Journal of the Japan Society of Powder and Powder Metallurgy

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The Structure of tungsten bar, K₂O- SiO₂- and Al₂O₃-doped or undoped, sintered and/or re-treated, was studied by means of optical and electron microscopes. Distribution of crystal grain size indicates the restraining effect of Al₂O₃ doping. Crystal boundary, sintering pore and etching pit are very characteristic. The relationship between the number or size of the pore and Al₂O₃ doping quantity is made clear, and confirmed by the apparent specific gravity. When SiO₂ and K₂O are removed by hydrofluoric acid washing, the effect of Al₂O₃ doping becomes prominent. When the restraining effect of Al₂O₃ doping is very strong, mean crystal grain size is usually small but sometimes, coarsening takes place in a particular region of the crystals, and the crystal boundry moves through the obstacles such as sintering pores or non-metallic inclusions, making huge crystal grains. Such phenomena was still further studied by the observation on the structure of bars heated up to just below the melting point in which the pore had become larger. The grain size of the doped re-treated bar is generally larger than that of the undoped re-treated bar, and the characteristics of boundary etc. are very similar to that of the sintered bar. A brief additional explanation is given on the recrystallized structure and recrystallization temperature of the wire worked out from these sintered bars.

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Iron powders were produced by reduction of iron oxide which prepared by calcination of iron oxalate, oxyhydrate, sulphate and formate, and also were produced by direct reduction of these salts at 400°C, 600°C and 800°C. Their morphological properties were determined by optical microscope and electron microscope.
Observation by electron microscope showed that iron particles produced, in all cases, consisted of small unit particle which were linked with each other, resulting in an aggregate. The size of the unit particle increased with increasing temperature and time of reduction. The aggregates of iron powders prepared at the lower reduction temperature, resemble, in shape, the original crystals of the mother salts or the particles of oxides, and thus the shape of aggregates prepared from oxalate was rod like, from oxyhydrate needle like, and from formate rod like. When the reduction temperature was high, however, material transport occured between aggregates and the aggregates lost their original shape. The effects of preparating method on the size and shape of the iron particle were also observed.

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