Temperature dependence of O solubility in liquid Na by atomistic simulation of Na(l)-Na2O(s) interfaces using corrected machine learning potential: a step towards simulating Na combustion.

Liquid Na combustion is a significant safety concern in sodium-cooled fast reactors. Atomistic simulations are an alternative to experiments for studying detailed mechanisms of complex combustion processes. However, accurate simulations of the interfaces involved in combustion are challenging even for density functional theory (DFT), because the systematic error between different chemical systems cannot be fully cancelled. Herein, we report the achievement of a key milestone in atomistic simulation of liquid Na combustion, which involves the development of a machine learning (ML) moment tensor potential that allows accurate simulation of interface systems between liquid Na and solid Na2O. The ML potential is trained by using supervised and active learning to ensure DFT-level accuracy. An empirical correction is then applied to achieve experimental accuracy by reducing systematic error. Consequently, the basic properties of liquid Na and solid Na2O are accurately simulated. In addition, with empirical correction, experimental O solubility data for liquid Na at 350-900 K are reproduced by using interface molecular dynamics simulations and a thermodynamic model. The temperature dependence of the enthalpy and entropy of the Na2O solution and their effect on O solubility are evaluated. The results show that, despite the increase in solution enthalpy with temperature, O solubility increases more rapidly than the linear Arrhenius plot due to the effect of solution entropy. The results of this study indicate that, with appropriate correction, ML potentials can achieve near-experimental accuracy, beyond the accuracy of DFT, in interface simulations and material properties calculations, paving the way for sodium combustion simulations in the future.

Accurate and Efficient Calculation of the Solution Enthalpy and Diffusivity of Solutes in Liquid Metals Using Machine Learning Potential.

Liquid metals (LMs) have various applications in energy systems, such as coolants in advanced nuclear reactors. In addition, room-temperature LMs are attracting attention as flexible components in robotics and electronics and as novel chemical reaction media to form low-dimensional materials. In many of these applications, the capabilities of LMs can be further enhanced if one can better understand and control the chemical reactivity of LMs, which is largely affected by the stability and mobility of solutes in LMs. Here, we propose an automated method using a machine learning moment tensor potential to efficiently calculate the solution enthalpy and diffusivity of solutes in LMs. From several test cases in liquid Na, we demonstrate that the method can achieve an accuracy comparable to that of a direct calculation using first-principles molecular dynamics, while significantly reducing the calculation cost to the order of 1/10 to 1/100. The method is expected to contribute to the advancement of LM chemistry and the development of new LMs.

Correction methods for first-principles calculations of the solution enthalpy of gases and compounds in liquid metals.

Liquid metals (LMs) have a wide range of engineering applications, such as in coolants, batteries, and flexible electronics. While accurate calculation methods for thermodynamic properties based on density functional theory (DFT) have been extensively developed for solid materials, including methods to correct identified systematic errors, almost no attempt has been made for LMs. In the present study, four correction methods for the first-principles calculation of the solution enthalpy of gases and compounds in LMs are proposed, namely, Correction-1, using the experimental binding energy of an impurity gas molecule; Correction-2, additionally using the experimental enthalpy of formation of a solid compound composed of LM and gas-impurity elements; Correction-3, using the concept of the fitted elemental-phase reference energies (FERE) method; and Correction-4, using the concept of the coordination corrected enthalpies (CCE) method. The performance of each method is examined with hydrogen, nitrogen, oxygen, and iodine gases and their sodium compounds in liquid sodium, and the operating principle of each method is clarified. In general, the four correction methods effectively reduce the calculation error, and Correction-2 reduces the error to less than 10 kJ mol-1, while the uncorrected errors are up to several tens of kJ mol-1. This study demonstrates that, with appropriate correction, the DFT calculation of the solution enthalpy of impurities in LMs can achieve the same level of accuracy as in precise experiments.

Hemocyte migration and expression of four Sox genes during wound healing in Pacific abalone, Haliotis discus hannai.

In molluscs, migration of hemocytes and epithelial cells is believed to play central roles in wound healing. Here, we assessed cellular and molecular mechanisms of wound healing in Pacific abalone, a marine gastropod. Light and electron microscopy in the wounds showed early accumulation of putative hemocytes, collagen deposition by fibroblasts, and further coverage of this tissue by migration of adjacent epithelial cells. Cell labelling technique allowed us to track hemocytes, which migrated to wound surface within 24 h. The migrated cells first expressed PCNA and SoxF weakly, and then the epithelial cells expressed abundant PCNA and SoxB1, SoxB2, and SoxC. These findings imply that abalone SoxF is involved in hemocyte migration or their differentiation into fibroblasts, and suggest that the migrated epithelia acquire stem cell-like property and undergo active proliferation. This study is the first to show direct evidence of hemocyte migration to wounds and expression of Sox genes in molluscan wound healing.

Solution enthalpy calculation for impurity in liquid metal by first-principles calculations: A benchmark test for oxygen impurity in liquid sodium.

