Drying Behavior of Colloidal Silica Gels

Observations of the drying behavior of thick-wall colloidal silica gel structures are reported. Various techniques are examined to prevent cracking during the drying of these highstrain viscoelastic materials. Experiments are described which illustrate the effect of relative humidity on the drying rate and on the shrinkage of various samples under isothermal conditions. Surface temperature measurements indicate that evaporation occurs at approximately the wet-bulb temperature of the surrounding atmosphere. Acoustic detection of the internal strain activity during drying leads to the conclusion that control of the sol-gel surface tension in the material preparation is crucial to reducing the interior stresses.     

Effects of Ammonium Chloride on the Rheological Properties and Sedimentation Behavior of Aqueous Silica Suspensions

The influence of ammonium chloride (NH₄Cl) on the rheological properties and sedimentation behavior of aqueous silica (SiO₂) suspensions of varying solids volume fraction (φ_s) was studied. SiO₂ suspensions with low NH₄Cl concentration (≤0.05 M , pH 5.2) exhibited Newtonian behavior and a constant settling velocity ( U ). The volume fraction dependence was well described by the Richardson–Zaki form, U = U ₀(1 −φ_s) ⁿ , where n = 4.63 and U ₀= 1.0419 × 10⁻⁵ cm/s. At higher NH₄Cl concentrations (0.07–2.0 M , pH 5.2), suspensions exhibited shear thinning and more complicated sedimentation behavior due to their aggregated nature. For all suspensions studied, however, the apparent suspension viscosity, characteristic cluster size, and initial settling velocity were greatest at ∼0.5 M NH₄Cl and exhibited a similar dependence on salt concentration. Above 0.5 M NH₄Cl, considerable restabilization was observed. This behavior cannot be explained by traditional DLVO theory.     

Colloidal Stability of Zirconia-Doped Yttria–Silica Binary Aqueous Suspensions

Minor amounts of zirconia (up to 5 wt%) were fused with yttria and their effect on water solubility of yttria, and colloidal stability of yttria–silica aqueous binary suspensions, was examined. It is shown that incorporated zirconia reduces the water sensitivity of yttria while maintaining the hydrophilic nature of the surface. This fusion procedure makes yttria particles suitable for production of longlasting yttria–silica aqueous suspensions.     

Polymer Effects on the Chemorheological and Drying Behavior of Alumina–Poly(vinyl alcohol) Gelcasting Suspensions

A new gelcasting system based on aqueous alumina (Al₂O₃)–poly(vinyl alcohol) (PVA) suspensions cross-linked via titanium ion complexation has been developed as a feedstock for bulk casting and solid freeform fabrication (SFF) routes. Its chemorheological properties, as measured by stress viscometry and oscillatory techniques, exhibited a strong dependence upon polymer hydrolysis and molecular weight. The gelation time of systems of constant PVA volume fraction (φ_PVA^soln) and cross-linker concentration decreased with increasing degree of hydrolysis and molecular weight, whereas their steady-state elastic modulus ( G' ) exhibited the opposite dependence. Stress evolution during drying of gelcast layers was measured in situ using a controlled-environment, cantilever deflection apparatus. Both the maximum and residual drying stresses increased with increasing degree of hydrolysis, with only a modest molecular weight effect observed.     

Elastic Properties of Porous Silica Derived from Colloidal Gels

The elastic properties were determined for gel-derived vitreous silica sintered from a relative density of 0.15 to full density. The gels were made from colloidal silica, potassium soluble silicate, and formamide. The soluble silicate was found to reinforce the gel network during gelation and its concentration relative to the colloidal silica affected Young's modulus for samples of relative density, 0.15 to 0.6 Young's modulus varied from 300 to 700 MPa at a relative density of 0.15 as a function of the soluble silicate concentration. Young's modulus at full density was measured to be 72.6 GPa. Poisson's ratio was found to be a function of relative density.     

