High Migration Energy Research Articles

Uncovering the combining V with Ni micro-addition alloying on the mechanical properties and multiphase transformation during solidification and homogenization of Al–Zn–Mg–Cu–Sc alloy

The ultimate tensile strength of the T6-aged Al–Zn–Mg–Cu–Sc alloy is improved by 24 MPa reached 667 MPa with the addition of 0.10 % V, 0.20 %Ni and decrease 0.1 %Sc. The thermodynamic and microalloying mechanisms of phase transformation during solidification and homogenization of Al–Zn–Mg–Cu–Sc–V–Ni alloy were clarified. The phase transformation model of Al3Sc (V, Ni) structure induced by V and Ni addition during homogenization was established. The calculation radius showed that coherent L12-ordered Al3Sc(V, Ni) nanoparticles were formed in the Al–Zn–Mg–Cu–Sc–V–Ni alloy, which had a small critical nucleation radius (22.0 nm). The large thermodynamic driving force and the high kinetic energy migration energy barrier of γ-Al7Cu4Ni and Al3Sc (V, Ni) phases led to the finer microstructure of grain. V element not only promoted the formation of Al21V2 phase containing Sc but also hindered the mutual diffusion of Al and (Sc, Ni) elements in the eutectic system via segregation at the α-Al/Al3Sc (V, Ni) interface, which improved the stability of the whole alloy. The edge nucleation of γ-Al7Cu4Ni phase upon its diffusion into a Sc-containing Al21V2 phase induced the formation of Al3Sc (V, Ni) structure, whereas the segregation of V and Ni further strengthened the alloy matrix.

Exploring the suppression methods of helium-induced damage in tungsten by investigating the interaction between beryllium and helium

Abstract Helium (He)-induced damage is a sensitive concern for the performance of tungsten plasma facing materials (W-PFMs). Recent experiments have revealed that trace impurities in He plasma can effectively prevent the formation of He bubbles and fuzz on W surfaces. To explore its plausibility and underlying mechanism, we performed a multiscale computational study that combines density functional theory calculations and object kinetic Monte Carlo simulations to investigate the effects of a small quantity of beryllium (Be) on the evolution of He bubbles. It is found that there is a strong attractive interaction between He and Be, which can be attributed to the decrease in electron density and the lattice distortion induced by embedded Be atoms. Therefore, the co-implantation of Be continuously introduces trapping centers for He. Due to the low implantation depth and high migration energy of Be, the Be atoms are located close to the surface, leading to the trapping of the majority of He within the near-surface region and the development of a shielding layer for He permeation. The presence of Be facilitates the dispersion of the trapped He, skewing the He clusters into smaller sizes. More importantly, the Be trapping centers bring the He clusters closer to the surface, significantly increasing the probability of bubble bursting and the release of He back to the vacuum. This ultimately leads to a lower retention of He in the case of He + Be co-irradiation, compared with the case of He-only irradiation. Consequently, our findings elucidate the suppressive effect of a low flux of Be atoms on the growth of He bubbles, highlighting the need to focus on synergetic effects between plasma species.

Insights into novel indium catalyst to kW scale low cost, high cycle stability of iron-chromium redox flow battery

Iron-chromium flow batteries (ICRFBs) have emerged as an ideal large-scale energy storage device with broad application prospects in recent years. Enhancement of the Cr3+/Cr2+ redox reaction activity and inhibition of the hydrogen evolution side reaction (HER) are essential for the development of ICRFBs and require a novel catalyst design. However, elucidating the underlying mechanisms for modulating catalyst behaviors remains an unresolved challenge. Here, we show a novel precisely controlled preparation of a novel thermal-treated carbon cloth electrode with a uniform deposit of low-cost indium catalyst particles. The density functional theory analysis reveals the In catalyst has a significant adsorption effect on the reactants and improves the redox reaction activity of Cr3+/Cr2+. Moreover, H+ is more easily absorbed on the surface of the catalyst with a high migration energy barrier, thereby inhibiting the occurrence of HER. The assembled ICRFBs have an average energy efficiency of 83.91% at 140 mA/cm2, and this method minimizes the electrodeposition process and cleans the last obstacle for industry long cycle operation requirements. The ICRFBs exhibit exceptional long-term stability with an energy efficiency decay rate of 0.011% per cycle at 1000 cycles, the lowest ICRFBs reported so far. Therefore, this study provides a promising strategy for developing ICRFBs with low costs and long cycle life.

