Freeze Casting as a Nanoparticle Material-Forming Method

Nanoparticle material forming is challenging because of loose packing and agglomeration issues intrinsic to nanoparticles. Liquid processing shows great potential to overcome such hurdles. This study is focused on nanoparticle colloidal processing and freeze-casting forming. Al₂O₃ nanoparticle suspensions are examined, and microstructure evolution of Al₂O₃ nanoparticle suspension during freeze casting is discussed. The "Fines" effect influences nanoparticle packing on freeze-cast sample surfaces. Trapped air bubbles in the suspension lead to a porous bulk microstructure. Prerest is necessary for dense and homogeneous green microstructure formation. The green strength, fracture mode, and ability to form fine features by freeze casting are also evaluated.     

Homogeneous Fabrication and Densification of Cordierite–Zirconia Composites by a Mixed Colloidal Processing Route

Based on the electrokinetic properties of aqueous silica, boehmite, and ZrO₂ dispersions, cordierite-ZrO₂ composites were fabricated by a mixed colloidal processing route. The fabricated composite was characterized by a dense and homogeneous microstructure and by a uniform spatial distribution of submicrometer-sized tetragonal ZrO₂ particles throughout the matrix. Increasing ZrO₂ content enhanced densification and resulted in a full density composite at 20 wt% ZrO₂. Fracture toughness was also increased with increasing ZrO₂ content. The enhanced toughening was partly attributed to the martensitic transformation of the dispersed tetragonal ZrO₂ particles in a cordierite matrix. The formation of zircon was suppressed by suitably adjusting the heating schedule during sintering.     

Surface Precipitation Route for the Development of Cordierite-Zirconia Composites

Based on the interfacial electrochemical properties of aqueous cordierite dispersion, a novel interfacial scheme for fabricating cordierite-ZrO₂ composite with a dense and homogeneous microstructure was proposed and tested. In the present processing route, cordierite particles were first uniformly coated with a thin layer (∼40 nm) of aluminum hydroxide by an in situ surface-induced coating. Then, a controlled surface precipitation of ultrafine zirconium hydroxide precursor onto the coated cordierite particles was performed to obtain a composite with a uniform spatial distribution of the dispersed ZrO₂ particles throughout the matrix. The coated layer acted to suppress the formation of zircon phase during sintering. The enhanced fracture toughness observed in the presence of ZrO₂ was partly attributed to the t → m transformation toughening of the dispersed tetragonal ZrO₂ particles.     

Fabrication of Fine YAG-Particulate-Dispersed Alumina Fiber

This paper involves novel fabrication processes for polycrystalline α-Al₂O₃-matrix composite fibers that contain nanosized yttrium aluminum garnet (YAG) particles. Dense α-Al₂O₃/YAG nanocomposite fibers with a fine and homogeneous microstructure can be successfully fabricated via a modified sol-gel process and α-Al₂O₃ seed-particle addition. YAG nanoparticles have been homogeneously dispersed within Al₂O₃-matrix grains as well as at grain boundaries. Effects of α-Al₂O₃ seed particles and YAG nanodispersions on crystallization and microstructure development of nanocomposite fibers are discussed.     

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 Physical Nature of Powders and Firing Atmosphere on ZnAI2O4 Formation

The rate of ZnA1₂O₄ formation for binary powder mixtures of ZnO and α-Al₂O₃ (dense coarse particles and weak agglomerates of fine powder) fired in air or O₂ atmospheres was measured and the microstructures of those systems were observed by scanning electron microscopy. With dispersed dense particles of α-Al₂O₃, the Al₂O₃ surfaces were covered with ZnO and the spinel grew into the particles maintaining essentially a constant reaction interface area. Calculations based on geometric measurements and use of Jander's equation gave a similar high activation energy, 354 kJ/mol, which corresponds to the activation energy of volume diffusion of Zn²⁺ in ZnAl₂O₄. An oxygen atmosphere had no effect. With a matrix of fine α-Al₂O₃ powder and dispersed granules of ZnO, a higher reaction rate occurred because of an increase in reaction interface area due to penetration of the powder compact matrix by ZnO vapor, which was enhanced by an O₂ atmosphere. The reaction layer grew into the alumina matrix adjoining the ZnO granules with a parabolic rate law. Apparent activation energies below ∼200 kJ/mol were calculated.     

