Tungsten Applications and Special Products

Ultrahigh Purity (UHP) Powders. Sputter targets for semiconductor manufacturing need ultrahigh purity (5N and 6N) tungsten metal powders. In particular, the incorporation of alpha emitters such as uranium and thorium, is very critical and has to be kept below 1 ppb. The mobile alkali ions (e.g., sodium and potassium) have to be kept below 0.1 ppm. Purification of preselected APT by multiple extraction and crystallization steps guarantees the required purity, where all production steps must be carried out and can be achieved only under clean room conditions (Ref 11, 27, 28).

Pure Tungsten Materials. For wire and sheet production, the tungsten powders are consolidated to rods by pressing and sintering the powder. In order to get a suitably high densification, high temperatures between 2500 and 3100 °C (Ref 2, 11, 29) are applied. Thereby, most impurities still present at this stage are largely removed by evaporation. The sintered tungsten bars thus obtained are swaged or rolled at elevated temperatures in order to produce either rods, which are further drawn to produce wires, or sheets. Such pure tungsten materials are widely used for high-temperature applications, for example in high-temperature furnace construction (Ref 2, 11).

Nonsag Tungsten. The "creeping" of tungsten wires in incandescent lamps during very high operation temperatures must be avoided. The filament should not sag. Accidentally and involuntarily discovered decades ago, this avoidance can be achieved by incorporating traces of potassium into the tungsten wires, thus successfully preventing the sagging of the tungsten filaments in light bulbs (Ref 2, 30).

Today's technology is to add a mixture of aluminum, silicon, and potassium into tungsten oxide prior to the reduction to tungsten metal. During this reduction, a certain part of the Al-, Si-, K-doping elements is enclosed into the tungsten metal powder particles. In the subsequent sintering of the pressed tungsten powder, aluminum and silicon evaporate, while potassium is too big to diffuse through the tungsten lattice. This procedure leads to an incorporation of potassium droplets as inclusions. During the wire drawing, the tiny potassium droplets are elongated and finally break into a multitude of tiny potassium bubbles in rows allocated parallel along the wire axis, which leads to an elongated interlocked grain structure. During recrystallization at elevated temperatures, the potassium bubbles stabilize very effectively by an effect analog to dispersion hardening, the movement of dislocations and thus the migration of the grain boundaries perpendicular to the wire axis. This particular structure is the reason for the nonsag properties of the tungsten wires in light bulbs (Ref 2, 11, 30, and 31).

Tungsten with Oxide Dispersions. Tungsten metal powder and thorium-containing compounds (such as ThO2 or Th-nitrate) are mixed, pressed, sintered, and further processed to wire or sheets. They have two major applications (Ref 2, 11):

• Welding electrodes, whereby the high electron emission capability of Th is used for easier electric arc formation and its stabilization (Ref 32, 33)

• Improved physical properties (dispersion hardening) in tungsten sheets and wires for (e.g., vibration-resistant automotive lamps) (Ref 34)

Due to environmental health and safety regulations associated with the use of the radioactive thorium, alternatives have been developed. Lanthanum and cerium have proved to be potential candidates to substitute thorium in welding electrodes (Ref 32, 33). For certain lighting applications, however, suitable substitutes are still missing (Ref 32, 34).

Tungsten Composite Materials. Tungsten metal powder mixed with other powders, such as iron, nickel, copper, and cobalt (both individually or in combination) can be liquid phase sintered at about 1500 °C, forming so called "heavy metals." Applications are based on the high density (17-19 g/cm3) and ductility of this composite and include counter weights, high energy penetrators, and radiation shields (Ref 11).

A porous skeleton of pressed tungsten metal powder can be infiltrated with copper alloys, or silver, leading to composite materials. W/Cu (and W/Ag) composites are used as electrical contacts (Ref 35, 36) due to their high electrical conductivity and their stability against damage caused by electrical arc formation. Recent developments (Ref 37, 38) describe the production of W/Cu-composite powders, which can be directly pressed to shape and sintered, thus preventing the infiltration and machining steps, which is of particular importance with respect to the application as heat sink in the semiconductor industry.

0 0

Post a comment