ORNL-TM-2047.txt

LOCKHEED MARTIN ENEFGY RESEARCH LIBRARIES

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LEGAL NDTICE

This .report was prepaced as an account of Gevernment 5punsor@d work, Meither the Umted anflss,_

nor the Commissian, nor ony person acting on behslf af fhe Commission: .

A. Makes any warranty or representution, sxpressed. o implisd, with respect to the securacy,
completeness, or usefulness of the information contained in this repott, or fhaf the use of
any infocmation, apporatus, methad, or protess diselosed in th-is reporf may not infringe
privately ownnd rights; or . ' ' '

B. Assumes . any liabilities with respest fo the wse of, or for damages resulting from the use of
‘any informution, apparotus, method, or process d:sciased in this report.

As used In the above, peraon pcting on behalf of the Comm:ssnon includes aay employes or

com‘mctnr a! the Commission, or. employes of such conrracror to fl-ae extent that such employes

or contracior of the Commission, or amployee of such . confractor prepares, dissemindfes, or
providés access ta, any information pursuvaat to his employment or confract with the Commission,

or his employment with such contractor.

 

 
LIST OF FIGURES

Fig. 1. Captive Liquid Cell,
Fig. 2. Fluoride Reflection Cell Concept.

Fig. 3. The Optical Transmittance of Several Metal
and Alloy Films Produced on Quartz Plate as a Function of
Wave Length.

Fig. 4. (a) Absorption Spectrum of a Type I Diamond.
(The Dashed Curve was Measured at Room Temperature and the
Full Curve was Measured at 80°K.) (b) Absorption Spectrum
for a Type I Diamond in the Infrared, Measured at Room
Temperature.

Fig. 5. Absorption Speéctra of Type II Diamond Measured
at Room Temperature., Curves A and B are for Two Different
Species.

OCKHEED
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MARTIN ENERGY RESEARCH LIBRARES

 

 

 

 

 

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INTRODUCTION

Molten fluoride spectroscopy has been very limited in the
past because of the unavailability of an optical cell which is
both resistant to fluoride attack and yet reasonably trans-
parent throughout the spectral range of interest.

With the increasing ewmphasis in molten fluoride programs
such as the molten salt breeder reactor, MSBR, spectroscopy
on fluoride systems has become a must in clarifying many of
the reactions which are taking place.

At present, the chemistry of Nb, Mo, and Ru (to name a

1 Equilibrium reactions such as:

few) is uncertain.

Nb + 5UF, = NbF; + 5UF, (1)
have been considered and are of prime concern. Spectroscopy
could aid in the investigation of reactions such as:

Nb + NbF; = 1lower fluorides (2)
because the lower fluorides would display visible and ultra-
violet absorption spectra due to their d-=—d transitions. As
the work progressed, the action of UF, on NbY% and UF; on NDbF,
could be examined to verify if equilibrium reactions between
niobium fluorides and uranium fluorides do actually exist as

predicted by thermodynamic data.

Before embarking upon any general program of fluoride
spectroscopy, a survey of the tools available for the work
should be made. All equipment but the optical cells would
be standard. The container problem can be divided into two
parts -— that of finding a non-optical corrosion-resistant
material (e.g., some metals, graphite, etc.) and that of

using good optical materials (e.g., quartz).
Windowless Containers

Captive Liquid Cells:

 

One approach to the problem has been to abandon the
search for a satisfactory optical material and to employ
corrosion-resistant containers so that the light beam could
pass through the melt and yet avoid the container. The most
successful application of this has been developed by Young?
in which he used a captive~liquid cell (Figure 1). He has
also tried wire loops wetted with fluoride melts but found
- that the "cell" principle worked bhest.

The cell has several "keeper' holes above the optical
path which insure that the cell is filled to the same level
every time. Some of the difficulties encountered with the
cell were changes in the liquid meniscus in the optical path.
and, as a result, some noticeable uncertainty in the real
optical path length. Problems such as these limit the accuracy
to within 10% for absorption coefficient measurements. 1In
addition, it is impossible to bubble reagent gases through
this kind of a cell. The only alternative is to use a reagent
gas mixed in with the helium cover gas that is incorporated
in the cell design and to rely upon gas diffusion through the
melt for adequate mixing.

