Combustion of the Fe-Si alloy in nitrogen gas

Combustion of the Fe-Si alloy in nitrogen gas was explored. The combustion was found to proceed in an autooscillating mode. Upon addition of nitrided ferrosilicon, magnesium fluoride, or ammonium chloride into a green mixture, the process becomes steady-state, while the extent of nitriding attains its maximum value. The phase composition and morphology of the product were found to depend on reaction conditions. The products formed in the presence of ferrosilicon or magnesium fluoride were found to largely contain columnar, platelike, and whiskerlike β-Si₃N₄ crystals (≥95%), while those in the presence of ammonium chloride contain silicon nitride with a granular structure and elevated content of the α-phase (<80%). Acid enrichment of combustion products was used to obtain silicon nitride with a Fe content of 0.01–0.1 wt %.      

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. __________ Translated from Steklo i Keramika, No. 2, pp. 28–30, February, 2007.     

Activated combustion of a silicon—carbon mixture in nitrogen and SHS of Si3N4—SiC composite ceramic powders and silicon carbide

It is established that Si₃N₄—SiC composites with a mass content of SiC 5–60% and a dominating content of the β-modification of silicon nitride can be produced by interaction of the components in the Si—C—N₂ system in the combustion regime. It is found that the fraction of α-Si₃N₄ can be increased by diluting the starting mixture with the end products, but this leads to the occurrence of a certain amount of unreacted silicon in the products. It is shown that the use of chemical activation allows one to perform a single-stage synthesis of Si₃N₄—SiC composites with any contents of the individual components (from 0 to 100%), including pure carbide silicon. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 5, pp. 56–62, September–October, 2006.     

Combustion of silicon powders containing organic additives in nitrogen gas under pressure: 1. Effect of dopants on combustion phenomenology

Introduction of organic dopants (in amounts up to 5 wt %) to Si powders was found to intensify their combustion in nitrogen gas under pressure without significant influence on the combustion temperature [T _c = 2270 ± 50 K at P(N₂) = 70 atm]. The addition of organic compounds also suppressed the coagulation of Si particles, improved the extent of conversion, and promoted combustion of coarse Si powders. This was associated with the reaction of organic species R (formed upon thermal decomposition of dopants) with solid Si(s) giving gaseous Si(g), which intensifies the processes of mass transport involving organic species R as gas transport agents.      

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.     

Synthesis of silicon oxynitride by infiltration-mediated combustion

Silicon oxynitride Si₂N₂ O was synthesized from a mixture of Si, SiO₂, and Si₃N₄ by infiltrationmediated combustion in nitrogen gas. The chemical/phase composition of product and process parameters (temperature and burning velocity) were studied upon variation in charge composition and initial pressure of gas reagent. Parameters of the reaction yielding single-phase Si₂N₂O have been optimized.      

A facile method to produce rodlike β-Si3N4 seed crystallites for bimodal structure controlling

β-Si₃N₄ rodlike seed crystallites were successfully produced by single-step heat treatment of commercial α-Si₃N₄ powder at 1900°C for 20 h under an N₂ gas pressure of 980 kPa. The average diameter, length, and aspect ratio of the seed crystallites were 0.73 μm, 1.37 μm, and 1.86, respectively. The α- ⇀ β-Si₃N₄ phase transformation proceeded mainly at 1900°C, and this temperature was lower than the theoretical α-Si₃N₄ dissociation temperature (1933°C) under N₂ gas pressure of 980 kPa. The formation of metastable solid solution due to the dissolution of O impurity into the α-Si₃N₄ crystal lattice was suggested as the driving force for the present oxide additive-free α- ⇀ β-Si₃N₄ phase transformation. β-Si₃N₄ ceramics were fabricated by liquid phase sintering promoted by an additive system of 1 wt% MgO with 3 wt% Gd₂O₃. Starting α-Si₃N₄ powder with 10 vol% rodlike β-Si₃N₄ seed crystallites prepared in this study and an extended sintering time for up to 20 h at 1950°C resulted in the formation of bimodal microstructure composed of fine matrix grains and large elongated grains originated from the seed crystallites. The β-Si₃N₄ ceramics exhibited improved fracture toughness and thermal conductivity of 5.9 ± 0.8 MPa m^−1/2 and 109.3 ± 0.4 W m⁻¹ K⁻¹, respectively, retaining a high fracture strength of about 1 GPa.     

