Bozzolo – Ferrante – Smith Research Articles

Modeling of In, Tl, Ga, Sb, and Pd as lanthanides binding agents in U-Zr metallic nuclear fuels

ABSTRACTAtomistic modeling is used to study the role of different alloying additions to metallic U-Zr nuclear fuels in terms of their ability to reduce lanthanide migration to the outer surface of the fuel and thus reduce their interaction with cladding. The Bozzolo-Ferrante-Smith (BFS) method for alloys is used to examine the behavior of each addition, the resulting phase structure, and the evolution of the fuel surface. Different behaviors are observed for each of the additives (In, Tl, Ga, Sb, Pd), all a result of the competition between the formation of bulk precipitates and the tendency of each additive to segregate to the surface. For each case, characteristic temperatures are determined indicating the range of temperatures in which each additive performs a different role. Sb and Pd additives are determined to be the most effective additions, properly balancing their ability to bind lanthanides in the fuel with their own segregating tendencies.

Lanthanides migration and immobilization in U–Zr nuclear fuels

Redistribution of lanthanides fission products during irradiation and migration to the surface of U–Zr based metallic fuels is a concern due to their interaction with the cladding. The existing remedy for preventing this effect is the introduction of diffusion barriers on the cladding inner surface or by adding thermodynamically stable compound-forming elements to the fuel. Exploring this second option, in this work atomistic modeling with the Bozzolo–Ferrante–Smith (BFS) method for alloys is used to study the formation of lanthanide-rich precipitates in U–Zr fuel and the segregation patterns of all constituents to the surface. Surface energies for all elements were computed and, together with the underlying concepts of the computational methodology and large scale simulations, the migration of lanthanides to the surface region in U–Zr fuels is explained. The role of additions to the fuel such as In, Ga, and Tl for immobilization of lanthanides is discussed.

Atomistic modeling and simulation of the role of Be and Bi in Al diffusion in U–Mo fuel

Within the RERTR program, previous experimental and modeling studies identified Si as the alloying addition to the Al cladding responsible for inhibiting Al interdiffusion in the UMo fuel. However, difficulties with reprocessing have rendered this choice inappropriate, leading to the need to study alternative elements. In this work, we discuss the results of an atomistic modeling effort which allows for the systematic study of several possible alloying additions. Based on the behavior observed in the phase diagrams, beryllium or bismuth additions suggest themselves as possible options to replace Si. The results of temperature-dependent simulations using the Bozzolo–Ferrante–Smith (BFS) method for the energetics for varying concentrations of either element are shown, indicating that Be could have a substantial effect in stopping Al interdiffusion, while Bi does not. Details of the calculations and the dependence of the role of each alloying addition as a function of temperature and concentration (of beryllium or bismuth in Al) are shown.

Atomistic Simulation of High‐Density Uranium Fuels

We apply an atomistic modeling approach to deal with interfacial phenomena in high‐density uranium fuels. The effects of Si, as additive to Al or as U‐Mo‐particles coating, on the behavior of the Al/U‐Mo interface is modeled by using the Bozzolo‐Ferrante‐Smith (BFS) method for alloys. The basic experimental features characterizing the real system are identified, via simulations and atom‐by‐atom analysis. These include (1) the trend indicating formation of interfacial compounds, (2) much reduced diffusion of Al into U‐Mo solid solution due to the high Si concentration, (3) Si depletion in the Al matrix, (4) an unexpected interaction between Mo and Si which inhibits Si diffusion to deeper layers in the U‐Mo solid solution, and (5) the minimum amount of Si needed to perform as an effective diffusion barrier. Simulation results related to alternatives to Si dispersed in the Al matrix, such as the use of C coating of U‐Mo particles or Zr instead of the Al matrix, are also shown. Recent experimental results confirmed early theoretical proposals, along the lines of the results reported in this work, showing that atomistic computational modeling could become a valuable tool to aid the experimental work in the development of nuclear fuels.

