Interaction between Dissolved Ba2+ and PAA-NH4 Dispersant in Aqueous Barium Titanate Suspensions

The interaction between dissolved Ba²⁺ and dissociated ammonium salt of poly(acrylic acid) (PAA-NH₄) in aqueous suspension has been studied. The dissolved Ba²⁺ causes flocculation of dissociated PAA-NH₄, thus degrading its dispersing effectiveness in aqueous BaTiO₃ suspensions. The concentration of PAA-NH₄ required to stabilize aqueous BaTiO₃ suspension increases with increasing Ba²⁺ concentration at a given pH. A stability map, which is determined by a rheological study, is constructed to describe the amount of PAA-NH₄ required to have well-dispersed aqueous BaTiO₃ suspensions as a function of Ba²⁺ concentrations at different pH values.     

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.     

Two-Step Sintering of Ceramics with Constant Grain-Size, II: BaTiO3 and Ni–Cu–Zn Ferrite

We investigated the preparation of bulk dense nanocrystalline BaTiO₃ and Ni–Cu–Zn ferrite ceramics using an unconventional two-step sintering strategy, which offers the advantage of not having grain growth while increasing density from about 75% to above 96%. Using nanosized powders, dense ferrite ceramics with a grain size of 200 nm and BaTiO₃ with a grain size of 35 nm were obtained by two-step sintering. Like the previous studies on Y₂O₃, the different kinetics between densification diffusion and grain boundary network mobility leaves a kinetic window that can be utilized in the second-step sintering. Evidence indicates that low symmetry, ferroelectric structures still exist in nanograin BaTiO₃ ceramics, and that saturation magnetization is the same in nanograin and coarse grain ferrite ceramics.     

Surface Chemistry, Dispersion Behavior, and Slip Casting of Ti3AlC2 Suspensions

The surface chemistry and dispersion properties of aqueous Ti₃AlC₂ suspension were studied in terms of hydrolysis, adsorption, electrokinetic, and rheological measurements. The Ti₃AlC₂ particle had complex surface hydroxyl groups, such as ≡Ti–OH,=Al–OH, and −OTi–(OH)₂, etc. The surface charging of the Ti₃AlC₂ particle and the ion environment of suspensions were governed by these surface groups, which thus strongly influenced the stability of Ti₃AlC₂ suspensions. PAA dispersant was added into the Ti₃AlC₂ suspension to depress the hydrolysis of the surface groups by the adsorption protection mechanism and to increase the stability of the suspension by the steric effect. Ti₃AlC₂ suspensions with 2.0 dwb% PAA had an excellent stability at pH=∼5 and presented the characteristics of Newtonian fluid. Based on the well-dispersed suspension, dense Ti₃AlC₂ materials were obtained by slip casting and after pressureless sintering. This work provides a feasible forming method for the engineering applications of MAX-phase ceramics, wherein complex shapes, large dimensions, or controlled microstructures are needed.     

Stabilization and Processing of Aqueous BaTiO3 Suspension with Polyacrylic Acid

Polyacrylic acid (PAA) has been chosen to stabilize BaTiO₃ powder in aqueous solution. Zeta potential studies show that the Ba-rich particle surface is positively charged in the pH range from 2 to 11 without PAA. Adsorption of PAA decreases the zeta potential in acidic solution and changes the surface to a negative charge in basic solution. Adsorption of PAA onto BaTiO₃ surfaces is found to be related to the pH-dependent PAA conformation in solution. The amount of PAA adsorbed at pH 1.5 is one order of magnitude more than at pH 10.5, due to a more compact polymer conformation. Colloid stability of aqueous BaTiO₃ suspension is related to the dissociation of PAA as a function of pH. BaTiO₃ suspensions can be stabilized at pH values higher than 7, where more than 99% of carboxylic groups are ionized. At pH 10.5, suspension stability is related to the PAA coverage on BaTiO₃ particle surface. Stabilization can be achieved only when conditions of both PAA ionization and BaTiO₃ surface coverage are satisfied, suggesting an electrosteric stabilization mechanism. Green density of 62% can be achieved by slip casting at a pH above 10, compared to the best density of 58.6% by isostatic pressing at 242 MPa.     

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.     

