# Effect of grain network modifications due to additives on critical current (Ic) in Bi-2212 superconductor

01 May 1999-Physica C-superconductivity and Its Applications (North-Holland)-Vol. 316, Iss: 1, pp 119-128

Abstract: The presence of weak links in high-Tc superconductors is a major drawback in achieving high critical current densities The present study deals with the effect of additives (Ag, Ag2O and BaBiO3) on the weak link behaviour of Bi-2212 in bulk form It is observed that the addition of silver (Ag) metal powder improves the connectivity between the grains by proximity-effect and thereby enhances the current carrying capability at temperatures very close to Tc De Gennes and Clarke models have been invoked to establish that Bi-2212 containing insulating additives (BaBiO3 and Ag2O) show a superconductor–insulator–superconductor (S–I–S) type grain network and thereby enhance the weak link behaviour, whereas metallic Ag addition shows a superconductor–normal metal–superconductor (S–N–S) behaviour and is, therefore, a potential candidate for large scale applications Further support to this data is obtained from ac susceptibility studies carried out at different magnetic field strengths (01–40 Oe)

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Abstract: We calculate the transport critical-current density in a granular superconductor in magnetic fields below about 5 x 10/sup -3/ T. The field dependence in this region is assumed to be controlled by intragranular or intergranular Josephson junctions. Various model calculations are fitted to transport critical-current data on bulk Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/..sqrt../sub delta/ ceramic superconductors, whose average grain size somewhat exceeds 10 ..mu..m. The results yield an average junction cross-sectional area (thickness x length) of 4--6 ..mu..m/sup 2/. If the junctions are at the grain boundaries, a London penetration depth of about 150--300 nm is inferred, consistent with other estimates. We conclude that Josephson junctions are limiting the transport critical current in these samples and that they lie at the grain boundaries. The parameters of the fit are not consistent with Josephson junctions at twinning boundaries.

165 citations

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Abstract: The effect of doping on the superconductivity in Bi/sub 2/Pb/sub 0.6/Sr/sub 2/Ca/sub 2-x/R/sub x/Cu/sub 3/O/sub /spl delta// (R=Ba,Y,V,Zn,Sn) has been investigated by resistivity, ac susceptibility measurements and x-ray diffraction (XRD) analysis. All the dopants were incorporated in the calcium site with x ranging from 0.00 to 0.10. The temperature dependence of electrical resistance and ac susceptibility measurements was studied for these samples. The nature of the temperature dependence of the resistance curves indicates the presence of a superconducting transition between grains coupled by weak links. From the XRD data, it is observed that the volume percentage of the 2223 phase decreases with the increasing doping concentration. The presence of low T/sub c/ phase is also visible in the real component (/spl chi/'), of the susceptibility data, while the imaginary component (/spl chi/"), shows a shift towards lower temperature in the intergranular coupling peak, T/sub p/, as the dopant concentration increases. All the doping elements do not enhance the T/sub c/ zero but decreases its value from /spl sim/104 K to 88 K, 53 K, 79.6 K, 58 K and 53 K for Ba, Y, V, Zn and Y respectively for x=0.10. Barium doped samples are least affected by compositional variation and the zero resistance values are higher than that of the other doped samples.

10 citations

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Abstract: The production of bulk Bi2+xSr3-yCayCu 2O8+D (Bi-2212) superconductors for fault current limiter application was developed via a partial-melting route. Aiming high Ic (critical current), which is the essential superconducting characteristic for application of this material in the construction of Fault Current Limiters (FCL), the produced blocks have predominance of Bi-2212 phase (83 wt%), which characterizes with high values of zero and onset transport critical temperature of 92K and 97.5K, respectively. A relatively low transition width, DT, from the superconducting to the normal state of 5.5K, revealed a good intergrain connectivity. Consequently, current measurements on the blocks of Bi-2212 show promising Ic values of 230A and 850A for direct and alternate current, respectively. It is expected that further increases in the Ic values will depend on the elimination of an observed amorphous phase and further reduction of amount and grain sizes of secondary phases, still present in the blocks obtained by the proposed partial-melting route. This may be achieved by a further optimization of the partial-melting processing parameters.

