Dispersion and Rheology of Aqueous Zirconium Diboride Nanosuspensions

The dispersion and rheology of aqueous ZrB₂ nanosuspensions were investigated by zeta potential measurements, particle size measurements, sedimentation tests, and rheology measurements, with poly (acrylic acid) (PAA) as dispersant. Results showed that the dispersion and rheology of nanosized ZrB₂ suspensions in aqueous media were dependent on pH value, PAA concentration, solid loading, and ball milling time. Concentrated (up to 30 vol% solid loading) and well‐stabilized aqueous ZrB₂ nanosuspension with low viscosity (0.485 Pa s at 60/s) was prepared at pH 10, with 1.0 wt% PAA.     

A PCE-based rheology modifier allows machining of solid cast green bodies of alumina

The performance of a poly(carboxylate ether) (PCE)-based superplasticizer to enable the machining of green bodies that are solid cast from suspensions of alumina was investigated. An alumina loading of 35 vol% in the presence of 1.25 wt% superplasticizer was established to be suitable for lathing and removal of significant amount of material through drilling. A reduction of 77% in the diameter of green bodies that corresponds to a 59% reduction in volume was achieved. The lathed green bodies exhibited smooth terraces without visible cracks. All of the green bodies were sintered without a polymer burnout step.     

Dispersion behavior of HfC-based nanopowders in ethanol

The dispersion behavior of two different types of ultrafine HfC-based powders in ethanol was investigated using polyethyleneimine (PEI) as a dispersant. The first type was synthesized by carbothermal reduction using a modified spark plasma sintering technique (d50: 125 nm). For the 10 wt% HfC suspensions, the highest zeta potential value (67.7 mV), the least sediment after sedimentation test for one day, and finest particle sizes were obtained when the concentration of PEI was 2.0 wt%. The concentrated HfC slurries with a solid loading of 40 vol% were achieved using 1.0 wt% PEI. The second type was mixing the HfC powder with HfSi2-C additives using a high-energy ball milling. The concentrated HfC slurry containing 20 wt% of HfSi2-C sintering additives was prepared up to 50 vol% solid loading using 0.50 wt% PEI. This is the first report for producing highly concentrated HfC-based nano slurries, which were highly suitable for the wet process of ultrahigh-temperature ceramic matrix composites (UTHCMCs).     

Preparation of concentrated barium titanate suspensions incorporating nano-sized powders

In order to reduce agglomeration and overcome the low packing density issues of working with nano-sized powders, a colloidal processing route has been chosen in this study. Commercial BaTiO₃ (BT) powders with a particle size in the range of 50 nm have been dispersed in the aqueous media. Rheological properties have been analyzed on suspensions with different solids loading, dispersant concentration, and pH conditions. Optimum dispersing conditions were obtained for suspensions prepared at basic pH (pH 10) with 0.646 wt% ammonium poly (acrylic acid) (NH₄PAA) as a dispersant. Suspensions have been centrifugally cast to obtain the green body, and the sintering conditions have been investigated by examining the phase evolution, microstructures and electrical properties of the sintered samples through XRD, SEM and dielectric measurements, respectively. The results show that for a 45 vol% suspension sintered at 1325 °C, the density of bulk ceramic can reach 5.85 g/cm³, nearly 97.0% of the theoretical density.     

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.     

Effect of heat treatment on the dispersion and sintering behaviour of tin doped indium oxide powders

Due to the importance of structural uniformity of ITO targets on the properties of ITO films, the untreated and heat treated tin doped indium oxide powders were used to study the effects of four different dispersants on the dispersion behaviour of nanosized ITO powders. The optimum dispersant is NH₄PAA and its optimum amount is 1.00?wt% when the pH value is 9.0. In addition, the effect of the treatment temperature of nanosized ITO powders on the dispersion and sintering behaviour was also studied by SEM, TEM and XRD. The solid loading of ITO slurries and the relative density of the sintered bodies prepared with ITO powders treated at 900?°C could reach 40?vol% (untreated, 25?vol%) and 98.53% (untreated, 95.04%), respectively. The results indicate that the heat treatment of powders at 900?°C allowed obtaining powders from which ITO aqueous suspensions with high solid loading could be prepared and dense bodies after sintering. In another word, the appropriate heat treatment process for tin doped indium oxide powders could reduce the sintering temperature by 50?℃ and refine the grain size.     

