Bcc Configurations Research Articles

Tailoring Mechanical Performance in Bulk Nanoparticle-Structured ZnO and Al₂O₃: Insights from Deep Learning Potential Molecular Dynamics Simulations

The rapid advancements in nanotechnology have created unique opportunities to manipulate material properties at the nanoscale, particularly through the design of nanoparticle-structured (NP-structured) materials. Due to their distinctive mechanical and chemical properties, materials composed of nanoparticles, such as ZnO and Al₂O₃, are of high interest for applications in structural reinforcements, electronics, and catalysis. However, accurately predicting the mechanical behavior of NP-structured materials remains a challenge. This study investigates the mechanical properties of bulk nanoparticle-structured (NP-structured) materials composed of ZnO and Al₂O₃ nanoparticles in FCC and BCC configurations. We used deep learning potentials (DLPs) derived from density functional theory (DFT) calculations to comprehensively analyze stress-strain behavior, local shear strain distributions, and elastic properties across various nanoparticle sizes and compositions. Our findings reveal that nanoparticle size and crystalline arrangement are crucial in determining the mechanical performance of these materials. Smaller ZnO nanoparticles exhibit higher yield stress, ultimate stress, and Young's modulus in both FCC and BCC configurations, attributed to their higher surface-to-volume (S/V) ratio. In contrast, larger Al₂O₃ nanoparticles demonstrated superior mechanical properties, especially in FCC configurations, due to the strong ionic bonding and effective stress distribution inherent in Al₂O₃. In ZnO/Al₂O₃ composite systems, we found that the mechanical properties could be precisely tuned by adjusting the nanoparticle size and ratio. Larger nanoparticles in FCC arrangements contributed to higher yield stress and stiffness, while smaller nanoparticles in BCC arrangements provided better overall mechanical performance. This work highlights the potential of DLPs in accurately predicting and optimizing the mechanical properties of NP-structured materials, offering valuable insights for the design of advanced materials with tailored mechanical characteristics.

Comparison of thermo-hydraulic performance among different 3D printed periodic open cellular structures

As additive manufacturing of periodic open cellular structures (POCS) is gaining interest in structured catalytic reactor research, this work seeks to thermohydraulically compare the well-known Kelvin lattice structure with the lesser-researched BCC and gyroid lattice structures. Using a combined CFD (Computational Fluid Dynamic) and experimental approach, the selected POCS are fabricated through Laser Powder Bed Fusion (LPBF), characterized, and subsequently subjected to numerical analysis. From the manufacturability point of view, the 3D printed samples closely matched their CAD designs, showing a maximum porosity deviation of 15% below design values. A CFD model, validated through pressure drop experiment, was employed to compare the POCS designs on shared geometric attributes such as specific surface area and porosity. While all structures exhibited comparable performance in term of heat and momentum transfer, our findings suggest that the Gyroid lattice may provide the optimal balance between momentum and heat transfer rates in low-velocity region. Conversely, the BCC configuration may be more favourable at higher velocity. An Ergun-like correlation was also developed and validated for each lattice type, with a Mean Absolute Percentage Error (MAPE) below 10%. Our pressure drop results align quite well with existing literature correlations, showing a MAPE under 20%. Concerning heat transfer, the values forecasted in this research show a reasonable alignment with literature’s results, though they tend to be on the lower spectrum.

Research on the effective thermal conductivity of nickel-based bi-porous capillary wicks: Modeling and validation

This study proposed an analytical model to predict the effective thermal conductivity (ETC) of bi-sized porous capillary wicks with both interstitial pores (formed inside nickel skeleton) and with large pores created by NaCl as the pore-forming agent. The interstitial-pore model was developed utilizing the sintering neck formation theory and thermal resistance network, which was validated by measured data obtained from samples of multiple particle sizes. It is shown that the model works well for fine nickel powders with the root mean square error (RMSE) of 13.8%, while a large deviation was observed when using the coarse powders. Based on the presented interstitial-pore model, an ETC model for samples containing formation pores was formulated. A total of six types of equations were proposed, considering three packing modes and two shapes of formation pores. Samples with NaCl of various granularities (54–75 μm, 88–125 μm) and proportions (2.5, 5.0, 7.5, and 10.0 wt.%) were made for the model validation. The results demonstrated that the bi-porous ETC models, with both interstitial pores and those formed by NaCl, exhibit good performance when applying the BCC configuration, while small and large formation pores can be characterized by spherical and cubic models respectively.

