Spherical Si Research Articles

Machine learning driven design of high-performance Al alloys

Aluminum (Al) alloys with both high strength and thermal conductivity (TC) are promising structural materials for wide application across different industries. Yet, design of such alloys is challenging, since strength and TC often share a trade-off. In this paper, we build prediction models for TC and ultimate tensile strength (UTS) of Al alloys using eXtreme gradient boosting (XGBoost) and support vector machine (SVM) algorithms, respectively. The models take physical descriptors from the alloy composition into account. Lasso and Gini Impurity algorithms were adopted for feature engineering. Guided by the models, an Al-2.64Si-0.43Mg-0.10Zn-0.03Cu alloy with TC over 190 W·m-1·K-1 and UTS over 220 MPa was designed. The alloy was fabricated and tested by experiment, and its UTS and TC are close to the model prediction. Microstructure characterization suggests that the fragmented and spherical Si phase, along with a few non-spherical Si phases, may be a key reason for the improved properties.

Cyclic deformation behavior of an overaged high-pressure die-cast aluminum alloy

The aim of this study is to identify the deformation behavior of a high-pressure die-cast Silafont®-36 aluminum alloy in an overaged T7 condition in relation to the significant microstructural changes caused by the overaging heat treatment. The T7 aluminum alloy exhibited a distinctive composite-like microstructure, consisting of coarser α-Al grains and larger spherical Si particles embedded in the matrix, in contrast to its as-cast and T4 counterparts. The softer α-Al matrix of the T7 alloy as revealed by the nanohardness and microhardness led to improved ductility and longer strain-controlled fatigue life at higher strain amplitudes, despite the lower strength. Cyclic stabilization was observed to be a salient feature of the T7 alloy, stemming from the equilibrium between cyclic hardening and cyclic softening. Strain-energy density-based model, by incorporating a material parameter called ‘intrinsic fatigue toughness’, could be used to predict the fatigue life of the alloy.

Nanoscale Al precipitation in the Si phase in AlSi10Mg alloy during electron beam powder bed fusion

Additive manufacturing of Al alloys has garnered attention in the aerospace and automobile industries. This is the first study on the formation of nanoscale Al precipitates in the Si phase of an AlSi10Mg alloy during electron beam powder bed fusion (EB-PBF). Spherical Si particles were homogeneously dispersed in the Al matrix, highlighting the difference from the laser beam PBF (LB-PBF) microstructures. Nanoscale Al phase was formed with a crystallographic orientation relationship with the surrounding Si phase: (111)Si//(111)Al and [11¯0]Si//[11¯0]Al. The formation of Al nanoprecipitates was attributed to an interplay between non-equilibrium solidification, wherein excess Al was dissolved in the Si particles, and the subsequent decomposition of the supersaturated Si phase during high-temperature exposure owing to the preheating procedure. To the best of our knowledge, such formation of Al nanoparticles has not been reported in AlSi10Mg produced through conventional processing or LB-PBF. Thus, the unique thermal history of EB-PBF provides novel opportunities for microstructural evolution, which may be beneficial for the development of novel Al-based alloys.

Realization of high-capacity coulombic efficiency in sodium alginate/carbon nanotube double network coated Si-anode for lithium-ion batteries

Silicon (Si) is considered as one of the most promising anodes for the next-generation lithium-ion batteries (LIBs) owing to its ultra-high specific capacity, low redox potential and the second abundance of elements in the earth's crust. However, drastic volume change will directly cause electrode pulverization, thus leading to low initial coulombic efficiency (ICE) and terrible cycling stability. In this work, a sodium alginate (SA)‑carbon nanotube (CNT) derived double carbon-coated Si composite (SA-CNT@Si) was prepared through freeze-drying technique followed by high-temperature carbonization, where SA was utilized to encase Si nanospheres, with CNT serving as the conductive framework that interconnects Si spheres. These composites exhibit improved electrical conductivity and stability throughout the charge and discharge cycles when employed as the anodes in LIBs, demonstrating superior electrochemical properties. The SA-CNT@Si electrode with a mass ratio of Si: SA: CNT = 5: 1: 0.75, achieved a first discharge specific capacity of 2200.8 mAh g−1 at a current density of 500 mA g−1 (with the initial three cycles at 100 mA g−1), along with an ICE of 86.2%. Even after 500 cycles, it maintained a capacity of 607 mAh g−1. This study presents a novel approach to designing Si-based anode materials characterized by both high electrical conductivity and structural stability.

