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.     

Processing of β-Silicon Nitride from Water-Based alpha-Silicon Nitride, Alumina, and Yttria Powder Suspensions

A water-based route for processing ß-Si₃N₄ from alpha-Si₃N₄, Al₂O₃, and Y₂O₃ powder mixtures was established. The surface charges and isoelectric points of the three different powders were investigated within the pH range from pH 3 to pH 12. Citric acid diammonium salt was found to be an effective deflocculant for shifting the isoelectric points to pH 3.5 for Al₂O₃ and to pH 6 for Y₂O₃. Aluminum hydroxide (Al(OH)₃) showed strong interaction with the Si₃N₄ powder, shifting the isoelectric point from pH 7 to pH 5.5. Low-viscosity, high-solids-loading suspensions (60-63 vol%) thus were possible at pH 9.7. The preparation of homogeneous and stable suspensions with a solids content of ≤61 vol% and a viscosity <1 Pa·s was limited to a pH regime between pH 9 and pH 10.5 because of the high solubility of yttria. The homogeneous suspensions were easily solidified by direct coagulation casting (DCC), using the urease-catalyzed decomposition of urea at pH 9 to pH 10, by forming salt. No shrinkage cracking, sedimentation, or phase separation was observed during coagulation or drying. The green-density distribution was homogeneous throughout all bodies, even for complex geometries.     

Dispersion of SiC in Aqueous Media with Al2O3 and Y2O3 as Sintering Additives

It has been well accepted that polyethylene imine (PEI) is an effective dispersant for silicon carbide (SiC) in aqueous media. However, after the addition of sintering additives (Al₂O₃ and Y₂O₃), this dispersing effect is reduced significantly. In this work, a second dispersant, citric acid, was used to resolve this problem. It was found that citric acid could decrease the slurry viscosity (without sintering additives) and enhance the PEI adsorption on SiC particle surface. The optimal amount of citric acid required to achieve a minimum viscosity for 55 vol% SiC suspensions was equal to ∼0.87 wt% (at pH ∼6.8). With the aid of citric acid, well-stabilized SiC suspensions (containing sintering additives) were realized, which exhibited slight shear thinning rheologies. After tape casting and SPS sintering, dense SiC samples were obtained with a homogeneous fine-crystalline microstructure. Results showed that citric acid was an effective dispersant for improving the dispersion of SiC particles containing sintering additives.     

Influence of Acidity on the Electrostatic Stability of Alumina Suspensions in Ethanol

Alumina (Al₂O₃) powders have been dispersed in ethanol (EtOH) solutions of different acidity. In aqueous media, acidity is defined by pH. This definition can be extended to nonaqueous media using ion-transfer functions. The electrical charge on the particle surfaces has been found to be acidity dependent. The electrostatic stability of Al₂O₃EtOH suspensions has been evaluated via electrophoresis and turbidity. An electrostatic stabilization mechanism is proposed and analyzed via Derjaguin–Landau–Verwey–Overbeek (DLVO) theory.     

Improvement of the Dispersion of Al2O3 Slurries Using EDTA-4Na

Polyacrylic acid (PAA) is known to be an effective dispersant for Al₂O₃ powder in aqueous media. However, at high solid loading (>55 vol%), the dispersion of the Al₂O₃ suspensions became difficult with only PAA as a dispersant. In this paper, ethylenediaminetetraacetic acid, tetrasodium salt, dihydrate (EDTA-4Na) was introduced to improve the dispersion of the Al₂O₃ suspensions. With the aid of EDTA-4Na, the adsorption amount of sodium polyacrylic acid (PAA-Na) increased, while the apparent viscosity of 60 vol% Al₂O₃ slurries decreased significantly. Particle size measurements showed that EDTA-4Na could help to reduce larger agglomerates, possibly by modifying the adsorbed layer thickness. The interactions between EDTA-4Na and PAA-Na were studied using Fourier-transform infrared spectroscopy analysis. Results showed that it was possible to introduce EDTA-4Na as the second dispersant to improve the dispersion of high solid content Al₂O₃ slurries.     

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.     

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.     

Alumina and Alumina/Zirconia Multilayer Composites Obtained by Slip Casting

The slip casting technique has been revealed as a powerful method to obtain multilayer composites close to theoretical density. From zeta potential and viscosity measurements of Al₂O₃ and Al₂O₃/ZrO₂ (4 vol% ZrO₂) suspensions, the corditions for the preparation of multilayer composites by slip casting have been determined. A microstructural analysis of the different layers by scanning electron microscopy is also reported.     

