Helium Ion Irradiation Research Articles

Damage evolutions of completely recrystallized W–Y2O3 composite evaluated using the dual effects of electron beam thermal shock and helium ion irradiation

Extreme service environments inevitably lead to the recrystallization of tungsten. This study investigates the behavior of completely recrystallized W–Y2O3 composite materials under transient thermal shock and helium irradiation conditions based on first wall service environment. The addition of Y2O3 particles exhibits a significant effect in terms of the crack resistance of the material. After helium ion irradiation, the aggregation of fuzz structures inside the cracks becomes increasingly evident as the crack width in the thermal shock region increases. Moreover, the resistance of Y2O3 particles to irradiation damage improves as the particle size decreases. The phase interface plays an important role in the helium ions irradiation process.

Defects introduced by helium irradiation at different temperatures in W and W-5wt%Re alloy

W and W-5wt%Re alloy were irradiated by 50 keV helium ions at RT, 200 °C, 350 °C and 500 °C to investigate the influence of temperature on the microdefects and surface morphology in the sample. The microdefects in the subsurface region and the surface damage caused by irradiation were characterized by Doppler broadening spectroscopy (DBS) of slow positron beam, transmission electron microscope (TEM) and scanning electron microscopy (SEM), respectively. The results show that a large number of vacancy-type defects were produced after helium ions irradiation, especially at 500 °C, the vacancy-type defects of W alloy are higher than that of W-5wt%Re alloy, which indicated that Re restricted the behavior of vacancy-type defects in tungsten to some extent. The surface irradiation damage such as crystal gap enlargement, grain fell off and pores were observed at different irradiation temperatures. Compared with the coral-like structure observed in W, the circular objects and Hilly-gully structure were observed in W-5wt%Re alloy, and both of them increased and grew with the increase of irradiation temperature. It was speculated that the presence of Re led to the difference by analyzing the formation mechanism of the coral-like structure and the Hilly-gully structure.

DETERMINATION OF CRITICAL DOSES OF RADIATION DAMAGE TO ALN CERAMIC UNDER IRRADIATION OF HELIUM AND HYDROGEN IONS

The work is devoted to the study of the radiation damage kinetics to heat-conducting, insulating and mechanical properties in polycrystalline ceramics based on aluminum nitride under irradiation of helium and hydrogen ions, as well as the determination of critical doses that cause maximum irreversible consequences. The choice of ions for irradiation is due to the ability to simulate the radiation damage processes during the accumulation of helium and hydrogen ions in the structure of the near-surface layer with the subsequent formation of gas-filled bubbles. During the studies carried out, it was found that at doses of irradiation with helium ions above 1х1017 ion/cm2 , there is a sharp deterioration in thermal conductivity and a decrease in ceramic resistance, which is associated with the onset of the formation of helium bubbles and partial embrittlement of the near-surface layer. However, an increase in the radiation dose above 5x1017 ion/cm2 does not lead to significant changes in thermal conductivity and insulation characteristics, which indicates the effect of radiation damage accumulation and a decrease in the ceramic degradation rate. In contrast to irradiation with helium ions, irradiation with hydrogen ions to doses higher than 1- 3х1017 ion/cm2 does not lead to significant changes in the thermal insulation characteristics, which indicates the ceramic resistance to hydrogen absorption processes.

Simulation study of effects of grain boundary and helium bubble on lattice thermal resistance of tungsten

Tungsten (W) as plasma facing material (PFM) has been used in ITER divertor. Many experiments indicate that due to helium (He) ion irradiation the thermal conductivities (TCs) of W can be significantly reduced. In this work, the lattice thermal transport property is studied based on the non-equilibrium molecular dynamics (NEMD) method. Firstly, the Kapitza resistance of grain boundaries (GBs) in W is obtained. It can be confirmed that the Kapitza resistance is positively correlated with GB energies, which reflect the lattice distortion degree in a sense. The result indicates that the distortion energy may play a key role in the thermal properties. Further simulation is performed to study the thermal resistance of GBs with nano He bubble. When the number of He atoms in bubble is less than a value, the thermal resistances are stable and change little. It implies that the effect of GBs is more significant than He bubble to block phonons diffusion. With the number of He atoms increasing, the property of heat conduction deteriorates further because the coupling of He bubble and GBs change the lattice distortion degree.

