In situ preparation of SiC/Si3N4-NW composite powders by combustion synthesis

Large-scale composite powders containing silicon carbide (SiC) particles and silicon nitride nanowires (Si₃N₄-NWs) were synthesized in situ by combustion synthesis (CS). In this process, a mixture of silicon, carbon black, polytetrafluoroethylene (PTFE) and a small amount of iron powders was used as the precursor. The products were characterized by XRD, SEM, EDS and TEM. The particles are equiaxed with diameters in the micron range, and the in situ formed nanowires are straight with uniform diameters of 20-350 nm and lengths of tens of microns. The Si₃N₄-NWs are characterized to be α-phase single crystals grown along the [1 0 1] or [1 0 0] direction. VLS and SLGS processes are proposed as the growth mechanisms of the nanowires. The as-synthesized powders have great potential for use in the preparation of high-performance SiC/Si₃N₄-NW composites.     

Transformation of polysilicon cutting waste into SiC/α-Si3N4 composite powders via electromagnetic induction heating

In order to use polysilicon cutting waste more efficiently, SiC/α-Si₃N₄ composite powders were prepared via electromagnetic induction heating at ambient pressure. Factors affecting synthesis and reaction mechanisms of the SiC/α-Si₃N₄ composite powders were investigated. Results show that the morphology of obtained α-Si₃N₄ consists of particles equiaxed with diameter in the micron range and one-dimensional Si₃N₄ nanowires with diameter of tens to hundreds of nanometers and length of up to tens of microns. It was found that the addition of NH₄Cl can promote nitridation by controlling reaction temperature and participating in the reaction. In addition, the existence of Fe₂O₃ impurities was found to be beneficial to the formation of α-Si₃N₄ in reaction process by removing the SiO₂ film from silicon and providing FeSi₂ liquid phase as effective pass for vapor phase. Mechanisms used to grow Si₃N₄-NWs were vapor-solid (VS) and vapor-liquid-solid (VLS) depositions.     

Combustion of silicon powders containing organic additives in nitrogen gas under pressure: 2. Composition of combustion products

Combustion of silicon powders containing organic dopants in nitrogen gas under pressure was found to yield a mixture of α-Si₃N₄, β-Si₃N₄, SiC, and Si₂N₂O. Relative amount of these compounds in combustion product was found to depend on the pressure of nitrogen gas, type and concentration of dopants, combustion geometry, and cooling rate. The formation of α-Si₃N₄ was found to occur in the presence of oxygen-containing dopants. The type of dopant was also found to affect the morphology of product particles.      

Preparation and characterization of porous Si3N4-bonded SiC ceramics and morphology change mechanism of Si3N4 whiskers

Porous Si₃N₄-bonded SiC ceramics with high porosity were prepared by the reaction-sintering method. In this process, Si₃N₄ was synthesized by the nitridation of silicon powder. The X-ray diffraction (XRD) indicated that the main phases of the porous Si₃N₄-bonded SiC ceramics were SiC, α-Si₃N₄, and β-Si₃N₄, respectively. The contents of β-Si₃N₄ were increased following the sintering temperature. The morphology of Si₃N₄ whiskers was investigated by scanning electron microscope (SEM), which was shown that the needle-like (low sintering-temperature) and rod-like (higher sintering-temperature) whiskers were formed, respectively. From low to high synthesized temperature, the highest porosity of the porous Si₃N₄ bonded SiC ceramic was up to 46.7%, and the bending strength was ~11.6?MPa. The α-Si₃N₄ whiskers were derived from the reaction between N₂ and Si powders, the growth mechanism was proved by Vapor–Solid (VS). Meanwhile, the growth mechanism of β-Si₃N₄ was in accordance with Vapor–Solid–Liquid (VSL) growth mechanism. With the increase of sintering temperature, Si powders were melted to liquid silicon and the α-Si₃N₄ was dissolved into the liquid then the β-Si₃N₄ was precipitated successfully.     