Stabilization of alumina with polyelectrolyte and comb copolymer in solvent mixtures of water and alcohols

Solvent mixtures of water and ethanol and water and isopropanol have been evaluated for processing of concentrated alumina suspensions. The addition of alcohols may increase the long-term stability of suspensions with soluble ceramic species such as magnesia, which is added as a sintering aid. A poly(acrylic acid) and a hydrophilic comb copolymer were used as dispersants for the different solvent mixtures. The aim was to compare the stabilization efficiency at normal processing conditions, pH 9–10, through rheological measurements and to develop a robust system including magnesia with long-term stability. The electrostatic stabilization of the dispersants in the different solvent mixtures was studied by zeta potential measurements. Highly negative zeta potentials were observed for the poly(acrylic acid) at pH 9–10 in the solvent mixtures. A charge contribution was also seen from the adsorbed comb copolymer, however smaller than for the poly(acrylic acid). Low viscosity was obtained for suspensions stabilized with poly(acrylic acid) in solvent mixtures with either 25 vol% ethanol or isopropanol. Higher alcohol to water ratio led to flocculation of the suspension when poly(acrylic acid) was used as dispersant. Alumina suspensions with added magnesia in isopropanol:water 25:75 and poly(acrylic acid) as dispersant showed long-term stability. The viscosity remained almost constant during 4 days of aging. Suspensions stabilized with the comb copolymer dispersant gave stable systems with ethanol and isopropanol concentrations between 25 and 75 vol%. The superior dispersing efficiency of the comb copolymer at alcohol contents above 25 vol% was believed to originate from steric stabilization in combination with low effective particle size, giving low viscosity through lower apparent solid contents of the suspension.     

Improved hydrophilicity of zinc oxide-incorporated layer-by-layer polyelectrolyte film fabricated by dip coating method

ZnO nanoparticles suspended in poly(acrylic acid) (PAA) were deposited onto layer-by-layer (LBL) polyelectrolyte (PET) films fabricated from poly(allylamine hydrochloride) (PAH) and PAA by dip coating method. Effect of etching time and concentration of ZnO suspension on hydrophilicity of the LBL-PET films before and after UV irradiation was examined using water contact angle measurement. 2.0 M PAH/PAA solutions with a dipping speed of 3.0 cm/min provided stable LBL-PET films with thickness sufficient for HCl etching. Glass substrates with the etched LBL-PET film dipped into 0.2 wt.% ZnO suspension exhibited the contact angle of 10° after irradiated by UV for 60 min.     

Stabilizing Highly Loaded Silicon Nitride Aqueous Suspensions Using Comb Polymer Concrete Superplasticizers

The stabilization of highly loaded silicon nitride suspensions will afford the processing of complex and near‐net shaped parts using methods such as injection molding or direct write additive manufacturing. In this study, aqueous silicon nitride suspensions up to 45 vol% solids loading were dispersed using commercially available comb‐type copolymer. These copolymers are used as superplasticizers in the concrete industry and are referred to as water‐reducing admixtures (WRAs). Four different WRA dispersants were examined and chemical analysis determined that each was made up of a sodium salt of polyacrylic acid (PAA‐Na) backbone with neutral polyethylene oxide (PEO) side chains that afford steric stabilization. The general structures of the WRAs were compared to each other by measuring the relative areas of their prominent FTIR peaks and calculating a PAA‐Na/PEO peak ratio. Suspensions were made with as‐received silicon nitride powders with 5 wt% aluminum oxide and 5 wt% yttrium oxide added as sintering aids. Three of the four WRA dispersants studied were able to produce suspensions with 43 vol% solids loading and 5 vol% polymer dispersant, while exhibiting a yield‐pseudoplastic behavior up to 30 s^?1. At higher solids loading (45–47 vol%), a shift to shear‐thickening behavior was observed at a critical shear rate for these WRAs. Those WRAs with a lower PAA‐Na/PEO peak ratio displayed better stabilization and diminished shear‐thickening behavior. The vol% of the dispersant was optimized producing yield‐pseudoplastic suspensions containing 45 vol% solids loading with yield stresses less than 75 Pa, no shear‐thickening behavior, and viscosities less than 75 Pa·s for shear rates in the range of 1–30 s^?1.     

