Reformulation of an Aqueous Alumina Slip Based on Modification of Particle-Size Distribution and Particle Packing

Six alumina casting slips with particle-size distributions varying from 44 to 0.1 μm were examined. Particle packing was calculated using the approach of Andreasen. Viscosity, green density, and pore-size distribution were measured. It was found that contouring the intermediate size distribution for particles finer than 15 μm provided the most desirable viscosity for slips composed of wide size distributions. For slips containing 50 vol% solids, the lowest viscosity obtained was 196 × 10⁻³ N · s/m² (with a two-component size distribution), and a green density of 2.52 g/cm³ (65% of theoretical) was achieved with a ternary system. These casts had bimodal pore-size distributions centered around approximately 1 and 0.1μm.     

Dispersion of Alumina and Silicon Carbide Powders in Alumina Sol

Dispersion of Al₂O₃ and SiC particles in an alumina sol has been investigated through determination of particle-size distribution, zeta potential, and agglomerate morphology. The particle size of Al₂O₃ and SiC (as determined by the particle-size analyzer) is strongly affected by the presence of alumina sol in the solution. The average agglomerate size is decreased by at least 50%. The zeta potential of Al₂O₃ in 1 M alumina sol increases slightly, whereas that of SiC reverses its sign over a wide range of pH values. It is proposed that these effects are caused by AlO₄Al₁₂(OH)₂₄(H₂O)⁷⁺₁₂ sol clusters (1-2 nm in size) that are absorbed on the surface of the large (1-5 µm) ceramic particles. The electrostatic and steric effects of the cluster absorption help to control the dispersion and stabilize the suspension of ceramic particles in the alumina sol during wet processing. It is expected that the alumina-sol clusters can be used as an efficient, clean dispersant for single-phase and composite ceramic powders.     

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.     

Preparation of Magnesium-Calcium Titanate Powders by Alkoxide Precursor Method

Magnesium-calcium titanate (Mg_0.95Ca_0.05)TiO₃ powders have been prepared via two semialkoxide methods that use nitrate and carbonate precursors instead of magnesium and calcium alkoxides. The thermal decomposition and crystallization behavior of the prepared gels and the morphology of the heat-treated powders have been investigated using thermal analysis, X-ray diffractometry, Fourier transform infrared spectroscopy, and scanning electron microscopy. The ilmenite structure starts to crystallize at a temperature of ∼500°C without forming an intermediate, spinel-structured phase. Powders obtained from carbonate precursors that have been dissolved in acetic acid reveal a more-spherical morphology and a narrower particle-size distribution than those from nitrate precursors.     

Rapid Preparation of Lead Titanate Sputtering Target Using Spark-Plasma Sintering

PbTiO₃ sputtering targets 8 cm in diameter were prepared using spark-plasma sintering (SPS) for relatively short periods, ∼2 min. Submicrometer-sized PbTiO₃ powders with a relatively large size distribution were densified to ∼86% of the theoretical X-ray density using the SPS process. In contrast, large-sized (8 cm in diameter) ceramics could not be prepared from starting powders with a relatively narrow particle-size distribution. Formation of cracks in the large PbTiO₃ targets was observed when samples were prepared under higher pressures (>50 MPa) or at higher temperatures (>900°C). Crack formation was attributed to unrelaxed internal stress originating from lower pore contents and from an inhomogeneous distribution of cations in the ceramics prepared under these conditions.     

Density and Permeability of Sintered Slip-Cast Magnesia

Fused magnesia was ball-milled in ethanol for 14, 30, 48, and 100 hours, and particle-size distributions were determined. Specimens were slip-cast from the suspensions and were fired in air at 1300°, 1400°, and 1470°C, for times up to 8 hours, and gas-permeability and bulk density determinations were performed on the fired pieces. For all specimens the gas flow was essentially of the Knudsen type. The maximum bulk density, 96.7% of theoretical, was obtained by sintering a specimen, cast from the magnesia ball-milled for 100 hours, at 1470° for 3 hours. At this density the permeability was zero. The relation between permeability and porosity could be represented by the equation K _M = 6.9 ° 10⁻¹⁰( P_F )^2.54, where P_F is 100( d ₀ - d/do ). Here d is the bulk density and d ₀ is a constant representing the bulk density at which the permeability becomes zero. The foregoing equation fits the data for all specimens if it is assumed that do is a function of the particle-size distribution in the slip, varying from 3.36 g per cm³ for the magnesia ball-milled for 100 hours to 3.58 g per cm ³ for the magnesia ball-milled for 14 hours.     

