A process is provided for producing a salt pill for use in very low temperature adiabatic demagnetization refrigerators (ADRs). The method can include providing a thermal bus in a housing. The thermal bus can include an array of thermally conductive metal conductors. A hydrated salt can be grown on the array of thermally conductive metal conductors. Thermal conductance can be provided to the hydrated salt

Canavan, Edgar R.

DiPirro, Michael J.

Shirron, Peter J.

NASA Technical Reports Server

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(12) United States Patent 	 (1o) Patent No.: 	 US 8,357,211 B2 
Shirron et al. 	 (45) Date of Patent: 	 Jan. 22, 2013 
(54) ADR SALT PILL DESIGN AND CRYSTAL 
GROWTH PROCESS FOR HYDRATED 
MAGNETIC SALTS 
(75) Inventors: Peter J. Shirron, Silver Spring, MD 
(US); Michael J. DiPirro, Silver Spring, 
MD (US); Edgar R. Canavan, 
Columbia, MD (US) 
(73) Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, DC (US) 
(*) Notice: 	 Subject to any disclaimer, the term of this 
patent is extended or adjusted under 35 
U.S.C. 154(b) by 319 days. 
(21) Appl. No.: 12/872,445 
(22) Filed: 	 Aug. 31, 2010 
(65) 	 Prior Publication Data 
US 2012/0049103 Al 	 Mar. 1, 2012 
(51) Int. Cl. 
COLD 1130 	 (2006.01)  
(52) 	 U.S. Cl . ........................................ 23/303; 23/295 R 
(58) Field of Classification Search .................... 23/303, 
23/295 R, 206, 300, 301, 302 R, 306 
See application file for complete search history. 
(56) References Cited 
U.S. PATENT DOCUMENTS 
	
3,086,850 A * 	 4/1963 Marino eta] . ................ 117/204 
	
5,082,582 A * 	 1/1992 Ames et al . ..................... 252/70 
* cited by examiner 
Primary Examiner Edward Johnson 
(57) ABSTRACT 
A process is provided for producing a salt pill for use in very 
low temperature adiabatic demagnetization refrigerators 
(ADRs). The method can include providing a thermal bus in 
a housing. The thermal bus can include an array of thermally 
conductive metal conductors. A hydrated salt canbe grown on 
the array of thermally conductive metal conductors. Thermal 
conductance can be provided to the hydrated salt. 
11 Claims, 2 Drawing Sheets 
https://ntrs.nasa.gov/search.jsp?R=20130009054 2019-08-31T00:00:17+00:00Z
U.S. Patent 	 Jan 22,2013 	 Sheet I of 2 	 US 8,357,211 B2 
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U.S. Patent 	 Jan. 22, 2013 	 Sheet 2 of 2 	 US 8,357,211 B2 
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US 8,357,211 B2 
1 
	
2 
ADR SALT PILL DESIGN AND CRYSTAL 	 producing a salt pill, is provided. According to various 
GROWTH PROCESS FOR HYDRATED 	 embodiments, the method of producing a salt pill can com- 
MAGNETIC SALTS 	 prise providing a thermal bus in a housing. The thermal bus 
can comprise an array of thermally conductive metal conduc-
STATEMENT OF GOVERNMENT INTEREST 
	
5 tors. In some embodiments, the housing can be sealed. 
According to some embodiments, thermal conductance can 
The invention described herein was at least in-part made by 	 be provided to the hydrated salt. The hydrated salt can be 
an employee of the United States Government and may be 	 grown on the array of thermal conductivity metal conductors 
manufactured and used by or for the Government of the 	 by flowing saturated solution into the housing and drawing 
United States of America for governmental purposes without 10 depleted solution away from the housing. In some embodi- 
the payment of any royalties thereon or therefor. 	 ments, crystallites can be nucleated first at the bottom of the 
housing. Controlled growth of crystallites can be established 
FIELD 
	
