Tungsten Products - Tungsten Blue Oxide (TBO, VV03.X)

TBO is manufactured by calcination of APT under more or less reducing conditions which vary from producer to producer. TBO is not a chemically defined compound, but consists of various different constituents, like trioxide, tungsten bronzes and different lower tungsten oxides. The relative amount of the se compounds in TBO depends on the calcination parameters.

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Tungsten Products - Tungsten Trioxide (W03)

In a technical scale Tungsten trioxide is almost exclusively manufactured by calcination of APT under oxidizing conditions (in air), W03 is one of the most important, highly pure intermediates for the production of other tungsten compounds and tungsten metal powder. In the latter application it was substituted to a large extent by tungsten by tungsten blue oxide. Because of its bright yellow colour it is us e d as a pigment in oil and water colours. It is employed in a wide variety of catalysts, most recently in catalysts for the control of air pollution and industrial hygiene (DeNOx).

Tungsten trioxide particles are pseudomorphous to APT; that means the particles have the same shape and size as the former APT crystal s, but consist of very small W03 grains.

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Tungsten Products - Ammonium Paratungstate (APT)

Ammonium Paratungstate (NH4)[H2W12042J,4H20 is the most important precursor for the majority of tungsten products. Exceptions are only product s of melting metallurgy and Menstrum WC produce d directly from ore concentrates.

All other intermediates such as tungsten trioxide, tungsten blue oxide, tungstic acid and ammonium metatungstate ca n be derived from APT, either by thermal decomposition or chemical conversion.

APT is a white crystallized powder having average crystal size between 30 and 100 im, Especially crucial for the quality is the purity, Typical levels of todays commercial APT are (upper limits in pg/g): Al 1-7, As 5-10, Bi 0.5-1, Ca 1-10, C0 1-10, Cr 1-10, Cu 1-3, Fe 3-10, K 2-10; Mg l-7, Mn 1-10, M0 5-30, Na 5-10, Ni 1-7, P 5-7, Pb 1-5, S 5-7, Si 1-10, Sn 1-10, Ti 3-10, and U 3-10, APT is packaged in polyethylene-lined drums.

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Tungsten Products - Concentrates

They are also called Primary Tungsten Sources, Marketable ore concentrates of Scheelite and Wolframite contain typically 65 – 70 % W03, Off grade concentrates having lower W03 concentrations are seldom on the market and if so, only for a lower price. In fully integrated companies lower grades (6-40% W03) are often preferred, because upgrading to high concentrations is mostly combined with a lower yield.

Concentrates in former times have been the main trading product, but were substituted today by APT. Concentrates are packaged either in large polyethylene “big bags” (1 t0 2 tons) or, alternatively, in steel drums and individual packaging (50 t0 200 kg).

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Tungsten Intermediates - 4. Cast Carbide

By melting tungsten metal and tungsten monocarbide (WC) together, a eutectic composition of WC and W2C is formed. This melt is cast and rapidly quenched to form extremeIy hard solid partied s having a fine crystal structure. A tough, feather-like structure is preferred over the brittle, blocky structure obtained by insufficient quenching. The solids are crushed and classified to various mesh sizes.

Tungsten Intermediates - 3. Tungsten Carbide (WC)

Most of the tungsten metal powder is converted to tungsten carbide (WC) by reaction with pure carbon powder, e,g. carbon black, at  900- 2,200℃ in pusher or batch furnaces, a process called carburization.

Tungsten carbide is, quantitatively, the most important tungsten compound. Because of its hardness, it is the main constituent in cemented carbide (hardmetal).

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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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SPECIAL TUNGSTEN FORMS AIND QUALITIES - Globular Tungsten Powder - Coarse Tungsten Powder

Good flowing tungsten powder can be obtained by:

Reduction of W03 0r blue oxide at high temperature, high humidity, and alkali metal doping, and subsequent classification;

Disintegration and classification of presintered parts;

Fine tungsten powder +0.2% Ni is compacted and sintered at 1840 0C for 3 hours. Afterward, the Ni is dissolved by nitric acid.

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SPECIAL TUNGSTEN FORMS AIND QUALITIES - Globular Tungsten Powder

SPECIAL TUNGSTEN FORMS AIND QUALITIES

Globular Tungsten Powder

This substance can be produce d from fine powder in a plasma jet (150-200 µm). Monodisperse single crystal powder (containing small amounts of Ni) can be obtained by acid leaching of sintered W:Ni-Cu parts.

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Plasma Spraying

Tungsten layers can be sprayed with suitable guns on different base materials. Easily flowing tungsten powder is used as spraying material Ar or Ar-N2 mixtures are preferred as protective atmosphere in closed chambers. Besides coatings, tungsten parts can be produced by spraying tungsten layers on an adequately formed nucleus until the desired thickness is reached.

