硅粉直接氮化反应合成氮化硅研究

研究了硅粉直接氮化反应合成氮化硅粉末的工艺因素(包括硅粉粒度、氮化温度、成型压力、稀释剂含量等),借助XRD,SEM等测试手段测定和观察了氮化产物的物相组成和断口形貌.研究结果表明:硅粉在流动氮气氛下,高于1200℃氮化产物中氮含量明显增加;在氮化反应同时还伴随着硅粉的熔结过程,它阻碍硅粉的进一步氮化,其影响程度与氮化温度、氮化速度,素坯成型压力及硅粉粒度等工艺因素有关.在硅粉素坯中引入氮化硅作为稀释剂,提高了硅粉的氮化率,使产物中残留硅量降低;同样在实际生产中可以通过控制适当热处理制度(如分段保温、慢速升温),达到硅粉的完全氮化.在生产中批量合成了含氮量为32.5%,残留硅量为0.05%,主要为α相,含少量β相的针状、柱状的氮化硅.     

Carbothermal Preparation of Silicon Nitride: Influence of Starting Material and Synthesis Parameters

C and SiO₂ of different types were mixed and heat-treated (1410° to 1550°C), according to a standardized temperature program, in flowing N₂ at different pressures. It was found that a starting material combination of C (115 m²/g) and SiO₂ (50 m²/g) yielded pure Si₃N₄ after about 2 h at 1550°C and 1.3 MPa. By adjustment of the pressure in the range 2 to 6 MPa, irreversible evaporation of SiO( g ) as well as formation of nonequilibrium phases was suppressed, and the amounts of residual C and O in the sample were controlled. Even a small amount of CO in the N₂ was observed to retard the nitridation. Possible explanations are discussed, based on thermodynamic calculations on the Si-O-N-C system.     

催化氮化制备氮化硅粉体

氮化硅陶瓷由于具有优良的机械性能、化学性能和物理性能而被广泛应用于化工、冶金及航天等领域.催化氮化法制备氮化硅可以有效避免“硅芯”及“流硅”等不完全氮化形为的发生;并促进氮化硅晶须的原位反应合成,改善氮化硅基材料界面的显微结构,提高最终制品的力学性能.本文综述了金属及金属氧化物催化剂催化氮化反应生成氮化硅的最新进展及一维氮化硅的原位生成机理,并在此基础上展望了催化氮化制备氮化硅工艺今后的发展方向.     

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 excess of C is necessary for complete conversion, and a simple sol–gelmixing technique provides excellent intermixing of the precursors. Copious flow of N₂ gas throughout the reactor bed is essential if pure Si₃N₄ is to be produced; small concentrations of CO and O₂ promote SiC and Si₂N₂O, respectively.     

Si3N4及其复合材料强韧化研究进展

简述了氮化硅陶瓷的结构、性能和制备工艺，并分别通过自增韧补强、纤维／晶须强韧化、层状结构强韧化、相变强韧化以及颗粒弥散强韧化等方法对氮化硅陶瓷的强韧化研究进行了分类叙述。     

氮化硅流延膜的制备

流延成型是一种制备高质量陶瓷基片的成型方法.氮化硅是一种高热导率的材料,有望在电子基片领域获得应用.本文利用流延成型制备了具有较好柔韧性和一定强度的氮化硅流延素坯膜.研究了无水乙醇、无水乙醇/丁酮作为溶剂时对浆料粘度的影响.通过优化流延浆料添加剂的各种配比,得出了适合氮化硅粉体(SN-E10)流延的最佳配方.     

