X-Message-Number: 5099
From: 
Date: Fri, 3 Nov 1995 00:11: -0500
Subject: SCI. CRYONICS heat of fusion

Yvan Bozzonetti (#5086) says he can build a large hyperbaric chamber with
accessories, but has limited funds. Cryonics Institute might be interested in
funding such work--IF adequate investigation suggests a genuine and
substantial potential usefulness.

He also (#5088) says (if I understand him correctly) not to worry about
dissipating heat of fusion when high pressure in a frozen specimen is
suddenly released, because temperature decreases when pressure does. Is this
always right?

In a sudden (adiabatic or isentropic process, no heat transfer) without a
phase change, certainly an increase in pressure will ordinarily produce a
smaller volume and a higher temperature, while a reduction of pressure will
produce a larger volume and lower temperature; the work done by (or on) the
system divides into two parts, change in temperature and change in that part
of the internal energy not defined by temperature. (In an ideal gas, of
course, all of the change in internal energy depends on the temperature
change alone; but for solids and liquids  temperature is not the only
variable affecting internal energy.)  But suppose there IS a phase change?

Unfortunately, I have forgotten most of my thermodynamics, and I don't have
my books unpacked to remind me, but let's see if I can figure out what
happens when a slight overpressure is suddenly applied to a piece of pure ice
at exactly the equilibrium temperature (close to zero C). 

Since the  melting point at higher pressure is lower for ice (in contrast to
most substances), applying an overpressure should melt the ice. (Let's bypass
the question of whether a SMALL overpressure, requiring only a little added
energy, could melt a large block of ice in a short time.) If the ice melts,
it absorbed sufficient heat of fusion, which (it seems to me) could only have
been supplied by the work done in building the pressure. 

Going the other way, releasing the overpressure on the slightly below zero
container of water, the water freezes. Where does the heat of fusion go?
Since the process is presumably adiabatic, it can only go into the work done
by the system as it expands.

A possible problem with thought experiments and text-book thermodynamics is
that in real life we are not dealing with reversible processes nor
instantaneous changes; we have to look at things like shock waves, for
example; and with water ice at high pressures we may have to deal with
several crystalline forms. Even so, the little discussion above is
suggestive.

What it suggests--it seems to me late at night--is that maybe dissipation of
heat of fusion would not be a problem, even if we do not go far below zero.
Perhaps Mr. Bozzonetti is right, at least in his conclusion. Or maybe someone
will point out errors or omissions.

Robert Ettinger


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