Positron Lifetime Research Articles

Positrons as chemically sensitive probes in interfaces of multicomponent complex materials: Nanocrystalline Fe90Zr7B3

Abstract The present paper reports on a combined analytical and structural study of nanocrystalline Fe90Zr7B3 by means of positron annihilation, (analytical) high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction. Particular focus is laid on the chemical nature of the intergranular amorphous matrix which occurs between the α-Fe nanocrystallites. Energy-dispersive X-ray measurements (EDX) with an electron nanobeam reveal an increased Zr content at the interface between the nanocrystallites and the intergranular amorphous phase. According to positron lifetime measurements, the intergranular amorphous phase and the interfaces between this phase and the nanocrystallites exhibit structural free volumes of the mean size slightly smaller than a lattice vacancy as in the amorphous precursor material. Coincident Doppler broadening measurements of the positron-electron annihilation photons show that the fraction of Zr in the neighborhood of the structural free volumes is higher in nanocrystalline Fe90Zr7B3 than in the amorphous state indicating an enhanced Zr concentration in the interfaces. These results are in good agreement with the HRTEM/ EDX studies and demonstrate the potentials of the coincident Doppler broadening technique for a chemical characterization of structurally complex materials on an atomistic scale.

Suppression of vacancy formation and hydrogen isotope retention in irradiated tungsten by addition of chromium

To study the effect of the content of chromium (Cr) in the tungsten (W) matrix on the vacancy formation and retention of hydrogen isotopes, the samples of the W-0.3 at.% Cr alloy were irradiated with 6.4 MeV Fe ions in the temperature range of 523–1273 K to a damage level of 0.26 displacement per atom (dpa). These displacement-damaged samples were exposed to D2 gas at a temperature of 673 K and a pressure of 100 kPa to decorate ion-induced defects with deuterium. The addition of 0.3 at.% Cr into the W matrix resulted in a significant decrease in the retention of deuterium compared to pure W after irradiation especially at high temperature (≥773 K). Positron lifetime in W-0.3 at.% Cr alloy irradiated at 1073 K was almost similar to that for non-irradiated one. These facts indicate the suppression of the formation of vacancy-type defects (monovacancies and vacancy clusters) by 0.3 at.% Cr addition, which leads to the significant reduction in deuterium retention in W-0.3 at.% Cr alloy.

Harvesting Light from BaHfO3/Eu3+ through Ultraviolet, X-ray, and Heat Stimulation: An Optically Multifunctional Perovskite.

Materials with optical multifunctionality such as photoluminescence (PL), radioluminescence, and thermoluminescence (TL) are a boon for a sustainable society. BaHfO3 (barium hafnium oxide [BHO]) under UV irradiation demonstrated visible PL endowed by oxygen vacancies (OVs). Eu3+ doping in BHO (BHOE) introduces f-state impurity levels just below the conduction band for both Eu@Ba and Eu@Hf sites, causing efficient host-to-dopant energy transfer, generating intense 5D0 → 7F1 magnetic dipole transitions (MDT) with internal quantum yield of ∼70%. X-ray photoelectron spectroscopy and electron paramagnetic resonance showed the formation of OVs in both BHO and BHOE samples with more vacancies in the doped sample. The positron lifetime measurements suggested that Eu3+ ions are distributed at both Ba2+ and Hf4+ sites. The association of OVs with Hf4+ and Eu3+ ions due to high charge/radius ratio is considered to be responsible for lowering the symmetry around Eu3+ ions to C4v in BHOE. Density functional theory studies of defect formation energy justified the same. Time-resolved emission spectroscopy showed distinct spectra for Eu@Ba and Eu@Hf sites corresponding to symmetric and asymmetric environments, respectively. This could be highly relevant in designing color tunable phosphor by forcing dopant ions at one specific site because Eu@Ba displayed orange emission whereas Eu@Hf displayed red emission. We could further harness BHOE for X-ray scintillator application by designing a thin film, which showed efficient conversion of high-energy X-ray into visible light. Under beta irradiation; both BHO and BHOE showed distinct TL glow curves as shallow traps were formed in the former and deep traps in the latter, which could have long-term implications in harnessing this material for persistent luminescence. We believe that BHO/BHOE demonstrated an extraordinary credential as a perovskite for multifunctional applications in the area of defect-induced light emission, UV phosphor, X-ray scintillator, and TL crystals.