Some measurements require the equilibration of the melt
and a solid phase and intermittent scans throughout this
period (e.g., excess metal in contact with melt in which a
higher valence solute is reduced.) This technique is, of
course, impossible using such a windowless cell.

These devices must not be passed over lightly for they
have accomplished many of the tasks for which they were
designed. It is only the objective here to indicate the
characteristics of the various techniques so that the spectro-

scopist can select the best one for his particular problem.
Screens for Windows:

 

Screens are another application of non-optical materials
to the solution of the cell problem. A single screen wetted
with molten salt has been used previously for infrared absorp-

10 7This or the alternative of using two

tion spectroscopy.
screens as windows on a cell could be employed for visible-
U.V. work. However the concept is not without flaw because
the liquid will tend to flow down to the lowest part of the
screen. Then the meniscus and path length will vary notice-
ably between the upper and lower portions of the screen, de-
tracting from the precision of the optical measurements.

Reflection Cells:

 

Reflection cells have also been used for spectroscopy,!?’1l?

The concept, illustrated in Figure 2, is that of reflecting
the incident light from a mirror surface on the bottom of the
melt. The window can then be situated above the melt so that
only vapors are in contact with it. These vapors may even be
controlled by a purge gas if necessary. The window material
problem is then solved because almost any optical material
would be suitable,

In fastening a window to a metal container, it may prove
difficult to obtain a gas-tight seal which would withstand
temperature changes of several hundred degrees. However, it
might not be absolutely necessary to have that good a seal.

The most serious faults with such a system would be at
the gas-liquid and liquid-mirror interfaces. Insoluble im-
purities and films would collect at both, causing high degrees
of light scatter. There would also be the problem of corro-
sion at the liquid-mirror interface. Furthermore, accurate
path length measurements would be difficult, if not impossible,
to make.

This cell could be useful if some of the problems with
it were overcome, but there are conceivably easier alterna-

tives which should first be investigated.
Cell Stabilities As Predicted by Thermodynamics

It is sometimes useful to speculate on the corrosion
reactions which are occurring and to calculate the free
energies of reaction for the process. For a silica container
it has been proposed? that

810z () + 2BeFy 4y = SiF4(g) + zBeo(S) (3)
Using standard free energies of formation® for the various
constituents involved, a AG for the reaction can be calculated.
Assuming that all activities are unity, the equilibrium press
sure of SiF, can be calculated. These calculations should be
extended to compare AG values of other reactions with experi-
mental results, for instance:

Si0, + 4LiF = §SiF, + 2Li,0 (4)

Table I lists the values of AGY® which have been calculated
for fluoride salts following the general scheme of:

: 4 - : 4
SlOZ + E MFX = S].F4 + E'S’— MYOXY/Z (5)

Table 1
Standard free energies of reaction for fluoride salts in the

equilibrium of equation (5).

 

MFX AGOReaction(Kcal/mole)
TT00YK 800V 1000°
LiF 112.930 108.004 103.292
BeF, 29.118 20.418 13.380
7rF, -6.005 ~12.699 ~17.915
AlF, ~105.126 ~93.812 -80.218

Assuming that SiF, and the oxide are insoluble in the
melt and that the oxide formed, Myoxy/z does not react with

the silica.

 

*These may be reasonable assumptions for melts like BeF,
and ZrF,, but it is well known? that the alkali oxides have a
1 to 2% solubility in their fluorides 2t the temperature in
question.
The AG? values would then predict that the order of decreasing
stability in quartz is: LiF, BeF,, ZrF,, and AlF,;. Actual
experimental evidence, however, indicates that the order is:
2LiF-BeF,>LiF-NaF-KF(eutectic)>LiF-ZrF, (30%) which is inter-
preted as BeF,>LiF>ZrF,. This suggests that the prevailing
reactions are not as simple as those suggested by the above
equilibria.

