd/Si, Ni/Si and Co implanted Si thin film systems are presented. In the case of Pd/Si, the evidence for the occurrence of divacancies across the Pd2Si/Si interface consequent to Pd2Si formation is shown. In Ni/Si system, the identification of a multitude of silicide phases of Ni2Si, NiSi and NiSi2 is obtained from depth-resolved S-parameter, which is corroborated with GIXRD and AES results. In the case of the Co implanted Si, the recrystallization of amorphized silicon and the formation of CoSi2 precipitates in Si are brought out clearly.
[en] Glass bonded sodalite is a promising waste form for immobilizing radioactive wastes, resulting from pyrometallurgical processing of the spent fuel. The synthesis procedure for the glass-bonded sodalite involves (a) incorporating the simulated chloride waste salt into dehydrated zeolite 4A by high temperature equilibration (b) blending the salt loaded zeolite with boroaluminosilicate glass and (c) sintering at high temperature and pressure to yield the glass-ceramic. This work is aimed at using positron lifetime spectroscopy to follow the changes in the free volume size as the zeolite is being transformed to the glass bonded sodalite. Positron lifetime measurements were carried out on various glass samples using lifetime spectrometer having a time resolution of 260 ps and the measured spectra were resolved into various lifetime components. Based on the Tao-Eldrup model, the longest positron lifetimes τ3 and τ4 (ns) which correspond to pick-off annihilation of o-Ps, are related to the average radii (R) of free volume holes. The variation in positron lifetime components and hence the deduced free volume radii for dehydrated zeolite 4A, pure sodalite, glass bonded sodalite and boroaluminosilicate glass are shown. The shortest lifetime component τ1 is attributed to positrons annihilating in the bulk and p-Ps annihilation. The intermediate component τ2, which is not listed in table, is due to positrons annihilating at the defect complexes. The longest lifetime components, τ3 and τ4, are attributed to o-Ps annihilation in the β and α cages, respectively. While dehydrated zeolite 4A exhibits 4 lifetime components, sodalite and glass bonded sodalite give rise to only three lifetime components. Absence of the fourth o-Ps lifetime component along with reduced average free volume radius from 2.56 Å to about 1.55 Å in sodalite implies that the chloride waste salt loaded in the zeolite has got incorporated possibly in both α and β cages. The fact that the o-Ps lifetime components for both sodalite and glass bonded sodalite are comparable implies that the positrons prefer to probe the sodalite rather than the shell like glass encapsulation. While τ1 remains unchanged going from zeolite to glass bonded sodalite, I1 is observed to systematically increase, pointing to a progressive densification of the matrix
[en] Variable low-energy positron beam is a unique defect characterization tool which uses positron as a probe to study vacancy type defects starting from near surface region up to a depth of a few hundreds of nm. In this technique, positrons of tunable energies are implanted into the material under study. The depth at which the positrons are implanted is determined by the positron beam energy. This energy tuning is achieved by programmable high voltage power supplies (0-30 kV). The existing setup was running on dedicated automation based on visual basic in old P-II system running on windows 98. Repeated failures and non availability of replacements lead to the development of state of the art Labview based automation program for positron beam experiment. The program does the following functions: controlling the remote programmable high voltage power supply to vary the positron energy, control of Windows based PCI MCA card (Fast Comtech 8k MCA), calculating the S, W, R and positronium fraction (fPs) parameters, errors, and other relevant parameters, saving the raw spectra, results file and log file and Dynamic display of raw spectrum, S, W, fPs vs E for easy visualization of progress of the experiment