Si Compacts Research Articles

Microstructure evolution and reaction behavior of Cu–Ni–Si powder system under solid-state sintering

Cu–Ni–Si powder compacts were firstly prepared by mechanical mixing and cold-pressing, and then held at 600, 700, 800, 900, 950 and 1000 °C for 5 min followed by water-quenching. Subsequently, X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and transmission electron microscope (TEM) were used to characterize the microstructure and phase composition of water-quenched samples, and competitive reactions among Ni–Si compounds were also discussed. Finally, a schematic diagram about the solid-state reaction process of Cu–Ni–Si powder system was proposed. Results showed that Ni and Si substances started to diffuse and reacted when the temperature was above 600 °C, resulting in the formations of Ni31Si12 phase at the primary powder boundary and Ni2Si precipitations inside the Cu powder, respectively. With the increase of temperature, Ni2Si phase dissolved into Cu matrix while the amount and size of Ni31Si12 phase increased along with the morphology changing from chrysanthemum-like shape to equiaxed structure. The final microstructure of Cu–Ni–Si powder system after sintering and water quenching consisted of Cu(Ni,Si) solid solution and equiaxed Ni31Si12 phase at the primary powder boundary, which was expected to achieve a combination of high strength and high electrical conductivity by subsequent heat treatments.

Characterization of porous sintered reaction-bonded silicon nitride containing three different rare-earth oxides

The effect of rare-earth oxides on the characteristics of porous sintered reaction-bonded silicon nitride (SRBSN) was investigated. Three types of raw Si powder mixtures containing different rare-earth oxide (La2O3, Er2O3, and Yb2O3) were prepared and nitrided in the form of compacts. The nitriding profiles of the respective raw powder mixtures with elevating temperature indicated that Yb2O3 clearly promoted the nitridation of Si compacts at low temperature compared with other rare-earth oxides, and the β-Si3N4 ratio after completion of the nitriding reaction was different at which temperature the major nitridation occurred. Yb2O3 was found to be the most effective additive to achieve a strong porous SRBSN, having the flexural strength of 441 MPa. The reason why Yb2O3 promotes the nitridation reaction and has excellent mechanical properties after post-sintering is that oxygen was removed during the nitriding reaction, which was supported by the analysis of oxygen content via the laser-induced breakdown spectroscopy.

Reactive synthesis of porous FeSi intermetallic compound

Porous FeSi intermetallic compound was prepared by reactive synthesis of elemental powders. The evolutions of phase composition, microstructure and pore structure of Fe–Si compacts during the synthesis procedure were investigated, as well as the corrosion resistivity of the porous material against sulfuric acid solution. The results show that the major phases are generated in the order of α-Fe, Fe3Si, η-Fe5Si3 and ε-FeSi during the sintering process. The skeletons of Fe5Si3 and FeSi phases are formed successively, resulting in a large number of connected pores in the compact. The final synthesized porous compound has the FeSi phase purity of 95.4 wt%. The open porosity, maximum pore size and gas permeability of porous FeSi are 53.8%, 14.1 μm and 92.20 m3 m−2 kPa−1·h−1, respectively. The radial dimension variation ratio, pore size and permeability show the linear change regulation with the sintering temperature with the change rate constants being 1.51 × 10−2%·°C−1, 5.3 × 10−2 μm·°C−1, 1.45 × 10−1 m3·m−2·kPa−1·h−1·°C−1, respectively. The pore size and permeability show the linear change law with the initial Fe powder size with the change rate constants being 5.4 × 10−2 μm·μm−1 and 3.05 × 10−1 m3·m−2·kPa−1·h−1·μm−1, respectively. The corrosion kinetics of the synthesized porous FeSi compound in 160 g·L−1 sulfuric acid solution shows a characteristic of two stages with the average corrosion rates being 2.11 × 10−3%·h−1 and 1.18 × 10−4%·h−1, respectively, in which porous FeSi shows good pore structure and microstructure stability.

