Spherical Si Research Articles

Body Centered Photonic Crystal

The photonic energy bands of body centered cubic photonic crystals formed from SiO2, GaP, Si, InAs, GaAs, InP, Ge and BaSrTiO3 dielectric spheres drilled in air and air holes drilled in these dielectric mediums were calculated using the plane wave expansion method. The filling factor for each dielectric material was changed until a complete energy gap was obtained and then the density of states was calculated. There were no complete band gaps for air spheres drilled in these eight dielectric mediums. The lattice constants were determined by using wavelengths in the region . The variation of the band gap widths with the filling factor and the variation of gap width to midgap frequency ratios with dielectric contrast were investigated. The largest band gap width of 0.021 for normalized frequency was obtained for GaP for the filling factor of 0.0736. The mode filed distributions were obtained by guiding a telecommunication wave with wavelength through a photonic cell formed from GaP spheres in air with a filling factor of 0.0736 for transverse electric and magnetic modes.

Body Centered Photonic Crystal

The photonic energy bands of body centered cubic photonic crystals formed from SiO2, GaP, Si, InAs, GaAs, InP, Ge and BaSrTiO3 dielectric spheres drilled in air and air holes drilled in these dielectric mediums were calculated using the plane wave expansion method. The filling factor for each dielectric material was changed until a complete energy gap was obtained and then the density of states was calculated. There were no complete band gaps for air spheres drilled in these eight dielectric mediums. The lattice constants were determined by using wavelengths in the region . The variation of the band gap widths with the filling factor and the variation of gap width to midgap frequency ratios with dielectric contrast were investigated. The largest band gap width of 0.021 for normalized frequency was obtained for GaP for the filling factor of 0.0736. The mode filed distributions were obtained by guiding a telecommunication wave with wavelength through a photonic cell formed from GaP spheres in air with a filling factor of 0.0736 for transverse electric and magnetic modes.

Inelastic shape changes of silicon particles and stress evolution at binder/particle interface in a composite electrode during lithiation/delithiation cycling

Inelastic shape changes of Si particles and stress evolution at binder/particle interface was modeled using coupled diffusion–stress framework available in finite element software. A simple model that contains two spherical Si particles with and without the polymer binder film was used to represent the composite electrode. The particles were lithiated and delithiated at two different rates: one representing a slow charging case which results in a uniform Li concentration throughout the Si particles and the other representing a fast charging condition which results in non-uniform lithium concentration within the spherical Si particles. The inelastic shape changes and associated contact forces predicted by the model are qualitatively consistent with experimental data. Further, the effect of binder mechanical properties and the binder fraction on the stress evolution in Si particles and at the binder/particle interface was calculated. The proposed model, although simple, can guide a battery design engineer to choose a proper binder, charge/discharge strategy, and binder fraction for a durable electrode design.

Effect of heat treatment on AlSi10Mg alloy fabricated by selective laser melting: Microstructure evolution, mechanical properties and fracture mechanism

The present paper systematically investigated the influence of solution and artificial aging heat treatments on the microstructures and mechanical properties of SLM-produced AlSi10Mg alloy parts. Due to the high cooling rate of SLM, an ultrafine eutectic microstructure in the as-built samples is characterized by spherical nano-sized network eutectic Si embedded in the Al matrix, which gives rise to significantly better tensile properties and Vickers micro-hardness. The solubility of Si atom in the Al matrix of as-built SLM samples is calculated to be 8.89at%. With the increase in the solution temperature, the solubility decreases rapidly. The artificial aging causes the further decrease of the solubility of Si atoms in the Al matrix. Upon solution heat treatment, Si atoms are rejected from the supersaturated Al matrix to form small Si particles. With increasing the solution temperature, the size of the Si particles increases, whereas their number decreases. After artificial aging, the Si particles are further coarsened. The variation in size of Si particles has a significant influence on the mechanical properties of the AlSi10Mg samples. The tensile strength decreases from 434.25±10.7MPa for the as-built samples to 168.11±2.4MPa, while the fracture strain remarkably increases from 5.3±0.22% to 23.7±0.84% when the as-built sample is solution-treated at 550°C for 2h. This study indicates that the microstructure and mechanical properties of SLM-processed AlSi10Mg alloy can be tailored by suitable solution and artificial aging heat treatments.