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Abstract: The properties of layered structures of superconducting (and non- superconducting) materials are studied, with emphasis on the related boundary- value problems. The experimental results are summarized, and theoretical interpretations are proposed for them. (T.F.H.)

1,031 citations

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TL;DR: It is concluded that Josephson junctions are limiting the transport critical current in these samples and that they lie at the grain boundaries.

Abstract: We calculate the transport critical-current density in a granular superconductor in magnetic fields below about 5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}3}$ T The field dependence in this region is assumed to be controlled by intragranular or intergranular Josephson junctions Various model calculations are fitted to transport critical-current data on bulk ${\mathrm{Y}}_{1}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ ceramic superconductors, whose average grain size somewhat exceeds 10 \ensuremath{\mu}m The results yield an average junction cross-sectional area (thickness x length) of 4-6 \ensuremath{\mu}${\mathrm{m}}^{2}$ If the junctions are at the grain boundaries, a London penetration depth of about 150-300 nm is inferred, consistent with other estimates We conclude that Josephson junctions are limiting the transport critical current in these samples and that they lie at the grain boundaries The parameters of the fit are not consistent with Josephson junctions at twinning boundaries

215 citations

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Abstract: THE critical current density across individual grain boundaries in thin films of the high-Tc superconductor YBa2Cu3O7–δ (YBCO) has been found1–4 to be inversely proportional to lattice misorientation for tilts up to ∼10°. Reports of impurity segregation5,6 at grain boundaries, and variations in the chemical stoichiometry7,8, have led to the view that deviations from the ideal composition are responsible for the depressed superconducting order parameter at the boundary. Here we present images of YBCO grain boundaries obtained by a scanning transmission electron microscope in Z-contrast mode9,10, which show that chemical segregation does not necessarily occur at these boundaries. A simple model of the strain associated with the grain-boundary dislocations provides a reasonable physical explanation of the suppressed superconductivity. The surprisingly large effect of strain implied by our model has implications beyond critical currents, for the physics and applications of any thin-film YBCO structures involving strained epitaxial layers.

188 citations

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Abstract: We calculate the transport critical-current density in a granular superconductor in magnetic fields below about 5 x 10/sup -3/ T. The field dependence in this region is assumed to be controlled by intragranular or intergranular Josephson junctions. Various model calculations are fitted to transport critical-current data on bulk Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/..sqrt../sub delta/ ceramic superconductors, whose average grain size somewhat exceeds 10 ..mu..m. The results yield an average junction cross-sectional area (thickness x length) of 4--6 ..mu..m/sup 2/. If the junctions are at the grain boundaries, a London penetration depth of about 150--300 nm is inferred, consistent with other estimates. We conclude that Josephson junctions are limiting the transport critical current in these samples and that they lie at the grain boundaries. The parameters of the fit are not consistent with Josephson junctions at twinning boundaries.

165 citations

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Abstract: The resistance of thin-film lead-copper-lead junctions has been studied with the lead in the superconducting state. The junctions will sustain a supercurrent up to a certain critical value above which a voltage appears, rising smoothly from zero as the current is increased. The effect of a magnetic field upon the critical current has demonstrated that the sandwiches behave phenomenologically as Josephson junctions. The critical current rises rapidly as the temperature is lowered, decreases exponentially with increasing thickness of copper and mcreases with increase of the mean free path of the copper. A simplified version of the de Gennes theory of the proximity effect has been used to account quantitatively for this behaviour. The experiments show that the coherence length of the paired electrons in the copper increases as the temperature decreases, implying that thermal fluctuations govern the decay of the pairs. From the value of the decay length, the interaction parameter in copper is estimated to lie between +0$\cdot$06 and +0$\cdot$14. The properties of these junctions are compared with those of junctions with insulating barriers.

159 citations