Positron Annihilation Doppler Research Articles

Positron annihilation study of lattice defects and nanoporous structures in Mn4+ doped K2SiF6 nanophosphors exhibiting high quantum yield

The present paper reports an application positron annihilation spectroscopy (PAS) to systematically study the formation of lattice defects, nanoporous structures and sites of Mn4+ ions doped into the K2SiF6 (KSFM) nanophosphors prepared using four different solvents. Three main techniques of PAS, including positron annihilation lifetime (PAL), Doppler broadening (DB) and electron momentum distribution (EMD) measurements, were simultaneously carried out under the same vacuum condition. The structural characteristics and quantum yield (QY) of KSFM nanophosphors were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), coupled plasma mass spectroscopy (ICP-MS), photonic multichannel analyzer-spectrophotometer (PMAS) and fluorescence spectrophotometer (PLS). The obtained results indicated that decreasing the dielectric constant of the solvents led to the reduction of the particle sizes in KSFM. The occupation of Mn4+ to the sites of Si4+ in the SiF62– octahedron structures of KSFM as well as the high concentration of lattice defects and nanopores (average size of 0.390–0.621 nm) in the doped materials that used isopropanol and diethyl ether solvents were indicated by PAS. Moreover, increasing the concentration of doped Mn4+ caused a decrease of the occupation of Mn4+ to the adjacent Si-vacancy and K-divacancy and at the same time, the number of Si-vacancies in KSFM increased. Combining PAS and QY measurements, we found that the QY of KSFM nanophosphors depended on not only the porous structures (nanopores) but also the concentration of doped Mn4+ in the sites of Si as well as the concentration of Si-vacancies.

Irradiation hardening behaviors of large-scale hot rolling potassium-doped tungsten alloy under synergistic irradiations of Fe11+ ion combined with deuterium and helium plasmas

In this study, a 25 kg potassium doped tungsten (KW) plate dispersed K bubbles was prepared by hot rolling firstly. In order to evaluate the mechanical properties of KW after irradiation, the synergistic irradiation of Fe11+ ion, He and D plasmas were performed on KW and pure tungsten (PW). 3.5-MeV Fe11+ ion irradiation was used to simulate displacement damage induced by neutrons. The total fluence and irradiation temperature were 5.31 × 1014 m−2 for 0.5 displacements per atom (dpa) damage and 400 K, respectively. Positron annihilation Doppler broadening spectrometry (PA-DBS) technique was applied to measure the type of defect in both undamaged and damaged PW & KW samples. The total ion fluence helium (He) and deuterium (D) were kept constant at 1 × 1025 He m−2 and 5 × 1024 D m−2, separately. Subsequently, the hardness value of PW and KW received from CSM nanoindentation technique was analyzed and compared. The results show that the increase of vacancy-type defects in KW is restrained after irradiation compared with PW. For un-irradiated PW and KW samples, there are almost the same pop-in events, which indicate that PW and KW a similar plastic deformation at room temperature. The KW shows a better irradiation hardening resistance than PW under the same irradiation condition. Besides, the evolution of irradiation hardening as well as their relationship with K bubbles were discussed in detail.

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.

On the Limitations of Positron Annihilation Spectroscopy in the Investigation of Ion-Implanted FeCr Samples

New materials for advanced fission/fusion nuclear facilities must inevitably demonstrate resistance to radiation embrittlement. Thermal and radiation ageing accompanied by stress corrosion cracking are dominant effects that limit the operational condition and safe lifetime of the newest nuclear facilities. To study these phenomena and improve the current understanding of various aspects of radiation embrittlement, ion bombardment experiments are widely used as a surrogate for neutron irradiation. While avoiding the induced activity, typical for neutron-irradiated samples, is a clear benefit of the ion implantation, the shallow near-surface region of the modified materials may be a complication to the post-irradiation examination (PIE). However, microstructural defects induced by ion implantation can be effectively investigated using various spectroscopic techniques, including slow-positron beam spectroscopy. This method, typically represented by techniques of positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy, enables a unique depth-profile characterisation of the near-surface region affected by ion bombardment or corrosion degradation. One of the best slow-positron beam facilities is available at the pulsed low-energy positron system (PLEPS), operated at FRM-II reactor in Munich (Germany). Bulk studies (such as high energy ion implantation or neutron irradiation experiments) can be, on the other hand, effectively performed using radioisotope positron sources. In this paper, we outline some basics of the two approaches and provide some recommendations to improve the validity of the positron annihilation spectroscopy (PAS) data obtained on ion-irradiated samples using a conventional 22Na positron source.

