Cryostats which are currently used to characterize the properties of superconductors between 4 K and 100 K are primarily single sample devices. The purpose of this paper is to present an instrument design which can hold up to five (5) one cm. diameter samples at a stable temperature (+ /-0.1K) within the above range specified while measurements of the sample properties are made

Burnside, G.

Hawk, Roger M.

McLeod, Paul C.

UARK (University of Arkansas )

Journal of the Arkansas Academy of Science Volume 47 Article 8 1993 Multiple Sample Cryostat for the Determination of Superconductor Properties G. Burnside University of Arkansas at Little Rock Roger M. Hawk University of Arkansas at Little Rock Paul C. McLeod University of Arkansas at Little Rock Follow this and additional works at: https://scholarworks.uark.edu/jaas  Part of the Other Electrical and Computer Engineering Commons Recommended Citation Burnside, G.; Hawk, Roger M.; and McLeod, Paul C. (1993) "Multiple Sample Cryostat for the Determination of Superconductor Properties," Journal of the Arkansas Academy of Science: Vol. 47 , Article 8. Available at: https://scholarworks.uark.edu/jaas/vol47/iss1/8 This article is available for use under the Creative Commons license: Attribution-NoDerivatives 4.0 International (CC BY-ND 4.0). Users are able to read, download, copy, print, distribute, search, link to the full texts of these articles, or use them for any other lawful purpose, without asking prior permission from the publisher or the author. This Article is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Journal of the Arkansas Academy of Science by an authorized editor of ScholarWorks@UARK. For more information, please contact scholar@uark.edu. 23AMultiple Sample Cryostat for the Determination ofSuperconductor PropertiesG. Burnside, R.M. Hawk, P.C. McLeodUniversity ofArkansas at Little RockDepartment of Electronics and Instrumentation2801 S. University Ave.Little Rock, AR 72204AbstractCryostats which are currently used to characterize the properties of superconductors between 4 Kand 100 Kare primar-ilysingle sample devices. The purpose of this paper is to present an instrument design which can hold up to five (5) onecm. diameter samples at a stable temperature (+ 0.1K) within the above range specified while measurements of the sam-ple properties are made.IntroductionThe characterization of potential superconductingmaterials requires an instrument which can maintain astable temperature within the superconductor's electro-magnetic transition range (usually between 4 K and 77K). Such devices, called Cryostats, use liquid helium, LHe,and/or liquid nitrogen, LN2> to provide a cold reservoirabove which the sample temperature is varied.Commercial Cryostats, while providing excellent tempera-ture stability, are usually single sample devices which costbetween $10,000 and $20,000 (Van Sciver, 1986). Theobject of this project is to design and build a multi-sampleCryostat for superconductor evaluation for a total cost of$5,000.The design of the Cryostat began with a survey of thepertinent literature regarding the design and construc-tion ofcommercial cryostats, from both practical and the-oretical considerations. The various methods of varyingtemperature were explored with the additional con-straints of producing an economical accessible multi-sam-ple device.DesignIHeat Load and Vacuum Considerations.