Defect Characterization Using Positron Annihilation Spectroscopy on Laser-Ablated Surfaces

The PULSTAR is a 1-MWth nuclear research reactor located at North Carolina State University. It is fueled by uranium dioxide assemblies enriched to 4% or 6% in U-235 and is currently under licensing for operation at 2-MWth power. The PULSTAR is a center for irradiation testing and pre/post irradiation examination of materials. Among its unique capabilities are positron annihilation spectroscopy (PAS) and neutron powder diffraction (NPD) facilities. The PAS facility provides an intense positron beam reaching 6 × 108 e+/s, which drives two spectrometers; the e+-PAS and the Ps-PAS, used for studies of defects in thin film materials. A Na-22 bulk PAS system is also operational, which is used for studying millimeter scale materials. All spectrometers are capable of performing Doppler Broadening Spectroscopy (DBS) and Positron Annihilation Lifetime Spectroscopy (PALS). To date, the PAS systems have been used to characterize various materials (unirradiated and irradiated) that include graphite, soft matter, and metal-organic frameworks (MOF). The NPD facility uses a double focusing single crystal silicon rotating monochromator producing neutron beams with different energies. A position sensitive detection bank covers a scattering angle of 5°-125°. The facility is used in the examination of unirradiated and irradiated materials including graphitic materials, magnetic materials without rare-earth elements, and anode materials used in lithium batteries. The collected diffraction patterns can be processed to produce atomic pair distribution functions. The PAS and NPD facilities are available through user programs including the US DOE’s Nuclear Science User Facilities (NSUF) and the US NSF’s Research Triangle Nanotechnology Network (RTNN).

La espectroscopía de aniquilación de positrones (PAS) ha demostrado ser una poderosa herramienta para el estudio de defectos en sólidos ya que posee características únicas debido a su alta sensibilidad a la presencia de defectos tales como vacancias, aglomerados de vacancias o nanohuecos. Asimismo, PAS posibilita la identificación y caracterización de cada uno de estos defectos. Existen diferentes variantes experimentales de PAS tales como la espectrometría temporal positrónica, que permite identificar y cuantificar los distintos tipos de defectos, y el ensanchamiento Doppler que brinda información no solo sobre los defectos sino, también, sobre las especies atómicas que decoran los sitios de atrapamiento de los positrones. Estas técnicas, acopladas a un haz de positrones lentos permiten estudiar, además, defectos sub-superficiales en materiales con espesores inferiores al micrómetro, films y coatings. En este trabajo, se presentan dos ejemplos que ponen de manifiesto la potencialidad de PAS para el estudio de defectos sub-superficiales en: i) vidrios de sílice implantados con iones de oro y ii) oro rugosado.

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.

Deuterated potassium dihydrogen phosphate (K(DxH1-x) 2PO4) crystals with different deuteration levels (x=0, 0.51, 0.85) were grown by conventional cooling method from deuterated solutions at Shandong University. Positron annihilation spectroscopy has been widely used to the study on micro-defects of semiconductors and other materials, which is very sensitive to the crystal structure, defect types, defect concentrations, and so on. In this paper, positron annihilation spectroscopies (positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy), combined with X-ray diffraction (XRD) are used to investigate micro-defects characterization in K(DxH1-x) 2PO4 crystals. Influences of deuteration degree on the crystal structure characteristics, defect types and concentrations are discussed. It can be concluded from XRD experiments that the lattice parameters of a and b increase with the increase in deuteration levels, while no obvious change occurs on the lattice parameter c. KH2PO4(KDP) crystals at low deuteration level and high deuteration level could be regarded as low deuterium-doped KDP crystal and low hydrogen-doped DKDP crystal respectively. It is indicated that the higher the replacement ratio in the crystals, the weaker the diffraction peak they show. Positron annihilation lifetimes increase clearly in the highly-deuterated KDP crystals. It is found that neutral interstitial defects and oxygen defects in the KDP crystal increase with increasing deuteration degree. And these types of defects can be attributed to lattice distortion effect. From positron annihilation lifetime results we can arrive at another conclusion that the compound defects will form and defects concentration is declined, when hydrogen vacancies, K vacancies and substitutional impurity defects continue to react by means of association reactions. These phenomena suggest that high deuteration plays a significant role in promoting association reaction of internal defects in the crystals. Furthermore, the polymerization reaction of the clusters and micro-cavities continue to occur, therefore defect concentrations will show a constant decrease. Doppler broadening spectra show that the internal defects in the crystals increase integrally with an increase of deuteration level; this agrees well with the results of positron annihilation lifetime. Moreover, Doppler broadening spectra indicate that the proportional change of these defects is synchronous and consistent with the actuality. To sum up, our experimental results suggest that the defect reaction is weak in low degree of KDP crystal deuteration growth (less than 50%), while reaction is enhanced in the high degree of deuteration growth (higher than 50%).

