Radiation-induced Lattice Defects Research Articles

A combined theoretical-experimental investigation of thermal transport in low-dose irradiated thorium dioxide

During reactor operation, nuclear fuels are subject to extreme temperature and irradiation conditions which can significantly degrade the fuel's thermal transport properties. The reduction in thermal conductivity of the fuel as a result of irradiation-induced lattice defects is arguably the most important fuel performance metric in regard to reactor efficiency and safety. Because thorium dioxide (ThO2) is suitable as a model system for more complex materials such as UO2 and its mixed oxides, we present a theoretical investigation of thermal conductivity reduction seen in defect-bearing thorium dioxide and compare directly to experimental measurements. Phonon-mediated thermal transport of the fuel is modeled by a solution to the Boltzmann transport equation (BTE) for phonons. A cluster dynamics (CD) model for lattice defect evolution during irradiation predicts defect densities which are used as input to the BTE for modeling phonon-defect scatterings. Phonon scatterings by lattice defects include those from point defects and vacancy clusters and interstitial clusters of various sizes. The CD model is benchmarked against structural defect characterization of irradiated thorium dioxide using electron microscopy. Thermal conductivity predicted by the BTE model is compared to measured values for irradiated thorium dioxide specimens below room temperature to isolate effects of phonon-defect scattering from intrinsic 3-phonon processes, which dominate at higher temperatures. The computed conductivity values are in partial agreement at temperatures close to room temperature while slight deviations are observed at the lowest measured temperatures, suggesting that implemented phonon-defect scattering cross-section expressions may not be adequate for low temperatures. The presented work provides a necessary investigation of the influence of irradiation induced defects on fuel performance and represents a first step toward a full characterization of phonon mediated thermal transport in irradiated materials with complex defect microstructure.

Full-stage precipitation during aging of Cu-0.55Cr-0.07Zr alloy for high heat flux fusion reactor technology

We have monitored the full-stage precipitate evolution in Cu-0.55Cr-0.07Zr alloy during aging at 480 °C (753 K) up to 14 days (20160 min), using micro-hardness, positron annihilation spectroscopy, electron microscopy and atom probe tomography. Cr-rich precipitates form during the early stages of aging, and after 5.5 min the distribution is characterised by a number density of 8×1023 m-3 and an average precipitate size of 2.5 nm. At that time, Zr segregation is also detected at the precipitate sites. Longer aging times lead to precipitate coarsening, simultaneously with the occurrence of a Zr-rich shell around precipitates potentially to help reduce local coherency strains. Peak aging of the alloy is attained after 120 min of aging, and overaging induces further precipitate coarsening and the transition from spherical to disc-like morphology. At the longest aging time of 14 days, the precipitates maintain a face-centred cubic symmetry, reaching an average size of ∼9 nm at a density of 0.2×1022 m-3. The presence of Zr atoms at the precipitate site and interface, and the disc-like morphology of overaged precipitates, should be considered when assessing the precipitate stability under irradiation and the precipitate/matrix interface as a potential sink for radiation-induced lattice defects.

Characterization of microstructure and microhardness of Neutron irradiated Inconel X-750

In this study, we characterized the microstructure evolution and hardness of Inconel X-750 samples that were irradiated in-reactor at several different dose rates and temperatures and at doses up to 84 dpa. The irradiation induced lattice defects, the stability of γ’ precipitates, and the formation of helium bubbles were studied. Detailed statistics regarding the size and density of those features were obtained. The combined effects from irradiation dose, dose rate, and temperature on the disordering and dissolution of precipitates and the formation of bubbles were discussed. The isolated contributions of those microstructural features to the total material strength were calculated based on the Dispersed Barrier Hardening (DBH) model. The spatial distribution of He bubbles, especially the visible large ones (>2 nm), were also found to either correlate or anti-correlate with the spatial distribution of solute elements, e.g., Ti, Fe, and Cr, at doses higher than 75 dpa.

