Synthesis of Low-Dielectric Silica Aerogel Films by Ambient Drying

A new ambient drying process that is simple, effective, and reproducible has been developed to synthesize low- k SiO₂ aerogel thin films for intermetal dielectric (IMD) materials. The SiO₂ aerogel films having a thickness of 9500 Å, a high porosity of 79.5%, and a low dielectric constant of 2.0 were obtained by a new ambient drying process using n -heptane as a drying solvent.     

Determination of the Degree of Mixing in Multicomponent Ceramic Powder Compacts by Micro-Raman Spectroscopy

Powder compacts of TiO₂ and ZrO₂ were prepared from their suspensions in water at different pH values, and micro-Raman spectroscopy was used to reveal the degree of mixing in each of them, with different sizes of the focused laser spot. The change in laser intensity and particle-size distributions led to a slight variation in the average composition, as determined using three different probe sizes. The Raman-estimated volume fractions of each constituent indicated that the aqueous suspensions in which the powders were well dispersed produced the most-uniform mixing. Powders that were flocculated in suspensions before mixing also resulted in a good mixture. The most-nonuniform mixing was observed when, in the individual suspensions, one of the constituents was dispersed and the other was flocculated. These results indicated that Raman spectroscopy can be useful for the rapid determination of the degree of mixing in powder mixtures.     

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 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 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.     

Rheological, Structural, and Stress Evolution of Aqueous Al2O3:Latex Tape-Cast Layers

The rheological, structural, and stress evolution of aqueous alumina (Al₂O₃):latex tape-cast layers of varying composition were studied by shear rheology, direct visualization, and a controlled environment stress measurement device. Their low shear viscosity was nearly independent of the alumina:latex ratio for binary mixtures whose particle size ratio (λ=̄_alumina:̄_latex) approached unity, but varied over an order of magnitude for systems with particle size asymmetry. Direct visualization of these mixtures revealed that particle flocculation occurred as their total solids loading increased. Their structure was characterized at intermittent points during the drying process by imaging freeze-dried samples using scanning electron microscopy (SEM). Their corresponding stress histories exhibited three distinct regions: an initial period of stress rise, followed by a stress maximum, and, finally, a period of stress decay. Pure alumina layers exhibited a maximum stress of ∼1 MPa and a residual stress below 0.01 MPa. Pure latex films exhibited a maximum stress of ∼0.1 MPa and only a slight stress decay. The ceramic phase dominated the initial period of stress rise, while the latex phase strongly influenced the residual stress of composite layers cast from alumina:latex suspensions. Their maximum drying stress increased with decreasing Al₂O₃ particle size, whereas their residual stress increased with increasing latex T _g.     

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.     

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.     

Rheological Property and Stress Development during Drying of Tape-Cast Ceramic Layers

Rheological property and stress development of tape-cast ceramic layers derived from nonaqueous alumina (A1₂O₃)-poly(vinyl butyral) (PVB) suspensions were observed during drying. Casting suspensions exhibited strong shear-thinning behavior, with a low shear Newtonian plateau apparent viscosity >10² Pa^.s. The apparent suspension viscosity displayed a power-law dependence on the A1₂O₃ volume fraction during the initial stage of drying (<30% solvent loss). Stress development, measured by a cantilever deflection method, and parallel weight loss measurements were performed during the drying of tape-cast layers and pure binder coatings. Maximum drying stresses (σ_max) of 1.37-0.77 MPa were observed for plasticized tapes cast at gap heights of 150-400 μm. In contrast, nonplasticized tapes of similar thickness displayed a more gradual stress increase, with σ_max values approximately an order of magnitude higher than their plasticized counterparts. The stress histories of the corresponding binder coatings were quite similar to the tape-cast layers, albeit slightly lower σ _max values were observed. Stresses decayed beyond σ_max with a logarithmic time dependence to an almost constant value of 0.2-0.4 MPa for the plasticized tapes. Based on these observations, process methodologies have been offered to minimize stress development and retention in tape-cast ceramic layers     

Consolidation Behavior of Flocculated Alumina Suspensions

The consolidation behavior of flocculated alumina suspensions has been analyzed as a function of the interparticle energy. Consolidation was performed by a centrifugal force field or by gravity, and both the time-dependent and equilibrium density profiles were measured by a gamma-ray absorption technique. The interparicle energy at contact was controlled by adsorbing fatty acids of varying molecular weight at the alumina/decalin interface. We found that strongly attractive interactions result in a particle network which resists consolidation and shows compressible behavior over a large stress range. The most weakly flocculated suspension showed an essentially incompressible, homogeneous density profile after consolidation at different centrifugal speeds. We also found a significant variation in the maximum volume fraction, φ_m, obtained, with φ_m∼ 0.54 for the most strongly flocculated suspension to φ_m∼ 0.63 for the most weakly flocculated suspension. The compresive yield stresses show a behavior which can be fitted to a modified power law. In this paper, we discuss possible correlations between the fitting parameters and physical properties of the flocculated suspensions.     

