Molecular dynamics simulations of displacement cascades in LiAlO2 and LiAl5O8 ceramics.

Molecular dynamics was employed to investigate the radiation damage due to collision cascades in LiAlO2 and LiAl5O8, the latter being a secondary phase formed in the former during irradiation. Atomic displacement cascades were simulated by initiating primary knock-on atoms (PKA) with energy values = 5, 10 and 15 keV and the damage was quantified by the number of Frenkel pairs formed for each species: Li, Al and O. The primary challenges of modeling an ionic system with and without a core-shell model for oxygen atoms were addressed and new findings on the radiation resistance of these ceramics are presented. The working of a variable timestep function and the kinetics in the background of the simulations have been elaborated to highlight the novelty of the simulation approach. More importantly, the key results indicated that LiAlO2 experiences much more radiation damage than LiAl5O8, where the number of Li Frenkel pairs in LiAlO2 was 3-5 times higher than in LiAl5O8 while the number of Frenkel pairs for Al and O in LiAlO2 are ~ 2 times higher than in LiAl5O8. The primary reason is high displacement threshold energies (Ed) in LiAl5O8 for Li cations. The greater Ed for Li imparts higher resistance to damage during the collision cascade and thus inhibits amorphization in LiAl5O8. The presented results suggest that LiAl5O8 is likely to maintain structural integrity better than LiAlO2 in the irradiation conditions studied in this work.

Uncertainty quantification for Bayesian active learning in rupture life prediction of ferritic steels.

Three probabilistic methodologies are developed for predicting the long-term creep rupture life of 9-12 wt%Cr ferritic-martensitic steels using their chemical and processing parameters. The framework developed in this research strives to simultaneously make efficient inference along with associated risk, i.e., the uncertainty of estimation. The study highlights the limitations of applying probabilistic machine learning to model creep life and provides suggestions as to how this might be alleviated to make an efficient and accurate model with the evaluation of epistemic uncertainty of each prediction. Based on extensive experimentation, Gaussian Process Regression yielded more accurate inference ([Formula: see text] for the holdout test set) in addition to meaningful uncertainty estimate (i.e., coverage ranges from 94 to 98% for the test set) as compared to quantile regression and natural gradient boosting algorithm. Furthermore, the possibility of an active learning framework to iteratively explore the material space intelligently was demonstrated by simulating the experimental data collection process. This framework can be subsequently deployed to improve model performance or to explore new alloy domains with minimal experimental effort.

Energy-dispersive X-ray spectroscopy and atom-probe tomography data quantifying component-ratios of multicomponent nano-precipitates in ion-irradiated ceria.

Samples of ∼1 µm films of CeO2 doped with 2 wt% Mo, 1.5 wt% Ru, 0.75 wt% Pd, 0.5 wt% Re and 0.25 wt% Rh grown with pulsed laser deposition were irradiated with I2+ ions (610 °C and 730 °C, 1016 and 5 × 1016 I2+/cm2). For selected samples post-irradiation heat treatment was conducted (900 °C, 1100 °C). The specimens were sectioned with focused ion beam milling and characterized in a transmission electron microscope with energy-dispersive x-ray spectroscopy, and with atom-probe tomography. Energy-dispersive x-ray spectroscopy was used to obtain elemental maps showing the distribution of dopants in the specimen after exposure. Some of these maps are discussed in detail in our companion article "Formation of multicomponent alloy particles in doped ceria under I2+ ion irradiation and thermal annealing" in the Journal of Nuclear Materials [1]. Advanced computational analysis could be used to more accurately quantify local compositions. Data is provided for additional regions of interest and one additional irradiation condition. The doped ceria film that was heat treated at 1100 °C delaminated from the substrate in most places. Samples were extracted from the underside of a delaminated piece and analyzed with atom-probe tomography. The resulting data show ceria and a Mo-rich particle and demonstrate that this approach is feasable in principle to study local compositions in a sample exposed to such extreme conditions.

Machine learning assisted prediction of the Young's modulus of compositionally complex alloys.