Helium ion irradiation enhanced precipitation and the impact on cavity formation in a HfNbZrTi refractory high entropy alloy

Refractory high entropy alloys have gained increasing research attention on their irradiation response for the potential applications in nuclear engineering. Considering the metastable nature of their single-phase solid solution structures, the phase stability under irradiation, especially at elevated temperatures, should be a critical concern during the long-term service. In the present study, a typical refractory high entropy alloy with a single-phase body-centered cubic (BCC) structure, HfNbZrTi, is irradiated with 1.5 MeV He ions under 700 °C to various fluences from 5 × 1015 to 1 × 1017 cm−2. Significant irradiation-enhanced precipitation with Hf and Zr enrichment is observed. Although most extended precipitates with tens of nanometers in size are with a hexagonal close-packed (HCP) structure, different structures, including HCP, face-centered cubic (FCC), and the close-packed structures containing highly mixed stacking sequences, are observed in the small precipitates with a few nanometers in size formed at the early-stage precipitation process, suggesting various precipitation paths. By carefully examining the interfaces between the matrix and precipitates, the precipitation mechanism is discussed with similar but slightly different orientation relationships from the Burgers path. Such irradiation-enhanced precipitation process has a strong impact on the cavity formation in this alloy. The cavities are dominantly formed through the heterogeneous nucleation and growth mechanisms, accompanied by the precipitation process. The cavities with low densities but large sizes are observed to be highly clustered with the precipitates. These findings not only unveil the detailed precipitation behavior in a model refractory high entropy alloy under irradiation, but also highlight the critical role that the phase stability plays on the irradiation damage of this novel alloy system.

Synergistic effect of irradiation and molten salt corrosion: Acceleration or deceleration?

The effect of helium ion irradiation with low and high ion fluences on the corrosion behaviour of nickel-based alloys in FLiNaK molten salt at 700 °C was investigated. The microstructure and element distribution were characterized to explore the corrosion behaviour of matrix and grain boundaries. The results showed that irradiation accelerates matrix corrosion in both Inconel 617 and GH3535 alloys. Moreover, the irradiation decelerated and accelerated intergranular corrosion were respectively observed under irradiation with low and high helium ion fluence, which is caused by the combined effects of the self-healing mechanism and diffusion of interstitials and helium atoms.

Selective Gas Permeation in Defect-Engineered Bilayer Graphene.

Defective graphene holds great potential to enable the permeation of gas molecules at high rates with high selectivity due to its one-atom thickness and resultant atomically small pores at the defect sites. However, precise control and tuning of the size and density of the defects remain challenging. In this work, we introduce atomic-scale defects into bilayer graphene via a decoupled strategy of defect nucleation using helium ion irradiation followed by defect expansion using hydrogen plasma treatment. The cotreated membranes exhibit high permeability and simultaneously high selectivity compared to those singly treated by ion irradiation or hydrogen plasma only. High permeation selectivity values for H2/N2 and H2/CH4 of 495 and 877, respectively, are achieved for optimally cotreated membranes. The method presented can also be scaled up to prepare large-area membranes for gas separation, e.g., for hydrogen purification and recovery from H2/CH4 and H2/N2 mixtures.

Effect of helium pre-implantation on the thermal shock performance of tungsten

• He plasma and ion irradiation improve the thermal stability and damage threshold of W. • Annealing decreases the cracking threshold of W, while not affects the roughening threshold. • He ion irradiation has a stronger influence than He plasma exposure. In ITER and future tokamaks, high heat flux loads will lead to recrystallization and decrease in toughness and thermal shock resistance of tungsten plasma facing materials, resulting in crack network formation and possible failure. In this study, thermal shock tests of blank, helium-plasma-exposed and helium-ion-irradiated rolled tungsten with/without isothermal annealing are conducted. Without annealing, cracks start to form in blank tungsten at the power density of 0.35 GW/m 2 , while no cracks are observed on the loaded surface of helium-plasma-exposed and helium-ion-irradiated ones. After annealing at 1473 K for 1 h, the recrystallization fractions of the three kinds of tungsten are around 50%, 18% and 8%, respectively. The following thermal shock tests of the annealed samples show that the cracking thresholds of blank and helium-plasma-exposed ones reduce to 0.3 GW/m 2 , while cracks do not appear in the helium-ion-irradiated one at this power density. Surface roughening thresholds are also improved in helium pre-implanted samples and do not change after annealing. It indicates that both helium plasma and ion irradiation improve the thermal stability and damaging threshold of tungsten, and helium ion irradiation has a stronger influence. This suggests a potential benefit of helium to extend the service lifetime of tungsten.

