 |
Magnetic Imaging of Superconducting Tapes
High temperature superconductors will likely play an important role in future electrical power transmission and generation applications because of their ability to conduct large amounts of current without power loss. The form of superconductor we are interested in is the coated conductor tape that is being produced by the Superconductivity Technology Center at Los Alamos.
Currently the STC makes YBa2Cu3O7-ub> coated conductor tapes greater than 1 meter long that carry nearly 200 amperes. An important aspect of continued YBa2Cu3O7-ub> (YBCO) coated conductor development is the identification and elimination of isolated regions of low Jc that sometimes occur in otherwise high Jc tapes. These "bad" regions limit the end-to-end critical current of the whole tape, even though they may comprise less than 1% of its length. As there are usually no obvious correlating defects in the substrate, these "bad" regions must occur during the growth and processing of the buffer layers or YBCO film.
 To understand these defective regions, we have developed a magnetic imaging system that allows us to see the transport current pathways and identify the nature of the defects that lead to low critical currents. Figure 1 shows a schematic of our apparatus.
Details of this apparatus are published elsewhere. Briefly, the tape sits at the bottom of a dewar full of liquid nitrogen while a magnetoresistive read head from a hard disk drive is scanned over its surface. The current flowing through the superconductor produces magnetic fields that induce proportional voltages in the head. These voltages are mapped as a function of scan position and since the response of the read head is linear for the fields we use, the maps represent magnetic field maps. We can then invert the magnetic field maps to current flow maps using the Biot-Savart law.
 Figures 2 and 3 show the results for a small test sample.
Figure 2, left, shows the field and magnetic field maps from a sample of YBCO for a low current (i.e., below the critical current).
 Figure 3, right, shows the results for a large current (i.e., above the critical current). The sample was 0.3 micron thick YBCO that was deposited on single crystal SrTiO3. If this were a conventional conductor, the current flow would be peaked in the middle. Since this is a superconductor however, current flows when magnetic vortices penetrate the material and this occurs from the edges inward since the vortices nucleate there first. If this were a thick film superconductor, the field and current profile would be triangular and step functions respectively because the current would flow only at the critical current density within the material. Since this is a thin film though, demagnetizing effects cause current flow in the center of the sample with peaks at the edges for current levels below the critical current. As the current is increased, the flow in the middle increases until a flat profile is achieved at the critical current. Above the critical current, the flow begins to peak in the middle as the vortices that nucleated on opposite sides meet in the middle and annihilate dissipatively. Defects in the materials at current levels below the critical current will hinder the current flow and cause it to bunch up or move into new areas.
 Figure 4, left, shows the field map, current map, and optical micrograph from a sample that had a known defect based on a low end-to-end critical current. The field map shows two glitches that correspond in the current flow maps to a region that has a low local critical current and a region that has an edge defect that is being avoided. The optical micrograph shows a cut in the substrate at the edge corresponding to the avoidance defect and extra pitting in the region corresponding to lower local critical current. It may be that this region is the result of the YBCO delaminating from the substrate.
 Figure 5, right, shows a critical current plot from a YBCO coated conductor sample that has an obvious low spot near the middle. We imaged this region at 20 amperes with the result shown below.
 Figure 6, left, is the current flow map for good and bad sections from the sample represented in Figure 5. Obviously the current transport problem is manifested as a constriction in the flow pattern that makes the sample effectively only 0.5 cm wide in the bad region. To understand why this occurs, we looked at high resolution x-ray diffraction from the bad region.
 Figure 7, right, shows one result from the bad region which plots the 005/004 peak ratio which is related to the ratio of c-axis to a-axis YBCO in the area. C-axis YBCO is known to be the optimum orientation for current flow along the tape direction. As we see from the plot then, one problem is that there is less c-axis YBCO in the bad region than there should be. This could be the result of a lower deposition temperature during film growth in the bad area that could have resulted from a slight buckling in the tape during deposition.
|
Related Links
|
Structure/Property Relations, MST-8