Nanosized Silicon Particles Research Articles

Formation of nanostructured weldments in the Al–Si system using electrospark welding

Electrospark welding (ESW) electrodes were manufactured from three binary aluminum–silicon alloys consisting of 12 and 17 wt% silicon, produced using chill and sand casting. The electrodes were used to assess the feasibility of producing aluminum–silicon weldments consisting of nano-sized silicon particles embedded in nanostructured aluminum matrix, using the ESW process. Line tests were performed to determine the optimal processing parameters resulting in a high quality deposit. X-ray diffraction (XRD) as well as optical and field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM) was performed to determine the composition and microstructure of the depositions. It was determined that a capacitance of 110 μF and a voltage of 100 V resulted in the highest quality deposition. Furthermore it was determined that the ESW process was capable of producing a microstructure consisting of an extremely fine-grained silicon phase ranging from ∼6 to 50 nm for the eutectic composition, and 10–200 nm for the hypereutectic compositions. Finally it was determined that the functional thickness limit of the aluminum–silicon deposit produced under these process parameters was 120 μm.

Synthesis of nanosized silicon particles by a rapid metathesis reaction

A solid-state rapid metathesis reaction was performed in a bed of sodium silicofluoride (Na 2SiF 6) and sodium azide (NaN 3) powders diluted with sodium fluoride (NaF), to produce silicon nanoparticles. After a local ignition of Na 2SiF 6+4NaN 3+ kNaF mixture (here k is mole number of NaF), the reaction proceeded in a self-sustaining combustion mode developing high temperatures (950–1000 °C) on very short time scales (a few seconds). Silicon nanoparticles prepared by the combustion process was easily separated from the salt byproducts by simple washing with distilled water. The structural and morphological studies on the nanoparticles were carried out using X-ray diffractometer (XRD) and field emission scanning electron microscope (FESEM). The mean size of silicon particles calculated from the FESEM image was about 37.75 nm. FESEM analysis also shows that the final purified product contains a noticeable amount of silicon fibers, dendrites and blocks, along with nanoparticles. The mechanism of Si nanostructures formation is discussed and a simple model for interpretation of experimental results is proposed.

Gas-Phase Synthesis of Silicon Nanoparticles and Mixing with Graphite Powders by Using Counter-Flow Injection

Mixing between nano-sized silicon particles and micro-sized graphite powders was performed for anode materials of lithium ion batteries, in which the silicon nanoparticles were synthesized by silane pyrolysis under optimized conditions and were subsequently coated onto the surface of graphite powders at mixing chamber by counter-flow injection. The counter-flow injection was employed as a dry particle mixing process in order to generate uniform silicon-coated graphite powders. Furthermore, this method also made it possible to combine particle synthesis and mixing into one single operation. Energy dispersive X-ray spectroscopy (EDS) was utilized to examine the degree of mixing through surface chemical composition analysis of the mixture. Experimental results indicate that the jet Reynolds number can be considered as the key parameter to determine the homogeneity of the mixture and its critical value may be in the range of 120 to 180 for good mixing performance, being in reasonable agreement with previous studies.

The structure and properties of a new type of nanostructured composite Si/C electrodes for lithium ion accumulators

The results obtained in studies of the structure and electrochemical properties of film electrodes prepared by magnetron plasma sputtering of silicon and graphite and working under the conditions of lithium injection and extraction are generalized. Composite silicon-carbon electrodes synthesized by depositing silicon and carbon nanolayers with the use of a magnetron plasma were films 100–500 nm thick. Part of them exhibited highly uniform nanogranular structure based on a carbon matrix with inserted silicon clusters of size below 6 nm. The nanogranular structure of Si/C composites was observed for the first time; such a morphology was not characteristic of not structured silicon layers deposited under equal conditions. The factors that determined the electrochemical charging-discharging behavior of new composites were the degree of uniformity of the nanogranular structure, the ratio between the silicon and carbon components, and film thickness. For two thin films, the initial composite capacitance was higher than that corresponding to the Li4.4Si stoichiometry for the silicon component and LiC6 stoichiometry for the carbon component, which was related to the special nanostructured state of silicon and carbon. The effects (luminescence band and absorption bands in the visible range) characteristic of nanosized silicon particles were observed.

