Si Powder Research Articles

Spark-plasma sintering of boron carbide–silicon carbide composites at 1400 °C from B4C+Si: Densification and sintering/reaction mechanisms

The feasibility of fabricating novel boron carbide–silicon carbide composites by spark-plasma sintering (SPS) of B4C+Si powder mixtures at only 1400 °C was investigated. First, it is shown that B4C can be fully densified at 1400 °C if ~20 vol% Si aids are used, leading to bi-particulate composites constituted by boron carbide (major phase) and SiC (minor phase). The formation of these composites is due to the fact that Si acts as a reactive sintering additive during SPS. Lower and higher proportions of Si aids are not optimal, the former leading to porous bi-particulate composites and the latter to dense triplex-particulate composites with some residual free Si. Importantly, it is also shown that these novel boron carbide–SiC composites are fine-grained, nearly-ultrahard, moderately tough, and more affordable to fabricate, a combination that makes them very appealing for many engineering applications. Second, it is demonstrated that during the heating ramp of the SPS cycles a eutectic melt is formed that promotes full low-temperature densification by transient liquid-phase sintering if sufficient Si aids are used. Otherwise, a subsequent stage of solid-state sintering is required at higher temperatures once the eutectic liquid has been consumed in the in-situ formation of SiC. And third, it is demonstrated that during SPS the original B4C undergoes a gradual isostructural crystallographic transition towards a Si-doped carbon-deficient boron carbide that is more relevant with increasing proportion of Si aids, and it is identified that the carbon source for the formation of SiC is almost exclusively the carbon exsoluted from the B4C crystals themselves during their isostructural transition. Finally, implications of interest for the ceramic and hard-material communities are discussed.

Effect of microstructural parameters of ball-milled Si powder on reactivity with water to produce H2: Way forward for on-demand H2 production

Silicon (Si) nanoparticles prepared by ball-milling technique can show many exciting properties including improved reactivity with water to produce hydrogen (H2) compared to that of the bulk. This is attributed to the change in microstructural parameters such as particle size and dislocation density of nano-crystalline Si during the milling process. The understanding of such effects is of great importance for the technological applications of these Si nanoparticles in H2 generation from water for fuel cells, and our present work deals with this crucial aspect. In our present investigation, we found that ball-milling technique can be used to tailor the microstructural parameters of milled Si sample where milling time can greatly influence the particle size and dislocation density. In water-splitting experiment, we noticed that these microstructural parameters as well as state of agglomeration of milled Si samples in water have profound effect on the reactivity of Si with water to evolve H2. Furthermore, selecting the right alkali can speed up such H2 release, making rapid on-demand production of H2 viable, particularly for fuel cell applications.

Fabrication and oxidation behavior of β-SiAlON powders in presence of trace Y2O3

β-SiAlON powders were fabricated using Al, Si, α-Al2O3 and trace Y2O3 powders as starting materials in flowing nitrogen atmosphere. The effect of Y2O3 on oxidation behavior of the prepared β-SiAlON powders has been investigated. The results show that the Y2O3 contributed to the generation of β-SiAlON during the fabricated process. The weight gain ratio of β-SiAlON in presence of Y2O3 is higher than that of without Y2O3 during non-isothermal oxidation up to 1723 K, which may be due to the Y2O3 dissolving into the lattice of β-SiAlON resulting in lattice deformation. At the initial oxidation period, the incorporation of Y2O3 increases the oxidation rate and decreases the oxidation activation energy (Ea) of β-SiAlON due to the lattice deformation increasing the reactivity of β-SiAlON. However, at the final oxidation period, denser glass film formed on the surface of β-SiAlON because of the incorporation of Y2O3, which reduces the opportunity for O2 contact with β-SiAlON, thus decreasing the oxidation rate and increasing the Ea of β-SiAlON.

