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Submitted on 1 Jan 1978
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SUPERCONDUCTIVE FIXED POINTS FOR TEMPERATURES ABOVE 0.5 K
J. Schooley
To cite this version:
J. Schooley. SUPERCONDUCTIVE FIXED POINTS FOR TEMPERATURES ABOVE 0.5 K. Jour-
nal de Physique Colloques, 1978, 39 (C6), pp.C6-1169-C6-1170. �10.1051/jphyscol:19786517�. �jpa-
00218002�
SUPERCONDUCTIVE FIXED POINTS FOR TEMPERATURES ABOVE 0.5 K
J.F. Schooley
National Bureau of Standards Washington, D.C. 20234, U.S.A.
Abstract.- A temperature fixed point device incorporating samples of the elements Cd, Zn, Al, In and Pb has been available from the NBS for several years. This paper describes current efforts to reduce the temperature uncertainties of the NBS device below the present one milli-kelvin level. It also notes that efforts to produce narrow transition widths in samples of Nb, V3Ga and Nb3Sn have resulted in. widths of 9mK, 40 mK and 17 mK, respectively.
A.INTRODUCTION.- The practice of using a superconduc- tive transition to obtain a fixed reference tempera- ture is by now well established. The National Bureau of Standards has made available for some five years a device, Standard Reference Material (SRM) 767, which employs high-purity samples of the supercon-
ductors Pb,In, Al, Zn and Cd. These samples can be expected to yield reference temperatures which are uniform within ± 1 mK from run to run and from devi- ce to device when operated according to the sugges- ted procedures/1/. Various experimenters have repor- ted measurements which include the use of the NBS SRM device/2/, and provisional temperatures have been assigned to the device's superconductive tran- sitions on the 1976 Provisional 0.5 K - 30 K Tem- perature Scale (EPT-76)/3/.
The following table shows these assignments and typical transition widths/4/ for the SRM 767 device :
Element T (76), K Width,mK
Pb 7.200 1-2
In 3.415
1-2
Al 1.179 1.5-3.5
Zn 0.851 1.5-6
Cd 0.519
1-3
Three major suggestions for improving the SRM 767 device have been received by the NBS staff.
These are as follows :
1) Extend the temperature range over which the superconductive fixed point principle may be applied ;
2) Reduce the temperature uncertainties of the SRM 767 ;
3) Attempt to verify the extent to which the individual samples of the device provide a referen-
ce temperature which is characteristic of the pure material.
R.J. Soulen, Jr. will report, in an accompa- nying paper, the progress which has been made to- wards obtaining superconductive reference tempera-
tures below 0.5 K. In this paper, we shall report the progress made at higher temperatures. There has been little progress, however, along the lines of
the third suggestion, we wish to commend to our col- leagues, particularly to those with access to super- conductive materials of different origins from the NBS samples, the usefulness of comparisons of refe- rence samples. Such comparisons would give confi- dence that the resulting reference temperatures can be obtained by preparing the material in any labo- ratory.
B.SUPERC0NDUCTIVE FIXED POINTS ABOVE 7.2 K / 5 / . ~ 1) Nb : Many superconductors have T values abo- ve that of Pb/6/. Among the elements, however, only Nb (T = 9.3 K) and Tc (T = 7 . 8 K) are in that ca-
c c tegory, and Nb is the only reasonable source of an elemental fixed point above 7.2 K.