Development of Short-Range Repulsive Potentials by Short-Chain Surfactants in Aqueous Slurries

The effect of both cationic ( n -trimethylammonium bromide, C _n TAB) and zwitterionic (phosphocholine, C _n C _n PC) surfactants on the properties of aqueous slurries of Si₃N₄ was studied. These surfactants were expected to adsorb to the surface of Si₃N₄ particles to produce short-range repulsive interparticle potentials that might be useful for the colloidal processing of advanced ceramic powders. Electrophoretic, adsorption, and viscosity measurements showed that longer-chain-length surfactants (C _n TAB, n ≥ 12, and C _n C _n PC, n ≥ 9) strongly adsorb. Surfactants with shorter chain lengths were highly soluble and did not adsorb. Although the C _n TAB, n ≥ 12, surfactants produced very weak particle networks with a low viscosity, the packing density during consolidation was very low, and the bodies were brittle (cracked before plastic deformation). The less-soluble, longer-chained C _n C _n PC, n ≥ 9, surfactants did produce high particle-packing densities but also produced brittle bodies. In all cases, it appeared that the surfactants could be pushed away from the surface during particle packing.     

Effect of the Presence of Ammonium Peroxodisulfate on the Direct Precipitation of Ceria and Ceria–Zirconia Solid Solutions from Acidic Aqueous Solutions

Direct precipitation of nanometer-sized particles of ceria–zirconia (CeO₂–ZrO₂) solid solutions with cubic and tetragonal structures was successfully attained from acidic aqueous solutions of cerium(III) nitrate (Ce(NO₃)₃) and zirconium oxychloride (ZrOCl₂) through the addition of ammonium peroxodisulfate ((NH₄)₂S₂O₈), because of promotion of the hydrolysis via the oxidation of Ce³⁺ ions, together with the simultaneous hydrolysis of ZrOCl₂ under hydrothermal conditions. Ultrafine CeO₂ particles also could be formed from relatively concentrated aqueous solutions of the same trivalent cerium salt in the presence of (NH₄)₂S₂O₈ via hydrolysis. The crystallite size and lattice strain of as-precipitated solid solutions varied, depending on the composition within the CeO₂–ZrO₂ system. Creation of a solid solution of ZrO₂ into a fluorite-type CeO₂ lattice clearly introduced lattice strain, as a consequence of the decreasing crystallite size. Both the direct precipitation process and the effectiveness of the presence of (NH₄)₂S₂O₈ for the synthesis of CeO₂–ZrO₂ solid solutions were discussed.     

Synthesis of Nanophased Metal Oxides in Supercritical Water: Catalysts for Biomass Conversion

Nanoparticles of zinc oxide-based materials (ZnO, ZnAl₂O₄) with various morphologies were synthesized in supercritical water (SCW) with a flow-type apparatus and in sub- and supercritical water with a batch reactor. In the flow-type apparatus, smaller particles were obtained. Depending on the precursors, the morphology of crystallites is rod, hexagonal, or rectangular shaped. ZnAl₂O₄ was synthesized with a high specific surface area ( S _BET) reaching 210 m²/g and nanocrystallite sizes ≤10 nm. The KOH concentration played a major role in the formation of ZnO and ZnAl₂O₄ phases. Then, the synthesized materials were used as catalysts for the biomass conversion by the oxidation process to produce hydrogen.     

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.     

Anatase-Type TiO2 and ZrO2-Doped TiO2 Directly Formed from Titanium(III) Sulfate Solution by Thermal Hydrolysis: Effect of the Presence of Ammonium Peroxodisulfate on their Formation and Properties

Anatase-type TiO₂ powder containing sulfur with absorption in the visible region was directly formed as particles with crystallite in the range 15–88 nm by thermal hydrolysis of titanium(III) sulfate (Ti₂(SO₄)₃) solution at 100°–240°C. Because of the presence of ammonium peroxodisulfate ((NH₄)₂S₂O₈), the yield of anatase-type TiO₂ from Ti₂(SO₄)₃ solution was accelerated, and anatase with fine crystallite was formed. Anatase-type TiO₂ doped with ZrO₂ up to 9.8 mol% was directly precipitated as nanometer-sized particles from the acidic precursor solutions of Ti₂(SO₄)₃ and zirconium sulfate in the presence and the absence of (NH₄)₂S₂O₈ by simultaneous hydrolysis under hydrothermal conditions at 200°C. By doping ZrO₂ into TiO₂ and with increasing ZrO₂ content, the crystallite size of anatase was decreased, and the anatase-to-rutile phase transformation was retarded as much as 200°C. The anatase-type structure of ZrO₂-doped TiO₂ was maintained after heating at 1000°C for 1 h. The favorable effect of doping ZrO₂ to anatase-type TiO₂ on the photocatalytic activity was observed.     

