ORNL-2106 - the Molten Salt Energy Technologies Web Site

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2.3. PHYSICAL PROPERTIES OF MOLTEN MATERIALS
P R E SS UR E -C OM P OS Ill ON -1 EM P E R At UR E
RELATIONS FOR THE SYSTEMS
KF-ZrF, AND RbF-ZrF,
S. Cantor
Among the important guiding principles in the
search for improved or modified fuel mixtures are
predictions regarding the changes in properties
that will occur as a result of the substitution of
one species of ion for another in an otherwise
similar melt. To a very considerable extent these
predictions can be based on a determination of
how the activity coefficients of the constituents
change with composition. One of the easiest
activities to measure, and a fairly important one
to know, is the activity of ZrF, in mixtures with
alkali fluorides at compositions such that the
vapor is essentially pure ZrF,. Mixtures con-
taining 45 mole % or more ZrF, produce a vapor
which, for practical purposes, is pure ZrF,, and
the activity of ZrF, is readily obtained from the
toto t vapor pressure, Accordingly, measurements
are being made of the pertinent vapor pressures
in alkali fluoride-ZrF, systems. It has been
found that the volatility of the ZrF, varies in-
versely with the size of the alkali cation. This
is a consequence of the more pronounced com-
plexing of ZrF, in the presence of the alkali
cations, which have less attraction for fluoride
ions. Lithium fluoride represents the case of an
outstandingly strong attraction of a cation for
ratio of the lithium i
vapor pressures fr
C. J. Barton, ANP Q
P Quar. Prog. Rep.
,K. A. Sense et al.. Vapor Pressures of the Sodium
Fluoride-Zirconium Fluoride System and Derived Jnfor-
mation, BMI-1064 (Jan. 9, 1956).
F. F. Blankenship G. M. Watson
E. R. Van Artsdalen
PERIOD ENDING JUNE 10, 7956
termined by the Rodebush-Dixon method that has
been used for other ZrF mixtures at ORNL.
This method depends on Ae “valve” action of
the salt vapor above a liquid held at constant
temperature; the valve action is manifested by a
differential manometer which registers the re-
sistance to flow of inert gas through a region
occupied by the refluxing salt vapor. The
“valve” becomes suddenly much more definite
in effect when the inert gas pressure is lowered
to a value corresponding to the salt vapor pressure.
An absolute manometer is used to measure the
pressure at which this occurs.
The System KF-ZrF,
The vapor pressure experiments initiated pre-
viously on the system KF-ZrF were continued.’
The Rodebush-Dixon method,46 previously de-
scri bed,S,7 was employed.
In the present study the temperature determi-
nations were carried out potentiometrically by
using calibrated platinum-platinum-rhodium ther-
mocouples. Pure ZrF, was prepared by subliming
hafnium-free ZrF, at 72OOC under a vacuum.
Translucent crystals were then hdnd-picked from
the sublimate. Spectrochemical analyses for ten
possible metallic contaminants showed, in every
case, less of the impurity than of the available
standard. Potassium fluoride was purified by
heating the reagent-grade product to 50°C above
the melting point, cooling it slowly, and selecting
clear fragments of the fused salt.
6W. H. Rodenbush and A. L. Dixon, Phys. Rev. 26,
851 (1925).
7R. E. Moore and C. J. Barton, ANP Quar. Prog. Rep.
Sept. 10, 1951, ORNL-1154, p 136.
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