Si Clusters Research Articles

Formation mechanisms of multiple Si/graphene nanocomposites through underwater pulsed discharge using different wire arrangements

Si/graphene nanocomposite is regarded as an advanced lithium-ion anode material, attributable to its advantageous microstructure and exceptionally high theoretical specific capacity. This study demonstrates a novel approach for the preparation of Si/graphene nanocomposites, achieved through the pulsed discharge method employing graphite and Si strips as precursors. We devised two distinct “one-step” synthetic routes through pulsed discharge of different wire arrangements. The characterization results of the obtained samples unveiled the presence of Si/graphene composite and Si@carbon layer composite. In addition, the formation mechanisms of Si/graphene nanocomposites have been proposed, different wire arrangements lead to the different energy deposition processes, inducing different robust mechanical and thermal effects on the products. The formed Si clusters either deposit on the surface of exfoliated graphene or become encapsulated within curling graphitic layers, forming a core@shell structure. This work not only presents a straightforward and innovative approach for synthesizing Si/graphene nanocomposites but also contributes valuable insights into the underlying mechanisms governing their formation.

Both the ionic and covalent bonds stabilize the W@Si12 cluster

Si is an important semiconductor material in the development of modern industry. With the miniaturization trend of semiconductor devices, the size of Si has reached the cluster size. The search for stable Si clusters is an important issue. In this work, the electronic structures and stability mechanism of the W@Si12 cluster are calculated by the first-principle calculations. Different from a C2v hexacapped trigonal prism structure of Si12, the W@Si12 cluster presents an embedded hexagonal prism structure with D6h symmetry. The addition of W atom leads to a higher stability. The molecular orbitals show obvious superatomic characteristics for the W@Si12 cluster, and their energy levels are more degenerate than that of the Si12 cluster. The population analysis indicates that a total charge of 2.21e is transferred from Si atoms to the center W atom, which suggests an ionic bond for W-Si. The electron localization function further proves a covalent bond for Si–Si. The enhanced stability of the W@Si12 cluster is attributed to the combination of the ionic and covalent bonds.

Damage microstructures in Ti3SiC2 under successive Xe-He-H ions irradiation and annealing process

Ti3SiC2 is recognized as a promising candidate material for nuclear energy applications; however, the microstructural defects resulting from complex irradiation processes remain elusive. This study investigates the microstructural evolution of Ti3SiC2 under sequential Xe-He-H ions irradiation at room temperature, followed by annealing at 900 °C, employing both experimental and first-principles computational approaches. A reverse phase transformation occurred following He irradiation, indicating that He facilitates the remote migration and recombination of Si vacancies through the formation of He-Si pairs during annealing. Post-annealing, the Xe-irradiated sample exhibited a widespread distribution of small Xe bubbles, whereas larger bubbles were prevalent at peak damage regions in the samples subjected to Xe+He and Xe+He+H irradiation. Concurrently, Si-rich precipitates with distinct distribution patterns were observed in samples irradiated with Xe, Xe+He, and Xe+He+H ions, significantly affecting the hardness. First-principles calculations reveal that the distinct damage profiles are primarily attributed to the dynamic behaviors of Si interstitials. The Ti-Si bonds are relatively weak and easily broken, leading to abundant Si interstitials in Xe ion irradiation, and He enhances Si diffusion. Conversely, H enhances Si segregation by preventing He from capturing Si interstitials. Within the TiC grains, He promotes Si migration, causing an approximately 700-fold increase in Si diffusivity at 900 °C. The Si segregation leads to the nucleation of 〈110〉 dislocation loops, culminating in the formation of distinctive strip-like Si clusters. This study elucidates the formation mechanism of Si segregation within Ti3SiC2, providing valuable insights for the design of irradiation-resistant materials.

