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1.
Composite powders containing silicon carbide (SiC) particles and silicon nitride nanowires (Si 3N 4-NWs) were synthesized by combustion synthesis, using elemental Si, carbon black, PTFE and small amount of metal powders as raw materials. The catalyst types and environmental gases and pressures have been altered to study their influence upon the crystal growth and the nature of the products. The products were characterized by X-ray diffraction, scanning and transmission electron microscopy. Results reveal that the metal/silicon liquid (e.g. Ni 2Si and Fe 3Si) formed during the combustion process is a key factor for the growth of Si 3N 4-NWs in nitrogen. For the process carried out in non-nitrogen gas (Ar, CO 2 or mixed CO 2/O 2), pure SiC particles were obtained. The rise in nitrogen pressure can promote the growth of Si 3N 4-NWs as well as large SiC particles. The growth of Si 3N 4-NWs could be explained by the SLGS mechanism, and the growth of SiC particles was involved in the gas-phase and liquid-phase mechanisms. 相似文献
2.
Reaction‐bonded Si 3N 4–SiC and Si 3N 4‐bonded ferrosilicon nitride, with Si powder, SiC particles and Fe 3Si–Si 3N 4 particles as raw materials, respectively, are prepared in flame‐isolation nitridation shuttle kiln with flowing N 2 at 1723K. There is columnar β‐Si 3N 4 in both Si 3N 4–SiC and Si 3N 4‐bonded ferrosilicon nitride. However, fibrous α‐Si 3N 4 is only observed in Si 3N 4–SiC and Si 3N 4‐bonded ferrosilicon nitride contains much more Si 2N 2O than Si 3N 4–SiC. By analyzing the oxidation thermodynamics of Si and Si 3N 4, it is known that in the process of producing Si 3N 4–SiC, Si is oxidized first to gaseous SiO and fibrous α‐Si 3N 4 is generated with SiO and N 2. The existence of SiO is the reason of low silicon nitridation rate. But in the process of producing Si 3N 4‐bonded ferrosilicon nitride, Si 3N 4 is easier to be oxidized than Si and Si 2N 2O is generated on the surface of Si 3N 4 hexagonal prisms in ferrosilicon nitride particles. Meanwhile, Si in raw materials forms new ferrosilicon alloys with Fe 3Si, which decreases the temperature of liquid appearance and blocks some open pores in the samples, which stops the matter loss of nitridation. Liquid ferrosilicon alloys favors β‐Si 3N 4 generation from Si direct nitridation and fibrous α‐Si 3N 4 transformation, which used to exist in ferrosilicon nitride raw materials. 相似文献
3.
Silicon nitride with 3% MgO powder mix is uniaxially and cold isostatically pressed to form a green Si 3N 4 crucible. Liquid phase sintering was applied to the green Si 3N 4 crucible at 1600 °C for 30 min under the nitrogen atmosphere. Intergranular Mg–Si–O–N glass remained between the silicon nitride grains which reacted with the molten metal during melting. This grain boundary glass was removed by vacuum heat treatment at 1575 °C for 5 h. The vacuum heat treated crucible was used to melt cast iron to examine reactions between the molten metal and Si 3N 4 ceramic crucible. EDX spectra across the Si 3N 4–cast iron interface and XRD for silicon nitride sample after cast-iron melting side surface analysis were carried out. Optical microscopy and SEM image analysis were made to examine the interaction between Si 3N 4 crucible and cast iron melt. Surprisingly, no reaction was observed between the vacuum heat treated crucible and melted cast iron. 相似文献
4.
