Doppler Broadening Positron Annihilation Spectroscopy Research Articles

Damage recovery stages revisited: Thermal evolution of non-saturated and saturated displacement damage in heavy-ion irradiated tungsten

Establishing the correlation between damage recovery and temperature in nuclear materials is vital to understanding material property degradation. Among the handful of topics related, one long-standing concern is the differences in damage recovery caused by different initial defect concentrations, with respect to ranges below and beyond the damage saturation limit. In this study, the temperature dependence of damage recovery has been comprehensively elucidated for pure tungsten, irradiated with 6 MeV copper ions at room temperature (RT), up to 0.05 dpa (non-saturated damage, n-SD) and 0.6 dpa (saturated damage, SD), and followed by isochronal annealing. Coupled transmission electron microscopy and Doppler broadening positron annihilation spectroscopy characterizations provided a full-probe-range picture of defect evolution. The varied initial defect concentrations caused retaining differences in damage recovery, but no substantial influence on temperature-dependent defect evolution. Moderate and dramatic damage recovery were identified below 973 K and above 1123 K in n-SD and SD tungsten, respectively. Most strikingly, nano-sized voids were confirmed at 473 K in both samples. Annealing at 1873 K saw the complete removal of defects in n-SD tungsten, but survival of a few dislocations via void pinning in SD tungsten. Full damage recovery in SD tungsten was projected to be ∼2000 K. The current revisit of damage recovery stages demonstrated the significance of encompassing scenarios of non-saturated and saturated defect concentrations. Accordingly, four damage recovery stages were redefined for heavy-ion irradiated tungsten, namely stage I (20 K–RT), stage II (RT–653 K), stage III (653–1123 K), and stage IV (1123–2000 K).

Influence of defect dynamics on the nanoindentation hardness in NiCoCrFePd high entropy alloy under high dose Xe+3 irradiation

We report the role of irradiation-induced defects and microstructure evolution in the wake of 1.05 MeV Xe3+ ion irradiation on the structural and mechanical properties of single-phase face-centered cubic (FCC) structured NiCoCrFePd high-entropy alloy (HEA). The defect evolution was investigated using positron annihilation Doppler broadening spectroscopy (PADBS), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), and nanoindentation techniques. Single-phase FCC structured NiCoCrFePd HEA remained structurally stable even upon irradiation to a very high fluence of 9 × 1016 ions/cm2. However, an anomalous reduction in the lattice constant, along with micro-strain relaxation and crystallite fragmentation was observed at the initial ion fluence of 1 × 1016 ions/cm2, which are recovered upon irradiation at the higher ion fluence due to collapse of small-sized point defects into the large-sized dislocations and simultaneously irradiation-induced recrystallization. Ion fluence-dependent PADBS analysis revealed the formation of mono-vacancies at lower fluence which were saturated by recombination and evolution to large-size defects at successive higher ion fluence that is consistent with electron microscopic investigations. Measurement of mechanical properties showed that the hardness was initially increased by 57% upon irradiation at fluence 1 × 1016 ions/cm2 and then it was slightly reduced (∼5%) at a higher fluence of 9 × 1016 ions/cm2 irradiation due to defect recombination and recrystallization which clearly shows the resistance of this HEA towards hardening. Thus, the present study provides a deeper understanding of the defect dynamics and their relation to the mechanical behavior of NiCoCrFePd HEA, which is important for the development of radiation-resistant alloys for nuclear energy systems.

Characterisation of defects in porous silicon as an anode material using positron annihilation Doppler Broadening Spectroscopy

Here we present Positron Annihilation Doppler Broadening Spectroscopy (PADBS) as a powerful method to analyse the origin and development of defect processes in porous silicon structures as a result of alloying with lithium for the use in battery anode applications. Several prepared anodes were lithiated (discharged against Li+/Li) and de-lithiated (charged) with different capacities followed by a distinct treatment procedure and an analysis using the Delft Variable Energy Positron Beam. The results presented here show that we can distinguish two different processes attributed to (1) structural changes in silicon as a result of the alloying process, and (2) the formation of defects that initiate degradation of the material. The limit at which the porous material can be used for at least the first two cycles without the occurrence of damage can thus be accurately determined by using the PADBS technique.

