Re: [Phys-l] 50 year incandescent bulb

[Joseph Bellina <jbellina@saintmarys.edu>]

Here is another possible explanation for the failure of an  
incandescent bulb.

The bulb filament isn't pure tungsten but an alloy to increase its  
ductility for drawing and to prevent the development of a bamboo  
looking structure in the filament.  With rapid heating and cooling,  
pure W polycrystalline wire recrystallizes to a  series of  
cylindrical single crystals with the planes of grain boundaries  
perpendicular to the wire, hence the name bamboo structure.  These  
boundaries are relatively weak and so have tendency to slip, leading  
to a local thinning of the wire and rapid failure.  Most of the  
alloys added to the wire have a lower vapor pressure than W, as I  
recall Rhenium is an exception, and so will slowly evaporate from the  
surface, to be replaced by material migrating from the bulk to the  
surface.  Over time the minority element concentration in the wire  
will decrease and the likelihood of bamboo formation increase.  That  
could be the mechanism for failure rather than simple evaporation of W.

joe

Joseph J. Bellina, Jr. Ph.D.
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556

On May 2, 2007, at 12:22 AM, Larry Woolf wrote:

> ________________________________
>
> From: Jason Alferness
> Sent: Tue 5/1/2007 8:59 PM
> Subject: Re: [Phys-l] 50 year incandescent bulb
> But wouldn't a beefier filament be an option?  Or is that also a
> resistivity compromise for ohmic heating?  If so, how bout makign some
> dirtier (presumably more resistive) W or perhaps some other metal  
> with a
> lower evaporation at the temp of interest?  (perhaps not, as W  
> seems to be
> the metal of choice)
> I'm sure at some point, the inner surface of the bulb would be pretty
> opaque, but that wouldn't seem to be a probelm for awhile since I  
> don't
> recall any noticable film on any bulb I've ever seen... but maybe  
> I'm just
> not lookign close enough..
> =================================================
> A thicker filament, with the same length, would have lower  
> resistance. Since bulbs operate at constant voltage, the power  
> dissipated would be higher (P=V^2/R), so the filament would be  
> hotter. Therefore the vaporization rate of the filament would  
> increase, as would the light output.
> If you made the metal used for the filament "dirtier," it would  
> likely lose ductility and not be able to be drawn into fine wire  
> and formed into a coiled coil. Also, tungsten is the most  
> refractory of the ductile metals.
> Surfaces of lamp housings do darken due to the tungsten vapor  
> condensing on the relatively cool glass housing.
>
> For more details about incandescent bulbs, experiments, and  
> references, you may want to look at the module I wrote:
> <http://www.sci-ed-ga.org/modules/materialscience/light/index.html>
>
> Larry Woolf
> General Atomics
>
>
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