P.M.G. Nambissan

Mechanism and kinetics of solid-state amorphization by mechanical alloying of Al65Cu35-xNbx

In the present study, we have synthesized a number of Al-based Al65Cu35u2009−u2009x Nbx ternary alloys by mechanical alloying and undertaken a detailed characterization of their microstructural evolution by X-ray diffraction, high-resolution transmission electron microscopy and positron annihilation spectroscopy. To predict the phase-equilibrium in a given ternary powder blend subjected to mechanical alloying, we have modified the Miedema model to incorporate the influence of interfacial energy contribution in solid-state amorphization and analytically calculate the enthalpy and Gibbs energy of ternary amorphous and nanocrystalline solid solutions. The predicted trend compares well with the experimental data. Finally, an attempt has been made to determine the mechanism of solid-state amorphization in Al65Cu35−xNbx alloys utilizing both experimental results and model-based thermodynamic calculations.

Cr3+-substitution induced structural reconfigurations in the nanocrystalline spinel compound ZnFe2O4 as revealed from X-ray diffraction, positron annihilation and Mössbauer spectroscopic studies

In an earlier work, the substitution of Zn2+ ions at the tetrahedral sites of nanocrystalline zinc ferrite (ZnFe2O4) by Ni2+ ions had been observed to cause a transformation from the normal spinel structure to the inverse one. The present study has been undertaken to explore the possibility of a similar change when the Fe3+ ions at the octahedral sites are replaced by Cr3+ ions. Concomitant lattice contraction and a steady decrease of the sizes of the nanocrystallites preceded and then resulted into the inversion of ZnFe2−xCrxO4 from normal spinel to inverse at x ≥ 0.8. Positron lifetime and coincidence Doppler broadening spectroscopic studies were carried out on the samples and a distinct third positron lifetime component emerged in the range of Cr3+ concentration 0.8 ≤ x ≤ 1.6. The new positron trapping sites were the result of the inversion of the spinel structure wherein the Cr3+ ions which substituted the Fe3+ ions at the octahedral sites got shifted to the tetrahedral sites, interchanging their positions with the Zn2+ ions. The incomplete success of inversion led to the generation of vacancy-type defects, which significantly trapped the positrons and the changes in their lifetimes indicated the occurrence of the process. The continued lattice contraction ensured an inverted spinel structure even for the final ZnCr2O4, which in coarse-grained form and at room temperature is a normal spinel. Mőssbauer spectroscopic studies also supported the idea of spinel inversion above x = 0.8 through definite changes in the isomer and quadrupole shifts.

Annealing behaviour of α-induced defects in tantalum studied by positron annihilation

Abstract Positron lifetime and Doppler broadening measurements have been carried out to study the isochronal annealing behaviour of defects induced in tantalum irradiated by 32 MeV α-particles. The vacancy migration stage is shifted towards higher temperatures (100°C) and the presence of hydrogen as a common contaminant in this metal is inferred to be the cause behind this observation. Below 300°C, processes such as dissociation of hydrogen-vacancy complexes, vacancy migration leading to the formation of three-dimensional vacancy clusters, and dislocation relaxation forming dislocation or vacancy loops take place. Above this temperature, multiple vacancy-helium complexes are formed. This is soon followed by the release of vacancies from the dissociating vacancy-impurity complexes. Annealing of the defects takes place above 650°C. The effects of isochronal annealing on the positron annihilation parameters demonstrate that helium, trapped inside the metal during the irradiation, plays a significant role in ...

Positron annihilation and scanning electron microscopy studies of α-irradiated aluminium crystals

Abstract Measurements of positron lifetime and annihilation γ lineshape have been made on 0–40 MeV α-irradiated Al single crystals and polycrystals to a total dose of about 5 × 1017 αcm−2. Scanning electron microscopy (SEM) observations were also made to study the surface morphology of the irradiated specimens. Although both sets of specimens were treated under the same irradiation conditions, significant differences were found between the defect structures of single crystals and polycrystals. A simple model is applied to the lifetime parameters to estimate the size and concentration of the He bubble. In polycrystals the size of bubbles formed is smaller and their concentration is higher than in single crystals. The effects of impurities and grain boundaries on bubble growth are discussed. The results of annihilation γ lineshape show evidence for positronium (Ps) formation in the single crystals. No Ps is formed in the polycrystals. A lower limit of about 36 A is estimated for the radius of the He bubble ...

Positron annihilation studies and complementary experimental characterization of xAg2O–(1 − x)(0.3CdO–0.7MoO3) metal oxide glass nanocomposites

