Elastic properties of cubic, tetragonal and monoclinic ZrO2 from first-principles calculations

In terms of first-principles calculations, elastic stiffness constants C_ij’s as well as the polycrystalline aggregates including the bulk, shear, Young’s moduli, Possion’s ratio, and anisotropy factors have been predicted for three technologically important polymorphs of ZrO₂, i.e., monoclinic m-ZrO₂, tetragonal t-ZrO₂, and cubic c-ZrO₂. Here, both the strain vs. stress (S-S) and the strain vs. strain energy (S-E) methods are adopted. In the first-principles calculations, both the local density approximation (LDA) and the generalized gradient approximation (GGA) are utilized. It is found that the more accurate structural and elastic properties are determined by LDA in comparison with experimental results and the S-S method is more effective than the S-E method although the two methods predict the similar results. The predicted negative values for C₁₆, C₃₆, and C₄₅ of m-ZrO₂ suggest that the certain normal or shear stress corresponds to an opposite shear strain for reducing the total energy. Small differences of shear and Young’s modulus between m-ZrO₂ and t-ZrO₂ suggest that their mechanical properties are comparable.     

Structural, electronic, and thermodynamic properties of UN: Systematic density functional calculations

A systematic first-principle study is performed to calculate the lattice parameters, electronic structure, and thermodynamic properties of UN using the local-density approximation (LDA)+U and the generalized gradient approximation (GGA)+U formalisms. To properly describe the strong correlation in the U 5f electrons, we optimized the U parameter in calculating the total energy, lattice parameters, and bulk modulus at the nonmagnetic (NM), ferromagnetic (FM), and antiferromagnetic (AFM) configurations. Our results show that by choosing the Hubbard U around 2 eV within the GGA+U approach, it is promising to correctly and consistently describe the above mentioned properties of UN. The localization behavior of 5f electrons is found to be stronger than that of UC and our electronic analysis indicates that the effective charge of UN can be represented as U^1.71+N^1.71−. As for the thermodynamic study, the phonon dispersion illustrates the stability of UN and we further predict the lattice vibration energy, thermal expansion, and specific heat by utilizing the quasiharmonic approximation. Our calculated specific heat is well consistent with experiments.     

First-principles calculations of phase transition,elasticity, and thermodynamic properties for TiZr alloy

Structural transformation, pressure dependent elasticity behaviors, phonon, and thermodynamic properties of the equiatomic TiZr alloy are investigated by using first-principles density-functional theory. Our calculated lattice parameters and equation of state for α and ω phases as well as the phase transition sequence of α → ω → β are consistent well with experiments. Elastic constants of α and ω phases indicate that they are mechanically stable. For cubic β phase, however, it is mechanically unstable at zero pressure and the critical pressure for its mechanical stability is predicted to equal to 2.19 GPa. We find that the moduli, elastic sound velocities, and Debye temperature all increase with pressure for three phases of TiZr alloy. The relatively large B/G values illustrate that the TiZr alloy is rather ductile and its ductility is more predominant than that of element Zr, especially in β phase. Elastic wave velocities and Debye temperature have abrupt increase behaviors upon the α → ω transition at around 10 GPa and exhibit abrupt decrease feature upon the ω → β transition at higher pressure. Through Mulliken population analysis, we illustrate that the increase of the d-band occupancy will stabilize the cubic β phase. Phonon dispersions for three phases of TiZr alloy are firstly presented and the β phase phonons clearly indicate its dynamically unstable nature under ambient condition. Thermodynamics of Gibbs free energy, entropy, and heat capacity are obtained by quasiharmonic approximation and Debye model.     

Ideal mechanical properties of vanadium by a first-principles computational tensile test

We employ first-principles total energy method based on the density functional theory with the generalized gradient approximation to investigate the ideal tensile strengths of a bcc vanadium (V) single crystal systemically. The ideal tensile strengths are calculated to be 19.1, 32.8 and 31.0 GPa for bcc V in the [1 0 0], [1 1 0] and [1 1 1] directions, respectively. We show that the [0 0 1] direction is the weakest direction due to the occurrence of structure transition at the lower strain and the [1 1 0] direction is strongest because of the stronger interaction of atoms between the (1 1 0) planes in comparison with the (0 0 1) and (1 1 1) planes. We derive the Young’s modulus formula V single crystal in different tensile directions and give detailed analysis. According to the elastic constants of V single crystal, we have estimated some mechanical quantities of polycrystalline V, which are the bulk modulus of B, the shear modulus of G, Young’s modulus of E and the Poisson’s ratio of ν. The results might provide a useful reference for V as a candidate structural material in the fusion Tokamak.     

Calculations of thermodynamic properties of PuO2 by the first-principles and lattice vibration

Plutonium dioxide (PuO₂) is a key compound of mixed oxide fuel (MOX fuel). To predict the thermal properties of PuO₂ at high temperature, it is important to understand the properties of MOX fuel. In this study, thermodynamic properties of PuO₂ were evaluated by coupling of first-principles and lattice dynamics calculation. Cohesive energy was estimated from first-principles calculations, and the contribution of lattice vibration to total energy was evaluated by phonon calculations. Thermodynamic properties such as volume thermal expansion, bulk modulus and specific heat of PuO₂ were investigated up to 1500 K.     