Nano-Porous-Silicon Powder as an Environmental Friend.

Nano-porous silicon (NPS) powder synthesis is performed by means of a combination of the ultra-sonication technique and the alkali chemical etching process, starting with a commercial silicon powder. Various characterization techniques {X-ray powder diffraction, transmission electron microscopy, Fourier Transform Infrared spectrum, and positron annihilation lifetime spectroscopy} are used for the description of the product’s properties. The NPS product is a new environmentally friendly material used as an adsorbent agent for the acidic azo-dye, Congo red dye. The structural and free volume changes in NPS powder are probed using positron annihilation lifetime (PALS) and positron annihilation Doppler broadening (PADB) techniques. In addition, the mean free volume (VF), as well as fractional free volume (Fv), are also studied via the PALS results. Additionally, the PADB provides a clear relationship between the core and valence electrons changes, and, in addition, the number of defect types present in the synthesized samples. The most effective parameter that affects the dye removal process is the contact time value; the best time for dye removal is 5 min. Additionally, the best value of the CR adsorption capacity by NPS powder is 2665.3 mg/g at 100 mg/L as the initial CR concentration, with an adsorption time of 30 min, without no impact from temperature and pH. So, 5 min is the enough time for the elimination of 82.12% of the 30 mg/L initial concentration of CR. This study expresses the new discovery of a cheap and safe material, in addition to being environmentally friendly, without resorting to any chemical additives or heat treatments.

Can nano PZT-OPC composites be used as a smart γ-ray attenuator?

Nano PbZr0.52Ti0.48O3-ordinary Portland cement (PZT-OPC) composites were fabricated by a ball milling technique with different PZT content (30, 50, 70, and 90% by weight). The prepared samples were annealed at 600 °C. The structure, morphology, thermal and electrical properties of these annealed samples were investigated using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and thermal gravimetric analysis (TGA). The effects of PZT on dielectric and piezoelectric properties of the composites were investigated. γ-ray spectroscopy was used to measure the γ-ray attenuation coefficients of the annealed nanocomposite samples using the 661.7 keV γ-ray of 137Cs source. The microstructure changes of the prepared composite samples were investigated using the positron annihilation lifetime (PAL) and Doppler broadening (DB) techniques. The XRD patterns show both PZT and cement peaks. Also, these patterns indicate the presence of PZT retards the hydration process of the Portland cement. The FTIR spectra show the major absorption bands for both PZT and cement. SEM images show the binding between calcium silicate hydrate and PZT which may be a better factor for improving the mechanical properties of the cement. TGA curves show a higher thermal weight loss for 50% PZT-OPC composite sample. The PZT-OPC composite sample with 50% PZT content shows the highest γ-ray mass attenuation coefficient compared with the other composite samples. The positron annihilation spectroscopy (PAS) results show that the size of large voids and the concentration of small defects increase by increasing PZT content. Also, these results reveal that the core electrons slightly reduced by increasing PZT content. The results indicate that the dielectric constant and piezoelectric coefficient values slightly and rapidly increase for composite samples with PZT content <50% and >50%, respectively. Finally, the 50% PZT-OPC composite sample showed novel shielding and acceptable piezoelectric properties to be used as a smart shield.

Amidoxime-functionalized polymer of intrinsic microporosity (AOPIM-1)-based thin film composite membranes with ultrahigh permeance for organic solvent nanofiltration

Advanced organic solvent nanofiltration (OSN) membranes with high permeance and stability are highly needed in petrochemical and pharmaceutical industries. Recently, polymers of intrinsic microporosity (PIMs) have attracted significant attention for the development of advanced membranes, because of their high porosity and solution processability. In this study, a thin film composite membrane based on an amidoxime-functionalized PIM (AOPIM-1) was prepared by spin coating followed by solvent activation. The activation approach could dramatically improve the solvent permeance of the membrane without compromising its rejection performance. The changes in porosity of the AOPIM-1 layer before and after activation were confirmed by positron annihilation Doppler spectroscopy. The optimum membrane exhibited an impressively high ethanol permeance of 15.5 L m−2 h−1 bar−1, nearly one orders of magnitude higher than that of Starmem240 (a commercial polyimide-based OSN membrane) with a comparable rejection of Rose Bengal (MW 1017 Da). In addition, the present membrane exhibited outstanding stability in polar solvents and a remarkably high rejection of active pharmaceutical ingredients (vitamin B12) from ethanol. This work could provide a new avenue for developing highly permeable OSN membranes and is expected to stimulate the use of PIM membranes in OSN applications.