Preparation of High-Purity ZrSiO4 Powder Using Sol–Gel Processing and Mechanical Properties of the Sintered Body

Effects of the concentration of ZrOCl₂, calcination temperature, heating rate, and the size of secondary particles after hydrolysis on the preparation of high-purity ZrSiO₄ fine powders from ZrOCl₂.8H ₂ O (0.2 M to 1.7 M ) and equimolar colloidal SiO₂ using sol–gel processing have been studied. Mechanical properties of the sintered ZrSiO₄ from the high-purity ZrSiO₄ powders have been also investigated. Single-phase ZrSiO₄ fine powders were synthesized at 1300°C by forming ZrSiO₄ precursors having a Zr–O–Si bond, which was found in all the hydrolysis solutions, and by controlling a secondary particle size after hydrolysis. The conversion rate of ZrSiO₄ precursor gels to ZrSiO₄ powders from concentrations other than 0.4 M ZrOCl₂.8H₂O increased when the heating rate was high, whereupon the crystallization of unreacted ZrO₂ and SiO₂ was depressed and the propagation and increase of ZrSiO₄ nuclei in the gels were accelerated. The density of the ZrSiO₄ sintered bodies, manufactured by firing the ZrSiO₄ compacts at 1600° to 1700°C, was more than 95% of the theoretical density, and the grain size ranged around 2 to 4 μm. The mechanical strength was 320 MPa (room temperature to 1400°C), and the thermal shock resistance was superior to that of mullite and alumina, with fairly high stability at higher temperatures.     

Plastic Flow During Hot-Pressing

The contribution of plastic flow to overall densification of a powder compact during hot-pressing was analyzed by incorporating the creep characteristics of materials at high temperatures into an equation applicable to hot-pressing conditions. When the particles are assumed to be spherical and when the effective stress acting at the points of contact under axial loading conditions is taken into consideration, the final form of the equation is: where α₁ and β are geometric constants which can be calculated from the packing geometry, A and n are material constants, D is the relative density of the compact, and R is the radius of the spheres at any stage of deformation in arbitrary units. Computerized plots of D vs t were obtained for several materials. Experimental verification of these plots using hot-pressing data for Pb-2% Sb, Ni, and Al₂O₃ spheres and coarse irregular Al₂O₃ particles showed that the experimental data fitted the theoretical prediction for orthorhombic packing well. A large deviation with respect to the initial packing density was encountered with very fine irregular Al₂O₃ powder, although its densification behavior with time was similar to that of the coarse particles.     

Fabrication of TiN/Si3N4 Ceramics by Spark Plasma Sintering of Si3N4 Particles Coated with Nanosized TiN Prepared by Controlled Hydrolysis of Ti(O-i-C3H7)4

TiN-coated Si₃N₄ particles were prepared by depositing TiO₂ on the Si₃N₄ surfaces from Ti(O- i -C₃H₇)₄ solution, the TiO₂ being formed by controlled hydrolysis, then subsequently nitrided with NH₃ gas. A homogeneous TiO₂ coating was achieved by heating a Si₃N₄ suspension containing 1.0 vol% H₂O with the precursor at 40°C. Nitridation successfully produced Si₃N₄ particles coated with 10–20 nm TiN particles. Spark plasma sintering of these TiN/Si₃N₄ particles at 1600°C yielded composite ceramics with a relative density of 96% at 25 vol% TiN and an electrical resistivity of 10⁻³Ω·cm in compositions of 17.5 and 25 vol% TiN/Si₃N₄, making these ceramics suitable for electric discharge machining.     

Preparation of TiO2-Coated Al2O3 Particles by Chemical Vapor Deposition in a Rotary Reactor

A process of coating Al₂O₃ particles with TiO₂ by hydrolysis of Ti(OC₄H₉)₄ using chemical vapor deposition in a rotary reactor has been developed. The process resulted in (1) a coating film of TiO₂ which was compact and uniform with the fraction of TiO₂ being 0.1%–10.0% and (2) an amorphous TiO₂ coating at a low reaction temperature converted to anatase at a reaction temperature higher than 673 K.     

