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Genesis of Halogen

Selection of halogen bulbs
Selection of halogen bulbs
As from the incandescent article the halogen bulb is an incandescent bulb but with additional features. It still uses the added inert gas to check the progress of evaporated tungsten.  For any particular gas filling, having more molecules in the path of wayward tungsten is desirable.  This is achieved by upping the filled gas pressure. Having more molecules coupled with weighty molecules such as krypton or xenon gives the best option, allowing for an even higher filament temperature. Higher gas pressure puts additional stress on the glass envelope so having a small strong case is necessary.

Still, the higher the temperature the more efficient the conversion to visible light. Estimates suggest that only 5% of the energy used by an incandescent tungsten filament bulb is emitted as visible light and the rest is radiated as invisible infrared light and heat.

To allow the element to run at a higher temperature a method to help recycle the lost tungsten is required. The recycling process used is known as the tungsten halide cycle. Highly reactive halogen, originally chlorine gas, works in conjunction with the higher filament temperature to recycle the tungsten. This recycling process allows the efficiency to double to roughly 10% and is accomplished within a bulb known as a halogen bulb.

Chemically, the elements collectively called the halogens sit in a specific region in the periodic table such as chlorine, fluorine, bromine and iodine. They are very reactive unlike the noble gasses, which are notably non-reactive. When added into the bulb gas mix the halogens are so reactive that they will attempt to react with anything including: the bulb capsule, any support structure and even the filament when it is switched off. To help balance this potentially damaging effect the added quantity of halogen is measured so there is only sufficient to deal with the evaporation of tungsten from the filament, thus keeping the halogen occupied with good work.

The filament temperature works at 3500°C and provides sufficient peripheral heat for the halogen gas to react with evaporated tungsten, sequestering it as a compound known as a halide, and preventing it from condensing on the envelope. As the captured tungsten halide compound circulates it meets the much hotter filament, which is hot enough to disassociate the compound thus, liberating both the tungsten and the halogen. The freed tungsten has the opportunity to condenses back onto the filament and the halogen is free to react with other expelled tungsten, starting the cycle again.

Bulb showing signs of tungsten deposits
Bulb showing signs of tungsten deposits
If the glass capsule is sufficiently hot, this recycling process can clear the glass of stray condensed tungsten. To allow the favourable restorative reaction to occur, the glass envelope needs to reach a temperature of 250 - 300°C. The problem is that high temperatures coupled with the high internal gas pressure can cause the weakening of the bulb.

To maintain physical integrity, the envelope needs to be fabricated from a material with a higher ductile point than the typical glass used in the manufacture of incandescent bulbs. Quartz is the envelope material of choice. The quartz envelope is usually reduced in size to ensure that it becomes sufficiently hot to facilitate the halide cycle. Quartz copes well with the combination of high pressure and temperatures generated by the tungsten filament.

The video clip shows a time-lapse of a bulb. First without halogen gas, which overtime becomes occluded and dull. Roughly half way through, halogen gas is added and the recycling process clears the envelope and recycles the tungsten back to the filament.

The recycling of tungsten via the halide cycle is not a perfect process and hot spots may appear on the filament. These hot spots are caused by a thinning and subsequent increase in localised resistance of the filament. As any particular spot on the filament thins, the localised electrical resistance increases and thus the localised temperature increases, hastening the evaporation of tungsten from that particular spot. Unfortunately, this self supporting deterioration means that the evaporation of the tungsten would be greater from these hot spots, thus reducing the life of the filament. Interestingly, the most reactive halogen, Fluorine, has a special ability to deposit more tungsten back to these specific hot spots resulting in the evening out the thickness of the element, so prolonging its life. It achieves this affect due to its relatively high halide stability compared to other halogens. The hottest parts of the filament, which are the thinner hot spots, naturally disassociate more of halide where condensing tungsten is required the most.

Sadly, fluorine is extremely reactive and is difficult to tame but its use was proposed as pre-shipping conditioning in the manufacturing process. Natural manufacturing variations in the thickness of the tungsten filament would be evened out by first using the fluorine halide and then bulb's gaseous contents changed to its less reactive cousin such as bromine before finally sealing and physical shipment to the consumer. Having a consistent filament at the point of manufacture increases the longevity as there would be no thinner and consequently hotter areas of the filament. A uniform filament coupled with the less aggressive halogen gas, would significantly extend the working life of the bulb.

Depending on which halogen had been added hydrogen can be added to help limit the highly reactive nature of the halogen and forms a compounds such as hydrogen bromide or hydrogen fluoride when the bulb is switched off, . When the halogen reacts with the tungsten, the hydrogen is temporarily disassociated and released. Sadly, hydrogen is difficult to contain and when free it can slowly defuse right through the quartz body. Once the halogen has reacted unfavourably it can traps itself and consequently be stopped from gainfully recycling the evaporated tungsten. Once this happens the tungsten will slowly build-up on the glass envelope as the balance changes in favour of the wayward condensed material.

Xenon filled halogen bulb
Xenon filled halogen headlamp bulb
So next time you buy a 50% brighter xenon halogen bulb for your car or house give a thought to its genesis and all the people who made it possible.

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