Tailoring the micropore structure of 6FDA-based polyimide membrane for gas permselectivity studied by positron annihilation

Two series of 6FDA-based co-polyimide membranes were synthesized by adding different diamine monomer containing special functional groups: APAF (containing OH) and DABA (containing -COOH). The ODA/APAF and ODA/DABA series were heat treated at 450 °C and 350 °C, respectively. Thermal rearrangement of polymer chain segments is confirmed in ODA/APAF membranes by infrared spectroscopy, while addition of DABA in ODA/DABA membranes causes decarboxylation crosslinking. Positron lifetime measurements reveal existence of ultramicropores in polyimides. The thermal rearrangement in ODA/APAF membranes induces remarkable increase in both size and number of ultramicropores and therefore increase in fractional free volume (FFV), but the crosslinking in ODA/DABA causes significant decrease in both size and number of ultramicropores. The increase in FFV is favorable for gas permeation, but too large size of the micropore drastically deteriorates the gas selectivity. Finally, the ODA/APAF(3:7) shows a high CO2 permeability of 127.4 Barrer with CO2/CH4 selectivity of 34.2, which surpasses the 1991 Robeson upper bound. Besides, the ODA/DABA(3:7) exhibits a H2 permeability of 22.6 Barrer with H2/CH4 selectivity of 256.8, which also exceeds the 1991 Robeson upper bound. Our results indicate that the gas permeability and selectivity of polymer membranes can be effectively adjusted by tailoring the ultramicropore structures, and positron can provide precise information about micropores.

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.

Using inverse Laplace transform in positronium lifetime imaging

Positronium (Ps) lifetime imaging is gaining attention to bring out additional biomedical information from positron emission tomography (PET). The lifetime of Ps in vivo can change depending on the physical and chemical environments related to some diseases. Due to the limited sensitivity, Ps lifetime imaging may require merging some voxels for statistical accuracy. This paper presents a method for separating the lifetime components in the voxel to avoid information loss due to averaging. The mathematics for this separation is the inverse Laplace transform (ILT), and the authors examined an iterative numerical ILT algorithm using Tikhonov regularization, namely CONTIN, to discriminate a small lifetime difference due to oxygen saturation. The separability makes it possible to merge voxels without missing critical information on whether they contain abnormally long or short lifetime components. The authors conclude that ILT can compensate for the weaknesses of Ps lifetime imaging and extract the maximum amount of information.

Preparation and characterization of low-κ polyhedral oligomeric silsesquioxane/polyimide hybrid films

A series of Polyimide (PI)/Phenyl-POSS (POSS) hybrid films with low dielectric constants and excellent comprehensive properties have been successfully prepared via physical blend methods by incorporating different proportions of POSS into the polyimide precursors (Pyromellitic dianhydride- 4, 4′-Diaminodiphenyl ether). An appropriate amount of POSS doping can effectively improve the optical and dielectric properties of hybrid films due to the contribution of POSS with hollow, cage-like molecular structure to weakening intermolecular charge transfer complexation and increasing nanopore volume fraction in hybrid films. Instead, excessive amount of POSS doping can reduce dielectric properties due to the interface polarization effect induced by POSS agglomeration. When the doping amount is 15%, the highest light transmission reaches to 84.5%. When the doping amount is 25%, the dielectric properties have been are best improved. The dielectric constant and loss at 104 Hz are reduced to 2.37 and 0.0018 respectively. However, the mechanical properties decrease in response to the increase of POSS content, because POSS molecular dispersing in the folded molecular chains hinders the interaction of molecular chains PI matrix. The correlation between the dielectric properties of PI/POSS films and their nonopore volume fraction has been explained with the help of positron lifetime spectra.

