Frozen metal spicules?

I am involved in the design and test of low-pressure, mercury vapor lamps
for
the air and water purification industry. Many lamps contain an
indium/mercury
amalgam to extend the high-temperature operational range.
After several on/off cycles we notice spicules/spikes/dendritic structures
growing up from
the applied amalgam. They don't affect the lamp's operation since they melt
back when the lamp
is on, but what actually causes them? The closest analogue I have yet found
is the growth of
ice spicules from ice made out of distilled water. Is this an analogous
process? Is there any
published work you can point me to. In this case Google wasn't to much of a
friend.
WEL
Reply to
lubarsky
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In whiskers? its a well known feature of Sn, in stressed electrodeposits. It wouldn't be so far out for In to have a similar behavior.
Reply to
beav
">>WEL
No. Not whiskers, they are rather thick. These things look like stalagmites. Or in some cases tiny pine trees. Freezing pure water in an ice cube tray will sometimes produce the same sort of thing. .
WEL
Reply to
lubarsky
It seems that the product is caused by the evaporation at high temperature and the subsequent cooling process. Because at low temperatures, the vapor pressure is small, so the extra atoms in the vapor phase have to be back to the surface of the applied amalgam.
lubarsky wrote:
Reply to
gtg116t
hmmmm. i'm stumped. got pictures?
Reply to
beav
Somehow I don't picture the evaporation and re-absorption of mercury atoms producing the thick spicules that rise above the general surface of the amalgam.
The most similar things I have found in any literature are the ice spicules that rise up occasionally from ice cubes. I had some beauties a few weeks back in my freezer which caused me to look them up on the internet.
Nice regular spicules of amalgam come along rarely, our photographs weren't very good. Now my camera technique is better and all I have seen for a long while are lumps which look much like collections of balls.
WEL
Reply to
lubarsky
Yep.
Not very spectacular ones, just lumps and bumps. Where would I send them?
WEL
Reply to
lubarsky
to my email address above or even just post them to the group. this is text, and unmoderated, so you may get a few whiners about posting jpgs to a text group. so what? you'd get more people looking at it.
if you send them to my emailaddress above, i'll post them here if you can't.
Reply to
beav
got your email. interesting pics.
the nearest thing that i've ever seen to things like that are when i sputtered bulk metals.
In and Hg have such high vapor pressure "blowing" it off the bulk piece, especially in a vacuum, shouldn't be too difficult. does it get red hot?
i'll post the pics here for everyone else's review.
Reply to
beav
The temperature is probably less than 80 C. The purpose of he amalgam is to try to hold the Hg vapor pressure to under 10 microns at an elevated (for a low-pressure lamp) wall temperature. I would have included a photo of the ice spikes found on the net but without contacting the owners it would be bad form.
WEL
Reply to
lubarsky
formatting link

if the link works, it'll show the first page of an abstracted article from 1962 that lists liquidus and solidus for many high % Hg Hg/In alloys and suggests that this stuff is very low melting.
does it melt in your hand?
you may in fact be sputtering , even at 80C.
i never really considered it before, but could you consider it physically "sputtering" if you took something like O2 and did it from the frozen bulk material at cryo temps? hmmm.
Reply to
beav

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