Structural Ceramic Microlaminates by Electrophoretic Deposition

Yttria-stabilized zirconia (YSZ)/Al₂O₃ laminar microcomposites with a total of 80 layers, each as thin as ∼2 μm, were fabricated by electrophoretic deposition. Positively charged YSZ or Al₂O₃ powders were deposited on a cathode, layer by layer from 10 wt% solids/ethanol suspensions. The suspensions were characterized by their rate of deposition as a function of voltage. The deposited samples had a green density of ∼60% of theoretical. The microstructure of the sintered, theoretically dense samples was characterized by optical and electron microscopy, and microindentation was used to explore the mechanical properties.     

Aqueous Colloidal Processing of ZTA Composites

Two different zirconia-alumina composites, ZTA-30 (70 wt% Al₂O₃+30 wt% ZrO₂) and ZTA-60 (40 wt% Al₂O₃+60 wt% ZrO₂), with potential for orthopedic applications, were processed in aqueous media and consolidated by slip casting (SC), hydrolysis-assisted solidification (HAS), and gelcasting (GC) from suspensions containing 50 vol% solids loading. For comparison purposes, the same ceramic compositions were also consolidated by die pressing of freeze-dried granules (FG). In the HAS process, 5 wt% of Al₂O₃ in the precursor mixture was replaced by equivalent amounts of AlN to promote the consolidation of the suspensions. Ceramics consolidated via GC exhibited higher green (three-point bend) strengths (∼17 MPa) than those consolidated by other techniques. Further, these ceramics also exhibited superior fracture toughness and flexural strength properties after sintering for 1 h at 1600°C in comparison with those consolidated by other techniques, including conventional die pressing (FG).     

Electric-Field-Induced Crack Growth Behavior in PZT/Al2O3 Composites

Hard lead zirconate titanate (PZT) and PZT/Al₂O₃ composites were prepared and the alternating-electric-field-induced crack growth behavior of a precrack above the coercive field was evaluated via optical and scanning electron microscopy. The crack extension in the 1.0 vol% Al₂O₃ composite was significantly smaller than that in monolithic PZT and the 0.5 vol% Al₂O₃ composite. Secondary-phase Al₂O₃ dispersoids were found both at grain boundaries and within grains in the composites. A large number of dispersoids were observed at the grain boundaries in the 1.0 vol% Al₂O₃ composite. It appears that the Al₂O₃ dispersoids reinforce the grain boundaries of the PZT matrix as well as act as effective pins against microcrack propagation.     

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.     

Novel Method to Prepare Electroconductive Titanium Nitride–Aluminum Oxide Nanocomposites

A novel method for the preparation of TiN–Al₂O₃nanocomposites was developed. TiN–Al₂O₃nanocomposite powders were prepared by the direct nitridation of TiO₂–Al₂O₃nanocomposite powders that were derived from the simultaneous hydrolysis of tetra-butyl titanate and precipitation of aluminum nitrate. Dense sintered bodies of these TiN–Al₂O₃nanocomposite powders were obtained by hot pressing at 1450°–1650°C and 30 MPa for 60 min. The resistivity of nanocomposite reaches a minimum (1.5 × 10⁻³Ω·cm) at 25 vol% TiN additions. The percolation concentration of nanocomposite is ∼10 vol% TiN.     

Rheology and Consolidation of Colloidal Alumina-Coated Silicon Nitride Suspensions

In an attempt to improve the colloidal processing of Si₃N₄ ceramics, we studied the rheology and consolidation of colloidal suspensions of Si₃N₄ particles (average particle size 0.7 μm) with small Al₂O₃ particles (average particle size 20 nm). It was found that at pH >7, the viscosity of the mixtures increased and then decreased with an increasing concentration of Al₂O₃. λpotential measurements, optical micrographs, and visible light absorptance measurement suggest that such viscosity behavior is due to clustering of Si₃N₄ particles bridged by the small Al₂O₃ particles. This is also supported by the Derjaquin-Landau-Verwey-Overbeek (DLVO) potential calculations that show the barrier height in the DLVO potential between Al₂O₃ and Si₃N₄ is small. The small barrier height under current experimental conditions stems from the small size of the Al₂O₃ particles. The small barrier height allows the thermal motion of the two kinds of particles to overcome the barrier and attach to each other. The adsorption of small Al₂O₃ particles on Si₃N₄ can occur even when both Al₂O₃ and Si₃N₄ carry the same sign of charges. The adsorption of Al₂O₃ on Si₃N₄ also increases the density of consolidated compacts.     

Biaxial Strength of a ZrO2/(Ni+Al2O3) Nanocomposite

Previous studies demonstrated that the strength of zirconia (ZrO₂) could be enhanced or reduced by respectively adding micrometer-sized alumina (Al₂O₃) or nickel (Ni) particles. In the present study, 5 vol% micrometer-sized Al₂O₃ particles and 1 vol% nanometer-sized Ni particles are incorporated into the ZrO₂ matrix, which is subsequently densified by pressureless sintering. The biaxial strength of the ZrO₂/(Ni+Al₂O₃) nanocomposite is nearly double that of the monolithic ZrO₂. The increase in strength correlated with a reduction in the critical flaw size and not with any change in toughness, which may be a result of grain boundary strengthening.     

