K. Sudarshan

Free volumes and structural relaxations in diglycidyl ether of bisphenol-A based epoxy–polyether amine networks

Two types of polyether diamines were used to prepare model rubbery and glassy epoxy–amine networks with diglycidyl ether of bisphenol-A; α,ω-diamino terminated polyoxypropylene (POP) diamines and α,ω-diamino terminated poly(oxypropylene)-block-poly(oxyethylene)-block-poly(oxypropylene)s (POP-POE-POP). The structural relaxations in the glassy and rubbery epoxy–amine networks at segmental (α relaxation) and local (β relaxation) levels were investigated by modulated differential scanning calorimetry (MDSC) and dynamic mechanical analysis (DMA). The characteristic length of glass transition of the networks ξ(Tg) was evaluated from MDSC using Donths thermal fluctuation approach. While the POP diamine networks showed ξ(Tg) values of 2.0 and 2.07 nm, for POP diamine molecular weights of 230 and 400, respectively, the corresponding values for POP-POE-POP diamine networks were found to be 1.41 and 1.58 nm for POP-POE-POP diamine molecular weights of 600 and 900. This implied diminishing size of the cooperatively rearranging regions with decreasing crosslink density. DMA measurements were used to evaluate the crosslink density of the networks, characteristic features of the α and β transitions in terms of the width, intensity of transitions, and activation energy of the β relaxation. The studies revealed highly cooperative sub-Tg β relaxations for the glassy networks and a truncated but pronounced β relaxation for the rubbery networks. Positron annihilation lifetime spectroscopy (PALS) was used to characterize the molecular topology of the networks in terms of the free volume nanohole sizes and their distribution. The difference of the average distance between crosslink points and the free volume nanohole size was seen to increase with the chain length of the diamines, indicating the fluctuational nature of the networks influenced by the sub Tg relaxation.

Luminescence of undoped and Eu3+ doped nanocrystalline SrWO4 scheelite: time resolved fluorescence complimented by DFT and positron annihilation spectroscopic studies

SrWO4 (SWO) and Eu3+ doped SrWO4 (SEWO) scheelite samples were synthesized using a polyol method. Crystallite sizes of the as prepared samples annealed at 300 and 500 °C are in a similar range (<20 nm) whereas those annealed at 700 and 900 °C are about ∼40–50 nm and 80–100 nm, respectively. Photoluminescence (PL) spectra of SWO samples show a broad peak corresponding to the oxygen to tungsten charge transfer transition. Along with the enhancement in emission intensity in the samples annealed at 700 and 900 °C, there is a blue shift in peak maxima. Our first principles quantum mechanical calculations showed that the break in symmetry of the unit cell of SrWO4 creates inherent defects in the lattice which are responsible for the reduction of the electronic band gap in the SrWO4 sample with decrease in size. On europium doping; energy transfer from the O2− → W6+ charge transfer band to Eu3+ takes place and the reason behind this is explained using density functional theory (DFT) calculations. Based on time resolved PL measurements, it is suggested that Eu3+ ions occupy two sites in SWO; a regular symmetric Sr2+ site and an asymmetric site (closer to charge compensating defects). With increase in annealing temperature, emission intensity as well as asymmetry around europium increased. The changes in asymmetry around europium and defect densities as determined from positron annihilation lifetime spectroscopy (PALS) suggest that though the overall vacancy concentration is reduced with an increase in annealing temperature, it is likely that vacancies closer to europium are slowly annealed out than the others.

Revealing the Nano‐Level Molecular Packing in Chitosan–NiO Nanocomposite by Using Positron Annihilation Spectroscopy and Small‐Angle X‐ray Scattering

Chitosan-NiO nanocomposite (CNC) is shown to be a potential dielectric material with promising properties. CNCs containing NiO nanoparticles (0.2, 0.6, 1, 2, 5 wt %) are prepared through chemical methods. The inclusion of NiO nanoparticles in the chitosan matrix is confirmed by scanning electron microscopy (SEM) and X-ray diffraction. The morphology of the NiO nanoparticles and the nanocomposites is investigated by transmission electron microscopy and SEM, respectively. Positron annihilation lifetime spectroscopy (PALS) and the coincidence Doppler broadening (CDB) technique are used to quantify the free volume and molecular packing in the nanocomposites. The triplet-state positronium lifetime and the corresponding intensity show the changes in nanohole size, density, and size distribution as a function of NiO loading. Small-angle X-ray scattering indicates that the NiO aggregates are identical in all the CNCs. The momentum density distribution obtained from CDB measurements excludes the possibility of a contribution of vacant spaces (pores) available in NiO aggregates to the free volume of nanocomposites upon determination by using PALS. The results show systematic variation in free-volume properties and nano-level molecular packing as a function of NiO loading, which is presumed to play a vital role in determining the various properties of the nanocomposites.