Atomistic structure and thermal stability of dislocation loops, stacking fault tetrahedra, and voids in face-centered cubic Fe

Irradiation-induced hardening and changes to mechanical properties can often lead to material degradation and can limit the operation of reactor components. In addition, changes to mechanical properties can be a precursor to other material environmental degradation. Dislocation loops, stacking fault tetrahedra (SFT), and voids are major irradiation-induced lattice defects that occur in structural materials and are often observed post-irradiation. However, these defects are often considered only as obstacles to dislocation motion, with hardening assumed to be dependent on the habit plane and Burgers vector of the defect. The stability and nature of dislocation loops and SFTs with temperature are often overlooked, and this may play a crucial role in the mechanical properties of structural materials. We combine high-resolution transmission electron microscopy (HR-TEM) and atomistic simulations to determine the atomistic configurations and thermal stability of dislocation loops and SFTs. Dislocation dissociations are observed in both vacancy and interstitial-type perfect dislocation loops as well as in vacancy-type Frank loops, whereas interstitial-type Frank loops remain stable and are the least influenced by temperature. Triangle-shaped intrinsic stacking faults are formed at the edges of Frank vacancy loops, and a direct transformation from triangle-shaped Frank vacancy loops to SFTs is observed in atomistic simulations. SFTs are energetically more stable than Frank vacancy loops based on formation energy calculations in pure face-centered cubic Fe, which may explain why vacancy-type Frank loops are infrequently observed experimentally. The motivation for this study is to develop a framework for the stability and nature of irradiation defects as a function of temperature, which will be subsequently used for more complex (alloy) systems.

The use of lattice defects in quartz to determine provenance and conditions for the formation of sedimentary deposits (using the example of quartz from sedimentary rocks of the Central Kyzylkum)

Research subject. The composition and distribution patterns of lattice defects in the clastic quartz of sedimentary rocks.Material and methods. Quartz collected from the core of two wells uncovering the Neogene, Paleogene and Cretaceous deposits in the rocks of the Central Kyzylkum. The registration and determination of the concentrations of lattice defects in detrital quartz were carried out by the EPR spectroscopy. Their stability in quartz under natural conditions was evaluated by comparing the concentrations of lattice defects in the near-surface and deep zones of quartz grains. The etching of quartz grains in HF was used to study the spatial location of lattice defects in the mineral. The genetic analysis of detrital quartz was carried out using the methods tested in the study of quartz from crystalline rocks.Results. The presence of substitutional Al, Ti, and Ge impurities, radiation induced E1-, Al–O−- and Ti-centers, as well as other paramagnetic centers possessing genetic information was found in quartz. The distribution of substitutional impurities in clastic quartz is close to their original distribution formed at the time of the mineral formation. This creates the prerequisites for identifying genetically similar quartz samples and attributing them to the certain provenance. The formation of sedimentary rocks of the Central Kyzylkum occurred due to two main provenance sources of terrigenous material. The expediency of using radiation induced centers with different thermal stability for studying the radiation prehistory of quartz and the conditions for the formation of uranium deposits is demonstrated.Conclusion. The clastic quartz of sedimentary deposits retains the main part of the genetically significant lattice defects formed during crystallisation. The most important of them are Al, Ti, and Ge isomorphic impurities. The regularities of their distribution, along with the distribution of radiation induced lattice defects in quartz, are able to carry information about the provenance of detrital material and the conditions of sedimentation and associated formation of mineral deposits.

Thermal conductivity of ThO2: Effect of point defect disorder

Thoria (ThO2) has lately gained attention due to its potential for use as a nuclear fuel. From a physics standpoint, ThO2 is an actinide-bearing material with no 5f electrons and is thus ideally suited as a baseline material for future studies of the physical properties of actinide systems with correlated electrons. Current investigations of ThO2 as a nuclear fuel focus on the influence of radiation-induced lattice defects on its thermal properties, especially the conductivity. This work presents a first investigation of the impact of point defect disorder on phonon thermal conductivity of ThO2 by solving the Boltzmann transport equation within the single-mode relaxation time approximation. The relaxation times of intrinsic, three-phonon scattering are calculated by a rigorous sampling of k-points within the irreducible Brillouin zone of the face-centered cubic crystal structure. The effect of point defects on the thermal conductivity of ThO2 is predicted using the classic model by Klemens for phonon relaxation times that result from the change in mass and induced lattice strain associated with point defects. Within this model, the change in force constants and atomic radii are computed using input from an atomistic model of ThO2. The defects considered are uranium substitution at a thorium site, oxygen vacancies and interstitials, and thorium vacancies and interstitials. The results show that the conductivity of ThO2 is highly sensitive to intrinsic point defects and less sensitive to U substitution on the cation sublattice.

