Sharon A. Nightingale

Sintering and grain growth of 3 mol % yttria zirconia in a microwave field

A comparative study of the sintering and grain growth of 3 mol% yttria zirconia using conventional and microwave heating was performed. Extensive measurements of grain size were performed at various stages of densification, and following isothermal ageing at 1500 °C for 1, 5, 10 and 15 h. Microwave heating was found to enhance densification processes during constant rate heating. The grain size/density relationship for the microwave-sintered samples was shifted in the direction of increased density for density values less than 96% of the theoretical value when compared to conventionally heated samples. This suggests that there may be a difference in the predominant diffusion mechanisms operating during the initial and intermediate stages of sintering. Results of the ageing experiments showed that once densification was near completion, grain growth was accelerated in the microwave field, and exaggerated grain growth occurred.

Strength and bonding in reduced ironsand–coal compacts

Abstract In this investigation, the strength and bonding within reduced ironsand–coal compacts were studied, with the aim of better understanding the binding mechanisms in the reduced compacts and, based on this understanding, to improve their strength. Ironsand ore and sub-bituminous coal were mixed and pressed into compacts, which were reduced by heating in a thermogravimetric furnace to temperatures between 1273 and 1573 K under argon. The progress of the reaction was monitored by measuring the weight loss with time. The reduced compacts were found to have low strength in compression testing. The main form of bonding between the reduced ironsand particles in the compact was by the formation of a slag-like material. Increasing the final reduction temperature was found to have a profound effect on the strength of the compacts by promoting the formation of this slag-like material.

Treatment of Transient Phenomena in Analysis of Slag-Metal-Gas Reaction Kinetics

Equations commonly used in describing reaction kinetics are examined and the problem of applying such equations to transient processes is discussed. Three examples of transient phenomena are examined in detail. It is shown that for carbon injection into slag, the reaction can be described by employing data for carbon oxidation in CO/CO2 by assuming reaction conditions approximately halfway between those in equilibrium with the slag and those in equilibrium with carbon. It is demonstrated that, when the time averaged interfacial area is employed, the rate of reaction between slag and iron-aluminum alloys can be described by a single first order rate equation, accommodating a 300% change in interfacial area. Creation of surface area in oxygen steelmaking is discussed and a method to determine the size distribution of droplets that are generated is proposed. It is concluded that changes in conditions during reaction complicate the analysis of kinetics. However, it should be possible to develop quantitative kinetic models to describe real processes.

Thermodynamic considerations of the corrosion of nickel ferrite refractory by Na3AlF6–AlF3–CaF2–Al2O3 bath

Abstract Thermodynamic analysis was carried out to interpret the results of corrosion testing of nickel ferrite samples in cryolite-based baths. The equilibrium between cryolite-based baths and nickel ferrite was considered. Isopleths between cryolite-based baths and nickel ferrite confirmed that for the temperature range of interest (1223–1273 K) there was limited solubility of nickel ferrite in the bath. To better understand the formation of the metal from nickel ferrite, the effect of reducing potentials on nickel ferrite and nickel ferrite–cryolite-based bath systems were considered. The formation of a metal phase was predicted at relatively high pO2. The metal phase was nickel-rich at higher pO2, becoming enriched in iron as the pO2 decreased. The oxide phases seen in corroded nickel ferrite samples corresponded to the spinel phase in the thermodynamic calculations. Penetration of aluminium oxides into the spinel phase seen in the experimental samples occurred only under a reducing potential.