A new empirical viscosity model for ceramic suspensions

This paper presents a new predictive viscosity model for ceramic suspensions. Starting from Einstein's model (1906), various theoretical, empirical, and phenomenological models have been proposed for different suspension systems. However, there is still a lack of reliable model for ceramic suspensions used in colloidal ceramic shape-forming methods. Here, the rheological properties of ceramic suspensions comprising NiO/YSZ (nickel oxide/yttria stabilized zirconia) as the ceramic powder, and furfuryl alcohol as the suspending media were measured over a range of shear-rates (between 1 and 1000 s⁻¹) and different solid volume fractions from 0 to 0.4010. An empirical equation was then developed for the ceramic suspensions using the mobility parameter (?/(?_m−?)), which links Einstein's model with the more recent relative viscosity models. The proposed model was used to predict the relative viscosity data, showing excellent agreement to the experimental data from this study and with reported data in literature for other ceramic systems. The model was also used to estimate the maximum solid volume fraction for the ceramic suspensions (?_m=0.571), with better accuracy than those estimated by existing models.     

Rheological behavior and microstructure of bimodal suspensions of core‐shell structured swollen particles

The rheological behavior and microstructure of bimodal suspensions of core‐shell structured swollen particles have been examined with changing volume ratio of two different sized particles. As the volume fraction of large particles increases, the viscosity, degree of shear‐thinning, and the critical shear stress σ_c decreases, while the interparticle distance ξ of the microstructure increases. The suspensions exhibit single mode rheological behavior and have a single diffraction peak in the SAXS profiles. These results suggest that the bimodal suspensions of the core‐shell structured swollen particles behave likely to unimodal suspensions of hard spheres with alloy like single mode microstructure composed of hypothetical intermediate size particle. The relationship between σ_c and ξ can be represented as σ_c = 3kT/4πξ³, which corresponds to the dynamics of the Brownian hard sphere model with ξ being the particle diameter. These findings indicate that the shear‐thinning of the suspensions can be attributed to dynamical competition between the thermal motion and the hydrodynamic motion under shear flow and that the mechanism can be applied to bimodal suspensions of the swollen particles as well as unimodal suspensions of hard spheres. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 102: 2212–2217, 2006     

Rheology of Concentrated Suspensions Containing Weakly Attractive Alumina Nanoparticles

The use of nanoparticles for the fabrication of new functional ceramics and composites often requires the preparation of concentrated fluid suspensions. However, suspensions containing nanoparticles are limited in solids content because of the excluded volume formed by the dispersant adlayer around the particles. We investigated the effect of the adlayer thickness on the rheological behavior of suspensions containing model alumina nanoparticles, using dispersant molecules with deliberately tailored chain length. The apparent viscosity and yield stress of the particle suspensions were markedly decreased by increasing the dispersant length, mainly due to a reduction of the attractive forces among particles. Fluid suspensions with solids content up to 35 vol% were prepared in toluene using a dispersant length of 2.5 nm. Our experimental results and viscosity predictions based on a hard sphere model indicate that fluid suspensions with up to 43 vol% of 65 nm alumina particles could be prepared using an optimum dispersant length of about 3.6 nm.     

Effect of Zwitterionic Surfactants on Interparticle Forces, Rheology, and Particle Packing of Silicon Nitride Slurries

Phosphocholine (PC) zwitterionic surfactants, with different hydrocarbon chain lengths (C₆C₆PC to C₉C₉PC), were absorbed on the surface of silicon nitride near the isoelectric point (pH 6). Adsorption of the surfactants changed the lateral and normal surface forces, the rheology, and the consolidation behavior of the particles. The normal force between two silicon nitride surfaces as a function of separation and the lateral (friction) forces were measured using an atomic force microscope (AFM). These measurements indicated that surfactant adsorption reduced the magnitude of the long-range attractive van der Waals force and produced a repulsive short-range force. Although the adsorbed layers provided a barrier to particle contact, they could be ejected with a critical force that increased with the hydrocarbon chain length. The effect of an adsorbed layer on the viscosity and consolidation of slurries was also measured. The viscosity of all slurries decreased with increasing shear rate, indicative of attractive particle networks. The highest viscosity was observed for slurries formulated at the isoelectric point without added surfactant. Much lower viscosities were observed when the surfactant concentration was greater than the critical micelle concentration (cmc). A relative density of 0.46 was obtained via pressure filtration at 4 MPa without a surfactant, and between 0.46 to 0.59 (C₆C₆PC to C₉C₉PC, respectively) for surfactant concentrations greater than the cmc. Comparing force measurements with rheology and packing density provides a basis for discussing the role of interparticle forces in ceramic powder processing via colloidal routes.     