Irradiation effects of tungsten-potassium alloy under 5 MeV proton

Large-size tungsten-potassium (WK) alloy was prepared by powder metallurgy and deformation processing technology. Spark plasma sintering (SPS) and rolling at high temperatures were used to develop this kilogram-scale WK ingot. In order to probe the irradiation-induced lattice defects in WK, proton irradiation with energy of 5 MeV and fluence rate of 1.11 × 1016 ion/cm2/h was conducted to produce step-wise displacement damage at different temperatures. Irradiation-induced dislocation loops were detected, and the size and density of dislocation loops changed progressively at different irradiation conditions. The average size of loops increased while the loop densities substantially decreased as irradiation temperature went up. With increasing the displacement damage, the loop size kept increasing slowly, while the density increased gradually. Compared with displacement damage, the evolution of dislocation loops was more sensitive to temperature. It was possibly due to the sinks in WK, such as the potassium bubble and the implanted hydrogen, altering interstitial tungsten atoms' diffusion.

Influence of copper precipitates on clustering behavior of alloying elements observed in Japanese reactor pressure vessel surveillance materials using atom probe tomography

In a highly irradiated reactor pressure vessel (RPV), solute Mn, Ni, and Si (MNS) atoms gather to form nanometer-sized microstructures, generally called MNS clusters. MNS often gather with Cu-rich precipitates, which can form in RPVs following lower dose irradiation. In this study, surveillance specimens provided from four nuclear power plants in Japan were analyzed using three-dimensional atom probe tomography (APT), and the nature of the solute enrichment was carefully compared. When analyzing the chemical composition of each cluster, a clear negative correlation was found between Si and Cu in all materials, but conversely, Mn was likely present in clusters with a high Cu concentration. Moreover, in a boiling water reactor material with high Cu, the ratio of MNS was shown to be similar to that of the Γ2 phase [Mn (Ni, Si)2]. In pressurized water reactor materials with medium and low Cu, however, Ni and Si enrichment was demonstrated to be higher than the ratio of the expected intermetallic compounds; such as a Γ2 phase and a G phase [Mn6 Ni16 Si7]. Ni and Mn atoms, once enriched in a copper-rich region, may elute out, and form an intermetallic compound with Si atoms within the vicinity. Particularly in highly irradiated RPV materials, such a structure may tend to be decorated by irradiation-induced lattice defects with Si segregation.

Macroporous SiC Formation in Anodizing Triggered by Irradiation-Induced Lattice Defects

Silicon carbide (SiC) attracts a lot of attention for several useful applications such as semiconductor devices and structural materials under severe conditions because of outstanding electronic pr...

Charge effect on the irradiation damage of silicon: Insights from phase-field simulation

We have developed a phase-field model to analyze the evolution of irradiation-induced lattice defects and variation of electrical properties of intrinsic single-crystal silicon by taking account of the charge effect on the formation energies of vacancies and interstitials. Significant difference in vacancy concentration at a long irradiation time has been found by comparing the defect behavior with and without charge. The simulated results indicate a critical carrier production rate at which the lattice defects kinetics speeds up obviously and thus leads to a higher vacancy concentration. The electrostatic potential and carrier density increase as more carriers are generated, which in turn would have important impact on the device performance.

Influence of Irradiation on Defect Spin Coherence in Silicon Carbide

Irradiation-induced lattice defects in silicon carbide (SiC) have already exceeded their previous reputation as purely performance-inhibiting. With their remarkable quantum properties, such as long room-temperature spin coherence and the possibility of downscaling to single-photon source level, they have proven to be promising candidates for a multitude of quantum information applications. One of the most crucial parameters of any quantum system is how long its quantum coherence can be preserved. By using the pulsed optically detected magnetic resonance (ODMR) technique, we investigate the spin-lattice relaxation time ($T_1$) and spin coherence time ($T_2$) of silicon vacancies in 4H-SiC created by neutron, electron and proton irradiation in a broad range of fluences. We also examine the effect of irradiation energy and sample annealing. We establish a robustness of the $T_1$ time against all types of irradiation and reveal a universal scaling of the $T_2$ time with the emitter density. Our results can be used to optimize the coherence properties of silicon vacancy qubits in SiC for specific tasks.

Atomistic simulation study of clustering and evolution of irradiation-induced defects in zirconium

Zirconium (Zr) alloys have been widely used as structural materials for in-core components in water-cooled nuclear reactors. During normal operation, these materials are exposed to a neutron flux in the core of the reactor, resulting in material degradation such as irradiation-induced anisotropic growth, thus affecting their performance in the long-term. Experimental and theoretical studies have shown that irradiation-induced defects in zirconium lead to the formation of defect clusters and loops. The anisotropy in the migration of defects has been suggested to play an important role in irradiation growth in pure Zr and its alloys. However, the mechanisms that govern the microstructural evolution that lead to the observed anisotropic growth of Zr is still unclear. In the present work, we perform a molecular dynamics simulation study of irradiation-induced lattice defects in Zr to investigate the formation of clusters and loops. Irradiation-induced damage is modeled by constrained stochastic formation of vacancies and self-interstitial atoms in bulk Zr. Using this approach, the formation and evolution of defect clusters and loops were determined. The dynamic properties of lattice defect structures were investigated through the evaluation of their migration and diffusivity. We found that the diffusivity of vacancy and interstitial clusters is anisotropic and slow, while the diffusivity of large loops is relatively high and confined to the ⟨a⟩ plane.