Ductility fracture mechanisms of Al-7Si-Mg casting alloys considering Si particles: A combined experimental and crystal plasticity study

Al-7Si-Mg casting alloys are widely utilized for their excellent formability, though they commonly exhibit poor ductility. This study comprehensively investigates the factors influencing the ductility of Al-7Si-Mg casting alloys, proposing a fracture mechanism and developing a model for ductile fracture of the alloy. Tensile tests were conducted on samples subjected to different aging times, with analysis of the alloy’s microstructure and tensile fracture surfaces. The findings reveal that fracture initiation occurs at clusters formed by the aggregation of Si particles, followed by the coalescence of cracks within these clusters, leading to macroscopic fracture. Different tensile strains were applied to specimens subjected to under-aging and over-aging treatments. The distribution of geometrically necessary dislocations (GNDs) in stretched samples indicates that the Si cluster area undergoes greater localized plastic deformation compared to the Al matrix, and this disparity increases with the applied strain. It has been observed that the reduced elongation observed in over-aged alloys is due to the coalescence of Si clusters under lower strain conditions. Representative volume elements with the characteristics of Si cluster is proposed, and the crystal plasticity finite element method is used for tensile simulations. The simulation results show that the proposed model is more consistent with experimental findings pertaining to elongation than with the results of a conventional void model. A longer aging time reduces ductility owing to a more intense strain localization under same tensile strain.

Influence of a Preageing Treatment on the Cluster Formation and the Further Ageing Route in Al–Mg–Si Extrusion Alloys

Herein, the influence of one‐ and two‐step preageing treatments prior to the artificial ageing step to peak age condition for three different 6xxx alloys, from low to high alloy content, is investigated. Analysis of the preaged conditions using atom probe tomography (APT) reveals a broad distribution of Mg and Si clusters, with some instances of Guinier‐Preston I zones present. Additionally, the high alloyed system exhibits a growth of precipitates toward a needle‐like shaped. These clusters formed during the preageing treatment serve as nucleation sites in the subsequent artificial ageing treatment, leading to an increase in hardness. Microstructural analysis conducted using transmission electron microscopy and APT demonstrates a reduction in the average precipitate length and a more narrow size distribution for T6 condition. This indicates that the preageing treatment results in a decreased portion of already overaged precipitates compared to the direct aged condition. Consequently, the more narrow size distribution of the precipitates leads to more effective hardening of the material, as a higher portion of precipitates is at or close to their optimum size. The investigations highlight the significant influence and positive effect of a preageing treatment at lower temperatures on the subsequent artificial ageing treatment.

Enhancing strength and ductility of extruded Ti-alloy-reinforced Al-Mg-Si composites: Roles of interface nanostructures and chemical bonding

A comprehensive understanding of interface nanostructures and chemical interactions is required for solving the strength-plasticity trade-off dilemma of Al/Ti composites. Continuously Ti-reinforced Al-matrix composites with superior strength and ductility are fabricated via a novel extrusion technique. The unique strain-hardening mechanism of the Al/Ti composite plates at low strains is attributed to the coupling effects of the strong interface constraint potency, the mechanical incompatibility between the soft Al and hard α-Ti, and a large elastic strain of hard α-Ti. Interlocking structures with atomic interdiffusion are observed at the Al/Ti interface. Mechanical and thermal effects induce lattice distortion and atomic superdiffusion, thus resulting in an interfacial intermixing zone with substantial dislocations and misfit strain. These characteristics produce a strong interface constraint effect during tensile, causing superior strain/dislocation hardening and uniform plasticity. Nanoclusters with cubic structures and dispersed dislocations are detected in the three-dimensional (3D) atomic intermixing zone at the Al/TiAl3 interface, accompanied by the aggregation of Mg. The nanostructures with numerous dislocations in the 3D Ti/TiAl3 transition zone feature lattices with partially tetragonal structures and ordered Ti-Al(Si) clusters. Ab initio calculations indicate that the strong chemical interaction between Si and Ti atoms not only improves the interface bonding strength, but also disturbs the lattices and broadens the transition zone. Moreover, the 3D interface transition zones, accompanied by dispersed dislocations, improve the interface constraint potency and hardening capability of the annealed Al/Ti composite plate. This work sheds light on the capability of the extrusion technique to produce fiber-reinforced Al-matrix composites with superior mechanical properties and provides new strategies for improving the chemical bonding strength and interface constraint effect via the interface invasion of specific elements.