Relative Colloidal Stability in Ethanol of Powders with Si-O Surface Species

The relative colloidal stability in ethanol of ceramic powders with Si-O surface groups (Si₃N₄, SiC, and MoSi₂) is reported. SiO₂/EtOH suspensions are also examined as a reference. The comparative relationships between pH, particle-surface charge, zeta potential, stability, and suspension rheology are discussed. The nature and influence of the surface species are determined, and the similarities and differences are compared. DLVO theory was used to define suspension stability, and it was found that non-oxide particles in EtOH are charge-stabilized and have viscosity and flow curves that agree with the electrokinetic measurements.     

A Mean Phi Model for Pressure Filtration of Fine and Colloidal Suspensions

A working model for engineering analysis of pressure filtration is presented. Based on the filtration characteristics of fine and colloidal suspensions, the process was divided into two stages. A time‐invariant spatially uniform volume fraction of solids approximation is invoked in the growing filter cake stage (stage 1). A time‐dependent spatially uniform volume fraction of solids assumption is made in the cake consolidation stage (stage 2). The two models, named collectively as Mean Phi (M‐P) model, have a common physical basis, seamless continuity between the stages and internal consistency. The M‐P model has only three parameters: terminal or equilibrium volume fraction of solids in the filter cake that is related to its compressive yield stress, critical volume fraction of solids, which joins stage 1 and stage 2, and a permeability factor, which is common to stages 1 and 2. The model is validated with a large number of colloidal suspensions filtered under highly diverse physical‐chemical process conditions. A Pareto profile is identified that relates the timescale of filtration and the extent of dewatering achieved, the two most important performance indices of the process.     

Rheological characterization of carbon black/polystyrene solution systems

Steady‐shear measurements of suspensions of carbon blacks (CB) in polystyrene (PS)/di‐(butyl phthalate) (DBP) solution were investigated as a function of volume fraction (?) of CB to clarify the effect of the primary particle size and the structure of CB aggregates on the rheological properties. The suspensions show a typical shear‐thinning behavior in the range of a shear rate studied. The Casson model was applied to evaluate the viscosity at infinite of shear rate η_∞ and the yield stress σ_y for the suspensions. Relative viscosity η_∞/η_m, (η_m: medium viscosity) thus obtained was compared to the high‐frequency viscosity for the ideal hard‐sphere silica suspensions to evaluate the effective volume fraction ?_eff of CB aggregates. The ?_eff value was larger for the higher‐structure CB with higher DBP absorption value, irrespective of the primary particle size. The yield stress σ_y had almost the same ?_eff dependence for neutral furnace CB/(PS/DBP) suspensions, although it was larger for acetylene black (AcB)/(PS/DBP) suspensions. These results demonstrated that the effective volume fraction is the most important quantity to characterize the CB aggregates on the rheological properties. It was also found that the correction of the medium viscosity changes due to polymer adsorption on the CB surface is important since neutral furnace CB adsorbs PS polymers but AcB hardly adsorbs PS polymers in the solution. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Mechanics and Microstructures of Concentrated Particle Gels

It is often assumed that the viscoelastic properties of dense colloids are determined by the colloid volume fraction, the interaction potential, as well as the particle size distribution and shape. The dependence of the viscoelastic behavior of particle suspensions and gels on these parameters has been widely studied, and is well understood in many cases. In contrast, our knowledge on the influence of microstructure on mechanical and rheological properties, in particular for high solid loading suspensions as used in ceramic processing, is much less developed. This aspect has been the focus of recent experiments, which show that small changes in microstructure can have dramatic effects on the mechanics and dynamics of concentrated colloidal gels. In this article, we attempt to give an overview of the influence of microstructure on the mechanical and rheological properties of colloidal systems. Particular attention is given to colloidal particle gels at high volume fractions.     