Irradiation-induced microstructure damage in He-irradiated 3C-SiC at 1000℃

The effect of the high-temperature helium irradiation on microstructural evolution of 3C-SiC was investigated by the combination of Raman spectroscopy, conventional transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). 3C-SiC wafers were irradiated with 130 keV He+ ions at fluences of 2 × 1016 He+/cm2, 4 × 1016 He+/cm2 and 2 × 1017 He+/cm2 at 1000℃. Helium bubbles, dislocation loops, and their interaction with the stacking faults were focused on and characterized by TEM. Helium bubbles preferentially nucleate and grow on stacking faults. Bubble links on the (100) plane in 3C-SiC are formed. In addition, stacking faults can effectively trap irradiation-induced lattice defects to enhance their recovery. The type of irradiation-induced lattice defects and elemental distribution are also investigated. The research results are valuable for the 3C-SiC used in the advanced nuclear energy systems.

In-situ study of heavy ion irradiation induced lattice defects and phase instability in β-Zr of a Zr–Nb alloy

In-situ heavy ion (1 MeV Kr2+) irradiation was carried out to investigate the evolution of irradiation induced lattice defects at room temperature and the stability of β-Zr under irradiation at temperatures ranging from 150 to 350 °C. It was found that the irradiation induced defects started to appear at a dose between 0.5 dpa and 0.75 dpa, which is relatively high compared to the case for α-Zr. The number density of dislocation loops increased exponentially with fluence, with an exponential index ∼3.52. The statistical results show that the electron beam with which in-situ observation was carried out had an influence on the size of the dislocation loops. The g·b analysis shows that dislocation loops with Burgers vector ½<1¯11> and <100> are both present in the sample after irradiation to 1.5 dpa. ω phase particles precipitate out during irradiation at 250 and 350 °C, as a result of the combination of the thermal treatment and irradiation enhanced concentration of point defects.

Evolution of radiation-induced lattice defects in 20/25 Nb-stabilised austenitic stainless steel during in-situ proton irradiation

We have monitored in situ the lattice defect evolution induced by proton irradiation in 20Cr-25Ni Nb-stabilised stainless steel, used as fuel cladding material in advanced gas-cooled reactors. At 420 °C, the damaged microstructure is mainly characterised by black spots and faulted a03〈111〉 Frank loops. Defect saturation is reached at only 0.1dpa. In contrast, at 460 °C and 500 °C proton bombardment induces the formation of a mixture of a03〈111〉 Frank loops and perfect a02〈110〉 loops. These perfect loops evolve into dislocation lines that form a dense network. This transition coincides with the saturation in the dislocation loop size and number density at 0.8dpa (460 °C) and 0.2dpa (500 °C), respectively. The presence of a high density of dislocation loops and lines at those two temperatures causes a vacancy supersaturation in the matrix, leading to the formation of voids and stacking fault tetrahedra.

Extreme radiation hard thin film CZTSSe solar cell

In this work, we have demonstrated the extreme radiation hardness of thin film CZTSSe solar cells. Thin film solar cells with CZTSSe, CZTS and CIGS absorber layers were irradiated with 3 MeV protons. No degradation in device parameters was observed until a displacement damage dose of 2 × 1010 MeV/g for CZTS and CZTSSe. CIGS solar cells degraded by 13% at the same dose. For the highest proton dose both the CZTSSe and CZTS degraded by 16% while CIGS suffered from 34% degradation in efficiency. The degradation in efficiency maybe attributed to the reduction in the minority carrier lifetime due to radiation induced lattice defects. Comparisons with previously available literature show that our CZTS technology has superior radiation hardness by about two orders of magnitude compared to existing state of the art Si and GaAs technology.

Atomically resolved chemical ordering at the nm-thick TiO precipitate/matrix interface in V-4Ti-4Cr alloy

We have used advanced analytical electron microscopy to characterise the local structure and chemistry at the interface between nm-thick TiO precipitates and the V-based matrix in a V-4Ti-4Cr alloy. Our results reveal the presence of an intergrowth between the fcc TiO and bcc vanadium structures, with a repeat lattice distance that equals 2.5 times the vanadium lattice parameter along the c-axis. Our atomic resolution analysis of the interface will impact the mechanistic understanding of its interaction with interstitials and radiation-induced lattice defects, and consequently trigger the development of improved alloy structures with interfaces engineered for enhanced radiation tolerance.

