Spherical Si Particles Research Articles

Microstructure and cavitation erosion characteristics of Al–Si alloy coating prepared by electrospark deposition

With a high-power electrospark deposition (ESD) processing unit, an Al-Si coating was prepared on ZL101 aluminum alloy substrate. The Al-Si coating was composed of many thin deposited layers with the thickness of 30-40 mu m and showed peculiar microstructural characteristics. The eutectic Si phase, which was homogenously distributed in the coating, existed in the form of latticed morphology with characteristics of being composed of many fine (similar to 50 nm size) spherical Si particles. Hardness tests showed that the microhardness of the coating was in the range of 100-110 HV with a small fluctuation, which implied that the deposited coating possess a uniform strength through the whole coating. After 4 hours' cavitation erosion tests, the cumulative volume loss of Al-Si coating (9.03 mm(3)) was only 31.5% of the substrate (28.66 mm(3)), which indicated that the AI-Si coating possessed better cavitation erosion resistance than ZL101 substrate. The excellent performance of Al-Si coating was mainly attributed to the peculiar microstructure and the formation of fine spherical Si particles. (C) 2008 Elsevier B.V. All rights reserved.

Corrosion Behavior of Al-7wt% Si-1.5wt% Cu Severely Deformed by Equal-Channel Angular Pressing

In this study, an Al-7 wt% Si-1.5 wt% Cu alloy was subjected to severe plastic deformation (SPD) by an equal-channel angular pressing (ECAP) technique. The ECAP process was repetitively carried out up to 8 passes using a strain introduction method of route BC, at a temperature of 25 °C and a pressing rate of 0.33 mm s-1. Microstructures of the samples before and after ECAP were observed by a scanning electron microscopy (SEM). Electrochemical properties of the Al-Si-Cu alloy fabricated by ECAP have been investigated in a borate-boric acid buffer solution containing Cl¯ ions at pH 8.3 and 25 °C by potentiodynamic polarization test. Corrosion pits on the sample surface after anodic polarization were investigated by means of SEM. The anodic polarization showed that as-cast Al-Si-Cu alloy with plate-shaped Si particles has poor resistance against pitting corrosion comparing to quenched sample without ECAP. Pitting potentials of ECAPed Al-Si-Cu alloy samples were higher than that of the sample without ECAP. In the Al-Si-Cu alloy, the corrosion pits were found in the region of Si particles and the size of pits formed on the ECAPed samples became smaller than that without ECAP. It is considered that the improvement of the pitting resistance of ECAPed Al-Si-Cu alloy is due to homogenous distribution of spherical Si particles generated during ECAP process.

Ultra-Hard Nanostructured Al-Si Thin Films

We show that it is possible to use high rate co-evaporation of Al and Si onto room temperature substrates to achieve a novel two-phase nanoscale microstructure. These nanocomposites have a hardness as high as 4GPa (Al-23at.%Si), and display noticeable plasticity. Films with compositions of Al-12at.%Si and pure Al (used as baseline) were analyzed using transmission electron microscopy (TEM). The scale of the Al-12at.%Si microstructure is an order of magnitude finer compared to that of pure Al films. It consists of a dense distribution of spherical nanoscale Si particles separating irregularly-shaped small Al grains. These new structures may have a mechanical performance advantage over conventional single phase nanomaterials due to the role of the dispersed hard phase in promoting strain hardening.

TEM Investigation of Spherical Primary Si Particles in As-Quenched Al60Si40 Ribbons

Al-Si alloys with high Si content are attractive in engineering application, especially in automobile industry. Their mechanical properties and machinability are quite sensitive to the morphology of the primary Si phase. In order to improve the mechanical properties of Al-Si alloys, optimizing the shape and distribution of the primary Si are quite important. In this paper, the microstructure of the primary Si in the Al 60 Si 40 alloys prepared by rapid solidification method has been studied used the transmission electron microscope (TEM).

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.

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.