The high degree of alkali oxide solubility would lead to
reactions such as:

Li,0 + Si0, = Li,SiO; (6)

Li,0 + 28i0, == Li,Si,0, (7)

at temperatures in the 400-800°C range. In support of this
hypothesis is the observation that the LiF-KF-NaF eutectic
melt in quartz is very clear for an hour or more at 454°C.
If the temperature is raised to 550°C, it is still clear.
But if the temperature is then dropped to 500°C the melt
becomes clouded by an insoluble species. It disappears when
the temperature is raised back to the maximum temperature.
Upon cooling, the quartz container indicates severe corrosion.
The stability of fluorides in contact with other materials
should also be estimated from thermodynamic data and using
similar qualifying assumptions. A few are listed in Table II
for common window materials. If Si0O, is satisfactory in accord-
ance with previous predictions, Al,0; (sapphire) would be much
better and MgO worse.
Mg0O has been used successfully for the LiF-KF-NaF eutec-
tic® to obtain the NiF, spectrum. It has also been tested
for use with 2LiF-BeF, melts and found to become coated with
a grey-white film which resisted all attempts to separate it
from the MgO crystal. The melt was analyzed for magnesium
and found to contain 0.5%. It was concluded that the opaque
coat on the MgO crystal was BeO.
Sapphire had been tested some years ago with the LiF-KF-
NaF eutectic.'?® It has traditionally been reported as "unsatis-

factory" as a result of these tests. However no comparison
Table II

Estimated AG® Values for Fluoride Melts in Contact with Al,0; and MgO Windows.

 

 

 

0
A Reaction

Reaction 600°K 800°K 1000°K

A1203(s) + 6L1F(1) v—2A1F3(g) + 3L120(1) 223.138 208.912 195.200

2(A1303)(s) + BZrF4(1) ¢=4A1F3(g) + BZrOZ(S) 86.111 55,715 26.773

MgO(S) + ZLlF(l) :=Mng(l) + leo(l) 35.692 35.744 35.510

~-6.714 ~-8.049 ~-9.447

MgO(S) + BEFZ(]_) ’«—‘Mng(l) + BeO(S)
10

has ever been attempted between gquartz and sapphire. Thermo-
dynamically, the latter would be predicted to be more stable.
Then if quartz proved to be of limited applicability, sapphire
would probably extend the range much further and permit
extensive use with Z2LiF-BeF,.

Wit hin the scope of crystalline windows, it is feasible
to use high-melting fluorides. Lithium and calcium fluorides
are two standard optical materials which are relatively in-
expensive. They might be useful if their solubilities in the
melt could be tolerated. At the melting point of a salt such
as 2LiF-BeF,, LiF would be in equilibrium with the melt and
hence not dissolve further. At higher temperatures, LiF
would dissoclve until it saturated the melt. Barring adverse
effects from such a change in composition and mass transfer of
LiF due to thermal gradients, it could prove to be a useful
material.

CaF, is higher melting and less soluble in salts like
ZLiF-BeF,. The melt should wet the crystal and thereby main-
tain optically fine conditions. The only impurity which would
be added to the system would be calcium ions. For some labora-
tory problems, they would not prove to be of any difficulty.
CaF, was tested with 2LiF-BeF, at 500°C and it behaved as
predicted. An analysis of the salt after a five-hour exposure
indicated that 4.53 wt. % of calcium had dissolved. During
the dissolution, the crystal remained perfectly clear. However,
upon cooling rapidly, it fractured into several pieces indicat-
ing one disadvantage of single crystals.