High-yield thermalized positronium at room temperature emitted by morphologically tuned nanochanneled silicon targets

Nanochanneled silicon targets with high positron/positronium (Ps) conversion rate and efficient Ps cooling were produced. Morphological parameters of the nanochannels, such as their diameter and length, were adjusted to get a large fraction of thermalized Ps at room temperature being emitted into vacuum. Ps cooling measurements were conducted combining single-shot positron annihilation lifetime spectroscopy and Doppler spectroscopy of the 13S → 23P transition. 2γ–3γ annihilation ratio measurements were also performed to estimate the positron/Ps conversion efficiency. In a converter with nanochannel diameter of 7–10 nm and depth of 3.89 μm, ∼28% of implanted positrons with an energy of 3.3 keV was found to be emitted as Ps with a transverse kinetic energy of 11 ± 2 meV. The reduction of the nanochannels depth to 1.13 μm, without changing the nanochannel diameter, was found to result in a less efficient cooling, highlighting the presence of Ps reflection from the bottom end of nanochannels.

Positron Annihilation Spectroscopy of Basaltic Rocks from Plio-Qaternary Volcanics, Sana’a-Amran Volcanic Field, Yemen

We focus a new application of positron annihilation lifetime spectroscopy and Doppler broadening techniques at the annihilation sites have been applied on some basaltic rocks from inactive volcano related to Sana’a-Amrana volcanic Field (SAVF). A brief geologic set up and the main petrographic signature of this inactive volcano are given. To gain an insight into geological environment, open nanospaces in the well-characterized basaltic rocks were investigated by positron annihilation spectroscopy. The analysis of the lifetime spectra yielded two lifetime components. The olivine basalt samples contain less mono- vacancies than those found in the trachybasalt and vesicular basalt samples. The overall defect (vacancy) concentration and its variation in the samples were estimated using the mean positron lifetime (τm). The Doppler broadening parameters (S and W-Parameters) were measured. A little variation of the S-parameter values has been observed for all the studied samples indicating that the valance electron contribution to annihilation is not significant.

Thermal kinetics of micro-defects in He-ion implanted W and W5Re alloys

To investigate the thermal evolution of vacancy-type defects in He-ion irradiated W and W5Re alloy, different isochronal annealing treatments from 373 to 1273 K were conducted on the irradiated materials. Positron annihilation spectroscopy including positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy were mainly used to characterize the micro-defects evolution. The results showed that the thermal evolution characteristics of defects in both W and W5Re were similar. After He-ion irradiation, mono-vacancies with positron annihilation lifetime of ~ 190 ps were detected in W, together with a large amount of dislocation loops with positron annihilation lifetime of ~ 150 ps in W5Re alloys. The coarsening of vacancy clusters at the expense of small vacancy clusters was the main thermal evolution feature of vacancy-type defects in both W and W5Re when annealing temperature increased to 1073 K. In this progress, the positron annihilation lifetime increased to ~ 350 ps (clusters composed of 4 –8 mono-vacancies) in both W and W5Re. As the temperature increased to 1273 K, the positron annihilation lifetime decreased to ~ 240 ps, which was attributed to a significant population reduction of the dislocation loops, the dissociation of large HenVm complexes and the annealing of micro-voids in both W and W5Re. The vacancy-type defects in W5Re were more susceptible to the annealing temperature because of the formation of vacancy cluster-Re complexes. Re clusters in irradiated W5Re alloy could serve as the nucleation sites of He bubbles, which promoted the swelling and protrusion formation on the surface.

