Abstract
The preparation and properties of ultrafine powders of tungsten, tungsten 18.7 % rhenium and thoriated tungsten (4.4% ThO2 for W) are described. The preparation consists of two processes, freeze-drying and reduction with hydrogen at relatively low temperature. According to the desired powder composition, the solution of ammonium paratungstate, the mixed solution of ammonium paratungstate and ammonium perrhenate or the solution of thorium nitrate suspending the fine particles of tungstic acid were frozen rapidly and dryed under the reduced pressure through sublimation. The products thus obtained consist of fine particles having a mean diameter of 0.3-3 microns. The more rapidly the solution was frozen, the finer the particles became. Freeze-dryed powders were reduced with hydrogen at 300-800°C. Metal particles of tungsten or tungsten alloy were obtained above 600°C, and the mean diameter was 0.02-0.1 microns. Inter-particle sintering was observed at higher reduction temperatures, resulting in larger particles.
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