Application of monosaccharides derivatives in colloidal processing of aluminum oxide

Obtaining highly loaded, time-stable and relatively low viscosity suspensions approaches colloidal processing to be very convenient and effective route of shaping of nanopowders. In order to obtain well dispersed, homogenous ceramic slurries, certain additives are given. Saccharides, particularly monosaccharides, as well as their derivatives, were found to be a group of effectively working processing agents in case of alumina, which has been used as a solid phase of highly loaded nanosuspensions. This class of chemical compounds can be described by a series of advantages – they are non-toxic, water-soluble, inexpensive, etc. In this paper suspensions of nano- and submicro-alumina powders with addition of d-fructose, 1-O-methyl-d-fructose, d-glucose and 3-O-acrylic-d-glucose have been studied in terms of their rheological properties, moreover the properties of as-received green bodies have been presented.     

UV Curable Systems for Tape Casting

The substitution of solvents by photopolymerizable binders in the tape casting process allows to achieve high ceramic loading and to eliminate the drying stage which is a critical step of the tape casting process. After the rapid UV polymerization of the resin, the high strength green tapes can be debinded and sintered. Ceramic suspensions containing alumina or zirconia powder, dispersant, UV curable binder and photoinitiator have been prepared. The use of a low viscosity organic vehicle allows to prepare low viscosity ceramic suspensions, containing about 50 vol% powder, which have a shear thinning behaviour. Because of the rapid attenuation of the incident light in UV curing systems containing ceramic particles it is important to estimate the thickness of the tape that can be treated. The effect of incident energy, of photoinitiator concentration and of powder volume fraction was studied. There is an optimal photoinitiator concentration which maximizes the cured depth and which depends on the volume fraction of solid. A theoretical model based on the Beer–Lambert’s law enables the prediction of cured depth for any volume fraction of solid. To prove the ability to manufacture ceramic sheets by tape casting, some suspensions were tape cast.     

Rheological Behavior of Slurries and Consolidated Bodies Containing Mixed Silicon Nitride Networks

Two particle networks, each formulated with a different interparticle potential, were mixed to control the rheological properties of ceramic slurries and to develop claylike plasticity in consolidated bodies. A weakly attractive network, containing silicon nitride powder, alkylated with hexadecanol, was mixed with a second slurry containing flocculated (nonalkylated) silicon nitride powder. The elastic modulus and apparent yield stress of concentrated suspensions containing each constituent and their mixtures were found to increase with volume fraction according to a previously reported power law function (exponents of 4.8 and 3.75, respectively). Because of the large difference in the relative strengths of the two networks, the flocculated network overwhelmingly controlled the behavior of the mixed slurries when its volume fraction (relative to total solids) exceeded 0.30. Slurries were consolidated by pressure filtration, and the saturated bodies were tested in uniaxial compression. Bodies containing only alkylated powder packed to a high volume fraction and deformed at a low flow stress. The addition of small amounts of the flocculated network increased the flow stress to produce a body with rheological properties similar to clay.     

Steady state flow of cement suspensions: A micromechanical state of the art

Fresh cementitious pastes can be viewed as suspensions of particles of many different sizes (from several tens of nm to 100 μm) in a continuous fluid phase. This broad poly-dispersity implies that various interactions such as surface forces (or colloidal interactions), Brownian forces, hydrodynamic forces or various contact forces between particles interplay. Depending on the volume fraction of the particles in the mixture, the use of admixtures or the magnitude of either the applied stress or strain rate, one or several of these interactions dominate. Our objective here is not to quantitatively predict rheology of cement pastes but rather to understand and classify the situations where, depending on composition and processing, one or other of the physical phenomena will control the macroscopic behavior. The result of this analysis is a conceptual diagram of predominant interactions in flowing cementitious suspensions under simple shear, as a function of shear rate and solid fraction.     