A1l of the materials mentioned above, quartz, sapphire,
LiF, CaF,, and MgO, possess excellent transmission character-
istics from 2400 to 200 mu and are relatively inexpensive. If
a metal cell were built to handle a variety of window materials,

the window could be changed as the task demanded.
11

Coated Quartz:

The list of satisfactory materials for fluoride spectro-

 

scopy is meager. FEach particular material has severe limita-
tions when applied to molten fluorides. To extend the useful-
ness of quartz, it might be possible to coat the Si0, surface
with a thin, unreactive layer. This coat must stick adequately,
be corrosion resistant, and be transparent in the optical range
of interest. These characteristics are most ideally obtained
with the noble metals. Ag, Au, and Pd. Because they have been
used previously for purposes related to these, their absorption
spectra are known and are shown in Figure 3.6 Silver is a well-
developed cogting material for glasses but it has a Fermi cut-
off at 3000 R which absorbs strongly. It has been studied by
Joos and Klopfer? who show the absorption at and below 3000 %
in detail.

The absorption curves of Figure 3 were obtained by coat-
ing quartz with an unmeasured thickness of metal until a satis-
factory transmission was obtained. For this reason two curves
for gold layers of different thickness appear in the figure.

It is seen that Pd and Pt-Ir (5%) offer the best coats as far
as uniform transmission through the UV visible range is cone
cerned.

The authors discuss problems associated with the non-
uniformity of the coat but do not mention which material gave
the best results. However, if the salts are non-wetting as
s ome fluorides have been observed to be, then the effect of
holes in the coat should not be noticed. Newertheless, the
coating should improve the corrosion resistance to some degree.
Then if quartz were found to be fairly resistant to attack by
particular fluorides i.e., 2LiF-BeF, — any extent of coverage
by an unreactive material would increase its usefulness.

Since it is more difficult to plate the inside of a container
than a plane surface,; a number of quartz windows could be

prepared and these mounted in the same metal cell used for
12

the crystalline windows mentioned above. Then a general pur-
pose cell for crystalline windows could be used for metal-
coated quartz windows as well,

Diamonds:

Although there are many alternatives to the partial
solution of the molten fluoride containment problem, the
material which fulfills the requirements best is diamond. It
possesses complete stability up to 700°C at which point it
begins to react with oxygen. However, in an inert atmosphere,
higher temperatures are conceivably available.

There are few reported applications of diamonds for use
as optical materials. One such case has been that for the I.R.
spectra of solids.!! Although the usage is not identical, it
indicates the feasibility of employing diamonds for spectro-
scopy.

The actual usage of diamonds for molten-fluoride cell
windows has been previously investigated by Cocks, et al and
is described in a report of limited circulation.!? It dis-
cusses the absorption spectroscopy of NaF-ZrF, doped with UF,.
Experience in the field of molten fluorides tells that this
melt is a most corrosive liquid, much more so than 2LiF-BeF, .
This article clearly demonstrates the feasibility of diamonds
as high-temperature cell windows.

The transmission properties are shown in Figures 4 and 5
for type I and II diamonds, respectively.® Type 11 is pre-
ferred but it is a rarer and hence more expensive variety.

One may then be forced to settle for type 1I.

The cell for diamond windows would necessarily be of
different design because smaller windows would have to be
used. It is assumed that a cell with windows of 1/8 to 1/4
inch diameter would suffice. Such a cell should be construct-

ed so as to protect the diamonds from the atmosphere.
13

CONCLUSIONS

If it is decided to develop a routine handling capability
of molten fluorides in the field of absorption spectroscopy,
then one is forced to accept the reality that none of the
above materials except perhaps diamond would be universally
applicable. It is assumed that fixed optical windows provide
the best system for accurate path length measurement and ease
of containment.

Therefore, quartz cuvettes offer the best starting point
for many reasons. After having thoroughly cleaned and degassed
the quartz at temperatures greater than 600°C, they can be
loaded with optically pure fluorides. "Optically pure'" would
describe in this case a fluoride which has been freed of alkali
oxide, HF, H,0, and then filtered to remove suspended part-
icles.* The MSRE salt, 2LiF-BeF,, handled in a drybox of <1
ppm H,O0 and stored in a degassed container behaves surprisingly
well in quartz under an atmosphere of helium. It shows no
sign of container corrosion after five hours at 500°C. Further
testing will describe its limits in detail.

