Colloidal Processing of a Very Fine BaTiO3 Powder—Effect of Particle Interactions on the Suspension Properties, Consolidation, and Sintering Behavior

The relation between the suspension state and the rheological properties, the consolidation, and packing of a very fine (nanosized) BaTiO₃ powder has been investigated. The BaTiO₃ powder was suspended in a nonaqueous medium by adsorbing fatty acids and a polymeric dispersant, poly(12-hydroxy stearic acid), (PHS), at the BaTiO₃/decane interface. Calculated interparticle energies imply that the suspension with PHS adsorbed is colloidally stable, while the suspensions with oleic and octanoic acid can be characterized as weakly and strongly flocculated, respectively. Analysis of settling experiments and rheological measurements at high concentrations confirmed these characteristics. Pressure filtration resulted in nearly identical green body densities in spite of the differences in colloidal properties, but the preliminary sintering experiments and microstructural characterization showed that the strongly flocculated suspension displays a significantly retarded sinterability compared to the colloidally stable and the weakly flocculated suspensions. The absence of a correlation between green density and sintering behavior was explained by considering both the volume taken by the adsorbed fatty acids and the PHS polymer—which can be substantial for nanosized powders—and the state of the suspension. While a decrease in the thickness of adsorbed surfactant or polymer layer will enable a higher particle packing density, such a thin adsorbed layer results in a more strongly flocculated suspension which will resist dense packing. Hence, it is suggested that the green bodies of the colloidally stable and the weakly flocculated suspensions correspond to a relatively homogeneous, but loosely packed, green body microstructure. The strongly flocculated suspension results in a green body with a more inhomogeneous microstructure.     

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.     

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.     

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.     

Influence of pressure on filtration of aqueous alumina suspensions

The dispersibility of colloidal alumina particles (median size 310 nm) was related to the surface potential, the solid concentration in a suspension and the pressure applied to the particles. The consolidation behavior of colloidal alumina particles with an isoelectric point pH 8.7 was examined using a developed pressure filtration apparatus at 1–10 MPa of applied pressure. The height of 7 or 20 vol% alumina suspensions at pH 3.0, 7.8 and 9.0 as a function of filtration time was fitted by a filtration model developed for a flocculated suspension rather than a traditional filtration model for a dispersed suspension. An increased pressure, a decrease of particle concentration and a porous microstructure of colloidal cake reduced the consolidation time of alumina suspension. The wet alumina compacts were significantly compressed during filtration but relaxed after the release of the applied pressure. However, the packing density of alumina compact after calcination at 700 °C was almost independent of the filtration pressure and controlled by the structure of network of alumina particles in a solution.     

Rheological Properties of Concentrated, Nonaqueous Silicon Nitride Suspensions

The rheological properties of nonaqueous silicon nitride powder suspensions have been investigated using steady shear and viscoelastic measurements. The polymeric dispersant, Hypermer KD-3, adsorbed strongly on the powder surfaces, and colloidally stable, fluid suspensions up to a volume fraction of φ= 0.50 could be prepared. The concentrated suspensions all displayed a shear thinning behavior which could be modeled using the high shear form of the Cross equation. The viscoelastic response at high concentrations was dominated by particle interactions, probably due to interpenetration of the adsorbed polymer layers, and a thickness of the adsorbed Hypermer KD-3 layer, Δ∼10 nm, was estimated. The volume fraction dependences of the high shear viscosity of three different silicon nitride powders were compared and the differences, analyzed by using a modified Krieger-Dougherty model, were related to effective volume effects and the physical characteristics of the powders. The significantly lower maximum volume fraction, φ_m= 0.47, of the SN E-10 powder was referred to the narrow particle size distribution and the possibility of an unfavorable particle morphology.     

Effect of Particle Size on Colloidal Zirconia Rheology at the Isoelectric Point

This paper examines the effects of particle concentration and size on the yield stress of ZrO₂ suspensions at a well-defined surface chemistry condition of the isoelectric point (IEP). At the IEP, the relationship between yield stress τ_Ymax and particulate volume fraction φ_s and mean particle size d was evaluated to be τ_Ymax = K φ_s^4.0/ d ^2.0. The difference in size distribution of the various ZrO₂ suspensions examined causes some degree of scatter in the data used to establish the τ_Ymax, φ_s, and d relation. The use of particle concentration n_t based on the fine size fraction instead of volume fraction φ_s provided a better correlation, because the fine particles govern the properties of the flocculated network structure.     