Effects of Sb, Sr, and Bi on the material properties of cast Al-Si-Cu alloys produced through heated mold continuous casting

Rare earth metals can create a fine eutectic Si structure in cast Al-Si10.6-Cu2.5 (ADC12) alloys produced through heated mold continuous casting. Fine and spherical eutectic Si phases are created in the ADC12 alloys through the addition of Sr0.04, and fine lamellar eutectic Si phases are created through Sb and Bi addition. Crystal orientation on the face perpendicular to the casting direction is formed by [110]; however, this uniform formation is collapsed in the ADC12 alloy with an increasing amount of Sr addition, such as Sr > 0.04%. The shape of the eutectic Si is statically analyzed, and the effects of the eutectic Si characteristics on the mechanical properties are examined experimentally. On the one hand, the mechanical properties of the ADC12-Sr alloy increase with increasing Sr content because of the fine eutectic Si, the randomly orientated crystal formation, and so on. On the other hand, the material ductility increases in the ADC12 alloy with increasing addition of Sb and Bi elements. A high fracture strain of approximately 14% is obtained for the ADC12-Bi1.5 alloy.

Microstructure and High Temperature Deformation of Extruded Al-12Si-3Cu-Based Alloy

The high temperature deformation behavior of commercial Al-12Si-3Cu-2Ni-1Mg alloy (DM104™) which was fabricated by casting and subsequent hot extrusion was evaluated by compressive tests over the temperature range of 250–470 °C and strain rate range of 0.001–1/s. The extruded alloy had equiaxed grains, spherical Si particles and fine intermetallic phases, such as δ(Al3NiCu) and Q(Al5Cu2Mg3Si6). The true stress-true strain curves from the compressive tests exhibited steady-state flow after reaching the peak stress. A close relationship between the steady-state stress and a constitutive equation for high temperature deformation was observed. Fine equiaxed grains and a dislocation structure within the equiaxed grains were observed in the deformed specimens, suggesting the occurrence of dynamic recrystallization during high temperature deformation.

Core-shell Si@TiO2 nanosphere anode by atomic layer deposition for Li-ion batteries

Silicon (Si) has been regarded as next-generation anode for high-energy lithium-ion batteries (LIBs) due to its high Li storage capacity (4200 mA h g−1). However, the mechanical degradation and resultant capacity fade critically hinder its practical application. In this regard, we demonstrate that nanocoating of Si spheres with a 3 nm titanium dioxide (TiO2) layer via atomic layer deposition (ALD) can utmostly balance the high conductivity and the good structural stability to improve the cycling stability of Si core material. The resultant sample, Si@TiO2-3 nm core–shell nanospheres, exhibits the best electrochemical performance of all with a highest initial Coulombic efficiency and specific charge capacity retention after 50 cycles at 0.1C (82.39% and 1580.3 mA h g−1). In addition to making full advantage of the ALD technique, we believe that our strategy and comprehension in coating the electrode and the active material could provide a useful pathway towards enhancing Si anode material itself and community of LIBs.

