Abstract: Mesoporous silica films on Si wafers were prepared via a sol-gel process using a triblock copolymer as the structural template. Spin-coated films were dried and subsequently subjected to Ar plasma treatments for decomposing the polymeric porogen. Mesopore formation in the silica film upon Ar plasma treatments was investigated by positron annihilation lifetime spectroscopy. In comparison with calcined silica film, much larger pores, more heterogeneous in size were found in silica film prepared with Ar plasma decomposition of the porogen.
Abstract: Positron annihilation lifetime (LT) measurements were used to study the influence of addition of iron on ortho-positronium (o-Ps) in micro and mesoporous media. Three types of porous samples containing iron were chosen for the investigation. LT spectra were decomposed to four components in the 0.1 - 50 ns lifetime range. An attempt is made to correlate the data with sites occupied by iron in the framework as derived from Mössbauer studies. No significant correlation could be detected in this respect. Instead, dominating role of presence or absence of iron can be deduced. Certain influence can also be attributed to the presence of adsorbed water.
Abstract: The annihilation behavior of positronium in N and P type porous silicon is investigated via positron annihilation lifetime (PAL) and positronium time-of-flight (Ps-TOF) measurements. The result shows the N type sample has smaller positronium annihilation fraction and more interconnected pores. The microstructure of Rapid Thermal Oxidation (RTO) treated N type porous silicon also studied using PAL spectroscopy, and the relationship between its photoluminescence properties and porous structure was discussed.
Abstract: Positron annihilation lifetime spectroscopy study of n-heptane adsorption and desorption on silica gel Si-60 gives insight into a free volume evolution during those processes. Analysis of the lifetime distribution allows to ascribe particular ortho-positronium components to silica walls, mesopores and n-heptane. Hysteresis loop of the intensities of the mesopore and n-heptane related components similar to classic adsorption/desorption isotherms is observed. Transition from an adsorbate multilayer structure to liquid when pores become completely filled, manifests itself as an intensity change. Kinetics of desorption and adsorption studies allows to determine the time of reaching the equilibrium by the Si 60–n-heptane system at various pressures.
Abstract: CoCrxFe2-xO4 samples with x = 0.0 to 2.0 were prepared by air oxidation of aqueous suspension containing Co2+, Cr3+ and Fe3+ ions and characterized by x-ray diffraction. Positron lifetime and coincidence Doppler broadening (CDB) measurements indicated three distinct stages of positron trapping in defects. Initially the vacancy-type defects located at the A-sites (tetrahedral) trapped positrons but, with the substitution of Fe by Cr in low concentrations (x <= 0.7), positrons are trapped by defects at the B-sites (octahedral). Mossbauer spectroscopic results indicated the cationic distribution at B-sites to be stoichiometry-dependent and, till x = 0.7, the deficiency of Fe3+(B) ions was compensated by interchange of Fe3+(A) ions with Co2+(B) ions. Between x = 0.9 and 1.7, the substitution resulted in continued decrease of Fe3+(B) ions and the structure got fully transformed into a normal spinel configuration during x = 1.8 to 2.0.
Abstract: We report in this work about the inversion of the spinel structure of ZnFe2O4 induced by the substitution of Zn2+ by Ni2+ ions. Positron lifetimes were measured in Zn1-xNixFe2O4 with different concentrations (x) of doped Ni2+ ions and a drastic change across x = 0.4 – 0.6 was observed, which is attributed to this transformation. The interchange of positions of the cations on doping leaves a fraction of them unoccupied and these vacancies act as positron trapping centres. Since Ni2+ is smaller in size than Zn2+, defects due to non-stoichiometry are less in NiFe2O4 than in ZnFe2O4. The increase in positron lifetime implies the trapping of positrons being shifted from A- to B-sites and is an indication of the transformation from inverse to normal spinel configuration. Coincidence Doppler broadening measurements supported these findings.
Abstract: The recovery of shallow donor states of the atoms of phosphorus in n-FZ-Si:P material irradiated at the room temperature with 15 MeV protons was studied in the course of isochronal annealing. This process was investigated by the positron annihilation lifetime (PAL) spectroscopy and by low-temperature electrical measurements. The positron traps of a vacancy type manifesting themselves as deep donors have been revealed. These defects begin to anneal at ~ 593 K– 613 K; roughly estimated activation energy of the process is Ea ≈ 0.59 eV under the first order of reaction. The results suggest the involvement, at least, of one vacancy and the impurity atom of phosphorus in the microstructure of the deep donor.
Abstract: In the present work hydrothermally grown ZnO single crystals were electrochemically charged with hydrogen. The influence of hydrogen on ZnO microstructure was investigated by positron annihilation spectroscopy (PAS) combined with X-ray diffraction (XRD) using synchrotron radiation. Hydrogen concentration in the samples was determined by nuclear reaction analysis (NRA). It was found that a high concentration of hydrogen can be introduced into ZnO by electrochemical loading. At low concentrations, absorbed hydrogen causes elastic volume expansion of ZnO crystal. At higher concentration, hydrogen-induced stresses exceed the yield stress in ZnO and plastic deformation of the crystal takes place leading to formation of a defected subsurface layer in the crystals.
Abstract: Hydrogen-induced defects of ZnO single crystals electrochemically charged with hydrogen have been investigated by positron beam-based Doppler broadening spectroscopy, X-ray diffraction (XRD) and optical microscopy (OM). XRD and OM results indicated that a deformation layer was formed due to hydrogen-induced structural change at the subsurface of ZnO single crystal. Slow positron beam measurements showed that this deformation layer contained many defects, such as dislocations and Zn vacancies, which led to increase of S parameter.
Abstract: A variable energy slow positron beam was utilised to investigate depth dependent effects of sintering on the tetragonal yttria stabilised zirconia nanopowders. Positron implantation was combined with the determination of Doppler broadened profiles of annihilation radiation. The results are consistent with recent positron lifetime data showing that sintering at elevated temperatures leads to a disappearance of pores and a significant grain growth, which is demonstrated by a strong suppression of positronium formation and a substantial decrease in concentration of open volume defects at triple points, respectively, with increasing sintering temperature. An existence of a subsurface layer of a relatively high content of defects was shown in sintered samples and tentatively attributed to arise from a diffusion of open volume defects from the sample interior toward the surface or from a sintering-induced surface modification.