Chemical quenching of positronium in OMC/SBA-15, OMC@SBA-15 and CuO@SBA-15 catalysts

Owing to highly ordered two-dimensional hexagonal structure, large surface area, variable pore size, high thermal stability and especially the electron delocalization energy determined by its frame structure, SBA-15 catalysts have received more and more researchers’ attention. By using the structure-directing agent of P123 and the silicon source of TEOS, we synthesize ordered mesoporous silica SBA-15. At the same time, ordered mesoporous carbon OMC is succefully synthesized with the template of SBA-15. The small angle X-ray diffraction, high resolution transmission electron microscopy and N<sub>2</sub> adsorption-desorption measurements are conducted to verify the highly ordered pore structure and relatively high specific surface area of SBA-15 and OMC, and their average pore radius are about 7.5 nm and 3.3 nm, respectively. Positron lifetime spectrum of SBA-15 catalyst is composed of two longer lifetimes and two shorter lifetimes: two longer lifetimes <inline-formula><tex-math id="M11">\begin{document}$ {\tau }_{3} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M11.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M11.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M12">\begin{document}$ {\tau }_{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M12.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M12.png"/></alternatives></inline-formula> are the annihilation in micropore and large pore of positronium (Ps), are 7.5 ns and 106 ns. However, there is nearly no longer lifetime component in OMC, which indicates that there might exist the quenching or inhibiting of positronium by carbon material. To verify this guess, we synthesize the catalysts of OMC/SBA-15, OMC@SBA-15 and CuO@SBA-15 by the solid state reaction and the impregnation filling method. With the increasing of OMC and CuO content, both the o-Ps lifetime <inline-formula><tex-math id="M13">\begin{document}$ {\tau }_{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M13.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M13.png"/></alternatives></inline-formula> and its intensity <inline-formula><tex-math id="M14">\begin{document}$ {I}_{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M14.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M14.png"/></alternatives></inline-formula> of these three compounds decrease. The annihilation rate of o-Ps lifetime varying with OMC and CuO content can be better fitted by one or two straight lines, The values of reaction rate constant K in OMC/SBA-15, OMC@SBA-15 and CuO@SBA-15 are <inline-formula><tex-math id="M15">\begin{document}$(2.39\pm $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M15.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M15.png"/></alternatives></inline-formula><inline-formula><tex-math id="M15-1">\begin{document}$ 0.44)\times {10}^{7}~{\mathrm{s}}^{-1}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M15-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M15-1.png"/></alternatives></inline-formula>/<inline-formula><tex-math id="M16">\begin{document}$(6.65\pm 0.94)\times {10}^{6}~{\mathrm{s}}^{-1}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M16.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M16.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M17">\begin{document}$(2.28\pm 0.19)\times {10}^{7}~{\mathrm{s}}^{-1}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M17.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M17.png"/></alternatives></inline-formula>, and <inline-formula><tex-math id="M18">\begin{document}$(8.76\pm 0.47)\times {10}^{6}~{\mathrm{s}}^{-1},$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M18.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M18.png"/></alternatives></inline-formula> respectively. Therefore, our results indicate that there are quenching effect and inhibition effect among the carbon, the CuO and the positronium, which lead <inline-formula><tex-math id="M19">\begin{document}$ {\tau }_{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M19.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M19.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M20">\begin{document}$ {I}_{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M20.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M20.png"/></alternatives></inline-formula>to decrease, and positronium is also a probe for detecting the pore structure of porous material.

