Consolidation Behavior of High-Performance Ceramic Suspensions

Colloidal processing has been shown to produce low defect and uniform ceramic microstructures from submicrometer ceramic powders. These concepts were applied to colloidal pressing to determine critical design relationships for uniaxial consolidation of dense and uniform green bodies from colloidal suspensions. Carefully controlled constant rate of strain consolidation experiments were carried out using alumina in water. The compression index decreased from 0.143 for a poorly dispersed alumina system to 0.077 for a well-dispersed alumina suspension compression curve, indicating that the well-dispersed system is stiffer in consolidation. The compression curves showed that, as the degree of dispersion decreases, increased consolidation stresses are required to achieve a given particle packing density. The compression index increased with increasing strain rate for well-dispersed alumina suspensions. Permeability through the sample ranged from 3 × 10^–8 to 4 × 10⁻⁷ cm/s, decreasing with decreasing void ratio during consolidation. Well-dispersed samples gave lower permeabilities than did poorly dispersed samples over a given consolidation increment. Coefficients of consolidation were nonconstant over the experimental effective stress range, invalidating the general solution to the linear consolidation equation. An approximate incremental solution was applied which indicated rapid pressing cycles are possible by starting with a suspension having a high solids concentration. Application of this consolidation data to nonlinear consolidation models is recommended for more exact prediction of consolidation time.     

Weakly Flocculated Aqueous Alumina Suspensions Prepared by the Addition of Mg(II) Ions

Well-dispersed aqueous alumina suspensions were prepared at an inherent pH via the addition of an anionic dispersant. With the addition of an appropriate amount of magnesium acetate to such a suspension, the surface charge of the particles was neutralized, which was reflected in the destabilization of the slurry. Because of the formation of coordinative bonds between the Mg ion and the two dissociated carboxylic groups of the dispersant, a thin neutral layer was formed on the surface of the particles, which established a nontouching particle network and resulted in a weakly flocculated suspension.     

Stabilization of Ethanol-Based Alumina Suspensions

Al₂O₃ powders have been successfully dispersed in ethanol by varying the suspension acidity. An operational pH (O.pH) was defined to measure the acidity of these ethanol-based suspensions. The isoelectric point of Al₂O₃ in ethanol was at an O.pH of 8. According to Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, suspensions between an O.pH of 3.5–10.5 possessed only attractive inter-particle potential. Suspensions below 3.5 had high ζ potential, fine particle size, and were Newtonian. However, suspensions at high pH were shear thinning and consisted of agglomerates, despite their high ζ potential. The use of citric acid as a dispersant has also been investigated. At an O.pH of 3, optimum additions of citric acid between 0.6 and 1.0 wt% decreased the particle size, resulted in repulsive inter-particle potentials and increased the solid loading capacity to 15 vol% from 2 vol% while maintaining Newtonian behavior and similar viscosity to suspensions at O.pH 2. Addition of citric acid created agglomerated suspensions that were negatively charged at O.pH 10.5 (obtained by adding NH₄OH), but positively charged suspensions at O.pH 13.6 (obtained by adding tetramethylammonium hydroxide).     

Compaction and Sintering Behavior of Bimodal Alumina Powder Suspensions by Pressure Filtration

The compaction behavior of fine alumina powders with different particle sizes or bimodal particle-size distributions that are undergoing pressure filtration was investigated. Three alumina powders—average particle sizes of 0.2—0.86 μm—were compacted to a solids fraction of 62—65 vol% from suspensions at pH 3, which was the pH level at which the suspensions showed their lowest viscosity. When the powders of different average sizes were mixed, the suspensions showed better flowability, and the lowest viscosity was obtained when the fraction of fines was ∼30 vol% and pH = 3. The mixed-sized powder suspensions were compacted to higher density than the suspensions of unmixed fine or coarse powders, and the maximum density was obtained for mixed suspensions that had the lowest viscosity, despite the different particle-size ratio. Maximum densities of 72.5% and 75.0% were attained when the size ratios were 2 and 5, respectively. The compacts that were pressure-filtered from mixed suspensions exhibited a single-peaked pore-size distribution and a homogeneous microstructure, whereas the pore-size distributions of dry-pressed compacts were double-peaked. The sintering behavior of the compacts that were pressure-filtrated from bimodal powders exhibited significantly better sinterability and much-less linear shrinkage than the coarser powders and the dry-pressed powder compacts.     

Ink-Jet Printing of Wax-Based Alumina Suspensions

Suspensions of fine alumina powder in a paraffin wax have been successfully formulated with viscosity values sufficiently low to allow ink-jet printing using a commercial printer. A commercial-grade paraffin wax, with stearylamine and a polyester, were used as the dispersant system. Suspensions with powder loadings up to 40 vol% were passed through the ink-jet printer head. Unfired ceramic bodies with a feature size of <100 μm have been successfully fabricated with waxes that had a powder loading of 30 vol%. The influence of suspension fluid properties on the ink-jet printing process has been studied, and the importance of the acoustic resonance within the ink-jet printing apparatus has been demonstrated.     

