Abstract
Iron powders having a high specific surface area, compared to current industrial iron powders, have been prepared by the decomposition and Subsequent reduction of ferrous oxalate, and by the reduction of a high purity grade of oxide, at 300 C, under a hydrogen atmosphere. The iron powders obtained were pyrophoric, so that all handling was carried out in glove boxes, under an argon atmosphere. The morphology of the powders was studied using electron microscopic and krypton sorption (BET) techniques. Both preparation routes yielded powders of similar shape and size although considerable differences existed in the oxides from which they were prepared. The sintering behaviour of the powders was studied at temperatures up to 900°C and for times up to 16 1/2 hours. A typical industrial iron powder viz carbonyl iron, was sintered alongside the fine powders to provide a basis for comparison. Marked differences were observed in sintering behaviour, the iron powder ex oxalate showing a rapid initial densification followed by an apparent cessation of densification. The other two powders showed more conventional behaviour up to 900 C. However on sintering at 900°C rapid drops in density were observed in both these powders, accompanied by an abnormal rate of grain growth at this temperature. The other powder, ex ferrous oxalate, showed a smaller drop in density and a more gradual rate of grain growth. Activation energies for sintering fell below the established values for grain boundary diffusion in iron, and this is interpreted in terms of the operation of more than one sintering mechanism acting in parallel with each other. Diffusion distances obtained from bulk diffusion data, are compared with actual grain sizes present in the materials, in order to discuss the relative contributions of both boundary and volume diffusion in terms of the proximity of grain boundary sinks. It was concluded that finer grain sizes resulted in a larger contribution of volume diffusion to sintering owing to a longer contact between pores and the grain boundary sink. Differences between the iron powder ex oxalate, and the other two materials are accounted for in terms of purity, since the much lower rate of grain growth in this former powder is primarily responsible for its densification characteristics, and such grain growth is markedly affected by purity. A dilatometric investigation was carried out on the sintered compacts, in order to determine the temperature of the alpha/gamma phase change in the materials. It was found that both carbonyl iron and iron powder ex oxalate would contain phase on sintering at 900°C, whereas transformation in the powder from high purity oxide occurred above this sintering temperature.