Padma Gopalan

The kinetics of formation and growth of TiC precipitates in Ti-modified stainless steel studied by positron annihilation spectroscopy

Abstract The formation and growth of TiC precipitates in Ti-modified austenitic stainless steel (D-9 alloy) is monitored by positron lifetime spectroscopy. From isochronal annealing studies various recovery stages are identified. TiC precipitates are found to be more stable in 20% cold worked alloy than in a 17.5% cold worked sample. From the isothermal annealing studies, it is found that TiC precipitation is controlled by dislocations. The limited temperature dependence of dislocation controlled TiC precipitation is governed by an apparent activation energy of 1.6 eV. In 20% cold worked alloy, TiC precipitates are found to be stable against growth even after 1000 h of annealing at 923 K. For higher annealing temperatures, TiC precipitate coarsening occurs due to recrystallisation.

Study of surface segregation of Si on palladium silicide using Auger electron spectroscopy

The transformation of Pd/Si to Pd2Si/Si is studied using Auger electron spectroscopy over a wide temperature range of 370–1020 K. The Pd film gets totally converted to Pd2Si upon annealing at 520 K, and beyond 570 K, Si starts segregating on the surface of silicide. It is found that the presence of surface oxygen influences the segregation of Si. The time evolution study of Si segregation reveals that segregation kinetics is very fast and the segregated Si concentration increases as the temperature is increased. Scanning electron microscopy measurements show that Pd2Si is formed in the form of islands, which grow as the annealing temperature is increased.

Anodisation time dependent shift of photoluminescence excitation and emission spectra in porous silicon

The photoluminescence excitation and emission techniques are used to monitor the absorption and emission characteristics of silicon nanocrystals. Contributions from various critical points are identified and their shift as a function of reduction in particle size is deduced. A red shift in the direct band gap energy of silicon with decreasing crystal size is experimentally observed for the first time. The results are explained on the basis of quantum confinement in silicon nanocrystals.

Studies on Fullerenes Using Positron Annihilation Spectroscopy

ABSTRACT Results of positron lifetime measurements as a function of temperature and pressure are presented. With the application of pressure, the lifetime in solid C60 is observed to decrease and show signatures of the structural transition from fcc to the simple cubic (sc) phase. Positron lifetime measurements as a function of temperature, carried out in both C60 and C70, indicate an increase in lifetime at elevated temperatures. This is accounted for in terms of thermally activated trapping of positrons from the interstitial regions to the open regions within the cage structure. In addition to studies in the solid state, the annihilation characteristics of the ortho positronium (O-Ps) formed in organic solvents has been monitored with increasing concentration of dissolved fullerenes, C60 and C70. The observed reduction of the ortho positronium lifetime is understood in terms of the formation of O-Ps- Fullerene complex prior to annihilation.

TDPAC studies of Hf doped YBCO

Time differential perturbed angular correlation measurements of the 133–482 keVγ-γ cascade of181Ta in Hf-doped YBa2Cu3O7−x are presented. The181Hf precurser nuclei are incorporated into the sample by thermal neutron irradiation. Two quadrupole interaction frequencies are observed in the as-irradiated sample:vQ1=161±10 MHz with intensityf1=75%, asymmetry parameterη1=0.32 and damping parameter Λ1=0.42, andvQ2=1108±40 MHz withf2=25%,η2=0.62, and Λ2=0.60. On annealing the sample in air at various temperaturesTa and quenching to room temperature,f1 remained nearly constant forTa<600°C andvQ1 for all annealing temperatures indicating that these are insensitive to oxygen stoichiometry. This frequency is interpreted to be due to181Hf substitutingY sites. BeyondTa=600°C,f1 increased and reached a constant value of 90% forTa=800°C. The value ofvQ2 showed a slight variation between 1086 and 1160 MHz, whilef2 remained nearly constant at 25% forTa<600°C. This component is identified to be due to181Hf substituting Cu 1 sites in the Cu-O chains of YBCO. Above 600°CvQ2 decreased and reached a value of 808 MHz beyond 750°C.

Positron trapping at precipitates in titanium-modified stainless steels

Abstract A comparative study of the result of Dryzek et al. and ours on the annealing behaviour of cold worked austenitic stainless steel using positron annihilation spectroscopy is presented. It is shown that the positron probes the TiC precipitates in titanium-modified stainless steels and not the M23C6 precipitates, as suggested by Dryzek et al.

Anodisation Time Dependence of Photoluminescence Properties of Porous Silicon

The photoluminescence excitation and emission techniques are used to monitor the absorption and emission characteristics of silicon nanocrystals. Contribution from various critical points are identified and their shift as a function of reduction in particle size is deduced. A red shift in direct band gap energy of silicon with decreasing crystal size is experimentally observed for the first time. The results are explained on the basis of quantum confinement in silicon nanocrystals.