Porous Silicon@Polythiophene Core–Shell Nanospheres for Lithium‐Ion Batteries

Polythiophene‐coated porous silicon core–shell nanospheres (Si@PTh) composite are synthesized by a simple chemical oxidative polymerization approach. The polythiophene acts as a flexible layer to hold silicon grains when they are repeatedly alloying/dealloying with lithium during the discharge/charge process. The long lifespan and high‐current‐density rate ­capability (at a current of 8 A g−1) of the Si@PTh composite are vastly improved compared with as‐prepared Si spheres. Typically, these Si@PTh composite electrodes achieve a reversible capacity of 1130.5 mA h g−1 at 1 A g−1 current density after 500 cycles, and can even possess a discharge capacity up to 451.8 mA h g−1 at 8 A g−1. The improved electrochemical performance can be ascribed to the synergy effects of the flexible PTh coating and the distinctive core–shell nanospheres with porous structure, which can largely alleviate the volume expansion of the Si during alloying with lithium.

The Correlation of the NA Measurements by Counting 28Si Atoms

An additional value of the Avogadro constant was obtained by counting the atoms in isotopically enriched Si spheres. With respect to the previous determination, the spheres were etched and repolished to eliminate metal contaminations and to improve the roundness. In addition, all the input quantities—molar mass, lattice parameter, mass, and volume—were remeasured aiming at a smaller uncertainty. In order to make the values given in Andreas et al. [Metrologia 48, S1 (2011)] and Azuma et al. [Metrologia 52, 360 (2015)] usable for a least squares adjustment, we report about the estimate of their correlation.

Morphology Tailoring of Sulfonic Acid Functionalized Organosilica Nanohybrids for the Synthesis of Biomass‐Derived Alkyl Levulinates

Morphology evolution of sulfonic acid functionalized organosilica nanohybrids (Si(Et)Si-Pr/ArSO3 H) with a 1D tubular structure (inner diameter of ca. 5 nm), a 2D hexagonal mesostructure (pore diameter of ca. 5 nm), and a 3D hollow spherical structure (shell thickness of 2-3 nm and inner diameter of ca. 15 nm) was successfully realized through P123-templated sol-gel cocondensation strategies and fine-tuning of the acidity followed by aging or a hydrothermal treatment. The Si(Et)Si-Pr/ArSO3 H nanohybrids were applied in synthesis of alkyl levulinates from the esterification of levulinic acid and ethanolysis of furfural alcohol. Hollow spherical Si(Et)Si-Pr/ArSO3 H and hexagonal mesoporous analogues exhibited the highest and lowest catalytic activity, respectively, among three types of nanohybrids; additionally, the activity was influenced by the -SO3 H loading. The activity differences are explained in terms of different Brønsted acid and textural properties, reactant/product diffusion, and mass transfer rate, as well as accessibility of -SO3 H sites to the reactant molecules. The reusability of the nanohybrids was also evaluated.

Improvements to the Volume Measurement of <sup>28</sup>Si Spheres to Determine the Avogadro Constant

In the determination of the Avogadro constant using the X-ray crystal density method with a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">28</sup> Si-enriched crystal prepared by the International Avogadro Coordination (IAC) project, the volume measurement and surface analysis of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">28</sup> Si spheres play a crucial role. For accurate volume determination, an optical interferometer has been improved taking account of the geometrical shape of the optical components. Furthermore, a new spectroscopic ellipsometer with a sphere rotation system has been developed for accurate surface analysis. The optical interferometer and the ellipsometer have been used for the volume determination and surface analysis of the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">28</sup> Si spheres repolished by the IAC, respectively. On the basis of the preliminary uncertainty estimation, the relative standard uncertainty of the apparent volume measurement has been reduced from 4.4 $\times 10^{-8}$ to 2.7 $\times 10^{-8}$ by the improvement of the interferometer. The number of ellipsometric measurement points on the sphere surface has been increased from 20 to 7782 by the installation of the new ellipsometer, yielding more reliable information on the sphere surface. Details of the improvements and the preliminary results of the measurements of the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">28</sup> Si spheres are given.