Diminishing the migration resistance of zinc ions by cation vacancy engineering in a spinel-framework

ZnMn2O4 with Zn vacancies has been constructed via alkaline etching, which shows lower diffusion energy barriers for Zn2+ than ZnMn2O4.

Enhancement of performance and kinetics of layered Li2MnO3 by Fe doping

Layered Li2MnO3 exhibits great potential as a high-capacity cathode used in lithium-ion batteries, but materials synthesized by traditional solid-phase methods exhibit poor activity and poor cycling stability. In this work, Li2MnO3 nanoparticles were synthesized by solid state method combined with high-energy ball milling, in which Fe was doped at transition metal sites to obtain a Li-rich cathode material (Li1.32Fe0.034Mn0.64O2). We systematically studied the effects of Fe doping and calcination temperature on the crystal structure and electrochemical properties and conducted kinetic studies using the galvanostatic intermittent titration technique (GITT), electrical impedance spectroscopy (EIS) and density functional theory (DFT) calculations. Our results demonstrate that the performance was significantly improved, its discharge capacity is about 235 mAh g−1 after 10 cycles compared 138 mAh g−1 of the pristine sample. The GITT and EIS results show that the Fe-doped sample had smaller charge transfer impedance, larger lithium-ion diffusion coefficient and lower internal polarization than the pristine. The activation energy results show the Fe-doped sample has larger activation energy at the late discharging period due to the anion redox reaction activation, indicting its O activation is more active than Li2MnO3. Moreover, the DOS results showed that the Fe-doped sample had a narrow band-gap, indicating good electrical conductivity. The CI-NEB results showed that the Fe-doped sample had a high transition metal ion migration energy barrier, indicating that its structure was stable. In conclusion, Fe doping is helpful to active the anion redox reaction in Li2MnO3 and improve its electrochemical performance as a low cost and high-capacity cathode materials.

Multiple-ring BP monolayer as adsorption material for lithium‑sulfur battery

The lithium‑sulfur batteries (LSBs) have drawn significant attention in the current energy research due to their higher energy density compared with the traditional rechargeable battery. However, the “shuttle effect” is one of the main obstacles and hinders practical application. In this work we investigated a boron‑phosphorus(BP) multiple-ring monolayer as an anchoring substrate for lithium‑sulfur batteries. The adsorption process of Li2Sn on BP monolayer was systematically studied by the physical and chemical adsorption, the analysis of charge density and the density of states(DOS) based on the first principle calculation. We found that the multiple-ring BP monolayer possesses desirable adsorption characteristics for Li2Sn species, ensuring the high-order Li2Sn can be adsorbed and suppress the shuttle effect effectively. Furthermore, the multiple-ring BP monolayer exhibits excellent electronic conductivity. The bandgap of BP monolayer decreased significantly with the lithiation process thus improving the conductivity. The migration process was investigated with a moderate energy barrier, facilitating the fast diffusion of Li2Sn on the BP monolayer. Such high adsorption performance, good conductivity, and moderate migration energy barriers indicate the BP monolayer can be a potential candidate for anchoring material of lithium‑sulfur battery.

SiC3N3 monolayer as a universal anode for alkali metal-ion batteries.

Our density functional theory calculations show that silicon doping in g-CN (SiC3N3) can improve the electrochemical performance of g-CN as an anode of alkali metal-ion batteries and solve the problems of too high adsorption ability and migration energy barrier commonly found in porous carbon nitride. The stability of SiC3N3 was verified by molecular dynamics simulations and phonon spectroscopy. Elastic constant calculations revealed that the Si doping in g-CN can improve its mechanical properties. Specifically, Li/Na/K has a suitable adsorption capability (-0.71/-0.52/-0.98 eV) and a lower migration barrier (0.73/0.43/0.21 eV) on SiC3N3, where the barrier of a single Li-ion is the lowest among the doped porous carbon nitride materials studied so far. Moreover, SiC3N3 exhibits a high theoretical capacity (253/1512/1512 mA h g-1) and a low open-circuit voltage (0.48/0.18/0.31 V) for Li/Na/K ion batteries. Compared with B-doped g-CN previously studied, Si doping can more effectively improve the electronic conductivity of g-CN owing to greater charge transfer between Si and g-CN; the migration energy barrier of alkali metal ions on SiC3N3 is reduced more significantly due to its puckered structure instead of a planar structure; and the capacity of SiC3N3 is nearly doubled for alkali metal ion batteries because it has more feasible adsorption sites for alkali metals. These results suggest that Si-doped g-CN can be a universal anode material for alkali metal ion batteries.