The solution enthalpy of oxygen in liquid Na was calculated as a test case for the computational method to evaluate the solution enthalpy in liquid metal using first-principles calculations. To obtain the necessary thermodynamic quantities at high temperatures, (i) first-principles molecular dynamics for pure and O-including liquid Na systems, (ii) vibration analysis for an O2 molecule, and (iii) phonon-based quasi-harmonic approximation for solid Na and Na2O were conducted. The calculation results were compared with available experimental data to validate the method. Consequently, the O2 solution enthalpy was calculated to be -387.1 kJ/mol at 600 K and -374.0 kJ/mol at 1000 K, comparable to the experimental data of -375.7 kJ/mol at 600 K and -369.3 kJ/mol at 1000 K. The Na2O solution enthalpy was calculated to be 28.6 kJ/mol at 600 K and 38.2 kJ/mol at 1000 K, while the experimental data gave a temperature-independent value of 46.9 kJ/mol. The possible causes of errors in the calculations were discussed. This work shows that computational calculations can contribute to establishing a fundamental database on the solubility of impurities in liquid metals.

Chemical origin of differences in steel corrosion behaviors of s-electron and p-electron liquid metals by first-principles calculation.

Steel corrosion is a key engineering issue in the development of advanced nuclear reactors using liquid metals. The present study demonstrates that the steel corrosion behaviors can be systematically understood and classified based on the types of valence electrons of liquid metals, namely, s-electron liquid metals (s-LMs), such as liquid Na and Li, and p-electron liquid metals (p-LMs), such as liquid lead-bismuth eutectic (LBE) and Pb, where the conduction band is composed of s and p valence electrons, respectively. Through a comparative analysis of the physiochemical states of 3d transition metal atoms dissolved in liquid Na and liquid LBE by means of first-principles molecular dynamics (FPMD), it is shown that the 3d and 4s orbitals of the transition metals hardly interact with the s band of s-LMs, while they strongly interact with the p band of p-LMs in a covalent manner. This fact is consistently seen in the electronic states and the atomic configuration and can be successfully used to explain the differences in the steel corrosion behaviors observed between the liquid metals by experiments. The present findings provide fundamental insights into the corrosion chemistry of liquid metals.

A comparative study on modeling of the ferromagnetic and paramagnetic states of uranium hydride using a DFT+U method.

Uranium hydride is a promising material for stationary hydrogen storage in fusion reactors. In this work, various material properties of uranium hydride in both ferromagnetic (FM) and paramagnetic (PM) states are calculated to determine the optimal first-principles calculation method. For the treatment of strongly correlated f-electrons, the PBE functional with a Hubbard U parameter of 0.6 eV is selected as the optimal method and provides accurate formation energies and reasonable structural properties of the FM state. Using this method, we test four model spin configurations to approximately simulate the PM state: FM, antiferromagnetic (AFM), special quasi-random structure (SQS) and nonmagnetic (NM) configurations. The FM and AFM configurations provide formation energy and lattice constants comparable to those of the SQS configuration, which is used as the reference PM state. In addition, the experimental results on thermal expansion and the bulk modulus in the PM states are well reproduced with the FM, AFM and SQS configurations. These results demonstrate that PBE+U with FM, AFM and SQS configurations can approximately simulate the PM states, although there are some properties that can only be qualitatively reproduced by DFT calculations, such as the magnetic transition. This study enables the design of multiscale modeling for uranium hydride while maintaining simultaneous efficiency and accuracy.

Structural and chemical analysis of second-row impurities in liquid lead-bismuth eutectic by first-principles molecular dynamics.

The structural and chemical states of the second-row impurities in liquid lead-bismuth eutectic (LBE) are studied by first-principles molecular dynamics. First, several structural quantities such as the number of first-neighboring atoms and the LBE-impurity-LBE characteristic angle are obtained to determine the impurity-LBE local structure. Next, the impurity charge states and the electronic density of states are analyzed to reveal the chemical states of the impurities in the liquid LBE. It is observed that for the majority of impurities the 2p-6p interaction specifies the local structure around the impurity as well as the chemical state of the impurity. The anisotropy in the 2p-6p covalent interaction causes the tetrahedron-wise structures for B, C, N, and O, and the ionic interaction is relatively strong for Li and F. The change in the interaction scheme over the second-row impurities can be explained from the downward shift of the 2s and 2p orbital energy levels as the atomic number increases. Further, some impurities indicate interaction preferences for either Pb or Bi. These findings can assist the understanding and prediction of the behavior of impurity atoms in liquid LBE.

Performance of exchange-correlation functionals in density functional theory calculations for liquid metal: A benchmark test for sodium.

The performance of exchange-correlation functionals in density-functional theory (DFT) calculations for liquid metal has not been sufficiently examined. In the present study, benchmark tests of Perdew-Burke-Ernzerhof (PBE), Armiento-Mattsson 2005 (AM05), PBE re-parameterized for solids, and local density approximation (LDA) functionals are conducted for liquid sodium. The pair correlation function, equilibrium atomic volume, bulk modulus, and relative enthalpy are evaluated at 600 K and 1000 K. Compared with the available experimental data, the errors range from -11.2% to 0.0% for the atomic volume, from -5.2% to 22.0% for the bulk modulus, and from -3.5% to 2.5% for the relative enthalpy depending on the DFT functional. The generalized gradient approximation functionals are superior to the LDA functional, and the PBE and AM05 functionals exhibit the best performance. In addition, we assess whether the error tendency in liquid simulations is comparable to that in solid simulations, which would suggest that the atomic volume and relative enthalpy performances are comparable between solid and liquid states but that the bulk modulus performance is not. These benchmark test results indicate that the results of liquid simulations are significantly dependent on the exchange-correlation functional and that the DFT functional performance in solid simulations can be used to roughly estimate the performance in liquid simulations.

