Positron Lifetime Component Research Articles

Effects of electrical conductivity on the formation and annihilation of positronium in porous materials.

In this paper we show the preliminary evidence that the formation of positronium depends on the electrical conductivity of porous materials. Porous nano γ-Al2O3 was chosen as the base material, and it was filled with carbon of different allotropes (commercial graphite, carbon black, carbon nanotubes and home-made ordered mesoporous carbon) by a mechanical mixing method. The positron lifetime and Doppler broadening of the annihilation radiation were measured for these composites. In the pure γ-Al2O3 sample, there are two long positron lifetime components τ3 (3.5 ns) and τ4 (101.3 ns) with intensities of 1.0% and 24.6%, which indicates the formation and annihilation of positronium in small and large pores, respectively. In the carbon filled γ-Al2O3 samples, the longest lifetime τ4 and its intensity I4 both show a continuous decrease with increasing carbon content. The Doppler broadening S parameter shows a similar tendency to τ4 and I4. This suggests that carbon has a quenching effect on positronium and also inhibits the formation of positronium. Among these four carbon allotropes, carbon nanotubes have the strongest quenching and inhibition effect, while graphite has the weakest effect. A detailed study further reveals that the decreasing rate of τ4 and I4 as well as the S parameter depend on the electrical conductivity of the carbon filled γ-Al2O3 and also the specific surface area of the filled carbon. Our results suggest that the formation and annihilation of positronium are strongly affected by the electrical conductivity of the materials.

Positron annihilation in mordenite zeolite

The theoretical study of the positron annihilation in complex material such as zeolite is greatly significant to support and increase the accuracy analysis of the material structure from the experimental data of the positron annihilation. The mordenite zeolite is a big and complicated structure consisting of channels and cavities. The analysis of the mordenite structure is studied by the PALS so depending on the selection of the positron lifetime components of the positron annihilation spectra fitting methods. Therefore, these positron life times in on TO4, Na, Ca, K, Fe, H2O and the rings which form the channels and cavities are sophisticatedly studied by the DFT calculation using Ab-initio. The mordenite and modified mordenite zeolite structures are precisely analyzed, and the physical behaviors of the positron in these are more understood by these theoretical results.

Optical and positron annihilation spectroscopic studies on PMMA polymer doped by rhodamine B/chloranilic acid charge transfer complex: Special relevance to the effect of γ-ray irradiation

Polymeric sheets of poly (methylmethaclyerate) (PMMA) containing charge transfer (CT) complex of rhodamine B/chloranilic acid (Rho B/CHA) were synthesized in methanol solvent at room temperature. The systematic analysis done on the Rho B and its CT complex in the form of powder or polymeric sheets confirmed their structure and thermal stability. The IR spectra interpreted the charge transfer mode of interaction between the CHA central positions and the terminal carboxylic group. The polymer sheets were irradiated with 70kGy of γ radiation using 60Co source to study the enhanced changes in the structure and optical parameters. The microstructure changes of the PMMA sheets caused by γ-ray irradiation were analyzed using positron annihilation lifetime (PAL) and positron annihilation Doppler broadening (PADB) techniques. The positron life time components (τi) and their corresponding intensities (Ii) as well as PADB line–shape parameters (S and W) were found to be highly sensitive to the enhanced disorder occurred in the organic chains of the polymeric sheets due to γ-irradiation.

Measurements of defect structures by positron annihilation lifetime spectroscopy of the tellurite glass TeO2–P2O5–ZnO–LiNbO3 doped with ions of rare earth elements: Er3+, Nd3+ and Gd3+

The objective of the study was the structural analysis of the TeO2-P2O5-ZnO-LiNbO3 tellurite glasses doped with ions of the rare-earth elements: Er3+, Nd3+ and Gd3+ based on the PALS (Positron Annihilation Lifetime Spectroscopy) method of measuring positron lifetimes. Values of positron lifetimes and the corresponding intensities may be connected with the sizes and number of structural defects, such as vacancies, mono-vacancies, dislocations or pores, the sizes of which range from a few angstroms to a few dozen nanometres. Experimental positron lifetime spectrum revealed existence of two positron lifetime components τ1 and τ2. Their interpretation was based on two-state positron trapping model where the physical parameters are the annihilation velocity and positron trapping rate.