Grain Growth Phenomena in the Interface Region of α-Alumina Bilayer Composites

Microstructural development in the interface region of α-Al₂O₃ bilayer composites has been systematically investigated in terms of the sintering additive CaO–SiO₂, residual impurity level in the starting powders (particularly MgO), and sintering conditions. The interfacial microstructure is strongly related to relative CaO–SiO₂ doping levels in the two constituting layers and to residual impurities in the starting powders. The presence of high levels of impurities in the starting powder can substantially modify the features of CaO–SiO₂-Al₂O₃ liquid at the interface region, thereby strongly influencing α-Al₂O₃ grain growth across the interface. Three grain growth modes in the interface region thus have been identified for different combinations of impurity level and CaO–SiO₂ dopant in the α-Al₂O₃ bilayer. This provides an important mechanism for controlling two-dimensional structures in coatings, films, and layered ceramic materials for various engineering applications.     

Consolidation of Alumina—Zirconia Mixtures by a Colloidal Process

The relationship between the dispersion of colloidal powder particles in Al₂O₃–ZrO₂ suspensions and the microstructures of consolidated compacts was examined. Suspensions were prepared from Al₂O₃ powder and ZrO₂ sol with average particle sizes of 390 and 62 nm, respectively. The dispersion was controlled by pH and salt concentration adjustments. The compacts composed of completely separated Al₂O₃ and ZrO₂ layers were obtained from well-dispersed suspensions with pH values below about 4 and salt concentration of 0.0527 M. An increase in pH or salt concentration resulted in macroscopically uniform compacts. The compacts made from suspensions with pH values above about 7, however, were composed of a mixture of Al₂O₃ and ZrO₂ agglomerates, with one acting as a matrix and the other a dispersed phase. Suspensions with a pH value of 4.5 and optimum salt concentrations resulted in compacts with microscopically uniform microstructure. Above or below these salt concentrations, ZrO₂ agglomerates were distributed in an Al₂O₃ matrix. The optimum concentration was dependent on solid content. In addition, the dispersion of mixed suspensions was compared with those of single-component suspensions. The ZrO₂ particles formed three-dimensional networks during agglomeration, which reduced the component separation in suspensions and during consolidation.     

Effect of Ultrasonication on the Microstructure and Tensile Elongation of Zirconia-Dispersed Alumina Ceramics Prepared by Colloidal Processing

To obtain dense, fine-grained ceramics, fine particles and advanced powder processing, such as colloidal processing, are needed. Al₂O₃ and ZrO₂ particles are dispersed in colloidal suspensions by electrosteric repulsion because of polyelectrolyte absorbed on their surfaces. However, additional redispersion treatment such as ultrasonication is required to obtain dispersed suspensions because fine particles tend to agglomerate. The results demonstrate that ultrasonication is effective in improving particle dispersion in suspensions and producing a homogeneous fine microstructure of sintered materials. Superplastic tensile ductility is improved by ultrasonication in preparing suspensions because of the dense and homogeneous fine microstructure.     

Stability of Aqueous α-Al2O3 Suspensions with Poly(methacrylic acid) Polyelectrolyte

Stability of aqueous α-Al₂O₃ suspensions with Na⁺ salt of poly(methacrylic acid) (PMAA-Na) polyelectrolyte was studied as a function of pH. At a given pH, the transition from the flocculated to the dispersed state corresponded to the adsorption saturation limit of the powders by the PMAA. As the pH was decreased, the adsorption saturation limit increased until insolubility and charge neutralization of the PMAA was approached. The critical amount of PMAA required to achieve stability is outlined in a stability map.     