Preparation of Ultrafine Silicon Nitride, and Silicon Nitride and Silicon Carbide Mixed Powders in a Hybrid Plasma

Ultrafine Si₃N₄ and Si₃N₄+ SiC mixed powders were synthesized through thermal plasma chemical vapor deposition (CVD) using a hybrid plasma which was characterized by the superposition of a radio-frequency plasma and an arc jet. The reactant, SiCl₄, was injected into an arc jet and completely decomposed in a hybrid plasma, and the second reactant, CH₄ and/or NH₃, was injected into the tail flame through multistage ring slits. In the case of ultrafine Si₃N₄ powder synthesis, reaction effieciency increased significantly by multistage injection compared to single-stage injection. The most striking result is that amorphous Si₃N₄ with a nitrogen content of about 37 wt% and a particle size of 10 to 30 nm could be prepared successfully even at the theoretical NH₃/SiCl₄ molar ratio of ∼ 1.33, although the crystallinity depended on the NH₃/SiCl₄ molar ratio and the injection method. For the preparation of Si₃N₄+ SiC mixed powders, the N/C composition ratio and particle size could be controlled not only by regulating the flow rate of the NH₃ and CH₄ reactant gases and the H₂ quenching gas, but also by adjusting the reaction space. The results of this study provide sufficient evidence to suggest that multistage injection is very effective for regulating the condensation process of fine particles in a plasma tail flame.     

Preparation of Silicon Nitride Whiskers from Diatomaceous Earth: I, Reaction Conditions

Whiskers and powder of silicon nitride were prepared by the carbothermal reduction and nitridation of diatomaceous earth in the presence of flowing N₂ and NH₃. The optimum temperature for the formation of Si₃N₄ whiskers was 1350°C and the yield reached almost 20% after 24 h. The α-Si₃N₄ content decreased with increasing nitridation temperature. Yields of the whiskers were dependent on NH₃ concentration and the total gas feed rate. The maximum yield of inside whiskers was obtained for a 25 vol% NH₃/N₂ mixture, while the maximum quantity of outside whiskers was produced for 75 vol% NH₃/N₂. The sum of the yield of the inside and outside whiskers increased with decreasing total gas feed rate. However, no nitridation of SiO₂ was observed at a feed gas rate below 0.18 mmol·min⁻¹. The yield of the inside whiskers increased gradually with increasing reaction time up to 36 h, whereupon a constant value was attained. Although the amount of outside whiskers produced was relatively small, the quantity seemed to increase until 60 h.     

硅藻土基调湿板材的水热合成试验研究

天然硅藻土中无定形二氧化硅的含量很高,以其为硅质材料,可用于制备硅酸盐制品.实验中采用铺浆注模、压制脱水成型、常压湿热养护等工艺措施制备硅藻土板材,考察了原材料掺量、脱水压力对硅藻土板材的力学性能影响,以及不同钙硅比对板材吸放湿性能的影响.试验结果表明:钙硅比在2:6时可获得最大抗折强度,随水泥掺量的增加样品抗折强度呈上升趋势,提高脱硫石膏掺量,样品的抗折强度随之增加,脱水压力的增加可提高样品的抗折强度,增加木质纤维掺量可提高制品抗折强度,外掺1％耐碱玻璃纤维可获得最大抗折强度;钙硅比的变化明显影响样品的吸放湿率,对样品的放湿速率影响较大,吸湿速率影响不大;样品的吸放湿率优于市面已有石膏板及硅酸钙板;SEM照片显示,硅藻土内部孔中水化产物随钙硅比的增加而增多.     

Reactivity of Aluminum Nitride Powder in Aqueous Silicon Nitride and Silicon Carbide Slurries

The reactivity of AlN powder with water in supernatants obtained from centrifuged Si₃N₄ and SiC slurries was studied by monitoring the pH versus time. Various Si₃N₄ and SiC powders were used, which were fabricated by different production routes and had surfaces oxidized to different degrees. The reactivity of the AlN powder in the supernatants was found to depend strongly on the concentration of dissolved silica in these slurries relative to the surface area of the AlN powder in the slurry. The hydrolysis of AlN did not occur if the concentration of dissolved silica, with respect to the AlN powder surface, was high enough (1 mg SiO₂/(m² AlN powder)) to form a layer of aluminosilicates on the AlN powder surface. This assumption was verified by measuring the pH of more concentrated (31 vol%) Si₃N₄ and SiC suspensions also including 5 wt% of AlN powder (with respect to the solids).     

