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1.
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 FeSix phase.  相似文献   

2.
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.  相似文献   

3.
Compacts of high-purity silicon powder were pretreated in hydrogen or argon to remove the surface silica and then nitrided in gas at atmospheric pressure and at 1623 K. The kinetics of nitridation were monitored continuously, the fracture surfaces of the nitrided samples examined using scanning electron microscopy and the alpha/beta nitride contents determined by X-ray diffraction. The experiments confirm that high-purity silicon powder, usually regarded as unreactive, can be rapidly nitrided to nearly complete conversion following pretreatments designed to remove the silica layer. The results suggest that nitridation occurred by the reaction of silicon vapour with nitrogen gas resulting in the deposition of massive Si3N4.  相似文献   

4.
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.  相似文献   

5.
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.  相似文献   

6.
The nitridation behavior of silicon powder with added Zr compounds was studied in order to assess the catalytic effect of zirconium on the formation of reaction bonded silicon nitride, using high purity silicon powder and monoclinic zirconia as starting materials. Thermogravimetric analysis revealed that the addition of ZrO2 to Si powder reduced the temperature of the main nitridation reaction, and increased the amount of silicon converted to silicon nitride at a given temperature. On the other hand, the nitridation rate at higher temperatures (1380-1400 °C) indicated similar values for both pure Si and Si with ZrO2 additions.  相似文献   

7.
Silicon nitride bonded silicon carbide foams have been produced by nitridation of the foamed compacts containing silicon carbide and silicon powders. When no nitridation additive was used the ceramic foams nitrided at all temperatures studied contained a significant amount of whisker phase α-Si3N4 formed both inside and outside the cell walls leading to a loose microstructure and a low mechanical strength. When the Al2O3 and Y2O3 were used as nitridation additives, the ceramic foams nitrided at temperatures of 1360 and 1395 °C containing certain amount of Si2N2O and whisker α-Si3N4 phases that are bonded by a glassy phase and behave as reinforcements for the ceramic foams exhibited a much higher mechanical strength. At nitridation temperature of 1430 °C, the ceramic foam showed the locally formed β-Si3N4 as the main nitrided phase that caused no increase in bonding area between the nitrided phase and the silicon carbide particles. Thus, a relatively lower mechanical strength was observed for the ceramic foam.  相似文献   

8.
Several salient factors influencing the formation of reaction-bonded silicon nitride (RBSN) compacts have been studied. These include the effects of mullite and alumina furnace tubes typically employed during high-purity nitridation studies, pre-sintering of green silicon compacts, free powder versus compact nitridation, and the influence of metal/metal oxide additions. The latter studies have provided experimental evidence for enhancement due to dissociated nitrogen, and suggest that (1) -Si3N4 formation does not necessarily require a liquid phase, (2) atomic nitrogen stimulates -phase formation, and (3) the liquid phase provides an efficient source for volatile silicon, promoting -Si3N4. These conclusions are consistent with accepted mechanisms for the formation of the two phases.  相似文献   

9.
Reaction behaviour of the mixed reactant powder of NiO and M2CO3 (M = Li, Na) for the formation of Ni1−x M x O (M = Li, Na) was investigated. Ni1−x M x O (M = Li, Na) compacts were fabricated by sintering green compacts of the mixed reactant powder at 1,173 K for 5 h in air. The results indicated that Ni1−x M x O (M = Li, Na) was successfully synthesized when the sintering temperature was approximately above 800 K. In addition, the resultant Ni1−x M x O (M = Li, Na) kept the same crystal structure with that of NiO (except for x = 0.4), but the diffraction peaks were found to shift to larger diffraction degrees compared with that of NiO. Interestingly, Li ions distribute in the lattice of NiO uniformly. Meanwhile, the electrical resistivity of the sintered compacts decreased and their semiconductor behaviour was always maintained with the increase of doping Li or Na. The power factor was increased significantly and exceeded 1.8 × 10−4 Wm−1 K−2 at 823 K.  相似文献   

10.
A mechanism for the nitridation of Fe-contaminated silicon   总被引:4,自引:0,他引:4  
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 FeSix liquid phase. The possible implications of the findings for the optimization of strength of reaction-bonded silicon nitride are briefly discussed.  相似文献   

11.
The technique for the fabrication of Si3N4 which was investigated involves the nitridation of Si:Si3N4 powder compacts containing additions of sintering aids (e.g. Y2O3 and Al2O3) followed by pressureless sintering. The development of microstructure during fabrication by this method has been followed by X-ray diffraction and analytical electron microscopy. As well as being important for the sintering process, it was found that the sintering aids promote nitridation through reaction with the surface silica on the powder particles. During nitridation extremely fine grained Si3N4 forms at silicon powder particle surfaces and at tunnel walls extending into the interior of these powder particles. Secondary crystalline phases which form during nitridation are eliminated from the microstructure during sintering. The- to-Si3N4 phase transformation is completed early in the sintering process, but despite this the fully sintered product contains fine-Si3N4 grains. The grains are surrounded by a thin intergranular amorphous film.  相似文献   

12.
A mechanism for the nitridation of silicon powder compacts   总被引:1,自引:0,他引:1  
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.  相似文献   

13.
《Advanced Powder Technology》2014,25(6):1667-1671
Crystalline silicon nitride (Si3N4) wires have been synthesized by means of ball milling and nitridation route. The influence of temperature of reaction and starting condition of the powder (milled or unmilled) on the synthesis of Si3N4 wires were studied. The reduced size of silicon particle during the milling process led to an increased degree of nitridation.Silicon powders with higher surface energy can react incessantly with nitrogen to form silicon nitride wires. The results show that the Si3N4 was fully formed with two kinds morphologies including globular and wire with a width of 100–300 nm and a length of several microns at temperature of 1300 °C for 1 h by employing the milled silicon powder. The infrared adsorption of wires exhibit absorption bands related to the absorption peaks of Si–N band of Si3N4.  相似文献   