Synthesis of nanosized Si particles via a mechanochemical solid–liquid reaction and application in Li-ion batteries

Abstract Nanosized silicon particles were obtained by reaction between SiBr4 solution and Mg granules during a ball-milling process. X-ray diffraction measurements show that crystallized silicon was obtained after 20 h of milling, together with MgBr2, non-reacted Mg and probably SiOx. The powder was subjected to a suitable cleaning treatment in order to minimize the proportion of residual products. The different analyses carried out after the treatment showed that the obtained silicon powder consists of nanosized Si particles with a limited oxidized fraction in regard to its high surface area. This material was considered for its application as the negative electrode for lithium-ion batteries. It delivers a large initial reversible capacity (1375 mAh g− 1). The capacity turns out to be almost stable after the fifth cycle, reaching around 900 mAh g− 1, which is approximately twice that obtained with an electrode made from commercial 100 nm Si particles.

Natural graphite coated by Si nanoparticles as anode materials for lithium ion batteries

Nano-sized crystalline silicon particles, prepared by a laser-induced vapour deposition method, were coated onto the surface of particles of a modified natural graphite (SSG) by sonicated dispersion and a subsequent heat-treatment process. The microstructure of the Si-coated SSG was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the nanometer-scale Si particles were uniformly and completely coated on the surface of SSG particles, and both the Si and SSG particles existed in the crystalline state. The Si-coated SSG exhibits a much higher reversible capacity than pristine SSG, while keeping the good cycling performance of SSG material. The higher capacity can be ascribed to the alloying of Si with lithium. Because of the heat-treatment at 600 °C, used to achieve a good combination of Si with the SSG base, the cycling of the composites is very satisfactory. As a result, Si-coated SSG is a promising anode material for lithium ion batteries.

Microstructure and performance of electroformed Cu/nano-SiC composite

The composite electroforming technology was propounded to fabricate nano-sized carbide silicon particles (nano-SiC) reinforced copper composite. The surface morphology, microstructure, microhardness, wear resistance and electrical resistivity ( E r) were examined. The results showed that nano-SiC were embedded in copper matrix uniformly and tightly. Comparing with the pure copper deposit, the surface of the composite was more fine, more compact and smooth, and the composite exhibited higher microhardness and better wear resistance. Besides, the change range of E r was small.

Preparation and photoluminescence of nanocrystalline Si-rich silicon oxide films by PECVD

Nanocrystalline Si-rich silicon oxide films were deposited using plasma enhanced chemical vapor deposition technique with the mixture of silane (SiH 4), nitrous oxide (N 2O) and hydrogen (H 2) as gas source on quartz glass substrate at the substrate temperature of 300 °C. The effect of the ratio N 2O/SiH 4 on the oxidation, microstructures and photoluminescence (PL) of the as-deposited Si-rich silicon oxide films was investigated with FTIR, XRD and HRTEM. The results reveal that with the increasing ratio of N 2O/SiH 4, more amounts of oxygen are incorporated in the as-deposited films and more nanosized silicon particles are embedded in the films, forming nanocrystalline Si-rich silicon oxide films. The quantum confinement effect or the cooperation of quantum confinement and luminescence center results in the nanocrystalline Si-rich silicon oxide films of higher PL intensity.

First Principles Calculations of the Absorption Spectrum of Si29H36

Density functional theory (DFT) and coupled-cluster (CC) calculations yield excitation thresholds for Si29H36 of 3.85 eV (BP DFT), 4.53 eV (B3LYP DFT), 6.09 eV (CCS), and 4.70 eV (CC2) as compared to the experimental value of 3.7 eV, indicating that the surface of the nanosized silicon particles is completely covered with hydrogen. The cluster is also found to possess a band structure with 212 (BP) and 135 (B3LYP) excited states below the ionization limit.

