Effect of Heating Rate on Sintering and Coarsening

The sintering of zinc oxide powder compacts has been investigated at constant rates of heating of 0.5° to 15°C/min. For samples with the same initial relative density (0.50), the temperature derivative of the densification strain versus density fits within a single, relatively narrow band. At low temperatures the densification rate as a function of temperature increases almost linearly with the heating rate. The data, covering a wide density range of 0.5 to 0.98, are consistent with an analysis that accounts for the coarsening (defined as an increase in the mean pore separation) in terms of two classes of microstructural coarsening processes: those associated with densifying and with nondensifying mechanisms.     

Reduction kinetics and microstructures of Al3+-containing cobalt ferrites

Dense, polycrystalline CoFe₂O₄, CoAl_0.02Fe_1.98O₄, and CoAl_0.1Fe_1.9O₄ were reduced in flowing H₂/0.01% H₂O at temperatures between 500 and 800° C. The reactions proceeded in a topochemical fashion. The rate of advance of the reaction interface was determined by direct measurement and by thermogravimetric analysis. An anomalous decrease in the reduction kinetics was observed for CoFe₂O₄ around 650° C and for CoAl_0.02Fe_1.98O₄ at 750° C. This reaction rate anomaly could be attributed to the appearance of a wüstite-type subscale. The effect of the substitutional Al³⁺ ions was to decrease the interfacial reaction rates. In the lower temperature range, the reaction was dominated by the interface reaction. With increasing temperatures, the importance of the gas transport resistance through the porous metal product scale increased. The microstructure of the scales was examined extensively. Pronounced grain-boundary attack was observed at lower temperatures leading to the formation of a distributed reaction interface. At higher temperatures, the reaction interface was better defined. The pore structure of the scales was examined after polishing and sputter etching. While changes in the pore morphology were observed, they were not correlated with the anomalous reaction rate effects.     

Metal-coated colloidal particles

Procedures are described for coating of submicrometre ceramic powders with copper and nickel. The process required precoating of the core particles with a palladium catalyst. A precoating procedure was developed in which palladium chloride is reduced by sodium hypophosphite or stannous chloride on the powder surface in aqueous suspensions. Commercial electroless solutions were used to deposit copper and nickel on the catalysed powders. The effect of various experimental parameters on the effectiveness of the surface conditioning of the particles was also investigated.     

Pore size distribution during compaction and early stage sintering of silicon nitride

The effect of a number of compaction parameters on the pore size distribution in silicon nitride powder compacts has been investigated by mercury porosimetry. The powder was compacted in a uniaxial die and the parameters of interest included uniaxial load, loading rate, polymer additives and stress relaxation. The use of a low-viscosity polymer solution as a lubricant, the allowance of stress relaxation following loading and heat treatment above the glass transition temperature of the polymer lead to a more uniform pore size distribution in the green samples. The more uniform green samples, in turn, showed a more uniform pore size distribution after initial stage sintering. The potential of the present techniques for improving the microstructural uniformity in ceramic bodies is discussed.     

Local electrode current density and flaw decoration of sodium-beta″ alumina

Simple decoration methods are described that can qualitatively reveal local electrode current densities and permit decoration of fine, sodium filled cracks in sodium beta″ alumina solid electrolytes.     

Atomic-resolution imaging of the nanoscale origin of toughness in rare-earth doped SiC

Ultrahigh-resolution transmission electron microscopy and atomic-scale spectroscopy are used to investigate the origin of the toughness in rare-earth doped silicon carbide (RE-SiC) by examining the mechanistic nature of the intergranular cracking events which we find to occur precisely along the RE-decorated interface between the SiC grains and the nanoscale grain-boundary phase. We conclude that, for optimal toughness, the relative elastic modulus across the grain-boundary phase and the interfacial fracture toughness are the most critical material parameters; both can be altered with judicious choice of rare-earth elements.     

Particulate composites from coated powders

A method is described for coating SiC whiskers with a thick layer of alumina to form homogeneous alumina matrix/silicon carbide whisker particulate composites. The method uses controlled heterogeneous precipitation in a relatively concentrated suspension of whiskers. The suspension-coated whiskers can be converted to a free-flowing powder after calcination. The processing parameters and the results of consolidation of the composite powder are reported. Nearly fully dense composites, containing over 40 vol.% whiskers, could be prepared.