Biennial Report 2005-2007 - Saha Institute of Nuclear Physics

54 Biennial Report 2005-07A remarkable defect evolution stage due to the diffusion of the boron atoms beyond their initialdepths of penetration was seen above 873K and it leaves the sample with defects still present evenat the highest annealing temperature 1273K used in this work.PMG Nambissan, PV Bhagwat†, MB Kurup†NAP2.4.1.6 Experiments on neon-irradiated MgB 2 superconductorsPositron annihilation spectroscopy and specific heat studies under magnetic field were carried outon 160 MeV Ne ion irradiated polycrystalline MgB 2 samples. There is an unusual decrease inpositron lifetime in the irradiated sample which may be due to neon ion implantation. This was alsoindicated by change in cell volume. Coincidence Doppler broadening spectra of Mg, B, irradiatedand unirradiated MgB 2 show that positrons primarily annihilate in boron sublattice in unirradiatedsample whereas there is some similarity of the spectrum of the irradiated sample with that of Mg.There is Mg deficiency in the unirradiated sample whereas predominantly boron vacancies exist inNe ion irradiated MgB 2 samples. Specific heat measurements show that there is a small increasein electronic part of the specific heat and electron-phonon coupling constant.A Talapatra†, SK Bandyopadhyay†, PMG Nambissan, Pintu Sen†, V Ganesan†NAP2.4.1.7 Positron lifetime and CDBS experiments on scandium-incorporated Al-6Mgalloy after isothermal and isochronal annealing treatmentsPositron lifetime studies were conducted on as-cast and isothermally annealed Al-6Mg-xSc alloys(x = 0, 0.2 and 0.4 wt.%) to gain insight into the mechanism of phase decomposition. The valuesof the measured lifetimes confirmed the presence of trapping sites larger than monovacancies andruled out the formation of GP zones. The positron lifetimes further increased on annealing thesamples, indicating different types of traps being formed. When scandium (Sc) is added, positronlifetimes are seen to decrease initially and then increase marginally after 0.2 wt.%. The latter is dueto the segregation of vacancy-Sc complexes over heterogeneous environments. Beyond 0.2 wt.% ofSc, Al 3 Sc also starts precipitating. Isothermal annealing of Sc-doped alloys led to precipitation andthen to particle coarsening. The precipitation takes place fast when annealed at larger temperaturesand overbalances the effect of defect annealing.R Sen†, PMG Nambissan, MK Mitra†, MK Banerjee†NAP2.4.1.8 Positron lifetime studies to assess the influence of zirconium in the phasedecomposition of Al-6Mg-0.4Sc alloyPositron lifetime measurements were carried out on Al-6Mg-0.4Sc (Zr-free) and Al-6Mg-0.4Sc-0.2Zr(Zr-rich) alloys after isothermal annealing at different temperatures to study the role of zirconiumas a partial substitute of scandium in the phase decomposition behavior. It is observed that theaddition of Zr lowers the positron lifetimes. This is attributed to the increased electron densityat the positron trapping sites due to the Zr atoms and is supported by a concurrent increase