絮凝处理后固相浓度对悬浮液粘度的影响研究

经絮凝处理后的固液悬浮体系,其粘度与固相浓度的关系可由笔者提出的四次多项式描述。该多项式是对只适用于低浓度体系的爱因斯坦线性模型的扩展,新的模型可分别体现流体介质、单个颗粒、线状絮体、面状絮体及体状絮体对体系粘度的贡献。经实验验证,所提出的四次多项式模型可很好地拟合实验数据。在固相浓度较低的时候,絮凝剂用量的增加并不改变体系的流变特性,只在固相浓度达到一定阈值后,絮凝作用才开始明显体现。在絮凝作用达到最大之前,随絮凝剂用量的增加,体系中单个颗粒对悬浮液粘度的贡献逐渐减小,而线状、面状及体状絮体的影响逐渐增大。絮凝剂过量时,絮凝作用下降,体现在体系粘度的降低。     

Osmotic Consolidation of Suspensions and Gels

An osmotic method for the consolidation of suspensions of ceramic particles is demonstrated. Concentrated solutions of poly(ethylene oxide) are separated from a suspension of ceramic particles by a semipermeable membrane, creating a gradient in solvent chemical potential. Solvent passes from the suspension into the polymer solution, lowering its free energy and consolidating the suspension. Dispersions of stable 8-nm hydrous zirconia particles were consolidated to over 47% by volume. Suspensions of α–alumina in three states of aggregation (dispersed, weakly flocculated, and strongly flocculated) were consolidated to densities greater than or equal to those produced in conventional pressure filtration. Moreover, the as–consolidated alumina bodies were partially drained of fluid during the osmotic consolidation process, producing cohesive partially dried bodies with improved handling characteristics.     

Cracking During Lateral Drying of Alumina Suspensions

The onset of cracking during drying of alumina suspensions cast onto a substrate has been studied experimentally. It is shown that the capillary pressure would impose a compressive stress on the particle array and cannot itself cause cracking. Based on experimental observations, it is proposed that the driving force for cracking arises from a misfit strain that occurs when repulsive layers between the particles collapse completely, after the particles have adhered to the substrate. This predicts that cracking should occur while the space between the particles is still filled with liquid, which is consistent with observations using laser speckle interferometry.     

Elastic and Yield Behavior of Strongly Flocculated Colloids

We have investigated theorectically the elastic and yield behaviors of strongly flocculated colloids by first examining the yield forces between two particles within the framework of Derjaguin-Landau-Verwey-Overbeck (DLVO) interactions. Under highly attractive conditions, i.e., in the absence of the secondary minimum in the DLVO potential, the radial (tensile) motion between particles is nonelastic because of the lack of an inflection point in the DLVO potential. However, the lateral (shear) motion is shown to be elastic up to a distance y_max, providing a mechanism for the observed elasticity in colloidal gels. If r ₀ and s ₀ are, respectively, the closest center-to-center and surface-to-surface distances between two particles, where zeta is the zeta potential of the particles and alpha a defined constant. Moreover, the yield force between two particles is much smaller in the lateral direction than in the radial direction. These results suggest that yielding of a particulate network is likely to occur through the lateral movements between particles. The yield strain can be approximated as that at which all the bonds in a certain direction have a perpendicular displacement > y _max, resulting in
The shear modulus of the network, G ', can be deduced by combining the elastic constant of the lateral movement with the existing elastic theory of a particulate network. The yield stress can be approximated as sigma where A is the Hamaker constant and R the particle radius. These predictions are shown to compare favorably with existing experiments.     

Effect of Particle Packing on the Filtration and Rheology Behavior of Extended Size Distribution Alumina Suspensions

The effect of particle packing on the rheology and casting behavior and cast cake characteristics of aqueous alumina suspensions (5O vol%) was investigated using the Andreasen approach. Varied packing was produced by blending two starting materials that differed in average size by a factor of 10. Formulations closest to ideal packing lowered viscosity. Specifically, the lowest viscosity suspension, 196 mPa s, was produced with the distribution closest to the ideal packing distribution. Typically, a well-dispersed suspension (characterized by low viscosity) casts slower than one of poorer dispersion given the same solids content. However, the suspension that provided the lowest viscosity cast at a rate of 0.41 mm²/s, which was the fastest rate of the compared formulations. Therefore, suspensions consisting of extended size distributions do not necessitate slow filtration rates. The colloidal properties of the individual starting materials, low shear rate rheology, and mercury porosimetry were used to explain the unexpected casting behavior. The dispersion and structure formation within the suspension were investigated using electrokinetic sonic amplitude measurements. Mercury porosimetry characterized the flow channels that developed in the casts. Low shear rate rheology confirmed the presence of flocculation that was first suggested by the porosimetry results.     