Formation of vacancies and metallic-like domains in photochromic rare-earth oxyhydride thin films studied by in-situ illumination positron annihilation spectroscopy

Rare-earth (RE) oxyhydride thin films show a color-neutral, reversible photochromic effect at ambient conditions. The origin of the photochromism is the topic of current investigations. Here, we investigated the lattice defects, electronic structure, and crystal structure of photochromic ${\mathrm{YH}}_{x}{\mathrm{O}}_{y}$ and $\mathrm{Gd}{\mathrm{H}}_{x}{\mathrm{O}}_{y}$ thin films deposited by magnetron sputtering using positron annihilation techniques and x-ray diffraction, in comparison with Y, ${\mathrm{YH}}_{\ensuremath{\sim}1.9}$, ${\mathrm{Y}}_{2}{\mathrm{O}}_{3}$, Gd, $\mathrm{Gd}{\mathrm{H}}_{\ensuremath{\sim}1.8}$, and ${\mathrm{Gd}}_{2}{\mathrm{O}}_{3}$ films. Positron annihilation lifetime spectroscopy (PALS) reveals the presence of cation monovacancies in the as-deposited Y and ${\mathrm{YH}}_{\ensuremath{\sim}1.9}$ films at concentrations of $\ensuremath{\sim}{10}^{\ensuremath{-}5}$ per cation. In addition, vacancy clusters and nanopores are found in the as-prepared ${\mathrm{YH}}_{x}{\mathrm{O}}_{y}$ and ${\mathrm{Y}}_{2}{\mathrm{O}}_{3}$ films. Doppler broadening positron annihilation spectroscopy (DB-PAS) of the Y- and Gd-based films reflects the transition from a metallic to an insulating nature of the RE metal, metal hydride, semiconducting oxyhydride and insulating oxide films. In-situ illumination DB-PAS shows the irreversible formation predominantly of di-vacancies, as PALS showed that cation mono-vacancies are already abundantly present in the as-prepared films. The formation of di-vacancies supports conjectures that ${\mathrm{H}}^{\text{--}}$ (and/or ${\mathrm{O}}^{2\text{--}}$) ions become mobile upon illumination, as these will leave anion vacancies behind, some of which may subsequently cluster with cation vacancies present. In addition, in RE oxyhydride films, partially reversible shifts in the Doppler parameters are observed that correlate with the photochromic effect and point to the formation of metallic domains in the semiconducting films. Two processes are discussed that may explain the formation of these metallic domains and the changes in optical properties associated with the photochromic effect. The first process considers the reversible formation of metallic nanodomains with reduced O : H composition by transport of light-induced mobile hydrogen and local oxygen displacements. The second process considers metallic nanodomains resulting from the trapping of photoexcited electrons in an ${e}_{g}$ orbital at the yttrium ions surrounding positively charged hydrogen vacancies that are formed by light-induced removal of hydrogen atoms from octahedral sites. When a sufficiently large concentration, on the order of \ensuremath{\sim}10%, is reached in a certain domain of the film, band formation of the ${e}_{g}$ electrons may occur, leading to an Anderson-Mott insulator-metal transition like the case of yttrium trihydride in these domains.

