Thallous halide materials for use in cryogenic applications

Thallous halides, either alone or in combination with other ceramic materials, are used in cryogenic applications such as heat exchange material for the regenerator section of a closed-cycle cryogenic refrigeration section, as stabilizing coatings for superconducting wires, and as dielectric insulating materials. The thallous halides possess unusually large specific heats at low temperatures, have large thermal conductivities, are nonmagnetic, and are nonconductors of electricity. They can be formed into a variety of shapes such as spheres, bars, rods, or the like and can be coated onto substrates.

This invention relates to nonmagnetic, dielectric compositions of matter 
which have large specific heats at low temperatures and their use in 
low-temperature, cryogenic applications. 
The development and use of low temperature processes has greatly expanded 
in recent years. The space program has spurred action in liquefaction of 
many different gases including nitrogen, oxygen, helium, and hydrogen. 
Additionally, the liquefaction of natural gas for large-scale ship 
transport has been greatly increased as demands for energy in this country 
have grown. 
In many cryogenic applications, the materials used must have large specific 
heats at the low operating temperatures encountered. For example, the 
solid packing material used as a heat exchange medium in the regenerator 
section of closed-cycle stirling-type refrigerators must not only be 
mechanically stable, but also must have a high specific heat at low 
temperatures to match closely the specific heat of the refrigerant being 
utilized for maximum operating efficiency. This is particularly true when 
helium gas is the refrigerant because at temperatures below 20.degree. K., 
its specific heat becomes very large. A specific heat mismatch between the 
solid packing material and refrigerant results in a loss of efficiency. 
Other cryogenic applications also require materials with a large 
low-temperature specific heat. The specific heats of all of the materials 
used as superconducting wires are quite small at low temperatures. 
Therefore, the application of a coating of a material with a large 
specific heat at low temperatures will result in improved thermal 
stability of the superconductor. Still other cryogenic applications may 
require materials with special combinations of properties. These 
properties include a large thermal conductivity at low temperatures, 
mechanical stability, resistance to cyclic fatigue or cryogenic 
embrittlement, a nonmagnetic nature, and a nonconductor of electricity. 
A large number of prior art materials have one or more of the above 
properties. These include lead (Pb) which is nonmagnetic and has a large 
low-temperature specific heat and neodymiun (Nd), europium selenide 
(EuSe), and alloys of erbium, gadolinium, and rhodium (Er-Gd-Rh). However, 
all of these materials are electrical conductors; in fact, lead is a 
superconductor at low temperatures. 
Even though lead is the most widely used material, it suffers from several 
shortcomings. It is a relatively soft material with poor creep and impact 
fatigue properties. In use in the regenerator section of cryogenic cooling 
systems it tends to degrade into a powder because of cyclic fatigue, and 
cryogenic embrittlement. Even when hardened by the addition of small 
amounts (up to 4%) of antimony and made into small spheres, longitudinal 
thermal conductance between spheres and the breakdown of the spheres into 
powder shortens the useful life of lead as a heat exchange material in a 
cryogenic regenerator. 
Thus, although some of the materials used by the prior art have one or more 
of the desirable properties, to my knowledge prior to my invention there 
were no nonmagnetic dielectric insulating materials having large 
low-temperature specific heats in use in the art. Accordingly, the need 
exists in the art for an improved material for use in cryogenic 
applications which has a large low-temperature specific heat as well as 
mechanical stability. Additionally, there is a need for a material which 
combines the above properties with those of being nonmagnetic and a 
nonconductor of electricity which can be adapted to a wider range of 
utilities at cryogenic temperatures. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, thallous halides, either alone or 
combined with other high specific heat ceramics such as those described in 
my copending application Ser. No. 29,554, filed Apr. 13, 1979, and 
entitled "Cryogenic Ceramic and Apparatus", can be utilized in a variety 
of cryogenic applications. The thallous halides are pure, singlephase, 
polycrystalline materials made by processes known in the art. They can 
easily be made 100% dense and are somewhat ductile in character. 