Recent Progress on the Positron Annihilation Experiments on Irradiated Ferritic/Martensitic Steels Containing Helium

The well-known and often acceptable radiation tolerance of ferritic/martensitic (f/m) steels can be severely diminished when neutron irradiation is accompanied by the production of helium. The presence of helium in the irradiated materials changes the kinetics of the nucleation, recombination, and clustering of the radiation-induced defects. High production rates of helium may lead to a non-negligible volumetric bubble swelling at relatively low temperatures. Extrapolation of the knowledge gained from neutron irradiation experiments to fusion or spallation environments is additionally complicated due to the unknown and comprehensive effects of dpa rate, temperature, the presence of sinks in the crystal lattice and others. To improve the understanding of the microstructure and irradiation parameters effects, close attention must be paid to the early stages of the radiation damage. It is expected that the pre-existing vacancy-type defects, attributed to lattice distortion at the grain/subgrain boundaries and oxide-matrix interfaces, are effective sinks for primary defects and helium, i.e. they control the formation and growth of helium-vacancy agglomerations. This early-stage radiation damage, however, cannot be captured by conventional transmission electron microscopy, and thus other experimental techniques are called for. One of the most perspective experimental approaches to investigate small vacancy-type defects, with a high sensitivity to confined helium, is to utilize positron annihilation spectroscopy (PAS). In particular, two spectroscopy techniques, positron annihilation lifetime spectroscopy (PALS) and Doppler broadening spectroscopy (DBS) of the annihilation line, can be beneficially used for the characterization of helium-vacancy clusters. This paper reviews the recent positron annihilation spectroscopy characterization of various irradiation experiments involving helium. Mainly two types of irradiation experiments are addressed, helium implantation and spallation neutron source irradiation experiments. Discussion is aimed at the potential of PAS in the early-stage formation of helium bubbles and the investigation of the effects of irradiation parameters in defect production and accumulation.

The positron and mechanical parameters of a cold-worked aluminum alloy (3004) Using PALT, PADBT and HV**

Abstract In the present work, the influence of plastic deformation on the properties of a 3004 Al-alloy was studied with different techniques. Crystallite size, dislocation density, defect density, micro-strain, and stored dislocation energy are presented and compared for different three techniques. Methodology Many techniques for detecting defects have been developed, such as the positron annihilation lifetime technique (PALT), the positron annihilation Doppler broadening technique (PADBT), and the Vickers hardness test (HV). Implications The positron mean lifetime value of a non-deformed sample is 173±4.8 ps, which increases until the thickness reduction reaches a 10% deformation then saturated at saturation trapping of the positron in defect states with a mean of 221±5 ps. At an S-parameter of 0.3709±0.0031, a W-parameter of 0.5885±0.0057 was obtained at zero deformation: this decreases until saturation at 10% deformation. Findings A good correlation between the three techniques is observed for mean crystallite size. A good correlation was also noted between PALT and PADBT from 0 to 10% thickness reduction. HV has good correlations with PALT and PADBT from 0 to 6% thickness reduction: then a clear difference was found from 6 to 15% thickness reduction. Originality such results confirm the fact that the used technique can effects on the obtained results in some limitations.

Blister-dominated retention mechanism in tungsten exposed to high-fluence deuterium plasma

To investigate the effect of blistering on hydrogen isotope (HI) retention, a series of deuterium plasma exposures were performed using recrystallized tungsten samples at 500 K with high fluences up to 1.0 × 1028 ions m−2 in the linear plasma device STEP. An increase of blister density and deuterium retention was observed with increasing plasma fluence. Based on the simulation of the thermal desorption spectra using TMAP, defects with different detrapping energies are found to be located at a depth of tens of microns, which coincides with the depth of the grain boundaries (GBs) close to the surface. The defect characterizations using transmission electron microscopy and positron annihilation Doppler broadening identified the defects as dislocation type and vacancy type, which were created by blistering. It is suggested that these defects can diffuse deep into the material, and the interaction between the diffusion of the defects and GBs causes a peculiar deuterium desorption spectrum over plasma fluences. Additionally, these blister-induced defects are the main source of deuterium retention. Regarding the effect of the blister-induced defects on deuterium retention, a blister-dominated retention mechanism is proposed to describe HI retention in conditions when blistering is severe as in this study. This investigation provides a new insight into the effect of blistering on retention and the modelling of retention in a tokamak edge plasma environment.