Colloidal Filtration and (Simultaneous) Sedimentation of Alumina and Silica Suspensions: Influence of Aggregates

Colloidal filtration and (simultaneous) sedimentation is studied for suspensions of alumina particles and monodisperse silica spheres. A comparison is made between stable suspensions and systems which are aggregated due to salt addition (silica) or absence of a deflocculant (alumina). From separate sedimentation experiments we conclude that the stable silica settles as an ordered particle array, that the unstable silica sediments as separate aggregates, and that aggregated aluina behaves as a densifying network. The filtration results show that settling of aggregates during filtration changes the filtration kinetics in accordance with our model for simultaneous filtration and sedimentation. Further, aggregates in suspension are shown to have little influence on the silica compact microstructure, whereas they clearly increase porosity and permeability of the alumina compact. We also find that for all suspensions porosities of compacts prepared by sedimentation are clearly larger then porosities of filter compacts.     

毫微结构的胶质二氧化硅对片状氧化铝浇注料性能的影响

最近几年溶胶作为结合剂一直用于低水泥、超低水泥和无水泥浇注料中。胶质二氧化硅是一种溶胶．由于其更高的热机械性能,它会影响到耐火浇注料的结构。若用胶质二氧化硅替代铝酸钙水泥（CAC）作为结合剂,则低水泥和超低水泥浇注料的所有优点都会表现出来,而消除了其许多缺点。研究了以不同比冽的胶质二氧化硅（如1％、3％和5％）替代CAC对片状氧化铝浇注料性能的影响。结果表明,当胶质二氧化硅从1％增加到5％时,体积密度增加,显气孔率则降低。另外,含5％胶质二氧化硅的试样在1400℃下的耐压强度与含1％或3％胶质二氧化硅或不合胶质二氧化硅的试样相比,耐压强度显著增加。     

特种陶瓷胶态成型技术中的干燥理论及应用

综述了最近几年国内外特种陶瓷胶态成型技术中干燥理论的研究进展.分析了干燥过程的基本步骤及水分流动的驱动形式.同时详细讨论了干燥过程中干燥应力的演化历史及其对薄膜或块体陶瓷生坯结构变化的影响.另外,分析了毛细管力对裂纹产生和扩展的作用.最后对干燥理论未来的发展趋势进行了展望.     

Effects of Silica Nanoparticles Content on the Properties and Corrosion Behavior of Electroless Ni-Ba-B Alloy Coatings

The beneficial role of silica nanoparticles addition as reinforcing agent on the various properties of the novel developed Ni-Ba-B coating was highlighted. Barium was considered as third element to act as an inhibiting alloying element for anodic passivation purposes. The ternary Ni-Ba-B coatings in three different concentrations of silica nanoparticles (0.5, 1.0 and 2.0 g/L) were coated on St 37 steel substrate in the presence of sodium dodecyl sulfate (SDS). The effect of nanoparticles on morphology and structure was investigated by FE-SEM, XRD and AFM tests. The nodularity and surface roughness of the coating increased by the presence of SiO2 nanoparticles in the electroless bath. The nanocomposite coating has amorphous and crystalline phases and its XRD peak at 44.50 is slightly sharper than the composite coating. DSC thermogram showed two exothermic peaks demonstrating its phase transformations. The WCA value of coating was confirmed its hydrophilicity property. Results also confirmed that the existence of silica nanoparticles results in an increase in the microhardness and corrosion resistance which may be attributed to the distribution of silica nanoparticles into Ni-Ba-B matrix.     

Polyelectrolyte Effects on the Rheological Properties of Concentrated Cement Suspensions

Polyelectrolyte species, known as superplasticizers, dramatically affect the rheological properties of dense cement suspensions. We have studied the influence of sulfonated naphthalene formaldehyde condensate (SNF) and carboxylated acrylic ester (CAE) grafted copolymers of varying molecular architecture on the surface (e.g., adsorption behavior and zeta potential) and rheological properties of concentrated cement suspensions of white portland cement and two model compounds, β-Ca₂SiO₄ and γ-Ca₂SiO₄. The adsorption of SNF species was strongly dependent on cement chemistry, whereas CAE species exhibited little sensitivity. The respective critical concentrations (Φ*) in suspension required to promote the transition from strongly shear thinning to Newtonian flow (flocculated → stable) behavior were determined from stress viscometry and yield stress measurements. Theoretical analysis of interparticle interactions suggested that only colloidal particles in the size range of ≤1 μm are fully stabilized by adsorbed polyelectrolyte species. Our observations provide guidelines for tailoring the molecular architecture and functionality of superplasticizers for optimal performance.     

