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SCIENCE 6 (1967) 388-390 0 North-Holland

LETTERS

SUPERCONDUCTING

Publishing Co., Amsterdam

TO THE EDITOR

CONTACTS

UNDER

Received 6 September

MECHANICAL

LOADS

1966

Knowledge of the variation of the real contact area between “flat” surfaces with applied normal load and surface roughness is needed in order to be able to predict electrical and thermal contact conductance behaviour of the interface. The real contact area is dictated by two factors, namely the number of individual areas of contact and their average size, which in turn depend on the surface finish. It has proved difficult to devise successful experimental techniques for differentiating between the variation of these two factors with load, though some work has been reported on silver chloride contactsr). This letter suggests a method of separating the effects of the two factors and presents some preliminary results. The contact between two superconducting solids can itself be superconductinga). Consider a model of a plane interface of area A between two superconductors, across which there are on average n bridges of electrical contact per unit area, each of average radius 6. If a current I (A) is passed across the interface, the magnetic field H (Oe) produced at the surface of each contact bridge will be given by 1-I = 2IjndA,

(1)

provided ti is large compared with the penetration depth 6. This condition is satisfied for the tin specimens discussed later because for these 6 equals 5 x lO-6 cm (ref. 3, whiled is of the order of 10e3 cm (ref.4). As the current is increased, H will exceed the critical field H, corresponding to the ambient temperature and the whole interface will undergo a transition from the superconducting to the normal state. Hence if 1, and A are measured, values of nti may be deduced from (1). The critical temperature for a superconductor is a function of the applied mechanical load, the coefficient of which is known for tins). If the critical temperature is measured under constant known load F and plastic deformation of the asperity tips is assumed, the total area of real contact (m&A) can be deduced from the known value2) of the flow pressure Y of tin at liquid 388

SUPERCONDUCTING

helium

temperatures

389

CONTACTS

by FIA PZ CY

real contact apparent

area

contact

area

= rcnfi2,

(2)

where C is approximately 3 for many practical cases l). Hence from equations (1) and (2) both il and d may be evaluated. Preliminary tests have been made to show the feasibility of the method using an apparatus similar to that described by Probert et a1.6). The specimens were very pure tin cylinders of diameter 2.01 cm and height 0.91 cm pressed end-to-end. The mating surfaces were turned to roughnesses of 1.88 x 10W4 cm CLA and overall flatness deviations of less than 2.5 x 10-j cm. No attempt was made to remove the oxide layers from the contacting surfaces. The specimens were held at 3.6 “K under a 10 kg load, and the current flowing across the interface was increased from zero: the corresponding potential drop was measured to lo-’ V with a Diesselhorst potentiometer. Fig. 1 shows the observed gradual transition from the superconducting to the normal behaviour which occurs between 4 x low4 and 8 x 10m4 A, and which is qualitatively similar to the curves obtained by Meissner 3, for a single contact between crossed tin wires, indicating that the basic assumptions are correct. Substituting the current values bounding the transition region in (1) we find that 13 x 10m6 cm-l A calculation

from the resistance I

x 10-6cn-‘.

in the normal

I

I

state using

r I

cl-

60

I

Helm’s’) I

‘-

2-

/ O.-.I0 0

2

L

I

I

I

I

6

0

10

12

I

Fig. 1.

ex-

~1o’LAMP)

Variation of the electrical resistance with current for a tin-tin contact.

390

T. R. THOMAS

AND

S. D. PROBER’I

pression for the constriction resistance of a single bridge gives nd= 1.8 x 10m6 cm-‘. This discrepancy suggests that the surface oxide films are insufficient to suppress the supercurrent: this phenomenon was also reported by Holm and Meissner for very clean tin contacts. We are indebted this work.

to the Science Research

School of Engineering, University College, Swansea,

Council

for financial

of

T. R. THOMAS and S. D. PROBERT

England References

1) 1. V. Kragelsky and N. B. Demkin, Wear 3 (1960) 170. 2) 3) 4) 5) 6) 7)

support

R. Holm and W. Meissner, Z. Physik 74 (1932) 736. H. Meissner, Phys. Rev. 109 (1958) 686. F. Boeschoten and E. F. M. Van der Held, Physica 23 (1957) 37. N. L. Muench, Phys. Rev. 99 (1955) 1814. S. D. Probert, T. Thomas and D. Warman, J. Sci. Instr. 41 (1964) 88. R. Helm, Electric Contacts Handbook (Springer-Verlag, Berlin, 1958) p. 4.

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