α-Si3N4 Content Research Articles

The Study on Porosity Controllable Filter Material for the Integrated Gasification Combined Cycle

The porous ceramic filter material is the most effective filter materials in the integrated gasification combined cycle. The porous silicon nitride due to its higher mechanical strength, as well as good corrosion resistance, is considered as a promising material in the integrated gasification combined cycle, and it were prepared directly though the SiO2 and α-Si3N4 through carbothermal reduction - pressureless sintering in nitrogen. There was a great lot weight loss in this reaction, so it was expected to create the high porosity materials. The rod-like β-Si3N4 grains and uniform pores were formed through changing the content of α-Si3N4, SiO2 and C. So high-performance and porosity controlled porous silicon nitride filter material was obtained. With an increasing in the α-Si3N4 content, the weight loss, the linear shrinkage, and the porosity decreased, the flexural strength increased accordingly. The porous silicon nitride filter material with addition of 50wt% α-Si3N4 showed a higher aspect ratio β-Si3N4 grains and better mechanical properties .

Performance improvement of Si3N4 ceramic cutting tools by tailoring of phase composition and microstructure

Silicon nitride (Si3N4) ceramic cutting tool with excellent cutting performance was successfully achieved through the control of phase composition and microstructure. The hardness of the tool with 49.7 wt% and 4.9 wt% α-Si3N4 is ~20.1 GPa and ~17.5 GPa, respectively. In continuous cutting of cast iron, the tool life significantly increases from ~1200 m to ~2400 m when α-Si3N4 content increases from 4.9 wt% to 49.7 wt%, respectively. The longer tool life is attributed to a better abrasive and adhesive wear resistance of α-Si3N4 phase. In addition, the chemical reaction between Si3N4 ceramic tool and cast iron would lead to the formation of an amorphous oxide layer on wear surface. It was noted that the higher α-Si3N4 content would lead to greater probability of chipping due to lower fracture toughness. Therefore, high-performance Si3N4 ceramic tool could be achieved using an α-Si3N4 matrix ceramic reinforced with elongated β-Si3N4 phase.

A novel route for the fabrication of porous Si3N4 ceramics with unidirectionally aligned channels

TBA-based freeze-drying and combustion synthesis (CS) are firstly combined to fabricate porous Si3N4 ceramics with controllably unidirectional channels. Uniformly unidirectional channels with variable pore diameters were successfully formed through unidirectional growth of ice. In addition, the residual α-Si3N4 content, development of grain morphology and mechanical property of sintered specimens were significantly influenced by the solid content of slurry. Porous Si3N4 ceramics with porosity of 62.5–78.8%, axial compressive strength of 7.3–61.6Mpa, and transverse compressive strength of 3.3–41.6 MPa were obtained with increasing solid content.

Low-Temperature Preparation of α-Si3N4 Powder via Combined High-Energy Ball Milling and Molten Salt Nitridation Method

In this work, α-Si3N4 powder was synthesized by a combined high-energy ball milling and molten salt nitridation method using silicon powder as raw materials, and NaCl-NaF binary molten salt as reaction medium and diluents. The effects of nitridation temperature, soaking time, and NaF content in the molten salt on the formation of α-Si3N4 were investigated. The results show that the nitridation of silicon powder was completed at NaF content of 10wt% in the molten salt after nitridation at 1200°C and held for 4 h. The α-Si3N4 content in the as synthesized powder is 96wt%. The α-Si3N4 whiskers with 40-280 nm in diameter and several microns to tens of microns in length appear in the final products. The crystal growth process of α-Si3N4 whiskers follows vapor-crystal (VC) mechanism.

Effect of Si particle size and NH4Cl additive on combustion synthesis of α-Si3N4

High α-phase content Si3N4 powders were fabricated by combustion synthesis method using coarse Si powder with NH4Cl additive. The effects of Si particle size and NH4Cl additive on the macrokinetic feature, phase composition, microstructure of combustion synthesized α-Si3N4 powder were investigated. Without the addition of NH4Cl, the content of α-Si3N4 in the products first decreases under a critical Si particle size of 7.8 μm, and then increases with increasing Si particle size. With the addition of 6 wt% NH4Cl, high α-phase content Si3N4 powder (>90 wt%) was obtained by using coarse (19.6 μm) Si powder. The addition of NH4Cl reduces the maximum combustion temperature and retention time of liquid Si by promoting the gasification of Si, which lead to the increase of α-Si3N4 content in the product. The use of coarse Si powder is of great significance for the low-cost production of high-quality α-Si3N4 powder.

