A.D. Caplin

Pinning mechanisms in Ag-sheathed Bi(Pb)SrCaCuO tapes

Abstract The pinning properties of Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3O10+y (2223) wires have been studied in comparison with the normal powder, and the powder scraped from the 2223 tape by the measurements of irreversibility lines (IL). The IL for both the tape and the powder scraped from the tape are nearly equally shifted to higher temperatures in comparison with that for the normal powder. This indicates that the enhanced IL is attributable to the defect pinning. TEM studies reveal that the grains within Ag-clad 2223 tape contain high concentrations of dislocations (1010/cm2) and stacking faults that may act as effective pinning sites. The results on the transport and magnetisation Jc suggest that the critical current is controlled by flux pinning at high temperatures, while the weak links limit the critical current at low temperatures.

Enhancement of critical current density of (Pb,Sn)-doped Bi-2212 superconductors at high temperature

Abstract The magnetic properties of ceramic samples of Bi 2−2 x (PbSn) x Sr 2 CaCu 2 O y ( x =0,0.1,0.2,0.3 and 0.4) superconductors have been investigated using a vibrating sample magnetometer from 20 to 50 K and up to 8 T. Phase analysis shows that most of the (Pb,Sn) dopant enters the Bi-2212 lattice and occupies Bi sites. The current-carrying scale-length analysis has been determined from the reverse leg of the magnetization hysteresis loops and compared to the actual dimensions of the sample. Magnetization hysteresis loops of the (PbSn)-doped sample exhibit strong pinning behaviour in the higher temperature range. The effect of (Pb,Sn) doping on intragranular critical current J c was found to be enhanced by one order of magnitude at 50 K. We discuss the experimental results within a model of (PbSn) cluster pinning in blocking layers. Doping by (PbSn) clusters into the Bi–O layers provides effective pinning centres, as well as a large reduction of the resistivity anisotropy.

‘Strong’ grain boundaries in Ag-BSCCO(2223) tapes

Abstract The remarkable improvement of the superconducting properties of polycrystalline BSCCO(2223) when processed into Ag-clad tapes has been studied magnetically. Two factors contribute to the excellent tape performance: Over the investigated range of field and temperature, the grain boundaries show no evidence of being ‘weak-links’. Also, intra granular flux pinning is much stronger than might be expected from single crystal data, and appears to determine the tape critical current.

Dissipation in granular high-temperature superconductors

The current-voltage characteristics of bulk-sintered YBa2Cu3O7 have been studied in detail. The onset of dissipation indicates that an intergranular critical current can be defined. The magnitudes of the resistance of a single grain boundary and of its critical current are estimated. These estimates lead to values of the rnic(0) product for the intergranular weak-links that are of the order of 100 μV, well within the wide range of rnic(0) product measured on bi-crystal grain boundaries by Dimos et al., but two to three orders of magnitude less than that expected from “classical” models of superconducting weak-links.

Microstructure and electrical properties of YBCO thin films

Microstructures of c-axis oriented YBCO thin films made by high-pressure d.c. sputtering on LaAlO3 and MgO substrates were examined by TEM. The a-axis oriented grains, second phases and micro-twins were frequently observed in the film. The a-axis oriented grains expanded along their c-axis directions during film growth. The a- and b-axis misorientations were observed in the film on MgO due to serious lattice mis-match between YBCO and MgO. The second phases were often accompanied with a-axis oriented grains suggesting they act as nuclei. These observed results were correlated with the measured Tc and Jc of the films.

Effect of mesoscopic inhomogeneities on the critical current of bulk melt-textured YBCO

Abstract The downsizing 211-inclusions and an increase of their density leads to rise in mean critical current value in Y-based melt-textured material. Very often 211-inclusion are spread in the material volume non-homogeneous, with typical scale 50–100 μm. Therefore it is difficult to find the real correlation between local critical current and the inclusions distribution. We performed a study of a local critical current using modified magneto-optic technique on a melt-textured YBaCuO ceramic, found the areas with constant current and studied the real structure of the material in the areas, inclusions distribution and their sizes, by scanning electron microscopy and X-ray microanalysis. The estimation of a pinning in these places, by taking into account the amount of inclusions and the length of their boundaries, and comparison with the value of local critical current reveals a strait correlation between the density of inclusions and the current but shows remarkable quantitative disagreement.

Non-destructive measurement of the critical current and the current carrying length scale in superconducting crystals and films

Abstract In superconductors having a plate-like geometry, valuable information can be obtained from detailed examination of the magnetic hysteresis loop. The length scale Λ on which screening currents flow, and the sheet critical current density K c can be evaluated independently. Because of internal weak-links, Λ may be less than the sample dimension, and will change with field and temperature. Experimental results on crystals and films confirming the validity of the analysis are presented.

Magnetic and transport studies of Bi2223/Ag tapes☆

Abstract The technological importance of the Bi2223/Ag tapes has been widely recognised and yet their electrical transport properties, particularly in an applied magnetic field, remain inconclusively. We present analysis of magnetisation loops and transport current-voltage characteristics taken on the same sample. Using the dynamic relaxation technique and the transport data we can extend our E - J curves over 5 orders of magnitude in electric field. The resultant E - J characteristics reveal clues as to the current limitation between the grains and the vortex dissipation within the grains. In an applied magnetic field J c behaves hysteretically giving further insight as to the granularity of these samples. We offer a comparative study of tapes with differing values of J c .

Crystal regrowth of YBCO thin films by ion implantation and rapid thermal annealing

Abstract The overall aim of this work is to engineer YBCO films with well-characterised crystal structures, by selective patterning, amorphisation, and annealing. Thin films (80 – 400 nm) of YBa 2 Cu 3 O 7-δ , deposited by sputtering onto LaAlO 3 and MgO substrates, have been implanted at 200 keV with either 20 Ne + or 18 O + ions, to a dose of 5x10 14 ions cm −2 . Both implantations amorphise the film, in some cases totally. Oxygen implantation produces less damage than the inert gas. Rapid Thermal Annealing (20 seconds, about 870°C, in oxygen) can induce recrystallisation, leading to a recovery of the superconducting 123 phase under certain conditions. Results from T c , J c , XRD, and TEM investigations are presented.

Variable range hopping conductivity in Si:As bolometers

Abstract The temperature dependent resistance R(T) of ion-implanted Si:As bolometers has been measured in the variable range hopping regime, with R(T) = CT 2m eχp( T m T ) m , for 1.6 > T > 0.2 K . The exponent m changed from 0.5 to 0.28 as the dopant concentration approached the metal-insulator transition. These sensitive bolometers, with an implant depth of 0.1 microns, are limited by the thermal contact between electrons and phonons below 0.1K.