Chemical states of 3d transition metal impurities in a liquid lead-bismuth eutectic analyzed using first principles calculations.

Steels are easily corroded in a liquid lead-bismuth eutectic (LBE) because their components, such as Fe, Cr and Ni, exhibit a high solubility in the liquid LBE. To understand the reason for such a high solubility of these 3d transition metals, we have performed first-principles molecular dynamics calculations and analyzed the pair-correlation functions, electronic densities of states, and Bader charges and volumes of the 3d transition metals dissolved in the liquid LBE as impurities. The calculations show that the 4s and 3d orbitals of the 3d impurity atoms largely interact with the 6p band of the LBE, which generates bonding orbitals. We suggest that the high stability of 3d metals in the liquid LBE is caused by the interactions of the 4s and 3d orbitals with the 6p band. Spin polarization is induced by V, Cr, Mn, Fe and Co impurity atoms in a similar manner to the Slater-Pauling curve of solid transition metals, which exhibits a downward shift in the atomic number by approximately two. Based on the degree of spin polarization and the shifted trend of the Slater-Pauling curve, we suggest that Ni exhibits a higher solubility than Cr and Fe because of the differences in their interaction strengths between their 3d orbitals and the 6p band. In addition, the 4s and 3d orbitals of the 3d impurity atoms were found to interact more favorably with the Bi 6p band than the Pb 6p band, which is consistent with the fact that liquid Bi is more corrosive to steels than is liquid Pb.

Study of intrinsic defects in 3C-SiC using first-principles calculation with a hybrid functional.

Density functional theory (DFT) with a tailored Hartree-Fock hybrid functional, which can overcome the band gap problem arising in conventional DFT and gives a valence band width comparable with experiment, is applied to determine formation energies and electronic structures of intrinsic defects in cubic silicon carbide (3C-SiC). Systematic comparison of defect formation energies obtained with the tailored hybrid functional and a conventional DFT functional clearly demonstrates that conventional DFT results are not satisfactory. The understanding on intrinsic defects, which were previously investigated mainly with conventional DFT functionals, is largely revised with regard to formation energies, electronic structures and transition levels. It is found that conventional DFT functionals basically lead to (i) underestimation of the formation energy when the defect charge is more negative and (ii) overestimation when the defect charge is more positive. The underestimation is mainly attributed to the well-known band gap problem. The overestimation is attributed to shrinkage of the valence bands, although in some cases such band shrinkage may lead to underestimation depending on how the defect alters the valence band structure. Both the band gap problem and the valence band shrinkage are often observed in semiconductors, including SiC, with conventional DFT functionals, and thus need to be carefully dealt with to achieve reliable computational results.

Nanoscale engineering of radiation tolerant silicon carbide.

Radiation tolerance is determined by how effectively the microstructure can remove point defects produced by irradiation. Engineered nanocrystalline SiC with a high-density of stacking faults (SFs) has significantly enhanced recombination of interstitials and vacancies, leading to self-healing of irradiation-induced defects. While single crystal SiC readily undergoes an irradiation-induced crystalline to amorphous transformation at room temperature, the nano-engineered SiC with a high-density of SFs exhibits more than an order of magnitude increase in radiation resistance. Molecular dynamics simulations of collision cascades show that the nano-layered SFs lead to enhanced mobility of interstitial Si atoms. The remarkable radiation resistance in the nano-engineered SiC is attributed to the high-density of SFs within nano-sized grain structures that significantly enhance point defect annihilation.

Prolactin: fishy tales of its primary regulator and function.

Prolactin (PRL) is an important regulator of multiple biological functions, and the control of PRL expression integrates a wide spectrum of molecules throughout vertebrates. PRL-releasing peptide (PrRP) seems to be an essential stimulator of PRL transcription and secretion in teleost pituitary and peripheral organs. In the amphibious euryhaline mudskipper, the localization of mRNA levels of PrRP and PRL as well as their regulation during acclimation to different environments are closely related. The presence of PrRP-PRL axes in the peripheral organs might suggest an ancient history of this axis prior to the evolution of the hypothalamus-pituitary, and it is possible that the PrRP is an original and primary regulator of PRL. In the euryhaline fishes, the permeability of gut of seawater-acclimated fish is generally greater than that of the freshwater (FW)-acclimated fish. The modification in the epithelial cell renewal system may play an important role in regulation of the permeability. PRL induces the cell proliferation during FW acclimation, whereas cortisol stimulates both cell proliferation and apoptosis. Indeed, a large proportion of the various actions of PRL seem to be associated directly or indirectly with cell proliferation and/or apoptosis, which might be a primary function of PRL.