Modeling of the Hydrolysis of α-Tricalcium Phosphate

Some of the formulations of apatitic calcium phosphate bone cements are based on the hydrolysis of α-tricalcium phosphate (α-Ca₃(PO₄)₂, α-TCP). In this work the hydrolysis kinetics of α-TCP are studied, taking into account the particle-size distribution of the initial powder, to identify the mechanisms that control the reaction in its successive stages. The temporal evolution of the depth of reaction is calculated from the degree of reaction data, measured by X-ray diffractometry. A kinetic model is proposed, which suggests the existence of two rate-limiting mechanisms: initially, the surface area of the reactants and, subsequently, the diffusion through the hydrated layer formed around the reactants. For the specific particle size and preparation used, the controlling mechanism changeover takes place after 16 h of reaction.     

Synthesis of Solid, Spherical CeO2 Particles Prepared by the Spray Hydrolysis Reaction Method

To avoid the formation of hollow particles during spray pyrolysis, a spray hydrolysis reaction method (SHRM) was studied. Unlike the conventional spray pyrolysis that uses metal salt as a precursor and dry air as a carrier gas, the SHRM introduces a mixture of metal salt and dimethyl oxalate (DMO) as precursors and a gas mixture of water vapor and air as the carrier gas. Spherical, solid CeO₂ particles characterized by SEM, BET, and density analysis were produced by the SHRM using Ce(NO₃)₃ and DMO as the precursors. DMO, as an internal precipitant, hydrolyzed and produced oxalic acid, which precipitated with cerium ions to form volume precipitation in the whole droplet at enough temperature and relative humidity. The volume precipitation induced by the in situ formation of oxalic acid in the whole droplet prevented Ce(NO₃)₃ nucleation at the droplet surfaces, thus avoiding the formation of hollow particles which usually occur in the conventional spray pyrolysis process. XRD and IR analysis showed that cerium oxalate was an intermediate product in the SHRM process.     

Effects of Amorphous and Crystalline SiO2 Additives on γ-Al2O3-to-alpha-Al2O3 Phase Transitions

This paper focused on the effects of various phases of SiO₂ additives on the γ-Al₂O₃-to-α-Al₂O₃ phase transition. In the differential thermal analysis, the exothermic peak temperature that corresponded to the theta-to-α phase transition was elevated by adding amorphous SiO₂, such as fumed silica and silica gel obtained from the hydrolysis of tetraethyl orthosilicate. In contrast, the peak temperature was reduced by adding crystalline SiO₂, such as quartz and cristobalite. Amorphous SiO₂ was considered to retard the γ-to-α phase transition by preventing γ-Al₂O₃ particles from coming into contact and suppressing heterogeneous nucleation on the γ-Al₂O₃ surface. On the other hand, crystalline SiO₂ accelerated the α-Al₂O₃ transition; thus, this SiO₂ may be considered to act as heterogeneous nucleation sites. The structural difference among the various SiO₂ additives, especially amorphous and crystalline phases, largely influenced the temperature of γ-Al₂O₃-to-α-Al₂O₃ phase transition.     

Experimental Analysis and Modeling of Slip Casting

The filtration mechanics of slip casting is extended to account for the filtrate transporting the finer particles to the bottom of the cake. Scanning electron micrographs of alumina (Al₂O₃) green microstructures illustrate that a higher concentration of fine particles can accumulate at the bottom section of a cake. The rheological behavior of alumina suspensions with different solids loadings, particle-size distributions, and amounts of deflocculant is discussed. Slip-casting experiments demonstrate that the rheology of a suspension greatly affects the green density and growth rate of the cake.     

Synthesis of Pure and Manganese-, Nickel-, and Zinc-Doped Ferrite Particles in Water-in-Oil Microemulsions

In recent years, the materials research focuses toward synthesis of finer and finer microstructural features. The unique properties of nanosized particles outweigh their higher production costs. Precipitation in microemulsion is one technique, which promises to produce small particles of controlled size and morphology at reasonable cost. The present study demonstrates the synthesis of nanocrystalline α-Fe₂O₃(hematite), Mn_0.5Zn_0.5Fe₂O_4, and Ni_0.5Zn_0.5Fe₂O₄ particles in a reverse micellar microemulsion system [water–iso-octane–AOT (sodium di-2ethylhexylsulfosuccinate)]. The synthesis of α-Fe₂O₃ is performed to obtain baseline data for the synthesis of Mn_0.5Zn_0.5Fe₂O₄ and Ni_0.5Zn_0.5Fe₂O₄ in the microemulsion system. Nanosized, spherical α-Fe₂O₃, Ni-Zn ferrite, and Mn-Zn ferrite particles (20–80 nm) with very narrow particle size distribution are synthesized. Crystallization of the particles is obtained at temperatures as low as 300°C.