Defect structure of thermoelectric Zn4Sb3

Thermoelectric samples prepared by spark plasma sintering method have been studied by resistivity measurement, X-ray diffraction (XRD) and positron annihilation spectroscopy (PAS). The existence of Zn vacancies in structure is confirmed by PAS, which contribute a positron lifetime component of ps. Positron lifetime calculation has been performed for different defect structural models of . By comparing the calculated lifetime values with the experimental result, we find that each Zn vacancy must have at least two close interstitial Zn atoms in the crystal structure of . Two separate phase transitions observed by resistivity measurement occur at about 232 and 252 K, whereas the positron annihilation measurements indicate they are about 225 and 240 K. Positron lifetime and Doppler broadening results reveal phase is a transition phase between and phases. The results also indicate the disordering of interstitial clusters into interstitial Zn sites during the to phase transition. The interstitial Zn clusters are found to act as shallow positron traps which apparently capture positrons below 200 K.

Defect production and deuterium retention in quasi-homogeneously damaged tungsten

To understand the deuterium (D) bulk retention behavior in plasma facing materials under reactor-relevant conditions, tungsten (W) foils were irradiated with 122MeV 20Ne ions and then exposed to D2 gas at 773K. A quasi-homogeneous distribution of atomic displacement damage to 0.3dpa within a depth of 50μm was produced in W using an energy degrader in the irradiation chamber. Results of positron annihilation lifetime spectroscopy showed a long positron lifetime component of ∼400ps appeared after irradiation, indicating the formation of vacancy clusters with up to 12 vacancies in W. Thermal desorption spectra showed a broad D desorption temperature range (730–1173K) with a high release peak at ∼1010K for the irradiated W specimens. The amounts of D retained in the irradiated W were significantly larger than the annealed ones, which could be attributed to the trapping effects of the vacancy clusters formed by cascade collisions and the clustering of mono-vacancies.

Charge-Induced Defect Formation in LixCoO2 Battery Cathodes: XRD and PA Spectroscopy Study

Lithium-ion batteries have developed into most advanced battery systems, e.g. laptops and mobile phones. LiCoO2 is a typical intercalation battery cathode material. However, reversible charge-discharge cycling of LiCoO2 is only possible down to 50% of the available Li-ions since further removal of Li-ions drastically reduces the capacity and cycle stability. The formation of vacancy-type defects during the charging process in LixCoO2 battery cathodes was investigated by XRD and position life-time spectroscopy and Doppler broadening of positron-electron annihilation (PA) radiation as defect specific techniques [1]. Li+-extraction, which in a battery mode corresponds to charging, was performed at 293 K under electrochemical control in a 3-electrode test-cell with a Maccor Series 4000 battery tester. The composition of the lithium-ion electrode material used was: 88wt.% LiCoO2 particles, 7 wt.% carbon black as conducting agent, 5 wt.% binder material (polyvinylidene difluoride hexafluoropropylene copolymer). Structural analysis of the electrode samples was performed by means of X-Ray diffraction using a Bruker D8 Advance diffractometer in Bragg-Brentano geometry with Cu-Kα radiation. Diffractograms were measured in the 2-Theta angle range from 150to 1300and were analysed by Rietveld refinement with the programs FULLPROF [2] and X'PertHighScorePlus (Panalytical). For positron annihilation measurements a positron source (22NaCl) was sandwiched between two identical LiCoO2 electrode samples. Positron lifetime measurements were performed with a fast-fast spectrometer with a time resolution of 221 ps. The spectra were analysed by using the program pfposfit [3]. Doppler broadening (DB) measurements were performed in a coincidence setup with two high purity Ge detectors.with energy resolution for the 511 keV annihilation γ-line in the detector system corresponds to ca. 0.88keV (FWHM). Both the Doppler broadening S parameter as well as the positron lifetime component τ1 exhibit a characteristic variation with increasing amount of Li+-extraction; the S-parameter and τ1 first increases upon decreasing x from 1 to 0.6. Further Li+-extraction causes a decrease of S and τ1 (x = 0.55), followed by a re-increase for x<0.55. Conclusions: The regime of reversible charging is dominated by vacancy-type defects on the Li+-sublattice the size of which increases with increasing Li+-extraction. Indication is found that Li+-reordering which occurs at the limit of reversible Li+-extraction (x = 0.55) causes a transition from the two-dimensional agglomerates into one-dimensional vacancy chains. Degradation upon further Li+-extraction is accompanied by the formation of vacancy complexes on the Co- and anion sublattice.