A selective laser melting and solution heat treatment refined Al–12Si alloy with a controllable ultrafine eutectic microstructure and 25% tensile ductility

This study shows that a eutectic Al–12Si alloy with controllable ultrafine microstructure and excellent mechanical properties can be achieved by using selective laser melting and subsequent solution heat treatment. This provides a novel and promising approach to the refinement of eutectic Al–Si alloys. Unlike Al–12Si alloys fabricated and refined by traditional methods, the as-fabricated Al–12Si in this study contains nano-sized spherical Si particles surrounding a supersaturated Al matrix. During solution heat treatment, precipitation and coalescence of the Si particles occur, which decreases the Si concentration in the matrix and sub-micron to micron-sized spherical particles embedded in an Al matrix form. The as-fabricated Al–12Si exhibits significantly better tensile properties than the traditionally produced counterparts; while the solution treated Al–12Si has an extremely high ductility of approximately 25%. Importantly, the mechanical properties of the Al–12Si can be tailored through controlling the precipitation and coalescence of the Si particles by varying the solution heat treatment time. A detailed transmission electron microscopy study was conducted to investigate this Al–12Si alloy with ultrafine eutectic microstructure. The excellent tensile properties have been attributed to the refined eutectic microstructure containing spherical Si particles. The formation of this unique microstructure is due to the super heating and an extremely high cooling rate during selective laser melting and the subsequent solution heat treatment, which enables Si to grow along its most stable plane {111}Si.

Reversible Lithium Storage in Carbon Encapsulated Hollow Silicon Nanospheres

One of the major challenges associated with the application of silicon as anode material for Li-ion batteries is the huge volume expansion/shrinkage during the lithiation/delithiation process. This volume change can result in the formation of crack in the particles and degrade the capacity. To solve this problem, we report a facile synthesis method which maintains the structural integrity of the material during cycling. Polystyrene beads was used as the templating agent and coated with silica through the sol-gel process. Then, the core was removed in the stepwise heating. Subsequently, the obtained shell was reduced to silicon via the magnesiothermic reduction. The final step is the coating of hollow Si spheres with carbon in the autoclave. SEM, TEM, XRD, FTIR, TGA and Raman were used to investigate the microstructure of the core-shell nanoparticles. Carbon encapsulated hollow spheres were mixed with carbon black, polyacrylic acid (PAA) and N-Methyl-2-pyrrolidone (NMP) to form a slurry. Copper foil as the current collector was covered with the slurry and dried at 120 ºC overnight. Half-cells were fabricated using a Li foil coupled with the carbon encapsulated Si hollow spheres in CR2032 coin cell cases. Charge and discharge were performed in the voltage window of 0.005-1.5 V with different current densities. The highest discharge capacity was about 2000 mAh/g at the first cycle. Cyclic voltammetry (CV) test confirms the reversibility of the reactions in both charge and discharge sections. Impedance spectroscopy measurements verify the decrease in impedance of the battery after cycling.

Study on the nanostructure formation mechanism of hypereutectic Al–17.5Si alloy induced by high current pulsed electron beam

This work investigates the nanostructure forming mechanism of hypereutectic Al–17.5Si alloy associated with the high current pulsed electron beam (HCPEB) treatment with increasing number of pulses by electron backscatter diffraction (EBSD) and SEM. The surface layers were melted and resolidified rapidly. The treated surfaces show different structural characteristics in different compositions and distribution zones. The top melted-layer zone can be divided into three zones: Si-rich, Ai-rich, and intermediate zone. The Al-rich zone has a nano-cellular microstructure with a diameter of ∼100nm. The microstructure in the Si-rich zone consists of fine, dispersive, and spherical nano-sized Si crystals surrounded by α(Al) cells. Some superfine eutectic structures form in the boundary of the two zones. With the increase of number of pulses, the proportion of Si-rich zone to the whole top surface increases, and more cellular substructures are transformed to fine equiaxed grain. In other words, with increasing number of pulses, more Si elements diffuse to the Al-rich zone and provide heterogeneous nucleation sites, and Al grains are refined dramatically. Moreover, the relationship between the substrate Si phase and crystalline phase is determined by EBSD; that is, (111)Al//(001)Si with a value of disregistry δ at approximately 5%. The HCPEB technique is a versatile technique for refining the surface microstructure of hypereutectic Al–Si alloys.