Spin conversion of positronium of ZIFs nanocrystalline

ZIFs crystal is composed of imidazolidyl bridging single metal ions, and its structure can be adjusted by flexibly selecting functional groups of imidazolidyl ligands, thereby possessing more new properties and functions. While, the pore structure and chemical environment of ZIFs crystals are closely related to their properties. In this work, ZIF nanocrystals are prepared by static reaction. The X-ray diffraction results confirm that the prepared crystals are typical of ZIF-8 crystals, and the regular rhomboidal structure can be observed by scanning electron microscopy. The N<sub>2</sub> adsorption-desorption test indicates that the ZIF crystal exhibits the larger specific surface area (2966.26 m<sup>2</sup>/g) and pore volume (3.01 cm<sup>3</sup>/g) . With the increase of Co content, specific surface area and pore volume of ZIFs crystal decrease, while the pore size remains nearly unchanged (around 12 Å). However, the pore size distribution calculated by N<sub>2</sub> adsorption/desorption isothermal curve does not show the ultra-micropore information of the six-membered ring composed of imidazole ligands (3.4 Å). The microstructure and surface properties of the crystal are investigated by positron annihilation lifetime and Doppler broadening. The positron lifetime spectrum has four components. The longer lifetimes <inline-formula><tex-math id="M8">\begin{document}$ {\tau }_{3} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M8.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M9">\begin{document}$ {\tau }_{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M9.png"/></alternatives></inline-formula> are the annihilation lifetimes of o-Ps in the microporous region and the regular angular gap of the crystal, respectively. With the increase of Co content, the lifetime <inline-formula><tex-math id="M10">\begin{document}$ {\tau }_{3} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M10.png"/></alternatives></inline-formula> hardly changes, while the longer lifetime <inline-formula><tex-math id="M11">\begin{document}$ {\tau }_{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M11.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M11.png"/></alternatives></inline-formula> decreases from 30.89 ns to 12.57 ns, and the corresponding intensities <inline-formula><tex-math id="M12">\begin{document}$ {I}_{3} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M12.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M12.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M13">\begin{document}$ {I}_{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M13.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M13.png"/></alternatives></inline-formula> decrease sharply from 12.93% and 8.15% to 3.68% and 0.54%, respectively. With the increase of Co content, the <i>S</i> parameter obtained by doppler broadening shows a continuous upward trend, and the p-Ps intensity also increases gradually, which is mainly due to the self-rotation effect of the electron element. Therefore, the decrease of <inline-formula><tex-math id="M14">\begin{document}$ {\tau }_{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M14.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M14.png"/></alternatives></inline-formula> in ZIFs nanocrystal is probably due to the self-rotation effect of positronium and Co ion on the crystal surface.

Positronium as a biomarker of hypoxia

Abstract In this review article, we present arguments demonstrating that the advent of high sensitivity total-body PET systems and the invention of the method of positronium imaging, open realistic perspectives for the application of positronium as a biomarker for in-vivo assessment of the degree of hypoxia. Hypoxia is a state or condition, in which the availability of oxygen is not sufficient to support physiological processes in tissue and organs. Positronium is a metastable atom formed from electron and positron which is copiously produced in the intramolecular spaces in the living organisms undergoing positron emission tomography (PET). Properties of positronium, such as e.g., lifetime, depend on the size of intramolecular spaces and the concentration in them of oxygen molecules. Therefore, information on the partial pressure of oxygen (pO2) in the tissue may be derived from the positronium lifetime measurement. The partial pressure of oxygen differs between healthy and cancer tissues in the range from 10 to 50 mmHg. Such differences of pO2 result in the change of ortho-positronium lifetime e.g., in water by about 2–7 ps. Thus, the application of positronium as a biomarker of hypoxia requires the determination of the mean positronium lifetime with the resolution in the order of 2 ps. We argue that such resolution is in principle achievable for organ-wise positronium imaging with the total-body PET systems.

Water intake of cellulose materials monitored by positron annihilation lifetime spectroscopy

In this study, for the first time, the experimental technique of positron annihilation lifetime spectroscopy (PALS) has been applied to monitor in situ the microstructural changes of cellulose-based materials, i.e. paper, during water intake. For three different cellulose samples, bleached fine paper without filler, Kraft paper without filler, and a viscose fiber sheet, the mean positron lifetime Delta tau _{mathrm {mean}} showed a strong increase with time in humid atmosphere, but exhibiting different trends depending on the type of sample. For all the cellulose samples investigated, the mean positron lifetime Delta tau _{mathrm {mean}} shows an initial strong increase simultaneously occurring (t<10 h) to the mass increase of the samples due to water intake. Interestingly, the variations of Delta tau _{mathrm {mean}} of the viscose fiber sheet and the Kraft paper sample both show a second increase on longer timescales (t>60 h in humid atmosphere) during which the mass increase of these samples has already been saturated. The results of this study show that by the means of PALS, water transport in paper can be reliably followed over a long timespan and it is even possible to distinguish between different types of cellulose materials. The second stage increase of the mean positron lifetime after long times in humid atmosphere for the Kraft paper sample and the pure viscose sheets even suggest that not only water intake itself can be monitored but also further atomistic processes in the material are accessible.

