Single Filament Tapes Research Articles

Density Enhancement for the Fabrication of Bi-2212 Superconducting Tapes

Abstract Bi2.1Sr1.96CaCu2.0O8+δ (Bi-2212) precursor powders were synthesized with co-precipitation process, and Bi-2212 single filament tapes were fabricated with powder-in-tube (PIT) technique. Based on SEM observation, the primary grain size of Bi-2212 precursor powders was 3∼5 μm. However, after sintering, Bi-2212 grains clustered together and the particle size distribution became quite wide, from 10μm to over 180 μm. By dividing the particles into four groups with different average particle sizes, the influences of average particle size on the filament density, microstructures and current capacity of final Bi-2212 tapes were investigated. Results show that with the decrease of particle size, the filament density increases over the entire cold-working process and heat treatment process. Therefore, the intergrain connection is enhanced, which leads to the obvious increase of current capacity.

Improving superconducting properties of Bi-2212 tapes by Ca nonstoichiometry

Single filament tapes with different Ca contents Bi2.1Sr1.96CaxCu2.0O8+δ (x = 0.90, 0.95, 1.00, and 1.05) were prepared by powder in tube (PIT) process. The introduction of Ca nonstoichiometry caused systematic changes in the phase compositions of the final tapes, thus changing the chemical compositions of Bi-2212 phase. The increase of critical temperature Tc from 81.2 to 84.2 K can be attributed to the carrier concentration change. The critical current density, Jc and flux pinning properties were measured and evaluated at 4.2 K and high field (0–20 T) with the variation of Ca content. Obvious improvements of Jc were obtained with the Ca content of x = 0.95 and 1.05 under perpendicular field, which implied that the chemical compositions changes of Ca could effectively lead to the formation of lattice defects, thus enhancing the flux pinning properties on the superconducting layers.

Influence of Cu Content in Precursor Powders on the Phase Evolution and Superconducting Properties of Bi-2212 Superconductors

Precursor powders of Bi 2.1 Sr 1.96 CaCu x O 8+δ (Bi-2212) high temperature superconductors with different nominal Cu ratios of x =2.0, 2.1, 2.2, 2.4 were fabricated by a modified co-precipitation process. The influences of Cu ratios on the phase evolution process during sintering were investigated. Bi-2212 thick films and single filament tapes were fabricated with dip-coating process and powder in tube process, respectively. Obvious influences of Cu ratios on the phase compositions, microstructures and superconducting properties were discussed. Results show that the initial Cu ratio can affect the thermodynamic properties of Bi-2212 phase, thus changing phase evolution process. With the increase of Cu ratio, the phase content of Bi-2201 decreases, while the content of AEC phase increases. The maximum critical current density is obtained in the x =2.2 film and tape simultaneously due to the proper phase composition and better texture structures.

Doping effects of ZrO2 nanoparticles on the superconducting properties of Bi-2212 tapes

Ag sheathed Bi-2212 single filament tapes with different content of ZrO2 nanoparticles addition were fabricated with powder-in-tube process. X-ray diffractions were performed to examine the lattice structure and phase composition changes of these tapes. With the increase of ZrO2 content, the lattice parameter c decreased, suggesting the change of chemical composition in Bi-2212 phase. Detectable Zr-containing particles can be observed in the tape with ZrO2 content of 3.0 wt% in both XRD patterns and SEM images due to the saturation of Zr4+ irons in Bi-2212 matrix. The thermopower values at room temperature were measured to estimate the carrier concentrations. Attributed to the doping effect of Zr4+, the carrier concentration decreased due to the introduction of excess electrons. Meanwhile, the resistivity measured at room temperature increased and the carrier mobility decreased, suggesting the enhanced carrier scattering mechanism. Due to the change of superconducting properties with ZrO2 doping, the current capacity of Bi-2212 tapes decreased. However, the (Ca, Sr)ZrO3 precipitations with proper size, which can work as effective pinning centers enhanced the flux pinning properties of Bi-2212 tapes. Thus the tape with ZrO2 content of 3.0 wt% regained the high current capacity under magnetic field and a higher Jc can be expected under the magnetic field higher than 25 T.