Investigation of Nanoscopic Free Volume and Interfacial Interaction in an Epoxy Resin/Modified Clay Nanocomposite Using Positron Annihilation Spectroscopy

Epoxy/clay nanocomposites are synthesized using clay modified with the organic modifier N,N-dimethyl benzyl hydrogenated tallow quaternary ammonium salt (Cloisite 10A). The purpose is to investigate the influence of the clay concentration on the nanostructure, mainly on the free-volume properties and the interfacial interactions, of the epoxy/clay nanocomposite. Nanocomposites having 1, 3, 5 and 7.5 wt. % clay concentrations are prepared using the solvent-casting method. The dispersion of clay silicate layers and the morphologies of the fractured surfaces in the nanocomposites are studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The observed XRD patterns reveal an exfoliated clay structure in the nanocomposite with the lowest clay concentration (≤1 wt. %). The ortho-positronium lifetime (τ(3)), a measure of the free-volume size, as well as the fractional free volume (f(v)) are seen to decrease in the nanocomposites as compared to pristine epoxy. The intensity of free positron annihilation (I(2)), an index of the epoxy-clay interaction, decreases with the addition of clay (1 wt. %) but increases linearly at higher clay concentrations. Positron age-momentum correlation measurements are also carried out to elucidate the positron/positronium states in pristine epoxy and in the nanocomposites. The results suggest that in the case of the nanocomposite with the studied lowest clay concentration (1 wt. %), free positrons are primarily localized in the epoxy-clay interfaces, whereas at higher clay concentrations, annihilation takes place from the intercalated clay layers.

A proposed k0 based methodology for neutron activation analysis of samples of non-standard geometry

An internal mono-standard method has been proposed for multi element analysis. This method gives the relative concentration of the elements in a sample of non-standard shape and size. It utilizes an in-situ relative efficiency calibration and hence, does not need the cumbersome procedures, otherwise required to correct for γ-attenuation in the sample. To validate this method, the relative concentration of elements in IAEA RMs SL-3 and Soil-7 were analyzed with sample amounts ranging from a few milligrams to grams. The samples were counted in different non-specific geometries. The results are in good agreement with the recommended values, suggesting that this methodology could be applied for the analysis of samples of non-standard size and shape, and in principle, for the analysis of large samples.

Role of various defects in the photoluminescence characteristics of nanocrystalline Nd2Zr2O7: an investigation through spectroscopic and DFT calculations

For the first time, visible photoluminescence could be seen in the blue and green regions in Nd2Zr2O7 nanocrystals synthesized by gel combustion at 800 °C. The nanocrystalline nature of samples was confirmed using X-ray diffraction (XRD), and transmission electron microscopy (TEM). The samples were further characterized using Fourier transform infrared spectroscopy (FTIR), diffuse reflectance spectroscopy (DRS). The photoluminescence emissions pointed to the presence of defects related to oxygen vacancies. DFT-based calculations showed that electronic transitions between defect states (which arise due to V1+O and V2+O defects) and the conduction band, as well as impurity states at the bottom of the CB, can lead to emissions in the green and blue regions. Samples were further annealed at higher temperatures of up to 1200 °C to observe the evolution of defects and its implications for the photophysical characteristics of Nd2Zr2O7. The emission intensity was found to increase with an increase in temperature. The increase in intensity upon annealing at a higher temperature was attributed to a reduction in the concentration of surface defects and cation vacancies as confirmed using positron annihilation lifetime spectroscopy (PALS). Based on positron annihilation gamma-ray coincidence Doppler broadening (CDB) measurements, it was observed that the nature of the defects probed by positrons did not change on annealing but their concentrations significantly changed. This was also reflected in the emission spectra, in which the spectral features remained the same but the intensity increased as the annealing temperature was increased. The value of the direct optical band did not change much either as a function of the annealing temperature, which further supports the trend in the emission spectra. In brief, it can be said that the characteristic emission in Nd2Zr2O7 samples is due to oxygen vacancies, whereas the increase in the emission intensity with temperature is due to a decrease in the concentration of cation vacancies and surface defects, which serve as alternative non-radiative paths.