from the bottom of the housing upwards. 
According to some embodiments, saturated solution can be 
The present invention relates generally to the field of adia-  15 flowed into the housing and depleted solution can be drawn 
batic demagnetization refrigerators (ADR), and particularly, 	 away from the housing simultaneously. In some embodi- 
a design encapsulating and growing hydrated paramagnetic 	 ments, each thermally conductive metal conductor of the 
salts for use in adiabatic demagnetization refrigerators. 	 array of thermally conductive metal conductors can have a 
spacing of from about 0.02 to about 0.20 cm, for example, and 
BACKGROUND 	 20 a diameter of about 0.04 cm to about 0.08 cm. 
According to some embodiments, each thermally conduc- 
High-resolution detectors used in space astronomy and in 	 tive metal conductor of the array of thermally conductive 
some laboratory and industrial applications must be cooled to 	 metal conductors can have a length of from about 2.0 cm to 
extremely low temperatures (<O.I K) in order to obtain the 	 about 8.0 cm, or from about 3.5 cm to about 5.5 cm. 
required sensitivity. For space applications, adiabatic demag-  25 	 According to some embodiments, the thermally conduc- 
netization refrigerators (ADRs) have an inherent advantage 	 tive metal conductors of the array can be bundled parallel to 
over other techniques because they don't rely on gravity to 	 one another. Each thermally conductive metal conductor of 
function, and because they have higher thermodynamic effi- 	 the array of thermally conductive metal conductors can be 
ciency. An ADR stage produces cooling (or heating) by the 	 spaced from one or more adjacent thermally conductive metal 
interaction of a magnetic field with the magnetic spins in a 30 conductors of the array by a spacing of about 5.0 cm or less, 
paramagnetic salt. Magnetizing the salt produces heating, and 	 for example, by 4.0 cm or less, by 2.0 cm or less, or by 1.0 cm 
demagnetizing the salt produces cooling. 	 or less. 
A conventional "single-shot" ADR consists of a "salt pill" 	 According to some embodiments, the saturated solution 
containing the magnetic salt, a superconducting magnet, and 	 can comprise an aqueous saturated solution of an appropriate 
a heat switch. The salt pill is located in the bore of the magnet, 35 ADR salt. 
and the heat switch links it to a heat sink. Regardless of the 	 In some embodiments, a design for a multi-stage, continu- 
initial conditions, the refrigeration cycle consists of the fol- 	 ous ADR can be employed, as described in A Multi-Stage 
lowing steps. First, the salt pill is magnetized, causing it to 	 Continuous-Duty Adiabatic Demagnetization Refrigerator," 
warm up. Second, when its temperature exceeds that of the 	 Shirron et al., Cryogenic Engineering Conference, Montreal, 
heat sink, the heat switch is powered into the on state. Third, 40 Canada, Jul. 13-16,1999, which is incorporated by reference 
the salt continues to be magnetized, generating heat which 	 in its entirety herein. The multi-stage continuous ADR can 
flows to the sink. This continues until full field is reached, 	 use a high heat capacity reservoir (a paramagnetic salt) con- 
which necessarily is strong enough to significantly align the 	 nected to a single-stageADR through a superconducting heat 
spins and suppress the entropy of the salt. Fourth, at full 	 switch. The reservoir can reduce temperature excursions as 
magnetic field, the heat switch is deactivated to thermally 45 the ADR is recycled. 
isolate the salt from the heat sink. Fifth, the salt is demagne- 
tized to cool it to the desired operating temperature. In gen- 	 BRIEF DESCRIPTION OF THE DRAWINGS 
eral, the salt will then be receiving heat from components 
parts. The heat is absorbed and operating temperature main- 	 The present teachings will be described with reference to 
tained by slowly demagnetizing the salt at just the right rate. 50 the accompanying drawings. The drawings are intended to 
Heat can continue to be absorbed until the magnetic field is 	 illustrate, not limit, the present teachings. 
reduced to zero, at which point theADR has run out of cooling 	 FIG. 1 shows an exploded view of a device to form a salt 
capacity. 	 pill according to various embodiments of the present teach- 
Conventional single-shot ADRs provide cooling for a 	 ings. 
period of time, then they must be taken off line and recycled. 55 	 FIG. 2 shows a perspective top view of a device to form a 
The periodic interruption in cooling leads to a number of 	 salt pill according to various embodiments of the present 
significant constraints on the design of ADRs for space mis- 	 teachings. 
sions. The two most important are that the hold time must be 
long, typically on the order of one day, and the time spent 
	 DETAILED DESCRIPTION OF THE PRESENT 