Densification and Mechanical Properties of B4C with Al2O3 as a Sintering Aid

The densification behavior and mechanical properties of B₄C hot-pressed at 2000°C for 1 h with additions of Al₂O₃ up to 10 vol% were investigated. Sinterability was greatly improved by the addition of a small amount of Al₂O₃. The improvement was attributed to the enhanced mobility of elements through the Al₂O₃ near the melting temperature or a reaction product formed at the grain boundaries. As a result of this improvement in the density, mechanical properties, such as hardness, elastic modulus, strength, and fracture toughness, increased remarkably. However, when the amount of Al₂O₃ exceeded 5 vol%, the level of improvement in the mechanical properties, except for fracture toughness, was reduced presumably because of the high thermal mismatch between B₄C and Al₂O₃.     

Tape Casting of Al2O3/ZrO2 Laminated Composites

Fracture toughness of ZrO₂-toughened alumina could he increased by macroscopic interfaces, such as those existing in laminated composites. In this work, tape casting was used to produce A/A or A/B laminates, where A and B can be Al₂O₃, Al₂O₃/5 vol% ZrO₂, and Al₂O₃/l0 vol% ZrO₂. An increase of toughness is observed, even in the Al₂O₃/Al₂O₃ laminates.     

Mechanical Properties and Fracture Toughness of α-Al2O3-Platelet-Reinforced Y-PSZ Composites at Room and High Temperatures

YPSZ/Al₂O₃-platelet composites were fabricated by conventional and tape-casting techniques followed by sintering and HIPing. The room-temperature fracture toughness increased, from 4.9 MPa·m^1/2 for YPSZ, to 7.9 MPa·m^1/2 (by the ISB method) for 25 mol% Al₂O₃ platelets with aspect ratio = 12. The room-temperature fiexural strength decreased 21% and 30% (from 935 MPa for YPSZ) for platelet contents of 25 vol% and 40 vol%, respectively. Al₂O₃ platelets improved the high-temperature strength (by 110% over YPSZ with 25 vol% platelets at 800°C and by 40% with 40 vol% platelets at 1300°C) and fracture toughness (by 90% at 800°C and 61% at 1300°C with 40 vol% platelets). An amorphous phase at the Al₂O₃-platelet/YPSZ interface limited mechanical property improvement at 1300°C. The influence of platelet alignment was examined by tape casting and laminating the composites. Platelet alignment improved the sintered density by >1% d _th , high-temperature strength by 11% at 800°C and 16% at 1300°C, and fracture toughness by 33% at 1300°C, over random platelet orientation.     

Rheological Behavior and Slip Casting of Al2O3–Ni Aqueous Suspensions

A strong effort has been devoted recently toward processing of metal–ceramic composites with tailored microstructure by colloidal methods. The aim of this work is to optimize the rheological behavior of concentrated Al₂O₃–nickel (Ni) aqueous suspensions and further slip casting in order to obtain dense green composites. Compositions with Ni relative contents ranging from 5 to 75 vol% were prepared from suspensions with high solids loadings (50 vol%) by adjusting the colloidal stability of each component in terms of pH, mobility, dissolution conditions, and influence of polyelectrolytes. The rheological properties were measured under controlled rate and controlled stress conditions at different basic pH conditions and contents of polyelectrolyte. Better rheological conditions of the mixtures were found for pH 10 and 1.0 wt% polyelectrolyte. Minimum viscosity was obtained for suspensions containing 15 vol% of Ni. The analysis of flow curves demonstrates that the suspensions form a structure at very low shear, hindering sedimentation. Homogeneous slip cast bodies with green densities up to 70% of theoretical and up to 75 vol% Ni were sintered in Ar to achieve dense biphasic composites.     

Microstructure and High-Temperature Mechanical Behavior of Alumina/Alumina–Yttria-Stabilized Tetragonal Zirconia Multilayer Composites

Layered composites of alternate layers of pure Al₂O₃(thickness of 125 μ m) and 85 vol% Al₂O₃-15 vol% ZrO₂ that was stabilized with 3 mol% Y₂O₃(thickness of 400 μ m) were obtained by sequential slip casting and then fired at either 1550° or 1700°C. Constant-strain-rate tests were conducted on these materials in air at 1400°C at an initial strain rate of 2 × 10^-5 s^-1. The load axis was applied both parallel and perpendicular to the layer interfaces. Catastrophic failure occurred for the composite that was fired at 1700°C, because of the coalescence of cavities that had developed in grain boundaries of the Al₂O₃ layers. In comparison, the composite that was fired at 1550°C demonstrated the ductility of the Al₂O₃+YTZP layer, but at a flow stress level that was determined by the Al₂O₃ layer.