Chromium catalysed silicon nitridation

Silicon powder compacts were fabricated with various amounts of chromium (0–5 at %) deposited onto the surface of the silicon powder by a solution-deposition process. These compacts were heated to several maximum temperatures in the range 1100–1250C in a flowing 10% H₂/90% N₂ atmosphere to evaluate the effect of the chromium content on the silicon nitridation. It was observed that silicon compacts containing 5 at % Cr were fully nitrided in approximately 3 h at 1150C, while less than 8% nitridation was achieved for pure-silicon compacts (with 0 at % Cr) compacts under the same conditions. Single-crystal silicon wafers with a 50 nm chromium layer were also nitrided; this provided a planar geometry, which facilitated our study of the catalysis mechanism. The rate-controlling process was shown to be first order, which may be indicative of a nucleation-and-growth mechanism, which is commonly observed for -silicon-nitride formation. This work demonstrates the feasibility of producing reaction-bonded silicon nitride at low temperatures using chromium catalysis, and it indicates the potential for fabricating fibre-reinforced silicon-nitride composites containing thermally sensitive fibres.     

Direct nitridation of aluminum compacts at low temperature

Direct nitridation of aluminum compacts (relative density 65%)consisting of commercial atomized powder was examined at temperaturesfrom 500 to 700°C, near or below the melting point ofaluminum, and under a pressured nitrogen atmosphere between0.5–7 MPa. Complete nitridation was achieved at a temperature as lowas 540°C by controlling the nitrogen pressure. Thenitridation process and the structures of generated aluminum nitride(AIN) were drastically influenced by nitrogen pressure. Consideringthe relations between the reaction conditions and the reactionprocesses, it is suggested that low nitridation temperature andpre-heating in vacuum have a good effect on the nitridation ratio.     

Fluoride accelerated nitridation of silicon

Fully characterized silicon powder has been nitrided alone, and in the presence of small amounts of aluminium, calcium and sodium fluorides. Comparisons of the overall extents of reaction, and individualα- andβ-silicon nitride phase yields, have been made. It is concluded that the principal action of these fluorides is the disruption of the native silica film on the silicon surfaces, thereby increasing the active site density at which nitridation can occur. The nitridation kinetics are shown to be consistent with a “mutually blocking pore” model.     

Quantitative kinetic analysis of silicon nitridation

Kinetic analysis of silicon nitridation requires intrinsic single-particle behaviour to be isolated from global or compact effects that typically manifest during the reaction-bonding process. These effects arise from the influence of adjacent particles, which modify the macropore structure as the reaction proceeds. Much of the variation in the published kinetic data can be attributed to compact effects, particle shape, and size distribution, resulting in a myriad of models being reported, each only applicable to the nitridation conditions in which the data were obtained. Our work clearly demonstrates that the intrinsic single-particle nitridation behaviour is well described by a sharp-interface model, with diffusion control (E _a = 301.5–310.0 kJmol^–1) through an expanding Si₃N₄ product layer developing on the individual grains. For the nitridation of silicon compacts, the reaction-bonding process can be divided into three fundamental stages: (1) initial devitrification/nucleation, (2) massive nitridation, and (3) termination by further sintering, densification, and coarsening of the Si₃N₄ product. Factors influencing and controlling each stage are summarized.     

A mechanism for the nitridation of Fe-contaminated silicon

The influence of iron impurity on both the oxidation and nitridation of high purity silicon has been investigated. It is shown that iron is effective in rapidly removing the protective silica film which normally covers silicon. Experimental evidence suggests that the removal is achieved by iron-induced devitrification and disruption of the silica, thus allowing the SiO (g) generated by the Si/SiO₂ interface reaction to escape. During the nitridation of iron-contaminated silicon powder compacts it is found that iron significantly enhances the extent of reaction for contamination levels of <1000 p.p.m. Fe (by weight). Above this level there is a decrease in the rate of formation of extra nitride. At all levels of contamination the percentage of silicon converted to -Si₃N₄ was observed to be directly proportional to the iron concentration, and it is shown that this -growth occurs within an FeSi_x liquid phase. The possible implications of the findings for the optimization of strength of reaction-bonded silicon nitride are briefly discussed.     

Effect of alkaline-earth fluorides on the nitridation of silicon

Effects of one per cent of alkaline earth fluorides (BaF₂, CaF₂, SrF₂) on the first stage nitridation of silicon have been studied. The fluorides enhances the reaction rate and changes the parabolic kinetics for silicon-nitrogen reaction to a linear kinetics. The accelerating effect being in the order BaF₂>CaF₂>SrF₂ with activation energies of 96.23, 146.44 and 251.04 kJ/mole respectively as compared to 385.43 kJ/mole for silicon-nitrogen reaction without additive. Thermodynamic consideration reveal that the fluorides accelerates the reaction by removing the SiO₂ layer formed on the silicon grain by the reaction SiO₂ (s) + 2MF₂ (g) ? SiF₄ (g) + 2MO (s). Evidences have been cited to show that the reaction Si (g) + MF₂ (g) ? SiF₂ (g) + M (g) also occurs.     