Metal oxide nanocomposites of the composition xAg2O–(1 − x)(0.3CdO–0.7MoO3) were prepared by a melt-quenching method and were characterized by different experimental techniques like X-ray diffraction, high resolution transmission electron microscopy and optical absorption spectroscopy. X-ray diffraction showed sharp diffraction peaks indicating large crystallites but transmission electron microscope images also showed crystallites of nanometer dimensions in appreciable concentrations, which confirmed the nanocomposite structure of the samples. Although the lattice constants did not show significant changes with the increase in concentration (x) of Ag2O, there is considerable relaxation of the growth-induced strain above x = 0.2. Interestingly this is also the concentration above which the optical band gap energy showed a mild decrease. One salient feature of this study is the use of positron annihilation spectroscopy for identifying and monitoring the structural defects such as vacancies and vacancy clusters as well as the free volume cavities during the change in concentration of Ag2O. Positron lifetime measurements indicated trapping of positrons initially in the interfacial defects within the 0.3CdO–0.7MoO3 nanocrystalline ensemble and then in the free volume defects within the amorphous Ag2O matrix. At higher Ag2O concentrations, positron trapping appeared to take place within the Cd2+-monovacancies in CdO and in the divacancies of neighbouring cationic and oxygen monovacancies in the α-MoO3 and CdMoO4 nanocrystallites. At x = 0.1–0.2, the effective positron trapping centres are translocated to the tetrahedral Mo6+-monovacancies instead of the Cd2+-monovacancies. The results of coincidence Doppler broadening spectroscopic measurements, which map the electron momentum distribution and its variations, indicated increasing trapping of positrons with increasing concentration of Ag2O, which again is attributed to the trapping sites in the increasing number of nanocrystallites being formed.

Positron annihilation spectroscopic studies of Mn substitution-induced cubic to tetragonal transformation in ZnFe2–xMnxO4 (x = 0.0–2.0) spinel nanocrystallites

The replacement of cations at the B-sites in the spinel ferrite ZnFe2O4 by Mn3+ ions brings in several interesting changes, the most striking among them being a transformation from the spinel cubic structure to a body-centered tetragonal one. Concomitantly, there are variations in the nanocrystallite sizes and also in the lattice parameters. These are examined through high-precision X-ray diffraction measurements and transmission electron microscopic analysis. A more interesting aspect is the success of positron annihilation spectroscopy comprising of the measurements of positron lifetime and coincidence Doppler broadening measurements in understanding the effects of cation replacement and the resultant generation of vacancy-type defects. There are definite changes in the positron lifetimes and intensities which show positron trapping in trivacancy-type defect clusters and the nanocrystallite surfaces. The presence of ortho-positronium atoms within the extended intercrystallite region is also identified, although in small concentrations. The cubic to tetragonal transformation is indicated through definite decrease in the values of the positron lifetimes. We also performed a model analysis to predict the expected effect of substitution on the positron lifetime in the bulk of the sample and the experimentally obtained positron lifetimes significantly differed, indirectly hinting at the possibility of a structural transformation. Finally, Mössbauer spectroscopic studies have indicated a ferromagnetic nature for one of the samples, i.e. the one with Mn3+ doping concentration x = 0.4, which incidentally had the lowest crystallite size ~10 nm.

Positron annihilation spectroscopic studies of multiferroic Bi1-xPrxFeO3 nanocrystalline compounds

Positron lifetime (PL) and coincidence Doppler broadening spectroscopic (CDBS) experiments were carried out on BiFeO3 samples doped with Pr(in place of Bi) in concentrations (x) = 0, 0.05, ..., 0.35. In the initial stages of doping, the existing Bi3+ vacancies are occupied by Pr3+ ions and positron trapping is reduced. From x = 0.15 to 0.35, more number of positrons diffused to the surfaces of the nanocrystallites. The CDBS ratio curves showed enhancement of the peak at pL = 10.2×10-3 m0c due to positron annihilation with the 2p electrons of oxygen. It suggested increased trapping of positrons in newly created cationic vacancy-type defects due to the mismatch of ionic radii of Pr3+ substituting Bi3+ ions. The lattice parameters decreased (3.9661-3.9248A) while the nanocrystallitesizes reduced and increased (28.6-34.2nm) during the doping.

Energy/time measurements with BaF2 and RCA C31024 PMT

Abstract Measurements have been made with 35 mm diameter × 20 mm long BaF2 scintillators coated with a 1 mg/cm2 layer of p-terphenyl wavelength shifter and RCA C31024 fast photomultiplier tubes. An energy resolution of 12.9% for the 137Cs gamma line and a time resolution of 400 ps for the two photopeaks of 60Co have been obtained. The results demonstrate that where quartz window PMTs are not readily available, the trouble of handling UV light can be avoided by the employment of wavelength shifters.

SEM observations of high energy alpha-irradiated metal surfaces

Abstract In this paper we present results from a study of the surface morphology of alpha-irradiated tantalum and tungsten using scanning electron microscopy (SEM). The surfaces of well polished polycrystalline Ta and W were irradiated with 32 MeV alpha-particles to a total dose of ~ 4.2 × 10 17 α/cm 2 . Although the irradiation conditions were same for both of these samples, the resulting surface deformations are strikingly different. A small number of unruptured blisters of irregular shape and ~ 20 to 40 μm in size is seen on the Ta surfaces. The W surfaces are characterized by a much higher number of square shaped flakes of different sizes varying from 5 to 50 μm. Possible reasons for the differences between the surface features seen on these samples are discussed.

Helium physisorption on internal surfaces of radiation-induced cavities in aluminum: a positron annihilation study

Abstract Positron lifetime spectra have been measured as a function of temperature in the range 10–295 K for aluminum single crystals irradiated with 0–40 MeV α particles to a total dose of ∼ 5 × 10 17 cm −2 . We present results for the temperature dependence of the lifetime component representing annihilations of the positrons trapped in helium-filled cavities. We discuss these results in terms of positron diffusion and helium physisorption on the internal surfaces of the irradiation-induced cavities.