First Principles Prediction of the Al-Li Phase Diagram

The phase diagram of the Al-Li system was determined by means of first principles calculations in combination with the cluster expansion formalism and statistical mechanics. The ground state phases were determined from first principles calculations of fcc and bcc configurations in the whole compositional range while the phase transitions as a function of temperature were ascertained from the thermodynamic grand potential and the Gibbs free energies of the phases. Overall, the calculated phase diagram was in good agreement with the currently accepted experimental phase diagram but the simulations provided new insights that are important to optimize microstructure of these alloys by means of heat treatments. In particular, the structure of the potential GP zones, made up of Al0.5Li0.5 (001) monolayers embedded in Al matrix, was identified. It was found that Al3Li is a stable phase although the energy barrier for the transformation of Al3Li into AlLi is very small (a few meV) and can be overcome by thermal vibrations. Moreover, bcc AlLi was found to be formed by martensitic transformation of fcc configurations and Al3Li precipitates stand for favorable sites for the nucleation of AlLi because they contain the basic blocks of such fcc ordering. Finally, polynomial expressions of the Gibbs free energies of the different phases as a function of temperature and composition were given, so they can be used in mesoscale simulations of precipitation in Al-Li alloys.

Strong ferromagnetic coupling and tunable easy magnetization directions of FexCo1−x layer(s) intercalated under graphene

We report the magnetic response of FexCo1−x layer(s) intercalated under graphene, compared with the bulk equiatomic FeCo(1 0 0) specimen. Graphene grown on Ir(1 1 1) can act as a protective membrane, allowing confined epitaxial growth of homogeneous FexCo1−x layer(s). Few intercalated FexCo1−x layers exhibit a significant enhancement of the spin (Seff) and orbital (L) moments with respect to both the single elements and the bulk equiatomic FeCo(1 0 0) configuration, as demonstrated by means of magnetic circular dichroism experiments. Reducing the intercalant thickness down to a single layer of FexCo1−x, the easy magnetization direction switches from parallel to perpendicular to the surface plane, with a magnetic response being influenced by the underlying metal. Element-sensitive hysteresis loops disclose identical magnetization response at the Fe and Co sites, thus constituting a single magnetic unit with enhanced ferromagnetic response, with Fe acting as a strong ferromagnet, differently from its bulk bcc configuration. This is a significant step towards the control at the atomic scale of the magnetic response in low dimensional 3d transition metal alloys, with tunable easy-magnetization direction, enhanced spin moments and magnetic anisotropy, in a inert and protected configuration thanks to the 2D graphene cover.

Recirculation zones induce non-Fickian transport in three-dimensional periodic porous media.

In this work, the influence of pore space geometry on solute transport in porous media is investigated performing computational fluid dynamics pore-scale simulations of fluid flow and solute transport. The three-dimensional periodic domains are obtained from three different pore structure configurations, namely, face-centered-cubic (fcc), body-centered-cubic (bcc), and sphere-in-cube (sic) arrangements of spherical grains. Although transport simulations are performed with media having the same grain size and the same porosity (in fcc and bcc configurations), the resulting breakthrough curves present noteworthy differences, such as enhanced tailing. The cause of such differences is ascribed to the presence of recirculation zones, even at low Reynolds numbers. Various methods to readily identify recirculation zones and quantify their magnitude using pore-scale data are proposed. The information gained from this analysis is then used to define macroscale models able to provide an appropriate description of the observed anomalous transport. A mass transfer model is applied to estimate relevant macroscale parameters (hydrodynamic dispersion above all) and their spatial variation in the medium; a functional relation describing the spatial variation of such macroscale parameters is then proposed.

Stability of iron crystal structures at 0.3–1.5 TPa

Ab initio molecular dynamics simulations carried out for tetragonal and orthorhombic distortions of iron closely follow the results of static-lattice electronic-structure calculations in revealing that the body-centered cubic (bcc) phase of Fe is mechanically unstable at pressures of 0.3–1.5 TPa and temperatures up to 7000 K. Crystal-structural instabilities originate in the static lattice for the bcc configuration, and are consistent with recent results from both static and dynamic high-pressure experiments. Both theory and experiment thus show that the close-packed (hexagonal, hcp and face-centered cubic, fcc) crystal structures of iron are those relevant to the cores of Earth-like planets.

Segregation into Layers: A General Problem for Structural Instability under Pressure, Exemplified by SnH4

When a molecular compound is thermodynamically unstable (but kinetically persistent) with respect to the elements, structures that contain segregated layers of the elements may be favored at moderate pressures, as a compromise between the potential stability of novel electronic configurations and decomposition into the elements (or other stable compounds). We use stannane, SnH(4), to approach this quite general problem theoretically, since the heat of formation of SnH(4) is so positive. Our ground-state DFT searches for optimal structures begin with slabs formed from 1-4 layers of tin atoms in the β-Sn and bcc configurations, and also slabs of molecular hydrogen or hydrogen atoms, preserving the overall SnH(4) stoichiometry. As argued, segregated layers are an important structural feature in the lower- and moderate-pressure regime (0 and 50 GPa). By 140 GPa (V/V(0)=0.21) the coordination of tin and hydrogen increases and the slabs disappear, as judged from the optimized structures.