Bulk Concentration Effects on the Structure and Orientation of Adsorbed Silane on the Surface of Nanosized SiO2 Particles

In this study, the concentration effects of 3-methacryloxypropyltrimethoxysilane (MEMO) on the surface coverage extent, orientation, and structures of MEMO silane grafted onto the surface of SiO₂ particles are investigated using Fourier transform infrared (FTIR) and ²⁹Si-NMR spectroscopy. The MEMO silane concentration effects on the rheological behaviors of the SiO₂ particles–1,6-hexanedioldiacrylate (HDDA) suspensions are discussed in terms of the surface coverage extent and the MEMO silane structures grafted onto the surface of SiO₂ particles. At low MEMO concentrations, many free silanol SiOH groups were observed for the MEMO grafted onto the SiO₂ surface and the adsorbed MEMO molecules tended to orient parallel to the SiO₂ surface due to the hydrogen bonding of the MEMO-carbonyl and the hydroxyl group of the oxides. At high MEMO concentrations, the condensation reactions between neighboring grafted MEMO molecules result in the predominance of a T² and T³ silicon atom structure and complete coverage of the SiO₂ surface by the grafted MEMO. The enhanced steric hindrance and compatibility between the MEMO-modified SiO₂ particles and HDDA monomer, which originate from the complete coverage of the SiO₂ surface by MEMO silane and a higher proportion of free carbonyl groups on the grafted MEMO, may improve the dispersibility of nanosized SiO₂ in acrylate suspensions.     

Basic Phosphate-Bonded Castables from Dolomitic-Magnesite Clinkers

Various basic refractory concretes, using dolomitic-magnesite clinkers, have been developed using phosphate binders and silica-rich additives. Three aggregate compositions with CaO/SiO₂ ratios varying from 2.01 to 2.31 wt% have been used, with two phosphate binders, differing by their chain length, and two silica additives, one colloidal, one crystalline. Maximum strengths, after curing, drying, and firing at various temperatures, have been attained by controlling the value of the CaO/(P₂O₅+ SiO₂) weight ratio of the concrete mixes close to unity. The value of this ratio has been found to be dependent upon the reactivity of the binder and the silica additive used.     

Effect of Particle Volume Fraction on the Gelation Behavior of the Temperature Induced Forming (TIF) Aqueous Alumina Suspensions

Stable aqueous Al₂O₃ suspensions that can be flocculated by increasing temperature were prepared using tri-ammonium citrate and poly(acrylic acid) (PAA). Effects of particle volume fraction on the rheological properties of Al₂O₃ suspensions were investigated. The modified Dougherty–Krieger equations were used to describe the volume fraction dependence of relative viscosity for both the PAA-addition and PAA-free suspensions. An equation using percolation model was proposed to analyze the temperature and volume fraction dependence of the relative viscosity of the suspensions. The calculated results were in good agreement with the experimental data.     

Dispersion of Aqueous Barium Titanate Suspensions with Ammonium Salt of Poly(methacrylic acid)

The colloidal stability of aqueous barium titanate (BaTiO₃) suspensions with the ammonium salt of poly(methacrylic acid) (PMAA-NH₄), as a function of pH, has been investigated. The concentration of PMAA-NH₄ required to stabilize aqueous BaTiO₃ suspensions decreases as the pH increases. A stability map that delineates the critical amount of PMAA-NH₄ necessary to achieve colloidal stability, as a function of pH, has been constructed.     

SiO2/Fe2O3 Mesoporous Composite Prepared With an Activated Carbon Template in Supercritical Carbon Dioxide

SiO₂/Fe₂O₃ mesoporous composites have been prepared with a nanoscale casting process using an activated carbon (AC) template in supercritical carbon dioxide (SC CO₂). The composite precursor and acetone solvent (for Fe₂O₃ precursor) were dissolved in SC CO₂, and then coated on the AC in the desired supercritical condition. After removal of the AC template by calcinations in air at 600°C, SiO₂/Fe₂O₃ mesoporous composite were obtained. Temperature, pressure, and composite precursor ratio effects were studied. Scanning electron microscopy result shows that the porous structure of AC template had been well replicated by the composite product. Transmission electron micrograph indicates that nano iron oxides were well dispersed in the composite product.     

Preparation of Porous Anatase Coating from Sol-Gel-Derived Titanium Dioxide and Titanium Dioxide-Silica by Water-Vapor Exposure

The effects of water vapor on the crystallization behavior of sol-gel-derived titanium dioxide (TiO₂) thin films that contained 0-50 mol% silica (SiO₂) were investigated. Anatase formed on exposure to water vapor at 60°-180°C, with a simultaneous decrease in the concentration of OH groups. An increase in the SiO₂ content of the exposed films led to an increase in the average crystalline size. Because crystallization of the exposed films of the films was not accompanied by shrinkage, porous anatase coatings were obtained via exposure at a relatively low temperature. Phase separation of the immiscible TiO₂-SiO₂ system was induced with water vapor, which resulted in acceleration of the crystallization of the sol-gel films.