We identify compositionally complex alloys (CCAs) that offer exceptional mechanical properties for elevated temperature applications by employing machine learning (ML) in conjunction with rapid synthesis and testing of alloys for validation to accelerate alloy design. The advantages of this approach are scalability, rapidity, and reasonably accurate predictions. ML tools were implemented to predict Young's modulus of refractory-based CCAs by employing different ML models. Our results, in conjunction with experimental validation, suggest that average valence electron concentration, the difference in atomic radius, a geometrical parameter λ and melting temperature of the alloys are the key features that determine the Young's modulus of CCAs and refractory-based CCAs. The Gradient Boosting model provided the best predictive capabilities (mean absolute error of 6.15 GPa) among the models studied. Our approach integrates high-quality validation data from experiments, literature data for training machine-learning models, and feature selection based on physical insights. It opens a new avenue to optimize the desired materials property for different engineering applications.

A machine learning aided interpretable model for rupture strength prediction in Fe-based martensitic and austenitic alloys.

The class of 9-12% Cr ferritic-martensitic alloys (FMA) and austenitic stainless steels have received considerable attention due to their numerous applications in high temperature power generation industries. To design high strength steels with prolonged service life requires a thorough understanding of the long-term properties, e.g., creep rupture strength, rupture life, etc., as a function of the chemical composition and processing parameters that govern the microstructural characteristics. In this article, the creep rupture strength of both 9-12% Cr FMA and austenitic stainless steel has been parameterized using curated experimental datasets with a gradient boosting machine. The trained model has been cross validated against unseen test data and achieved high predictive performance in terms of correlation coefficient ([Formula: see text] for 9-12% Cr FMA and [Formula: see text] for austenitic stainless steel) thus bypassing the need for additional comprehensive tensile test campaigns or physical theoretical calculations. Furthermore, the feature importance has been computed using the Shapley value analysis to understand the complex interplay of different features.

A new non-diffusional gas bubble production route in used nuclear fuel: implications for fission gas release, cladding corrosion, and next generation fuel design.

A novel relationship between noble metal phase particles and fission gas bubble production in used nuclear fuel is described. The majority of Te atoms within noble metal phase undergo radioactive decay to form stable Xe within a few hours after particle formation. This results in the production of clusters of Xe atoms contained within the solid metal matrix exhibiting an equivalent gas bubble pressure approaching 1 GPa. These high pressure bubbles are stabilized by the UO2 within the bulk of the fuel. However, when these bubbles form near the fuel/cladding interface, in combination with local and temporal damage caused by fission recoil, they are capable of overcoming the fracture strength of the UO2 and rupturing catastrophically. The force of the resulting bubble rupture is sufficient to eject noble metal phase particles several microns into the cladding. This proposed mechanism explains the observance of noble metal phase in cladding and is consistent with a host of morphological features found near the fuel/cladding interface.

Highly Selective Supported Graphene Oxide Membranes for Water-Ethanol Separation.

A polyethersulfone (PES)-supported graphene oxide (GO) membrane has been developed by a simple casting approach. This stable membrane is applied for ethanol/water separation at different temperatures. The 5.0 µm thick GO film coated on PES support membrane showed a long-term stability over a testing period of one month and excellent water/ethanol selectivity at elevated temperatures. The water/ethanol selectivity is dependent on ethanol weight percentage in water/ethanol feed mixtures and on operating temperature. The water/ethanol selectivity was enhanced with an increase of ethanol weight percentage in water/ethanol mixtures, from below 100 at RT to close to 874 at a 90 °C for 90% ethanol/10% water mixture. Molecular dynamics simulation of water-ethanol mixtures in graphene bilayers, that are considered to play a key role in transport, revealed that molecular transport is negligible for layer spacing below 1 nm. The differences in the diffusion of ethanol and water in the bilayer are not consistent with the large selectivity value experimentally observed. The entry of water and ethanol into the interlayer space may be the crucial step controlling the selectivity.

Molecular Dynamics Simulations Reveal that Water Diffusion between Graphene Oxide Layers is Slow.