SHS of α-Si<Subscript>3</Subscript>N<Subscript>4</Subscript> from fine Si powders in the presence of blowing agents

Explored was the effect of blowing agent (NH₄Cl + NH₄F mixtures) on the SHS of α-Si₃N₄ from a mixture of fine Si powder and α-Si₃N₄ used as an inert diluent. A prerequisite for a maximal specific surface of resultant Si₃N₄ powder was found to be perfect homogenization of starting Si-Si3N4 mixtures.     

Synthesis of monophase two-dimensional α-Si3N4 nanoplatelets via an ionothermal route

Monophase α-Si₃N₄ was prepared by a salt melt strategy using Si as a starting material. The results showed that molten salt enhanced the conversion of Si to monophase α-Si₃N₄, and some as-synthesized α-Si₃N₄ nanoparticles had a hexagonal platelet morphology. The α-Si₃N₄ hexagonal nanoplatelets had an average lateral dimension of about 170 nm and a thickness of about 5.2 nm. The nitridation of Si in molten salt was investigated by a thermal quenching method, and the silicon halide intermediate products were detected by X-ray photoelectron spectroscopy. Silicon halides possessing higher reactive activity facilitated the nitridation of Si and nucleation of α-Si₃N₄. Furthermore, the lattice mismatch between α-Si₃N₄ and the salts was calculated, suggesting that the lateral growth of α-Si₃N₄ nanoplatelets had been guided by the salt crystal structure. Based on the results, the relevant formation mechanism of α-Si₃N₄ nanoplatelets was proposed.     

Defect structures and dopant solution states of Hf-doped Si3N4 ceramics

Transition-metal-doped silicon nitride ceramics have attracted much attention as gate materials for semiconductors because of their electrical properties as well as chemical and thermal stability. The present study aims to clarify the defect structures of Hf-doped β-Si₃N₄ by theoretical calculations and scanning transmission electron microscopy (STEM). First-principles calculations based on a hybrid functional method were performed. It was found that Hf dopants are mainly substituted for the Si sites and can be occasionally located at interstitial sites in the lattice of β-Si₃N₄. The substitution sites of Hf dopants predicted by the first-principles calculations were also confirmed by the high-resolution STEM images.     

Synthesis of porous biomorphic α/β-Si<Subscript>3</Subscript>N<Subscript>4</Subscript> composite from sea sponge

Biomorphic α/β-Si₃N₄ composites were produced from natural sea sponge via replication method. The sponges were impregnated with a Si-containing slurry via dip-coating. After coating, the sponges were submitted to oxidation at 600°C for 1 h to decompose the bio-polymers followed by burning out of carbon, leading to a Si-skeleton. Subsequent thermal treatment at 1,450°C for 5 h under flowing nitrogen promoted the nitridation of the Si resulting in α/β-Si₃N₄ with an α/β fraction of 67%. The ceramic composite maintained the original morphology of the sea sponge and exhibited a porosity of 88%. The microstructure comprised whiskers, small irregular shaped particles and rod-like hexagonal grains.     

Sintering of SHS-produced α-Si<Subscript>3</Subscript>N<Subscript>4</Subscript>, α-SiAlON,and β-SiAlON

Explored was the sintering of SHS-produced α-Si₃N₄, α-SiAlON, and β-SiAlON in the presence of sintering aids such as Y₂O₃ and Al₂O₃. Process conditions were optimized to prepare sintered sialon ceramics with a high relative density and good strengths characteristics.     

Effect of the diluent on combustion synthesis of silicon nitride

Si₃N₄ powders were prepared by combustion synthesis with 1- and 3-μm α-Si₃N₄, β-Si₃N₄ diluent and BN inert diluent. The maximum temperatures of samples with boron nitride (BN) as a diluent are about 1500–1600°C lower than that of samples with α-Si₃N₄ and β-Si₃N₄ as diluents are about 1600–1800°C. Moreover, the newly formed α-Si₃N₄ contents in the synthesized products with BN as diluent over 90 wt% are much higher than those with α-Si₃N₄ and β-Si₃N₄ as diluent about 20–40 wt%. The strip-like α-Si₃N₄, rod-like β-Si₃N₄ grains, and radiative shaped grains can be observed in the synthesized products. Finally, the effect of the diluent on the α-phase content of combustion synthesized Si₃N₄ is discussed, which provides key guidance for preparing Si₃N₄ powders with high α-phase content.     