Role of nano-ZrO2 powder in in-situ formation of ceramic whiskers in Al2O3-C slide plate materials

The effects of nano-ZrO2 powder on the evolution of phase composition and microstructure of Al2O3–C materials during firing have been investigated using tabular alumina, Si powder, Al powder, graphite and nano-ZrO2 powder as starting materials. The residual content of Al and Si gradually decreased with increasing temperature, and the added nano-ZrO2 facilitates the reactions involving Si and Al. The quantity of in-situ synthesized AlN and SiC whiskers exhibiting continuous interlocking networks increased gradually with temperature. Nano-ZrO2 facilitates more ceramic whiskers generated in samples, which play a strengthening and toughening role in the materials, thus improving the thermal shock resistance and high temperature strength of Al2O3–C materials. In addition, physical properties are improved due to the nano-ZrO2 filling the pores and increasing the density of materials.

Reduction of core loss for FeSi soft magnetic composites prepared using atomic layer deposition-based coating and high-temperature annealing

Using atomic layer deposition and high-temperature annealing to fabricate coatings for Fe-6.5 wt%Si powder reduce the hysteresis losses of the resultant composites drastically without sacrificing the permeability properties, and keep the eddy current losses at a relatively low value. The core loss of annealed FeSi SMCs is 39.67% lower than that of composites prepared using raw Fe-6.5 wt%Si powder, meanwhile the permeability remains at relatively high value (at B = 100 mT, f = 50 kHz). The decreasing of the core loss is mainly attributed to hysteresis loss reduction caused by grain growth. And the low eddy current loss is guaranteed by the uniform, complete, compact coating layer formed the surface of FeSi particles, which transforms to a protective and insulating layer containing silicate and metal oxide after annealing. On the premise of ensuring the integrity of the insulation layer, the annealing temperature of powder is significantly increased by surface modification, so as to reducing the hysteresis loss meanwhile controlling the eddy current loss. • Low core loss Fe-Si soft magnetic composites were prepared. • Nano-Al 2 O 3 was deposited on the surfaces of Fe-Si powders by ALD. • A protective and insulating layer formed at the interface after annealing. • Hysteresis loss reduced obviously due to the grain growth. • Decline of hysteresis loss dominates the dramatic reduction of total loss.

Separation of silicon carbide and silicon powders in kerf loss slurry of loose abrasive sawing by means of phase transfer separation method with mixed cationic-anionic surfactant collectors addition

Kerf loss slurry generated from the slicing process of silicon wafer by using loose abrasive sawing method contains silicon carbide (SiC) and silicon (Si) powders. This study attempted to separate SiC and Si powders in kerf loss slurry through phase transfer separation method with mixed cationic-anionic surfactant collectors addition. Isooctane and water were employed as the two phases. Dodecylamine acetate (DAA) and sodium oleate (NaOL) was used as the cationic and anionic surfactant collector, respectively. The effects of single surfactant collector addition (DAA/NaOL) and mixed cationic-anionic surfactant collectors addition (DAA + NaOL) on the transfer of individual SiC and Si powders from the water phase to the isooctane phase, as well as on the separation of SiC and Si powders in kerf loss slurry, were investigated and compared.The results indicated that DAA could improve the transfer of both SiC and Si powders to the isooctane phase, and the improvement for SiC powder was higher than for Si powder. In contrast, NaOL could only improve the transfer of SiC powder to the isooctane phase, and the improvement for SiC powder was less than DAA did. Compared to single DAA/NaOL addition, mixed DAA + NaOL addition could improve the separation of SiC and Si powders in kerf loss slurry at lower surfactant collector dosages. In addition, the dosage of NaOL in mixed DAA + NaOL addition had more significant effect on improving their mutual separation than that of DAA did. The optimal separation results were obtained with mixed surfactant collectors addition of 0.01 kg/ton DAA and 0.05 kg/ton NaOL at pH 5. The purity and recovery of SiC powder recovered from the isooctane phase was 90.0% and 92.0%, respectively, and those of Si powder recovered from the water phase was 89.5% and 87.0%, respectively. The total dosage of surfactant collectors was 40% reduced.