A Computational Study of 3‐D Helium Clusters

AbstractPhysical and thermodynamic properties have been calculated and analyzed for the best and optimized geometries of the 3‐D clusters with N = 3 to N = 10 atoms and unit cells of three types of crystalline systems using ab initio RHF/6–31G** method. Dependence of the lattice binding energy on the cluster parameter, R, has been studied. Similar behavior observed for the binding energies for all clusters shows that probabilities of their existence in the condensed phase are more or less the same. In the next step, thermodynamic properties have been calculated and analyzed for He27 3‐D helium clusters with simple cubic, body centered cubic (bcc), trigonal and hexagonal (hcp) configurations. The results show that the hexagonal cluster is more favored over other clusters. It is found that these clusters are electronically stable over a limited range of the values for the lattice parameter. ΔfH is constant in this stability region and thus the ΔfG exactly follows the variations of TΔfS. Surface effects have been investigated by comparing the square and hexagonal He9 2‐D lattices with the cubic and hexagonal He27 3‐D lattices, respectively. The lattice parameters, densities and molar volumes calculated for the clusters with hcp and bcc configurations have satisfactory agreement with the available experimental values. Properties of the He13, He34 and He104 hcp clusters have also been calculated and analyzed.

Structural transformations in Fe-Ni-alloy nanoclusters: Results of molecular-dynamic-simulation

Size effect in Fe-Ni-alloy nanoclusters has been studied by the molecular-dynamics (MD) method using multiparticle interatomic interaction potentials. It is shown that the α-γ transformation in nanoparticles with sizes d>3.5 nm proceeds by the mechanism of nucleation at grain boundaries and propagation of fcc-phase plates. As a result of the transformation, a twinned lamellar domain structure is formed. In particles with sizes 3.0<d<3.5 nm, the α-γ transformation is accompanied by radial-symmetry atomic movements that are close to those characteristic of the Bain scheme. This results in the formation of a single-domain fcc phase. In nanoparticles with sizes 1.5<d<3.0 nm, the α-γ transformation proceeds via an intermediate state that is retained within a temperature range of a few hundreds of kelvins and is characterized by an incomplete phase transformation. It has been found that in Fe-Ni clusters with sizes d≤1.5 nm, the α-γ transformation does not occur. During heating, the initial bcc configuration turns into an icosahedral one through polytetrahedral or amorphous-like configuration.

Simulations of Spontaneous Phase Transitions in Large, Deeply Supercooled Clusters of SeF6

The crystallization and subsequent solid-state transitions in a series of large clusters of SeF6 of two sizes have been studied by molecular dynamics simulations at constant temperature. Several diagnostic methods were applied to monitor molecular details of the clusters' structures and their evolution with time. The behavior of 12 liquid clusters with 725 molecules and 10 with 1722 molecules was examined at 140 and 130 K. During the nanosecond runs of the simulations all of these clusters froze, initially to the bcc or a related but distorted structure. At the higher temperature all but one of the larger clusters underwent a transition to the monoclinic structure whereas all but one of the smaller clusters remained bcc. At the lower temperature all of the smaller clusters ultimately transformed, usually quite abruptly, to the monoclinic structure. In the case of the larger clusters a transition to the monoclinic phase was observed at 140 K whereas at 130 K, besides the monoclinic structure, the orthorhombic or a mixture of orthorhombic and monoclinic phases was obtained in a few clusters. Many of the larger frozen clusters were polycrystalline while the smaller ones were single crystals. How these results relate to Kashchiev's criterion for mononuclear vs polynuclear growth is discussed, and the time dependence of crystal growth was found to agree well with the Kolmogorov−Johnson−Mehl−Avrami equations. Growth rates of the bcc phase were in reasonable agreement with Turnbull's theory. Simulations of solid-state transitions from clusters prepared to have a well-ordered bcc configuration clearly indicate a lower nucleation rate for the low-energy phase than in a cluster with grain boundaries and/or solid−liquid interfaces. A striking result was that nucleation invariably occurred at or near the clusters' surfaces despite the fact that surfaces of clusters tend to be disordered and melt at significantly lower temperatures than their cores. Such a behavior has also been reported for simulations of monatomic clusters.

Rh polarization in ultrathin Rh layers on Fe(001).

We discuss the origin of the Rh polarization at the interface with a semi-infinite Fe(001) crystal obtained through spin-resolved valence-band and core-level spectroscopy. Due to the fact that it is not yet known how Rh grows on Fe(001), we discuss different epitaxial arrangements of Rh atoms on Fe(001), i.e., not only the usual fcc and bcc configurations but also face-centered tetragonal (fct) and body-centered tetragonal (bct) configurations which conserve the atomic volume. The fcc and bcc configurations do not agree with the experimental results because the polarization of the Rh is long ranged for fcc and too small for bcc. fct and bct configurations appear to be more favorable. Moreover, in all cases investigated an oscillatory polarization of the Rh atoms has been obtained.

Lattice relaxation in α-iron containing small xenon-vacancy clusters

Binding energies and atomic configurations for small xenon-vacancy clusters in α-iron have been computed using a model based on a two-body potential between the interacting atoms. Various clusters were placed near the center of a sphere containing about 150 iron atoms initially at regular positions in bcc configuration. The results of these calculations are of importance for the interpretation of hyperfine interaction measurements on implanted radioactive sources.