Silver-sheathed Tapes Research Articles

Explosive Consolidation of (Bi, Pb)-Sr-Ca-Cu-0 Superconductor Powders

Superconducting (2212 phase) Bi-Sr-Ca-Cu-0 (BSCCO) and BSCCO-Ag composites were explosively consolidated in silver tubing and then drawn and rolled into tapes. These silver-sheathed tapes were then subjected to repeated cycles of pressing and heat treatment, which resulted in enhanced texturing and grain growth and a subsequent increase in critical current density (Jc). The effect of Ag powder additions to the superconducting powder further increased texturing and Jc, with optimal properties occurring with 10 vol.% silver powder additions. Scanning electron microscopy indicated that grain alignment increased and grain structure became refined after the thermomechanical treatment. X-ray diffraction studies indicated that grain orientation and conversion of 2212 phase to 2223 phase are improved when explosive consolidation is introduced before the drawing step in the powder-in-tube process.

Aspects of the current understanding of the supercurrent transport in (Bi,Pb)2Sr2Ca2Cu3O10 silver-sheathed Tapes—the “railway-switch” model

The “railway-switch” model describes the superconducting current transport in (Bi,Pb)2Sr2Ca2Cu3O10 silver-sheathed tapes under the basic assumption that small-angle c axis tilt grain boundaries (“railway-switch”) constitute strong intergrain links for the supercurrent in the textured filament [B. Hensel, J.-C. Grivel, A. Jeremie, A. Perin, A. Pollini, and R. Flukiger,Physica C 205, 329 (1993)]. We give an overview of the model and some recent experimental results with the objective to identify the mechanisms that limit the critical current density. The measurements have been performed on monofilamentary “powderin-tube” samples [Jc(T < 77K, B < 0 T) < 20..30 kA/cm2] that were prepared in long lengths by rolling as the only tape-forming process. We conclude that the low intragrain critical current density jc c along the c axis (or the even lower critical current density jc t across twist boundaries or intergrowths) is the dominant limitation for the transport critical current in high-quality tapes. We discuss possible starting points for a performance improvement of the (Bi,Pb)2Sr2Ca2Cu3O10 silver-sheathed tapes for applications.

Anomalous behaviour in the critical current density of a (Bi2-xPbx)Sr2Ca2Cu3O10- delta tape below 10 K

The critical current density of a (Bi2-xPbx)Sr2Ca2Cu3O10- delta silver-sheathed tape has been measured in fields up to 15 T and at temperatures from 2 K up to Tc. Measurements were made for three different field current orientations of the tape. Below 10 K there is a dramatic anomalous rise in the critical current as the magnetic field is decreased below a value BA. The value of BA is temperature dependent but orientation independent. At 4.2 K and 2 K, BA is 1.5 T and 2 T respectively. At 10 K and above this anomalous behaviour disappears. The effect is attributed to the existence of a low-Tc ( approximately 10 K) superconducting phase at the grain boundaries in the tape and clearly demonstrates the importance of intergrain connectivity in obtaining high critical current densities in (Bi2-xPbx)Sr2Ca2Cu3O10- delta tapes.

Hot deformation effects on critical currents in (Bi,Pb) 2Sr 2Ca 2Cu 3O x silver-sheathed tapes

We have subjected fully reacted high-quality pressed silver-sheathed (Bi,Pb) 2Sr 2Ca 2Cu 3O x tapes to a hot deformation treatment at various temperatures and times. The critical current density j c of these tapes before deformation is about 34 kA/cm 2 in self-field at 77 K. Some dependence exists upon the length of the deformation treatment but the deformation-induced effects can be summarized as follows: The self-field j c at 77 K is enhanced to about 42 kA/cm 2, about a 25% increase. The field dependence of j c is weakened significantly and thus we still measure about 1000 A/cm 2 at 77 K, 0.5 T ( H⊥ tape), which is the highest values measured in tapes up to date. Thus even though these tapes do not have the highest self-field j c values they mark new standards for j c values in high magnetic fields for non-irradiated tapes. At lower measuring temperatures strongly enhanced in field j c values are also observed. The irreversibility line appears to be shifted to higher field for both orientations of the tape with respect to the magnetic field. Using transmission electron microscopy we find an enhanced density of basal plane dislocations generated by the deformation at high temperature.

