Microstructural and superconducting properties of Yb-substituted (Bi,Pb)-2212 superconductor sintered at different temperatures

We have examined the effect of cooling rates on oxygen content of Bi-2223 phase samples with and without silver sheating. Two sets of samples with and without silver sheating were annealed under identical conditions and cooled with rates of 10 °C/h, 25 °C/h, 50 °C/h, 75 °C/h, and 100 °C/h. XRD examination of the samples showed that a high percentage of Bi-2223 was obtained. Microstructure examinations were performed by scanning electron microscopy. Resistive and magnetic transitions of the samples were studied. All the reported data were discussed and related.

Effect of cooling rates on bare bulk and silver wrapped pellets of Bi-2223 superconductor

Transport current density of Ag-sheathed superconductor tapes using Bi-Sr-Ca-Cu-O powders prepared by the co-precipitation method

Dense Bi2223 superconductor discs with a highly oriented structure were prepared by the sinter-forging method. A detailed investigation of the processing parameters has been carried out on the phase assemblage, the particle size of the calcined precursor powder and the sinter-forging time and temperature. A high transport critical current density, Jc, was obtained from a starting powder rich in Bi2212 and secondary phases. Using fine powder, a high degree of orientation, larger than those of multifilamentary tapes, and excellent intergrain connection were observed. The optimal sinter-forging temperature was found to be in the range of 845–850 °C and the transport critical current densities reached values of 12 700 A cm−2 at 77 K in self-field. The combination of textural and microstructural investigations testify that a nucleation-growth mechanism from the Bi2212 particles is responsible for the Bi2223 phase formation, rather than an intercalation process.

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Sinter-forging of strongly textured Bi2223 discs with large Jcs: nucleation and growth of Bi2223 from Bi2212 crystallites

The melting and solidification behavior of Bi2Sr2Ca2Cu3 O10 (Bi-2223) precursors has been studied. Nominal compositions corresponding to excess of liquid, Ca2CuO3 and CuO have been investigated. Each sample was made by packing a precursor powder into a silver crucible, in order to approximately simulate the situation found in 2223 silver-sheathed tapes. The samples were partially melted and then slow-cooled, being quenched from different temperatures and analyzed through X-ray diffraction (XRD) and scanning electron microscopy (SEM/EDS). The precursors decomposed peritectically during melting, forming liquid and solid phases. Very long plates with compositions falling in the vicinity of the 2223 primary phase field formed upon slow-cooling. The 2223 phase may have been formed and the results suggest that long grains of this phase might be obtained by melting and crystallization if the exact peritectic region and the optimum processing conditions are found.

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Phase relations study on the melting and crystallization regions of the Bi-2223 high temperature superconductor

In this work, an alternative method denominated as continuous cooling sintering is proposed, for Bi-2223/Ag tape processing. By this method, tapes are heated to a temperature above the single 2223 phase region then sintered over the whole Bi-2223 phase formation range by continuous cooling at a low rate, without holding at any fixed temperature. The advantages of this method are (1) reducing the total sintering time and (2) increasing dramatically the Bi-2223/Ag formation range. The optimum processing temperature window extends from 815°C to 855°C with variation of critical current density I c remaining within 15%. This advantage facilitates the preparation of long length tapes that usually occupy considerable space in furnaces and, therefore, inevitably are inhomogeneously heated during the sintering process.

Continuous cooling sintering: a new method for Bi-2223/Ag tape processing

In this work, we successfully prepared Bi-2223/Ag tapes by sintering at temperatures above the Bi-2223 decomposition temperature. The continuous cooling sintering (CCS) process, which was proposed in our previous work, was used as the sintering regime. Using this method, Bi-2223/Ag tapes are heated to temperatures above the 2223 stability region followed by cooling at a low rate without lengthy annealing at any fixed temperature. The results indicate that, upon cooling, the liquid phase and other secondary phases produced at high temperatures react forming the Bi-2223 phase. The tapes exhibit better grain connectivity and less porosity in comparison with those prepared by the common method. The critical current density Jc and Jc-H behaviour are improved.

Bi-2223/Ag tapes prepared at temperatures above the Bi-2223 decomposition temperature

In this work, Bi-2223/Ag tapes were prepared by the continuous cooling sintering (CCS) method. The results indicate that this method can dramatically extend the optimized sintering temperature window as much as 40°C. Moreover, the total sintering time can be greatly reduced. The J c of the tapes sintered for total 40 h (including first and second sintering cycles) is similar to those sintering for total 97 h without significant effect on the J c – H behavior. The mechanisms of the increase in sintering processing window and the decrease in total sintering time under the CCS process are extensively discussed.

Enhancement of heat-treatment temperature window and reduction of sintering time of Bi-2223/Ag tapes by continuous cooling sintering

Influence of processing parameters of partial-melting process on phase assemblage, microstructure and superconducting properties of Bi-2212 bulk material

The effects of the precursor powder on Bi-2223 phase formation and the superconducting properties of Ag sheathed Bi-2223 tapes were studied. Three precursors were prepared. One was made by the “one-powder” method and the other two were prepared by the “two-powder” method, in which two compounds were mixed. The results showed that not only the phase assemblage, but also the microstructure of a precursor could influence the 2223 formation and the final morphology of the tapes.

S.K. Xia

Papers

Continuous cooling sintering: a new method for Bi-2223/Ag tape processing

Bi-2223/Ag tapes prepared at temperatures above the Bi-2223 decomposition temperature

Enhancement of heat-treatment temperature window and reduction of sintering time of Bi-2223/Ag tapes by continuous cooling sintering

Influence of processing parameters of partial-melting process on phase assemblage, microstructure and superconducting properties of Bi-2212 bulk material

Preparation of Bi-2223/Ag tapes by controlling precursor powders