Micro Electrical Discharge Machining of Ultrafine Particle Type Tungsten Carbide Using Dielectrics Mixed with Various Powders.

Electrical discharge machining (EDM) is widely used to machine hard materials, such as tungsten carbide; however, the machining rate and surface quality are low. In this research, the effects of mixing electrically conductive carbon nanofiber (CnF), semiconductive silicon (Si) powder, and insulative alumina powder (Al2O3) at different concentrations in a dielectric fluid were studied by observing single discharge craters and hole machining performance in the EDM of ultrafine particle type tungsten carbide. Craters obtained using carbon nanofiber and alumina were much smaller than in oil-only conditions. In contrast, The results show that adding CnF significantly improved the material removal rate under all conditions. Si and Al2O3 powders only improved the machining performance at a high discharge energy of 110 V. Furthermore, improvement in surface roughness was observed prominently at high voltages for all the powders. Among the three powders, alumina was found to improve the surface roughness the most.

Mo-Si-B Alloy Formed by Optional Laser Melting Process.

In this research article, the molybdenum alloy was prepared by solid-solid doping, selecting pure Mo powder, amorphous Si powder, and B powder as the experimental raw materials for SLM molding. The density and mechanical properties of Mo-Si-B alloys prepared by SLM technology under different processes and compositions were explored, and at the same time, the microstructure of the obtained alloy was observed. The result shows that, with a laser power of 250 W, a scanning speed of 500 mm/s, and a scanning distance of 60 μm, the Mo4.5-Si2-B (at.%) alloy has the highest forming rate under the 120°parameters of the rotating scanning strategy, and the highest density is 94.22%.

Novel (Zr, Ti)(C, N)–SiC ceramics via reactive hot-pressing at low temperature

Novel (Zr, Ti)(C, N)–SiC ceramics were fabricated by reactive hot-pressing at 1500–1700 °C using ZrC, TiC0.5N0.5, and Si powders as raw materials for the first time. The effects of Si addition on the microstructures and mechanical properties were investigated. The reaction completely proceeded to generate SiC with an Si addition of 5 mol%. The grain refinement by SiC and solid solution strengthening of (Zr, Ti)(C, N) improve the mechanical properties. A high flexural strength of 522 ± 20 MPa was obtained in the Z5T5S ceramics (ZrC with 4.75 mol% TiC0.5N0.5 and 5 mol% Si additional). When the Si addition increases to 10 mol%, the residual Ti-stabilized β-ZrSi phase appears, which significantly reduces the flexural strength. The Vickers hardness and fracture toughness monotonically increase with increasing Si addition. The fine-grained microstructure without a residual phase and abundant intrinsic lattice defects in the Z5T5S composite endow it with the potential to obtain excellent radiation resistance.

Synthesis of Ti3SiC2 coatings onto SiC monoliths from molten salts

Coatings with composition close to Ti3SiC2 were obtained on SiC substrates from Ti and Si powders with the molten NaCl method. In this work, the growth of coatings by reaction in the salt between monolithic SiC substrates and titanium powder is obtained between 1000 and 1200 °C. At 1000 °C, a coating of 8 µm thickness is formed in 10 h whereas a thin coating of 0.5 µm has been grown in 2 h. A lack in silicon was first found in the coatings prepared at 1100 and 1200 °C. For these temperatures, the addition of silicon powder in the melt had a favorable effect on the final composition, which is found very close to the composition of Ti3SiC2. The reaction mechanism implies the formation of TiCx layers in direct contact with the SiC substrate and the presence of more or less important quantities of Ti3SiC2 and Ti5Si3Cx in the upper layers.

