Silicon Powder Mixtures Research Articles

Effect of Silicon on the Surface Modification of Al-Cr Powder Cathodes Subjected to Vacuum Arc Treatment

Al-Cr and Al-Cr-Si composite cathodes were obtained by the hot compaction of aluminum, chromium, and silicon powder mixtures. The phase transformations in the surface layer of the Al-Cr-Si composite cathodes subjected to the arc heating were considered. The elemental and phase compositions of the modified cathodes’ surfaces were studied using X-ray diffraction, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). The effect of the silicon addition on the structural evolution in the cathode surface during arc evaporation is shown. It was found that an arc impact on the cathode surface resulted in the melting and consequent crystallization of the multiphase mixture of intermetallic compounds and eutectic in the cathode surface layer. Cathode surface layers were found to be depleted of aluminum and silicon due to the ejection of these elements in drop form from the Al-Si liquid layer on the cathode surface. This can result in the change in the elements ratio in the deposited coating as compared with that in the cathode and thus influence the coated tools’ durability.

Reversible kinetics and rapid tunnelling characteristics of silicon doped magnesium-titanium nanocomposites prepared by mechanical alloying route for nickel-metal hydride batteries

Magnesium-Titanium-Silicon (Mg-Ti-Si) hybrid nanocomposites have been synthesized by using high energy ball milling. Magnesium, titanium and silicon powder mixtures are taken in the stoichiometric molar ratio of Mg0.8-xTi0.2Six (x = 0, 0.10, 0.15, 0.20 and 0.25) and mechanically milled for 30 h. The phase transformation, micro-strain and microstructural changes during mechanical alloying are examined by X-ray diffraction (XRD) and scanning electron microscope (SEM). Band gap engineering is a process done by high energy ball milling of Mg0.8Ti0.2, Mg0.8-xTi0.2Six powders, which have been tailored from 0.629 to 1.685 eV, which are obtained from the optical absorption spectra. The band gap energy is the most substantial parameter to understand the electrochemical charge-discharge activation behaviour. The high planar diffusion with high reversible reaction has been occurred only for the 30 h ball milled Mg0.70Ti0.20Si0.10 and Mg0.65Ti0.20Si0.15 powders. During the oxidation behaviour, they have been gained the higher diffusion coefficient (1.87 × 10−08 cm2 s−1 and 1.53 × 10−08 cm2 s−1), high exchange ion current density (1841 mA g−1 and 1113 mA g−1), the patronizing maximum discharge capacity (329 mAh g−1 and 307 mAh g−1), maximum energy density (0.265 Wh g−1 and 0.219 Wh g−1) with high quasi-reversible coulombic efficiency (65.80% and 61.40%) and high cyclic stability (87.23% and 82.08%) respectively. The addition of silicon concentration increases (i.e. x = 0.10–0.25) in magnesium-titanium based ball milled powders enhance the activation behaviour and robustly reduce the Gibbs free energy from −127 to −64 kJ mol−1.

Yolk-shell silicon/carbon composites prepared from aluminum-silicon alloy as anode materials for lithium-ion batteries

Silicon is an attractive anode material for lithium-ion batteries due to its ultrahigh theoretical specific capacity. However, its commercial application is largely limited by the poor cycling stability due to its huge volume change during lithiation and delithiation. A low-cost method is developed to prepare yolk-shell silicon@void@carbon composite particles in this study. The synthetic phenol-formaldehyde resole resin was used to bind the graphite to the surfaces of the aluminum-silicon alloy particles, and then was thermal cross-linked and carbonized; aluminum was gently dissolved into ferric chloride etchant, leaving void space between the carbon shell and silicon core. Owing to the presence of the robust carbon shell and internal void space, the yolk-shell composites exhibited significantly better cycling stability than the powder mixture of silicon and graphite. The capacity retention of the silicon component in the powder mixture dropped below 60% after only one cycle, whereas silicon in the composite still has about 70% remaining capacity after 100 cycles.

The study of pressing mechanically activated niobium and silicon powder mixtures

The paper presents the results obtained in the study of the pressing process of mechanically activated powder mixtures of the niobium – silicon system. Pressing often precedes the process of self-propagating high-temperature synthesis and the initial density of the compressed samples can have a significant impact on the regime of subsequent high-temperature combustion, on the composition and properties of the final product (niobium silicide). The obtained dependences of porosity on the time of mechanical activation showed the features of pressing, which are associated with the formation of layered agglomerates in the process of mechanical activation. It was found that the compression curves of mechanoactivated powders differ qualitatively from the standard compression curves of fine powders. The paper considers the influence of mechanical activation time on the density of the compressed samples obtained by varying the pressing pressure and the time spent under the press. The relationship between the density of compresses and layered agglomerates, which are formed during mechanical activation, is considered.

