Ac Inductive Method Research Articles

第３高調波誘導法による高温超電導薄膜・コート線材の 臨界電流密度と<i>n </i>値の磁界角度依存性の測定

Nondestructive, AC inductive methods detecting third-harmonic voltages are widely used to measure the distribution of local critical current densities Jc of superconducting films deposited on large-area single-crystal substrates. We have extended this method to determine the electric field E versus current density J relation and the n-value (index of the power-law E-J characteristics) by evaluating the dependence of Jc on measurement frequency. The method to determine Jc with an electric-field criterion has been established as an international standard. This convenient method can also be applied to coated conductors with metallic substrates. Magnetic-field angular dependent Jc measurements are possible, and such measurements are of practical importance in applying coated conductors to superconducting coils. They are also useful in investigating the flux pinning mechanism. In this focused review, we briefly introduce the measuring method that has been based on our long years of research, and describe the precautionary points when recent coated conductors with high critical currents per unit width are measured.

Nondestructive inductive measurement of local critical current densities and n-values in coated conductors

Nondestructive, AC inductive methods detecting third-harmonic voltages are widely used to measure the distribution of local critical current densities Jc of large-area superconducting films. We have extended this method to determine the electric field versus current density relation by evaluating the dependence of Jc on measurement frequency. This powerful method can also be applied to coated conductors whose critical currents per unit width are larger than 25kA/m. However, the n-values may be underestimated due to the skin effect and the obtained Jc values may be inaccurate when a thick metallic stabilizing layer of Ag or Cu (⩾10μm) covers the film of (RE)Ba2Cu3O7 (abbreviated to (RE)BCO, where RE stands for a rare-earth element). To overcome this problem, a useful method is to limit the measurement frequencies to less than ∼8kHz. We illustrate the feasibility of this approach with several examples of measurements in YBCO- and GdBCO-coated conductors. We also demonstrate that magnetic-field angle dependent Jc measurements are possible for coated conductors. Such measurements are of practical importance in designing superconducting coils and are also useful in investigating the flux pinning mechanism.

Estimation of Critical Current Densities in Polycrystalline ${\rm Sr}_{0.6}{\rm K}_{0.4}{\rm Fe}_{2}{\rm As}_{2}$ Superconductors

The superconducting Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.6</sub> K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.4</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (122 type) polycrystalline iron-based pnictides with Pb or Ag addition were prepared by a normal PIT method. The critical current densities were investigated by ac inductive (Campbell's) method. It was found that a fairly large critical current density over 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> A/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> even at the relatively high temperatures flows locally with the perimeter size similar to the average grain size of the bulk samples, while an extremely low global critical current density of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> A/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> flows through the whole sample. Furthermore, a unique history dependence of global critical current density was observed, i.e., it shows a smaller critical current density in the increasing-field process than that in the decreasing-field process. Basing on the experimental results of the penetrating ac magnetic flux profile and the characteristic of ac magnetic field vs. penetration depth, the inter- and intra-granular super-current, the weak-link and the history effect in these materials were discussed.

Critical current characteristics and history dependence in superconducting SmFeAsOF bulk

The superconducting SmFeAsO1−xFx (x=0.2) polycrystalline bulks were prepared by the powder-in-tube (PIT) method. The magnetic field and temperature dependences of critical current densities in the samples were investigated by resistive and ac inductive (Campbell's) methods. It was found that a fairly large shielding current density over 109 A/m2, which is considered to correspond to the local critical current density, flows locally with the perimeter size similar to the average grain size of the bulk samples, while an extremely low transport current density of about 105 A/m2 corresponding to the global critical current density flows through the whole sample. Furthermore, a unique history dependence of global critical current density was observed, i.e., it shows a smaller value in the increasing-field process than that in the decreasing-field process. The history dependence of global critical current characteristic in our case can be ascribed to the existence of the weak-link property between the grains in SmFeAsO1−xFx bulk.

The defect detection in HTS films on third-harmonic voltage method using various inductive coils

We purposed and tested the contactless ac inductive method to estimate the critical current density of high temperature superconducting (HTS) films by detecting a third-harmonic voltage. In this study, a minimum diameter of the HTS film to estimate the correct critical current density was experimentally investigated using three miniature inductive coils with various sizes. It was clarified that the correct critical current density could be measured and estimated when the diameter of HTS films was about 1.2 times larger than outer diameter of each tested inductive coil. The accuracy of detection for micro-scale defects with various shapes in HTS films was studied using miniature inductive coils by the third-harmonic voltage method. And it was possible to detect the defects in HTS films by obtained the slope values of initial region at generated third-harmonic voltage traces.