Preparation of α-Si3N4 by direct nitridation using polysilicon waste by diamond wire cutting

With the rapid development of the semiconductor industry and solar photovoltaic industry, a large number of polysilicon wastes from diamond wire cutting are accumulated, which not only pollute the environment, but also cause safety problems due to the ultrafine particle size and high reactivity. The diamond wire cutting polysilicon waste was used to prepare α-Si₃N₄ by direct nitridation method. This method could not only fully recycle the waste and reduce environmental pollution, but also could reduce the production cost of α-Si₃N₄. Furthermore, the effects of FeCl₃, NaCl, and metal Cu on the nitridation of polysilicon waste are investigated in detail, respectively. It is found that FeCl₃ and NaCl are not ideal additives for the preparation of α-Si₃N₄. However, α-Si₃N₄-dominated Si₃N₄ can be obtained via adding 5 wt% Cu after nitridation at 1250°C for 8 hours, and the relative content of α-Si₃N₄ reaches 92.37%.     

Coarse-grained β-Si3N4 powders prepared by combustion synthesis

Coarse-grained β-Si₃ N₄ powders were prepared by combustion synthesis under N₂ pressure of 6 MPa, with a low diluent content of not more than 10 wt.% and high reaction temperature of >1900°C. β-Si₃ N₄ was obtained as the major phase in the products, except for a small amount of residual Si. The addition of carbon black was effective to reduce the residual Si, but resulted in the formation of β-SiC when too much carbon black was used. The coarse-grained β-Si₃ N₄ powders consisted of β-Si₃ N₄ crystals with an average thickness of more than 10 µm, and some crystals were thicker than 20 µm. The growth mechanism of the coarse β-Si₃ N₄ crystals was discussed, associated with the particular reaction conditions in combustion synthesis.     

CEramics based on α-sialon

Ceramics containing pure α-sialon and α-sialon in combination with β-Si₃N₄, produced from two types of silicon nitride powder, are described. It is shown that the high-temperature strength of the ceramics increases with increase in the content of α-sialon in the composition. Data on the microstructure, phase composition, and mechanical characteristics of the synthesized materials are presented.     

Phase composition of the products of nitriding ferrosilicon with zircon concentrate additives

The process of nitride formation during combustion of iron silicon with the addition of zircon concentrate in nitrogen gas is investigated. It is determined that in the presence of zircon (5–70%) ferrosilicon burns in a surface regime, and the products of combustion are multiphase. For the optimal parameters of synthesis (nitrogen pressure 4–7 MPa and sample diameter 35–40 mm) the products of combustion are a composition consisting of a ceramic constituent (Si₃N₄-ZrO₂-Si₂N₂O) and iron (α-Fe). The method of acidic enrichment is used to obtain a composite ceramic powder with iron mass content 0.5%     

In situ synthesis of cobalt silicide particle reinforcing and coloring Si3N4 ceramics

In this paper, silicon nitride (Si₃N₄) ceramics with black color and high toughness were fabricated by gas pressure sintering and characterized by X-ray diffraction, Raman, scanning electron microscopy, EDS, and transmission electron microscopy. The in situ formed cobalt silicide was confirmed to contribute to the black color through the introduction of CoO. Due to the addition of CoO, the growth of β-Si₃N₄ grains is promoted, forming elongated grains, and eventually forms the self-reinforcing microstructure. However, with adding excessive CoO, interfacial debonding is found between cobalt silicide and Si₃N₄ matrix and a decrease in strength was resulted. The optimum composition is 1 mol% CoO in Si₃N₄, with the fracture toughness of 9.9 ± 0.3 MPa m^1/2, flexural strength of 826.1 ± 46.0 MPa, and a much darker black color. The mechanism of color formation is discussed where the black color derives mainly from the metallic silicon and additionally the porosity.     

Formation mechanisms of Si3N4 microstructures during silicon powder nitridation

Silicon nitride (Si₃N₄) 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-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.     

Combustion synthesis of well-dispersed rod-like β-Si3N4 crystals by the addition of carbon

The well-dispersed rod-like β-Si₃N₄ crystals have been prepared by combustion synthesis with the addition of carbon. The added carbon helps to separate Si particles and remove the SiO₂ oxide layer, and thus reduces the agglomeration of β-Si₃N₄ crystals. By adding carbon, the reaction temperature and the width of β-Si₃N₄ crystals are decreased, but the aspect ratio of β-Si₃N₄ crystals is increased. The well-dispersed β-Si₃N₄ crystals with an average width of 0.84 μm and aspect ratio of 2.3 are produced by adding 2 wt% carbon. When 5 wt% carbon is added, the reaction temperature is too low and the nitridation of Si becomes incomplete, and at the same time much SiC occurs in the product.