Synthesis of Si3N4 whiskers by rapid nitridation of silicon droplets

Belt-like β-Si₃N whiskers were successfully synthesized by nitriding of liquid silicon without catalysts at 1500°C by using micron-sized silicon powders within 10 minutes. Silicon droplets formed by the melting of silicon particles greatly facilitates the diffusion of nitrogen. Several whiskers cling together to form a whisker-cluster. The whisker-clustermorphology results from nitriding of separate silicon droplets. The growth of the belt-like β-Si₃N₄ whisker was controlled by vapor-liquid-solid mechanism. The synthesis of silicon nitride whiskers can be effectively improved by nitriding liquid phase silicon.     

Quantitative model for the viscous flow and composition of two-phase silicon nitride-based particles in plasma-spray deposition

Silicon nitride does not melt but decomposes at 1900 °C and so thermal spraying of pure silicon nitride powder is impracticable. However, the use of silicon nitride and other non-oxide ceramics as thick, thermally sprayed coatings has considerable engineering potential owing to their unique combination of properties. This research shows that embedding fine silicon nitride particles within an oxide matrix to form composite feedstock particles enables the formation of silicon nitride composite coatings with little decomposition of the silicon nitride. Successful deposition of the coatings depends critically on the flow of the feedstock particles on impact with the substrate. This paper concerns the design of oxide matrix systems for the deposition of silicon nitride composite coatings by thermal spraying. A quantitative model is developed for the viscous flow of two-phase feedstock particles at impact. A number of matrix systems are investigated, including a series of yttria–alumina and yttria–alumina–silica compositions. The research shows that certain oxide matrices can provide the required viscous flow and protect the silicon nitride from decomposition.     

Study on rapid nitridation process of molten silicon by thermogravimetry and in situ Raman spectroscopy

The melt of silicon, hindering nitridation for its agglomeration, should be avoided in the direct nitridation of silicon to synthesize silicon nitride powders, although liquid phase facilitates nitridation. Therefore, we proposed a method to nitride molten silicon without agglomeration. Thermogravimetric and in situ Raman studies on the nitridation process of molten silicon were performed. The as-prepared silicon nitride samples were found to be micron clusters composed of submicron grains with high α-Si₃N₄ content. The nitridation of molten silicon at 1500°C was completed after 500 s and 109 times faster than the nitridation of solid silicon at 1350°C. β-Si₃N₄ is produced dominantly by α–β-phase transition. Less nitridation time and low temperature can decrease the β-Si₃N₄ content. The rapid nitridation was owning to core–shell structure Si@Si₃N₄, which was formed after the initial nitridation of silicon particles and hindered the agglomeration of molten silicon.     

Infiltration of Reaction-Bonded Silicon Nitride with Equilibrium Y-Si-O-N Melt

An equilibrium Y-Si-O-N melt was infiltrated to eliminate the open porosity of reaction-bonded silicon nitride at 1600–1800°C. This oxynitride melt contained two equilibrium phases, a β-Si₃N₄ solid phase and a liquid phase at high temperatures. Before infiltration, porous reaction-bonded silicon nitride compacts were heat-treated to completely transform to the β-Si₃N₄ phase. After infiltration, the flexural strength of the reaction-bonded silicon nitride material increased from 200 to 600 MPa at 25°C, from 200 to 300 MPa at 1400°C in air.     

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.     

Mechanical and fatigue strengths of silicon nitride ceramics in liquid aluminum alloys

Although the silicon nitride ceramic (Si₃N₄) has a high mechanical strength even at high air temperature, much reduction in the material strength occurs in liquid aluminum alloys. The bending strength of the ceramic in molten Al alloys is about 20% lower than that obtained in an air temperature of 750 °C. Moreover, a significant reduction of the fatigue strength occurs in the Al alloy melt. The change of mechanical properties of Si₃N₄ ceramic in the melt also depends on the amount of iron in the molten aluminum alloy; a large amount of iron makes the fatigue strength low. The reduction in the material strength is attributed to the change of material properties caused by the chemical reaction between iron and silicon nitride. Details of the chemical reaction in Si₃N₄ ceramic are discussed in the present work.     

