Re: rapid boiling at the very end with liquid nitrogen

[Roger Haar <haar@PHYSICS.ARIZONA.EDU>]

Hi,
        As several other have said the rapid boiling at
the end of the cooling is associated with the
collapse of the gas envelope around the sample.

        Once we tried measuring the heat capacity of an
aluminum block as a function of temperature to see
if it was a possible experiment for our students.
We were looking to reproduce the Debye curve.

        We had a thermocouple stuck into the AL block and
the LN2 + block + container were on a digital
balance ( Delta Heat is proportional to Delta
mass).  Every 20 seconds, one person would call
out, another would record the temperature and
third the reading on the balance.  Things were
going along fine for 7 or 8 minutes, until at
about 150 K, this rapid boiling kicked in.  The
next temperature reading was about 85K.  We all
were just staring blankly at each other.

        There is a double whammy trying to get this last
measurement in that in addition to the enhanced
heat transfer between the block and the LN2, the
heat capacity of the AL block was decreasing
quickly.

        The experiment was made usable for the students
by putting the sample into a baggie the was tied
tightly.   Now if only we could solve the problem
of digital balances evaporating.

Thanks,
Roger Haar  U of AZ

*************************************************************************************
"Michael J. Moloney" wrote:
> My students and I have seen metal samples cooled to liquid nitrogen
> temperature, and for all metals we tried (copper, aluminum, iron, and
> lead) we saw a rapid boiling at the end, when we think the metal had
> reached the same temperature as the liquid.
>
> I've played around with the ideas of heat exchange, heat capacity and
> thermal conductivity but have no satisfactory explanation for this
> phenomenon.
>
> Any enlightenment would be greatly appreciated.
>
> Mike
>
> --
> Mike Moloney, Physics & Optical Engineering Department
> Rose-Hulman Inst of Tech, 812 877 8302
> moloney@rose-hulman.edu   http://www.rose-hulman.edu/~moloney