Synthesis of Si3N4–TiN–SiC composites by combustion reaction under high nitrogen pressures

Si₃N₄–TiN–SiC composites were synthesized from TiSi₂ and SiC mixtures via the combustion reaction under high nitrogen pressure. The nitridation mechanism of TiSi₂ was analyzed. The results show that the nitridation of TiSi₂ produced TiN and Si firstly, and Si₃N₄ phase was formed by the further nitriding of Si. The molten eutectic phase and its agglomeration between Si and TiSi₂ formed one core-shell structure and affected the nitridation process. Under higher nitrogen pressure, the nitridation reaction was complete and the relatively dense Si₃N₄–TiN–SiC composites obtained. TEM observation revealed inhomogeneous Si₃N₄ grain size, amorphous phase, cavities, microcracks and dislocations, and graphite from the nitridation of SiC in the microstructure.     

Crystal Growth in the Combustion Synthesis of α‐Si3N4 Using Si with Different Particle Sizes

In this present work, Si₃N₄ powders with high α‐phase contents and distinct crystal morphologies were prepared via a promising approach of combustion synthesis (CS), using Si powders with different particle sizes as reactants. The influence of Si particle size on phase composition and crystal morphologies in the products was systematically investigated. Two unique crystal morphologies, radial‐spheroidal‐cluster and flowerlike, were observed in the Si₃N₄ products. The crystal growth mechanisms of Si₃N₄ granules in the CS system with disparate Si have been proposed based on experiments and thermokinetic calculations. As conclusion, the α‐phase content in the final product was synergetically dominated by the vaporization process of Si particles and the α–β phase transformation of Si₃N₄ during the after‐burn period. Si₃N₄ powders with high α‐phase content can be obtained from Si powders with an appropriate particle size.     

Combustion synthesis of α-Si3N4 powders using in-situ nano-SiO2 coated Si and Si3N4 reactants

Alpha phase silicon nitride (α-Si₃N₄) powders were synthesized by combustion reaction of the in-situ nano-SiO₂ coated Si and Si₃N₄ reactants with pressurized nitrogen. The combustion temperature profile as well as the product phase composition and morphology were investigated. Regardless of the combustion temperature reached as high as 1800 °C, up to 86 wt% α-Si₃N₄ was obtained in the combustion-synthesized product from the reactants with only 6 wt% SiO₂ addition, which is three times higher than that of without SiO₂ coating, meanwhile, the morphology of Si₃N₄ grains changed from rod-like to equiaxed grain，indicating the in-situ coated SiO₂ tailored the nitridation reaction path of Si successfully by enhancing the silicon monoxide (SiO) gas phase formation.     

Silicon nitridation mechanism in reaction‐bonded Si3N4–SiC and Si3N4‐bonded ferrosilicon nitride

Reaction‐bonded Si₃N₄–SiC and Si₃N₄‐bonded ferrosilicon nitride, with Si powder, SiC particles and Fe₃Si–Si₃N₄ particles as raw materials, respectively, are prepared in flame‐isolation nitridation shuttle kiln with flowing N₂ at 1723K. There is columnar β‐Si₃N₄ in both Si₃N₄–SiC and Si₃N₄‐bonded ferrosilicon nitride. However, fibrous α‐Si₃N₄ is only observed in Si₃N₄–SiC and Si₃N₄‐bonded ferrosilicon nitride contains much more Si₂N₂O than Si₃N₄–SiC. By analyzing the oxidation thermodynamics of Si and Si₃N₄, it is known that in the process of producing Si₃N₄–SiC, Si is oxidized first to gaseous SiO and fibrous α‐Si₃N₄ is generated with SiO and N₂. The existence of SiO is the reason of low silicon nitridation rate. But in the process of producing Si₃N₄‐bonded ferrosilicon nitride, Si₃N₄ is easier to be oxidized than Si and Si₂N₂O is generated on the surface of Si₃N₄ hexagonal prisms in ferrosilicon nitride particles. Meanwhile, Si in raw materials forms new ferrosilicon alloys with Fe₃Si, 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‐Si₃N₄ generation from Si direct nitridation and fibrous α‐Si₃N₄ transformation, which used to exist in ferrosilicon nitride raw materials.     

