SPECIAL TUNGSTEN FORMS AIND QUALITIES - High-Purity Tungsten - 02

Of all impurity elements, in particular’ uranium and thorium contents must be extremely low. These naturally radioactive elements cause “soft errors” in memory circuits due to a-particle emission.

The raw material is always APT with different purity depending on the uranium and thorium content. It depends on the ore deposit whether these two elements are present or not. If the concentrations are unacceptably high after dissolution, a selective solvent extraction process removes the main portion of these two elements are present or not. lf the concentrations are unacceptably high after dissolution a selective solvent extraction process removes the main portion of these two elements.

The APT is calcined to AMT at 220℃ and dissolved in water while adding hydrogen peroxide and sulfuric acid, The traces of uranium and thorium are extracted selectively by 0.5 M di-2-ethylhexyl phosphoric acid and 0.125 M tri-n-octyl-phosphine oxide dissolved in kerosene.

APT, having acceptably low uranium/thorium concentrations, can be further purified by double or multiple crystallization to remove other impurities.

The W0₃ gained after calcinations in both cases is reduced by hydrogen to tungsten powder of high purity. This can be used for the production of tungsten, tungsten silicide or TiW sputtering targets.

If further purification is desired, rods are produced by compacting and sintering and are subjected to electron beam floating zone melting. This procedure does not further reduce the concentrations of uranium and thorium, but does reduce the concentrations of many other elements. Moreover, residual impurity elements are distributed homogeneously.

Powders having sufficient purity can be processed to sputter targets directly. Tungsten sputter targets have usually been manufactured by hot pressing. However, it was not possible to maintain the high-purity level of the powder. A much better way to achieve high purity in the sputter target is to employ a special sintering process which reduces the contents of gaseous impurities and provides a homogeneous distribution of all remaining.

In general, it is important to maintain clean room conditions throughout the production steps.

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