Effect of volume fraction and particle size on wall slip in flow of polymeric suspensions

For especially highly concentrated suspensions, slip at the wall is the controlling phenomenon of their rheological behavior. Upon correction for slip at the wall, concentrated suspensions were observed to have non‐Newtonian behavior. In this study, to determine the true rheological behavior of model concentrated suspensions, “multiple gap separation method” was applied using a parallel‐disk rheometer. The model suspensions studied were polymethyl methacrylate particles having average particle sizes, in the range of 37–231 μm, in hydroxyl terminated polybutadiene. The effects of particle size and solid particle volume fraction on the wall slip and the true viscosity of model concentrated suspensions were investigated. It is observed that, as the volume fraction of particles increased, the wall slip velocity and the viscosity corrected for slip effects also increased. In addition, for model suspensions in which the solid volume fraction was ≥81% of the maximum packing fraction, non‐Newtonian behavior was observed upon wall slip correction. On the other hand, as the particle size increased, the wall slip velocity was observed to increase and the true viscosity was observed to decrease. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 439–448, 2005     

Particle Packing in Ceramic Injection Molding

The shrinkage of ceramic injection molding suspensions caused by pyrolytic removal of the organic vehicle was measured for initial ceramic volume fractions from 0.48 to 0.64. Shrinkage was inversely related to initial ceramic volume fraction V , and maximum volume fraction after pyrolysis V *,_max was 0.65. The factors which restrict shrinkage are discussed. The maximum volume fraction, obtained from semiempirical equations relating the viscosity of suspensions to volume fraction of powder ( V _max) was 0.73 to 0.76. Both the viscosity of the suspensions and the shrinkage on removal of the organic vehicle can be interpreted in terms of a free volume of fluid over and above that needed to fill interstices between contacting particles. Thermomechanical measurements also show that the free-volume concept helps to interpret the transition from fluid to quasi-solid properties as the organic vehicle is removed by pyrolysis from a molded body.     

硅藻土光固化成型浆料和多孔陶瓷的制备

本文成功开发了用于光固化成型的树脂基硅藻土浆料,系统探讨了分散剂种类、含量及固含量对硅藻土浆料流变特性的影响,并对其作用机理进行了分析,获得了用于光固化成型的高固含量、低黏度的硅藻土浆料,并采用3D陶瓷光固化设备制备了结构复杂的硅藻土多孔陶瓷。结果表明,BYK2009为硅藻土浆料的最佳分散剂,且当分散剂相对粉体质量为3%时,浆料黏度最低。成功制备出粉体体积分数为40%的硅藻土浆料,在剪切速率为30 s^-1时,硅藻土浆料黏度为17.30 Pa·s。在900 ℃烧结时得到显气孔率为51.30%、抗弯强度为(46.28±2.63) MPa的硅藻土多孔陶瓷。本研究为光固化成型具有复杂多级孔结构的硅藻土载体提供了参考。     

Modelling high concentration settling slurry flows

The well‐known two‐layer model for predicting friction losses for pipeline flows of settling slurries has been extended to solids concentrations above 35% by volume. This has been achieved by incorporating new experimental results to account for increases in friction which have been observed with “settling” slurries at high concentrations. The kinetic (fluid‐like) friction appears to increase with solids concentration in a manner which suggests that particle‐wall contact plays an important role. The experiments also suggest that the fraction of the total solids which contributes Coulombic (velocity insensitive) friction also increases to some extent at high concentrations. This effect is expressed in a tentative correlation which employs a slurry Reynolds number. In addition, new experimental measurements of delivered and in‐situ concentrations have been used to test predictions made with the previous version of the model. The measurements show that the interracial friction factor proposed recently by Wilson and coworkers is preferable for slurries of coarse particles.     