A New Method to Increase the Doping Efficiency of Proton Implantation in a High-Dose Regime

Proton-implantation doping offers a feasible method to modify the doping profile in semiconductor grade silicon in depths ranging up to several hundred micrometers at comparatively low thermal budgets. The hydrogen-related donors are radiation-induced defect complexes decorated by hydrogen. During the proton implantation, typically in the energy range of several 100 keV to several MeV and with fluences in the range between about 1013 cm-2 and 1015 cm-2, intrinsic radiation defects are induced. During a successive anneal in the temperature range between about 300—500 °C the implanted hydrogen diffuses from its projected range through the radiation damage profile and decorates the radiation-induced defects or defect complexes, thus activating, the hydrogen-related donors. Due to this basic mechanism, the profile shape of the hydrogen-related donor profiles is closely correlated to the initial damage profile of the radiation-induced lattice defects. The profiles, hence, exhibit an extended penetrated range with an approximately constant concentration followed by an expressed peak near the end-of-range of the protons.The achievable doping concentration of the hydrogen-related donors induced by the proton implantation and successive annealing is, however, typically limited to a few 1015 cm-3. Earlier studies, based mainly on co-implantations of helium and hydrogen, have led us to propose this to be due to an over-decoration effect of the hydrogen-related donors by excess hydrogen. Based on this assumption, we present a pre-conditioning method based on vacancy-related gettering sites in order to reduce the concentration of excess hydrogen. The procedure comprises a pre-implant with protons and an annealing step at elevated temperatures above the range typically used for proton-implantation doping and is thus fully applicable to a commercial manufacturing environment. During this pre-conditioning, thermally stable higher-order defect complexes are created that are for the most part electrically inactive. These defect complexes act as gettering centers and may in effect reduce the concentration of free hydrogen after a second implant. Experimental results based on spreading resistance measurements are presented in this report that clearly show the beneficial effect of the pre-conditioning and its ability to help overcome several limitations of proton-implantation doping of high-purity silicon.The figure illustrates the effect of the new pre-conditioning method on two magnetic Czochralski-grown silicon wafers. The dashed line represents the carrier concentration profile after a 3-MeV implantation with a proton dose of 4×1014 cm-2 and subsequent annealing at 490 °C for 5 h. The solid line depicts a profile resulting from the same main process steps where the wafer has been pre-conditioned before. It appears that the peak concentration could be increased by a factor of 3 to approximately 2×1015 cm-3. For consecutive studies it may also be of interest that the pre-conditioning results in a steadily increasing carrier concentration up to a depth of 80 µm from the surface.

Radiation induced microstructures in ODS 316 austenitic steel under dual-beam ions

An ODS 316 austenitic steel was fabricated and irradiated using dual ion beams (1MeV Kr+ and 15keV He+) with in-situ transmission electron microscope (TEM) observation. Cavities formed at a low dose in samples irradiated with simultaneous helium injection. It was found that Y–Ti–O particles acted as strong traps for cavity formation at low doses. Helium exhibited a significant effect on cavity development. Cavities were also preferentially nucleated along grain boundaries, phase boundaries and twin boundaries. Irradiation induced lattice defects mainly consisted of small 1/2〈110〉 perfect loops and 1/3〈111〉 Frank loops. An increment of helium injection rate also greatly enhanced the Frank loop growth. Small (<10nm) Y–Ti–O particles were found to be unstable after irradiation to high doses. M23C6 precipitates were observed after irradiation and helium might play a major role in their formation.

Evaluation of Degradation due to Electron Irradiation of Si&lt;sub&gt;1-x&lt;/sub&gt;C&lt;sub&gt;x&lt;/sub&gt; S/D n-type MOSFETs

To solve the problem of the limitation to improve device performance in standard Si integration technologies and to develop radiation-harsh devices, the irradiation effects of Si1-xCx source/drain (S/D) n-type metal oxide semiconductor field effect transistors (n-MOSFETs) have been investigated. It is shown that the drain current and the maximum electron mobility of Si1-xCx n-MOSFETs decrease by electron irradiation. The reduction of the device performance can be explained by the radiation-induced lattice defects in the devices. However, the electron mobility enhancement effect by adding C remained after an electron irradiation up to 5×1017 e/cm2.