He-ion Irradiation Research Articles

Optical characteristics and surface dendritic growth of Nd,Y:CaF2 crystals irradiated by high-energy He ions

This study aims to investigate the various responses of the optical characteristics to He-ion irradiation at different fluences and energies in Nd,Y:CaF2 crystals. Through optical absorption and photoluminescence spectra, we observe that electrons excited by He-ion irradiation are trapped by anionic vacancies, He ions and Nd3+ to form F-centers, He atoms and Nd2+, respectively. At an energy of 5 MeV, F-centers decrease with increasing fluence, whereas at a fluence of 1×1013 ions/cm2, increasing the energy promotes Nd2+ formation, and a further increase in the fluence based on an energy of 16 MeV can significantly promote Nd2+ production. After the irradiation, the electrons from F-centers are also captured by Nd3+, leading to the decay of F-centers and an increase in Nd2+ with time. In addition, the growth of CaF2 dendrites with multiple morphologies on the surface by high-energy irradiation was observed. It was explained by the surface recrystallization between the escape of F and surface metallization due to high-energy irradiation. This research provides significant insights into the conditions of irradiation modulating defect generation and evolution, as well as laying the groundwork for future applications of radiation-exposed crystals as radiation dosimeters and lasers.

Effects of deuterium plasma exposure and helium ions irradiation on nanoindentation hardness of tungsten

Irradiation hardening is one of the service performance concerns for tungsten in fusion reactors. This work investigated the effect of deuterium (D) plasma exposure and helium (He) ions irradiation on the hardening behavior of the same tungsten sample using nanoindentation. The results demonstrate that the hardness of tungsten increases after D plasma exposure, He ions irradiation, and synergistic irradiation of He-ion and D-plasma. In general, the degree of hardening is He-ion + D-plasma irradiation, individual He-ion irradiation, and D plasma exposure in descending order. D plasma exposure results in an increase in hardness of more than 10 %, which is attributed to the pinning of dislocations by D plasma-induced defects and D-defect complexes (clusters). In the case of the He-ion irradiated tungsten, a large number of defects such as He nanobubbles induced by He-irradiation result in a 70 % increase in hardness. The superposition effect on the hardening of tungsten by D plasma exposure after He ions irradiation was observed, which essentially remains an increase in hardness due to D plasma exposure. This implies that the degradation of mechanical properties caused by D plasma exposure on tungsten will not be overlapped by other particle irradiation. Moreover, He bubbles in tungsten remain stable and grow slightly after annealing at 1173 K, resulting in a limited decrease in hardness.

Solid-State Transformation of Uranyl Peroxide Materials through High-Level Irradiation.

The solid-state transformation of sodium uranyl triperoxide (Na4(UO2)(O2)3·9H2O, NaUT) to sodium uranyl tricarbonate (Na4(UO2)(CO3)3) by radiolysis was observed for the first time. The exposure of NaUT to 3 MGy gamma irradiation resulted in partial breakdown of the peroxides forming a mixed peroxide and carbonate species. The effects of He-ion irradiation on NaUT were also investigated up to 225 MGy using both hydrated argon and dry argon. The complete conversion to the uranyl tricarbonate phase by 56 MGy was done using hydrated argon, while dry argon did not fully convert showing the importance of water in the system. He-ion irradiated NaUT samples all convert to the tricarbonate phase with time in air post radiation exposure. This transition was monitored via Raman spectroscopy, infrared spectroscopy (IR), and powder X-ray diffraction (PXRD) to further confirm the identity of the final product as the sodium uranyl tricarbonate, čejkaite. This transformation outlines a mechanism for the mobility of uranyl in natural environments and in the Hanford tanks.

