Alkali silica reactivity of agglomerated silica fume

It is commonly accepted that replacement of a portion of cement in mortar or concrete with well-dispersed silica fume reduces expansion caused by alkali silica reaction. Recently there has been much discussion that large, agglomerated particles of silica fume may actually act as alkali silica reactive aggregates, thereby increasing expansion rather than reducing it. The data in the literature, from both field and laboratory studies, are inconsistent. This prompted an extensive laboratory investigation into the alkali silica reactivity of silica fume. Results from accelerated expansion testing and microscopic investigations are presented. It was seen that some agglomerated silica fumes participate in ASR while others do not. Factors determining the reactivity of silica fume agglomerates are suggested.     

Flame retardant mechanism of silica gel/silica

Various types of silica, silica gel, fumed silicas and fused silica were added to polypropylene and polyethylene oxide to determine their flame retardant effectiveness and mechanisms. Polypropylene was chosen as a non‐char‐forming thermoplastic and polyethylene oxide was chosen as a polar char‐forming (slight) thermoplastic. Flammability properties were measured in the cone calorimeter and the mass loss rate was measured in our radiative gasification device in nitrogen to exclude any gas phase oxidation reactions. The addition of low density, large surface area silicas, such as fumed silicas and silica gel to polypropylene and polyethylene oxide significantly reduced the heat release rate and mass loss rate. However, the addition of fused silica did not reduce the flammability properties as much as other silicas. The mechanism of reduction in flammability properties is based on the physical processes in the condensed phase instead of chemical reactions. The balance between the density and the surface area of the additive and polymer melt viscosity determines whether the additive accumulates near the sample surface or sinks through the polymer melt layer. Fumed silicas and silica gel used in this study accumulated near the surface to act as a thermal insulation layer and also to reduce the polymer concentration near the surface. However, fused silica used in this study mainly sank through the polymer melt layer and did not accumulate near the surface. The heat release and the mass loss rate of polypropylene decreased nearly proportionally with an increase in mass loading level of silica gel up to 20% mass fraction. Polyethylene oxide samples with fumed silicas and silica gel formed physically strong char/silica surface layers. This layer acted not only as thermal insulation to protect virgin polymer but also acted as a barrier against the migration of the thermal degradation products to the surface. Published in 2000 by John Wiley & Sons, Ltd.     

以硅灰、白炭黑、硅溶胶为硅源合成碳化硅晶须的研究

以硅灰、白炭黑、硅溶胶为硅源,炭黑为碳源,采用碳热还原法合成碳化硅晶须,通过XRD及SEM对合成产物的物相及形貌进行分析,探讨了合成温度(分别为1 400、1 450、1 500、1 550℃)、硅源、n(C)∶n(SiO2)对合成碳化硅晶须的影响.结果表明:n(C)∶n(SiO2)为2.4～3.6,合成温度为1 500 ℃,保温3 h时,硅溶胶与炭黑反应没有生成碳化硅晶须,硅灰、白炭黑与炭黑反应均生成碳化硅晶须;以硅灰为硅源合成碳化硅晶须的质量及数量明显优于以白发黑为硅源合成碳化硅晶须;合成碳化硅晶须的最佳n(C)∶n(SiO2)为3.3.     

Synthesis of mesoporous silica using a mineral silica source

Mesoporous silica has been applied in catalysis, separations, and drug delivery. It has generally been made using organosilicon precursors such as tetraethyl orthosilicate. For sustainability, it is necessary to find readily accessible mineral sources for making mesoporous silica. In this work, we demonstrate the successful synthesis of mesoporous silica with 10 nm average pore size using the mineral forsterite (Mg₂SiO₄) as a silica source, providing a potentially cheaper and more Earth-friendly route to making technologically important porous silica materials. Pure forsterite was synthesized by a solid-state chemistry route at 1000°C and underwent dissolution–reprecipitation in aqueous hydrochloric acid containing the soft template surfactant, Pluronic P123. Variations of initial reaction pH (−0.2 to 0.6), reaction time (12–24 h), reaction temperature (50 to 90°C), and silica precursor (forsterite and fumed silica) were performed. The mesoporous silica aged at 70°C for 24 h had the highest porosity, with a surface area of 735 m²/g and a pore volume of 1.4 ml/g, comparable to mesoporous silica made using conventional starting materials. This novel geomimetic synthesis route supports the possibility of analogous formation of structured (mesoporous or zeolitic) silica in nature under abiotic or prebiotic conditions.     

Novel epoxy–silica nano-composites using epoxy-modified silica hyper-branched structure

Epoxy-based nano-composite films with organically modified silica reinforcement have been synthesized and characterized for their thermal and mechanical properties. Silica network generated from the sol–gel process using tetraethoxysilane (TEOS) was suitably modified to have epoxy groups within an extended silica network structure. For this purpose glycidoxypropyltrimethoxysilane and aminophenyltriethoxysilane were reacted to produce epoxide chains having alkoxy end-groups. The precursor molecules having both epoxy and alkoxide functionalities were then mixed with TEOS to produce epoxymodified silica network within a diglycidal ether of bisphenol-A (DEGEBA) matrix. Cross-linking of the matrix using jeffamine D-400 and the sol–gel process was carried out simultaneously. Epoxy–silica hybrids with different silica contents were characterized by scattering electron and atomic force microscopic techniques. Visco-elastic properties, thermal stability and coefficient of thermal expansion of the resulting hybrids were measured through dynamical thermal mechanical, thermogravimetric and thermal mechanical analyses. Large interfacial interaction between the organic and inorganic phases and the distribution of silica at nano-level using epoxy-modified silica in the matrix results in considerable improvement in mechanical properties whereas the coefficient of thermal expansion is reduced in contrast to the neat epoxy system.     