Physical Properties of ZnS Thin Films Prepared by Low-temperature Sulfidation

In the article, Zn thin films were deposited on the glass substrates at room temperature by magnetron sputtering from a zinc target. And ZnS thin films were prepared by annealing Zn thin films in sulfur vapor and Ar gas at 200 and 400 ℃. The microstructure defects, crystallizations, surface morphology and optical properties of the samples were analyzed by PAT(positron annihilation technique), XRD(X-ray diffraction), SEM(scanning electron microscopy) and UV-VIS spectrophotometer. The resultant ZnS thin films exhibited a high optical transmittance of about 80% in the visible region. With the increase of sulfidation time, the band-gap value was increased from 3.55 to 3.57 eV, and the S/Zn atomic ratio was enhanced from 0.54 to 0.89, implying an obvious improvement of ZnS film quality. This demonstrated that the excess-sulfur problem in the ZnS films was well solved compared with those samples prepared by sulfidation in the vacuum-sealed quartz-glass ampoules. Besides, the structural defects of the thin films before and after sulfidation were investigated by positron annihilation Doppler broadening measurements. It was found that the S parameter of the samples after sulfidaton was greater than that before sulfidation, implying the higher structural defect concentration for the former.

Synergistic CO2‐Sieving from Polymer with Intrinsic Microporosity Masking Nanoporous Single‐Layer Graphene

AbstractHigh‐flux nanoporous single‐layer graphene membranes are highly promising for energy‐efficient gas separation. Herein, in the context of carbon capture, a remarkable enhancement in the CO2 selectivity is demonstrated by uniquely masking nanoporous single‐layer graphene with polymer with intrinsic microporosity (PIM‐1). In the process, a major bottleneck of the state‐of‐the‐art pore‐incorporation techniques in graphene has been overcome, where in addition to the molecular sieving nanopores, larger nonselective nanopores are also incorporated, which so far, has restricted the realization of CO2‐sieving from graphene membranes. Overall, much higher CO2/N2 selectivity (33) is achieved from the composite film than that from the standalone nanoporous graphene (NG) (10) and the PIM‐1 membranes (15), crossing the selectivity target (20) for postcombustion carbon capture. The selectivity enhancement is explained by an analytical gas transport model for NG, which shows that the transport of the stronger‐adsorbing CO2 is dominated by the adsorbed phase transport pathway whereas the transport of N2 benefits significantly from the direct gas‐phase transport pathway. Further, slow positron annihilation Doppler broadening spectroscopy reveals that the interactions with graphene reduce the free volume of interfacial PIM‐1 chains which is expected to contribute to the selectivity. Overall, this approach brings graphene membrane a step closer to industrial deployment.

Study the Properties of 6061 Aluminum Alloy which used as Cladding for Nuclear Reactor Fuel by Using Positron Annihilation Doppler Broadening Technique .

Doppler ­ broadening spectroscopy is a technique that is used to know the properties of matter by means of gamma quanta. The entire energy of an annihilation positron – electron pair includes the energy according to its mass (= 2 x 511 Kev) and kinetic energy corresponding to the motion of the particles causes an energy shift of the annihilation line. The line shape has two parameters S and W parameters,but the W parameter is more sensitive to the chemical surrounding of the annihilation site than the S parameter, because the core electron having high momentum are contributing. Mainlyin the region of large deviations from the annihilation energy of 511 kev. Both the S and w parameter are sensitive to the concentration and kind of defect.The goal of our work is to investigate the dependence of annihilation parameters on crystal defect the content of Mg atoms in a series of commercial Al – alloys by Doppler broadening technique . Position parameter like (crystallite size – trapping rate – defect density – dislocation density Storedenergy).

Positron annihilation spectroscopy study of vacancy-type defects in He implanted polycrystalline α-SiC

The evolution of He implantation-induced defects in polycrystalline α-SiC has been investigated by a combination of positron annihilation Doppler broadening spectroscopy (DBS) and transmission electron microscopy (TEM). Samples implanted at room temperature with 230 keV He+ ions to fluences of 1 × 1015, 5 × 1015 and 1 × 1016 ions/cm2 were isochronally annealed in vacuum at temperatures up to 1400 °C with a step of 100 °C. Excellent correlation between the calculated damage profile and the positron annihilation depth profile were obtained at low annealing temperatures, after considering the positron depth distribution given by so-called Makhovian profiles.The DBS data from the peak region revealed a reasonable correlation between positron trapping at vacancy-type defects and the He-to-dpa ratio. Thermal annealing at 1400 °C resulted in the incomplete recovery of the microstructure, where large cavities distributed along grain boundaries in the damage peak region are surrounded by small agglomerations of (helium)vacancy-type defects.

