Studies on thermally induced structural changes in silicon based polymers by positron annihilation

Temperature dependence of positron annihilation lifetime spectra of polysilanes such as, poly(methyl-n-propylsilane) (PMPrS) and poly(di-n-hexylsilane) (PDHS) has been investigated. The τ₃ in PMPrS is seen to increase monotonically around the solid–liquid transition temperature. The transition temperature and free volume are observed to depend on the molecular weight and/or packing of the backbones. For PDHS, a sharp change in τ₃ and I₃ is seen at the solid–solid transition temperature. Free volume radius probability density functions, above and below the transition temperature, are presented in PMPrS and PDHS. Positron studies are complimented by conventional thermal analysis studies.     

Foreword

Electrochemical behaviors of U⁴⁺ in LiCl–KCl–UF₄ eutectic and deposition of U metal were investigated. It was found that the presence of F^? has influence on the diffusion of U³⁺ and U⁴⁺ as comparing to data obtained in pure chloride molten salts. Electrochemical deposition of U was carried out by using pulse current electrolysis. Characterization results indicate that U metal was obtained at the cathode, implying U metal can be directly deposited from LiCl–KCl–UF₄ eutectic in this case and the extractive ratio is calculated to be 98%. Our results demonstrate feasible separation of U from LiCl–KCl–UF₄ molten salt by electrochemical method.     

Cross-linked DNA generated by "bis-click" reactions with bis-functional azides: site independent ligation of oligonucleotides via nucleobase alkynyl chains

Template-free cross-linking of single-stranded DNA bearing octadiynyl side chains at the 7-position of 8-aza-7-deazapurine moieties with bisfunctional azides is reported employing a Cu(I)-catalyzed azide-alkyne "bis-click" reaction. Bis-adducts were formed when the bis-azide:oligonucleotide ratio was 1:1; monofunctionalization occurred when the ratio was 15:1. Four-stranded DNA consisting of two cross-linked duplexes was obtained after hydridization. Cross-linked duplexes are as stable as individual duplexes when ligation was introduced at terminal positions; ligation at a central position led to a slight duplex destabilization.     

Direct evidence of Cd vacancies in CdSe nanoparticles: positron annihilation studies

Poly vinyl alcohol (PVA) capped CdSe nanoparticles having size in the range of 7–17 nm have been synthesized through chemical route and characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Positron coincidence Doppler broadening (CDB) measurements have been carried out in these nanoparticles. It is observed that the electron momentum distributions show a variation in the core electron momentum region with the particle size. In order to examine the influence of defects, first principle calculations of electron momentum distributions in bulk CdSe and in the presence of Cd as well as Se vacancy defects have been performed. Comparison of experimental data with the calculated momentum distribution reveals the presence of Cd vacancy defects, the concentration of which decreases with the increase in the particle size. The present study also indicates possible Se enrichment on the surface of the nanoparticles with the decrease in the particle size.     

Self-injection length in La0.7Ca0.3MnO3-YBa2Cu3O7-δ ferromagnet-superconductor multilayer thin films

We have carried out extensive studies on the self-injection problem in barrierless heterojunctions between La_0.7Ca_0.3MnO₃ (LCMO) and YBa₂Cu₃O_7-δ (YBCO) thin films. The heterojunctions were formed in situ by sequentially growing LCMO and YBCO films on 〈100〉 LaAlO₃ (LAO) substrate using a pulsed laser deposition (PLD) system. YBCO micro-bridges with 64 μm width were patterned both on the LAO (control) and LCMO side of the substrate. Critical current, I _c, was measured at 77 K on both the control side as well as the LCMO side for different YBCO film thickness. It was observed that while the control side showed a J _c of ∼ 2 × 10⁶ A/cm², the LCMO side showed about half the value for the same thickness (1800 ?). The difference in J _c indicates that a certain thickness of YBCO has become ‘effectively’ normal due to self-injection. From the measurement of J _c at two different thicknesses (1800 ? and 1500 ?) of YBCO films both on the LAO as well as the LCMO side, the value of self-injection length (at 77 K) was estimated to be ∼ 900 ?. To the authors’ best knowledge, this is the first time that self-injection length has been quantified. A control experiment carried out with LaNiO₃ deposited by PLD on YBCO did not show any evidence of self-injection.     