High-heat flux Cu-0.8Y alloys investigated by positron annihilation spectroscopy

This work studies the thermal stability of the microstructure and the evolution of the defects of two high-heat flux Cu-0.8 wt%Y alloys fabricated following two alternative powder metallurgy routes. One batch was produced by direct hot isostatic pressing (HIP) consolidation of Cu-0.8 wt%Y pre-alloyed atomized powders while an additional ball milling processing step was introduced before HIP sintering for the second alloy. The stability and recovery characteristics of the vacancy type defects in these alloys in the as-produced state and after processing by severe equal channel angular pressing to achieve a refine microstructure have been investigated by positron lifetime and coincidence Doppler broadening measurements in samples subjected to isochronal annealing from room temperature to 900 °C. Microhardness measurements and electron transmission microscopy analysis have also been performed to support the results obtained from the positron annihilation spectroscopy analysis techniques. The recovery curves of the positron lifetime and S-W plots show a recovery stage in agreement with the recovery stage V for Cu. However, a full recovery is not accomplished, and a stage that reverts the previous recovery takes place after annealing above ~600 °C, that leads to the formation of very stable defects at temperatures up to 900 °C, identified as vacancy aggregates and nanocavities. The characteristic shape of the coincidence Doppler broadening indicates that the dispersed Y-O particles in the Cu matrix appear to be responsible for stabilizing the vacancy aggregates and nanocavities for temperatures above 600–700 °C.

Micro-defects and luminescence of thulium-doped yttrium aluminum garnet single crystals

Thulium-doped yttrium aluminum garnet (YAG) single crystals with composition (TmxY(1-x))3Al5O12 (x = 0.001–0.02) were prepared by the optical floating zone method. The crystals were of high quality with no cracks, bubbles, or inclusions, and with >80% visible light transmittance. The lowest value for the mean positron lifetime was observed with crystals which had x = 0.01, which demonstrates that this composition had the fewest number of defects. This sample also produced the highest emission intensities in the PL spectra, and had narrower lines than those previously reported for Tm3+-doped yttria-stabilized zirconia. However, with higher Tm3+ concentrations, interactions between neighboring Tm3+ ions resulted in partial quenching of the luminescence. Overall, Tm-doped YAG single crystals synthesized by the optical floating zone (OFZ) method contained few defect centers and exhibited narrow luminescence linewidths, which demonstrate potential uses in high resolution applications.

Humidity effect on the transport properties of per‐fluorinated sulfonic acid/PTFE proton exchange membranes: Positron annihilation study

AbstractThe positron annihilation lifetime (PAL) technique was used to investigate the yield and lifetime of positronium (Ps) in three different samples of per‐fluorinated sulfonic acid/polytetrafluoroethylene (PTFE) proton exchange membranes. The high probability of Ps formation enables the study of the hole volume size by the PAL technique and explains the mechanism of both methanol permeation and proton conductivity of the membranes in relation to the relative humidity (RH). The influence of various membrane parameters, including water uptake, proton conductivity, and mechanical properties, was studied. The concentration of water molecules and SO3−–H3O+groups is correlated with the ion exchange capacity (IEC) of the membranes where higher IEC leads to high absorbance of water. The proton conductivity is enhanced by increasing the IEC value of the membrane. The activation energies of decomposition temperature for the F‐950 membrane are the lowest (Eb = 2.856 and 1.030 eV for the 1st and 2nd decomposition peaks, respectively, implying that the membrane with high IEC is thermally less stable. With increasing the IEC of the membranes, the concentration of the water molecules/SO3 group increases. In addition, the proton conductivity of the membrane is positively influenced by increasing water content at high RH. The fact that the variations in hole volume size and both methanol permeability and proton conductivity are highly dependent on RH suggests that hole volume size plays an important role in many parameters of hydrated perfluorinated sulfonic acid/PTFE proton exchange membranes. The outcomes data were introduced and interpreted appropriately with a theoretical model and compared with the literature.

Recovery and recrystallization in vanadium foil studied by positron annihilation and X-ray methods

The pure vanadium cold-rolled foils were sequentially annealed at elevated temperature, followed by measurements using positron annihilation and XRD techniques. It was found that in the recovery stage in the temperature range from 100 to 450 °C, a significant rearrangement occurs in the dislocation microstructure, which confirms the reduction of the first positron lifetime component. The recrystallization process was beginning at 650 °C and proceeds to 800 °C or more what is associated with the reduction of the intensity of the second positron lifetime associated with vacancy clusters preassembly located at the boundaries of new grains. The determined activation energy of grain migration was equal to 3.12 ± 0.38 eV, and the recrystallization temperature was established at 700 °C. The significant change in the pole figures indicating recrystallization was noticed only at 1100 °C, however, the recrystallization process has been confirmed by the differential thermal analysis. In the temperature range from 100 to 1100 °C, a continuous decrease of the crystalline lattice strain was observed.

