Superconducting contacts for top-loading dilution refrigerators L.G. Gunderson and R.C. Richardson Cornell Materials Science Center, Ithaca, NY 14853, USA
Received 5 May 1989 Superconducting contacts made of niobium capillary supported by a brass sleeve were made for use in top-loading dilution refrigerators. The critical current of the joints produced in this manner varied between 3 and 40 mA, with a lower limit of 0.7 mA.
Keywords: superconductivity; SQUIDs; dilution refrigerators; superconducting contacts Since it is frequently necessary to have a completely superconducting loop for use with SQUIDs, a method was developed for making external and internal connections of niobium-titanium (Nb-Ti) wire to niobium slug rings, for use in the Oxford Instruments Model 200 top-loading dilution refrigerator. Normal-metal contacts were also made, in which the internal connections are made by soldering copper wires into small holes in the rings. For niobium rings and for Nb-Ti wire, however, soldering is not an option due to the poor wetting of niobium metal by the solder. The internal connections were made by crimping 0.1 mm Nb-Ti wires into a piece of niobium capillary and then spot welding this to the inner surface of the ring. These contacts were mechanically stable and easy to make.
crimping, it was found that some reshaping with pliers was necessary for it to fit into its support. After reshaping, the capillary was inserted until it reached the bottom of the larger hole in the sleeve, and then the protruding end was clipped close to the brass sleeve with wire cutters, producing a sharp edge which cuts through the oxide. At this point the contact was spring loaded into a brass piece which simulated that at the bottom of the top-loader: a spring behind the insulator kept the contact held firmly against the appropriate ring when the slug was screwed into the bottom of the piece. For the tests in question, two niobium rings were used which were connected internally by spot welding the two Nb-Ti wires together. Externally, each had two copper wires spot welded to the piece of capillary to make a four wire measurement of the joint resistance.
Results The entire apparatus was immersed in liquid helium, where resistance and critical current measurements were taken. These measurements were repeated several times, with the joint brought back up to room temperature and the slug unscrewed and then screwed into place again. When the joint became fully superconducting, the critical current was found to range from 3 to 40 mA. On some of the trials there was a measurable resistance at 4 K, ranging from 0.15 to 10 Q. For the case in which the residual resistance was 0.15 f~, a critical current of 0.7 mA was measured, which sets a minimum for the critical current given a fully superconducting joint. After a few trials, these 'bad' contacts were easily recognizable by their erratic resistance changes Upon cooling. Superconducting joints made in this manner were easily constructed, and the niobium tips showed no sign of bending or rotation after the experiments were finished. They also have the advantage that the niobium tips are cut from capillary, alleviating the need for machining bulk niobium. Figure 1 is a diagram of the slug and the spring-loaded contacts.
Acknowledgements Experimental s e t - u p As the slug is screwed into the bottom of the top-loader, spring loaded contacts complete the circuit. To penetrate the niobium's oxide, the contacts were made as follows: an insulating piece identical to those used with copper contacts was made, as was a cylindrical piece of brass, 1 mm shorter than the corresponding copper contact, and of the same diameter. In this brass piece, a small hole was drilled for the wires to pass through, as well as a larger hole into which a piece of 1 mm o.d. niobium capillary would be inserted. The diameter of this second hole was 0.05 mm larger than the diameter of the capillary, and extended about three quarters of the way down the piece. To assemble the contacts, two wires were passed through the insulating piece, the back end of the brass sleeve, and then crimped and spot welded them in the end of a piece of niobium capillary, about 4in long. Since the capillary was deformed appreciably due to the
This work was supported by the National Science Foundation, Grant No. DMR88-8516616 AO2.
Niobium Nylon Brass Copper
Figure 1 Diagram of slug and spring-loaded contacts. Points designated A are the superconducting contacts, while point B shows the position of the sample cell
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Cryogenics 1989 Vol 29 December