Promising anode materials for alkali metal ion batteries: a case study on cobalt anti-MXenes.

There is continuous demand for energy storage devices with high energy densities in consumer electronics, electric vehicles, and the grid energy market. Although commercial lithium-ion batteries (LIBs) satisfy the current needs, the limited availability of their raw materials and the moderate specific charge capacities (SCCs) of LIBS have motivated scientists to search for alternate anode materials for LIBs and create technologies beyond LIBs. In this work, we studied the potential of six cobalt anti-MXenes (CoAs, CoB, CoP, CoS, CoSe, and CoSi), a class of newly discovered 2D materials, as anode materials for lithium, sodium, and potassium ion batteries (LIBs, NIBs, and KIBs). We found that these materials are good electrical conductors and have high adsorption stability for alkali metal ions, which helps to prevent the formation of dendrites and increase the cycle life of the battery. They also show moderate to low migration energy barriers (MEBs), indicating the potential for faster charge-discharge kinetics. We also explain the slightly counter-intuitive result of observing low MEBs along with high adsorption stability. Furthermore, Co-anti-MXenes can adsorb multiple alkali atoms per formula unit, resulting in high specific charge capacities and low average anodic voltages. For example, as anode materials for lithium-ion batteries, CoP and CoSi have SCC values of 1075.4 mA h g-1 and 934 mA h g-1, and anodic voltages as low as 0.28 V and 0.43 V, respectively. Moreover, even the maximally metalated Co-anti-MXenes did not show agglomeration tendency at room temperature. Furthermore, the volume expansion of these materials is minimum for both Li and Na adsorption. As a whole, we find that Co-anti-MXenes are promising as anode materials for alkali metal ion batteries.

Reversible proton co-intercalation boosting zinc-ion adsorption and migration abilities in bismuth selenide nanoplates for advanced aqueous batteries

Rechargeable aqueous zinc-ion batteries present low-cost, safe, and environmentally-friendly battery technology but suffer from the limited choice of cathode materials because of the sluggish kinetics of divalent zinc-ion associated with the high adsorption and migration energy barrier. Herein, a reversible zinc/bismuth selenide mild aqueous system was demonstrated for the first time, where bismuth selenide nanoplate cathode delivers a high specific capacity of 263.2 mA h g−1 at 0.1 A g−1 and robust rate capability of 100.6 mA h g−1 even at 10 A g−1 with long-term lifespan (82.3% retention after 1000 cycles). Benefiting from the layered structure and nanoplate morphology of the bismuth selenide cathode, surface-dominated ion storage is verified by a quantitative kinetics analysis, particularly at high current rates. Notably, unlike conventional batteries with only the reversible intercalation of alkali ions into metal chalcogenides, zinc/bismuth selenide aqueous batteries possess a sequential proton and zinc-ion insertion/extraction process, identified by in situ synchrotron radiation-based X-ray diffraction. Density functional theory analysis approves the low adsorption energy and preferential embedding process of protons, and that can further optimize Zn2+ adsorption and migration abilities in bismuth selenide nanoplate, which is mainly responsible for the excellent performance.

Hydrogen induced slowdown of spallation in high entropy alloy under shock loading

Hydrogen embrittlement is ubiquitous in metals and alloys exposed to hydrogen, which has been extensively studied over a century. In contrast to traditional alloys, mechanisms of hydrogen embrittlement under shock loading are poorly understood, especially in recently emerging multi-principle element and chemically disordered high entropy alloys (HEAs). By using a specially designed double-target technique, an unexpected phenomenon of hydrogen-retarded spallation was observed in CrMnFeCoNi HEA under plate impact loading. To reveal the underlying mechanism, a trans-scale statistical damage mechanics model was developed based on microstructural characterization and first principles calculations. The hydrogen-retarded nucleation of micro-voids is attributed to hydrogen-vacancy complexes with high migration energy, while formation of nano-twins with high resistance reduces their growth rate. These results shed light on the better understanding of hydrogen embrittlement in chemically complex HEAs.

Theoretical calculation assisted materials screening of BiOX (X = F, Cl, Br, I) for electrochemical absorption of cesium ions.

