A positron annihilation spectroscopic investigation of europium-doped cerium oxide nanoparticles

Doping in ceria (CeO2) nanoparticles with europium (Eu) of varying concentrations (0, 0.1, 0.5, …, 50 atom%) is studied using complementary experimental techniques and novel observations were made during the investigation. The immediate observable effect was a distinct reduction in particle sizes with increasing Eu concentration attributed to the relaxation of strain introduced due to the replacement of Ce4+ ions by Eu3+ ions of larger radius. However, this general trend was reversed in the doping concentration range of 0.1–1 atom% due to the reduction of Ce4+ to Ce3+ and the formation of anion vacancies. Quantum confinement effects became evident with the increase of band gap energy when the particle sizes reduced below 7–8 nm. Positron annihilation studies indicated the presence of vacancy type defects in the form of vacancy clusters within the nanoparticles. Some positron annihilation was also seen on the surface of crystallites as a result of diffusion of thermalized positrons before annihilation. Coincidence Doppler broadening measurements indicated the annihilation of positrons with electrons of different species of atoms and the characteristic S–W plot showed a kink-like feature at the particle sizes where quantum confinement effects began

Mossbauer, XRD and positron annihilation studies on natural magnetite and hematite ore from Ari Dongri, central India

Natural magnetite and hematite samples taken from iron ore deposits associated with Precambrian banded iron-formation (BIF) at Ari Dongri (20°23'N:81°3°E), Bastar district in Central India have been studied by Mossbauer, XRD and positron annihilation techniques. Three magnetite samples show a genetic association with a-Fe2O3 with a wide range of variations in Fe3O4:α-Fe2O3 ratio. The fourth sample, a typical specular hematite, shows α-Fe2O3 content of the order of 90%, the rest being magnetite. The magnetite present in the samples was found to be stoichiometric. None of the samples contains maghemite (γ-Fe2O3). Some geological implications of the observed variation in the oxidation states of the samples are considered

Mechanism and kinetics of solid-state amorphization by mechanical alloying of Al<SUB>65</SUB>Cu<SUB>35-x</SUB>Nb<SUB>x</SUB>

In the present study, we have synthesized a number of Al-based Al65Cu35-x Nbx ternary alloys by mechanical alloying and undertaken a detailed characterization of their microstructural evolution by X-ray diffraction, high-resolution transmission electron microscopy and positron annihilation spectroscopy. To predict the phase-equilibrium in a given ternary powder blend subjected to mechanical alloying, we have modified the Miedema model to incorporate the influence of interfacial energy contribution in solid-state amorphization and analytically calculate the enthalpy and Gibbs energy of ternary amorphous and nanocrystalline solid solutions. The predicted trend compares well with the experimental data. Finally, an attempt has been made to determine the mechanism of solid-state amorphization in Al65Cu35-xNbx alloys utilizing both experimental results and model-based thermodynamic calculations

Amorphisation and intermetallic nanophase formation in ball-milled Al-Ti-Si studied through positron lifetime spectroscopy

Alloys of Al-Ti-Si of different compositions were prepared by ball-milling and were characterised by X-ray diffraction. A characteristic transformation to an amorphous phase during continued ball-milling is highlighted in the context of the production of excess free volume at the grain boundaries and interfaces, as the grains undergo nanometrical size reduction. Nanocrystalline intermetallic alloys appear to develop when the alloys are milled for 20-30 h. Positron lifetime measurements are carried out on samples of various compositions and after different hours of milling. The observed annihilation characteristics are found to differ from the normally expected trend during the reduction of the grain sizes. It is observed that the alloy with larger Al content is less susceptible to amorphisation whereas the said transformation is achieved with less hours of milling with increasing Ti content. The amorphisation process in fact begins when the average grain size falls to typical nanometric values where the fractional excess free volume associated with the atoms on the grain boundaries increases asymptotically. The nanoparticles of the intermetallic alloys appear to agglomerate due to the energy dissipated by the thermalising positrons

Investigation of silver - glass nanocomposites by positron lifetime spectroscopy

Nanocrystalline silver particles were grown in a glass medium by ion-exchange and reduction techniques and studied by positron lifetime spectroscopy. The particle sizes varied from 5 to about 25 nm as observed by transmission electron microscopy. The positron lifetime spectra of all the samples could be decomposed into three components having lifetimes of around 160 ps, 400 ps and 1500 ps. The first is ascribed to positron annihilation at the interfaces of the nanocrystalline silver and the glass matrix, and it decreases and stabilizes as the silver grain size increases. The second component is explained as arising from positrons trapped and annihilated at the free-volume defects in the glass matrix. The third component arises because of the annihilation of orthopositronium at large free-volume defects. The effects of temperature on the interfacial defects and the processes leading to the formation of additional positron trapping centres are discussed

Annihilation characteristics of positrons in a polymer containing silver nanoparticles

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 semiconductor like behavior

A positron annihilation study of vacancy-type defects in Al-Cu-Fe quasicrystals

A positron annihilation study of the icosahedral Al-Cu-Fe alloy system has been carried out using both Doppler broadening and lifetime measurement techniques. Isochronal annealing studies show that a nearly unchanged two-component lifetime spectrum is obtained up to the highest annealing temperature of 450 degrees C used in the present work, indicating the stability of the structure and the defect state in this quasicrystalline phase up to this annealing temperature. The vacancy defect concentration is about 8 ppm which is higher than that observed in the authors' earlier study on icosahedral Al-Mn-Si, though the vacancy size is lower-approximately a monovacancy size on average in Al-Cu-Fe against a divacancy size in Al-Mn-Si. The existence of such a high concentration of vacancy defects even in quasicrystalline Al-Cu-Fe, which has little phason disorder, should be significant from the point of view of developing a structural model for quasicrystals. It appears that a two-component positron lifetime spectrum indicative of a high concentration of vacancy-type defects is characteristic of the icosahedral quasicrystalline phase. The possibility of a central hole in the icosahedral building block in this phase is ruled out from the authors' positron annihilation experiments