GROWTH MODE OF Sn ON A AU(111) SUBSTRATE

Atomistic simulations of low coverage Sn growth on Au(111) using the Bozzolo–Ferrante–Smith (BFS) method for alloys are presented. Simulated annealing and atom-by-atom analysis of the energetics explains the close competition between the experimentally observed surface alloying and layer-by-layer growth.

Surface structure and composition of Ni–Ti alloys and their influence on the temperature-dependent segregation behavior

Atomistic modeling of segregation in Ni–Ti alloys is performed using the Bozzolo–Ferrante–Smith (BFS) method for alloys. It is found that Ti segregation to the surface decreases rapidly even for small increases in Ni concentration. A competing process is identified in the vicinity of surface defects, leading to excess Ti coverage of the exposed surface and the creation of Ti-covered pits. A simple explanation based on an atom-by-atom energy analysis is found to satisfactorily explain the observed effects.

Catalyst design for carbon nanotube growth using atomistic modeling

The formation and stability of bimetallic catalyst particles, in the framework of carbonnanotube growth, is studied using the Bozzolo–Ferrante–Smith (BFS) method for alloys.Monte Carlo–Metropolis simulations with the BFS method are utilized in order to predictand study equilibrium configurations for nanoscale catalyst particles which are directlyrelevant to the catalyst state prior to growth of carbon nanotubes. At the forefront ofpossible catalyst combinations is the popular Fe–Mo bimetallic catalyst, whichwe have recently studied experimentally. We explain our experimental results,which indicate that the growth observed is dependent on the order of co-catalystdeposition, in the straightforward interpretation of BFS strain and chemical energycontributions toward the formation of Fe–Mo catalyst prior to growth. We find that thecompetition between the formation of metastable inner Mo cores and clustersof surface-segregated Mo atoms in Fe–Mo catalyst particles influences catalystformation, and we investigate the role of Mo concentration and catalyst particle size inthis process. Finally, we apply the same modeling approach to other prominentbimetallic catalysts and suggest that this technique can be a powerful tool tounderstand and manipulate catalyst design for highly efficient carbon nanotubegrowth.

Modeling of the deposition of Ni and Pd on Mo(1 1 0)

Recent experimental work on the deposition of fcc metals on a bcc substrate motivates this atomistic modeling analysis of Ni and Pd deposition on Mo(1 1 0). A detailed atom-by-atom analysis of the early stages of growth, focusing on the formation of surface alloys and 3D islands is presented, identifying the interactions leading to each type of behavior. Further analysis describes the growth pattern as a function of coverage. Temperature effects are studied via Monte Carlo simulations using the Bozzolo–Ferrante–Smith (BFS) method for alloys for the energetics.

The BFS Method Combined with Chemical Cluster Interactions for the Study of Order-Disorder Transitions

First Principles (FP) methods are invoked to improve the accuracy of Bozzolo-Ferrante- Smith (BFS) model, one of the quantum-approximate modeling techniques for the computation of thermodynamic properties that involve a large number of particles. The BFS method calculates the energy of an atom in an alloy in two steps [1]. A first term pertains to the structural contribution. A recent improvement [2] allows to calculate the strain energy depending on the local environment [1,2] and this involves only pure element properties of the different atomic species. In the second step, binary chemical interactions are taken into account. This was originally done by only two interaction parameters for each atom pair in an alloy. In contrast, the adaptable parameterization of Cluster Expansion Methods (CEM) routinely incorporates any number of FP data to describe ordering in alloy systems. But in standard CEM calculations, no explicit information on local atomic displacements is used. In this work, the BFS chemical energy term is successfully replaced by a CEM chemical term to combine the ability of BFS to account for local displacements and the ability of CEM to include as many FP results as needed for the correct evaluation of alloying effects.