It has been found that the thallous halides possess a combination of 
properties which render them admirably suitable for use as heat exchange 
material in the regenerator section of cryogenic refrigerating systems, as 
stabilizing coatings for superconducting transmission lines, and as 
dielectric insulating materials. The thallous halides have large heat 
capacities which compare favorably with those of lead at low temperatures. 
They have thermal conductivities of approximately half that of lead at 
temperatures between 7.degree. and 15.degree. C. and closely approach the 
thermal conductivity of lead below 7.degree. K. Additionally, the thallous 
halides have good mechanical stability, a nonmagnetic nature, and are 
nonconductors of electricity. They may be used in cryogenic devices as 
powders, spheres, bars, or plates, or may be coated directly onto other 
surfaces. If formed into spheres, the spheres should have a diameter 
preferably of from about 0.001 to 0.015 inches. 
Accordingly, it is an object of the present invention to provide a class of 
materials useful in low temperature applications and possessing a 
combination of properties not attainable in the prior art and to provide 
methods for using such materials in cryogenic processes. These and other 
objects and advantages of the invention will become apparent from the 
following description, the accompanying drawings, and the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The thallous halides of the present invention and their methods of 
preparation are per se known. The thallous fluorides, chlorides, bromides, 
and iodides are available as crystalline materials and have melting points 
of from 327.degree. C. to 430.degree. C. Because of their ductility and 
flexibility, they can easily be densified and formed into spheres or other 
shapes utilizing standard ceramic methods. Individual thallous halide 
compounds or mixtures of them may be formed into structural shapes by 
pressing finely divided powders in a die at room temperature and then 
firing at sintering temperatures. Well known fugitive organic binders may 
be added to the powders to aid in the plastic formability of the 
compositions. Such organic binders are oxidized at the sintering 
temperatures utilized and form no part of the final structure. 
Additionally, the thallous halides of the present invention may be hardened 
by the addition of effective amounts (i.e., less than about 10% by weight) 
of a valency controlled dopant material. Such dopants and their hardening 
effects on alkali halides are known. Examples of such dopants are silver 
chloride, cesium iodide, and tin chloride. 
In an alterative embodiment, the thallous halides of the present invention 
may be mixed with the family of large low-temperature specific heat 
ceramic materials disclosed in my copending U.S. application Ser. No. 
29,554, filed Apr. 13, 1979, and entitled "Cryogenic Ceramic and 
Apparatus." The ceramic materials there disclosed consist of crystalline 
metal oxides defined by one of the following molar formulas: 
1. AB.sub.2 O.sub.4, where A is selected from one or more of Group 2B metal 
ions alone or in combination with one or more of other divalent metal ions 
where at least about 90 mole % of A is a Group 2B metal ion or ions, and B 
is either chromium or chromium plus one or more other trivalent metal ions 
where at least about 90 mole % of B is chromium; 
2. AB.sub.2 O.sub.6, where A is selected from one or both of manganese or 
nickel ions alone or in combination with one or more other divalent metal 
ions, where at least 90 mole % of A is manganese or nickel and B is 
selected from one or both of niobium or tantalum ions; and 
3. A.sub.2 BCO.sub.6, where A is selected from lead ion alone or in 
combination with one or more other divalent metal ions where at least 
about 90 mole % of A is lead ion, B is either gadolinium or manganese 
alone or in combination with one or more other trivalent metal ions where 
at least about 90 mole % of B is gadolinium or manganese ion, and C is 
selected from one or both of niobium and tantalum ions. 
This family of ceramics has been demonstrated to be dielectric insulators 
having values of specific heat at least as great as that of lead at 
temperatures below 15.degree. K. These ceramics can be easily fabricated 
as taught in the above copending application by mixing powders of the 
oxides of the metals in proper molar proportions and then calcining and 
sintering at temperatures in the range of from 900.degree. to 1500.degree. 
C. 