Critical current density normal to the tape plane and current-transfer length of (Bi,Pb) 2Sr 2Ca 2Cu 3O 10 silver-sheathed tapes

We have measured the critical current density parallel ( j c) and normal (j c n ) to the plane of monofilamentary (Bi,Pb) 2Sr 2Ca 2Cu 3O 10 silver-sheathed tapes prepared on the laboratory scale ( l ≤ 10 m) by the powder-in-tube technique with rolling as the only tape-forming process. For the range of critical current densities along the tape j c( T = 77 K, B = 0 T) = 20…30 kA/cm 2 we obtain j c n ) = 2…3 kA/cm 2 normal to the tape plane. We have a directly measured the current-transfer length L c, i.e. the length along the tape that is necessary for the distribution of the current over the filament cross-section. As an upper limit we find L c ≤ 2 mm at T = 77 K for tapes with j c( T = 77 K, B = 0 T) > 20 kA/cm 2. Transmission electron microscopy of the polycrystalline (Bi,Pb) 2Sr 2Ca 2Cu 3O 10 filament confirms the dominant role of the frequent small-angle c-axis tilt grain boundaries (“railway switches”) for the current transport, and the observed low anisotropy ratio j c j c n ≈ 10 as well as the magnetic-field dependence j c n (B) agree well with the “railway-switch” model.

Limits to the critical transport current in superconducting (Bi,Pb)2Sr2Ca2Cu3O10 silver-sheathed tapes: The railway-switch model.

We have proposed the ``railway-switch'' model to describe the superconducting transport current in (Bi,Pb${)}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ silver-sheathed tapes. The model assumes that in the textured polycrystalline filament the frequent small-angle c-axis tilt grain boundaries (``railway switches'') constitute strong links for the supercurrent. With the objective to identify the mechanisms that limit the critical-current density in the tapes we present measurements of the transport critical current normal to the tape plane and of the current-transfer length along the tape axis. From I-V curves we obtain the longitudinal critical-current distribution and compare it to the thickness variation of the filament. The experiments have been performed on monofilamentary powder-in-tube samples prepared in long lengths by rolling as the only tape-forming process. For all investigated samples the critical-current density at T=77 K in self-field was in the range ${\mathit{j}}_{\mathit{c}}$=20--30 kA/${\mathrm{cm}}^{2}$. We conclude that the dominant limitation for the transport critical current in the tapes is imposed by the low intragrain critical-current density ${\mathit{j}}_{\mathit{c}}^{\mathit{c}}$ along the c axis (as compared to the in-plane critical-current density ${\mathit{j}}_{\mathit{c}}^{\mathit{a}\mathit{b}}$) and by the even lower critical-current density ${\mathit{j}}_{\mathit{c}}^{\mathit{t}}$ across twist boundaries or intergrowths. Possibilities for an improvement of the performance of the (Bi,Pb${)}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ silver-sheathed tapes are discussed.

Silver-sheathed YBa2Cu4O8 tapes: decomposition, characterization, and critical current enhancement

Decomposition of YBa2Cu4O8 into YBa2Cu3O7- delta +CuO in the silver-sheathed tape form has been studied. In these tapes, the excess CuO is expected to provide additional flux pinning centres and consequently to increase the critical current density beyond that of an equivalent conventional YBa2Cu3O7- delta . Silver-sheathed YBa2Cu4O8 tapes were produced by first using an oxalate solution method to coprecipitate (submicrometre to nanometre size) precursor powder that was then drawn into tapes by a powder in tube technique. From a series of investigations of the parameters for decomposition of YBa2Cu4O8, the optimal time and temperature for processing these tapes were found to be 25 min and 915 degrees C, with oxygenation for 48 h at 450 degrees C. This procedure yielded the best Jc of 2.2*104 A cm-2 in self-field at 5 K. This critical current value is an enhancement of two orders of magnitude from the value obtained in as-drawn tape.