Coagulation Method of Aqueous Concentrated Alumina Suspensions by Thermal Decomposition of Hydroxyaluminum Diacetate

Consolidation of aqueous concentrated suspensions was used to shape alumina green bodies because it enabled us to obtain complex-shape components with accurate sizes. A high state of alumina particle dispersion was achieved by using (HO)₂C₆H₂(SO₃Na)₂ (Tiron), which allowed us to obtain stable alumina suspensions at pH 9 with a powder concentration higher than 60 vol%. The addition to the suspension of hydroxyaluminum diacetate, (CH₃CO₂)₂AlOH, which decomposed as the temperature increased, permitted us to coagulate an alumina suspension dispersed with Tiron efficiently. Adsorption measurements, electrokinetic mobility, and the rheological behavior of the suspensions provided useful methods to characterize each processing stage. Dense green bodies with sufficient cohesion could be demolded and dried, demonstrating that the dispersant and the flocculant agent chosen permit one to optimize the direct coagulation casting processing of alumina components.     

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.     

Processing of Homogeneous Zirconia-Toughened Alumina Ceramics with High Dry-Sliding Wear Resistance

The preparation of dense homogeneous zirconia-toughened alumina (ZTA) with high dry-sliding wear resistance is described. These ZTA ceramics are obtained by sintering green compacts, made by colloidal filtration of well-defined ZrO₂-Al₂O₃ particle suspensions, for 2 h at 1400°C. The optimum solid and stabilizer concentrations for the filtration process were determined. The sintered ZTA microstructure consists of a homogeneous distribution of zirconia grains in an alumina matrix with grain sizes of 0.2 and 0.5 µm, respectively. In pin-on-disk tribological measurements at relatively high initial contact pressures (1130 MPa) and sliding speeds (0.5 m/s) a very low specific wear rate (about 5 × 10^-8 mm³/(N·m)) and a coefficient of friction of 0.45–0.55 were found. It is shown that, in this case, wear is dominated by abrasion and polishing.     

Aggregation Effects on the Compressive Flow Properties and Drying Behavior of Colloidal Silica Suspensions

The influence of aggregation phenomena on the compressive flow properties and drying behavior of nonaqueous and aqueous silica (SiO₂) suspensions of varying electrolyte (NH₄Cl) concentrations were studied. Compressive rheology measurements, including sedimentation and centrifugal consolidation, were first conducted to investigate consolidation behavior in the absence of solvent evaporation. The volume-fraction-dependent osmotic pressure and compressive yield stress were determined for dispersed and flocculated SiO₂ suspensions, respectively. Consolidation behavior then was studied in situ by simultaneously measuring stress evolution and solvent loss as a function of drying time. The observed drying stress histories of the films were complex, consisting of several characteristic regions. First, there was an initial period of stress rise to a maximum drying stress. These measured stress values exhibited good agreement with the osmotic pressure and compressive yield stress at equivalent SiO₂ volume fractions for the dispersed and flocculated systems, respectively. Beyond the maximum drying stress there was a subsequent region of stress decay, which coincided with the draining of liquid-filled pores. No residual drying stress was detected for films prepared from salt-free SiO₂ suspensions, whereas salt-containing films exhibited residual drying stresses likely due to salt-bridging effects. Microstructural characterization of dried films prepared from aqueous SiO₂ suspensions revealed nonuniformities in the spatial distribution of colloidal particles and precipitated salt, with the highest concentrations located at the outer edges of the films. Such features result from capillary-induced transport of these species during drying, and they have important implications on colloidal processing of ceramic thick films and bulk forms.     

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.     

Homogeneous Distribution of Sintering Additives in Liquid-Phase Sintered Silicon Carbide

The addition of sintering additives to silicon carbide particles by electrostatic adsorption of colloidal A1₂O₃ and Y₂O₃ sols has been studied as a way to achieve an optimum homogeneity in the microstructure. The adsorption behavior of the sol particles was examined by electrophoretic measurements and X-ray fluorescence analysis. Both A1₂O₃ and Y₂O₃ sols could simultaneously be adsorbed on the SiC particle surfaces. Viscosity measurements showed that the colloidal sol particles had a stabilizing effect on the slip, and hence slips with relatively high solid loadings could be prepared without adding extra dispersing agent. Liquid-phase-sintered silicon carbide materials (LPS-SiC) with 2 wt% A1₂O₃ and 1 wt% Y₂O₃ were prepared by freeze granulation/ pressing and sintering at 1880^deg;C for 4 h. The homogeneity of the green compacts was quantified using a spot analysis technique in an electron probe microanalyzer. It was clearly shown that the addition of sols gave a more homogeneous microstructure than the reference sample with Y₂O₃ and A1₂O₃ added as powders. The addition of sintering additives as sols also enhanced the sintering behavior.     

Synthesis of LiAlO2 Powder by Hydrolysis of Metal Alkoxides

Crystalline fine powders of LiAlO₂ can be prepared by hydrolysis of metal alkoxides for application to the breeder blanket in nuclear fusion. The precipitates were fine crystalline particles of β-LiAlO₂of ∼0.1-μm size under controlled conditions. The transformation from β- to γ-phase was confirmed at temperatures between 700° and 750°C. The microstructure of a sintered body at 1000°C consisted of homogeneous fine particles and pores suitable for the blanket material. A fully dense sintered body could be prepared at 1400°C.     