Facile synthesis of Fe-6.5wt%Si/SiO2 soft magnetic composites as an efficient soft magnetic composite material at medium and high frequencies

Fe-6.5wt%Si/SiO2 soft magnetic composites (SMCs) were successfully designed to reduce the core loss and improve the magnetic properties of Fe-6.5wt%Si alloy via the fluidized in-situ chemical vapor deposition method combined with the subsequent spark plasma sintering (SPS) process. Most conductive Fe-6.5wt%Si particles were firstly coated with SiO2 during chemical vapor deposition, and the highly compact and insulated Fe-6.5wt%Si/SiO2 SMCs were then obtained quickly by the spark plasma sintering. A 470 nm thick SiO2 insulating layer was found to exist uniformly and densely around Fe-6.5wt%Si particles in Fe-6.5wt%Si/SiO2 SMCs with the fluidized chemical vapor deposition (FCVD) process. It caused these SMCs to exhibit much higher electrical resistivity, lower core loss, and better frequency stability of permeability than those of raw Fe-6.5wt%Si compacts and Fe-6.5wt%Si/SiO2 SMCs with the conventional chemical vapor deposition (CVD) process. This new method provides a promising way to optimize the magnetic properties of soft magnetic composite materials, which can possess larger energy conversion efficiency for the electric-magnetic switching devices.

Nitridation behavior of silicon powder compacts of various thicknesses with Y2O3 and MgO as sintering additives

AbstractThe effects of the nitriding temperature (1300 and 1350°C), holding time (0‐4 hours), and thickness of Si powder compacts on the nitridation behavior of silicon were investigated by examining the nitridation rates, analyzing phase compositions, and observing the microstructures of nitrided compacts. Si powder compacts doped with Y2O3 and MgO as sintering additives were prepared with thicknesses of 3, 6, and 9 mm. The phases of nitrided compacts were transformed from Si to α‐Si3N4 and β‐Si3N4 with an increase in the nitriding temperature and holding time. The degree of nitridation increased with the nitriding temperature and holding time. The β/(α+β) ratio increased with the nitriding temperature and holding time, and with a decrease in the thickness of the Si powder compacts. However, all compacts exhibited the same tendency for a higher β/(α+β) ratio at the compact surface than in the bulk of the compact. The variation in the β/(α+β) ratio for each compact decreased with an increase in the nitriding temperature and holding time.

Nickel and Copper-Free Sintered Structural Steels Containing Mn, Cr, Si, and Mo Developed for High Performance Applications

AbstractIn an attempt to study the sinterability of potential high-strength nickel-free sintered structural steels containing Mn, Cr, Si and Mo compacts were prepared based on sponge and water atomised iron powders and on Astaloy prealloyed powders. To these were admixed ferromanganese, ferroslicon, and graphite. The samples were sintered at temperatures 1120 and 1250°C in laboratory tube furnaces in hydrogen, hydrogen-nitrogen atmospheres with dew points better than -60°C or in nitrogen in a semiclosed container in a local microatmosphere. After sintering the samples were slowly cooled or sinterhardened. Generally resultant microstructures were inhomogeneous, consisted of pearlite/ bainite/martensite, but were characterised by an absence of oxide networks. Sintering studies performed over a range of compositions have shown that superior strength, ranging beyond 900 MPa, along with reasonable tensile elongation, can be achieved with these new steels.

Effects of Mechanical Milling and Sintering Temperature on the Densification, Microstructure and Tensile Properties of the Fe–Mn–Si Powder Compacts

This work reported on the effects of mechanical milling and sintering temperature on the densification, microstructure and mechanical properties of the Fe–28Mn–3Si (wt%) alloy. Elemental Fe, Mn and Si powders were used as the starting materials, and two batches of powder mixture were prepared: one was blended elemental (BE) powder mixture; the other was mechanically milled (MM) powder mixture milled for 5 h using planetary ball milling. Both powder mixtures were pressed under a uniaxial pressure of 400 MPa, and subsequently sintered in a high vacuum furnace for 3 h at 1000, 1100, 1200 and 1300 °C. It was found that Mn depletion region (MDR) was formed on the surface of all the sintered samples. The sintered BE compacts had a low density ( γ -austenite and minor e -martensite. In comparison, additional (Fe, Mn) 3 Si phase was observed in the BE alloys sintered at 1000 °C, and a single α -Fe phase was identified in the BE compact sintered at 1300 °C. The tensile properties of the sintered MM compacts increased significantly with the temperature and were significantly higher than those of their BE counterparts.

