Silicon Nitride Derived from an Organometallic Polymeric Precursor: Preparation and Characterization

Partially crystalline Si₃N₄, with nanosized crystals and a specific surface area greater than 200 m²/g, is obtained by pyrolysis of a commercially available vinylic polysilane in a stream of anhydrous NH₃ to 1000°C. This polymer does not contain N initially. Crystallization to high-purity α-Si₃N₄ proceeds with additional heating above 1400°C under N₂. The changes in crystallinity, powder morphology, infrared spectra, and elemental compositions, for samples annealed from 1000° to 1600°C under N₂, are consistent with an amorphous-to-crystalline transformation. Although macroscopic consolidation and local densification occur at 1400°C, volatilization and accompanying weight loss limit bulk densification. The effect of temperature on specific surface area is examined and related to the sintering process. These results are applicable to pyrolysis, decomposition, and crystallization studies of ceramics synthesized by polymeric precursor routes.     

氮化制度对Si-Al-Al2O3体系合成Sialon的影响

以SiC、α-Al2O3微粉、Si粉、Al粉为原料,在1450℃流动氮气中制备Sialon/SiC材料。研究了不同温度保温氮化对Sialon合成的影响。研究结果表明:有部分硅粉残余没有氮化;在1100℃保温氮化的氮化率高于在1150℃、1200℃保温氮化;1100℃保温氮化,残余Si量少、分布均匀,残余硅熔聚现象较轻,利于Sialon的合成。     

利用粉煤灰生产SiC陶瓷原料

利用粉煤灰部分或全部代替石英砂,配以适量的锯末,在高温炉中特定的烧成制度下,采用SiO2-C还原法将粉煤灰中的二氧化硅与锯末中的碳结合制取高性能的SiC陶瓷原料。利用正交实验对合成工艺中的合成温度、保温时间、碳过量数进行了设计和探索性实验,并对合成产物中的碳化硅的含量进行了测试。研究表明,在经催化剂(FeCl26H2O)浸泡12小时,加入2%矿化剂,在1380℃的合成温度下,保温时间2小时,碳过量10%的情况下,生成的SiC陶瓷原料中SiC的含量在53.6%以上。     

Sol-Gel Alumina Coating for Improved Cyclic Oxidation Resistance of Silicon Nitride

A 25 nm thick α-alumina layer was deposited on a turbine-grade silicon nitride by sol-gel dip coating and subsequent heat treatment in air at 1200°C. This layer had a nanometer grain structure. Silicon nitride protected by this thin layer showed a significant improvement in oxidation resistance over its uncoated counterpart after 200 cyclic exposures in air at 1250°C. The oxide layer grown on the coated silicon nitride also exhibited superior surface morphology, compared with the uncoated silicon nitride.     

浅谈一次盐水的工艺指标控制

介绍了一次盐水的各项工艺指标对生产的影响以及稳定控制这些指标的方法，强调了在生产中控制好一次盐水的工艺指标是影响产品质量及离子交换膜使用寿命的关键。     

Microstructure and Mechanical Properties of Hot-Pressed Silicon Carbide-Aluminum Nitride Compositions

A flexural strength of up to 1 GPa was achieved in SiC-AIN materials and is attributed to a dense, equiaxial grain structure of the 2H(δ) SiC-AIN solid solution, with a relatively uniform grain size of ∼ 1 μm. The strength was found to decrease with increasing grain size. While the β→α phase transformation and the formation of various metastable polytypes make microstructural control difficult in SiC materials, excellent control is facilitated in SiC-AIN materials as a result of the stable 2H solid solution. Several mechanisms of grain refinement during the β→ 2H transition were observed, most notably the direct formation of several 2H grains from a single β grain. In addition, grain growth is limited by the diffusion-controlled nature of the transition. These mechanisms could be utilized to achieve even higher strength values, with potentially higher reliability of the materials in structural applications.     

Molybdenum Oxycarbide and Tungsten Oxycarbide Coatings on Silicon Carbide Fibers

The strength and toughness of fibrous composites depend on the interface properties which control the bonding between the fibers and matrices. One method of controlling the interface involves coating the fiber with an appropriate material. In a previous study, it was found that there is a definite advantage in using low coating temperatures to prevent fibers from degrading. We therefore were interested in a report that Mo₂C could be deposited from Mo(CO)₆ at temperatures as low as 300° to 475°C. Our studies indicated that the material was not Mo₂C, but an oxycarbide, which, with an analogous tungsten oxycarbide coating, was applied to SiC yarns. Both oxycarbides could be converted to the metals by heat-treating in N₂.     

Consequences of different cutting regimes on regrowth and nutrient stoichiometry of Sparganium erectum L. and Potamogeton natans L.

Aquatic vegetation forms an essential component in freshwater ecosystems but due to changed environmental and anthropogenic conditions often needs management to reduce nuisance for human land‐use. In this paper, the authors looked at the regrowth of two macrophyte species (Potamogeton natans and Sparganium erectum) in two lowland rivers under different cutting treatments. After an initial cross‐sectional transect was manually removed from bank to bank at the beginning of the growth season, a monthly repetitive removal of biomass in plots on that transect was done during the rest of the growth season (testing frequency of mowing). Additional new transects were also cut in subsequent months (testing timing of mowing). Finally, biomass was repetitively removed in plots in those additional transects too (testing frequency of mowing × timing of mowing). The biomass at the end of the growth season was analysed for C, N, P, and Si. It was demonstrated that timing and frequency of vegetation cutting has an important effect on the capacity and rate of species' recovery and therefore on the efficiency of the applied management. Nutrient stoichiometry of the regrown biomass was directly affected by cutting. Caused by differences in the applied timing and frequency of the cutting, C/N and N/P ratios and BSi concentrations were highly variable. Yet, overall, there was a clear tendency towards a higher C/N ratio and BSi concentration and lower N/P ratio in biomass that recovered after cutting. This human impact on the quantity and quality of autochthonous organic matter may have knock‐on effects on the decomposers food web and mineralization process.     

Solubility of Si3N4 in Liquid SiO2

The nitrogen solubility in the SiO₂-rich liquid in the metastable binary SiO₂-Si₃N₄ system has been determined by analytical TEM to be 1%–4% of N/(O + N) at 1973–2223 K. Analysis of the near edge structure of the electron energy loss peak indicates that nitrogen is incorporated into the silicate network rather than being present as molecular N₂. A regular solution model with a positive enthalpy of mixing for the liquid was used to match the data for the metastable solubility of N in the presence of crystalline Si₃N₄ and to adjust the computed phase diagram. The solubility of Si₃N₄ in fused SiO₂ is far less than reported in liquid silicates also containing Al, Mg, and/or Y. Apparently, these cations act as modifiers that break anion bridges in the silicate network and, thereby, allow further incorporation of Si₃N₄ without prohibitive amounts of network cross-linking. Finally, indications emerged regarding the diffuse nature of the Si₃N₄-SiO₂ interface that leads to amorphous regions of higher N content.     

Mechanical Properties of Alumina/Silicon Carbide Whisker Composites

The improvement of mechanical properties of Al₂O₃/SiC whisker composites has been studied with emphasis on the effects of the whisker content and of the hot-pressing temperature. Mechanical properties such as fracture toughness and fracture strength increased with increasing whisker content up to 40 wt%. In the case of the high SiC whisker content of 40 wt%, fracture toughness of the sample hot-pressed at 1900° decreased significantly, in spite of densification, compared with one hot-pressed at 1850°. Fracture toughness strongly depended on the microstructure, especially the distribution of SiC whiskers rather than the grain size of the Al₂O₃ matrix.