Effect of local loading on microstructure and enhanced mechanical property of large complex castings prepared by Al–Si–Fe–Mn–Mg–Cu alloy during squeeze casting

Flywheel shells with a complex structure and large wall-thickness difference, as key components in heavy trucks, serve to connect the engine and transmission. Formability and mechanical performance control of such components should be taken into consideration. In this work, an Al–Si–Fe–Mn–Mg–Cu alloy was used to manufacture the flywheel shell via squeeze casting. The role of local loading on microstructure and mechanical property at thick-walled positions was investigated. Furthermore, the effect of the squeeze casting specific pressure and heat treatment on the microstructure and mechanical property of the Al–Si–Fe–Mn–Mg–Cu alloy flywheel shells was also analyzed. The results showed that at the thick-walled positions, local loading not only helped eliminate the solidification defects, but also refined the microstructure including α-Al grains and secondary dendrite arm spacing. With increasing the squeeze casting specific pressure from 24 MPa to 32 MPa, microstructure refinement and mechanical property enhancement of squeeze casting flywheel shells were obtained. After T6 heat treatment, the yield strength and ultimate tensile strength of flywheel shells were further increased to 261.8 and 318.4 MPa, respectively, owing to the formation of spherical eutectic Si phases and nano-sized β'', Q and S precipitates.

Towards high performance Al–Si–Fe alloy castings via rheo-diecasting: Effect of injection velocity and heat treatment on microstructure evolution and property

The mechanical properties and thermal conductivity of Al–Si–Fe castings must meet high requirements in the automobile, aerospace, and communication fields. Traditional high-pressure diecasting (HPDC) castings often have dendritic structures and pore defects that deteriorate their overall performance. Therefore, an advanced rheo-diecasting (RDC) technique assisted by a vibrating contraction sloping plate was proposed to address this problem. However, the process optimization strategy, microstructural evolution, and performance improvement mechanisms remain unclear. The influence of the injection velocity on the microstructure, mechanical properties, and thermal conductivity of the RDC tensile specimens was first investigated. A high-performance cover plate was fabricated via RDC using an optimized injection velocity. The results showed that when the injection velocity increased to 2.8 m/s, the porosity decreased to a minimum value, and the mechanical performance and thermal conductivity of the RDC tensile specimens reached their maximum values. Furthermore, spherical eutectic Si phases and nanosized Mg2Si precipitates for the RDC cover plate were observed after T6 heat treatment, which further improved the mechanical properties and thermal conductivity owing to Orowan strengthening and decreased lattice distortion, respectively. The ultimate tensile strength, yield strength, elongation, and thermal conductivity for the RDC cover plate were 358.2 MPa, 279.9 MPa, 12.5%, and 180.72 W/(m·K), respectively, which were higher than that of reported similar alloys. From engineering perspectives, this study had the application value for produce of high-quality RDC castings.

Development of Spherical Eutectic Si Particles through M-SIMA Process and Its Effects on the Tensile Properties of Al-7Si-0.05Sr Alloy

Development of Spherical Eutectic Si Particles through M-SIMA Process and Its Effects on the Tensile Properties of Al-7Si-0.05Sr Alloy

Deposition of uniform diamond films on three dimensional Si spheres by using faraday cage in MPCVD reactor