Referring now to FIG. 1, it can be seen that the specific heats of the 
thallous halides are equal to or in excess of the literature reported 
values for lead. The specific heats shown in the Figures are plotted on a 
volumetric basis which is the most demanding basis of comparison with lead 
because of its extremely high density. The data for lead shown in FIGS. 1 
and 2 was estimated by using the following specific heat expression for 
metals: 
EQU C=C.sub.D (.theta..sub.D / T)+.delta.T 
where C.sub.D is the Debye function, .theta..sub.D is the Debye 
temperature, and .delta. is the coefficient of electronic contribution. 
Values for .theta..sub.D of 108.degree. K and .delta. of 
3.36.times.10.sup.-3 J.multidot.mole.sup.-1 .multidot.K.sup.-2 were taken 
from Gopal, Specific Heats at Low Temperatures, p.63 (Plenum Press, 1965). 
As illustrated in FIG. 2, solid solutions of mixtures of thallous halides 
also possess large specific heat values. The specific heat of a solid 
solution of 60 mole % thallous chloride and 40 mole % thallous bromide is 
shown to have a specific heat in excess of that of lead and temperatures 
below above 10.degree. K. 
The thallous halides also have high thermal conductivities at low 
temperatures. FIG. 3 illustrates the comparative thermal conductivities of 
thallous chloride, lead, and copper at temperatures below about 15.degree. 
K. As can be seen, although the thermal conductivity of thallous chloride 
is not as large as that of lead, it is at least 50% of value for lead over 
the range illustrated and approaches the value for lead at temperatures 
below 5.degree. K. Thermal conductivity data for both lead and copper were 
taken from Childs et al, NBS Monograph 131, U.S. Department of Commerce 
(September, 1973). 
Referring now to FIG. 4, the volumetric specific heats of four exemplary 
ceramic compositions from my above-mentioned copending application Ser. 
No. 29,554 are shown in comparison with that of lead. The ceramic 
composition labeled A is MnNb.sub.2 O.sub.6, composition B is NiNb.sub.2 
O.sub.6, Composition C is Cd.sub.2 Cr.sub.3 NbO.sub.9, and D is Zn.sub.2 
Cr.sub.3 NbO.sub.9. As can be seen, each individual ceramic composition 
has a maximum specific heat at a slightly different temperature. For 
example, the specific heat of ceramic C has a maximum at about 8.degree. K 
of about 0.7 Joules per cubic centimeter per degree Kelvin. 
As shown in FIG. 5, the volumetric specific heats of thallous chloride and 
ceramic C are significantly greater than those reported by Hartwig, Paper 
U-9, Cryogenic Engineering Conference, Queens' University, Kingston, 
Ontario (1975), for various unfilled epoxy resins. As illustrated in FIG. 
5, the open circles signify data from an epoxy resin identified at 
CY221-HY979 by Hartwig; closed circles, X183/2476-HY905; and crosses, 
CY221-HY956. As shown, at 8.degree. K., the specific heat of thallous 
chloride is 4.4 times larger than that of epoxy resins and the specific 
heat of ceramic C is 28 times larger on a volumetric basis. 
These properties illustrate the significant advantages which are obtained 
by using thallous halides alone or in a composite solid solution mixture 
with the ceramics disclosed in my copending application Ser. No. 29,554. 
This is because the windings most often utilized to insulate 
superconducting wires presently are epoxy resins such as Araldite epoxy 
resin available from General Electric Co., Schenectady, N.Y. The materials 
of the present invention not only having much greater specific heats at 
low temperatures than do the presently utilized epoxy resins, they 
additionally possess much greater dielectric constants, thermal 
conductivities, and enthalpies which will serve to provide better thermal 
damping of temperature fluctuations, better electrical insulation, and 
improved enthalpy stabilization of the superconducting wires. The thallous 
halide materials of the present invention can also be combined with such 
epoxy resins in forming insulation for superconducting wires. 
The dielectric constants of the thallous halides and ceramic C are 
unusually large, approximately 37 for thallous chloride and approximately 
300 for ceramic C. By comparison, the dielectric constants of glasses and 
epoxies are in the range of from 3 to 5. Moreover, the enthalpies of both 
the thallous halides and the ceramics disclosed in my copending 
application Ser. No. 29,554 are substantially greater than the presently 
used epoxy resins. Examplary enthalpy data relative to 4.degree. K. for 
thallous chloride and ceramic C are reported in Table I below which 
illustrate the significant differences relative to an Araldite epoxy 
resin. 