Influence of Various Consolidation Techniques on the Green Microstructure and Sintering Behavior of Alumina Powders

Submicrometer alumina powders, of both standard and narrow particle-size distributions, were consolidated by dry-pressing and colloidal-forming techniques. The resulting green compacts were characterized in terms of pore-size distribution, green density, shrinkage behavior, and sintered density. The interrelationships between powder characteristics, green compact microstructure, and sintering behavior are discussed. There are distinct differences between the characteristics of compacts that were directly consolidated from powder suspensions and those that were dry pressed. Dense microstructures can be achieved at low temperatures using narrow-sized, well-dispersed powders with colloidal-forming techniques.     

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.     

Plasticizing Aqueous Suspensions of Concentrated Alumina with Maltodextrin Sugar

Aqueous suspensions of submicrometer, 20 vol% Al₂O₃ powder exhibited a transition from strongly flocculated, thixotropic behavior to a low-viscosity, Newtonian-like state upon adding small amounts of maltodextrin (0.03 g of maltodextrin/(g of Al₂O₃)). These suspensions could be filter pressed to highly dense (57%) and extrudable pastes only when prepared with maltodextrin. We analyzed the interaction of maltodextrin with Al₂O₃ powder surfaces and quantitatively measured the resulting claylike consolidation, rheological, and extrusion behaviors. Benbow extrusion parameters were comparable to, but higher than, those of kaolin at approximately the same packing density of 57 vol%. In contrast, Al₂O₃ filter cakes without maltodextrin at 57 vol% density were too stiff to be extruded. Measurements of rheological properties, acoustophoresis, electrophoresis, sorption isotherms, and diffuse reflectance Fourier infrared spectroscopy supported the hypothesis that sorbate-mediated steric hindrance, rather than electrostatic, interparticle repulsion, is important to enhancing the consolidation and fluidity of maltodextrin–Al₂O₃ suspensions. Viscosity measurements on aqueous maltodextrin solutions indicated that free maltodextrin in solution does not improve suspension fluidity by decreasing the viscosity of the interparticle solution.     

Ultrasonic and Mechanical Behavior of Green and Partially Sintered Alumina: Effects of Slurry Consolidation Chemistry

Green and partially sintered compacts of alpha-Al₂O₃ powder were made by filtration of aqueous suspensions under three conditions: ( i ) electrostatic stabilization without any organic additive, ( ii ) strong flocculation near the isoelectric point without any organic additive, and ( iii ) weak flocculation by the use of maltodextrin or oxalic acid additives. We evaluated relationships between the macroscopic and interparticle mechanical behavior of these compacts using model correlations with measurements of diametral compression, ultrasonic velocity, and ultrasonic attenuation. Although type iii green specimens were less dense than type i , type iii exhibited significant increases in velocity, macroscopic Young's modulus, interparticle-contact stiffness, and diametral compressive strength, suggesting that the mechanism of stiffening/strengthening entailed interparticle bridging of maltodextrin or oxalic acid. These properties were significantly reduced upon heating type iii specimens to 500°C, suggesting that pyrolysis of surface-adsorbed maltodextrin and oxalic acid may have reduced the interparticle stiffness and strength. In contrast, negligible changes in these properties occurred upon heating type i specimens to the same temperature. Despite small increases in packing density, significant decreases in attenuation and significant increases in velocity, interparticle-contact stiffness, and Young's modulus occurred upon heating all specimens to greaterthan equal to700°C, suggesting the formation of interparticle necks by solid-state sintering.     

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.     

Suspensions and Sediments. Part II. Behavior of Concentrated Suspensions

Abstract

The behavior of concentrated suspension, the phenomenon of hindered settling, and structural characteristics of the settling suspensions are described.

The use of interface sedimentation rate the settled volume in calculating the volume of associated fluid carried down on sedimenting particles is outlined by employing a theory which assumes that particle-particle association is the factor responsible for variation of sedimentation behaviour from one suspension system to another.     

pH Valaue Effects in Shear Rheology of Concentrated Alumina Suspensions

1 INTRODUCTIONThe suspensions employed in ceramic processing arehighly concentrated.These suspensions are normallyknown as slip or slurry in materials science and engi-neering.It is of great interest that concentrated sus-pension exhibit many of the phenomena of very dilutecolloidal suspensions.This is probably because boththe suspension systems have charged particles,so thatthe principles of colloid chemistry can be applied forthem.     

Relationship Between Water Mobility and Viscosity of Nanometric Alumina Suspensions

Addition of fructose has been shown to reduce the viscosity of aqueous suspensions of nanometric alumina powders. Highly concentrated and flowable aqueous nanometric alumina suspensions were achieved by fructose addition. Oxygen-17 nuclear magnetic resonance spectroscopy was used to investigate the relaxation time and the molecular mobility of water in the suspensions with and without fructose. It was found that a significant fraction of water is bound to the surface with lower mobility. Average water mobility increases and the viscosity of the suspension decreases with fructose addition, both of which are because of displacement of water from the alumina surface by fructose molecules. The results were related to rheological behavior of the suspensions.