Deposition of a consistent diamond coating on intricate surfaces has historically posed a difficulty. This article presents the advancement in research on depositing a uniform diamond coating on three-dimensional (3D) silicon spheres, utilizing a faraday cage within a microwave plasma chemical vapor deposition (MPCVD) reactor. A self-consistent three-dimensional multi-physics field model has been established for simulating the microwave electric field and plasma inside a reactor with or without a faraday cage. Results indicate the cage effectively reduces the discharge at the top of the silicon sphere and improves uniformity of the distribution of the microwave electric field and plasma electron density on the sphere's surface. MPCVD trials have demonstrated that a powerful discharge takes place at the apex of the silicon sphere resulting in an excessive thermal gradient if the cage is not added. The apex of the silicon sphere is covered with a blend of diamond and graphite phases. Due to the excessive internal stress of the coating, approximately 50 % of the samples experience coating detachment. After the addition of the cage, the strong discharge is transferred to the top of the cage, effectively suppressing the microwave electric field and plasma inside the cage. Improved quality, reduced internal stress, and enhanced uniformity of the polycrystalline diamond coating are achievable on the silicon spheres within the cage. Moreover, this technique offers a novel approach to coat diamond on complex 3D geometries.

Magnetron sputter deposition of ultrathick boron carbide coatings on spherical substrates for inertial confinement fusion

Most designs of inertial confinement fusion (ICF) targets require ablators in the form of hollow spherical shells with an outer diameter of ∼1−3 mm and a wall thickness of ∼20−200 microns. Vapor deposition onto rolling spherical substrates (templates) is the generally preferred method for the fabrication of ICF ablators. Amorphous boron carbide (B4C) is a promising material for use as an ablator due to its favorable properties. However, the challenge lies in creating ultrathick B4C coatings with submicron-scale density uniformity on rolling spherical substrates. Here, we show examples from our ongoing systematic study of the deposition of B4C onto 2-mm-diameter Si spheres by radio-frequency magnetron sputtering (RFMS). We start with a RFMS recipe developed for depositing planar amorphous B4C films with close-to-zero residual stress and transfer the deposition process from stationary planar substrates to rolling spherical substrates. A major challenge of the deposition onto rolling spheres is preventing the formation of so-called nodular defects, attributed to the incorporation of particulates into the growing film. To address this challenge, we explore ways to minimize the generation and accumulation of particulates on the substrate holder by a combination of enhanced film adhesion to the substrate holder surface and removal of particulates from the holder area where spherical substrates are rolling. With this approach, we demonstrate the deposition of an ∼75-micron-thick B4C coating on a Si sphere.

Hydrophobic deep eutectic solvents with anti-wear properties for MEMS/NEMS

Most known deep eutectic solvents (DESs) are hydrophilic but, recently, hydrophobic DESs have been developed with some interesting features that make them potential alternative lubricants. The advantage of this type of compounds is their stability relative to the presence of water vapor in humid environments during industrial application. In this work, we report the use of hydrophobic DESs based on combinations of salts and acids or natural components (NADES) to lubricate Si surfaces. Two sets of previously reported DESs were prepared and studied: ionic DESs containing ammonium salts, and non-ionic DESs based on menthol. The friction coefficients were measured using steel and Si spheres against Si surfaces and their values were compared to those obtained with hexadecane, the reference lubricant. Non-ionic hydrophobic DESs presented poor lubrication for Si surfaces when compared to the reference lubricant, while all ionic hydrophobic DESs outperformed hexadecane. Among the most promissory hydrophobic DESs, [Aliquat]Cl:menthol (1:2) stood up showing CoF < 0.1 and the best surface protection against wear, independently of its water content, under stresses up to 1.3 GPa. The resistance of the adsorbed boundary layer might be justified by the strong interactions between its HBD and HBA components.

Fabrication of SiC reticulated porous ceramics with dense struts by in-situ generation of SiC

SiC reticulated porous ceramics have been widely utilized as filters in the smelting industry. To address the problem of hollow ceramic struts and improve the strength and thermal shock resistance of SiC reticulated porous ceramics, a one-step coating process was carried out using slurries with SiC, metallic Si and various carbon sources (carbon spheres, carbon black, and flake graphite). The effect of the carbon sources on the rheological behavior of the coating slurries was investigated, as well as the relationship between the rheological behavior of the slurries and the structure of the green bodies. The hollow struts and flaws resulting from the combustion of polymer templates were then filled with SiC whiskers formed by the reaction between Si and carbon sources during heating. Among the carbon sources, the carbon spheres significantly stimulated the growth of the SiC whiskers, which filled the voids and flaws. After heating at 1550 °C, the SiC reticulated porous ceramics prepared with Si and carbon spheres reached a porosity of 80%, compressive strength of 1.85 MPa, and residual compressive strength rate of 81%. In industrial tests, the fabricated SiC reticulated porous ceramics were used as filters to effectively remove inclusions in molten copper. Importantly, the structure of the SiC filters remained intact after use.