Combustion synthesis of α-Si3N4 assisted by molten salt additives

Combustion synthesis (CS) of silicon nitride (Si3N4), assisted by molten salt additives under high N2 pressure, is reported. The effect of salt additives (NaCl, MgCl2, and MgCl2∙6H2O) on the reaction temperature and the final α-Si3N4 content is studied. The maximum reaction temperature (Tmax) decreased with an increase in the amount of the salt additives. NaCl is found to be the most suitable as it results in 57.8% α-Si3N4 at 30 mass% concentration. MgCl2 is strongly hygroscopic, and MgCl2∙6H2O decomposes at very low temperature. Therefore, they absorb heat at low temperatures, which makes it difficult to reach the ignition temperature, thereby hindering the reaction propagation. Si3N4 is necessary as a diluent for creating pores in the raw materials to allow effective penetration and contact of N2 gas with the Si particles.

Combustion synthesis of α-Si3N4 with green additives

This paper reports the combustion synthesis of α-Si3N4 using green additives of water and alcohol. The addition of water and alcohol is demonstrated to be effective in controlling the reaction kinetics and improving the formation of α-Si3N4 by vapor reactions. With increasing proportion of additives in the starting composition, the content of α-Si3N4 in the product is clearly enhanced. In contrast to the ammonium halides usually employed as additives in the combustion synthesis of α-Si3N4, the green additives used here are nontoxic, noncorrosive, and thus more attractive for industrial applications.

Combustion synthesis of α-Si3N4 with the addition of NH4Cl

This paper reports combustion synthesis of α-Si3N4 with the addition of NH4Cl. The effect of NH4Cl on the phase composition of the products is investigated, and the reaction mechanism in the nitridation of Si with the participation of NH4Cl is discussed. The content of α-Si3N4 in the products increases with increasing content of NH4Cl in the starting compositions, and reaches 93% when 10 wt% NH4Cl is added. It is proposed that the addition of NH4Cl promotes the nitridation of Si by gaseous reaction and improves the formation of α-Si3N4. In the reaction, NH4Cl decomposes into HCl and NH3, and HCl reacts with Si to form SiCl4, which is later nitridized into Si3N4. In the nitridation of Si, HCl acts as a catalyst and a carrier to transfer Si atoms from solid and liquid Si particles into gas phase. The contribution of NH3 to the nitridation of Si is limited, and most N atoms required for the synthesis of Si3N4 are provided by N2.

Formation of Different Si3N4 Nanostructures by Salt-Assisted Nitridation.

Silicon nitride (Si3N4) products with different nanostructure morphologies and different phases for Si3N4 ceramic with high thermal conductivity were synthesized by a direct nitriding method. NaCl and NH4Cl were added to raw Si powders, and the reaction was carried out under a nitrogen gas flow of 100 mL/min. The phase composition and morphologies of the products were systemically characterized by X-ray diffraction, field emission scanning electron microscopy, and high-resolution transmission electron microscopy. At 1450 °C, the NaCl content was 30 wt %, the NH4Cl content was 3 wt %, and the maximum α-Si3N4 content was 96 wt %. The process of Si nitridation can be divided into three stages by analyzing the reaction schemes: in the first stage (25-900 °C), NH4Cl decomposition and the generation of stacked amorphous Si3N4 occurs; in the second stage (900-1450 °C), NaCl melts and Si3N4 generates; and in the third stage (>1450 °C), α-Si3N4 → β-Si3N4 phase change and the evaporation of NaCl occurs. The products are made of two layers: a thin upper layer of nanowires containing different nanostructures and a lower layer mainly comprising fluffy, blocky, and short needlelike products. The introduction of NaCl and NH4Cl facilitated the evaporation of Si powders and the decomposition of Al2O3 from porcelain boat and furnace tube, which resulted in the mixing of N2, O2, Al2O, and Si vapors and generated Al xSi yO z nanowires with rough surfaces and lead to thin Si3N4 nanowires, nanobranches by the vapor-solid (VS), vapor-liquid-solid (VLS), and the double-stage VLS base and VS tip growth mechanisms.

Formation mechanisms of Si3N4 microstructures during silicon powder nitridation

Silicon nitride (Si3N4) was synthesized under a nitrogen gas flow (100mL/min) using a molten salt nitriding method to investigate the effects of the temperature and NaCl content on the α-Si3N4 content in products and their micro-morphologies. Adding NaCl and β-Si3N4 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-Si3N4 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 Si3N4 nanowires, Si3N4 nanobelts, and clastic oxide impurities; the lower one contained short needle-like and blocky Si3N4. 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-Si3N4-NaCl mixtures and the different concentrations of raw materials resulted in different morphologies.