Minimum critical current density of 10 5 A/cm 2 at 77 K in the thin layer of Bi 1.8Pb 0.4Sr 2.0Ca 2.2Cu 3.0O y superconductor near the Ag in Ag-sheathed tapes

We report for the first time a minimum critical current density in a BSCCO/Ag polycrystalline sample of 1.1. × 5 A/cm 2 at 77 K in a self-field. For practical applications of (Bi, Pb) 2Sr 2Ca 2Cu 3O y (BSCCO-2223) high-temperature superconducting material, the critical current density ( J c) is a limiting factor. A requirement for a J c of 10 5 A/cm 2 in long length wires and tapes, at 77 K and high magnetic field, has been established. The transport J c were measured on samples that retained a 10 μm thickness of the superconducting core after they were removed from rolled tapes. The thin superconducting layer region next to the silver sheath was shown to be the actual current carrying region in the tape. These results confirm the presence of a high critical current density region adjacent to the silver-sheathed tapes, and provide a basis for further improvements in J c. Regions of silver were also found to be present in the superconducting core.

Critical-current characteristics of c-axis-oriented (Bi,Pb)2Sr2Ca2Cu3Ox silver-sheathed tapes from 10 K to Tc0.

The dependence of the transport critical-current density on an applied magnetic field and temperature of a high-${\mathit{J}}_{\mathit{c}}$ (Bi,Pb${)}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$ silver-sheathed tape has been measured in the range from 10 to 110 K. It is found that the transport critical current follows the relation ${\mathit{J}}_{\mathit{c}}$=${\mathit{J}}_{\mathit{c}0}$(1-aT+${\mathit{bT}}^{2}$). For T40 K, b=0, while for T&gt;40 K, b&gt;0. ${\mathit{J}}_{\mathit{c}}$ and the applied field follow a power-law relation ${\mathit{J}}_{\mathit{c}}$\ensuremath{\propto}${\mathit{B}}^{\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\alpha}}}$ (\ensuremath{\alpha}\ensuremath{\approxeq}0.3) for T40 K and an exponential-law relation ${\mathit{J}}_{\mathit{c}}$\ensuremath{\propto}exp(-\ensuremath{\beta}B) for T&gt;65 K. For 40 KT65 K, there is a transition field for ${\mathit{J}}_{\mathit{c}}$(B,T). This represents a transition of the above two relations for ${\mathit{J}}_{\mathit{c}}$(B). The experimental results can be explained within the framework of the flux-creep model in terms of different potential-barrier forms. The limitation of ${\mathit{J}}_{\mathit{c}}$ from 10 K to ${\mathit{T}}_{\mathit{c}0}$ seems to be attributed to the flux creep for high-${\mathit{J}}_{\mathit{c}}$ textured Bi(2223) Ag-sheathed tapes. The flux lattice seems to experience a transition that is similar to the flux-lattice melting in this material in both applied-magnetic-field orientations near 40 K.

Electrical and magnetic properties of hot extruded Bi-2223/Ag concentric tapes

In this work we report on the anisotropic physical properties of silver-sheathed Bi-2223 tapes fabricated by means of hot extrusion and repeated pressing and sintering processes. The obtained Bi-2223/Ag short tapes, having critical current densitiesJ c of 20–30 kA/cm2 at 77 K, 0 T, were measured in external magnetic fields up to 0.5T applied in two different orientations (i.e. µ0 H ∥ (a,b)-planes and µ0 H ⊥ (a,b)-planes). The magnetic characterizations were performed in a wide range of temperatures and magnetic fields to study the first magnetization curve of tapes evaluating the lower critical fields µ0 H c1 ∥a b and µ0 H c1 ⊥a b and their dependences on temperature. TheJ c values at different fields in the temperature range 4.6–90 K, calculated from the magnetization data by the critical state model, are also presented.