—The Cryostatcomposed of two concentric dewers (Fig. 1), a liquidlium,LHe, reservoir which is surrounded by a reservoirliquid nitrogen, LN2. The LN2 at 77 K, reduces theiiational heat load from room temperature into the>re expensive liquid helium by three orders of magni-le since this load is proportional to the fourth power ofiabsolute temperature (White, 1968). The relation isen by Stephan-Boltzmann's law and is of the formQ^oCiy-T^/V,where a is the Stephan-Boltzmann constant (5.67 x 10'4W/m2-K4), and V is the view factor which relates the areas andemmisivities of surface two as seen by surface one.To further reduce heat loss, the volume external toeach of these dewers is evacuated to decrease the conduc-tive heat loss of the gases and to reduce the losses due toexternal condensation on the LN2 dewer. Bylowering thepressure, the distance a molecule travels before collidingwith another (called the mean free path) increases to thepoint where the dominant method of thermal energyexchange is due to collision between the remaining gasmolecules and the dewer walls. Pressure reductions of 10'4torr correspond with a mean free path of 100 cm (Rose-Innes, 1973), which is much less than the interwall dis-tance of the dewers. Because the pressure in these areas isdetermined by the statistical interactions of the molecules(molecular flow), the ports to the external diffusion pumpmust be made as large as possible. This provides a largecross-sectional area through which the particles may passand be trapped.The location of welds and venting of blind threadedholes also becomes important when considering molecu-lar flow. Ifa weld between two plates is on the pressur-ized side of a vacuum-pressure interface, as shown in Fig.2, the molecules within the seam willonly 'see' the faceof the seam to pass through and the time to pump downto sufficient vacuum increases (Green, 1986).The diameter of the tubes which are used to fillthecryogens into their respective dewers are important inthat if the tube is sufficiently small (less than 2.5 cm) andis long (over 30 cm) the tube may develop powerful lowfrequency vibration due to the thermosiphoning of vaporfrom the dewer to the atmosphere (Green, 1986). Filltubes are made as short as possible to eliminate theseProceedings Arkansas Academy of Science, Vol.47, 199323Journal of the Arkansas Academy of Science, Vol. 47 [1993], Art. 8Published by Arkansas Academy of Science, 199324problems.Fig. 1. Cryostat DewerA safety pressure relief valve is mounted on the outershell of the Cryostat in case of a leak of one of the cryo-jens into the vacuum jacket. Upon contact with theroom temperature wall of the jacket, the cryogen wouldexpand and increase the pressure which might destroythe Cryostat.Stainless steel was chosen as the material from whichhe Cryostat would be constructed since it does not out-gass in a vacuum, has low thermal conductivity (16 W/m-{), high yield strength (130 kpsi), and is readily welded.Where a stainless steel plate is to be welded to a length ofube to form a dewer, a groove is cut into the plate tohalfof its thickness, as shown inFig. 2. This reduces the stresson the weld by creating a much smaller area across whichthermal shrinkage occurs during dewer cooling as thepure tensile weld stress is reduced by the cantelever thuscreated (Barron, 1966). Additionally, a seating groove iscut into the tube tominimalize shear stresses on the weld.Fig. 2. Welded connection between dewer tube and endplate showing improper weld location.Temperature Variability.—Superconducting samplesmay be placed directly in contact with either the LN2 orthe LHe to determine their properties at those tempera-tures, although the energy involved in measuring theresistance of the material will tend to raise its tempera-ture above that of the surrounding cryogen. If the sam-ple holder is made to be in good thermal contact withthe cryogen, then any heat added to the holder willquick-lybe passed to the liquidfied gas and willnot significantlyraise the temperature of the sample. To vary the temper-ature of the sample then, the sample must first be cooledto cryogenic temperatures, and then the sample shouldbe thermally isolated from the cryogen. Any heat which issubsequently added willprimarily raise the temperatureof the sample and not be wasted in boiling the liquifiedgas (Holma, 1981).An exchange gas method (Obert et al., 1982) for tem-perature variation was chosen to modify the temperatureof the sample, as shown inFig. 