Electrokinetic and Rheological Properties of Aqueous TiN Suspensions with Ammonium Salt of Poly(methacrylic acid)

The study illustrates how rheology and electrokinetics can be utilized in the investigation and optimization of the properties of ceramic suspensions. Zeta potential studies show that the isoelectric point of TiN particulates is at pH=4, and for the pH range used, PMAA–NH₄ addition results in a more negative zeta potential value. The rheological behavior of titanium nitride in the presence of PMAA–NH₄ is strongly dependent on pH and three rheological types have been defined.     

Stability of Aqueous Barium Titanate Suspensions for MLCC Inkjet Printing

BaTiO₃‐based materials are currently used for the fabrication of multilayer ceramic capacitors (MLCC) because of their high dielectric properties. The inkjet printing (IJP) process can be used to fabricate MLCC of complex configurations by integrating internal electrodes and dielectric layers in a single step using a multi printing‐head system. Stabilized aqueous suspensions of BaTiO₃‐based powders are required to obtain dielectric inks adapted to IJP. This study investigates the influence of BaTiO₃ powder hydrolysis in water on the surface chemistry and stability in relationship with the milling step used to adjust the powder grain size to IJP. Optimum parameters for a good stability of BaTiO₃ suspensions are identified. The selected dispersant is a polyacrylate (PAA) for which the content is adjusted to minimize the sedimentation rates as required by IJP. Moreover, the addition of ethylene glycol is shown to be necessary to avoid the formation of a gel structure which could result from the interaction of borates ions leached from the surface of BaTiO₃ with the PAA dispersant. A mechanism of gel formation is proposed.     

Effects of Poly(acrylic acid) on Rheological and Dispersion Properties of Aqueous TiO2 Suspensions

The poly(acrylic acid) was used as dispersant to prepare aqueous TiO₂/poly(acrylic acid) suspensions. The poly(acrylic acid) was adsorbed on the surface of the TiO₂ particles. The zeta potential of the TiO₂ particles in TiO₂/poly(acrylic acid) suspensions was higher than that of the TiO₂ particles in TiO₂ suspensions, and the zeta potential of the TiO₂ particles increased with increasing poly(acrylic acid) content. At the same shear rate, the viscosity of TiO₂/poly(acrylic acid) suspensions was lower than that of TiO₂ suspensions, and the liquidity was improved. The dispersion of TiO₂ particles in TiO₂/poly(acrylic acid) suspensions was improved compared with that of TiO₂ particles in TiO₂ suspensions.     

Dissolution and Dispersion Behavior of Barium Carbonate in Aqueous Suspensions

Dissolution of BaCO₃ and its effect on the dispersion behavior of aqueous BaCO₃ suspensions at various pH values have been investigated. The amount of leached Ba²⁺ decreases with increasing pH value, which agrees with thermodynamically calculated results. The dissolution of BaCO₃ also causes an increase in pH value of the suspension, but the change decreases with increasing initial pH value. The isoelectric point (IEP) of leached BaCO₃ powder is at a pH of ∼10–10.5 and remains unchanged with increasing solids loading. The IEP of BaCO₃ shows no significant change with added KCl or K₂CO₃, but shifts to a higher pH with increasing concentration of added BaCl₂.     

Fabrication of Barium Strontium Titanate (Ba0.6Sr0.4 TiO3) 3D Microcomponents from Aqueous Suspensions

This study reports on the fabrication of BST (Ba_0.6Sr_0.4TiO₃)‐based 3D microcomponents through an aqueous colloidal approach. The BST powder was synthesized by solid‐state reaction assisted by thermal analysis and XRD diffraction. Well crystalline cubic BST single phase was obtained upon heat‐treating the precursor powder mixture at 1200°C–1250°C. The reactivity of the BST powder toward water was evaluated by the changes in pH and concentration of leached ionic species along aging time. Hydrolysis reactions were suppressed by chemisorbing a suitable phosphate‐based protective agent and enabled an excellent dispersion ability to be achieved. Aqueous suspensions with solids loadings as high as 50 vol% that are stable against hydrolysis for several days could easily be prepared from the surface‐treated BST powder in aqueous solutions of an epoxy resin. After adding a suitable hardener, the colloidal suspensions could easily be cast into soft silicon rubber molds to obtain homogeneous and high dense green BST bodies.     