Structure of nanometersized silicon particles prepared by various gas phase processes

We have explored various gas phase processes for the fabrication of nanometersized Si and SiO x particles and measured their structural properties (agglomeration, size, shape, crystallinity, surface roughness and internal structure) by conventional and high resolution electron microscopy. Agglomerated amorphous Si particles, 10–30 nm in size, were prepared by gas phase reactions including cluster growth processes in a low pressure silane plasma. Annealing at 900°C resulted in almost complete crystallisation of nearly spherical particles covered by an amorphous oxide shell. Inert gas arc evaporation of silicon yielded single crystalline, spherical Si particles, 4–16 nm in size, in which no defects were detected. These particles, agglomerated into chains and tangles, are covered entirely by a thin amorphous oxide layer. Thermal evaporation of solid SiO in an inert gas atmosphere produced agglomerated, nearly spherical amorphous SiO x particles, 8–24 nm in size, with considerable surface roughness. Upon annealing at 900°C, the formation of 3–6 nm sized Si crystallites in the interior of these particles was observed.

Synthesis and structural characterisation of Si and SiOx particles of nanometer sizes

AbstractThe possibilities of various gas phase processes for fabrication of nanosized Si and SiOx particles have been explored in a comparative Study. Conventional and high resolution electron microscopy was used to investigate in detail agglomeration, size, shape, crystallinity, surface roughness and internal structure of the particles, respectively. Accretions of nearly spherical crystalline Si particles of 2‐8 nm size mostly embedded in amorphous matrix were prepared by means of a gas evaporation cluster source. Their structural characteristics depend on the aggregation conditions applied. Inert gas are evaporation of silicon yielded single crystalline, spherical Si particles 4‐16 nm in size being completely free of defects. These particles, agglomerated into chains and tangles, are covered entirely by a thin amorphous oxide layer. Thermal evaporation of solid SiO in an inert gas atmosphere produced highly agglomerated, nearly spherical amorphous SiOx, particles 8‐24 nm in size with a considerable surface roughness. Upon annealing at 900°C the formation of 3‐6 nm sized Si crystallites in the interiro of these particles were observed.

The apparent activation energy for the dissolution of spherical Si-particles in AlSi-alloys

The apparent activation energy for the dissolution of spherical silicon particles in an aluminium matrix is analysed using a numerical dissolution model in which local equilibrium at the particle–matrix interface is assumed. The model takes into account long range diffusion, the temperature dependent solid solubility of silicon in aluminium, the shape of the particle, the finite cell size in which the particle can dissolve and the statistical distribution of the particle size. It is shown that the apparent activation energy can deviate substantially from the activation energy for diffusion, which is the rate controlling process. The model is validated using isoconfigurational annealing experiments at various temperatures for a high purity Al-1.35 mass% Si alloy. An excellent agreement between theory and experiments is obtained. With minor modifications the model can be adjusted to predict the apparent activation energy of (spherical) particle dissolution in other binary systems too.

Internal stress and solid solubility effects on the thermal expansivity of Al-Si eutectic alloys

Thermal expansion measurements are reported for a number of as-cast Al-Si eutectic alloys including a Sr-modified alloy which gives nearly spherical Si particles. The measurements were obtained by heating and cooling over repeated temperature cycles between room temperature and 500°C. In general, lower expansivity values were measured on the cooling cycle as compared with the heating cycle, resulting in a net positive permanent deformation at room temperature. Analytical solutions are described for the thermal expansivity of a concentric-spheres model for a Si particle contained within an Al matrix. The effect of plastic flow in the Al is included. Overall, the predictions show reasonable agreement with the measured expansivities. The observed differences between heating and cooling are of the same order as that which is predicted. At high temperatures, the measured increase in expansivities is smaller than calculated. The latter effect is explained by the decrease in expansivity which results from an increasing solid solubility of silicon in aluminum with increasing temperature.

Fine Particles of Silicon. I. Crystal Growth of Spherical Particles of Si

Fine crystals of Si having spherical shapes of less than 200 nm, as well as hybrid particles of Si and β-SiC were prepared by a gas evaporation method using an arc discharge as the heat source. Si particles always contain stacking faults and twins which are categorized as growth faults. All the planar faults in a particle occur in the same orientation (parallel to a [110] direction). The nature of these planar faults was examined in terms of crystal growth using an electron microscope. As a result, it has been concluded that these spherical Si particles are crystallized from liquid droplets which condense from Si vapors.

球状シリコン超微粒子の結晶成長

球状シリコン超微粒子の結晶成長