Atomistic Modeling of Multicomponent Systems

The need to accelerate materials design programs based on economical and efficient modeling techniques provides the framework for the introduction of approximations in otherwise rigorous theoretical schemes. Several quantum approximate methods have been introduced through the years, bringing new opportunities for the efficient understanding of complex multicomponent alloys at the atomic level. As a promising example of the role that these methods might have in the development of complex systems, in this work we discuss the Bozzolo-Ferrante-Smith (BFS) method for alloys and its application to a variety of multicomponent systems for a detailed analysis of their defect and phase structure and their properties. Examples include the study of the phase structure of new Ru-rich Ni-base superalloys, the role of multiple alloying additions in high temperature intermetallic alloys, and interfacial phenomena in nuclear materials, highlighting the benefits that can be obtained from introducing simple modeling techniques to the investigation of complex systems.

Structure of ultrathin Ag films on theAl(100)surface

The structure for submonolayer amounts of Ag deposited on the $\mathrm{Al}(100)$ surface at room temperature has been studied using low-energy electron diffraction (LEED) and low-energy ion-scattering spectroscopy (LEIS/ISS). The Ag coverage was determined using Rutherford backscattering spectroscopy. We conclude that the Ag atoms form two domains of a buckled, quasihexagonal coincident lattice structure on the $\mathrm{Al}(100)$ surface, having a repeat distance of 5 Al interatomic spacings in the [110] direction. The LEED pattern shows a double-domain $(5\ifmmode\times\else\texttimes\fi{}1)$ structure with additional intensity in those spots corresponding to a (111) close-packed hexagonal layer. The analysis of the ISS results suggests that the heights of the adsorbed Ag atoms above the Al surface are not all the same, leading to the proposed buckling model that is in agreement with recent scanning tunneling microscopy measurements. In addition, some Al atoms move from the substrate up into the Ag adlayer to form a surface alloy. Model calculations using the quantum approximate Bozzolo-Ferrante-Smith (BFS) method indicate that the hexagonal layer is energetically preferred as a result of increased nearest-neighbor coordination within the Ag layer.

Atomistic modeling of Zn deposition on the low index faces of Cu

The formation process of Zn/Cu surface alloys is investigated using the Bozzolo–Ferrante–Smith (BFS) method for alloys. The effects of the crystallographic orientation on the deposition process, formation of surface alloys as a function of temperature and coverage, Zn surface migration, and interdiffusion in the Cu substrate, are modeled and discussed with atom-by-atom energy analyses and large scale simulations.

Determination of structural alloy equilibrium properties from quantum approximate methods

In this paper we address basic issues, not investigated previously, concerning the relationship between ab initio methods and the Bozzolo-Ferrante-Smith (BFS) method for alloys, and its ability to model the process of alloy formation and reproduce structural alloy properties near and at equilibrium. Based on perturbation theory, the method requires single element parameters and their binary combinations, even for multicomponent systems. A direct comparison of BFS predictions of equilibrium alloy properties against ab initio results, and the handling and influence of the parameterization on the accuracy with which the method reproduces the process of alloy formation are presented. Besides establishing a range of validity for the method in reference to first-principles results, a simple algorithm for the determination of equilibrium properties of ordered alloy systems is introduced and illustrated with applications to binary and higher order systems, maximizing the flow of information carried in the first-principles-based parameters.

Atomistic analysis of surface segregation in Ni–Pd alloys

Atomistic analysis and large-scale atomistic simulations using the Bozzolo–Ferrante–Smith (BFS) method for alloys were performed to investigate surface segregation in Ni–Pd alloys as a function of temperature, crystal face, and composition. Pd-enrichment of the first layer was observed in (1 1 1) and (1 0 0) surfaces, and enrichment of the top two layers occurred for (1 1 0) surfaces. In all cases, the segregation profile shows alternating planes enriched and depleted in Pd in a decreasing oscillatory profile. Based on atom-by-atom static energy calculations, an explanation for the observed surface features is presented in terms of competing chemical and strain energy effects.