Understanding the interactions between anticancer active molecules Cp2TiCl2 and Cp2VCl2 and E@Al12 (E = C, Si) clusters using density functional theory calculations

This work investigated the interaction between E@Al12 (E = C, Si) clusters and titanocene dichloride (TDC) and vanadocene dichloride (VDC) anticancer drugs using BP86. Corrected interaction energy values of the complexes were computed. Interaction effect on the frontier orbital energy values of the pure nano-cluster and drugs were illustrated. Also, electrophilicity-based charge transfer (ECT) was indicated charge flow from anticancer drug to nano-cluster. Cl⋅⋅⋅Al weak non-covalent interactions in the studied systems were explored with Independent gradient model analysis based on Hirshfeld partition of molecular density (IGMH) and Non-covalent interaction (NCI). QTAIM analysis was considered to exploring of bond critical point of Al…Cl.

Random Green’s Function Method for Large-Scale Electronic Structure Calculation

We report a linear-scaling random Green’s function (rGF) method for large-scale electronic structure calculation. In this method, the rGF is defined on a set of random states and is efficiently calculated by projecting onto Krylov subspace. With the rGF method, the Fermi–Dirac operator can be obtained directly, avoiding the polynomial expansion to Fermi–Dirac function. To demonstrate the applicability, we implement the rGF method with the density-functional tight-binding method. It is shown that the Krylov subspace can maintain at small size for materials with different gaps at zero temperature, including H2O and Si clusters. We find with a simple deflation technique that the rGF self-consistent calculation of H2O clusters at T = 0 K can reach an error of ∼ 1 meV per H2O molecule in total energy, compared to deterministic calculations. The rGF method provides an effective stochastic method for large-scale electronic structure simulation.

Molecular dynamics to model carbon infiltration into a porous silicon matrix: An experimental and computational approach

In this work, a porous silicon matrix (PSm) and its functionalization with carbon atoms are modeled by molecular dynamics (MD); the simulation is correlated experimentally to analyze the model results. Computationally, the Tersoff potential for the silicon carbon system is used to build a model by MD using Large-scale Atomic/Molecular Massively Parallel Simulator free-source code (LAMMPS). First, a model of a PSm with an aleatory distributed macroporous was built; then, C atoms were injected into the PSm model to functionalize the matrix; finally, the model shows the functionalization of the PSm, forming carbon-silicon composites, which are Si clusters surrounded by C atoms. Experimentally, PSm was fabricated using samples of a p-type crystalline silicon wafer by electrochemical etching. A colloidal suspension of graphene oxide (GOs) was used as a source of carbon atoms: the GOs was added to the electrolyte, during the etching process, to functionalize the PSm with C atoms. AFM images show that the PSm pores are not obstructed after functionalization. Carbon-silicon composites were formed into the PSm samples according to X-ray diffraction and EDS characterizations. The results of the modeling agree with the results of the experiments.

Dielectric Behavior and Prolate Growth Patterns of Silicon Clusters SiN with N = 12-30 by Cryogenic Electric Beam Deflection.

We present a comprehensive investigation of the dielectric behavior and geometric structures of cold neutral SiN clusters of intermediate size with N = 12-30 atoms. For this, cryogenic electric beam deflection experiments were carried out for the first time for Si clusters at nozzle temperatures below 30 K. In combination with quantum chemical calculations based on density functional theory and classical trajectory simulations of the rotational dynamics in the electric field, the geometric structures of the clusters are discriminated. Clusters with N < 15 favor a single-capped square antiprism as a nucleus for cluster growth, forming compact geometries in the molecular beam. Starting with 15 atoms, a prolate-like growth is observed. The prolate structures are based on stable building blocks which reappear for numerous sizes throughout the cluster growth. Finally, the transition from prolate to quasi-spherical shapes is shown to take place around Si29/Si30 as predicted theoretically by the literature. The influence of the exchange-correlation functional on the predicted structure and dielectric properties is discussed in detail for some clusters. Relaxation of the electric-dipole moment and therefore quenching of the observed electric response due to vibrational excitation and collisions with the background gas are also considered, which explains deviations between experiment and theory.