3. As seen in this figure,the gas exchange insert fits into the Cryostat heliumdewer filltube. The temperature of the sample is variedby indirectly coupling the sample to the LHe by varyingthe pressure of helium gas in the sample compartment.When the pressure of the gas is high (760 torr), the ther-mal link between the LHe and the sample is good and thesample approaches 4K. As the pressure in the samplecompartment is reduced, it becomes thermally isolatedProceedings Arkansas Academy of Science, Vol. 47, 199324Journal of the Arkansas Academy of Science, Vol. 47 [1993], Art. 8https://scholarworks.uark.edu/jaas/vol47/iss1/825from the cryogen and the sample temperature can beraised by a small heater imbedded in the sample mount-ing block. A thermocouple mounted on the sample blockis used to determine the average block temperature.Fig.3. Gas Exchange Insert for CryostatCryostat Operation.—Initial loading of the Cryostatbegins with purging of the device with nitrogen gas,which flushes the room air and concomitant moisturefrom the walls of the instrument. The evacuation valve isthen attached to a rotary pump and the pressure is'roughed in1. Once the pressure is reduced to approxi-mately 10"2 torr, the diffusion pump is started and thepressure is reduced to 10"4 torr. The evacuation valve isclosed and the LN2 is the introduced into the dewer.This brings the temperature of the device down to 77K,whereupon the LHe is siphoned from a storage dewer.The LHe cryopumps the pressure in the vacuum jacket to10~ 6 torr which provides an ideal storage environment.Estimates on the heat load indicate that the instrumentwillrequire 15 liters of LN2 for initial cooling.Multiple samples which are to be introduced into theCryostat are prewired and screwed to the sample mount.The samples are cleaned to remove any oils which wouldoutgas under reduced pressure, preventing the attain-ment of a sufficient vacuum. Next, the samples aremounted in the gas exchange insert, which is floodedwith room temperature helium gas. The entire insert isprecooled in the nitrogen dewer and then the insert isplaced in the Cryostat LHe filltube where it cools to 4K.After equilibrating, measurements on the samples aretaken to determine ifany of the samples are supercon-ducting. The insert pressure is then lowered bypumpingwhich thermally isolates the sample mount from theCryostat sufficiendy tobegin heating the sample block fortemperature manipulation.Literature CitedBarron, R. 1966. Cryogenic Systems. McGraw-Hill. NewYork. 255 pp.Green, G. W. 1986. The Design and Construction ofSmall Vacuum Systems.Chapman and Hall,London. 99 pp.Holma,J. P. 1981. Heat Transfer. McGraw-Hill,NewYork. 132 pp.Obert, W. Coupland, J. R., Hammond, D.D., Cook,T.and Harwood, K.1982. Advances inCryogenicEngineering. Vol.25, p. 293.Rose-Innes, A.C. 1973. Low Temperature LaboratoryTechniques, English Universities Press, London. 118 pp.Scott, R.B. 1963. Cryogenic Engineering, Met-ChemResearch Inc., Boulder. 98 pp.Siegel, R. and Howel,J. R. 1981. Thermal RadiationHeat Transfer, McGraw-Hill,New York.52 pp.White, G. K.1968. Heat Transfer. Oxford UniversityPress, London. 233 pp.Van Sciver, S. W. 1986. Helium Cryogenics. PlenumPress, New York. 85 pp.Proceedings Arkansas Academy of Science, Vol.47, 199325Journal of the Arkansas Academy of Science, Vol. 47 [1993], Art. 8Published by Arkansas Academy of Science, 1993