Optimization of Bi-2212 high temperature superconductors by potassium substitution

Polycrystalline bulks of Bi2Sr2Ca1−xKxCu2.0O8+δ (Bi-2212) with x = 0, 0.05, 0.10 and 0.15 were fabricated by the spark plasma sintering technique. The influences of K doping on the microstructures, electronic structures, as well as the related superconducting properties, were systematically investigated. The XRD analyses confirmed that K+ ions have successfully substituted into the matrix of Bi-2212, and lead to a systematical change of lattice parameters. Due to the change of thermodynamic properties, bulks with higher density, larger grain size and better texture structures were obtained after doping. Therefore, ac susceptibility measurement revealed the optimization of intergrain connections, which lead to the optimization of both self- and in-field critical current density, Jc of this system. The optimization of microstructures also caused the enhancement of surface pinning. Based on the enhancements of both intergrain connections and flux pinning properties, an obvious improvement of critical current density was obtained with the optimal doping content of K = 0.05. Meanwhile, Bi-2212 single filament tapes with K doping content of 0 and 0.05 were also fabricated by the powder-in-tube process. The XRD patterns also proved the successful doping of K ions in the Bi-2212 matrix. The critical current density Jc, measured by the transport method under the magnetic field from 0 to 20 T at 4.2 K, proved the effectiveness of K doping on the enhancement of flux pinning properties of Bi-2212.

Optimization of Current Carrying Capacity in Bi-2212 High Temperature Superconductors

Bubbles were considered to be a serious problem happened during the partial melting process of Bi-2212 superconducting wires and tapes, which can interrupt the textural alignment of Bi-2212 grains, block the current transport in single or several filaments, and even cause great damage of the entire wire. Therefore, it is necessary to suppress the formation of bubbles by optimization the traditional powder in tube process. In order to decrease the volume of absorbed H2O and CO2, an extra milling process was adopted on the precursor powders in oxygen atmosphere, as well as the filling process of precursor powders into Ag sheath. The influences of oxygen milling process on the lattice parameter, microstructure, phase evolution mechanism and transport properties were systematically analyzed. Due to the enhanced texture structures, optimized critical current was achieved in both single filament tapes and multi-filament wires. Further optimization of the modified powder in tube process is on the way in order to further improve the transport properties of Bi-2212 wires for practical applications.

Properties of ${\rm MgB}_{2}$ Tapes Prepared by Using MA In-Ex Situ Powder

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <formula formulatype="inline"><tex Notation="TeX">${\rm MgB}_{2}$</tex> </formula> single-filament tapes and wires with Monel/Nb sheath were produced by the Powder in Tube (PIT)—technique using MA <emphasis emphasistype="boldital">in-ex situ</emphasis> powder. This powder prepared by mechanical alloying (MA) of Mg and B was finally reacted to <formula formulatype="inline"><tex Notation="TeX">${\rm MgB}_{2}$</tex></formula> at 700<formula formulatype="inline"><tex Notation="TeX">$^{\circ}{\rm C}$</tex></formula>. The amount of <formula formulatype="inline"><tex Notation="TeX">${\rm MgB}_{2}$</tex></formula> after annealing was about 91 wt%. Additionally 9 wt% MgO secondary phase was found. The primary crystallite size was about 25 nm. </para>

Manufacture of (Bi,Pb)2Sr2Ca2Cu3O10-based tapes with a composite sheath

(Bi,Pb)2Sr2Ca2Cu3O10-based single-filament tapes were prepared using a pure Ag protective sheath in contact with the ceramic core and an external sheath consisting of Ni. The influence of the composite sheath geometry on the (Bi,Pb)2Sr2Ca2Cu3O10 phase formation kinetics was studied by in-situ synchrotron x-ray diffraction in a 8.5% O2 – 91.5% N2 gas mixture and was found to depend strongly on the sheath architecture as a consequence of differences in the oxygen supply between the ceramic core and the outer atmosphere. Owing to the efficient protection of the pure Ag layer against Ni diffusion, the critical temperature of the (Bi,Pb)2Sr2Ca2Cu3O10 phase is unaffected. Critical current densities as high as 35 kA/cm2 (77K, self field) were obtained in a single heat treatment when a slow cooling rate was used. The ceramic density was significantly improved in comparison with that of tapes prepared using only pure Ag as sheath material and heat treated once. This effect can be attributed to the higher mechanical strength of the composite sheath at elevated temperatures, which prevents the development of an otherwise significant porosity during the growth of the superconducting crystallites.