Diversity of Behavior after Collisions of Sn and Si Nanoparticles Found Using a New Density Functional Tight-Binding Method.

We present a new approach to studying nanoparticle collisions using density functional based tight binding (DFTB). A novel DFTB parametrization has been developed to study the collision process of Sn and Si clusters (NPs) using molecular dynamics (MD). While bulk structures were used as training sets, we show that our model is able to accurately reproduce the cohesive energy of the nanoparticles using density functional theory (DFT) as a reference. A surprising variety of phenomena are revealed for the Si/Sn nanoparticle collisions, depending on the size and velocity of the collision: from core-shell structure formation to bounce-off phenomena.

A novel method to characterize the residual stress on the fused silica surface based on the evolution of the atomic point defects

The residual stress would have a significant impact on the service performance and fatigue life of the fused silica (non-crystal material) components (e.g., Hemispherical Shell Resonator). However, there is still no effective method for characterizing the residual stress on their working surfaces. Therefore, it is essential to develop a novel method for characterizing the residual stress on fused silica surfaces. Herein, the residual stress is found to significantly affect the mechanical properties of fused silica surfaces. On the basis of the study on the photoluminescence (PL) properties of residual stress zones, atomic point defects are found to be closely related to the residual stress. Based on an investigation of the PL properties of different types of mechanically processed surfaces, four types of point defects (ODCII, STE, Si clusters and NBOHCI) have been determined. On the basis of the molecular dynamics (MD) simulation results and the PL experiments on different types of mechanically processed surfaces, STE point defects at the atomic scale are found to be closely related to the residual stress for the first time. Based on this, PL-detection method is proposed to characterize the residual stress on the material surface for the first time. Besides, the relative amount of STE point defect (K = S/S0) is first proposed to quantitatively characterize the residual stress. In sum, this work determines the structure at the atomic scale which is closely related to the residual stress and opens a convenient door to quantitatively characterize the residual stress with PL-detection method. It could also provide a reference for the characterization and even the alleviation of the residual stress on the surfaces of other non-crystal materials.

Structural organization of phase-separated bioactive glasses and the clustering of Si, P, B, Na and F atoms investigated by solid-state NMR and Monte Carlo simulations

In this paper we studied the effect of NaBO2 addition to a phase-separated alkali-free bioactive glass with a composition of 38.49 SiO2 • 36.07 CaO • 19.24 MgO • 5.61 P2O5 • 0.59 CaF2. Microscopy reveals binodal phase separation involving two Si-containing microphases with a droplet size of ∼200 μm, driven by the thermodynamic LLPS mechanism. The local environments and spatial distribution of silicate, phosphate, and fluoride ions in this phase-separated system were studied, using 29Si, 31P, 11B, 19F, 25Mg, and 23Na nuclear magnetic resonance (NMR) and infrared spectroscopy. The silicate units are dominantly of the metasilicate (Si2) type. The phosphate units exist mostly as orthophosphate (P0) while the borate is present in the form of pyroborate (B1). Multinuclear dipolar re-coupling experiments indicate that the minority components F, P, B and Na all occur within a common phase. Thus, atomic distribution scenarios involving the separation of these components into separate phases can be excluded. The 31P spin echo decay (SED) method was used along with Monte Carlo simulations to characterize the spatial distribution of the phosphate component. Based on the analysis, the phosphate component forms clusters of sizes 1-4 nm, which are embedded in an environment more dilute in phosphate, having a random distribution. While 19F SED results indicate that the fluoride ions do not form clusters and are close to randomly distributed, dipolar recoupling of 31P suggests a local environment resembling that of fluorapatite.