Nanoscale imaging of He-ion irradiation effects on amorphous TaOx toward electroforming-free neuromorphic functions

Resistive switching in thin films has been widely studied in a broad range of materials. Yet, the mechanisms behind electroresistive switching have been persistently difficult to decipher and control, in part due to their non-equilibrium nature. Here, we demonstrate new experimental approaches that can probe resistive switching phenomena, utilizing amorphous TaOx as a model material system. Specifically, we applied scanning microwave impedance microscopy and cathodoluminescence (CL) microscopy as direct probes of conductance and electronic structure, respectively. These methods provide direct evidence of the electronic state of TaOx despite its amorphous nature. For example, CL identifies characteristic impurity levels in TaOx, in agreement with first principles calculations. We applied these methods to investigate He-ion-beam irradiation as a path to activate conductivity of materials and enable electroforming-free control over resistive switching. However, we find that even though He-ions begin to modify the nature of bonds even at the lowest doses, the films' conductive properties exhibit remarkable stability with large displacement damage and they are driven to metallic states only at the limit of structural decomposition. Finally, we show that electroforming in a nanoscale junction can be carried out with a dissipated power of <20 nW, a much smaller value compared to earlier studies and one that minimizes irreversible structural modifications of the films. The multimodal approach described here provides a new framework toward the theory/experiment guided design and optimization of electroresistive materials.

Effects of He-ion irradiation on microstructures of low activation Ti-Ta-V alloy from atomic simulations and irradiation experiments

Most conventional titanium alloys contain nuclides with long half-lives, which is not conductive to material recovery after decommissioning. To recover components from post-service reactors safely and effectively, it is necessary to use materials with low activation and rapid induction of radioactive decay. In this work, a low-activation α-titanium alloy Ti-5Ta-1.6 V was prepared and irradiated with 80 keV helium ions to 6.8 dpa at room temperature. The mechanical properties and irradiation swelling properties of Ti-5Ta-1.6 V alloy were mainly investigated. The results showed that compared with the commercially pure titanium alloy (CP-Ti), the mechanical properties of Ti-5Ta-1.6 V were substantially improved. After ion irradiation, the irradiation swelling rate of Ti-5Ta-1.6 V (in mass ratio) was found to be significantly lower than that of CP-Ti by atomic force microscopy (AFM) technology. Molecular dynamic (MD) simulations were used to study the irradiation behavior and cascade collisions process, and the Ti-5Ta-1.6 V produced a small number of stable defects with cluster size and number density, which is consistent with the results of irradiation experiments. The results indicate that Ti-5Ta-1.6 V is expected to be a promising new titanium alloy for nuclear applications, which provides a new idea for the selection and optimization of low-activation and radiation-resistant nuclear materials.

Characterization of microstructure and hardening of SiC ceramic brazed joint using TiZrCuNi filler irradiated by He ions

SiC ceramic brazed joints for fissile fuel cladding tube have attracted significant interest, but the lack of evaluation of the irradiance resistance has brought serious obstacles to their application in nuclear power. Herein, we propose the experimental study of He-ion irradiation on the interface phases between SiC substrate and TiZrCuNi brazing filler. The microstructure and hardness evolution of each phase at the interface were investigated under different irradiation fluences at room temperature, and the irradiation effects caused by He ion irradiation, such as amorphous and helium bubble, were elucidated. It was revealed that SiC and (Ti,Zr)2(Cu,Ni) were amorphous with the increase of irradiation fluence, while (Zr,Ti)C was not amorphous but produced a large number of dislocation defects. In terms of irradiation hardening, it was affected by the coupling effect of amorphous and helium bubble defects.