Enhanced nitridation of silicon compacts by Yb2O3 addition

The catalytic effect of ytterbium oxide (Yb2O3) on the nitriding reaction of Si compacts was investigated. Si powder mixtures containing Yb2O3 were prepared and nitrided in the form of compacts with a multi-step heating schedule over the range of 1200°C–1450°C. The nitriding profiles of the powder mixture with increasing temperature indicated that Yb2O3 clearly promoted the nitridation of Si compacts at 1200°C compared with the pure Si compact containing no additives. The critical role of Yb2O3 on the nitridation of Si, was elucidated that Yb2O3 promotes the loss of initial SiO2 of the raw Si powder via the measurement of the weight changes at low temperature (1100°C) and thermogravimetric analysis under N2 atmosphere. It was also found that the β-ratio of fully nitrided Si was closely related to the intermediate degree of nitridation at 1200°C and 1300°C.

Application of Metal Injection Molding to Al Powder

Metal injection molding (MIM) was applied to Al powders. Sintered Al compacts were prepared by MIM in this study. At first, sintered pure-Al compacts were prepared under various process conditions. The relative densities of the compacts debound in Ar gas and sintered at 650 °C for 2 hr in vacuum of 10−3 Pa order were 86 %, 90 % and 96 % when the average particle size were 20 μm, 10 μm and 3 μm, respectively. The 3 μm powder compact had the excellent tensile strength of 120 MPa and elongation at fracture of 19 % at room temperature. Next, the addition of Si powder to Al powder was examined to improve sinterability of Al powder. Regardless of Al powder size, the addition of Si powder widely enhanced densification of sintered Al compacts. Using Al powder of 35 μm, the relative density of the sintered pure-Al compact was only 66 %, while that of the sintered Al-1 mass%Si compacts went up to 89 %. When Al powder of 11 μm was used, the sintered Al-1Si compacts had the relative density of 97 % and the tensile properties close to annealed wrought pure-Al.

High resistivity and low core loss of intergranular insulated Fe–6.5 wt.%Si/SiO2 composite compacts

Intergranular insulated Fe–6.5wt.%Si/SiO2 composite compacts were prepared by in-situ chemical deposition combined with a subsequent spark plasma sintering process. Most of the conductive Fe–6.5wt.%Si alloy particles were coated with SiO2 insulating layer in Fe–6.5wt.%Si/SiO2 composite compacts. The presence of SiO2 insulating layer increased the resistivity by two orders of magnitude for intergranular insulated Fe–6.5wt.%Si/SiO2 composite compacts (4.8×10−5Ω·m) compared with that of Fe–6.5wt.%Si compacts without SiO2 insulating layer (9.7×10−7Ω·m). In addition, most of the eddy currents could be confined in the SiO2-coated Fe–6.5wt.%Si particles, which equated to reducing the effective radius of eddy current in Fe–6.5wt.%Si alloy. So, the intergranular insulated Fe–6.5wt.%Si/SiO2 composite compacts presented a lower core loss compared with that of Fe–6.5wt.%Si compacts without SiO2 insulating layer or traditional Fe–6.5wt.%Si electrical steels at medium and high frequencies. Good soft magnetic properties were also obtained for the intergranular insulated Fe–6.5wt.%Si/SiO2 composite compacts, with a saturation magnetization of 164.6emu/g, a coercive force of 12Oe, and a relative permeability of 3.2×103 at 50Hz. The intergranular insulated Fe–6.5wt.%Si/SiO2 composite compacts provide valuable inspiration for developing new high silicon electrical steel cores with low core loss and excellent magnetic and electric properties.

High pressure infiltration sintering of cBN–Si composites

Polycrystalline cubic boron nitride (PcBN) compacts with high hardness, high thermal stability and high cutting performance have been prepared through method of high pressure infiltration of silicon melt. The scanning electron microscopy (SEM) and metallographic observations indicates that cBN–cBN bonding appeared extensively in the sintered samples. The hardness tests give an average Vickers hardness of 43GPa for the well-sintered cBN–Si compacts, which is much higher than the conventional PcBN composites (~30GPa). The onset temperature of oxidation can reach to 1300°C, about 300°C than that of the commercial PcBN (~1000°C). According to the results of turning test, the cutting performance of our sintered samples is also superior to the commercial PcBN.

