Positron Lifetime Values Research Articles

Defect driven tunable optical properties of Li+ and Zr4+ co-doped Eu3+: MgF2 compounds

A novel approach of defect engineering in Eu3+ doped MgF2 compounds has been carried out in order to tune the optical properties of Eu3+ ion. Upon doping various co-dopant ions such as Li+ and Zr4+ ions, it has been observed for the first time in the photoluminescence (PL) study that such co-dopant ion would control the characteristics transition lines of Eu3+ ions and results in tunable color characteristics of the phosphor materials. While Li+ co-doping results in a higher intense magnetic dipole (MD) line (5D0 → 7F1), doping with Zr4+ results in an intense electric dipole (ED) line (5D0 → 7F2). Further, Li+ co-doping also results in an unusual highly intense 5D0 → 7F4 line. From PL decay profile it has been observed that although for both cases of the co-dopant ions two different Eu3+ components are found to exist, their contribution is completely different. From positron annihilation lifetime study it was observed that various Mg-vacancies and their cluster forms are responsible for the tunable emission characteristics. First principle based theoretical study has also been carried out to understand the configuration of various cluster vacancies as suggested by their respective positron lifetime values. The short-lived Eu3+ component, which gives rise to highly intense 5D0 → 7F4 line, originates from a Mg-site at the surface of the particle and close to a vacancy cluster VC1. While the long lived Eu3+ component is responsible for highly intense ED line and close to Mg mono- or di-vacancies in the bulk of the particle. Therefore, defects (both dopant impurity ions and lattice vacancies) play a significant role in tuning the emission characteristics of Eu3+ doped MgF2 compounds.

Defects in H/He neutral beam irradiated potassium doped tungsten alloy by positron annihilation technique

Pure tungsten (W) is a primary plasm-facing material (PFM) candidate because of its superior properties, but it still has some drawbacks. In order to solve these problems, various methods have been used to improve the performances of tungsten-based materials. Potassium (K) doping, as one of the typical dispersion-strengthening methods for W materials, improves low temperature brittleness, reduces the ductile-brittle transition temperature, and suppresses the recrystallization. Meanwhile, it also improves the thermal shock resistance and fracture toughness of the material by introducing nano-sized K bubbles. However, this method brings a large number of defects inevitably. In fact, the K bubbles and the dislocations which are pinned by these K bubbles can affect the morphology and evolution of hydrogen (H) and helium (He) atoms in the alloys. As a very sensitive method to detect vacancy-type defects in materials, positron annihilation spectroscopy is used to study the irradiation damage caused by H and He atoms in this paper. The calculation of positron lifetime shows that positrons are more sensitive to the vacancy-type defects. Bounding of H and He with vacancies reduces the positron lifetime because of the increase of valence electron density. Combining the calculated results with the positron annihilation lifetime spectrum (PLAS) results shows that the W-K alloy is easier to promote the H atoms to release. Besides, it also more likely to form larger He bubbles which can be estimated by positron lifetime values. The result is also confirmed by the measurements from the scanning electron microscope (SEM) and slow positron Doppler broadening spectroscopy (DBS). The defects in the W-K alloy such as K bubbles and their pinned dislocations can act as diffusion channels to promote the H atoms to release, which gives rise to a smoother surface under the pure H irradiation. Meanwhile, under the condition of the H+6%He irradiation, the K bubbles and their pinned dislocations in the W-K alloy become the capture center of He atoms, promote their nucleation and grow into larger He bubbles. Moreover, under the action of stress and temperature gradient, some of the He bubbles migrate to the surface and release, this process is conducive to the recovery of related defects and the reduction of radiation damage.

Gradient Microstructure Induced by Surface Mechanical Attrition Treatment (SMAT) in Magnesium Studied Using Positron Annihilation Spectroscopy and Complementary Methods.

Surface mechanical attrition treatment (SMAT) was used to generate a gradient microstructure in commercial grade magnesium. Positron annihilation lifetime spectroscopy and variable energy positron beam measurements, as well as microhardness tests, electron backscatter diffraction, X-ray diffraction, and electrochemical corrosion tests, were used to investigate the created subsurface microstructure and its properties. It was found that SMAT causes an increase in dislocation density and grain refinement which results in increased hardness of the subsurface zone. The mean positron lifetime values indicate trapping of positrons in vacancies associated with dislocations and dislocation jogs. The increase of the SMAT duration and the vibration amplitude influences the depth profile of the mean positron lifetime, which reflects the defect concentration profile. Electrochemical measurements revealed that the structure induced by SMAT increases the susceptibility of magnesium to anodic oxidation, leading to the enhanced formation of hydroxide coverage at the surface and, as a consequence, to the decrease in corrosion current. No significant effect of the treatment on the residual stress was found.