Mechanical spectroscopy connected to creep and stress relaxation in a high resistant silicon nitride

Silicon nitride processed by gas pressure sintering contains a very small amount of glassy phase and consequently exhibits a strong resistance to deformation until 1450 °C. Above this temperature, both relaxation kinetics and creep rate rapidly increase. To explain such a behaviour, the formation of a liquid phase by dissolution of YSiAlON phases was proposed. The present paper shows that mechanical spectroscopy argues for the existence of such a liquid phase at high temperature. The mechanical loss is very low in the as-sintered material. Nevertheless, the internal friction peak generally observed in silicon nitride, and attributed to the glass transition in the glassy pockets, is also observed in the gas pressure sintered silicon nitride. Moreover, the peak is much higher in annealed and “quenched” specimens and it increases with annealing time. These results show that the annealed and “quenched” material contains much more glassy phase and so argues for the dissolution of crystalline phases at high temperature.     

High pressure synthesis of cubic boron nitride from Si-hBN system

Cubic boron nitride (cBN) was synthesized from hexagonal boron nitride (hBN) in the presence of silicon. The cBN forming pressure-temperature region was determined at pressures up to 7.7 GPa. Near perfect octahedral cBN crystals could be synthesized under the P-T conditions near the low temperature boundary of the cBN-forming region. When the temperature was above 1700°C at 6.5 GPa, the transformation rate of cBN from hBN was very high and the cBN crystals had an oriented columnar morphology. This suggests that silicon has strong catalytic ability for cBN formation from hBN. The energy-disperse X-ray analysis (EDXA) identified that silicon was homogeneously distributed in the cBN crystals.     

Catalytic Effects of Metals on Direct Nitridation of Silicon

Catalytic effects were investigated on the direct nitridation of silicon granules, impregnated with 0.125–2.0% by mass of calcium, yttrium, iron, copper, silver, chromium, or tungsten, in a stream of nitrogen with 10% hydrogen, using a tubular flow reactor operated at temperatures ranging from 1200° to 1390°C. Calcium and yttrium suppressed the formation of β-silicon nitride while iron enhanced the formation of β-silicon nitride over the temperature range investigated. An addition of 0.125% calcium resulted in about 99% overall conversion with 100%α-phase and a 2.0% yttrium addition yielded an overall conversion over 98% with an α-phase content above 97%. Copper promoted not only the nitridation but the formation of α-silicon nitride at 1200°C, but enhanced the β-phase formation at higher temperatures. The role of liquid phases on the formation of α-/β-silicon nitride was also discussed based on the nitridation of silicon impregnated with copper, calcium, silver, chromium, and tungsten.     

Thermal shock resistance of the porous boron nitride/silicon oxynitride ceramic composites

The thermal shock resistance of the porous boron nitride/silicon oxynitride (BN/Si₂N₂O) ceramic composites were tested by the quenching‐strength method with temperature differences of 600‐1400°C. The residual flexural strength of the composites decreased with increasing temperature difference from 600°C to 900°C. This weakening in flexural strength was attributed to the formation of microcracks in the matrix caused by thermal stress damage. Afterward, as the formation of a dense oxidized layer sealed the surface and hindered further oxidation, the residual flexural strength increased with the further increase of temperature difference from 900°C to 1100°C. Finally, when the temperature differences were above 1100°C, the residual flexural strength gradually decreased with increasing temperature difference, which was attributed to the further oxidation and large thermal stress damage. And the thermal shock resistance of the porous BN/Si₂N₂O ceramic can be improved by the introduction of high contents of sintering aids and h‐BN.     

The role of an SiC interlayer at a graphite–silicon liquid interface in the solution growth of SiC crystals

SiC crystal growth using the top seeded solution growth (TSSG) method involves the precipitation of solid SiC from carbon that is dissolved in a silicon melt. The growth rate of SiC is strongly influenced by the solubility of C in liquid Si, which is quite low. In this study, the dissolution of C from graphite to the Si melt was explored by observing the formation of an SiC interlayer at a graphite – Si liquid interface. The SiC interlayer was observed to become thickened during the several hours needed to reach a certain thickness at 1500 °C. Assuming that the SiC interlayer is a direct C source, a pre-formed SiC layer was coated on the graphite crucible to evaluate its effect on the concentration of C in the Si melt. As a result, the concentration of C in the Si melt increased within a short time, especially at low temperatures. By applying the SiC coated crucible to the TSSG process for SiC crystal growth, we confirmed that the development of a pre-formed SiC layer enhanced the growth rate of SiC crystals, especially at the initial stage of crystal growth at low temperatures.     

