A. Ingram

Positronics of subnanometer atomistic imperfections in solids as a high-informative structure characterization tool.

Methodological possibilities of positron annihilation lifetime (PAL) spectroscopy applied to characterize different types of nanomaterials treated within three-term fitting procedure are critically reconsidered. In contrast to conventional three-term analysis based on admixed positron- and positronium-trapping modes, the process of nanostructurization is considered as substitutional positron-positronium trapping within the same host matrix. Developed formalism allows estimate interfacial void volumes responsible for positron trapping and characteristic bulk positron lifetimes in nanoparticle-affected inhomogeneous media. This algorithm was well justified at the example of thermally induced nanostructurization occurring in 80GeSe2-20Ga2Se3 glass.

Structural studies of spinel manganite ceramics with positron annihilation lifetime spectroscopy

The new transition-metal manganite Cu0.1Ni0.8Co0.2Mn1.9O4 ceramics for temperature sensors with improved functional reliability are first proposed. It is established that the amount of additional NiO phase in these ceramics extracted during sintering play a decisive role. This effect is well revealed only in ceramics having a character fine-grain microstructure, while the monolithization of ceramics caused by great amount of transferred thermal energy reveals an opposite influence. The process of monolitization from the position of evolution of grain-pore structure was studied in these ceramics using positron annihilation lifetime spectroscopy.

Water-sensitive positron trapping modes in nanoporous magnesium aluminate ceramics

The water-sensitive positron trapping modes in nanoporous MgAl2O4 ceramics with a spinel structure are studied. It is shown that water-sorption processes in magnesium aluminate ceramics leads to corresponding increase in positron trapping rates of extended defects located near intergranual boundaries. This catalytic affect has reversible nature, being strongly dependent on sorption water fluxes in ceramics. The fixation of all water-dependent positron trapping inputs allow to refine the most significant changes in positron trapping rate of extended defects.

PALS as characterization tool in application to humidity-sensitive electroceramics

It is shown that positron annihilation lifetime spectroscopy is a quite promising tool for nanostructural characterisation of humidity-sensitive spinel-type MgAl2O4 ceramics, the results being achieved provided four-component fitting procedure with arbitrary lifetimes is applied to treat mathematically the measured spectra. It is shown that Tao-Eldrup model is adequate for calculation of nanopore size in MgAl2O4 ceramics using lifetime values of the third and four components.

Influence of phase separation on the devitrification of 45S5 bioglass.

The devitrification of the 45S5 variety of bioactive glasses (BGs) in relation to phase separation is studied with scanning electron microscopy, X-ray powder diffraction, Fourier transform infrared spectroscopy, differential scanning calorimetry and positron annihilation lifetime spectroscopy techniques. It is shown that the type of phase separation (such as spinodal vs. droplet-like) has a pronounced effect on the activation energy of viscous flow and crystallization, the onset temperature of crystallization and the void size distribution at the nanoscale. Generally, the Johnson-Mehl-Avrami (JMA) relation does not describe crystallization kinetics in bulk 45S5 BG. However, for powder samples (<300 μm) the difference in crystallization kinetics, which is surface-driven for the two kinds of glasses, becomes much smaller, and can be described with the JMA relation under some circumstances.

Initial stage of physical ageing in network glasses

An atomistic view on Johari–Goldstein secondary β-relaxation processes responsible for structural relaxation far below the glass transition temperature (Tg ) in network glasses is developed for the archetypal chalcogenide glass, As20Se80, using positron annihilation lifetime, differential scanning calorimetry, Raman scattering and nuclear magnetic resonance techniques. Increased density fluctuations are shown to be responsible for the initial stage of physical ageing in these materials at the temperatures below Tg . They are correlated with changes in thermodynamic parameters of structural relaxation through the glass-to-supercooled liquid transition interval. General shrinkage, occurred during the next stage of physical ageing, is shown to be determined by the ability of system to release these redundant open volumes from the glass bulk through the densification process of glass network.