We have devoted considerable effect to the stu- dy of superconductive transition widths and tempera- tures of Nb samples from various sources and follo- wing various preparation methods. The results range in T values from 9.2 K to 9.3 K and in widths from
c
1 mK to 45 mK or more. Easily the most reproducible of the NBS-prepared samples were those which were outgassed and annealed in vacuum by the passage of alternating current sufficient to raise their tem- perature to about 1600°C. Some thirty of these sam- JOURNAL DE PHYSIQUE
Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-1169
Résumé.- Depuis plusieurs années le NBS possède un dispositif qui donne des températures fixes en utilisant les éléments Cd, Zn, Al,. In et Pb. Cet article mentionne les efforts actuels entrepris pour réduire les incertitudes en température de ce dispositif au-dessous de sa valeur actuelle de ± 0,001 K. Ces efforts ont permis d'avoir des transitions à faibles largeurs (en température) de 9 mK, 40 mK et 17 mK pour des échantillons Nb, V3Ga et Nb3Sn respectivement.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786517
ples showed widths of about 9 mK and Tc values within a range of a few millikelvins of each other. However, no experiment-to-experiment reproducibility studies have been undertaken as yet with the electrically annealed samples. We expect to commence these tests soon.
2) Nb3Sn, V3Ga : Of the many compound and al- loy superconductors possessing T values above 7.2 K, we have selected Nb3Sn and V3Ga, because they offer not only useful reference temperatures but also trac- table preparation methods.
Samples of each of these materials can be pre- pared simply. Our technique in each case has been the method of vapor diffusion of the low-melting component into the surface of a 1 mm diameter x 3 cm long rod of the appropriate transition metal.This process can be accomplished at temperatures from 700°C to 1100°C. The resulting sample is a composite in which the original wire core is surrounded by a thin layer (our metallographic examinations indica- te a thickness of some 10 pm ) of the high-T com- pound Nb3Sn or VsGa, as the c'ase may be. In the mea- surement scheme which is used, these composite rods appear to be solid rods of the high-Tccompound.
The number of NbsSn and V3Ga samples exhibi- ting superconductive transition widths as narrow as 17 mK and 40 mKz respectively, is relatively small, and even these show a substantial variation in T values.
CSEDUCTION OF SRM 767 REFERENCE ZlEMPERATURE UNCER- TAINTIES.- One of the basic premises upon which the development of the SRM 767 rested was that of ease of sample preparation/l/. We initially supposed that each experimenter might ultimately be able to pre- pare hGs own samples according to recipes deduced from our work. This supposition proved to be unwor- kable because of the delicacy .of the samples ; more- over, we have noted no desire among our colleagues to prepare their own samples. Nevertheless, the em- ployment of simple pGeparation methods for the sam- ples has resulted in a device cost which is suffi- ciently low to permit almost any interested labora- tory to acquire a device for its own continuing ca- libration requirements.
In view of the reference to the Tc values of SRM 767 in the EPT-76 scale, we have attempted to produce more uniform samples of Pb, In, Al, Zn and Cd by careful annealing and by this means, to mini- mize the experimental uncertainties in the T values.
Samples of each of the five SRM 767 elements so pre- pared have been found with superconductive transi- tion widths less than 0.5 mK. The question of sample -toLsample and experiment-to-experiment reproducibi- lity at the 0.1 mK level, is, however, still unresol- ved.
References
/I/ Schooley,J.F., Soulen,R.J.,Jr. and Evans,G.A., Jr., NBS Special Publication 260-44, Dec. 1972 Soulen,R.J. Jr., Schooley,J.F. and Evans,G.A.Jr.
Rev. Sci. Inst. 44 (1973) 1537
-
/ 2 / Besley,L.M. and Kemp,W.R.G., Metrologia
2
(1977) 35.Martin,D.L., Rev. Sci. Inst.
46
(1975)1670
/3/ Durieux,M., Astrov,D.N., Kemp,W.R. and Swenson, C.A., Metrologia, to be published
/4/ We define the superconductive transition width as the temperature range within which the cen- tral 80 % of the total mutual inductance change takes place.
/5/ Two students, Mr. J.H. Day of Howard University, Washington,D.C., and Dr. K. Mohan of Georgetown University, Washington,D.C., performed most of the experiments described in Section B
/6/ Roberts,B.W., J. Phys. Chem. Ref. Data
2
(1976)58 1