English

Multiple Sample Cryostat for the Determination of Superconductor Properties

ScholarWorks@UARK

Journal of the Arkansas Academy of ScienceVolume 47 Article 81993Multiple Sample Cryostat for the Determination ofSuperconductor PropertiesG. BurnsideUniversity of Arkansas at Little RockRoger M. HawkUniversity of Arkansas at Little RockPaul C. McLeodUniversity of Arkansas at Little RockFollow this and additional works at: http://scholarworks.uark.edu/jaasPart of the Other Electrical and Computer Engineering CommonsThis article is available for use under the Creative Commons license: Attribution-NoDerivatives 4.0 International (CC BY-ND 4.0). Users are able toread, download, copy, print, distribute, search, link to the full texts of these articles, or use them for any other lawful purpose, without asking priorpermission from the publisher or the author.This Article is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Journal of the Arkansas Academyof Science by an authorized editor of ScholarWorks@UARK. For more information, please contact scholar@uark.edu.Recommended CitationBurnside, G.; Hawk, Roger M.; and McLeod, Paul C. (1993) "Multiple Sample Cryostat for the Determination of SuperconductorProperties," Journal of the Arkansas Academy of Science: Vol. 47 , Article 8.Available at: http://scholarworks.uark.edu/jaas/vol47/iss1/823AMultiple Sample Cryostat for the Determination ofSuperconductor PropertiesG. Burnside, R.M. Hawk, P.C. McLeodUniversity ofArkansas at Little RockDepartment of Electronics and Instrumentation2801 S. University Ave.Little Rock, AR 72204AbstractCryostats which are currently used to characterize the properties of superconductors between 4 Kand 100 Kare primar-ilysingle sample devices. The purpose of this paper is to present an instrument design which can hold up to five (5) onecm. diameter samples at a stable temperature (+ 0.1K) within the above range specified while measurements of the sam-ple properties are made.IntroductionThe characterization of potential superconductingmaterials requires an instrument which can maintain astable temperature within the superconductor's electro-magnetic transition range (usually between 4 K and 77K). Such devices, called Cryostats, use liquid helium, LHe,and/or liquid nitrogen, LN2> to provide a cold reservoirabove which the sample temperature is varied.Commercial Cryostats, while providing excellent tempera-ture stability, are usually single sample devices which costbetween $10,000 and $20,000 (Van Sciver, 1986). Theobject of this project is to design and build a multi-sampleCryostat for superconductor evaluation for a total cost of$5,000.The design of the Cryostat began with a survey of thepertinent literature regarding the design and construc-tion ofcommercial cryostats, from both practical and the-oretical considerations. The various methods of varyingtemperature were explored with the additional con-straints of producing an economical accessible multi-sam-ple device.DesignIHeatLoad and Vacuum Considerations.—The Cryostatcomposed of two concentric dewers (Fig. 1), a liquidlium,LHe, reservoir which is surrounded by a reservoirliquid nitrogen, LN2. The LN2 at 77 K, reduces theiiational heat load from room temperature into the>re expensive liquid helium by three orders of magni-le since this load is proportional to the fourth power ofiabsolute temperature (White, 1968). The relation isen by Stephan-Boltzmann's law and is of the formQ^oCiy-T^/V,where a is the Stephan-Boltzmann constant (5.67 x 10'4W/m2-K4), and V is the view factor which relates the areas andemmisivities of surface two as seen by surface one.To further reduce heat loss, the volume external toeach of these dewers is evacuated to decrease the conduc-tive heat loss of the gases and to reduce the losses due toexternal condensation on the LN2 dewer. Bylowering thepressure, the distance a molecule travels before collidingwith another (called the mean free path) increases to thepoint where the dominant method of thermal energyexchange is due to collision between the remaining gasmolecules and the dewer walls. Pressure reductions of 10'4torr correspond with a mean free path of 100 cm (Rose-Innes, 1973), which is much less than the interwall dis-tance of the dewers. Because the pressure in these areas isdetermined by the statistical interactions of the molecules(molecular flow), the ports to the external diffusion pumpmust be made as large as possible. This provides a largecross-sectional area through which the particles may passand be trapped.The location of welds and venting of blind threadedholes also becomes important when considering molecu-lar flow. Ifa weld between two plates is on the pressur-ized side of a vacuum-pressure interface, as shown in Fig.2, the molecules within the seam willonly 'see' the faceof the seam to pass through and the time to pump downto sufficient vacuum increases (Green, 1986).The diameter of the tubes which are used to fillthecryogens into their respective dewers are important inthat if the tube is sufficiently small (less than 2.5 cm) andis long (over 30 cm) the tube may develop powerful lowfrequency vibration due to the thermosiphoning of vaporfrom the dewer to the atmosphere (Green, 1986). Filltubes are made as short as possible to eliminate theseProceedings Arkansas Academy of Science, Vol.47, 199323Journal of the Arkansas Academy of Science, Vol. 47 [1993], Art. 8Published by Arkansas Academy of Science, 199324problems.Fig. 1. Cryostat DewerA safety pressure relief valve is mounted on the outershell of the Cryostat in case of a leak of one of the cryo-jens into the vacuum jacket. Upon contact with theroom temperature wall of the jacket, the cryogen wouldexpand and increase the pressure which might destroythe Cryostat.Stainless steel was chosen as the material from whichhe Cryostat would be constructed since it does not out-gass in a vacuum, has low thermal conductivity (16 W/m-{), high yield strength (130 kpsi), and is readily welded.Where a stainless steel plate is to be welded to a length ofube to form a dewer, a groove is cut into the plate tohalfof its thickness, as shown inFig. 