Defect dynamics in Li substituted nanocrystalline ZnO: A spectroscopic analysis

Very recently, vacancy-type defects have been found to play a major role in stabilizing d0 ferromagnetism in various low dimensional ZnO systems. In this context, the evolution of vacancy-type defects within the ZnO nanocrystals due to the doping of ZnO by alkali metal lithium (Li) is investigated using X-ray photoelectron (XPS), photoluminescence (PL) and positron annihilation spectroscopy (PAS). Li-doping is found to have significant effects in modifying the vacancy-type defects, especially the Zn vacancy (VZn) defects within the ZnO lattice. XPS measurement indicated that initially the Li1+ ions substitute at Zn2+ sites, but when Li concentration exceeds 7at%, excess Li starts to move through the interstitial sites. The increase in positron lifetime components and the lineshape S-parameter obtained from coincident Doppler broadening spectra with Li-doping indicated an enhancement of VZn defect concentration within the doped ZnO lattice. The vacancy type defects, initially of the predominant configuration VZn+O+Zn got reduced to neutral ZnO divacancies due to the partial recombination by the doped Li1+ ions but, when the doping concentration exceeded 7at% and Li1+ ions started migrating to the interstitials, positron diffusion is partly impeded and this results in reduced probability of annihilation. PL spectra have shown intense green and yellow-orange emission due to the stabilization of a large number of VZn defects and Li substitutional (LiZn) defects respectively. Hence Li can be a very useful dopant in stabilizing and modifying significant amount of Zn vacancy-defects which can play a useful role in determining the material behavior.

Effects of process parameters on the defects in graphene oxide–polyaniline composites investigated by positron annihilation spectroscopy

Graphene oxide (GO)-polyaniline (PANI) composites were prepared with different relative abundance of PANI and GO by in situ polymerization of aniline in the presence of GO and ammonium persulphate at different temperatures. In the process, GO also got reduced to graphene. Positron lifetimes and coincidence Doppler broadening of the electron-positron annihilation gamma ray spectra originating from the composite samples were measured and the results are reported. The positron lifetimes indicated the presence of very large size defects in the form of vacancy clusters within the samples. Another interesting observation was the increase of relative intensity of the defect specific positron lifetime component when an increase in relative abundance of PANI led to increased reduction of GO to graphene. The reduction also shrank the volume occupied by GO and the free volume thereby released added to the overall defect concentration, resulting in a simultaneous increase of the intensity of the positron lifetime component. The variation of the positron lifetime and its intensity with the synthesis temperature suggested an optimum temperature suitable for the process. The above observations are corroborated by other experimental investigations like electron microscopy, X-ray diffraction and electrical conductivity.

Positron lifetimes in pores of some low-silica zeolites: influence of water content, crystal size and structural type

Positron annihilation lifetime spectroscopy has been used for observation of the free voids in low silica zeolites, of LTA and FAU structure and different crystal size, before and after their heating at different temperatures below 200 °C. Sizes of crystals of the zeolite samples, as well as their structures and water contents, were characterized by X-ray diffraction, scanning electron microscopy, thermogravimetry, and particle size distribution measurements. The number of positron lifetime components as well as their values and intensities, after water removal from the zeolite structure, vary. However, changes are not uniform in all investigated zeolites. The differences are compared and discussed in the dependence of different structure types and crystal sizes of investigated zeolites.

Effect of Hydrogen on Vacancy Formation in Sputtered Cu Films Studied by Positron Annihilation Spectroscopy

As a part of the LSI interconnect fabrication process, a post-deposition high-pressure annealing process is proposed for embedding copper into trench structures. The embedding property of sputtered Cu films has been recognized to be improved by adding hydrogen to the sputtering argon gas. In this study, to elucidate the effect of hydrogen on vacancy formation in sputtered Cu films, normal argon-sputtered and argon–hydrogen-sputtered Cu films were evaluated by positron annihilation spectroscopy. As a result, monovacancies with a concentration of more than 10-4 were observed in the argon–hydrogen-sputtered Cu films, whereas only one positron lifetime component corresponding to the grain boundary was detected in the normal argon-sputtered Cu films. This result means monovacancies are stabilized by adding hydrogen to sputtering gas. In the annealing process, the stabilized monovacancies began clustering at around 300 °C, which indicates the dissociation of monovacancy-hydrogen bonds. The introduced monovacancies may promote creep deformation during high-pressure annealing.

