Fred F. Lange

Hydrothermal synthesis of KNbO 3 and NaNbO 3 powders

Orthorhombic KNbO 3 and NaNbO 3 powders were hydrothermally synthesized in KOH and NaOH solutions (6.7-15 M) at 150 and 200 °C. An intermediate hexaniobate species formed first before eventually converting to the perovskite phase. For synthesis in KOH solutions, the stability of the intermediate hexaniobate ion increased with decreasing KOH concentrations and temperatures. This led to significant variations in the induction periods and accounted for the large disparity in the mass of recovered powder for different processing parameters. It is also believed that protons were incorporated in the lattice of the as-synthesized KNbO 3 powders as water molecules and hydroxyl ions.

Relation between percolation and particle coordination in binary powder mixtures

Abstract Deformation processing of composite materials is often affected by the percolation of inclusions, which may form a connective network supporting a significant part of the applied pressure and to hinder the densification of the matrix powder. By numerical simulation it was found that the critical volume fraction of inclusions required to form the first percolative cluster strongly depends on the inclusion to matrix particle size ratio. Moreover the fraction of inclusions belonging to a connective cluster is a size-ratio-insensitive function of the inclusion-inclusion coordination number. Using this correlation and an analytical model for particle coordination, a simple scheme is proposed to predict the percolation within bimodal powder mixtures. Interest of these results for understanding pressure transmission during processing of particulate composites is also discussed.

Epitaxial growth of PbTiO 3 thin films on (001) SrTiO 3 from solution precursors

A mixed alkoxide liquid precursor was used to form epitaxial PbTiO 3 thin films by spin-coating on cubic (001) SrTiO 3 substrates. The films were heat-treated at temperatures between 380 °C/1 h and 800 °C/1 h. X-ray diffraction, atomic force microscopy, scanning and transmission electron microscopy were used to characterize the microstructure of the films and to evaluate the epitaxial phenomena. At ∼400 °C/1 h, a polycrystalline, metastable Pb-Ti fluorite crystallizes from the pyrolyzed amorphous precursor. At slightly higher temperatures (∼420 °C/1 h), the thermodynamically stable phase with the perovskite structure epitaxially nucleates at the film/substrate interface. A small number of epitaxial grains grow through the film toward the surface and consume the nanocrystalline fluorite grains. Coarsening of the perovskite grains leads to a reduction in mosaic spread during further heating. Pores, which concurrently coarsen with grain growth, produce a pitted surface as they disappear from within the film. At 800 °C/1 ha dense epitaxial PbTiO 3 film with a smooth surface is observed. Parameters governing the formation of a - and c -domains are discussed as well as the small tilts of the domain axes away from the substrate normal.

Processing of damage-tolerant, oxidation-resistant ceramic matrix composites by a precursor infiltration and pyrolysis method

Damage-tolerant, continuous fiber ceramic matrix composites have been produced by an inexpensive method. According to this method, the space between the fibers is filled with a powder. The powder particles are heat treated to form a porous framework without shrinkage, which is then strengthened with an inorganic synthesized from a precursor. High particle packing densities can be achieved within the fiber preform provided that the particle-to-fiber diameter ratio is small. Filling the interstices with a powder increased the composite density and also limits the size of the crack-like voids within the matrix. In this review we describe the mechanical properties of partially dense materials produced from powders to show that a porous matrix can be strong. We demonstrate that the packing density of particles around fibers is highest when the particle-to-fiber diameter ratio is small. The kinetics and mechanical behavior of composite systems is summarized to demonstrate the requirements of damage-tolerant properties. An all-oxide ceramic matrix composite produced by this method is discussed.

Deformation consolidation of metal powders containing steel inclusions

Abstract Uniaxial consolidation experiments have been conducted at room temperature for two deformable metal powders (1100 Al and Pb5%Sb) containing various amounts of spherical steel inclusions. The experiments illustrate that the inclusion phase offers little constraint to matrix deformation at volume fractions

Monodisperse micrometer-scale spherical assemblies of polymer particles

Uniform spherical colloidal assemblies of closely packed monodisperse colloidal particles have been prepared by injecting an aqueous suspension of polymer spheres into a surfactant-laden oil phase through a micropipette. The size of these assemblies can be controlled by varying the injection pressure or particle concentration. The Figure shows 6.3 μm diameter assemblies composed of 230 nm particles.

Hydrothermal synthesis of perovskite and pyrochlore powders of potassium tantalate

Potassium tantalate powders were hydrothermally synthesized at 100 to 200 °C in 4 to 15 M aqueous KOH solutions. A defect pyrochlore, Kta_(2)O_(5)(OH). nH2O (n ≈ 1.4), was obtained at 4 M KOH, but at 7–12 M KOH, this pyrochlore was gradually replaced by a defect perovskite as the stable phase. At 15 M KOH, there was no intermediate pyrochlore, only a defect perovskite, K_(0.85)Ta_(0.92)O_(2.43)(OH)_(0.57) 0.15H_(2)O. Synthesis at higher KOH concentrations led to greater incorporation of protons in the perovskite structures. The potassium vacancies required for charge compensation of incorporated protons could accommodate water molecules in the perovskite structure.

Bifurcation in alumina plates produced by a phase transformation in central, alumina/zirconia thin layers

Abstract Alumina plates with a center, thin layer composed of Al 2 O 3 and ZrO 2 (equal volume fractions) have been produced by sequential slip casting. Central layers as thin as 4 μm were fabricated using a low solid content slurry, but were observed to segregate the two oxides due to segregation of the consolidated layers during slip casting. Thicker layers were homogenous. The ZrO 2 tetragonal to monoclinic transformation generated residual, compressive stresses in the thin, center layer. Edge cracking and crack bifurcation was observed when the compressive layer thickness was ⩾ 25 μm.

Mechanical behavior of saturated, consolidated, alumina powder compacts: effect of particle size and morphology on the plastic-to-brittle transition

The effects of particle size and morphology on the mechanical behavior of pressure consolidated, saturated, alumina powder bodies, were determined with uniaxial compression experiments of cylindrical specimens at a fixed displacement rate. Five different α-Al2O3 powders, from the same manufacturer, were used. The slurries were dispersed at pH 4 and then either coagulated with additions of NH4Cl to produce weakly attractive particle networks with short-range repulsive potentials or flocculated at the isoelectric point (iep=pH 9). These slurries were consolidated by pressure filtration using pressures ranging from 2.5 to 150 MPa. Larger particles packed to higher relative densities when compared to smaller particles. Blocky particles packed at a lower relative density when compared to particles with roundish surfaces. Bodies were plastic when consolidated below a critical consolidation pressure; above this pressure, the body was brittle. Bodies formed with large particles were brittle at a lower consolidation pressure. The effect of particle size is discussed with respect to the number of particle–particle contacts per unit volume at a given relative density. Namely, for a given applied pressure, larger forces exist between larger particles because of the smaller number of contacts per unit volume relative to smaller particles.

COLLOIDAL PROCESSING OF POWDER FOR RELIABLE CERAMICS

Abstract Interparticle forces can be controlled by barriers formed around particles (clouds of ions and/or chem-adsorbed molecules) that produce repulsive forces to counter the persistent, attractive van der Waals forces. How these forces control the mechanical properties of powder compacts, saturated with a liquid, is new and critical to new forming technologies being developed for more reliable ceramics.