Effect of the Silicon Nitride Coating of Quartz Crucible on Impurity Distribution in Ingot‐Cast Multicrystalline Silicon

Multicrystalline silicon ingots were fabricated via the directional solidification method in a vacuum induction furnace using silicon nitride (Si₃N₄) as quartz crucible coating. The effect of Si₃N₄ coating on the impurity content, transference, and distribution in Si ingots was investigated. The results can be summarized as follows: The impurities contained in Si ingots, including Fe, Ca, Mn, Cu, P, and B, were found to decrease at varying degrees when the inner surface of the quartz crucible was coated with Si₃N₄. A mixed layer composted by Si₃N₄ and Si was observed on the surface of the Si ingot. This mixed layer provided microdefects for the nucleation of metallic impurities. The Fe and N contents in the Si/Si₃N₄ mixed layer changed with the same tendency, and FeSi₂ particles formed on the Si/Si₃N₄ crystalline boundary. Both Fe and Ca precipitated in the SiC particles near the Si ingot/Si₃N₄‐coating interface.     

In-situ formation of BN layers by nitriding boron powders in BN/Si3N4-based composite ceramics

Boron nitride/silicon nitride (BN/Si₃N₄) composite ceramics were fabricated via the in-situ nitridation of boron (B) and silicon (Si) powders in forming gas (95%N₂/5%H₂) at 1390?°C. The effect of the B content on the phase composition, microstructure, density/porosity, machinability as well as mechanical properties of nitridized BN/Si₃N₄ composite ceramics was investigated. The addition of B slightly increased the nitridation degree of the Si and B powders mixture, and improved the ratio of the β-Si₃N₄ phase significantly at low B contents. B powders may have acted as a nucleating agent to promote the formation of β-Si₃N₄ crystals. A core-shell Si₃N₄/BN structure was revealed by the TEM technique, and the number of BN layers increased with the increase of the B content. The in-situ BN formed by the nitridation of B played a similar role with the BN directly added in enhancing the machinability of the BN/Si₃N₄ composite ceramics. The method of the in-situ nitridation of B is also effective to prepare SiC fiber-reforced BN/Si₃N₄ ceramic matrix composites.     

3D printing Si3N4-bonded SiC refractories fabricated using colloidal films-containing slurries based on non-spherical SiC and Si powders

Stable slurries for Si₃N₄-bonded SiC refractories for direct ink writing (DIW) were successfully prepared from a mixture of non-spherical silicon carbide (SiC) and silicon (Si) powders with an average particle size of D₅₀ = 41.98 μm. The rheological properties and printability of slurries prepared using polyvinyl alcohol (PVA; 4–16 wt %) or hydroxypropyl methylcellulose (HPMC, 0.5–2 wt.%) were investigated with the effect of sintering temperature on the mechanical performance, phase, and microstructure of Si₃N₄-bonded SiC refractory products. The results indicated that slurries prepared with the HPMC solution showed better printability than those prepared with the PVA solution because colloidal films formed by HPMC in slurries play a role in encasing particles, preventing solid−liquid separation and contributing to plasticity and lubrication, which guarantees the smooth extrusion and homogeneity of slurries. The successful printing of SiC–Si slurries is not only related to proper viscosity, yield value, and shear thinning characteristics but it is also crucial for maintaining the homogeneity of slurries under extrusion pressure. Optimal SiC–Si slurries containing 52 vol % SiC–Si and 1.5 wt% HPMC exhibited proper viscosity, shear thinning, and homogeneity characteristics during printing. The obtained specimens achieved the best printing performance with height and section retention rates of 98.7% and 97.6%, respectively. When sintered at 1450 °C, Si₃N₄ fibres grow further and reach a diameter of 342.5 nm, the nitriding rate is 92.43%, the fibres tend to form a full network structure, and the mechanical properties of Si₃N₄-bonded SiC products are the best.     

