关键词： Silicon nitride  

摘要： The improvement of mechanical properties by microstructural control has been one of the main topics of interest in the development of silicon nitride ceramics. Toughening, by developing an in situ composite or self-reinforced microstructure, has attracted particular *** design is a key factor in the optimization of processing parameters. The microstructures of sintered materials are composed of silicon nitride grains and grain boundaries, which can be either crystalline, amorphous, or partially crystalline, depending on the composition, amount of sintering additives, and processing parameters. Silicon nitride ceramics have been fabricated with an addition of metal oxides and rare-earth oxides that form a liquid phase during sintering and accelerate grain boundary diffusion. The effect of composition of the glassy phase on the mechanical properties of ceramics is presented by Becher et al. and Hoffmann elsewhere in this issue. This article focuses specifically on the design and control of grain *** it is well recognized, many processing parameters affect grain growth behavior and the resulting microstructure. During sintering, the α- to β-phase transformation leads to a self-reinforcing microstructure on account of the anisotropic grain growth of the stable hexagonal β- Si3N4 phase. Therefore, α-rich powders are widely used for starting materials. Phase transformation accelerates anisotropic grain growth, resulting in an increase in the fracture toughness of Si3N4 ceramics. Kang and Han discuss the effect of phase transformation on nucleation and grain growth in an article in this issue. The effect of the grain-size distribution on microstructural development is described in this article, based on studies conducted mostly with β-Si3N4 powders.

关键词： Silicon carbide  

摘要： Application of silicon carbide (SiC) as a structural material has been limited thus far by its low fracture toughness, even though, in comparison to other ceramic materials, SiC has superior high-temperature strength and creep, wear, corrosion, and oxidation resistance. For automotive applications, a higher fracture toughness is required. For example, the brittleness and catastrophic fracture behavior of SiC materials have resulted in limited use in automobile exhaust-valve systems and turbocharger rotors. High-density SiC bodies can be produced by a pressureless sintering process. However, the sintered bodies often include flaws which are related to processing, primarily, the presence of agglomerates and crystallographic defects in the starting powders. The importance of grain size and shape refinement in the improvement of mechanical properties has been recognized, and thus, processing procedures and sintering aid compositions have been examined extensively. However, one of the key factors is the “as-received” powder characterization (distribution of grain sizes, polytypes, and impurities) for producing sintered bodies of SiC with consistent physical properties.A complexity in SiC materials is that SiC can form various crystal structures having essentially the same chemical composition but a differing number of stacking layers in the unit cell. This is commonly called a polytype. There is only one crystal structure with cubic symmetry, which is identified as 3C or the β-phase. At high temperature, the β-phase transforms to α-phases with hexagonal or rhombohedral symmetry, with 4H, 15R, and 6H (Ramsdell notation) being the major polytypes observed in SiC materials. Preference of the polytype selection during the β- to α-phase transformation is dependent on the chemistry of the sintering aids and metallic impurities in the grain boundaries.

关键词： Cements  

摘要： A computer-based numerical model is presented, with which hydration and micro-structural development in Portland cement-based materials can be simulated. The model, called HYMOSTRUC, the acronym for HYdration, Morphology and STRUCtural development, enables the prediction of hydration curves as a function of the particle size distribution and chemical composition of the cement, the water/cement ratio and the actual reaction temperature. The influence of microstructural development, i.e. the formation of interparticle contacts, on the rate of hydration is modelled explicitly. The background and structure of the simulation model are discussed in Part I of the series. Characteristic model features and the predictive potential of the model as regards the hydration process, hydration temperatures and microstructural development in both bulk cement paste and in the matrix-aggregate interfacial zone will be dealt with in Part II. A detailed explanation of the model is found in (2).

关键词： Silicon carbide  

摘要： The machinability of a heterogeneous silicon carbide with weak interphase boundaries, elongated grains, and high internal stresses is evaluated relative to a homogeneous control material with a well-bonded, equiaxed, and unstressed grain structure, Drilling and grinding rates for the silicon carbide are substantially enhanced by the microstructural heterogeneity-the weak boundaries enable easy grain-scale dislodgement in place of the more conventional macrofracture chipping mode of removal. At the same time, the residual machining damage in the machined surfaces is significantly less strength degrading in the heterogeneous material. Implications concerning the microstructural design of flaw-tolerant ceramics for enhanced machinability are considered.

关键词： SrO   ZrO2-MgO-SiO2-SrO   MgO   四元系   显微结构   SiO  

摘要： 对MgO-SuO2-SrO（CaO）三元系的相平衡关系和ZrO2-MgO-SiO2-SrO（CaO）四元系的相平衡关系进行了比较。研究了SrO/SiO2比对MgO质耐火材料各种次生相形成的影响。相平衡和显微结构的研究结果预示了含有SrO的材料具有较高的使用温度的潜力,而且凡是需要避免CaO存在的地方,选择这种材料可能是恰当的。

关键词： .radiation-induced   Irradiated   Structural Changes   soft rays   strands   chemically   cytoplasm   epidermal   Chlorella   tomato   Study the Ultrastructure   Living Biological   light microscopy   vacuole   abscission   protonema  

摘要： Imaging biological specimens with soft x rays offers several potential benefits over electron microscopy, and these are briefly reviewed. The disadvantages, most notably radiation-induced structural changes, have been investigated and images of irradiated algal cells (Chlorella) are presented. In soft x-ray contact microscopy the image is recorded rapidly to avoid both natural and radiation-induced movement and this technique has been used to study the ultrastructural effects of electron microscopy fixatives. In the epidermal hairs of tomato plants there are numerous strands of cytoplasm which, by light microscopy, appear to traverse the vacuole but are rarely seen by electron microscopy. However, by soft x-ray contact microscopy these strands and the organelles within them can be successfully imaged. Moreover, examination by soft x-ray contact microscopy of the cytoplasm in a fixed material shows that these strands are not present in chemically fixed material. This paper also reports the use of soft x-ray contact microscopy to examine the abscission cells found within the protonema of a moss (Bryum tenuisetum) and compares the images to those obtained by light and electron microscopy. © 1995 IOS Press. All rights reserved.

关键词： 烧结助剂Ni   C-B4C-SiC复合材料   显微结构   性能  

摘要： 本文研究了烧结助剂Ｎｉ对Ｃ－ＢＣ－ＳｉＣ复合材料显微结构与性能的影响．Ｘ射线衍射表明Ｎｉ在烧结温度下与ＳｉＣ，ＢＣ人发生反应在晶界生成ＮｉＳｉＢ，并产生液相，有效地促进了Ｃ－ＢＣ－ＳｉＣ复合材料的烧结，抑制了晶粒的长大，使复合材料密度与强度大幅度地增加，电阻率下降；同时ＮｉＳｉＢ在氧化时生成致密的２ＮｉＯ·ＢＯ，包裹了易氧化的ＢＣ和Ｃ，有效地防止了复合材料的氧化，从而大大提高了制品的抗氧化性能．

关键词： 瓷绝缘子   材料   显微组织  

摘要： 对１５家中小厂２５种电瓷试样进行了显微结构分析。阐述了提高原料质量和加强工艺控制对改善瓷质显微结构的重要性。