14.
The oxidation behaviour of attrition-milled Al88Si12/Al2O3 powder mixtures was investigated for the formation of mullite/Al2O3 composites by the reaction bonded alumina (RBAO) process. Cylindrical powder compacts were heated at 5°C min–1 to temperatures between 450 and 1400°C. Oxidation occurred rapidly between ca. 400 and 750°C. Dense, outer reaction layers which formed at the lower temperatures inhibited complete oxidation and led to fracture of the body during continued heating to higher temperatures (above ca. 850°C) While the incorporation of ZrO2 improved the oxidation of the samples, X-ray analysis indicated that the Si in the alloy reacted with the ZrO2 to form phases which prevented the formation of mullite at the temperatures used in the experiments.  相似文献   

15.
Cerium oxide (CeO2) is a promising material that has potential for use in a number of applications, such as resistive-type oxygen sensors and solid oxide fuel cells. In this work, the sintering behavior of hydrothermal synthesized nano-size CeO2 powders and chemical precipitated and commercial micron-size CeO2 powders were investigated by continuous monitoring of the shrinkage kinetics. The results demonstrated that during the high temperature sintering process a partial redox reaction of ceria occurred, i.e., a fraction of Ce4+ was reduced to Ce3+, and oxygen gas was released. The redox reaction influenced the sintering behavior of CeO2, resulting in a decrease in density and microcracking for the hydrothermal synthesized nano-size CeO2 powder compacts and sagged points in the sintering curves for the chemical precipitated and commercial micron-size CeO2 powder compacts. It was found by scanning electron microscopy that the partial redox reaction of ceria produced additional pores in the powder compacts during the sintering process and thus much higher temperatures were needed to achieve high density.  相似文献   

16.
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.  相似文献   

17.
Nitridation of whisker-reinforced reaction bonded silicon nitride ceramics   总被引:1,自引:0,他引:1  
Ceramic matrix composites were fabricated from silicon carbide whisker-reinforced reaction bonded silicon nitride. Optimal dispersion of the SiC whiskers in the silicon powder slip was achieved by milling and pH control; a pH range of 4–5 giving the best results. Only a slight drop in green density was observed for a 30 wt% addition of SiC whiskers. The effects of the whisker additions on the nitridation kinetics of reaction bonding were investigated and the additions were found to increase the induction period before nitridation and to slightly decrease the nitridation rate but green density and temperature were still found to be the main factors controlling nitridation. Modulus of Rupture measurements for the composites showed a decrease in strength compared to the monolithic material.  相似文献   

18.
The influence of changes in the surface chemistry and surface composition of colloidal BaTiO3, due to its dissolution and adsorption/precipitation of Ba2+ in an aqueous medium, on the microstructure and permittivity of sintered powder compacts was investigated. For BaTiO3 powder with Ba-deficient (Ti-excess) surface prepared at pH 3, grain growth was enhanced at 1350 °C (above the eutectic) and permittivity was reduced (relative to stoichiometric BaTiO3 prepared at pH 9) with increasing sintering temperature due to the liquid phase formed at grain boundaries. This same sample showed minimal grain growth and moderate enhancement of sinterability at 1300 °C (below the eutectic) attributed to sliding of the Ti-excess surface phase. BaTiO3 powder treated at pH 3 and subsequently adjusted to pH 10 results in a core-shell structure with a varying near-surface stoichiometry, and produced abnormal grain growth for the compact sintered at 1350 °C. Permittivity of this sample was significantly reduced at 1350 °C due to the formation of the liquid phase, while exhibiting a similar permittivity to that of the stoichiometric sample when sintered at 1300 °C, despite significant microstructural coarsening. We conclude that changes in the surface-phase Ba/Ti ratio of particulate precursors, due to dissolution, adsorption and precipitation reactions in aqueous media, are as significant in determining the mechanical and electronic properties of the sintered material as are variations in the bulk stoichiometry of BaTiO3.  相似文献   

19.
The atomic environment of iron impurities is investigated during the processing cycle of reaction-bonding silicon nitride (RBSN). Several analysis techniques are utilized, including X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), and electron spin resonance (ESR), to examine iron impurities in the starting silicon powder, in sintered silicon compacts, and in RBSN materials. Results indicate that iron impurities in as-received metallurgical grade silicon powder are incorporated in the silicon bulk as a highly distorted FeSi2 compound. No surface iron or iron-based particulate is observed in the starting material. Upon sintering, the iron environment becomes an ordered FeSi2 structure. In the RBNS material, the FeSi2 structure is again distorted, as observed by both EXAFS and ESR.  相似文献   

20.
Hot pressed AIN without additives was oxidized et 1100 to 1400°C in dry air, wet air and wet nitrogen gas atmospheres with 1.5 to 20 kPa of water vapour pressure. AIN was oxidized by both air and water vapour, and formed -Al2O3 film on the surface above 1150°C. The oxidation kinetics in air were parabolic end were promoted by water vapour. On the other hand, the oxidation kinetics in wet nitrogen were linear below 1250°C and parabolic above 1350°C. The oxidation rate in wet nitrogen was much greater than that in wet air. The rate of oxidation increased with increasing temperature until 1350°C, and then decreased. The parabolic rate constant decreased with increasing temperature and increased linearly with increasing water vapour pressure. The linear rate constant at 1150 to 1250° C increased with increasing the temperature with the apparent activation energy of 250 kJ mol–1. The relation between the linear rate constant and water vapour pressure was of the Langmuir type.  相似文献   

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