Lattice contraction in nanosized silicon particles produced by laser pyrolysis of silane

We used laser-induced decomposition of silane for the fabrication of nanosized Si particles and studied in detail their structural characteristics by conventional and high resolution electron microscopy. The silane gas flow reactor incorporated in a molecular beam apparatus was operated without size selection to achieve a broad size distribution. Deposition at low energy on carbon substrates yielded single crystalline, spherical Si particles almost completely free of planar lattice defects. The particles, covered by thin amorphous oxide shells, are not agglomerated into larger aggregates. The lattice of diamond cubic type exhibits deviations from the bulk spacing which vary from distinct contraction to dilatation as with decreasing particle size the oxide shell thickness is reduced. This effect is discussed in terms of the strong Si/oxide interfacial interaction and compressive stresses arising upon oxidation. A negative interface stress, as determined from the size dependence of the lattice spacing, limits the curvature of the interface, i.e., at small sizes Si oxidation must be considered as a self-limiting process.

In-situ Raman spectroscopy and laser-induced fluorescence during laser chemical vapor precipitation of silicon nanoparticles

Raman scattering in combination with laser induced fluorescence (LIF) has been used to identify molecules and intermediates during the nucleation and growth of nanosized silicon particles by CO 2 laser excitation of silane. Upon switching on the CO 2 laser, the silane Raman peak height decreases due to a decrease in species number density and LIF peaks emerge due to presence of SiH 2 radicals. The dissociation temperature has been determined inside the reaction zone and a value of 605 K is obtained. The nucleation and growth mechanism starts with the insertion of the SiH 2 radicals into the Si-H bond of silane which is demonstrated by the formation of disilane. Amorphous nanosized silicon particles are obtained at reactor pressures of 50 torr and are collected on a cold finger.

Nanosized silicon particles by inert gas arc evaporation

We have explored the possibilities of inert gas arc evaporation for fabrica tion of nanosized Si particles and studied agglomeration, size, shape, crystallinity, surface roughness and internal structure of these particles by conventional and high resolution electron microscopy. The rarely used technique yielded single crystalline, spherical Si particles 3 – 16 nm in size completely free of planar lattice defects. The particles, covered by thin amorphous oxide shells, are agglomerated into chains and tangles. The lattice of diamond cubic type exhibits contractions -which decrease as with decreasing particle size the oxide shell thickness is reduced. This effect is ascribed to compressive stress at the Si/oxide interface.

Growth, microstructure and sintering behavior of nanosized silicon powders

The growth and properties of nanosized silicon particles produced by gas-phase reaction in a low-pressure silane plasma has been studied. In situ ion-mass spectroscopy and Mie scattering measurements were used to monitor the formation of powders. High resolution transmission electron microscopic (HREM) studies confirmed the observations made by Raman spectroscopy that the nanoparticles grow from medium range ordered clusters. Onset of crystallization was ∼ 700°C when structures ranging from very small crystalline ordered regions of 2.5–3.5 nm in size to fast-grown multiply twinned crystallites were formed. Size and surface roughness of the as-prepared powders were widely preserved throughout all stages of heating. It was observed that the powder morphology influences the sintering behavior. Silicon clusters which are formed during the powder synthesis acted as seeds for the crystallization process which led to the formation of polycrystalline particles. Classical sintering models offer inadequate explanation of sintering behavior, but hard-core/sinterable coating or particle sliding models can explain the sintering rate of this powder satisfactorily.

Laser-driven flow reactor as a cluster beam source

A novel technique for the production of expansion-cooled cluster beams from materials with low vapor pressure is presented. The clusters are produced in a flow reactor from gas phase reactants by aggregation of CO2-laser-induced decomposition products. By introducing a conical nozzle into the reaction zone, they are extracted into a molecular beam apparatus and analyzed with a time-of-flight mass spectrometer. Depending on the type of CO2-laser employed, the source can be operated in the pulsed or continuous mode. The generation of carbon and silicon clusters is demonstrated by decomposing gaseous C2H2 and SiH4, respectively. The laser-driven cluster course is also employed to generate fullerenes and nanosized silicon particles.