The effects of impurity iron content on characteristics of sintered reaction‐bonded silicon nitrides were examined by adding iron powder to a high purity raw Si powder. Powder compacts of the raw Si powder doped with 2 mol% Y 2O 3 and 5 mol% MgSiN 2 as sintering additives and Fe as impurity (0 mass%, 0.1 mass%, 1.0 mass% and 5.0 mass%) were nitrided at 1400°C for 8 h under a N 2 pressure of 0.1 MPa, followed by post‐sintering at 1900°C for 6 h under a N 2 pressure of 0.9 MPa. All the SRBSN (Sintered Reaction‐Bonded Silicon Nitride) specimens had about the same 4‐point bending strength of 730–770 MPa. The fracture toughness of the specimens was gradually decreased with increasing Fe additive amount due to the inhibition of development of rodlike β‐Si 3N 4 grains by SiFe x particles formed during nitridation process. The thermal conductivity was also decreased with an increase in Fe amount. It seems that the increasing oxygen in grain‐boundary phase caused by the oxidation of Fe during milling resulted in the increase in lattice oxygen of β‐Si 3N 4 grains, which caused phonon scattering and thereby decreased thermal conductivity of β‐Si 3N 4. There was little change in the dielectric breakdown strength of the specimens: 24, 22, 22, and 21 kV/mm for the specimens without Fe, and with 0.1 mass%, 1.0 mass% and 5.0 mass% Fe, respectively. The surface resistivity of the specimens with 0 mass%, 0.1 mass% and 1.0 mass% Fe was in the range of 10 13 Ω, but the specimen with 5 mass% Fe was about one order lower than the others. 相似文献
5.
Herein, Si 3N 4 powders of comparatively high α‐phase but with distinct morphologies, especially α‐Si 3N 4 fibers, were successfully prepared by a developed combustion synthesis (CS) strategy. Different proportions of Fe and Fe 2O 3 were innovatively doped in reactants as additives to control the phase constitution and their relative percentage, as well as morphologies of final microstructures. One step further, the effects of Fe‐contained impurities on the CS process were rationally proposed and verified based on a series of meticulous designed experiments. It turns out that two contradictory effects of metal Fe on the formation of α‐Si 3N 4 synergistically play vital roles in the CS reaction. The existence of metal Fe can accelerate the crystallization of the amorphous SiO 2, which act as protection layer outside the Si powders and subsequently promote the generation of gaseous SiO. These gaseous SiO easily reacts with N 2 and eventually form α‐Si 3N 4. On the other hand, the formation of β‐Si 3N 4 will be promoted by the assistance of some liquid phases, and in this case, they mainly come from the reaction between Fe and Si. For this study, when the content of doped Fe is below 2 mol%, the prior effect on promoting α‐phase content is pronounced. Otherwise, the latter dominates the CS process as the content of Fe additive is further increased above 2 mol%. In a different way, Fe 2O 3 mainly encourages the formation of β phase through the large amount of newly generated liquid phases, although the reduced SiO 2 and Fe may still promote the α/β ratio on some extent. 相似文献
6.
Fused silica (SiO 2) ceramic crucibles with α-Si 3N 4 coating are commonly used for smelting photovoltaic silicon (Si). However, SiO 2 ceramics will inevitably undergo crystallization and large volume change during the high-temperature service, which will lead to crucible cracking and deteriorate the quality and yield of the Si ingot. In this work, α-Si 3N 4/SiO 2 ceramics are fabricated by introducing α-Si 3N 4 into SiO 2 ceramics to inhibit crystallization. The results show that the introduction of α-Si 3N 4 can effectively inhibit crystallization of SiO 2 ceramics at temperature higher than 1450 °C. Only 5 wt% cristobalite form in SiO 2 ceramic with 20 wt% α-Si 3N 4 (heated at 1550 °C for 30min). The crystallization activation energy of SiO 2 ceramic containing 20 wt% α-Si 3N 4 increases by 2.27 times to 931.2kJ/mol compared with that of pure SiO 2 ceramic (409.6kJ/mol). The inhibition crystallization effect and increased activation energy derive from the in-situ formation of O–Si–N chemical bond and physical isolation of SiO 2 particles by α-Si 3N 4 powders. 相似文献
7.
The Si 3N 4 coating and Si 3N 4 coating with Si 3N 4 whiskers as reinforcement (Si 3N 4w-Si 3N 4) were prepared by chemical vapor deposition (CVD) on two-dimensional silicon nitride fiber reinforced silicon nitride ceramic matrix composites (2D Si 3N 4f/Si 3N 4 composites). The effects of process parameters of as-prepared coating including the preparation temperature and volume fraction of Si 3N 4w on the microstructure and mechanical properties of the composites were investigated. Compared with Si 3N 4 coating, Si 3N 4w-Si 3N 4 coating shows more significant effect on the strength and toughness of the composites, and both strengthening and toughening mechanism were analyzed. 相似文献
8.