Preparation of graphene supported porous Si@C ternary composites and their electrochemical performance as high capacity anode materials for Li-ion batteries

Graphene supported porous Si@C ternary composites had been synthesized by various routes and their structural, morphological and electrochemical properties were investigated. Porous Si spheres coated with carbon layer and supported by graphene have been designed to form a 3D carbon conductive network. Used as anode materials for lithium ion batteries, graphene supported porous Si@C ternary composites demonstrate excellent electrochemical performance and cycling stability. The first discharge capacity is 2184.7mAh/g at a high current density of 300mA/g. After 50 cycles, the reversible capacity is 652.4mAh/g at a current density of 300mA/g and the coulomb efficiency reaches at 98.7%. Due to their excellent electrochemical properties, graphene supported porous Si@C ternary composites can be a kind of promising anode materials for lithium ion batteries.

Synthesis of silicon dioxide, silicon, and silicon carbide mesoporous spheres from polystyrene sphere templates

Amberlite XAD 16N mesoporous polystyrene spheres were used as a template to create silicon dioxide (SiO2), silicon, and silicon carbide (SiC) mesoporous spheres. Polystyrene spheres, infiltrated with either hydrochloric acid catalyzed tetraethyl orthosilicate or dimethylethylamine catalyzed tetramethyl orthosilicate, were heated to 550 °C to induce oxidation and/or decomposition of the polystyrene template and yielded SiO2 spheres. To create Si and SiC spheres, SiO2 and SiO2-infiltrated spherical polystyrene templates, respectively, were distributed in finely grated magnesium before heating to 675 and 700 °C each in an argon atmosphere. Mg by-products in the form of magnesium silicates and residual SiO2 were removed by washing the spheres with hydrochloric acid and hydrofluoric acid, respectively. X-ray diffraction, Brunauer–Emmet–Teller model specific surface area analysis, Barrett–Joyner–Halenda model pore diameter analysis, transmission electron microscopy and scanning electron microscopy were employed to investigate the microstructure and porosity during and after synthesis of the spheres. All three types of spheres maintained high porosity and their spherical shape throughout the synthesis. SiO2 spheres had a surface area of 700 m2 g−1, Si spheres a surface area of 160 m2 g−1, and SiC spheres a surface area of 215 m2 g−1. SiO2 spheres with dispersed Ag nanoparticles were also successfully created by adding AgNO3 to the precursor solution; they had a surface area of 220 m2 g−1. To prove the versatility of this infiltration method, Dy2O3 spheres were also fabricated, though they were not porous. This infiltration method is not only versatile, as it is suitable for the preparation of numerous types of mesoporous spheres, but it is also a simple synthesis method that guarantees a well-defined spherical shape and narrow particle size distribution, primarily while maintaining a high surface area.

Multipole analysis of unidirectional light scattering from plasmonic dimers

We analyze unidirectional scattering produced by sub-wavelength plasmonic dimers formed by two silver strips separated by a thin dielectric spacer and embedded in a uniform dielectric medium. Achieving the Kerker condition, which requires matching the strengths of the electric and magnetic-type contributions of the same multipolar order, is possible with such structures for both forward and backward unidirectional scattering by matching the geometric shape-leveraged resonant magnetic dipolar response with the off-resonant electric dipolar contribution. However, unidirectionality is strongly affected by coupling between the two elements in the dimer structure, leading to the manifestation of the electric quadrupole response in the far field. We develop an approach allowing for an easy inverse scattering retrieval of various multipole contributions to the far-field pattern produced by this type of geometry. The retrieval shows unambiguously that the electric quadrupole response contributes up to 30% of the scattered far-field intensity, in addition to strong manifestation of both electric and magnetic dipolar modes. A modified condition for unidirectionality can be developed based on the principle that suppression of radiation in either the forward or backward direction can be achieved whenever the combined strength of multipolar modes of a certain parity, radiating along the propagation direction, matches that of an opposite parity, and noting that parities of electric and magnetic modes interchange with increasing multipole order. With this condition satisfied, unidirectionality of 26 dB/17 dB for forward/backward scattering, respectively, can be achieved with dimer geometries. We also perform a detailed quantitative analysis of scattering cross sections of dimer structures compared to those of Si and gold spheres, accounting for the actual material losses. We show that dimer structures allow for improving backscattering unidirectionality by 10 dB compared to what is achievable with Si spheres, owing to their reliance on off-resonant electric dipolar response; they also allow for a significant (below ) reduction of the size of a unidirectionally scattering nanoelement.

