Halogen Cycle

gafftaper;56331 said:
...Gases inside the Lamp: Edison knew that normal oxygen rich atmospheric gas caused the rapid destruction of the filament, so his experimental lamps were usually vacuum sealed. In 1913, it was discovered that filling the lamp with a inert gases not found in typical atmospheric gas could actually prolong the life of the filament. Inert gases like argon and Krypton don't react to the tungsten and cause deterioration.
Later in time it was discovered that adding a Halogen to the gas mix had a unique property. As the lamp heats up, tungsten molecules are driven off of the filament by the intense heat. In a non-halogen lamp, these molecules eventually bump into the envelope and are stuck there since it is much cooler than the rest of the lamp. Over time, a dark build up on the envelope occurs and eventually the filament weakens and fails. In a halogen lamp, the Tungsten molecules combine with the halogen to make a form of salt. This new molecule floats around the lamp and once it comes back near the filament, the heat breaks the molecule apart and the tungsten is deposited back on the filament. This prolongs the life of the lamp significantly. ...

From http://www.sylvania.com/ConsumerProducts/AutomotiveLighting/Products/Halogen/HowHalogenWorks.htm:
Halogen Cycle
In conventional gas-filled tungsten-filament lamps, tungsten molecules evaporate from the hot filament, are carried by convection currents of the inert fill gas to the relatively cool inner surface of the bulb, and are deposited to form a thin film which gradually increases in thickness during the life of the lamp. These phenomena cause depreciation of lumen output and efficacy in two ways. First, deposition of the evaporated tungsten on the bulb wall builds up a film of increasing opacity which absorbs increasing portions of the light produced by the filament and thus reduces the total light output. Second, evaporation of tungsten from the filament reduces the filament wire diameter, increasing the resistance and thus (at constant voltage) decreasing the amperes, wattage, lumens, lumens per watt, and color temperature.

In tungsten-halogen lamps, the effects described above are reduced or retarded by the regenerative action of the halogen cycle, which operates by virtue of the temperature gradient between the filament and the bulb. As a general concept:
a. The filament, fill gas, and bulb are initially at some low temperature (e.g., ambient, for a cold start).
b. When power is applied, the filament rapidly rises to its operating temperature (2800K to 3400K depending on application), heating the fill gas and the bulb. The bulb wall rises to an operating temperature of 400°C to 1000°C, and the fill gas rises to temperatures ranging from that at the filament to that at the bulb wall. This temperature gradient causes convection currents in the fill gas.
c. As the bulb wall rises above temperatures in the range 200°C to 250°C (depending on nature and amount of halogen vapor), the halogen cycle begins to operate. Tungsten molecules evaporated from the filament combine with the halogen vapor to form a tungsten halide (e.g., tungsten iodide or tungsten bromide). The halide does not condense on the hot wall of the bulb but is circulated by convection back to the region of the filament.
d. At the filament where the temperature exceeds 2500°C, the tungsten halide dissociates, the tungsten is deposited on the filament, and
e. The free halogen vapor is recirculated to continue the regenerative cycle. This cycle thus keeps the bulb wall clean by preventing deposition of tungsten and results in much higher lumen maintenance over the life of the lamp than that obtained for conventional tungsten-filament lamps.

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