From: neufeld@helios.physics.utoronto.ca (Christopher Neufeld)
Subject: Re: Metals in liquid Nitrogen !!
Organization: University of Toronto Physics/Astronomy/CITA
Date: Fri, 11 Dec 1992 18:34:21 GMT

In article <1992Dec11.171454.3158@magnus.acs.ohio-state.edu> kpearce@magnus.acs.ohio-state.edu (BULLDAWG) writes:
>In article <10DEC199219275429@trentu.ca> lchardon@trentu.ca (PETIT PAPA NOEL.) writes:
>>
>>	I'm sorry if this has been discussed here before, but I'm quite 
>>puzzled by the following:
>>
>>Sure enough, somebody pulled out a penny and dropped it in a 
>>full styrofoam cup. What happens is the following:
>>The nitrogen will boil for some time, like it boils when you drop anything 
>>in it, 
>>but then suddenly it will boil really hard, about 3 times more 
>>violently than it did before, for about half a second, and then it will 
>>just gently bubble away (When the metal is in thermal equilibrium with it, 
>>i guess...).
>
>But at this point, the metal makes a phase transition from one kind of
>metal crystal to another (bcc to alpha).
>
   I doubt that. It's very quick in the case of copper in liquid
nitrogen. You need to move atoms around the crystalline lattice on the
time scale of seconds, and at quite low temperatures. I'd expect the
deformation of the lattice to powder many metals.
   Also, the later bubbling is much stronger than even the room
temperature copper produces when first dropped into the liquid nitrogen.
It seems that this is consistent only with your theory if the phase
transition heats the copper up to substantially more than room
temperature. I'm trying to imagine a phase diagram where phase A is
stable at T1, and at T2 it goes to phase B, liberating enough energy to
heat it to T3 > T1. It seems unlikely.

>Whenever a substance under goes a phase trnsition it gives up heat.
>
   Demonstrably false. Ice doesn't give up heat when changing phase to
water.

>>
>>	WHY ???????????????
>
>
>>	WHY JUST METALS ???
>
>Well, just about anything will work, but metals have phase transitions at 
>lower temperatures so that most of the bubbling from cooling off is gone.
>
   I disagree.

   Here's what I believe, and I put copper sample holders into liquid
nitrogen sometimes several times a day, always seeing this effect. As
another poster remarked, when the copper is first put into the liquid
nitrogen it forms a vapour barrier which prevents the liquid nitrogen
from coming into good mechanical contact with the copper. To do this it
need only balance the hydrostatic pressure of the liquid nitrogen, a
couple of percent of atmospheric pressure for common laboratory dewar
depths (unlike your counterexample of trying, I believe, to levitate an
iron block on a drop of nitrogen). The copper cools until the liquid
nitrogen can come touch it. At this point the heat transfer rate goes way
up (as evidenced by the thermocouple in my experiment (which does not,
BTW, show some huge heat spike from the latent heat of a phase
transition)). Nitrogen boils furiously for a while, and with my
experiment, in which the copper isn't fully immersed in the liquid
nitrogen, you can see that the liquid is now touching the metal, where
before it obviously wasn't.
   Why does this happen only with metals? In order to be obvious there
has to be a long time between immersion of the object in the nitrogen and
the time when the surface of the object reaches a temperature which
allows nitrogen to touch it. This implies a high heat capacity (not
really true of metals) and/or a high thermal conductivity (very true of
metals). The high thermal conductivity of the copper means that the whole
penny has to be cooled down before the nitrogen can touch it, rather than
just a thin surface layer, as would be the case for styrofoam or wood.


-- 
 Christopher Neufeld....Just a graduate student  | Entropy isn't what it
 neufeld@helios.physics.utoronto.ca    Ad astra  | used to be.
 utzoo.utoronto.ca!generic!cneufeld              |
 "Don't edit reality for the sake of simplicity" |
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