Vacancy‐Type Defects in H+6%He Neutral Beam Irradiated WK Alloy Probed by Slow Positron Beam

H+6%He mixed neutral beam with a power density of 8 MV m−2 and fluence of 1025 m−2 is produced to induce irradiation damage at surface temperatures of 400, 800, and 1300 °C in WK alloy, and at 400 and 800 °C in pure W samples. Doppler‐broadening positron annihilation spectroscopy (DB‐PAS) and X‐ray diffraction (XRD) are used to probe the evolution of vacancy‐type defects in the samples after irradiation. The results show that for the WK alloy, the S‐parameters for the 400, 800, and 1300 °C samples increase sequentially because of the increase in vacancy‐type defect concentrations or volumes. The WK sample at 1300 °C exhibits a wider defect damage layer and more complicated vacancy‐type defects than those in the WK samples at 400 and 800 °C. For pure W, the S‐parameter of the 800 °C samples is larger than that of the 400 °C samples, which is attributed to the aggregation and growth of vacancy‐type defects. Based on a comparison of the WK and pure W samples, the pinning effect of K bubbles in the WK samples has an influence on the transportation of H/He atoms and the migration and aggregation of vacancy‐type defects.

Zinc Oxide Defect Microstructure and Surface Chemistry Derived from Oxidation of Metallic Zinc: Thin-Film Transistor and Sensor Behavior of ZnO Films and Rods.

Zinc oxide thin films are fabricated by controlled oxidation of sputtered zinc metal films on a hotplate in air at temperatures between 250 and 450 °C. The nanocrystalline films possess high relative densities and show preferential growth in (100) orientation. Integration in thin‐film transistors reveals moderate charge carrier mobilities as high as 0.2 cm2 V−1s−1. The semiconducting properties depend on the calcination temperature, whereby the best performance is achieved at 450 °C. The defect structure of the thin ZnO film can be tracked by Doppler‐broadening positron annihilation spectroscopy as well as positron lifetime studies. Comparably long positron lifetimes suggest interaction of zinc vacancies (VZn) with one or more oxygen vacancies (VO) in larger structural entities. Such VO‐VZn defect clusters act as shallow acceptors, and thus, reduce the overall electron conductivity of the film. The concentration of these defect clusters decreases at higher calcination temperatures as indicated by changes in the S and W parameters. Such zinc oxide films obtained by conversion of metallic zinc can also be used as seed layers for solution deposition of zinc oxide nanowires employing a mild microwave‐assisted process. The functionality of the obtained nanowire arrays is tested in a UV sensor device. The best results with respect to sensor sensitivity are achieved with thinner seed layers for device construction.

Vacancy-Hydrogen Interaction in Niobium during Low-Temperature Baking

A recently discovered modified low-temperature baking leads to reduced surface losses and an increase of the accelerating gradient of superconducting TESLA shape cavities. We will show that the dynamics of vacancy-hydrogen complexes at low-temperature baking lead to a suppression of lossy nanohydrides at 2 K and thus a significant enhancement of accelerator performance. Utilizing Doppler broadening Positron Annihilation Spectroscopy, Positron Annihilation Lifetime Spectroscopy and instrumented nanoindentation, samples made from European XFEL niobium sheets were investigated. We studied the evolution of vacancies in bulk samples and in the sub-surface region and their interaction with hydrogen at different temperature levels during in-situ and ex-situ annealing.

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.

Point defects in BaSi2 thin films for photovoltaic applications studied by positron annihilation spectroscopy

Barium di-silicide (BaSi2) is a very promising absorber material for high-efficiency thin-film solar cells, due to its suitable bandgap, high light absorption coefficient, and long minority-carrier lifetime. In this study, we compare the nanostructure, layer composition, and point defects of BaSi2 thin films deposited by Radio Frequency (RF) sputtering, Thermal Evaporation (TE), and Molecular Beam Epitaxy (MBE), using Doppler Broadening Positron Annihilation Spectroscopy (DB-PAS) depth profiling, Raman spectroscopy, and x-ray diffraction. Our DB-PAS study on thermally annealed RF-sputter deposited and on TE-deposited BaSi2 layers, in a comparison with high quality BaSi2 films produced by MBE, points to the presence of vacancy-oxygen complexes and Si or Ba mono-vacancies, respectively, in the (poly)crystalline BaSi2 films. The degree of near-surface oxidation increases, going from MBE and TE to the industrially applicable RF-sputtered deposition synthesis. The use of a-Si capping layers on the thermally annealed RF-sputtered BaSi2 films leads to a clear reduction in sub-surface oxidation and improves the quality of the BaSi2 films, as judged from DB-PAS.