The electrochemical switching ion exchange (ESIX) technique has been widely used for the separation and recovery of radioactive cesium ions (Cs+) from wastewater. In this study, a series of BiOX (X = F, Cl, Br, I) materials were first evaluated for their absorption properties to Cs+ through density functional theory (DFT) calculations. The calculations predict that BiOBr has the best absorption performance among the four materials, BiOF, BiOCl, BiOBr, and BiOI, due to its high absorption energy and low ion migration energy barrier to Cs+. Simultaneously, the selectivity calculations revealed that BiOBr also showed the best selectivity for Cs+ compared with Li+ and Na+. Subsequently, four materials were prepared using the hydrothermal synthesis method and their electrochemical absorption performance was tested. The results showed that BiOBr has the highest electroactivity, and its absorption capacity was up to 16 mg Cs+/g BiOBr in a solution mixture of 50 ppm Li+, Na+, and Cs+. Based on our theoretical calculations and experiments, our findings provide prospective insights for predicting the electrochemical absorption performance of materials using first-principles calculations.

Intrinsic defect migration in Be12Ti

Be12Ti is a leading candidate neutron-multiplier material for fusion breeder blankets; yet the evolution of the crystal defects under irradiation is poorly understood. Here, the migration of intrinsic defects in tetragonal Be12Ti was predicted using atomic scale computer simulation. Transport of titanium and beryllium through the interstitial, interstitialcy and vacancy-mediated models was considered, along with the migration of divacancy clusters, previously identified as important to the defect chemistry of Be12Ti. It was found that titanium defects exhibit much higher migration energies than beryllium for most migration pathways, leading to a dramatic difference in the self-diffusivity of the two species. Both beryllium vacancy and interstitial diffusion is close to isotropic with vacancy transport exhibiting the highest self-diffusion coefficient. Migration of beryllium di-vacancies is also isotropic with activation energy equal to that of isolated vacancies. The titanium interstitial exhibits significantly lower migration energy than its vacancy (1.00 eV and 6.75 eV respectively), with both mechanisms strongly anisotropic: the activation energy for [001] migration is at least 5 eV lower than other directions. Even the more exotic mixed titanium beryllium vacancy migration exhibits a much higher migration energy than [001] titanium interstitial transport. The framework used for predicting defect transport kinetics, including vacancy-mediated, interstitial and interstitialcy mechanisms, can be applied to any complex-structured intermetallic compound.

Simulation study of helium bubble coalescence in tungsten at various temperatures relevant to fusion conditions

Molecular dynamics (MD) methods are used to study nanosized helium (He) bubble coalescence process in tungsten (W) at various temperatures relevant to fusion conditions, on an atomistic scale. Bubble coalescence in W is observed at a higher temperature and He/V ratio, while the calculated internal bubble pressure due to virial stress increases with the increase in the He/V ratio; bubble coalescence is significantly dependent on the bubble distance. In these MD simulations, coalescence occurs, only when the surface distance between the two bubbles is equal to 1a0, where a0 denotes the lattice constant and is approximately 0.317 nm at 2100 K. On the other hand, a bubble diameter between 1a0 and 3a0 may have relatively limited effect on the coalescence, although larger-sized bubbles may have higher migration energy. Local stress profile calculated indicates that initial bubbles can interact with each other, which may enhance the He atoms diffusion between bubbles and their coalescence. Physical contact at the initial stage of coalescence may occur between two nearby bubbles accompanied by W lattice distortion because of the limited displacement of W atoms near the bubbles and rapid migration of He atoms within the two bubbles. These results are beneficial for understanding the evolution of He bubbles in bulk W.