Core loss reduction in Fe–6.5 wt.%Si/SiO2 core–shell composites by ball milling coating and spark plasma sintering

The Fe–6.5wt.%Si/SiO2 core–shell composite powders and compacts have been prepared by simple ball milling coating and spark plasma sintering process. The microstructure, magnetic and electrical properties of the compact samples obtained at different sintering temperatures were investigated by differential scanning calorimetry, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrometry, vibrating sample magnetometer, and impedance analyzer. The relative densities of Fe–6.5wt.%Si/SiO2 composite compacts first increase with sintering temperature from 800°C to 1150°C, and then keep nearly unchanged with higher temperature, while the coercivity and electrical resistivity change in the opposite direction. The saturation magnetization fluctuates around an identical value with increasing the sintering temperature from 800°C to 1250°C. The Fe–6.5wt.%Si/SiO2 composite compact sintered at 1150°C displays excellent soft magnetic properties: saturation magnetization of 153.9emu/g, coercivity of 14Oe, relative permeability of 8.6×103 at 50Hz, respectively. The Fe–6.5wt.%Si/SiO2 core–shell composite compacts clearly exhibit much higher electrical resistivity and lower core loss (1.4×10−5Ωm and 18.1W/kg (P5/400)) than those of Fe–6.5wt.%Si compacts without core–shell structures (9.6×10−7Ωm and 37.9W/kg (P5/400)). The core–shell structures introduced here is a good potential method to increase electrical resistivity and reduce core loss of soft magnetic materials, which can obviously improve energy conversion efficiency of electric–magnetic switching device.

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.

Microstructure and mechanical properties of h-BN–SiC ceramic composites prepared by in situ combustion synthesis

► h-BN–SiC ceramic composites were fabricated by one-step SHS under high nitrogen pressure. ► Si 3 N 4 is the preferential formation phase under high nitrogen pressure of 120 MPa. ► h-BN–SiC ceramic composites have good bond strength and mechanical properties. h-BN–SiC ceramic composites were prepared by in situ combustion synthesis of B 4 C and Si powder compacts under high nitrogen pressure. The high reaction nitrogen pressure was beneficial to the improvement of the relative density and mechanical properties of the products, but higher nitrogen pressure of 120 MPa was favorable to the formation of Si 3 N 4 , which was consistent with the theoretical thermodynamic calculation. The h-BN–SiC ceramic composites have good mechanical properties over those of conventionally processed ones because of the in situ formed h-BN and SiC phases fabricated under high temperature and pressure. The bending strength and fracture toughness of the h-BN–SiC ceramic composites prepared under 100 MPa nitrogen pressure were 82 MPa and 1.9 MPa m 1/2 , respectively.

ChemInform Abstract: Nitridation Kinetics and Thermodynamics of Silicon Powder Compacts.

The nitridation kinetics of Si powder compacts were studied by measuring the flow rate dependence of N2 and N2–H2 gas mixtures during slow heating of Si compacts while simultaneously monitoring the oxygen partial pressure of the egress gas. The reaction was found to occur in two distinct stages. In pure nitrogen the initial stage was interpreted in terms of devitrification of the native silica layer, catalyzed by Fe impurities, and the exposure of the underlying Si. The reaction sequence at that point has been unequivocally shown to be followed by the formation of oxynitride according to as evidenced by a large increase in the oxygen partial pressure simultaneously with the onset of the initial stage. In the absence of hydrogen, both reactions are rapidly suppressed as the oxygen liberated competes with the nitrogen for the exposed silicon surface and reoxidizes it. However, in the presence of hydrogen, the oxygen liberated reacts with the hydrogen and prevents the reoxidation of the Si. The net reaction in this case is and/or with the second reaction being thermodynamically favored. The main nitridation reaction occurring between 1300° and 1400°C appears to be diffusion controlled and depends on the nature of the passivating layer that forms during the initial stage.

Binder-Free Pellet Fabrication of Ultra-Fine Poly-Si Powder for Economical Production of Solar Grade Si Ingot

The consolidation process of ultra fine Si powders, generated as by-product during the decomposition process of silane gases, was systematically investigated for use as economical solar-grade feedstock. Si powder compacts were tried to fabricate by a consolidation process without a binding agent and then their density ratio and strength were evaluated. The Si powders in as-received state were not pure enough to be used alone as solar grade feedstock material. After the adequate chemical treatments, a sufficiently high purity above solar-grade was able to be achieved.