Preparation of Si3N4-TaC and Si3N4-ZrC composite ceramics and their mechanical properties

Si3N4-TaC and Si3N4-ZrC composite ceramics with sintering additives were consolidated in the sintering temperature range of 1500–1600°C using a resistance-heated hot-pressing technique. The addition of 20–40mol% carbide improved the sinterability of the ceramics. The ceramics were densely sintered under 0–40mol% TaC or ZrC at 1500°C, 0–80mol% TaC at 1600°C, and 0–60mol% ZrC at 1600°C. In ceramics sintered at 1500°C, the proportion of α-Si3N4 was larger than that of β-SiAlON; α-Si3N4 transformed mostly to β-SiAlON at 1600°C. Carbide addition was effective in inhibiting α-Si3N4-to-β-SiAlON phase transformation. Young's modulus for the dense Si3N4-TaC and Si3N4-ZrC ceramics increased with the carbide amount, and the hardness of dense Si3N4-ZrC and Si3N4-TaC ceramics increased from 14GPa to 17GPa with increasing α-Si3N4 content. Dense Si3N4-TaC and Si3N4-ZrC ceramics, with larger quantities of α-Si3N4 sintered at 1500°C, exhibited high hardness; the fracture toughness of these ceramics decreased with increasing α-Si3N4 proportion. Both the hardness and fracture toughness of the dense Si3N4-TaC and Si3N4-ZrC ceramics were strongly related to the proportion of α-Si3N4 in the sintered body.

Phase composition of thin silicon carbonitride films obtained by plazma endanced chemical vapour deposition using organosilicon compounds

High-temperature silicon carbonitride films are synthesized by plasma decomposition of gas mixtures of 1,1,1,3,3,3-hexamethyldisilazane (HMDS) (the synonym used by IUPAC bis(trimethylsilyl) amine) with ammonia or helium in the temperature range of 673–1273 K. It is shown that silicon carbonitride films, obtained in high temperature processes of the plasma decomposition of organosilicon compounds, are nanocomposite. In their amorphous matrix the crystals belonging to the phases of the α-Si3−nCnN4 family and impurity graphite are embedded. To clarify the previously obtained data by means of the X-ray diffractometry using synchrotron radiation, they are compared with the published results of modeling the structure of these phases. It is shown that nanocrystals belonging to the phases of α-Si3N4, α-Si2CN4, α-SiC2N4, and α-C3N4 are present in the films. An increase in the ammonia concentration in the initial gas mixture causes a decrease in the film hardness from 24 GPa to 16 GPa due to the increased content of α-Si3N4 and α-Si2CN4 nanocrystals having a lower hardness compared to α-C3N4 and α-SiC2N4.

Effect of dilution and additive on direct nitridation of ferrosilicon

Abstract Ferrosilicon nitrides were fabricated by direct nitridation of ferrosilicon, and the effects of ferrosilicon nitride and α-Si3N4 dilution and ammonium chloride (NH4Cl) additive on characteristics of the nitride products were investigated. The phases of final nitride products consist of α-Si3N4, β-Si3N4 and iron silicides with various compositions. Long columnar crystals, equiaxed crystals and flower-like crystals were observed in the final products, and fiber or whisker-like crystals with a low degree of crystallization were also found in the nitrides. Both the diluents and additive can improve the nitridation degree, but only the additive can increase the content of α-Si3N4 in the nitride products. As a diluent, ferrosilicon nitride can decrease the cost of nitride products, and increase their nitridation degree more than α-Si3N4. NH4Cl made the products fluffy and expansive, while the dimensions of the compact without addition of NH4Cl remain unchanged during nitridation.

Effects of whisker-like β-Si3N4 seeds on phase transformation and mechanical properties of α/β Si3N4 composites using MgSiN2 as additives

α/β Si3N4 composites with β-Si3N4 content ranging from 26% to 100% were hot-pressed with or without β-Si3N4 seeds, using MgSiN2 as additives, and their mechanical properties were investigated. When the α-Si3N4 content was over 58%, the microhardness of α/β Si3N4 composites was in the range of 23–24GPa, and then the indentation hardness decreases with decreasing the content of α-Si3N4, whether with β-Si3N4 seeds or not. The toughness increased with increasing elongated β-Si3N4 grains, which improved fracture resistance by crack bridging, pull out or the crack deflection mechanism, and reached the maximum value of 7.0MPam1/2 with 1wt% β-seeds. In comparison with α/β Si3N4 composite with a similar phase composition, the fracture strength was improved by adding β-Si3N4 seeds because of the relatively smaller grain sizes and higher toughness. The α/β Si3N4 composite with 5wt% β-seeds showed a high strength of 1253MPa, a high hardness of 20.9GPa and a toughness of 6.9MPam1/2.