Optimization of a Nanoparticle Suspension for Freeze Casting

Poly(acrylic acid) (PAA) dispersant concentration, suspension pH, and Al₂O₃ solids loading effects on PAA adsorption onto Al₂O₃ nanoparticles were studied; the stability and rheology of the Al₂O₃ nanoparticle suspensions were examined. The most desirable suspension conditions were 7.5–9.5 for pH and 2.00–2.25 wt% of Al₂O₃ for the PAA concentration. Electrical double-layer thickness and PAA adsorption layer thickness comparison showed that electrosteric stabilization was dominant. 45.0 vol% Al₂O₃ solids loading can be achieved for freeze casting. The maximum solids loading was predicted to be 50.7 vol%. The freeze-cast sample showed that pre-rest before freezing was critical for achieving desirable microstructures.     

Preparation and properties of AlN ceramic suspension for non-aqueous gel casting

A non-aqueous gel casting process based on the mixed solvent (ethanol and polyethylene glycol) and low-toxicity N,N-dimethylacrylamide (DMAA) was developed for an aluminum nitride (AlN) ceramic. In the present work, rheological properties of non-aqueous concentrated AlN suspensions were investigated in the presence of mixed solvent, dispersant, milling time, monomer and solid loading, in order to screen for the most suitable experimental conditions to obtain a good rheological behavior for gel casting. The results showed that the 50 vol% slurry with 0.2 wt% dispersant concentration, 2 h milling time, 6 wt% -monomer content, and a solvent ratio of 3:1, can meet the requirements for the casting process of AlN ceramic slurries. After being dried at 100 °C for 1 h, the optimum bulk density and maximum flexural strength of the AlN green bodies were as high as 1.97 g/cm³ and 18.68 MPa, respectively. SEM photographs revealed that the green body had a relative uniform microstructure when the solid loading was 50 vol%. The shrinkage and deformation of shaped sintered bodies prepared through gel casting were small after sintering. The sintering shrinkage, apparent porosity, bulk density and flexural strength were 14.8%, 0.22%, 3.21 g/cm³ and 310 MPa, respectively.     

Processing of Highly Concentrated Aqueous α-Alumina Suspensions Stabilized with Polyelectrolytes

Stability and rheology of aqueous α-Al₂O₃ suspensions with poly(methacrylic acid) and poly(acrylic acid) polyelectrolytes were studied as a function of pH, solids loading, and molecular weight. Past work has found polyelectrolyte-stabilized suspensions to be fairly pH independent at low (e.g., 20 vol%) solids loadings. However, we now show that the effective pH range to provide dispersed and fluid suspensions narrows as the concentration of solids increases as related to interparticle forces. At high solids levels, the presence of excess polymer in solution has detrimental effects on stability, which increases as the molecular weight increases. Finally, with knowledge of these concepts, highly concentrated fluid suspensions of 60 vol% Al₂O₃ (>85 wt%) with submicrometer-size particles have been processed. Higher consolidated densities and reduced sintering temperatures are attained when compared with more conventional processing techniques.     