Measurement of k0-Factors in Prompt Gamma-Ray Neutron Activation Analysis

The guided thermal neutron beam at 100 MW Dhruva research reactor facility of Bhabha Atomic Research Centre (BARC) was used to carry out prompt gamma-ray neutron activation analysis (PGNAA). The prompt k0-factors have been determined for the isotopes of the elements H, B, K, Co, Cu, Ca, Ti, Cr, Cd, Ba, Hg and Gd with respect to 1951 keV gamma-line of 36Cl. The prompt k0-factors for H, Cl and Cu were also measured with respect to the 1381 keV gamma-line of 49Ti. Different samples like NH4Cl, Ti metal, cobalt chloride and other stoichiometric compounds and pure metals were used for this purpose. Prompt gamma-rays were accumulated using a 22% HPGe detector connected to a PC based 8k MCA in single mode counting. The energy calibration in the range of 100–8500 keV was carried out using gamma-rays from 152Eu and 60Co, and the prompt gamma-rays from 36Cl whereas the absolute detection efficiency for this energy range was determined using 152Eu and prompt gamma-rays from 36Cl and 49Ti.

Air quenching of positronium in mesoporous materials: positron porosimetry

The o-Ps lifetimes in mesoporous materials in vacuum and in the presence of air have been measured. The measured lifetimes are examined in terms of various available models for positron porosimetry. The o-Ps lifetimes measured in vacuum agree well with those predicted from the extended Tao–Eldrup model. In the presence of air, the o-Ps lifetimes are seen to reduce. The enhancement in the annihilation rate of positronium caused by air is seen to increase with decrease in the pore size. The positronium lifetimes measured in the presence of air in the present experiment and the literature data are fitted to a model based on the statistical probability of positronium interacting with the pore wall. This describes the increase in the quenching rate with decrease in pore size by taking into account the reactivity of air as well as the reduction in the average pore size due to adsorption of air on the pore surface. This simple model can be used in positron porosimetry for measurements carried out in the presence of air.

Capture γ-rays from 60Co as multi γ-ray efficiency standard for prompt γ-ray neutron activation analysis

Abstract The absolute emission probabilities of 49 γ-rays from thermal neutron capture in 59 Co were measured in the energy range of 150–7500 keV. Two independent approaches were used to arrive at the number of captured neutrons. It has been proposed to use cobalt as a standard for absolute efficiency calibration of the detector over a wide energy range and as a suitable comparator in the k 0 -standardisation method of prompt γ-ray neutron activation analysis work.

Investigation of free volume characteristics of the interfacial layer in poly(methyl methacrylate)–alumina nanocomposite and its role in thermal behaviour

The free volume characteristics (free volume size, density and size distribution) of the interfacial layer (IL) in PMMA–alumina (0.3 wt%) nanocomposites were investigated using the well-established positron annihilation lifetime spectroscopy (PALS) technique. Differential scanning calorimetry (DSC) measurements were carried out to investigate the glass transition process of the PMMA matrix in these nanocomposites. PALS results show significant variations in the free volume structure of the nanocomposite, contrary to the assumptions made in earlier reports, i.e., free volume in PMMA remains unaffected on alumina loading. The study shows that as a result of strong interfacial interaction between PMMA and alumina, a denser region is created in the vicinity of alumina particles (interfacial layer), having smaller-sized nanoholes with narrower size distribution compared to bulk PMMA. This observation is consistent with the studies wherein an interfacial layer with constrained segmental motion was inferred from spectroscopic and thermal analysis techniques. The glass transition temperature, Tg, determined from DSC is observed to vary with alumina loading qualitatively similar to the variation in the fractional free volume of PMMA. The study indicates that the critical free volume for PMMA to undergo glass transition process depends on the interfacial layer fraction and hence varies as a function of alumina loading.