Synthesis of silicon quantum dots showing high quantum efficiency.

Quantum efficiencies of Si quantum dots (QDs) have been investigated from the reaction of magnesium silicide and ammonium chloride. The change of quantum yield and optical characterization of Si QDs are measured depending on the reaction time. Highly luminescent Si QDs were obtained as the reaction time increased. Absorption measurement indicated that the Si QDs consisted of only silicon and hydrogen atom. Optical characterizations of Si QDs were measured by UV-Vis and PL spectroscopy. The size distribution and orientation of Si QDs were measured by TEM and XRD. TEM image displays the spherical Si QDs with the size of 3-4 nm. As the reaction time increased, Si QDs grew and their emission wavelength shifted to the longer wavelength. The monotonic shift of the PL as a function of excitation wavelength resulted in the excitation of different sizes of QDs that had different optical transition energies. Photoluminescence quantum yields exceeding 60% have been achieved.

Gradient microstructure and multiple mechanical properties of AlSi9Cu alloy ribbon produced by melt spinning

The microstructure and mechanical properties of AlSi9Cu alloy ribbons produced by melt spinning were investigated by field emission gun scanning electron microscope (FEGSEM), X-ray diffraction (XRD), tensile and microhardness tests. The solidification time and cooling rate of the 54μm thick ribbon were estimated to be 9×10−5s and 5.72×106Ks−1 respectively. XRD results revealed supersaturated solid-solubility of Si in Al matrix to be 3.03 and 3.57at.% for the wheel side and free surface of the ribbon separately. Microstructure evolution of three gradient regions along the cross-section of the ribbon was studied by SEM in detail, and fine spherical Si particles ranged from 12 to 290nm were found to be distributed both in the Al matrix and the α-Al boundary. The ultimate tensile strength of the ribbon was as high as 383MPa and 1.8times higher than that of the conventional cast ingot, and the breaking elongation of the ribbon was about 1.5% with cleavage fracture feature, while the microhardness of the ribbon was 2.5times higher than that of the ingot.

Low pressure preparation of spherical Si@C@CNT@C anode material for lithium-ion batteries

Si@C@CNT@C composite is prepared by preliminary low-pressure forming Si@C and Si@C@CNT precursors from Si powder. After pyrolysis from glucose, acetylene and pitch, Si@C@CNT@C shows a spherical multi-phase composite structure. By using as lithium storage material, Si@C@CNT@C shows an initial discharge capacity of 620.5mAhg−1 with an initial coulombic efficiency of 82.2%. After 60 cycles, this spherical sample can maintain a reversible capacity of 563.5mAhg−1 at 100mAg−1, corresponding to a capacity retention of 90.8%. For comparison, the reversible capacities for Si powder, Si@C and Si@C@CNT are 10.9, 380.3 and 494.6mAhg−1, respectively. Even cycled at 400mAg−1, Si@C@CNT@C can deliver a reversible lithium storage capacity of 389.2mAhg−1. It indicates that spherical Si@C@CNT@C can be used as a high performance anode material for lithium-ion batteries.