Non-sag tungsten

Non-sag tungsten wires are the most creep-resistant wires, with the exception of monocrystalline tungsten. They are therefore used for sag-free lamp filaments at service temperatures of up to 3000 °C (0.88 r ,), and shear stresses in the range of 0.5 to 10 MPa [1.66]. 

J. P. Momiroli, Low Tenrperature Embrittlement of Undoped and Doped Timgsten, in The Metallurgy of Doped/Non-sag Tungsten (E. Pink and L. Baiha, eds.), Elsevier, London (1989). 

Besides chemical conversion and chemical vapor transport, the reduction process is a purification step, too. Trace impurities, always present in the oxide, may evaporate. On the other hand, foreign phases can be incorporated during the CVT growth of tungsten, finally leading to inclusions in the tungsten powder particles. This is of special interest in the production of non-sag tungsten wire used for incandescent lamps. 

CS structures have the ability to build-in foreign metal ions, especially in the reduced form. This seems to play an important role in the dopant uptake of tungsten blue oxide in non-sag tungsten production (for more information see Section 5.4.5). 

A schematic representation of the non-sag tungsten wire fabrication process is shown in Fig. 6.4. 

FIGURE 6.6. Typical non-sag tungsten wire processing schedule [6.2]. 

FIGURE 6.7. SEM image of a non-sag tungsten filament (a) before use, (b) after several hundred hours of operation in a 60-W incandescent lamp. Facetting of the originally round filament occurred due to evaporation of tungsten at the high operation temperature (2400-2500 °C). 

More than 90% of non-sag tungsten is used for incandescent lamps. Small amounts are used as defroster heating wires in automobile windshields and as heating wire coils for almninum evaporation in metallizing applications. For the latter application, thick wires are used (0.5-1 mm). The service life of these wires is not only determined by their mechanical properties but also by their corrosion resistance, which is higher for coarser-grained doped materials with a minimum of grain-boundary volume [6.18]. 

Horascek, O. (1989) The Metallurgy of Doped/Non-Sag Tungsten. Elsevier, London, p. 251. 

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