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Relevant academic research and scientific papers

Hydrolysis-induced aqueous gelcasting of β-SiAlON-SiO2 ceramic composites: The effect of AlN additive

Dense β-SiAlON-SiO2 (SiO2520, 40, 50, 60, and 80 wt%)ceramic composites have been prepared from β-Si4Al 2O2N6 and fused silica by sintering at 15001-1750°C for 3-4 h. For comparison purposes, a powder mixture consisting 60 wt% β-Si4Al2O2N6 and 40 wt% fused silica has been consolidated following a new near-net shape technique based on hydrolysisinduced aqueous gelcasting (GCHAS) and sintered for 3 h at 1750°C. In the GCHAS process, consolidation of suspensions containing 50 vol% solids was achieved by adding a polymerization initiator, a catalyst, and AlN powder equivalent to 1-5 wt%Al2O3. Thin-wall radomes consolidated by GCHAS (using AlN equivalent to 5 wt% Al2O3 in the suspension) have exhibited green strengths 420 MPa. The sintered materials were characterized for various properties including hardness, fracture toughness, mass loss, shrinkage, coefficient of thermal expansion, and dielectric constant. The Si2N2O formed from a powder mixture of 60 wt% β-Si4Al2O2N6 and 40 wt% fused silica at 1750°C for 3 h exhibited a flexural strength of B140 MPa, Young's modulus of 214 GPa, coefficient of thermal expansion of 3.5×10 -6°C-1, hardness of 1390 kg/mm2, fracture toughness of 4.2MPa .m1/2, and a dielectric constant of 5.896 and tan δ of 0.002 at 17 GHz.

The Preparation of Silicon Oxynitride, Si2N2O, as a High Surface Area Powder by Reaction of Silica with Ammonia at 1100 degC

The title reaction is found to occur with an amorphous, non-porous but high surface area form of silica; other silica phases, differing in crystallinity and porosity, give substantially lower degrees of nitrogen incorporation on reaction with ammonia.

Spark plasma sintering of nanosized amorphous silicon nitride powder with a small amount of sintering additive

Dense and fine-grained β-Si3N4 ceramics were successfully obtained with a small amount of sintering additives, 1.5 mass% Y2O3 and 0.5 mass% Al2O3, using nanosized amorphous Si3N4 powder by spark plasma sintering at temperatures of 1500°-1800°C and a pressure of 30 MPa under N 2. The β-Si3N4 ceramics were composed of equiaxed grains with an average size of 300 nm. A higher sintering temperature was required for the densification of submicrometer-sized α-Si 3N4 powder with the small amount of the additives. The use of nanosized amorphous Si3N4 powder accelerated the densification and the transformation to the β-phase.

Synthesis of silicon nitride nanorods using carbon nanotube as a template

A method to prepare silicon nitride nanoscale rods using carbon nanotube as a template has been presented in this letter. The products of the reaction of carbon nanotubes with a mixture of Si and SiO2 powder in nitrogen atmosphere are β-Si3N4, α-Si3N4, and Si2N2O nanorods. The sizes of the nanorods are 4-40 nm in diameter and up to several microns in length. The formation mechanism of the nanorods has also been discussed.

Carbothermal synthesis of silicon nitride: Effect of reaction conditions

Conditions for carbothermal synthesis of α-Si3N4 are presented with special emphasis on the reaction temperature, C:SiO2 ratio, and precursor mixing. With pure precursors, the conversion temperature is 1500° to 1550 °C. An

Influence of thermal conditions on the combustion synthesis of Si2N2O phase

This work is a contribution to the study of Si2N2O synthesis via Self-Propagating High-temperature Synthesis (SHS). The elaboration of Si2N2O was carried out using two kinds of experimental equipments and differences both in reaction temperatures and in products compositions were evidenced. In order to determine the reaction mechanisms, several compositions and thermal conditions of the reactant mixture were studied. Considering our results, two kinds of reaction mechanisms for the combustion synthesis of Si2N2O could be suggested. Furthermore, the results previously reported in the literature have been explained in the light of this work. The SHS samples were characterized by X-ray diffraction and scanning electron microscopy (SEM) analysis. Their quantitative compositions were determined by Rietveld refinement.