It has been suggested that SiF, atmospheres would improve
the quartz stability? due to a displacement of the equilibrium
depicted in equation (3). This should be thoroughly tested
with reference samples under helium to determine their relative
stability. If it does behave favorably, SiF, would provide a
simple modification to the silica container. However, special
attention should be paid to avoid subtleties such as changes
in the spectra due to SiF, solubility, etc.

A metal-coated quartz container would then follow as a

further modification to the quartz system.

 

¥*Very satisfactory filtration of 2LiF-BeF,; has been
obtained with silica frits under helium cover gas.
14

To study very reactive fluorides such as Zr¥F,;, a metal
cell should be built to accommodate windows of metal-coated
quartz, sapphire, magnesium oxide, and fluoride single crys-
tals. Finally for ultimate stability, a diamond window cell
should be constructed.

The steps to the proposed solution are numerous and some
are perhaps not worth the limited gains which they might yield.
The sequence of quartz, coated-quartz, sapphire, and diamond
is preferred because it would save much time in developing
procedures. If diamond proves to be reasonably priced, coated
quartz and sapphire could be omitted from the sequence.

Therefore, quartz is favored from the standpoint of con-
venience and economy although it does have limited applic-
ability. Diamond should prove best in the long-term work
because it is most nearly ideal with réspect to corrosion

resistance and transparency.
15

REFERENCES

1. W. R. Grimes, "Chemical Research and Development of Molten-
Salt Breeder Reactors,'" ORNL-TM-1853, pp. 61-65.

2. J. P. Young, Anal. Chem. 36, 390 (1964).

 

3. JANAF Tables of Thermochemical Data.

4. J. H. Shaffer, ANP Quar. Progress Report, September 30,
1957, ORNL-2387, pp. 139-141.

5. J. P. Young and J. C. White, Anal. Chem. 32, 799 (1960).

 

6. Z, Nagy, Z. Samsoni, and K. Benko, Spectrochimica Acta,
19, 2057 (1963). |

 

7. G. Joos and A. Klopfer, Z. Physik. 138, 251 (1954).

8. R. Berman, ed., Physical Properties of Diamonds, Clacerdon
Press, Oxford, 1965, pp. 295-300.

 

9. C. E. L., Bamberger, J. P. Young, and C. F. Baes, Jr.,
"Containment of Molten Fluorides in Silica," Unpublished,
August, 1967,

10. J. Grenberg and L. J. Hallgren, Rev. Sci. Instr. 31, 44
(1960). T

 

11. E. R. Lippincott et al,, Anal. Chem. 33, 137 (1961).

 

 

12. G. G. Cocks, J. B. Schroder, and C. M. Schwartz, "The
Spectroscopy of Fused Salts," Progress Relating to ANP
Applications, February — April, 1957, Battelle Memorial
Institute Report No. BMI — 1185, p. 13.

 

13. J. P. Young, private communication.
16

ORNL- LR-DWG. 79851

 

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17

ORNL-DWG 67-11149
LIGHT PATH

QUARTZ * *

W —— g F B

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Fig. 2. Fluoride Reflection Cell Concept.
18

ORNL-DWG 67-11146

 

 

 

 

 

 

 

 
 
 

 

 

 

 

 

 

 

 

 

 

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2200 4000 6000 8000 10,000

A, WAVELENGTH (A)

Fig. 3. The Optical Transmittance of Several Metal and Alloy Films
Produced on Quartz Plate as a Function of Wave Length.
19

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25

Fig. 4. (a) Absorption Spectrum of a Type I Diamond. (The Dashed
Curve was Measured at Room Temperature and the Full Curve was Measured
at 80°K.) (b) Absorption Spectrum for a Type I Diamond in the Infrared,
Measured at Room Temperature.
ABSORPTION COEFFICIENT (ecm™Y)

Temperature.

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WAVELENGTH (10734)

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Fig. 5. Absorption Spectra of Type II Diamond Measured at Room

Curves A and B are for Two Different Species.
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Distribution

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67.

68-82.

83.

C. J. Barton

D. M. Moulton

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