Some magneto-dynamic properties of type II superconductors
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The possibility of making a refrigerator by mechanically induced flux flow to transfer heat in a type II superconductor has been examined. Heat generation due to flux flow was sufficient to destroy the cooling effect over most of our experimental temperature range, although some cooling was obtained near T[sub]c. The dissipation due to flux pinning was determined by the pinning strength of the specimen. The pinning forces in Nb and Pb-In were determined by mechanically sweeping a magnetic field over them and measuring the resulting force. Three different forces due to the interaction of the magnetic field, with (a) the trapped flux lines, (b) the Meissner screening current, and (c) the pinning sites of the specimen, were identified. The magnitude of the force due to the screening current was proportional to the field strength, and was reduced by the trapped, flux lines inside the specimen. The magnitude of the bulk pinning force was in agreement with that calculated from the magnetization via the Irie-Yamafujii model; F[sub]p = 𝛼B<p>[super] 𝛾. The dependence of the pinning force and the magnetic properties on the temperature and on the surface treatment of the specimen was also studied. The temperature dependence of the pinning force was found to be given by F[sub]p(T) = F[sub]p(o)(1 -T/T[sub]c ) it is suggested that this can be understood in terms of Anderson's flux creep model. The critical current of various specimens was obtained from the pinning force and the magnetization. In applying the Irie-Yamafuji model, the demagnetization and the surface effects of a specimen had to be taken into account, and methods of doing these have been suggested and experimentally checked. The role of the magneto-caloric effect on the mixed state specific heat was discussed. A 'two-fluid' type equation was proposed and used to calculate the mixed state specific heat. The results were compared with the experimental measurements. With the above experiments, the dissipation associated with flux flow was studied in terms of the pinning and viscous forces. It was shown that dissipation due to pinning was dominant in low fields and that viscous forces gradually became important as the field increased. It was found that the motion of the flux lattice inside a type II superconductor could not be induced by moving the source of a uniform magnetic field. This showed that the concept of 'lines of flux' has no meaning for the case of a uniform field.
Thesis, PhD Doctor of Philosophy
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