2.2 Direct Sintering. The tungsten bars are commonly mounted in pairs or several units in a vertical, free-hanging position in a water-cooled chamber. They are clamped at the top using water cooled copper contacts with tungsten inserts. The bottom clip is made of tungsten and must be able to move as the bar shrinks. An electric current of several thousand ampere is passed thro ugh it in order to heat it up to about 3000℃by Joule’s energy. Sintering is conducted under dry hydrogen. Several such sintering aggregates are commonly connected in series so that several bars can be sintered simultaneously.
In order to achieve an appreciable strength of the green compact, which is necessary for handling, it must be presintered. Presintering is done by heating to 1100-1300℃in muffle furnaces under hydrogen atmosphere. By reducing the oxide film covering the powder particles, the cohesive strength between them is increased.
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