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Positron beam measurements carried out on high temperature Ni+ implanted FeCrCoNi show no void formation but only presence of monovacancies. With thermal treatment, evidence for the formation of nano sized σ-CoCr are obtained from positron based ratio curve analysis in the temperature range 973–1123 K. Corroborative evidence for the presence of these precipitates are obtained from EBSD and GIXRD measurements. Further annealing beyond 1150 K leads to coarsening of these precipitates and they no longer trap the positrons.

Positron beam-based Doppler broadening results on He ion-implanted FeCrCoNi show distinct changes in defect-sensitive S-parameter corresponding to the formation of helium-vacancy complexes, nucleation/ growth of helium bubbles, and finally the helium bubble rupture, with isochronal annealing. Data analyses of the positron beam-based ratio curves provide clear information on the kinetics of bubble growth and rupture. The difference data in S-parameter (ΔS), which corresponds to the bubble growth, displays a maximum at 1123 K, with annealing temperature. Comparing the ΔS value to that reported for nickel, it is concluded that FeCrCoNi has higher bubble growth resistance than nickel.

Defect characterization of room temperature 1.5 MeV Ni ion implanted high entropy FeCrCoNi alloy for two fluences (1 × 10 15 ions/cm 2 and 5 × 10 16 ions/cm 2 ) was carried out using the variable low energy positron beam. The FCC solid solution remains robust and stable under 100 dpa irradiation and high temperature annealing. The change in the defect sensitive S-parameter upon implantation reveals the presence of monovacancies for both the doses. The changes in the defect microstructure upon thermal annealing are found to be dose dependent. The high dose shows the formation of stable stacking fault tetrahedrons (SFT's) from the aggregates of monovacancies at higher annealing temperatures while the low dose shows the annealing of monovacancies with temperature.

Defect annealing in FeCrCoNi high entropy alloy is studied using positron lifetime and Doppler broadening spectroscopic techniques. Variation of positron lifetime with temperature show the annealing of defects beyond 770 K. Theoretical positron lifetime and electron momentum distributions were computed for the alloy to understand the nature of defects present in the arc melted alloy. X-ray diffraction measurements show that the arc melted alloy forms FCC solid solution and the phase remains stable even after annealing at 1373 K. Recrystallization assisted grain growth occurs beyond 973 K.

Structural stability studies of boron doped 40:60 iron phosphate glasses under humid storage conditions, using Raman spectroscopy is reported. Raman measurements indicate that increased boron doping in these glasses maintaining oxygen/phosphorous ratio constant serves to strengthen the iron phosphate glass network, while for samples with iron/phosphorous ratio a constant, a systematic weakening of the network was observed with increased boron content. Raman measurements on glasses exposed to ambient laboratory conditions (300 K; RH: 70 -80%) for three years reveal excellent structural stability against atmospheric corrosion. Incidentally the sample with the strongest network - 20 mol% boron doped iron phosphate glass with O/P constant, indicated devitrification of the glass surface on storage. Definitive evidence of attack by atmospheric water vapour on the glass surface leading to the above devitrification has been obtained using Raman spectroscopy. Possible pathways for the enhanced atmospheric corrosion of this glass are discussed that would help mitigate this detrimental degradation of boron doped iron phosphate base glasses.

Detailed Raman spectroscopic studies on 40:60 iron phosphate glasses (IPG) and cesium doped IPG under ambient conditions revealed an evident disproportionation of the pyrophosphate network beyond 18 mol% Cs doping. In situ high pressure Raman spectroscopic studies were carried out on IPG and Cs doped IPG up to 40 GPa for the first time in order to investigate the structural stability of these glasses under pressure. Although the parent glass is found to be robust to such high pressures, pressure quenched samples loaded with cesium beyond 18 mol% concentration were observed to permanently cross link leading to new high density forms of the glass. The extent of cross linking is observed to systematically increase with the cesium content in the parent matrix.