Electrostatic Adsorption of a Colloidal Sintering Agent on Silicon Nitride Particles

Pressureless sintering of silicon nitride requires addition of sintering agents. The main part of this study was done in order to homogenize the distribution of sintering agents, in this case Y₂O₃, in a silicon nitride matrix. Colloidal 10-nm Y₂O₃ Particles were electrostatically adsorbed on Si₃N₄ particle surfaces. The adsorption was studied by X-ray fluorescence analysis and electrophoretic measurements. Addition of Y₂O₃ sol to a Si₃N₄ suspension decreased the viscosity of the suspension. The slip casting properties of Si₃N₄ suspensions with added Y₂O₃ sol were examined, and the homogeneity of Y₂O₃ in the green compacts was compared with conventionally prepared samples. An improved microstructural homogeneity was obtained when Y₂O₃ sol particles were adsorbed on the Si₃N₄ particle surfaces.     

Spark Plasma Sintering Behavior of Nano-Sized (Ba, Sr)TiO3 Powders: Determination of Sintering Parameters Yielding Nanostructured Ceramics

Nano-powders of BaTiO₃, SrTiO₃, Ba_0.6Sr_0.4TiO₃ (BST64), and a mixture of the composition (BaTiO₃)_0.6(SrTiO₃)₀₄ with particle sizes in the range of 60–80 nm were consolidated by spark plasma sintering (SPS). An experimental procedure is outlined that allows the determination of a "kinetic window," defined as the temperature interval within which the densification process can be kinetically separated from the grain growth one, enabling preparation of dense nanostructured ceramics. The width of this window varied from almost zero for BST64 to 125°C for the (BaTiO₃)_0.6(SrTiO₃)_0.4 mixture. During the densification (sintering) of the (BaTiO₃)_0.6(SrTiO₃)₀₄ mixture, BST64 is formed. The main part of this reaction occurs in a fully densified body through which suggesting that the constitutional phase(s) have a self-pinning effect on the grain growth.     

Effects of Solids Loading, pH, and Polyelectrolyte Addition on the Stabilization of Concentrated Aqueous BaTiO3 Suspensions

Colloidal stability of concentrated aqueous BaTiO₃ suspensions with ammonium salts of poly(acrylic acid) (PAA-NH₄) and poly(methacrylic acid) (PMAA-NH₄) as a function of pH and solids loading is investigated. For suspensions with solids loading less than 40 vol%, the required polyelectrolyte concentration to stabilize aqueous BaTiO₃ suspensions decreases with increasing pH, but remains relatively unchanged with increasing solids loading. As the solids loading continuously increases (e.g., >50 vol%), the required amount of polyelectrolyte increases, but exhibits a minimum at pH ∼ 9.2. The critical amount of polyelectrolyte needed to achieve colloidal stability of aqueous BaTiO₃ suspensions as a function of pH and solids loading is summarized in a three-dimensional stability map.     

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.     

Formation, Packing, and Sintering of Monodisperse TiO2 Powders

Monodisperse TiO₂ powders were synthesized by the controlled hydrolysis of dilute alcoholic solutions of titanium alkoxides. The state of powder aggregation in dispersions and powder packing in the green bodies strongly affected the sintering behavior. The sintering of uniformly packed powder compacts resulted in finegrained microstructures with >99% of theoretical density nt temperatures much lower than those required to sinter conventional TiO₂ powders.     

Grain-Boundary Migration and Dielectric Properties of Semiconducting SrTiO3 in the SrTiO3-BaTiO3-CaTiO3 System

Chemically induced grain-boundary migration and its effects on the interface and dielectric properties of semiconducting SrTiO₃ have been investigated. Strontium titanate specimens that had been doped with 0.2 mol% of Nb₂O₅ were sintered in 5H₂/95N₂. The sintered specimens were diffusion annealed at 1400°C in 5H₂/95N₂ with BaTiO₃ or 0.5BaTiO₃-0.5CaTiO₃ (mole fraction) packing powder. The grain boundaries of the annealed specimens were oxidized in air. In the case of BaTiO₃ packing, grain-boundary migration occurred with the diffusion of BaTiO₃ along the grain boundary. The effective dielectric constant of the specimen decreased gradually as the temperature increased but showed two peaks, possibly because of barium enrichment at the grain boundary and an oxidized Sr(Ba)TiO₃ layer. In the case of 0.5BaTiO₃-0.5CaTiO₃ packing, although barium and calcium were present at the grain boundary of the specimen, no boundary migration occurred, as in a previous investigation. With the diffusion of barium and calcium, the resistivity of the specimen increased and the variation of the effective dielectric constant with temperature was much reduced, in comparison to those without solute diffusion. These enhanced properties were attributed to the solute enrichment and the formation of a thin diffusional Sr(Ba,Ca)TiO₃ layer at the grain boundary.     