TABLE I 
______________________________________ 
Enthalpy Improvements Over Araldite Epoxy Resin 
Enthalpy Ratios to Epoxy 
Temperature Thallous 
(.degree.K.) Chloride Ceramic C 
______________________________________ 
6 6.7 8.2 
7 6.5 9.0 
8 6.3 17.7 
9 6.2 16.9 
______________________________________ 
As can be seen, the enthalpies of thallous chloride vary from 6.2 to 6.7 
times greater than that of an Araldite epoxy resin at typical operating 
temperatures for superconducting wires. The enthalpies of Ceramic C are 
even greater. 
The excellent low-temperature specific heat and thermal conductivity 
properties of the thallous halides and the unusually high dielectric 
constants and enthalpies for the family of ceramic materials reported in 
my copending application Ser. No. 29,554 can be combined advantageously to 
provide a series of materials having optimum properties for operation at a 
given temperature. Windings for superconducting wires made of composites 
of the thallous halide materials and the ceramics can be made, for 
example, by spraying a superconducting wire with the desired composite 
mixed with a fugitive organic binding material which is subsequently burnt 
out. Alternatively, the wire may be dipped in a mixture of the composite 
and organic binder. In still another alternative method, the composite may 
be vacuum deposited on the surface of the wire using known techniques. The 
final thickness of the coating may be 2 to 50 times the diameter of the 
wire. 
Referring now to FIG. 6, another important utility for the thallows halide 
materials of the present invention is illustrated. As shown in FIG. 6, the 
major components of a closed-cycle cryogenic refrigeration system 10, 
having a compressor section 12, a regenerator section 14, an expander 
section 16, and a refrigeration section 18. When a refrigerant fluid 
undergoes compression in compressor section 12, heat energy is generated 
and dissipated to an adjoining heat sink (either atmosphere or previous 
refrigeration section). The compressed fluid refrigerant is then passed 
through regenerator 14 where it is cooled by giving up heat to the heat 
exchange material packed therein. The chilled refrigerant is then expanded 
while doing some work in expander section 16 and is further chilled. It is 
then circulated through the refrigeration section 18 where it cools a 
thermal load and maintains the load at a desired service temperature. The 
refrigerant is then passed back through regenerator 14 and cools the heat 
exchange material therein by taking up the heat energy stored there from 
the passage of the compressed refrigerant. This cycle is repeated 
continuously during operation. 
Although lead or a lead-antimony alloy have been the most commonly used 
heat exchange materials in such regenerators, lead suffers from many 
disadvantages. Spheres of lead tend to degrade after repeated cycling at 
low temperatures which affects their performance. There also tends to be 
bonding between the spheres which increases axial thermal conductance. 
Moreover, when helium gas is used as the refrigerant, its large specific 
heat at low temperatures causes a mismatch with the specific heat of lead 
and prevents optimum heat exchange from occurring. 
The thallous halides of the present invention have specific heats greater 
than lead at temperatures below 20.degree. K. Additionally, they can be 
formed easily into spheres or other shapes such as bars, rods, honeycombs, 
or the like. When formed into spheres, they preferably have a diameter of 
between about 0.001 and 0.015 inches. Moreover, because they are 
dielectric materials, they can be used for the complete construction of 
the regenerator section of a closed-cycle refrigeration system. In 
combination with selected ceramic materials disclosed in my copending 
application Ser. No. 29,554, the thallous halides can provide unusually 
high specific heats which can be maximized for almost any desired 
operating temperature below 20.degree. K. 
While the compositions, methods, and apparatus herein described constitute 
preferred embodiments of the invention, it is to be understood that the 
invention is not limited to these precise embodiments, and that changes 
made be made in either without departing from the scope of the invention, 
which is defined in the appended claims.