235 research outputs found
Interface modelling for {\it ab initio} evaluation of contact angle on a metallic surface
Controlling the contact angles of the wettability is an important issue
especially in industrial applications. Establishing its {\it ab initio}
predictions is hence a topic of great interest. For the predictions, it is
required to setup a model of the adsorption structure of liquid molecules on a
surface. The appropriate setting is expected to depend on whether the surface
is of insulating or metallic materials, the latter of which is the target of
the present study while all preceding {\it ab initio} studies have worked on
the former. Since the feasibility of {\it ab initio} evaluations relies on the
approximation of the liquid-gas interface energy evaluated roughly by the
crystal ice, it would be a natural choice to take the periodic honeycomb array
of the water molecules as the adsorbing model of water on the surface. Although
the periodic model have successfully been used for the preceding treatments of
insulating surfaces, we found for the case with metallic surfaces that the
periodic model gives worse prediction to reproduce experimental values. Rather
than that, the models with isolated water multimers are found to give better
predictions. The ambiguity of the models about the size of multimers and the
coverage is found to be small (), and is averaged over to
give a plausible value based on the Boltzmann weight with the adsorbing
energies. The procedure we are providing can generally be applicable to any of
wettability on the surfaces of metallic materials.Comment: 4 page
Stability of crystallographic texture in laser powder bed fusion: Understanding the competition of crystal growth using a single crystalline seed
In metal additive manufacturing, crystallographic orientation control is a promising method for tailoring the functions of metallic parts. However, despite its importance in the fabrication of texture-controlled functional parts, the stability of the crystallographic texture is not widely discussed. Herein, the crystallographic texture stability under laser powder bed fusion was investigated. Two methodologies were employed. One is that a laser scanning strategy was alternately changed for a specific number of layers. The other is a “seeding” experiment in which single-crystalline substrates with controlled crystallographic orientations in the building (z-) direction and the xy-plane (perpendicular to the building direction) were used as the starting substrate. The transient zone width, where the crystallographic orientation was inherited from the layer beneath, was analyzed to evaluate the texture stability. The crystallographic direction of the seed within the xy-plane, rather than the building direction, determined the transient zone width, i.e., the texture stability. In particular, the texture in the newly deposited portion was stable when the laser scanning direction matched the orientation in the underneath layer, otherwise the crystal orientation switched rapidly, such that the orientation was parallel to the scanning direction. Interestingly, the crystallographic orientation along the building direction in the underneath layer hardly impacted the stability of the texture. Therefore, for the first time, it has been clarified that the orientation in the scanning direction, rather than the building direction, was preferentially stabilized, whereas the orientation in the other directions secondary stabilized.Ishimoto T., Hagihara K., Hisamoto K., et al. Stability of crystallographic texture in laser powder bed fusion: Understanding the competition of crystal growth using a single crystalline seed. Additive Manufacturing, 43, 102004. https://doi.org/10.1016/j.addma.2021.102004
Towards chemical accuracy using the Jastrow correlated antisymmetrized geminal power ansatz
Herein, we report accurate atomization energy calculations for 55 molecules
in the Gaussian-2 (G2) set using lattice regularized diffusion Monte Carlo
(LRDMC). We compare the Jastrow-Slater determinant ansatz with a more flexible
JsAGPs (Jastrow correlated antisymmetrized geminal power with singlet
correlation) ansatz. AGPs is built from pairing functions, which explicitly
include pairwise correlations among electrons and hence, this ansatz is
expected to be more efficient in recovering the correlation energy. The AGPs
wave functions are first optimized at the variational Monte Carlo (VMC) level,
which includes both the Jastrow factor and the nodal surface optimization. This
is followed by the LRDMC projection of the ansatz. Remarkably, for many
molecules, the LRDMC atomization energies obtained using the JsAGPs ansatz
reach chemical accuracy (1 kcal/mol) and for most other molecules, the
atomization energies are accurate within 5 kcal/mol. We obtained a mean
absolute deviation of 1.6 kcal/mol with JsAGPs and 3.2 kcal/mol with JDFT
(Jastrow factor + Slater determinant with DFT orbitals) ansatz. This work shows
the effectiveness of the flexible AGPs ansatz for atomization energy
calculations and electronic structure simulations in general.Comment: 15 pages, 4 figures, JCTC accepted version after peer-revie
Potential high- superconductivity in YCeH and LaCeH under pressure
Lanthanum, yttrium, and cerium hydrides are the three most well-known
superconducting binary hydrides, which have gained great attention in both
theoretical and experimental studies. Recent studies have shown that ternary
hydrides composed of lanthanum and yttrium can achieve high superconductivity
around 253 K. In this study, we employ the evolutionary-algorithm-based crystal
structure prediction (CSP) method and first-principles calculations to
investigate the stability and superconductivity of ternary hydrides composed of
(Y, Ce) and (La, Ce) under high pressure. Our calculations show that there are
multiple stable phases in Y-Ce-H and La-Ce-H hydrides, among which
-YCeH, -LaCeH, -YCeH, and
-LaCeH possessing H or H clathrate structures
can maintain both of the thermodynamic and dynamic stabilities. In addition, we
also find that these phases also maintain a strong resistance to decomposition
at high temperature. Electron-phonon coupling calculations show that all of
these four phases can exhibit high-temperature superconductivity.
-YCeH is predicted to have a superconducting transition
temperature () as high as 246 K at 350 GPa. The value of
-LaCeH at 250 GPa is about 233 K, which is slightly smaller
than that of -YCeH. However, it is found that
-LaCeH can be stabilized at 200 GPa, making the high-pressure
synthesis of LaCeH easier.Comment: 5 figure
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