Calculating the Thermal Conductivity of Heated Powder Compacts

This work is devoted to the theoretical understanding of the microstructure and thermal conductivity relationships of compacted ceramic powders in the initial, nondensifying stage of sintering. A model based on surface diffusion of vacancies for the growth of the neck between particles is combined with numerical fully three-dimensional code calculations, which solve for the effective heat conductivity coefficient of lightly sintered particles. The predictions of the model are in agreement with experimental observations. The approach presented can be applied to solve a series of related problems, like dielectric properties, with arbitrary microstructure and multicomponent composite of the powders.     

气相法白炭黑生产技术的研究

本文介绍了气相法白炭黑的制备原理与生产工艺,以及在生产过程中的技术难题.     

气相法白炭黑生产技术的研究

本文介绍了气相法白炭黑的制备原理与生产工艺,以及在生产过程中的技术难题。     

Binder Evolution from Powder Compacts: Thermal Profile for Injection-Molded Articles

Binder evolution information generated using thermal analysis techniques is used along with microstructural information to define a thermal cycle for debinderizing injection-molded articles. In addition, the roles of binder chemistry, powder morphology, binder loading, article size, heating rate, and environmental conditions in determining a satisfactory thermal cycle are investigated. Major binder evolution events and types of defects generated are identified. An improved binder removal cycle is developed from this evaluation for organics elimination of a honeycomb structure.     

气相二氧化硅的表面改性

简要介绍了气相二氧化硅的表面改性、改性原理、改性工艺、常用改性剂以及改性气相二氧化硅的应用。概述了我国气相二氧化硅工业的发展现状。     

Particle Sizes of Fumed Silica

Fumed silica is a synthetic amorphous silicon dioxide produced by burning silicon tetrachloride in an oxygen-hydrogen flame. Surface areas range from 50–400 m²/g. Using particle sizing techniques, fumed silica shows micron sized particles leading to surface areas markedly lower than expected. Fumed silica appears as a fluffy solid with bulk densities down to 0.03 g/cm³, being invariant over the wide range of surface areas. Attempts to relate the variation of the surface area directly to the performance of fumed silica in technical applications, such as its thickening efficiency in fluids, mainly fail and remain ambiguous.     

气相法二氧化硅的表面改性

以气相法二氧化硅为原料,六甲基二硅氮烷为改性剂,采用干法工艺对气相法二氧化硅表面进行改性,探讨了六甲基二硅氮烷用量、反应温度、反应时间对气相法二氧化硅表面改性效果的影响。结果表明,当六甲基二硅氮烷用量为气相法二氧化硅质量的25%、反应温度为150℃、反应时间为40 min时,二氧化硅表面的硅羟基数量最少,比改性前减少97%。     

气相法白炭黑的市场分析

简述了世界气相法白炭黑生产厂的生产能力、国内外差距和国内需求预测。     

Reaction of Ammonia with Fumed Silica

The reaction between fumed silica and ammonia has been studied between 600° and 1200°C. Up to 24 wt% nitrogen can be introduced into vitreous silica by adjusting the partial pressure of ammonia, temperature, and reaction time. The experiments show that the amount of nitrogen incorporated into silica has a linear dependence on the partial pressure of ammonia and a square root dependence on the reaction time. The activation energy for the reaction was found to be 76.0 kJ/mol. The nitrided powders as well as several hotpressed samples were crystallized at 1550° to 1650°C. Silica glasses containing 0 to 1 wt% nitrogen crystallize to cristobalite. Glasses containing 2 to 10 wt% nitrogen are stable against devitrification in nitrogen, even at 1650°C. Glasses with 12 to 28 wt% nitrogen crystallize to fine-grained Si₂N₂O with residual glass.     

气相法二氧化硅在硅橡胶中的应用

介绍了气相法二氧化硅在硅橡胶中的用量对其拉伸强度、伸长率及硬度的影响。此外,分析了硫化后产生气泡的各种原因。     

气相法白炭黑在消泡剂中的应用

论述了气相法白炭黑的结构特点及在消泡剂中的作用,表明比表面积为200～380m2/g的气相法白炭黑具有最佳应用效果。接着,讲述了它的应用特点、配比和应用体系。     

合成条件对气相法白炭黑性能的影响

以甲基三氯硅烷、氢气、空气为原料,制备气相法白炭黑。考察了原料配比、骤冷介质温度、合成炉温度、原料气速对白炭黑的比表面积、平均粒径的影响。结果表明：甲基三氯硅烷、氢气的量之比与白炭黑的比表面积成反比;合成炉温、骤冷介质温度与白炭黑的比表面积、平均粒径成反比;原料气速与白炭黑的比表面积成正比,与平均粒径成反比。较佳工艺为：甲基三氯硅烷、氢气与空气的量之比为0.33∶0.47∶1,冷却介质温度80～90℃,炉温800～850℃,气体流速5～6 m/s。     