Membranes made of stacked layers of graphene oxide (GO) hold the tantalizing promise of revolutionizing desalination and water filtration if selective transport of molecules can be controlled. We present the findings of an integrated study that combines experiment and molecular dynamics simulation of water intercalated between GO layers. We simulated a range of hydration levels from 1 wt.% to 23.3 wt.% water. The interlayer spacing increased upon hydration from 0.8 nm to 1.1 nm. We also synthesized GO membranes that showed an increase in layer spacing from about 0.7 nm to 0.8 nm and an increase in mass of about 15% on hydration. Water diffusion through GO layers is an order of magnitude slower than that in bulk water, because of strong hydrogen bonded interactions. Most of the water molecules are bound to OH groups even at the highest hydration level. We observed large water clusters that could span graphitic regions, oxidized regions and holes that have been experimentally observed in GO. Slow interlayer diffusion can be consistent with experimentally observed water transport in GO if holes lead to a shorter path length than previously assumed and sorption serves as a key rate-limiting step.

Ab initio molecular dynamics simulation of proton hopping in a model polymer membrane.

We report the results of ab initio molecular dynamics simulations of a model Nafion polymer membrane initially equilibrated using classical molecular dynamics simulations. We studied three hydration levels (λ) of 3, 9, and 15 H2O/SO3(-) corresponding to dry, hydrated, and saturated fuel cell membrane, respectively. The barrier for proton transfer from the SO3(-)-H3O(+) contact ion pair to a solvent-separated ion pair decreased from 2.3 kcal/mol for λ = 3 to 0.8 kcal/mol for λ = 15. The barrier for proton transfer between two water molecules was in the range from 0.7 to 0.8 kcal/mol for the λ values studied. The number of proton shuttling events between a pair of water molecules is an order of magnitude more than the number of proton hops across three distinct water molecules. The proton diffusion coefficient at λ = 15 is about 0.9 × 10(-5) cm(2)/s, which is in good agreement with experiment and our previous quantum hopping molecular dynamics simulations.

Insight from molecular modelling: does the polymer side chain length matter for transport properties of perfluorosulfonic acid membranes?

We present a detailed analysis of the nanostructure of the short side chain (SSC) perfluorosulfonic acid membrane and its effect on H(2)O clustering, H(3)O(+) and H(2)O diffusion, and mean residence times of H(2)O near SO(3)(-) groups based on molecular dynamics simulations. We studied a range of hydration levels (λ) at temperatures of 300 and 360 K, and compare the results to our findings in the benchmark Nafion® membrane. The water cluster diameter is nearly the same in the two membranes, while the extent of SO(3)(-) clustering is more in the SSC membrane. The calculated cluster diameter of about 2.4 nm is in excellent agreement with the recently proposed cylindrical water channel model of these membranes. The diffusion coefficients of H(2)O and H(3)O(+) are similar in SSC and Nafion membranes. Raising the temperature of the SSC membrane from 300 to 360 K provides a much bigger increase in proton vehicular diffusion coefficient (by a factor of about 4) than changing the side chain length. H(3)O(+) ions are found to exchange more frequently with SO(3)(-) partners at the higher temperature. Our key findings are that (a) the hydrophobic-hydrophilic separation in the two membranes is surprisingly similar; (b) at all hydration levels studied, the long side chain of Nafion is bent and is effectively equivalent to a short side chain in terms of extension into the water domain; (c) vehicular proton transport occurs mainly between SO(3)(-) groups; and (d) changing the size of the simulation cell does not change the results significantly. The simulations are validated in good agreement with the corresponding experimental values for the simulated membrane density and diffusion coefficients of H(2)O.

Molecular structure and transport dynamics in perfluoro sulfonyl imide membranes.

We report a detailed and comprehensive analysis from classical molecular dynamics simulations of the nanostructure of a model of hydrated perfluoro sulfonyl imide (PFSI) membrane, a polymeric system of interest as a proton conductor in polymer electrolyte membrane fuel cells. We also report on the transport dynamics of water and hydronium ions, and water network percolation in this system. We find that the water network percolation threshold for PFSI, i.e. the threshold at which a consistent spanning water network starts to develop in the membrane, is found to occur between hydration levels (λ) 6 and 7. The higher acidity of the sulfonyl imide acid group of PFSI compared to the sulfonic acid group in Nafion, as computationally characterized in our earlier ab initio study (Idupulapati et al 2010 J. Phys. Chem. A 114 6904-12), results in a larger fraction of 'free' hydronium ions at low hydration levels in PFSI compared to Nafion. However, the calculated diffusion coefficients of the H(3)O(+) ions and H(2)O molecules as a function the hydration level are observed to be almost the same as that of Nafion, indicating similar conductivity and consistent with experimental data.