Superior strength–ductility synergy of (TiC + Ti5Si3)/Ti composites with nacre-inspired architecture

Inspired by the ultrastrong and tough nacre structure, in this study, we prepared in-situ titanium (Ti) matrix composites (TMCs) containing laminated Ti, planarly aligned TiC and vertically distributed Ti5Si3 phases, which is close to the unique “brick-and-mortar” architecture of nacre. This concept was realized via electrophoretic depositing Si powders and graphene oxides (GOs) on pure Ti foils followed by spark plasma sintering. Results showed that dramatically higher yield strength (YS) of 829 MPa could be achieved in the composite, which is 178% higher than that of pure Ti counterpart while maintaining good plasticity of 8.1% in elongation. Compared with the laminated TiC/Ti composite with higher strength and moderate plasticity, simultaneous improvement in strength (19%) and ductility (33%) was attained in the nacre-like composites, leading to superior strength–ductility synergy. The strengthening and toughening mechanism were discussed based on microstructures and fractographs. Our study offers a facile approach to fabricating large-sized nacre-like artificial composites with outstanding properties.

Effect of impurities of Fe2O3 and TiO2 in bauxite on oxidation kinetics of β-SiAlON powders

The oxidation kinetics of β-SiAlON with and without impurities synthesized using α-Al2O3, Si, Al, Fe2O3 or TiO2 powders at 1573 K in microwave sintering furnace, were investigated. The initial oxidation temperature of β-SiAlON with impurities was lower than that sample without impurities. In the initial oxidation stage at 1373–1473 K, the lattice deformation accelerated oxidation rate of β-SiAlON with impurities. However, in the final oxidation stage, the oxidation rate of β-SiAlON with impurities was much lower than that of reference sample, because impurities promote glass film formation on surface of β-SiAlON particles, reducing the chance of β-SiAlON reacting with O2.

Microstructure and properties of pressure-less joining of SiCf/SiC composites by Ti–Si alloys

Prepared with pure Ti and Si powders as raw materials, the Ti16Si84 alloy (at. %) was obtained through refining in an intermediate frequency magnetic induction furnace. In this study, a novel method to join SiC fiber-reinforced SiC matrix composites (SiCf/SiC composites) under non-pressurized conditions is proposed using Ti16Si84 alloy foil as the joining material. The joining process was optimized, and the wettability, microscopic morphology, and bonding reaction of the joint were studied, along with the fracture characteristics and the bonding mechanism. Additionally, the corrosion resistance of the joints was examined in pure static water with high-dissolved oxygen (246 ppm) at 360 °C and 18.6 MPa. The results showed that Ti16Si84 alloy has good wettability with SiCf/SiC, enabling it to join SiCf/SiC successfully. The joint was composed of SiC, TiSi2, and Si, and there was no intermediate between them. When the brazing temperature was 1410 °C, the shear strength of the joint reached 42.5 MPa, and the joint integrity was favorable. After the shear test, numerous SiCf/SiC composites were torn by the solder, indicating their advantageous shear properties and high joint strength. After three days of corrosion, the joint samples' weight showed almost no changes. The corrosion depth was less than 20 μm and the residual shear strength of the corroded joined samples was 38.37 MPa, indicating excellent hydrothermal corrosion resistance of the joint material.

The two-tier tissue reinforcing and toughening mechanism caused by dual-granularity Si powders ingredient, and dielectric properties in reaction bonded and sintered reaction bonded Si3N4 porous ceramics

With the rapid development of hypersonic vehicles and broadband wave-transparent radome, Si3N4 porous ceramics (Si3N4-PC) have attracted attention due to their excellent intrinsic properties of Si3N4 and high porosity. However, its high porosity results in low strength and toughness, which are fundamental properties for radome. Reaction bonded (RB) Si3N4-PC has advantages of dielectric properties and cost over general phase transformation sintering (PTS) and sintered reaction bonded (SRB) Si3N4-PC while it has been neglected in recent years. In this study, RB and SRB Si3N4-PC prepared by non-aqueous gelcasting and the influence of Si powders ingredient on their properties are discussed in order to illuminate the potential of RB Si3N4-PC in wave-transparent materials. The results show that RB Si3N4-PC with dual-granularity ingredients of 5 μm & 45 μm produces a two-tier tissue of framework of coarse whiskers enhanced by a network of tiny whiskers. SRB Si3N4-PC evolves into a two-tier tissue of framework of columnar and rod-like grains joining together to brace each other. The two-tier microscopic tissue strongly reinforces and toughens the structure and results in higher σF and γwof. As a result, the RB and SRB Si3N4-PC of dual-granularity of 5 μm & 45 μm obtain the maximum σF of 109.94 MPa and 119.56 MPa as well as maximum γwof of 990.74 J m-2 and 1167.88 J m-2, respectively. Furthermore, the ε′ and tanδ of RB and SRB Si3N4-PC of dual-granularity of 5 μm & 45 μm are about 4.20 and 4.52 as well as 7.01 × 10-3 and 22.90 × 10-3, respectively. It is concluded that RB Si3N4-PC of dual-granularity has good mechanical and dielectric properties, which are favorable for radome.