recycling the ADR must be short, typically one hour or less. 60 	 INVENTION 
A need exists for an ADR salt pill design and a method for 
making the same that overcome these constraints. 	 According to various embodiments, a new process for pro- 
ducing a salt pill is provided for use in very low temperature 
SUMMARY 	 adiabatic demagnetization refrigerators (ADRs). According 
65 to various embodiments, the method of producing a salt pill 
According to various embodiments of the present teach- 	 can comprise providing a thermal bus in a housing. The 
ings, a new process for growing hydrated magnetic salts, or 	 thermal bus can comprise an array of thermally conductive 
US 8,357,211 B2 
3 
metal conductors. A hydrated salt can be grown on the array 
of thermally conductive metal conductors. The housing com-
prising the array of thermally conductive metal conductors 
can be sealed. In some embodiments, thermal conductance 
can be provided to the hydrated salt during the formation 5 
thereof. 
According to some embodiments, the hydrated salt can be 
grown on the array of thermally conductive metal conductors 
by exposing the thermal bus to a saturated solution. This 
process can comprise a continuous counter flow technique io 
where saturated solution flows into the salt pill and depleted 
solution is withdrawn from the salt pill. The salt pill can be 
grown using a small peristaltic circulating pump with two 
pumping lines. One pumping line can deliver saturated solu-
tion at room temperature from a remote reservoir into the salt 15 
container and the other pumping line can withdraw depleted 
solution and return it to the reservoir. The return line's inlet 
can be initially positioned close to the bottom of the salt 
container. According to some embodiments, the return line 
can comprise larger tubing than the supply line to achieve a 20 
four times higher flow rate. According to some embodiments, 
the solution can be maintained at a level just below the return 
line's inlet. According to some embodiments, the return line 
can be raised as the salt grows, thereby producing salt pill 
with no voids or pockets of spent solution. 25 
According to some embodiments, the high thermal con-
ductivity metal conductors can be made from copper. The 
thermal bus can be sealed in a housing. The housing can be 
hermetically sealed. The housing can comprise stainless 
steel, or can comprise any other suitable metal. 30 
According to various embodiments, automated machining 
processes can be employed to implement the process for 
producing a salt pill. According to some embodiments, an 
array of conductors can be generated that is connected to an 
external bolting surface. The size and spacing of the array of 35 
thermal conductors can provide very high thermal conduc-
tance to the salt while minimizing complications due to sur-
face tension. 
According to various embodiments and with reference to 
FIG. 1, a method of producing a salt pill can comprise posi- 40 
tioning a thermal bus 10 in a housing 12. According to some 
embodiments, the housing 12 can comprise a cylindrical case 
12a having a closed bottom end 12b and an open top end 12c. 
As shown, the housing can comprise an end cap 14 for closing 
open top end 12c of cylindrical case 12a. According to some 45 
embodiments, thermal bus 10 can be formed by cutting a 
single piece of metal into an array of thermally conductive 
metal conductors 11. Thermal bus 10 can be cut using a wire 
EDM (electric discharge machine) technique. Each thermally 
conductive metal conductor of the array of thermally conduc- 50 
tive metal conductors can have a diameter of about 0.04 cm to 
about 0.08 cm, for example, of 0.04 cm, 0.05 cm, 0.06 cm, 
0.07 cm, or 0.08 cm. The diameter of each conductor in the 
array of thermally conductive metal conductors 11 can be 
equal to, less than, or greater than, the spacing between each 55 
conductor. In some embodiments, the diameter of each con-
ductor in the array of thermal conductivity metal conductors 
11 can be equal to the spacing between the conductors. 
According to some embodiments, each thermal conductivity 
metal conductor of the array of thermal conductivity metal 60 
conductors can have a length of about 2.0 cm to about 8.0 cm, 
or from about 3.5 cm to about 5.5 cm. According to some 
embodiments, housing 12 can be sized and dimensioned to 
completely encompass thermal bus 10. 
According to various embodiments, the salt pill can be 65 
from about 4 cm to about 7 cm in length. The salt pill can be 
about 5.0 cm, 5.2 cm, 5.4 cm, 5.6 cm, 5.8 cm, or 6.0 cm, in  
4 
length. According to some embodiments, the salt pill can have 
a diameter of from about 1 cm to about 4 cm. In some embodi-
ments, the salt pill can have a diameter of about 1.1 cm, 1.3 
cm, 1.5 cm, 1.7 cm, or 1.9 cm. In some embodiments, a fill 
tube 16 adjacent end cap 14 can be used to provide access for 