Growth mechanisms of Li dendrite and the suppression strategies

<p indent="0mm">Lithium metal battery (LMB) is widely acknowledged as one of the most promising battery technologies for nextgeneration energy storage systems due to its ultra-high energy density. Unfortunately, the practical application of LMB has been plagued by the uncontrollable growth of Li dendrites during electrochemical cycling, which would not only consume the electrolyte irreversibly and reduce its cycling lifespan, but also cause the internal short-circuit and result in severe safety issues. Aiming to effectively suppress or even eliminate the growth of Li dendrites, a great deal of strategies such as adding electrolyte additives, employing superconcentrated electrolytes, forming artificial solid electrolyte interphase (SEI) layers, physical modification of Li anodes and/or designing advanced structured Li anodes have been proposed. Although the cycling performance of LMBs has been significantly improved by above-mentioned approaches, yet their overall performance is still inferior to that of the commercialized lithium ion batteries (LIBs) counterpart. To further enhance the performance of LMBs so as to accelerate their research and development process, an in-depth and comprehensive understanding of the fundamental growing mechanisms of Li dendrites is highly desirable. To shed lights on designing novel strategies to suppress or eliminate the growth of Li dendrites, a comprehensive overview of the current understanding of the growth mechanisms of Li dendrites during electrochemical cycling is provided in the current article. Herein, several models proposed from both theoretical derivation and experimental observation are reviewed to fundamentally illustrate the formation and growth behaviors of Li dendrites. The space-charge model that is proposed to describe Li dendrite nucleation is firstly introduced. In this model, the dendritic Li is formed because of a very large space charge near the electrode surface that is created by the anion depletion during Li ion deposition. The SEI-induced growing model, which illustrates that the cracking/pinholes at the defects of SEI films could cause electrochemical hot spots and triggers the dendritic Li growth, is then presented. In addition, the heterogeneous nucleation and growth model, which considers both the thermodynamic and kinetic conditions to rationalize the nucleation and growth of Li dendrites, is also explained. After that, the surface nucleating and diffusion model, which indicates that lower surface energy and higher migration energy are two important factors for Li dendrite growth on the surface of Li metal, is briefly presented. The following is the tip-induced nucleation model, which states that the Li ion deposition occurs easily on the protrusions of metal tips due to the enhanced electric and ionic fields therein. Finally, the film growth model, which is inspired by the plasma enhanced chemical vapor deposition (PECVD) process, is introduced to explain the underlying growing behaviors of Li dendrites. In addition to these models proposed to illustrate the growing mechanisms of Li dendrites, the corresponding strategies, which are derived from these models and aimed to suppress the growth of Li dendrites, are also concisely presented. This review article ends with an insightful discussion together with some suggestions for future research directions of the Li dendrites. In brief, the current article tends to not only summarize the fundamental understandings regarding the growth mechanisms of Li dendrites but also provide a few instructive suggestions for future research activities of the LMBs.

Lithium vacancy migration in Li2O2: From first principles studies

Li ion diffusion in the discharge products (Li2O2) of the Li-O2 batteries is relevant to the performance of the battery. In this paper, Li-vacancy migration in the bulk and at the surfaces of Li2O2 are studied from first principles calculations. Three low index surfaces (0001), (112¯0) and (12¯00) are considered. Results show that Li-vacancy migration in the Li12O2 bulk is slow with the energy barrier of 0.41 eV. On the other hand, it is found that Li-vacancy migration on the (0001) and (11¯00) surface is very difficult with the high migration energy barriers of 1.22 eV and 1.06 eV, respectively, while on the (112¯0) surface, Li-vacancy migration is fast with the low energy barrier of 0.34 eV. Moreover, the formation energy of Li-vacancy on the (112¯0) surface is 2.65 eV, suggesting that the formation of Li-vacancy is the bottleneck for creating fast enough Li diffusion at the Li2O2 (112¯0) surface. These results will provide the fundamental understanding of Li ion diffusion in the discharge products of the Li-O2 battery.

The charge transfer of intercalated Li atoms around islands on Li-halide (F, Br, Cl) surface of SEIs: A first principles calculation

The solid electrolyte interface (SEI) not only play the role of surface protective film for lithium metal anodes in lithium-sulfur batteries but also use to suppress the growth of lithium dendrites during the charging process. The relationship between the morphology of the SEI and the deposition behavior of lithium atoms determines the effect of homogenization of the dendrites. The Li-halide (LiF, LiCl, and LiBr) solid electrolyte interface is comparatively investigates by comprehensive first-principles calculations on Li metal anodes. Surface adsorption, surface diffusion, and charge transfer of Li atoms during the deposition process on the Li-halide SEIs with aggregated Li–Ha2Li2 island configurations is systematically calculates to achieve the orientation of Li atoms from deposition. However, the Li atom has high migration energy regardless of the uniformity of the surface of LiF-SEI, which has high stability making the charge transfer of Li atoms less affected by the subneighbor and subsurface Ha atoms. The Li atom deposition behavior makes the charge transfer and lithium halide (LiHa) SEI potential tend to be evenly distributed, which in turn leads to uniform deposition of Li atoms.