Hyperfine structures, isotope shifts, and transition rates of C II, N III, and O IV from relativistic configuration interaction calculations

Energy levels, specific mass shift parameters, hyperfine interaction constants, Landé g_J factors, and transition probabilities between computed levels are reported for C II, N III, and O IV. Results include levels belonging to 2s²2p,2s2p²,2p³,2s²3s,2s²3p,2s²3d,2s2p3s and, in the case of C II, the 2s²4s and 2s²4p configurations. Wavefunctions were determined using the multiconfiguration Dirac-Hartree-Fock method and account for valence, core-valence, and core-core correlation effects.     

Thermal and mechanical analysis of welded structures

The prediction of the residual state of stress and deformation in a welded structure is one of the most interesting, challenging, and complex problems in structural mechanics. The wide spectrum of physical phenomena provides the interest; economic and safety considerations provide the challenge; and the essential nonlinearity of the analytical models provides the complexity. Even if it is assumed that reasonable prediction of tje transient temperature field in the structure is possible, determining the residual mechanical state is both a difficult and an expensive task. The analysis inevitably involves temperature-dependent mechanical properties and, in addition, the severe thermal gradients and high temperatures generic to the welding process induce irrecoverable inelastic creep and plasticity in the structure.In spite of this, the stress analysis is now considered to be a straightforward application for general purpose, nonlinear finite element structural programs. A few special features of such analyses, however, will be discussed: (1) the legitimacy of time-dependent plasticity theories for treating the residual stress problem; (2) criteria for choosing plane stress, plane strain, generalized plane strain, or fully three-dimensional models; (3) methods for coping with possible floating solid regions during cooling; and (4) the use of linear constraints to treat weld metal deposition and intermittent contact.Since the most important parameters in the welding process that pertain to the stress analysis are the cooling rate and the welding torch efficiency, the heat transfer problem seems to require a critical look. The dominant features of this problem are: (1) source (torch) characterization; (2) radiation from surfaces that are heated to high temperatures; (3) latent heat effects; and (4) subsidiary considerations, such as enforced convection heat transfer modes that are designed to control the cooling rate. Motion of the welding torch, even at low speeds, is not usually a critical factor in determining the residual mechanical state. Again, finite element analysis is applicable, provided that the solution accuracy can be adequately estimated. Several alternative, two-step, implicit time integration schemes will be compared, especially with regard to accuracy and numerical stability for welding-type problems. The efficacy of ‘flux correction’ will also be discussed and the application of these ideas to typical industrial welding problems will then be outlined.     

Molecular dynamics characterization of thermodynamic and mechanical properties of Pu as dependent upon alloying additions and defects concentration. Part I

The paper presents results of molecular dynamics (MD) simulations which were performed to investigate mobility of defects in the δ-PuGa alloy. The defects diffuse through thermal fluctuations and MD results provided parameters for the Arrhenius law describing defect diffusion versus temperature. On the basis of this information a model of radiation defect accumulation allowing for different types of defects and grain size was constructed.The annealing of the defects at elevated temperatures and the effect of accelerated ageing due to adding small quantities of Pu-238 upon defect accumulation were evaluated.     

Evolving density and static mechanical properties in plutonium from self-irradiation

Plutonium, because of its self-irradiation by alpha decay, ages by means of lattice damage and helium in-growth. These integrated aging effects result in microstructural and physical property changes. Because these effects would normally require decades to measure, studies are underway to assess the effects of extended aging on the physical properties of plutonium alloys by incorporating roughly 7.5 wt% of highly specific activity isotope ²³⁸Pu into the ²³⁹Pu metal to accelerate the aging process. This paper presents updated results of self-irradiation effects on ²³⁸Pu-enriched alloys measured by immersion density, dilatometry, and tensile tests. After nearly 90 equivalent years of aging, both the immersion density and dilatometry show that the enriched alloys continue to decreased in density by ∼0.002% per year, without void swelling. Quasi-static tensile measurements show that the aging process increases the strength of plutonium alloys.     

High-temperature thermodynamic properties of hypo- and hyper-stoichiometric uranium carbides

Measurements of carbon activity over univariant and bivariant UC_x compositions, including the difficult region near stoichiometry, have been made in the temperature range 2155–2455°K. A flowing-gas technique was used with H₂-CH₄ mixtures in order to fix the carbon potential in the gas stream. The shape of the isotherms showed a sharp decrease in carbon activity near the stoichiometric composition as the composition of the condensed phase was reduced toward the lower phase boundary in the hypostoichiometric region. With argon as the carrier gas, a congruent vaporization composition of C/U ≈ 1.05 was obtained. Values for the free energy and heats of formation, relative partial molar enthalpies and entropies of solution, enthalpy of vaporization and uranium activity are presented.     