Ti3AlC2, a candidate structural material for innovative nuclear energy system: The microstructure phase transformation and defect evolution induced by energetic heavy-ion irradiation

For the structure materials applied in the innovative nuclear energy system, the strongly environment radiation source is always a big concern which will severely degrade the materials performance especially at high temperature. To explore the mechanisms of the anti-irradiation properties in Ti3AlC2, a typical MAX phase material showing excellent irradiation damage tolerance and resistance to amorphization, we conducted a series of 1 MeV C4+ ions irradiation experiments on them at different temperatures (RT, 300°C, 500°C and 800°C). Through Grazing Incidence X-ray Diffraction (GIXRD), Raman spectra (Raman), slow positron annihilation Doppler Broadening Spectroscopy (DBS) and high resolution Transmission Electron Microscopy (HRTEM), the anti-irradiation properties were systematically investigated. For the first time, an entire microstructure phase transformation process of Ti3AlC2 from α to β to γ and to perfect fcc structure phase induced by irradiation at RT and it is inverse (recovery) process of phase transformation under high temperature (≥300 °C) irradiation conditions are found and confirmed. And lots of simple vacancies are induced by irradiation and the density of them gets saturated above 5 × 1015 ions/cm2 fluences. These processes of phase transformation and recovery and vacancy saturation phenomenon are the primary reasons for why Ti3AlC2 has excellent irradiation damage tolerance and resistance to amorphization. In addition, the micro strain and lattice parameters are also affected by microstructure transformation and have been discussed. In a word, Ti3AlC2 materials show good anti-irradiation properties especially at high temperature, and now it is a primary candidate as the coating of the cladding material and the spallation target beam windows material in Chinese ADS project. And studies on this kind of materials provide a promising new concept and way for designing the structural materials for innovative nuclear energy system.

Characterization of Enzymatically Synthesized Titania Thin Films Using Positron Annihilation Spectroscopy Reveals Low‐Cost Approach for Organic/Inorganic Photovoltaic Cells

AbstractA new method is developed to produce mesoporous titania thin films at room temperature using the enzyme papain in a dip‐coating procedure, providing low‐cost titania films in a sustainable manner. Quartz crystal microbalance, positron annihilation Doppler broadening and lifetime spectroscopy, scanning electron microscopy, and X‐ray diffraction are used to determine the deposition and structural properties of the films. As‐deposited films have low densities ρ ≈ 0.6 g cm−3, contain small micropores and proteins, and exhibit corrugated surfaces. Annealing at temperatures of 300 °C or higher leads to the destruction and evaporation of most of the organic material, resulting in a thickness decrease of 50–60%, more pure titania films with increased density, an increase in micropore size and a decrease in the concentration and size of atomic‐scale vacancies. Up to 50 layers could be stacked, allowing easy control over the total layer thickness. Based on these titania films, first test devices consisting of natural dye‐sensitized solar cells are produced, that show photovoltaic activity and indicate possibilities for low‐cost, accessible, organic production of solar cells. Given the wide range of other applications for titania, this new method is a promising candidate for improving the fabrication of those products with respect to cost, sustainability, and production speed.

Vacancy cluster growth and thermal recovery in hydrogen-irradiated tungsten

The thermal evolution of vacancies and vacancy clusters in tungsten (W) has been studied. W (100) single crystals were irradiated with 200 keV hydrogen (H) ions to a low damage level (5.8×10−3 dpa) at 290 K and then annealed at temperatures in the range of 500–1800 K. The resulting defects were characterized by positron annihilation lifetime spectroscopy (PALS) and positron annihilation Doppler broadening spectroscopy (DBS). Annealing at 700 K resulted in the formation of clusters containing 10–15 vacancies, while at 800 K and higher temperatures clusters containing about 20 vacancies or more were formed. Reduction of the defect concentration likely accompanied by further coarsening of the clusters started at 1300 K and ended at 1800 K with the complete defect recovery. The determined cluster sizes at 700 K and 800 K were larger than the estimated minimum cluster sizes that are thermally stable at these temperatures, indicating that the migration and ensuing coalescence of small clusters plays an important role in cluster growth.