Effect of Silica Nanoparticle Size on the Stability of Alumina/Silica Suspensions

The influence of the addition of silica particles (5, 15, 25, and 300 nm) on the zeta potential and viscosity of aqueous alumina slurries (250 nm particles) was investigated in a pH range where the surface charge was positive for alumina and negative for silica. For slurries formulated with the smaller silica particles, the isoelectric point shifted from pH 9.0 to pH ∼3 (depending on the particle size of the silica) with increasing volume fraction of silica particles. At pH 9, the original isoelectric point for the alumina alone, these mixed slurries had a shear-rate-independent, low viscosity (Newtonian behavior). Both of these results show that the smaller (≤25 nm) silica particles adsorb to the surface of alumina. The fraction of silica adsorbed to the alumina surface was dependent on the size of the silica particles, and was consistent with surface coverage calculations based on the effect of surface curvature on the limits of dense random parking. The larger silica particles (300 nm) could not physically cover the surface of the alumina particles, and simply formed a mixed, attractive particle network that exhibited a much higher viscosity with non-Newtonian (viz., shear rate thinning) behavior.     

Phase Behavior, 3-D Structure, and Rheology of Colloidal Microsphere–Nanoparticle Suspensions

A new route for tailoring the behavior of colloidal suspensions through nanoparticle additions is reviewed. Specifically, the interparticle interactions, phase behavior, 3-D structure, and rheological properties of microsphere–nanoparticle mixtures that possess both high charge and size asymmetry are described. Negligibly charged microspheres, which flocculate when suspended alone, undergo a remarkable stabilizing transition upon the addition of highly charged nanoparticles. The formation of a dynamic nanoparticle halo around each colloid induces an effective repulsion between the microspheres that promotes their stability. With increasing nanoparticle concentration, the colloids again undergo flocculation because of the emergence of an effective microsphere attraction, whose magnitude exhibits a quadratic dependence on nanoparticle volume fraction. The broader impact of these observations on colloidal stabilization and assembly of advanced ceramics is highlighted.     

Rheology and Consolidation of Colloidal Alumina-Coated Silicon Nitride Suspensions

In an attempt to improve the colloidal processing of Si₃N₄ ceramics, we studied the rheology and consolidation of colloidal suspensions of Si₃N₄ particles (average particle size 0.7 μm) with small Al₂O₃ particles (average particle size 20 nm). It was found that at pH >7, the viscosity of the mixtures increased and then decreased with an increasing concentration of Al₂O₃. λpotential measurements, optical micrographs, and visible light absorptance measurement suggest that such viscosity behavior is due to clustering of Si₃N₄ particles bridged by the small Al₂O₃ particles. This is also supported by the Derjaquin-Landau-Verwey-Overbeek (DLVO) potential calculations that show the barrier height in the DLVO potential between Al₂O₃ and Si₃N₄ is small. The small barrier height under current experimental conditions stems from the small size of the Al₂O₃ particles. The small barrier height allows the thermal motion of the two kinds of particles to overcome the barrier and attach to each other. The adsorption of small Al₂O₃ particles on Si₃N₄ can occur even when both Al₂O₃ and Si₃N₄ carry the same sign of charges. The adsorption of Al₂O₃ on Si₃N₄ also increases the density of consolidated compacts.     