Efficient dispersants for the dispersion of gallium zinc oxide nanopowder in aqueous suspensions

Appropriate dispersants for the dispersion of gallium zinc oxide (GZO) nanopowder in aqueous suspensions were identified in this study. The dispersion efficiencies and stabilization mechanisms of water‐based dispersants ammonium poly(acrylic acid) (PAA–NH₄), an anionic polyelectrolyte, and polyethylenimine (PEI), a cationic polyelectrolyte, were compared. The experimental analyses of rheology and sedimentation showed that both PAA–NH₄ and PEI were good dispersants for the dispersion of GZO. Theoretical calculations based on Derjaguin‐Landau‐Verwey‐Overbeek theory revealed that the stabilization mechanism of PEI was mainly related to the steric effect, and a very low molecular weight of 1800 g/mol was insufficient for powder stabilization. GZO was well dispersed by PEI with high molecular weight of 10 000 g/mol, but agglomeration occurred when too much PEI was added. Compared with PEI, PAA–NH₄ was more efficient because of its high contribution to the increase in electrostatic repulsion. Based on theoretical considerations on both steric and electrostatic effects, namely, the electrosteric stabilization mechanism, PAA–NH₄ is optimal for the dispersion of GZO nanopowder in aqueous suspensions.     

SLA 3D printing of high quality spine shaped β-TCP bioceramics for the hard tissue repair applications

β-TCP has excellent biodegradability and bioabsorption properties, and is regarded as an ideal hard tissue repair material. In the present study, 3D printing β-TCP green bodies was realized using the stereo lithography apparatus (SLA) technology. The effects of the KH-560 dispersant and solid loading on the slurry properties were investigated systematically. The optimized KH-560 addition amount and the solid loading of the slurry were 2.0 wt% and 48 wt%, respectively, and the corresponding slurry for the subsequent SLA 3D printing exhibited good fluidity, uniform dispersion and good stability. The sintering schedule of the printed β-TCP green bodies was optimized by the DSC-TG analysis. By sintering the green bodies at 1050 °C for 8 h, high quality β-TCP bioceramics without crack or deformation were fabricated. It was found that increasing the solid loading of the slurry would decrease the porosity while reducing the shrinkage degree of the β-TCP ceramics. However, the slurry could hardly be printed when its solid loading was higher than 50 wt%.     

Digital light processing-stereolithography three-dimensional printing of yttria-stabilized zirconia

Digital light processing (DLP)-stereolithography three-dimensional (3D) printing is a well known technique for fabricating components with complex geometries. However, the application of DLP 3D printing to functional ceramics such as 8 mol% yttria-stabilized zirconia (8YSZ), which is one of the most extensively used electrolyte materials for solid oxide fuel cells, is still a great challenge. Therefore, the fabrication of fully 8YSZ monoliths via DLP 3D printing was attempted herein, including the preparation of UV-curable ceramic suspensions, shaping of green bodies, and debinding and sintering. The results show that intact green bodies printed using a 30 vol% 8YSZ-photosensitive resin suspension with 0.1 wt% oleic acid as the dispersant under the optimized printing conditions was sufficiently dense without connected pores after vacuum debinding and sintering in air. The successful fabrication of 8YSZ monoliths with design flexibility via 3D printing provides a simple method for preparing functional ceramic components and may expand the application of 3D printing technology to the energy field.     

Roles of polymeric dispersant charge density on lead zirconate titanate aqueous processing

Poly(acrylic acid) (PAA) and poly(acrylic acid-co-maleic acid) (PACM) were used as dispersants in preparation of lead zirconate titanate (PZT) aqueous suspensions. The effects of dispersant structure on particle stabilization were investigated through properties of the suspensions. Viscosity and sedimentation height measurements showed that addition of the dispersants improved particle stabilization. The dispersant concentrations to obtain the lowest viscosity were 0.4 wt% for PAA and 0.2 wt% for PACM based on powder dried weight basis. Furthermore, effects of pH were studied on the suspensions prepared with 0.2 wt% dispersants. Viscosity and sedimentation behaviors indicated the improvements of particle dispersion and suspension stability with an increasing pH. Particle dispersion revealed by laser light scattering and scanning electron microscopy supported an improvement of particle dispersion at alkaline pHs. Detailed analysis of these data indicated that the PACM exhibited higher dispersant efficiency for PZT aqueous suspension in all conditions. The results were discussed based on the concentrations of anionic –COO^? groups at various pHs and charge density along polymeric backbone of the dispersants.     