Evolution of vacancy defects in heavy ion irradiated tungsten exposed to helium plasma

Evolution of vacancy-type defects has been investigated in undamaged and copper ion pre-damaged tungsten exposed to low-energy and high-flux helium plasma (60 eV, 1 × 1022 He/m2s). The results measured by Doppler broadening positron annihilation spectroscopy (DB-PAS) indicate that helium-vacancy complexes generate due to intense self-trapping in the undamaged tungsten after helium plasma exposure. In contrast, the occupation of pre-existing vacancies and vacancy clusters, caused by the presence helium atoms, plays a dominant role in the pre-damaged tungsten, but the density of vacancy-type defects in the pre-damaged case is still higher than that in the undamaged case. This means that the dominating process of helium-vacancy complexes formation in tungsten changes from self-trapping to vacancy-trapping in the case of high vacancy density. Meanwhile, the elastic recoil detection analysis (ERDA) and transmission electron microscopy (TEM) results reveal that these pre-existing defects can increase helium retention and helium nano-bubble size. However, it is surprising that in the case of pre-damaged sample, the density/volume of vacancy-type defects also decreased even outside of the helium distribution depth. We attribute this phenomenon to the interstitial dislocation loops punched by helium clusters preferentially diffusing into the pre-damage regions of tungsten far beyond the helium distribution depth, resulting in the significant recombination with vacancies or vacancy clusters. This intrinsic mechanism is further verified by TEM observations and molecular dynamics (MD) simulations.

Photochromic YOxHy Thin Films Examined by <i>in situ</i> Positron Annihilation Spectroscopy

Doppler broadening positron annihilation spectroscopy depth profiles were collected on photochromic YOxHy thin films. In situ UV illumination of photochromic semiconductor YOxHy films leads to an increase in S-parameter and a large reduction in W-parameter, possibly caused by a change in the charge state of vacancies or the growth of hydrogen-rich metallic Y(Ox)Hy clusters, albeit that vacancy formation or changes in positronium formation during illumination might also play a role. Intriguingly, both the S- and W-parameters increase during thermal bleaching, indicating that another process takes place. The Doppler parameters do not return to their initial values after complete thermal bleaching, suggesting that persistent local rearrangements of vacancies and possibly hydride ions have occurred during the full photodarkening-thermal bleaching cycle. Positron annihilation lifetime spectroscopy shows that a small fraction of positronium is formed in as-deposited YOxHy films, indicating that the films contain some nanopores.

Ion irradiation induced changes in defects of iron thin films: Electron microscopy and positron annihilation spectroscopy

Single crystal Fe thin films (∼250 nm) were grown on MgO substrates and irradiated with 2.0 MeV Fe+ ions at 10 and 50 dpa, and the defect evolution was studied using high resolution Transmission Electron Microscopy (HR-TEM) and Doppler Broadening Positron Annihilation Spectroscopy (PAS). It was shown that irradiation induced or exacerbated a thin oxide layer at the outer interface and produced substantial Fe/Mg mixing at the film/substrate interface, particularly for the higher dose. Modeling of the PAS data allowed interpretation of the defect types at different distances from the Fe surface, and included several types of MgO substrate damage and annihilation condition changes, indicative of damage due to ballistic effects of the Fe atoms as well as chemical changes due to implantation and subsequent diffusion. This detailed PAS study compared with TEM and energy dispersive spectroscopy (EDS) provides significant insight into depth-dependent defect creation. These results will be useful for predicting defect creation in Fe-based materials under irradiation conditions, for extension to neutron irradiated structural materials.