Silicon nitride (Si 3N 4) was synthesized under a nitrogen gas flow (100 mL/min) using a molten salt nitriding method to investigate the effects of the temperature and NaCl content on the α-Si 3N 4 content in products and their micro-morphologies. Adding NaCl and β-Si 3N 4 in silicon powders resulted in Si nitridation products divided into two layers. Analysis of the lower product using X-ray diffraction revealed a change in the α-Si 3N 4 content with changes in the temperature and NaCl content. Analysis of the lower and upper layers using scanning electron microscopy revealed that the upper layer contained Si 3N 4 nanowires, Si 3N 4 nanobelts, and clastic oxide impurities; the lower one contained short needle-like and blocky Si 3N 4. From the microstructures of the products, the product morphology related to that the dry mixing procedure did not correspond to homogenization of the starting Si-Si 3N 4-NaCl mixtures and the different concentrations of raw materials resulted in different morphologies. 相似文献
9.
A dense β-Si 3N 4 coating toughened by β-Si 3N 4 nanowires/nanobelts was prepared by a combined technique involving chemical vapor deposition and reactive melt infiltration to protect porous Si 3N 4 ceramics in this work. A porous β-Si 3N 4 nanowires/nanobelts layer was synthesized in situ on porous Si 3N 4 ceramics by chemical vapor deposition, and then Y–Si–Al–O–N silicate liquid was infiltrated into the porous layer by reactive melt infiltration to form a dense composite coating. The coating consisted of well-dispersion β-Si 3N 4 nanowires/nanobelts, fine β-Si 3N 4 particles and small amount of silicate glass. The testing results revealed that as-prepared coating displayed a relatively high fracture toughness, which was up to 7.9 ± 0.05 MPa m 1/2, and it is of great significance to improve thermal shock resistance of the coating. After thermal cycling for 15 times at ΔT = 1200 °C, the coated porous Si 3N 4 ceramics still had a high residual strength ratio of 82.2%, and its water absorption increased only to 6.21% from 3.47%. The results will be a solid foundation for the application of the coating in long-period extreme high temperature environment. 相似文献
10.
Using Si and BN powders as raw materials, silicon nitride/hexagonal boron nitride (Si 3N 4/BN) ceramic composites were fabricated at a relatively low temperature of 1450 °C by using the reaction bonding technology. The density and the nitridation rate, as well as the dimensional changes of the specimens before and after nitridation were discussed based on weight and dimension measurements. Phase analysis by X-ray diffraction (XRD) indicated that BN could promote the nitridation process of silicon powder. Morphologies of the fracture surfaces observed by scanning electron microscopy (SEM) revealed the fracture mode for Si 3N 4/BN ceramic composites to be intergranular. The flexural strength and Young's modulus decreased with the increasing BN content. The reaction-bonded Si 3N 4/BN ceramic composites showed better machinability compared with RBSN ceramics without BN addition. 相似文献
11.
This report describes an investigation of the synthesis of β-Si 3N 4 particles from α-Si 3N 4 particles. The β fraction of Si 3N 4 particles was found to depend on temperature, heating time, and the type of crucibles in which the Si 3N 4 particles were heated. When Si 3N 4 particles were heated in a crucible made of carbon, most α-Si 3N 4 particles converted to β-Si 3N 4 after heating at 2000°C for 90 min in an atmosphere of N 2 of 9 kgf/cm 2. The morphology of the resulting β-Si 3N 4 particles appeared as a whisker shape. When Si 3N 4 particles were heated in a crucible made of boron nitride, most α-Si 3N 4 particles converted to β-Si 3N 4 after heating at 2000°C for 480min in an atmosphere of N 2 of 9kgf/cm 2. The resulting morphology was equiaxed. It is suspected that the transformation occurs via the gas phase and is affected by the partial pressure of oxygen in the atmosphere. 相似文献
12.
The influence of the Schmidt number on the incorporation of SiC, Si2N2O and Si3N4 precipitate formation during directional solidification of mc-silicon was investigated. SiC, Si2N2O and Si3N4 particles that precipitate in multi-crystalline silicon for PV application have detrimental effects on the wafer sawing process and solar cell performance. Time-dependent numerical modelling of the mass transport phenomena has been used with a diffusion model. There is still no study on the Schmidt number during the mc-silicon growth process. So, the numerical study has been investigated on the formation of second phase inclusions in mc-silicon for various Schmidt numbers. It may be used to control the precipitate of SiC, Si2N2O and Si3N4 in a mc-silicon ingot. In this paper, the second phase inclusions that precipitate in the mc-silicon ingot have been simulated and analyzed for three different Schmidt numbers Sc = 1, Sc = 10 and Sc = 100. From the obtained results, Schmidt number Sc = 1 is found to grow better quality mc-silicon crystals. 相似文献
13.