Preparation and Characterization of Barium Titanate Suspensions for Stereolithography

Ferroelectric photoactive suspensions for stereolithography have been developed by dispersing a high volume fraction of barium titanate powder in hexanediol diacrylate (HDDA) with the aid of effective photoinitiators and dispersants. Rheological properties showed a shear thinning behavior and a low viscosity at a shear rate adequate for the recoating process. The barium titanate–HDDA suspension showed poor curing behavior due to the large refractive index difference between the ceramic and the resin. The coarse barium titanate–HDDA suspension showed a smaller surface reflectance and a larger cure depth than the fine barium titanate–HDDA suspension.     

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.     

Effects of Excess Barium Ions on Aqueous Barium Titanate Tape Properties

The effects of excess free barium ions in aqueous barium titanate slip on the resulting BaTiO₃ tape properties were investigated in terms of the slip behavior, green/sintered tape density and morphology, and dielectric properties. The excess free barium ions expressed by means of the Ba/Ti ratio adversely affected most tape properties. Increase in the slip viscosity, green porosity, and agglomeration along with a decrease in mechanical properties and green/sintered density were found with the increase in the Ba/Ti ratio. However, dielectric permittivity was increased with increase in the Ba/Ti ratio. An effort was made to correlate these phenomena with Ba²⁺ leaching in water for realistic multilayer ceramic capacitor applications.     

Dispersion of Boron Nitride Powders in Aqueous Suspensions with Cellulose

The dispersion of nitride and carbide ceramic particles in water is difficult, due to the absence of hydroxyl groups on their surface. Boron nitride (BN) is not an exception and despite its numerous applications, no effective dispersant has been identified so far. We demonstrate here the dispersion properties of two cellulose derivatives for hexagonal BN powders, hydroxyl ethyl cellulose and methyl cellulose. The effect of particle size and cellulose concentration was investigated. The adsorption of cellulose onto the surface of the BN particles was confirmed by isotherm adsorption. Zeta potential measurements show a charge screening effect of the cellulose The suspensions obtained were highly loaded and stable versus pH, and thus could lead to homogeneous codispersion of BN particles and sintering additives.     

Room-Temperature Solubility Behavior of Barium Titanate in Aqueous Media

In the present study, the incongruent dissolution of barium from barium titanate (BaTiO₃) has been studied as a function of dispersion pH and powder volume fraction for two different BaTiO₃ powders. In alkaline dispersions, the barium solubility strongly increases as the pH increases, as suggested by thermodynamic considerations. At pH <7, the barium solubility reaches a plateau, the height of which is dependent on the surface area of the powder and the solids loading of the slip. The BaTiO₃ surface is completely depleted of barium in this region.     

Solid Freeform Fabrication of Aqueous Alumina–Poly(vinyl alcohol) Gelcasting Suspensions

Solid freeform fabrication of aqueous alumina–poly(vinyl alcohol) (Al₂O₃–PVA) gel-casting suspensions was conducted using a computer-controlled extrusion apparatus fitted with a two-nozzle delivery system. The impact of casting parameters on the shear rate profiles experienced during deposition was evaluated via conventional flow analysis and computer simulations. In addition, the influence of these parameters on line resolution/uniformity, printability, and as-cast component properties was studied using laser profilometry, optical microscopy, and scanning electron microscopy. Continuous printablity was achieved for tip diameters ranging from 0.254 to 1.370 mm for all mixing rates and suspension compositions studied. Printed lines were uniform with good edge definition, and line dimensions were independent of mixing rate for these process conditions. The Al₂O₃ volume fraction (φ_Al2O3) in the as-deposited layers depended on casting conditions and cross-linking agent concentration, where (φ_Al2O3 increased with decreased tip diameter and increased cross-linking agent concentration. The free-formed Al₂O₃ components exhibited uniform particle packing, with minimal macrodefects (e.g., slumping or staircasing) and no discernable microdefects (e.g., bubbles or cracking).     