Extended defects in insulating MgAl2O4 ceramic materials studied by PALS methods

Extended positron-trapping defects in technological modified insulating nanoporous MgAl2O4 ceramics are characterized by positron annihilation lifetime spectroscopy. The results are achieved using three-component fitting procedure with arbitrary lifetimes applied to treatment of measured spectra. Within this approach, the first component in the lifetime spectra reflects microstructure specificity of the spinel structure, the second component responsible to extended defects near intergranual boundaries and the third component correspond to ortho-positronium pick-off decaying in nanopores of ceramics. It is shown that in ceramics of different technological modifications the same type of positron traps prevails.

Positron annihilation lifetime study of polyvinylpyrrolidone for nanoparticle-stabilizing pharmaceuticals.

Positron annihilation lifetime spectroscopy was applied to characterize free-volume structure of polyvinylpyrrolidone used as nonionic stabilizer in the production of many nanocomposite pharmaceuticals. The polymer samples with an average molecular weight of 40,000 g mol(-1) were pelletized in a single-punch tableting machine under an applied pressure of 0.7 GPa. Strong mixing in channels of positron and positronium trapping were revealed in the polyvinylpyrrolidone pellets. The positron lifetime spectra accumulated under normal measuring statistics were analysed in terms of unconstrained three- and four-term decomposition, the latter being also realized under fixed 0.125 ns lifetime proper to para-positronium self-annihilation in a vacuum. It was shown that average positron lifetime extracted from each decomposition was primary defined by long-lived ortho-positronium component. The positron lifetime spectra treated within unconstrained three-term fitting were in obvious preference, giving third positron lifetime dominated by ortho-positronium pick-off annihilation in a polymer matrix. This fitting procedure was most meaningful, when analysing expected positron trapping sites in polyvinylpyrrolidone-stabilized nanocomposite pharmaceuticals.

Microstructure Hierarchical Model of Competitive e+-Ps Trapping in Nanostructurized Substances: from Nanoparticle-Uniform to Nanoparticle-Biased systems

Microstructure hierarchical model considering the free-volume elements at the level of interacting crystallites (non-spherical approximation) and the agglomerates of these crystallites (spherical approximation) was developed to describe free-volume evolution in mechanochemically milled As4S4/ZnS composites employing positron annihilation spectroscopy in a lifetime measuring mode. Positron lifetime spectra were reconstructed from unconstrained three-term decomposition procedure and further subjected to parameterization using x3-x2-coupling decomposition algorithm. Intrinsic inhomogeneities due to coarse-grained As4S4 and fine-grained ZnS nanoparticles were adequately described in terms of substitution trapping in positron and positronium (Ps) (bound positron-electron) states due to interfacial triple junctions between contacting particles and own free-volume defects in boundary compounds. Compositionally dependent nanostructurization in As4S4/ZnS nanocomposite system was imagined as conversion from o-Ps trapping sites to positron traps. The calculated trapping parameters that were shown could be useful to characterize adequately the nanospace filling in As4S4/ZnS composites.

Effect of rare-earth doping on the free-volume structure of Ga-modified Te20As30Se50 glass

By exploring the positron–electron annihilation technique in positron lifetime measuring mode, it is shown that principal rare-earth (RE) induced structural reconfiguration in Ga-codoped TAS-235 glass (that is a glassy Te20As29Ga1Se50 alloy) is related to occupation of intrinsic free-volume voids by embedded RE ions tightly connected with Ga-based tetrahedrons via strong covalent RE-Se/Te–Ga links. A gradual decrease in the intensity of the second component of two-term decomposed lifetime spectra of annihilating positrons accompanied with a detectable increase in the defect-related positron lifetime (thus inducing essentially a depressed rate in positron trapping) is evidenced by the example of Pr3+ ions added homogeneously to Te20As29Ga1Se50 glass in the amount of 500 ppmw. Observed changes in positron lifetime spectra are explained in terms of the competitive contribution of different occupancy positions in Ga-codoped glass available for RE ions and trapped positrons.