Research on the Sintering of Doped Tungsten Powder

Abstract

Although no satisfactory explanation of the action of doping can be given, its effect on sintering seems to be that when doping reagents volatilise, they separate grains resulting in finer grain size than pure tungsten ingot. Moreover, the impurities as FeO may appreciably be reduced by the volatilisation of doping reagents. We prepared two lots of tungsten powder which contain 0.4%SiO₂, 0.2%K₂O and 0.03%Al₂O₃ to WO₃ and sintered the pressed ingots using sintering currents of 65%, 75%, 80%, 85%, 88%, 90%, 92% and 95% to fusion current. The changes in shrinkage, hardness,apparent density, electrical resistivity, grain size and decrease of nonvolatile residue and FeO during sintering have been studied.
(1) Sample No. 1, which is finer than No. 2, showed the easy sintering.
(2) From the hardness and grain size measurement, it was supposed that the temperature was the highest at central part and then upper and lower part of an ingot.
(3) The effect of doping reagents on the restraining grain growth during sintering is remarkable—the grain size of pure tungsten ingot sintered at 90% current is about 100 times as large as that of doped ingot. In consequence of the fact that grain growth takes place rapidly between 65∼80% current, it is possible to control the gain size at this sintering current. But we could not find conspicuous purifying effect during sintering on the decrease of impurity (FeO). The composition of residue is about 50%SiO₂ and 40%Al₂O₃.