Counterion Effect of Cationic Surfactants Upon the Interaction with Poly(methacrylic acid)

This work investigates decylammonium salts, with inorganic (chloride and thiocyanate) and organic (acetate and butyrate) counterions, in interaction with poly(methacrylic acid) (PMA). The study is conducted by means of surface tension, relative viscosity and pH measurements. The role of the methyl group in PMA appears by comparison with analogous mixtures of poly(acrylic acid) (PAA). In aqueous solution, a non-cooperative interaction between the surfactant and the hydrophobic microdomains (HMD) of PMA starts first. With surfactant addition, micellar aggregates bound onto the polymer chain appear at a concentration T ₁, which is higher than for the analogous PAA systems, but lower than the respective CMC. T ₁ is lower for chloride than for thiocyanate and lower for acetate than for butyrate. Depending on the counterion, the HMD conceal or expose carboxyl groups, with influence upon the interaction strength and the bulk and surface behavior. Stronger interactions and maxima of surface tension and relative viscosity appear for the systems with organic counterions. An estimation of the interaction strength shows weaker surfactant interactions with PMA than with PAA, and that acetate induces the strongest interaction while thiocyanate the weakest.     

Aqueous Colloidal Characterization and Forming of Multimodal Barium Titanate Powders

Aqueous suspensions of two barium titanate (BaTiO₃) powders, with 100 and 500 nm nominal size, were found to be stabilized by a predominantly steric mechanism when using a polyelectrolyte surfactant, an ammonium salt of poly(methacylate) (PMA-NH₄). Considerable amounts of barium ions were shown to readily dissolve from BaTiO₃ powders in an aqueous environment, even without the addition of acid or base. This barium dissolution obscured the accurate measurement of the isoelectric point of the two powders. Suspension stabilization was observed to occur at basic pH when using PMA-NH₄. Concentrated suspensions of each individual powder exhibited shear thinning behavior, with the onset of shear thickening occurring at relatively high shear rates. Suspensions containing 85 vol% coarse/15 vol% fine powders demonstrated the lowest apparent viscosity for a given solids loading, while the highest sintered density was obtained with a mixture of 70 vol% coarse/30 vol% fine, when sintering at 1300°C for 2 h (97.4% of theoretical, slip cast from a suspension containing 50 vol% solids).     

Adiabatic compressibility of polyelectrolytes in aqueous solutions: Poly(methacrylic acid)

The adiabatic compressibility of dilute aqueous solutions of methacrylic acid, poly-(methacrylic acid), and three poly(sodium methacrylates) obtained by neutralizing the polyacid with sodium hydroxide to different extents were determined from soundvelocity and density data. The ultrasonic velocity at 25°C. was measured by employing a precision ultrasonic interferometer, and the density was measured with Ostwald-type pycnometers. The plots of the decrease of compressibility per unit concentration, (β₁ ? β)/c versus c shows that there is a marked difference between the curves of monomer and of polymer solutions. In case of the monomer there is a proportional decrease with increase in concentration, whereas in polymer in the dilute region (0.1?0.5g./dl.) the curve rises sharply, then shows down, and finally approaches a constant value at comparatively higher concentrations. The nature and number of the free counterions and the shape and the concentration of the polymer molecules are responsible for the compressibility of polymer solutions. However, the contribution of the size and shape and concentration of the polymer seem to be less than that of the nature and number of the counterions. The apparent molal volume ΦV₂ and apparent molal compressibility ΦK₂ for polymer repeat units show a sharp decrease with increase in concentration and finally attain a constant value at higher concentrations; this has been explained by the fact that in the dilute region the polymer, being extended by coulombic repulsion between similar charges situated on the side chain, enhances the formation of water clusters around it, and the free counterions are solvated, leading to a decrease to these values. The number of free counterions proportionately increases with concentration, causing a proportional decrease of the ΦV₂ and ΦK₂ values, until the concentration reaches a definite stage, above which the so-called condensation of ions occurs, and the number of free counterions does not increase further at higher concentrations.     