Combustion synthesis of ultra-fine SiC powders in low pressure N2-atmosphere

Ultra fine SiC nano-powders (100–300 nm) of high purity were successfully produced by combustion of a powder mixture of Si and C, with the addition of poly-tetra-fluoro-ethylene (PTFE) as a chemical stimulator, in a moderately pressurized nitrogen atmosphere (1–10 MPa). The experimental results showed that with the aid of mechanical activation of the starting powders, a small amount of PTFE (1.5 wt%) can effectively stimulate the reaction between Si and C. Both the experimental results and thermodynamic calculations indicate that the formation of Si₃N₄ plays a key role in the process. The optimum conditions for producing the aforementioned SiC fine powders were 1.5 wt% PTFE, 1 MPa N₂ pressure and no addition of diluents of SiC powder.     

Combined effect of Fe-Si alloys and carbon on Si3N4 stability at elevated temperatures

The combined effect of carbon and Fe-Si alloys on Si₃N₄ was explored by heat treating Si₃N₄ materials at 1500?°C and 1600?°C in flowing nitrogen. The phase compositions and microstructures were characterized by XRD and SEM, respectively. The reaction degree was analysed based on the mass variation in the system. Combined with a thermodynamic assessment, the reaction mechanism was studied and proposed. The results show that the coexistence of Fe-Si alloys and carbon accelerates the phase transformation from Si₃N₄ to SiC and worsens the strength of Si₃N₄ materials. Fe-Si alloys accelerate the deposition of CO gas to free carbon and accelerate the decomposition of Si₃N₄ to Si. The in situ-formed Si can react with carbon, thus accelerating the thermodynamic and kinetic formation of SiC. Along with the growth of pores and the deterioration of the wettability of Fe-Si alloys during this process, the microstructure changes from a network constituted by Si₃N₄ columns/whiskers to porous SiC particles with weak linkages, which leads to the failure of Si₃N₄ materials. Therefore, the combined effect of Fe-Si alloys and carbon is harmful for Si₃N₄ materials at 1500–1600?°C.     

Silicon nitride–silicon carbide micro/nanocomposites: A review

This paper reviews investigations of silicon nitride–silicon carbide micro–nanocomposites from the original work of Niihara, who proposed the concept of structural ceramic nanocomposites, to more recent work on strength and creep resistance of these unique materials. Various different raw materials are described that lead to the formation of nanosized SiC within the Si₃N₄ grains (intragranular) and at grain boundaries (intergranular). The latter exert a pinning effect on the amorphous grain boundary phases in the silicon nitride and also act as nucleation sites for β-Si₃N₄, which limits grain growth during sintering. This finer microstructure results in strengths higher than for the monolithic silicon nitride. Intragranular SiC particles enhance strength and fracture toughness as a result of residual compressive thermal stresses within the nanocomposites. High temperature strength and creep resistance are also much higher than for monolithic silicon nitride and a few investigations of these topics are briefly reviewed and the proposed mechanisms are described. Within the context of other studies cited, work on Si₃N₄–SiC micro–nanocomposites by the current authors describes an aqueous processing route for better dispersion of commercial powders prior to sintering.     

Fabrication of Si3N4–SiC/SiO2 composites using 3D printing and infiltration processing

Si₃N₄–SiC/SiO₂ composites were prepared by employing three-dimensional (3D) printing using selective laser sintering (SLS) and infiltration processing. The process was based on the infiltration of silica sol into porous SLS parts, and silicon carbide and silicon nitride particles were bonded by melted nano-sized silica particles. To optimize the manufacturing process, the phase compositions, microstructures, porosities, and flexural strengths of the Si₃N₄–SiC/SiO₂ composites prepared at different heat-treatment temperatures and infiltration times were compared. Furthermore, the effects of the SiC mass fraction and the addition of Al₂O₃ and mullite fibers on the properties of the Si₃N₄–SiC/SiO₂ composites were investigated. After repeated infiltration and heat treatment, the flexural strength of the 3D-printed Si₃N₄–SiC/SiO₂ composite increased significantly to 76.48 MPa. Thus, a Si₃N₄–SiC/SiO₂ composite part with a complex structure was successfully manufactured by SLS and infiltration processes.     