Irradiation behaviors of two novel single-phase bcc-structure high-entropy alloys for accident-tolerant fuel cladding

High-entropy alloys (HEAs) are potential alternative materials for accident-tolerant fuel cladding due to their excellent irradiation resistance and high-temperature corrosion resistance. In this work, two novel body-centered cubic (bcc) structured Mo0.5NbTiVCr0.25 and Mo0.5NbTiV0.5Zr0.25 HEAs were fabricated. Helium-ion irradiation was performed on the two HEAs to simulate neutron irradiation, and the crystal structure, hardness, and microstructure evolution were investigated. The crystal structure of the Mo0.5NbTiVCr0.25 HEA remained stable at low fluences, while amorphization may occur at high fluences in the two HEAs. The irradiation hardening value of the Mo0.5NbTiVCr0.25 was 0.77 GPa at fluences of 1 × 1017 ions/cm2 and 1.49 GPa at fluences of 5 × 1017 ions/cm2, while the hardening value of the Mo0.5NbTiV0.5Zr0.25 was 1.36 GPa at ion fluences of 5 × 1017 ions/cm2. In comparison with most of the conventional alloys, the two HEAs showed slight irradiation hardening. The helium bubbles and dislocation loops with small size were observed in the two HEAs after irradiation. This is the first time to report the formation of a dislocation loop in bcc-structure HEAs after irradiation. However, voids and precipitates were not observed in the two HEAs which could be ascribed to the high lattice distortion and compositional complexity of HEAs. This research revealed that the two HEAs show outstanding irradiation resistance, which may be promising accident-tolerant fuel cladding materials.

Role of helium ion irradiation in the corrosion of CVD−SiC in molten LiF−NaF−KF salt

The role of helium ion irradiation in the corrosion of CVD−SiC in molten LiF−NaF−KF salt was studied. Results indicate that the corrosion of SiC relates to the diffusion of element Si. Impurities in molten LiF−NaF−KF salt react with SiC to drive the outward diffusion of element Si and graphitic carbon is further formed. Energetic helium ions break down the SiC bonds to form defects. Slight disorder has little effect on the corrosion of SiC, while the amorphous induced by helium ion irradiation significantly enhances the diffusion of element Si and thus increases the corrosion of SiC.

Gate-Switchable Arrays of Quantum Light Emitters in Contacted Monolayer MoS2 van der Waals Heterodevices.

We demonstrate electrostatic switching of individual, site-selectively generated matrices of single photon emitters (SPEs) in MoS2 van der Waals heterodevices. We contact monolayers of MoS2 in field-effect devices with graphene gates and hexagonal boron nitride as the dielectric and graphite as bottom gates. After the assembly of such gate-tunable heterodevices, we demonstrate how arrays of defects, that serve as quantum emitters, can be site-selectively generated in the monolayer MoS2 by focused helium ion irradiation. The SPEs are sensitive to the charge carrier concentration in the MoS2 and switch on and off similar to the neutral exciton in MoS2 for moderate electron doping. The demonstrated scheme is a first step for producing scalable, gate-addressable, and gate-switchable arrays of quantum light emitters in MoS2 heterostacks.

The Raman band shift of suspended graphene impacted by the substrate edge and helium ion irradiation

Non-through and through nanopores were introduced to study the Raman band shift of suspended graphene by the substrate edge and the helium ion beam irradiation during the fabrication of nanopore in graphene. Before the ion beam irradiation, there is a blue-shift in the G band and G’ band of suspended graphene on the micro-scale non-through and through holes edge because of the n-type mixing for suspended graphene from the translocation. After different doses of the helium ion irradiation, G’ band Raman of suspended graphene on through are blue-shift, and the G band positions are red-shift. Helium ion irradiation introduces n-type doping during the graphene nanopore fabrication. The observed Raman shifts help us to gain more intrinsic properties of the graphene nanopore. Thus, Raman spectroscopy can be used as a quantitative diagnostic tool to character graphene-based nanopore.

