NetNews Usenet Archive 1992 #30/NN_1992_30.iso/spool/sci/physics/20899

Text File | 1992-12-11 | 6.7 KB | 145 lines

Newsgroups: sci.physics
Path: sparky!uunet!caen!zaphod.mps.ohio-state.edu!magnus.acs.ohio-state.edu!kpearce
From: [e-mail] (BULLDAWG)
Subject: Re: Metals in liquid Nitrogen !!
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Date: Fri, 11 Dec 1992 20:03:21 GMT
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In article <[e-mail]> [e-mail] (Christopher Neufeld) writes:
>In article <[e-mail]> [e-mail] (BULLDAWG) writes:
>>In article <[e-mail]> [e-mail] (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.

Sorry, but it is true.  Phase transitions can (and are observed in labs
down the hall from mine) and do occur in microseconds. The rearrangements that
you refer to are on the order of _tenths_ of _angstroms_.

Go back to fundimental chemistry.  Phase changes are generally exothermic.


>
>>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.

You have missed the point.  Water going to ice gives up heat (delta H of
freezing).  A phase change in copper will give up heat (going to a
thermodynamically more stable form at lower temperature).

To assert that the copper must _heat_ _up_ (become hotter) because of the
phase change (and thus be impossible) is strictly ignorant of phase
change processes that have been well characterized for over 100 years!
When a phase change occurs, heat is liberated (or absorbed) in exactly
the amount that is being removed (or added) to the sytem until the entire
system completes the phase change.  This is the origin of the so-called
phase transition temperature halt.  (note that an ice-water mixture is always
pretty close to 0 C at equilibrium regardless of the proportion of water/ice).

Anyway, I can go on until I am blue in the face about thermodynamics.  Go pick
up a book on it, _Physical_Chemistry_ by Arfkin is a good start.

>
>>>
>>>    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,

But at this point you would be right back to "levitating" the LN2 away from
the object and you would see according to your assertion, cycles of fast
bubbling, slow bubbling, fast, slow, etc which is _not_ seen.

>BTW, show some huge heat spike from the latent heat of a phase
>transition)).

As above, you would not see a heat spike, you would see a slowing down of the
cooling momentarilly (with the new fast bubbles).
In fact, since the situation is far from equilibrium,
you probally wouldn't see the eutectic halt unless you had a _very_ fast
thermocouple and meter and indeed I would be hard pressed to say how you could
seperate the fast cooling from the LN2 from the eutectic halt that adds some
heat to the over all process.

> 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.

I have done the same experiment myself in the lab (today, I just happen to
use LN2 as a coolant for my experiments), and I only see the situation
you describe when the sample is near LN2 temparture.  But if you look
carefully, you will see that _between_ where the bubbles are breaking the
surface, the LN2 is touching the surface.

>   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.
>

Ken Pearce,
just a grad student in Physical Chemistry  }:)


>
>--
> Christopher Neufeld....Just a graduate student  | Entropy isn't what it
> [e-mail]    Ad astra  | used to be.
> utzoo.utoronto.ca!generic!cneufeld              |
> "Don't edit reality for the sake of simplicity" |