3D Investigation of Damage During Strain‐Controlled Thermomechanical Fatigue of Cast Al–Si Alloys

The strain‐controlled thermomechanical fatigue behavior is investigated for three cast near‐eutectic Al–Si alloys with different Ni, Cu, and Mg contents. Synchrotron tomography and neutron diffraction experiments are used to correlate 3D microstructural features with damage initiation and evolution. The results show that the alloy with lower Cu, Ni, and Mg concentrations has up to 45% higher thermomechanical fatigue resistance for cooling/heating rates of 5 and 15 K s−1. In addition, this alloy also exhibits damage formation at later stages during thermomechanical fatigue and slower damage accumulation compared to other alloys. This difference in behavior is a consequence of its higher ductility, which is a result of the lower volume fraction and global interconnectivity of the 3D hybrid networks formed by Si and intermetallics and the absence of large primary Si clusters which act as preferred crack initiation sites during the early stages of thermomechanical fatigue.

Effect of Ge addition on the clustering of naturally-aged Al–Mg–Si–Cu alloy

Ge addition effect on the delaying natural aging of Al–Mg–Si–Cu alloy was investigated. Natural aging kinetic and cluster size was suppressed by the Ge addition. Using atom probe tomography, it was found that Ge refines cluster evolution by disturbing Mg-enrichment of Si–Si cluster as well as enhancing vacancy trapping. Segregation of Ge at the interface of cluster was also regarded as potential barrier to growth of the clusters.

Influence of powder layer thickness on microstructure and T5 heat treatability of F357 alloy fabricated by laser powder bed fusion process

Laser powder bed fusion (LPBF) is one of the most widely used additive manufacturing (AM) methods to produce metal parts with new functionalities, but it still suffers from a low built rate. Increasing the powder layer thickness is an efficient way to augment LPBF productivity. The objective of this study is to investigate the influence of the powder layer thickness during the LPBF of F357 alloy on the microstructure and age-hardening response during T5 heat treatment. Scanning electron microscope (SEM) and transmission electron microscope (TEM) observations show that microstructure differences are negligible between samples printed at 30 µm and 50 µm layer thicknesses in the as-built condition. Samples built with both layer thicknesses and subjected to T5 heat treatments at 150 °C, 160 °C, and 170 °C also show similar microstructures and their α-Al cells contain large Si precipitates and small Si clusters and no β’’-Mg2Si precipitates. Therefore, β’’-Mg2Si precipitation hardening is not the dominant mechanism, and strengthening appears to be mainly due to the small Si clusters. With the help of the Johnson-Mehl-Avrami (JMA) model, the activation energy of the precipitation process of the samples built at 30 and 50 µm layer thicknesses is calculated to be 142 kJ/mol and 139 kJ/mol, respectively. These comparable values suggest a similar evolution of the precipitation hardening for both layer thicknesses, which is consistent with the observed microstructures. Consequently, the production efficiency of LPBF-printed F357 alloys can be enhanced by using higher layer thickness and the T5 heat treatment.

Formation of Ni–Si clusters and their relationship with dislocation loops in irradiated stainless steels

Ni–Si cluster is a major contributor to the irradiation hardening in irradiated stainless steels. The heterogeneous nucleation of Ni–Si clusters is associated with radiation-induced segregation at sinks (e.g., dislocation loops), and the possible formation of Ni–Si clusters on dislocation loops may influence the evolution process of the two types of defects. Besides, the nature of Ni–Si clusters is unclear yet, reflected by the discrepancies in the reported cluster composition. In this study, model alloys of 316L stainless steel with different Si contents were irradiated to 5 dpa at 450°C. Ni–Si clusters were analyzed by atom probe tomography and were compared with the loops observed by transmission electron microscopy. Results showed that higher Si content promoted cluster number density and size but did not change cluster composition. Discontinuous torus-shaped, continuous torus-shaped, and disk-shaped clusters were discovered. The cluster shape, size, and crystallographic orientation supported the formation of torus-shaped clusters on Frank loops and perfect loops. In large torus-shaped clusters, Si and Ni tended to concentrate in the outer and inner regions of the torus, respectively, and it may be attributed to the stress field of the interstitial-type Frank loops on which the torus-shaped clusters were formed. The constant composition of large Ni–Si clusters indicated the possible existence of a preferential Ni–Si phase. Combining the results of the 1D concentration profile and the MSM using Si atoms and Ni atoms, the Ni/Si ratio of the large clusters was between 1.6–3.0.