白炭黑与单分散二氧化硅粒子补强橡胶的差异

比较白炭黑与单分散二氧化硅粒子补强橡胶的差异。单分散纳米二氧化硅粒子表面光滑，粒子之间作用较弱，有利于分子链问的相对运动；与单分散二氧化硅粒子相比，纳米二氧化硅粒子链具有4个方面的特性：①纳米粒子链的弹性，②纳米粒子链的重聚性，③链的粒子粗糙表面的活性，④链的粒子问“脖颈”和支链的形态结构性。纳米二氧化硅粒子链补强橡胶的分散结构比单分散二氧化硅粒子补强橡胶更加复杂；高分子链与二氧化硅粒子链组成的复合团聚体对橡胶的补强、增韧起关键作用。     

介孔中空二氧化硅及硅基微球制备研究进展

介孔中空材料有特定孔道结构,具有中空、密度小、比表面积大的特点,因而具有较好的渗透性、吸附性、筛分分子能力和光学性能,成为具有广泛应用前景的热点研究材料。详细地介绍了国内外新型功能材料介孔中空二氧化硅及硅基微球主要制备方法,包括表面沉积法、层层组装法、原子转移自由基聚合法、喷雾法、微乳液法。同时介绍了合成的介孔、中空二氧化硅和硅基微球的形貌特点及应用。指出不同方法、不同制备条件对材料的形貌、孔径大小、孔形状及材料晶形有很大影响,条件温和、步骤简单、环境友好的制备方法是发展趋势。     

Synthesis of ultrabright nanoporous fluorescent silica discoids using an inorganic silica precursor

The templated sol-gel synthesis of ultrabright fluorescent nanoporous silica particles based on the use of organic silica sources has previously been reported. The use of organosilanes as the main silica precursors has a number of issues, in particular, the low robustness of the synthesis due to instability of the organic silica source. Here we report on a novel synthesis of ultrabright fluorescent nanoporous silica discoids (a specific shape in-between the sphere and disk) of 3.1 ± 0.7 microns in size, which were prepared using a stable inorganic sodium silicate silica source. Organic fluorescent dye Rhodamine 6G (R6G) was physically (non-covalently) entrapped inside cylindrical nanochannels of ～4-5 nm in diameter. In contrast to the synthesis with organic silica precursors, the obtained particles showed an excessive leakage of dye. To prevent this leakage, we modified the synthesis by adding a small amount of a secondary silica source. The synthesized particles show virtually no leakage, high photostability, and a brightness equivalent to the fluorescence of up to 7 × 10(7) free R6G molecules. This is about 7 times higher than the fluorescent brightness of particles of the same size made of CdSe/ZnS quantum dots, and 420 times higher than the brightness of the same volume of aqueous solution of free R6G dye.     

白炭黑性剂性能探讨

对醇类（甘油和PEG－4000）、脲类及胺类活性剂的透明性、变色情况、加工性能及硫化性能进行了对比试验。结果表明，PEG－4000配料方便，混炼时易分散，硫化胶的物理性能最佳，变色程度居中，是最的白黑活性剂；脲类活性剂最透明，变色最轻，可以减小用量与PEG－4000并用；甘油的活化性能较好，但配料误差大，工艺性能不稳定，胺类活性剂变色最重，活化性能不好，故二者不宜使用。     

Rapid firing of silica

Summary It was shown to be possible to fire silica in 200-ton periodic kilns in 160–170 h without worsening their properites.Experiments on the rapid heating and cooling of silica need to be continued both in test and in industrial furnaces.In an experimental furnace the firing time was reduced by about 80%, but this schedule was not proven in an industrial kiln.The obtained thermal-physical properties of the green goods permit a more accurate calculation of the permitted rated of heating. It is necessary to explain the influence of grading, batch composition and pressing conditions on the thermal-physical properties of green silica goods.     

Preparation of amorphous silica

Amorphous silica having reinforcing properties for styrene butadiene rubber (SBR) was obtained by contacting silica aquagel seed, containing 1-3% SiO₂ by weight, with a freshly prepared silica sol. Reinforcing by the isolated silica was greatly affected by the pH of the aquagel-sol system and moderately affected by temperature and by the silica content of the aquagel seed. Maximum tensile strengths for the vulcanizates were found using the product obtained from a 2 to 2.5% silica aquagel seed-sodium silicate system acidified with sulphuric acid to pH 3 at 85 to 95°C.     

Transparent PMMA/silica nanocomposites containing silica nanoparticles coating under supercritical conditions

This work reports the preparation of PMMA/silica nanocomposites with high optical transparency and enhanced mechanical properties using a melt compounding method. The surface of SiO₂ particles was modified with a γ-methacryloxypropyltrimethoxy silane coupling agent in a supercritical carbon dioxide-ethanol mixture and by conventional procedure. Dispersion of silica nanoparticles in ethanol at low temperatures plays an important role in deagglomeration and dispersion of nanosilica, which leads to the optimal particle-matrix bonding in composites. The optimal mechanical and optical properties were found for composites loaded with 5 wt% silica nanoparticles treated under supercritical coating method.