Machinability of Silicon Nitride/Boron Nitride Nanocomposites

The machinability and deformation mechanism of Si₃N₄/BN nanocomposites were investigated in the present work. The fracture strength of Si₃N₄/BN microcomposites remarkably decreased with increased hexagonal graphitic boron nitride ( h -BN) content, although machinability was somewhat improved. However, the nanocomposites fabricated using the chemical method simultaneously had high fracture strength and good machinability. Hertzian contact tests were performed to clarify the deformation behavior by mechanical shock. As a result of this test, the damage of the monolithic Si₃N₄ and Si₃N₄/BN microcomposites indicated a classical Hertzian cone fracture and many large cracks, whereas the damage observed in the nanocomposites appeared to be quasi-plastic deformation.     

X-ray Diffraction Study of the Structure of Silicon Nitride Fiber Made from Perhydropolysilazane

Continuous stoichiometric silicon nitride fiber was produced by the pyrolysis of perhydropolysilazane. This high-purity silicon nitride fiber is colorless and has high strength, modulus of elasticity, and thermal stability which are properties suitable for reinforcing plastics, metals, glasses, and even ceramics. The structures of three kinds of amorphous silicon nitride fibers (with stoichiometric, Si-excess and O-excess composition) were investigated by X-ray diffraction followed by calculating the radial distribution functions. Radial distribution functions were calculated from several possible crystal structures and compared with observed radial distribution functions.     

氮化硅/羟基磷灰石复合材料的制备研究

主要用超声分散结合滴定工艺制备了氮化硅／羟基磷灰石(Si3N4/HAp)复合粉体，并经冷压成型、冷等静压成型及无压烧结制备了氮化硅／羟基磷灰石复合材料。X射线衍射(XRD)及扫描电镜(SEM)研究发现，氮化硅的加入促进了羟基磷灰石的分解，细化了晶粒，可降低羟基磷灰石的烧结温度。     

Preparation of Mesoporous Silicon Nitride via a Nonaqueous Sol–Gel Route

A silicon diimide gel Si(NH) _x (NH₂) _y (NMe₂) _z was prepared by an acid-catalyzed ammonolysis of tris(dimethylamino)silylamine. Pyrolysis of the gel at 1000°C under NH₃ flow led to the formation of an amorphous silicon nitride material without carbon contamination. All of the gel and pyrolyzed products exhibited a mesoporous structure with a high surface area and narrow pore-size distribution. The effective surface area of the pyrolyzed silicon nitride residues decreases with increasing temperature, but the heating rate during pyrolysis has little influence on the surface area and pore-size distribution of the final mesoporous ceramic Si₃N₄ products because of the highly cross-linked structures of the precursor silicon diimide gel.     

Fabrication and Microstructure of Silicon Nitride/Boron Nitride Nanocomposites

A chemical process for fabrication of Si₃N₄/BN nanocomposite was devised to improve the mechanical properties. Si₃N₄/BN nanocomposites containing 0 to 30 vol% hexagonal BN ( h -BN) were successfully fabricated by hot-pressing α-Si₃N₄ powders, on which turbostratic BN ( t -BN) with a disordered layer structure was partly coated. The t -BN coating on α-Si₃N₄ particles was prepared by reducing and heating α-Si₃N₄ particles covered with a mixture of boric acid and urea. TEM observations of this nanocomposite revealed that the nanosized hexagonal BN ( h -BN) particles were homogeneously dispersed within Si₃N₄ grains as well as at grain boundaries. As expected from the rules of composites, Young's modulus of both micro- and nanocomposites decreased with an increase in h -BN content, while the fracture strength of the nanocomposites prepared in this work was significantly improved, compared with the conventional microcomposites.     