Atomic-scale study of He ion irradiation-induced clustering in α-Zirconium

Zircaloy-4 alloy specimens consisting of α-Zr grains were irradiated on a tandem accelerator with 5 MeV He ions to a fluence of 5 × 1021 ions m − 2 at different temperatures. Defect clusters and He bubbles were observed in the TEM micrographs of the irradiated microstructure. The small defect clusters and He bubbles were found to exhibit spherical shapes whereas the large defect clusters showed plate-like shapes. Molecular dynamics simulations and crystallography analyses revealed that the defect clusters are mainly composed of Zr interstitials. During He-ion irradiation, the clustering of Zr dumbbell interstitials results in the formation of small and spherical defect clusters. With further irradiation, the Zr interstitials prefer to occupy the octahedral sites that align along the {12¯14} planes with other site interstitials, forming cluster sections. These initial planar clusters then grow by extending along the <101¯0> directions and by forming more cluster sections along the <12¯16> directions, leading to large plate-like defect clusters on the {1¯21¯1}planes. He bubbles nucleate by the clustering of He atoms and vacancies that are formed by kicking out the lattice atoms, and then grow by the migration and coalescence of helium-vacancy clusters. Small He atoms have many interstitial sites to occupy in the α-Zr HCP crystal, allowing He bubbles to grow to spherical shapes. In addition, grain boundaries induce the accumulation of primary point defects and He atoms and high irradiation temperature accelerates diffusion, and hence larger defect clusters and He bubbles form in the grain boundary regions and at higher irradiation temperature.

Effects of He-ion irradiation on the microstructures and mechanical properties of the novel Co-free VxCrFeMnNiy high-entropy alloys

High-entropy alloys (HEAs), as a promising candidate structural material for advanced nuclear energy system, have been widely studied recently due to the combination of the outstanding irradiation tolerance and enhanced mechanical properties at elevated temperatures. In this study, an FCC-structured CrFeMnNi2 HEA and a BCC-structured VCrFeMn HEA without Co element were designed and prepared. The effects of He-ion irradiation on the microstructures and mechanical properties were investigated, so as to preliminarily evaluate the irradiation tolerance of the novel Co-free HEAs. At fluences of 3 × 1016 and 6 × 1016 ions/cm2, the CrFeMnNi2 and VCrFeMn HEAs showed good structural stability at 1023 K; the transmission electron microscope results revealed no amorphization/precipitations. The average sizes and number densities of He bubbles increased as the fluence improved for the two alloys. At each fluence, the average size of the He bubbles in the VCrFeMn HEA was larger than that in the CrFeMnNi2 HEA, while the number density and volume fraction were lower. The CrFeMnNi2 HEA possessed a higher hardening fraction (77%), and an irradiation-induced hardening saturation was observed in the VCrFeMn HEA with a lower hardening fraction of 33%. The experimental results indicated that the novel low-activation VCrFeMn HEA might be a competitive candidate structural material for advanced reactors. The mechanisms of the irradiation tolerance of Co-free HEAs were also discussed.

Composition-dependent changes in mechanical property of borosilicate glass induced by electron-ion sequential irradiation

Mechanical property and microstructure changes induced by 500-keV He2+ have been investigated in three kinds of 1.2-MeV electron pre-irradiated Na2O-B2O3-SiO2 (labeled as NBS) glasses. The glasses before and after irradiation are analyzed by nanoindentation methods and infrared spectroscopy. The hardness and modulus variation of sequential He-ion irradiation in the electron pre-irradiated NBS1 and NBS2 specimens exhibited damage promotion. On the contrary, a pronounced recovery effect is observed in the behavior of NBS3 glass. Both the hardness and modulus changes are correlated with the microstructure evolution, and the corresponding micro-mechanism is discussed. Moreover, the pre-existing responses induced by electron irradiation differ from the ion irradiation caused, which can alter the damage equilibrium level of modulus on borosilicate glass under He-ion sequential irradiation. This study of electron-ion sequential irradiation effects will provide new insights for understanding the irradiation response of nuclear waste glass matrix under mixed radiation fields.