Production of silicon nitride by gas nitridation of ultra-fine silicon powder

Ultra-fine silicon powder of diameter 20 to 50 nm was gas nitrided at 1373 K. Contamination with air increased the nitriding temperature, which was lower than that found in previous work for larger particle-size powders.     

A mechanism for the nitridation of silicon powder compacts

A mechanism for the nitridation of silicon powder is proposed, based on an interpretation of the microstructure of partially reacted compacts. It is observed that the reaction does not occur at the solid-state interface between the silicon and the nitride product layer. Both silicon and nitrogen are transported through this layer and the removal of silicon results in the formation of pores in the silicon crystals at the nitride-silicon interface. The nitridation reaction takes place within these pores, which subsequently migrate into the silicon grains, and within the original voidage of the compact.     

The role of hydrogen in the nitridation of silicon powder compacts

High-purity silicon powder compacts have been nitrided in nitrogen atmospheres containing varying partial pressures of hydrogen. The accelerated nitridation rates observed are interpreted in terms of the interaction of the hydrogen with the natural oxide film on the surface of the silicon particles. A model is presented for reactions taking place during the nitridation of these compacts in the presence of furnace atmospheres contaminated by low partial pressures of water vapour.     

Effect of Fe and Al additions on nitridation of silicon

The effect of Fe and AI on the nitridation of Si was investigated under controlled O₂ partial pressures in the range 1.2×10^–12 to 1.2×10^–15 atm at 1420° C. The volatilization of Si during nitridation is attributed to SiO gas formation. Addition of Fe promoted the nitridation to phase at lower temperatures than the melting point of Si, whereas addition of Al increased the amount of phase. The stabilization of the structure is explained by Al₂O₃ formation and the dissolution of it in Si₃N₄ to form (Si, AL)₃ (N, O)₄ (sialon).     

Effects of additives on the nitridation process of foamed materials containing silicon and silicon carbide powders

Without using any additive, the nitridation process of silicon powder was slow and the main product was α-Si₃N₄ due to the cycling production of SiO species. The addition of Al₂O₃ and Y₂O₃ could facilitate the nitridation process resulting in a higher β-Si₃N₄ content presumably due to the liquid phase formed between Al₂O₃, Y₂O₃ and surface silica on silicon powder. When a small amount of 1.76% Fe₂O₃ was added, the accelerated nitridation process was attributed to the FeSi₂ liquid phase produced by reaction Fe element with surface silica at a lower temperature of 1212 °C, but the Al₂O₃ and Y₂O₃ additives could still be active for sustaining the nitridation process at higher temperature. At a higher Fe₂O₃ concentration of 3.46%, the nitridation process was mainly controlled by the formed FeSi₂ liquid phase. This study has demonstrated the active role of using Al₂O₃ and Y₂O₃ combination and Fe₂O₃ on the nitridation process, which could be helpful for further investigation on reaction bonding of SiC and Si₃N₄ ceramics.     

聚碳硅烷氮化热解法制备Si3N4纤维

将PCS电子束交联丝在氨气氛中氮化热解、脱碳氨化,继在氮气氛中高温热引发缩合/转氨基反应,生成硅氮烷并最终形成氮化硅(Si3N4)纤维。所制备的Si3N4纤维白色透明,横截面和表面均光滑致密,无明显缺陷和孔洞。还研究了氮化热解的反应机理以及热解工艺对氮化硅(Si3N4)纤维结构和性能的影响。红外光谱和元素分析的结果显示,氮化热解脱碳彻底,Si3N4纤维C含量<1%;烧结温度提高,N含量随之增加,O含量则先增后减;烧结温度不超过1500℃,纤维为无定型。力学性能结果分析表明,随热解温度的提高,纤维力学性能先提后降,1300℃时达到最大值。氮化热解过程是采用NH3进行脱碳氨化,并在N2气氛下高温热引发缩合/转氨基反应产生硅氮烷并最终形成Si3N4的过程。     

Optimization of time-temperature schedule for nitridation of silicon compact on the basis of silicon and nitrogen reaction kinetics

A time-temperature schedule for formation of silicon-nitride by direct nitridation of silicon compact was optimized by kinetic study of the reaction, 3Si + 2N₂ = Si₃N₄ at four different temperatures (1250°C, 1300°C, 1350°C and 1400°C). From kinetic study, three different temperature schedules were selected each of duration 20 h in the temperature range 1250°-1450°C, for complete nitridation. Theoretically full nitridation (100% i.e. 66.7% weight gain) was not achieved in the product having no unreacted silicon in the matrix, because impurities in Si powder and loss of material during nitridation would result in 5–10% reduction of weight gain. Green compact of density < 66% was fully nitrided by any one of the three schedules. For compact of density > 66%, the nitridation schedule was maneuvered for complete nitridation. Iron promotes nitridation reaction. Higher weight loss during nitridation of iron doped compact is the main cause of lower nitridation gain compared to undoped compact in the same firing schedule. Iron also enhances the amount of Β-Si₃N₄ phase by formation of low melting FeSi_x phase.