The influence of mechanochemical activation on combustion synthesis of Si3N4

This study addresses itself in the performance of Si₃N₄ combustion synthesis, occurred in the presence of Si₃N₄ and NH₄Cl powders in N₂ atmosphere of 6 MPa. Mechanochemical activation of Si powder, achieved via high-energy attrition milling up to 24 h, increases the intensity and the efficiency of the reactions between Si and N₂ as well as combustion temperature. Benign processing conditions, anticipated with lower mechanochemical activation of Si powder, low N₂ pressures, and low combustion temperatures, favor formation of α-Si₃N₄.     

Properties of SiC,Si3N4 and SiO2 ceramic powders produced by vibration ball milling

SiC, Si₃N₄ and SiO₂ powders were ground by a vibration ball mill, the pot and balls of which were made of Si₃N₄ ceramics, in a purified methanol medium. The mean particle diameters of SiC and SiO₂ powders were ca. 0.1 μm and thus ultrafine particles were manufactured. The increasing rate of specific surface area by fine grinding was largest on SiO₂ powders, and the rate decreased gradually with an increase in grinding time. The specific surface area of Si₃N₄ powders increased proportionally with an increase in grinding time. The grindability was obtained in the following order: SiO₂ > SiC > Si₃N₄. The ground SiC powders showed a large lattice strain and little fragmentation of crystallites. The strain and the crystallite size of ground Si₃N₄ powders were moderate.     

Preparation of silicon carbide–silicon nitride composite foams from pre-ceramic polymers

A new method of forming silicon carbide–silicon nitride composite foams is presented. These are prepared by immersing a polyurethane foam in a polysilane precursor solution mixed with Si₃N₄ powder to form a pre-foam followed by heating it in nitrogen at >900°C. X-ray diffraction patterns indicate that a SiC–Si₃N₄ composite was formed after sintering the ceramic foam at >1500°C. Micrographs show that most of these foams have well-defined open-cell structures and macro-defect free struts. The shrinkage is reduced considerably due to the addition of Si₃N₄ particles.     

Effect of additives on slip casting rheology,microstructure and mechanical properties of Si3N4/SiC composites

The SiC/Si₃N₄ composites were fabricated with sintering process. To produce SiC/Si₃N₄ composite components, slurry mixtures containing Si/SiC powders were used by the slip casting method. In order to investigate the effect of dispersants and additives on the rheological properties and the body casted, slurries with concentration of 70% solid weight were prepared. It included a mixture of silicon and silicon carbide with weight ratios of 30 wt% and 70 wt%, respectively, and various weight percentages of Ball clay as lubricant and Tiron (sodium salt of benzene disulfonic acid) as dispersant at pH value of 7. After preparing the green bodies by slip casting method by using plaster mold, the samples were sintered at 1450 °C inside an atmospheric-controlled furnace under a pressure of 0.12 MPa of nitrogen gas for 2 h. By examining the rheological properties of the slurry and the sintering properties, it was concluded that the best slurry was obtained in terms of viscosity, density, porosity and strength using 5 wt% Ball clay and 0.5 wt% Tiron. Phase transformations, microstructure and morphology of the sintered specimens were accomplished by Field Emission Scanning Electron Microscopy (FESEM) examination and X-ray diffraction experimental analysis. XRD and FESEM results demonstrated that the composite fabricated by slurry containing 5 wt% Ball clay and 0.5 wt% Tiron had the least porosity without SiO₂ phase.     

燃烧合成制备Si3N4-TiN-SiC陶瓷的显微结构

以TiSi2为反应原料,SiC作稀释剂,燃烧合成制备Si3N4-TiN-SiC陶瓷.利用燃烧波"淬熄"法使反应各个阶段的物相得以保留,通过X射线衍射及扫描电镜分析TiSi2在燃烧合成中的反应过程及显微组织转化.结果表明:完全反应后产物的主相为Si3N4,其余为TiN和SiC.在燃烧过程中,TiSi2首先受热熔化,包覆于SiC颗粒表面,随后与N2反应生成TiN和Si.Si在高热作用下发生熔化、汽化,液态Si与未反应的TiSi2互溶.生成的Si与氮气发生反应,形成Si3N4晶核,并不断长大.燃烧合成反应过程中,Si3N4晶须的生长十分复杂,由气-液-固机制、气-固机制及蒸发凝聚的气相生长机制共同作用.