The effect of Co nanoparticles (NPs) on the nitridation of silicon (Si) was studied. Co NPs were deposited homogeneously on the surfaces of Si powders using an in situ reduction method using NaBH 4 as a reducing reagent. Si powders impregnated with 0.5–2.0 wt% Co NPs were nitrided in 1200°C–1400°C for 2 h. The resultant silicon nitride powders were characterized by XRD, FE‐SEM, TEM, and EDS. The results showed that: (1) Co NPs significantly decreased the Si nitridation temperature, and the nitridation could be completed at 1300°C upon using 2 wt% Co NPs as catalysts. For comparison, the Si conversion could not be completed even at a temperature as high as 1400°C in the case without using a catalyst; (2) many Si 3N 4 whiskers with 80–320 nm in diameter and tens micrometers in length were generated and uniformly distributed in the final products. They were single‐crystalline α‐Si 3N 4 grown along the [101] direction. The enhanced nitridation in the case of using Co NPs as a catalyst was attributed two following factors, the increased bond length and weakened bond strength in N 2 caused by the electron donation from the Co atoms to the N atoms. 相似文献
14.
We have combined first-principle calculations of charge transfer at the Si/Si 3N 4 interface with the interaction potential models for bulk Si and Si 3N 4 to produce a model for the Si/Si 3N 4 interface. Using these interatomic potentials, million atom molecular dynamics simulations have been performed to characterize the structure of Si(111)/Si 3N 4(0001) and the Si(111)/a-Si 3N 4 interfaces. Ten million-atom simulations are performed using multiresolution molecular-dynamics method on parallel computers. Atomic stress distributions are determined in a 54 nm nanopixel on a 0·1 μm silicon substrate. Effects of surfaces, edges, and lattice mismatch at the Si(111)/Si 3N 4(0001) interface on the stress distributions are also investigated. Stresses are found to be highly inhomogeneous in the nanopixel—the top surface of silicon nitride has a compressive stress of +3 GPa and the stress is tensile, −1 GPa, in silicon below the inter-face. These simulation methods can also be applied to other semiconductor/ceramic interfaces as well as to metal/ceramic and ceramic/ceramic interfaces. 相似文献
15.
Si 2N 2O ceramics were prepared by plasma activated sintering using nanosized amorphous Si 3N 4 powder without sintering additives within a temperature range of 1400°C–1600°C in vacuum. A mixed Si–N 4?n–O n ( n = 0, 1…4) amorphous structure was formed in the process of sintering, and Si 2N 2O crystals were nucleated where the local structure was similar with Si 2N 2O. After sintering at 1600°C, the Si 2N 2O ceramic was composed of elongated plate‐like Si 2N 2O grains and amorphous phase. The Si 2N 2O grains showed a width of less than 100 nm and a very high aspect ratio. 相似文献
16.
In this work atmospheric plasma spraying of SiC and Si 3N 4 was investigated. Plasma spraying of these ceramics raises several problems since they would tend to decompose instead of melting at elevated temperatures during the process. To circumvent this problem the nonoxide ceramics were deposited as a composite powder mixed with nonoxide ceramic particles resulting in a ceramic/ceramic composite structure. Our findings were that using such a composite feedstock powder both oxidation and decomposition of the nonoxide particles could be avoided. A vitrified phase was also developed in the coating. 相似文献
17.
Silicon nitride-monoclinic barium feldspar (Si 3N 4-m-BAS) composite possesses great dielectric properties, low density, and low thermal expansion coefficient (CTE). Preparing dense Si 3N 4-m-BAS coating on porous Si 3N 4 ceramic is an effective strategy to improve its water resistance and ensure its dielectric performances. However, this promising coating has not been reported yet, because the synthesis of m-BAS is difficult, and the densification of Si 3N 4-BAS composite requires very high temperature. Here, the BaO-Al 2O 3-SiO 2 glass/Si 3N 4-BAS coating was first fabricated by a manual spray method and pressureless sintering at 1450°C. Combining the influence of Si 4+ on the crystal phase composition of BAS and the volume expansion effect of silicon in N 2, an effective coating structure design scheme was proposed. By changing the content of silicon powder, the CTE and horizontal shrinkage of the coating during sintering were controlled. Besides, the prepared coatings exhibited low water absorption and high bonding strength. During the thermal shock tests, SiO 2 produced by the oxidation of Si 3N 4 healed the cracks in the coating, thus delaying the degradation of the properties. The coating prepared in this work is expected to be applied to radome in extreme service environments. 相似文献
18.