A New Synthesis Method of α-Silicon Nitride Powder-Reductive Combustion Synthesis from Silicon and Silicon Dioxide

A new synthesis method of Si3N4 powder with a high α-phase content, namely reductive combustion synthesis, was pioneered in the present work. Some fundamental experiments were carried out and the result indicated that the content of α-Si3N4 increases with the addition of SiO2 and C, which considerably enhances the efficiency and drastically reduces the cost. The new method not only confers the advantage of carbothermal reduction and nitridation technology, but overcomes the shortcomings of a conventional combustion synthesis process.

Combustion Synthesis of Si<sub>3</sub>N<sub>4</sub> from Si/NH<sub>4</sub>Cl at Low Nitrogen Pressure

Combustion synthesis (CS) of Si3N4 was accomplished by using as-milled Si/NH4Cl as reactants at low nitrogen pressure. The additive of NH4Cl decreased the combustion temperature and promoted the Si nitridation. Full nitridation of Si was achieved by burning Si in pressurized nitrogen with 10 ~ 25 wt. % NH4Cl as additives while no Si3N4 diluent added. The maximum combustion temperature (Tc), the combustion velocity (u) together with the α-Si3N4 content and mean particle size (d50) of the powder products were found to be great dependent on the NH4Cl content added in the reactants. Fine Si3N4 powder products with α-phase content up to 85 wt. % were obtained via steady combustion mode. A mathematical approach named combustion wave velocity methods for the analysis of temperature profiles in CS was proposed and the reaction kinetics was discussed. The apparent activation energy calculated according to the temperature profile analysis method is 29.7 kJ/mol, which agrees well with the corresponding low temperature nitriding combustion of Si.

Phase composition and morphology of products of combustion of ferrosilicon in nitrogen

The results of a study of silicon nitride phase formation in combustion of ferrosilicon in gaseous nitrogen are reported. It was shown that formation of α-or β-modifications of silicon nitride is basically determined by the composition of the batch for self-propagating high-temperature synthesis. When ammonium chloride was added to the initial ferrosilicon, a combustion product with a high (up to 80%) α-Si3N4 content is formed, while dilution with the final product and magnesium fluoride results in predominant (more than 95%) formation of β-Si3N4. The particle size and shape are a function of the conditions of synthesis and are primarily determined by the temperature and the additives incorporated in the initial alloy.

Thermal conductivity of reaction bonded Si 3N 4

To investigate the effect of α- and β-Si3N4 content on the thermal conductivity of silicon nitride ceramics, six samples containing various quantities of α-Si3N4 were prepared by the reaction bonding process and the thermal conductivity of the reaction bonded Si3N4 (RBSN) was measured by the transient hot wire method at various temperatures from room temperature to 1000°C. The RBSN samples containing 1, 25, 33, 45, 55 and 66% α-Si3N4 were produced by nitriding the compacts of pure silicon powder in a nitrogen atmosphere at 1480°C for prescribed periods. Bulk density, true density and total porosity of samples were 2.51±0.02g/cm3, 3.08±0.03g/cm3 and 18.6±1.0%, respectively, regardless of the α-content. No significant change in microstructure of RBSN samples was observed by scanning electron microscopy With an increase in the content of α-Si3N4, the thermal conductivity (λ) of BRSN samples decreased from λ=17.4Wm-1K-1 for the sample with α-Si3N4 of 1% (viz. consisted mainly of β-Si3N4) to λ=9.3Wm-1K-1 for that of 66%. It was considered that the α-Si3N4 solid solutions by which the thermo-elastic wave carrying the heat energy in the non-electro-conductive solid materials such as Si3N4 ceramics is scattered, might be formed with the impurities included in silicon powder as a starting material. The thermal conductivity of all RBSN samples decreased simply with increasing temperature due to the increase of the mutual scattering of the thermo-elastic wave, and the decreasing rate of thermal conductivity for the RBSN with lower α-contents was found to be larger than that for those with higher α-contents.