硅微粉对有机硅电子灌封胶性能的影响

以端乙烯基硅油为基胶、含氢硅油为交联剂、硅微粉为填料制得有机硅电子灌封胶.研究了经硅烷偶联荆γ-甲基丙烯酰氧基丙基三甲氧基硅烷(KH-570)处理后的硅微粉及用量对有机硅电子灌封胶的黏度、力学性能、导热性能和电学性能的影响.结果表明,硅微粉经硅烷偶联剂处理后有利于提高有机硅电子灌封胶的性能,当采用KH-570质量浓度为...     

Adsorption Effects on the Rheological Properties of Aqueous Alumina Suspensions with Polyelectrolyte

The influence of adsorption coverage and free polymer on the rheological properties of aqueous alumina suspensions with polyelectrolyte was studied. The flow curves of the suspensions followed the Casson model very well in the observed range of shear rate. The Casson yield value, τ_c, was used to evaluate the stability. The coverage exerted a profound influence on the rheological properties by affecting the interaction between particles. The zeta potential increased and the value of τ_c decreased as the coverage increased. The free polymer could cause weak flocculation as its concentration was increased to a certain level. The opposite effects of the adsorbed polymer and free polymer on stability resulted in the presence of optimum coverage for stabilization. The concentration of free polymer increased rapidly as the adsorption coverage approached the saturation limit, and then the free polymer started to dominate the stability. Stabilization could be achieved at a condition of unsaturated adsorption and was related to the non-high-affinity adsorption at alkaline pH (9.2). The impact of adsorption affinity on stability was discussed.     

Compressive Properties and Fracture Behavior of Two-and-a-Half-Dimensional C/SiC Composites

Uniaxial compression tests for two-and-a-half-dimensional carbon fiber-reinforced silicon carbide composites along the warp and weft directions were conducted at room temperature. The results show that the stress–strain behaviors for both loading directions exhibited great anisotropy. The compressive strength was greater in the weft direction than in the warp direction, which is attributed to good confinement of the weft yarns and resisting roles of the warp yarns in the former. Microscopic examinations reveal that the fracture surfaces had good correlation with the weave architecture, and the failure mechanism was characterized by transverse shear resulting from fiber microbuckling in the axial yarns.     

Effects of Boehmite-Coating Thickness on the Consolidation and Rheological Properties of Boehmite-Coated SiC Suspensions

We examined the effect of the boehmite coating thickness on the rheology and consolidation of boehmite-coated SiC suspensions. The thickness of the boehmite coating on SiC was varied by adjusting the boehmite concentration relative to SiC. For boehmite concentrations less than 10 wt%, the coating thickness increased with increasing boehmite concentration. For boehmite concentrations higher than 10 wt%, the coating thickness saturated. Further increase in the boehmite concentration led to the presence of small boehmite particles in the suspensions. All boehmite-coated suspensions gelled near their isoelectric points and the storage moduli of the gels with respect to pH exhibited a maximum near the isoelectric points. Below 10 wt% boehmite, the suspensions had very few small boehmite particles. The maximum storage modulus, G ^'_0,max, of the boehmite-coated SiC gel decreased with increasing coating thickness, t , as G ^'_0,max∝ t ⁻², in good agreement with our earlier theoretical prediction. Meanwhile, the maximum sedimentation densities, φ_max, of the coated suspensions occurred at around pH ∼ 4.0 and increased with increasing coating thickness from under φ_max= 25 vol% with 1 wt% boehmite to above φ_max= 65 vol% with 10 wt% boehmite due to increased zeta potential with increasing coating thickness. Above 10 wt% boehmite, the excess boehmite particles in the suspension increased the maximum suspension storage modulus, G ^'_0,max, and decreased the maximum sedimentation density, φ_max.     

添加剂对二氧化硅包裹碳化硅材料性能的影响

本文利用碳化硅颗粒表面氧化生成的二氧化硅将碳化硅颗粒包裹起来的方法,制备了二氧化硅包裹碳化硅材料;借助XRD、SEM分析了加入不同添加剂对材料抗折强度及抗热震性等性能的影响。结果表明添加硅粉能提高二氧化硅包裹碳化硅材料的抗弯强度和抗热震性,而Cu粉和V2O5对材料的性能影响不大。