Jump to content

Source of carbon in sealed incondecent light bulb


Bushranger

Recommended Posts

I had to change a brake light in my vehicle yesterday.  The filament had burned out.  A apparent carbon stain was on the inside of the glass bulb which for me begged the question: Where did the carbon come from?  The filament is made of elemental tungsten...no carbon compound in it to release carbon when it burns out.  There is usually some inert gas inside the bulb also...usually elemental argon, I believe.  Other than that there is just the glass stalks that hold the filiment in the void of the bulb.  Therefore, just where does the carbon come from that stains the inside of the glass when the filament burns out?

The only thing I can postulate is that if the filament were not pure tungsten, but something like tungsten carbide, that would be a source for the carbon stain.

So, what is the source for that apparent carbon stain?

Link to comment
Share on other sites

3 hours ago, MigL said:

As Strange has implied...
What makes you think the 'stain' is carbon ?

( I recall doing vacuum deposition of thin metal films on glass substrates in 2nd year Uni )

I was assuming it was carbon from the fact that it was Black.  So, maybe not carbon at all, but vaporized tungsten.

Link to comment
Share on other sites

15 hours ago, Bushranger said:

So, maybe not carbon at all, but vaporized tungsten.

It's a pretty continual vaporization process over the life of the bulb. If there was a LOT of tungsten residue, the bulb could have been an older style that used a vacuum inside the bulb instead of inert argon. Argon greatly reduces the loss of tungsten atoms to heat, but there's always a bit of evaporation going on. 

Link to comment
Share on other sites

Quote

Wikipedia

An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated to such a high temperature that it glows with visible light (incandescence). The filament is protected from oxidation with a glass or fused quartz bulb that is filled with inert gas or a vacuum. In a halogen lamp, filament evaporation is slowed by a chemical process that redeposits metal onto the filament, thereby extending its life.

https://en.wikipedia.org/wiki/Incandescent_light_bulb

 

Note the halogen scavengers against filament evaporation.

Link to comment
Share on other sites

Additionally in the bulb is a little bit iodine in, which reacts at the high temperature of the filament to WI2 . This deposit on the filamnet again and get decompsed again, it enhance the life time of the filament. But some of the vapour will deposit also on the glass it self. 

Edited by chenbeier
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.