Positron Lifetime Spectroscopy Research Articles

Positron Trapping Rate in Some Ferroelectrics, Semiconductors and Glasses

This paper presents comparative results of positron spectroscopy studies in ferroelectrics, semiconductors and glasses. The positron lifetime spectroscopy study is focused on the investigation of the positron trapping rate in the ferroelectrics, the Triglycine Sulphate (TGS) (NH3CH2COOH)3H2SO4, Rochelle Salt (RS) NaKC4H4O64H2O and Potassium Dihydrogen Phosphate (KDP) KH2PO4 structures, semiconductors and glasses. The positive positron charge opens the possibility for determining the changes in charge states in technologically important order-disorder ferroelectrics. The positron lifetime spectroscopy (PLS) results show that the second lifetime component as a function of temperature in order-disorder ferroelectrics TGS, RS and KDP is due to the positron trapping in negatively charged defects of ferroelectric. The PLS analysis has been applied for examination of positron trapping in the electronicdefect structure in selected ferroelectrics materials, semiconductors and glasses.

Comparative study of electron beam and gamma ray irradiation effects in RPC glass detector materials by positron lifetime spectroscopy

This work is intended to compare the electron beam and gamma ray irradiation induced microstructural effects and electrical conductivity in glass Resistive Plate Chamber (RPC) detector materials with different doses. Positron annihilation lifetime spectroscopic (PALS) technique was used to analyze the irradiation induced structural modifications in the RPC detector materials. PALS analysis at lower doses indicates the increased void size with the creation of additional sites in the glass network upon both the irradiation sources. At higher irradiation doses, the reduced void size shows the enhanced chemical bonding between the tetrahedral sites of Si-O-Si and hence increases the short range order in the silica glass. The variation of DC conductivity upon both the irradiation techniques correlates well with the measured void size. Gamma ray irradiation produced more structural changes in the glass samples as compared to the electron beam irradiation.

Positron Annihilation Study of RPV Steels Radiation Loaded by Hydrogen Ion Implantation.

Specimens of 15Kh2MFAA steel used for reactor pressure vessels V-213 (VVER-440 reactor) were studied by positron annihilation techniques in terms of their radiation resistance and structural recovery after thermal treatment. The radiation load was simulated by experimental implantation of 500 keV H+ ions. The maximum radiation damage of 1 DPA was obtained across a region of 3 µm. Radiation-induced defects were investigated by coincidence Doppler broadening spectroscopy and positron lifetime spectroscopy using a conventional positron source as well as a slow positron beam. All techniques registered an accumulation of small open-volume defects (mostly mono- and di-vacancies) due to the irradiation, with an increase of the defect volume ΔVD ≈ 2.88 × 10−8 cm−3. Finally, the irradiated specimens were gradually annealed at temperatures from 200 to 550 °C and analyzed in detail. The best defect recovery was found at a temperature between 450 and 475 °C, but the final defect concentration of about ΔCD = 0.34 ppm was still higher than in the as-received specimens.

On the Sc-rich core of Al3(Sc,Er,Zr) precipitates

The development of microstructure in an Al–Sc–Er–Zr alloy during aging at 600 °C was investigated utilizing microhardness measurements, positron annihilation spectroscopy (PAS) and (scanning) transmission electron microscopy ((S)TEM). The results of positron lifetime (LT) spectroscopy and coincidence Doppler broadening (CBD) indicate that Sc atoms segregate at dislocations. Two major groups of precipitates were observed after 4 h of aging at 600 °C, one having the usual Er-rich core, Sc-rich inner shell and Zr rich outer shell structure and the other having a core rich in Sc.

