Characterisation of Ion Implantation-induced Defects in Certain Technologically Important Materials by Positron Annihilation (Review Paper)

The application of positron annihilation spectroscopy for the studies of defects produced by different types of charged particles and ions in a variety of materials is discussed with specific examples. The ability to detect and quantify the information through the characteristic parameters of the annihilation radiation in a totally non-destructive method has made the fundamental process of electron-positron annihilation a powerful spectroscopic probe for investigating the structure and properties of materials. Ion implantation produces defects in the structure of solids and the latter can be recovered from the defects by annealing at high temperatures. Here the annealing is done in sequential steps so that the different stages of evolution of defects and their interaction with impurity atoms can be studied systematically. Defects produced by irradiation by particles like protons, alpha, boron and neon ions in materials ranging from simple metals to binary alloys are discussed.A detailed evaluation of the positron lifetimes in terms of the popular positron trapping models is also presented. Further as a special case, the method of extraction of values of several useful physical parameters of inert gas bubbles inside a metal matrix is explained with the help of a model analysis.Defence Science Journal, 2009, 59(4), pp.329-341, DOI:http://dx.doi.org/10.14429/dsj.59.153

Position lifetime amd coincidence doppler broadening studies of graphene oxide - polyaniline nanocomposite

In this work, we report about the results of a detailed investigation comprising of measurements of positron lifetimes and coincidence Doppler broadening of the electron positron annihilation gamma ray spectra on graphene oxide – polyaniline nanocomposite samples prepared with different ratios of weight and at different temperatures. The two experimental techniques are capable of providing information respectively on the elec- tron density and momentum distribution at the specific sites of annihilation. This makes them effective in identifying the different types of defects present in the composite matrix. The magnitudes of the positron lifetimes were commensurate with the expectation of very large size defects in the form of vacancy clusters within the atomic composition of the composite material. The average defect size apparently reduced when the relative abundance of polyaniline increased that led to increased reduction of graphene oxide to graphene. The reduction also resulted in the shrinkage of the graphene oxide matrix and the free volume thereby released added to the overall defect concentration. The variation of the positron lifetime and its intensity with the temperature of synthesis suggested an optimum temperature suitable for the process. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/2062

Annihilation characteristics of positrons in a polymer containing silver nanoparticles

9 págs.; 8 figs.; 6 tabs.Composites of silver particles of diameters in the range 16.4-33.3 nm and polyacrylamide were prepared by a chemical method. Positron lifetime and Doppler broadening measurements of these samples were carried out. The positron lifetime spectra of all the samples could be decomposed into three components having lifetimes around 200, 500, and 1800 ps. These are believed to arise due to vacancy clusters on the grain surfaces, the open spaces between the grain surface and the surrounding polymer layer and the annihilation of orthopositronium at the free-volume defects, respectively. The lifetime of positrons trapped at the grain surface defects and the grain-polymer interface is found to decrease as the grain size is increased. Doppler broadening measurements were carried out from 13 K to 300 K on a silver-polyacrylamide nanocomposite containing silver particles of diameters in the range 2-20 nm. The line-shape parameter S is decomposed by a mixture rule to obtain the contribution of electrons from the nanoparticles. This shows a sharp increase at around 80 K that is adduced as evidence for the splitting of the electron energy levels in the nanosized silver particles leading to a semiconductorlike behavior. © 1998 The American Physical SocietyM. Mukherjee and D. Chakravorty acknowledge the support from a foreign research Grant No. N00014- 93-1-0040 by the Office of Naval Research, Virginia, U.S.A.Peer Reviewe

Precipitation Behavior Investigated Through Positron Annihilation in Sc-doped Al-6Mg Followed by the Effects of Zr-addition

AbstractPhase decomposition in Al–6Mg alloy doped with Sc up to 0.6wt.% was first investigated through positron lifetime and coincidence Doppler broadening spectroscopic (CDBS) measurements. The results varied significantly with the degrees of doping and heat treatment conditions due to the entrapment and annihilation of positrons at vacancies and lattice irregularities like coherent and semi-coherent precipitate-matrix interfaces. Sc-vacancy complexes helped in fine scale precipitation of Al3Sc during the annealing. The substructure stabilization is effected more at low annealing temperature and shorter annealing times. The precipitation behaviour in 0.2wt.% Zr-doped Al-6Mg-0.4Sc alloy under different annealing conditions was also studied. Although Sc has better diffusivity in Al-6Mg than Zr, the latter appeared to be an ideal additive to generate new precipitates of the form Al3Sc1-xZrx and the differences are reflected in the positron lifetimes and CDBS ratio curves. Transmission electron microscopy showed spheroidal precipitates with complete absence of facets, implying the modification of the surface morphology of the precipitates

Gadolinium substitution induced defect restructuring in multiferroic BiFeO3: case study by positron annihilation spectroscopy

Positron annihilation spectroscopy (PAS) comprising of the measurements of positron lifetime and coincidence Doppler broadening spectra has been carried out to understand and monitor the evolution of the vacancy-type defects arising from the ionic deficiencies at lattice points of the multiferroic perovskite bismuth ferrite (BiFeO3) doped with 1, 5 and 10 at% gadolinium (Gd3+) ions. Negatively charged defects in the form of Bi3+ monovacancies $(V_{{\rm Bi}}^{3-} )$ were present in the undoped nanocrystallites, which strongly trapped positrons. During the successive doping by Gd3+ ions, the positron trapping efficiency decreased while the doped ions combined with the vacancies to form complexes, which became neutral. A fraction of the positrons got annihilated at the crystallite surfaces too, being evident from the very large positron lifetimes obtained and confirming the nano-size-specific characteristics of the samples. Further, the intercrystallite regions provided favourable sites for orthopositronium formation, although in minute concentrations. The dopant ion-complex formation was also depicted clearly by the defect characteristic S–W plot. Also, the large change of electrical resistivity with Gd concentration has been explained nicely by invoking the defect information from the PAS study. The study has demonstrated the usefulness of an excellent method of defect identification in such a novel material system, which is vital information for exploiting them for further technological applications