Thoriated tungsten filament

Use Ceramics (high temperature), gas mantles, nuclear fuel, flame spraying, crucibles, medicine, nonsilica optical glass, catalyst, thoriated tungsten filaments. 

Use Reagent for determination of fluorine, thoriated tungsten filaments. 

The reaction occurring in thoriated tungsten filaments was also discussed. [1963ALL/MCD]... 

Oxide-coated cathodes that are indirectly heated are more efficient ( 1 eV) than thoriated tungsten filaments, but they cannot sustain the high-voltage high-current operating conditions associated with power grid tubes. When the maximum output power required is below a few hundred watts, they are the device of choice and find application in low-power high-frequency transmitters. They are also widely used by audiophiles who feel that vacuum-tube power amplifiers produce a quality of sound output that is superior to that produced by semiconductor devices. 

At a typical operating temperature of 1600-1800° C, a thoriated tungsten filament will produce an average emission of 40-430 mA/cm. Peak current ranges up to 10 A/cm or more. 

The beam of ionizing electrons is produced by thermionic emission from a resis-tively heated metal wire ox filament typically made of rhenium or tungsten. The filament reaches up to 2000 °C during operation. Some reduction of the working temperature without loss of electron emission (1-10 mA mm ) can be achieved by use of thoriated iridium or thoriated rhenium filaments. [22] There is a wide variety of filaments available from different manufactures working almost equally well, e.g., the filament can be a straight wire, a ribbon, or a small coil (Fig. 5.9). 

Studies by McCulloh and Hlford57 found that dual tungsten filaments exhibited better linearity and also exhibited sensitivities which were in closer agreement to those stated by the manufacturer. Filaments made out of thoriated iridium demonstrated linearity differences as great as 30%. These differences were evident in standard as well as nude gauges. 

The end of useful life for a thoriated tungsten tube occurs when most of the carbon has evaporated or has combined with residual gas, depleting the carbide surface layer. Theoretically, a 3% increase in filament voltage will result in a 20 K increase in cathode temperature, a 20% increase in peak emission, and a 50% decrease in tube life because of carbon loss. This cycle works in reverse, too. For a small decrease in temperature and peak emission, the fife of the carbide layer—and hence, the tube may be increased. 

Extending the life of a microwave tube begins with accurate adjustment of filament voltage. The filament should not be operated at a reduced voltage in an effort to extend tube hfe. UnHke a thoriated tungsten grid tube, reduced filament voltage may cause uneven emission from the surface of the cathode with little or no improvement in cathode life. 

Similarly, timgsten electrodes used in tungsten inert gas welding and filaments used in light bulbs and vacuum tubes are strengthened by dispersions of thoria (Th02) particles (thoriated timgsten). The presence of thoria in W also lowers work function and is used to improve the efficiency of electron emitters in x-ray tubes and other electron beam devices. 

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