Effects of swift heavy ions at different fluencies on WC-6Co hard metal alloy

Tungsten carbide hard metal alloy with 6% by weight cobalt was studied before and after irradiation at different fluencies with 167 MeV132Xe ions. Raman spectroscopy, X-ray diffraction, neutron diffraction and positron lifetime spectroscopy were employed in order to assess the microstructural evolution in the material upon irradiation fluence increase. Analysis of the Raman spectrum for the pristine, non-irradiated material unveils that the surface is composed of a graphite-like phase and highly oxidized tungsten atoms spread in the carbon matrix. All characteristic peaks of tungsten carbide (WC) and possible cobalt phases are either missing or strongly overlapped in all Raman spectra. Bonding between tungsten and oxygen atoms broke upon irradiation and total deoxidation of the surface is detected for the two highest fluencies investigated at 5 × 1013 ions/cm2 and 3,83 × 1014 ions/cm2. Increasing the irradiation dose causes amorphization of the carbon phase on the surface accompanied by “up and down” trend of change in carbon cluster size. The Raman spectra analysis also unveils, that molecular nitrogen (N2) from the atmosphere penetrates the carbon matrix upon irradiation. The results from the X-ray and neutron diffraction reveal that the main phase in the material is δ-WC and also give information about changes of the lattice parameters with increasing fluence. Reorganization of the induced point defects to dislocation defects as a function of the irradiation dose is discussed, but no phase transition of the main δ-WC phase is detected. Steady increase of compressive internal stress with increasing irradiation dose is noted by XRD. The tendency is not monotonic and the stress leans towards saturation at the highest fluence, with the highest value of −5.26 GPa. The Positron lifetime spectroscopy measurements show the presence of short lifetime component ranging from 170 ps to 190 ps, interpreted as small vacancy clusters. The intensities of the different positron lifetime components vary with the irradiation dose non-monotonically.

Characterization of Lattice Defects in Refractory Metal High‐Entropy Alloy HfNbTaTiZr by Means of Positron Annihilation Spectroscopy

Lattice defects in refractory metal high‐entropy alloy HfNbTaTiZr are investigated using positron lifetime spectroscopy combined with coincidence Doppler broadening (CDB) spectroscopy. Annealing of HfNbTaTiZr alloy at 1000 °C results in full recovery of defects and formation of single‐phase random solid solution characterized by the bulk positron lifetime of 141 ps. This value is in accordance with the theoretical estimation. Quenching of the alloy from high temperature prevents recovery of thermally equilibrium vacancies and enables to retain a high concentration of vacancies in the sample at ambient temperature. For single vacancies in the HfNbTaTiZr alloy, a characteristic positron lifetime of 212 ps is measured, which is significantly shorter than the value estimated by theoretical calculations. It indicates that there is considerable ion relaxation around vacancies. The CDB spectroscopy investigations reveal that vacancies in HfNbTaTiZr alloy are not distributed randomly but are preferentially coupled with Hf solutes.

Positron lifetime spectroscopy applied to pure sulphur and selenium

High purity crystalline orthorhombic Sulphur and hexagonal Selenium have been investigated using Positron Lifetime Spectroscopy technique. Annealed for 3 h at 120 °C Selenium lifetime (308(1) ps) has been compared with previous Positron Lifetime Spectroscopy knowledge in order to validate the measurements and the analysis procedure, and also compared with theoretical calculations available in literature, including different methods and parametrizations, resulting on the best match of LMTO-ASA method with GGA parametrization. Annealed for 3 h at 80 °C Sulphur samples were studied by the same means providing a single lifetime component with value 300(1) ps that has been assigned to the positron annihilation in the bulk that is not present in the current literature.

Temperature studies using positron lifetime spectroscopy of the subsurface zone in pure titanium exposed to a dry sliding test

The report presents the positron annihilation studies of subsurface zone generated in pure titanium exposed to a long period dry sliding test. The total depth of the subsurface zone induced is detected at about 250 µm. Only dislocations and vacancy clusters which consist of two or three vacancies are observed in this zone. Their concentration decreases with the depth, especially at the depth above 100 µm. Despite the long duration of the sliding test, no clear signs indicating the presence of a tribolayer were observed. This was confirmed also by observation of annealing of defects in this zone at different depths. The lack of the tribolayer is in contrast to the research done so far for other metals.