Fabrication and properties of in situ silicon nitride nanowires reinforced porous silicon nitride (SNNWs/SN) composites

The in situ silicon nitride nanowires reinforced porous silicon nitride (SNNWs/SN) composites were fabricated via gelcasting followed by pressureless sintering. SNNWs were well distributed in the porous silicon nitride matrix. The tip-body appearance suggested a VLS growth mechanism. The flexural strength and elastic modulus of the prepared composites can achieve 84.3?±?3.9?MPa and 23.3?±?2.0?GPa respectively (25?°C), while the corresponding porosity was 40.7?vol.%. Remarkably, the strength retention rate of the composites at 1400?°C was up to 66.1%. This is due to the excellent thermal stability of SNNWs and silicon nitride matrix. Also, the fracture toughness of the composites was improved to ～42% larger than pure porous silicon nitride ceramics because of the bridging effect of the NWs and the interlocking effect of β-Si₃N₄ crystals. In addition, a good thermal shock resistance and dielectric properties were indicated. The good overall performance made SNNWs/SN composites promising candidate for advanced high-temperature applications.     

Synthesis of Scandium Nitride Crystals from Indium–Scandium Melts

The synthesis of scandium nitride (ScN) nanoscale crystals by dissolving N₂ into In‐Sc melts is demonstrated for the first time. The crystallization mechanism of ScN from In‐Sc melts is investigated. In the N₂ pressure of 0.3 MPa, the ScN yield increases with the Sc concentration in flux and the growth temperature within the range 900–1100°C, achieving a maximum value of about 70% at temperatures above 1100°C. Scanning electron microscopy evidences the growth of round‐shaped ScN crystals with increasing average size from 48 to 860 nm in the temperature range of 900–1300°C. This study shows that Sc effectively promotes the dissolution of N₂ in the In‐Sc melt, and In‐Sc is a promising flux for the synthesis of ScN nanocrystals.     

Silicon carbide-based foams derived from foamed SiC-filled phenolic resin by reactive infiltration of silicon

Macro-cellular porous silicon carbide-based foams were fabricated by reactive infiltration of melt silicon into porous carbonaceous preforms pyrolyzed from foamed SiC-filled phenolic resins (PF). The SiC-filled PF foams were prepared at 80 °C with different heating rate. The effect of heating rate on the foaming behavior of the liquid SiC-filled PF mixture and the microstructure of the foams were investigated. The foamed SiC-filled PF was then pyrolyzed at 1000 °C and infiltrated by melt Si at 1600 °C, leading to the formation of open macro-cellular structure. At a heating rate of 6 °C min⁻¹, Si-infiltrated foams with a porosity of ~72% and a mean pore size of ~0.5 mm were obtained. The Si-infiltrated foams with dense struts mainly inherited the pore structure of pyrolyzed preforms. The main phases of SiC-based foams were α-SiC, β-SiC and the remnant Si, which contributed to high compressive strength of the SiC-based foams.     

The effect of Al2O3-MgO additives on the microstructure of spark plasma sintered silicon nitride

It was shown that spark plasma sintered silicon nitride with a high content of Al₂O₃ and MgO consists of α and β silicon nitride, the main phase being α silicon nitride. The increase in the sintering temperature did not lead to significant changes in the phase composition as occurs in silicon nitride added with Al₂O₃-Y₂O₃. It was found that increasing in SPS temperature above 1650 °C leads to an insignificant increase in the density. A complex shaped equiaxed grain microstructure was shown in both cases. However, doping with aluminum and yttrium oxides allows obtaining an elongated grain microstructure. The Hall-Petch effect was observed for the microhardness of the investigated SPSed silicon nitride. The microhardness of the described ceramics was rather high and more than 1900 HV compared to the pressureless sintered at 1800 °C silicon nitride with the microhardness equal to 1511 HV.     