Pressure Filtration Model of Ceramic Nanoparticles

The consolidation behavior of nanometer-sized particles at 20–800 nm was examined using a pressure filtration apparatus at a constant compressive rate. The relation of applied pressure (Δ P _t)–volume of dehydrated filtrate ( V _f) was compared with the established filtration theory for the well-dispersed suspension. The theory was effective in the early stage of the filtration but deviation between the experiment and the theory started when Δ P _t exceeded a critical pressure (Δ P _tc). It was found that this deviation is associated with the phase transition from a dispersed suspension to a flocculated suspension at Δ P _tc. The factors affecting Δ P _tc are the ζ potential, concentration, and size of the particles. Based on the colloidal phase transition, a new filtration theory was developed to explain the Δ P _t– h _t (height of suspension) relation for a flocculated suspension. Good agreement was shown between the developed theory and experimental results.     

Aggregation Effects on the Compressive Flow Properties and Drying Behavior of Colloidal Silica Suspensions

The influence of aggregation phenomena on the compressive flow properties and drying behavior of nonaqueous and aqueous silica (SiO₂) suspensions of varying electrolyte (NH₄Cl) concentrations were studied. Compressive rheology measurements, including sedimentation and centrifugal consolidation, were first conducted to investigate consolidation behavior in the absence of solvent evaporation. The volume-fraction-dependent osmotic pressure and compressive yield stress were determined for dispersed and flocculated SiO₂ suspensions, respectively. Consolidation behavior then was studied in situ by simultaneously measuring stress evolution and solvent loss as a function of drying time. The observed drying stress histories of the films were complex, consisting of several characteristic regions. First, there was an initial period of stress rise to a maximum drying stress. These measured stress values exhibited good agreement with the osmotic pressure and compressive yield stress at equivalent SiO₂ volume fractions for the dispersed and flocculated systems, respectively. Beyond the maximum drying stress there was a subsequent region of stress decay, which coincided with the draining of liquid-filled pores. No residual drying stress was detected for films prepared from salt-free SiO₂ suspensions, whereas salt-containing films exhibited residual drying stresses likely due to salt-bridging effects. Microstructural characterization of dried films prepared from aqueous SiO₂ suspensions revealed nonuniformities in the spatial distribution of colloidal particles and precipitated salt, with the highest concentrations located at the outer edges of the films. Such features result from capillary-induced transport of these species during drying, and they have important implications on colloidal processing of ceramic thick films and bulk forms.     

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.     

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.     

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.     

Influence of Powder Agglomerates on the Structure and Rheological Behavior of Injection-Molded Zirconia–Wax Suspensions

Surface-modified zirconia powders with 20% to 100% of the particle surface area covered with stearic acid was blended with 50 vol% of an organic wax vehicle for rheological study. These fractions of surface coverage gave rise to different degrees of powder agglomeration. A further adsorption of the wax on partially modified particle surfaces was examined, which was likely to exhibit different degrees of solid surface-wax affinity depending upon whether the particle surfaces were "bare" or "premodified." The rheological behavior of the suspensions revealed that the shear viscosity as well as the yield stress increased appreciably with decreased fractions (or surface coverage) of the pre-adsorption. The observed suspension rheology due to incomplete surface modification can be accounted for by the formation of agglomerates which suppress suspension flowability to a significant extent. The formation of the agglomerates alters the suspension structure by reduction of the maximum solid concentration (φ_max) that is attainable for a given powder. This change in suspension structure (decrease in φ_max) leads to a restriction of particle mobility, reflected as a linear function of the yield stress of the suspensions.     

Macroscopic dynamics of flocculated colloidal suspensions

The macroscopic dynamics of strongly flocculated colloidal suspensions are of both fundamental interest and practical significance across a range of applications from soil mechanics to solid–liquid separation. Over 30 years ago, Buscall and White [Buscall, R., White, L.R., 1987. The consolidation of concentrated suspensions. Part 1. The theory of sedimentation, Journal of the Chemical Society: Faraday Transactions I 83 (3), 873–891] proposed a one-dimensional (1D) theory of sedimentation and consolidation of such suspensions, which spawned suspension characterization tools and process modeling techniques to successfully predict 1D separation behavior. However, many applications such as continuous thickening or cross-flow filtration are multidimensional in practice, involving suspension transport, deformation and separation, and so cannot be described by the 1D BW theory. Multidimensional suspension mechanics has received a little attention over the past 30 years, requiring development of a tensorial rheology of suspensions, capable of describing arbitrary loadings and simultaneous shear and compression. In this paper, we develop a multidimensional theory of the macroscopic mechanics of strongly flocculated colloidal suspensions to describe suspension transport, deformation and separation in terms of the bulk suspension velocity field and local average solids volume fraction alone. This is achieved by consistent extension of the fundamentals of BW theory to multiple dimensions, with constitutive models justified and expressed in terms of experimentally measurable material functions. Constitutive aspects which are outstanding due to lack of empirical evidence are clearly identified, pointing to potential research directions regarding the tensorial rheology of colloidal suspensions. Utility of the theory is demonstrated in an example comparison between 1D and 2D continuous gravity thickener operation.     