Zone melting-refining in Bi-Pb-Sr-Ca-Cu-O superconductor prepared through the sol-gel method

A zone melting-refining technique, with a horizontal sample-holding pattern for incorporating grain orientation in sintered bismuth-based superconductor (BPSCCO) is presented. Acrylate precursor-derived BPSCCO powder was uniaxially compacted into strips of 60 mm×10 mm ×2mm and sintered at 845°C for 20 h to obtain 90% density. These strips were zone melted in a three-zone furnace which had an independent temperature control system for each zone. The grain orientation in the sample was studied by scanning electron microscopy and X-ray diffraction. In the case of a BPSCCO-Ag (5%) composite, the silver was seen to flow along the intergrannular boundaries of the BPSCCO grains. The present technique is an easy and fast method to introduce orientation of grains with large aspect ratio into sintered BPSCCO or Ag-BPSCCO composite under partial melting-refining conditions. The possibility of extending the present technique to silver-sheathed wire and tapes is also indicated.

Strain dependence of the critical current density of Bi-2223 multifilament tape conductors

Measurements of the critical current density at 77 K as a function of bend strain (0–2%) are reported for Bi-2223 silver-sheathed tapes. The irreversible strain limit, ϵ irr , for the onset of strain damage to the superconductor core, increases from 0.2–0.3% for monofilament tapes (∽75 μm filament thickness) to values up to 1.5% for 49-filament tapes (∽5 μm filament thickness).

The effect of potential contact position on AC loss measurements in superconducting BSCCO tape

AC losses in (Bi, Pb) 2Sr 2Ca 2Cu 3O x silver-sheathed tape were measured as a function of frequency and amplitude of a sinusoidall y varying transport current. Experimental results show that energy losses are purely hysteretic in nature in the range of applied frequencies for transport currents below the critical value I c. The lossy component of the measured voltage is very dependent on the position of the potential taps placed onto the surface of the tape. This can be explained using the theory of Norris [J. Phys. D 3 (1970) 489] for ellipsoidal conductors. It means that neither voltage gives an accurate measurement of the loss. The error is least for the side contacts but still varies from 40% at I c to a factor of four at 0.1 I c.

AC losses in (Bi,Pb) 2Sr 2Ca 2Cu 3O x tapes

A double peak structure is observed in the AC losses of (Bi,Pb) 2Sr 2Ca 2Cu 3O x silver-sheathed tapes using a torsion-pendulum oscillator. The low-temperature peak is associated to the intragrain flux expulsion, while the high-temperature peak results from a macroscopic current path around the whole sample due to a well-coupled fraction of the grains. The flux pinning by the dislocations forming small-angle grain boundaries is suggested to control the transport current.

Preparation and properties of highJc Tl(2223) Ag-sheathed tape

The techniques of preparation of silver-sheathed tape of the TlBaCaCuO superconductor with high Jc ( = 1.5 × 104 A/cm2, 77 K, 0 T) is reported. The experimental results show that powder packing densities and annealing temperature have important influence on Jc. The character of Jc under applied magnetic field is analyzed.

Effect of processing parameters on superconducting properties of Ag-sheathed Bi-2223 tape

Silver-sheathed tapes were fabricated using a Bi-2223 system. The tapes were processed to their final thickness by direct rolling as well as by repeated rolling with intermediate heat treatment. The rolling reduction rate was also varied to find its effect on the final properties. The critical current J c measured at 77 K could be enhanced to 12 500 A cm −2 for repeatedly rolled tapes from 2770 A cm −2 obtained in directly rolled tapes processed under identical conditions. In repeatedly rolled tapes the J c- B variations were found to be much lower in comparison to the directly rolled tapes. Texture evaluation using the angular dependence of the magnetoresistance showed the a- b plane orientation to be along the rolling direction.