Atomistic simulations of perfluoro phosphonic and phosphinic acid membranes and comparisons to Nafion.

We used classical molecular dynamics simulations to investigate the morphology and proton transport properties of perfluoro phosphonic (FPA) and phosphinic acid (FPA-I) membranes that have potential applications in low-temperature fuel cells. We systematically investigated these properties as a function of the hydration level. We examined changes in structure, transport dynamics of water and hydronium ions, and water network percolation relative to those in Nafion membrane to examine the effect of functional group acidity on these properties. Phosphonic and phosphinic acid moieties in FPA and FPA-I have lower acidity than sulfonic acid in Nafion, yet the diffusion of water was faster in FPA and FPA-I than in Nafion, particularly at low hydration levels. However this did not give rise to notable differences in hydronium ion diffusion and water network percolation for these membranes over Nafion. These results, along with similar findings from our recent study of perfluoro-sulfonyl imide membranes carrying stronger superacids than the sulfonic acid of Nafion, suggest that there is no strong correlation between the acidity of the functional groups and the dynamics of water and hydronium ions in hydrated polymer electrolyte membranes with similar fluorocarbon backbones and side chains.

Modeling the nanophase structural dynamics of phenylated sulfonated poly ether ether ketone ketone (Ph-SPEEKK) membranes as a function of hydration.

Solvated phenylated sulfonated poly ether ether ketone ketone (Ph-SPEEKK) membranes in the presence of hydronium ions were modeled by classical molecular dynamics simulations. The characterization of the nanophase structure and dynamics of such membranes was carried out as a function of the water content λ, where λ is the number of water molecules per sulfonate group, for λ values of 3.5, 6, 11, 25, and 40. Analysis of pair correlation functions supports the experimental observation of membrane swelling upon hydration as well the increase in water and hydronium ion diffusion with increasing λ. Whereas the average number of hydrogen bonds between hydronium ions and sulfonate groups is dramatically affected by the hydration level, the average lifetime of the hydrogen bonds remains essentially constant. The membrane is found to be relatively rigid, and its overall flexibility shows little dependence on water content. Compared with Nafion, water and ion diffusion coefficients are considerably smaller at lower hydration levels and room temperature. However, at higher λ values of 25 and 40, these coefficients are comparable to those in Nafion at a λ value of 16. This study also shows that water diffusion in Ph-SPEEKK membranes at low hydration levels can be significantly improved by raising the temperature with important implications for proton conductivity.

Atomistic simulation of water percolation and proton hopping in Nafion fuel cell membrane.

We have performed a detailed analysis of water clustering and percolation in hydrated Nafion configurations generated by classical molecular dynamics simulations. Our results show that at low hydration levels H(2)O molecules are isolated and a continuous hydrogen-bonded network forms as the hydration level is increased. Our quantitative analysis has established a hydration level (λ) between 5 and 6 H(2)O/SO(3)(-) as the percolation threshold of Nafion. We have also examined the effect of such a network on proton transport by studying the structural diffusion of protons using the quantum hopping molecular dynamics method. The mean residence time of the proton on a water molecule decreases by 2 orders of magnitude when the λ value is increased from 5 to 15. The proton diffusion coefficient in Nafion at a λ value of 15 is about 1.1 × 10(-5) cm(2)/s in agreement with experiment. The results provide quantitative atomic-level evidence of water network percolation in Nafion and its effect on proton conductivity.

Ab initio study of hydration and proton dissociation in ionomer membranes.

We present a comparative study of proton dissociation in various functional acidic units that are promising candidates as building blocks for polymeric electrolyte membranes. Minimum energy structures for four acidic moieties with clusters of 1-6 water molecules were determined using density functional theory at the B3LYP/6-311G** level starting from chemically rational initial configurations. The perfluoro sulfonyl imide acid group (CF(3)CF(2)SO(2)NHSO(2)CF(3)) was observed to be the strongest acid, due to the substantial electron withdrawing effect of both fluorocarbon groups. The hydrophilic functional group (CH(3)OC(6)H(3)OCH(3)C(6)H(4)SO(3)H) of sulfonated polyetherether ketone (SPEEK) membrane was found to be the strongest base, with the acidic proton dissociation requiring the addition of six water molecules and the hydrated proton being more tightly bound to the conjugate base. Even though both perfluoro sulfonyl imides and sulfonic acids (hydrophilic functional groups for sulfonyl imide and Nafion ionomers, respectively) required only three water molecules to exhibit spontaneous proton dissociation, the largest possible solvent-separated hydronium ion was attained only for the sulfonyl imide moiety. These results provide a rationale for the enhanced conductivity of perfluorinated sulfonyl imide-based membranes relative to that of the widely used Nafion membrane.