Unusual He-ion irradiation strengthening and inverse layer thickness-dependent strain rate sensitivity in transformable high-entropy alloy/metal nanolaminates: A comparison of Fe50Mn30Co10Cr10/Cu vs Fe50Mn30Co10Ni10/Cu

In this work, we prepare transformable HEA/Cu nanolaminates (NLs) with equal individual layer thickness (h) by the magnetron sputtering technique, i.e., Fe50Mn30Co10Cr10/Cu and Fe50Mn30Co10Ni10/Cu, and comparatively study He-ion irradiation effects on their microstructure and mechanical properties. It appears that the as-deposited HEA/Cu NLs manifest two size h-dependent hardness regimes (i.e., increased hardness at small h and hardness plateau at large h), while the He-implanted ones exhibit monotonically increased hardness. Contrary to the fashion that smaller h renders less irradiation hardening in bimetal NLs, the Fe50Mn30Co10Cr10/Cu NLs manifest the trend that smaller h leads to greater irradiation hardening. By contrast, the Fe50Mn30Co10Ni10/Cu NLs exhibit the maximum irradiation hardening at a critical h = 50 nm. Below this critical size, smaller h results in lower radiation hardening (similar to bimetal NLs), while above this size, smaller h results in greater radiation hardening (similar to Fe50Mn30Co10Cr10/Cu NLs). Moreover, these transformable HEA/Cu NLs display inverse h-dependent strain rate sensitivity (SRS m) before and after He-ion irradiation. Nevertheless, compared with as-deposited samples, the irradiated Fe50Mn30Co10Cr10/Cu NLs display reduced SRS, while the irradiated Fe50Mn30Co10Ni10/Cu NLs display enhanced SRS. Such unusual size-dependent irradiation strengthening and inverse h effect on SRS in irradiated samples were rationalized by considering the blocking effects of He bubbles on dislocation nucleation and motion, i.e., dislocations shearing or bypassing He bubbles.

Composition-dependent mechanical property changes and damage recovery in He-ion-irradiated borosilicate glasses

This study explored the mechanical property changes in three kinds of pristine and Au3+ pre-irradiated Na2O-B2O3-SiO2 (labeled as NBS) glasses induced by 0.5-MeV He2+ ions over a broad fluence range. The glass samples were prepared with different R values = [Na2O]/[B2O3] (0.40, 0.75 and 1.34). After He-ion irradiation, the hardness diminished and finally reached saturation, the modulus showed a slight decrease for the NBS1 and NBS2 glasses, but the NBS3 glass exhibited no significant change. The mono He2+ ion irradiation scenario did not show the self-recovery effect of Au3+ ion irradiation scenario. The sequential He-ion irradiation can lead to damage recovery in the Au-ion pre-irradiated glasses. The final equilibrium state was affected by the composition of pristine glass, which can be attributed to lower thermal stability and the phase separation characteristic of NBS3 glass. Such results hold great significance for understanding the radiation damage caused by multiparticle irradiation in the glass matrix of nuclear wastes.

Achieved improvement of damping capacity in Ti Ni shape memory alloy through He-ion irradiation

TiNi shape memory alloys have been widely used to eliminate noise and mechanical vibrations. However, narrow working temperatures and low damping performance limit its applications. Here, a high damping capacity with the relaxational internal friction close to 0.0524 at 5 Hz and the intrinsic internal friction close to 0.04512 are detected after He ion irradiation. The internal friction value greater than 0.02 in a wide working temperature range of 200 K is reported. Further analyses indicate the critical role of He ion irradiation in increase of damping property. We demonstrate that the high broad internal friction could be derived from the interaction between He/H atoms and martensite twinning interface, and the change in twinning type (111¯) type I twinning to (001) compound twinning in the irradiated layer. The He ion irradiation strategy disclosed in the current work is expected to be a promising way to design high damping alloys.

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.

Irradiation Hardening Behavior of He-Irradiated V–Cr–Ti Alloys with Low Ti Addition

