Hydrogen reduction of cobalt ferrite
- PDF / 2,211,312 Bytes
- 11 Pages / 603.28 x 783.28 pts Page_size
- 92 Downloads / 236 Views
I. I N T R O D U C T I O N
IN a recent study of the effect of aluminum doping on the gaseous reduciton kinetics of cobalt ferrite, Rey and De Jonghe found that, for both doped and undoped material, the reaction rate decreased with increasing reduction temperature over a certain temperature range? They reported that the temperature corresponding to the minimum rate was dependent on the level of aluminum doping, and correlated the onset of the rate with the appearance of a wtistite-type subscale. Similar effects have been noticed for the reduction of iron oxides. For example, Turkdogan and Vinters 3 reported that the reduction rate of hematite went through a minimum at around 843 K, which is the minimum temperature at which wiistite is thermodynamically stable. Quets, Wadsworth, and Lewis 4 found the same effect for the reduction of magnetite. A reaction rate minimum has also been reported for the reduction of cobalt oxide by Lilius? In this case, the reaction rate decrease could be attributed to the changes in the scale morphology at different reduction temperatures. Mathematical models describing the kinetics of reduction reactions have been widely used to differentiate between the possible rate controlling steps that affect reduction rates. The model developed by Spitzer, Manning, and Philbrook ~allowed for the possibility of three different phenomena to contribute to the resistance to J. R. PORTER is Postdoctoral Research Associate and L. C. DE JONGHE is Senior Staff Scientist at the Materials and Molecular Research Division, LawrenceBerkeleyLaboratory. Dr. De Jonghe is also Associate Professor-in-Residence,Department of Materials Scienceand Mineral Engineering,Universityof California, Berkeley,CA 94720. Manuscript submitted December27, 1979. METALLURGICALTRANSACTIONSB
reaction; namely, the mass transfer resistance of the reacting gases between the bulk gas stream and the specimen surface, the diffusion through the porous reduced metal scale, and the chemical reaction itself at the scale/oxide interfaces. Although their model was developed to describe the reduction of spheres of oxide, a similar approach can be used to develop equations applicable to more than one specimen geometry. More sophisticated models have been worked out by Szekeley, Evans, and Sohn, 6 applicable to different specimen geometries. These authors developed a model, using reduction parameters in dimensionless form, which can be applied to the reduction of spheres, cylinders, and slabs of both porous and nonporous solids. However, as pointed out by Turkdogan and Vinters, 3 no single rate equation can be expected to accurately describe reduction behavior over a very wide range of reaction conditions and, for a full understanding of the reduction of any oxide, further characterization of the individual steps of the reaction sequence is essential. For our example of cobalt ferrite reduction, further examination of the reaction interface is most likely to lead to an improved understanding of the reaction rate anomaly, since it appears to be asso
Data Loading...