Formation process of calcium-α SiAlON hollow balls composed of nanosized particles by carbothermal reduction-nitridation

Carbothermal reduction-nitridation (CRN) of SiO2-Al 2O3-CaO powders was performed under various firing conditions to investigate the formation process of Ca-α sialon hollow balls composed of nanosized particles. Scanning electron microscopy and transmission electron microscopy observations of the samples obtained at different firing temperatures confirmed that solid spherical particles were formed at the early stage of the reaction, and nanosized particles were subsequently produced on the surface of these solid balls. From X-ray diffraction and energy-dispersive spectrometry analyses, it was found that the solid balls initially formed at 1450°C were mainly amorphous and contained Si, Al, Ca, O, and a small amount of N. Further nitridation at 1450°C gradually converted the solid balls into Ca-α sialon hollow balls over time. The results revealed that the formation of Ca-α sialon hollow balls depends on the formation of solid balls from the Si-Al-Ca-O liquid phase at the initial stage of the CRN process.

Alloy Engineering of a Polar (Si,Ge)2N2O System for Controllable Second Harmonic Performance

Although silicon oxynitrides are important semiconductors for many practical applications, their second-order nonlinear optical (NLO) properties have never been systemically explored. Using the first-principles calculations, in this article, we discover that the sinoite (e.g., silicon oxynitride Si2N2O) can simultaneously exhibit wide optical band gap, strong second-harmonic generation (SHG) effect, and large birefringence, which are further confirmed by our preliminary experimental measurement. Importantly, we propose that alloying engineering can be further applied to control the balanced NLO properties in the Si2N2O system. Based on first-principles cluster expansion theory, we demonstrate that alloying Ge into Si2N2O can form low formation-energy Si2(1-x)Ge2xN2O alloys, which can in turn achieve controllable phase-matching harmonic output with high SHG efficiency at different energy ranges. Therefore, alloy engineering could provide a unique approach to effectively control the NLO performance of Si2(1-x)Ge2xN2O, making this polar alloy system hold potential applications in tunable laser conversion and controllable optical devices.

Rapid crystallization process of amorphous silicon nitride

The crystallization of nanosized amorphous silicon nitride powder is one of the methods to produce sub-micrometer/nanosized α-Si3N 4 powder. The application of this method is still limited by the long crystallization time, low output, and high cost. This article invents a new crystallization method of amorphous silicon nitride involving the addition of Si powder. The new process reduces the complete crystallization time from more than 6 h to 30 min, thus allowing efficient production of sub-micrometer/ nanosized α-Si3N4 powder. Effects of factors such as additive, temperature, and duration on the crystallization process are investigated using XRD and FTIR. The experimental results showed that the added Si powder accelerates the crystallization process effectively. The final product is a mixture of α-Si3N4 and Si2N 2O. In this article, amorphous silicon nitride powder with added Si is annealed at 1450°C. Powder of nearly 100% crystalline phase content is produced either by adding 10% Si and annealing for 15 min or by adding 5% Si and annealing for 30 min.

Direct electrochemical preparation of nanostructured silicon carbide and its nitridation behavior

Silicon carbide was synthesized from mixtures of SiO2 and graphite by applying the concept of the FFC-Cambridge process and several fundamental aspects of the synthesis route were investigated. Porous disks composed of powders of SiO2 and graphite in molar ratios of 1:0.5, 1:1 and 1:1.5 were prepared by sintering in inert atmosphere and subjected to electro-deoxidation in molten CaCl2 at 1173 K under a range of experimental conditions. Disks of molar ratio 1:1.5, reduced at an applied voltage of 2.8 V for a duration of 6 h, yielded exclusively phase-pure SiC of nanowire morphology as the reaction product, while the other precursor compositions provided significant amounts of calcium silicides. Voltages lower than 2.8 V gave mixtures of SiC with elemental Si and graphite, and voltages higher than that gave CaSi alone. Shorter electro-deoxidation times led to incomplete reduction and allowed for the identification of CaSiO3 as a transient phase. Based on the experimental results a multipath reaction mechanism is proposed, consisting of the electrochemical reduction of SiO2 and CaSiO3 to Si and the subsequent in-situ carbonization of the Si formed to SiC. The effect of N2 at high temperature on the electrochemically synthesized SiC was investigated and the formation of nanowire Si2N2O was observed. Overall, the process presented is a facile single-step and low-temperature method for the synthesis of SiC with possible commercial prospects.