Dispersion and Rheology of Aqueous Suspensions of Nanosized BaTiO3

The zeta potential and the rheological behavior of aqueous suspensions of a commercial nanosized BaTiO₃ powder were studied. The influence of the volume fraction of solids, the kind and concentration of dispersant, the homogenization technique, and times were investigated. Because the as-received powder contained ∼5 wt% organics, the powder was calcined to 350°C/0.5 h. After calcination, a good dispersion was achieved for suspensions with up to 80 wt% solids using a polyacrylic-based dispersant combined with tetramethylammonium hydroxide and 10 min of ultrasound. Green bodies obtained by slip casting had relatively low densities (2.6 g/cm³, 43% TD), but homogeneous, agglomerate-free microstructures.     

Dispersion Behavior of Barium Titanate Nanoparticles Prepared by Using Various Polycarboxylic Dispersants

The effects of the polycarboxylic dispersant structures on the crystallinity and sedimentation behavior of prepared BaTiO₃ nanoparticles were analyzed using four types of dispersants—ethylenediaminetetraacetic acid dipotassium salt (EDTA), trans -aconitic acid (TAA), ammonium acrylate–methyl acrylate co-polymer (PAA50), and sodium polyacrylate (PAA100). In the case of EDTA and TAA, the adsorbed ratio of the dispersants on BaTiO₃ nanoparticles was relatively low, and only slight improvement of sedimentation behavior was observed. On the other hand, in the case of PAA50 and PAA100, the adsorbed ratio was high, and the sedimentation behavior was gratefully improved. Next, in order to analyze the relationships among the additive amount of polycarboxylic dispersants, crystallinity, and sedimentation behavior, various amounts of PAA100 or PAA50 were treated in the synthesis solution. The sedimentation behavior of BaTiO₃ nanoparticles improved with increasing amounts of PAA100 and PAA50 while their crystal phase became amorphous. Adding PAA50 at a molar ratio of COO⁻/Ba²⁺=0.266 resulted in BaTiO₃ nanoparticles with the best dispersion stability in an aqueous media.     

Influence of Solids Concentration on the Isoelectric Point of Aqueous Barium Titanate

We report on findings that the particle surface charge is influenced by solids concentration in aqueous suspensions of BaTiO₃. Three decades in solids concentration were analyzed by combining results from two different electrokinetic methods. Combined results demonstrate a systematic acidic shift in the isoelectric pH with decreasing solids concentration. The shift is attributed to the development of a Ba-depleted, TiO₂-rich surface layer. Using kinetic arguments, it is shown that the thickness of this layer will be proportional to the surface-to-volume ratio.     

Rapid Electroacoustic Method for Monitoring Dispersion: Zeta Potential Titration of Alumina with Ammonium Poly(methacrylate)

The zeta potential (ζ) of 2-vol% and 10-vol% suspensions of a commercial α-alumina has been measured using the electroacoustic technique and found to be in good agreement with values reported previously from microelectrophoresis measurements. An increasingly negative ζ value was observed through semiautomatic addition of solutions of a ammonium salt of poly(methacrylic acid). The point of complete surface coverage was inferred from simultaneous conductivity measurements. The average particle size obtained from the electroacoustic dynamic mobility spectrum showed the effect of coagulation near the isoelectric point. The expected correlations among ζ, the viscosity, and the sedimentation volume were observed. The results illustrate the efficacy of the electroacoustic technique for examining colloidal dispersions.