A set of V–(4–8)Cr–(0–4)Ti alloys was fabricated to survey an optimum composition to reduce the radioactivity of V–Cr–Ti alloys. These alloys were subjected to nano-indenter tests before and after 2-MeV He-ion irradiation at 500 °C and 700 °C with 0.5 dpa at peak damage to investigate the effect of Cr and Ti addition and gas impurities for irradiation hardening behavior in V–Cr–Ti alloys. Cr and Ti addition to V–Cr–Ti alloys for solid–solution hardening remains small in the unirradiated V–(4–8)Cr–(0–4)Ti alloys. Irradiation hardening occurred for all V–Cr–Ti alloys. The V–4Cr–1Ti alloy shows the highest irradiation hardening among all V–Cr–Ti alloys and the gas impurity was enhanced to increase the irradiation hardening. These results may arise from the formation of Ti(CON) precipitate that was produced by He-ion irradiation. Irradiation hardening of V–Cr–1Ti did not depend significantly on Cr addition. Consequently, for irradiation hardening and void-swelling suppression, the optimum composition of V–Cr–Ti alloys for structural materials of fusion reactor engineering is proposed to be a highly purified V–(6–8)Cr–2Ti alloy.

Study on the interaction between He and defects induced by He-ion irradiation in W and W5Re alloy

To investigate the interaction between He atom and irradiation-induced defects in W and W5Re, multiple energetic He ions with damage dose ranging from 0.84 dpa to 1.63 dpa were selected in the present study to implant into W and W5Re samples. Doppler broadening spectroscopy (DBS) and coincidence Doppler broaden spectroscopy (CDB) based on slow positron beam and synchrotron X-ray diffraction technology were employed in the current work. Under the same dose (≤ 1 × 1021 ions m−2) irradiation at room temperature, the depth distribution of vacancy-type defects in W and W5Re is similar. With the increase of irradiation dose, the vacancy-type defects in W continue to grow through absorbing and merging. However, Re atoms are more likely to hinder this growth mechanism, leading to an accumulation of high density but small size vacancy-type defects in W5Re. The HenVm complexes in W continuously attract the mobile interstitial He atom and then induce the trap mutation in the implanted region. Micro-voids and large He bubbles (∼ μm) exceed the recognition of positron. Re atoms has strong the attraction to vacancies and positron affinity. The possible compound of Re and HenVm complexes makes it easier for positron to characterize the He-related and Re-related information around the defect sites. The lattice swelling is similar for the W and W5Re because the structure of HenVm cluster or He bubble can destroy the lattice periodicity and increase the lattice stress in the implanted region.

Anisotropic He-ion irradiation damages in nanocolumnar W thin films

The effects of He-ion irradiation on the microstructures and the mechanical, thermal properties of sputter-deposited nanocolumnar tungsten thin films have been studied. 200 keV He+ ion irradiation with a fluence of 2x1017 ions/cm2 was performed in the growth direction of the W thin films. Small scale mechanical testing methods, such as nanoindentation and square membrane deflection experiments, were carried out, and the thermal conductivity measurement was performed based on the electrical resistivity measurement and the Wiedemann–Franz law for the unirradiated and irradiated W thin films. It was revealed that the properties in the out-of-plane direction are not changed much, but a significant degradation occurs in the in-plane direction after the He-ion irradiation. The microstructure of the film and the distribution of He-ion induced damages are responsible for the anisotropic property changes by He-ion irradiation.

Atomistic defects as single-photon emitters in atomically thin MoS2

Precisely positioned and scalable single-photon emitters (SPEs) are highly desirable for applications in quantum technology. This Perspective discusses single-photon-emitting atomistic defects in monolayers of MoS2 that can be generated by focused He-ion irradiation with few nanometers positioning accuracy. We present the optical properties of the emitters and the possibilities to implement them into photonic and optoelectronic devices. We showcase the advantages of the presented emitters with respect to atomistic positioning, scalability, long (microsecond) lifetime, and a homogeneous emission energy within ensembles of the emitters. Moreover, we demonstrate that the emitters are stable in energy on a timescale exceeding several weeks and that temperature cycling narrows the ensembles' emission energy distribution.