Effect of 2-Phosphonobutane-1,2,4-tricarboxylic Acid Adsorption on the Stability and Rheological Properties of Aqueous Nanosized 3-mol%-Yttria-Stabilized Tetragonal-Zirconia Polycrystal Suspensions

2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA) was evaluated as a dispersant for nanosized 3-mol%-Y₂O₃-stabilized tetragonal-ZrO₂ polycrystal (3Y-TZP) suspensions. The adsorption of PBTCA was characterized using the decolorization method of ferric 5-nitrosalicylate complexes. Maximum adsorption of the dispersant on the 3Y-TZP powder was found to occur at pH 3.0. At pH >3.0, the adsorbed amount decreased with increased pH. Semiquantitative analysis using auger electron spectroscopy showed that PBTCA adsorbed irreversibly on the powder. The surface charge of the powder was evaluated by measuring the zeta-potential in dilute powder suspensions. The suspension was most effectively stabilized at high pH by the high charge induced by the adsorption of PBTCA. Rheological properties of the suspension were evaluated as a function of dispersant amount and solids loading. The optimum amount of dispersant increased with increased solids loading for solids loading >20 vol%. A stable suspension of 35 nm 3Y-TZP particles with a solids loading as high as 32 vol% was obtained using PBTCA as dispersant, in contrast to 28 vol% when using ammonium polyacrylate (NH₄PAA). Theoretical calculations of the interaction between 3Y-TZP particles showed that the stabilization of the suspensions was attributed to a combination of the electrostatic repulsion and a steric barrier caused by the adsorbed PBTCA. Induced coupling plasma analysis showed that PBTCA could be completely burned out during sintering, which confirmed its suitability as a dispersant for 3Y-TZP.     

Preparation of SiO2 Glass from Model Powder Compacts: I, Formation and Characterization of Powders, Suspensions, and Green Compacts

Dense SiO₂ glass was produced at ∼1000°C by using highly ordered compacts of spherical, nearly monosized, amorphous SiO₂ particles. In Part I of this study, the formation and characterization of powders, suspensions, and green bodies are described. Thermogravimetry and DTA revealed that substantial loss of bound water occurs in powders calcined at temperatures as low as 200°C. Surface area and density measurements were used to show that the water loss occurs without micropore formation. FTIR spectroscopy revealed that residual silanol groups persist to the highest temperatures (1050°C) studied. The state of particulate dispersion in suspensions was modified by pH adjustment and monitored by rheological measurements. Flocculated suspensions (low pH) produce inhomogeneous, low-density powder compacts with highly bimodal pore-size distributions. Uniform green bodies (with higher packing densities) were prepared using well-dispersed suspensions (high pH). Two-dimensional, close-packed hexagonal arryas of particles were observed in these compacts. Pore-size distributions were narrower, but still bimodal due to the presence of three-particle and four-particle pore channels. The sintering behavior of these compacts is described in part II.     

Hydrothermal Synthesis of Nanometer-Sized Barium Titanate Powders: Control of Barium/Titanium Ratio, Sintering, and Dielectric Properties

Nanometer-sized BaTiO₃ powders have been synthesized hydrothermally from Ba(OH)₂ and titanium alkoxide at 150°C for 2 h, and the Ba/Ti ratio has been measured with an accuracy of ±0.003. Stoichiometric powders can be obtained by adjusting the Ba/Ti ratio of the reactants to a value of 1.018. At a lower Ba/Ti ratio, the solubility of Ba(OH)₂ prevents full incorporation of barium, and barium-deficient powders result. A higher Ba/Ti ratio leads to the incorporation of excess barium in the powder. K _s(BaTiO₃,-25°C) = 7 × 10^-8 has been calculated for the equilibrium reaction. From this result, two reproducible processes for the synthesis of stoichiometric BaTiO₃ are proposed. The processes rely only on very accurate control of the chemical composition (Ba/Ti ratio) of the precursor suspension. The sintering behavior of powders having Ba/Ti ratio values between 0.965 and 1.011 is described from results of dilatometric measurements and isothermal sintering. Room-temperature dielectric constants as high as 5600 and losses as low as 0.009 have been obtained for a stoichiometry slightly less than 1.000. It is expected that optimum sintering behavior and electrical properties are obtained in the stoichiometry range 0.995-1.000.     