A new macromolecular coupling agent butyl acrylate (BA)‐methyl methacrylate (MMA)‐vinyl triethoxy silane (VTES) tercopolymer was synthesized using solution polymerization initiated by free radical initiator benzoyl peroxide (BPO) and dicumyl peroxide (DCP). Dodecylthiol is choosed as the chain transfer to control the molecule weight of this tercopolymer. The terpolymer's molecular structure was confirmed by FTIR and NMR, and its average molecular weight was determined by GPC. In this work, the tercopolymer BA–MMA–VTES is used for surface modification of silicon nitride (Si 3N 4) nanopowder. The structure surface properties and thermal stability of modified nano‐Si 3N 4 were systematically investigated by FTIR, TGA, TEM, and size distribution analyzer. The results show that the macromolecular coupling agent bonds covalently on the surface of nano‐sized Si 3N 4 particles and an organic coating layer is formed. The optimum loading of this macromolecular coupling agent BA–MMA–VTES tercopolymer is 5% (wt %) of nano‐sized Si 3N 4. TEM also reveals that modified nano‐Si 3N 4 possesses good dispersibility. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 相似文献
19.
In this work, a macromolecular coupling agent (BA‐MAA‐AN tercopolymer) was used for surface modification of native nano‐sized silicon nitride (Si 3N 4) powder. This modification strategy was designed for preparing nano‐Si 3N 4/NBR composites. The structure and surface properties of modified nano‐Si 3N 4 were systematically investigated by FTIR, XPS, TGA, TEM, Size Distributions Analyzer, and Contact Angle Measurement. It was found that, the optimum loading of BA‐MAA‐AN tercopolymer coated on the surface of nano‐sized Si 3N 4 is 10% of nano‐Si 3N 4. According to the spectra of FTIR, XPS and TGA, it can be inferred that this macromolecular coupling agent covalently bonds on the surface of nano‐sized Si 3N 4 particles and an organic coating layer is formed. The contact angle experiments show that the hydrophobic property of nano‐sized Si 3N 4 modified with macromolecular coupling agent is improved obviously. TEM reveals that modified nano‐Si 3N 4 possesses good dispersibility and the average diameter in NBR is less than 100 nm. It has also been found that the oil resistance of NBR based nanocomposites is improved greatly due to the modified nano‐Si 3N 4. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 相似文献
20.
Post-reaction sintering of a powder compact of Si and sintering aids is a useful technique for fabricating silicon nitride (Si 3N 4) ceramics at low costs. In order to inhibit the inhomogeneous and uncontrollable exothermic nitridation of Si in the powder compact, Si–Y 2O 3–Al 2O 3 nanocomposite particles are designed as an aid for post-reaction sintering. These Si–Y 2O 3–Al 2O 3 nanocomposite particles are prepared via mechanical treatment applying high shear stress. Scanning electron microscopy (SEM) observations show that Y 2O 3 and Al 2O 3 particles are homogenously dispersed, and fixed to the Si particles. A green compact prepared using the Si–Y 2O 3–Al 2O 3 nanocomposite particles results in lower electrical resistivity than that prepared using a powder mixed by wet ball-milling, which suggests that Si particles in the green compact prepared using the nanocomposite particles are isolated by Y 2O 3 and Al 2O 3 particles. The isolation of Si particles by the sintering aids successfully prevents the Si particles from melting and agglomerating during the nitridation process, resulting in a higher nitridation ratio and higher α-Si 3N 4 phase content due to the inhibition of rapid heat transfer caused by the exothermic reaction. The nitridation ratio also increases with the applied power during mechanical treatment. As a result of firing the homogeneously nitrided powder compacts at high temperatures, Si 3N 4 ceramics with homogeneous microstructure and improved density are successfully fabricated in this manner. 相似文献
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