Effect of Aqueous Processing Conditions on the Microstructure and Transformation Behavior in Al2O3─ZrO2(CeO2) Composites

Aqueous processing of Al₂O₃─ZrO₂ (123 mol% CeO₂) composites, combined with sintering conditions, was used to control the microstructure and its influence on the martensitic transformation temperature of t -ZrO₂ and the transformation-toughening contribution at room temperature. The resultant ZrO₂ grain sizes in the dense composites were related to the transformation-toughening behavior of t -ZrO₂. The data show that (1) the best processing conditions exist when the electrophoretic mobilities of the two solids are positive, adequately high to ensure colloidal stability, efficient packing,and uniform ZrO₂ distribution but differ greatly in magnitude, (2) the colloidal stability of ZrO₂ controls the overall stability and the rheological and processing behavior of this mixture, (3) the grain size distribution in dense pieces sintered for 1 h at 1500°C is comparable to the particle size distribution of the powders, (4) the martensite start temperature for the tetragonal to-monoclinic transformation in Al₂O₃ containing 20 and 40 vol% ZrO₂ increases and can approach 0°C with increasing average ZrO₂ grain size, and as a result, (5) the fracture toughness values at room temperature are raised from 4–5 MPa.m^1/2 to 9–12 MPa.m^1/2 for these two compositions.     

Processing of Silicon Carbide-Mullite-Alumina Nanocomposites

Nanocomposite materials in the form of nanometer-sized second-phase particles dispersed in a ceramic matrix have been shown to display enhanced mechanical properties. In spite of this potential, processing methodologies to produce these nanocomposites are not well established. In this paper, we describe a new method for processing SiC-mullite-Al₂O₃ nanocomposites by the reaction sintering of green compacts prepared by colloidal consolidation of a mixture of SiC and Al₂O₃ powders. In this method, the surface of the SiC particles was first oxidized to produce silicon oxide and to reduce the core of the SiC particles to nanometer size. Next, the surface silicon oxide was reacted with alumina to produce mullite. This process results in particles with two kinds of morphologies: nanometer-sized SiC particles that are distributed in the mullite phase and mullite whiskers in the SiC phase. Both particle types are immersed in an Al₂O₃ matrix.     

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.     

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.     

Low-Temperature Sintering of Seeded Sol—Gel-Derived, ZrO2-Toughened Al2O3 Composites

Seeding a mixture of boehmite (AIOOH) and colloidal ZrO₂ with α-alumina particles and sintering at 1400°C for 100 min results in 98% density. The low sintering temperature, relative to conventional powder processing, is a result of the small alumina particle size (∼0.3 μm) obtained during the θ-to α-alumina transformation, homogeneous mixing, and the uniform structure of the sol-gel system. Complete retention of pure ZrO₂ in the tetragonal phase was obtained to 14 vol% ZTA because of the low-temperature sintering. The critical grain size for tetragonal ZrO₂ was determined to be ∼0.4 μm for the 14 vol% ZrO₂—Al₂O₃ composite. From these results it is proposed that seeded boehmite gels offer significant advantages for process control and alumina matrix composite fabrication.     

Hot Isostatic Pressing of YBa2Cu3O7-δ and YBa2Cu3O7–δ/Ag Composites

Hot isostatic pressing (HIP) can be used to produce fully dense shapes of high-temperature ceramic superconductors. Densification modeling of monolithic YBa₂Cu₃O_7-δ and the composite YBa₂Cu₃O_7-δ/Ag systems allows an understanding of the HIP process and has led to the development of successful protocols for HIP of these materials. Ag metal is the best encapsulation material found for both systems. HIP of monolithic YBa₂Cu₃O_7-δ requires a slow ramp of pressure in order to prevent decomposition into more basic oxides such as Y₂BaCuO₅ and CuO. HIP of composite YBa₂Cu₃O_7-δ/Ag requires careful powder processing to obtain dense material with a fine dispersion of Ag.     

Effect of Ionic Strength on the Stability of Binary Ceramic Suspensions

The colloidal stability of aqueous Al₂O₃/ZrO₂ and Al₂O₃/SiC suspensions in the presence of electrolyte was investigated by rheological measurement. It was observed that the stability of binary systems improved with increasing ionic strength, where the solution pH value was maintained in the middle range of the two isoelectric points of the constituent powders. In this case, the electrostatic attractive interaction between dissimilar particles became more prominent when their number ratio approached unity. In addition, the salt-dependent stability was studied over the entire range of pH and relative component fraction. The results showed that the role of ionic strength in promoting flocculation or stabilization of colloidal dispersions was mainly determined by the predominant electrostatic interaction between particles, and the experimental stability maps were used to distinguish the stable and unstable suspensions with the addition of an electrolyte.