He-ion irradiation effects on the microstructure stability and size-dependent mechanical behavior of high entropy alloy/Cu nanotwinned nanolaminates

High entropy alloys (HEAs) have attracted extensive attention due to their excellent properties in harsh environments. In this work, we prepare HEA/Cu (HEA = FeCoCrNi) nanotwinned nanolaminates (NTNLs) with equal individual layer thickness (h) and study the He-ion irradiation effects on their deformation behavior and mechanical properties. With decreasing h the microstructure of HEA/Cu NTNLs transits from the nanolayered structure with nanotwins confined in the isolated layers at large h ≥ 25 nm to the columnar grained single crystal-like structure with nanotwins penetrated across the coherent layer interface(s) at small h < 25 nm. Compared with the as-deposited HEA/Cu NTNLs showing notable detwinning behavior, in particular at small h, the nanotwins in irradiated samples are difficult to detwin owing to the He defects pinning effect on twin boundaries. It appears that both as-deposited and irradiated HEA/Cu NTNLs manifest the trend of “smaller is stronger”. Unlike the reported monotonically increased/reduced irradiation hardening with h in conventional bimetal nanolaminates, a minimum irradiation hardening occurs in the present HEA/Cu NTNLs. Unexpectedly, the as-deposited HEA/Cu NTNLs manifest positive reduced strain rate sensitivity (SRS) m with decreasing h, while their irradiated siblings exhibit the transition from positive to negative SRS m. The size-dependent mechanical properties of HEA/Cu NTNLs are rationalized by the partial-based mechanism in combination with the stability of nanotwins.

Enhancing resistance to radiation hardening and radiation thermal conductivity degradation by tungsten/graphene interface engineering

Radiation damage could be effectively alleviated by metal/graphene interfaces, which act as sinks to absorb defects spontaneously. In this study, four indispensable properties of tungsten nanofilms with inserted graphene monolayers were investigated, including changes in the thermal and mechanical properties as well as their respective radiation responses. We demonstrated that after the introduction of monolayer graphene, the hardness of the tungsten nanofilm was enhanced significantly by graphene interfaces with different densities. Molecular dynamics simulations showed that the presence of graphene interfaces can effectively inhibit dislocation propagation and delay the plastic deformation of tungsten. The cross-plane thermal conductivity of the tungsten nanofilms decreased after graphene was inserted, and this trend became more gradual as the density of the graphene interfaces increased. Uniquely, the thermal conductivity of the tungsten–graphene multilayered nanofilm showed a reduction of ∼20% after He-ion irradiation at 12.8 dpa compared to the greater than 50% reduction in the pure tungsten nanofilm. Moreover, the hardness of the pure tungsten nanofilm showed an increase of ∼160%, while the multilayer nanofilms exhibited almost no irradiation hardening. Transmission electron microscopy was used to analyze the effect of defects on the hardness and heat transport. Our results suggest that constructing tungsten/graphene interfaces has great potential in enhancing resistance to radiation thermal conductivity reduction and radiation hardening.

D2 retention behavior and microstructural evolution of W-2wt.%Y2O3 alloy during He-ion irradiation at high temperatures

In this study, we carried out the pre-irradiation of a dispersion-strengthened W-2wt.%Y2O3 alloy with good mechanical properties with high-energy Fe-ions instead of neutrons. The effect of this pre-irradiation on the D2 retention behavior of the alloy was investigated. A maximum of 1022 D/m2 was irradiated onto the samples using 5-keV D2 ions, and Fe pre-irradiation was carried out up to an ion fluence of 4 × 1018/m2 with 1 MeV energy. For comparison, commercial pure W was also studied. The Fe-ion pre-irradiation process increased the D2 retention in both pure W and the W–Y2O3 alloy. However, this D2 retention was independent of the pre-irradiation dose. In addition, the formation of He bubbles in the W–Y2O3 alloy caused by the irradiation at 1.8 × 1021 He/m2 and high temperatures of 773, 973, and 1173 K was also investigated. When the irradiation dose exceeded 1.5 × 1020 He/m2, visible He bubbles were formed, and He bubbles grew and void swelling increased with an increase in the irradiation dose. Compared to pure W, the W–Y2O3 alloy showed suppressed void swelling. This can be attributed to the dispersion of vacancies and He atoms due to the He irradiation.