High-temperature performances of Si-HfO2-based environmental barrier coatings via atmospheric plasma spraying

To improve high-temperature bearing capability of coatings, novel agglomerated Si-HfO2 powders were prepared by adding HfO2 powders into original Si powders by spray drying method. Three-layer environmental barrier coatings (EBCs) with Si-HfO2 bond layer, Yb2Si2O7 intermediate layer and Yb2SiO5 surface layer were prepared on SiC ceramic substrates by atmospheric plasma spraying (APS). The high temperature properties of coatings were systematically investigated. The results indicated that the coatings had good high temperature oxidation resistance, and remained intact after being oxidized or steam corrosion at 1400 °C for 500 h, so the addition of HfO2 improved the thermal cycling performances of the coating. The HfO2 in Si bond coating could effectively inhibit the growth of thermal grown oxide at high temperatures. This work indicates that the high temperature properties of the coatings are improved by this novel EBCs using the novel agglomerated Si-HfO2 powders.

Effect of modified coal tar pitch addition on the microstructure and properties of Al2O3–SiC–C castables for solid waste incinerators

Al2O3–SiC–C (ASC) castables were performed using brown alumina, SiC, ultrafine α-Al2O3, microsilica, Si powder, calcium aluminate cement and modified coal tar pitch (CP) as starting materials. Effect of CP content and firing atmosphere on the microstructure and properties of ASC castables were investigated. The results show that cold strength, hot strength, thermal shock resistance and slag resistance of ASC castables were slightly improved with increasing of CP content. As an example of castables containing 2 wt% CP, it can be found that the cold strength and hot strength of the sample after firing in oxidizing atmosphere and reducing atmosphere were higher than that of in weak oxidizing atmosphere, and the sample after firing in reducing atmosphere possess overall properties including higher strength, good thermal shock resistance and slag resistance. The differences in properties of ASC castables with different CP addition were closely related to their phase compositions, microstructure including pore size distribution, and the amount of in-situ formed SiC whiskers.

Preparation of hollow SiC spheres by combustion synthesis

AbstractHollow spherical β‐SiC was successfully prepared in argon by combustion synthesis using Si powder and polytetrafluoroethylene (PTFE) powder. The phase composition and morphology of spherical products can be controlled by adjusting the Si/C2F4 molar ratio (MSi/(C2F4)). When MSi/(C2F4) = 3, the phase content of β‐SiC is the highest (up to 85.54%), and hollow spherical products obtained; When MSi/(C2F4) ≥ 5, the Si/SiC microspheres are solid. The synthesis mechanism of hollow β‐SiC microspheres is as follows: Si particles react with PTFE releasing heat. Then unreacted Si absorbs heat to form liquid phase microspheres, which is equivalent to the core template to form β‐SiC microspheres by reaction with cracked C. Meanwhile, the silicon diffuses from the core to the shell to form the cavity. This method can synthesize the hollow spherical β‐SiC in a simple way without prearranged spherical template and long synthesis cycle.