Reaction between NiO and Al2O3 in NiO/γ-Al2O3 catalysts probed by positronium atom

NiO/γ-Al2O3 catalysts with NiO content of 9wt% and 24wt% were prepared by solid state reaction method. They are annealed in air at temperatures from 100°C to 1000°C. Positron lifetime spectra were measured to study the microstructure variation during annealing process. Four positron lifetime components were resolved with two long lifetime τ3 and τ4, which can be attributed to the ortho-positronium lifetime in microvoids and large pores, respectively. It was found that the longest lifetime τ4 is rather sensitive to the chemical environment of the large pores. The NiO active centers in the catalysts cause decrease of both τ4 and its intensity I4, which is due to the spin-conversion of positronium induced by NiO. However, after heating the catalysts above 600°C, abnormal increase of the lifetime τ4 is observed. This is due to the formation of NiAl2O4 spinel from the reaction of NiO and γ-Al2O3. The generated NiAl2O4 weakens the spin-conversion effect of positronium, thus leads to the increase of o-Ps lifetime τ4. Formation of NiAl2O4 is further confirmed by both X-ray diffraction and X-ray photoelectron spectroscopy measurements.

Mn SUBSTITUTION EFFECTS AND ASSOCIATED DEFECTS IN ZnO NANOPARTICLES STUDIED BY POSITRON ANNIHILATION

Nanocrystalline ZnO particles substituted with different concentrations (0–30%) of Mn were synthesized by using a modified ceramic route and characterized by X-ray diffraction, transmission electron microscopy, selected area electron diffraction and energy dispersive X-ray analysis methods. Positron lifetime and coincidence Doppler broadening measurements were used as probes to identify the vacancy-type defects present in them and monitor the changes while doping. The predominant positron trapping center in the undoped ZnO is identified as the trivacancy-type cluster V Zn + O + Zn , which is negatively charged, and it transformed to the neutral divacancy V Zn + O on doping with Mn 2+ ions. The intensity of the defect-specific positron lifetime component got reduced initially indicating partial occupancy of the vacancies by the doped cations but then recovered on further doping due to the additional Zn vacancies created as a result of the increasing strain introduced by the Mn ions of larger radius. The creation of a new phase ZnMn 2 O 4 thereafter changed the course of variation of the annihilation parameters, as the positrons got increasingly trapped in the vacancies at the tetrahedral and octahedral sites of the spinel nanomanganite.

Positron lifetime in piezoelectric PbNb2O6 and BaTiO3 polycrystals

AbstractQuenching induced orthorhombic distortion makes quenched (Q) lead meta‐niobate (PbNb2O6), to be called PNQ, piezoelectric with a high Curie temperature (Tc) of 570 °C, while its slow‐cooled rhombohedral phase, PNS, remains non‐piezoelectric. So, positron lifetime components ((i and Ii) have been measured for these pellets and also for tetragonal BaTiO3 or BT pellets (Tc = 127 °C), prepared by 6h and (6+4)h firings. In our 3‐component fit, (1 is different in PNQ (152.9 ± 9.8) ps and PNS (136.6 ± 9.2) ps. Fraction (related to I2) of trapped positrons (showing (2) is higher in PN‐samples (∼74 to 79%) than in BT‐samples (40 to 53%). This is tentatively attributed to a structural peculiarity of PN tungsten‐bronze structure. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Back Cover: Phys. Status Solidi C 6/11

AbstractPositron annihilation, as a well‐established experimental tool in the materials science community, provides unique information about the behavior of the material microstructure after radiation treatment. The experimental simulation of radiation damage by charged particle implantation is, as a first approach, a very reasonable way to study radiation embrittlement. The article presented by Vladimir Kršjak et al. on p. 2339 ff. discusses this timely topic on recent experiments on helium implanted Fe–Cr model alloys. Simultaneous results from positron beam and conventional depth profiled positron lifetime techniques are demonstrated. In the cover picture, the intensity of the positron lifetime component I3 characterizes the presence of large open volume defects (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Positron trapping model for point defects and grain boundaries in polycrystalline materials

The exact solution of a diffusion-reaction model for the trapping and annihilation of positrons in grain boundaries of polycrystalline materials with competitive trapping at intragranular point defects is presented. Closed-form expressions are obtained for the mean positron lifetime and for the intensities of the positron lifetime components associated with trapping at grain boundaries and at intragranular point defects. The closed-form solutions allow direct insight in the physical details of the positron annihilation characteristics and can be conveniently applied for the analysis of experimental data. It turns out that the model is not only essential for positron annihilation studies which aim at issues of grain-boundary physics or nanoscaled material but is also of relevance for studies of point defects in polycrystalline materials when grain sizes are in the micrometer range.