Simulation of Ranges and Sputtering of Low Energy Helium and Argon Ions Irradiation on Tungsten Surface

Simulation of Ranges and Sputtering of Low Energy Helium and Argon Ions Irradiation on Tungsten Surface

Modification of thermal transport in few-layer MoS2 by atomic-level defect engineering.

Molybdenum disulfide (MoS2) has attracted significant attention due to its good charge carrier mobility, high on/off ratio in field-effect transistors and novel layer-dependent band structure, with potential applications in modern electronic, photovoltaic and valleytronic devices. Despite these advantages, its thermal transport property has often been neglected until recently. In this work, we probe phonon transport in few-layer MoS2 flakes with various point defect concentrations enabled by helium ion (He+) irradiation. For the first time, we experimentally show that Mo-vacancies greatly impede phonon transport compared to S-vacancies, resulting in a larger reduction of thermal conductivity. Furthermore, Raman characterization shows that the in-plane Raman-sensitive peak E2g1 was red-shifted with increasing defect concentration, corresponding to the gradual damage of the in-plane crystalline networks and the gradual reduction in the measured thermal conductivity. Our work provides a practical approach for atomic-level engineering of phonon transport in two-dimensional (2D) layered materials by selectively removing elements, thus holding potential applications in designing thermal devices based on various emerging 2D materials.

Helium ions irradiation analysis of W0.5(TaTiVCr)0.5 for application as a future fusion plasma-facing material

The irradiation resistance of a reduced activation tungsten-containing high-entropy alloy-based material W0.5(TaTiVCr)0.5 was analyzed using He ions irradiation. Mirror-polished samples were exposed to 200 keV He+ ions at a maximum fluence of 1 × 1021 ions/m2. The nano-indentation hardness of W0.5(TaTiVCr)0.5 increased from 11.6 GPa to 14.9 GPa (at 200 nm depth) following He ion irradiation. Examination of the sample's internal microstructure, using transmission electron microscopy, revealed some randomly dispersed black spots and precursors of He bubbles and dislocations causing irradiation hardening. Relatively better irradiation resistance of W0.5(TaTiVCr)0.5 compared with W, under the given experimental conditions suggests its potential fusion plasma facing applications.

Repair Kinetics of DNA Double Strand Breaks Induced by Simulated Space Radiation.

Radiation is unavoidable in space. Energetic particles in space radiation are reported to induce cluster DNA damage that is difficult to repair. In this study, normal human fibroblasts were irradiated with components of space radiation such as proton, helium, or carbon ion beams. Immunostaining for γ-H2AX and 53BP1 was performed over time to evaluate the kinetics of DNA damage repair. Our data clearly show that the repair kinetics of DNA double strand breaks (DSBs) induced by carbon ion irradiation, which has a high linear energy transfer (LET), are significantly slower than those of proton and helium ion irradiation. Mixed irradiation with carbon ions, followed by helium ions, did not have an additive effect on the DSB repair kinetics. Interestingly, the mean γ-H2AX focus size was shown to increase with LET, suggesting that the delay in repair kinetics was due to damage that is more complex. Further, the 53BP1 focus size also increased in an LET-dependent manner. Repair of DSBs, characterized by large 53BP1 foci, was a slow process within the biphasic kinetics of DSB repair, suggesting non-homologous end joining with error-prone end resection. Our data suggest that the biological effects of space radiation may be significantly influenced by the dose as well as the type of radiation exposure.

Effects of high-energetic 3He+ ion irradiation on tungsten-based composites

The interaction of plasma and its encompassing materials with one another is one of the principle designing issues of fusion reactors. Tungsten is considered one of the primary candidate materials in fusion applications due to its superior properties. In this work, we doped vanadium carbide powders to the tungsten matrix to enhance the properties of tungsten alloys. Tungsten-based composites, which irradiated with 2.5 MeV 3He+ ions at room temperature, were analyzed by atomic force microscopy (AFM) in order to obtain surface morpohologies after irradiation. Helium ion irradiated tungsten-based composites were studied by using XRD, neutron diffraction technique, raman spectroscopy, and positron annihilation spectroscopy to reveal the microstructural changes. XRD and ND analyses clarified the changes in the crystal structures of the tungsten-based materials after He ion irradiation. The simulation of radiation damage and the calculation of displacements per atom (DPA) was also determined by the SRIM code. SRIM showed that the maximum helium concentration in the specimens takes place in the depth range of 58–65 nm. The crystallite size of tungsten-based composites slightly increased after helium ion irradiation. AFM results revealed that the maximum size of bubbles on the surface of tungsten-based composites shape under 100 nm. Positron annihilation spectroscopy studies of the specimens have been discussed before and after 3He+ ion irradiation.