Sustaining strength-ductility synergy of SLM Fe50Mn30Co10Cr10 metastable high-entropy alloy by Si addition

Metastable high entropy alloys possesses excellent ductility and toughness, but its low strength hinders its engineering applications. Therefore, many studies have been carried out to improve the strength of metastable high entropy alloys while maintaining high ductility. In this paper, a new high entropy alloy is fabricated by selective laser melting (SLM) with the addition of Si powder into Fe50Mn30Co10Cr10 metastable high entropy alloy powders, with compositions (Fe50Mn30Co10Cr10)100-xSix (x = 0, 1, 3, 5). The results show that the yield strength and ultimate tensile strength of Si-containing metastable high-entropy alloys gradually increase with the increase of Si elements, and the performance is far better than that of without Si addition. The Si element increases the metastability of the γ(FCC) phase in the high-entropy alloy, and promotes the phase transformation-induced plasticity by adjusting the twinning strain during the deformation process; so that the total elongation of the material prepared by SLM can reach 30% and above. The combined effect of multiple strengthening mechanisms such as dislocation, deformation twinning, and phase transformation has greatly increased the strength of high-entropy alloys. In addition, the excellent strength-ductility combination of the alloy containing Si element indicates that the composition has good printability for laser additive manufacturing, which can provide a reference for expanding the diversification of high-entropy alloys in the additive manufacturing process.

Si@C anode materials decorated with higher manganese silicides for enhanced rate capability and cycle stability

The poor cycle stability and high price are primary drawbacks of Si-based anode materials to their wide applications. Partially alloying silicon with transition metals together with carbon coating is effective in improving the cycle performance. Herein, higher manganese silicides (HMS) are decorated on carbon coated silicon, denoted as Si@HMS@C, by coating microscale waste Si powders recovered from photovoltaic industry with in situ polymerized resorcinol formaldehyde (RF) resin, followed by absorbing manganese acetate tetrahydrate and thermal pyrolizing in flowing N2 gas. As compared to the carbon coated silicon anode (Si@C), the Si@HMS@C anodes exhibit greatly enhanced cycle performance and rate capability. A reversible capacity of 1070 mAh g−1 is maintained after 400 cycles at 0.4 A g−1. The ex situ XRD measurements as well as microstructure investigation before and after cycle test reveal that the phase and morphology of HMS keep unchanged. Meanwhile, reversible redox reactions associating to the reaction of Li with HMS or with its derivatives induced by the initial irreversible side reaction of HMS with electrolyte are identified. These results suggest that the HMS might have played multifunctional roles on improving the Si anode’s electrochemical performance, including stabilizing the structure, increasing the electronic conductivity and contribute additional capacity.

Comparison of LPBF processing of AlSi40 alloy using blended and pre-alloyed powder

Elemental powders or pre-alloyed powders can be blended to form a powder stock with new compositions. In-situ alloying of blended powder mixtures during LPBF processing speeds up the alloy development process as it allows composition adjustment. Pre-alloyed powder is typically used in industrial settings to ensure consistency of the built material, whereas blended powders are used to experiment with new alloy compositions. In this research, we compare a range of binary alloy compositions built from blended elemental powder and pre-alloyed powder to evaluate how the feedstock powder affects the built material. The alloy is a lightweight alloy with a low coefficient of thermal expansion, comprised of Al and Si. The following compositions were formed by mixing pure Al and Si powder: AlSi38, AlSi40 and AlSi42, and pre-alloyed AlSi40 powder was also used. Parametric studies were performed for both the blended and pre-alloyed powders. It was found that the optimum process parameters for the blended and pre-alloyed powder were different. The line energy for pre-alloyed powder was selected to be 0.150 J/mm (300 W, and 2000 mm/s) which yielded a density of 99.5% of the theoretical density. The optimal density for the blended powders was 99.07-99.16% which was also achieved with 0.150 J/mm (375 W, 2500 mm/s). The alloys are brittle, with Young's modulus 86-99 GPa. Coefficient of thermal expansion (CTE) reduced with increasing Si content, material built from pre-alloyed powder had 5% higher CTE than material built from blended powder.