Critical current transport measurements

Critical current measurements have been made with a variety of techniques. The indirect technique, that of obtaining the critical current from the magnetization response is discussed in Chapter 18. Direct transport measurements, using attached current and voltage leads, and indirect measurements requiring macroscopic current circulation will be discussed. Critical currents are desired as a function of both temperature and applied magnetic field since a variety of theories discuss the functional relationship. And applications may require either or both of these data. 

The comparison of critical current density obtained from direct transport measurements (termed Jct) with that inferred from... 

Figure 14, Transport of a 24 m silver-sheathed Tlo,5Pbo,5Bao.4Sri,6Ca2Cu308.25 tape. Measurements were made under liquid N2 by the four-point method using the second and third contact points as voltage taps. The same criterion of AF = 1 pV was used for all distances to determine the critical current, which decreased from 8.5 A to 7.0 A as the distance d between the voltage taps increased from 1 cm to 2375 cm.

Direct transport measurements are generally employed on thin films, tapes and coated conductors. They quickly reach their natural limits as the currents go up and/or the cross-section of the conductor increases. They are fast, i.e., can be started immediately after the magnetic field is set, and trace the I-V characteristics, which is of course most valuable if flux creep or thermally assisted flux flow (Kes et al. 1989) prevails, because the electric field is directly determined. On the other hand, must be defined by a criterion e.g., 1 [xV/cm. It will be affected by the presence of stabilising materials, the silver tube or conducting substrates, and thus generally refers to the overall Jc, wfrich can be converted to the critical current density of the superconductor if the shape and the volume fraction of the superconducting material are known. 

Fig. 43. Effects of heavy-ion irradiation under different angles on flux pinning in Y-I23 superconductors (a) hysteresis loops at (top) 5 K and (bottom) 70 K of an Y-123 single crystal irradiated with 580 MeV Sn ions under an angle of 30° with respect to the c-axis and measured with the field at +30° or -30° from the c-axis (cf. the schematic of the field and track orientations, from Civale 1997) (b) angular dependence of the transport critical current densities in Y-123 thin films irradiated under various angles with respect to the c-axis (top parallel to the c-axis, 340 MeV Xe middle 30°, 770 MeV Pb bottom 60°, 340 MeV Xe from Kraus et al. 1994b) (c) angular dependence of the transport critical current densities in an Y-123/Pr-123 multilayer system irradiated by 770 MeV Pb ions under an angle of 30° with respect to the c-axis (Kraus et al. 1994b).

The electrode is a critical component in the electrochemical measurement of diffusion coefficients. The general type of electrode to be used (i.e., UME or conventional electrode, see Chapters 5,6, and 11 for more detail) has a fundamental impact on the type of measuranent to be made. As shovra in Figure 19.1a, at a conventional electrode diffusion normally is planar (i.e., comes from essentially one direction toward the electrode surface). With planar diffusion there is a depletion of the redox species close to the surface, resulting in a current that decays with time. As such, under planar diffusion control the current is measured as a function of time. With an RDE, forced convection causes efficient, steady-state transport of species to the electrode surface (Figure 19.1b) and time-independent current results. With... 

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