Electrical conductivity of NaSi at elevated temperatures

Abstract The conductivity of sodium silicide (NaSi) was measured from 100 °C to 740 °C using a four-point DC method. NaSi was synthesized by heating a mixture of silicon powder and sodium liquid in an argon atmosphere at 550–650 °C. The formation of NaSi was followed by monitoring the conductivity and confirmed by post-experiment XRD analysis. The results showed semiconductor behaviour, with a shift in the conduction activation energy occurring near 460 °C, from 0.032 eV at the lower temperatures to 1.42 eV at higher temperatures. The room temperature conductivity of 0.22 S cm−1 was obtained by extrapolation, closely matching the results from simple two-point conductivity measurements.

INFLUENCE OF ENERGY ON PHASE COMPOSITION OF END-PRODUCT OBTAINED BY VACUUM-FREE ELECTRIC ARC SYNTHESIS OF CUBIC SILICON CARBIDE

The paper describes the scientific and technical basis of the vacuum-free plasma method for obtaining silicon carbide realized by DC arc discharge between graphite electrodes. In a series of experiments the energy supplied to the system was changed by increasing the duration of arc discharge with the constant value of current intensity (165 A); two precursor types were used: a mixture of silicon powder with X-ray amorphous carbon in the microfiber form in the first case and with carbon powder in the second case; the mass ratio in the initial mixture was Si:C = 2:1. As a result of the evaluation of the synthesis product quantitative composition, the experimental parameters that allow to achieve the maximum content of the target silicon carbide phase (up to 45%) are determined. Moreover, it was possible to determine the parameters when the only impurity phase in the product was graphite; as a result, the purification of the product from unbound carbon and thereby obtaining silicon carbide with ~99% content was successfully performed by atmospheric furnace heating at a temperature of 900 °C. This result is ensured by two factors: the presence of carbon fibers in the initial reagents mixture and a sufficient level of the supplied energy of about 216 kJ/g.

Surface alloying of A2618 aluminum with silicon and iron by TIG process

In this study, surface alloying of A2618 aluminum was carried out by pre-placing powder mixture of silicon and iron followed by subsequent tungsten inert gas (TIG) surface melting. This process resulted in the formation of 1.4 to 2.1mm thick alloyed layers containing Al-Fe-Si intermetallic compounds on aluminum substrate. The fabricated layers were then studied using optical and scanning electron microscopes equipped with energy dispersive X-ray spectroscopy (EDS) analyzer, X-ray diffractometer, microhardness tester and pin on disc wear testing machine. The hardness of the alloyed layer reached values as high as 210HV0.2, as compared with 60HV0.2 for A2618 aluminum base material, due to the formation of Al13Fe4, Fe5Si3, Al8Fe2Si, and Al5FeSi compounds in the alloyed region. The wear rate of the surface alloyed specimen was almost half of the wear rate of the untreated base material due to the presence of the aforementioned hard intermetallic compounds in the alloyed layer.

Siliconization of titanium oxycarbides by silicon monoxide

Siliconization of titanium oxycarbides TiCxOy by gaseous SiO at 1350°C was studied. The reaction source of SiO was a powder mixture of silicon and silicon dioxide. It was found that the main siliconization product is Ti5Si3 and the siliconization of high-carbon (x > 0.70) oxycarbides also gives Ti3SiC2 and TiSi2.

Synthesis of CsCl-type single-phase RuSi under shock compression

Shock recovery experiments were performed on ruthenium–silicon powder mixtures by a flyer plate impact technique. The flyer velocities were in the range of 0.46–2.73 km/s, and the incident shock pressures were calculated to be ∼2.9–∼40.4 GPa by the impedance matching method. The recovered samples were characterized by X-ray diffraction and scanning electron microscopy. Results indicate that shock could induce a reaction between ruthenium and silicon. The shock pressure was found to affect reaction kinetics and microstructure of the recovered sample significantly. The dynamic reaction has a threshold pressure, and the samples loaded above threshold pressure almost completely reacted to a single-phase intermetallic compound of CsCl-type RuSi. These results indicate that shock compression could be an effective way to synthesize RuSi.

The Effect of Si3N4Addition on Nitriding and Post-Sintering Behavior of Silicon Powder Mixtures

Nitriding and post-sintering behavior of powder mixture compacts were investigated. As mixture compacts are different from simple Si compacts, the fabrication of a sintered body with a mixture composition has engineering implications. In this research, in specimens without a pore former, the extent of nitridation increased with Si₃N₄ content, while the highest extent of nitridation was measured in Si₃N₄-free composition when a pore former was added. Large pores made from the thermal decomposition of the pore former collapsed, and they were filled with a reaction product, reaction-bonded silicon nitride (RBSN) in the Si₃N₄-free specimen. On the other hand, pores from the decomposed pore former were retained in the Si₃N₄-added specimen. Introduction of small Si₃N₄ particles (d 50 = 0.3 ㎛) into a powder compact consisting of large silicon particles (d 50 = 7 ㎛) promoted close packing in the green body compact, and resulted in a stable strut structure after decomposition of the pore former. The local packing density of the strut structure depends on silicon to Si₃N₄ size ratio and affected both nitriding reaction kinetics and microstructure in the post-sintered body.