Atomistic simulation of track formation by energetic recoils in zircon.

We have performed classical molecular dynamics simulations of fission track formation in zircon. We simulated the passage of a swift heavy ion through crystalline zircon using cylindrical thermal spikes with energy deposition (dE/dx) of 2.5-12.8 keV nm( - 1) and a radius of 3 nm. At a low dE/dx of 2.55 keV nm( - 1), the structural damage recovered almost completely and a damage track was not produced. At higher values of dE/dx, tracks were observed and the radius of the track increased with increasing dE/dx. Our structural analysis shows amorphization in the core of the track and phase separation into Si-rich regions near the center of the track and Zr-rich regions near the periphery. These simulations establish a threshold dE/dx for fission track formation in zircon that is relevant to thermochronology and nuclear waste immobilization.

Energetic recoils in UO2 simulated using five different potentials.

This report presents the results of classical molecular dynamics simulations of the diffuse premelting transition, melting, and defect production by 1 keV U recoils in UO(2) using five different rigid ion potentials. The experimentally observed premelting transition occurred for all five cases. For all the potentials studied, dynamic defect annealing is highly effective and is accompanied by replacement events on the anion sublattice. The primary damage state after approximately 15 ps consists of isolated Frenkel pairs and interstitial and vacancy clusters of various sizes. The average displacement energy varies from approximately 28 to approximately 83 eV and the number of Frenkel pairs is different by a factor of 3 depending on the choice of potential. The size and spatial distribution of vacancy and interstitial clusters is drastically different for the potentials studied. The results provide statistics of defect production. They point to a pressing need to determine defect formation, migration, and binding energies in UO(2) from first principles and to develop reliable potentials based on this data for simulating microstructural evolution in nuclear fuel under operating conditions.

First-principles study of defects and phase transition in UO(2).

Defect properties and phase transition in UO(2) have been studied from first principles by the all-electron projector-augmented-wave (PAW) method. The generalized gradient approximation with empirical self-interaction correction, (GGA)+U, formalism has been used to account for the strong on-site Coulomb repulsion among the localized U 5f electrons. The Hubbard parameter U(eff), magnetic ordering, chemical potential and heat of formation have been systematically examined. By choosing an appropriate U(eff) = 3.0 eV it is possible to consistently describe structural properties of UO(2) and model the phase transition processes. The phase transition pressure for UO(2) is about 20 GPa, which is less than the experimental value of 42 GPa but better than the LDA+U value of 7.8 GPa. Meanwhile our results for the formation energies of intrinsic defects partly confirm earlier calculations for the intrinsic charge neutral defects but reveal large variations depending on the determination of the chemical potential and whether the environment is O-rich or U-rich. Moreover, the results for extrinsic defects of Xe, which are representative of mobile insoluble fission product in UO(2), are consistent with experimental data in which Xe prefers to be trapped by Schottky defects.

Atomistic simulations of hydrated nafion and temperature effects on hydronium ion mobility.

The effects of hydration level and temperature on the nanostructure of an atomistic model of a Nafion (DuPont) membrane and the vehicular transport of hydronium ions and water molecules were examined using classical molecular dynamics simulations. Through the determination and analysis of structural and dynamical parameters such as density, radial distribution functions, coordination numbers, mean square deviations, and diffusion coefficients, we identify that hydronium ions play an important role in modifying the hydration structure near the sulfonate groups. In the regime of low level of hydration, short hydrogen bonded linkages made of water molecules and sometimes hydronium ions alone give a more constrained structure among the sulfonate side chains. The diffusion coefficient for water was found to be in good accord with experimental data. The diffusion coefficient for the hydronium ions was determined to be much smaller (6-10 times) than that for water. Temperature was found to have a significant effect on the absolute value of the diffusion coefficients for both water and hydronium ions.