Positrons localize in trapped states at a variety of defect sites in solids, from which they subsequently annihilate with unique observable characteristics. As such, the positron is a valuable probe for the study of these defects. Positron annihilation spectroscopy (PAS) has made significant contributions in recent years to the determination of atomic defect properties in metals and alloys, and in molecular solids as well. It has also been used extensively in the monitoring and characterization of vacancy-like microstructure development, as occurs during post-irradiation annealing. The characterization of defects using PAS is selectively reviewed and some possibilities for using the positron as a localized probe of the atomic and electronic structure of atomic defects and their aggregates are discussed.

Positron annihilation spectroscopy (PAS) is a novel method that provides molecular-level information about complex macromolecular structure in a manner different from, but complementary to, conventional physical and chemical methodology. This paper presents a perspective of PAS in polymeric systems covering 12 aspects: historical, spacial, spherical quantum model, anisotropic structure, voids, positronium chemistry, time, positron annihilation lifetime spectroscopy and data analysis, variable monoenergetic slow positron beam techniques and depth profiling, elemental analysis, multidimensional instrumentation advances in PAS, and free volume and free-volume theories.

Investigation on the variation in the fine structure of plasma-polymerized composite membrane by positron annihilation spectroscopy

In this paper, a systematic study is provided of the chemical environment inside free volume holes in a series of halogenated polystyrenes (p-position), −[CH2CH C6H5X]n– (X = F, Cl, Br, I), by using positron annihilation spectroscopy. In such polymers it was determined that the chemical environment is the major effect on Doppler broadening of two 511 keV γ photons from positron–electron annihilation. Doppler broadening energy spectroscopy (DBES) and positron annihilation lifetime spectroscopy (PALS) were combined in a novel approach to study the chemical environment in a polymer system. A highly linear relationship between Doppler broadenings caused by electron kinetic energies of valence electrons and the ionization potentials of halogen elements was obtained, as well as a similar correlation with the results from ab initio molecular orbital (MO) calculations for monohalogenobenzenes, C6H5X (X = F, Cl, Br, I). The results demonstrate that the combination of DBES and PALS may provide an effective way to study the chemical environment inside free volume holes in polymers.

Iron(Fe)-based alloys, such as austenitic stainless steels, form protective surface oxide layers when exposed to oxygen, aqueous electrolyte, and water vapor, which effectively inhibit corrosion [1]. One attribute that underpins the electrochemical reactivity of surface oxide layers is the presence and properties of charged point defects (e.g. vacancies and interstitials charge compensated by electrons or holes), which govern the rate and mechanism of ionic transport when exposed to an oxidizing environment such as an aqueous electrolyte [2,3,4]. However, the effect of radiation on the relationship between point defects, oxide microstructure, and oxidation behavior is poorly understood. Structural materials used in nuclear applications are particularly susceptible to radiation damage, which can degrade the underlying surface oxides. To investigate this phenomenon, thermally oxidized pure Fe was selected as a model material system to study radiation-induced defects in iron oxide scales [3]. Iron oxides were fabricated by thermal oxidation of pure Fe metal in air at 400°C, 600oC, and 800°C for 1 hour. Each oxides layer exhibits an outer hematite (α-Fe2O3) and inner magnetite (Fe3O4)/wüstite (FeO) layers with varying thickness depending on the oxidation conditions. These oxides were then irradiated with 200 keV, 0.03 dpa protons at room temperature. The oxide microstructure and phase compositions were characterized with transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS). These results were corroborated with X-Ray diffraction (XRD) and Raman spectroscopy analysis. The relative vacancy concentrations and size distributions in the oxide layers were investigated using positron annihilation spectroscopy (PAS) and positron annihilation lifetime spectroscopy (PALS) techniques. Lastly, these oxides were exposed to a 0.1 M borate buffer solution (pH 9.2) at applied potential where corrosion reactivity was governed by interfacial reactions and ionic defects to regulate the growth processes observed. The potentials were cycled over the ranges where oxides growth and reduction occurred involving Fe/Fe(II)/Fe(III) reactions at a pH which suppressed chemical reactions at the oxide/electrolyte interface and would lead to solubilization of Fe species. The subsequent oxidation, reduction, and semiconductor properties of the oxides were evaluated using AC and DC electrochemical methods, both with and without prior irradiation. Current results demonstrate that radiation damage can manifest itself in the form of vacancy clustering and possible maghemite (γ-Fe2O3) formation. These phenomena were corroborated with donor density and diffusivity values calculated using Mott-Schottky analysis and electrochemical impedance spectroscopy (EIS) analysis and equivalent circuit model (ECM) interpretation, respectively. Radiation damages to the outer rate-regulating α-Fe2O3 layer correlates with enhanced anodic current density corresponding to Fe/Fe(II)/Fe(III) reactions. However, proton radiation may also interact with the metal/oxide and oxide/oxide interfaces depending on the oxide structure and H+ beam penetration depth, leading to either degradation or improvement in the Fe-oxide corrosion responses. Nevertheless, the damage persists and can continue to impact corrosion mechanisms long after the irradiation source is removed. The multiple techniques adopted, each characterizing a material property of a different length scale (from meso-, micro-, to macroscopic) enable us to hypothesize the roles that point defects played after irradiation and during corrosion and how the radiation-induced changes in the defect characteristics influence corrosion electrochemistry. Acknowledgment Research primarily supported as part of the fundamental understanding of transport under reactor extremes (FUTURE), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Utilization of the Malvern-Panalytical Empyrean diffractometer were supported by Nanoscale Materials Characterization Facility with National Science Foundation under award CHE-2102156. The TEM sample preparation and characterization was performed at the Analytical Instrumentation Facility at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (award number ECCS-1542015). LANL is operated by Triad National Security, LLC, for the National Nuclear Security Administration of the U.S. DOE (Contract No. 89233218CNA000001).