Grain Growth Control and Dopant Distribution in ZnO-Doped BaTiO3

ZnO additions to BaTiO₃ have been studied in order to determine the role of this dopant on sintering and microstructure development. As a consequence of a better initial dopant distribution, samples doped with 0.1 wt% zinc stearate show homogeneous fine-grained microstructure, while a doping level of 0.5 wt% solid ZnO is necessary to reach the same effect. When solid ZnO is used as the dopant precursor, ZnO is redistributed among the BaTiO₃ particles during heating. Since no liquid formation has been detected for temperatures below 1400°C in the system BaTiO₃-ZnO, it is proposed that dopant redistribution takes place by vapor-phase transport and grain boundary diffusion. Shrinkage and porosimetry measurements have shown that grain growth is inhibited during the first step of sintering for the doped samples. STEM-EDX analysis revealed that solid solubility of ZnO into the BaTiO₃ lattice is very low, being strongly segregated at the grain boundaries. Grain growth control is attributed to a decrease in grain boundary mobility due to solute drag. Because of its effectiveness in controlling grain growth, ZnO appears to be an attractive additive for BaTiO₃ dielectrics.     

Electrophoretic Deposition and Characterization of Micrometer-Scale BaTiO3 Based X7R Dielectric Thick Films

Uniform and relatively dense BaTiO₃ thick films of 1–5 μm were prepared by an electrophoretic deposition process using submicrometer BaTiO₃ powders (mean particle size: ∼0.2 μm). Two different BaTiO₃ powders and solvent media were used to investigate the film quality and thickness control. The surface charge mechanism of BaTiO₃ particles was explained according to the observed results. The microstructures were examined by means of SEM. The experimental results show that the thickness could be controlled independently of suspension concentration by keeping a constant applied voltage and a constant current drop in a given suspension. BaTiO₃ thick films have good insulation resistance and dielectric properties such as a dielectric constant and a dissipation factor that are compatible with the data from conventional tape-cast BaTiO₃ thin layers.     

Effect of Dispersant Concentration on Preparation of an Ultrahigh Density ZnO–Al2O3 Target by Slip Casting

The rheological behavior of concentrated ZnO–Al₂O₃ aqueous suspensions has been studied in order to obtain an ultrahigh-density ZnO–Al₂O₃ composite ceramic target by slip casting. The influence of the mass fraction of polyacrylic acid (PAA) on the fluidity of slurries and the density and strength of the green and sintered bodies was investigated. The slurries exhibited a near-Newtonian flow behavior and had a lower viscosity with 0.3 wt% PAA. The excess of PAA enhanced the green strength and the density and strength of the sintered bodies. An ultrahigh density sintered body (>99.7% theoretical density) could be obtained after pressureless sintering at 1400°C. The Al species were well distributed in the sintered bodies, which showed a homogeneous, defect-free microstructure with no abnormal grain growth.     

Poly(acrylic acid)–Poly(ethylene oxide) Comb Polymer Effects on BaTiO3 Nanoparticle Suspension Stability

We have studied the effects of poly(acrylic acid)–poly(ethylene oxide) (PAA–PEO) comb polymers on the stability of aqueous BaTiO₃ nanoparticle suspensions over a wide pH range in the presence and absence of mono- and divalent salt species. The comb polymer architecture consists of charge-neutral PEO teeth attached at random intervals along an ionizable PAA backbone. Potentiometric titrations, light scattering, and turbidity measurements were conducted on pure PAA and PAA–PEO solutions to assess their degree of ionization, radius of hydration, and stability. Adsorption isotherm and rheological measurements were conducted on BaTiO₃ nanoparticle suspensions to determine the effectiveness of both PAA and PAA–PEO dispersants. Our observations indicate that the presence of PEO teeth effectively shield the underlying PAA backbone from ion interactions, e.g., counterion-screening or ion-bridging effects, thereby allowing PAA–PEO dispersants to impart stability to BaTiO₃ nanoparticle suspensions over a wide range of pH, ionic strength, and ion valency conditions where pure PAA fails.     

Core-Shell Structure of Acceptor-Rich, Coarse Barium Titanate Grains

Heavily doped barium titanate (BaTiO₃) powders, one with a donor-rich and the other with an acceptor-rich composition, were prepared. After sintering, the donor-rich specimen exhibited a fine-grained microstructure but significant grain growth occurred in the acceptor-rich specimen. A stable dielectric behavior was observed only in the donor-rich fine-grained specimen over the temperature range studied. However, in both specimens, an undoped core region several hundred nanometers in size was detected. The core-shell structure appeared to be maintained in BaTiO₃ under the conventional sintering conditions_.