悬浮体中团聚体的产生机理及其对材料结构与性能的影响(英文)

研究了陶瓷胶态成型过程悬浮体中产生团聚体的微观机理,根据液体介质中陶瓷粉体颗粒之间的作用势,建立了团聚体产生的微观结构模型.利用环境扫描电子显微镜对悬浮体中粉体团聚的显微结构进行"准"直接观察,发现悬浮体中存在2种团聚体形态:Ⅰ型团聚体(硬团聚)和Ⅱ型团聚体(软团聚),揭示了团聚体的产生是由于浆料的固相含量偏离了稳定固相含量.研究发现:悬浮体中的团聚体会遗传到陶瓷体中,并对陶瓷构件的力学性能有显著影响.探讨了团聚体对烧结体的结构及力学性能影响的原因.当悬浮体中固相含量低于稳定固相体积分数ψ0时,悬浮体中会形成松散团聚即软团聚;当固相含量超过ψ0时,悬浮体中会形成紧密团聚即硬团聚:当固相含量等于ψ0时,悬浮体具有均匀分散的稳定结构.由于悬浮体固相含量偏离稳定周相含量ψ0而在悬浮体中产生的团聚体,会由于原位周化而遗传到坯体之中,影响烧结体结构的均匀性.     

Characterization of rheological properties of colloidal zirconia

Dynamic viscosity of aqueous suspensions of nanosized zirconia (ZrO₂) have been studied for the low volume fraction range. The specific surface area of dry powder was determined from the BET method. The zeta potential of zirconia particles as a function of pH was measured by the microelectrophoretic method. The isoelectric point found in this way was 4.7. The particle density in aqueous suspensions was found by the dilution method. The dynamic viscosity of suspensions was measured by using a capillary viscometer that eliminated the sedimentation effects. Experimental data showed that for dilute zirconia suspension, the relative viscosity increased more rapidly with the volume fraction than that the Einstein formula predicts. This allowed one to calculate the specific hydrodynamic volume of particles in the suspensions and their apparent density. It was found that particles forming zirconia suspensions were composed of aggregates having porosity of 40–50%. The size of the primary particles forming these aggregates was 0.2 μm that agrees well with the BET specific surface data. The influence of an anionic polyelectrolyte:polysodium 4-styrenesulfonate (PSS) on zirconia suspension viscosity also was studied. First the PSS viscosity alone was measured as a function of its volume fraction for various ionic strength of the solutions. The data were interpreted in terms of the flexible rod model of the polyelectrolyte. Then, the viscosity of ZrO₂ in PSS solutions of fixed concentration was measured as a function of the concentration of zirconia. It was revealed that the viscosity of the mixtures was proportional to the product of the zirconia and polyelectrolyte viscosities taken separately.     

Powder-Based Tin Oxide Microcomponents on Silicon Substrates Fabricated by Micromolding in Capillaries

With the introduction of soft lithography and micromolding in capillaries, low-cost microfabrication with liquid materials has become possible. In this article, we demonstrate how to fabricate porous ceramic lines of 10 μm width and several millimeter length on silicon wafer substrates by using colloidal suspensions of tin oxide. Microchannels of poly(dimethylsiloxane) (PDMS) served as molds that were spontaneously filled owing to capillary forces with suspensions of 0.1–40 vol% solid loading. The resulting ceramic lines have a height of about 7 μm and therefore differ from the usual ceramic thin film coatings. The capillary filling characteristics were observed under the microscope, and the implications of rheology and suspension chemistry are discussed and evaluated. Using the same capillaries, even smaller lines (2–3 μm width) of powder particles could easily be prepared by adjusting only the solid content of the suspensions.     

Fractions design of irregular particles in suspensions for the fabrication of multiscale ceramic components by gelcasting

A new approach for the preparation of suspensions with a high solid loading and low viscosity by using irregular particles was proposed. These suspensions were prepared for the fabrication of multiscale ceramic components by gelcasting. Based on the Funk-Dinger function and fractal theory, the closest packing theory was applied to optimize the volume fractions of different particles. The maximum solid loading of slurries prepared for gelcasting was 62 vol%, and the viscosity at a shear rate of 100 s^?1 was only 0.29 Pa s. By as-prepared suspensions, a decimeter-scale ceramic part with submillimeter features was fabricated successfully by gelcasting, which verify the feasibility of the proposed method.