Automatic Measurement of the Distribution of $J_{\rm c}$ and n-Values in Large-Area Superconducting Films Using Third-Harmonic Voltages

Nondestructive, ac inductive methods are widely used to measure critical current densities Jc of large-area superconducting films. However, conventional methods are not accurate because they have not considered the electric-field E criterion of the Jc measurement. In these methods, ac magnetic fields are generated with ac currents Io cos wt in a small coil mounted just above the film. Jc is calculated from the threshold coil current Ith, at which full penetration of magnetic field is achieved. We recently estimated the induced electric fields in the superconducting film at Io = Ith, which are proportional to the frequency, and proposed a method to determine the E - J characteristics by measuring Ith at various frequencies. In this paper we propose a standard method to precisely measure Jc with an electric-field criterion by detecting third-harmonic voltages. We also demonstrate a machine that can automatically measure the distribution of Jc and the n-values of the power-law E - J characteristics. We show several examples of measurements of co-evaporated YBCO films and low-cost metal organic deposition YBCO films, which were used in switching experiments for resistive fault-current limiters.

Precise Determination of the Threshold Current for Third-Harmonic Voltage Generation in the ac Inductive Measurement of Critical Current Densities of Superconducting Thin Films

In an ac inductive method to measure critical current densities J/sub c/ of superconducting films by detecting third-harmonic voltages V/sub 3/ cos 3/spl omega/t, ac magnetic fields are generated with ac currents I/sub 0/ cos /spl omega/t in a small coil mounted just above the film. J/sub c/ is calculated from the threshold coil current I/sub th/, at which full penetration of magnetic field is achieved and V/sub 3/ starts to emerge. For convenience, I/sub th/ has been usually determined by a constant-voltage criterion, e.g., V/sub 3///spl radic/2 = 50 /spl mu/V. However, V/sub 3/ is proportional to I/sub th/, and the determination of I/sub th/ by the constant-voltage criterion inherently causes a systematic error. We confirmed this by comparing the obtained J/sub c/ with those obtained from dc magnetization and by a transport method. We propose to use a constant-resistance criterion, such as V/sub 3//I/sub 0/ = 2 m/spl Omega/, as an alternative method to determine I/sub th/ accurately.

Evaluation of E– J characteristics of YBCO coated-conductor by AC inductive method using third-harmonic voltage

The E– J characteristics were measured for a YBCO coated-conductor by using AC inductive method of third-harmonic voltage. The results are compared with the measurements obtained by the four probe method and DC relaxation method of magnetization. The results by three kinds of measurements meet approximately on a single curve and E– J curve in a wide range of electric field is obtained. Hence, the third-harmonic voltage method is useful for a continuous measurement of J c property of long coated tape. The obtained E– J characteristics are argued.

Nondestructive inductive measurement of critical current densities in melt-textured YBa 2Cu 3O 7 and SmBa 2Cu 3O 7 using third-harmonic voltages

Local critical current densities J c of large melt-textured YBa 2Cu 3O 7 and SmBa 2Cu 3O 7 samples were nondestructively measured using a previously developed ac inductive technique [Appl. Phys. Lett. 83 (2003) 3972]. In this method an ac magnetic field is generated by an ac drive current ( I 0cos ωt) in a small flat coil placed just above the superconductor, and a third-harmonic voltage ( V 3cos3 ωt) generated in the same coil due to the nonlinear magnetic response of type-II superconductors is measured. Because V 3 is proportional to I 0 2 and inversely proportional to J c, J c can be measured in the surface region (typically down to ∼0.1 mm beneath the surface) of the bulk superconductor by measuring V 3 as a function of I 0. The magnetic-field dependence of J c agreed reasonably well with that measured by using dc magnetization, if the effect of induced electric fields on J c is taken into account. By this ac inductive method J c can be measured by electric-field criteria ( E c≈10 −4–10 −2 V/m) that are higher than those generally used in transport measurements of J c ( E c=10 −4 V/m). We can measure the distribution of J c in a large superconducting sample by scanning the measuring coil over the entire sample surface.

Possible superconductivity in Au(dmit) 2 Langmuir–Blodgett films

The complex magnetic susceptibility was measured for the Langmuir–Blodgett (LB) films of ditetradecyldimethylammonium-Au(dmit) 2 (2C 14-Au(dmit) 2) using an ac inductive method. A diamagnetic transition of the real part ( χ′) was observed at around 4 K under a weak ac field of 0.1 mT (80 Hz) and it was shifted toward the lower side for about 0.6 K with superposing a dc field of 60 mT. These results suggest existence of a superconducting phase in the 2C 14-Au(dmit) 2 LB system.

Use of the ac inductive method to assess the quality of bulk melt textured superconductors

Through a measurement of flux profiles, the inductive method of Campbell has not only been used for determining the bulk critical current density of large sized samples but has also been extended to study the coupling strength of the domains in melt textured NdBa 2Cu 3O 7− δ superconductors. An analysis of the profiles measured for the slow (1°C/h) and fast cooled (40°C/h) samples through peritectic temperature, T p clearly showed that the domain boundaries in the fast cooled samples are decoupled in the presence of external dc fields. The observed features of flux profiles are correlated to the microstructural features of the samples. It is proposed that this technique can be used routinely to assess the quality of large size melt textured samples for high current applications like magnetic bearings or quasi permanent magnets.

