Tungsten Intermediates - 2. Tungsten Metal Powder (W)

Yellow or blue oxide is reduced to tungsten metal powder by hydrogen. The reduction is carried out either in pusher furnaces, in which the powder passes through the furnace in boats, or in rotary furnaces, at 700-1,000℃.

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Tungsten Intermediates - 1. Ammonium Paratungstate (APT)

APT [(NH4)₁₀[H₂W₁₂0₄₂]•4H₂0] is the main intermediate and also the main tungsten raw material traded in the market. APT is usuaIIy calcined to yellow (W03) or blue oxide (W03-X; a slightly substoichimetric trioxide with varying oxygen content).

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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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SPECIAL TUNGSTEN FORMS AIND QUALITIES - High-Purity Tungsten - 01

High-purity or ultrahigh-purity tungsten (5N or 6N) is used in the form of thin films in the electronic industry as the preferred material for gate electrodes, interconnects, and barrier metal because of its high resistance to electron migration, high temperature stability, an d tendency to form stable silicides. In principle, tungsten thin films can be produced either by CVD or by PVD. For PVD-produced tungsten thin films, sputter targets of high purity (6N) are commercially available today.

This is partly due to improvements in production methods, which in fact caused higher purity. But it is also somewhat a consequence of increased sensitivity of the analytical methods and the equipment used nowadays. For example, some years ago it was only possible to say that the concentration is less than 1µg/g or less than 0.1µg/g. Meanwhile, application of more advanced techniques has changed the situation drastically. In high-purity tungsten, metal levels of trace elements are in the ng/g level or even below, which affords special analytical requirements such as working in clean room laboratories, prevention of any other contamination, and correct calibration. The most important analytical methods applied today for that purpose are glow discharge mass spectrometry, secondary ion mass spectrometry, isotope dilution, and trace matrix separation combined with inductively coupled plasma atomic emission spectro-metry.

As an example, it was possible by isotope dilution analysis to determine 15ppq of uranium in an ultrapure tungsten sample. This corresponds to /5vx 10-15%. These methods are generally very complex and expensive. Comparative analyses often show significant scatter.

SPECIAL TUNGSTEN FORMS AIND QUALITIES - Porous Tungsten

  The production of structural parts from porous tungsten affords special technological procedures.

  Starting materials are mainly coarse or globular powders, which are compressed by special methods, sometimes with the addition of pore-forming materials which evaporate during sintering. Also in sintering, special procedures such as infiltration, evaporation, surface treatment, etc. are necessary. Powder grain size, compacting pressure, sintering temperature, and time determine pore size and permeability. At densities between ’50 and 78% all pores are open; at >90% density, all are closed.

  Parts with ran open coherent porosity of”>20% are used in vacuum tubes and aeronautics because of refractoriness, shape stability, and chemical resistance. Examples are porous cathodes impregnated with alkaline earth oxides as electron-emitting sources in special tubes, thermoionic converters heated by nuclear energy, and ionic propulsion units with porous tungsten plates as ionic sources for Cs vapor of high temperature. Those emitter plates are  made of globular tungsten powder (7µm) density 75-85%, pore size 2-30 µm and pore number  l.4 x 10⁶-8 x10⁶ •cm^-3.

  Porous tungsten can be infiltrated either to machine it to rigid dimensional tolerances, or to fabricate composite materials, Copper, which is used as infiltrant for machining, can be completely removed afterward by heating in vacuum by evaporation.

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SPECIAL TUNGSTEN FORMS AIND QUALITIES - Tungsten Thin Films

The preparation of tungsten thin films is possible by different techniques, such as electrolysis from salt melts, CVD, PVD and plasma spraying, diffusion, pIating, electrophoresis, and metallizing. The technically important techniques are only CVD, PVD, and plasma spraying.

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SPECIAL TUNGSTEN FORMS AIND QUALITIES - Globular Tungsten Powder - Single Crystals

These crystals can be produced by:

Annealing of deformed wires or rods by secondary recrystallization and extreme crystal growth; thoriated tungsten wires (cross-section, e.g., 100µm) are pulled through a narrow zone at a temperature of above 2000℃ at the same time the crystal grows, forming a single crystal up to several meters long (Pintsch procedure).

Zone melting.

Single crystals, due to their high cost are used only in special cases like basic investigations. The outstanding properties are a lack of grain boundaries and a very small number of lattice defects. They are therefore easy to deform, show great ductility, and possess no embrittlement up to the highest temperature.

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