Carbon doping - A key for the substitute of thoriated tungsten

Thoriated tungsten is the state-of-the-art cathode material for short arc lamps used in cinema lighting and semiconductor lithography. But the radioactive nature of thorium is driving the development of a substitute.

Thoriated tungsten is the state-of-the-art cathode material for short arc lamps used in cinema lighting and semiconductor lithography. The major effect of thorium is the temperature reduction at the cathode tip due to a lowered electron work function. The radioactive nature of thorium is driving the development of a substitute. At the 19th Plansee Seminar Hannes Traxler from Plansee and Maria Nilius from Osram presented lanthanated tungsten cathodes doped with carbon as an eco-friendly alternative. Two beneficial effects of carbon are found. High-temperature diffusion couples prove that carbon activates the diffusion of lanthanum. Thus flickering due to depletion of lanthanum at the cathode tip is prevented. Additionally, highly instrumented lamp tests prove the prevention of lanthanum containing deposits close to the cathode tip. Such deposits trigger the flickering since their low work function attracts the spot of the arc. Tests with commercial lamps confirm a stable light output by carbon doping but show increased blackening of the lamp bulb

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