Effect of sintering temperature on phase composition and J/sub c/ of Ag-sheathed Bi-2223 single and multifilamentary tapes

The sintering temperature window of Ag sheathed Bi-2223 tapes was studied by a temperature gradient method for fabrication of tapes. The I/sub c/ of the tapes was found to be very sensitive to the sintering temperature. For the single filament tapes, the I/sub c/ reaches the maximum value at a temperature higher than that at which the 2212 percentage reaches the minimum. Around 6.5% of 2212 still exists in the best single filamentary tapes, while just a half of this 2212 fraction (about 3.5%) remains in the best 19 filamentary tapes. The J/sub c/ was found to be more sensitive to Bi-2212 phase content in the multifilamentary tapes than that in the single core tapes. A J/sub c/ of 4.5/spl times/10/sup 4/ A/cm/sup 2/ (J/sub c/ 1/spl times/10/sup 4/ A/cm/sup 2/) for single core tapes and of 3.3/spl times/10/sup 4/ A/cm/sup 2/ (J/sub c/ 0.65/spl times/10/sup 4/ A/cm/sup 2/) for 19 core multifilament tapes were achieved reproducibly with sintering time of 100 hours and one intermediate pressing. 70% of maximum I/sub c/ can be retained within 4 degrees sintering temperature window for both single and 19 filamentary tapes.

Comparison of Bi-system 2223 and 2212 thick superconducting tapes: Grain alignment, current density, and strain effects

Bi-Sr-Ca-Cu-O (2212) and Bi-Pb-Sr-Ca-Cu-O (2223) superconductors were fabricated into silver-sheathed tapes by cold rolling and sintering. The resulting samples are compared in terms of microstructural alignment, critical current density, and strain dependence. Although both samples show good grain alignment, that of the 2212 phase is superior due to partial melting during heat treatment. This microstructural superiority is reflected in superior Jc as well. The single-filament and multifilament 2212 tapes have Jc ≳ 5 × 104 A/cm2 and Jc ≳ 1.5 × 104 A/cm2 at 20 T, respectively. Each material shows a similar dependence of Jc upon strain, with the 2223 phase exhibiting slightly more degradation. Furthermore, multifilamentary tapes were less affected by strain than single-filament tapes. A multifilamentary tape at 4.2 K, 0.5 T showed only a weak dependence upon strain, indicating that the strain dependence of the Bi system at low temperature may not be severe.

Fabrication, current density and strain dependence of sintered, Ag-sheathed BiSrCaCuO (2212) single filament and multifilamentary tape superconductors

Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/ (2212) superconductors have been fabricated into silver (Ag)-sheathed single-filament, multifilament, and wire-in-tube (WIT) tapes. Calcining, annealing and sintering conditions have been studied to improve the critical current density J/sub c/(B, 4.2 K) at high magnetic field (B). The optimization is discussed, and resulting properties are related to the microstructure of the superconductor. Superconductor J/sub c/ approximately 1-5*10/sup 4/ A/cm/sup 2/ has been obtained in fields >20 T at 4.2 K in both single and multifilamentary tapes. The dependence of J/sub c/ upon mechanical strain ( in ) has been investigated. No prestraining of the superconductor occurred in any of the samples, most likely due to yielding of the Ag sheath. Although strain resistance was good in all samples, the WIT tapes were significantly more resistant than the single-filament tapes. For the single-filament tapes, in /sub irr/ approximately 0.24%, while for the WIT tapes, in /sub irr/ approximately 0.36%, where in /sub irr/ is the irreversible strain limit.