Temperature dependent positron annihilation studies in Nafion-117 polymer

We have measured the S-parameter and life-time as a function of temperature from 15 to 300 K in acid Nafion-117. In addition, high temperature measurements from 300 to 550 K have been carried out in acid as well as Cs neutralised Nafion membrane. The results are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved

Understanding microstructural evolution and soft magnetic properties in nanocrystallized metallic glass Fe68.5Cu1Nb3Si18.5B9

Microstructural evolution during nanocrystallization of amorphous metallic glass Fe68.5Si18.5Nb3B9Cu1 has been studied. The effect of microstructural features of nanocrystalline phases on soft magnetic properties have been evaluated and rationalized with existing theoretical models. The TEM and XRD studies have shown the presence of single Fe3Si nanocrystalline phase at 550 degrees C (size range 13-16 nm) and three nanocrystalline phases Fe3Si, Fe3B, and Fe2B at 800 degrees C (size range 18-100 nm). Increase in annealing time at 800 degrees C resulted in decomposition of metastable Fe3B phase to equilibrium Fe2B phase. Positron annihilation spectroscopy revealed the presence of nanovoids in amorphous samples. Theoretical estimations showed that these nanovoids were having free volume equivalent to that of a vacancy defect consisting of five or more atoms vacancy cluster present in the amorphous samples. Nature of interfaces associated with nanocrystalline phases could be characterized using positron annihilation spectroscopy. This study showed that metallic nanoparticles have very low concentration of thermal vacancies. Effects of nature of phases, particle density, and nanoparticle size on saturation magnetization and coercivity have been studied

Study on secondary phase precipitate behavior in Zircaloy-2 by positron annihilation spectroscopy

It is well established that in Zr alloys, secondary phase precipitates (SPP) have a major influence on the corrosion and mechanical properties. Thus it is important to understand the kinetics and growth of precipitates under thermal treatments. In this study, secondary phase precipitates were generated by suitable heat treatment at 800 degrees C and 840 degrees C for different duration (1-1440 minutes). Positron lifetime spectroscopy and Doppler broadening measurements were carried out to examine the nucleation and growth aspects of the precipitates. The variation in lifetime and S-parameter were seen to be consistent. From the S-W curve it is seen that the defects associated with precipitation at 800 degrees C and 840 degrees C are different. Possible mechanism for precipitation of SPP's has been proposed based on the positron annihilation data. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Positron annihilation study of recrystallization behaviour in Zr2.5%Nb alloy

In CANDU reactors Zr 2.5%Nb alloy is used as pressure tube material due to low neutron absorption cross section, a high resistance to corrosion in water, high strength and good creep resistance. The two phase alloy consists of alpha-fcc anb beta-bcc. The recrystallization behavior of Zr2.5Nb strongly depends on morphology and distribution of second phase. In this study, the material was deformed to 60% and then was recrystallized in alpha + beta region for different duration (i.e. 2 minutes to 14 days at 700 degrees C). The recrystallized microstructure was characterized by Orientation Imaging Microscopy (OIM). The evolution of microstructures was examined by Positron Lifetime Spectroscopy and Doppler Broadening measurement for each heat treated samples. Results from positron anhilation studies and other microstructural examination have been used to rationalize the recrystallization aspects, defects and beta-phase distribution. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Characterizing the nanoclay induced constrained amorphous region in model segmented polyurethane-urea/clay nanocomposites and its implications on gas barrier properties

There has been an increasing recognition of the fact that purely geometric factors associated with clay platelet dispersion in a polymer matrix cannot adequately explain the barrier properties of polymer/clay nanocomposites. The objective of the present work is to understand the nanoclay induced structural changes in a polyurethane-urea matrix and clay dispersion at different length scales using segment-specific characterization techniques and implications of the same in gas barrier properties using He, N-2 and CO2 as probe molecules. Wide angle X-ray diffraction (WAXD) and positron annihilation life time spectroscopy (PALS) studies revealed nanoclay induced alterations in the chain packing of the amorphous soft segments of the polyurethane matrix at a molecular scale of a few Angstroms. The hard segment organization and the phase morphology of the nanocomposites, spanning length scales of several nanometers, were investigated by small angle X-ray scattering (SAXS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Furthermore, the presence of a constrained amorphous region surrounding the nanoclay was confirmed from AFM, WAXD and PALS results. Several pertinent structural variables from the gas transport point of view were deduced from these characterization techniques to understand the effect of the barrier properties in tandem with the clay dispersion morphology

Correlation of soft magnetic properties with free volume and medium range ordering in metallic glasses probed by fluctuation microscopy and positron annihilation technique

Amorphous ribbons of different thicknesses of Co64.5Fe3.5Si16B14Ni2 alloy were synthesized using the melt spinning technique by varying wheel speed. The effect of cooling rate on the ribbon thickness and their soft magnetic properties have been studied. The amorphous structure has been characterized in terms of structural free volume and medium range order (MRO) by positron annihilation spectroscopy and fluctuation electron microscopy techniques. Positron lifetime spectra of amorphous samples showed two lifetime components. The first component was found to be correlated with MRO whereas, the second lifetime component was found to be associated with nanovoid type of defects, and the second component was strongly dependent on processing conditions. It could be established that the coercivity of the amorphous samples produced by the rapid solidification technique mainly depends on the defects formed during processing rather than change induced in MRO. (C) 2012 Elsevier B.V. All rights reserved

Defect profiling in organic semiconductor multilayers

Defect depth profile study has been carried out in organic semiconductor (OSC) multilayers to characterize the buried interfaces and layers using beam based positron annihilation spectroscopy. The bilayer and trilayer heterostructures (p-n, p-p and n-p-n) comprise of organic-organic and organic-inorganic (substrate) interfaces. Our study reveals the presence of defects at the interfaces whose concentration is seen to vary with the layer thickness. The S-W correlation has been used to examine the effect of organic materials as well as thickness of the layers on the defect microstructure in multilayers. The nature and type of defects in p-p bilayer are seen to be different as compared to p-n and n-p-n multilayers. Positron mobility in OSC layers has been calculated from the fitted diffusion length which is seen to be of the same order as the effective mobility of charge carrier obtained from the measured current density-voltage (J-V) characteristics. The role of structural defects and the intrinsic electric field at the interfaces on positron systematics is also examined. Positron diffusion modeling together with experimental data suggests that the defect at the interfaces has a stronger influence on the positron systematics than the intrinsic electric field across organic-organic interfaces. (C) 2012 Elsevier B.V. All rights reserved

Grain fragmentation and twinning in deformed Zircaloy 2: Response to positron lifetime measurements

Microstructural developments in deformed Zircaloy 2 were studied using techniques ranging from positron annihilation spectroscopy (PAS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). Grain fragmentation during plastic deformation, which is expected to involve dislocation realignment and recovery, showed a clear correlation with drop in mean PAS lifetime (tau(m)). An increased tau(m), on the other hand, was associated with annihilation of deformation twins. The deformation twins, (1 0 (1) over bar 2) type tensile twins, were identified from axis-angle relationship: 94.8 degrees ((1) over bar 2 (1) over bar 0). A deviation of more than 5 degrees from the exact twin orientation relationship was considered as annihilation. Such annihilation, caused by developments of in-grain misorientations, is expected to involve accommodation through interfacial dislocations. Presence of such dislocations, and associated point defects, offers an explanation for the observed increase in tau(m). (C) 200