Synergetic Optimization of Electrical and Thermal Transport Properties by Cu Vacancies and Nanopores in Cu2Se.

In this study, a series of Cu2+x-yInySe (-0.3 ≤ x ≤ 0.2 and 0 ≤ y ≤ 0.05) samples were prepared by melting and the spark plasma sintering method. X-ray diffraction measurements indicate that the Cu-deficient samples (x = -0.3 y = 0 and x = -0.2 y = 0) prefer to form the cubic phase (β-Cu2Se). Adding excessive Cu or introducing In atoms into the Cu2Se matrix triggers a phase transition from the β to α phase. Positron lifetime measurements confirm the reduction in Cu vacancy concentration by adding excessive Cu or introducing In atoms into Cu2Se, which causes a dramatic decrease in carrier concentration from 1.59 × 1021 to 5.0 × 1019 cm-3 at room temperature. The samples with In contents of 0.01 and 0.03 show a high power factor of about 1 mW m-1 K-2 at room temperature due to the optimization of the carrier concentration. Meanwhile, the excess Cu content and doping of In atoms also favor the formation of nanopores. These pores have strong interaction with phonons, leading to remarkable reduction in lattice thermal conductivity. Finally, a high ZT value of about 1.44 is achieved at 873 K in the Cu1.99In0.01Se (x = 0 and y = 0.01) sample, which is about twice that of the Cu-deficient sample (Cu1.7Se). Our work provides a viable insight into tuning vacancy defects to improve efficiently the electrical and thermal transport performance for copper-based thermoelectric materials.

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.

Analysis of Surface Properties of Nickel Alloy Elements Exposed to Impulse Shot Peening with the Use of Positron Annihilation.

The paper presents the results of experimental studies on the impact of impulse shot peening parameters on surface roughness (Sa, Sz, Sp, Sv), surface layer microhardness, and the mean positron lifetime (τmean). In the study, samples made of the Inconel 718 nickel alloy were subjected to impulse shot peening on an originally designed stand. The variable factors of the experiment included the impact energy, the diameter of the peening element, and the number of impacts per unit area. The impulse shot peening resulted in changes in the surface structure and an increase in surface layer microhardness. After the application of impulse shot peening, the analyzed roughness parameters increased in relation to post-milling values. An increase in microhardness was obtained, i.e., from 27 HV 0.05 to 108 HV 0.05 at the surface, while the maximum increase the microhardness occur at the depth from 0.04 mm to 0.08 mm. The changes in the physical properties of the surface layer were accompanied by an increase in the mean positron lifetime τmean. This is probably related to the increased positron annihilation in point defects. In the case of small surface deformations, the increase in microhardness was accompanied by a much lower increase in τmean, which may indicate a different course of changes in the defect structure consisting mainly in modification of the dislocation system. The dependent variables were subjected to ANOVA analysis of variance (it was one-factor analysis), and the effect of independent variables was evaluated using post-hoc tests (Tukey test).

Extremely low thermal conductivity of β−Ga2O3 with porous structure

Due to the ultrawide bandgap (4.9 eV), high carrier mobility (300 cm2V−1s−1), and high thermal stability, β−Ga2O3 can be a potential candidate for high-temperature thermoelectric materials. However, the intrinsically high thermal conductivity may hinder its application for thermoelectric conversion. In this work, porous β−Ga2O3 was prepared by the solvothermal method together with spark plasma sintering technology. Positron lifetime measurement and N2 adsorption confirm the introduction of pores by adding sucrose in the sample preparation. The sucrose-derived β−Ga2O3 sintered at a relatively low temperature of 600 °C remains highly porous, which results in an extremely low thermal conductivity of 0.45 W m−1K−1 at room temperature, and it further decreases to 0.29 W m−1K−1 at 600 °C. This is the lowest thermal conductivity for β−Ga2O3 reported so far. Our work provides an avenue to reduce the thermal conductivity for β−Ga2O3 and is believed to be widely applicable to many other thermoelectric materials.

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.