CHANGES IN THE MECHANICAL, MICRO-, AND NANO-STRUCTURAL PROPERTIES OF REINFORCED VULCANIZED NATURAL RUBBER COMPOUNDS: THEIR DEPENDENCE ON THE SiO2/CB RATIO

ABSTRACT A systematic study on the influence of fillers on the structural properties at micro and nanoscale of vulcanized compounds based on a natural rubber matrix reinforced with silica and/or carbon black is presented. Several compounds with different SiO2/CB ratios (1.5/1, 2.25/1, and 3/1) and total filler contents (55, 60, and 65 phr) were prepared and vulcanized at 150 °C. The experimental techniques used were rheometric, swelling and dynamic mechanical tests, and positron lifetime spectroscopy. From these techniques, cure reaction parameters, the fraction at the maximum degree of swelling, storage modulus, loss tangent, and fractional free volume were measured. Using a recent model based on a hydrodynamic description and the percolation of aggregates in a rubber matrix, it was found that regardless of the filler combinations, the dynamic storage modulus is well represented as a function of the filler volume fraction. Besides, beyond a critical SiO2/CB ratio (2.25/1) in the formulations of the compounds, the loss tangent does not depend on the SiO2/CB ratio. The results obtained show a direct correlation among mechanical properties, swelling and fractional free volume, and the type and amount of fillers in the reinforced compounds.

Positron lifetime spectroscopy applied to pure Tellurium

High-purity crystalline Tellurium has been investigated using positron lifetime spectroscopy technique in order to determine basic information missing in the current experimental knowledge of the positron annihilation spectroscopy field. Three different pairs of samples have been studied in the as-received state and, in order to eliminate the vacancy-type defects, after consecutive isothermal treatments at 300 °C. Lifetime corresponding to the annihilation in the Tellurium bulk has been determined as 282(1) ps. Previous theoretical calculations present in the bibliography that used different methods and parameterization provided a wide range of values for the annihilation lifetime of the positron in the bulk of Tellurium. The obtained result has been used to identify the most accurate results among them.

Low molecular weight ‘liquid’ polymer extended compounds, impact on free volume and crosslink density studied by positron lifetime spectroscopy and stress-strain analysis according to Mooney-Rivlin

Liquid polymers are used in elastomeric compounds to modify their processing and final characteristics. The material is considered as a polymeric plasticizer and is used to substitute or supplement conventional plasticizer like paraffinic or aromatic oils, or hydrocarbon resins. Their plasticizing effect and contribution to the crosslinking in sulfur cured SSBR mixtures is studied. Aromatic oil, aliphatic resin and different liquid polymers, varied in type, molecular weight and microstructure are discussed. Different plasticizing effects are shown with corresponding curing torques and Shore hardness values. The free volume – measured by positron annihilation lifetime spectroscopy - increases or decreases according to the liquid polymer's incorporation in the main polymer.

Effects of dislocations and hydrogen concentration on hydrogen embrittlement of austenitic 316 stainless steels

The effects of dislocations and hydrogen concentration on the hydrogen embrittlement (HE) in 316 austenitic stainless steels (316SS) were systematically investigated in the present study. The results revealed a decrease in tensile strength and ductility of 316SS after hydrogen charging. Furthermore, the severity of HE was dependent on the hydrogen concentration. The results of x-ray diffraction revealed the presence of martensite in the hydrogen-charged (50 mA /cm2) specimen annealed at 1000 ℃. This was attributed to the internal stress originating from hydrogen-induced lattice expansion. The analysis of the fracture surface of all the specimens revealed an increase in the brittleness with the absorption of hydrogen. An increase in the hydrogen concentration induced a transformation of the fracture mode i.e., from ductile dimple fracture to cleavage and intergranular fractures. The evolution of hydrogen in 316SS was associated with the defects induced by deformation. The shift in the major desorption peaks of the annealed specimen to the high-temperature region indicated the formation of effective hydrogen-trapping sites during hydrogen-charging. The specimens were subjected to positron lifetime spectroscopy and thermal desorption spectroscopy. The results for the raw specimen, charged at 20 mA /cm2, revealed the gradual escape of hydrogen atoms and recovery of vacancy defects with an increase in the annealing temperature. In addition, the hydrogen-induced and cold rolling-induced dislocations was preserved.