Molecular Dynamics Study on Crack Propagation in Al Containing Mg-Si Clusters Formed during Natural Aging.

The crack propagation behavior of Al containing Mg-Si clusters is investigated using molecular dynamics (MD) simulations to demonstrate the relationship between the natural aging time in Al-Si-Mg alloys and ductility. Experimental results show that the elongation at failure decreases with natural aging. There are few studies on the relationship between natural aging and ductility because of the difficult observation of Mg-Si clusters. To solve the difficulty, cracked Al containing Mg-Si clusters of varying sizes are assumed for the MD simulations. A larger Mg-Si cluster in Al results in earlier crack opening and dislocation emission. Moreover, as the Mg-Si cluster size increases, the stress near the crack tip becomes more concentrated. This causes rapid crack propagation, a similar effect to that of crack tip sharpening. As a result of long-term natural aging, the cracks expand rapidly. The influence of geometry is also investigated. Crack lengthening and thickness reduction negatively impact the fracture toughness, with the former having a larger impact than the latter. Although there are several discrepancies in the practical deformation conditions, the simulation results can help to more thoroughly understand natural aging in Al-Si-Mg alloys.

First-principles investigation of defective graphene anchored with small silicon clusters as a potential anode material for lithium-ion batteries

The interplay between vacant graphene and silicon (Si) provides a viable way to tune the properties for successful implementation as a promising anode material of lithium-ion batteries. Therefore, understanding the interaction of such defects when coupled with silicon clusters is of particular importance. Using first-principles methods, we investigate the atomic adsorption on graphene with a single vacancy and a double vacancy, as well as the structural and electrical properties. The presence of defects strongly enhances the interaction between the defective graphene and Si clusters, dependent on Si type. We observe that SiO strongly adsorbed on the divacancy graphene with the adsorption energy of -4.49 eV could provide extra intercalation places for Li atoms, augmenting the adsorption energy of Li from -1.18 eV to -4.15 eV compared to the pristine graphene. Moreover, it exhibits a threefold increase in the lithiation /de-lithiation potential and the superior Li storage capacity. The enrichment of Li adsorption and uptake is also observed in the hybrid Si6/single-vacancy graphene. Thus, the interface engineering via deposition of various Si clusters on the vacant graphene could be a new strategy to achieve a promising anode material for Li-ion batteries.

A donor-supported silavinylidene and silylium ylides: boroles as a flexible platform for versatile Si(ii) chemistry.

Electron-deficient, anti-aromatic 2,5-disilyl boroles are shown to be a flexibly adaptive molecular platform with regards to SiMe3 mobility in their reaction with the nucleophilic donor-stabilised precursor dichloro silylene SiCl2(IDipp). Depending on the substitution pattern, selective formation of two fundamentally different products of rivalling formation pathways is achieved. Formal addition of the dichlorosilylene gives the 5,5-dichloro-5-sila-6-borabicyclo[2.1.1]hex-2-ene derivatives. Under kinetically controlled conditions, SiCl2(IDipp) induces 1,3-trimethylsilyl migration and adds exocyclically to the generated carbene fragment giving an NHC-supported silylium ylide. In some cases interconversion between these compound classes was triggered by temperature or NHC-addition. Reduction of silaborabicyclo[2.1.1]hex-2-ene derivatives under forcing conditions gave clean access to recently described nido-type cluster Si(ii) half-sandwich complexes of boroles. Reduction of a NHC-supported silylium ylide gave an unprecedented NHC-supported silavinylidene which rearranges to the nido-type cluster at elevated temperatures.