硅藻土在聚烯烃色母粒中的应用研究

将硅藻土与有机蓝颜料在双辊开炼机上直接混合制成色母粒,并对聚烯烃进行着色。利用加硫成型试验机制备色板和薄膜,利用测色仪测定色板的反射率、L*、a*、b*值、着色力以及色差,通过光学电子显微镜观察了薄膜中颜料粒子的分散情况,研究硅藻土对色母粒着色性能、分散性能及耐热性能的影响。结果表明,硅藻土可替代部分有机蓝颜料,当替代质量分数为5%~25%时,制成的色母粒能够提高塑料制品的着色性能。此外,当硅藻土替代有机蓝颜料比例为25%时,颜料的着色性能最佳,着色聚烯烃制品的耐热温度提高10℃左右。     

Recycling of waste flat glass in the fabrication of Si3N4 powder by the carbo-thermal reduction and nitridation

The recycle rate of waste flat glass is quite low in spite of higher SiO₂ content and its pre-melted characteristic. This study searched a novel approach for the fabrication of Si₃N₄ powder by carbo-thermal reduction and nitridation from waste glass. For comparison, starting powder mixtures were prepared from silica and waste SLS (soda-lime-silica) glass. Carbo-thermal reduction and nitridation process was conducted using two temperatures and different atmospheres. The results showed that equiaxed, micron size β-Si₃N₄ particle could be synthesized from waste glass. The use of waste glass reduced the amount of fiber-like Si₃N₄ particles and SiC phase by increasing the amount of the liquid phase formed. This study proved the utilization of waste flat glass as a low cost and environment friendly SiO₂ source.     

Texture in Silicon Nitride Seeded with Silicon Nitride Whiskers of Different Sizes

Silicon nitride ceramics seeded with 3 wt%β-Si₃N₄ whiskers of two different sizes were prepared by a modified tape casting and gas pressure sintering. The fine whiskers had a higher aspect ratio than the coarse whiskers. Quantitative texture analysis including calculation of the orientation distribution function (ODF) was used for obtaining the degrees of preferred orientation of sintered samples. The maximum multiples of random distribution (mrd) values of samples seeded with the fine and coarse whiskers were large, greater than 15 and 9, respectively. Meanwhile, the mrd value of a sample seeded with fine whiskers was only 9 when it was prepared by conventional tape casting. The microstructures and the XRD data revealed that the well-aligned whiskers grew significantly after sintering and dominated the texture. Differences among the degrees of preferred orientation of the samples were explained using Jeffrey's model on rotation of elliptical particles carried by a viscous fluid.     

In Situ Synthesis of Silicon Carbide Whiskers from Silicon Nitride Powders

Silicon carbide whiskers were synthesized in situ by direct carbothermal reduction of silicon nitride with graphite in an argon atmosphere. Phase evolution study reveals that the formation of β-SiC was initiated at 1400° to 1450°C; above 1650°C silicon was formed when carbon was deficient. Nevertheless, Si₃N₄ could be completely converted to SiC with molar ratio Si₃N₄:C = 1:3 at 1650°C. The morphology of the SiC whiskers is needlelike, with lengths and diameters changing with temperature. SiC fibers were produced on the surface of the sample fired at 1550°C with an average diameter of 0.3 μm. No catalyst was used in the syntheses, which minimizes the amount of impurities in the final products. A reaction mechanism involving the decomposition of silicon nitride has been proposed.     

Insights into the Adsorption of Heavy Metals from Wastewater using Diatomaceous Earth

The release of large quantities of heavy metals (e.g., Zn, Cu, Pb, Ni, Cd, etc.) into the natural environment has resulted in a significant number of environmental problems, disorderliness in human physiology, and other biological systems due to their high toxicity to plant, animal, and human life. This review paper explores the feasibility of diatomaceous earth for the adsorption of heavy metals from wastewater, its application in metal adsorption, and wastewater purification. Impregnating the diatomite surface with 0.38 g/g of manganese oxide gave a 2.4-fold increase in the adsorbent surface area. A number of equilibrium studies demonstrated that treated diatomite has a higher removal capacity for heavy metals from water than untreated diatomite. The future challenges and prospect of diatomite were also addressed. Conclusively, it was established that the use of diatomaceous earth is a promising technology in the removal of heavy metals from wastewaters and environment