Alan Bleier

Nucleation and Growth of Uniform m-ZRO 2

Hydrothermal treatment of zirconyl salt solutions produces uniform m-ZrO/sub 2/ powder on the order of 80 nm. This powder is porous, has a 3-nm crystallite size, and exhibits an unusually high degree of crystallographic alignment within particles. The generation of this powder occurs via a complex process involving nucleation, growth, and controlled agglomeration of primary particles. Particle formation, crystallographic alignment and particulate uniformity are explained in terms of solution reactions and colloidal behavior.

Secondary minimum interactions and heterocoagulation encountered in the aqueous processing of alumina—zirconia ceramic composites

Abstract Heterocoagulatien calculations are used to explain the various colloidal stability phenomena encountered aqueous binary suspensions that contain alumina and zirconia. Differential sedimentation of the particulates leads to a non-uniform oxide distribution in the pellets prepared by the pressure filtration of stable systems, yet consolidation under the conditions that promote a weak secondary minimum association of the dissimilar oxides makes the microstructures in the pellets uniform. This effect depends on the filtration pH. The mutual association is favored in stable systems if the zeta potentials of the unlike particles arc of the same sign but differ significantly in magnitude. The best consolidation conditions for this binary system occur in the pH range 5–6, where each oxide is positively charged. The critical depths of the secondary minimum depend on the particle sizes and may exceed 1O kT . Pre-exposure of zirconia to aluminum chloride and pre-exposure of each oxide to polyacrylic acid can alter the material distribution in pellets made from colloidally stable systems. Either additives effect depends on its concentration and the filtration pH. Adsorption of either aluminum-containing species or polymer makes the heteroassociation of alumina and zirconia particles less favorable.

Rheological and Related Colloidal Aspects of Aqueous Processing that affect the Development of Microstructure

Shear flow in {alpha}-Al{sub 2}O{sub 3} suspensions having a volume fraction of solids ({phi}) in the range between 0.17 and 0.50 was investigated between pH 4 and 12. It is Newtonian if the magnitude of the zeta potential exceeds a critical value which depends on {phi}; its value is 39 mV if {phi} = 0.40 and 74 mV if {phi} = 0.50. If this potential is less than the critical value, shear flow is pseudoplastic; its yield value markedly changes (e.g., 0 to {gt}100 Pa) in a slightly {phi}-dependent, narrow pH range ({lt}0.5 units). If a second oxide, t-ZrO{sub 2}, is present, its pH-dependent colloidal behavior governs the overall rheology, though its concentration may be only 11 % that of {alpha}-Al{sub 2}O{sub 3}. Scanning electron microscopy of composite pieces indicates that a detrimental, rheologically detectable interaction between {alpha}-Al{sub 2}O{sub 3} and t-ZrO{sub 2} can be avoided and the distribution of t-ZrO{sub 2} can be optimized during pressure casting by control of pH to ensure Newtonian flow or a very low yield value when the flow is pseudoplastic.

Chemical and Physical Principles of Processing that affect Microstructure of Al2O3-ZrO2 Composites

Aqueous processing of 1- to 20-% v/v binary suspensions containing 0.50-μm α-Al 2 O 3 and 0.71-μm m-ZrO 2 was investigated in the presence and absence of AlCl 3 from pH 2 to 12 using sedimentation, rheological, electrokinetic, and potentiometric techniques. Differential sedimentation of the oxide particulates leads to a nonuniform distribution of ZrO 2 when colloidally stable slurries are used. However, processing under conditions that promote weak association of the oxides improves uniformity, with optimal conditions in the absence of AlCl 3 being in the pH range 5 to 6. Thus, prefired microstructure depends on the pH at which processing occurs, behavior derived from the different, pH-dependent surface properties of the oxides. Pre-exposure of m-ZrO 2 to AlCl 3 in the pH range of 4 to 8 alters the final composite microstructure via adsorption of Al-species and renders the two solids more similar electrostatically, thereby reducing the forces responsible for the weak association and uniform distribution.

Particle size and solvent effects in the processing of silicon

Abstract A dispersibility model previously developed for silicon powder that does not possess surface charge in non-aqueous media is extended to predict the behavior when surface charge exists, for instance as a result of surface oxidation. The role of particle size is also examined. In the absence of surface charge, a critical particle size exists below which silicon suspensions are colloidally stable and above which they are not. This value depends on the dielectric permittivity of the liquid medium and becomes infinite if it matches that of silicon. In the presence of surface charge, the critical size also depends on the electrostatic potential at the particle—liquid interface. Steric dispersants that stabilize suspensions may affect the critical size also. Diagrams which show the transitions between conditions that lead to useful, stable suspensions and those that promote extensive agglomeration of silicon are developed.

Effects of Surface Charge on the Processing of Nonaqueous Silicon Slurries

The influence of surface charge on the dispersion aspects of the nonaqueous processing of silicon powder is explained by the relationship between the liquid mediums dielectric constant and the maximum particle size that is stabilized by colloidal forces. The effects of surface charge, derived from the chemistry of the solid-liquid pair or the adsorption of ionic processing aids, are either great, significantly increasing the maximum particle size that is stable against agglomeration from the nanometer range to the micrometer range, or virtually undetectable, contributing nothing to enhanced stability. Guidelines for the processing of silicon powder when surface charge exists are discussed.