Phase relations and thermodynamic properties of the uranium-nitrogen system

The present state of knowledge of the phase relations and thermodynamic properties of the uranium-nitrogen system is given. Emphasis is placed on the nonstoichiometric and thermodynamic properties of uranium sesquinitride. The present paper consists of two parts. The first part is on the phase relations; an equilibrium phase diagram is proposed, and the relationship between structure and lattice parameter for the α-U₂N₃-UN₂ system is shown. The second part deals with thermodynamic properties of UN and α-U₂N₃; the data on decomposition pressures, specific heats, and heats and free energies of formation are summarized and evaluated.     

Atomistic properties of helium in hcp titanium: A first-principles study

First-principles calculations based on density functional theory have been performed to investigate the behaviors of He in hcp-type Ti. The most favorable interstitial site for He is not an ordinary octahedral or tetrahedral site, but a novel interstitial site (called FC) with a formation energy as low as 2.67 eV, locating the center of the face shared by two adjacent octahedrons. The origin was further analyzed by composition of formation energy of interstitial He defects and charge density of defect-free hcp Ti. It has also been found that an interstitial He atom can easily migrate along 〈0 0 1〉 direction with an activation energy of 0.34 eV and be trapped by another interstitial He atom with a high binding energy of 0.66 eV. In addition, the small He clusters with/without Ti vacancy have been compared in details and the formation energies of He_nV clusters with a pre-existing Ti vacancy are even higher than those of He_n clusters until n ? 3.     

Surface segregation and adsorption effects of iron-technetium alloys from first-principles

Surface properties of Tc-rich and Fe-rich portions of the Tc-Fe binary alloy phase diagram were computed in this work on the basis of density functional theory. Tc and Fe were found to have minimal degrees of mixing in the parent phases, consistent with the experimentally derived phase diagram. The influence of oxygen on surface phase stability was also studied, with no significant impact on surface segregation or degree of surface mixing. Oxygen adsorption was shown to change the ordering of surface facets in Tc, such that the pyramidal phase becomes lower in energy than the prismatic phase, even with low coverage of oxygen. No evidence for increased surface segregation upon oxidation was found for the solid-solution phases. A potential-pH surface Pourbaix diagram was derived for Tc and H, OH and O adsorbed sub-monolayers were shown to be precursors to oxide formation. While Tc and Fe have similar reactivities and properties in their parent phases, and hence, also in solid-solution, the properties of the intermetallic are expected to be significantly different due to the size-mismatch between the elements.     

Quantum mechanical calculations of uranium phases and niobium defects in γ-uranium

Depleted uranium (U) from fuel enrichment processes has a variety of applications due to its high density. With the addition of a small concentration of niobium (Nb), U becomes stainless. Nb is fully miscible with the high-temperature γ phase of U and tends to segregate upon cooling below 1050 K. The starting point of segregation is the configuration of Nb substitutional or interstitial defects. Using quantum mechanical calculations, the authors find that the formation energy of a single vacancy is 1.08 eV, that of Nb substitution 0.59 eV, that of Nb interstitial at octahedral site 1.58 eV, and that of Nb interstitial at tetrahedral site 2.35 eV in the dilute limit of isolated defects; all with reference to a reservoir of the pure γ phase U and pure Nb. The analysis of electronic structures reveals the correlation of formation energies of Nb defects with the local perturbations of electron distribution. Higher formation energy of Nb defects correlates with larger perturbation. Based on this study, Nb atoms thermodynamically prefer to occupy substitutional sites in the γ phase U.     

Evaluation of phosphate thermodynamic properties for spent electrolyte recycle

Adaptation of the phosphate conversion technique was undertaken and evaluated for application to the recycle process of the spent electrolyte generated from metal electrorefining process. In order to confirm the conversion behaviors of fission product (FP) chlorides to the phosphates, conversion experiments were carried out for some alkali metal, alkaline earth metals and rare-earth elements and their results were compared with that of thermodynamic calculations of previous study [I. Amamoto, H. Kofuji, M. Myochin et al., in: Proceedings of Global 2007, Boisi, Idaho, USA, 2007]. Among these elements, rare-earth chlorides were converted into phosphates and Cs was not, according to the prediction by the calculation. As for alkaline earth metals, their equilibrium constants were nearly 1 based on the results of the calculations, the conversion reactions were difficult to occur. In addition, it was clarified that phosphates were thermally unstable, easily to decompose at higher temperature, through the measurements of their heat flow and vapor pressure.     

Electrochemical and thermodynamic properties of ytterbium trichloride in molten caesium chloride

This work presents the electrochemical study of YbCl₃ in molten CsCl in the temperature range 973-1073 K. Transient electrochemical techniques have been used in order to investigate the reduction mechanism, transport parameters and thermodynamics properties of the reaction YbCl₂+1/2Cl₂=YbCl₃. The results obtained show that the reduction reaction is reversible being controlled by the rate of the mass transfer. The diffusion coefficient of [YbCl₆]^3- complex ions was determined. The apparent standard electrode potential of the soluble-soluble redox system Yb³⁺/Yb²⁺ was obtained by cyclic voltammetry.