One may distinguish between three basic types of substrates:

Expendable substrates are removed after spraying by selective chemical etching, melting, etc;

Reusable substrates are used for mass production;

Permanent substrates become an integral part of the article.

Density ranges from 50—95% of the theoretical value. Higher density is attained by uniform substrate temperature and Iow deposition rate. Density and strength might be increased by post-sinterning in hydrogen. Low-pressure plasma spraying is mainly used for producing intricate shapes, which otherwise cannot be manufactured.

Electron-Bean Zone Melting

Electron-beam zone melting is the preferred method to produce larger, single crystals of tungsten. A polycrystalline tungsten rod is mounted in a vertical position in a stainless steel bell jar and connected as anode. An electron gun (cathode) traverses along the rod by means of a screw drive mechanism. On melting in a high vacuum, a narrow, liquid zone is formed and held between the two solid rod parts by surface tension forces. As the zone slowly moves through the rod several times (zone floating), impurities are concentrated on both the starting and terminal end of the rod. Since the molten zone is quite narrow and the temperature gradient in its vicinity rather steep, nucleation starts essentially at one point and a single crystal is the result. The concentration of many impurity elements can be decreased considerably by zone melting. Al, Ca, Co, Cu, Fe, Mg, Mn, Ni, Si, Na, Ti, Ta, Re, Mo, and Nb concentrations can be reduced below 1 pg/g Interstitials remain at the following pg/g levels: O₂ 2, N₂5, C10, and H1.

Single crystals produced by this technique represent the most ductile tungsten currently available. They are free of grain boundaries, which cause brittleness.

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POWDER METALLURGY-3. Fabrication of Tungsten -3.3 Bonding of Tungsten-02

Welding. Tungsten possesses only moderate welding properties. Welding must be carried out under controlled weld atmospheres, preferably in a dry box, since any contamination by oxygen will reduce the ductility of the joint. Before welding, the metal has to be degreased and pickled, commonly with a mixture of nitric acid and hydrofluoric acid (90/10 vol%). The weld seams have a coarse-grained structure in the hot-fusion zone, owing to recrystallization, and can therefore withstand only low mechanical stresses. Tungsten can be welded by tungsten inert gas (TIG) welding, laser beam welding (for thin parts), plasma welding, friction welding and electron beam welding, the latter being the preferred fusion-welding method. Tungsten can be welded to W-Re and Mo-Re alloys. W-Re alloys (in particular W-26Re) are recommended as filler metal for welding tungsten. Tungsten can be diffusion welded to tungsten and other metals. Despite the high melting point of tungsten, temperatures of 1300-2000℃ and pressures of 2—20N•mm^-2 give satisfactory joints. Intermediate layers of Ni, Pt, Rh, Ru, and Pd accelerate the diffusion process. Tungsten wires can be spot-welded to other metals under a protective gas blanket.

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POWDER METALLURGY-3. Fabrication of Tungsten -3.3 Bonding of Tungsten

3.3 Bonding of Tungsten

Mechanical Joining. Mechanical joining, such as riveting, tacking, or lacing, comprises the simplest methods of joining tungsten, provided the joint need not be impermeable to liquids and/or gases. Tungsten and molybdenum parts can be used. Mechanical fastening is used for constructional parts, such as heating elements, containers, large shields, etc.

Brazing. The parts to be joined must be free from grease, oils, oxides, or other impurities. As tungsten is very sensitive to oxidation, brazing must be carried out under a protective gas, hydrogen, or in vacuum. If maximum strength of the joint is required, tungsten has to be brazed below its recrystallization temperature. Typical brazing metals and temperatures are; Rh (1970℃), Ni (1430℃), or AgCu20Pd15 (700-900℃). More derailed information is available. Brazing of tungsten to ceramics, graphite, and silicon has gained importance for the fabrication of refractory-metal composites.

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POWDER METALLURGY-3. Fabrication of Tungsten -3.2 Shaping-02

Tungsten can be drilled, turned, milled, planed and ground. However, machining operations require experience and close adherence to optimum conditions. Preheating to about 200℃ and the use of cemented carbide tools are recommended. Complex shapes and holes can be formed by spark erosion; the tungsten workpiece forms the anode and the working electrode the cathode.

Tungsten (pure W, doped W, W-Re) is commercially available as wrought products (sintered bars, billets, forgings, rods, pins, disks, cylinders, tubes, sheet, strips, ribbons, wires) as well as in the form of a broad variety of shaped parts(crucibles, springs, heating elements, rivets, etc.).

Cold-rolled sheet is supplied in thicknesses from 0.025 to 0.4mm (up to 500mmxl000mm). Hot-rolled tungsten is available with thicknesses from 0.4 to 8.0mm (maximum lengths, 1500mm). Tungsten wires are manufactured from about I mm down to 5 µm.

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