Effect of Solids Loading and Dispersant Concentration on Strain Mismatch and Deformation of Slip-Cast Green Bodies

A method has been developed to identify low levels of strain mismatch present in bodies in the green state. High-aspect-ratio beams of alumina were slip cast at 35–50 vol% solids over a range of poly(acrylic acid) concentrations. After sintering in an unconstrained manner, the beams deform into an arc. The magnitude and direction of curvature was observed to depend on both the solids loading and the concentration of polyelectrolyte. Model calculations indicate that strain mismatch within the beams of 1%, which corresponds to green density variations of up to 2.5%, would account for the observed degree of bending. The direction of bending was rationalized in terms of the competing effects of cake compressibility and particle velocity.     

Rheological behavior of aqueous mullite-albumin-methylcellulose systems

In this work, the thermogelling behavior of aqueous mullite-bovine serum albumin (BSA) suspensions was studied by dynamic rheology in order to determine the experimental conditions that must be used to form mullite green bodies by thermal consolidation. Viscoelastic properties (G′ and G′′) as a function of temperature (30–95 °C) and time were determined by temperature sweep tests and time sweep tests, respectively. On the other hand, the influence of methylcellulose (MC) (2 wt%) as a binder on the viscoelastic properties of the aqueous mullite-protein system as a function of both experimental parameters (temperature and time) was also studied. In addition, shear flow properties of aqueous mullite (40 vol%; 0.45 wt% of polyacrylic polyelectrolyte as a dispersant)–BSA (10 and 15 vol%)-MC (2 wt%) suspensions were analyzed to obtain information on the rheological behavior of the suspensions at room temperature. The results obtained showed that the presence of mullite particles and MC changed the onset temperature of gelation of the protein and increased the gelation time. Thus, both the mullite particles and methylcellulose intervened in the formation of the developed protein gel.     

Aqueous tape casting of lithium metazirconate (Li2ZrO3) thin sheets

Tape cast Li₂ZrO₃ thin sheets were successfully made from aqueous suspensions with 30 vol% solid loading, using 0.5 wt% of ammonium polyacrylate and 0.2 wt% of tetramethyl ammonium hydroxide as organic stabilizers. The addition of 15 wt% of acrylic binder emulsion into the slip enabled to cast green tapes with suitable strength and plasticity for their easy folding. Zeta potential and rheological flow curves were analyzed to optimize the slip stability and composition, and the thermal behavior, phase composition and microstructure of the tapes were evaluated. After sintering at 1200 °C/15 h, tapes reached bulk densities close to 81% vs TD and exhibited a smooth flawless surface. SEM images showed an average particle size of 6.2 µm, and the XRD phase analysis revealed monoclinic Li₂ZrO₃ and only 6% of ZrO₂. Overall, this detailed work will be relevant for further research on the manufacture of solid-state electrolytes based on Li₂ZrO₃ tapes.     

Influence of solid loading on densification behavior,micromorphology and dielectric properties of Ba0.67Sr0.33TiO3 ceramics fabricated by a novel DCC-HVCI method

Ba_0.67Sr_0.33TiO₃ (BST) ceramics with highly improved dielectric performance were fabricated by a novel direct coagulation casting via high valence counter ions (DCC-HVCI) method. The influence of solid loading on densification behavior, micromorphology, and dielectric performance of the samples was investigated. With the increase of solid loading from 40 to 50 vol%, the maximum densification rate of BST ceramics increased from 0.090 to 0.122 s⁻¹, and the densification temperature decreased from 1424 to 1343 °C, which indicated that high solid loading could promote the densification behavior of samples during sintering. BST ceramics fabricated by the DCC-HVCI method showed uniform grain size and microstructure, which was beneficial for the dielectric properties of BST ceramics. Samples obtained from 45 vol% suspensions possessed the lowest dielectric permittivity (ε_r ≈ 2801), and the dielectric loss (tanδ≈0.0262) was about 1/10 of that of dry-pressed samples (tanδ≈0.301), which could be attributed to the composition homogenization.