Synthesis of spherical Si3N4 powders by liquid‐phase modified carbothermal reduction and nitridation

Abstractβ‐Si3N4 powder with well‐dispersed spherical particles was synthesized by liquid‐phase modified carbothermal reduction and nitridation. The influence of the type and content of additives on the particle morphology was investigated. The liquid phase formed at the reaction temperature modifies the morphology of the Si3N4 powder. The Si3N4 products precipitated along the liquid‐solid interface, thus forming a spherical morphology and promoting the transformation from α‐Si3N4 to β‐Si3N4. The type of additives has a strong influence on the formation temperature and viscosity of the liquid phase and hence affects the product morphology. In the samples with CaF2 and CaO additives, sufficient liquid phase occurred and spherical particles were obtained. In the sample with Y2O3, however, the liquid phase was insufficient due to the higher formation temperature, thus resulting in an irregular particle morphology.

Effects of solution treatment on microstructure and mechanical properties of (SiC+TiB2)/Al–Zn–Mg–Cu composite fabricated by laser powder bed fusion

This study investigated the effects of solution treatment (ST) on the microstructure and mechanical properties of (SiC+TiB2)/Al–Zn–Mg–Cu composite produced using laser powder bed fusion (LPBF). Microstructural analysis reveals that the grid-like distribution of eutectic precipitates within the as-printed composite disappeared, giving way to the formation of uniformly distributed precipitates. As the ST temperature increased, the content of spherical Si particles decreased, while the content of elongated Al4SiC4 precipitates increased. Matrix grains recrystallization occurred, resulting in a gradual increase in the average grain size from 3.79 μm to 5.23 μm. The preferred crystallographic orientation features along the printing direction, <001> {001} plate texture, were weakened, gradually exhibiting a more disordered growth. The microhardness and tensile strength of the composites exhibited a slight decrease and exhibited a trend of initially increasing and then decreasing with increasing ST temperature. Additionally, the heterogeneity in microhardness along different directions has been reduced. When the ST temperature was 460 °C, the elongation of the composite experienced a significant increase, escalating from 6.12 ± 0.36% to 10.3 ± 0.25%. However, with the increase in ST temperature, it displayed an obvious downward trend. The maximum tensile strength was 449 ± 8 MPa, and the elongation was 9.6 ± 0.4% with the ST temperature of 500 °C. The variations in mechanical properties were primarily attributed to the elimination of grid-like substructure, the uniform distribution of precipitates, and the gradual coarsening of grains. This research was expected to contribute to developing advanced lightweight materials with improved performance, opening up new opportunities for their application in various industries.

Recycling silicon scrap for spherical Si–C composite as high-performance lithium-ion battery anodes

The growth of the semiconductor and solar industry has been exponential in the last two decades due to the computing and energy demands of the world. Silicon (Si) is one of the main constituents for both sectors and, thus, is used in large quantities. As a result, a lot of Si waste is generated mainly by these two industries. For a sustainable world, the circular economy is the key; thus, the waste produced must be upcycled/recycled/reused to complete the circular chain. Herein, we show that an upcycled/recycled Si can be used with carbon as a composite anode material, with high Si content (∼40 wt%) and loading of 3–4 mAh/cm2 for practical use in lithium-ion batteries. The unique spherical jackfruit-like structure of the Si–C composite can minimize the total lithium inventory loss compared to the conventional Si–C composite and pure Si, resulting in superior electrochemical performance. The superior electrochemical performance of Si–C composites enables the cell energy density of ∼325 Wh kg−1 (with NMC cathode) and ∼260 Wh kg−1(with LFP cathode), respectively. The results demonstrate that Si-based industrial waste can be upcycled for high-performance Li-ion battery anodes through a controllable, scalable, and energy-efficient route.