Microstructure and Positron Lifetimes of Aluminide Coatings on Inconel 713

The microstructure of the palladium modified and non-modified aluminide coatings was examined by the EDS and the positron annihilation spectroscopy methods. Both coatings have a double layer structure: β-NiAl phase or β-(Ni,Pd)Al phase on the top and the interdiffusion zones with the chromium and molybdenum rich phases in the β-NiAl or or β-(Ni,Pd)Al phase below. Palladium, that forms the β-(Ni,Pd)Al phase and substitutes for nickel atoms causes the increase of the positron lifetime value due to the increase in the number of open volume defects in the lattice which are jogs or vacancies on dislocation lines.

Detection of tribolayer in different metals using positron lifetime spectroscopy

Investigation of the positron lifetime depth profile in pure iron, copper, stainless steel and silver alloy subjected to a dry sliding test have been performed. Near the worn surface, at the depth less than 10 μm the so called tribolayer is visible. Its characteristic feature is the presence of the large vacancy clusters but only in the pure studied metals. A deeper, deeply plastically deformed zone is observed, where positron lifetime values remain almost constant. Beyond, the third zone is found which is characterized by the almost exponential decays of the positron lifetime value up to the bulk value. The total depth of the subsurface zone depends on the material and significantly exceeds 200 μm.

Synthesis and characterization of aluminophosphate glasses with unique blue emission

The aluminophosphate glasses were prepared by the melting-quenching technique and their optical and structural properties were investigated. The 56P2O5-20Al2O3 glass exhibits unique blue emission at 406 nm due to the phosphorus defect in the glass matrix. Addition of SiO2, ZnF2, or Li2O to 56P2O5-20Al2O3 glass matrix has different influence on formation of phosphorus defect resulting in different intensities of blue emission at 406 nm, though SiO2, ZnF2 or Li2O mostly enters the glass network as modifier. Structural analysis of P2O5-Al2O3 based aluminophosphate glasses by positron annihilation lifetime spectroscopy indicates that values of positron lifetimes and the corresponding intensities may be connected with the sizes and number of structural defects. Under excitation of 251 nm, a broad blue emission band and a red emission band were observed in the Mn-doped 56P2O5-20Al2O3-15SiO2-10ZnF2 glass, indicating the operation of energy transfer from the phosphorus defect to octahedrally coordinated Mn in the glass host.

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.

Positron annihilation measurements in high energy alpha irradiated undoped indium antimonide

Positron annihilation lifetime spectroscopy and Doppler broadening annihilation line-shape measurements were carried out in 40 MeV alpha-irradiated undoped InSb. After irradiation the sample was subjected to an isochronal annealing over temperature region of 25°C–400°C with annealing time of 30 min at each set temperature. After each annealing the positron measurements were carried out at room temperature. Formation of radiation induced defects and their recovery with annealing temperature were investigated. A three component positron lifetime analysis was undertaken to observe the trapping of positrons in the sample after irradiation and during annealing. The average positron lifetime value τ avg = 313 ps at room temperature after irradiation indicated the presence of defects and the high value of τ2 at room temperature suggested that the probable defects were divacancies. A two stage recovery of defects was observed during post irradiation isochronal annealing over the temperature region 25°C–400°C. The variations in line-shape parameter (S) and defect specific parameter (R) during annealing in the temperature region 25°C–400°C resembled the behavior of τ avg indicating the migration of vacancies, formation of vacancy clusters and the disappearance of defects between 300°C to 400°C.

Positron Momentum Calculation of Defects in α-Fe and Fe-Cr Containing Hydrogen and Helium

Lattice defects that contain hydrogen and helium in Fe and Fe-Cr have been studied by positron model calculations using Density Functional Theory (DFT) in its Local Density Approximation (LDA) implementation. It was found that the value of positron lifetime for perfect Fe lattice is 101.6 ps. and for mono vacancy is 176.6 ps. which a close to the experimental values. The calculated values of Positron Life-Time (PLT) for Fe-2.5% wt Cr is 104.7 ps., for Fe-5% wt Cr is 127.6 ps., for Fe-8% wt Cr is 128.9 ps. and for Fe-9% Cr is 128.2 ps. A correlation between the positron lifetime and H and He atom concentration has been established. The model calculations for PLT and Coincidence-Doppler-Broadening (CDB) for Fe-Cr containing vacations in the presence of different numbers of hydrogen and helium atoms will be presented.