Further evidence that the critical current density of (Bi, Pb) 2Sr 2Ca 2Cu 3O x silver-sheathed tapes is controlled by residual layers of (Bi, Pb) 2Sr 2CaCu 2O y at (001) twist boundaries

Field-dependent AC susceptibility measurements are correlated with the transport critical current density ( J ct) and microstructure for some 20 Ag-clad (Bi, Pb) 2Sr 2Ca 2Cu 3O x (2223) tapes having J ct (0 T, 77 K) values ranging from 0–20 000 A/cm 2. Fields of 0.1-1 mT induced a double transition in the real component of the susceptibility of all samples. The field dependence of the lower temperature transition ( T' ( H)) exhibited a marked kink at 80 K and was always very sensitive to the applied field above 80 K, indicating an electromagnetically granular behavior. High-resolution transmission electron microscopy often revealed (Bi, Pb) 2Sr 2CaCu 2O x (2212 phase) intergrowths at the numerous (001) twist grain boundaries. Samples with a high J ct (0 T, 77 K) had only occasional half layers of 2212 at the (001) boundaries and required correspondingly higher fields to induce a kink in the T'( H) characteristic. The universal appearance of a kink at 80 K in all samples and the strong correlation of the field required to produce the kink to the zero-field, 77 K transport critical current density strongly suggests that c-axis transport across (001) twist boundaries within grain colonies controls the critical current density, even in relatively high J c BSCCO-2223 tapes.

Mechanically induced flux pinning in (Bi,Pb) 2Sr 2Ca 2Cu 3O x tapes

It is shown that mechanical deformation enhances intragrain flux pinning in silver-sheathed tapes of the (Bi, Pb) 2Sr 2Ca 2Cu 3O x (2223) material. Tapes with zero transport critical current density ( J ct) - but nonzero magnetic hysteresis - were produced with prereacted 2223 powders. Comparisons of the hysteresis in tapes directly after a heat treatment and after a subsequent pressing step show a clear increase of the hysteresis, almost a factor of three at 20 K and 3 T, corresponding to a five-fold increase in J c. The J c values can be lowered again by an additional heat treatment. Bent platelets can be observed in the microstructure after, but not before, pressing, suggesting that dislocations that could have been introduced in the pressing step may act as pinning centers.

Texture relationships and interface structure in Ag-sheathed Bi(Pb)-Sr-Ca-Cu-O superconducting tapes

We have studied the phase composition, alignment, and bonding at the Ag/Bi-Sr-Ca-Cu-O (BSCCO) interface for silver-sheathed tapes of both the 2212 and 2223 compositions which have been processed under different conditions. Strong texturing of the (001) planes of the BSCCO phases parallel to the Ag interface was seen, regardless of the Ag orientation. Short facets of non-(001) BSCCO planes accommodate local misalignments between the Ag and the (001) planes of the BSCCO phases. Whether the 2212 composites are processed by complete melting or by sintering below the melting point, there is always a residual half-layer of 2201 at the interface. By contrast, 2223 composites exhibit 2223 layers right up to the interface without any transition phases. Both compounds bond strongly to Ag and cracks, when present, occur preferentially away from the interface.

Phase composition and local grain alignment at the Ag/superconductor interface in Ag-sheathed Bi-Sr-Ca-Cu-O tapes

Texturing of the superconductor phase and proximity to a sourse of silver appear to be crucial to the development of high critical current density tapes of the Bi-Sr-Ca-Cu-O (BSCCO) high-temperature superconductors, yet relatively little is understood about the mechanism(s) by which the [001] texture develops and the complex role that the silver sheath material plays. We have studied the phase composition, alignment, and bonding at the Ag/BSCCO interface for silver-sheathed tapes of both the 2212 and 2223 phases that have been processed under fundamentally different conditions. High-resolution TEM imaging revealed details of the interface that give insight into these important questions.Ag-sheathed BSCCO tapes with cation compositions Bi2Sr2CaCu2Ox (2212) and Bi1.8Pb0.4Sr2.0Ca2.2Cu3Ox (2223) were prepared by standard powder-in-tube methods. 2212 sample A was heat-treated at 920°C for 15 minutes to melt the superconductor core, then cooled at 240°C/h from 920°C to 840°C and annealed at 840°C for 70 hours.