Experimental Analysis and Modeling of Slip Casting

The filtration mechanics of slip casting is extended to account for the filtrate transporting the finer particles to the bottom of the cake. Scanning electron micrographs of alumina (Al₂O₃) green microstructures illustrate that a higher concentration of fine particles can accumulate at the bottom section of a cake. The rheological behavior of alumina suspensions with different solids loadings, particle-size distributions, and amounts of deflocculant is discussed. Slip-casting experiments demonstrate that the rheology of a suspension greatly affects the green density and growth rate of the cake.     

Effect of Calcination on the Sintering of Gel-Derived, Zirconia-Toughened Alumina

The densification behavior of ZrO₂ (+ 3 mol% Y₂O₃)/85 wt% Al₂O₃ powder compacts, prepared by the hydrolysis of metal chlorides, can be characterized by a transition- and an α-alumina densification stage. The sintering behavior is strongly determined by the densification of the transition alumina aggregates. Intra-aggregate porosity, resulting from calcination at 800°C, partly persists during sintering and alumina phase transformation and negatively influences further macroscopic densification. Calcination at 1200°C, however, densifies the transition alumina aggregates prior to sintering and enables densification to almost full density (96%) within 2 h at 1450°C, thus obtaining a microstructure with an alumina and a zirconia grain size of 1 μm and 0.3–0.4 μm, respectively.     

Pressure Sensitivity for Particle Packing of Aqueous Al2O3 Slurries vs Interparticle Potential

The relation between relative density and applied network pressure has been determined for aqueous Al₂O₃ slurries prepared with different interparticle potentials and consolidated by centrifugation. Attractive interparticle potentials were obtained by either changing the pH to the isoelectric point or adding an excess electrolyte. A range of centrifugal speeds produced consolidation pressures between 10^–3 and 10 MPa. At lower speeds, the density gradient was determined with an X-ray absorption technique. The maximum packing density (0.62 ± 0.02) was achieved for both dispersed and coagulated slurries at network pressures >0.5 MPa. At lower pressures, thepacking density of these slurries was pressure-dependent, where the coagulated (salt added) slurries had a relative density between that of the dispersed and flocced slurries. Slurries flocculated by adjusting the pH to the isoelectric point never reached the highest packing density at the largest pressure.     

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.     

Coagulation Method of Aqueous Concentrated Alumina Suspensions by Thermal Decomposition of Hydroxyaluminum Diacetate

Consolidation of aqueous concentrated suspensions was used to shape alumina green bodies because it enabled us to obtain complex-shape components with accurate sizes. A high state of alumina particle dispersion was achieved by using (HO)₂C₆H₂(SO₃Na)₂ (Tiron), which allowed us to obtain stable alumina suspensions at pH 9 with a powder concentration higher than 60 vol%. The addition to the suspension of hydroxyaluminum diacetate, (CH₃CO₂)₂AlOH, which decomposed as the temperature increased, permitted us to coagulate an alumina suspension dispersed with Tiron efficiently. Adsorption measurements, electrokinetic mobility, and the rheological behavior of the suspensions provided useful methods to characterize each processing stage. Dense green bodies with sufficient cohesion could be demolded and dried, demonstrating that the dispersant and the flocculant agent chosen permit one to optimize the direct coagulation casting processing of alumina components.     

Anodic Spark Deposition from Aqueous Solutions of NaAlO2 and Na2SiO3

Coatings were deposited on Cu⁰, Ni⁰, and Al⁰ anodes from aqueous solutions of NaAlO₂ and Na₂SiO₃ by anodic spark deposition. Some coatings formed at constant current density (1.0 A/cm²) were characterized by scanning electron microscopy, electron beam microprobe analysis, X-ray diffraction, and emission spectroscopy. A relation for the current decay during a multiple-spark, constant-voltage process was determined. This relation, which describes the behavior of most of the systems studied, is

where φ_t, φ₀, and φ_∞ are the current densities at time t , at the start of the decay ( t =0), and at the steady state, respectively. K is a constant that is voltage and system dependent.