Voids in mixed-cation silicate glasses: Studies by positron annihilation lifetime and Fourier transform infrared spectroscopies

PALS in comparison with FTIR studies have been applied to investigate the structure of different oxide glasses. Three components of the positron lifetime τ (τ1 para- and τ3 ortho-positronium and τ2 intermediate lifetime component) and their intensities were obtained. The results of the calculation of mean values of positron lifetimes for the investigated glasses showed the existence of a long-living component on the positron annihilation lifetime spectra. From the Tao–Eldrup formula we can estimate the size of free volume. On the basis of the measurements we can conclude that the size and fraction of free volume reaches the biggest value for the fused silica glass. The degree of network polymerisation increases void size.

Monovacancies and Na substitutional defects in PbTe: theoretical positron annihilation study

The electron structures of cubic PbTe and substitutional defects are investigated using the state of the art pseudo-potential plane wave method, in the framework of the density functional theory within the generalized gradient approximation. The structural dependent positron annihilation in cubic PbTe was calculated and the positron lifetimes of the bulk, the Pb mono-vacancy, the Te mono-vacancy and the Na atom substituting on Pb were calculated respectively. The study provides evidence that positrons can distinguish between mono-vacancies and in situ substitutional Na doping. The positron lifetime values were found to be too similar to be distinguished. But calculations of the positron annihilation momentum distribution spectra did show clear differences between the two types of defect and may be exploited to characterize substitutional defects.

Defects structure characterization of NiMnGa alloys by PALS

We have studied the behaviour of defects in off-stoichiometric Ni-Mn-Ga ferromagnetic shape memory alloys by means of positron lifetime spectroscopy. The measurements presented in this work have been performed in six ternary alloys. The studied samples cover a large composition range. Positron experiments have been performed at room temperature after subsequent isochronal annealing at different temperatures up to a maximum temperature of 700°C. Results show a large variation of the average positron lifetime value with the isochronal annealing temperature in three of these samples, with significant differences between them. In the other three, the response is quite different. The results are discussed in terms of different types of positron trapping defects and their evolution with the annealing temperature. The present work shows a high dependence of recovery behaviour with composition in NiMnGa ferromagnetic shape memory alloys.

Positron annihilation characteristics in multi-wall carbon nanotubes with different average diameters

Positron lifetime spectroscopy was used to study multi-wall carbon nanotubes. The measurements were performed in vacuum on the samples having different average diameters. The positron lifetime values depend on the nanotube diameter. The results also show an influence of the nanotube diameter on the positron annihilation intensity on the nanotube surface. The change in the annihilation probability is described and interpreted by the modified diffusion model introducing the positron escape rate from the nanotubes to their external surface.

Effects of copper precipitates on microdefects in deformed Fe-1.5 wt%Cu alloy

As-annealed Fe–1.5 wt%Cu alloys were deformed by cold rolling. Positron annihilation lifetime spectroscopy (PALS) and Doppler broadening spectra (DBS) were used to analyze the microdefects in the alloy induced by deformation. Cu precipitates were formed uniformly in the alloy after annealing at 1173 K. To investigate the effects of Cu precipitates on the formation and migration of defects, well-annealed pure Fe samples were prepared as a reference. PALS results show that the value of positron lifetime (both long lifetime τ2 and mean lifetime τm) in the alloy is smaller than in pure Fe during deformation. This indicates that Cu precipitates restrain the growth of vacancy clusters. Results of DBS show that the S parameter in pure Fe is larger than in the alloy, and more deformation is needed for the S parameter to be saturated in the alloy.

Positron Annihilation Spectroscopy Study of Ni-Mn-Ga Ferromagnetic Shape Memory Alloys

Abstract We have studied the role that vacancy type defects play in the martensitic transformation of Ni-Mn-Ga ferromagnetic shape memory alloys by means of positron lifetime spectroscopy. The measurements presented in this work have been performed in five ternary alloys. Three of them transform to modulated and two to non-modulated martensitic phases. With these five samples we cover a large range in composition. Positron experiments have been performed at room temperature after subsequent isochronal annealing at different temperatures and up to a maximum temperature of 600°C. Results show a large variation of the average positron lifetime value with the isochronal annealing temperature in non-modulated samples. However, the response in the modulated samples is quite different. The results are discussed in term of different type of positron trapping defects and their evolution with the annealing temperature. The present work shows a correlation between vacancy concentration and martensitic transformation temperature of ferromagnetic shape memory alloys.