Evaluating comparative wear behaviour of Al-15%Si based alloy/composites reinforced with zinc and zirconium oxide

ABSTRACT The tribological characteristics of stir cast Al-15%Si-10%Zn matrix with 15%ZrO2 reinforced composite were investigated. The microstructure of the Al-15Si alloy consisted of coarse primary Si and acicular eutectic Si phases randomly distributed in the Al dendrites. The Al-15%Si-10%Zn/ZrO2 composite showed spherical Si with a size between 25 and 75 µm; fragmented eutectic Si phase and uniform dispersion of ZrO2 particles in the matrix. The wear test was conducted using in a pin-on-disc method at different loads and sliding velocities. It was found reduced coefficient of friction, significant increase in the hardness and wear resistance of the composite compared to the matrix alloy. High wear resistance in the composite due to the effect of solid lubrication provided by Zn, and good fracture toughness and wear resistance by the reinforced ZrO2 particles. The predominant oxidative and abrasive wear were identified as the mechanisms leading to material failure through plastic deformation and delamination.

Hierarchical pomegranate-structure design enables stress management for volume release of Si anode

Si is a promising anode material for lithium-ion batteries owing to its high theoretical capacity. However, large stress during (de)lithiation induces severe structural pulverization, electrical contact failure, and unstable solid-electrolyte interface, which hampers the practical application of Si anode. Herein, a Si-based anode with a hierarchical pomegranate-structure (HPS-Si) was designed to modulate the stress variation, and a sub-micronized Si-based sphere was assembled by the nano-sized Si nanospheres with sub-nanometer-sized multi-phase modification of the covalently linked SiO2–x, SiC, and carbon. The sub-micronized HPS-Si stacked with Si nanospheres can avoid agglomerates during cycling due to the high surface energy of nanomaterials. Meanwhile, the reasonable pore structure from SiO2 reduction owing to density difference is enough to accommodate the limited volume expansion. The Si spheres with a size of about 50 nm can prevent self-cracking. SiO2–x, and SiC as flexible and rigid layers, have been synergistically used to reduce the surface stress of conductive carbon layers to avoid cracking. The covalent bonding immensely strengthens the link of the modification with Si nanospheres, thus resisting stress effects. Consequently, a full cell comprising an HPS-Si anode and a LiCoO2 cathode achieved an energy density of 415 Wh kg−1 with a capacity retention ratio of 87.9% after 300 cycles based on the active materials. It is anticipated that the hierarchical pomegranate-structure design can provide inspiring insights for further studies of the practical application of silicon anode.

Preparation of Hollow Nanostructured Si Spheres by Zincothermic Reduction of SiO2 to Si for Lithium-Ion Batteries

The choice of SiO2 precursor plays an important role in designing and synthesizing specific nanostructured Si materials with improved Li-storage performance. Herein, various SiO2 precursors are used to prepare nanostructured Si spheres via zincothermic reduction and their effects on the reduction process are systematically investigated. It is found that the size and structure of SiO2 precursors have a significant effect on the zincothermic reduction process. Among various precursors, nano-SiO2 and bio-SiO2 (e.g., SiO2 derived from rice husk (RH-SiO2) and bamboo leaves (BL-SiO2)) are successfully reduced to Si spheres, while crystalline bulk SiO2 (i.e., micro-SiO2) cannot be reduced. The ease of zincothermic reduction of nano- and bio-SiO2 is due to the high surface area of SiO2 that allows sufficient and facile chlorination–reduction reactions. In addition, the hollow Si spheres prepared from nano-SiO2, RH-SiO2, and BL-SiO2 are used as anode materials for lithium-ion batteries, all of which exhibit good electrochemical performances, delivering the specific discharge capacities of 947.6, 1130.7, and 1050 mAh g–1 over 300 cycles at 1 A g–1, respectively. Overall, this work offers a deeper understanding of zincothermic reduction and demonstrates the broad applicability of zincothermic reduction for the preparation of nanostructured Si from a variety of Si sources.