Experimental Study on the Relationship between Carrier Gas Temperature and Entrainment Characteristics of Ultrafine Silicon Powder in Fluidized Bed

The effect of carrier gas temperature on entrainment characteristics of Geldart group C powder from solid mixtures (groups A and C) were investigated in a fluidized bed, which was 40mm in inner diameter and 150mm in height. Silicon powder of mean size 2.2μm was adopted as entrained material (group C), hollow alumina pellets of mean size 775μm were used as coarse particles (Group A), and industrial nitrogen (99.5%) was applied as carrier gas. mixture of silicon powder (50g) and alumina pellets (50g) were used as bed materials. The nitrogen flow rate varied from 1.5m3/h to 2.5m3/h, and the nitrogen temperature varied from 60°C to 120°C. The experimental results revealed that the bed materials were in good fluidizaton state at all of the experimental conditions. At given nitrogen flow rate, the entrainment rate constant and powder gas ratio increased at the same rate as the nitrogen temperature increased. When nitrogen temperature increased from 60°C to 120°C, the entrainment rate constant and powder gas ratio increased over 140%. The phenomena indicted that increasing carrier gas temperature was an effective method to improve entrainment characteristics of ultrafine powders.

Carbon-assisted synthesis of silicon nanowires

Carbon-assisted synthesis of silicon nanowires has been accomplished with silicon powders as well as solid substrates. The method involves heating an intimate mixture of silicon powder and activated carbon or a carbon coated solid substrate in argon at 1200–1350 °C, and yields abundant quantities of crystalline nanowires. Besides being simple, the method eliminates the use of metal catalysts.

High-pressure, high-temperature sintering of diamond–SiC composites by ball-milled diamond–Si mixtures

A new method of sintering diamond-silicon carbide composites is proposed. This method is an alternate to the liquid silicon infiltration technique and is based on simultaneous ball milling of diamond and silicon powder mixtures. Composites with fine-grain diamonds embedded in a diamond–SiC nanocrystalline matrix were sintered from these mixtures. Scanning electron microscopy, x-ray diffraction, and Raman scattering were used to characterize the ball-milled precursors and the sintered composites. It was found that the presence of diamond micron-size particles in the initial powder mixture promotes milling of silicone particles and their transformation into the amorphous state. Mechanical properties of the composites, sintered from mixtures of different ball-milling history at different pressure–temperature conditions, (6 GPa/1400 °C and 8 GPa/2000 °C) were studied.

レーザ合金化法による工業用純チタンの表面硬化処理

In order to improve a surface hardness of commercially pure titanium, surface alloying with laser was carried out. Pure silicon, nickel, aluminum powder and a mixture of silicon and 9 mass%WSi2 powder were used for surface alloying. These powders were spread on surfaces of titanium plates and reacted with titanium by CO2 laser beam. Laser irradiation was carried out in argon atmosphere. As a result, when the silicon powder was used, a hard silicide layer (a principal ingredient was Ti5Si3) was formed over thickness 600 μm. In the case of the nickel powder, an alloying layer was consisted of TiNi and Ti2Ni, and its highest hardness was HV600. When the aluminum powder was used, needle-like shapes of precipitates of TiAl3 were observed in aluminum-rich matrix phase. Moreover, in alloying with the powder mixture of silicon and WSi2 powder, concentration phase of tungsten that had over HV1500 hardness were observed locally.

Si nanowires grown from silicon oxide

Bulk-quantity Si nanowires have been synthesized by thermal evaporation of a powder mixture of silicon and SiO 2. Transmission electron microscopy showed that, at the initial nucleation stage, silicon monoxide vapor was generated from the powder mixture and condensed on the substrate. Si nanoparticles were precipitated and surrounded by shells of silicon oxide. The Si nanowire nucleus consisted of a polycrystalline Si core with a high density of defects and a silicon oxide shell. The growth mechanism was proposed to be closely related to the defect structure and silicon monoxide.