LiInSe2 polycrystalline material was successfully synthesized from melt and temperature oscillation method (MTOM). The crystalline phase was confirmed by powder X-ray diffraction pattern. Crack free LiInSe2 single crystal of size 12 mm diameter and 32 mm length was grown using two zone tubular resistive heated furnace by modified vertical Bridgman–Stockbarger method with steady ampoule rotation. The grown LiInSe2 crystal was subjected to single crystal XRD, inductively coupled plasma-optical emission spectroscopy (ICP-OES), positron annihilation spectroscopy (PAS), Hall effect and dielectric measurements. Single crystal XRD measurement confirms the orthorhombic crystal system. ICP-OES analysis gives the crystal composition as Li0.81In1.01Se2.18. The average lifetime 278.03 ps in PAS measurements corresponds to vacancy clusters present in LiInSe2 crystal. Hall effect measurement confirms the n-type semiconductor nature. The dielectric permittivity and dielectric loss were obtained to be 9.8 and 0.108 respectively by capacitance measurements at room temperature for the frequency of 2 MHz.

Fatigue crack growth behaviour of a near α titanium alloy Timetal 834 has been studied at maximum dynamic strain ageing temperature (450 °C) and at the service temperature (600 °C). Compact-tension type fracture mechanics specimens have been tested with stress ratio and frequency of 0.1 and 1 Hz, respectively, under constant amplitude loading. The alloy showed higher crack growth rate at 450 °C due to dynamic strain ageing which causes higher stress concentration at the crack tip as compared to 600 °C. The pronounced effect of oxide and roughness induced crack closure revealed from Auger electron spectroscopy and surface profiler, respectively, have been attributed to reduce the stage I as well as stage II fatigue crack growth rate at 600 °C. Fractographic and crack path observations confirmed that the crack growth mechanism remained highly faceted in nature at low ΔK levels which changes subsequently to striation mode of fracture at relatively higher ΔK levels.

Raman and infrared spectroscopic measurements were carried out on 40 mol% Fe 2 O 3 :60 mol% P 2 O 5 iron phosphate glass samples to study the effect of increasing external pressure on the local structure of both anhydrous and water trapped glasses. In-situ high‐pressure Raman measurements, done up to 24.2 GPa on anhydrous samples, revealed hardening of the phonon modes accompanied by a loss of Raman intensity and ultimately leading to complete smearing of modes at very high pressures. These changes were found to be reversible upon quenching the sample back to ambient pressure. Raman and infrared spectroscopy of pressure quenched IPG samples containing trace amounts of water have revealed definite signatures of devitrification at room temperature.

Iron phosphate glasses have been investigated using positron lifetime and micro Raman spectroscopy to study the crystallization kinetics and defect evolution across the glass and crystallization transitions. Positron lifetime parameters show distinct changes at temperatures corresponding to both the transitions. These changes have been understood as the signature of the evolution of open volume defects and their clustering across the amorphous to crystalline interfaces. Micro Raman measurements on annealed samples have identified the crystalline phases as Fe3(P2O7)2 and FePO4. Ab-initio positron lifetime computations reveal that the positron probes the Fe/P sites in defect free crystals. The nature of open volume defects produced upon annealing is identified by computing positron lifetimes for various possible defect clusters. A combination of both experimental and theoretical techniques has provided insights in understanding the evolution of defects across the phase transition in these glasses.

The void swelling behaviour of (15Ni–14Cr) Ti-modified steels simulated by heavy ion irradiation, has been investigated using step height measurements and positron annihilation spectroscopy. 18% cold-worked samples were pre-implanted with helium at 170 keV and 275 keV energies so as to create a uniform helium concentration of 100 appm around the 588 nm depth region. This was followed by a 2.5 MeV Ni ion irradiation to create a peak damage of ~ 84 dpa (displacement per atom) at various irradiation temperatures between 700 and 970 K. The gross swelling which has occurred throughout the ion range is determined by step height measurements. Defect-sensitive positron annihilation lineshape S-parameter measurements show clear changes consequently to void formation as a function of sample depth. From the variation of the average S-parameter as a function of irradiation temperature, the peak swelling temperature has been deduced. The changes observed in step height measurements are compared with positron annihilation measurements and the results are discussed.