Analytical bond-order potential for silver, palladium, ruthenium and iodine bulk diffusion in silicon carbide

The analytical bond-order potential has been developed for simulating fission product (Ag, Pd, Ru, and I) behavior in SiC, especially for their diffusion. We have proposed adding experimentally available elastic constants and physical properties of the elements as well as important defect formation energies calculated from density functional theory simulation to the list of typical properties as the extensive fitting database. The results from molecular dynamics simulations are in a reasonable agreement with defect properties and energy barriers of their experimental/computational counterparts. The successful validation of the new potential has established a good reliability and transferability of the potentials, which enables the ability of simulation in extended scale. The kinetic behavior such as diffusion of different interstitials is then realized by applying the new interatomic potentials. The bulk diffusion is less likely to dominate the transport of the four fission products under pure thermal condition, when we refer to their extremely small values of the effective diffusion coefficients. The interstitial mechanism is hard for Pd, Ru, and I to access due to the high formation energy and high migration energy. However, it is found that the migration energy of silver is relatively low, which indicates Ag diffusion via an interstitial mechanism being feasible, especially under irradiation condition, where massive interstitials can be formed in high-temperature nuclear reactors.

Polaronic Resistive Switching in Ceria‐Based Memory Devices

AbstractResistive random access memory (RRAM), or memristors, operating with a voltage‐controlled low‐ and high‐resistance state (ON/OFF), are a critical component for next‐generation nanoelectronics. Most memristors are based on oxides, but the underlying working mechanism remains generally unclear. Using first‐principles calculations, it is revealed that it is polaron that acts as the conducting species to mediate the resistive switching process in CeO2, while the commonly believed oxygen vacancy (VO2+) plays only a secondary role in assisting polaron formation. Importantly, polaron and related complexes have desired low formation energies (≈−0.3 eV) and extremely small migration barriers (≈0.1 eV), to synergistically form conductive filaments in the CeO2 matrix with shallow electronic states near Fermi level, while VO2+ has a much higher migration energy and does not change the insulating nature of CeO2. A switching field is also estimated of ≈3 V between the ON/OFF states from the relative stability of VO2+, Hint+/Hsub+ (institutional/substitutional hydrogen) and polaron complexes in reference to Fermi level, which agrees with experiments. The proposed polaron‐based switching mechanism is general, paving the way for future understanding and design of multifunctional electronic nanodevices beyond RRAM.

Fracture Along Deformation Twin Boundary in Small‐Volume Fe40Mn40Co10Cr10 High Entropy Alloy

High‐entropy alloys (HEAs) with extraordinary combination of strength, ductility, and toughness have drawn extensive attention in recent years. One of widely accepted explanations for the marked mechanical performance in bulk HEAs is due to the tendency of deformation twinning. Here, the authors perform in situ tensile test on small‐volume non‐equiatomic Fe40Mn40Co10Cr10 HEA single crystals, and find that deformation twinning is a typical intrinsic factor for accelerating fracture. Profuse needle‐like deformation twins are nucleated under loading, while their thickening is very difficult due to high twin boundary migration energy, thus nanoscale surface crack is easily produced, which triggers catastrophic fracture.

Helium in-plane migration behavior on 〈1 0 0〉 symmetric tilt grain boundaries in tungsten

We present the results of an atomistic modeling study of small helium cluster migration in the plane of symmetric tilt grain boundaries. The relevant migration pathways and energies were determined by way of temperature accelerated dynamics and the nudged elastic band method. We find that small helium clusters show much higher migration energies when bound to the grain boundary than in the bulk for all types of grain boundaries, indicating strongly-impeded helium transport behavior. Larger helium clusters (up to three helium atoms) tend to have higher migration energies compared with smaller clusters. Longer-distance migrations also tend to have higher migration energies, but helium cluster migration is highly affected by the structure of the grain boundary. The binding energy of the grain boundaries studied is high enough that helium clusters would be unlikely to leave the grain boundary plane. However, vacancy migration energies are relatively low compared to the bulk, and are also much lower than helium cluster migration energies on the grain boundary plane. This suggests that helium cluster migration on the grain boundary is actually governed by the rate of vacancy migration: in the bulk, helium clusters are mobile, but they become bound to and immobilized by grain boundaries, forming bubbles. Bubbles, however, are likely more mobile on the grain boundary than they are in the bulk due to the increased rate of vacancy migration on the grain boundary. We expect similar migration behavior for other types of grain boundaries because of the increased excess volume found near all grain boundaries.