2. This reduces the stresson the weld by creating a much smaller area across whichthermal shrinkage occurs during dewer cooling as thepure tensile weld stress is reduced by the cantelever thuscreated (Barron, 1966). Additionally, a seating groove iscut into the tube tominimalize shear stresses on the weld.Fig. 2. Welded connection between dewer tube and endplate showing improper weld location.Temperature Variability.—Superconducting samplesmay be placed directly in contact with either the LN2 orthe LHe to determine their properties at those tempera-tures, although the energy involved in measuring theresistance of the material will tend to raise its tempera-ture above that of the surrounding cryogen. If the sam-ple holder is made to be in good thermal contact withthe cryogen, then any heat added to the holder willquick-lybe passed to the liquidfied gas and willnot significantlyraise the temperature of the sample. To vary the temper-ature of the sample then, the sample must first be cooledto cryogenic temperatures, and then the sample shouldbe thermally isolated from the cryogen. Any heat which issubsequently added willprimarily raise the temperatureof the sample and not be wasted in boiling the liquifiedgas (Holma, 1981).An exchange gas method (Obert et al., 1982) for tem-perature variation was chosen to modify the temperatureof the sample, as shown inFig. 3. As seen in this figure,the gas exchange insert fits into the Cryostat heliumdewer filltube. The temperature of the sample is variedby indirectly coupling the sample to the LHe by varyingthe pressure of helium gas in the sample compartment.When the pressure of the gas is high (760 torr), the ther-mal link between the LHe and the sample is good and thesample approaches 4K. As the pressure in the samplecompartment is reduced, it becomes thermally isolatedProceedings Arkansas Academy of Science, Vol. 47, 199324Journal of the Arkansas Academy of Science, Vol. 47 [1993], Art. 8http://scholarworks.uark.edu/jaas/vol47/iss1/825from the cryogen and the sample temperature can beraised by a small heater imbedded in the sample mount-ing block. A thermocouple mounted on the sample blockis used to determine the average block temperature.Fig.3. Gas Exchange Insert for CryostatCryostat Operation. —Initial loading of the Cryostatbegins with purging of the device with nitrogen gas,which flushes the room air and concomitant moisturefrom the walls of the instrument. The evacuation valve isthen attached to a rotary pump and the pressure is'roughed in1. Once the pressure is reduced to approxi-mately 10"2 torr, the diffusion pump is started and thepressure is reduced to 10"4 torr. The evacuation valve isclosed and the LN2 is the introduced into the dewer.This brings the temperature of the device down to 77K,whereupon the LHe is siphoned from a storage dewer.The LHe cryopumps the pressure in the vacuum jacket to10~ 6 torr which provides an ideal storage environment.Estimates on the heat load indicate that the instrumentwillrequire 15 liters of LN2 for initial cooling.Multiple samples which are to be introduced into theCryostat are prewired and screwed to the sample mount.The samples are cleaned to remove any oils which wouldoutgas under reduced pressure, preventing the attain-ment of a sufficient vacuum. Next, the samples aremounted in the gas exchange insert, which is floodedwith room temperature helium gas. The entire insert isprecooled in the nitrogen dewer and then the insert isplaced in the Cryostat LHe filltube where it cools to 4K.After equilibrating, measurements on the samples aretaken to determine ifany of the samples are supercon-ducting. The insert pressure is then lowered bypumpingwhich thermally isolates the sample mount from theCryostat sufficiendy tobegin heating the sample block fortemperature manipulation.Literature CitedBarron, R. 1966. Cryogenic Systems. McGraw-Hill. NewYork. 255 pp.Green, G. W. 1986. The Design and Construction ofSmall Vacuum Systems.Chapman and Hall,London. 99 pp.Holma,J. P. 1981. Heat Transfer. McGraw-Hill,NewYork. 132 pp.Obert, W. Coupland, J. R., Hammond, D.D., Cook,T.and Harwood, K.1982. Advances inCryogenicEngineering. Vol.25, p. 293.Rose-Innes, A.C. 1973. Low Temperature LaboratoryTechniques, English Universities Press, London. 118 pp.Scott, R.B. 1963. Cryogenic Engineering, Met-ChemResearch Inc., Boulder. 98 pp.Siegel, R. and Howel,J. R. 1981. Thermal RadiationHeat Transfer, McGraw-Hill,New York.52 pp.White, G. K.1968. Heat Transfer. Oxford UniversityPress, London. 233 pp.Van Sciver, S. W. 1986. Helium Cryogenics. PlenumPress, New York. 85 pp.Proceedings Arkansas Academy of Science, Vol.47, 199325Journal of the Arkansas Academy of Science, Vol. 47 [1993], Art. 8Published by Arkansas Academy of Science, 1993

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Multiple Sample Cryostat for the Determination of Superconductor Properties

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