Positron annihilation spectroscopy along with glancing incidence X-ray diffraction have been used to investigate tin oxide thin films grown on Si by pulsed laser deposition. The films were prepared at room temperature and at 670 K under oxygen partial pressure. As-grown samples are amorphous and are found to contain large concentration of open volume sites (vacancy defects). Post-deposition annealing of as-grown samples at 970 K is found to drastically reduce the number of open volume sites and the film becomes crystalline. However, film grown under elevated temperature and under partial pressure of oxygen is found to exhibit a lower S -parameter, indicating lower defect concentration. Based on the analysis of experimental positron annihilation results, the defect-sensitive S -parameter and the overlayer thickness of tin oxide thin films are deduced. S – W correlation plots exhibit distinct positron trapping defect states in three samples.

The defect recovery in proton irradiated Ti-modified D9 steel has been studied by positron annihilation isochronal and isothermal annealing measurements. D9 samples have been irradiated with 3 MeV protons followed by isochronal annealing at various temperatures in the range of 323 to 1273 K. The dramatic decrease in positron annihilation parameters, viz. positron lifetime and Doppler S-parameter, around 500 K indicates the recovery of vacancy-defects. A clear difference in the recovery beyond 700 K is observed between solution annealed and cold worked state of D9 steel due to the precipitation of TiC in the latter. Isothermal annealing studies have been carried out at the temperature wherein vacancies distinctly migrate. Assuming a singly activated process for defect annealing, the effective activation energy for vacancy migration is estimated to be 1.13 ± 0.08 eV.

Automation of positron beam experiment with a single chip embedded design using a programmable system on chip (PSoC) which provides easy interfacing of the high-voltage DC power supply is reported. Virtual Instrument (VI) control program written in Visual Basic 6.0 ensures the following functions (i) adjusting of sample high voltage by interacting with the programmed PSoC hardware, (ii) control of personal computer (PC) based multi channel analyzer (MCA) card for energy spectroscopy, (iii) analysis of the obtained spectrum to extract the relevant line shape parameters, (iv) plotting of relevant parameters and (v) saving the file in the appropriate format. The present study highlights the hardware features of the PSoC hardware module as well as the control of MCA and other units through programming in Visual Basic.

Silicide formation in Co/Si thin structures synthesized using thermal evaporation, sputter deposition and ion implantation, has been investigated using depth-resolved positron annihilation spectroscopy (PAS) together with other corroborative experimental techniques. S vs. E p curves and S – W correlation plots have revealed important processes such as defect annealing, interdiffusion, silicide formation and recrystallization of amorphous Si. These studies have shown that there exist differences in the formation temperature of the silicide phases, the sequence of silicide phase formation and defect generation owing to the nature of the deposition methods employed.

Germanium atomic (Ge1) and molecular ions (Ge2) of equivalent energy are implanted in silicon at an elevated temperature. The ion induced damage has been characterized by RBS channeling (RBS/C) and positron annihilation spectroscopy. The RBS/C studies indicate that the molecular ion implantation has produced more defects in the near surface regions compared to the atomic ion implantation. This paper reports a first time observation of an enhanced production of vacancy related defects in silicon implanted with molecular ions.

Depth resolved positron annihilation studies on Pd/Si thin film system have been carried out to investigate silicide phase formation and vacancy defect production induced by thermal annealing. The evolution of defect sensitive S-parameter clearly indicates the presence of divacancy defects across the interface, due to enhanced Si diffusion beyond 870 K consequent to silicide formation. Corroborative glancing incidence X-ray diffraction (GIXRD), Auger electron spectroscopy (AES) and Rutherford backscattering spectrometry (RBS) have elucidated the aspects related to silicide phase formation and Si surface segregation.