Probing Phase Evolution Behavior during Nanocrystallization of Metallic Glass Using Positron Annihilation Spectroscopy

Positron annihilation lifetime spectroscopy and coincidence Doppler broadening (CDB) techniques have been used to study the nanocrystallization behavior of soft magnetic materials, using Metglas 2826MB as an example. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) characterization techniques revealed the presence of two phases, γ-(Fe,Ni) and (Fe,Ni,Mo)23B6, as well as the size and volume fraction of the phases following predetermined thermal annealing. Two distinct positron lifetime components have been observed in both amorphous and nanocrystallized samples. In the nanocrystallized samples, it has been demonstrated unambiguously that small and large lifetime components were due to positron annihilation with crystalline nanophases and with amorphous-crystalline or intercrystalline interfaces, respectively. First-principle calculation of the positron lifetime and electron momentum distribution in crystalline phases, supplemented by TEM and XRD studies, helped in the unambiguous interpretation of the experimental observation. This study throws new insights into positron behavior in metallic glasses, especially in the presence of single or multiple nanophases embedded in the amorphous matrix.

Positron annihilation study on ZnO-based scintillating glasses

Positron lifetime of ZnO-based scintillating glasses (55 − x)SiO 2–45ZnO– xBaF 2 ( x = 5, 10, 15 mol%) were measured with a conventional fast–fast spectrometer. Three positron lifetime components τ 1, τ 2, and τ 3 are ∼0.23 ns, ∼0.45 ns, and ∼1.6 ns, respectively. All the three positron lifetime components first increase with increasing BaF 2 concentration from 5 mol% to 10 mol%, then decreases as BaF 2 further increases to 15 mol%. The result suggests that the glass sample with 10 mol% BaF 2 contains the highest defect density, and is in excellent agreement with glass chemistry, glass density, thermal properties, and calculated crystallinity. Therefore, positron annihilation lifetime measurement is an effective tool for analyzing defects in ZnO-based scintillating glasses.

Investigation of free volume and the interfacial, and toughening behavior for epoxy resin/rubber composites by positron annihilation

An epoxy resin (EP) matrix has been modified with carboxyl-randomized butadiene-acrylonitrile (CRBN) rubber and hydroxyl-terminated butadiene-acrylonitrile rubber (HTBN). When the rubber content was low (⩽5%), the free volume size varied slowly, meanwhile, the free volume size of EP/HTBN system was smaller than EP/CRBN system. With further addition of rubber component (⩾7%), the free volume size of the composite increased quickly. In this case, the free volume size of EP/HTBN system was larger than EP/CRBN system. The intermediate positron lifetime component ( I 2) was affected by the structure of the interface layer between rubber particles and EP matrix. The different free volume properties could be attributed to the compatibility between rubber and EP.

Study of C 6+ (80 MeV) ion induced effects on CdS:Mn system

Manganese-doped CdS nanocrystals have been synthesized by using aqueous solution precipitation method. These samples of CdS:Mn have been subjected to irradiation using C 6+ (80 MeV) under fluence of 1 × 10 13 ions/cm 2. For characterization X-ray diffraction (XRD), optical absorption, photoluminescence (PL), surface morphology and positron annihilation spectroscopy (PALS) have been performed. The system has been found in hexagonal phase having crystalline size distribution of 18–20 nm. The PL intensity has been found to be decreased, while optical band gap has been found to be increased of irradiated samples. Positron lifetime component τ 1 (210–183 ps) is due to annihilation from diffused vacancies on the grain surfaces and τ 2 (399–367 ps) is due to positrons directly trapped in the vacancy cluster like defects. The third component τ 3 (2453–2064 ps) is due to annihilation of orthopositronium, at interfaces and boundaries within the nanocrystals.