The effect of dislocations on MnNi-rich clusters in self-ion irradiated FeMnNi alloy

The effect of dislocations on irradiation-induced MnNi-rich clusters in self-ion irradiated FeMnNi alloy was investigated using X-ray diffraction (XRD), nanoindentation, positron annihilation Doppler broadening spectroscopy (PADBS), and atom probe tomography (APT). The nanoindentation results showed that irradiation-induced hardening decreased with dislocation density increasing. The results of APT and PADBS revealed that high-energy Fe-ion irradiation generates high-density MnNi-rich clusters and large vacancy-type defects in irradiated FeMnNi alloy. Note that dense dislocations suppress the formation of large vacancy-type defects and MnNi-rich clusters. By analyzing the effect of dislocations on point defects (PDs), diffusion of Mn and Ni atoms, and nucleation of MnNi-rich clusters, we found that nucleation mechanism of MnNi-rich clusters is strongly dependent on the flux of PDs. However, high-density dislocations can facilitate to annihilate irradiation-induced PDs and cause the decline of defect-solute coupling fluxes, which are responsible for a reduction in number density and mole fraction of MnNi-rich clusters.

On the effect of atomic layer deposited Al2O3 on the environmental degradation of hybrid perovskite probed by positron annihilation spectroscopy

Doppler broadening-positron annihilation spectroscopy (DB-PAS) provides insights into the structural degradation of pristine and ALD Al2O3 capped CH3NH3PbI3−xClx under ambient air exposure.

Radiation-induced changes of vacancy-type defects in ferroelectric capacitors as revealed by Doppler broadening positron annihilation spectroscopy

Thin film ferroelectric capacitors of composition Pb(Zr0.52Ti0.48)O3 were exposed to Fe3+ radiation (1011 to 1013 ions/cm2), and the change in the defect structure was investigated by Doppler broadening positron annihilation spectroscopy and other characterization techniques. As the radiation fluence increases, a systematic drop of the S parameter of the positron annihilation photopeak is observed and attributed to an increase in the Zr- and Ti-site related vacancies relative to the Pb-sites. The results demonstrate that the radiation has a more significant influence on the Zr- and Ti-sites relative to the Pb-sites. It is also observed that the S parameter of the Mn-doped samples is higher than the undoped counterparts. Coupled with ferroelectricity measurements and X-ray diffraction, the results suggest that the Mn dopant modifies the initial structure of the material and leads to a different functional response.

Investigation of X-ray-induced Defects on Metals and Silicon by Using Coincidence Doppler Broadening Positron Annihilation Spectroscopy

The mechanical properties of Al, Ti, Fe, and Cu metals p-type Si, and n-type Si were investigated by using coincidence Doppler broadening (CDB) positron annihilation spectroscopy. The samples in this experiment were irradiated by using X-rays at generating powers for up to 9 kW. The data taken after the irradiation showed all the characteristic features predicted from defects with vacancies. The S parameter values of the metals were generally less than those of semiconductors such as p-type Si and n-type Si. The relationship between n-type Si and p-type Si were more affected when n-type Si rather than p-type Si was irradiated with X-rays.

Evolution and role of vacancy clusters at grain boundaries of ZnO:Al during accelerated degradation of Cu(In,Ga)Se2 solar cells revealed by positron annihilation