气相法白炭黑的研究进展

概述了气相法白炭黑的制备、结构与性能，重点介绍了气相法白炭黑的改性方法以及在高分子材料方面的应用进展，并展望了气相法白炭黑的应用前景。     

气相二氧化硅胶体电解质的研究进展

文章介绍了阀控密封胶体蓄电池用气相二氧化硅电解质的研究进展。分别从气相二氧化硅的性质,胶体电解质的配制工艺,气相二氧化硅的粒径,胶体电解质的含量及添加剂等几个方面进行了综述。最后指出了胶体蓄电池的存在的主要问题及其发展前景。     

SiO2微粉悬浮液的剪切流变行为

利用流变仪对SiO2微粉悬浮液进行稳态剪切和动态剪切实验,通过同轴旋转剪切、小振幅振荡剪切、大振幅振荡剪切3种方式对悬浮液的剪切流变行为进行表征,得到悬浮液结构稳定性的动力学条件及结构破坏条件。结果表明:同轴旋转法可对悬浮液结构稳定性的强弱进行定性判断,非牛顿系数n能够表征悬浮液结构稳定性的强弱;小振幅振荡法可对其强弱进行定量分析,确定了悬浮液的线性黏弾区,获得了悬浮液结构开始破坏、开始流动时的应变和剪切应力;大振幅振荡法实验下,Cox-Merz经验公式适用于SiO2微粉悬浮液,能够确定存储模量为零的状态,进而得到悬浮液结构完全被破坏时所需的剪切应力和剪切速率。     

气相二氧化硅的制备方法及其特性

简述了气相二氧化硅的发展情况和制备方法,介绍了气相二氧化硅的特性、标准及其相关表征方法。     

Reformation of Fumed Silica Gel After Being Broken

The reorganization of fumed silica gel formed in sulphuric acid medium after being broken has been studied using rheological measurements including thixotropy, gelling time, and gel strength determination according to the falling rod method. The original gel formed by fumed silica particles was also studied for comparison. The results showed that the segments yielded after the original gel was broken can reorganize into a new gel although its ability was weaker than that of fresh particles. The dispersed size of silica particles influenced both the formation of the original gel and the reorganization behavior. The smaller silica particles favored a prolonged gelling time but yielded a gel with relatively low strength. Comparatively, the tendency to reorganize was weaker than in the original gel formation. The standing temperature during the fresh gel formation showed less of an effect on the reorganization process.     

Fracture Toughness of Spray-Dried Powder Compacts

The strengths and fracture toughness values were measured for alumina powder compacts containing two different binder systems. Diametral compression was used to measure both the tensile strength and the fracture toughness (through-thickness notch). This methodology was very useful in linking processing parameters, such as binder choice and compaction stress, to the quality of the green bodies. Observations of the compact structure before and after fracture showed that the binders segregated to the region between the spray-dried granules. The presence of the excess binder in this region was linked to both the failure mode and the creation of secondary cracks.     

Liquid Precursor Infiltration Processing of Powder Compacts: II, Fracture Toughness and Strength

Si₃N₄ powder compacts were infiltrated with liquid precursors which produce either Zr(Y)O₂ (3 mol% Y₂O₃) solid solution or amorphous Si₃N₄ after pyrolysis at relative low temperatures and without shrinkage. Results show that cracks which occur within a thin, surface layer of the precursor during pyrolysis can extend into the powder compact. As suggested by theory, this cracking phenomenon could be avoided either by making the powder compact stronger before infiltration or by removing the thin precursor layer before pyrolysis. The mechanical properties of these materials were studied as a function of residual porosity. It was observed that crack extension occurred within the second phase produced by infiltration and pyrolysis. The second phase appeared to govern the critical stress intensity factor ( K_c ) of the material. K_c was found to be a linear function of the change in residual, relative porosity divided by the initial, relative porosity in the powder compact. Reasonable flexural strengths (∼300 MPa) could be achieved despite considerable residual porosity.