Pulsed laser ablation is a process in which an intense laser pulse interacts with the matter producing plasma. The present work describes the theoretical work being conducted in the past for the pulsed laser ablation phenomenon. It incorporates the theoretical models being proposed by various researchers around the globe for pulsed laser ablation. The main processes involved in the laser–matter interaction leading to plasma plume formation are the absorption and the reflection of the incident laser light by the condensed phase, ablation of the condensed phase, absorption of the laser light within the expanding plume, expansion of the vapor-plasma plume, and in the case of pulsed laser desorption, the interaction of the ablated species with the substrate surface. Thus the plasma, produced in the laser–matter interaction, plays an important role in the pulsed laser ablation (PLA) process.

Positron annihilation spectroscopy characterization results for neutron-irradiated $3C$-SiC are described here, with a specific focus on explaining the size and character of vacancy clusters as a complement to the current understanding of the neutron irradiation response of $3C$-SiC. Positron annihilation lifetime spectroscopy was used to capture the irradiation temperature and dose dependence of vacancy defects in $3C$-SiC following neutron irradiation from 0.01 to 31 dpa in the temperature range from $380{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ to $790{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. The neutral and negatively charged vacancy clusters were identified and quantified. The results suggest that the vacancy defects that were measured by positron annihilation spectroscopy technique contribute very little to the transient swelling of SiC. In addition, coincidence Doppler broadening measurement was used to investigate the chemical identity surrounding the positron trapping sites. It was found that silicon vacancy-related defects dominate in the studied materials and the production of the antisite defect ${\mathrm{C}}_{\mathrm{Si}}$ may result in an increase in the probability of positron annihilation with silicon core electrons.

The type of silane coupling agent (SCA) has an important influence on carbon fiber (CF) modification efficiency and the properties of the obtained CF-based polymer composites. To quantitatively reveal the effects of SCA type, three kinds of SCA (γ-aminopropyl triethoxylsilane, γ-glycidoxypropyl trimethoxylsilane, and γ-methacryloxy propyl trimethoxylsilane)-modified CF-incorporated silicon rubber (SR) composites were prepared. The microstructure (free volume characteristic and interfacial interaction) of the obtained CF/SR composites was revealed by positron annihilation lifetime spectroscopy (PALS). Based on the results of mechanical, electrical, and thermal properties, the relationship between microstructure and performance was established. This investigation provides a powerful approach to the quantitative description of polymer composite microstructures, which will benefit the construction of structure-property relationships and high-performance polymer composites.

Free volume evolution in polyoxymethylene studied by positron annihilation spectroscopy

Probing vacancy-type free-volume defects in Li2B4O7 single crystal by positron annihilation lifetime spectroscopy

Microstructural changes in grade 2 titanium generated by surface mechanical attrition treatment (SMAT) were studied using positron annihilation lifetime spectroscopy and complementary methods. A significant increase in the mean positron lifetime indicated many lattice defects introduced by SMAT. Two positron lifetime components were resolved in the positron lifetime spectra measured. The longer lifetime revealed the presence of vacancy clusters containing about 3 or 4 vacancies, while the shorter one was attributed to the annihilation of positrons trapped at dislocations. The changes of the positron lifetime indicated a decreasing dislocation density and the presence of a deeper layer with a higher concentration of vacancy clusters at the distance from the treated surface for which the microhardness approached the value for the strain-free matrix. Electrochemical impedance spectroscopy showed the positive effect of SMAT on the corrosion resistance of the titanium studied in a saline environment also after removal of the original oxide layer that was formed during the SMAT.