Measurement of Critical Current

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 current transfer problem that had been identified with low temperature superconducting composites deserves additional mention for the high temperature superconductors, that in the bulk material are frequently not fully dense. Making the electrical connection in such a manner as to obtain uniform current distribution throughout the cross section of the material is difficult. The method described by Jin, et al. (24) with embedded wires or particles may provide for a significant improvement but the present techniques used to determine the critical current by a surface contact on the ceramic sample are subject to this problem. A discussion for the multifilamentary wire of NbsSn is provided by Goodrich and Fickett (30) and this discussion is likely to be similar to the high temperature materials that are not fully dense. 

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Figure 8 Pulsed current technique for the measurement of critical current. From Jones, T., McGinnis, W., Boss, R., Jacobs, E., Schindler, J., Rees, C., Tech. Doc. 1306 July 1988 Naval Ocean Systems Center, San Diego, CA.

Earlier work (1987-1989) took no account of these factors, and thus may contain serious errors. In more recent investigations, improved physical characterization has become more common practice in electrochemical studies on the HTSC problem, and the range of methods used has expanded rapidly, e.g., measurement of critical currents, magnetic-field tests, studies of anisotropy, etc. 

Measurements of critical current densities in solutions containing known concentrations of additives are available only for the growth of Ag whiskers in AgNOg solutions containing gelatin. At 25°C the constant in... 

Temperature measurement(s), 24 433-467, 75 469, 77 783-784 of critical current density, 23 847-848 fixed-point thermometer calibration,... 

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

Some disadvantages are associated with this system. A first disadvantage is the disappearance of the continuous character of the sensor since a certain period of time elapses between the measurement of electrical current at the sensor surface and the moment in time when the sample to be analysed leaves the process bath. Whereas the developed sensor is intrinsically a continuous working system, it is clear that a basically discontinuous system can be considered as virtually continuous when the duration of the measurement is situated below a specific critical threshold, the dead time for every considered application. One of the tasks of the research is to keep the dead time as short as possible, and if necessary take this into account when the global process is directed by means of the output signal of the sensor expanded with a FIA system. 

In this work, we have measured the critical current density of YBa2Cu307 thin film in strong magnetic field H up to 10 T at various angle 6 between H and the crystallographic c-axis. We have compared our results to the intrinsic pinning model proposed by Tachiki and Takahachi [7],... 

The resonance frequency of mixer-amplifier tank circuit is measured to be 1.5 GHz. The SQUID amplifier has the following experimentally measured parameters critical current and normal state resistance per junction of front end SQUID are Ic=40 pA and Rn=4 Q, squid inductance is 40 pH, mutual input inductance is 400 pH and input inductance is 5 nH providing the amplifier noise temperature Tn=90 K. 

A portion of the film on SrTi03 shown in fig. 13(b) was patterned into a 16 urn wide line by reactive ion etdiing to measure the critical current density and for other magnetotransport measurements. Figure 14 shows the temperature dependence of the critical current density, J. It can been seen that at 77 K and in zero field, a current density of 0.69 x 10 A/cm is measured. The film critical current density is greater than 4 x 10 A/cstr at 50 K and in a field of 14 Tesla. More detailed magnetotransport results will be mentioned later. 

In this paper, some results of critical current and magnetization measurements are discussed. The tests were carried out to obtain actual values of the components of the critical current density. These values can be used to calculate losses in multifilamentary composites irrespective of the magnetic field configuration relative to the axis of the composite. 

Undesired side reactions occur under starvation conditions, as discussed in detail in Section 20.4. These cell states are caused by an inhomogeneous distribution of reactant gases, temperature, and local water content across the active area during fuel-cell operation. Measurement of the current density distribution allows the detection of these critical conditions. However, gradients of the electrochemical reaction rates are mostly compensated by in-plane currents within the catalyst layer, gas diffusion layer, and bipolar plates. Therefore, for measurement of the current density distribution, the local current flow has to be separated into individual pathways. 

In contrast to a direct injection of dc or ac currents in the sample to be tested, the induction of eddy currents by an external excitation coil generates a locally limited current distribution. Since no electrical connection to the sample is required, eddy current NDE is easier to use from a practical point of view, however, the choice of the optimum measurement parameters, like e.g. the excitation frequency, is more critical. Furthermore, the calculation of the current flow in the sample from the measured field distribution tends to be more difficult than in case of a direct current injection. A homogenous field distribution produced by e.g. direct current injection or a sheet inducer [1] allows one to estimate more easily the defect geometry. However, for the detection of technically relevant cracks, these methods do not seem to be easily applicable and sensitive enough, especially in the case of deep lying and small cracks. 

Application of this method or Eq. (3-25 ) in the presence of stray currents is conceivable but would be very prone to error. It is particularly valid for good coating. Potential measurement is then only significant if stray currents are absent for a period, e.g., when the source of the stray current is not operating. In other cases only local direct measurements with the help of probes or test measurements at critical points can be considered. The potential test probes described in Section 3.3.3.2 have proved true in this respect. 

As shown in Fig. 25, an example of the extrapolation of the current transient obtained from the potential sweep yields the critical potential after ascertaining that the data obtained are independent of the sweep rate. Figure 26 exhibits the results of the critical pitting potential measurement for the majority salt of NaCl and the minority ion of Ni2+when the concentration of NaCl is varied under the condition of constant Ni2+ionic concentration. From the plot in Fig. 26, it follows that... 

Fig. 4 shows the current density over the supported catalysts measured in 1 M methanol containing 0.5 M sulfuric acid. During forward sweep, the methanol electro-oxidation started to occur at 0.35 V for all catalysts, which is typical feature for monometallic Pt catalyst in methanol electro-oxidation [8]. The maximum current density was decreased in the order of Pt/CMK-1 > Pt/CMK-3 > Pt/Vulcan. It should be noted that the trend of maximum current density was identical to that of metal dispersion (Fig. 2 and Fig. 3). Therefore, it is concluded that the metal dispersion is a critical factor determining the catalytic performance in the methanol electro-oxidation.

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