Structural Defects in TiNi-Based Alloys after Warm ECAP

The microstructure, martensitic transformations and crystal structure defects in the Ti50Ni47.3Fe2.7 (at%) alloy after equal-channel angular pressing (ECAP, angle 90°, route BC, 1–3 passes at T = 723 K) have been investigated. A homogeneous submicrocrystalline (SMC) structure (grains/subgrains about 300 nm) is observed after 3 ECAP passes. Crystal structure defects in the Ti49.4Ni50.6 (at%) alloy (8 ECAP passes, angle 120°, BC route, T = 723 K, grains/subgrains about 300 nm) and Ti50Ni47.3Fe2.7 (at%) alloy with SMC B2 structures after ECAP were studied by positron lifetime spectroscopy at the room temperature. The single component with the positron lifetime τ1 = 132 ps and τ1 = 140 ps were observed for positron lifetime spectra (PLS) obtained from ternary and binary, correspondingly, annealed alloys with coarse-grained structures. This τ1 values correspond to the lifetime of delocalized positrons in defect-free B2 phase. The two component PLS were found for all samples exposed by ECAP. The component with τ2 = 160 ps (annihilation of positrons trapped by dislocations) is observed for all samples after 1–8 ECAP passes. The component with τ3 = 305 ps (annihilation of positrons trapped by vacancy nanoclusters) was detected only after the first ECAP pass. The component with τ3 = 200 ps (annihilation of positrons trapped by vacancies in the Ti sublattice of B2 structure) is observed for all samples after 3–8 ECAP passes.

Systematics of positron lifetime spectroscopy measurements in ferroelectrics

In the paper is demonstrated the use of positron lifetime spectroscopy (PLS) in a comparative study of the results in ferroelectrics. The first part is devoted to the application of PLS as a probe for measurements of ferroelectrics. In Triglycine Sulphate (TGS) (NH3CH2COOH)3H2SO4, Rochelle Salt (RS) NaKC4H4O64H2O and Potassium Dihydrogen Phosphate (KDP) KH2PO4 at different temperatures and gamma irradiation doses is established the existence of three positron lifetime components. This experimental results show that the second long lifetime component as a function of temperature in order-disorder ferroelectrics TGS, RS and KDP is due to the positron trapping in negatively charge defects of ferroelectric. The positron spectroscopy method is effective for measurements of extremely small defects and voids, which is important for the next exploitation period of the ferroelectrics. PLS measurements of ferroelectrics show that about the third lifetime component is due to the local formation of orhto-positronium state on the boundary between the domains. In the second part the PLS analysis has been applied for examination the electronic and defect structure in selected ferroelectrics materials. In gamma irradiated TGS positrons are trapped in the defect states of the oxygen ions of the two radicals CH2COO- and NH3CHCOO-. In RS positrons are trapped also in the defect states of the oxygen ions and OH groups.

Free Volume Controlled Ionic Conductivity in Poly Vinyl Alcohol/Zinc Acetate Solid Polymer Electrolytes

Poly vinyl alcohol (PVA)/Zinc acetate (ZnAc) Solid Polymer Electrolytes (SPEs) of different ZnAc weight percentage (5%, 10%, 15%, 20%) were prepared. Positron Lifetime Spectroscopy (PLS) studies reveal the increased fractional free volume (Fv) from 1.93% to 2.16% for 20 wt% of ZnAc loading. The increased ionic conductivity of PVA/ZnAc SPEs for 20 wt% of ZnAc loading suggests the increased concentration of Zn2+ cations and acetate anions. The ionic conductivity of PVA/ZnAc SPEs increases at higher frequencies of the applied electric field due to the increased polarization of dipoles. The increased ionic conductivity of PVA/ZnAc SPEs with increased fractional free volume suggests that the mobility of Zn2+ and acetate ions is favored by the amorphous phase of the SPEs. The XRD results justify the increased amorphousness of PVA/ZnAc SPEs. FTIR studies confirm the formation of polymer salt complexes due to chemical bonding between C–H group of PVA and carbonyl group of (C=O) of ZnAc salt.