Use of Si 3N 4 as a reactant in preparation of TiN–Ti 5Si 3 composites by solid-state SHS reactions

An experimental study on in situ formation of TiN–Ti 5Si 3 composites with a wide range of compositions by self-propagating high-temperature synthesis (SHS) was conducted in two reaction systems. One adopted silicon nitride (Si 3N 4) as the solid source of nitrogen in a solid-state combustion system. The other employed both solid and gaseous reagents and performed the reactions of the Ti–Si powder compact under nitrogen pressures. Self-sustained reactions featuring a distinct combustion front were observed in both systems. TiN–Ti 5Si 3 composites containing Ti 5Si 3 from 20 to 80 mol% were effectively produced by solid-state combustion synthesis of Ti–Si 3N 4 and Ti–Si 3N 4–Si powder compacts. This result verifies the contribution of Si 3N 4 to the formation of TiN and Ti 5Si 3. However, the solid/gas reaction of Ti–Si compacts with gaseous nitrogen was proved to be an inappropriate route in terms of product composition. For the production of a TiN-rich composite such as 80 mol% TiN, it was found that the nitrogen uptake by the Ti–Si sample compact was inadequate, thus resulting in formation of a great amount of Ti 2N in the final product. On the contrary, when preparing the TiN–Ti 5Si 3 composite with a low TiN content like 20 mol% or even 50 mol%, Ti–Si powder compacts in gaseous nitrogen were subjected to excessive Ti nitridation and yielded a Si-rich silicide phase TiSi 2 in addition to TiN and Ti 5Si 3.

Fabrication of Si3N4 Micro-Components by a Combined Microfabrication Process

Si-based high-temperature ceramics are attractive materials for power microelectromechanical systems (power MEMS), such as microscale gas turbines, micro-combustors and micro-reactors. This presentation introduces a novel process for the microfabrication of Si3N4 ceramics, which mainly consists of pre-sintering of Si powder compacts, micromachining of pre-sintered Si preforms and reaction sintering of the micromachined Si preforms. The present process has its high potential for Si3N4 3-dimensional microfabrication because it combines the machinablity of pre-sintered Si powder compacts and near-net shaping characteristic of S3N4 reaction sintering. Si3N4 micro-components such as micro nozzle arrays and micro-rotor were fabricated by using the present process.

Thermal Behavior of AZ31 and Si in Solid State Synthesis of Mg2Si

Mg2Si/Mg composite which shows high mechanical properties was expected to be produced via solid-state synthesis using AZ31 powder and Si powder as initial material. For the first step of this research, thermal behavior of AZ31 and Si was investigated in this paper.AZ31/Si compacts with various compositions were examined by differential scanning calorimeter and analyzed by X-ray diffraction to confirm reactability between them.The detailed result indicated that Si could completely react with AZ31 and synthesize Mg2Si when heated at 753 K, which is almost the same temperature as in the case of using purity Mg. The starting temperature of synthesis became lower and exothermic heat increased as Si content increased. Repeated plastic working (RPW) was also effective to decrease their synthesizing temperature. That is because the reaction between Mg and Si occurred easily due to the mechanisms of RPW process such as (1) breakage of oxide film on the surface of Mg particles and (2) refinement of both of the Mg and Si particles. In addition, it was observed as the result of optical microstructure that Mg2Si particles were dispersed into matrix and they existed at the interface of Mg grain.

Microstructures and Fracture Characteristic of Si<SUB>3</SUB>N<SUB>4</SUB>-O’SiAlON Composites using Waste-Si-Sludge

Si 3 N 4 -O'SiAlON composites using waste-Si-sludge was fabricated by gas-pressure-sintering (GPS) process at 1950°C for 3.5 h. The percent nitridation of waste Si compacts showed a lower value than that of using commercial Si powders caused by the high contents of oxygen in the waste Si particles. Some amounts of Si 2 N 2 O and Y 2 Si 2 O 7 phases were also detected in the reaction-bonded Si 3 N 4 (RBSN) although main components were α-Si 3 N 4 , β-Si 3 N 4 and residual Si phases. In the post-sintered body by GPS, no residual Si, α-Si 3 N 4 , Si 2 N 2 O and Y 2 Si 2 O 7 phases were detected except the β-Si 3 N 4 and O'SiAlON. The grain growth of rod-like Si 3 N 4 grains was inhibited by the dispersion of fine O'-SiAlON particles. The fracture toughness and strength of Si 3 N 4 -O'SiAlON composites were 5.6 MPa.m 1/2 and 456 MPa, respectively.