High-temperature properties and interface evolution of silicon nitride fiber reinforced silica matrix wave-transparent composite materials

In order to improve the high-temperature performance of wave-transparent materials especially for the high-speed aircrafts application, filament winding combined with sol-gel method was adopted to the fabrication of unidirectional silicon nitride fiber reinforced silica matrix composites. The mechanical properties and the interface evolution at high temperatures were investigated. The results show that the composite sintered in N₂ maintains a flexural strength of 210MPa at up to 1200°C, while its counterpart prepared in air experiences a dramatic reduction to about 73MPa. The degradation is due to the partial oxidation of silicon nitride fibers at the fiber matrix interface. Besides, it is also notable that the bending strength of these two composites undergoes a similar growth from about 160 to 210MPa when tested under 900°C, which can be explained by the release of thermal stress on the silicon nitride fibers.     

Conversion of methanol on silicoaluminophosphate—formation of hexamethylbenzene and olefins

The catalytic conversion of methanol over new generation molecular sieve silicoaluminophosphate of type 5 (SAPO-5) has been studied. The formation of gaseous, liquid and solid hydrocarbons have been observed. While the gaseous hydrocarbon contains 32·5–56·7 wt% of olefins, the liquid product comprises hydrocarbons in the boiling range 98°C-320°C as determined by gas chromatographic distillation technique. The solid hydrocarbon was identified as hexamethylbenzene by physical, chemical and spectral analysis.     

Pyrolysis of kraft lignin in the presence of molten ZnCl2-KCl mixture

Pyrolysis of kraft lignin was carried out using small reactors with an added ZnCl₂-KCl mixture at three levels of temperature, 500°C, 550°C and 600°C, and three levels of salt-to-lignin ratio (SR), 1, 2 and 3. Nineteen kinds of phenolic compound were identified and their quantitative determinations were made by gas chromatography. The yield of gaseous products determined gravimetrically exhibited a strong dependence on the added amount of salt mixture, and its lowest value was obtained at SR of 1. The formation of gaseous products, apparently stemming from decomposition of liquid products and tar, was observed at reaction times longer than 30 min. At SR of 1 and a temperature of 550°C, all phenolic compounds attained their maximum yields almost synchronously at a reaction time of 15 min. Cresol's formation was slightly increased with the amount of added salt mixture. At a temperature of 550°C, cresols and guaiacols decomposed according to a pseudo-first-order lumping model, while catechols did not follow the similar model.     

Preparation of silicon carbide-silicon nitride fibers by the pyrolysis of polycarbosilazane precursors: A review

The development of silicon carbide-silicon nitride (SiC-Si₃N₄) fibers by the pyrolysis of polycarbosilazane precursors that was carried out in this laboratory is reviewed. Precursor resin, which was prepared by heating tris(N-methylamino)methylsilane or tris(N-methylamino)phenyl-silane to about 520°C, was drawn into fibers from the melt and then made unmeltable by humidity conditioning at 100°C and 95 percent relative humidity. The humidity-treated precursor fibers were pyrolyzed to ceramic fibers with good mechanical properties and electrical resistivity. For example, SiC? Si₃N₄ fibers derived from tris(N-methyl-amino)-methylsilane had a tensile rupture modulus of 29 × 10⁶ psi and electrical resistivity of 6.9 × 10⁸ Ω-cm, which is 10¹² times greater than a value obtained for graphite fibers.     

Conversion of brown coals in aqueous and toluene-containing media under supercritical conditions in the presence of catalysts

The conversion of brown coals from the Borodino and Kangalas deposits in an aqueous medium and in a mixture of toluene with water was studied under supercritical conditions over the temperature range of 375–550°C and at pressures from 22 to 40 MPa. It was found that the methanation, hydrolysis, and oxidation reactions of brown coals with the predominant formation of gaseous products (methane, carbon dioxide, and hydrogen) prevailed in an aqueous medium. Liquid substances were formed in an insignificant amount. In the toluene solvent under supercritical conditions at 440°C, the addition of a small water amount (15%) stimulated the degradation of coals with the predominant formation of liquid products and moderate gas formation. The use of calcium oxide and sodium hydroxide as catalysts increased the yields of liquid products. It was noted that the reactivity of Kangalas coal in this process was higher than that of Borodino coal.