Analysis of surface and bulk behavior in Ni–Pd alloys

The most salient features of the surface structure and bulk behavior of Ni–Pd alloys have been studied using the Bozzolo–Ferrante–Smith (BFS) method for alloys. Large-scale atomistic simulations were performed to investigate surface segregation profiles as a function of temperature, crystal face, and composition. Pd enrichment of the first layer was observed in (1 1 1) and (1 0 0) surfaces, and enrichment of the top two layers occurred for (1 1 0) surfaces. In all cases, the segregation profile shows alternated planes enriched and depleted in Pd. In addition, the phase structure of bulk Ni–Pd alloys as a function of temperature and composition was studied. A weak ordering tendency was observed at low temperatures, which helps to explain the compositional oscillations in the segregation profiles. Finally, based on atom-by-atom static energy calculations, a comprehensive explanation for the observed surface and bulk features will be presented in terms of competing chemical and strain energy effects.

Theoretical description of the interdiffusion of Al in the U–Mo solid solution

The Bozzolo–Ferrante–Smith (BFS) method for alloys was applied to the analysis of Al interdiffusion in the U–Mo solid solution system as a function of Mo concentration. The binary Al/U and Al/Mo systems show opposite behavior, which in the ternary case Al/U–Mo, translates into the role of regions rich in Mo acting as interdiffusion barriers to Al, in excellent agreement with experimental evidence.

Experimental verification of the theoretical prediction of the phase structure of a Ni–Al–Ti–Cr–Cu alloy

The Bozzolo–Ferrante–Smith (BFS) method for alloys was applied to the study of NiAl-based materials to assess the effect of alloying additions on structure. Ternary, quaternary and even pentalloys based on NiAl with additions of Ti, Cr and Cu were studied and experimental verification of the theoretical predictions including the phase structure of a Ni–Al–Ti–Cr–Cu alloy is presented. Two approaches were used, Monte Carlo simulations to determine low energy structures, and analytical calculations of the energy of high symmetry configurations which give physical insight into preferred structures. The energetics for site occupancy in ternary and quaternary systems were calculated leading to an indirect determination of solubility limits at 0 K. Precipitate formation with information concerning structure and lattice parameter were also ‘observed’ computationally and the general characteristics of a Ni–Al–Ti–Cr–Cu alloy were correctly predicted. The results indicate that the BFS method for alloys can be a useful tool for alloy design and can be used to complement experimental alloy design programs.

Atomistic modeling of quaternary alloys: Ti and Cu in NiAl

The change in site preference in NiAl(Ti,Cu) alloys with concentration is examined experimentally via ALCHEMI and theoretically using the Bozzolo-Ferrante-Smith (BFS) method for alloys. Results for the site occupancy of Ti and Cu additions as a function of concentration are determined experimentally for five alloys. These results are reproduced with large-scale BFS-based Monte Carlo atomistic simulations. The original set of five alloys is extended to 25 concentrations, which are modeled by means of the BFS method for alloys, showing in more detail the compositional range over which major changes in behavior occur. A simple but powerful approach based on the definition of atomic local environments also is introduced to describe energetically the interactions between the various elements and therefore to explain the observed behavior.

Modeling of 3C-SiC( [formula omitted]) using the BFS method for alloys

The Bozzolo–Ferrante–Smith (BFS) method for alloys is extended to elemental semiconductors, and in this introductory work, to 3C-SiC(1 0 0) surfaces. In spite of the differences between metallic systems and semiconductors, the formulation of the BFS method for metallic alloys remains unaltered, thus providing a computationally simple and physically sound tool to deal with semiconductors. First-principles methods are used for the determination of the parameters of Si, C and SiC. These are fully transferable and applicable to any surface problem dealing with the 3C-SiC structure. Basic features regarding the surface energy and structure of Si, C and 3C-SiC are determined and compared to experiment.

Site occupancy of ternary additions to B2 alloys

In this broad-based survey study, the substitutional site preference of ternary alloying additions to B2 compounds (stable at room temperature and 50/50 composition) is determined using the Bozzolo–Ferrante–Smith (BFS) method for alloys. The method is applied to Ni, Al, Ti, Cr, Cu, Co, Fe, Ta, Hf, Mo, Nb, W, V and Ru additions to NiAl, FeAl, CoAl, CoFe, CoHf, CoTi, FeTi, RuAl, RuSi, RuHf, RuTi, and RuZr. The results are compared, when available, to experimental data and other theoretical results.