A moving-load controlled-displacement fracture-toughness testing machine

In conventional fracture-toughness testing, the line of application of the loads remains fixed with respect to specimen geometry. In this testing machine, the load moves with the advancing crack front, and displacement is used as the controlled variable to propagate and arrest a crack. The energy-release rate at the onset of crack propagation and, hence, the plane-strain fracture toughnessK _Ic can be measured directly without compliance calibration or stress-intensity evaluation. The specimen is in the form of a flat plat 25 by 50 cm which is simple to machine and provides about 30 values ofK _Ic. The versatility of the machine is demonstrated by making a statistical analysis ofK _Ic for 7075-T6 Al by showing the effect of plate thickness on the fracture toughnessK _c using a tapered specimen, and by evaluatingK _c in 7075 Al as a function of aging temperature in a thermal-gradient-treated specimen.     

Free volume related fluorescent behavior in electron beam irradiated chalcone doped PVA

Effect of electron irradiation on the free volume related microstructural and optical properties of chalcone doped Poly(vinyl alcohol) composite films have been studied using FTIR, UV-Visible, XRD and Positron Annihilation techniques. The FTIR spectral study shows that the irradiation induces the crosslinking within the composite. Using UV-Visible absorption spectra the optical energy band gap and activation energies were estimated and the variation of these parameters suggests the existence of defects and molecular ordering within the irradiated composite. XRD diffractograms reveal that the crosslinking enhances the crystallinity of the sample. In this cross-linked polymer composite the fluorescence enhancement has been observed in the fluorescence spectral study. The Positron annihilation result suggests that the irradiation affects the free volume properties and crosslinking hinders the chalcone chromophore molecular rotation. Under this restricted condition the chromophore molecules likely to emit enhanced fluorescence and its mobility is directly related to the free volume around it.     

Study of magnetoresistance and conductance of bicrystal grain boundary in La0.67Ba0.33MnO3 thin film

La_0.67Ba_0.33MnO₃ (LBMO) thin film is deposited on a 36.7°C SrTiO₃ bicrystal substrate using laser ablation technique. A microbridge is created across bicrystal grain boundary and its characteristics are compared with a microbridge on the LBMO film having no grain boundary. Presence of grain boundary exhibits substantial magnetoresistance ratio (MRR) in the low field and low temperature region. Bicrystal grain boundary contribution in MRR disappears at temperature T>175 K. At low temperature, I-V characteristic of the microbridge across bicrystal grain boundary is nonlinear. Analysis of temperature dependence of dynamic conductance-voltage characteristics of the bicrystal grain boundary indicates that at low temperatures (T<175 K) carrier transport across the grain boundary in LBMO film is dominated by inelastic tunneling via pairs of manganese atoms and tunneling through disordered oxides. At higher temperatures (T>175 K), magnetic scattering process is dominating. Decrease of bicrystal grain boundary contribution in magnetoresistance with the increase in temperature is due to enhanced spin-flip scattering process.     

Molar extinction coefficients of some carbohydrates in aqueous solutions

Molar extinction coefficients of some carbohydrates viz. l-arabinose (C₅H₁₀O₅), d-glucose (C₆H₁₂O₆), d-mannose (C₆H₁₂O₆), d-galactose (C₆H₁₂O₆), d(-) fructose (C₆H₁₂O₆) and maltose (C₁₂H₂₄O₁₂) in aqueous solutions have been determined at 81, 356, 511, 662, 1173 and 1332 keV by gamma ray transmission method in a narrow beam good geometry set-up. These coefficients have been found to depend upon the photon energy following a 4-parameter polynomial. These extinction coefficients for different sugars having the same molecular formula have same values varying within experimental uncertainty. Within concentration ranges studied, Beer-Lambert law is obeyed very well.     

Correlations of fission fragment angular momentum with collective and intrinsic degrees of freedom

In the present work angular momenta of the fragments corresponding to¹³²I^m,g have been deduced from the radiochemically determined independent isomeric yield ratios and statistical model based analysis in neutron induced fission of²³⁵U,²³⁹Pu and²⁴⁵Cm and spontaneous fission of²⁵²Cf. These data along with similar data on¹³⁴I, reported earlier from this laboratory, bring out the effects of deformed 66n and spherical 82n shells on fragment angular momentum showing also an inverse correlation of the latter with elemental yields. Quantitative estimates of fragment scission point deformation and the coefficient of change of fragment angular momentum with kinetic/excitation energy have been deduced and are seen to be in good agreement with the expected theoretical estimates.