Measurement of the transition temperature distribution and magnetic field profile of large superconducting films

A novel apparatus has been developed to measure the transition temperature (T/sub c/) distribution and magnetic field profile of large superconducting films. Two techniques were used. The first one is an ac inductive method which provides a contactless measurement of T/sub c/. The second technique employs a Hall device to measure magnetic field. Driven by two step motors, the measuring probe, either a two-coaxial-winding coil or a Hall sensor, can move over the sample on a polar plane. As the probe scans the sample step by step, various points on the sample are measured one by one. Experiments were carried out on a 30 mm diameter YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin-film. The T/sub c/ distribution and the surface profile of trapped magnetic field were obtained.

Homogeneity measurements of transition temperature and critical current density of large area HTSC thin films

An apparatus for measuring the homogeneity of the transition temperature (Tc) and the critical current density (Jc) of large area HTSC thin films has been developed. Two step motors are used to control the movement of the scanning probe on a polar plane. There are two techniques included. The first is an ac inductive method which provides a nondestructive measurement for determining Tc. Based on the Bean model, the second technique, for detecting the uniformity of Jc, uses the surface magnetic field distribution of the material to derive the Jc profile. Measurements were carried out on a 30mm diameter YBCO sample. A Tc map and surface distribution of the trapped field after field cooling which responds to the Jc profile were obtained.

Temperature dependences of the critical current density and the pinning potential near the irreversibility temperature in YBa2Cu3O7- delta

In order to clarify the mechanism of the irreversibility line in high-Tc oxide superconductors, we measured temperature dependences of the critical current density, Jc, and the averaged pinning potential, Up, using an ac inductive method and investigated the flux motion in the pinning potential well in the vicinity of the irreversibility temperature, Tirr. From our experimental results, it has been found that Up decreases with increasing temperature and becomes nearly equal to the thermal activation energy, kB T, at Tirr. This result suggests that the irreversibility line is the thermally induced depinning line.

AC response and losses in high-Tc superconductors at power frequency

Using ac inductive method, magnetization curves of some high-Tc superconductors, ordinary ceramic YBCO and BPSCCO as well as melt processed YBCO, were obtained in the frequency range 10~500Hz with field amplitudes below 0.3T. Then ac losses were calculated as a function of a field amplitude and were discussed in terms of critical state model, granularity and flux diffusion.

Granular effects in superconducting YBa 2Cu 4O 8 ceramic

Two characteristic transition temperatures have been observed in superconducting YBa 2Cu 4O 8 ceramic by means of an ac inductive method. Our susceptibility data from a powdered sample, and a bulk sample at different magnetic field strengths, show that the two temperatures correspond to superconducting transition at the intergrain region T c(inter) and at the intergrain region T c(intra) ; the latter gives a weak diamagnetic signals. We found that T c(inter) was gradually lowered when the sample was subjected to increasing quenching temperatures, while T c(intra) was unaffected by such treatment, the latter being consistent with the oxygen stability of YBa 2Cu 4O 8. The small susceptibility signals occuring at T c(intra) will be discussed.

Ｂｉ‐Ｐｂ‐Ｓｒ‐Ｃａ‐Ｃｕ‐Ｏ超電導テープ線材における臨界電流密度の履歴効果と弱結合

Critical current density and its history effect in Bi-based high-Tc superconducting tapes were measured by ac inductive method in various magnetic fields and temperatures. A rapid decrease in the critical current density with elevating temperature and the history effect were observed in the same low temperature region between 4.2K and 30K. The coincidence suggests that the two phenomena are caused by “low temperature weak links” in specimens. Weak link region is considered to lose superconductivity at temperature higher than 30K. Above 30K, strongly bonded area retains superconductivity and Jc changes slowly with increasing temperature showing no history effect.

An investigation of inter- and intragranular effects in superconducting YBCO specimens using AC inductive methods

The complex AC susceptibility ( chi (T,bo,B)) for two cylindrical YBa2Cu3O7-x samples has been measured. The critical current densities for the inter- and intragranular components are extracted from these results and the volume fractions of the components are calculated using various models. The solvent used during milling of the starting powders seems to have a marked effect on both of these parameters.

Axial anisotropy of intergrain critical current in Bi-Pb-Sr-Ca-Cu-O

The intergrain critical current densities, Jc, parallel and perpendicular to c-axis were measured as a function of the magnetic field at 4.2 and 77.3K by an ac inductive method for the fairly oriented Bi1.6Pb0.4Sr2Ca2Cu3Oy specimens. The Jc's perpendicular to c-axis were higher than those parallel c-axis, about ten times within 5T at 4.2K, and about three times within 0.1T at 77.3K. Our results suggect the orientation of each grain to be especially worthwhile for performing the high Jc at the low temperature.

Nonsaturated Global Pinning Force in Superconducting Nb–Ta with Normal Precipitates

Nonsaturated pinning characteristic and elastic and plastic properties of a fluxoid lattice are measured by an ac inductive method for superconducting Nb–Ta tapes with normal Nb 2 N precipitates. The interaction distance proportional to a yield strain of the fluxoid lattice is shown to be approximately constant except in the vicinity of the upper critical field, indicating that the fluxoid lattice is stabilized by strong pinning interactions by normal precipitates. The dependence of the pinning force density on the pinning parameters estimated from electron micrographs agrees better with the linear summation theory than the theory of Larkin and Ovchinnikov.