Direct-Write of Nanoscale Domains with Tunable Metamagnetic Order in FeRh Thin Films.

We have directly written nanoscale patterns of magnetic ordering in FeRh films using focused helium-ion beam irradiation. By varying the dose, we pattern arrays with metamagnetic transition temperatures that range from the as-grown film temperature to below room temperature. We employ transmission electron microscopy, X-ray diffraction, and temperature-dependent transport measurements to characterize the as-grown film, and magneto-optic Kerr effect imaging to quantify the He+ irradiation-induced changes to the magnetic order. Moreover, we demonstrate temperature-dependent optical microscopy and conductive atomic force microscopy as indirect probes of the metamagnetic transition that are sensitive to the differences in dielectric properties and electrical conductivity, respectively, of FeRh in the antiferromagnetic (AF) and ferromagnetic (FM) states. Using density functional theory, we quantify strain- and defect-induced changes in spin-flip energy to understand their influence on the metamagnetic transition temperature. This work holds promise for in-plane AF-FM spintronic devices, by reducing the need for multiple patterning steps or different materials, and potentially eliminating interfacial polarization losses due to cross material interfacial spin scattering.

Tribological properties improvement of H-DLC films through reconstruction of microstructure and surface morphology by low-energy helium ion irradiation

The hydrogenated diamond-like carbon (H-DLC) films were deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) technique and implanted with 60 keV He ion (He+) over a wide dose ranges. A two-steep model for the evolution of tribological properties of H-DLC films after irradiation has been proposed and discussed. Firstly, at the He+ doses range from 0 to 3 × 1016 cm−2, irradiation treatment induced structural transformation of graphitization in the H-DLC films modification layer due to the increase of sp2/sp3 ratio, which led to the upgradation of tribological properties owing to the lubrication effect of graphite. The decreases of the root mean square roughness (Sq) led to a decrease in the resistance to the relative movement of the friction pair. Therefore, the reduction of friction coefficient and wear rate of H-DLC was contributed to be the combined effect of graphite phase transition and low Sq value. As the dose was increased to 1 × 1017 cm−2, the degradation of tribological properties was evidenced, which could be ascribed to the formation of irradiation-induced He bubbles and holes on the surface of films. In addition, when the irradiation dose increased from 1 × 1016 cm−2 to 3 × 1016 cm−2 and then to 1 × 1017 cm−2, the variation of H/Ef value also indicated that the anti-wear properties of H-DLC film first increased and then decreased. Based on the experimental results, the H-DLC film that irradiated by the He+ at the dose of 3 × 1016 cm−2 shows the best tribological properties with low friction coefficient and high anti-wear properties while maintaining agreeable mechanical properties.

Experimental and theoretical analysis of equilibrium segregation and radiation-induced segregation of Cr at grain boundaries in a reduced activation ferritic/martensitic (RAFM) steel

Helium ion irradiation at 350 °C was performed to study equilibrium segregation and radiation-induced segregation (RIS) of Cr at grain boundaries in reduced activation ferritic/martensitic steels. Cr concentration at grain boundary was measured by scanning transmission electron microscopy with an energy-dispersive spectrometer. The measured Cr concentration at grain boundaries in heat treated zone was 11.7 and 12.8 wt.% in irradiated zone, respectively, which matched well to the calculated results from Mclean and modified Perk model. Equilibrium segregation and RIS of Cr mechanisms were theoretically analysed. The analysis indicates that as temperature rises, equilibrium Cr segregation decreases monotonically, while RIS of Cr has a bell-shape profile, which increases first and then decreases. It is also shown that at low and high temperatures, equilibrium segregation of Cr is higher than that of RIS; at intermediate temperatures, equilibrium Cr segregation is lower than RIS.