Reaction synthesis of refractory disilicides by mechanical alloying and shock reactive synthesis techniques

The reaction synthesis of disilicides of Group IVA–VIA transition metals by mechanical alloying and shock reactive synthesis techniques has been investigated. All nine disilicide compounds were directly produced from their elemental powder mixtures by mechanical alloying. Moreover, the formation of some disilicides, such as MoSi2 and NbSi2, proceeded by mechanically induced self-propagating reactions, the mechanism of which is analogous to that of the self-propagating high-temperature synthesis (SHS). Shock reactive synthesis of MoSi2, NbSi2, and TiSi2 was conducted with a one-stage gun. Powder samples used for the shock study were prepared from metal–silicon powder mixtures that had been subjected to high-energy ball milling. The explosive driven flyer plate struck the sample-containing capsule at a velocity of 1 km s−1. In most cases, shock-induced reactions went to completion. The effect of thermochemical driving force on the reactive formation of the disilicide compounds is discussed.

Shock-induced chemical reactions in titanium–silicon powder mixtures of different morphologies: Time-resolved pressure measurements and materials analysis

The response of porous titanium (Ti) and silicon (Si) powder mixtures with small, medium, and coarse particle morphologies is studied under high-pressure shock loading, employing postshock materials analysis as well as nanosecond, time-resolved pressure measurements. The objective of the work was to provide an experimental basis for development of models describing shock-induced solid-state chemistry. The time-resolved measurements of stress pulses obtained with piezoelectric polymer (poly-vinyl-di-flouride) pressure gauges provided extraordinary sensitivity for determination of rate-dependent shock processes. Both techniques showed clear evidence for shock-induced chemical reactions in medium-morphology powders, while fine and coarse powders showed no evidence for reaction. It was observed that the medium-morphology mixtures experience simultaneous plastic deformation of both Ti and Si particles. Fine morphology powders show particle agglomeration, while coarse Si powders undergo extensive fracture and entrapment within the plastically deformed Ti; such processes decrease the propensity for initiation of shock-induced reactions. The change of deformation mode between fracture and plastic deformation in Si powders of different morphologies is a particularly critical observation. Such a behavior reveals the overriding influence of the shock-induced, viscoplastic deformation and fracture response, which controls the mechanochemical nature of shock-induced solid-state chemistry. The present work in conjunction with our prior studies, demonstrates that the initiation of chemical reactions in shock compression of powders is controlled by solid-state mechanochemical processes, and cannot be qualitatively or quantitatively described by thermochemical models.

Formation of Reaction‐Bonded Silicon Nitride Using Microwave Heating

Rod‐shaped silicon compacts were nitrided in a microwave applicator using a minimum of insulation in order to maximize the temperature gradients to effect an inside‐out reaction. These specimens exhibited very nonuniform conversion to Si3N4, and fully nitrided areas had poor microstructures consisting of alternating regions of high and low density. The key factor was found to be sintering of the silicon powder during initial heating which caused the specimens to be electrically conducting during the early stages of the reaction and led to large changes in the microwave heating behavior of the specimens as they nitrided. The temperature and composition profiles in the specimens were simulated using a numerical model. The results of the model indicated that the observed microstructures were caused by high temperatures and temperature gradients in the areas of maximum nitridation rate which caused silicon vapor to diffuse within the specimens. Some compacts were made from a mixture of silicon powder and silicon nitride powder to avoid sintering of the silicon particles. These specimens nitrided uniformly with inside‐out composition profiles, indicating that microwave heating would be beneficial for the nitridation of pure silicon powder compacts if sintering could be avoided.

Production of Hypereutectic Al-Si Alloys by P/M Route

In this work hypereutectic aluminium silicon alloys with high silicon content that are difficult to produce by casting, are produced by powder metallurgy (P/M) route. Different alloys are produced with varying silicon content. Aluminium silicon alloys are produced by both hot pressing and sintering of aluminium and silicon powder mixtures for the comparison. The production of hypereutectic Al-Si alloys by sintering of powder compacts is found to be unfeasible in terms of obtaining high density. The hardness and the density values of the alloys containing same amount of silicon, are higher in the alloy produced by hot pressing than the sintered alloys. The increase in hardness of the alloys is observed by increasing the silicon content. 100% theoretical density and 422 kg mm{sup -2} MHV hardness values are reached for the Al-67%Si composite which is hot pressed under 500 MPa and at 600 C. (orig.)

Phase evolution and formation process of compound during ball milling of TiSi powder mixtures

Titanium and silicon powder mixtures (Ti−3.67−80)at.%Si) were vibratory ball milled to investigate the effect of milling conditions and initial powder compositions on the phase formation during milling. During milling, the temperature of a mill container was measured with a thermocouple attached to the surface of the container. An exothermic temperature spike was observed only when the powder with Ti−40at.%Si composition was milled for 60 h, aged for 48 h and milled again for 32.5 h. During this temperature spike, an amorphous phase which had been formed after the secondary milling time of 32.5 h rapidly changed into a crystalline phase and fine particles were formed. The temperature spike suggests the occurrence of the combustion synthesis reaction during milling. On the contrary, no temperature spike was observed in continuous milling condition.