Study of Co-containing Ni-Mn-Ga by positron annihilation

Positron annihilation spectroscopy measurements have been carried out in the ferromagnetic shape memory alloy Ni-Mn-Ga-Co. Positron experiments have been performed at room temperature after subsequent isochronal annealing up to 600 °C. Positron lifetime results have been compared with Differential Scanning Analysis experiments performed in the same samples. Experiments show a large variation of the average positron lifetime values with the annealing temperature of the samples, indicating a clear relation between the concentration of vacancies and the properties of the martensitic transformations of these alloys.

Evidence of formation of tetravacancies in uniformly oxygen irradiated n-type silicon

High purity n-type silicon single crystal with resistivity in the order of 4000 Ω cm has been irradiated with high-energy oxygen ions at room temperature up to a fluence of 5E15 ions/cm 2. The energy of the beam was varied from 3 to 140 MeV using a rotating degrader to achieve a depthwise near-uniform implantation profile. Radiation induced defects and their dynamics have been studied using positron annihilation spectroscopy along with isochronal annealing up to 700 °C in steps of 50 °C for 30 min. After annealing the sample at 200 °C for 30 min, formation of silicon tetravacancies has been noticed. The formation of the tetravacancies was found to be due to agglomeration of divacancies present in the irradiated sample. An experimentally obtained positron lifetime value of 338±10 ps has been reported for silicon tetravacancies, which has a very close agreement with the value obtained from recent theoretical calculations. The tetravacancies were found to dissociate into trivacancy clusters upon further annealing. The trivacancies thus obtained were observed to agglomerate beyond 400 °C to form larger defect clusters. Finally, all the defects were found to anneal out after annealing the sample at 650 °C.

Application of positron annihilation and Raman spectroscopies to the study of perovskite type materials

Defect properties of perovskite type materials, Ba3B′Nb2O9 (where B′=Mg, Zn, or Co), with near-stoichiometric compositions were studied by positron annihilation and Raman spectroscopies. Theoretical simulations of stoichiometric perovskites revealed a dependence of the positron bulk lifetime on the degree of ordering. In Ba3MgNb2O9 (BMN) the positron bulk lifetime for a completely disordered structure is 195 ps versus 237 ps for a completely ordered one. The predicted bulk lifetimes for Ba3ZnNb2O9 (BZN) and Ba3CoNb2O9 (BCN), with Pm3¯m symmetries are 193 ps and 194 ps, respectively. It was found that deviation from stoichiometry results in the appearance of secondary Ba- and Nb-rich phases, which according to theoretical simulations have bulk lifetimes much longer than that of the host material. Positron lifetime spectroscopy was used to monitor changes in the concentration of these second phases. The difference between predicted defect lifetimes and the bulk values for the studied perovskites was less than 70 ps. This and the likely small concentrations made it impossible to discern the presence of point defects in the samples. Raman measurements demonstrated the presence of a particular mode that could be attributed to the formation of a 1:1 phase, the size of which is limited by requirements for charge compensation. The existence of an internal electric field between charged 1:1 nanoregions and the rest of material creates conditions for preferential positron annihilation that influence the obtained positron lifetime values. For BZN type materials it was found that the degree of 1:2 cation ordering decreases by increasing the sintering temperature to above 1400 °C.

Positron lifetime calculation of defect in palladium containing hydrogen and helium

Lattice defects containing hydrogen and helium in Pd have been studied by positron model calculations using Density Functional Theory [DFT] in its local density approximation [LDA] [1] implementation. A correlation between the positron lifetime, positron ground stage energy and number atoms of H and He has been established. It was found that the value of positron lifetime for perfect Pd lattice is 106 ps and for mono vacancy is 184 ps which corresponds to the experimental values.

Positron annihilation studies on bulk cobalt silicides

AbstractBulk cobalt silicides Co2Si, CoSi and CoSi2 have been investigated using experimental positron lifetime, coincidence Doppler broadening and depth‐resolved positron beam studies. Psitron lifetime and coincidence Doppler ratio curves have shown distinct features for the silicides as compared to that of pure Co and Si. Ab‐initio calculations of positron life time have been carried out using the DOPPLER program. The experimental positron lifetime values are higher than the theoretical estimates for Co2Si and CoSi2, suggesting that these silicides contain vacancy defects. This is further supported by positron diffusion lengths deduced from the VEPFIT analysis of positron beam results. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)