the flow of solution. Fill tube 16 can be sealed after crystal is 
formed in housing 12. End cap 14 can be welded to open top 
end 12c of cylindrical case 12a. According to some embodi-
ments, cylindrical case 12a can be hermetically sealed. 
According to various embodiments, end cap 14 can be 
provided with a thermometer mount 18. Thermometer mount 
18 can be used to mount a thermometer. Thermometer mount 
18 can be made from copper, or other suitable metal. Ther-
mometer mount 18 can be in intimate thermal contact with 
crystals formed in housing 12. 
According to various embodiments, the method can com-
prise growing a hydrated salt on the array of thermally con-
ductive metal conductors 11. A saturated solution can be 
flowed into the housing and a depleted solution can be drawn 
away from the housing. In some embodiments, saturated 
solution can be flowed into the housing and depleted solution 
can be drawn away from the housing simultaneously. 
According to some embodiments, the saturated solution 
can comprise hydrated salt and water. According to some 
embodiments, the hydrated salt can comprise any suitable 
hydrated salt, such as, for example, chromium potassium 
alum, chromic cerium alum (CCA), or combinations thereof 
and the like. In some embodiments, crystallites can be nucle-
ated first at the bottom of housing 12. Controlled growth of 
crystallites can be established from the bottom of housing 12 
upwardly. Thermal conductance can be provided to the 
hydrated salt in housing 12. 
In various embodiments, the array of thermally conductive 
metal conductors can be connected to an external bolting 
surface. In some embodiments, the array of thermal conduc-
tivity metal conductors can be integrally connected to an 
external bolting surface. 
According to various embodiments, fill fraction of from 
about 95% to about 100% can be achieved. 
FIG. 2 depicts an alternative embodiment of salt pill grow-
ing device 101. Salt pill growing device 101 is identical to salt 
pill growing device 100 from FIG. 1 except that salt pill 
growing device 101 comprises an attachment means 20 on 
end cap 14 and at least two fill ports 22 for salt growth. 
Other embodiments of the present invention will be appar-
ent to those skilled in the art from consideration of the present 
specification and practice of the present invention disclosed 
herein. It is intended that the present specification and 
examples be considered as exemplary only with the true 
scope and spirit of the invention being indicated by the fol-
lowing claims and equivalents thereof. 
What is claimed is: 
1. A method of producing a salt pill comprising: 
providing a thermal bus in a housing, the thermal bus 
comprising an array of thermally conductive metal con- 
ductors; 
sealing the housing comprising the array of thermally con-
ductive metal conductors; 
providing thermal conductance to the array of thermally 
conductive metal conductors; and 
growing a hydrated salt on the array of thermally conduc-
tive metal conductors by flowing saturated solution into 
the housing and drawing depleted solution away from 
the housing, whereby crystallites are nucleated first at 
the bottom of the housing and controlled growth of 
crystallites is established from the bottom of the housing 
in an upward direction. 
US 8,357,211 B2 
5 
2. The method of claim 1, wherein the sealing comprises 
hermetically sealing the housing. 
3. The method of claim 1, wherein the flowing saturated 
solution into the housing is conducted simultaneously with 
the drawing depleted solution away from the housing. 	 5 
4. The method of claim 1, wherein each thermally conduc-
tive metal conductor of the array of thermally conductive 
metal conductors has a diameter of from about 0.04 cm to 
about 0.08 cm. 
5. The method of claim 1, wherein each thermally conduc- 10 
tive metal conductor of the array of thermally conductive 
metal conductors has a length of from about 3.5 cm to about 
5.5 cm. 
6. The method of claim 1, wherein the thermally conduc-
tive metal conductors of the array of thermally conductive 
metal conductors are spaced apart from each other.  
6 
7. The method of claim 1, wherein the saturated solution 
comprises chromium potassium alum, chromic cerium alum, 
or a combination thereof. 
8. The method of claim 1 further comprises connecting the 
array of thermally conductive metal conductors to an external 
bolting surface. 
9. The method of claim 1, wherein the array of thermally 
conductive metal conductors is integrally connected to an 
external bolting surface. 
10. The method of claim 1, wherein the fill fraction 
achieved comprises from about 95% to about 100%. 
11. The method of claim 1, wherein the housing comprises 
stainless steel. 


ADR salt pill design and crystal growth process for hydrated magnetic salts

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