Silicide formation induced by thermal annealing in Ni/Si thin film system has been investigated using glancing incidence X-ray diffraction (GIXRD) and Auger electron spectroscopy (AES). Silicide formation takes place at 870 K with Ni 2 Si, NiSi and NiSi 2 phases co-existing with Ni. Complete conversion of intermediate silicide phases to the final NiSi 2 phase takes place at 1170 K. Atomic force microscopy measurements have revealed the coalescence of pillar-like structures to ridge-like structures upon silicidation. A comparison of the experimental results in terms of the evolution of various silicide phases is presented.

Pore structure in polypropylene membranes has been investigated using slow positron beam and small-angle X-ray scattering (SAXS) studies. The pore structure has been modified by chemically grafting methylmethacrylate in polypropylene. Depth-resolved Doppler lineshape S-parameter and positronium (Ps) fraction (3γ/2γ ratio) measurements have been carried out using a variable low energy positron beam. Lineshape S-parameter and Ps fraction are found to decrease with the extent of grafting. The deduced positron diffusion length is also seen to decrease with the extent of pore grafting. The power law dependence of SAXS intensity over a wide range of the wave vector transfer reveals the fractal nature of the pore–membrane surface and, the surface fractal dimensions are seen to increase with amount of pore grafting. The value of the negative exponent of the power law size distribution of the pores has been found to be increasing with increase in grafting revealing reduction in the pore size and narrower pore size distribution. The deduced pore size from SAXS measurements is correlated with positron diffusion length and S-parameter from the positron beam measurements.

The transformation of Co/Si to CoSi 2 /Si in the temperature range of 300–1170 K has been investigated using depth-resolved positron annihilation and Glancing incidence X-ray diffraction (GIXRD). The different silicide phases formed are identified from the experimental positron annihilation characteristics, which are consistent with the GIXRD results. The present study clearly indicates the absence of vacancy defects in the silicide overlayer.

GaN epilayer grown on Sapphire by MOCVD technique has been irradiated with 75 MeV Sn 5+ ions to a fluence of 1×10 11 , 10 12 , 10 13 and 10 14 cm −2 and studied using depth-resolved positron annihilation spectroscopy. High-energy ion-induced defects are found to be Ga mono vacancies, which have been identified from the defect sensitive S -parameter values as well as from R -parameter values, for all fluence beyond 10 11 cm −2 . It is also found that the concentration of vacancy defects increases with ion fluence.

The effect of doping of the polyaniline emeraldine base (PEB), with Ni as well as Ni over layer coating has been investigated using variable low energy positron beam. Depth-resolved Doppler S-parameter measurements have been performed on undoped, Ni-doped polyaniline (PANI), and Ag (40 nm) film deposited PANI samples. Significant variation in S-parameter is observed for undoped and Ni-doped PANI. The size of the free volume hole has shifted to lower values upon doping with Ni as compared to that of undoped PANI, which is consistent with the conductivity measurements. For Ag-coated PANI systems, the S vs. Ep curves show distinct changes at the surface and interior regions. These results are discussed in the light of changes in free volume hole size distribution.

The void swelling behavior of heavy ion irradiated D9 steel has been investigated using variable low energy positron beam. The normalized defect-sensitive S-parameter shows up a large increase in the depth region corresponding to the maximum radiation damage as a function of irradiation temperature. From the variation of S-parameter as a function of irradiation temperature, the peak swelling temperature has been deduced and the results are discussed.

Bulk nickel silicides (NiSi and NiSi2) have been studied using the experimental positron lifetime and depth-resolved positron beam measurements. Ab-initio calculations of positron lifetime for the silicides have also been carried out using the atomic superposition method. For NiSi phase, it is found that the theoretically computed positron lifetime compares favourably with the experimentally deduced value indicating that NiSi is defect free. However, for NiSi2, the experimental positron lifetime does not compare well with the theoretical value suggesting that NiSi2 contains vacancy defects. This is further supported by the positron diffusion lengths deduced from the VEPFIT analysis of the positron beam results.