Positron annihilation lifetime spectroscopy (PALS) and Doppler broadening positron annihilation spectroscopy DB-PAS) depth profiling demonstrate pronounced growth of vacancy clusters at the grain boundaries of as-deposited Al-doped ZnO films deposited as transparent conductive oxide (TCO) on Cu(In, Ga)Se2 (CIGS) solar cells upon accelerated degradation at 85 ◦C/85% relative humidity. Quantitative fractions of positrons trapped either in the vacancy clusters at the grain boundaries or in Zn monovacancies inside the grains of ZnO:Al were obtained by detailed analysis of the PALS data using a positron trapping model. The time and depth dependence of the positron Doppler depth profiles can be accurately described using a planar diffusion model, with an extracted diffusion coefficient of 35 nm2/hour characteristic for in-diffusion of molecules such as H2O andCO2 into ZnO:Al TCO films via the grain boundaries, where they react with the ZnO:Al. This leads to increased open volume at the grain boundaries that imposes additional transport barriers and may lead to charge carrier trapping and nonradiative recombination. Simultaneously, a pronounced increase in series resistance and a strong reduction in efficiency of the ZnO:Al capped CIGS solar cells is observed on a remarkably similar timescale. This strongly indicates that these atomic-scale processes of molecular in-diffusion and creation of open volume at the grain boundaries play a key role in the degradation of the solar cells. PhySH: Solar Cells, Positron Annihilation Spectroscopy, Grain Boundaries, Vacancies, Thin Films, Diffusion, Electrical Properties, Solid State Chemistry, Optoelectronics

Effects of stress-relief pre-annealing on deuterium trapping and diffusion in tungsten

Positron annihilation Doppler broadening spectroscopy (PA-DBS) with slow positrons was utilized to characterize the change of vacancy-type defects induced by a stress-relief annealing (1273K for 1h in vacuum) as pre-annealing can remove not only residual stresses but also most intrinsic defects of the material. Deuterium behavior after low energy (40eV/D) and high-flux (1024D/m2s) plasma exposure was analyzed by a combination of PA-DBS, elastic recoil detection analysis (ERDA) and thermal desorption spectroscopy (TDS). It is found that the vacancy defects density in tungsten is largely decreased by annealing and some intrinsic vacancies clusters to be multi-vacancies during the annealing process. With deuterium plasma exposure, a general decrease of S parameter found in both non- and pre-annealed tungsten suggests the deuterium occupation of vacancy-type defects. In combination of the S parameter change in the non- and pre-annealed tungsten, it is implied that deuterium trapping ability of vacancy-type defects is constrained due to the residual stresses. This confinement effect concentrates on the top surface and it becomes weaker with increasing depth. A similar deuterium distribution in the near surface of non- and pre-annealed tungsten supports this confinement effect. In addition, comparing the amount of deuterium retained in the near surface and in the bulk suggests that the residual stress facilitate deuterium to diffuse into a larger depth in tungsten.

The Evolution Behavior of Defects in the Nanofilms of W/Cu and W Probed by Doppler Broadening Positron Annihilation Spectroscopy

W/Cu multilayer nanofilms and pure W nanofilms were prepared in pure Ar and He/Ar mixing atmosphere by radio frequency magnetron sputtering method. The defect evolution of the samples was characterized by Doppler broadening positron annihilation spectroscopy (DB-PAS).The results show that plenty of defects can be produced by introducing helium (He) into W/Cu multilayer nanofilms. With the natural storage time increasing, the helium located in the near surface of W/Cu multilayer nanofilm would be released gradually and induce the coalescence of the helium related defects due to the diffusion of the helium and defects. In addition, the pure W nanofilms were irradiated by 30 keV helium ions and 40 keV hydrogen (H) ions in sequence at room temperature. From the DB-PAS analysis, it can be shown that a large number of vacancy-type defects are produced due to the He and/or H irradiation. H ions would be trapped by He related defects and produced He-H-V complexes.

Evolution of the Helium-Related Defect in Polycrystalline Tungsten

The evolution of the helium-related defect in polycrystalline tungsten was studied by Doppler Broadening Positron Annihilation Spectroscopy and Scanning Electron Microscope as functions of annealing temperature and implantation fluence. The experimental results showed that the defect type was not changed at lower annealing temperature, and the decline of S parameter manifested that the lower temperature annealing led the decrease of defect concentration. In addition there existed the same defect type with the increment of the implantation fluence, thus the rise of S parameter implied the increase of the defect concentration.