Formation of Si nanoparticles by pulsed discharge of Si strips in distilled water

Si nanoparticle features multiple excellent properties, such as high theoretical capacity of 4200 mAh/g and low volume expansion effect, and it is regarded as an outstanding anode electrode material for Li-ion batteries. In this study, we obtained Si nanoparticles through pulsed discharge of Si strips and analyzed the pulsed discharge process based on recorded current data. The recovered samples were characterized by various techniques, such as XRD, Raman spectroscopy, SEM, and TEM. The characterization results confirm that the recovered samples are smooth spherical Si nanoparticles smaller than 200 nm. Our investigation reveals that the charging voltage is a key factor to adjust the size distribution of recovered Si nanoparticles. In the charging voltage range of 4–7 kV, the increase of charging voltage value decreases D90 (the particle size at the 90% undersize point in the size distribution) of recovered Si nanoparticles from 48.7 to 24.9 nm. In the charging voltage range of 7–12 kV, the increase of charging voltage value increases D90 of recovered Si nanoparticles from 24.9 to 66.5 nm. Thus, the critical charging voltage value is 7 kV, at which condition D90 of formed Si nanoparticles is the minimum (24.9 nm). In addition, the analysis of discharge current curves indicates three discharge stages, including semiconductor joule heating, conductor joule heating, and plasma discharge, which possess correlation to the size distribution of formed Si nanoparticles.

Cavity-enhanced magnetic dipole resonance induced hot luminescence from hundred-nanometer-sized silicon spheres

Abstract In this paper, we demonstrate the first example of phonon-assisted hot luminescence (PAHL) emission from silicon (Si) spheres (diameter &gt; 100nm) without using the plasmonic effect or quantum confinement effect. Instead, we excite the hot luminescence of Si by a strong thin-film-cavity-enhanced magnetic dipole resonance. The thin-film cavity (80 nm SiO2/Ag) shows a strong co-enhancement with the magnetic dipole resonance of Si sphere (diameter = 120 nm). The concentrated electromagnetic fields induce significant light–matter interaction. Our Si sphere coupled with a thin-film cavity achieves a 10-fold field enhancement relative to the Si sphere without an enhancement substrate. Furthermore, we experimentally use cavity-enhanced magnetic dipole resonance to a 50-fold enhancement in PAHL. The measured internal quantum efficiency for the visible light emitted from the Si spheres was approximately 2.4%. Furthermore, we demonstrate the tunability of emission peaks merely by adjusting the sizes of Si spheres using thermal oxidation and etching processes. For comparison, we calculated the peak wavelength (λ peak) sensitivities (Δλ peak/ΔDiameter) of Si spheres and Si QDs through Mie theory and effective mass approximation, respectively. The predicated peak sensitivities of the Si spheres ranged from 1.3 to 3.2; they were much more controllable than those of the Si QDs (200–400). Thus, the peak wavelengths of the PAHL of the Si spheres could be modulated and controlled much more precisely and readily than that of the Si QDs. With the tunability and strong electromagnetic field confinement, the cavity-enhanced magnetic dipole resonance appears to have great potential in the development of all-optical processing based on Si photonics.

Absorption enhancement in visible range from Fano resonant silicon nanoparticle arrays embedded in single crystal Mg:Er:LiNbO3 synthesized by direct ion implantation

Fano resonant Si nanoparticles (NPs) are synthesized in single-crystal Mg:Er:LiNbO3 using ion implantation and subsequent thermal annealing. The structural and optical properties of the Si NPs embedded in the crystal have been investigated. Spherical particles with radius of about 60 nm are observed by cross-sectional transmission electron microscope, while ion beam analysis are used to characterize the NPs formation process. The absorption of the Mg:Er:LiNbO3 crystals have been enhanced significantly due to the embedded Si NPs, which are induced by the Fano resonance effect in the visible light wavelength band. Periodic structures of spherical Si particles model is proposed and analyzed using the Mie theory to study the optical response features and local fields. As a result, numerical simulations demonstrate that periodicities of the array of Si NPs can yield narrow resonant peaks connected with multiple light scattering by the NPs and displaying a Fano-type resonant profile. The wavelengths of the absorption peak show clear red shift with increasing the radius of NPs and the peak intensity can be enhanced by decreasing the array period. This work opens an avenue to modulate the optical filed by embedding Fano resonant Si NPs for potential application in optical devices.