Effects of oxalic acid concentration on the microstructures and properties of nano-VO2(B)

In this work, a series of VO2(B) samples were synthesized via a hydrothermal process by reducing V2O5 with different concentrations of oxalic acid. The prepared samples were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, cyclic voltammetry, and galvanostatic charge–discharge. Furthermore, the microdefects of VO2 samples were characterized by positron lifetime spectroscopy. The results revealed that VO2(B) was successfully prepared and the concentration of reducing agent had a certain influence on the microstructures and properties. The electrochemical performance measurements showed that all samples had good cycle stability in 2 mol/L KOH solution. The x = 1.50 sample displayed higher discharge capacitance of 149.5 F g−1 at 30 mA g−1 and the discharge capacitance remained about 67% even after 80 charge/discharge cycles. The positron lifetime spectra revealed that the main defects in VO2 samples were microvoids, and the defect concentration and size were affected by the C2H2O4 concentration.

Size effect in nonstoichiometric titanium monoxide and vanadium carbide nanocrystals measured by positron lifetime spectroscopy

A size effect in nonstoichiometric titanium monoxide TiOy and vanadium carbide VCy nanocrystals containing high concentrations of structural vacancies was detected by electron-positron annihilation. The effect consists of a considerable increase in the positron lifetime with the specific surface area. An analysis of the intensity of a long lifetime surface component in the positron annihilation spectra and the average positron lifetime for different nonstoichiometric compounds indicated a universal character of the observed size effect.

Influence of beam current on microstructure of electron beam melted Ti-6Al-4V alloy

The defect microstructure of the samples manufactured from Ti-6Al-4V powder was studied using electron beam melting (EBM) in the beam current range of 17 - 13 mA. The hybrid digital complex combined positron lifetime spectroscopy and coincidence Doppler broadening spectroscopy was used to characterize the defect structure of the materials. The microstructure and defects were also analyzed by transmission electron microscopy. It has been established that the main type of the defects in the EBM manufactured samples is dislocations. According to the conducted measurements and calculations, the dislocation density in the EBM manufactured samples exceeds by two orders the similar value for the cast Ti-6Al-4Valloy. Formation of Ti-Ti-Al nanoscale clusters has been found in the EBM manufactured samples.

Influence of Zn2+ doping on the lattice defects and photoluminescence studies of Sr2CeO4:Eu3+ nanophosphor: Applications for data encryption strategies

In the present work, luminescent Sr2CeO4:Eu3+ (5 mol %), Zn2+ (0.25–3 mol %) nanophosphors were fabricated by the ultrasound irradiated sonochemical method. The samples prepared with 3 h ultrasound treatment exhibit well crystalline and single phase Sr2CeO4. The dumbbell shaped morphology of the prepared samples was elucidated from both SEM and TEM results. The energy band gap of the prepared samples was estimated and found to be in the range ∼3.18–3.63 eV. The incorporation of Zn2+ greatly influences the defect and emission intensities, as revealed from photoluminescence and positron lifetime spectroscopy measurements. The enhancement in the photoluminescence emission intensity after Zn2+ incorporation was observed, which may be due to the creation of defects and efficient energy transfer between Zn2+ and Eu3+ or defects that can act as emission centers. The positron lifetime spectroscopy qualitatively explains the concentration of defects (vacancy and voids) which are induced by Zn2+ co-doping. The encryption strategies are provided using the photoluminescent chalk for high level information protection. We believe that the versatile, convenient and user-friendly strategy demonstrated herein will open a new insight for on-site information protection.

Defects and their range in pure bismuth irradiated with swift Xe ions studied by positron annihilation techniques

Investigations of defects and their spatial distribution in Bi irradiated with 167 MeV Xe26+ ions of different doses have been performed using conventional positron lifetime spectroscopy and variable energy positron beam. In an implanted layer, in which ions are traveling, interacting with atoms and stopping, only clusters which consist of more than eight vacancies were found. It was assigned from ab initio theoretical calculations of positron lifetime in vacancy clusters in Bi. The thickness of this layer corresponds to the range of implanted ions calculated from the SRIM code. However, beyond this layer, an extended layer with such defects has also been found. Its thickness is comparable to the thickness of the implanted layer and it depends on the dose. Defects induced by implantation are also present near the entrance surface, and their concentration depends on the dose of implanted ions as well. Three methods for reconstructing the actual mean positron lifetime and thus the induced depth defect distribution have been proposed, two of them are used in current research.